Geochemistry Geophysics G Geosystemsceoas.oregonstate.edu/people/files/...g3_alkenones.pdf · improving analytical methods or by developing complementary data sets to confirm or reject

Alkenones and multiproxy strategies in

paleoceanographic studies

A. C. MixCollege of Oceanic and Atmospheric Sciences, Oregon State University, Oceanography Administrative Building 104,

Corvallis, Oregon 97331-5503 ([email protected])

E. BardCentre Europe'en de Recherche et d'Enseignement en Ge'osciences de l'Environnement, CNRS-UniversiteÂ d'Aix-

Marseille III, Aix-en-Provence, France ([email protected])

G. EglintonEarth Sciences Department, University of Bristol, Bristol, England, United Kingdom ([email protected])

L. D. KeigwinWoods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 ([email protected])

A. C. RaveloInstitute of Ocean Sciences, University of California, Santa Cruz, California 95060 ([email protected])

Y. RosenthalInstitute of Marine and Coastal Studies, Rutgers University, New Brunswick, New Jersey 08903

([email protected])

[1] Abstract: We assess multiproxy strategies involving alkenone-based and other paleoceanographic

indicators in an effort to define uncertainties and to advance understanding of past oceanic and climatic

changes. Looking to the future, we recommend a parallel strategy of applying proxies to the geologic

record while continuously seeking refinements in methods and understanding of mechanisms that

control each proxy. A multiproxy strategy increases confidence in results and provides oceanographic

context for more effective interpretation of processes. A combination of proxies allows derivation of

additional properties, such as upper ocean paleosalinity or paleo-pCO2, for a more complete view of the

oceanographic and climate mechanisms involved in changing Earth's environment.

Keywords: Alkenone; isotope; foraminifera; paleotemperature; carbon dioxide; paleosalinity.

Index terms: Paleoceanography; paleoclimatology; geochemistry; organic marine chemistry.

Received February 22, 2000; Revised October 22, 2000; Accepted October 23, 2000;

Published November 22, 2000.

Mix, A. C., E. Bard, G. Eglinton, L. D. Keigwin, A. C. Ravelo, and Y. Rosenthal, 2000. Alkenones and multiproxy

strategies in paleoceanographic studies, Geochem. Geophys. Geosyst., vol. 1, Paper number 2000GC000056 [11,165

words, 4 figures, 1 table]. Published November 22, 2000.

Theme: Alkenones Guest Editor: John Hayes

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

Review

Volume 1

November 22, 2000

Paper number 2000GC000056

ISSN: 1525-2027

Copyright 2000 by the American Geophysical Union

1. Introduction

[2] Measures of concentration, relative abun-

dance, and other properties of alkenones, spe-

cific long-chain organic molecules produced in

the ocean by haptophyte algae (such as certain

species of Coccolithophoridae), offer great

potential as proxy indices of climate change

in marine sediments. Paleoceanographers rely

on proxy data, which in the geologic record

substitute for the kind of instrumental measures

made in today's world. No proxies are perfect.

All have analytical uncertainties, and all have

biases associated with real-world recording

processes. Although an ideal proxy would be

dependable and predictable based on primary

control by laws of thermodynamics, this situa-

tion is rarely realized because most paleocea-

nographic proxy data are at least partly

controlled by poorly understood biological

processes.

[3] Biological processes in the water or sedi-

ment column create some biases associated

with the ecological or physiological responses

of the organisms with which a proxy is asso-

ciated. Such factors underscore the value of

multiple measurements. Because secondary

influences on proxies such as the biological

or postdepositional effects are likely to differ,

multiproxy studies can help to identify the

primary signals of interest. Here we review

the status of such multiproxy studies in the

context of alkenone-based measurements. Our

assessment of future research strategies stems

from discussion at the National Science Foun-

dation sponsored workshop Alkenone-Based

Paleoceanographic Indicators, hosted by

Woods Hole Oceanographic Institution and

the U.S. National Academy of Sciences in

October 1999.

[4] We begin with the conviction that all of the

proxies discussed here are useful. Where they

agree, they confirm and increase confidence in

results. Where they disagree, they lead us to

assess the reasons for their differences, yielding

new insights into underlying processes. Con-

sistent with our focus on alkenone-based

proxies, we ask the question, `̀ How can other

proxies, combined with alkenone indices, help

to constrain the interpretation of paleoceano-

graphic and paleoclimatic processes?'' We seek

to identify key research opportunities that

exploit multiple proxies to gain confidence in

results with complementary data or in combi-

nation to derive important properties that can-

not be reconstructed with a single proxy

measure.

[5] Following early studies that identified the

promise of alkenones as paleoceanographic

proxies [e.g., Volkman et al., 1980a, 1980b;

Marlowe et al., 1984; Brassell et al., 1986a,

1986b; Prahl and Wakeham, 1987] measure-

ments made in modern (core top) sediments

have supported the concept of using the Uk037

index, (e.g., [C37:2]/([C37:2]+[C37:3]), as a rea-

sonable measure of mean annual sea surface

temperature (SST) [Herbert et al., 1998; MuÈller

et al., 1998]. No significant diagenetic effects

on the Uk037 index have been detected [Prahl et

al., 1989; Madureira et al., 1995; Teece et al.,

1998]. Patterns of temperature change based on

the index appear to behave in systematic ways

through the last few hundred thousand years

[Schneider et al., 1996, 1999; Bard et al.,

1997]. The alkenones thus provide powerful

tools for estimating past changes in ocean

temperatures on a global scale, except for

regions in which alkenone productivity and

contents in sediments are very low, such as

the centers of subtropical gyres, and areas in

which temperatures may exceed the limits of

temperature calibration, such as the polar

oceans or tropical warm pools.

[6] As with all geologic proxies, care must be

taken to evaluate potential biases and errors on

a case-by-case basis. Second-order problems

GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056

regarding the use of alkenone-based measure-

ments as indices of sea surface properties

include the following:

1. Observations of peak alkenone production

at subsurface depths suggest that alkenone

indices may not record sea surface condi-

tions [Prahl et al., 1993; Okouchi et al.,

1999].

2. Observations of seasonal blooms in alke-

none production suggest that alkenone

indices may not record annual-average

conditions [Prahl et al., 1993].

3. The finding of different relationships of

Uk037 to temperature in different species or

strains of haptophyte algae in laboratory

cultures leads to uncertainties in the

choice of temperature calibrations applied

to the geologic record in some areas

[Conte et al., 1998].

4. Kinetic effects of cellular growth, observed

to affect both the calibration of the Uk037

index relative to temperature and d13C

values within the alkenone fraction, suggest

that these measures may be subject to

ecological biases [Bidigare et al., 1997;

Epstein et al., 1998].

5. Owing to erosion and redeposition at the

seafloor, fine-grained particles (likely car-

riers of alkenones in the geologic record)

may in some areas such as sediment drifts

be contaminated with material that is older

or from another geographic area [Laine

and Hollister, 1981; Keigwin and Jones,

1989].

6. Bioturbation in the sediment column,

which appears to be strongest for fine

particles that carry alkenones [Wheatcroft,

1992], may disrupt the stratigraphic

record of alkenone indices relative to

other proxy data carried on larger sedi-

mentary particles.

[7] Alkenone-based proxies are not the only

ones subject to the sorts of challenges noted

above. Essentially all paleoceanographic proxy

data are affected by these or analogous pro-

blems or biases. Paleoceanographers often live

with these problems, limiting their choice of

study materials and interpretations to the parts

of the record that are robust. They constantly

strive to reduce or mitigate such problems by

improving analytical methods or by developing

complementary data sets to confirm or reject

inferences based on a single proxy. Over the

years this mode of operation has yielded dra-

matic improvements in proxies and deep

insights into the operation of a constantly

changing Earth system.

2. Opportunities for Multiproxy

Studies

[8] In the following sections we outline oppor-

tunities for multiproxy studies that combine

alkenone-based indices with other proxies in a

way that provides unique information. This can

be viewed in two ways. First, application of

two or more proxies thought to record similar

aspects of the environment (SST, for instance)

enhances confidence in a result. Analysis of

differences in results offers the potential of

richer, more complete insights into each proxy

by helping to identify specific conditions asso-

ciated with preferred habitats. Second, analysis

of a combination of proxies that examine

different aspects of the environment provides

an oceanographic context, which can help to

reveal biases or overprints on the primary

signal. Other strategies for combining proxies

allow derivation of new indices, such as paleo-

pCO2 or paleosalinity. The discussion begins

by addressing opportunities to mitigate or capi-

talize on some of the challenges discussed

above. This is not a comprehensive review.

Our intent is to provide key examples to

illustrate a general strategy of using multiple

proxies to gain richer understanding of the

environment.


3. Water Column Structure

[9] Tests or calibrations of paleoceanographic

proxies often compare measures from modern

(core top) sediments to historical atlas data or

from sediment traps to coeval water column

data. The strength of core top tests is the broad

geographic range of available test sites. How-

ever, high-quality cores that preserve the sedi-

ment-water interface and have sufficiently high

accumulation rates to verify (based on 14C

dating) the presence of modern sediments

remain limited. Sediment trap materials are

guaranteed to represent the modern sediment

rain within a given year but offer less geo-

graphic coverage, sometimes lack suitable

water column data, and are unlikely to repre-

sent long-term average conditions at a site.

Nevertheless, both strategies are useful.

[10] When observed at large spatial scales,

alkenone measurements in modern (core top)

sediments support the concept of using the Uk037

index as a reasonable measure of SST [Herbert

et al., 1998; MuÈller et al., 1998]. In some

regions, however, detailed analysis of core tops

suggests that geologic records of Uk037 may be

biased due to seasonal and/or subsurface alke-

none production [e.g., Okouchi et al., 1999].

Water column and sediment trap samples pro-

vide evidence for production of alkenones in

association with subsurface chlorophyll max-

ima, which are often observed within the pyc-

nocline or thermocline [Prahl et al., 1993]. For

example, off Oregon, Uk037 growth temperatures

were 58±88C lower than SST values during a

summer-dominated bloom in September 1998

(Figure 1). Confirming this pattern, tempera-

tures recorded by Uk037 in core top sediments

here are similar to sea surface temperatures in

the winter [Prahl et al., 1995; Doose et al.,

1997]. Little biological production occurs here

in winter, but these findings are consistent with

a well-known regional pattern of phytoplankton

growth 40±80 m below the sea surface in

summer, associated with a deep chlorophyll

maximum [Prahl et al., 1993]. In the northeast

Pacific this biological feature is often found

near the top of the so-called Shallow Salinity

Minimum water, which forms near the North

Pacific Subpolar Front in winter [Kenyon,

1978; Talley, 1985].

[11] Among planktonic organisms that carry

paleoceanographic information, subsurface

growth is not unique to alkenones. In a transect

similar to that studied by Prahl et al. [1993],

some species of foraminifera were found living

in roughly the same depths and seasons as the

alkenone producers [Ortiz and Mix, 1992; Ortiz

et al., 1995]. For example, living (protoplasm

full) shells of the planktonic foraminifer Neo-

globoquadrina dutertrei, captured in plankton

tows off Oregon in September 1989, were most

abundant in subsurface habitats offshore (Fig-

ure 1). Subsurface maxima such as these reflect

this species need for both food (most abundant

in a subsurface maximum) and light (for algal

symbionts, most abundant at the sea surface).

The broad similarity of depths occupied by the

phytoplankton and zooplankton offers hope

that multiproxy comparison of tracers in these

different organisms is possible.

[12] Information from other proxies can place

Uk037 temperature estimates into the context of

vertical and seasonal gradients of temperature

within the euphotic zone and may open oppor-

tunities to estimate important water column

properties such as thermocline temperature.

For example, planktonic foraminifera and radi-

olarians, the zooplankton groups most com-

monly preserved in the geologic record,

include many different species with distinct

depth and seasonal preferences [Deuser et al.,

1981; Fairbanks et al., 1982; Sautter and

Thunell, 1989; Ortiz and Mix, 1992; Welling

and Pisias, 1998]. Distribution of species in the

water column and their seasonal succession

clearly depend on water column structure and


food supply in addition to SST [Mix, 1989a;

Ravelo and Fairbanks, 1992; deVernal et al.,

1993; Ortiz et al., 1995, 1996; Kohfeld et al.,

1996; Watkins et al., 1996, 1998; Pisias et al.,

1997; Ravelo and Andreasen, 1999].

[13] Statistical methods for estimating water

column properties from an array of species

assemblages build on the pioneering work of

Imbrie and Kipp [1971]. The strength of such

whole-fauna methods lies in their ability to

predict large-scale patterns of ocean properties,

using statistical relationships to `̀ see through''

local biases such as the depth habitat of indi-

vidual species. In theory, statistical transfer

functions that estimate mean annual SST do

1216

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100200300400500600

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Distance from coast (km)

September 1989: Temperature, 42oN

Dep

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572 km 220 km289 km 121 km

Figure 1. (top) Water column temperature and salinity profiles in a conductivity-temperature-depth transectalong 428N off southern Oregon in September 1989 [Ortiz et al., 1995]. Labeled points note the apparentdepths recorded by Uk0

37 temperatures [Prahl et al., 1993] in sediment trap samples collected in September,1988. G is the `̀ gyre'' site (9.38C), M is the `̀ midway'' site (11.38C), and N is the `̀ nearshore'' site (10.48C).Uk0

37 temperatures suggest subsurface formation of alkenones, consistent with the location of deep chlorophyllmaxima along a density surface of 25.0±25.2 st. Solid triangles denote locations of depth-stratified planktontow samples. (bottom) Abundance of living (protoplasm full) shells of the planktonic foraminiferNeogloboquadrina dutertrei, captured in plankton tows along 428N (distances offshore noted) in September1989 [Ortiz et al., 1995], records subsurface habitats offshore that are broadly similar to those recorded bythe alkenones.


not require the fauna or flora to live at the sea

surface, in all seasons, or even to be controlled

mechanistically by temperature. The method

does require that whatever properties actually

control species distributions are linearly related

to SST. If this relationship varies through time,

then estimates may be biased. In spite of nearly

30 years of continuous study, new statistical

methods continue to be developed [Hutson,

1979; Prell, 1985; Pflaumann et al., 1996;

Ortiz and Mix, 1997; Malmgren and Nordlund,

1997; Waelbroeck et al., 1998; Mix et al.,

1999]. Analysis of transfer functions based on

multiple fossil groups provides a useful check

on the results from any individual proxy [Mol-

fino et al., 1982; Pisias and Mix, 1997].

[14] Identification of fossil species assemblages

and geochemical measurements on several spe-

cies of foraminifera are commonly used to

provide information regarding water mass

properties (such as temperature) at the sea sur-

face, seasonal ranges in temperature, biological

productivity, and pycnocline depth or structure

[Imbrie and Kipp, 1971; Mix, 1989a, 1989b;

Abrantes, 1992; Watkins et al., 1996, 1998;

Andreasen and Ravelo, 1997; Mulitza et al.,

1997; Pisias and Mix, 1997; Cayre et al.,

1999a; Wolff et al., 1999]. Theory suggests that

prediction of multiple environmental properties

is possible from rich, multispecies data sets

[Imbrie and Kipp, 1971] but does not offer

definitive guidance about which environmental

properties are most important. Debate con-

tinues regarding the potential for biases in

estimates and the difficulty in uniquely assign-

ing variations in the fauna to any particular

environmental property, in part because many

environmental properties covary in the modern

ocean [Watkins and Mix, 1998]. Nevertheless,

most paleoceanographers accept that species

assemblages can yield useful quantitative or

semiquantitative indices of upper ocean tem-

perature, as well as estimates of either biologi-

cal productivity (as reflected in food supply) or

pycnocline structure (noting that these two

variables tend to be correlated in the modern

ocean). In addition, stable isotope and trace

metal/calcium ratio measurements made on

different species of planktonic foraminifera

(such as those known to tolerate oligotrophic

conditions and others limited to food-rich con-

ditions associated with nutrient-replete subsur-

face waters) can be used to detect changes in

the position of nutricline relative to the thermo-

cline and water column stratification [Fair-

banks et al., 1982; Ravelo and Fairbanks,

1992; Farrell et al., 1995].

[15] Faunal or isotopic estimates related to

pycnocline depth or structure would add value

to alkenone-based temperature estimates by

placing the Uk037 index into an appropriate

oceanographic context [Chapman et al.,

1996]. Consider the effects of thermocline

depth on the Uk037 record of temperature (Figure

2). Here we use water column data for annual-

average distributions of temperature, nitrate,

and chlorophyll (World Ocean Atlas -98,

[Ocean Climate Laboratory, 1999]) to model

such effects. We assume, for purposes of this

sensitivity test, that alkenones are produced in

the water column proportional to observed

chlorophyll concentrations. We calculate the

temperature expected to be recorded by alke-

nones as the chlorophyll-weighted average

temperature in the water column. This exercise

is intended as a sensitivity test. Although chlor-

ophyll is a reasonably good qualitative measure

of phytoplankton concentration and production

at low latitudes, subsurface chlorophyll max-

ima are not always associated with highest total

production [Cullen, 1982], and it is unclear

how chlorophyll data relate to total alkenone

production in many settings.

[16] In our sensitivity tests, at sites in which the

thermocline is always deeper than the euphotic

zone (Figure 2a±2c) the chlorophyll-weighted

temperatures approximate the contrast in SST


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between sites (or by inference, through time at

a site). Absolute temperature estimates are

slightly colder than surface values because such

sites generally have a deep chlorophyll max-

imum associated with a deep nutricline. In such

oceanographic settings the Uk037 index would

complement species-based estimates of paleo-

temperatures and agreement would yield con-

fidence in the result.

[17] At sites with a shallow thermocline (Figure

2d±2f), the chlorophyll-weighted temperatures

significantly underestimate both absolute SSTs

and the SST difference between the sites (or by

inference, through time at a site). The Uk037

index could, however, yield insight into tem-

peratures within the thermocline. This context

would present an exciting opportunity to under-

stand the properties of water masses that venti-

late the pycnocline and act as source waters in

upwelling systems, for example, in comparison

to studies of paleoproductivity.

[18] In a test case comparing sites with dif-

ferent thermocline depths and similar SSTs

(Figure 2g±2i) the chlorophyll-weighted tem-

peratures misrepresent the lack of SST con-

trast between sites. If a site changed through

time between these two conditions, alkenones

would likely provide insight into the sense of

pycnocline change (apparent cooling would

imply pycnocline shoaling). This application

could be used to confirm estimates based on

other proxies such as the comparison of

shallow-dwelling and deep-dwelling species

of foraminifera.

[19] In each of the hypothetical cases noted

above, the use of alkenone measurements in

concert with other proxies would add value and

convert a potential problem of interpretation

into an opportunity for rich insight into ocea-

nographic processes responsible for changes in

the observations.

4. Ecosystem Structure

[20] Laboratory cultures show that different

species or strains of alkenone-producing algae

yield significantly different relationships

between Uk037 and temperature [Conte et al.,

1998]. Although evidence for some of the

culture-based temperature relationships is rare

in sediment core tops, it remains possible that

ecosystem changes in the past would favor one

strain or another. Such changes, if undetected,

could lead to biases in the geologic record of

temperature change.

Figure 2. Water column profiles of annual-average temperature T, nitrate N, and chlorophyll C (opensymbols [from Ocean Climate Laboratory, 1999]); are used to explore hypothetical influences of thermoclinedepth on temperatures recorded by alkenone proxies. This exercise is intended as a sensitivity test. Atlaschlorophyll data are limited to 100-m depth, and we assume exponential decrease at greater depths. Here weassume that alkenones are produced in the water column proportional to observed chlorophyll distributions,and calculate the chlorophyll-weighted average temperature in the water column (solid symbols). (a±c) T, N,and C at two sites with a deep pycnocline, with warm SST (squares, 108S, 1358W), and cool SST (diamonds,288N, 1378W). Here chlorophyll-weighted temperatures (solid symbols, shown at chlorophyll-weighteddepths) approximately record the SST contrast of 78C between sites, but slightly underestimate absolute SSTdue to subsurface production. (d±f) T, N, and C at two sites with a shallow thermocline, one with warm SST(squares, 98N, 918W), and one with cool SST (diamonds, 38S, 828W). Here chlorophyll-weightedtemperatures (solid symbols) underestimate the SST contrast of 58C between sites as well as the absolute SSTvalues due to subsurface production. (g±i) T, N, and C at two sites with similar SST but contrast inthermocline depth, one deep (squares, 108S, 1358W), and one shallow (diamonds, 98N, 918W). Herechlorophyll-weighted temperatures (solid symbols) would predict unrealistic SST contrast of 58C betweensites, but if the primary control by pycnocline depth was recognized based on other proxies, the recordedtemperatures would provide a useful confirmation of changes in the pycnocline.


[21] Attempts to assess changes in the domi-

nance of alkenone producers based on the

preserved record of coccolith species abun-

dances [MuÈller et al., 1997; Weaver et al.,

1999] so far suggest no significant biases in

Uk037 temperature estimates related to species

dominance changes. Such tests have only been

made in a few locations, however, and should

be extended. Species abundance data from

other fossil groups such as foraminifera, radi-

olarians, diatoms, and others may also add

value as a record of the structure of ecosystems

or water column processes that influence the

dominant strain of algae that produce alke-

nones. As a preliminary step, we see value in

investigating relationships of residuals in Uk037

temperature estimates (i.e., the difference

between inferred temperatures and atlas values)

with species distributions (based on raw spe-

cies data in all fossil groups in addition to

derived properties such as transfer-function

temperature estimates) in the same core top

sediment samples.

[22] This strategy for assessing ecosystem com-

position can also be applied to multimolecular

proxies, i.e., those based on the abundances of

a range of organic compounds analyzed in the

same samples. Alkenone distributions (e.g.,

unsaturation in n-C37, C38, and C39) and those

of other biosynthetically related compounds

(e.g., alkenonates, alkenes) probably vary

among the present-day species (and strains)

that synthesize them. Several hundred biomar-

kers are now known to occur virtually ubiqui-

tously in marine sediments [Brassell, 1993].

Some are rather specific in their origin, as are

the alkenones (e.g., dinosterol comes from

dinoflagellates), while others are of rather gen-

eral biological origin (e.g., cholesterol is pre-

sent in a broad range of zooplankton and other

animal groups). Some biomarkers are clearly of

terrigenous origin (e.g., long straight chain

lipids such as n-C29 and n-C31 alkanes and n-

C28 and n-C30 alkanols, from land plants).

Relative or absolute abundances and fluxes of

biomarkers such as these can provide valuable

indications of the inputs of the various groups

of organisms through time (molecular stratigra-

phy) or space (molecular mapping). However,

relationships between biomarker production

and sediment input remain poorly calibrated

for many organic compounds.

[23] Opportunities exist to collect major sets of

biomarker data as part of the alkenone analyses

with comparatively little additional effort. The

requirements are that the analytical steps,

which can be largely automated, be restricted

to extraction, simple fractionation, derivatiza-

tion, identification, and quantification by gas

chromatography mass spectrometry (GCMS) or

other methods. A conservative estimate of the

resolving power of this approach is that 50±100

biomarkers could be quantified in semiauto-

matic fashion, with little additional cost relative

to the measurement of just a few compounds

(Table 1).

[24] Multimolecular procedures of this type are

routinely employed in large numbers of ana-

lyses for pollutants, as in Environmental Pro-

tection Agency (EPA) studies. Extensive data

sets on molecular abundance may be manipu-

lated statistically in conjunction with other

types of proxy data in attempts to relate eco-

system components (species and strains of alga

or other organisms) with covarying biomarker

distributions. Such chemometric strategies

[e.g., Brassell et al., 1986a] mirror those used

for multivariate statistical analyses and transfer

function calculations based on other fossil

groups [e.g., Imbrie and Webb, 1981; Imbrie

et al., 1989]. Challenges will arise as these

strategies are developed, in part because some

organic compounds are much more vulnerable

to diagenetic alteration than are others. Never-

theless, examination of a variety of compounds

with different diagenetic properties may yield

insights into these secondary processes, in a


way that will benefit primary environmental

reconstructions.

[25] Looking farther into the future, we see

great promise in rapidly improving methods

of genetic sequence analysis, which may even-

tually be able to detect the genetic makeup of

the organisms that produce alkenones or other

organic compounds. Suitable methods that

would be applicable to sedimentary materials

do not exist at present, but development may be

possible. If such a strategy could identify

unambiguously the relative abundance of the

various strains of haptophyte algae responsible

for making the biomarker molecules preserved

in deep-sea sediments, it would be relatively

straightforward to choose the proper tempera-

ture calibration scheme for a paleotemperature

estimate.

5. Paleosalinity

[26] For many years, paleoceanographers have

sought viable proxies of regional salinity varia-

tions in the ocean. The combination of salinity

and temperature estimates would enable calcu-

lation of water densities, important in the high

latitudes for discerning sources of intermediate-

and deep-water ventilation. In the low latitudes,

salinity estimates also help to constrain the

balance between precipitation and evaporation

and to infer freshwater transports between

ocean basins. Near coastlines, paleosalinity

estimates would help record the history of river

fluxes associated with climate changes on land.

The importance of having a proxy for paleosa-

linity cannot be overstated. In models, even

small changes in the flux of fresh water to the

ocean in the high latitudes, or the transport of

water vapor between ocean basins in the low

latitudes, have dramatic effects on global ther-

mohaline circulation and global climate [Rahm-

storf, 1995]. Even a semiquantitative salinity

proxy would be valuable.

[27] Presently, there is no direct geologic proxy

for paleosalinity. Estimates are derived from

multiple proxies, for example, by combining

measurements of d18O in the calcite shells of

planktonic organisms such as foraminifera with

independent estimates of paleotemperature

[Imbrie et al., 1973; Broecker, 1989; Duplessy

et al., 1993; Rostek et al., 1993; Wang et al.,

1995; Cayre et al., 1999b] after correction for

the imprint of changing ice volume on global

oceanic d18O budgets [Labeyrie et al., 1987;

Mix, 1987; Shackleton, 1987]. For example,

comparison of Uk037 temperature estimates with

foraminiferal d18O data led Rostek et al. [1993]

to propose large changes in Indian Ocean

salinity related to monsoon strength (Figure

3). In practice, rigorous estimation of paleosa-

linity has proven quite difficult, as propagation

of errors from each part of the proxy adds

uncertainties to the result [Schmidt, 1999a,

1999b]. Comparison of multiple independent

proxies for salinity would provide important

crosschecks on results.

[28] Two primary challenges have so far limited

the application of isotopic salinity estimates:

uncertainties in the constancy of the relation-

ship between salinity and the oxygen isotopic

composition of water, and the precision of

Table 1. Classes of Biomarkers and Numbers of Compounds as Candidates of Multimolecular Proxies

Biomarker Class Compounds

C37 ± 39 alkenones, related alkenoates, and alkenes �15Sterols (e.g., dinosterol, cholesterol) �20Long chain n-alkanes, alkanols, and alkanoics (methyl esters), diols, and ketols �40Phytol, diplopterol, and other triterpenoids (e.g., amyrins, hopenes) �10


temperature estimates applicable to foraminif-

eral shells [Rohling and Bigg, 1998]. Progress

is being made on the first challenge in model-

ing studies that include both water and isotopic

transports in the atmosphere and ocean [Juillet-

Leclerc et al., 1997; Schmidt, 1998; Jouzel et

al., 2000; Delaygue et al., 2000]. Multiproxy

estimates of temperature may improve esti-

mates of paleosalinity by narrowing the range

of uncertainty for temperature corrections. Uk037

indices will play a role in this effort. Other new

methods will also help. For example, estimates

of calcification temperature based on proxies

such as Mg/Ca within the same shells as the

isotope measurements now appear promising

[NuÈrnberg et al., 1996; Hastings et al., 1998;

Lea et al., 1999, 2000; Mashiotta et al., 1999;

Rosenthal et al., 2000a; Elderfield and Gans-

sen, 2000].

[29] Similar strategies to derive paleosalinity

estimates from temperature estimates and oxy-

gen isotope data may come from the combina-

tion of the Uk037 index and d18O in the coccolith

fraction in deep-sea sediments. Isotopic dise-

quilibrium effects in coccoliths from oceanic

sediments remain poorly known, however,

because the small size of coccoliths makes it

nearly impossible to isolate species [Dudley

and Goodney, 1979]. New technologies for

analyses of isotopes and trace metals of small

particles by laser ablation may overcome such

difficulties in the future.

[30] Preliminary data on the distribution of the

alkenone n-C37:4 suggest an intriguing correla-

tion to sea surface salinity [Rosell-MeleÂ, 1998].

The cause of this relationship remains un-

known, but one option is a biological effect

based on differences in the saturation index of

n-C37 alkenones of species that survive in

conditions of low salinities relative to species

prevalent at higher salinities. At present, the

cause of this relationship is little more than

speculation, and other methods to identify the

28

27

26

25

-3

-2

-1

0

-3

-2

-1

39

37

35

330 50 100 175125 1507525

AGE (kyr BP)

SS

T(U

k’ 3

7o C

)δ18

OG

.ru

be

r

(o /oo

PD

B)

δ18O

(o /oo

PD

B)

Sal

inity

(o /oo

PS

U)

a

b

c

d

Figure 3. The combination of Uk037 temperature

estimates and foraminiferal d18O data estimatechanges in paleosalinity in Indian Ocean coreMD900963, 5804^0N, 73853^0E [Rostek et al.,1993]. (a) Uk0

37 SST estimates, based on thecalibration of Prahl and Wakeham [1987]. (b) Thed18O values of the surface-dwelling foraminiferaGlobigerinoides ruber. (c) Smoothed d18O of G.ruber (solid line), compared to an estimate of globaloceanic d18O history associated with changing icevolume (dashed line, following Labeyrie et al.[1987]). (d) Estimates of surface-water paleosalinityat the site of MD900963 based on temperature andice-volume corrections to the d18O of G. ruber, anda modern salinity of 35 PSU. Upper and lower linesreflect the range of slopes in the relationship of d18Oand salinity observed in surface waters of themodern Indian Ocean [Rostek et al., 1993].


organisms creating the alkenone molecules

(genetic sequencing, or additional molecular

information) may be needed. Nevertheless, we

see value in exploring relationships of this and

other types of organic molecules to salinity and

to a rich array of environmental properties in

general. If the relationships to salinity involve

the presence of different species or strains of

alkenone producers, resolving these effects

may yield further insights into interpretations

of temperature from unsaturation indices.

6. Paleo-pCO2

[31] The carbon isotopic composition of alke-

nones or other organic compounds combined

with those of foraminifera offer the potential to

reconstruct the distribution of aqueous CO2

concentrations in the upper ocean (Figure 4).

In concert with paleotemperature measure-

ments these data can be used to estimate atmo-

spheric pCO2 values that would be in

equilibrium with surface waters. This is com-

monly referred to as paleo-pCO2 [Jasper and

Hayes, 1990; Jasper et al., 1994]. The calcula-

tion of paleo-pCO2 from the carbon isotopic

composition of alkenones depends, among

others, on an accurate estimation of the tem-

perature at the precise season and depth of the

alkenone production. This is best done with

Figure 4. Multiproxy measurements of paleo-pCO2 in core W8402A-14GC from the equatorialPacific Ocean. (a) Equatorial Pacific paleo-pCO2

estimates based on d13C and temperature data (opensquares from Jasper et al. [1994]) and similarvalues corrected for growth rate effects estimatedfrom Sr/Ca (solid circles from Stoll and Schrag[2000], error bars reflect the estimated range ofgrowth rates). Solid line without symbols is atmo-spheric pCO2 history as recorded in the Vostok icecore [Petit et al., 1999]. (b) �pCO2, the differencebetween equatorial Pacific paleo-pCO2 estimatesand atmospheric values (open squares withoutgrowth rate corrections, solid circles with growthrate corrections and error bars as in Figure 4a).Positive values of �pCO2 indicate that theequatorial Pacific was a consistent source of CO2

to the atmosphere over the past �250 ka. (c) Uk037

temperature estimates in core W8402A-14GC [Lyleet al., 1992]. (d) Sea surface PO4 concentrations,estimated from Sr/Ca in the coccolith fraction ofcore W8402A-14GC, used for growth rate correc-tions on alkenone d13C [Stoll and Schrag, 2000].


alkenone proxy Uk037. Even if Uk0

37 records a

subsurface or seasonally biased temperature

estimate, it is the most appropriate temperature

to use in estimating pCO2 from alkenone iso-

topic data because it is carried in the same

molecule and thus is likely to record corre-

sponding environmental conditions.

[32] Metabolic processes within cells and

within populations may affect alkenone indices.

For example, some studies suggest potential

impacts of cell growth rate on alkenones [Bidi-

gare et al., 1997], which complicate estimation

of dissolved CO2 concentrations based on d13C

[Popp et al., 1999], and perhaps also tempera-

ture estimates based on the Uk037 index [Epstein

et al., 1998]. Application of additional proxies

offer the potential to constrain growth rates in

the geologic record and thus to correct for such

effects. Empirical evidence from the water

column suggests that population growth rates

are related to nutrient contents of water masses

[Bidigare et al., 1999]. On the basis of these

relationships, molecular indices of growth rate

are under development.

[33] Several geologic proxies offer the potential

to constrain the environmental or ecosystem

controls on growth rates of the biota. Proxies

exist for upper ocean nutrient concentrations

based on trace metal/calcium ratios in plank-

tonic foraminifera [Boyle, 1981; Keigwin and

Boyle, 1989], although effects of algal sym-

bionts [Mashiotta et al., 1997] and temperature

[Rickaby and Elderfield, 1999] may also influ-

ence such estimates. Nutrient utilization esti-

mates have been based on d15N of sedimentary

organic matter [Altabet and Francois, 1994].

Primary productivity estimates are based on

transfer function approaches using planktonic

foraminiferal, calcareous nannofossil, diatom,

or dynocyst species data [Mix, 1989a; Abrantes,

1992; Cayre et al., 1999a; Beaufort et al.,

1999], benthic foraminiferal species percen-

tages [Loubere and Fariduddin, 1999] or

abundance [Herguera and Berger, 1991], or

mass accumulation rates of biogenic materials

or chemical tracers [Dymond et al., 1992; Lyle

et al., 1992].

[34] Preliminary culture experiments suggest

that Sr/Ca in calcite liths may reflect the growth

rate of the coccolithophorid Emiliania huxleyi

[Rosenthal et al., 2000b], perhaps owing to a

kinetic effect on calcification rate. Because E.

huxleyi is a known alkenone producer, Sr/Ca in

this species may, in turn, reflect the growth rate

of the alkenone-producing flora. A recent study

of surface sediments across a latitudinal trans-

ect in the equatorial Pacific suggests that Sr/Ca

of the bulk coccolith fraction, cleaned to the

extent possible of contaminants associated with

Fe and Mn oxyhydroxides, covaries with mod-

ern spatial patterns of primary productivity

[Stoll and Schrag, 2000].

[35] Under an assumption that Sr/Ca variations

in the bulk coccolith fraction track growth rates

of the alkenone-producing haptophytes, Stoll

and Schrag [2000] use downcore data to cor-

rect for growth rate changes on the carbon

isotopic paleo-pCO2 estimates of Jasper et al.

[1994], which include estimates of temperature

based on the Uk037 index [Lyle et al., 1992].

These new calculations confirm the earlier

inference that the equatorial Pacific was always

a regional source of CO2 to the atmosphere;

i.e., local paleo-pCO2 values were consistently

higher than atmospheric pCO2 values in the

Vostok ice core [Petit et al., 1999]. The new

calculations suggest even greater CO2 outgas-

sing from the region and a long-term drift from

much higher pCO2 values in the past (espe-

cially >150 kyr BP) to lower values with the

last 100 ka. Significant uncertainties in the Sr/

Ca growth rate corrections include possible

changes in the Sr content of seawater through

time, which remains controversial [Stoll and

Schrag, 1998; Martin et al., 2000]. Clearly,

more work is needed to test the efficacy of


these multiproxy approaches in constraining

regional sources and sinks of CO2 in the

paleoceanographic record. Nevertheless, we

are encouraged that qualitative or semiquanti-

tative corrections for growth rates on alkenone

d13C data and paleo-pCO2 estimates may be

possible.

[36] Relatively little data exist on the chemical

or isotopic composition of coccolith species in

the geologic record. This reflects the difficulty

of isolating the calcite plates formed by indi-

vidual species of coccoliths, because of their

small size. New technologies for isolating such

particles or for analyzing them individually

within a mixed sample may be possible, based

on advances in small sample techniques such as

microprobe or laser ablation inductively coup-

led plasma±mass spectrometer (ICP±MS).

Recently developed analytical methods also

offer the opportunity to measure multiple ele-

mental ratios rapidly (e.g., Cd/Ca, Ba/Ca, Mg/

Ca, and others), thereby making the multiproxy

approach much easier to apply [Rosenthal et al.,

1999]. With these new technologies we expect

further progress on development of growth

rate corrections and other isotopic proxies

related to paleo-pCO2 estimates. In spite of

large uncertainties in several aspects of these

measurements and their interpretation, estab-

lishing regional patterns of paleo-pCO2 to

assess changes in sources and sinks of atmo-

spheric CO2 in the ocean would be of such

enormous benefit that these studies are worth

pursuing, recognizing that estimates will im-

prove as knowledge of controls on the proxies

is refined.

7. Seafloor Processes

[37] Alkenones, as hydrophobic compounds

originally part of the cells of coccolithophores,

should have an affinity for fine-grained sedi-

ments through adsorption on clays and silts, or

absorption in organic particles. Although rea-

sonable, this conjecture is unproven, and one of

our recommendations is for studies to confirm

which phases in the sediment actually carry the

alkenone signal.

[38] A present focus of paleoceanographers is

on rapidly accumulating sequences in sediment

drifts and continental margin sediments as a

means of obtaining high-resolution paleoceano-

graphic records [Sachs and Lehman, 1999]. We

worry, however, that artifacts may be intro-

duced into alkenone-based (and other) environ-

mental histories in such settings because fine-

grained sediments in drift deposits may be

transported downslope or across an ocean basin

by turbidity currents or may be advected by

bottom currents in deep recirculating gyres.

[39] For example, more than 30 years ago, it

was recognized that fine-grained sediments rich

in hematite all along the U.S. Atlantic conti-

nental margin were introduced by the St. Lawr-

ence river system and redistributed by deep-sea

contour currents [Heezen et al., 1966]. These

reddish sediments (which also include pre-

Pleistocene coccoliths) extend as far as Ber-

muda Rise, where excess fines accumulate in

drifts [Laine and Hollister, 1981; McCave et

al., 1982; Suman and Bacon, 1989]. Rapid

changes in carbonate contents of Bermuda Rise

sediments reflect changing input of fine-

grained terrigenous sediments from eastern

Canada [Keigwin and Jones, 1989]. Thus

changes in the fine sediments at Bermuda Rise

reflect variability of high-latitude climate and

deep-water circulation, rather than local condi-

tions in the subtropical gyre. If alkenones were

carried along with these fine-grained sedi-

ments, an intermittent cold artifact associated

with waters from the continental slope could

occur that might mimic high-latitude climate

changes [Sachs and Lehman, 1999]. Likewise,

the presence of shallow-water microfossils in

deep-water sequences from continental margins

provides clear evidence of downslope transport


that could carry an anomalous signal. Measures

of reworked tracers appropriate to each site

(such as shallow-water fossils in continental

margin settings or mineralogical tracers of fine-

grained sediment sources) will help to identify

problems associated with sediment transport.

[40] Bioturbation induced by benthic organisms

moves clay- and silt-sized particles more read-

ily than sand-sized particles [Wheatcroft,

1992]. Thus there is a potential for bias in

alkenone records relative to records based on

sand-sized foraminifera. For example, in ultra-

high resolution studies the alkenone record of

brief climate change events may be smoothed

or even shifted in depth compared to estimates

based on foraminiferal species or stable isotope

records. This would not be a large problem in

laminated sediments that have not been biotur-

bated or for low-resolution studies in which the

depth spacing between samples is >10 cm.

Indeed, downcore scanning based on reflec-

tance spectroscopy suggests that significant

information can be preserved at the scale of a

few centimeters in spite of bioturbation effects.

Barring effects of sediment transport noted

above, meaningful records of climatic variabil-

ity on the scale of a few centuries can be

produced where sedimentation rate exceeds

10 cm kaÿ1 [Chapman and Shackleton, 1998].

[41] Deconvolving the effects of bioturbation

on sedimentary signals is not straightforward.

In part this reflects a lack of quantitative indices

of bioturbation intensity through time, except

in isolated cases such as where thin ash layers

are present [Ruddiman and Glover, 1972].

Pisias [1983] argued based on time series

analysis of geologic records that bioturbation

intensity increases with sedimentation rate,

implying a result that even sediments accumu-

lating slowly would preserve a useful geologic

record. Some progress in developing tracers of

bioturbation intensity has come with studies

that suggest a link to total food supply (i.e.,

biological productivity of overlying waters)

[Trauth et al., 1997], which also may leave

an imprint on the benthic species assemblages

[Loubere, 1989].

[42] Bioturbation models typically treat only the

part of mixing that can be simulated as a

diffusive process [Peng et al., 1979], which

occurs typically within a few centimeters of the

sediment-water interface. In addition, burrows

are commonly preserved below the well-mixed

surface layer. The presence of burrows and

other bioturbation structures may be detected

by careful visual description or digital imaging

of the sediments [Ortiz and Rack, 1999] or by

systematic X-ray analysis of sediment cores. In

many cases, such data can help investigators to

avoid major artifacts associated with burrows at

the time of sampling.

[43] Bioturbation is potentially damaging when

materials carrying a chemical tracer are moved

from intervals of high abundance into barren or

nearly barren zones. This is called the `̀ biotur-

bation-abundance couple'' [Broecker et al.,

1984]. Its effects have been explored in the

context of radiocarbon dating [Bard et al.,

1987; Broecker et al., 1999] and stable isotope

records [Mix and Ruddiman, 1985]. Such stu-

dies require precise data on abundance (relative

to total sediment) of the material carrying a

paleoceanographic tracer. Even if the effects of

bioturbation cannot be removed completely

from measured records using mathematical

models, potential problems can be identified

if reliable abundance data are available. Thus

we recommend that that absolute abundances

of alkenones (and other proxy carriers such as

microfossil shells) be reported for studies in

which bioturbation may be a problem.

8. Summary and Recommendations

[44] Great opportunities exist for application of

multiple proxies to the record of past changes


in the oceans and global climate. Multiproxy

strategies can help one to `̀ see'' through the

biological processes that mediate most paleo-

ceanographic records. In many cases, the exact

biological mechanisms or chemical pathways

controlling paleoceanographic proxies are un-

certain. We suggest a strategy of application of

proxies to the geologic record in parallel with

continuous efforts to refine understanding of

mechanisms and calibration of methods. Our

experience is that this two-pronged approach

leads to rapid advancement of knowledge.

[45] In the simplest case, application of multiple

proxies to estimate a single environmental

property, such as SST, helps gain confidence

in the result by reducing errors. Differences

between estimates based on multiple proxies

are equally important, as they lead to deeper

understanding based on the oceanographic con-

text of each measurement. Thus we recommend

analysis of residuals between estimates based

on multiple proxies or between proxy estimates

and actual (atlas or measured) environmental

values as a path toward greater insight into the

true nature of each proxy. In an example of

oceanographic context we note that faunal or

isotopic estimates of pycnocline depth or struc-

ture would add value to alkenone-based tem-

perature estimates by placing the Uk037 index into

an appropriate oceanographic context. Informa-

tion on overall ecosystem structure based on

faunal and floral species assemblages, or iden-

tification of alkenone producers via a biomar-

ker or genetic strategy, would constrain the

choice of temperature calibrations and thus

improve paleotemperature estimates.

[46] A more complicated multiproxy strategy

involves the use of dissimilar proxies to derive

oceanic properties that cannot be assessed with

any single measurement. Examples in which

progress is likely are estimating paleosalinity

and paleo-pCO2. Significant uncertainties are

involved in both properties. Nevertheless, both

are so important that continued study is war-

ranted. For paleosalinity estimates, multiproxy

temperature estimates (including faunal trans-

fer functions, Uk037, and trace metal indices)

coupled to oxygen isotope data in foraminifera

are needed. We also recommend detailed study

of the coccolith fraction, using emerging tech-

nologies for small-sample chemical and isoto-

pic analysis. On the basis of preliminary

relationships between alkenone n-C37:4 and

salinity we believe exploration of full chro-

matograms for multimolecular geochemical

evidence of salinity change and other envir-

onmental effects is warranted. Estimates of

paleo-pCO2 call for a geologic proxy for cell

or population growth rate, and we suggest

further explorations of proxies for nutrients,

nutrient utilization, biological productivity,

and calcification rate.

[47] As a first step toward identifying biases in

the sedimentary record associated with sedi-

ment transport and bioturbation, we recom-

mend research to identify the particles that

carry alkenones, as well as a practice of report-

ing absolute abundances of alkenones (and

other particles carrying environmental signals

for other proxies).

[48] Confirmation that multiproxy strategies

actually work in the geologic record requires

coordinated study of core top sediments. This is

easier to recommend than to accomplish, as

high-quality core top samples are relatively rare

commodities. We thus recommend that field

programs involving coring make special efforts

to archive and share high-quality samples of the

sediment-water interface. The best available

tool for this purpose is the multicore, which is

fortunately available in many institutions.

Where possible, high-quality core tops should

be dated using 14C and other stratigraphic

methods to verify the presence of essentially

modern sediments and if possible to establish

sedimentation rates.


[49] Finally, we note that the level of scrutiny

the alkenone community is applying to their

proxies, as exemplified by the Woods Hole

workshop of 1999, serves as a model for other

communities. One of our recommendations is

for workers on other data types to join together

in a similar fashion to intercalibrate, refine, and

revise their proxies as needed for a richer and

more complete view of ocean and climate

history.

Acknowledgments

[50] We thank T. I. Eglinton, M. H. Conte, G. Eglinton,

and J. M. Hayes, organizers of the NSF sponsored

workshop Alkenone-Based Paleoceanographic Indica-

tors. We also thank the workshop participants for

spirited discussions and the U.S. National Academy of

Sciences and Woods Hole Oceanographic Institution for

facilitating this effort. Reviews by W. Curry, M.

Feldberg, L. Labeyrie, F. Prahl, and R. Thunell improved

the manuscript.

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