Mineralogical and geochemical analyses of Antarctic lake ...current sediment study to facilitate identiﬁcation of biologic activity and geochemical cycles in the lake ecosystem

PII S0016-7037(00)00651-2

Mineralogical and geochemical analyses of Antarctic lake sediments: A study ofreflectance and Mossbauer spectroscopy and C, N, and S isotopes with applications

for remote sensing on Mars

JANICE L. BISHOP,1,* A NDRE LOUGEAR,2 JASON NEWTON,3 PETER T. DORAN,4 HEINZ FROESCHL,5 ALFRED X. TRAUTWEIN,2

WILFRIED KORNER,6 and CHRISTIAN KOEBERL6

1SETI Institute/NASA-Ames Research Center, MS-239-4, Moffett Field, CA 94035, USA2Institute of Physics, Medical University of Luebeck, Ratzeburger Allee 160, D-23538 Luebeck, Germany

3Earth and Marine Sciences C514, University of California, Santa Cruz, CA 95064, USA4Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA

5Austrian Research Centers Seibersdorf, Chemical Analytics, Arsenal, Object 214, Faradaygasse 3, A-1030 Vienna, Austria6Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

(Received March7, 2000;accepted in revised form April4, 2001)

Abstract—We analyzed lake-bottom sediments from the Dry Valleys region of Antarctica to study theinfluence of water chemistry on the mineralogy and geochemistry of these sediments, as well as to evaluatetechniques for remote spectral identification of potential biomarker minerals on Mars. Lakes from the DryValleys region of Antarctica have been investigated as possible analogs for extinct lake environments on earlyMars. Sediment cores were collected in the present study from perennially ice-covered Lake Hoare in theTaylor Valley. These sediments were taken from a core in an oxic region of the lake and another core in ananoxic zone. Differences between the two cores were observed in the sediment color, Fe(II)/Fe(III) ratio, thepresence of pyrite, the abundance of Fe, S, and some trace elements, and the C, N, and S isotope fractionationpatterns. The results of visible-infrared reflectance spectroscopy (0.3–25mm), Mossbauer spectroscopy (77and 4 K), and X-ray diffraction are combined to determine the mineralogy and composition of these samples.The sediments are dominated by plagioclase, K-feldspar, quartz, and pyroxene. Algal mats grow on the bottomof the lake and organic material has been found throughout the cores. Calcite is abundant in some layers ofthe sediment core from the shallow, oxic region, and pyrite is abundant in the upper sediment layers of thecore from the deep, anoxic region of Lake Hoare. Analysis of the spectroscopic features due to organics andcarbonates with respect to the abundance of organic C and carbonate contents was performed in order to selectoptimal spectral bands for remote identification of these components in planetary regoliths. Carbonate bandsnear 4 and 6.8mm (;2500 and 1500 cm21) were detected for carbonate abundances as low as 0.1 wt% CO2.Organic features at 3.38, 3.42, and 3.51mm (2960, 2925, and 2850 cm21) were detected for organic Cabundances as low as 0.06 wt% C. Thed13C andd15N trends show a more complex organic history for theanoxic region sediments than for the oxic region sediments. The biogenic pyrite found in the core from theanoxic zone is associated with depletedd34S values and high organic C levels and could be used as a potentialbiomarker mineral for paleolakes on Mars.Copyright © 2001 Elsevier Science Ltd

1. INTRODUCTION

The McMurdo Dry Valleys in Antarctica provide a uniqueopportunity for studying mineral formation and alteration pro-cesses in a closed and relatively pristine ecosystem. Water andsediments from the Antarctic Dry Valleys region have beenstudied previously to gain information about relationships be-tween microorganisms and water chemistry (Green et al.,1986b, 1988; Craig et al., 1992; Wharton et al., 1993), paleo-limnology (Doran et al., 1994a), chemical alteration, and sed-imentation processes in perennially ice-covered lakes and colddeserts (Gibson et al., 1983; Nedell et al., 1987; Squyres et al.,1991) and spectroscopic detection of minerals and organics innatural sediments (Bishop et al., 1996). Lake Hoare is partic-ularly interesting for biogeochemical study because of the algalmats at the bottom of the lake and the oxygen-rich (oxic) zoneabove;27 m and the oxygen-poor (anoxic) zone below;27 m(Wharton et al., 1993; Andersen et al., 1998). Differences in

major element and trace element composition havebeen ob-served in the water and sediments from these oxic andanoxic regions (Green et al., 1986a,b, 1988; Bishop et al.,1996).

Sediment cores from Lake Hoare are analyzed here to buildon the results of previous studies (Green et al., 1986a,b, 1988;Bishop et al., 1996). Because of the differences in water chem-istry and sediment composition found in these previous studiesfor regions of the lake above and below the oxic–anoxic bound-ary, the current study focused on the differences in chemistryand mineralogy between sediments from these two regions.Visible to infrared reflectance spectra were measured of thebulk and,125-mm size fractions of these sediments. Chemicaland mineralogical analyses were performed on the homoge-nized ,125-mm material to ensure consistency among mea-surements. A core particularly enriched in carbonate and or-ganic material from the oxic region of the lake was selected forstudy here to evaluate the spectral features for these speciesacross a range of compositions. The current study includesMossbauer spectroscopy to better characterize the iron-bearingminerals and C, N, and S isotopic ratios to evaluate the influ-

*Author to whom correspondence should be addressed ([email protected]).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 65, No. 17, pp. 2875–2897, 2001Copyright © 2001 Elsevier Science LtdPrinted in the USA. All rights reserved

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2875

ence of the water chemistry and biologic activity on the sedi-ments.

Low-temperature Mo¨ssbauer spectroscopy has been effec-tive in measuring Fe(II)/Fe(III) ratios and identifying iron-bearing minerals in sediment cores (Drodt et al., 1997, 1998;Konig et al., 1997) and in hydrothermal vent systems (Wade etal., 1999). This technique is applied here to look for differencesin the mineralogy and Fe oxidation state in sediments from theoxic and anoxic zones of the lake.

1.1. Lake Hoare, Taylor Valley, Antarctica

The McMurdo Dry Valleys along the western coast of theRoss Sea in Antarctica remain relatively ice-free because thelocal ablation rates exceed the annual snowfall. These dryvalleys and their ice-covered lakes have been the focus ofprevious studies as analogs for dry valleys on Mars (Gibson etal., 1983; Agresti et al., 1986; Squyres et al., 1991). An inter-esting feature of Lake Hoare is the presence of microbial matsand organic material in the lake-bottom sediments (Nedell etal., 1987). Previous radiocarbon dates for Lake Hoare sedi-ments indicate a sedimentation rate of 0.15 mm per year forsediments near dive hole 2 (oxic region) (Doran et al., 1999).

Green et al. (1988) studied the geochemistry of Lake Hoareand noted trends in cation and anion concentrations and se-lected major and minor elements in the lake water. Bishop et al.(1996) analyzed the geochemistry, mineralogy, and reflectance

spectra of samples from three sediment cores from Lake Hoareand found distinct compositional differences between sedi-ments collected under the oxic and anoxic zones of the lake.Higher calcite abundances were found in the sediments fromthe most shallow dive hole (DH-2, 15-m depth) (Bishop et al.,1996), which is consistent with the findings of Green et al.(1988) that the lake water from the surface down to;20 m indepth is supersaturated in calcite. Related trends in dissolvedCO2 and O2 have been observed in the Lake Hoare watercolumn: the decreases in O2 level and the increases in CO2

level with depth are thought to be due to sedimentation oforganic material (Andersen et al., 1998).

1.2. Importance of Sediment Analysis for Exobiology

Characterizing the relationships between microorganisms,water chemistry, and sediment composition in the AntarcticDry Valleys will provide important information for interpreta-tion of sediment processes on Mars. The vertical nutrient pro-files observed in lake water and sediments are directly relatedto microbial activities (Nealson, 1997). Therefore, biologicactivity may greatly influence the chemical environment ofsediments. Isotopic trends for C, N, and S are included in thecurrent sediment study to facilitate identification of biologicactivity and geochemical cycles in the lake ecosystem. Methanewas found in the lake water just above the surface sediments in

Fig. 1. Location of Lake Hoare in the Dry Valleys region of Antarctica. The sample cores were collected from the lakebottom at dive holes DH-2 and DH-4 in Lake Hoare. (Upper right inset) Study location with respect to the Antarcticcontinent. (Upper left inset) Taylor Valley, which contains Lake Hoare.

2876 J. L. Bishop et al.

the anoxic zone, suggesting the presence of methane-producingbacteria there (Andersen et al., 1998).

Previous studies have shown that isotope ratios for C and Nin lake water can provide information about biologic vs. abio-logic activity in the ice-covered lakes of the Antarctic DryValleys region (Doran et al., 1998; Lawrence and Hendy, 1989;Wharton et al., 1993). Analysis ofd13C for carbonate andorganic separates of Lake Hoare sediments in a study by Doranet al. (1994b) gave a range of 3.2 to 7.5‰ for the carbonatesthroughout the core (0–32 cm) and a range of21.9 to225.7‰for the organic carbon. A recent study has evaluated the Ant-arctic paleolakes as potential models for regions of extinct lifeon Mars (Doran et al., 1998). Carbon isotopic trends are re-ported for several lakes in the McMurdo Dry Valleys region,and sedimentation processes are investigated. Lacustrine sandmounds were found to contain abundant authigenic carbonateand organic matter and have good conditions for paleolimno-logical records. Doran et al. (1998) recommended evaluatingdeltas on Mars through surface images and searching forsmaller lacustrine sand mounds for potential evidence of exo-biological activity.

1.3. Applications to Future Mars Missions

The Mars exploration rovers currently planned to launch in2003 are based on the Athena rovers that were planned for theMars surveyor missions (Squyres et al., 1998). These roverswill use a combination of instruments, including a thermalinfrared emission spectrometer (known as a MINI-TES), ana-proton–X-ray spectrometer, and a Mo¨ssbauer spectrometer(Squyres et al., 2001) to characterize the surface and subsurfaceat two sites on Mars.

2. MATERIALS AND METHODS

2.1. Sample Collection and Preparation

The samples studied here are sediments from lake-bottom cores thatwere collected through dive holes at DH-2 and DH-4 of Lake Hoare, asdescribed in Nedell et al. (1987) and Wharton et al. (1993). A map ofthe Dry Valleys region and Lake Hoare, indicating the locations ofthese dive holes, is shown in Figure 1. Dive hole DH-2 is located;190m from the lake shore. The bottom of the lake is;15 m below thesurface of the ice near this dive hole, and the lake-bottom sedimentshere are in contact with oxic water. Dive hole DH-4 is farther fromshore (;300 m), where the lake is deeper (;30 m) and extends into theanoxic zone. Several cores have been retrieved from a distance of atleast 5 m from each dive hole. The sediment cores were kept frozenuntil sampled in the laboratory. Two cores are evaluated in this study:core E is number 49 from DH-2 (oxic) and core H is number 13 fromDH-4 (anoxic).

The sediment cores were thawed in an argon atmosphere to selectsamples for the Mo¨ssbauer measurements without oxidizing the iron-bearing minerals. Individual samples were removed from the core atone place for each layer and kept frozen until iron transmission Mo¨ss-bauer measurements were performed at 4.2 and 77 K. The cores werethen thawed to room temperature, and all other measurements wereperformed on these air-dried samples. The sediment layers in each corewere separated according to texture and color and are shown in Figure2. The layers shown in Figure 2 were separated according to approx-imate median grain size: fine sand (less than;250-mm particle size),medium sand (;300–500-mm particle size), and coarse sand (;500–1000-mm particle size). The samples measured by Mo¨ssbauer wereselected from one specific site for each layer but are assumed to befairly representative of that layer. In figures where the Mo¨ssbauer dataare compared with the other results, the exact position of the Mo¨ssbauer

samples is given, whereas the range or central point for each sedimentlayer is given for the other measurements.

The samples were alternately ground in a mechanical ball-millgrinder in air and dry-sieved to,125-mm particle size to avoid excessgrinding of the softer minerals. Each sample aliquot was ground com-pletely until all of the material passed through the,125-mm sieve toensure that the initial composition is represented in the homogenizedpowders.

2.2. Analytical Techniques—Chemistry and Mineralogy

The chemical analyses were carried out on pressed powders with aPhilips PW2400 wavelength-dispersive X-ray spectrometer and follow-ing typical procedures for X-ray fluorescence (XRF). All chemical dataare based on 105°C dried samples, and the loss on ignition wasdetermined at 850°C.

Measurement of organic and inorganic C and N concentrations in thesamples was performed with an LECO RC-412 multiphase determina-tor, which enables oxidation of the sample in an O2 atmosphere atelevated temperatures. The organic C-H bonds are oxidized at temper-atures,500°C, whereas the inorganic C bonds are oxidized at highertemperatures (;.700°C). The sum of organic and inorganic C values

Fig. 2. Sediment stratigraphy for the sample cores showing relativeparticle size and the presence of organic material. Core E is from divehole DH-2, which is located under an oxic region of Lake Hoare, andcore H is from dive hole DH-4, located under an anoxic region of thelake.

2877Mineralogy and geochemistry of lake sediments from Antarctica

measured by this technique compare favorably with similar C valuesdetermined from the loss on ignition data.

X-ray diffraction (XRD) measurements were performed on pressedpowders with a Philips PW 1710 diffractometer. These measurementswere made by CuKa radiation, a 0.1-mm receiving slit, and a graphitemonochromator.

2.3. Analytical Techniques—Isotopes

C and N isotope measurements (d13C andd15N) were performed byDumas combustion techniques on the bulk organic material in eachsample from cores E (DH-2, oxic region) and H (DH-4, anoxic region)as in previous studies (DesMarais et al., 1989; Wharton et al., 1993;Doran et al., 1998). Samples were pretreated for 48 h in a mild aceticacid solution to separate and purify the organic material. This techniqueremoves the calcite without destroying labile organic matter in themicrobial mats. After this pretreatment, samples were rinsed threetimes, dried at 105°C for 24 h, and cooled in a desiccator. Duplicateswere run for all samples. Samples were prepared and measured fol-lowing normal procedures (e.g., Wong et al., 1992). Briefly, sampleswere combusted at 1000°C along with a pulse of pure O2, transportedvia a carrier gas through a series of chemical scrubbers, and finallytransported through a second furnace held at 650°C (contains elementalcopper to remove any excess oxygen and reduce nitrogen oxides tonitrogen). Water vapor and the remaining pure N2 and CO2 are sepa-rated by chemical or cryogenic traps and are moved directly into the ionsource of a Micromass Sira Series 2 mass spectrometer for isotopicmeasurement.

All carbon isotopic results are reported as per mil (‰) values,relative to the Peedee belemnite carbonate standard from the Viennaconference (V-PDB) according to normal procedures. The standardanalyzed with our samples had ad13C of 29.0‰ with respect toV-PDB. The15N/14N value of each sample was reported as the relativeper mil difference between sample and atmospheric N2, and the stan-dard analyzed with our samples had ad15N value of 6.7‰ with respectto atmospheric N2.

The S isotope compositions were determined with a continuoushelium flow Carlo Erba elemental analyzer coupled with isotope ratio–mass spectrometry (EA-IRMS) with a Micromass Optima instrument,as in other studies (Giesemann et al., 1994; Grassineau et al., 1998).The samples were weighed into a 53 8–mm tin capsule along with afew mg of V2O5 to facilitate complete combustion. This mixture wasthen compacted and loaded into an autosampler, which introduces thesamples into the elemental analyzer.

Shortly after introduction of a sample into the combustion furnace,oxygen is admitted, producing flash combustion at;1800°C. Thecombustion gases are swept through a column of WO3 granules andthen reduced copper, which ensures that the S-containing gases are pureSO2, and that any surplus of oxygen is trapped. Water is removed in asubsequent trap containing magnesium perchlorate, which is thenpassed through a chromatographic column where it is separated overtime from CO2 and N2.

A small percentage of the gas chromatography (GC) column effluentis passed into the ion source of the mass spectrometer. The S compo-nent of the sample is compared with an SO2 reference gas that isdelivered via the inlet. Ion currents of masses 64 and 66 were recordedand the areas under these peaks were integrated. Silver sulfide stan-dards approved by the International Atomic Energy Agency, Vienna,were routinely analyzed with the samples (approximately one standardper eight samples) to ensure that there is no fractionation of thereference gas. The isotope ratios were normalized to Can˜on Diablotroilite (V-CDT) and are given in terms ofd34S as per mil relative toV-CDT.

EA-IRMS provides major advantages over the classical techniquesof reduced sample weights and much reduced analysis time. The mainlimitation of this technique is that the S content must be at least 300ppm, due to a maximum sample weight of 20 mg. For samples with Sconcentrations less than 300 ppm, a chemical means of concentratingthe sulfur is necessary (e.g., Sasaki et al., 1979). In this study, we ranresidues as indicated above after HCl digestion of the samples. Thisremoves soluble sulfates and allows for measurement of pyrite abun-dance (and also other insoluble S phases, if present).

Fig. 3. Relative abundances of major minerals in the oxic region sediments (core E, DH-2-49) and the anoxic regionsediments (core H, DH-4-13). Mineral abundances were determined by XRD. Pyroxene was identified in these samples fromthe reflectance spectra and the fits of the high-resolution Mo¨ssbauer spectra; however, the plagioclase XRD peaks maskthose of pyroxene, making it difficult to uniquely identify pyroxene in these samples by XRD. The mica is probably biotite.The detection limit of these minerals is;3 wt%.


2.4. Mossbauer Spectroscopy

All samples were kept frozen at 77 K after sampling to preventoxidation and preserve the natural Fe(II)/Fe(III) ratios. The sampleswere prepared;1 mm thick in 10-mm-diameter Delrin cups. Thicknesseffects for these sediments are assumed to be negligible as in previoussediment studies (Drodt et al., 1997, 1998; Ko¨nig et al., 1997) on thebasis of calculations for materials of related composition (Long et al.,1983; Rancourt et al., 1993). Initial Mo¨ssbauer spectra were measuredon the samples kept at 77 K in an Oxford Instruments refrigerator. Thesource was57Co diffused in a Rh foil and retained at room temperature.Isomer shifts are given relative toa-Fe at room temperature. Spectrawere recorded for samples from the oxic (core E, DH-2) and anoxic(core H, DH-4) regions of the lake in transmission geometry with a512-channel analyzer over the velocity range211.8 to 111.8 mm/s(termed “normal resolution”), as in previous studies (Drodt et al., 1997,1998; Konig et al., 1997). Additional “high-resolution” Mo¨ssbauerspectra were measured at 77 K for selected anoxic region samples overthe velocity range24 to 14 mm/s. Because pyrite has nearly the sameMossbauer parameters as high-spin Fe(III) (isomer shift and quadru-pole splitting), identification of diamagnetic pyrite in mixtures that alsocontain paramagnetic minerals is facilitated by low-temperature mea-surements in the presence of applied external fields (Montano andSeehra, 1976; Gu¨tlich et al., 1978). Mo¨ssbauer spectra were thereforealso measured at 4.2 K (normal resolution) in a field of 1 T (appliedperpendicular to theg-beam) for the anoxic region samples.

2.5. Reflectance Spectroscopy

Bidirectional reflectance spectra were measured from 0.3 to 3.6mmunder ambient conditions with the standard viewing geometry of 0°incidence and 30° emergence angles and from 1 to 25mm with aNicolet Fourier transform interferometer (FTIR) spectrometer in aH2O- and CO2-purged environment, as in previous experiments (e.g.,Bishop et al., 1996). Sample preparation for the spectral measurementsinvolved pouring the particulate material into a sample dish 12 mm indiameter and 3 mm in depth. For the,125-mm samples, the dish wastapped gently on a hard surface to settle the particles without smooth-ing, so that a natural surface texture was present. The bidirectional datawere calibrated to absolute reflectance using Halon (e.g., Pieters, 1983),and the FTIR data were scaled to the bidirectional data near 1.2mm.

3. RESULTS

3.1. Mineralogical Composition

3.1.1. X-ray Diffraction

Semiquantitative mineralogical analyses were performed byXRD techniques on pressed powders and are summarized inFigure 3. The dominant minerals include plagioclase, K-feld-spar, quartz, and pyroxene. This is similar to the mineralogyobserved for surface sediments from the Antarctic Dry Valleysregion (Gibson et al., 1983). Pyroxene was difficult to uniquelyidentify with XRD alone because plagioclase is abundant andhas XRD peaks that mask those of pyroxene. The samples fromthe anoxic region (core H) contain pyrite, but there is noevidence of pyrite in the oxic region (core E) samples. Second-ary amounts of mica, amphibole, and chlorite are presentthroughout both cores. Calcite is present at high abundance insample E-3 and in much lower quantities elsewhere.

3.1.2. Mossbauer Spectroscopy

Mossbauer spectra are shown in Figure 4 for selected oxicregion (core E) samples at 77 K and in Figure 5 for selectedanoxic region (core H) samples measured at 77 K and addi-tionally at 4.2 K in an applied field of 1 T. The spectra wereanalyzed by performing mathematical fits to separate Fe(III),

pyrite, and additional Fe(II) species. The nonpyrite Fe(II) con-tribution is present as two forms of pyroxene, Fe rich and Mgrich, and very likely as one other phase, which is probably asilicate mineral such as chlorite, mica, or amphibole (McCam-mon, 1995). In a first attempt, we have analyzed the spectrarecorded at 77 K with only two subspectra for Fe(II) speciesand one subspectrum for Fe(III)1 pyrite species (note thatpyrite is inseparable from Fe(III) at 77 K). The obtained isomershift (d) and quadrupole splitting (DEQ) values and the relativeabundances of the Fe phases at 77 K are given in Table 1 for theoxic and anoxic region samples. The Mo¨ssbauer parameters fortwo pyroxene compositions (FeSiO3 and Mg0.85Fe0.15SiO3)

Fig. 4. Mossbauer spectra of selected samples from the oxic region(core E) measured at 77 K under high resolution. The spectral characterof samples E-2, E-6 and E-10 is fairly representative of the samplesstudied from this core. The spectrum of sample E-3 is shown as well,because it differs from the others.


Fig. 5. Mossbauer spectra of selected anoxic region (core H) samples. (a) Measured at 77 K under high resolution. (b)Measured at 4.2 K with an applied field of 1 Tesla normal to theg-ray direction. These Mo¨ssbauer measurements werenecessary for the anoxic region samples to separate out the contributions from pyrite and Fe(III). Spectra are shown herefor samples H-3, H-4, and H-9, which exhibit the spectral range observed for the anoxic region samples.


from (McCammon, 1995) are also shown in Table 1 for com-parison.

This comparison indicates that none of the obtained Fe(II)-a/Fe(II)-b ratios in Table 1 resembles either FeSiO3 (50/50ratio) or Mg0.85Fe0.15SiO3 (80/20 ratio). We therefore repeatedour analysis of the anoxic region (core H) samples with threesubspectra for Fe(II) species instead of only two. The results ofthese analyses are shown in Table 2 and Figure 5a. Estimates ofthe FeSiO3 and Mg0.85Fe0.15SiO3 were obtained by comparingthe relative amounts of the Fe(II)-1 and Fe(II)-2 sites (in Table2). Mg-rich pyroxenes are more prevalent, except near 15 cm,in core H.

The spectra recorded at 4.2 K in an applied field of 1 T (Fig.5b) allow for separation of paramagnetic Fe(III) from diamag-netic pyrite contributions. In our analysis, we have approxi-mated the subspectrum representing pyrite by a quadrupoledoublet with fixed parameters—that is,d 5 0.41 mm/s andDEQ 5 0.63 mm/s. These values are results from investigationson pure pyrite and pyrite in mixtures (Lougear, 2000). Thisapproximation is justified because the small field of 1 T causesonly a slight broadening of the absorption lines of the diamag-netic pyrite. It is, however, strong enough to cause magnetichyperfine splitting in the Mo¨ssbauer spectrum of paramagneticFe(III) species. This contribution is represented by a hyperfinefield distribution with a fixed isomer shift of 0.48 mm/s and aquadrupole splitting of zero. These values are typical for ferriciron in clay minerals found in sediments (Drodt et al., 1997,

1998). The hyperfine field distribution in this analysis takescare of the fact that Fe(III) is inhomogeneously distributed,causing variations in exchange interactions.

The Fe(II) contributions in the spectra recorded at 4.2 K in afield of 1 T are divided into two subspectra, as described incorresponding Mo¨ssbauer studies of sediments from the PeruBasin (Drodt et al., 1997, 1998). The paramagnetic Fe(II) partis assigned by a broad quadrupole doublet because an appliedfield of 1 T is not strong enough to cause appreciable spinexpectation values of the ferrous high-spin iron (Trautwein etal., 1991) and therefore prevents resolved magnetic hyperfinesplitting. The second Fe(II) part in iron oxide species exhibitsmagnetic ordering and therefore resolved magnetic hyperfinesplitting. We have approximated this by a sextet with lineintensities of 3:4:1, as is expected for a ferromagnetic powdersample with an external field applied perpendicular to theg-beam; additionally, we have used a fixed isomer shift of 1.28mm/s, which is typical for ferrous high-spin iron, and a fixed

Table 1. Mossbauer parameters for core E (collected from the oxicregion of the lake) and core H (collected from the anoxic region of thelake) samples measured at 77 K.

Sediment depth(cm)

Ironspecies

d(mm/s)

DEQ

(mm/s)Relative amount of

Fe species (%)

0 (E-1) Fe(II)-a 1.28 2.16 44Fe(II)-b 1.27 2.91 30Fe(III) 0.47 0.79 26










Table 1. (Continued).

Sediment depth(cm)

Ironspecies

d(mm/s)

DEQ

(mm/s)Relative amount of

Fe species (%)

1 (H-1) Fe(II)-a 1.3 2.21 54Fe(II)-b 1.3 2.94 31Fe(III) 0.45 0.63 15






24.5 (H-7) Fe(II)-a 1.29 2.18 52Fe(II)-b 1.29 2.94 33Fe(III) 0.44 0.64 15




77 K spectra of mineral standards for comparison from McCammon(1995)

HpyroxeneFeSiO3

J Fe(II)-1 1.26 2.00 50Fe(II)-2 1.30 3.13 50

H pyroxeneMg0.85Fe0.15SiO3

JFe(II)-1 1.28 2.16 80Fe(II)-2 1.29 3.06 20

Note:d refers to isomer shift vs.a-Fe andDEQ refers to quadrupolesplitting. Typical error margins are 0.02 mm/s ford andDEQ, 2% forFe(II)-a, and 1% for Fe(II)-b and Fe(III). The relative abundance ofFe(III) in this analysis includes pyrite contributions.


quadrupole splitting of22.5 mm/s, with 2.5 mm/s representingthe average of all quadrupole splittings obtained for the ferroushigh-spin species of the anoxic region (core H) samples (Table2). The line width as well as the hyperfine field were freeparameters. The angle between the main axis of the electricfield gradient and the hyperfine field was kept at 90° (Drodt etal., 1997). The overall contributions of the high-spin Fe(II)subspectra were held constant (to within62%) to those ob-tained from the 77 K measurements; this is a reasonable ap-proximation because the Lamb-Mo¨ssbauer factor (f factor)does not change significantly between 4.2 K and 77 K and fromone Fe(II) containing oxide to another.

The parameters resulting from this analysis of the anoxicregion (core H) samples, recorded at 4.2 K in an applied fieldof 1 T, are summarized in Table 3. This analysis providesquantitative information about the composition of the iron-bearing species in these sediments. The iron-bearing speciesinclude pyrite, paramagnetic and magnetically ordered Fe(II)species, and Fe(III) species of which the hyperfine field distri-bution covers paramagnetic as well as magnetically orderedFe(III) sites.

3.1.3. Reflectance Spectroscopy

Visible to infrared reflectance spectra of the,125-mm par-ticle size fractions of all samples are shown in Figure 6. Thevisible to near-infrared (NIR) region spectra are shown inFigure 6a as a function of wavelength in microns. Pyroxene

bands are observed near 0.93mm and;2 mm in all samples.These features are characteristic of low-Ca (high-Fe) pyroxene.Broadening of the;1-mm band for the anoxic region samplesH-1,2,3 may be due to pyrite. Also observed for all samples isa strong water band near 3mm; this feature is stronger androunded for spectra containing organic (;3.4 mm) or calcite(;3.4 and;4 mm) features. The spectra of samples E-2 andE-3 exhibit particularly strong calcite bands. Weak features dueto mica, clay minerals, or both are observed in some samplesnear 1.4, 1.9, and 2.2mm. Samples H-1,2,3,4,5 exhibit sharpchlorophyll bands at 0.67mm. These spectra also show organicC-H stretching bands near 3.4mm. (For detailed visible/NIRspectra of minerals, see Burns, 1993, and Gaffey et al., 1993.)In the oxic region core (E), organics and calcite are evident inthe surface layers, and calcite is observed deeper in the core forsamples E-6 and E-8 on the basis of the reflectance spectra inFigure 6a. The upper segments of the anoxic region core (H)contain organic C-H and chlorophyll spectral features, whereasthe lower layers contain calcite features. It is interesting that theanoxic region sediments have strong chlorophyll bandsthroughout the upper;20 cm of sediment. In a previous studyof lake Hoare sediments, chlorophyll absorptions are also ob-served in the surface layers of sediments in both the oxic andanoxic regions, and again at a;15-cm depth in the anoxicregion core (Bishop et al., 1996).

The mid-infrared region spectra are shown in Figure 6b as afunction of wavenumber (inverse centimeters) to better displaythe features. Quartz and pyroxene peaks are observed for mostsamples. The quartz features occur near 1200 cm21 (;8–9mm), and the pyroxene features occur near 800 to 1100 cm21

(;9–12mm) and near 500 to 550 cm21 (;18–20mm). Spec-tral evidence of carbonates is strongest for the oxic regionsamples E-2 and E-3. Carbonate bands are observed near 1800cm21 (sharp) and 1470 cm21 (broad) as volume-scattering,absorption features (troughs) and near 880 cm21 as surface-scattering features (peaks). The mid-infrared spectral propertiesof minerals depend on the particle size and texture of thesamples, as shown for quartz, pyroxene, and calcite (Salisburyand Wald, 1992; Moersch and Christensen, 1995; Wenrich andChristensen, 1996; Mustard and Hays, 1997; Lane, 1999),making it difficult to identify minerals in mixtures. For thisreason, the confidence of spectral mineral identification is in-creased by use of the entire spectral region (visible, NIR,mid-infrared).

Because grain size plays an important role in the reflectancespectra of minerals, spectra were measured of the originalsamples as well as the,125-mm powders. Shown in Figure 7are mid-infrared reflectance spectra of both the original mate-rial and the,125-mm powders for three sediments and theminerals quartz, pyrite, and calcite. Spectral features due toquartz can be seen throughout the spectra of samples E-5 andH-9 in Figure 7 and features due to calcite dominate the spectraof sample E-3 in Figure 7. Pyrite exhibits a sharp band near 440cm21 and a broad doublet near 1500 and 1630 cm21 (Fig. 7).The spectra of calcite and quartz both have features in the range1500 to 2000 cm21, which complicates identification of otherminerals in this spectral region. Neither of these pyrite featurescan be uniquely identified in spectra of any of the anoxic regionsamples; however, a broad band centered near 1650 cm21 insamples H-1,2,3 (Fig. 6) is consistent with the presence of both

Table 2. Mossbauer parameters of Fe(II) species for the anoxicregion (core H) samples at 77 K obtained from a fit with four subspec-tra (Fe(III) contributions are not listed).

Sample(cm)

Fe (II)doublet

d(mm/s)

DEQ

(mm/s)

Relativeamount ofFe species

(%)

Relativeamount ofFe(II)-1,2

(%)

9 (H-3) Fe(II)-1 1.28 2.14 35 83Fe(II)-2 1.26 3.06 7 17Fe(II)-3 1.25 2.77 13 —

15 (H-4) Fe(II)-1 1.28 2.22 23 51Fe(II)-2 1.26 2.95 22 49Fe(II)-3 1.28 2.63 13 —

20 (H-5) Fe(II)-1 1.28 2.15 49 77Fe(II)-2 1.28 3.05 15 23Fe(II)-3 1.26 2.70 17 —

21 (H-6) Fe(II)-1 1.27 2.11 46 78Fe(II)-2 1.27 3.04 13 22Fe(II)-3 1.28 2.68 18 —

24.5 (H-7) Fe(II)-1 1.27 2.14 54 83Fe(II)-2 1.28 3.10 11 17Fe(II)-3 1.28 2.77 21 —

29 (H-8) Fe(II)-1 1.27 2.15 61 85Fe(II)-2 1.28 3.10 11 15Fe(II)-3 1.29 2.78 17 —

34 (H-9) Fe(II)-1 1.28 2.14 58 77Fe(II)-2 1.28 3.07 17 23Fe(II)-3 1.29 2.69 17 —

36 (H-10) Fe(II)-1 1.27 2.13 58 78Fe(II)-2 1.27 3.04 16 22Fe(II)-3 1.27 2.67 14 —

Note:d refers to isomer shift vs.a-Fe, andDEQ refers to quadrupolesplitting. Mossbauer spectra of selected samples are shown in Figure5a.


calcite and pyrite and a weak feature at 440 cm21 in the E-3spectrum might be due to pyrite as well. Additional mid-infrared spectra of minerals can be found in Farmer (1974) andSalisbury et al. (1991). Recent reflectance spectroscopy studiesof Martian meteorite ALH 84001 also show reflectance spectraof multiple grain sizes of pyroxenes (Bishop et al., 1998a,b).

In addition to band centers that are characteristic of specificminerals, another spectral parameter, called the Christiansenfeature, is diagnostic for silicate minerals (Salisbury, 1993).The Christiansen feature is the reflectance minimum or emis-sion maximum that occurs on the short-wavelength flank of thestrong band due to the silicate stretching vibration. The Chris-tiansen features are given in Table 4 for the samples in thisstudy. Samples E-2 and E-3 do not have strong silicate featuresin this region but do have reflectance minima at somewhatshorter wavelengths (longer wavenumbers) and are listed inTable 4 in brackets. Although the mid-infrared spectral features(Figs. 6 and 7) are dominated by quartz bands for manysamples, the Christiansen feature for most samples is charac-teristic of K-feldspar. The Christiansen feature is influenced byall the components in the mixture and has been shown to becorrelated to the silica content of rocks and soils (Salisbury,1993).

3.2. Chemical Composition

Major element abundances for samples from the oxic region(core E, DH-2) and the anoxic region (core H, DH-4) weredetermined by XRF and are given in Table 5. The iron has beendivided into Fe(II) and Fe(III) on the basis of the Mo¨ssbauermeasurements. Samples E-1, H-1, and H-2 are the small, or-ganic-rich regions from the tops of the cores, and insufficientmaterial was available for chemical analyses. The oxic andanoxic region samples exhibit similar major element chemistryoverall. Sample E-3 is dominated by calcite and thereforecontains elevated Ca abundance and reduced levels of theremaining major elements compared with the other samples. Ingeneral, the Fe content is higher and the Al and Si contents arelower for the anoxic region samples (core H) than for the oxicregion samples (core E). The anoxic region samples also havesignificantly higher S abundances than those from the oxicregion. The minor elements Cr, Ni, and V have higher abun-dances in the anoxic core samples, and the Sr content is higherin the oxic core samples. The Al2O3 to TiO2 ratio is lower andthe Th/Cu, Th/Ni, and Th/Y ratios are higher for the anoxicregion samples compared with those from the oxic region. Therelative abundances of Fe, Al, and Si coupled with the differ-

Table 3. Mossbauer parameters for anoxic region (core H) samples from spectra measured at 4.2 K spectra with an applied field of 1 Teslaperpendicular to theg-beam.

Sample(cm) Subspectrum

G(mm/s)

d(mm/s)

DEQ

(mm/s)Bhf

(Tesla)Relative amount of

Fe species (%)

9 (H-3) pyrite 0.47 0.41 20.63 — 32Fe(II)-doublet 0.66 1.33 2.48 — 13Fe(II)-sextet 2.16 1.28 22.50 11 43Fe(III)-distribution 0.60 0.48 0.0 — 12




24.5 (H-7) pyrite 0.40 0.41 20.63 — 7Fe(II)-doublet 0.71 1.33 2.35 — 32Fe(II)-sextet 2.24 1.28 22.50 11 53Fe(III)-distribution 0.60 0.48 0.0 — 8




Note: G refers to line width,d refers to isomer shift vs.a-Fe, DEQ refers to quadrupole splitting, and Bhf refers to the magnetic hyperfine fieldmeasured at the57Fe nucleus. Mo¨ssbauer spectra of selected samples are shown in Figure 5b. Typical error margins for the relative amount of Fespecies are 2% for the largest values of pyrite and 3% for the other values of pyrite, 2% for the Fe(II)-doublet, and 3% for the Fe(II)-sextet and forthe Fe(III)-distribution.


Fig. 6. Reflectance spectra from 0.3 to 25mm of sediments from the oxic and anoxic regions of Lake Hoare. The visibleto near-infrared region is shown in terms of wavelength (A) and the mid-infrared region is shown in terms of wavenumber(B) to facilitate visualization of the spectral features.


ences in the ratio of Al2O3 to TiO2 and the ratios of the mobileelement Th with the immobile elements Cu, Ni, and Y areconsistent with a more mafic source for the anoxic regionsamples, or that these samples have experienced less weather-ing, or both (Nesbitt and Markovics, 1997).

The sediments from the oxic and anoxic region cores containboth organic C and calcite, as observed for related samples ina previous study (Bishop et al., 1996). The results of the Canalyses and inorganic and organic C determinations are givenin Table 6. The volatile component is also listed for eachsample and includes carbonates, organic materials, and water.

3.3. Isotope Compositions

The results of C and N isotopic analyses are given in Table7 for the oxic region (core E, DH-2) and anoxic region (core H,DH-4) samples. Sufficient sulfur for isotope determinations ispresent only in the anoxic region samples and the results ofthesed34S measurements are given in Table 8.The low Sabundance for the oxic region samples is consistent with theXRD and Mossbauer data that show evidence of pyrite only inthe anoxic region core. The quantity of S present in the anoxic

samples after dissolution of soluble sulfates is shown in Table8. These insoluble S values are less than the total S measuredby XRF (Table 5) for most samples.

4. DETECTION OF CARBONATES AND ORGANICMATTER

Relatively strong spectral bands due to carbonates occur near3.4 and 4mm, and those due to aliphatic hydrocarbons (organ-ics) occur near 3.4mm; these bands are frequently used inremote sensing for identification of carbonates and organics ingeologic samples. Difficulty arises in semiquantitative analysisif both carbonates and organic materials are present becauseboth of these species exhibit multiple bands in the 3.3- to3.5-mm region. To test correlations of the spectroscopic fea-tures with the organic and calcite abundances for remote de-tection, a reflectance continuum was removed from the infraredspectra in the range 3.2 to 3.7 and 3.7 to 4.2mm. A line is fittedto the spectrum at two points, and the spectrum is divided bythe line, giving normalized spectra for the region of interest.Band depths and band ratios are frequently used in remotesensing for mineral identification when absolute reflectance isnot possible. These spectra are shown in Figure 8, along withspectra of ethanol on a glass slide and fine-grained calcite froma previous study (Bishop et al., 1998a). C-H stretching featuresdue to hydrocarbons in organic material are observed at 3.38mm (;2960 cm21) for asymmetric CH3– vibrations, at 3.42mm (;2925 cm21) for asymmetric –CH2– vibrations, and at3.505 mm (;2850 cm21) for symmetric –CH2– vibrations.Strong calcite bands are observed at 3.87 and 3.98mm (;2590and 2510 cm21), and medium-strength calcite bands occur at3.36, 3.42, and 3.48mm (;2975, 2925, and 2875 cm21). Banddepths were measured (e.g., Clark and Roush, 1984) at 3.36,3.38, 3.42, 3.48, 3.505, and 3.98mm, and band areas weredetermined by integrating the area of overlapping bands for thecalcite features near 4mm (3.8–4.1mm) and the multipleorganic and carbonate bands near 3.4mm (3.3–3.6mm). Manyof these values are given in Table 9.

The band depths for organic features at 3.38, 3.42, and 3.505mm and the spectral area from 3.3 to 3.6mm should correlatewell with the abundance of organic carbon if no carbonates are

Table 4. Position of Christiansen features (CF) for the oxic (core E)and anoxic (core H) region samples.

Sample CF Sample CF Mineral CF

E-1 1275 cm21 H-1 1235 cm21 quartz 1350 cm21

E-2 (1400 cm21) H-2 1270 cm21 plagioclase 1300 cm21

E-3 (1390 cm21) H-3 1275 cm21 K-feldspar 1280 cm21

E-4 1280 cm21 H-4 1275 cm21 high-Capyroxene

1190 cm21

E-5 1275 cm21 H-5 1280 cm21 low-Capyroxene

1170 cm21

E-6 1280 cm21 H-6 1275 cm21

E-7 1280 cm21 H-7 1280 cm21

E-8 1275 cm21 H-8 1275 cm21

E-9 1280 cm21 H-9 1275 cm21

E-10 1280 cm21 H-10 1275 cm21

Note: The Christiansen feature values for the sediment samplesshown here were determined from the spectra in Figure 6 and those forthe minerals shown here are from Salisbury (1993).

Fig. 7. Reflectance spectra from 400 to 2700 cm21 (;3.7–25mm) ofthe original material and the,125 mm powders for three sedimentsamples and,75 and 75 to 250mm size separates for quartz, pyrite,and calcite (from Salisbury et al., 1991).


present. However, calcite is present in many samples, espe-cially in the oxic region core. As seen in Figure 8, if organicsdominate this spectral region, then the 3.42mm band will be thestrongest feature, and if carbonates dominate, then the 3.48mmband will be the strongest. Ratios of the band depths at 3.42mmand 3.48mm were used to characterize the relative abundanceof hydrocarbons and calcite. Shown in Figure 9a are the spec-tral band depths associated with organic C (3.38, 3.42, and3.505 mm) and the spectral area from 3.3 to 3.6mm vs. theorganic C content for most samples in this study; sampleshaving high carbonate abundances were removed from consid-eration by requiring that the 3.48mm band depth/3.42mm banddepth ratio be,1.2. The best correlation here is observed forthe 3.42mm band depth, although the other features exhibitgood correlation as well. The correlations with organic Ccontent are decreased when samples containing high calciteabundance are included.

The correlation between the spectral area from 3.3 to 3.6mmvs. the inorganic C content for all samples in this study is poorbecause overlapping organic and calcite features are presentand because the carbonate abundance is very low in somesamples. For samples where the 3.48mm band depth/3.42mmband depth ratio is.1, calcite dominates this spectral region.Shown in Figure 9b are band depths at 3.48 and 3.98mm vs.inorganic C content for the calcite-bearing samples. The cor-relation for both band depths is very good. If both features areavailable, the 3.98-mm band depth would be better to usebecause this band is stronger and there are no organic featureshere (see Fig. 8); however, applying this technique of pre-screening samples by comparing the 3.48-mm/3.42-mm ratioenables improved spectral analysis in the region of carbonateand hydrocarbon overlap.

Similarly, the 3.8- to 4.1-mm band area compares well withthe inorganic C abundance. The calcite band near 4mm (;2500cm21) is detected for 0.1 wt% CO2. Mid-infrared features near

6.8 and 11.5mm (1470 and 850 cm21) are detected for calciteabundances as low as 0.3 wt% CO2; however, detection ofthese features is dependent on which other minerals are presentand on the relative particle size of the mineral grains.

5. CORRELATION OF SEDIMENT VARIATIONS ANDBIOLOGICAL ACTIVITY

5.1. Depth Profiles for Major Elements and Mineralogy

Variations in the major element compositions from Table 5and trends in Fe(II) species from the Mo¨ssbauer analyses(Tables 1 and 3) are shown in Figure 10 for the oxic regionsamples (core E, DH-2) and Figure 11 for the anoxic regionsamples (core H, DH-4). For the sediments in the shallow, oxicpart of the lake (Fig. 10), there is a significant change in allelements near 5 cm (E-3) where calcite is extremely abundant.Otherwise, the Al, Ca, and K contents tend to follow trends inthe Fe content for the oxic region samples, with slight decreasesnear 22 cm (E-6) and slight increases near 35 cm (E-9). Exceptfor the dip due to calcite, Si abundance is inversely related tothese elements. Na and Mg exhibit trends inversely related toeach other.

For the sediments in the deep, anoxic part of the lake (Fig.11), changes in the chemistry are observed from;15 to 21 cm,near 32 cm, and near 36 cm. The amounts of Si and Al drop,whereas the Mg and Ca levels increase below;25 cm. Arelative change in the abundance of Fe(II) and Fe(III) occursnear 15 cm (Table 3 and Fig. 11) and is accompanied bysmaller changes in the other elements. The elevated Fe(III)level near 15 cm could be due to redox changes during sedi-mentation and/or biologic activity. Samples H-7 (24.5–29 cm)and H-8 (29–34 cm) have fairly similar chemical compositionsexcept for the change in iron oxidation state. The increasedferric oxide abundance near 15 cm is accompanied by higherFe-rich pyroxene levels compared with other sediment layers,

Table 5. Major (wt%) and minor (mg/kg) elements for Antarctic sediments from the oxic (core E) and anoxic (core H) regions of Lake Hoare.

Sample SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOIa Total

E-2 55.4 0.43 13.2 0.9 3.2 0.073 3.2 12.3 2.5 2.4 0.14 5.6 99.7E-3 26.4 0.31 5.5 1.2 1.0 0.026 1.8 32.8 0.9 1.1 0.17 29.1 100.3E-4 59.4 0.55 15.1 1.0 3.7 0.085 3.9 7.8 2.8 2.6 0.15 2.2 99.6E-5 60.8 0.53 15.3 0.8 3.9 0.093 4.1 6.4 3.0 2.6 0.14 1.3 99.5E-6 64.1 0.44 14.1 0.8 3.4 0.079 3.4 5.4 3.4 2.4 0.13 1.3 99.3E-7 64.4 0.58 14.2 1.0 3.7 0.090 3.9 4.9 3.1 2.5 0.16 0.7 99.6E-8 60.0 0.61 15.2 1.1 3.8 0.090 4.2 6.4 2.9 2.7 0.17 1.5 99.3E-9 61.5 0.62 15.3 0.9 4.5 0.100 3.9 6.0 2.9 3.0 0.13 0.6 99.9E-10 62.3 0.50 15.8 1.2 3.1 0.079 3.4 5.5 3.4 2.7 0.15 1.1 99.5

H-3 59.7 0.51 14.3 0.9 5.5 0.085 3.8 4.6 2.8 2.4 0.18 4.0 99.3H-4 60.7 0.61 14.9 2.1 3.7 0.098 4.3 5.5 2.8 2.5 0.17 1.5 99.3H-5 63.9 0.50 13.8 0.8 4.2 0.085 4.4 4.9 2.6 2.2 0.16 1.5 99.5H-6 62.3 0.48 14.6 0.7 4.1 0.093 4.0 6.1 3.0 2.4 0.13 1.4 99.8H-7 58.8 0.75 13.0 0.6 6.4 0.130 5.9 7.3 2.8 2.1 0.18 1.5 100.1H-8 59.4 0.73 13.6 3.4 6.2 0.130 5.9 6.7 2.5 2.1 0.16 1.1 99.7H-9 59.5 0.73 12.9 0.8 6.2 0.130 6.4 7.2 2.5 2.0 0.17 0.9 99.9H-10 60.1 0.68 13.1 0.4 6.1 0.115 5.9 6.8 2.7 2.1 0.18 1.1 99.9

SO-1b 55.0 0.87 17.6 (8.6) 0.110 3.8 2.5 2.7 3.2 0.15 4.5 99.0SO-1c 55.9 0.89 17.4 (8.8) 0.121 4.0 2.4 2.7 3.1 0.14 4.7 100.1

a Loss on ignition at 850°C.b Certified values of the soil standard SO-1 from Potts et al. (1992).c Measured values.


which suggests that variations occurred over time in the lakewater iron concentrations.

The trends for pyrite abundance as determined from Mo¨ss-bauer, XRD, and the insoluble S data are similar overall,indicating that most of the insoluble S is due to pyrite. Both theinsoluble S and pyrite abundance are lower and less variablefrom ;20 cm downward in the anoxic region sample core

(Table 8, Figs. 3 and 12). There is a rapid increase in pyritecontent to a maximum of 2.8 wt% S near the 10-cm depth,followed by a partial decrease in insoluble S content for the toptwo layers. The weight percent S as pyrite was calculated fromthe relative abundance of Fe as pyrite from the Mo¨ssbauermeasurements (Table 3) and the FeO contents from XRF (Ta-ble 5) for the anoxic region sediments. The abundances of thisS due to pyrite and the insoluble S are shown in Figure 12. Asmall increase in pyrite abundance at 36 cm (H-10) is observedalong with increases in Si, Al, K, and Na abundances anddecreases in Fe(III), Mg, and Ca abundances from H-9 to H-10(see Fig. 1). The presence of pyrite in the lake-bottom sedi-ments from the anoxic region in our study is one of the moststriking differences between these sediments and the surfacesediments studied by Gibson et al. (1983). A ratio of Corg to theinsoluble S content has been determined and is given in Table

Table 5. Continued

Sample Cl F S Ba Co Cr Cu Ga Mo Nb Ni Pb Rb Sr Th U V Y Zn Zr

E-2 510 440 320 460 18 103 14 14,4 ,4 46 10 62 420 7 ,5 67 11 28 112E-3 910 1640 1120 200 7 52 22 3,4 6 21 13 32 860 ,5 ,5 45 6 15 71E-4 860 240 420 570 18 100 16 15,4 13 46 17 81 340 10 ,5 94 16 31 123E-5 1120 ,150 400 570 17 114 15 14 ,4 19 52 10 84 340 15 ,5 84 14 37 121E-6 ,100 ,150 110 580 19 154 14 14 ,4 9 60 22 77 380 11 ,5 85 11 47 104E-7 160 450 160 600 22 147 14 15,4 12 55 24 75 340 13 ,5 94 13 51 120E-8 150 660 490 660 21 112 15 16,4 11 52 18 92 390 ,5 ,5 91 18 36 134E-9 140 ,150 400 640 17 117 14 16 ,4 8 55 12 87 350 10 5 94 17 35 122E-10 110 ,150 450 650 16 109 14 16 ,4 9 50 19 98 430 11 ,5 74 18 30 142

H-3 200 970 20,200 580 23 123 26 14 6 13 46 15 76 280 2 5 113 10 27 90H-4 310 540 9200 560 22 144 20 14,4 7 60 23 78 300 11 ,5 115 14 34 117H-5 230 490 9000 470 19 135 21 13,4 15 57 13 76 300 ,5 ,5 108 12 28 105H-6 300 220 5200 550 19 142 16 14,4 13 63 14 78 340 6 ,5 96 14 30 111H-7 580 1220 8100 410 27 185 17 13,4 12 71 13 67 270 8 ,5 159 14 44 137H-8 620 1010 4400 450 27 195 18 12 4 9 71 16 66 250 14,5 152 14 44 132H-9 520 790 2600 410 28 209 13 15,4 10 79 9 65 260 6 ,5 165 13 44 140H-10 600 940 4200 470 27 194 17 13,4 7 77 13 67 260 10 ,5 137 13 41 122

SO-1b 150 850 103 870 29 170 61 24 2 12 92 20 141 330 12 2 133 25 144 84SO-1c 180 1050 ,100 880 34 166 56 24 ,4 13 92 24 134 330 12 ,5 128 28 138 91

Table 6. Abundance of volatile components in Antarctic sedimentsfrom Lake Hoare.

Sample

Cinorganic

(wt%)

CO2

carbonate(wt%)

Corganic(wt%)

H2O(wt%)

S(CO2 1 Corg

1 H2O)

E-1 0.10 0.37 0.23 1.28 1.88E-2 1.04 3.81 0.38 1.03 5.22E-3 6.33 23.2 1.80 3.61 28.60E-4 0.34 1.24 0.14 0.44 1.82E-5 0.12 0.43 0.19 0.28 0.90E-6 0.14 0.52 0.16 — 0.68E-7 0.01 0.04 0.08 — 0.12E-8 0.20 0.72 0.10 0.31 1.13E-9 0.01 0.04 0.06 0.61 0.71E-10 0.09 0.34 0.11 0.82 1.27

H-1 0.06 0.23 3.49 5.75 9.47H-2 0.06 0.22 3.38 5.48 9.08H-3 0.03 0.11 1.02 1.52 2.65H-4 0.03 0.10 0.38 1.16 1.64H-5 0.01 0.04 0.41 0.98 1.43H-6 0.10 0.36 0.34 0.05 0.75H-7 0.12 0.45 0.27 0.74 1.46H-8 0.07 0.26 0.11 0.42 0.79H-9 0.10 0.35 0.10 — 0.45H-10 0.10 0.35 0.21 0.23 0.79

Note: Some H2O values were slightly negative due to changes inrelative humidity; these are indicated with a dash.

Table 7.d13C and d15N isotope data from the oxic (core E) andanoxic (core H) regions of Lake Hoare.

Core E (oxic region), DH2-49 Core H (anoxic region), DH4-13

Sample d13C s d15N s Sample d13C s d15N s

E-1 220.3 0.04 3.3 0.22 H-1 229.9 0.07 1.2 0.44E-2 220.4 0.08 3.2 0.20 H-2 230.1 0.07 24.1 0.30E-3 220.0 0.09 2.6 0.32 H-3 226.3 0.03 25.6 0.04E-4 219.5 0.44 3.0 0.10 H-4 226.0 0.10 23.8 0.28E-5 219.6 0.12 2.7 0.22 H-5 222.5 0.08 25.6 0.28E-6 220.5 0.16 3.4 0.06 H-6 222.9 0.02 25.2 0.30E-7 220.8 0.06 3.4 0.29 H-7 225.7 0.05 22.6 0.28E-8 219.9 0.05 3.2 0.21 H-8 222.9 0.10 23.3 0.31E-9 220.2 0.03 3.2 0.24 H-9 225.8 0.02 0.1 0.38E-10 220.7 0.08 3.9 0.26 H-10 227.9 0.03 1.3 0.20

Note: d13C in ‰ relative to V-PDB;d15N in ‰ relative to atmo-spheric nitrogen.


8 and is highest in layers H-1 (0–3 cm), H-5 (20–21 cm), andH-10 (36–38 cm). These layers may indicate locations ofhigher bacterial reduction in the sediments.

5.2. Depth Profiles for Isotopic Compositions

Depth profiles are shown in Figure 12 for thed13C andd15Npatterns in the oxic (core E, DH-2) and anoxic (core H, DH-4)region samples and for thed34S pattern in the anoxic regioncore. For the sediments collected in the oxic region of LakeHoare, thed13C values fall near220‰ and thed15N near 3‰,and there is little variation in either isotope with depth in thesediment core. For the sediments collected in the anoxic regionof the lake thed13C values are about230‰ near the surfaceand tend to become more positive with depth. Thed15N valuesare ;1‰ for the surface sediments in the anoxic region andbecome more negative with depth, whereas thed34S values arein the range216 to220‰ near the surface and more positiveat depth. Variations in the isotopic trends are much moresignificant in the anoxic sediments than in the oxic sediments,and changes in thed13C, d15N, andd34S tend to parallel eachother with depth in the sediment core. This suggests thatbiologic activity is primarily responsible for these changes—and further, that this activity is reflected in all three isotopes.

Thed13C values observed here for the oxic region sedimentsare consistent with the C isotopic composition of organicmatter in sediments and are typically due to the activity ofphotoautotrophes (Quay et al., 1986; Fogel and Cifuentes,1993; Wharton et al., 1993; Seewald et al., 1994). The isotopicvariation observed for N parallels the trends in C isotopes forthe oxic region sediments. Nitrogen fixation, ammonia uptake,and nitrification all potentially contribute to thed15N values inthe oxic region sediments. There is no evidence for sulfatereduction in the oxic regions of Lake Hoare as no pyrite isformed even though Fe(III) is present.

The higher abundance of organic material in the sedimentsfrom the anoxic region, coupled with the larger isotopic vari-ations observed for these sediments, may indicate that addi-tional microbial processes are more active here than in thesediments from the oxic region. Typically, in sediments,d13Cvalues for dissolved organic carbon are near215 to218‰ andfor dissolved inorganic carbon are within25 to 17‰

(O’Leary, 1981; Rau et al., 1989; Fogel and Cifuentes, 1993;Seewald et al., 1994; Laws et al., 1995). Thed13C valuesobserved in the anoxic sediments are more negative than thevalues often found for dissolved organic carbon and can beexplained by respiration (e.g., Quay et al., 1986).

Other processes, such as methanogenesis, may also be influ-encing the isotopic composition of the organic carbon in theanoxic region of the lake. Andersen et al. (1998) observedmethane just above the sediment surface in the deeper, anoxicregions of Lake Hoare. Galchenko (1994) measured methano-gensis in another Antarctic lake, and preliminary data suggestthat methanogenesis is taking place in Lake Hoare as well(Galchenko, personal communication). Methanogens are nor-mally active below the sulfate zone in sediments, as observedin a study of anoxic sediments in Aarhus Bay (Jørgensen,1996). In that study, methane bubbled up through the sedimentsto the water. If methanogens are present in the anoxic regionsediments of Lake Hoare, this could explain the methane ob-served. Methanogenesis (and also respiration by methanotro-phes) may also be contributing to the depletions in13C near 26

Fig. 8. Reflectance spectra from 3.25 to 4.15mm for calcite andethanol (A), continuum-removed reflectance spectra for the oxic region(core E) samples (B), and anoxic region (core H) samples (C). Thesespectra are normalized to more readily compare the band strengths ofthe organic and calcite components. Multiple organic C-H stretchingfeatures occur in the range 3.3 to 3.5mm. A strong carbonate bandoccurs near 4mm and weaker bands occur near 3.4 and 3.8mm.

Table 8. S abundance andd34S isotope data from the anoxic regions(core H) of Lake Hoare.

Sample d34S s Sinsol, wt% sCorg/Sinsol

ratio

H-1 216.4 1.45 1.51 0.31 2.3H-2 220.9 0.81 2.27 0.12 1.5H-3 218.6 0.92 2.56 0.20 0.4H-4 24.45 0.30 0.47 0.11 0.8H-5 211.2 0.66 0.28 0.17 1.4H-6 22.32 0.09 0.13 0.01 2.5H-7 20.47 0.37 0.16 0.07 1.7H-8 24.27 0.42 0.09 0.00 1.2H-9 26.56 NA 0.07 NA 1.4H-10 214.3 0.75 0.09 0.01 2.2

Note:d34S in ‰ relative to V-CDT; Sinsol is S due to pyrite and acidinsoluble sulfates, and Corg is wt% organic C from Table 6. NA, notavailable.


and 36 cm depth in these sediments. Dissolved methane in thewater column of a Norwegian fjord was also thought to haveinfluenced the depletedd13C values in the anoxic region sedi-ments in that study (Velinsky and Fogel, 1999).

Large variations have been observed in N isotopic fraction-ations in natural environments, due in part to fluctuations in theN abundance, and isotopic discrimination during uptake ofNH4

1 can result in depletions in thed15N of particulate N by 0to 27‰ (Fogel and Cifuentes, 1993). Thed15N values for thesediments in the anoxic region of our study varied from;1‰near the surface to23 to 25‰ at many places along the core.These more negatived15N values are probably due to thepresence of higher NH4

1 levels at depth and more denitrificationoccurring in these sediments. A recent study ofd13C andd15Ntrends in both sediments and water above and below the oxic–anoxic transition from a Norwegian fjord found that bothisotopes were more depleted in the anoxic region relative to theoxic region (Velinsky and Fogel, 1999), as we observed in thisstudy. They observedd15N levels of;4 to 6‰ in oxic regionsas compared with;1 to 4‰ in anoxic regions, whereas weobserved;3 to 4‰ in the oxic region sediments vs.11 to26‰ for the anoxic region sediments. Velinsky and Fogel(1999) attribute the nitrogen fractionation between the oxic andanoxic sediments to a balance between inorganic N utilizationduring photosynthesis, the reduction of nitrate to form ammo-nia, ammonia uptake by bacteria, and denitrification as oxygenlevels are reduced with depth. A combination of these factors isprobably influencing thed15N values observed in our sedimentsas well. Through biogeochemical analysis of the water columnand sediments, Velinsky and Fogel (1999) show that the de-pleted N isotope values due to bacterial primary production inthe water column are incorporated into the sediments, whereasthe higher isotope values associated with algal production arenot. If this is occurring in our anoxic region sediments as well,

Fig. 9. Band depths and integrated areas of spectral features arecorrelated with organic and inorganic C abundances. Good correlationis observed between spectral band depths at 3.42mm and measuredorganic C abundance, and between spectral band depths at 3.48 and3.98mm and measured inorganic C abundance. Ratios of band depthsfor carbonate and organic features were used to identify samples thatare spectrally dominated by organics or carbonates in this region. Theerror bars are typically smaller than the plot symbols and are thereforenot included.

Table 9. Band depths for infrared spectral features due to organics and calcite in the Lake Hoare sediments.

SampleBand depth

3.42 mmBand depth

3.48 mmArea (107)3.3–3.6mm

Band depth3.98 mm

Area (107)3.8–4.1mm

s (spectra);4 mm

Band depth;11.5 mm

Band depth;14 mm

E-1 4.3 2.7 4.9 2.7 2.2 43 1024 — 5E-2 10.8 15.6 20.9 31.4 37.8 33 1024 24 7E-3 29.7 36.3 56.5 59.0 94.1 43 1024 41 51E-4 2.1 4.0 4.3 11.7 12.0 43 1024 — 7E-5 2.2 2.5 3.6 5.7 5.3 43 1024 — —E-6 2.1 2.7 3.6 6.3 6.2 23 1024 4 —E-7 0.8 0.7 0.9 — 0.8 43 1024 — 7E-8 2.0 3.3 3.7 8.5 9.0 43 1024 3 2E-9 0.6 0.7 1.0 — 0.7 33 1024 — —E-10 1.3 1.8 2.2 4.8 4.2 23 1024 3 3

H-1 28.4 14.9 32.8 1.0 0.7 33 1024 — —H-2 26.0 13.4 30.0 — 0.2 13 1024 — —H-3 15.6 6.6 15.6 — 0.0 23 1024 — —H-4 6.4 2.7 6.1 0.7 0.6 23 1024 — —H-5 9.3 3.4 8.5 — 0.4 23 1024 — —H-6 5.0 3.7 6.2 4.4 4.2 33 1024 3 —H-7 2.4 2.1 3.5 4.0 4.9 33 1024 — 2H-8 — 1.4 1.6 3.7 3.7 23 1024 2 —H-9 — 1.4 1.3 3.7 3.8 23 1024 — —H-10 2.9 2.7 3.9 4.8 5.6 33 1024 — 3

Note: The error margins for the reflectance spectra are negligible based on the standard deviations measured for at least 10 data points near 4mm.Typical error margins for the band depth and area measurements are on the order of 1 to 5%.


then the depletedd15N values in the anoxic sediments can beexplained by bacterial activity.

The S isotope values vary at several points along the anoxiccore H from ;0 to 221‰ relative to V-CDT weighted forinsoluble S content). Thed34S profiles indicate a marked dif-ference between the upper layers (top 13 cm) and the lowerportion of the core. Thed34S values are depleted for theseupper sediment layers (26.4, 220.9 and218.6‰) and areenriched for samples H-6 (21–24.5 cm,22.3‰) and H-7(24.5–29 cm,20.5‰). Relatively depletedd34S values are alsoobserved for sample H-5 at 20 to 21 cm depth (211.2‰) andfor the lowest portion of the sediment core, where the S isotopiccompositions increase again to214.3‰. The isotopic compo-sition of S in pyrite formed through bacterial reduction ofsulfate has been measured from;0 to 240‰ (Schidlowski etal., 1983). Because microbial sulfate reduction kinetically fa-vors cleavage of the lower energy of32S-16O bonds relative tothose of the heavier S and O isotopes (Harrison and Thode,1958), the resulting pyrite hasd34S values that are fractionatedtowards the lighter isotope.

Jørgensen (1982) showed the important role of bacterialsulfate reduction (BSR) in degradation of organic material insediments, and Sagemann et al. (1998) further showed thatsulfate reduction can constitute an important metabolic processin cold sediments. The sediments on the surface of the anoxicregion exhibitd34S values that are;10‰ lighter than thosedeeper in this core. Laboratory cultures produce enrichments of32S significantly less than BSR in the natural environment

(Habicht and Canfield, 1997). However, because 90% of thesulfide produced by this process is reoxidized by both bacterialand abiotic pathways (Jørgensen, 1982), the large34S deple-tions observed in many marine sulfides can be attained via arepeated cycle of sulfide oxidation by iron oxides to elementalsulfur and subsequent disproportionation.

According to geochemical budgets of Lake Hoare, calculatedfrom comparing stream inputs of sulfate over the “chlorideage” (moles Cl in the lake divided by the annual Cl load inmoles per year) of the lake with the total sulfate concentration(Green et al., 1988), it is estimated that 60% of the total sulfatehas been removed over recent geological time. An obviousprimary cause of sulfate removal is BSR. As shown in Figure12, the minimumd34S occurs at a similar depth to the maxi-mum concentration of sulfides, which are indicated by Sinsol

and pyrite. Thed34S values of the uppermost sediments in theanoxic region (Fig. 12) show that the top layers are enriched inthe lighter isotope. Such profiles are similar to those in otherlakes where BSR is taking place—that is, sulfate is removedfrom the lake water and deposited in the sediment as ironsulfides, mainly pyrite (White et al., 1989; Fry et al., 1995).

5.3. Summary of Sediment Depth Profiles

Lake Hoare contains primarily oxic water with a smallregion that falls below the oxic–anoxic transition at 27 m depthdown to 34 m. The sediments in oxic lake regions containlarger amounts of calcite than the sediments in anoxic lake

Fig. 10. Major element composition of the oxic region samples (core E). Variations in two Fe(II) components are shownfor comparison with Fe2O3, FeO, SiO2, Al2O3, CaO, MgO, Na2O, and K2O abundances.


regions. Elevated levels of organic carbon were found in thesurface layers of sediments from both regions, and these areparticularly high for the anoxic region sediments. For thesediments in the anoxic part of the lake, a transition occurs near10 to 15 cm depth, dividing the surface sediments that areactive in BSR from the deeper sediments. A transition isobserved at this depth in the pyrite abundance and the isotopicratios of C, N, and S for the anoxic region sediments (Fig. 12).The S abundance in the anoxic region sediments is a factor of10 higher than in the oxic region sediments and the changes inC and N isotope ratios with depth are significantly greater forthe anoxic than for the oxic region sediments.

Doran et al. (1998) proposed three “photosynthetic zones” inLake Hoare to account for the varied isotopic fractionation. Inthe deep zone of the lake, there is unlimited CO2 supply andrelatively low rates of photosynthesis, so that uptake of theplentiful CO2 is slow and the autotrophs can discriminatebetween the carbon isotopes and preferentially utilize12C.These results, together with the presence of bacteria such asmethanogens and methanotrophes, can explain the highly13C-depleted organic C observed in the anoxic region sediments inour study. In the shallow zone, CO2 becomes undersaturated,creating a diffusion-limited situation in which more13C mustbe incorporated into the production of photosynthate. In theclear, shallow moat waters, photosynthetic rates must be highduring the period of constant sunlight in summer. Doran et al.(1998) proposed that dissolved inorganic carbon is assimilated

at the maximum rate at which it can diffuse into the mat, almosteliminating isotopic fractionation. This is observed in the oxicregion sediments in the current study where the isotopic frac-tionation is lowered.

The photosynthetic production rate is dependent on the avail-ability of solar radiation and nutrients (e.g., Fogel and Cifu-entes, 1993). Variations ind13C down the core may reflectchanges in productivity, lake level, or both over time. In otherwords,d13C in the sediment core should be inversely propor-tional to the thickness of the overlying water/ice column, ratesof production at the time of deposition, or both. The icethickness influences the penetration depth of visible and infra-red light necessary for photosynthesis. Thinner ice would en-able more photosynthetic organisms to live in the lake, thusdepleting the lighter C and producing more O2. As the CO2

supply becomes limited, the organisms must use the less de-sirable heavier C, which may be what is occurring in the watercolumn when the ice is thin. If the O2 produced throughphotosynthesis were allowed to accumulate, then the oxic–anoxic boundary would be expected to lower. However, whenO2 becomes available in natural systems, an increase in theabundance of O2-utilizing organisms is typically observed.These organisms then deposit more Corg, and the result is thatthe oxic–anoxic boundary is elevated in the lake. Conversely,thicker ice would reduce the number of photosynthetic organ-isms, reduce the number of O2-utilizing organisms, and lowerthe oxic–anoxic boundary in the lake. This scenario is compli-

Fig. 11. Major element composition of the anoxic lake samples (core H). Variations in two Fe(II) components are shownfor comparison with Fe2O3, FeO, SiO2, Al2O3, CaO, MgO, Na2O, and K2O abundances.


cated by the relative balance of O2 produced by photosyntheticorganisms and used up by others.

Possibly, then, parts of the current anoxic region were in anoxic zone at the time of sediment deposition. Changes in the icethickness on top of Lake Hoare have been observed and areattributed to climate variations (e.g., Doran et al., 1994a).Another feature of Lake Hoare that could contribute to shifts inthe oxic–anoxic boundary is the topography of the lake bottom.Rather than one large anoxic basin, there are several smallpools of anoxia. As these individual anoxic pools fill up andoverflow into surrounding regions of the lake bottom, mixing ofoxic and anoxic water would occur. If this is happening in LakeHoare, then formerly oxic regions could have become anoxicrather quickly, which could explain the changes in C, N, and Sisotope fractionation observed in the anoxic sediment corestudied here. Velinsky and Fogel (1999) also described a shiftin the oxic–anoxic boundary over time as one explanation forthe changes ind15N with depth in their sediments.

For the anoxic region core, the highest levels of organic C(Corg) are observed in the top layers (0–13 cm depth), whichare correlated with increases in pyrite levels and depletedd34Svalues. It is assumed that surface sediments in the anoxic regioncore encompass the active BSR layer. The two lower layers,around 20 and 35 cm in depth, may have been times ofheightened BSR activity, relative to;15- and;25-cm depth,and are associated with slightly elevated pyrite levels. Markedchanges also occurred in the C and N isotopes at these layers,which indicates that other processes may have also occurred.Perhaps most interesting is that an enrichment ind13C near 20to 21 cm is accompanied by a depletion ind15N and that the

opposite trend is observed at 36 to 38 cm. This suggests thatthere may have been changes in nutrient availability over time.

The differences between the S content profiles of sedimentsfrom Lake Hoare in this study and those of nearby Lake Fryxell(Howes and Smith, 1990) are not easily explained. In the LakeFryxell sediment profiles, the chromium-reducible S (equiva-lent to pyrite with minor elemental S) abundance is againhighest in the top 5 cm, but is fairly constant between depths of5 to 20 cm. Pyrite levels in Lake Hoare are elevated deeperbelow the surface than in Lake Fryxell, and more variability inpyrite abundance was observed with depth in the sediments.Green et al. (1988) found that Lake Fryxell contains signifi-cantly higher sulfate losses and has a much larger anoxic zonethan Lake Hoare, which suggests that BSR is taking place to agreater extent in the former lake. Possibly, Lake Hoare wassubjected to changes in sulfate input, or climate variations inAntarctica caused shifts in the oxic–anoxic boundary that in-fluenced the biologic activity in the lake. Additional samplesfrom the anoxic regions of these lakes should be analyzed toconfirm the trends characteristic of anaerobic microbes ob-served here.

6. APPLICATIONS TO ASTROBIOLOGY AND REMOTESENSING ON MARS

Characterizing the relationships between organisms, waterchemistry, and putative sediment composition in the lakes fromthe Antarctic Dry Valleys provides important information forinterpreting possible sedimentary processes on Mars. Gibson etal. (1983) showed the importance of chemical and physical

Fig. 12. Depth profiles for isotopes.d13C andd15N trends for the oxic and anoxic region cores;d34S trends for the anoxiccore; and wt% S due to pyrite from the Mo¨ssbauer measurements and due to insoluble S (mostly pyrite) from the EAmeasurements.


alteration processes on sediments from cold desert environ-ments. Chemical analyses of the rocks and soils on Marsindicate complex mixing (McLennan, 2000). Enrichment of Feand depletion of Ti in this study raise the possibility of sedi-ment transport. Mobile sedimentary units with distinct spectralproperties have been identified on Mars near thePathfinderlanding site (Greeley et al., 1999; Bell et al., 2000).

Biogeochemical interactions play an important role in ter-restrial sediment alteration when microorganisms are present,as shown in this study. In other studies of organisms in coldsediments, Rivkina et al. (1998) found that anaerobic microor-ganisms are viable in sediments in near-freezing environments.Sagemann et al. (1998) further observed that organisms livingin near-freezing arctic sediments actually have an ideal growthtemperature of;25 to 30°C and have apparently adapted to thecold environment. Biogeochemical analyses of sediments fromthe ice above Lake Vostok, Antarctica, imply that this lake maysupport a microbial population, although it has an ice layermore than 3600 m thick that isolates the lake from the atmo-sphere (Karl et al., 1999; Priscu et al., 1999). These resultssuggest that there is a possibility, however remote, of microbiallife in permafrost and paleolakes on Mars and other planets. Weare not speculating here on how extreme this possibility mightbe at any given time on Mars or elsewhere. Unique identifica-tion of Martian biosignatures, if present, is likely to be ex-tremely difficult and will probably only be possible for returnedsamples. Remote sensing on Mars, however, will be an impor-tant tool for selection of which samples to bring back. Identi-fication of minerals and other materials by use of visible, NIR,thermal infrared, and Raman spectroscopy will be important onlanded missions to Mars in determining the locations for otherin situ studies for astrobiology and for collection of samples toreturn to Earth for additional study.

Sedimentary features on Mars such as paleolakes (Whartonet al., 1995) and lacustrine basins and sand mounds (Scott et al.,1991; Doran et al., 1998) have been suggested as potential sitesof interest for exobiology. Paleolakes and lacustrine features onMars have also been studied to explain the surface processes onMars (Baker et al., 1991; Goldspiel and Squyres, 1991; Scott etal., 1991, 1992, 1995; Rice, 1992). An example of a potentialMartian paleolake is Palos Crater Lake, which is shown inFigure 13. Palos Crater Lake is one of many sites nominated asa landing site on Mars (e.g., Forsythe and Blackwelder, 1999;Zimbelman and Rice, 1999). As described by Zimbelman andRice (1999), this crater, located in the ancient highlands ofnorthern Hesperia Planum, has a smooth crater floor, and itscrater rim is dissected by three valley networks. The chemicaland mineralogical correlations with biologic activity observedin this study for the Lake Hoare sediments are further supportfor the importance of chemical and mineralogical investiga-tions of potential sedimentary sites on Mars. Understanding theevolution of Martian sedimentary systems (whether they arebiologic or nonbiologic) will help us better understand earlysedimentary processes on the Earth.

Evidence of fluvial features, such as valley networks andoutflow channels, have been characterized in detail on Mars(Carr, 1981). Analysis of the topography and geologic units inwestern Arabia and Sinus Meridiani on Mars led Edgett andParker (1997) to suggest that sedimentary processes may beoccurring there. More recently, Malin and Edgett (2000) and

Carr and Malin (2000) have shown evidence for erosionalactivity attributed to liquid water that may have occurred inrecent times on Mars. Crystalline hematite has now been iden-tified at Sinus Meridiani with the Thermal Emission Spectrom-eter (TES) on the Mars global surveyor (MGS) (Christensen etal., 2000a); this is further evidence that sedimentary processesmay have occurred on Mars. The presence of potential paleo-lakes, oceans, and other sedimentary processes on Mars aresupported by the recent high-resolution imaging and topogra-phy data measured by other instruments on MGS (e.g., Cliffordand Parker, 1999; Edgett and Malin, 1999; Head et al., 1999).These sites would be ideal locations to look for biomarkerminerals and other evidence of organisms or habitats capable ofsupporting life on early Mars. Some images of polar regionstaken by the Mars observer camera on MGS show observationsof ice layering (see http://mars.jpl.nasa.gov/mgs/msss/camera/images/grl_99_shorelines/index.html), where freeze–thaw cy-cles may be transporting fresh sediment to the surface. Thesewould also be areas of interest for evidence of former life, ifpresent, on Mars.

Biologic processes are generally characterized by depletionsin the heavy isotopes and obvious biomarker trends in terres-trial sediments are lighterd13C, d15N, and d34S values fororganic matter relative to inorganic material, as observed in thisand other studies. Because atmospheric sulfur on Mars canmimic a biologic signature (Farquhar et al., 2000), interpreta-tion of isotopic trends for isolated samples would be difficult.Variations in Fe(II)/Fe(III) levels would be easier to identifyremotely but could be attributed to multiple factors, not justbiologic activity. Observation of the mineral pyrite in potentialsedimentary regions or paleolakes on Mars could be used toidentify regions of possible biologic activity. Unfortunately,

Fig. 13. Viking image of Palos Crater Lake located in the ancienthighlands of northern Hesperia Planum, Mars. This 55-km-diametercrater is centered near 2.5° S and 249.5° W (Zimbelman and Rice,1999).


pyrite on the surface would likely be reoxized to sulfates overMartian time scales, and biogenic pyrite would likely be finegrained, which would have reduced infrared spectral featuresand be more difficult to detect by means of infrared spectro-scopy, but still possible with Raman spectroscopy, which is lesssensitive to particle size. Better methods of finding evidence ofbiologic activity on Mars or other planetary surfaces, if present,would involve combining multiple measurements either in situon a rover or in the laboratory on Earth after sample return.Observation of correlations in abundance of pyrite, ferric ox-ides/oxyhydroxides, organic C, and lighter isotopes of C, N, O,or S relative to these isotopes occurring in clearly inorganicregions elsewhere on the planet would be an indicator thatbiologic processes may have taken place. Such observationsmay be used for sample selection.

Carbonates are also frequently associated with organisms, asin the oxic region sediments in this study. Remote detection ofcarbonates through NIR or thermal infrared imaging on Mars orother planetary surfaces would be an indicator of a site withpotential for life. Identification of carbonate-rich sediments, ifpresent on Mars, would be of interest to astrobiology whetheror not life evolved on that planet. Carbonate bands near 4 and6.8mm (;2500 and 1500 cm21) were detected in this study forcarbonate abundances as low as 0.1 wt% CO2. Carbonates canform under a range of temperatures and do not necessarilyimply an environment favorable to life. Carbonates have notbeen detected on Mars by TES (Christensen et al., 2000b),which suggests that they are not pervasive on the surface,although Pollack et al. (1990) did observe a band at 6.7mm inthe atmospheric dust that was attributed to carbonates.

Organic hydrocarbons are another species frequently associ-ated with living systems. Raman spectra, infrared spectra, orboth obtained from a rover would be capable of identifyingorganic material in rocks or sediments if the organics arepresent near the surface. Organic features at 3.38, 3.42, and3.505 mm (2960, 2925, and 2850 cm21) were detected fororganic C abundances as low as 0.06 wt% C; high spectralresolution would be required to separate these hydrocarbonfeatures from those of carbonates that also occur in this region.Raman spectra in the range 100 to 3500 cm21 have been usedto identify endolithic microbial communities in Antarctic rocks(Russell et al., 1998). Spectral features from 1200 to 1600cm21 and near 3000 cm21 are particularly useful for identifi-cation of organic species in infrared and Raman spectra. ARaman spectrometer on a rover would be useful in identifyingminerals (and also microorganisms if present) in possible pa-leolake environments on Mars. Raman, Mo¨ssbauer, and ther-mal infrared spectrometers were components of the AthenaRover (Klingelhofer, 1998; Squyres et al., 1998; Jolliff et al.,1999), a modified version of which is currently scheduled to flyon the Mars exploration rovers in 2003. Unfortunately, theRaman spectrometer is no longer planned for the 2003 Marsexploration rovers, but we hope it will be added in futurelanded missions to Mars. Although the Viking biology tests didnot find any evidence of organics in the dust on Mars (e.g.,Klein, 1978), some Martian meteorites have been found tocontain organics (e.g., McKay et al., 1996). A lander sent toone of the paleolakes on Mars with the capability of measuringthe infrared and Raman spectra of rocks, particularly the un-

exposed surfaces, could be more successful in detecting organ-ics on Mars, if present.

Sediment cores from the bottom of Lake Hoare, Antarctica,were analyzed in this study for their mineralogical and geo-chemical compositions. Comparison of the sediment chemistryand mineralogy with the water chemistry and biologic activityled to explanations of the biogeochemical cycling in this lakeenvironment and provided implications for astrobiology onMars.

6.1. Oxic vs. Anoxic Lake Regions

The lake water is supersaturated in O2 in the shallowerregions and anoxic below a 27-m depth, as measured downfrom the ice surface (Craig et al., 1992; Wharton et al., 1993).The sediments collected from DH-2 at a depth of;15 m belowthe ice surface (core E) measured in this study contain plagio-clase, K-feldspar, calcite, quartz, pyroxene, mica, amphibole,and chlorite. These lake-bottom sediments are mineralogicallysimilar to surface sediments from the Dry Valleys region (Gib-son et al., 1983). The sediments collected from DH-4 at a lakedepth of;30 m below the ice surface (core H) contain plagio-clase, K-feldspar, quartz, pyroxene, pyrite, mica, amphibole,and chlorite. The calcite abundance is minimal in these anoxicregion sediments, where,0.5 wt% inorganic CO2 was de-tected. The difference in calcite abundance for the two cores isattributed to calcite supersaturation in the shallower waters(Green et al., 1988). A recent study of mats from Lake Hoarefound calcite present at;50 wt% abundance in mats collectedat depths from 5 to 10 m, whereas only;5 to 10 wt% calcitewas found in mats collected at lower regions of the lake (Hawesand Schwarz, 1999).

6.2. Correlation of Chemistry and Mineralogy withSediment Depth

Variations in chemistry and mineralogy were fairly insignif-icant for the sediments collected from the shallower, oxicregion of the lake, with the exception of a particularly calcite-rich mat layer at 3 to 5 cm depth. In contrast, the sedimentscollected from the deeper, anoxic lake region exhibited largervariations in mineralogy, elemental trends, and isotopic pat-terns. Organisms were associated with calcite mat layers in theoxic region sediments, whereas organisms were correlated withincreased pyrite abundance and depletedd34S in the anoxicregion sediments.

6.3. Summary of C, N, and S Isotope Analyses

The isotope trends in the sediments studied here depend onthe abundance of organically derived hydrocarbons and howmicroorganisms interact with their environment. Thed13Cdepth profile for the oxic region sediment core in our studyshows little variation, with values near220‰ throughout thecore. In contrast, thed13C depth profile for the anoxic regionsediment core varies from222 to 230‰. Thesed13C valuesare all consistent with organically derived hydrocarbons insediments (Fogel and Cifuentes, 1993). Thed15N measure-ments for the oxic region core are relatively constant at;3‰,whereasd15N varies from about11 to 26‰ for the anoxicregion core. This suggests that N is more active in biologic


cycling in the anoxic region sediments. As N is a limitingnutrient and the source of N for these sediments is the NH4

1

from the water column, which is highest below 15 m (Whartonet al., 1993), the abundance of N in the water could be animportant factor in isotopic variation. Thed34S data are onlyavailable for the anoxic region sediment core. Thed34S depthprofile is depleted for the upper sediment layers (216.4,220.9,and218.6‰) that are correlated with high pyrite, ferric oxide,and organic C abundance and are thought to have active sulfatereduction. Lowd34S values (211 to 214‰) are observed foradditional sediment layers that are also correlated with evi-dence of sulfate reduction, which may have taken place at anearlier time when these layers occupied the sediment surface.The heaviestd34S values are22 to 0‰ and are consistent withS isotopes observed for aqueous sulfides (MacFarlane andShimizu, 1991). Isotope analyses should be considered in con-junction with other geochemical and mineralogical techniqueswhenever possible to reduce the possibility of incorrect inter-pretation of complex systems. Associated variations in C, N,and S isotope ratios for the anoxic region sediments mayindicate changes in the nutrient balance with time and/or shiftsin the oxic–anoxic boundary of the lake as climate changeinfluences the ice thickness.

6.4. Implications for Astrobiology

Biogeochemical interactions were shown in this study toplay an important role in sediment alteration when microorgan-isms are present. The abundance of nutrients, rather than coldtemperatures, appears to limit organism growth in the Antarcticsediments studied here, as well as in cold sediments from otherstudies. In anoxic regions where sulfate is plentiful, sulfate-reducing bacteria are active and biogenic pyrite is observed.Observation of correlations in abundance of pyrite, ferric ox-ides/oxyhydroxides, organic C, and/or lighter isotopes of C, N,O or S, relative to the isotopes occurring in clearly inorganicregions, may be an indicator of biologic processes. The bestmethods of detecting biologic activity on Mars or other plan-etary surfaces would involve combining multiple chemical andmineralogical measurements either in situ on a rover or in thelaboratory on Earth after sample return.

Acknowledgments—Support during initiation of this project was pro-vided to JLB by the University of Vienna, Institute for Geochemistryand from the National Research Council. Technical assistance from R.Hamid (XRF), T. Hiroi (reflectance spectroscopy) and M. Drodt (Mo¨ss-bauer) is much appreciated. This article benefited from the helpfulsuggestions of J. Bell, K. Habicht, S. McLennon, N. Ostrom, J. Riceand an anonymous reviewer. RELAB is a multi-user facility supportedby NASA grant NAG5 to 3871. Thanks are due to the AustrianUniversitat fur Bodenkultur, Institute of Applied Geology for makingtheir XRD facilities available. The isotopic analyses were supported bythe Austrian FWF, project Y58-GEO, and the NASA ExobiologyProgram, grant NAG5 to 9427. The Mo¨ssbauer measurements werefunded by the German Federal Ministry of Education, Science, Re-search and Technology (grant 03G0106I). The samples were obtainedthrough funding provided by NASA’s Exobiology Program(NAGW1947) and the NSF Office of Polar Programs (OPP-92117733,OPP9810219).

Associate editor:N. E. Ostrom

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Mineralogical and geochemical analyses of Antarctic lake ...current sediment study to facilitate identiﬁcation of biologic activity and geochemical cycles in the lake ecosystem