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Trace metal and carbon isotopic variations in cave dripwater and stalagmite geochemistry from northern Borneo: TRACE METALS/CARBON STALAGMITES-BORNEO
Abstract
1] We investigate stalagmite trace metal ratios and carbon isotopic composition (13 C) as potential paleoclimate proxies by comparing cave dripwaters, stalagmites, and bedrock composition from Gunung Mulu and Gunung Buda National Parks in northern Borneo, a tropical rainforest karst site. Three year long, biweekly time series of dripwater Mg/Ca, Sr/Ca, and 13 C from several drips at our site are not correlated with rainfall variability, indicative of a relatively weak relationship between hydroclimate and dripwater geochemistry at our site. However, combining all of the dripwater geochemical data gathered over four field trips to our site (N > 300 samples), we find that drips with highly variable Mg[Sr]/Ca have relatively invariable 18 O values close to the mean. We hypothesize that increased residence times translate into reduced variance in dripwater 18 O through mixing in the epikarst as well as increased Mg[Sr]/Ca values through increased calcite precipitation in the epikarst. Mg/Ca, Sr/Ca, and 13 C time series from three overlapping stalagmites that grew over the last 27 kyrs are characterized by strong centennial-scale variations, and bear little resemblance to previously published, well-reproduced 18 O time series from the same stalagmites. The only shared signal among the three stalagmites' geochemical time series is a relative decrease of 1% in 13 C from the Last Glacial Maximum to the Holocene, consistent with a transition from savannah (C4) to rainforest (C3) conditions documented in nearby records. Taken together, our study indicates that stalagmite Mg[Sr]/Ca ratios are poor indicators of hydroclimate conditions at our site, while stalagmite 13 C exhibits some reproducible signals on glacial-interglacial timescales. Components: 11,578 words, 12 figures, 7 tables. (2013), Trace metal and carbon isotopic variations in cave dripwater and stalagmite geochemistry from northern Borneo, Geochem. Geophys. Geosyst., 14, 3567–3585, doi:10.1002/ ggge.20215.
... External factors such as temperature, pH of rainwater and transfer time from the surface all influence the δ 18 O signal preserved in the speleothem (Denniston and Luetscher, 2017;Guo and Zhou, 2019;McDermott, 2004). Karst processes such as kinetic isotope fractionation from the degassing of CO 2 during speleothem formation, prior calcite precipitation, karst hydrological processes and seasonal fluctuations in cave ventilation have also been identified as variables that can affect the δ 18 O signal (Partin et al. 2013 and references therein; Treble et al. 2022). To overcome this, contemporary studies of the modern cave system are used to better inform interpretation of palaeoenvironments. ...
... Speleothem data for SEA includes records from Flores, East Java and Borneo (Ayliffe et al. 2013 (Fig. 3e); Griffiths et al. 2009Griffiths et al. , 2016Lewis et al. 2011;Partin et al. 2013;Westaway et al. 2007). Not all of these studies associate their palaeoenvironmental findings with the archaeological record of the site. ...
... Separating the input from atmospheric, soil and microbial processes and vegetation type is a particular problem in using δ 13 C records to interpret climate changes at a given site (Blyth et al. 2013a;Wong and Breecker, 2015). Partin et al. (2013) investigated δ 13 C stable isotope data from Gunung Mulu and Gunung Buda National Parks, northern Borneo. Utilising Mg/Ca and Sr/Ca elemental analysis alongside δ 13 C, Partin et al. (2013) found no connection between the δ 13 C of the bedrock and dripwater or speleothem δ 13 C. ...
We review palaeoenvironmental applications of stable isotope analysis to Late Pleistocene archaeological sites across Southeast Asia (SEA), a region critical to understanding the evolution of Homo sapiens and other co-existing Late Pleistocene (124–11.7 ka) hominins. Stable isotope techniques applied to archaeological deposits offer the potential to develop robust palaeoenvironmental reconstructions, to contextualise the occupational and non-occupational history of a site. By evaluating the published research in this field, we show that sediments, guano, tooth enamel, speleothem and biomolecular material such as leaf waxes have great potential to provide site-specific palaeoenvironmental records and local and catchment-scale landscape context to hominin dispersal in the region. However, stable isotope techniques used in these contexts are in their infancy in SEA, and the diagenetic controls associated with hot and humid environments that typify the region are not yet fully understood. Additionally, availability of sources of stable isotopes varies between sites. Nonetheless, even the limited research currently available shows that stable isotope analyses can aid in developing a better understanding of the role of the environment on the nature and timing of dispersals of our species eastwards into SEA and beyond.
... Additional factors that govern the rainfall-to-drip water transformation include the size of the vadose zone (Ford & Williams, 2007), the degree of interaction between different reservoirs across the vadose zone (Fairchild et al., 2006), and the preferred recharge flow pathway (Fairchild & Baker, 2012). Karst water routing can vary appreciably through time, with a wide range of water transit times implied by cave drip water δ 18 O (hereinafter δ 18 O dw ) variability within the same cavern (Partin, Cobb, et al., 2013;Treble et al., 2013;Zhang & Li, 2019). Indeed, long-term monitoring of rainfall and cave drip waters remain crucial to quantify the robustness of stalagmite δ 18 O-based climate reconstructions at individual paleoclimate sites. ...
... This study extends one rainfall and three cave drip water time series first presented by Moerman et al. (2014) from Gunung Mulu National Park in N. Borneo (4°06′N, 114°53′E) (supporting information Figure S1). For a detailed description of the geologic and climatic setting of Gunung Mulu, the reader is referred to previous studies (Carolin et al., 2016;Cobb et al., 2007;Moerman et al., 2013;Partin, Cobb, et al., 2013). Rainfall samples were collected by Mulu Airport Meteorological staff using a splayed-bottom, copper rain gauge (Casella model M1144003), following the sampling protocol outlined in Moerman et al. (2013). ...
... Interannual variations from ENSO-driven rainfall δ 18 O are evident in all three Mulu cave drip water δ 18 O time series, reflecting rainfall δ 18 O variations with karst residence times between~3 and 18 months. Indeed, all three cave δ 18 O dw time series are consistent with a relatively simple transformation of δ 18 O R to cave δ 18 O dw , supporting previous descriptions of diffuse-seepage flow at Mulu Moerman et al., 2014;Partin, Cobb, et al., 2013). The shorter karst residence times at Wind cave (4-5 months) implies less homogenization of δ 18 O R and/or less vadose zone mixing with other reservoirs of karst waters, resulting in higher amplitude δ 18 O dw variability (−3.7‰ to −11.7‰). ...
Time series of cave drip water oxygen isotopes (δ¹⁸O) provide site‐specific assessments of the contributions of climate and karst processes to stalagmite δ¹⁸O records employed for hydroclimate reconstructions. We present ~12‐year‐long time series of biweekly cave drip water δ¹⁸O variations from three sites as well as a daily resolved local rainfall δ¹⁸O record from Gunung Mulu National Park in northern Borneo. Drip water δ¹⁸O variations closely match rainfall δ¹⁸O variations averaged over the preceding 3–18 months. We observe coherent interannual drip water δ¹⁸O variability of ~3‰ to 5‰ related to the El Niño–Southern Oscillation (ENSO), with sustained positive rainfall and drip water δ¹⁸O anomalies observed during the 2015/2016 El Niño. Evidence of nonlinear behavior at one of three drip water monitoring sites implies a time‐varying contribution from a longer‐term reservoir. Our results suggest that well‐replicated, high‐resolution stalagmite δ¹⁸O reconstructions from Mulu could characterize past ENSO‐related variability in regional hydroclimate.
... Here we present a new 6.5 year, biweekly collected time series of dripwater δ 18 O from the Mulu karst system in northern Borneo, where tropical hydroclimate has been reconstructed over the past 500,000 years Meckler et al., 2012;Carolin et al., 2013]. Building upon previous work by Cobb et al. [2007] and Partin et al. [2013a], this paper seeks to characterize Mulu karst hydrology and its impact on the dripwater δ 18 O variability that is ultimately archived in Borneo speleothem δ 18 O. By comparing Mulu rainfall and dripwater δ 18 O time series, we identify the dominant karst transformation processes at Mulu, estimate water transit times, and demonstrate how different residence times can influence speleothem δ 18 O. ...
... Drip rate was measured manually via stopwatch. Two of the dripwater time series were collected from Wind Cave and the third from Lang's Cave, approximately 5 km to the south [Partin et al., 2013a]. The two Wind Cave drips are located 50-75 m from the entrance and overlain by~100 m of bedrock. ...
... While similarly large dripwater δ 18 O variations are reported elsewhere [e.g., Ayalon et al., 1998;Oster et al., 2012], these sites typically involve rapid flow through karst conduits and joints, such that dripwater δ 18 O time series reflect discrete rainfall events. Flow to the temporally monitored drips at Mulu, however, is likely delivered by diffuse seepage flow Partin et al., 2013a]. Given the dominance of seasonal variability at most other cave monitoring sites with diffuse flow, large seasonal variations in rainfall δ 18 O (>4‰) are largely smoothed out by residence times greater than a year, generating dripwater δ 18 O variability of only~1‰ [e.g., Mickler et al., 2004;Cruz et al., 2005b;Pape et al., 2010;Partin et al., 2012;Treble et al., 2013;Genty et al., 2014]. ...
Speleothem oxygen isotopes (δ18O) are often used to reconstruct past rainfall δ18O variability, and thereby hydroclimate changes, in many regions of the world. However, poor constraints on the karst hydrological processes that transform rainfall signals into cave dripwater add significant uncertainty to interpretations of speleothem-based reconstructions. Here we present several 6.5-year, biweekly dripwater δ18O timeseries from northern Borneo and compare them to local rainfall δ18O variability. We demonstrate that vadose water mixing is the primary rainfall-to-dripwater transformation process at our site, where dripwater δ18O reflects amount-weighted rainfall δ18O integrated over the previous 3–10 months. We document large interannual dripwater δ18O variability related to the El Niño-Southern Oscillation (ENSO), with amplitudes inversely correlated to dripwater residence times. According to a simple stalagmite forward model, asymmetrical ENSO extremes produce significant offsets in stalagmite δ18O timeseries given different dripwater residence times. Our study highlights the utility of generating multi-year, paired timeseries of rainfall and dripwater δ18O to aid interpretations of stalagmite δ18O reconstructions.
... Treble et al. (2005a) suggest that their 20th century δ 13 C record could partially reflect water stress on leaf stomata (Farquhar et al., 1988), based on correlation with Mg concentrations. In Indonesia speleothem records, δ 13 C has been found to often co-vary with Mg/Ca and Sr/Ca ratios, suggesting that these proxies respond to prior calcite 30 precipitation (Griffiths et al., 2010, Partin et al., 2013. Prior calcite precipitation (PCP) occurs in dry conditions, when less water is transported or stored in the vadose zone, which leads to degassing of CO 2 into fractures in the bedrock (Fairchild et al., 2000). ...
... In addition, Australian studies of Sr/Ca has found the ratios to represent temperature-regulated fluctuations in bioproductivity above the cave (Desmarchelier 15 et al., 2006) or a blended signal of PCP and/or growth rate, depending on the time scale examined (McDonald et al., 2004, Treble et al., 2003. Indonesian studies suggest that drip rate and residence time influence Mg/Ca and Sr/Ca ratios in locations where PCP is not a constant driver (Partin et al., 2013). P/Ca has been suggested to be a palaeorainfall proxy (Fairchild et al., 2001, Treble et al., 2003 because of the release of phosphorous from soils and subsequent transport and incorporation associated with heavy rainfall events. ...
Non-annually resolved palaeoclimate records in the Australasian region were compiled to facilitate investigations of decadal to centennial climate variability over the past 2000 years. A total of 661 lake/wetland, geomorphic, marine, and speleothem records were identified and then assessed against a set of a priori criteria based on temporal resolution, record length, dating methods, and confidence of the proxy-climate relationship over the Common Era. A high quality subset of 22 records across Australasia met the criteria and they are endorsed for subsequent analyses. New chronologies based on progressive Bayesian techniques were constructed for the high quality records to ensure a consistent approach to age modelling and quantification of age uncertainties. Chronological uncertainty was the primary reason why records did not meet the selection criteria. Despite present limitations, existing proxies and reconstruction techniques that successfully capture climate variability in the region show potential to address spatial gaps and expand the range of climate variables covering the last 2000 years from the Australasian region.
... During this time, groundwater allowed effective mixing and damping of δ 18 O, indicating longer residence times in the dry season than in the rainy season. This process led to a prolonged period of "mixed effects" of groundwater in the dry season, which resulted in sufficient mixing (Genty et al., 2014;Partin et al., 2013). In the rainy season, some of the δ 18 O and δD of groundwater had distributions similar to those of the hydrogen and oxygen isotopes of soil water and surface water (Fig. 7b), indicating that surface water, soil water and groundwater had the same source and might have experienced similar physiochemical processes (such as evaporation) (Leaney and Herczeg, 1995). ...
Groundwater recharge supports sustainable development for drinking water and irrigated agriculture in vast areas worldwide. Human activities and climate change (e.g., uneven precipitation) have significantly impacted groundwater dynamics. A long term monitoring and sampling campaign was conducted for the major water bodies to investigate the evolution of groundwater and its capacity to support sustainable development in a subtropical agroforestry catchment in the middle of the Sichuan Basin with horizontal sedimentary bedrock (red beds), where seasonal droughts are notable. Major hydrochemical indicators, including δD and δ 18 O were measured to trace the water movement. The results showed that the chemical type of shallow groundwater was Ca•Mg-HCO 3 or Ca-HCO 3 , which was mainly controlled by the weathering and hydrolysis of rocks. The interaction processes in the rainy season were stronger than those in the dry season. The isotopic signature in rainwater showed distinct seasonal pattern was strengthened by the local basin climate. The isotopic signals of groundwater responding to rainfall showed that recharge cycles of shallow groundwater in the rainy season lasted approximately 45-75 days. The optimized estimation based on mixing model and chloride ion balance (CMB) showed that the rainfall recharge ratio to shallow groundwater averaged about 27.83% (4.3%-58.0%) at event scale and the annual rainfall recharge ratios averaged about 19.36% (12.2%-45.7%). Groundwater in this region was renewed rapidly, as its groundwater storage capacity could be low with horizontal sedimentary bedrock. By revealing the water chemistry dynamics of shallow groundwater in the study area, this work preliminarily identified the groundwater recharge sources and estimated the recharge ratios by rainfall in a typical hilly area with red beds, thus providing a scientific basis for further regional assessment of groundwater resources.
... 11 and 12). Previous works have revealed that the δ 18 O d (drip-water δ 18 O) inherited the signal of atmospheric precipitation on the monthly timescale, and the influence of the "mixing effect" with "old water" in the epikarst zone (Pape et al., 2010;Li et al., 2011;Partin et al., 2013;Zhang and Li, 2019) seriously damp or even suppress the rainfall seasonal characteristics (e.g. heavier in summer and lighter in winter) (Genty et al., 2014), however, other Table 1. ...
... However, when comparing the mean daily calcite precipitation rate to the mean daily rainfall amount, no significant correlation was observed, although there may be a lag between rainfall and calcite precipitation rate, or spatial variations in rainfall events for the island of Eleuthera. We note that previous studies have demonstrated a minimal lag between rainfall and drip rate (Partin et al., 2013), while other studies have shown the response of drip rate to rainfall can be variable within a cave (Wong et al., 2011). ...
... The stable isotopes of hydrogen and oxygen in the moisture and the vapor were fractionated several times in the processes of transportation and revaporization, resulting in a larger slope and intercept in the LMWL (Li et al., 2010;Zhou and Li, 2017). On the other hand, no clear seasonal change in δ 18 O and δD of drip water was observed ( Fig. 2F and G), which was attributed to the "mixing effect" of the groundwater when it flowed through the vadose zone (Pape et al., 2010;Li et al., 2011;Partin et al., 2013;Genty et al., 2014). The thickness of overlying strata was 300-500 m above Furong Cave, which resulted in a long residence time of the precipitation and groundwater in the bedrock, smoothening the seasonal signals of δ 18 O and δD in the precipitation . ...
Cave monitoring is crucial for the interpretation of climatic and environmental significances of various geological proxies in speleothem. Therefore, the hydrochemical and stable isotopic compositions (δ ¹⁸ O, δD, Mg/Ca, Sr/Ca, and Ba/Ca) of karst cave waters during 2005–2016 CE were constantly monitored in Furong Cave, Chongqing City, Southwest China. A comparison with local hydrological conditions led to 4 main conclusions as follows: (1) the Mg/Ca ratio is significantly responsive to the changes in drought/wet conditions outside the cave, which increased in drought years and decreased in wet years, respectively. Seasonal variation of Sr/Ca is more significant than those of Mg/Ca and Ba/Ca ratios. (2) Prior calcite precipitation (PCP), incongruent calcite dissolution (ICD), water–rock interaction (WRI), and pCO 2 of soil and cave air, may account for the changes in trace element ratios in the epikarst, which resulted in a complex variation of element ratios in the cave drip water. In general, WRI in drought years is stronger than that in wet years, and that in low discharge sites is stronger than in high discharge sites. Seasonal variation of Ca ²⁺ concentration, induced by PCP, exerts significant impact on the evolution of Sr/Ca ratio in drip water. (3) δ ¹⁸ O and δD of drip water are influenced by the “mixing effect” leading to the result that their seasonal variations are less significant than that of precipitation. (4) At least in the study period, the ratios of trace elements in the drip water in Furong Cave mainly reflected the variations of local hydrological conditions (drought or wet) dominated by precipitation. Because of the “mixing effect” of groundwater, high-resolution δ ¹⁸ O record (e.g., seasonal and annual) of speleothem in Furong Cave may not be recommendable. However, the δ ¹⁸ O is potentially a reliable proxy in speleothems to record the change of rainfall on decadal and longer timescales.
... During this period, most drip water comes from water which was recharged to the unsaturated zone in the preceding rain season, and due to PCP has higher Sr [Ba]/Ca. This contrasts with tropical climates, such as at Gunung Mulu and Gunung Buda National Parks in northern Borneo, where stalagmite Mg [Sr]/Ca ratios are poor indicators of hydroclimate conditions, due to a wetter climate and less strong seasonality 35 . Note the y-axis of Sr/Ca is inverted, so that the peaks of Sr/Ca in the graph corresponds to the lower value of Sr/ Ca. ...
Stalagmites play an important role in paleoclimatic reconstructions from seasonal to orbital time scales as230Th-dating can provide an accurate absolute age. Additionally, seasonal trace element and optical layers can provide a precise age. We analyzed the seasonal variability of multiple trace elements on a stalagmite (XMG) in Shihua Cave, Beijing and compared them with results from laminae counting. The results show that (1) the polished section of the topmost part of XMG has obvious bi-optical layers under a conventional transmission microscope, however, laminae are not observed using this method in the rest of the sample, and (2) The variations of P/Ca, Sr/Ca, Ba/Ca, U/Ca and Mg/Ca show seasonal cycles throughout the sample. The PC1 in the Principal Component Analysis (PCA) of five trace elements represents the annual cycle. This stalagmite was deposited over 150 ± 1 years through PC1 peak counting. This result corresponds well with the annual layers and U-Th dating. Trace element cyclicity of PC1 can increase the accuracy of stalagmite dating, especially in the absence of obvious laminae and are a powerful method to identify seasonal changes in a strongly contrasting wet-dry monsoon climate region.
... In the past, many studies were focused on dripwater discharge and its relation to rainfall (Baker et al. 1997;Genty and Deflandre 1998;Baker and Brundson 2003;Tooth and Fairchild 2003;Baldini et al. 2006;Fairchild et al. 2006b;Ban et al. 2008). Both dripwater isotopic composition and isotope transfer mechanism into calcite have been studied in detail (Cruz et al. 2005;Dreybrodt 2008;Fuller et al. 2008;Bradley et al. 2010;Fohlmeister et al. 2010;Dreybrodt and Scholz 2011;Frisia et al. 2011;Polk et al. 2012;Partin et al. 2013). Recently, much attention is focused on trace element content and trace element partitioning (Huang and Fairchild 2001;Gabitov and Watson 2006;Sinclair 2011;Wong et al. 2011;Baldini et al. 2012;Hartland et al. 2012). ...
Hydrological/hydrochemical properties of drip points in Kateřinská Cave (Moravian Karst) were studied with focus on their relation to climatic variables, especially rainfall and temperature. The discharges of two drips (marked as meteorologically sensitive drips) showed statistically significant positive correlations with rainfalls with respective lags of −5 and −2 days. In addition, the discharges showed statistically significant positive correlations with specific electrical conductivity, pH, and supersaturation with respect to calcite. Other drips in the same cave (marked as meteorologically insensitive drips) did not show these correlations. Because the climatic conditions are identical, the drip types must differ in their reaction/flow paths. This indicates that the paths may control some proxies possibly preserved in speleothems, which should be taken into consideration for paleoenvironmental reconstructions. In case of the speleothems formed by the meteorologically sensitive drips, we believe that it would be possible to distinguish the intra-annual climatic events via individual laminas and their composition.
... As such, long term datasets of stable oxygen-isotope ( 18 O) and geochemistry (trace element concentrations and ratios) of rainfall and cave dripwater provide an empirical basis for identifying factors influencing trace element variability ultimately 25 recorded in speleothems (Baldini et al., 2002;Treble et al., 2003;Riechelmann et al., 2011;Oster et al., 2012;Frappier, 2013;Partin et al., 2013). For example, McDonald et al. (2004) demonstrated Mg/Ca and Sr/Ca ratios in the dripwater doubled in response to an El Niño event which occurred during a 2.5 year baseline monitoring study at Wombeyan Caves, SE Australia. ...
Speleothems (cave deposits), used for paleoenvironmental reconstructions, are deposited from cave dripwaters. Differentiating climate and karst processes within a dripwater signal is fundamental for the correct identification of paleoenvironmental proxies and ultimately their interpretation within speleothem records. We investigate the potential use of trace element and stable oxygen-isotope (δ18O) variations in cave dripwaters as paleorainfall proxies in an Australian alpine karst site. This paper presents the first extensive hydrochemical and δ18O dataset from Yarrangobilly Caves, in the Snowy Mountains, south-east Australia. Using a 7-year long rainfall δ18O and dripwater Ca, Cl, Mg / Ca, Sr / Ca and δ18O dataset from three drip sites, we determined that the processes of mixing, dilution, flow path change, carbonate mineral dissolution and Prior Calcite Precipitation (PCP) accounted for the observed variations in the dripwater geochemical composition. We identify that the three monitored drip sites are fed by fracture flow from a well-mixed epikarst storage reservoir; supplied by variable concentrations of dissolved ions from soil and bedrock dissolution. We constrained the influence of multiple processes and controls on dripwater composition in a region dominated by ENSO. During the El Niño and dry periods, enhanced PCP, a flow path change and dissolution due to increased soil CO2 production occurred in response to decreased rainfall, in distinction to the La Niña phase where dilution dominated and reduced PCP was observed. We present a conceptual model, illustrating the key processes impacting the dripwater chemistry. We identified a robust relationship between ENSO and dripwater trace element concentrations and propose variations in speleothem Mg / Ca and Sr / Ca ratios may be interpreted to reflect paleorainfall conditions. These findings inform paleorainfall reconstruction from speleothems regionally and provide a basis for paleoclimate studies globally, in regions where there is intermittent recharge variability.
... Our goal is to quantify the time evolution of ENSO variance over the Holocene. Our sub-annual sample resolution allows for comparisons of the 2-7 yr variance (above the Nyquist frequency), A relatively long transit time of diffuse seepage flow that feeds the drip water is proposed to attenuate the rainfall δ 18 O signal in the drip water in the Mulu cave system Partin et al., 2013), which would tend to reduce interannual variability in the speleothems (Moerman et al., 2014). This process would help explain the weaker interannual features we observe in our high-resolution δ 18 O time series from BA03 as compared to the Niño3.4 ...
The El Niño-Southern Oscillation (ENSO) is the primary driver of interannual climate variability in the tropics and subtropics. Despite substantial progress in understanding ocean–atmosphere feedbacks that drive ENSO today, relatively little is known about its behavior on centennial and longer timescales. Paleoclimate records from lakes, corals, molluscs and deep-sea sediments generally suggest that ENSO variability was weaker during the mid-Holocene (4–6 kyr BP) than the late Holocene (0–4 kyr BP). However, discrepancies amongst the records preclude a clear timeline of Holocene ENSO evolution and therefore the attribution of ENSO variability to specific climate forcing mechanisms. Here we present results from a U–Th dated speleothem in Malaysian Borneo sampled at sub-annual resolution. The of Borneo rainfall is a robust proxy of regional convective intensity and precipitation amount, both of which are directly influenced by ENSO activity. Our estimates of stalagmite variance at ENSO periods (2–7 yr) show a significant reduction in interannual variability during the mid-Holocene (3240–3380 and 5160–5230 yr BP) relative to both the late Holocene (2390–2590 yr BP) and early Holocene (6590–6730 yr BP). The Borneo results are therefore inconsistent with lacustrine records of ENSO from the eastern equatorial Pacific that show little or no ENSO variance during the early Holocene. Instead, our results support coral, mollusc and foraminiferal records from the central and eastern equatorial Pacific that show a mid-Holocene minimum in ENSO variance. Reduced mid-Holocene interannual variability in Borneo coincides with an overall minimum in mean from 3.5 to 5.5 kyr BP. Persistent warm pool convection would tend to enhance the Walker circulation during the mid-Holocene, which likely contributed to reduced ENSO variance during this period. This finding implies that both convective intensity and interannual variability in Borneo are driven by coupled air-sea dynamics that are sensitive to precessional insolation forcing. Isolating the exact mechanisms that drive long-term ENSO evolution will require additional high-resolution paleoclimatic reconstructions and further investigation of Holocene tropical climate evolution using coupled climate models.
... Please cite this article in press as: Wong, C.I., Breecker, D.O., Advancements in the use of speleothems as climate archives, Quaternary Science Reviews (2015), http://dx.doi.org/10.1016/j.quascirev.2015.07.019 speleothems, which have been interpreted to record transition from mixed C 3 eC 4 savanna to C 3 rainforest in northern Borneo (Partin et al., 2013) or not been interpreted (Shakun et al., 2007) can be entirely explained by pCO 2 . Interestingly, if rainforest did replace savanna in northern Borneo during the last deglaciation, our interpretation suggests that the d 13 C values of speleothem CaCO 3 were not sensitive to this transition, which is consistent with a bias toward deeply rooting C 3 trees in savanna ecosystems (Breecker et al., 2012; Meyer et al., 2014). ...
... Stagmalites have been widely used to reconstruct paleoclimate over the world (Li et al., 2000). Stalagmites form in karstified sites around the world, and are valuable archives of terrestrial paleoclimate; they can be precisely dated, and they afford high-resolution reconstructions on glacial-interglacial timescales (Partin et al., 2013). Formation of stagmalites in karst systems is generally controlled by several factors, and a detailed understanding of individual hydrological processes is needed to convert these data into measures of paleoclimate (Fairchild et al., 2000). ...
Understanding of hydrological processes in caves is important to help us interpret paleoclimate records from speleothems. In this study, we integrated hydrogeological and hydrogeochemical properties to characterize the hydraulic behavior of karst waters in the Heshang Cave, Central China. Using geological and topographical analyses, we identified regional watershed boundaries and hydrogeological connections that were controlled by anticlinal geometry and faults. Water samples were collected from the Heshang Cave and potential recharge sites. Geochemical data of c(Sr2+)/c(Ca2+) and c(Mg2+)/c(Ca2+) ratios suggest that the drainage system in Heshang Cave consists of two flow paths. For vadose waters, including drip water and rimstone pool water, c(Sr2+)/c(Ca2+) ratio ranges from 0.000 6 to 0.001 1, and c(Mg2+)/c(Ca2+) ratio ranges from 0.97 to 1.1, indicating that recharge was mainly from rainfall infiltration through the overlying Cambrian dolomite stratum. In contrast, slope current and underground river waters have higher c(Sr2+)/c(Ca2+) ratios (values from 0.002 2 to 0.002 8), and lower c(Mg2+)/c(Ca2+) ratios (values from 0.50 to 0.64). These waters show homogeneous isotopic composition (δ18O: −7.15‰-−6.95‰; δD: −52.73‰-−51.31‰), implying recharge of allogeneic water from the Xiaoer River via karst conduits that pass through Ordovician limestone and Cambrian dolomite stratum.
Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide out-of-sample evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ¹⁸O, δ¹³C) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at https://doi.org/10.17864/1947.147.
We present a meta-analysis of data from 22 caves and 96 drip sites from 4 continents where both the cave drip water d18O and the weighted mean d18O of precipitation have been measured. Drip water d18O is similar to the weighted mean d18O of precipitation (within ± 0.3 ‰) for sites where mean annual temperature (MAT) is less than 15 °C (85% of drips where MAT < 15 °C) and an aridity index (annual precipitation (P) / annual potential evapotranspiration (PET)) is greater than 0.65 (74% of drips at sites where P/PET > 0.65). In contrast, at warmer locations with increased water deficit, drip water d18O deviates from the weighed mean precipitation d18O by +3 ‰ and -1.5 ‰. We argue that this is due to evaporation in the soil and shallow vadose zone (thereby increasing drip water d18O) and lower water storage in the vadose zone, leading to relatively less mixing (thereby increasing the range in drip water d18O to more closely reflect recharge water d18O). Speleothems that have formed close to isotopic equilibrium are likely to have an oxygen isotope composition that contains a mixed signal of cave air temperature and precipitation d18O only in cool and temperate regions (T < 15 °C), or very wet climates where P/PET >> 0.65. In contrast, in warmer and drier environments, speleothems which have formed close to equilibrium will have d18O that reflects cave air temperature and a seasonal bias toward the d18O composition of rain in periods of high recharge, as well as the extent of evaporative fractionation of stored karst water.
Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide “out-of-sample” evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ¹⁸O, δ¹³C) measurements are referenced by distance from the top or youngest part of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information about the full range of measurements carried out on each speleothem and information about the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at http://dx.doi.org/10.17864/1947.139.
Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in midlatitude caves have observed seasonal variability in cave pCO2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it has been proposed that caves will ventilate on daily time scales, preventing cave pCO2 from increasing substantially above atmospheric pCO2. By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8 year cave monitoring study, including observations of cave pCO2 and carbonate chemistry, at Jinapsan Cave, Guam (13.4°N, 144.5°E). We find that cave pCO2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO2 ranging from 500 to 5000 ppm. The seasonality of cave pCO2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO2. Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO2. These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave affect the interpretation of speleothem-based paleoclimate records.
Speleothems (cave deposits), used for palaeoenvironmental reconstructions, are
deposited from cave drip water. Differentiating climate and karst processes
within a drip-water signal is fundamental for the correct identification of
palaeoenvironmental proxies and ultimately their interpretation within
speleothem records. We investigate the potential use of trace element and
stable oxygen-isotope (δ18O) variations in cave drip water as
palaeorainfall proxies in an Australian alpine karst site. This paper presents
the first extensive hydrochemical and δ18O dataset from Harrie Wood
Cave, in the Snowy Mountains, south-eastern (SE) Australia. Using a 7-year long
rainfall δ18O and drip-water Ca, Cl, Mg / Ca, Sr / Ca and
δ18O datasets from three drip sites, we determined that the
processes of mixing, dilution, flow path change, carbonate mineral
dissolution and prior calcite precipitation (PCP) accounted for the observed
variations in the drip-water geochemical composition. We identify that the
three monitored drip sites are fed by fracture flow from a well-mixed
epikarst storage reservoir, supplied by variable concentrations of dissolved
ions from soil and bedrock dissolution. We constrained the influence of
multiple processes and controls on drip-water composition in a region
dominated by El Niño–Southern Oscillation (ENSO). During the El Niño and dry periods, enhanced PCP, a
flow path change and dissolution due to increased soil CO2 production
occurred in response to warmer than average temperatures in contrast to
the La Niña phase, where dilution dominated
and reduced PCP were observed.
We present a conceptual model, illustrating the key processes impacting the
drip-water chemistry. We identified a robust relationship between ENSO and
drip-water trace element concentrations and propose that variations in speleothem Mg / Ca and Sr / Ca ratios may be interpreted to reflect
palaeorainfall conditions. These findings inform palaeorainfall reconstruction
from speleothems regionally and provide a basis for palaeoclimate studies
globally, in regions where there is intermittent recharge variability.
Robust chronologies are crucial for the correct interpretation of climate proxy records and for detailed reconstructions of palaeoclimate. Stalagmites have garnered strong interest as recorders of past climate in part due to their amenability to U-series dating. However, many stalagmites are not dateable using this technique due to low 238U and/or high detrital Th concentrations (e.g., many tropical cave systems (Adkins et al., 2013)), and occasionally these issues affect stalagmites across wide geographical regions (e.g., large parts of Australia (Green et al. 2013)) complicating the use of stalagmites in these areas. Radiocarbon (14C) offers an alternative method of dating stalagmites, but issues associated with the 'dead carbon fraction' (DCF) have historically hindered this approach. Here, a novel 14C-based method for dating stalagmites is presented and discussed. The technique calculates a best-fit growth rate between a time-series of stalagmite 14C data and known atmospheric 14C variability. The new method produces excellent results for stalagmites that satisfy four requirements: i) the absence of long-term secular variability in DCF (i.e., stalagmite DCF varies around a mean value with no long-term trend), ii) stalagmite growth rate does not vary significantly (the technique identifies stalagmites with substantial growth rate variability), iii) the stalagmite record is long enough that measurable 14C decay has occurred, and iv) one 'anchor' point exists where the calendar age is known. The model produces good results for a previously U-Th dated stalagmite from Heshang Cave, China, and is then applied to an undated stalagmite from southern Poland. The new method will not replace high-precision U-Th measurements, because the precision of the technique is difficult to quantify. However, it provides a means for dating certain stalagmites undateable by conventional U-Th methods and for refining coarse U-Th chronologies.
The International Atomic Energy Agency (IAEA), in cooperation with the
World Meteorological Organization (WMO), has been conducting a
world-wide survey of hydrogen (2H/1H) and oxygen
(18O/16O) isotope composition of monthly
precipitation since 1961. At present, 72 IAEA/WMO network stations are
in operation. Another 82 stations belonging to national organizations
continue to send their results to the IAEA for publication. The paper
focuses on basic features of spatial and temporal distribution of
deuterium and 18O in global precipitation, as derived from
the IAEA/WMO isotope database. The internal structure and basic
characteristics of this database are discussed in some detail. The
existing phenomenological relationships between observed stable isotope
composition of precipitation and various climate-related parameters such
as local surface air temperature and amount of precipitation are
reviewed and critically assessed. Attempts are presented towards
revealing interannual fluctuations in the accumulated isotope records
and relating them to changes of precipitation amount and the surface air
temperature over the past 30 years.
The relationship between climate variability and rainfall oxygen isotopic (δ18O) variability is poorly constrained, especially in the tropics, where many key paleoclimate records rely on past rainfall isotopes as proxies for hydroclimate. Here we present a daily-resolved, 5-yr-long timeseries of rainfall δ18O from Gunung Mulu National Park, located in northern Borneo (4°N, 114°E) in the heart of the West Pacific Warm Pool, and compare it to local and regional climatic variables. Daily rainfall δ18O values range from +0.7‰ to −18.5‰ and exhibit a weak but significant inverse relationship with daily local precipitation amount (R=−0.19, p<0.05), consistent with the tropical amount effect. Day-to-day δ18O variability at Mulu is best correlated to regional precipitation amount averaged over the preceding week (R=−0.64, p<0.01). The inverse relationship between Mulu rainfall δ18O and local (regional) precipitation amount increases with increased temporal averaging, reaching R=−0.56 (R=−0.72) on monthly timescales. Large, negative, multi-day rainfall δ18O anomalies of up to 16‰ occur every 30–90 days and are closely associated with wet phases of the intraseasonal Madden–Julian Oscillation. A weak, semi-annual seasonal cycle in rainfall δ18O of 2–3‰ bears little resemblance to seasonal precipitation variability, pointing to a complex sequence of moisture sources and/or trajectories over the course of the year. Interannual rainfall δ18O variations of 6–8‰ are significantly correlated with indices of the El Niño Southern Oscillation, with increased rainfall δ18O during relatively dry El Niño conditions, and vice versa during La Nina events. We find that Mulu rainfall δ18O outperforms Mulu precipitation amount as a tracer of basin-scale climate variability, highlighting the time- and space-integrative nature of rainfall δ18O. Taken together, our results suggest that rainfall δ18O variability at Mulu is significantly influenced by the strength of regional convective activity. As such, our study provides further empirical support for the interpretation of δ18O-based paleo-reconstructions from northern Borneo stalagmites as robust indicators of regional-scale hydroclimate variability, where higher δ18O reflects regional drying.
The rate of long-term (2 m.y.) base-level lowering estimated in an extensive sequence of limestone caves in Sarawak, Malaysia, from uranium series, electron spin resonance, and paleomagnetic dating is 0.19 +0.03/-0.04 m/ka. This rate has remained constant over at least the last 700 ka, as shown by comparison of the number and spacing of wall notches formed during phases of interstadial and interglacial aggradation with peaks in the deep-sea oxygen isotope curve. It is argued that base-level lowering occurs in response to epirogenic uplift of the more resistant limestones due to regional denudation of the softer shales, and to flexural isostacy associated with high rates of offshore sedimentation.
A 30-month study of drip hydrochemistry from Kooringa Cave, eastern Australia, revealed a clear geochemical response to the 2002-2003 El Niño. Through the drought, drip discharge fell to base flow, drip Ca2+ concentrations fell by half and drip Mg/Ca and Sr/Ca increased in a co-varying pattern. Calcite Mg/Ca and Sr/Ca predicted from drip waters displayed a marked increase through, and just beyond, the period of greatest moisture deficit. The results suggest that stalagmites from shallow caves in drought-sensitive eastern Australia potentially preserve a valuable record of El Niño-La Niña history.
This study aims to establish evidence for the widespread existence of preserved high-resolution trace element variations in speleothems that may have climatic significance. Ion microprobe analysis of speleothems reveals that annual to sub-annual variations in element chemistry exist at five, shallow western European cave sites (Crag Cave, County Kerry and Ballynamintra, County Waterford. Ireland: Uamh an Tartair, Sutherland, Scotland: Grotte Pere-Noël, Belgium: Grotta di Ernesto, NE Italy) with widely varying climatic, geomorphic and geological settings. The variations are not restricted to species (Mg. Sr and Ba) known to substitute directly for Ca in the calcite lattice, but include H, F, Na and P, Phosphorus (as phosphate) displays the greatest variability and may have the most significance as a proxy for the seasonal temperature cycle because of its role as a nutrient element. The technique allows estimation of growth rate of speleothems at any interval of interest, which is one of several possible uses in palaeoclimatology.
Variations in growth rates of speleothem calcite have been hypothesized to reflect changes in a range of paleoenvironmental variables, including atmospheric temperature and precipitation, drip-water composition, and the rate of soil CO2 delivery to the subsurface. To test these hypotheses, we quantified growth rates of modern speleothem calcite on artificial substrates and monitored concurrent environmental conditions in three eaves across the Edwards Plateau in central Texas. Within each of two caves, different drip sites exhibit similar annual cycles in calcite growth rates, even though there are large differences between the mean growth rates at the sites. The growth-rate cycles inversely correlate to seasonal changes in regional air temperature outside the caves, with near-zero growth rates during the warmest summer months, and peak growth rates in fall through spring. Drip sites from eaves 130 km apart exhibit similar temporal patterns in calcite growth rate, indicating a controlling mechanism on at least this distance. The seasonal variations in calcite growth rate can be accounted for by a primary control by regional temperature effects on ventilation of cave-air CO2 concentrations and/or drip-water CO2 contents. In contrast, site-to-site differences in the magnitude of calcite growth rates within an individual cave appear to be controlled principally by differences in drip rate. A secondary control by drip rate on the growth rate temporal variations is suggested by interannual variations. No calcite growth was observed in the third cave, which has relatively high values of and small seasonal changes in cave-air CO2. These results indicate that growth-rate variations in ancient speleothems may serve as a paleoenvironmental proxy with seasonal resolution. By applying this approach of monitoring the modern system, speleothem growth rate and geochemical proxies for paleoenvironmental change may be evaluated and calibrated.
The variations of tropical precipitation are antiphased between the hemispheres on orbital timescales. This antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres, which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the Hadley cells. Here we compare an oxygen isotopic record recovered from a speleothem from northeast Brazil for the past 26,000 years with existing reconstructions of precipitation in tropical South America. During the Holocene, we identify a similar, but zonally oriented, antiphasing of precipitation within the same hemisphere: northeast Brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high, whereas the rest of southern tropical South America shows opposite characteristics. Simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation-driven monsoon activity. Our results suggest that convective heating over tropical South America and associated adjustments in large-scale subsidence over northeast Brazil lead to a remote forcing of the South American monsoon, which determines most of the precipitation changes in the region on orbital timescales.
A detailed stable-isotope record is presented for the full length of the Greenland Ice-core Project Summit ice core covering the last 250,000 years according to a graduated timescale. It appears that the climatic stability of the Holocene is the exception rather than the rule; the last interglacial is also noted to have lasted longer than is implied by the deep-sea SPECMAP record. This discrepancy may be accounted for if the climate instability at the outset of the last interglacial delayed the melting of the Saalean ice sheets in America and Eurasia.
OXYGEN isotope measurements in Greenland ice demonstrate that a series of rapid warm-cold oscillations---called Dansgaard-Oeschger events---punctuated the last glaciation1. Here we present records of sea surface temperature from North Atlantic sediments spanning the past 90 kyr which contain a series of rapid temperature oscillations closely matching those in the ice-core record, confirming predictions that the ocean must bear the imprint of the Dansgaard-Oeschger events2,3. Moreover, we show that between 20 and 80 kyr ago, the shifts in ocean-atmosphere temperature are bundled into cooling cycles, lasting on average 10 to 15 kyr, with asymmetrical saw-tooth shapes. Each cycle culminated in an enormous discharge of icebergs into the North Atlantic (a 'Hein-rich event'4,5), followed by an abrupt shift to a warmer climate. These cycles document a previously unrecognized link between ice sheet behaviour and ocean-atmosphere temperature changes. An important question that remains to be resolved is whether the cycles are driven by external factors, such as orbital forcing, or by inter-nal ice-sheet dynamics.
Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave.
The origin and environmental dependencies of lamination in stalagmites from Katerloch, common in speleothems from other cave sites, are examined in detail. Petrographic observations and chemical analyses (including isotopes) of stalagmites and modern calcite were combined with multi-annual cave monitoring. All investigated stalagmites are composed of low-Mg calcite and show white, porous laminae and typically thinner, translucent dense laminae. The binary lamination pattern results from changes in the calcite fabric: white, porous laminae are characterized by a high porosity and abundant fluid inclusions and also by enhanced vertical growth and thinning towards the flanks. Translucent, dense laminae exhibit a compact fabric and constant thickness of individual growth layers. U-Th dating supports an annual origin of the lamination and the seasonally changing intensity of cave ventilation provides a robust explanation for the observed relationships between lamination, stable C isotopic compositions and trace elements (Mg, Sr and Ba). The seasonally variable air exchange, driven by temperature contrasts between the cave interior and outside atmosphere, modulates the rate and amount of CO2 degassing from the drip water and affects the hydrochemistry and consequently the fabric of the precipitating calcite. Although cave air composition and drip rate are both major variables in controlling CO2 degassing from the drip water, the seasonally changing ventilation in Katerloch exerts the primary control and the results suggest a secondary (amplifying/attenuating) influence of the drip rate. Drip rate, however, might be the controlling parameter for lamina development at cave sites experiencing only small seasonal cave air exchange. Importantly, the seasonally variable composition of drip water does not reflect the seasonal cycle of processes in the soil zone, but results from exchange with the cave atmosphere. The alternating porous and dense calcite fabric is the expression of a variable degree of lateral coalescence of smaller crystallites forming large columnar crystals. The white, porous laminae represent partial coalescence and form during the warm season: low calcite δ13C values are linked to low δ13C values of cave air and drip water during that time. This observation corresponds to times of reduced cave ventilation, high pCO2 of cave air, low drip water pH, lower calcite supersaturation and typically high drip rates. In contrast, the translucent, dense laminae represent more or less complete lateral coalescence (inclusion-free) during the cold season (high calcite, drip water and cave air δ13C values), i.e. times of enhanced cave ventilation, low cave air pCO2, increased drip water pH, relatively high calcite supersaturation and typically low drip rates. In essence, the relative development of the two lamina types reflects changes in the seasonality of external air temperature and precipitation, with a strong control of the winter air temperature on the intensity of cave-air exchange. Thick translucent, dense laminae are favoured by long, cold and wet winters and such conditions may be related closely to the North Atlantic Oscillation mode (weak westerlies) and enhanced Mediterranean cyclone activity during the cold season. Studies of speleothem lamination can thus help to better understand (and quantify) the role of seasonality changes, for example, during rapid climate events.
There is a paucity of experimental data on calcite precipitation from waters at low ionic strength and low ratios of Mg/Ca and Sr/Ca, using controlled and constant precipitation rates. Such data are particularly needed for studies of speleothem geochemistry in relation to palaeoclimates.We carried out a series of experiments using a karst-analogue set-up in a chamber of constant temperature and 100% humidity. A steady flow of NaHCO3 and CaCl2 solutions at PCO2 around 10−3.2 were mixed just before passage through a tube (analogous to a soda-straw stalactite) and allowed to drip onto a surface, analogous to a stalagmite. Growth rates were comparable with linear extension rates of natural speleothems.Analytical spots gave reproducible analyses in later analytical cycles after ablation of surface calcite with Na and Mg contamination. Different crystals from the same experiment tended to show positive covariation of Na and Mg with negative covariation with Sr. This may be due to the presence of growth hillocks with vicinal faces with differential partitioning behaviour.The result for the partition coefficient for Mg (DMg) at 25°C is 0.031 ± 0.004, which is quantitatively in good agreement with the trends of previous workers. At 15°C, the result is 0.019 ± 0.003. The temperature dependency is higher than experimental data on seawater-analogue solutions, but lower than a previous estimate based on a comparison of speleothem chemistry with single water analyses.Data for DSr are mainly in the range of 0.057 to 0.078, with a possible weak dependency on growth rate, consistent with previous experimental work. Absolute values are higher than studies in Mg-free saline solutions, which is attributed mainly to salinity effects. Values of DSr are nevertheless somewhat lower than in natural caves, which may relate to crystal growth factors.Mg partition coefficient values should allow robust determination of solution Mg/Ca compositions in enclosed caves, which are at constant temperature on the decadal timescale. The inferred sensitivity of DSr to growth rate factors implies that Sr values should be interpreted more cautiously. Muted changes could relate entirely to growth rate variations, whereas changes of large magnitude imply a control by solution composition. The absence of local (tens of micron scale) antipathetic variations in Sr and Mg in studied natural speleothems, implies that intracrystalline zoning phenomena, if present, are on a finer scale in those natural materials compared with experimental products.
1] A 50 kyr-long exceptionally well-dated and highly resolved stalagmite oxygen (d 18 O) and carbon (d 13 C) isotope record from Sofular Cave in northwestern Turkey helps to further improve the dating of Greenland Interstadials (GI) 1, and 3 – 12. Timing of most GI in the Sofular record is consistent within ±10 to 300 years with the ''iconic'' Hulu Cave record. Larger divergences (>500 years) between Sofular and Hulu are only observed for GI 4 and 7. The Sofular record differs from the most recent NGRIP chronology by up to several centuries, whereas age offsets do not increase systematically with depth. The Sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern Mediterranean to GI, whereas a phase lag of $100 years between climate and full ecosystem response is evident. Finally, results of spectral analyses of the Sofular isotope records do not support a 1,470-year pacing of GI. Citation: Fleitmann, D., et al. (2009), Timing and climatic impact of Greenland interstadials recorded in stalagmites from northern Turkey, Geophys. Res. Lett., 36, L19707, doi:10.1029/ 2009GL040050.
Geological and biological archives showing an annually laminated internal structure are currently top priority in palaeoclimate research, as they are recognized as very high-resolution archives of environmental change. Also, the annual origin of laminations can be validated by absolute age dating or by using instrumental data for the most recent period. Microscopic laminae may span several hundreds to thousands of years and frequently reveal a high degree of internal growth variability. Quantitative examination of laminations using transmitted-light or epifluorescence microscopy is thus a tedious task and may be partly automated. We developed a software (WinGeol Lamination Tool) using C'/'/ capable of semi-automatically detecting and measuring individual lamina thicknesses in archives showing large internal growth variability. The Lamination Tool enables the operator to efficiently and quantitatively examine laminae down to the micron scale and it was successfully tested on a variety of annually banded samples.
High-resolution oxygen isotope (d 18 O) profiles of Holocene stalagmites from four caves in Northern and Southern Oman and Yemen (Socotra) provide detailed information on fluctuations in precipitation along a latitudinal transect from 121N to 231N. d 18 O values reflect the amount of precipitation which is primarily controlled by the mean latitudinal position of the ITCZ and dynamics of the Indian summer monsoon (ISM). During the early Holocene rapidly decreasing d 18 O values indicate a rapid northward displacement in the mean latitudinal position of the summer ITCZ and the associated ISM rainfall belt, with decadal-to centennial-scale changes in monsoon precipitation correlating well with high-latitude temperature variations recorded in Greenland ice cores. During the middle to late Holocene the summer ITCZ continuously migrated southward and monsoon precipitation decreased gradually in response to decreasing solar insolation, a trend, which is also recorded in other monsoon records from the Indian and East Asian monsoon domains. Importantly, there is no evidence for an abrupt middle Holocene weakening in monsoon precipitation. Although abrupt monsoon events are apparent in all monsoon records, they are short-lived and clearly superimposed on the long-term trend of decreasing monsoon precipitation. For the late Holocene there is an anti-correlation between ISM precipitation in Oman and inter-monsoon (spring/autumn) precipitation on Socotra, revealing a possible long-term change in the duration of the summer monsoon season since at least 4.5 ka BP. Together with the progressive shortening of the ISM season, gradual southward retreat of the mean summer ITCZ and weakening of the ISM, the total amount of precipitation decreased in those areas located at the northern fringe of the Indian and Asian monsoon domains, but increased in areas closer to the equator.
1] The Madden-Julian Oscillation (MJO) is the dominant component of the intraseasonal (30 – 90 days) variability in the tropical atmosphere. It consists of large-scale coupled patterns in atmospheric circulation and deep convection, with coherent signals in many other variables, all propagating eastward slowly ($5 m s À1) through the portion of the Indian and Pacific oceans where the sea surface is warm. It constantly interacts with the underlying ocean and influences many weather and climate systems. The past decade has witnessed an expeditious progress in the study of the MJO: Its large-scale and multiscale structures are better described, its scale interaction is recognized, its broad influences on tropical and extratropical weather and climate are increasingly appreciated, and its mechanisms for disturbing the ocean are further comprehended. Yet we are facing great difficulties in accurately simulating and predicting the MJO using sophisticated global weather forecast and climate models, and we are unable to explain such difficulties based on existing theories of the MJO. It is fair to say that the MJO remains an unmet challenge to our understanding of the tropical atmosphere and to our ability to simulate and predict its variability. This review, motivated by both the acceleration and gaps in our knowledge of the MJO, intends to synthesize what we currently know and what we do not know on selected topics: its observed basic characteristics, mechanisms, numerical modeling, air-sea interaction, and influences on the El Niño and Southern Oscillation.
Consideration of a range of evidence from geomorphology, palynology, biogeography and vegetation/climate modelling suggests that a north-south ‘savanna corridor’ did exist through the continent of Sundaland (modern insular Indonesia and Malaysia) through the Last Glacial Period (LGP) at times of lowered sea-level, as originally proposed by Heaney [1991. Climatic Change 19, 53–61]. A minimal interpretation of the size of this corridor requires a narrow but continuous zone of open ‘savanna’ vegetation 50–150 km wide, running along the sand-covered divide between the modern South China and Java Seas. This area formed a land bridge between the Malaysian Peninsula and the major islands of Sumatra, Java and Borneo. The savanna corridor connected similar open vegetation types north and south of the equator, and served as a barrier to the dispersal of rainforest-dependent species between Sumatra and Borneo. A maximal interpretation of the available evidence is compatible with the existence of a broad savanna corridor, with forest restricted to refugia primarily in Sumatra, Borneo and the continental shelf beneath the modern South China Sea. This savanna corridor may have provided a convenient route for the rapid early dispersal of modern humans through the region and on into Australasia.
Oxygen isotope records of five stalagmites from Hulu Cave near Nanjing bear a remarkable resemblance to oxygen isotope records from Greenland ice cores, suggesting that East Asian Monsoon intensity changed in concert with Greenland temperature between 11,000 and 75,000 years before the present (yr. B.P.). Between 11,000 and 30,000 yr. B.P., the timing of changes in the monsoon, as established with 230Th dates, generally agrees with the timing of temperature changes from the Greenland Ice Sheet Project Two (GISP2) core, which supports GISP2's chronology in this interval. Our record links North Atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer East Asian Monsoon originates.
The Mulu Park contains a range of limestone hills which are among the most cavernous in the world. Most of the known caves are fossil phreatic conduits, some occupied by modern rivers, and watertable development is mostly restricted to the hill margins. A repeated cycle of development opened with deep phreatic trunk caves and closed with lateral solution at the watertable. The large size of the caves is due to high erosion rates over a long period of time, and cavern collapse is proposed as a partial origin on a large, closed depression. A new small-scale solution feature, consisting of daylight-orientated slots and christened photokarren, is described.
Modern rainwater, cave dripwater and cave stalagmite geochemical time
series from a cave in Guam (13°38'N, 144°53'E) are used to
better understand how changes in cave stalagmite geochemistry relate to
aboveground changes in rainfall at a tropical location. A scientific
field team based in Guam collects ˜monthly samples from multiple
sites for geochemical analyses at a cave and aboveground rainfall from a
nearby location. We compute a transfer function between rainfall amount
and oxygen isotopic composition (δ18O) of a decrease
(increase) of 0.94 ± 0.3 m/year for every 1‰ increase
(decrease) in rainfall δ18O, based on data extracted
from the International Atomic Energy Agency (IAEA) Global Networks of
Isotopes in Precipitation (GNIP) database and from data generated in
this study. Dripwater δ18O and Mg/Ca ratios show annual
cyclicity at some, but not all sites, accentuating the complex nature of
cave hydrology. A stalagmite δ18O record for the last
˜160 years indicates the existence of droughts of decadal length,
when rainfall is estimated to be ˜0.65 ± 0.3 m/year less
than average conditions. This estimate of rainfall reduction most likely
refers to wet season months, as these months preferentially contribute
to groundwater recharge. The proxy-based climate record at Guam provides
new evidence highlighting how a rainy site in the Western Pacific Warm
Pool today can experience considerable changes in rainfall on decadal
timescales.
Positive correlation between Sr/Ca and Mg/Ca in cave dripwaters and speleothem is often attributed to ‘prior calcite precipitation’ (PCP), where exclusion of Mg and Sr from calcite precipitated upstream of the dripwater/speleothem site results in enrichment in the Mg/Ca and Sr/Ca ratios of residual effluent. Modelling predicts that PCP will produce a straight line correlation in graphs of ln(Sr/Ca) vs. ln(Mg/Ca) with a theoretical slope that is given by (KdSr − 1) / (KdMg − 1) (calculated to be ∼ 0.88 ± 0.13) regardless of rock or absolute dripwater composition (Sinclair et al., submitted for publication). While this makes the correlation slope potentially diagnostic for PCP, Mg and Sr are also preferentially released from calcite during incongruent calcite dissolution (ICD), and it is possible that ICD produces a similar correlation between ln(Sr/Ca) and ln(Mg/Ca).
Although the effects of sedimentation in caves have been recognised for many years, its role in speleogenesis is frequently overlooked. Influxes of sediment into a cave system fundamentally alter the way cave passages develop, either by alluviation in a vadose environment, forcing lateral corrosion and the development of notches, or by upwards dissolution in a phreatic environment through a process known as paragenesis. Sediment influxes affect the hydrological functioning of a karst aquifer by changing the way conduits behave and subsequently develop both in plan and long section.Here we give an overview of the mechanisms of cave sedimentation and describe how the process of alluviation and paragenesis affect speleogenesis. A characteristic suite of meso- and micro-scale dissolutional features can be used to recognise paragenetic development, which is reviewed here. In a vadose environment these include alluvial notches, whilst in a phreatic environment, half tubes, anastomoses and pendants, bedrock fins and paragenetic dissolution ramps result. Using these to identify phases of sedimentation and paragenesis is crucial for reconstructing denudation chronologies from cave deposits. We suggest that sedimentation and paragenesis are most likely to occur in certain geomorphological situations, such as ice marginal and periglacial environments, beneath thick residual soils and where rivers can transport fluvial sediment into a cave, either via stream sinks or back-flooding.
A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes.
The coupled δ13C-radiocarbon systematics of three European stalagmites deposited during the Late Glacial and early Holocene were investigated to understand better how the carbon isotope systematics of speleothems respond to climate transitions. The emphasis is on understanding how speleothems may record climate-driven changes in the proportions of biogenic (soil carbon) and limestone bedrock derived carbon. At two of the three sites, the combined δ13C and 14C data argue against greater inputs of limestone carbon as the sole cause of the observed shift to higher δ13C during the cold Younger Dryas. In these stalagmites (GAR-01 from La Garma cave, N. Spain and So-1 from Sofular cave, Turkey), the combined changes in δ13C and initial 14C activities suggest enhanced decomposition of old stored, more recalcitrant, soil carbon at the onset of the warmer early Holocene. Alternative explanations involving gradual temporal changes between open- and closed-system behaviour during the Late Glacial are difficult to reconcile with observed changes in speleothem δ13C and the growth rates. In contrast, a stalagmite from Pindal cave (N. Spain) indicates an abrupt change in carbon inputs linked to local hydrological and disequilibrium isotope fractionation effects, rather than climate change. For the first time, it is shown that while the initial 14C activities of all three stalagmites broadly follow the contemporaneous atmospheric 14C trends (the Younger Dryas atmospheric 14C anomaly can be clearly discerned), subtle changes in speleothem initial 14C activities are linked to climate-driven changes in soil carbon turnover at a climate transition.
We present bulk average Sr and Mg data for 13 speleothems from different locations in the western equatorial Pacific (Guam, Solomon Islands, and Vanuatu). These data plot on a single straight line in a graph of ln(Sr/Ca) vs ln(Mg/Ca) with a slope of ~ 0.9. A 22,000 year record of Sr and Mg in one of these samples from Guam also plots with the same slope, suggesting that the process partitioning Mg and Sr within the Guam speleothem is the same as the one that partitions Mg and Sr between the different Pacific speleothems. We rule out temperature, growth rate, detrital phases, and sea-spray as likely mechanisms for this correlation. We construct mathematical models of limestone diagenesis and show that this cannot explain the slope of the correlation.
The reconstruction of the palaeoclimate of the eastern Mediterranean region for the last 60 ka BP is based on the&dgr;18O and&dgr;13C variations of speleothems from Soreq Cave, Israel. Climatic conditions during most of the time interval between 60 and 17 ka BP (the period equivalent to the last glacial) were relatively cold and dry, while they were warmer and wetter from 17 ka BP to the present. At ~17 ka BP, there was a major climatic change with a sharp increase in annual rainfall and temperature and a very wet period occurring between 8.5 and 7.0 ka BP. During the colder and drier period, large, detritus-free, preferentially oriented calcite crystals were deposited from slow-moving water. As a result of a sharp change in the hydrological regime at ~17 ka BP, fast-moving water started entrainment of the soil and carrying detrital material into the cave, and the calcite crystals deposited became small and anhedral. Coinciding with the petrographic and isotopic changes, a sharp drop occurred in the concentrations of strontium, barium and uranium, and in the ratios 87Sr/86Sr and (234U/238U)0, which reached minimum values during the wettest period. This drop reflects enhanced weathering of the soil dolomite host rock. During colder and drier periods, higher trace-element concentrations and higher isotopic ratios reflect an increase in the contribution of salts derived from exo genic sources (sea spray and aeolian dust), and a reduced contribution of weathering from the host dolo mites.
The Melinau carbonate platform initiated during the Mid-Eocene on a rotating slice of the Rajang accretionary prism. The differential sedimentary loading enhanced a rotation of the mobile substratum and created an elongated, asymmetrical wedge-top basin. The extensional southern margin of the basin consists of a 2100–2200-m-thick section of Eocene-to-Oligocene carbonates. These thin laterally towards the northern margin of the basin, where a carbonate factory was active on a postulated underlying thrust. Backstepping and dismemberment of the carbonate system started during the latest Oligocene and deep-marine sedimentation became prevalent over the entire region during the Early Miocene.
Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid–rock interaction in the host limestone, degassing of CO2 and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water–rock interaction and soil processes in controlling stalagmite δ13C. This approach uses the dead carbon proportion (dcp) estimated from coupled 14C and 230Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterranean-type climate, to determine the roles of water–rock interaction and soil processes in determining stalagmite δ13C. Increases in stalagmite dcp between 16.5 and 8.8 ka are coincident with decreased δ13C, indicating a varying yet substantial contribution from the soil organic matter (SOM) reservoir, likely due to significantly increased average age of SOM in the soil veneer above the cave during wet climatic intervals.
Closely-spaced transects measured by excimer laser ablation inductively-coupled plasma mass spectrometry (ELA-ICPMS) at 5 and 32 μm spatial resolution are used to generate trace element composition maps (Ba, Sr, Mg, U, Na, P and Al) from MND-S1, a previously studied modern stalagmite from southwest Australia (Treble et al., 2003: EPSL216: 141). Rainfall at the site is highly seasonal, and trace elements in MND-S1 show strong seasonal variation. Trace element maps show that Ba, Sr, U and Na concentrations coherently follow annual growth layers identified from Scanning Electron Microscopy (SEM) images. The SEM images also reveal that stalagmite growth did not proceed uniformly: growth layers vary in thickness and locally pinch out. Highly preferential crystal growth, determined by nucleation sites left by the previous year’s growth, may be responsible for this uneven growth layering. Differential crystal growth apparently causes variability of trace element concentrations along each annual layer, although additional disequilibrium processes affect Mg, which is less distinctly banded than Ba, Sr, U and Na. Uneven and discontinuous growth layers influence the number of annual cycles, their wavelengths and seasonal amplitudes measured in any one transect. This has clear implications for studies that use annual trace element cycles as chronological markers, growth rate or seasonality proxies.
Twenty-one 14C accelerator mass spectrometric analyses were obtained for three Holocene stalagmites from the Uamh-an-Tartair cave (Sutherland, Scotland) in order to estimate the past dead carbon proportion (dcp). Results show that the dcp increases from 22 to 38% from 3780 years ago to the present. Because δ13C variation is small within each sample, it is concluded that this dcp increase is the product of the ageing of soil organic matter related to peat bog development above the cave that produced older soil CO2 and not from a more intense dissolution of the surrounding carbonates, which would have led to a δ13C increase.Comparison with samples from other sites in Europe shows no intersite correlation between dcp and δ13C, but a relatively good intersite correlation is observed between dcp and average site temperature. Thus, temperature may be a major factor controlling the production of old soil organic matter CO2 and, therefore, the dead carbon content of seepage water. In contrast to the Scotland stalagmites, two other Holocene samples from sites in southern France and Belgium exhibit a good correlation between δ13C and dcp, which can be explained in terms of variations in the intensity of limestone dissolution. Consequently, δ13C variations observed in stalagmites are not always due to changes in the vegetation type (C3/C4) as has been commonly assumed; 13C/12C variations in speleothem calcite may also be controlled by the soil organic matter age and, in some cases, by the intensity of the limestone dissolution. Conversely, a largely constant speleothem δ13C signal, as observed for the Scotland stalagmites, does not necessarily imply that surface climate and vegetation conditions were stable since the dcp variations, in this case, are clearly related to the peat bog development during stalagmite growth.
A method for rapid determination of high-precision Sr/Ca ratios in
scleractinian corals is presented. Using an inductively coupled plasma
atomic emission spectrophotometer, samples are corrected for instrument
drift using a reference solution, similar to the approach used for
analysis of stable isotopes using gas-source mass spectrometry. Further
correction for variation of the Sr/Ca ratio with Ca concentration is
accomplished using internal standards. The precision, once all
corrections have been made, is better than 0.1% (relative standard
deviation, 1σ) for samples of similar Ca concentration and better
than 0.2% for samples with variable Ca concentrations. This method
increases the sample throughput by approximately a factor of 20 relative
to thermal ionization mass spectrometry and significantly reduces
instrument and per sample costs. Comparison of Sr/Ca data for a coral
from the Galapagos Islands with an instrumental temperature record shows
excellent agreement and demonstrates the potential for application of
this technique to samples of modern and fossil scleractinian corals and
other marine carbonates, including foraminifera.
Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ18O and δ13C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.
Nearly ten years of daily rawinsonde data for Canton Island (3S, 172W) have been subjected to spectrum and cross-spectrum analysis. In the course of this analysis a very pronounced maximum was noted in the co-spectrum of the 850- and 150-mb zonal wind components in the frequency range 0.0245-0.0190 day1 (41-53 days period). Application of a posteriori sampling theory resulted in a significance level of 6% (0.1% prior confidence level). This type of significance test is appropriate because no prior evidence or reason existed for expecting such a spectral feature. Subsequent analysis revealed the following structure of the oscillation. Peaks in the variance spectra of the zonal wind are strong in the low troposphere, are weak or non-existent in the 700-400 mb layer, and are strong again in the upper troposphere. No evidence of this feature could be found above 80 mb, or in any of the spectra of the meridional component. The spectrum of station pressure possesses a peak in this frequency range and the oscillation is in phase with the low tropospheric zonal wind oscillation, and out of phase with that in the upper troposphere. The tropospheric temperatures exhibit a similar peak and are highly coherent with the station pressure oscillation; positive station pressure anomalies are associated with negative temperature anomalies throughout the troposphere. Thus, the lower-middle troposphere appears to be a nodal surface with u and P oscillating in phase but 180° out of phase above and below this surface. Evidence for this phenomenon was found in shorter records at Kwajalein (9N, 168E) but not at Singapore (1N, 104E) or Balboa, Canal Zone (9N, 79w). We speculate that the oscillation is a large circulation cell oriented in zonal planes and centered in the mid-Pacific.
At two caves (Clamouse, S France and Ernesto, NE Italy), cave drip and pool waters were collected and sampled at intervals over a 2–3 year period. Mg/Ca and Sr/Ca concentration ratios, corrected for marine aerosols, are compared with those of bedrocks and, in some cases, aqueous leachates of soils and weathered bedrocks. Cave waters do not lie along mixing lines between calcite and dolomite of bedrock carbonate, but typically show enhanced and covarying Mg/Ca and Sr/Ca.Four factors are considered as controlling processes. (1) The much faster dissolution rate of calcite than dolomite allows for the possibility of increase of Mg/Ca if water–rock contact times are increased during drier conditions. A theoretical model is shown to be comparable to experimental leachates. (2) Prior calcite precipitation along a flow path is a powerful mechanism for generating enhanced and covarying Mg/Ca and Sr/Ca ratios. This mechanism requires the solution to lose CO2 into pores or caverns. (3) Incongruent dolomite dissolution has only limited potential and is best regarded as two separate processes of dolomite dissolution and calcite precipitation. (4) selective leaching of Mg and Sr with respect to Ca is shown to be important in leachates from Ernesto where it appears to be a phenomenon of calcite dissolution. In general selective leaching can occur whenever Ca is sequestered into precipitates due to freezing or drying of soils, or if there is derivation of excess Sr and Mg from non-carbonate species.The Ernesto cave has abundant water supply which in the main chamber is derived from a reservoir with year-round constant PCO2 of around 10−2.4 and no evidence of calcite precipitation in the karst above the cave. Two distinct, but overlying trends of enhanced and covarying Mg/Ca and Sr/Ca away from the locus of bedrock compositions are due to calcite precipitation within the cave and, at a variable drip site, due to enhanced selective leaching at slow drip rates. Mg-enhancement in the first chamber is due to a more dolomitic bedrock and longer residence times.The Clamouse site has a less abundant water supply and presents geochemical evidence of prior calcite precipitation, both in the cave and in overlying porous dolomite/dedolomitized limestone bedrock. Initial PCO2 values as high as 10−1 are inferred. Experimental incubations of Clamouse soils which generated enhanced PCO2 and precipitated CaCO3 had compositions similar to the karst waters. Calcite precipitation is inferred to be enhanced in drier conditions.Hydrological controls on cave water chemistry imply that the trace element chemistry of speleothems may be interpretable in palaeohydrological terms. Drier conditions tends to promote not only longer mean residence times (enhancing dolomite dissolution and hence Mg/Ca), but also enhances degassing and calcite precipitation leading to increased Mg/Ca and Sr/Ca.
Carbon isotopes in speleothems may serve as indicators of vegetative change, climatic conditions, and karst processes. In many recent studies of Chinese stalagmites, however, carbon isotopes have often been neglected or underutilized in interpreting paleoenvironments. Here, we present a continuous decadal-scale δ13C record (819 measurements) of the mid- to late-Holocene from a precisely-dated (10 230Th dates) aragonite stalagmite from Lianhua Cave, Hunan Province, China. Compared to coeval stalagmites from other Chinese caves, the average δ13C value (−3.6‰) of stalagmite A1 is higher by ~2–7‰. Variations in the δ13C values (0.1‰ to −6.0‰) reflect changes in both vegetative productivity and inorganic processes, which respond to climatic processes. The δ13C record of stalagmite A1 can be subdivided into three intervals: 1) warm–humid stage (6.6 to 3.8 ka); 2) transitional stage (3.8 to 1.6 ka); and 3) cool–arid stage (1.6 ka to present). Comparisons with other stalagmite and paleoclimatic records demonstrate that these intervals are generally consistent with changes in regional vegetation and climatic conditions.
There has been considerable uncertainty about the nature of Pleistocene environments colonised by the first modern humans in Island SE Asia, and about the vegetation of the Last Glacial Maximum (LGM) in the region. Here, the palynology from a series of exposures in the Great Cave of Niah, Sarawak, Malaysian Borneo, spanning a period from ca. 52,000 to 5,000 BP is described. Vegetation during this period was climate-driven and often highly unstable. Interstadials are marked by lowland forest, sometimes rather dry and at times by mangroves. Stadials are indicated by taxa characteristic of open environments or, as at the LGM, by highly-disturbed rather open forest. Stadials are also characterised by taxa now restricted to 1000-1600 m above sea level, suggesting temperature declines of ca. 7-9⁰ C relative to present, by comparison with modern lapse rates. The practice of biomass burning appears associated with the earliest human activity in the cave.
Diverse interpretations have been made of carbon isotope time series in speleothems, reflecting multiple potential controls. Here we study the dynamics of 13C and 12C cycling in a particularly well-constrained site to improve our understanding of processes affecting speleothem δ13C values. The small, tubular Grotta di Ernesto cave (NE Italy) hosts annually-laminated speleothem archives of climatic and environmental changes. Temperature, air pressure, pCO2, dissolved inorganic carbon (DIC) and their C isotopic compositions were monitored for up to five years in soil water and gas, cave dripwater and cave air. Mass-balance models were constructed for CO2 concentrations and tested against the carbon isotope data. Air advection forces winter pCO2 to drop in the cave air to ca. 500 ppm from a summer peak of ca. 1500 ppm, with a rate of air exchange between cave and free atmosphere of approximately 0.4 days. The process of cave ventilation forces degassing of CO2 from the dripwater, prior to any calcite precipitation onto the stalagmites. This phase of degassing causes kinetic isotope fractionation, i.e. 13C-enrichment of dripwater whose δ13CDIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ13C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO2 evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic 13C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO2 between cave water and cave air. This implies that speleothem δ13C values may not be simply interpreted either in terms of hydrology or soil processes.
Cave air PCO2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO2 production, resulting in high summer PCO2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO2 degassing rates from drip water, by considering both the seasonal and high-frequency cave air PCO2 variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 μm h− 1) and minima in late June/early July (0.033 μm h− 1). For well-mixed ‘diffuse flow’ cave drips such as those that feed CC-Bil, high summer cave air PCO2 depresses summer calcite deposition, while low winter PCO2 promotes degassing and enhances deposition rates. In stalagmites fed by well-mixed drips lacking seasonal variations in δ18O, integrated annual stalagmite calcite δ18O is unaffected; however, seasonality in cave air PCO2 may influence non-conservative geochemical climate proxies (e.g., δ13C, Sr/Ca). Stalagmites fed by ‘seasonal’ drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air PCO2 may increase relative to cave air PCO2 due to higher soil temperatures. This in turn may bias stalagmite calcite δ18O records towards isotopically heavier summer drip water δ18O values, resulting in elevated calcite δ18O values compared to the ‘equilibrium’ values predicted by calcite–water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air PCO2 variability and the resulting intra-annual variability in calcite deposition rates.
Simultaneous high-precision measurement of 24 Mg, 25 Mg and 26 Mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry (MC-ICP-MS) relative to the international standard SRM980. Data are presented on low-Mg calcite speleothems and their associated host rocks and waters from four caves, one in the French Alps and three in Israel, covering various climate conditions. In addition, data are presented on three dolostones and three limestones from the Himalaya. The overall variation is 4.13x and 2.14x in N 26 Mg and N 25 Mg, respectively. This is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of Mg is not unique. Each speleothem shows a characteristic range of N 26 Mg values that are attributed to the isotopic composition of the local water. Differences between the isotopic composition of Mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to Mg isotopic fractionation during carbonate precipitation in the temperature range of 4^18C. The low-Mg calcite is enriched in light isotopes by 1.35x/AMU and the dependence on temperature has been found to be less than 0.02x/AMU/C. Despite various geological settings, the N 26 Mg of the studied dolostones is 2.0 þ 1.2x higher than the N 26 Mg of the limestones. All together, these results suggest a strong mineralogical control and a weak temperature effect on the Mg isotopic composition of carbonate. ß 2002 Elsevier Science B.V. All rights reserved.
The response of a climate proxy against measured temperature, rainfall and atmospheric circulation patterns at sub-annual resolution is the ultimate test of proxy fidelity but very few data exist showing the level of correspondence between speleothem climate proxies and the instrumental climate record. Cave sites on the Gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with the longest known available precipitation isotopes and instrumental records. An actively growing speleothem sampled from New St. Michaels Cave in 2004 is composed of paired laminae consisting of light columnar calcite and a darker microsparitic calcite. Stable isotope analysis of samples micromilled in 100 μm steps at the equivalent of bi-monthly intervals reveals fabric-correlated annual cycles in carbon isotopes, oxygen isotopes and trace elements responding to seasonal changes in cave microclimate, hydrology and ventilation patterns. Calcite δ 13 C values reach a minimum in the light columnar fabric and evidence from trace element behaviour and cave monitoring indicates that this grows under cave 'winter' conditions of highest pCO 2 , whereas the dark microsparitic calcite, characterised by elevated δ 13 C and δ 18 O values grows under low 'summer' pCO 2 conditions. Drip water δ 13 C DIC reaches a minimum in March–April, at which time the attenuated δ 18 O signal becomes most representative of winter precipitation. An age model based on cycle counting and the position of the 14 C bomb carbon spike yields a precisely dated winter oxygen isotope proxy of cave seepage water for comparison with the GNIP and instrumental climate record for Gibraltar. The δ 18 O characteristics of calcite deposited from drip water representing winter precipitation for each year can be derived from the seasonally resolved record and allows reconstruction of the δ 18 O drip water representing winter precipitation for each year from 1951–2004. These data show an encouraging level of correspondence (r 2 = 0.47) with the δ 18 O of rainfall falling each year between October and March and on a decadal scale the δ 18 O of reconstructed winter drip water mirrors secular change in mean winter temperatures.
In chapter 2 the isotopic fractionation of water in some simple condensation-evaporation processes are considered quantitatively on the basis of the fractionation factors given in section 1.2. The condensation temperature is an important parameter, which has got some glaciological applications. The temperature effect (the δ's decreasing with temperature) together with varying evaporation and exchange appear in the “amount effect” as high δ's in sparse rain. The relative deuterium-oxygen-18 fractionation is not quite simple. If the relative deviations from the standard water (S.M.O.W.) are called δD and δ18, the best linear approximation is δD = 8 δ18.Chapter 3 gives some qualitative considerations on non-equilibrium (fast) processes. Kinetic effects have heavy bearings upon the effective fractionation factors. Such effects have only been demonstrated clearly in evaporation processes, but may also influence condensation processes. The quantity d = δD −8 δ18 is used as an index for non-equilibrium conditions.The stable isotope data from the world wide I.A.E.A.-W.M.O. precipitation survey are discussed in chapter 4. The unweighted mean annual composition of rain at tropical island stations fits the line δD = 4.6 δ18 indicating a first stage equilibrium condensation from vapour evaporated in a non-equilibrium process. Regional characteristics appear in the weighted means.The Northern hemisphere continental stations, except African and Near East, fit the line δD = 8.0 δ18 + 10 as far as the weighted means are concerned (δD = 8.1 δ18 + 11 for the unweighted) corresponding to an equilibrium Rayleigh condensation from vapour, evaporated in a non-equilibrium process from S.M.O.W. The departure from equilibrium vapour seems even higher in the rest of the investigated part of the world.At most stations the δD and varies linearily with δ18 with a slope close to 8, only at two stations higher than 8, at several lower than 8 (mainly connected with relatively dry climates).Considerable variations in the isotopic composition of monthly precipitation occur at most stations. At low latitudes the amount effect accounts for the variations, whereas seasonal variation at high latitudes is ascribed to the temperature effect. Tokyo is an example of a mid latitude station influenced by both effects.Some possible hydrological applications are outlined in chapter 5.
Each of two calcitic stalagmites from Grotte de Clamouse, Herault, southern France, displays a discrete aragonite layer dated at around 1100 yr BP. The layer of fanning aragonite ray crystals is immediately preceded by calcite with Mg and Sr compositions that are uniquely high for the past 3 kyr. Trace element compositions close to the boundary between original aragonite and calcite are consistent with quasi-equilibrium partitioning of trace elements between the phases. Study of modern dripwaters demonstrates that pronounced covariation of Mg/Ca and Sr/Ca ratios in dripwater occurs owing to large amounts of calcite precipitation upflow of the drips that fed the stalagmites. Trace element to Ca ratios are enhanced during seasonally dry periods. Ion microprobe data demonstrate a pronounced covariation of trace elements, including Mg and Sr in calcite, and Sr, U and Ba in aragonite. The mean peak spacing is close to the long-term mean of annual growth rates determined by differences in U-series ages and so the trace element peaks are interpreted as annual. The trace element chemistry of the stalagmites on annual to inter-annual scales thus directly reflects the amounts of prior calcite precipitation, interpreted as an index of aridity. The longer-term context is a multi-decadal period of aridity (1200–1100 yr BP) possibly correlated with an analogous episode in Central America. The arid period culminated in the nucleation of aragonite, but within a decade was followed by a return to precursor conditions. Copyright © 2005 John Wiley & Sons, Ltd.
Distribution coefficients, as a function of precipitation rate, were determined for the metals Sr2+, Co2+, Mn2+ and Cd2+in calcite. A pH-stat was used to maintain a constant degree of-saturation, and hence precipitation rate, during each coprecipitation run. The precipitation rate was proportional to the degree of supersaturation and the mass of seed crystal introduced. Distribution coefficients (λ) as a function of rate were determined using radioactive isotopes for solutions with saturations Ω = 1 to Ω = 5.5. Strontium distribution coefficients increased with increasing precipitation rate, while Co, Mn and Cd distribution coefficients decreased with increasing precipitation rate. The following rate expressions (at 25°C) were derived: logλSr = 0.249 log R −1.57logλMn = −0.266 log R + 1.35logλCo = −0.173 log R + 0.68logλCd = −0.194 log R + 1.46 where R is the observed precipitation rate in nmoles CaCO3 per mg seed crystal per min.In separate experiments the uptake of radioactive isotopes was monitored during the recrystallization of calcite seed crystals. Rates of recrystallization were from 100 to 10, 000 times slower than the pH-stat experiments, but yielded distribution coefficients consistent with the above rate expressions.Using gross estimates of biogenic crystal growth rates, aragonite to calcite transformation rates, and the above Sr rate expression, biogenic calcite and diagenetic calcite Sr contents are estimated. These experiments indicate that in addition to solution composition, precipitation rate is a significant factor influencing the trace metal content of naturally occurring calcite.
Uranium-series dating and stable isotope analyses of two speleothems from northwest Nelson, New Zealand, record changes in regional climate and local forest extent over the past 31,000 years. Oxygen isotope variation in these speleothems primarily represents changes in the meteoric waters falling above the caves, possibly responding to latitudinal changes in the position of the Subtropical Front in the Tasman Sea. Seven positive excursions can be identified in the oxygen isotope record, which coincide with periods of glacier advance, known to be sensitive to northward movement of the Subtropical Front. Four glacier advances occurred during oxygen isotope stage 2, with the most extreme glacial conditions centered on 19,000 cal yr B.P.2An excursion in the oxygen isotope record from 13,800 to 11,700 cal yr B.P. provides support for a previously identified New Zealand glacier advance at the time of the Younger Dryas Stade, but suggests it began slightly before the Younger Dryas as recorded in Greenland ice cores. Carbon isotope variations in the speleothems record changes in forest productivity, closely matching existing paleovegetation records. On the basis of vegetation changes, stage 2 glacial climate conditions terminated abruptly in central New Zealand, from 15,700 to 14,200 cal yr B.P. Evidence of continuous speleothem growth at one site suggests that depression of the local treeline was limited to 600–700 m below its present altitude, throughout the last 31,000 years.
A better knowledge of dripwater hydrology in karst systems is needed to understand the palaeoclimate implications of temporal variations in Mg/Ca and Sr/Ca of calcareous cave deposits. Quantitative modelling of drip hydrology and hydrochemistry was undertaken at a disused limestone mine (Brown's Folly Mine) in SW England overlain by 15 m of poorly karstified Jurassic limestones, with sub-vertical fracturing enhanced by proximity to an escarpment. Discharge was monitored at 15 sites intermittently from the beginning of 1996, and every 10–20 days from later 1996 to early 1998. Samples for hydrochemical parameters (pH, alkalinity, cations, anions, fluorescence) were taken corresponding to a sub-set of these data and supplemented by bedrock and soil sampling, limited continuously logged discharge, and soil water observations. Three sites, covering the range of discharge (approximately 1 μL s⁻¹ to 1 ml s⁻¹ maximum discharge) and hydrochemical behaviours, were studied in more detail. A quantitative flow model was constructed, based on two parallel unit hydrographs: responsive and relatively unresponsive to discharge events, respectively. The linear response and conservative mixing assumptions of the model were tested with hydrogeochemical data.