Palaeo-climate reconstruction from stable isotope variations in speleothems: a review

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Palaeo-climate reconstruction from stable isotope variations in speleothems: 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 ‘8200year’ 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 highresolution palaeo-climate records from continental settings to test and validate general circulation models (GCMs) at a higher spatial resolution, and to investigate possible leads and lags between different components of the climate system. Speleothems are multi-proxy palaeoclimate archives with the potential to provide such data. In carefully chosen sites they can record key aspects of climate variability such as mean annual temperature, rainfall variability, atmospheric circulation changes and vegetation response in a variety of measurable parameters that include stable isotope ratios, inter-annual thickness variations of growth laminae, growth-rate changes, variations in trace element ratios, organic acid contents and the nature of trapped pollen grains. This review focuses on the use of stable isotopes in *Tel.: +353-1-706-2328; fax: +353-1-283-7733. E-mail address: [email protected] (F. McDermott). 0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2003.06.021

speleothems as palaeo-climatic indicators, and the emphasis is on developments and data sets that have been reported since previous reviews of the subject (Schwarcz, 1986; Gascoyne, 1992). The focus is primarily on oxygen isotopes, but carbon isotopes are included whenever they have contributed significantly to palaeoclimatic interpretations. Several unresolved issues remain, but recently there have been important insights into the interactions between component parts of the system (e.g. marine sources, isotopic evolution in the hydrological system and isotopic effects during infiltration through the unsaturated zone) that now underpin the interpretation of O isotopes in speleothems. Systematic studies of stable isotopes in speleothems commenced more than three decades ago (Hendy and Wilson, 1968; Thompson et al., 1974), but progress was hampered by the need for large samples (ca 10 g) for alpha-spectrometric U-series dating. The development of thermal ionisation mass-spectrometry (TIMS) techniques to measure U-series isotope ratios rejuvenated the subject (Edwards et al., 1988; Li et al., 1989). TIMS

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can provide 230Th/U dates that are almost 10 times more precise than conventional alpha-spectrometry, with a reduction in sample size by about the same magnitude. Recently, a new generation of plasma-ionisation magnetic-sector mass spectrometers (PIMMS) characterised by high ionisation efficiency promise further improvements in sample size requirements and analytical precision relative to TIMS (Shen et al., 2002). The latter instruments offer vastly improved ionisation efficiency for thorium, and with further refinement are likely to become the method of choice, especially for lowuranium Holocene speleothems that contain relatively little radiogenic 230Th. With these new technological developments, speleothems offer advantages over many other continental palaeo-climate records because they can be dated in calendar years with a precision approaching 70.5% (2s), circumventing radiocarbon age calibration and reservoir correction problems that hamper other continental climate archives such as lake sediments and peat records. Indeed it is likely that speleothem records will increasingly be used to refine the chronology of the Greenland ice-core records, assuming that regional synchroneity for the major early Holocene and last glacial Dansgaard–Oeschger (D/O) O isotope shifts can be demonstrated (e.g. McDermott et al., 2001; . and Mangini, 2002; Genty et al., Wang et al., 2001; Spotl 2003). It should be noted, however, that U-series dates depend critically on the accuracy with which the mixed 229 Th/236U spikes have been calibrated with respect to known secular equilibrium standards, and there is currently a need to undertake systematic inter-laboratory comparisons to ensure that U-series dates produced by different laboratories are directly comparable. Stalagmite growth rates vary by at least two orders of magnitude (typically in the range 0.01–1.0 mm/year), depending on factors such as temperature and the calcium ion concentration of the drip-waters (Baker et al., 1998; Genty et al., 2001a, b). Thus, the time interval represented by individual stable isotope measurements depends critically on the growth rate of the speleothem chosen for analysis. Using conventional sampling techniques (e.g. a dental drill to remove 0.5 mm samples), the time interval averaged by stable isotope measurements would typically range from a few years to several decades. The detection of short-lived climatic events and the resolution of low-amplitude climatic signals therefore require the use of rapidly deposited speleothems, assuming that conventional sampling and analytical techniques are employed. In slowly deposited speleothems serious damping of the isotope signal may occur, with the result that significant but short-lived climatic events (e.g. the 8200-year cooling event) might not be detected (McDermott et 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 of sampling for O and C isotope analyses. McDermott et al. (2001) employed a laser-ablation gas-chromatography isotope ratio mass spectrometry (LA-GC-IRMS) system with a 25 W CO2 laser to thermally release CO2 by 400 ms laser bursts. Using a system of forward and reverse profiling along the central growth axis of a Holocene stalagmite (CC3) a spatial resolution 250 mm was achieved (see Section 4.2). Analysis of standards gives similar d13C values to those obtained by conventional acid digestion, but d18O values that are systematically lowered by 2 per mil. Replicate analyses of standards indicate that the isotope data are reproducible to better than 0.1% for d13C and 0.2% for d18O. Following the 2 per mil correction, the laser data accurately reproduce the first-order features of a previously published coarse resolution O isotope record for this speleothem (McDermott et al., 1999). The spatial resolution achievable by this system represents about a four-fold improvement relative to that of conventional dental drilling methods, but the data acquisition is rapid and automated, thereby offering significant 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 20 mm, corresponding to a weekly to monthly temporal resolution in a recently deposited stalagmite from Belize. These highresolution data exhibit high amplitude (11%), rapid (subseasonal) fluctuations in d13C that appear to reflect variations in the El Nin˜o/Southern Oscillation (ENSO) system. A similar spatial resolution (25 mm) was achieved recently by Kolodny et al. (2003) using an ion microprobe. This method offers excellent spatial resolution, but the relatively poor analytical precision that characterises the 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 speleothems it may be possible in the future to reconstruct the annual hydrological cycle of d18O variability, offering both a chronological tool (cycle counting) and new insights into changes in the amplitude of seasonal d18O variability in rainfall. For the case of a speleothem growth rate of 0.5 mm/year for example, it should be possible to obtain a temporal resolution better than 1 month using an ion-probe technique (25 mm spot size), and such a study would be best carried out in a region where a relatively large seasonal d18O cycle is anticipated. In cases where the sampling resolution is subannual, but insufficient to resolve a clear seasonal cycle, care must be taken to avoid unresolved seasonal effects that could lead to a noisy signal (e.g. comparing ‘winter calcite’ in one analysis with ‘summer calcite’ in an adjacent analysis).

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A number of issues relating to the interpretation of stable isotope data in speleothems remain unresolved. The most challenging of these has been to decipher the various, usually competing factors that drive oxygen isotope variations, in order to recover unambiguous palaeo-climatic signals. An early goal was to reconstruct absolute changes in mean annual air temperature (e.g. Gascoyne et al., 1980), but this is increasingly seen as unrealistic, because of the plethora of effects that influence the d18O of cave drip-waters (d18Odw), and therefore the d18O of the precipitated speleothem calcite (d18Oct). These effects are discussed below, but recently there is renewed confidence that reliable stable isotope data 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, this volume). In principle, these developments should allow the original goal of palaeo-temperature estimation to be attained in situations where it can be demonstrated that calcite was deposited in isotopic equilibrium with the cave drip-waters. In addition, the fluid inclusion data can be used to reconstruct temporal and spatial variability in the d18O of palaeo-meteoric water, and in the future these data will test the validity of GCMs that incorporate the ‘water isotopes’ in their hydrological cycles. Despite the intricacies of data interpretation, caves remain attractive targets for palaeo-climate studies because they preserve relatively pure calcium carbonate (typically calcite), precipitated from meteoric water in environments where it is protected from erosion for long periods of time (often 104–106 years). Speleothems typically consist of macro-crystalline calcite, although aragonite occurs occasionally, particularly in association with high-Mg calcite or dolomite host-rocks, and/or associated with relatively dry periods when long water– rock contact times facilitate relatively more dolomite dissolution in partially dolomitised limestone hostrocks. Petrographic studies of speleothems prior to analysis are essential to avoid analysing re-crystallised specimens, to identify possible growth hiatuses (usually marked by thin detrital-rich layers), to recognise shifts and offsets in the growth axis and to identify changes in carbonate mineralogy. The possible palaeo-environmental significance of the mineralogy and crystal morphology 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 may provide additional constraints to aid the interpretation of stable isotope data. Denniston et al. (2000), for example, interpreted the presence of aragonite layers in speleothems from a dolomitic cave in central Nepal as reflecting reduced monsoonal precipitation and increased cave aridity. In many cases, petrographic information such as this aids the interpretation of stable

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isotope data, but it is important to demonstrate that petrographic changes are regionally synchronous, to avoid mis-interpretations that could result from localised 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 (typically 71 C) throughout the year, and are similar to the mean annual air temperature of the region above the cave. Second, in cool temperate regions, cave air is characterised by high-humidity levels (typically 95–99%), minimising evaporation that might otherwise cause kinetic isotope fractionation. The mechanisms of speleothem deposition have been discussed in detail elsewhere (Schwarcz, 1986; Ford and Williams, 1989), but a critical point is that, in cave interiors, calcite deposition typically occurs by degassing of CO2 from carbonate-saturated drip-waters on entering the cave atmosphere, and not by evaporation of water. Degassing is driven by the difference between the pCO2 of the soil and that of the cave air (typically in the ranges 0.1–3.5% and 0.06–0.6%, respectively). In high-humidity cave interiors where evaporation is negligible, it can often be demonstrated that stalagmite calcite is deposited at, or very close to, isotopic equilibrium with the cave drip-water. Under these conditions, the d18O of the freshly precipitated calcite reflects both the d18O of the drip-water and the temperature dependent fractionation between the drip-waters and the deposited calcite. Thus, in order to interpret correctly the oxygen isotope fluctuations in the calcite, it is critical to understand the factors that influence oxygen isotope ratios in the cave waters of individual drip systems. The hydrological characteristics (e.g. Smart and Friedrich, 1987) of individual drip-sites influence the transfer of the meteoric water stable isotope signal to the cave dripwater. Ideally, the d18O of cave drip-water should record the weighted mean d18O of the meteoric water that falls on the surface above the cave site. The latter requirement is likely to be met by seepage-flow drip-sites in shallow 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 a result of near-surface evaporative processes (Bar-Matthews et al., 1996; Denniston et al., 1999a). An additional complication is that soil pCO2 and dripwater Ca contents may vary seasonally, with the result that calcite deposition rates also vary seasonally (e.g. Genty et al., 2001a, b). One consequence is that the recorded d18O and d13C signal in speleothems can preserve a seasonal bias, but this possibility could be detected by detailed seasonal monitoring of the chosen drip sites to understand the factors controlling intraannual variability in growth rates. These issues highlight the need for detailed site-specific present-day monitoring studies 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/semiarid regions, evaporative processes that modify d18Op at the surface prior to infiltration and in the upper part of the vadose zone. The present-day pattern of spatial and seasonal variations in d18Op is well documented (Rozanski et al., 1982, 1993; Gat, 1996) and is a consequence of several so-called ‘‘effects’’ (e.g. latitude, altitude, distance from the sea, amount of precipitation, surface air temperature). A critical requirement for the recovery of d18Op from d18Oct is that isotopic equilibrium is maintained between the cave drip-water and the calcite deposited therefrom. The criteria for recognising conditions of equilibrium deposition have been discussed previously (Hendy, 1971; Schwarcz, 1986). Briefly, the conditions are (i) that d18O remains constant along a single growth layer while d13C varies irregularly, and (ii) that there is no correlation between d18O and d13C along a growth layer. In practice, consistent sampling along single growth layers is often difficult to achieve, not least because visible layers are often thinner along the flanks of stalagmites compared with their central growth axis. Nonetheless, the so-called ‘Hendy criteria’ are used widely by researchers as a check that calcite was deposited at or close to isotopic equilibrium with cave drip-waters. In some cases it can be demonstrated that calcite deposited along the flanks of speleothem exhibit kinetic fractionation effects, but that the material deposited close to the central growth axis may have been deposited in isotopic equilibrium with . the cave drip-waters (e.g. Talma and Vogel, 1992; Spotl and 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 than dd18Oct/dT (approximately 0.24%  C 1 at 25 C, O’Neill et al., 1969), the equilibrium fractionation that accompanies calcite deposition from drip-waters inside a cave. In a review of long-term changes in the O isotopic composition of precipitation over the mid- to high latitudes, Rozanski et al. (1993) calculated an average modern-day dd18Op/dT of approximately 0.6%  C 1, but such averages clearly mask considerable site-specific variability (e.g. Fricke and O’Neill, 1999), and the relationship may have been different in the past. In principle therefore, d18Oct could increase, decrease or fortuitously remain invariant to an increase in mean annual air temperature. The latter response would require that dd18Op/dT cancelled out dd18Oct/dT, and such 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; Hellstrom et al., 1998; Frumkin et al., 1999a, b) have been reported. This illustrates the difficulty in unambiguously relating changes in d18Oct to changes in mean annual temperature, particularly over time intervals where temperature changes may have been small, and firstorder climate transitions (e.g. glacial to interglacial transitions) are not represented in the record. These uncertainties underline the need for additional proxy information from the same stalagmite (e.g. annual layer thickness variations, growth-rate changes, fluid inclusion data) to underpin the interpretation of d18O. On centennial to millennial timescales, factors other than mean annual air temperature may cause temporal variations in d18Op (e.g. McDermott et al., 1999 for a discussion). These include: (i) changes in the d18O of the ocean surface due to changes in continental ice volume that accompany glaciations and deglaciations; (ii) changes in the temperature difference between the ocean surface temperature in the vapour source area and the air temperature at the site of interest; (iii) long-term shifts in moisture sources or storm tracks; (iv) changes in the proportion of precipitation which has been derived from non-oceanic sources, i.e. recycled from continental 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 might provide quantitative temperature estimates to the more attainable goal of providing precise chronological control on the timing of major first-order shifts in d18Op, that can be interpreted in terms of changes in atmospheric circulation patterns (e.g. Burns et al., 2001; McDermott et al., 2001; Wang et al., 2001), changes in the d18O of oceanic vapour sources (e.g. Bar Matthews et al., 1999) or first-order climate changes such as D/O . and Mangini, events during the last glacial (e.g. Spotl 2002; Genty et al., 2003). Future technical developments that may allow direct measurement of d18O in speleothem fluid inclusions will reduce the uncertainties in the interpretation of d18Oct and may ultimately allow calculation of absolute temperature changes. Reliable fluid inclusion data can provide invaluable constraints on palaeohydrological conditions, particularly in regions where speleothem deposition is continuous through the glacials. Using independent palaeo-temperature estimates, Matthews et al. (2000) calculated the d18O of fluid inclusions using the values from coexisting calcite. Combining these calculated d18O values with D/H measurements carried out using a vacuum thermal extraction technique enabled these authors to compare the fluid inclusion data for four fossil speleothems with the present-day cave waters and meteoric water lines. The most

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significant result was that fluid inclusions from two speleothems deposited during the glacial period plot close to the global meteoric water line (MWL), in contrast with present-day precipitation and cave dripwaters that plot on the Mediterranean meteoric water line (MMWL). These data highlight the important insights into climate-driven changes in the operation of the meteoric water cycle that can be gained from studies of speleothem fluid inclusions. In a study of fluid inclusions from three caves in Israel, McGarry et al. (2004, this volume) used both the MWL and MMWL to calculate the d18O of fluid inclusions from measured D/ H values. The resulting palaeo-temperature estimates are in good agreement with alkenone and modern analogue-based estimates from the eastern Mediterranean Sea for the past 140,000 years. These data also indicate that whereas the dD–d18O relationships for meteoric water in the region follow the MMWL in the present-day and the last interglacial there was a strong short-lived shift towards the MWL during the time interval corresponding to the last glacial. In the near future it is likely that there will be further developments of the fluid inclusion extraction and measurement techniques that will underpin the interpretation of oxygen isotope ratios in speleothems. Meanwhile, the emphasis is on developing well-dated high-resolution d18O records that can be correlated with better understood (but often more poorly dated) records such as the Greenland ice cores, and on mapping out the geographical 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 offer productive targets for fluid inclusion studies in the future.

3. Carbon isotopes in speleothems At pH values typical of karst waters, equilibrium constants for the relevant reactions dictate that bicarbonate is the dominant species in solution, and so the large (ca 10%) bicarbonate-CO2 fractionation factor dominates the equilibrium fractionation process. Two endmember models, which describe the processes by which percolating groundwaters acquire calcium carbonate in the soil and host-rocks above a cave, have been described (e.g. Hendy, 1971; Salomons and Mook, 1986). In an open-system model, continuous equilibration occurs between the seepage water and an infinite reservoir of soil CO2. This drives a monotonic increase in bicarbonate content as the water progressively acquires more solutes in the unsaturated zone. Under these conditions, the d13C of the dissolved species reflects the isotopic composition of the soil CO2, with no detectable isotopic imprint from the carbonate host-rock. For a C3 plant system, the d13C of the dissolved inorganic carbon (DIC) in the percolat-

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ing solution is predicted to be in the range 14% to 18% when the solution reaches saturation with respect to CaCO3, depending on soil pCO2 and temperature (Hendy, 1971; Salomons and Mook, 1986; Dulinski and Rozanski, 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 since CO2 is consumed in the carbonation reaction H2O+CO2=H2CO3 the extent of limestone dissolution is limited by the finite CO2 reservoir. Under these conditions the isotopic composition of the carbonate host-rock influences the isotopic composition of the DIC. For a C3 system with soil gas d13C of ca 23% and a host limestone with d13C of +1%, the DIC d13C is typically ca 11%. In practice most natural systems are likely to be partially open, and a mathematical description of such variability has been formulated (Dreybrodt, 1988). In arid regions, large shifts in the d13C values of speleothem calcite have been ascribed to climate-driven changes in vegetation (e.g. C3 versus C4 dominated plant assemblages, Dorale et al., 1992; Bar-Matthews et al., 1997). Data from pedogenic carbonates often support such interpretations (e.g. Cerling, 1984; Cerling et al. 1991). In these regions, relatively large shifts in d13C can occur, because soil respired CO2 in equilibrium with a C3 dominated plant assemblage has d13C in the range 26% to 20%, while that in equilibrium with C4 vegetation is significantly heavier (d13C of 16% to 10%). These differences are preserved as distinctive ranges in d13C in secondary carbonates (typically 14% to 6% for carbonates deposited in equilibrium with CO2 respired from C3 plants, and 6% to +2% for that from C4 plants). However, many temperate-zone speleothems also exhibit d13C values > 6%. These values are higher than those predicted to be in equilibrium with the prevalent C3 vegetation in temperate regions (Baker et al., 1997). In situations where the soil–water residence times may be relatively short, complete isotopic equilibration may not occur between soil CO2 and the percolating H2O, with the result that the water may retain a component of (isotopically heavier) atmospheric CO2 in solution. Experimental studies (e.g. Liu and Dreybrodt, 1997) have confirmed that the hydration of CO2 is relatively slow, and that the kinetics of the reaction CO2+H2O=H2CO=H++HCO3 is controlled by the CO2 hydration step. Other processes including evaporation, rapid degassing of cave dripwaters, kinetic fractionation, CO2 degassing of dripwaters and consequent calcite precipitation in the vadose zone above a cave have been offered as possible explanations for these relatively heavy carbon isotope signatures (Baker et al., 1997; Genty and Massault, 1997).

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As an example from the recent literature, Genty et al. (2003) noted that stalagmites deposited during the late glacial in the south of France exhibit d13C values that are much higher than in those deposited during the Holocene. These differences were attributed to changes in the relative proportions of atmospheric and biogenic (light) carbon. This interpretation implies that periods of climatic amelioration promote the production of soil biogenic CO2, resulting in isotopically lighter carbon isotope ratios in the speleothem calcite. In summary, the interpretation of carbon isotopes in regions where switches in the proportions of C3 and C4 plants can be independently verified (e.g. from pollen data) is relatively straightforward. In temperate regions that lack a natural C4 vegetation however, the interpretation of carbon isotopes in speleothems remains difficult, and the data are often interpreted on an ad hoc basis. So far, the geochemical criteria for distinguishing between the processes that might be responsible for carbon isotope variations have not been established, yet these are essential if reliable palaeoclimatic information is to be inferred from the d13C record of temperate-zone speleothems. If, for example, incomplete equilibration between soil CO2 and percolating water is the primary factor responsible for elevated d13C in some temperate-zone speleothems, then elevated d13C should be associated with wetter periods, when the water/soil gas contact times are shorter. If, on the other hand, seasonal evaporation of water in the undersaturated zone or perhaps within the cave itself is the dominant processes, then high d13C should be associated with drier periods. One promising line of research is to combine trace element and carbon isotope data, because depending on the nature of the co-variations, several possible mechanisms for changes in d13C can be ruled out. In a study of a 31,000-year-old speleothem from New Zealand for example, Hellstrom and McCulloch (2000) were able to rule out a reduction in cave seepage water flow rates as an explanation for elevated d13C. Barium concentrations exhibited a strong negative correlation with d13C, the opposite to that predicted if high d13C was caused by enhanced prior calcite precipitation in the flow-path as a result of slower flow rates. Future research should seek to develop further these geochemical and petrographic criteria and to underpin these arguments with theoretical modelling and with systematic measurements on present-day dripwaters.

studies for which good chronological control (i.e. TIMS or PIMMS U-series dates) is available are discussed below. 4.1. Isotope stage 6 and the penultimate deglaciation Speleothem records from Late Pleistocene mid- to high-latitude sites are discussed first, because these are likely to be sensitive to glacial–interglacial transitions, and they illustrate an important feature of speleothems, namely that calcite deposition slows down or ceases during glacials. Fig. 1 is a 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 absence of speleothem deposition in the mid- to high latitudes of the Northern Hemisphere during isotope stage 2 is striking, consistent with results from previous compilations based on less precise alpha-spectrometric dates (e.g. Gordon et al., 1989; Baker et al., 1993; Hercmann, 2000). By contrast, speleothem deposition appears to have been essentially continuous through the glacial periods 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 to 60 ka (Winograd et al., 1992; Ludwig et al., 1992). Unlike speleothems (sensu-stricto), the Devils Hole calcite (DH-11) was deposited in a phreatic open fault zone by calcite-supersaturated groundwaters. Its O isotopic composition therefore reflects changes in the

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4. Case study review The following case study review is structured around the new insights that studies of stable isotopes in speleothems have provided in some of the key issues in palaeo-climatology. The major results from those

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Age kyr B.P. 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.

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O isotopic composition of regional meteoric water that recharged the aquifer, in turn reflecting changes in the average winter–spring surface temperature in the southern Great Basin. Unlike speleothems there is a significant, but poorly constrained transit time between the recharge zone and the site of calcite deposition. As a result, the DH-1 record provides minimum estimates for the 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 SPECMAP record that largely reflects Northern Hemisphere ice volume (Fig. 2) indicates that both clearly record the first-order glacial–interglacial transitions. In detail, however, there are several important differences between the DH-11 and SPECMAP curves (Winograd et al., 1992), and most attention has focused on the differences in the timing of Termination II (Fig. 2), because it is argued that the timing of this termination is crucial for testing the Milankovitch hypothesis. In the DH-11 record, Termination II occurs at 14073 ka, predating by some 12 ka the timing of Termination II in the SPECMAP record (12873 ka). While the interpretation of the DH-11 record remains controversial, recently published independent data sets from both the continental and marine realm (see below) appear to corroborate the inference that Termination II pre-dated by several ka the timing of maximum insolation at 65 N. A study from Spannagel Cave in the high Austrian . et al., 2002) provides compelling new Alps (Spotl evidence that climatic conditions had ameliorated sufficiently by 13571.2 ka, to allow flowstone deposition to re-commence following the penultimate (Isotope Stage VI) glaciation. Results from high-altitude continental sites such as this Alpine site are important, because they are likely to be sensitive to glacial/ interglacial transitions. Calcite deposited at 13571.2 ka exhibits very low d18O (ca 12.571.5%), probably indicating that deposition occurred from low

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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).

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d18O glacial melt-waters. By contrast, calcite deposited in the interval 122–116 ka has d18O values of about 9%, similar to those for Holocene stalagmites from the site, indicating a switch from glacier derived to ‘normal’ meteoric water sources. The critical result is that liquid water was available for speleothem deposition at this high-altitude Alpine site by 13571.2 ka, indicating that deglaciation had clearly commenced. This result corroborates other lines of evidence from the marine realm (e.g. Esat et al., 1999; Henderson and Slowey, 2000; Gallup et al., 2002) that the timing of termination II occurred at least 8000 years before the 65 N insolation maximum at 128 ka. The new result also corroborates previously published interpretations based on compilations of alpha-spectrometric U-series dates for speleothems (Baker et al., 1993) that speleothem deposition had re-commenced by at least 133 ka, and support a TIMS U-series date of 13371.2 ka for the base of a speleothem in N England (Baker et al., 1995). However, as noted by Baker et al. (1996), site-specific effects can influence speleothem growth, and while the presence of speleothems can be taken to indicate liquid water availability, their absence at any particular site does 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 Hamarnesgrotta near Rana, 20 km south of the Arctic Circle in northern Norway (Linge et al., 2001a, b), are relevant here, because the high latitude of the cave site makes it sensitive to the onset of periglacial conditions, associated with the accumulation of ice in northern Scandinavia. TIMS U-series ages show that speleothem deposition at this high-latitude site occurred during isotope sub-stages 5e–5a (123.5–73.3 ka). An important result is that conditions favourable for speleothem growth existed during isotope stage 5e, and that the slowdown in calcite precipitation rate marks the termination of the interglacial climate sometime between 119.5 and 107.7 ka. During isotope stage 5 the growth rate of the speleothem apparently responded to progressively deteriorating climatic conditions above the cave site. Thus, the growth rate was relatively rapid in the 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 in this speleothem exhibited low-amplitude (ca 0.5%) variability on centennial to millennial timescales during isotope stage 5, and the values were similar to those for Holocene speleothems from the region, reflecting relatively stable conditions, with evidence for cooler conditions between 122.05 and 121.7 ka. Previously published alpha-spectrometric U-series dates for speleothems from cave sites in northern Norway are critical

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to the debate about the timing of the penultimate deglaciation. In particular, a previously reported alphaspectrometric U-series date of 14575 ka (1s) (Lauritzen, 1995) from Okshola, a lowland site close to the Arctic Circle in northern Norway is pivotal to the debate, and perhaps should be refined using more precise 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. The stalagmite was deposited from approximately 155 to 100 ka and exhibits highest growth rates (B 61.5 mm/ ka) during a relatively short time interval between 129.271.6 and 122.172.0 ka. This time interval coincides with prolific coral growth along the Western Australian coast and marks the onset and duration of full interglacial conditions. Since speleothem growth rates reflect precipitation rather than temperature in this region, it was argued that the highest rates of precipitation on land occurred during the period when full interglacial sea-levels were attained. Periods of lower effective precipitation prior to 129.2 ka (lower speleothem growth rates) were attributed to latitudinal shifts in the location of the subtropical highs and associated westerly circulation. Based on the pattern of the growth rate of the speleothem, Zhao et al. (2001) argued that the penultimate deglaciation was underway by about 142 ka, but that the full interglacial conditions (highest coral and speleothem growth rates) coincide broadly with the 65 N summer insolation peak that occurred at 128–126 ka. Thus, while the maintenance of full interglacial conditions can be explained by insolation (Milankovitch) forcing, an additional forcing mechanism is required to trigger the onset of deglaciation 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 and 74 ka. Significantly, all of the growth phases of stalagmite Cla4 correspond to humid periods during which sapropels were deposited in the eastern basin of the Mediterranean, and most of the growth phases correspond to relatively warm periods of high sea stands during isotope stages 5 and 7. As discussed by Plagnes et al. (2002), several European Cave sites show evidence for speleothem growth during MIS 6 (Fig. 1), indicating significant periodic climatic amelioration. Speleothem deposition between 169.171.5 and 162.3471.5 ka (MIS sub-stage 6.4) was interpreted to reflect the S6 sapropel event that occurred in the eastern Mediterranean. In a study that provides additional evidence for the hydrological conditions during MIS 6, Bard et al. (2002) demonstrated that d18O in a stalagmite from 19 m below present-day sea-level at Argentarola Cave on the Tyrrhenian 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 in d18O could be accounted for by changes in the isotopic composition of the vapour source, but the remaining 1–2% was interpreted as reflecting the so-called ‘amount’ effect, reflecting wetter conditions in the region during MIS 6.5. The inferred change to wetter conditions during sapropel 6 is consistent with the pluvial events during this and later sapropel events (S1–S6) inferred independently on the basis of decreases in d18O in speleothems from Israel (Bar-Matthews et al., 2000; Ayalon et al., 2002). Taken together these results yield important new insights. In particular, it is clear that wetter conditions associated with the formation of sapropel 6 were not confined to the eastern basin of the Mediterranean through increased Nile discharge. Instead the western Mediterranean was also apparently wetter 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 isotope shifts interpreted to reflect sapropel S6 in the studies of Bard et al. (2002) and Plagnes et al. (2002). . Spotl and Mangini (2002) demonstrated that a stalagmite from the Austrian Alps, deposited in the time interval between 57 and 46 ka preserves evidence for high d18O events that appear to be coeval with the MIS 3 interstadial events 15a, 15b, 14 and 12, recognised previously in the Greenland ice cores. This is an important result because it is the first time that precisely dated D/O events have been identified in terrestrial climate records in the mid-latitudes. The study also provides evidence that mean annual temperatures at this high-altitude (2165 m a.s.l.) site remained close to that of the present-day (2 C) over the 11 ka interval of speleothem growth during isotope stage 3. Shifts to higher d18O of approximately 2% centred at 55.4 ka and between 54.3 and 51.1 ka are interpreted to reflect interstadial events 15b and 14 of the GRIP record, respectively, with a smaller (o1%) shift at 54.9 ka BP corresponding to event 15a of the GRIP record (Fig. 3). These results are in good agreement with data for five partially overlapping stalagmites from Hulu Cave in eastern China (Wang et al., 2001) that appear to show a strong coherence with O isotope variability in the GRIP and GISP2 ice cores (Fig. 4). In the latter study oxygen isotope records from five stalagmites define overlapping O isotope trends, indicating that these speleothem records preserve a precisely dated signal of changes in the d18O of precipitation. Taken together these data indicate that further refinements to the Greenland icecore chronology are required. It is clear from revisions of the GRIP chronology (compare curves B and C in Fig. 3) that the dating uncertainties in the ice cores now present a major obstacle in assessing the latitudinal leads and lags in the timing of the interstadial events of the last glacial.

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

In a significant recent study, Genty et al. (2003) argued that D/O events during the last glacial are recorded by carbon and oxygen isotopes in a well-dated stalagmite 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 the occurrence of the D/O events in mid-latitude western Europe, it offers perhaps the best available chronological control on the timing and occurrence of such events, and provides an improved chronological framework for

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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).

the GRIP and GISP2 ice-core records (Fig. 4). The stalagmite has three growth hiatuses that occurred between 78.8–75.5 ka (hiatus D2), 67.4–61.2 ka (hiatus D3) and 55.7–51.8 ka (hiatus D4). Carbon isotope ratios increase prior to and after hiatus D3 and are interpreted to reflect a cooling event (the ‘Villars Cold Phase’) rather than a localised drip-water or other cave hydrological effect. Hiatus D3 is also tentatively correlated with Heinrich event H6 (Fig. 4). A remarkably coherent picture of continental climate Late Pleistocene variability with close links to the oceanic realm has emerged from studies of speleothems from the eastern margin of the Mediterranean (Fig. 5). Particularly impressive is the well-dated composite d18O record for the past 185 ka based on 21 speleothems from Soreq 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 be made between different coeval speleothems in this composite record is that the shifts in d18O are relatively large (several per mil), indicating a strong climatic signal in the d18O record. The Soreq d18O record appears to reflect predominantly two effects: (i) changes in the d18O of the oceanic vapour source and (ii) the ‘amount’ effect (Bar Matthews et al., 1996, 1997, 1999, 2000; Kaufman et al., 1998; Ayalon et al., 1998, 2002). These studies are important because they establish a critical link between the oceanic realm and continental climate in this region. Thus, d18O minima in speleothems from Soreq coincide exactly with the occurrence of sapropel events in the Mediterranean Sea, and recently it has been shown that this is true for glacial as well as for interglacial conditions (Ayalon et al., 2002). The dominance of the ‘amount effect’ on d18O in stalagmites in this region

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-9

-8

δ18 O (‰, PDB)

-7

-6

-5

-4 -3

-2 200

150

100

50

0

Age (ka) 18

Fig. 5. Composite d O 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).

allows reliable reconstruction of arid and pluvial phases (Bar Matthews et al., 1996, 1997, 1999, 2000; Ayalon et al., 2002; Bard et al., 2002). d18O and deposition rate changes in speleothems from Hoti Cave in northern Oman (Burns et al., 1998; Burns et al., 2001) have provided new palaeo-climatic insights in this part of the tropics. Low d18O in speleothem calcite associated with episodes of enhanced deposition during marine isotope stages 5e, 7a and 9 was interpreted to reflect pluvial phases linked to increased monsoon rainfall. Thus, rapid speleothem growth occurred during five well-dated intervals 6–10.5, 78–82, 120–135, 180–200 and 300–325 ka with evidence for non-deposition of calcite, reflecting relatively arid conditions during the intervening episodes. Each pluvial episode was characterised by d18O values that are significantly lower (by several %) than those of modern stalagmites, consistent with a northwards shift in the monsoon rainfall, as a result of a northwards shift in the inter-tropical convergence zone during peak interglacial periods. Holmgren et al. (1995) presented oxygen and carbon isotope data for a stalagmite from Lobatse II Cave in southeastern Botswana in which deposition was restricted to two phases. During the first phase of deposition (51–43 ka), warm humid conditions associated with C3 vegetation were inferred (low d18O, low d13C), whereas the second phase (27–21 ka) was dominated by drought-adapted C4 plants (higher d13C and d18O), indicating cooler conditions. Correlations between the Lobatse II Cave study and that for Cango

Caves (Cape Province, South Africa) are hindered by problems of imprecise age constraints, but the available data tentatively indicate that cooling was recorded at both sites in the period prior to the last glacial maximum (Holmgren et al., 1995). Further TIMS or PIMMS U-series dates are clearly required to investigate further the possible regional significance of such cooling. Dorale et al. (1998) presented oxygen and carbon isotope data for four stalagmites that were deposited in the interval between 75 and 25 ka at Crevice Cave, Missouri, USA. The study is significant because the four coeval stalagmites exhibit coherent oxygen and carbon isotope records. High d18O values between 59 and 55 ka were inferred to reflect warm conditions, and this period coincided high d13C, interpreted to reflect the expansion of the prairie-type C4 vegetation. 4.2. Holocene records In a study of a Holocene speleothem from S^ylegrotta, northern Norway, Lauritzen and Lundberg (1999) calibrated the temperature dependence of d18Oc at this site, using independently derived estimates of present-day and Little Ice Age (LIA) mean annual temperatures. Data from other sources (e.g. tree-line changes at 3700 years, evidence from early to midHolocene diatoms and inferred climatic conditions at 10,000 years BP) are in broad agreement with the relationship between d18Oct and temperature based on these two data points, indicating that this calibration may 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 4000 years exhibits heavier d18O than that of stalagmite SG93, and the pattern of d18O correlates with that of SG93 over some intervals of the Holocene only. Surprisingly, a third stalagmite (SG92–4) exhibits d18O variability that correlates remarkably well with that of SG93, despite the fact that it was collected from close to the cave entrance and might be expected to have been affected by a more variable micro-climate. Studies such as these that employ several coeval stalagmites are invaluable to assess the reliability of speleothems as palaeo-climatic recorders and they highlight the danger of relying on a single stalagmite d18O record. It seems likely that processes other than climate variability (e.g. variable water ingress routes and water mixing) can substantially affect the d18O values recorded in stalagmites, particularly at high temporal resolutions). McDermott et al. (2001) presented a laser-ablation high-resolution O isotope time-series record for a Holocene stalagmite (CC3) from Crag Cave, a coastal site in SW Ireland. The main result of this study was that subtle higher frequency (century scale) d18O variations in the early to mid-Holocene appear to correlate with

those in the GISP2 ice core, suggesting that the latter reflect regional Holocene climate signals. Approximately 1640 laser-ablation d18O measurements were carried out along the growth axis of this 465 mm long stalagmite, resulting in an exceptionally high-resolution Holocene d18O record. The ‘8200-year’ cold event was defined by eight data points centred on 8.3270.12 ka and it exhibited a very large (ca 8%) decrease in d18O (Fig. 6). Since the speleothem d18O signal may in part reflect temporal changes in the vapour source and/or cloud trajectories it was not possible to calculate temperature changes from these d18O data. The timing of the ‘8200-year’ event in speleothem CC3 is within the dating uncertainties of the GISP2 core. Thus, the maximum amplitude occurs at 8.3270.12 ka compared with 8.2170.10 ka in GISP2, and is coeval with faunal evidence for cooling at 8.3070.06 ka in core 28-03 from the Norwegian Channel. Significantly, the amplitude of this large shift to lower d18O was too large to ascribe solely to a reduction in mean annual air temperature. Instead it was attributed to freshening of the surface of the adjacent N Atlantic by isotopically depleted melt-water. The absence of a clear shift in d18O in the speleothem data during the

Fig. 6. Comparison of the O isotope record from stalagmite CC3 (south-west Ireland) with the GISP2 curve (McDermott et al., 2001). A timing of the large (ca 8%) shift in d18O in the CC3 record is indistinguishable (within the 2s dating uncertainty) of the ‘8200-year’ cold event in the GISP2 record. Other post-8200-year oscillations in d18O in the early part of the Holocene appear to occur synchronously (within dating uncertainties) in the two records (McDermott et al., 2001 for a discussion). RWP, DACP, MWP and LIA are possible expressions of the ‘Roman Warm Period’, ‘Dark Ages Cold Period’, ‘Medieval Warm Period’ and ‘Little Ice Age’, respectively.

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other N Atlantic ice-rafting events at ca 5.9, 4.3, 2.8 and 1.4 ka despite the high resolution of the data (7–18 years per analysis) is significant. It indicates that unlike the ‘8200-year’ event, the later Holocene ice-rafting events failed to trigger large changes in d18O, and by implication failed to establish a detectable melt-water cap on the mid-latitude N Atlantic. In the mid-western United States a coherent picture of climate-driven vegetation changes in the Holocene is beginning to emerge from speleothem studies, supplemented by data from pollen records. Dorale et al. (1992) presented stable isotope data for a TIMS U-series dated 7800-year-old stalagmite (1 s) from Cold Water Cave in northeast Iowa. The O isotope data were interpreted in terms of palaeo-temperature changes, while the carbon isotope data were taken to reflect climate-driven changes in the nature of the vegetation above the cave. A midHolocene warming of about 3 C relative to the early Holocene, followed by a cooling of 3–4 C was inferred for the period between 4 and 1 ka, assuming a simple relationship between d18Oct and mean annual air temperature. d13C exhibited strong unidirectional shifts during the past 6000 years in this record, and in particular a marked shift to higher d13C between 5.9 and 3.6 ka indicates a replacement of forest by a C4-rich prairie vegetation, consistent with published pollen records from the region. The shift from forest to prairie-type vegetation about 5900 years ago appears to have occurred rapidly, probably within a century, offering insights into the rates at which climate-driven changes in vegetation type can occur in regions close to ecotone 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 that appear to have been influenced strongly by site-specific effects. While d13C varied coherently between the three stalagmites, d18O did not, indicating that local nearsurface evaporative enrichment of 18O had variably modified the d18O signal prior to infiltration. The O isotope variations in the stalagmite with lowest d18O (sample 2SS), and therefore the one least affected by evaporative enrichment effects appear to reflect temporal changes in the moisture source region rather than temperature changes. Thus, the shift to lower d18O during the mid-Holocene, accompanied by an increase in stalagmite growth rate, is best explained by a switch to 18O-depleted moisture sources derived from the Gulf of Mexico or the Pacific Ocean. The observation that low d18O was accompanied by enhanced speleothem growth in the mid-Holocene is puzzling in view of the overall drier mid-Holocene climate that must have accompanied the forest–prairie transition in the region. One explanation is that increased infiltration was possible because most of the precipitation occurred during the cool season (Denniston et al., 1999a).

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Speleothems such as these represent good candidates for fluid inclusion studies, because if the preferred interpretation is correct, trapped fluids should exhibit d18O variations similar to the calcite values. The authors conclude that the previously calculated temperature estimates, based on stalagmite 1 s from the Cold Water Cave may have overestimated the amount of midHolocene warming. This study highlights the difficulties inherent in interpreting speleothem d18O as a quantitative palaeo-temperature proxy, particularly in regions where surface water deficits occur seasonally, and underlines the need to analyse several coeval stalagmites to ensure that a robust regional climate signal is recorded. 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 from studies of lake palaeo-levels in the Sahel region of Africa (Gasse and Street, 1978; Ritchie et al., 1985). The speleothem results offer enhanced chronological control, firmly placing the early–mid-Holocene transition from pluvial to the present arid to semi-arid conditions at 6.2 ka BP. Significantly, proxies for monsoon strength derived from marine sediment proxies from the Indian Ocean and Arabian Sea (e.g. Anderson and Prell, 1993; Rosteck et al., 1997) that effectively monitor wind strength rather than monsoon rainfall amount, differ from the record derived from speleothems. One explanation (Burns et al., 2001) is that speleothems offer a more direct and reliable signal of monsoon rainfall signal over the continents, whereas the marine records primarily record monsoon wind strength variations that are not necessarily accompanied by increased rainfall. This arises because factors other than monsoon wind strength, such as changes in the sea surface temperature of the tropical Indian Ocean that supplies the moisture, may strongly influence moisture transport to the Arabian Peninsula (Burns et al., 2001). A more detailed O isotope analysis for a Holocene speleothem from this cave was presented by Neff et al. (2001). An important finding of the latter study was that speleothem d18O correlates with D14C, interpreted as reflecting solardriven changes in the monsoon in this region on centennial to millennial timescales. Thus, while it appears that on relatively long timescales the northwards transport of moisture to the Arabian Peninsula may be driven by glacial to interglacial cycles, similar changes may occur on much shorter timescales in response to variable insolation. The latter study is one of the few that has successfully demonstrated a link between solar variability and climatic conditions in the Holocene. In a multi-proxy study, Xia et al. (2001) presented stable isotope data for a well-dated 1.13 m long stalagmite that grew continuously from 9180 to 5060 years BP in Lynds Cave, northwestern Tasmania. The

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interval between 8000 and 7400 years BP was characterised by high d18O, relatively low growth rates and high initial (234U/238U) ratios, reflecting conditions that were probably warmer and drier than those of the present-day. By contrast, calcite deposited in the interval between 7400 and 6600 years BP exhibited lower d18O, high growth rates and low initial (234U/238U) ratios. These characteristics were interpreted to reflect relatively wet conditions, and coincide with the so-called mid-Holocene climatic optimum that had been recognised in earlier studies of pollen sequences and lake levels in the region. The most recent part of the record, from 6100 to 5100 years, is characterised by the lowest growth rates and dramatic fluctuations in both O and C isotopes as a result of kinetic fractionation processes in response to cooler, drier conditions and a reduction in the humidity of the cave air. In a study of a 2.7 m long stalagmite from Cango Caves, Cape Province, South Africa, Talma and Vogel (1992) used data for 14C-dated groundwater to estimate of the d18O of recharge, and therefore of cave drip-water in the past. On this basis, they calculated that temperatures were approximately 6–7 C colder during the last glacial maximum compared with the present. Lower temperatures were also inferred for parts of the mid- to late Holocene, centred on approximately 4500 and 3000 14C years BP. This study is noteworthy because it is one of the few where an independent estimate of the d18O of cave drip-waters could be provided, allowing quantitative temperature estimates. In the future material such as this would represent an excellent target for fluid inclusion studies as a means to test the inferences from the isotopic composition of 14 C-dated groundwater. Repinski et al. (1999) analysed a speleothem from Cold Air Cave (Northern Province, South Africa) and attributed the lower d18O values that occurred between about 800 and 400 years ago to the LIA. Higher d18O values between about 4400 and 4000 years ago were interpreted to reflect a warmer period, assuming that the overall positive correlation between temperature and d18O was valid through the late Holocene. Further work is clearly required to explore how the inferences drawn by Repinski et al. (1999) for the period around 4000 years ago in the Northern Province relate in a regional context to the results of Talma and Vogel (1992) for the Cape Province.

5. Summary and pointers for future research So far, the major contribution of stable isotope studies on speleothems for palaeo-climatic reconstruction has been the development of well-dated highresolution d18O records that can be correlated with better understood records such as the Greenland ice

cores, thereby defining the geographical extent of regionally synchronous O isotope ‘events’ such as the D/O events, regional pluvial events, and late glacial to early Holocene oscillations. The major strength of speleothem studies has been in the provision of robust chronologies that are independent of both the orbitally ‘tuned’ marine records and the ice-core chronologies. In some regions where speleothem deposition appears to continue uninterrupted across glacial–interglacial transitions (e.g. Israel), remarkably detailed land–sea correlations have emerged, that in turn provide important new insights into the operation of the hydrological system under different climatic regimes. A weakness of speleothem stable isotope studies has been the difficulty in providing unambiguous palaeo-climatic interpretations of the data. The development of reliable analytical protocols to recover both the D/H and the O isotope ratios of trapped fluid inclusions would greatly facilitate the interpretation of stable isotope data and is the subject of ongoing research at several laboratories. This capability would allow unambiguous estimation of palaeo-temperatures, and would provide valuable tests for the output of ‘water-isotope enabled’ GCMs. Critically, stable isotope measurements on fluid inclusions may allow, for the first time, realistic estimates of the uncertainties associated with reconstructed climate parameters (e.g. palaeo-temperature, palaeo-precipitation). Many of the O isotope ‘events’ that have been defined by recent studies will offer productive targets for fluid inclusion studies in the future. In the past decade there has been a trend towards the provision of ever more detailed, higher-resolution records (e.g. use of lasers and micro-drilling). Very high-resolution (e.g. close to annual) data sets may prove difficult to interpret however, because seasonal noise may dominate the signal, particularly in Holocene speleothems where climate variability is likely to be subtle. Future studies are likely to make further use of ion-probe instruments to measure O isotope ratios at a sub-annual resolution, while accepting a reduced analytical precision on individual measurements. In carefully chosen, rapidly deposited material it may be possible to define the annual cycle in d18O in speleothem calcite, thereby simultaneously providing a chronology for short intervals and a measure of changes in seasonal cyclicity as a function of different climate regimes. A critical aspect of future speleothem-based O isotope records will be the provision of ever more reliable chronologies for which realistic uncertainties are explicitly expressed, in order to compare with other calendar-year-based proxies (e.g. ice cores) and forcing mechanisms (e.g. insolation). This in turn will require the use of more realistic and statistically constrained age–depth relationships in U-series-dated speleothems. Currently, there is little effort to realistically represent

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and report age uncertainties associated with interpolations between dated intervals of speleothems, and different models (e.g. linear interpolation, linear regression and polynomial curve-fitting) are used without any rigorous exploration of the chronological consequences of choosing one model over other equally plausible models. This is particularly an issue in the case of Holocene speleothems for which interpolation uncertainties typically exceed the 2s uncertainties associated with individual U-series age determinations. Most chronologies are based on U-series dating techniques and with the advent of different analytical approaches (e.g. TIMS and PIMMS) it is imperative that systematic laboratory inter-comparison programmes are carried out in the near future, because the accuracy of all U-series ages depends ultimately on the accuracy with which the mixed U–Th spikes have been calibrated in different laboratories. These issues relating to chronology are increasingly important as researchers seek to establish correlations between different speleothem records and with other independently dated archives such as ice cores. Speleothems offer perhaps the best opportunity to accurately constrain the timing of clearly defined climate signals (e.g. glacial–interglacial transitions, D/O oscillations, the ‘8200-year’ event). By focusing on these times of high signal to noise ratio it should be possible to assess inter-hemispheric and latitudinal leads and lags, providing that a carefully constructed chronology is available. It is noteworthy that at present the lowlatitudes in both hemispheres, and the Southern Hemisphere in particular are under-represented in the currently available database of reliably dated speleothem stable isotope records (Fig. 1). Increasingly, multi-proxy studies are being undertaken on individual speleothems (e.g. combining stable isotopes with trace elements, petrographic information and growth-rate information). These approaches help to narrow the uncertainties associated with the interpretation of stable isotope data from individual speleothems, but are not a substitute for replication of records within individual cave sites. Clearly, a balance must be found between the conflicting requirements to replicate records and to conserve cave sites for aesthetic purposes and for scientific investigations in the future. Finally, studies should include more systematic seasonal monitoring of present-day precipitation and cave drip-waters. Monitoring is both expensive and time consuming. It is, nonetheless, essential to provide constraints on the O isotopic composition of cave drip-waters, to assess seasonal biases in calcite deposition rates, to investigate the extent to which drip-waters reflect the weighted mean d18O value of precipitation and to understand better the factors that control the d13C of the DIC.

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Acknowledgements The author thanks Ian Fairchild and an anonymous reviewer for their constructive comments that helped to improve the manuscript. Mira Bar-Matthews kindly provided Fig. 5. Various aspects of the material presented here have evolved as a result of discussions with colleagues and acquaintances that include Ian Fairchild, Andy Baker, Peter Rowe, Tim Atkinson, Mira Bar-Matthews, Alan Matthews, Chris Hawkesworth, Silvia Frisia, Andrea Borsato, Dominique Genty, Tim Heaton, Dave Mattey, James and Lisa Baldini. Melanie Leng is sincerely thanked for her patient editorial advice.

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