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doi:10.1016/j.qua

Quaternary Science Reviews 23 (2004) 901–918

Palaeo-climate reconstruction from stable isotope variations inspeleothems: a review

Frank McDermott*

Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland

Received 30 April 2003; accepted 13 June 2003

Abstract

Speleothems are now regarded as valuable archives of climatic conditions on the continents, offering a number of advantages

relative to other continental climate proxy recorders such as lake sediments and peat cores. They are ideal materials for precise

U-series dating, yielding ages in calendar years, thereby circumventing the radiocarbon calibration problems associated with most

other continental records. Stable isotope studies in speleothems have shifted away from attempting to provide palaeo-temperature

reconstructions to the attainable goal of providing precise estimates for the timing and duration of major O isotope-defined climatic

events characterised by high signal to noise ratios (e.g. glacial/interglacial transitions, Dansgaard–Oeschger oscillations, the ‘8200-

year’ event). Unlike the marine records, speleothem data sets are not ‘tuned’, and their independent chronology offers opportunities

to critically assess leads and lags in the climate system, that in turn can provide important insights into forcing and feedback

mechanisms. Improved procedures for the extraction and measurement of stable isotope ratios in fluid inclusions trapped in

speleothems are likely to provide, in the near future, a much enhanced basis for the quantitative interpretation of O isotope ratios in

speleothem calcite. The latter developments open up once again the tantalising prospect of palaeo-temperature estimates, but more

importantly perhaps, provide a direct test for a new generation of general circulation models whose hydrological cycles will

incorporate the ‘water isotopes’. The literature is reviewed briefly to provide for the reader a sense of the current state-of-the-art, and

to provide some pointers for future research directions.

r 2004 Elsevier Ltd. All rights reserved.

1. Introduction

Increasingly there is a need for well-dated high-resolution palaeo-climate records from continentalsettings to test and validate general circulation models(GCMs) at a higher spatial resolution, and to investigatepossible leads and lags between different components ofthe climate system. Speleothems are multi-proxy palaeo-climate archives with the potential to provide such data.In carefully chosen sites they can record key aspects ofclimate variability such as mean annual temperature,rainfall variability, atmospheric circulation changes andvegetation response in a variety of measurable para-meters that include stable isotope ratios, inter-annualthickness variations of growth laminae, growth-ratechanges, variations in trace element ratios, organic acidcontents and the nature of trapped pollen grains. Thisreview focuses on the use of stable isotopes in

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s: frank.mcdermott@ucd.ie (F. McDermott).

front matter r 2004 Elsevier Ltd. All rights reserved.

scirev.2003.06.021

speleothems as palaeo-climatic indicators, and theemphasis is on developments and data sets that havebeen reported since previous reviews of the subject(Schwarcz, 1986; Gascoyne, 1992). The focus is primar-ily on oxygen isotopes, but carbon isotopes are includedwhenever they have contributed significantly to palaeo-climatic interpretations. Several unresolved issues re-main, but recently there have been important insightsinto the interactions between component parts of thesystem (e.g. marine sources, isotopic evolution in thehydrological system and isotopic effects during infiltra-tion through the unsaturated zone) that now underpinthe interpretation of O isotopes in speleothems.Systematic studies of stable isotopes in speleothems

commenced more than three decades ago (Hendy andWilson, 1968; Thompson et al., 1974), but progress washampered by the need for large samples (ca 10 g) foralpha-spectrometric U-series dating. The developmentof thermal ionisation mass-spectrometry (TIMS) tech-niques to measure U-series isotope ratios rejuvenatedthe subject (Edwards et al., 1988; Li et al., 1989). TIMS

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can provide 230Th/U dates that are almost 10 times moreprecise than conventional alpha-spectrometry, with areduction in sample size by about the same magnitude.Recently, a new generation of plasma-ionisation mag-netic-sector mass spectrometers (PIMMS) characterisedby high ionisation efficiency promise further improve-ments in sample size requirements and analyticalprecision relative to TIMS (Shen et al., 2002). The latterinstruments offer vastly improved ionisation efficiencyfor thorium, and with further refinement are likely tobecome the method of choice, especially for low-uranium Holocene speleothems that contain relativelylittle radiogenic 230Th. With these new technologicaldevelopments, speleothems offer advantages over manyother continental palaeo-climate records because theycan be dated in calendar years with a precisionapproaching 70.5% (2s), circumventing radiocarbonage calibration and reservoir correction problems thathamper other continental climate archives such as lakesediments and peat records. Indeed it is likely thatspeleothem records will increasingly be used to refine thechronology of the Greenland ice-core records, assumingthat regional synchroneity for the major early Holoceneand last glacial Dansgaard–Oeschger (D/O) O isotopeshifts can be demonstrated (e.g. McDermott et al., 2001;Wang et al., 2001; Sp .otl and Mangini, 2002; Genty et al.,2003). It should be noted, however, that U-series datesdepend critically on the accuracy with which the mixed229Th/236U spikes have been calibrated with respect toknown secular equilibrium standards, and there iscurrently a need to undertake systematic inter-labora-tory comparisons to ensure that U-series dates producedby different laboratories are directly comparable.Stalagmite growth rates vary by at least two orders of

magnitude (typically in the range 0.01–1.0mm/year),depending on factors such as temperature and thecalcium ion concentration of the drip-waters (Bakeret al., 1998; Genty et al., 2001a, b). Thus, the timeinterval represented by individual stable isotope mea-surements depends critically on the growth rate of thespeleothem chosen for analysis. Using conventionalsampling techniques (e.g. a dental drill to remove0.5mm samples), the time interval averaged by stableisotope measurements would typically range from a fewyears to several decades. The detection of short-livedclimatic events and the resolution of low-amplitudeclimatic signals therefore require the use of rapidlydeposited speleothems, assuming that conventionalsampling and analytical techniques are employed. Inslowly deposited speleothems serious damping of theisotope signal may occur, with the result that significantbut short-lived climatic events (e.g. the 8200-yearcooling event) might not be detected (McDermottet al., 2001).A feature of stable isotope studies on speleothems

during the past decade has been efforts to improve the

spatial, and therefore the temporal resolution ofsampling for O and C isotope analyses. McDermottet al. (2001) employed a laser-ablation gas-chromato-graphy isotope ratio mass spectrometry (LA-GC-IRMS)system with a 25W CO2 laser to thermally release CO2

by 400ms laser bursts. Using a system of forward andreverse profiling along the central growth axis of aHolocene stalagmite (CC3) a spatial resolution 250 mmwas achieved (see Section 4.2). Analysis of standardsgives similar d13C values to those obtained by conven-tional acid digestion, but d18O values that are system-atically lowered by 2 per mil. Replicate analyses ofstandards indicate that the isotope data are reproducibleto better than 0.1% for d13C and 0.2% for d18O.Following the 2 per mil correction, the laser dataaccurately reproduce the first-order features of apreviously published coarse resolution O isotope recordfor this speleothem (McDermott et al., 1999). Thespatial resolution achievable by this system representsabout a four-fold improvement relative to that ofconventional dental drilling methods, but the dataacquisition is rapid and automated, thereby offeringsignificant advantages over conventional analyses.A different approach has been the use of micro-milling

techniques to improve the spatial resolution of sampling.A recent study by Frappier et al. (2002), for example,achieved a sampling resolution of just 20mm, correspond-ing to a weekly to monthly temporal resolution in arecently deposited stalagmite from Belize. These high-resolution data exhibit high amplitude (11%), rapid (sub-seasonal) fluctuations in d13C that appear to reflectvariations in the El Nino/Southern Oscillation (ENSO)system. A similar spatial resolution (25mm) was achievedrecently by Kolodny et al. (2003) using an ion micro-probe. This method offers excellent spatial resolution, butthe relatively poor analytical precision that characterisesthe current generation of instruments (ca 70.5%)restricts its use to the study of high-amplitude isotopic‘events’ and/or climate transitions.In principle though, with carefully chosen spe-

leothems it may be possible in the future to reconstructthe annual hydrological cycle of d18O variability,offering both a chronological tool (cycle counting) andnew insights into changes in the amplitude of seasonald18O variability in rainfall. For the case of a speleothemgrowth rate of 0.5mm/year for example, it should bepossible to obtain a temporal resolution better than 1month using an ion-probe technique (25 mm spot size),and such a study would be best carried out in a regionwhere a relatively large seasonal d18O cycle is antici-pated. In cases where the sampling resolution is sub-annual, but insufficient to resolve a clear seasonal cycle,care must be taken to avoid unresolved seasonal effectsthat could lead to a noisy signal (e.g. comparing ‘wintercalcite’ in one analysis with ‘summer calcite’ in anadjacent analysis).

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A number of issues relating to the interpretation ofstable isotope data in speleothems remain unresolved.The most challenging of these has been to decipher thevarious, usually competing factors that drive oxygenisotope variations, in order to recover unambiguouspalaeo-climatic signals. An early goal was to reconstructabsolute changes in mean annual air temperature (e.g.Gascoyne et al., 1980), but this is increasingly seen asunrealistic, because of the plethora of effects thatinfluence the d18O of cave drip-waters (d18Odw), andtherefore the d18O of the precipitated speleothem calcite(d18Oct). These effects are discussed below, but recentlythere is renewed confidence that reliable stable isotopedata can be extracted from speleothem fluid inclusions,albeit at a relatively coarse temporal resolution (e.g.Matthews et al., 2000; Dennis et al., 2001; Genty et al.,2002; Serefiddin et al., 2002; McGarry et al., 2004, thisvolume). In principle, these developments should allowthe original goal of palaeo-temperature estimation to beattained in situations where it can be demonstrated thatcalcite was deposited in isotopic equilibrium with thecave drip-waters. In addition, the fluid inclusion datacan be used to reconstruct temporal and spatialvariability in the d18O of palaeo-meteoric water, andin the future these data will test the validity of GCMsthat incorporate the ‘water isotopes’ in their hydro-logical cycles.Despite the intricacies of data interpretation, caves

remain attractive targets for palaeo-climate studiesbecause they preserve relatively pure calcium carbonate(typically calcite), precipitated from meteoric water inenvironments where it is protected from erosion for longperiods of time (often 104–106 years). Speleothemstypically consist of macro-crystalline calcite, althougharagonite occurs occasionally, particularly in associa-tion with high-Mg calcite or dolomite host-rocks, and/orassociated with relatively dry periods when long water–rock contact times facilitate relatively more dolomitedissolution in partially dolomitised limestone host-rocks. Petrographic studies of speleothems prior toanalysis are essential to avoid analysing re-crystallisedspecimens, to identify possible growth hiatuses (usuallymarked by thin detrital-rich layers), to recognise shiftsand offsets in the growth axis and to identify changes incarbonate mineralogy. The possible palaeo-environmen-tal significance of the mineralogy and crystal morphol-ogy of speleothems has been discussed elsewhere (e.g.Gonzalez et al., 1992; Frisia et al., 2000; Frisia et al.,2002), and in well-characterised karst systems these mayprovide additional constraints to aid the interpretationof stable isotope data. Denniston et al. (2000), forexample, interpreted the presence of aragonite layers inspeleothems from a dolomitic cave in central Nepal asreflecting reduced monsoonal precipitation and in-creased cave aridity. In many cases, petrographicinformation such as this aids the interpretation of stable

isotope data, but it is important to demonstrate thatpetrographic changes are regionally synchronous, toavoid mis-interpretations that could result from loca-lised cave- or drip-specific hydrological routing effects.Two features of the cave environment facilitate the

use of stable isotopes in palaeo-climate reconstruction.First, cave air temperatures remain relatively constant(typically71�C) throughout the year, and are similar tothe mean annual air temperature of the region abovethe cave. Second, in cool temperate regions, caveair is characterised by high-humidity levels (typically95–99%), minimising evaporation that might otherwisecause kinetic isotope fractionation. The mechanisms ofspeleothem deposition have been discussed in detailelsewhere (Schwarcz, 1986; Ford and Williams, 1989),but a critical point is that, in cave interiors, calcitedeposition typically occurs by degassing of CO2 fromcarbonate-saturated drip-waters on entering the caveatmosphere, and not by evaporation of water. Degas-sing is driven by the difference between the pCO2 ofthe soil and that of the cave air (typically in the ranges0.1–3.5% and 0.06–0.6%, respectively). In high-humid-ity cave interiors where evaporation is negligible, it canoften be demonstrated that stalagmite calcite is depos-ited at, or very close to, isotopic equilibrium with thecave drip-water. Under these conditions, the d18O of thefreshly precipitated calcite reflects both the d18O of thedrip-water and the temperature dependent fractionationbetween the drip-waters and the deposited calcite. Thus,in order to interpret correctly the oxygen isotopefluctuations in the calcite, it is critical to understandthe factors that influence oxygen isotope ratios in thecave waters of individual drip systems. The hydrologicalcharacteristics (e.g. Smart and Friedrich, 1987) ofindividual drip-sites influence the transfer of themeteoric water stable isotope signal to the cave drip-water. Ideally, the d18O of cave drip-water should recordthe weighted mean d18O of the meteoric water that fallson the surface above the cave site. The latter require-ment is likely to be met by seepage-flow drip-sites inshallow temperate-zone caves (Young et al., 1985;McDermott et al., 1999), but in arid and semi-arid sites,seasonally variable isotopic enrichment may occur as aresult of near-surface evaporative processes (Bar-Mat-thews et al., 1996; Denniston et al., 1999a). Anadditional complication is that soil pCO2 and drip-water Ca contents may vary seasonally, with the resultthat calcite deposition rates also vary seasonally (e.g.Genty et al., 2001a, b). One consequence is that therecorded d18O and d13C signal in speleothems canpreserve a seasonal bias, but this possibility could bedetected by detailed seasonal monitoring of the chosendrip sites to understand the factors controlling intra-annual variability in growth rates. These issues highlightthe need for detailed site-specific present-day monitoringstudies to understand better the relationship between the

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palaeo-d18O signal preserved in speleothem calcite(d18Oct) and palaeo-climatic variability.

2. Oxygen isotopes in precipitation

As discussed above, d18O in cave drip-waters reflect (i)the d18O of precipitation (d18Op) and (ii) in arid/semi-arid regions, evaporative processes that modify d18Op atthe surface prior to infiltration and in the upper part ofthe vadose zone. The present-day pattern of spatial andseasonal variations in d18Op is well documented(Rozanski et al., 1982, 1993; Gat, 1996) and is aconsequence of several so-called ‘‘effects’’ (e.g. latitude,altitude, distance from the sea, amount of precipitation,surface air temperature). A critical requirement for therecovery of d18Op from d18Oct is that isotopic equili-brium is maintained between the cave drip-water andthe calcite deposited therefrom. The criteria for recog-nising conditions of equilibrium deposition have beendiscussed previously (Hendy, 1971; Schwarcz, 1986).Briefly, the conditions are (i) that d18O remains constantalong a single growth layer while d13C varies irregularly,and (ii) that there is no correlation between d18O andd13C along a growth layer. In practice, consistentsampling along single growth layers is often difficult toachieve, not least because visible layers are often thinneralong the flanks of stalagmites compared with theircentral growth axis. Nonetheless, the so-called ‘Hendycriteria’ are used widely by researchers as a check thatcalcite was deposited at or close to isotopic equilibriumwith cave drip-waters. In some cases it can bedemonstrated that calcite deposited along the flanks ofspeleothem exhibit kinetic fractionation effects, but thatthe material deposited close to the central growth axismay have been deposited in isotopic equilibrium withthe cave drip-waters (e.g. Talma and Vogel, 1992; Sp .otland Mangini, 2002).The temperature dependence of d18O in rainfall

(dd18Op/dT) is variable and site dependent. In principle,dd18Op/dT could be greater than, equal to, or less thandd18Oct/dT (approximately �0.24% �C�1 at 25�C,O’Neill et al., 1969), the equilibrium fractionation thataccompanies calcite deposition from drip-waters inside acave. In a review of long-term changes in the O isotopiccomposition of precipitation over the mid- to highlatitudes, Rozanski et al. (1993) calculated an averagemodern-day dd18Op/dT of approximately 0.6% �C�1,but such averages clearly mask considerable site-specificvariability (e.g. Fricke and O’Neill, 1999), and therelationship may have been different in the past. Inprinciple therefore, d18Oct could increase, decrease orfortuitously remain invariant to an increase in meanannual air temperature. The latter response wouldrequire that dd18Op/dT cancelled out dd18Oct/dT, andsuch cases appear to be rare in the literature. A broadly

similar number of cases where dd18Oct/dT is positive(e.g. Goede et al., 1990; Burns et al., 2001; Onac et al.,2002) and negative (e.g. Gascoyne, 1992; Hellstromet al., 1998; Frumkin et al., 1999a, b) have beenreported. This illustrates the difficulty in unambiguouslyrelating changes in d18Oct to changes in mean annualtemperature, particularly over time intervals wheretemperature changes may have been small, and first-order climate transitions (e.g. glacial to interglacialtransitions) are not represented in the record. Theseuncertainties underline the need for additional proxyinformation from the same stalagmite (e.g. annual layerthickness variations, growth-rate changes, fluid inclu-sion data) to underpin the interpretation of d18O.On centennial to millennial timescales, factors other

than mean annual air temperature may cause temporalvariations in d18Op (e.g. McDermott et al., 1999 for adiscussion). These include: (i) changes in the d18O of theocean surface due to changes in continental ice volumethat accompany glaciations and deglaciations; (ii)changes in the temperature difference between the oceansurface temperature in the vapour source area and theair temperature at the site of interest; (iii) long-termshifts in moisture sources or storm tracks; (iv) changesin the proportion of precipitation which has beenderived from non-oceanic sources, i.e. recycled fromcontinental surface waters (Koster et al., 1993); and (v)the so-called ‘‘amount’’ effect.As a result of these ambiguities there has been a shift

from the expectation that speleothem d18Oct mightprovide quantitative temperature estimates to the moreattainable goal of providing precise chronologicalcontrol on the timing of major first-order shifts ind18Op, that can be interpreted in terms of changes inatmospheric circulation patterns (e.g. Burns et al., 2001;McDermott et al., 2001; Wang et al., 2001), changes inthe d18O of oceanic vapour sources (e.g. Bar Matthewset al., 1999) or first-order climate changes such as D/Oevents during the last glacial (e.g. Sp .otl and Mangini,2002; Genty et al., 2003).Future technical developments that may allow direct

measurement of d18O in speleothem fluid inclusions willreduce the uncertainties in the interpretation of d18Oct

and may ultimately allow calculation of absolutetemperature changes. Reliable fluid inclusion data canprovide invaluable constraints on palaeohydrologicalconditions, particularly in regions where speleothemdeposition is continuous through the glacials. Usingindependent palaeo-temperature estimates, Matthewset al. (2000) calculated the d18O of fluid inclusions usingthe values from coexisting calcite. Combining thesecalculated d18O values with D/H measurements carriedout using a vacuum thermal extraction techniqueenabled these authors to compare the fluid inclusiondata for four fossil speleothems with the present-daycave waters and meteoric water lines. The most

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significant result was that fluid inclusions from twospeleothems deposited during the glacial period plotclose to the global meteoric water line (MWL), incontrast with present-day precipitation and cave drip-waters that plot on the Mediterranean meteoric waterline (MMWL). These data highlight the importantinsights into climate-driven changes in the operation ofthe meteoric water cycle that can be gained from studiesof speleothem fluid inclusions. In a study of fluidinclusions from three caves in Israel, McGarry et al.(2004, this volume) used both the MWL and MMWL tocalculate the d18O of fluid inclusions from measured D/H values. The resulting palaeo-temperature estimatesare in good agreement with alkenone and modernanalogue-based estimates from the eastern Mediterra-nean Sea for the past 140,000 years. These data alsoindicate that whereas the dD–d18O relationships formeteoric water in the region follow the MMWL in thepresent-day and the last interglacial there was a strongshort-lived shift towards the MWL during the timeinterval corresponding to the last glacial.In the near future it is likely that there will be further

developments of the fluid inclusion extraction andmeasurement techniques that will underpin the inter-pretation of oxygen isotope ratios in speleothems.Meanwhile, the emphasis is on developing well-datedhigh-resolution d18O records that can be correlated withbetter understood (but often more poorly dated) recordssuch as the Greenland ice cores, and on mapping out thegeographical extent of regionally synchronous O isotope‘events’ such as the D/O events and the early Holocene‘8200-year’ event. Many of these ‘events’ will offerproductive targets for fluid inclusion studies in thefuture.

3. Carbon isotopes in speleothems

At pH values typical of karst waters, equilibriumconstants for the relevant reactions dictate that bicarbo-nate is the dominant species in solution, and so the large(ca 10%) bicarbonate-CO2 fractionation factor dom-inates the equilibrium fractionation process. Two end-member models, which describe the processes by whichpercolating groundwaters acquire calcium carbonate inthe soil and host-rocks above a cave, have been described(e.g. Hendy, 1971; Salomons and Mook, 1986). In anopen-system model, continuous equilibration occursbetween the seepage water and an infinite reservoir ofsoil CO2. This drives a monotonic increase in bicarbonatecontent as the water progressively acquires more solutesin the unsaturated zone. Under these conditions, the d13Cof the dissolved species reflects the isotopic compositionof the soil CO2, with no detectable isotopic imprint fromthe carbonate host-rock. For a C3 plant system, the d13Cof the dissolved inorganic carbon (DIC) in the percolat-

ing solution is predicted to be in the range �14% to�18% when the solution reaches saturation with respectto CaCO3, depending on soil pCO2 and temperature(Hendy, 1971; Salomons and Mook, 1986; Dulinski andRozanski, 1990).Under closed system conditions by contrast, the

percolating water becomes isolated from the soil CO2

reservoir as soon as carbonate dissolution commences(Hendy, 1971; Salomons and Mook, 1986), and sinceCO2 is consumed in the carbonation reactionH2O+CO2=H2CO3 the extent of limestone dissolutionis limited by the finite CO2 reservoir. Under theseconditions the isotopic composition of the carbonatehost-rock influences the isotopic composition of theDIC. For a C3 system with soil gas d13C of ca �23%and a host limestone with d13C of +1%, the DIC d13C istypically ca �11%. In practice most natural systems arelikely to be partially open, and a mathematical descrip-tion of such variability has been formulated (Dreybrodt,1988). In arid regions, large shifts in the d13C values ofspeleothem calcite have been ascribed to climate-drivenchanges in vegetation (e.g. C3 versus C4 dominatedplant assemblages, Dorale et al., 1992; Bar-Matthewset al., 1997). Data from pedogenic carbonates oftensupport such interpretations (e.g. Cerling, 1984; Cerlinget al. 1991). In these regions, relatively large shifts ind13C can occur, because soil respired CO2 in equilibriumwith a C3 dominated plant assemblage has d13C in therange �26% to �20%, while that in equilibrium withC4 vegetation is significantly heavier (d13C of �16% to�10%). These differences are preserved as distinctiveranges in d13C in secondary carbonates (typically �14%to �6% for carbonates deposited in equilibrium withCO2 respired from C3 plants, and �6% to +2% forthat from C4 plants).However, many temperate-zone speleothems also

exhibit d13C values >�6%. These values are higherthan those predicted to be in equilibrium with theprevalent C3 vegetation in temperate regions (Bakeret al., 1997). In situations where the soil–water residencetimes may be relatively short, complete isotopicequilibration may not occur between soil CO2 and thepercolating H2O, with the result that the water mayretain a component of (isotopically heavier) atmosphericCO2 in solution. Experimental studies (e.g. Liu andDreybrodt, 1997) have confirmed that the hydration ofCO2 is relatively slow, and that the kinetics of thereaction CO2+H2O=H2CO=H++HCO3

� is con-trolled by the CO2 hydration step. Other processesincluding evaporation, rapid degassing of cave drip-waters, kinetic fractionation, CO2 degassing of drip-waters and consequent calcite precipitation in thevadose zone above a cave have been offered as possibleexplanations for these relatively heavy carbon isotopesignatures (Baker et al., 1997; Genty and Massault,1997).

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F. McDermott / Quaternary Science Reviews 23 (2004) 901–918906

As an example from the recent literature, Genty et al.(2003) noted that stalagmites deposited during the lateglacial in the south of France exhibit d13C values thatare much higher than in those deposited during theHolocene. These differences were attributed to changesin the relative proportions of atmospheric and biogenic(light) carbon. This interpretation implies that periodsof climatic amelioration promote the production of soilbiogenic CO2, resulting in isotopically lighter carbonisotope ratios in the speleothem calcite.In summary, the interpretation of carbon isotopes in

regions where switches in the proportions of C3 and C4plants can be independently verified (e.g. from pollendata) is relatively straightforward. In temperate regionsthat lack a natural C4 vegetation however, theinterpretation of carbon isotopes in speleothems re-mains difficult, and the data are often interpreted on anad hoc basis. So far, the geochemical criteria fordistinguishing between the processes that might beresponsible for carbon isotope variations have not beenestablished, yet these are essential if reliable palaeo-climatic information is to be inferred from the d13Crecord of temperate-zone speleothems. If, for example,incomplete equilibration between soil CO2 and percolat-ing water is the primary factor responsible for elevatedd13C in some temperate-zone speleothems, then elevatedd13C should be associated with wetter periods, when thewater/soil gas contact times are shorter. If, on the otherhand, seasonal evaporation of water in the under-saturated zone or perhaps within the cave itself is thedominant processes, then high d13C should be associatedwith drier periods. One promising line of research is tocombine trace element and carbon isotope data, becausedepending on the nature of the co-variations, severalpossible mechanisms for changes in d13C can be ruledout. In a study of a 31,000-year-old speleothem fromNew Zealand for example, Hellstrom and McCulloch(2000) were able to rule out a reduction in cave seepagewater flow rates as an explanation for elevated d13C.Barium concentrations exhibited a strong negativecorrelation with d13C, the opposite to that predicted ifhigh d13C was caused by enhanced prior calciteprecipitation in the flow-path as a result of slower flowrates. Future research should seek to develop furtherthese geochemical and petrographic criteria and tounderpin these arguments with theoretical modellingand with systematic measurements on present-day drip-waters.

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Age kyr B.P.

0 50 100 150 200

Fig. 1. Compilation of approximately 750 TIMS U-series speleothem

dates that have been published during the past decade, plotted against

the latitude of the relevant cave site. The timing and duration of the

marine isotope stages (MIS) 1–6 (Martinson et al., 1987) are also

shown.

4. Case study review

The following case study review is structured aroundthe new insights that studies of stable isotopes inspeleothems have provided in some of the key issues inpalaeo-climatology. The major results from those

studies for which good chronological control (i.e. TIMSor PIMMS U-series dates) is available are discussedbelow.

4.1. Isotope stage 6 and the penultimate deglaciation

Speleothem records from Late Pleistocene mid- tohigh-latitude sites are discussed first, because these arelikely to be sensitive to glacial–interglacial transitions,and they illustrate an important feature of speleothems,namely that calcite deposition slows down or ceasesduring glacials. Fig. 1 is a compilation of approximately750 TIMS U-series speleothem dates that have beenpublished during the past decade, plotted against thelatitude of the relevant cave site. The absence ofspeleothem deposition in the mid- to high latitudes ofthe Northern Hemisphere during isotope stage 2 isstriking, consistent with results from previous compila-tions based on less precise alpha-spectrometric dates(e.g. Gordon et al., 1989; Baker et al., 1993; Hercmann,2000). By contrast, speleothem deposition appears tohave been essentially continuous through the glacialperiods at lower latitudes in the Northern Hemisphere(Fig. 1).Perhaps the best known Late Pleistocene continental

O isotope record is from the Devils Hole calcite vein(Nevada) that was deposited continuously from 566 to60 ka (Winograd et al., 1992; Ludwig et al., 1992).Unlike speleothems (sensu-stricto), the Devils Holecalcite (DH-11) was deposited in a phreatic open faultzone by calcite-supersaturated groundwaters. Its Oisotopic composition therefore reflects changes in the

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O isotopic composition of regional meteoric water thatrecharged the aquifer, in turn reflecting changes in theaverage winter–spring surface temperature in the south-ern Great Basin. Unlike speleothems there is asignificant, but poorly constrained transit time betweenthe recharge zone and the site of calcite deposition. As aresult, the DH-1 record provides minimum estimates forthe timing of climate-driven changes in O isotope ratios.A comparison of the DH-11 record with the Vostok(Antarctica) ice-core deuterium record and the SPEC-MAP record that largely reflects Northern Hemisphereice volume (Fig. 2) indicates that both clearly record thefirst-order glacial–interglacial transitions. In detail,however, there are several important differences be-tween the DH-11 and SPECMAP curves (Winogradet al., 1992), and most attention has focused on thedifferences in the timing of Termination II (Fig. 2),because it is argued that the timing of this termination iscrucial for testing the Milankovitch hypothesis. In theDH-11 record, Termination II occurs at 14073 ka, pre-dating by some 12 ka the timing of Termination II in theSPECMAP record (12873 ka). While the interpretationof the DH-11 record remains controversial, recentlypublished independent data sets from both the con-tinental and marine realm (see below) appear tocorroborate the inference that Termination II pre-datedby several ka the timing of maximum insolation at65�N.A study from Spannagel Cave in the high Austrian

Alps (Sp .otl et al., 2002) provides compelling newevidence that climatic conditions had amelioratedsufficiently by 13571.2 ka, to allow flowstone deposi-tion to re-commence following the penultimate (IsotopeStage VI) glaciation. Results from high-altitude con-tinental sites such as this Alpine site are important,because they are likely to be sensitive to glacial/interglacial transitions. Calcite deposited at13571.2 ka exhibits very low d18O (ca �12.571.5%),probably indicating that deposition occurred from low

200100 600500300 4000

VOSTOK

DH-11

SPECMAP

+2

0

-2

-2

0

+2

-2

0+2

δ18O

δD

Age kyr. B.P.

Fig. 2. Devils Hole O isotope, SPECMAP and Vostok ice-core records

compared. The dashed vertical line represents termination II in the

Vostok and the Devils Hole (DH-11) records. Terminations are defined

as the mid-points of deglaciations. The records have been normalised

to standard deviation units for the portions of each record shown.

Diagram redrawn after Winograd et al. (1992).

d18O glacial melt-waters. By contrast, calcite depositedin the interval 122–116 ka has d18O values of about�9%, similar to those for Holocene stalagmites fromthe site, indicating a switch from glacier derived to‘normal’ meteoric water sources. The critical result isthat liquid water was available for speleothem deposi-tion at this high-altitude Alpine site by 13571.2 ka,indicating that deglaciation had clearly commenced.This result corroborates other lines of evidence from themarine realm (e.g. Esat et al., 1999; Henderson andSlowey, 2000; Gallup et al., 2002) that the timing oftermination II occurred at least 8000 years before the65�N insolation maximum at 128 ka. The new result alsocorroborates previously published interpretations basedon compilations of alpha-spectrometric U-series datesfor speleothems (Baker et al., 1993) that speleothemdeposition had re-commenced by at least 133 ka, andsupport a TIMS U-series date of 13371.2 ka for thebase of a speleothem in N England (Baker et al., 1995).However, as noted by Baker et al. (1996), site-specificeffects can influence speleothem growth, and while thepresence of speleothems can be taken to indicate liquidwater availability, their absence at any particular sitedoes not necessarily imply permafrost conditions.Data from high-latitude sites can also yield useful

insights into the timing of the penultimate deglaciation.U-series ages for stalagmite Ham85-2 from Hamarnes-grotta near Rana, 20 km south of the Arctic Circle innorthern Norway (Linge et al., 2001a, b), are relevanthere, because the high latitude of the cave site makes itsensitive to the onset of periglacial conditions, asso-ciated with the accumulation of ice in northernScandinavia. TIMS U-series ages show that speleothemdeposition at this high-latitude site occurred duringisotope sub-stages 5e–5a (123.5–73.3 ka). An importantresult is that conditions favourable for speleothemgrowth existed during isotope stage 5e, and that theslowdown in calcite precipitation rate marks thetermination of the interglacial climate sometime between119.5 and 107.7 ka. During isotope stage 5 the growthrate of the speleothem apparently responded to pro-gressively deteriorating climatic conditions above thecave site. Thus, the growth rate was relatively rapid inthe period between 123.35 and 119.5 ka (B46 mm/year),declining to about 0.7 mm/year in the period 119.5–107.7 ka, marking the end of interglacial conditions.Speleothem growth became exceedingly slow(B0.07 mm/year) in the period 107.7–73.3 ka. d18O inthis speleothem exhibited low-amplitude (ca 0.5%)variability on centennial to millennial timescales duringisotope stage 5, and the values were similar to those forHolocene speleothems from the region, reflectingrelatively stable conditions, with evidence for coolerconditions between 122.05 and 121.7 ka. Previouslypublished alpha-spectrometric U-series dates for spe-leothems from cave sites in northern Norway are critical

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to the debate about the timing of the penultimatedeglaciation. In particular, a previously reported alpha-spectrometric U-series date of 14575 ka (1s) (Laur-itzen, 1995) from Okshola, a lowland site close to theArctic Circle in northern Norway is pivotal to thedebate, and perhaps should be refined using moreprecise TIMS or PIMMS methods.Evidence for the timing and duration of the last

interglacial was also presented by Zhao et al. (2001)based on data for an interglacial stalagmite (NEW-B)from Newdegate Cave in southern Tasmania. Thestalagmite was deposited from approximately 155 to100 ka and exhibits highest growth rates (B 61.5mm/ka) during a relatively short time interval between129.271.6 and 122.172.0 ka. This time interval coin-cides with prolific coral growth along the WesternAustralian coast and marks the onset and duration offull interglacial conditions. Since speleothem growthrates reflect precipitation rather than temperature in thisregion, it was argued that the highest rates of precipita-tion on land occurred during the period when fullinterglacial sea-levels were attained. Periods of lowereffective precipitation prior to 129.2 ka (lower spe-leothem growth rates) were attributed to latitudinalshifts in the location of the subtropical highs andassociated westerly circulation. Based on the pattern ofthe growth rate of the speleothem, Zhao et al. (2001)argued that the penultimate deglaciation was underwayby about 142 ka, but that the full interglacial conditions(highest coral and speleothem growth rates) coincidebroadly with the 65�N summer insolation peak thatoccurred at 128–126 ka. Thus, while the maintenance offull interglacial conditions can be explained by insola-tion (Milankovitch) forcing, an additional forcingmechanism is required to trigger the onset of deglacia-tion at ca 142 ka.Plagnes et al. (2002) presented stable isotope data for

a stalagmite (Cla4) from Grotte de Clamouse (S France)that was deposited discontinuously between 189 and74 ka. Significantly, all of the growth phases ofstalagmite Cla4 correspond to humid periods duringwhich sapropels were deposited in the eastern basin ofthe Mediterranean, and most of the growth phasescorrespond to relatively warm periods of high sea standsduring isotope stages 5 and 7. As discussed by Plagneset al. (2002), several European Cave sites show evidencefor speleothem growth during MIS 6 (Fig. 1), indicatingsignificant periodic climatic amelioration. Speleothemdeposition between 169.171.5 and 162.3471.5 ka (MISsub-stage 6.4) was interpreted to reflect the S6 sapropelevent that occurred in the eastern Mediterranean. In astudy that provides additional evidence for the hydro-logical conditions during MIS 6, Bard et al. (2002)demonstrated that d18O in a stalagmite from 19m belowpresent-day sea-level at Argentarola Cave on theTyrrhenian coast of Italy exhibits a 2–3% shift to lower

values between 180 and 170 ka (MIS sub-stage 6.5).Approximately 0.8–1.5% of the observed 2–3% shift ind18O could be accounted for by changes in the isotopiccomposition of the vapour source, but the remaining1–2% was interpreted as reflecting the so-called‘amount’ effect, reflecting wetter conditions in the regionduring MIS 6.5. The inferred change to wetter condi-tions during sapropel 6 is consistent with the pluvialevents during this and later sapropel events (S1–S6)inferred independently on the basis of decreases in d18Oin speleothems from Israel (Bar-Matthews et al., 2000;Ayalon et al., 2002). Taken together these results yieldimportant new insights. In particular, it is clear thatwetter conditions associated with the formation ofsapropel 6 were not confined to the eastern basin ofthe Mediterranean through increased Nile discharge.Instead the western Mediterranean was also apparentlywetter during this part of the penultimate glacial,although it is noted that there are significant (ca 10 ka)unresolved differences in the timing of the O isotopeshifts interpreted to reflect sapropel S6 in the studies ofBard et al. (2002) and Plagnes et al. (2002).Sp .otl and Mangini (2002) demonstrated that a

stalagmite from the Austrian Alps, deposited in thetime interval between 57 and 46 ka preserves evidencefor high d18O events that appear to be coeval with theMIS 3 interstadial events 15a, 15b, 14 and 12, recognisedpreviously in the Greenland ice cores. This is animportant result because it is the first time that preciselydated D/O events have been identified in terrestrialclimate records in the mid-latitudes. The study alsoprovides evidence that mean annual temperatures at thishigh-altitude (2165m a.s.l.) site remained close to that ofthe present-day (2�C) over the 11 ka interval ofspeleothem growth during isotope stage 3. Shifts tohigher d18O of approximately 2% centred at 55.4 ka andbetween 54.3 and 51.1 ka are interpreted to reflectinterstadial events 15b and 14 of the GRIP record,respectively, with a smaller (o1%) shift at 54.9 ka BPcorresponding to event 15a of the GRIP record (Fig. 3).These results are in good agreement with data for fivepartially overlapping stalagmites from Hulu Cave ineastern China (Wang et al., 2001) that appear to show astrong coherence with O isotope variability in the GRIPand GISP2 ice cores (Fig. 4). In the latter study oxygenisotope records from five stalagmites define overlappingO isotope trends, indicating that these speleothemrecords preserve a precisely dated signal of changes inthe d18O of precipitation. Taken together these dataindicate that further refinements to the Greenland ice-core chronology are required. It is clear from revisionsof the GRIP chronology (compare curves B and C inFig. 3) that the dating uncertainties in the ice cores nowpresent a major obstacle in assessing the latitudinal leadsand lags in the timing of the interstadial events of thelast glacial.

ARTICLE IN PRESS

Fig. 3. Lower curve (D) shows d18O variations in stalagmite SPA 49 from Kreegruben Cave in the Austrian Alps (after Sp .otl and Mangini, 2002).

The O isotope records from the GISP2 core (curve A), and two GRIP d18O records using the 1995 timescale (curve B) and the 2001 timescale (curve

C) are also shown. Tentative correlations between the SPA 49 record and the GRIP curve have been suggested by the authors (dashed lines).

Greenland Interstadials (GIS) events 15 and 14 occurred at 55.6 and 54.2 ka, significantly earlier than in the older (1995) GRIP chronology, and only

slightly later than indicated by the more recent (2001) GRIP chronology. These data illustrate the potential importance of well-dated speleothem

records in refining the chronology of the high-latitude ice cores.

F. McDermott / Quaternary Science Reviews 23 (2004) 901–918 909

In a significant recent study, Genty et al. (2003)argued that D/O events during the last glacial arerecorded by carbon and oxygen isotopes in a well-datedstalagmite from Villars Cave (Vil9) in south-east France,deposited between 83 and 32 ka (Fig. 4). This study is

important, because in addition to demonstrating theoccurrence of the D/O events in mid-latitude westernEurope, it offers perhaps the best available chronologi-cal control on the timing and occurrence of such events,and provides an improved chronological framework for

ARTICLE IN PRESS

Fig. 4. Comparison of the carbon isotope record from stalagmite Vil9 (Villars Cave, south-west France) with O isotope records from the Hulu Cave

site in China (Wang et al., 2001) and the GISP2 record (after Genty et al., 2003). Vertical stippled boxes in the Villars record denote the presence of

depositional hiatuses (D2–D4). Genty et al. (2003) correlate hiatus D3, the so-called ‘Villars cold phase’ between 67.470.9 and 61.270.6 ka ago, with

Heinrich event H6. Significantly, d13C values remain high for several millennia after hiatus D3, indicating that this cold phase probably had a severe

impact on local vegetation and soils. The possible correlation of Heinrich events H1–H6 in the Hulu Cave record (upper part of the diagram) with the

GISP2 record are from Wang et al. (2001).

F. McDermott / Quaternary Science Reviews 23 (2004) 901–918910

the GRIP and GISP2 ice-core records (Fig. 4). Thestalagmite has three growth hiatuses that occurredbetween 78.8–75.5 ka (hiatus D2), 67.4–61.2 ka (hiatusD3) and 55.7–51.8 ka (hiatus D4). Carbon isotope ratiosincrease prior to and after hiatus D3 and are interpretedto reflect a cooling event (the ‘Villars Cold Phase’)rather than a localised drip-water or other cavehydrological effect. Hiatus D3 is also tentativelycorrelated with Heinrich event H6 (Fig. 4).A remarkably coherent picture of continental climate

Late Pleistocene variability with close links to theoceanic realm has emerged from studies of speleothemsfrom the eastern margin of the Mediterranean (Fig. 5).Particularly impressive is the well-dated composite d18Orecord for the past 185 ka based on 21 speleothems fromSoreq Cave in Israel (Bar Matthews et al., 1996, 1997,1999, 2000; Ayalon et al., 1998, 2002; Kaufman et al.,

1998). One of the reasons that robust matches can bemade between different coeval speleothems in thiscomposite record is that the shifts in d18O are relativelylarge (several per mil), indicating a strong climatic signalin the d18O record. The Soreq d18O record appears toreflect predominantly two effects: (i) changes in the d18Oof the oceanic vapour source and (ii) the ‘amount’ effect(Bar Matthews et al., 1996, 1997, 1999, 2000; Kaufmanet al., 1998; Ayalon et al., 1998, 2002). These studies areimportant because they establish a critical link betweenthe oceanic realm and continental climate in this region.Thus, d18O minima in speleothems from Soreq coincideexactly with the occurrence of sapropel events in theMediterranean Sea, and recently it has been shown thatthis is true for glacial as well as for interglacialconditions (Ayalon et al., 2002). The dominance of the‘amount effect’ on d18O in stalagmites in this region

ARTICLE IN PRESS

-9

-8

-7

-6

-5

-4

-3

-2

050100150200

δ18O

(‰

, PD

B)

Age (ka)

Fig. 5. Composite d18O curve constructed from 21 overlapping speleothem records for the past 185 ka from Soreq Cave in Israel (after Ayalon et al.,

2002). Filled circles along the top of the diagram illustrate the position of the 95 TIMS U-series dates that provide the chronology for this long

record. Note that speleothem deposition was continuous during the glacials at this eastern Mediterranean site, in contrast with northern latitude sites

where speleothem deposition ceased (Fig. 1).

F. McDermott / Quaternary Science Reviews 23 (2004) 901–918 911

allows reliable reconstruction of arid and pluvial phases(Bar Matthews et al., 1996, 1997, 1999, 2000; Ayalonet al., 2002; Bard et al., 2002).

d18O and deposition rate changes in speleothems fromHoti Cave in northern Oman (Burns et al., 1998; Burnset al., 2001) have provided new palaeo-climatic insightsin this part of the tropics. Low d18O in speleothemcalcite associated with episodes of enhanced depositionduring marine isotope stages 5e, 7a and 9 wasinterpreted to reflect pluvial phases linked to increasedmonsoon rainfall. Thus, rapid speleothem growthoccurred during five well-dated intervals 6–10.5, 78–82,120–135, 180–200 and 300–325 ka with evidence fornon-deposition of calcite, reflecting relatively aridconditions during the intervening episodes. Each pluvialepisode was characterised by d18O values that aresignificantly lower (by several %) than those of modernstalagmites, consistent with a northwards shift in themonsoon rainfall, as a result of a northwards shift in theinter-tropical convergence zone during peak interglacialperiods.Holmgren et al. (1995) presented oxygen and carbon

isotope data for a stalagmite from Lobatse II Cave insoutheastern Botswana in which deposition was re-stricted to two phases. During the first phase ofdeposition (51–43 ka), warm humid conditions asso-ciated with C3 vegetation were inferred (low d18O, lowd13C), whereas the second phase (27–21 ka) wasdominated by drought-adapted C4 plants (higher d13Cand d18O), indicating cooler conditions. Correlationsbetween the Lobatse II Cave study and that for Cango

Caves (Cape Province, South Africa) are hindered byproblems of imprecise age constraints, but the availabledata tentatively indicate that cooling was recorded atboth sites in the period prior to the last glacial maximum(Holmgren et al., 1995). Further TIMS or PIMMSU-series dates are clearly required to investigate furtherthe possible regional significance of such cooling.Dorale et al. (1998) presented oxygen and carbon

isotope data for four stalagmites that were deposited inthe interval between 75 and 25 ka at Crevice Cave,Missouri, USA. The study is significant because the fourcoeval stalagmites exhibit coherent oxygen and carbonisotope records. High d18O values between 59 and 55 kawere inferred to reflect warm conditions, and this periodcoincided high d13C, interpreted to reflect the expansionof the prairie-type C4 vegetation.

4.2. Holocene records

In a study of a Holocene speleothem from S^yle-grotta, northern Norway, Lauritzen and Lundberg(1999) calibrated the temperature dependence of d18Oc

at this site, using independently derived estimates ofpresent-day and Little Ice Age (LIA) mean annualtemperatures. Data from other sources (e.g. tree-linechanges at 3700 years, evidence from early to mid-Holocene diatoms and inferred climatic conditions at10,000 years BP) are in broad agreement with therelationship between d18Oct and temperature based onthese two data points, indicating that this calibrationmay be robust outside the calibration interval. Another

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Holocene stalagmite (SG95) from the same cave system(Linge et al., 2001a, b) deposited during the past 4000years exhibits heavier d18O than that of stalagmiteSG93, and the pattern of d18O correlates with that ofSG93 over some intervals of the Holocene only.Surprisingly, a third stalagmite (SG92–4) exhibits d18Ovariability that correlates remarkably well with that ofSG93, despite the fact that it was collected from close tothe cave entrance and might be expected to have beenaffected by a more variable micro-climate. Studies suchas these that employ several coeval stalagmites areinvaluable to assess the reliability of speleothems aspalaeo-climatic recorders and they highlight the dangerof relying on a single stalagmite d18O record. It seemslikely that processes other than climate variability (e.g.variable water ingress routes and water mixing) cansubstantially affect the d18O values recorded in stalag-mites, particularly at high temporal resolutions).McDermott et al. (2001) presented a laser-ablation

high-resolution O isotope time-series record for aHolocene stalagmite (CC3) from Crag Cave, a coastalsite in SW Ireland. The main result of this study was thatsubtle higher frequency (century scale) d18O variationsin the early to mid-Holocene appear to correlate with

Fig. 6. Comparison of the O isotope record from stalagmite CC3 (south-wes

the large (ca 8%) shift in d18O in the CC3 record is indistinguishable (within

record. Other post-8200-year oscillations in d18O in the early part of the Holo

two records (McDermott et al., 2001 for a discussion). RWP, DACP, MWP

Ages Cold Period’, ‘Medieval Warm Period’ and ‘Little Ice Age’, respective

those in the GISP2 ice core, suggesting that the latterreflect regional Holocene climate signals. Approximately1640 laser-ablation d18O measurements were carried outalong the growth axis of this 465mm long stalagmite,resulting in an exceptionally high-resolution Holocened18O record. The ‘8200-year’ cold event was defined byeight data points centred on 8.3270.12 ka and itexhibited a very large (ca 8%) decrease in d18O(Fig. 6). Since the speleothem d18O signal may in partreflect temporal changes in the vapour source and/orcloud trajectories it was not possible to calculatetemperature changes from these d18O data. The timingof the ‘8200-year’ event in speleothem CC3 is within thedating uncertainties of the GISP2 core. Thus, themaximum amplitude occurs at 8.3270.12 ka comparedwith 8.2170.10 ka in GISP2, and is coeval with faunalevidence for cooling at 8.3070.06 ka in core 28-03 fromthe Norwegian Channel.Significantly, the amplitude of this large shift to lower

d18O was too large to ascribe solely to a reduction inmean annual air temperature. Instead it was attributedto freshening of the surface of the adjacent N Atlanticby isotopically depleted melt-water. The absence of aclear shift in d18O in the speleothem data during the

t Ireland) with the GISP2 curve (McDermott et al., 2001). A timing of

the 2s dating uncertainty) of the ‘8200-year’ cold event in the GISP2

cene appear to occur synchronously (within dating uncertainties) in the

and LIA are possible expressions of the ‘Roman Warm Period’, ‘Dark

ly.

ARTICLE IN PRESSF. McDermott / Quaternary Science Reviews 23 (2004) 901–918 913

other N Atlantic ice-rafting events at ca 5.9, 4.3, 2.8 and1.4 ka despite the high resolution of the data (7–18 yearsper analysis) is significant. It indicates that unlike the‘8200-year’ event, the later Holocene ice-rafting eventsfailed to trigger large changes in d18O, and byimplication failed to establish a detectable melt-watercap on the mid-latitude N Atlantic.In the mid-western United States a coherent picture of

climate-driven vegetation changes in the Holocene isbeginning to emerge from speleothem studies, supple-mented by data from pollen records. Dorale et al. (1992)presented stable isotope data for a TIMS U-series dated7800-year-old stalagmite (1 s) from Cold Water Cave innortheast Iowa. The O isotope data were interpreted interms of palaeo-temperature changes, while the carbonisotope data were taken to reflect climate-driven changesin the nature of the vegetation above the cave. A mid-Holocene warming of about 3�C relative to the earlyHolocene, followed by a cooling of 3–4�C was inferredfor the period between 4 and 1 ka, assuming a simplerelationship between d18Oct and mean annual airtemperature. d13C exhibited strong unidirectional shiftsduring the past 6000 years in this record, and inparticular a marked shift to higher d13C between 5.9and 3.6 ka indicates a replacement of forest by a C4-richprairie vegetation, consistent with published pollenrecords from the region. The shift from forest toprairie-type vegetation about 5900 years ago appearsto have occurred rapidly, probably within a century,offering insights into the rates at which climate-drivenchanges in vegetation type can occur in regions close toecotone boundaries (Denniston et al., 1999b).In a follow-up study of two additional Holocene

stalagmites from Cold Water Cave, northeast Iowa,Denniston et al. (1999) presented O isotope data thatappear to have been influenced strongly by site-specificeffects. While d13C varied coherently between the threestalagmites, d18O did not, indicating that local near-surface evaporative enrichment of 18O had variablymodified the d18O signal prior to infiltration. The Oisotope variations in the stalagmite with lowest d18O(sample 2SS), and therefore the one least affected byevaporative enrichment effects appear to reflect tempor-al changes in the moisture source region rather thantemperature changes. Thus, the shift to lower d18Oduring the mid-Holocene, accompanied by an increasein stalagmite growth rate, is best explained by a switchto 18O-depleted moisture sources derived from the Gulfof Mexico or the Pacific Ocean. The observation thatlow d18O was accompanied by enhanced speleothemgrowth in the mid-Holocene is puzzling in view of theoverall drier mid-Holocene climate that must haveaccompanied the forest–prairie transition in the region.One explanation is that increased infiltration waspossible because most of the precipitation occurredduring the cool season (Denniston et al., 1999a).

Speleothems such as these represent good candidatesfor fluid inclusion studies, because if the preferredinterpretation is correct, trapped fluids should exhibitd18O variations similar to the calcite values. The authorsconclude that the previously calculated temperatureestimates, based on stalagmite 1 s from the Cold WaterCave may have overestimated the amount of mid-Holocene warming. This study highlights the difficultiesinherent in interpreting speleothem d18O as a quantita-tive palaeo-temperature proxy, particularly in regionswhere surface water deficits occur seasonally, andunderlines the need to analyse several coeval stalagmitesto ensure that a robust regional climate signal isrecorded.The data for the Holocene portions of stalagmites

from Hoti Cave in northern Oman (Burns et al., 1998;Burns et al., 2001) are consistent with inferences fromstudies of lake palaeo-levels in the Sahel region of Africa(Gasse and Street, 1978; Ritchie et al., 1985). Thespeleothem results offer enhanced chronological control,firmly placing the early–mid-Holocene transition frompluvial to the present arid to semi-arid conditions at6.2 ka BP. Significantly, proxies for monsoon strengthderived from marine sediment proxies from the IndianOcean and Arabian Sea (e.g. Anderson and Prell, 1993;Rosteck et al., 1997) that effectively monitor windstrength rather than monsoon rainfall amount, differfrom the record derived from speleothems. One ex-planation (Burns et al., 2001) is that speleothems offer amore direct and reliable signal of monsoon rainfallsignal over the continents, whereas the marine recordsprimarily record monsoon wind strength variations thatare not necessarily accompanied by increased rainfall.This arises because factors other than monsoon windstrength, such as changes in the sea surface temperatureof the tropical Indian Ocean that supplies the moisture,may strongly influence moisture transport to theArabian Peninsula (Burns et al., 2001). A more detailedO isotope analysis for a Holocene speleothem from thiscave was presented by Neff et al. (2001). An importantfinding of the latter study was that speleothem d18Ocorrelates with D14C, interpreted as reflecting solar-driven changes in the monsoon in this region oncentennial to millennial timescales. Thus, while itappears that on relatively long timescales the north-wards transport of moisture to the Arabian Peninsulamay be driven by glacial to interglacial cycles, similarchanges may occur on much shorter timescales inresponse to variable insolation. The latter study is oneof the few that has successfully demonstrated a linkbetween solar variability and climatic conditions in theHolocene.In a multi-proxy study, Xia et al. (2001) presented

stable isotope data for a well-dated 1.13m longstalagmite that grew continuously from 9180 to 5060years BP in Lynds Cave, northwestern Tasmania. The

ARTICLE IN PRESSF. McDermott / Quaternary Science Reviews 23 (2004) 901–918914

interval between 8000 and 7400 years BP was char-acterised by high d18O, relatively low growth rates andhigh initial (234U/238U) ratios, reflecting conditions thatwere probably warmer and drier than those of thepresent-day. By contrast, calcite deposited in the intervalbetween 7400 and 6600 years BP exhibited lower d18O,high growth rates and low initial (234U/238U) ratios.These characteristics were interpreted to reflect rela-tively wet conditions, and coincide with the so-calledmid-Holocene climatic optimum that had been recog-nised in earlier studies of pollen sequences and lakelevels in the region. The most recent part of the record,from 6100 to 5100 years, is characterised by the lowestgrowth rates and dramatic fluctuations in both O and Cisotopes as a result of kinetic fractionation processes inresponse to cooler, drier conditions and a reduction inthe humidity of the cave air.In a study of a 2.7m long stalagmite from Cango

Caves, Cape Province, South Africa, Talma and Vogel(1992) used data for 14C-dated groundwater to estimateof the d18O of recharge, and therefore of cave drip-waterin the past. On this basis, they calculated thattemperatures were approximately 6–7�C colder duringthe last glacial maximum compared with the present.Lower temperatures were also inferred for parts of themid- to late Holocene, centred on approximately 4500and 3000 14C years BP. This study is noteworthybecause it is one of the few where an independentestimate of the d18O of cave drip-waters could beprovided, allowing quantitative temperature estimates.In the future material such as this would represent anexcellent target for fluid inclusion studies as a meansto test the inferences from the isotopic composition of14C-dated groundwater.Repinski et al. (1999) analysed a speleothem from

Cold Air Cave (Northern Province, South Africa) andattributed the lower d18O values that occurred betweenabout 800 and 400 years ago to the LIA. Higher d18Ovalues between about 4400 and 4000 years ago wereinterpreted to reflect a warmer period, assuming that theoverall positive correlation between temperature andd18O was valid through the late Holocene. Further workis clearly required to explore how the inferences drawnby Repinski et al. (1999) for the period around 4000years ago in the Northern Province relate in a regionalcontext to the results of Talma and Vogel (1992) for theCape Province.

5. Summary and pointers for future research

So far, the major contribution of stable isotopestudies on speleothems for palaeo-climatic reconstruc-tion has been the development of well-dated high-resolution d18O records that can be correlated withbetter understood records such as the Greenland ice

cores, thereby defining the geographical extent ofregionally synchronous O isotope ‘events’ such as theD/O events, regional pluvial events, and late glacial toearly Holocene oscillations. The major strength ofspeleothem studies has been in the provision of robustchronologies that are independent of both the orbitally‘tuned’ marine records and the ice-core chronologies. Insome regions where speleothem deposition appears tocontinue uninterrupted across glacial–interglacial tran-sitions (e.g. Israel), remarkably detailed land–seacorrelations have emerged, that in turn provide im-portant new insights into the operation of the hydro-logical system under different climatic regimes. Aweakness of speleothem stable isotope studies has beenthe difficulty in providing unambiguous palaeo-climaticinterpretations of the data. The development of reliableanalytical protocols to recover both the D/H andthe O isotope ratios of trapped fluid inclusions wouldgreatly facilitate the interpretation of stable isotope dataand is the subject of ongoing research at severallaboratories. This capability would allow unambiguousestimation of palaeo-temperatures, and would providevaluable tests for the output of ‘water-isotope enabled’GCMs. Critically, stable isotope measurements on fluidinclusions may allow, for the first time, realisticestimates of the uncertainties associated with recon-structed climate parameters (e.g. palaeo-temperature,palaeo-precipitation). Many of the O isotope ‘events’that have been defined by recent studies will offerproductive targets for fluid inclusion studies in thefuture.In the past decade there has been a trend towards the

provision of ever more detailed, higher-resolutionrecords (e.g. use of lasers and micro-drilling). Veryhigh-resolution (e.g. close to annual) data sets mayprove difficult to interpret however, because seasonalnoise may dominate the signal, particularly in Holocenespeleothems where climate variability is likely to besubtle. Future studies are likely to make further use ofion-probe instruments to measure O isotope ratios at asub-annual resolution, while accepting a reduced analy-tical precision on individual measurements. In carefullychosen, rapidly deposited material it may be possible todefine the annual cycle in d18O in speleothem calcite,thereby simultaneously providing a chronology for shortintervals and a measure of changes in seasonal cyclicityas a function of different climate regimes.A critical aspect of future speleothem-based O isotope

records will be the provision of ever more reliablechronologies for which realistic uncertainties are ex-plicitly expressed, in order to compare with othercalendar-year-based proxies (e.g. ice cores) and forcingmechanisms (e.g. insolation). This in turn will requirethe use of more realistic and statistically constrainedage–depth relationships in U-series-dated speleothems.Currently, there is little effort to realistically represent

ARTICLE IN PRESSF. McDermott / Quaternary Science Reviews 23 (2004) 901–918 915

and report age uncertainties associated with interpola-tions between dated intervals of speleothems, anddifferent models (e.g. linear interpolation, linear regres-sion and polynomial curve-fitting) are used without anyrigorous exploration of the chronological consequencesof choosing one model over other equally plausiblemodels. This is particularly an issue in the case ofHolocene speleothems for which interpolation uncer-tainties typically exceed the 2s uncertainties associatedwith individual U-series age determinations.Most chronologies are based on U-series dating

techniques and with the advent of different analyticalapproaches (e.g. TIMS and PIMMS) it is imperativethat systematic laboratory inter-comparison pro-grammes are carried out in the near future, becausethe accuracy of all U-series ages depends ultimately onthe accuracy with which the mixed U–Th spikes havebeen calibrated in different laboratories. These issuesrelating to chronology are increasingly important asresearchers seek to establish correlations betweendifferent speleothem records and with other indepen-dently dated archives such as ice cores.Speleothems offer perhaps the best opportunity to

accurately constrain the timing of clearly defined climatesignals (e.g. glacial–interglacial transitions, D/O oscilla-tions, the ‘8200-year’ event). By focusing on these timesof high signal to noise ratio it should be possible toassess inter-hemispheric and latitudinal leads and lags,providing that a carefully constructed chronology isavailable. It is noteworthy that at present the low-latitudes in both hemispheres, and the Southern Hemi-sphere in particular are under-represented in thecurrently available database of reliably dated spe-leothem stable isotope records (Fig. 1).Increasingly, multi-proxy studies are being under-

taken on individual speleothems (e.g. combiningstable isotopes with trace elements, petrographic in-formation and growth-rate information). These ap-proaches help to narrow the uncertainties associatedwith the interpretation of stable isotope data fromindividual speleothems, but are not a substitute forreplication of records within individual cave sites.Clearly, a balance must be found between the conflictingrequirements to replicate records and to conserve cavesites for aesthetic purposes and for scientific investiga-tions in the future.Finally, studies should include more systematic

seasonal monitoring of present-day precipitation andcave drip-waters. Monitoring is both expensive and timeconsuming. It is, nonetheless, essential to provideconstraints on the O isotopic composition of cavedrip-waters, to assess seasonal biases in calcite deposi-tion rates, to investigate the extent to which drip-watersreflect the weighted mean d18O value of precipitationand to understand better the factors that control thed13C of the DIC.

Acknowledgements

The author thanks Ian Fairchild and an anonymousreviewer for their constructive comments that helped toimprove the manuscript. Mira Bar-Matthews kindlyprovided Fig. 5. Various aspects of the materialpresented here have evolved as a result of discussionswith colleagues and acquaintances that include IanFairchild, Andy Baker, Peter Rowe, Tim Atkinson,Mira Bar-Matthews, Alan Matthews, Chris Hawkes-worth, Silvia Frisia, Andrea Borsato, Dominique Genty,Tim Heaton, Dave Mattey, James and Lisa Baldini.Melanie Leng is sincerely thanked for her patienteditorial advice.

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