Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions

In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ¹⁸O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ¹⁸O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ¹³C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ¹³C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ¹³C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO₂ derived from overlying C₃-type vegetation and marine dolomite host rock.The δ¹⁸O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ¹³C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ¹⁸O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ¹³C and δ¹⁸O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ¹³C–δ¹⁸O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ¹³C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO₂-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C₃ to C₄-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ¹³C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types.Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ¹⁸O ranges. In particular, the higher δ¹⁸O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ¹⁸O–δ¹³C range representative of any given climatic period.     

Speleothem calcite farmed in situ: Modern calibration of δO and δC paleoclimate proxies in a continuously-monitored natural cave system

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.Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s⁻¹ in winter and 0.4 m s⁻¹ in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO₂. A clear relationship is found between calcite δ¹³C and cave air ventilation rates estimated by proxies pCO₂ and ²²²Rn. Calcite δ¹³C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ¹³C_CaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ¹³C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ¹³C records in ancient speleothems. Calcite δ¹³C_CaCO3 may be a proxy not only for atmospheric CO₂ or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows.Farmed calcite δ¹⁸O was found to exhibit a +0.82 ± 0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ¹³C_CaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ¹⁸O_CaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water-calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water-calcite oxygen isotope fractionation across a range of temperatures and cave environments:

1000lnα=16.1(10³T^-1)-24.6     

CO clumping in speleothems: Observations from natural caves and precipitation experiments

The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ¹⁸O. Interpreting speleothem δ¹⁸O records in terms of absolute paleotemperatures and δ¹⁸O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ₄₇ (a geochemical variable reflecting the statistical overabundance of ¹³C¹⁸O bonds in CO₂ evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ₄₇ value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation.Here we report Δ₄₇ values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ₄₇ and heavier δ¹⁸O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ₄₇ disequilibrium and that of δ¹⁸O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ¹⁸O and Δ₄₇ values, probably inherited from prior degassing within the cave system.In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ₄₇ in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ₄₇ in inorganic carbonates.     

Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ¹³C and δ¹⁸O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO₂ at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ¹³C and δ¹⁸O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ¹⁸O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ¹⁸O and highly variable δ¹³C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.     

Modelling fractionation of stable isotopes in stalagmites

High resolution δ¹³C and δ¹⁸O profiles recorded in precisely dated speleothems are widely used proxies for the climate of the past. Both δ¹³C and δ¹⁸O depend on several climate related effects including meteorological processes, processes occurring in the soil zone above the cave and isotope fractionation processes occurring in the solution layer on the stalagmite surface. Here we model the latter using a stalagmite isotope and growth model and determine the relationship between the stable isotope values in speleothem calcite and cave parameters, such as temperature, drip interval, water p_CO2 and a mixing coefficient describing mixing processes between the solution layer and the impinging drop.The evolution of δ¹³C values is modelled as a Rayleigh distillation process and shows a pronounced dependence on the residence time of the solution on the stalagmite surface and the drip interval, respectively. The evolution of δ¹⁸O values, in contrast, is also influenced by buffering reactions between the bicarbonate in the solution and the drip water driving the δ¹⁸O value of the bicarbonate towards the value expected for equilibrium isotope fractionation between drip water and calcite. This attenuates the dependence of the δ¹⁸O values on drip interval. The temperature dependence of δ¹⁸O, however, is more pronounced than for δ¹³C and in a similar range as expected for fractionation under equilibrium conditions.We also investigate the isotopic enrichment of the δ¹³C and δ¹⁸O values along individual growth layers and, thus, the slopes expected for Hendy tests. The results show that a positive Hendy test is only possible if isotope fractionation occurred under disequilibrium conditions. However, a negative Hendy test does not exclude that isotope fractionation occurred under disequilibrium conditions. A more reliable indicator for disequilibrium fractionation is the enrichment of the δ¹³C values along an individual growth layer.     

洞穴滴水地球化学的空间和时间变化及其控制因素--以北京石花洞为例

通过对北京石花洞滴水地球化学一个水文年的观测,揭示了洞穴滴水水文地球化学季节变化与外界气候变化的关系,3个滴水点的滴率随降雨量的增加都有明显的变化,但不同滴水点滞后时间不同。滴水滴率、Mg²⁺和SO^{2-₄含量的季节变化数据显示,雨季洞穴滴水主要来源于当季降水,但也存在岩层滞留水的混入。滴水中Mg/Ca比值存在明显季节变化,旱季较低而雨季较高,但在雨季初期出现较大的波动。分析洞穴上覆土壤和洞内裂隙土壤数据,认为雨季初期滴水中Mg/Ca比值的波动是由土壤中Mg2+的快速淋溶造成的,上覆土壤结构性质和组分变化均影响滴水地球化学特征。}     

Stable isotope variations in modern tropical speleothems: Evaluating equilibrium vs. kinetic isotope effects

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.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ¹³C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The ¹³C depletions are likely caused by kinetically driven departures in the fractionation between HCO₃⁻ (aq) and CaCO₃ from equilibrium conditions, caused by rapid calcite growth. ¹³C enrichments can be accounted for by Rayleigh distillation of the HCO₃⁻ (aq) reservoir during degassing of ¹³C-depleted CO₂.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in ¹⁸O relative to equilibrium values. δ¹⁸O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO₃⁻ (aq) and CaCO₃ from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ¹³C, δ¹⁸O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO₂ hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO₃⁻ (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in ¹⁸O enrichment in the HCO₃⁻ (aq) reservoir and ultimately in the precipitated CaCO₃.     

Speleothem record of the last 180 ka in Villars cave (SW France): Investigation of a large δ18O shift between MIS6 and MIS5

The Vil-car-1 flowstone core from Villars cave (SW France) provides one of the first European speleothem records extending back to 180 ka, based on U–Th TIMS and MC-ICP-MS measurements. The core offers a continuous record of Termination II and the Last Interglacial. The penultimate deglaciation is characterized by a prominent 5‰ depletion in calcite δ¹⁸O. Determining which specific environmental factors controlled such a large oxygen isotopic shift offers the opportunity to assess the impact of various factors influencing δ¹⁸O variations in speleothem calcite.Oxygen isotope analyses of fluid inclusions indicate that drip water δ¹⁸O remained within a very narrow range of ±1‰ from Late MIS6 to the MIS5 δ¹⁸O optimum. The possibility of such a stable behaviour is supported by simple calculations of various effects influencing seepage water δ¹⁸O.Although this could suggest that the isotopic shift in calcite is mainly driven by temperature increase, attempts to quantify the temperature shift from Late MIS6 to the MIS5 δ¹⁸O optimum by assuming an equilibrium relationship between calcite and fluid inclusion δ¹⁸O yield unreasonably high estimates of ～20 °C warming and Late MIS6 cave temperatures below 0 °C; this suggests that the flowstone calcite precipitated out of thermodynamic equilibrium at this site.Using a method proposed by Guo et al. (submitted for publication) combining clumped isotope measurements, fluid inclusion and modern calcite δ¹⁸O analyses, it is possible to quantitatively correct for isotopic disequilibrium and estimate absolute paleotemperatures. Although the precision of these absolute temperature reconstructions is limited by analytical uncertainties, the temperature rise between Late MIS6 and the MIS5 optimum can be robustly constrained between 13.2 ± 2.6 and 14.6 ± 2.6 °C (1σ), consistent with existing estimates from Western Europe pollen and sea-surface temperature records.     

Holocene monsoon climate documented by oxygen and carbon isotopes from lake sediments and peat bogs in China: a review and synthesis

There has been much recent debate about Holocene climate variation in the monsoon region of China, especially the temporal pattern of variations in precipitation, the time-transgressive nature of the Holocene precipitation maximum, and the extent to which variations in regions influenced by the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) have been synchronous. We summarize and compare carbonate oxygen-isotope records (δ¹⁸O_carb) from ten lakes within the present-day ISM region. We discuss their paleoclimate significance considering the present-day moisture source, isotopic composition of precipitation and the hydrological setting. The δ¹⁸O_carb records are controlled mainly by the isotopic composition of lake water, which in turn is a function of regional Precipitation/Evaporation (P/E) balance and the proportion of precipitation that is monsoon-derived. We normalized the δ¹⁸O_carb data and used these records to generate an integrated moisture index. This index, along with oxygen-isotope records from speleothems and carbon-isotope records (δ¹³C_org) from peats within the monsoon region, suggests that Holocene climate was broadly synchronous across the monsoon region and, within the limits of accuracy of the existing age models, provides no strong evidence for previously-proposed anti-phasing of the ISM and the EASM. Stable-isotope records from lake sediments and peat bogs have excellent potential for providing high-quality paleoclimate data for monsoon Asia, and complement high-resolution speleothem sequences, which are only found in certain localities.     

Changing moisture sources over the last 330,000 years in Northern Oman from fluid-inclusion evidence in speleothems

Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δD_FI) are between −60 and −20‰ (VSMOW) and δ¹⁸O values of speleothem calcite (δ¹⁸O_C) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ¹⁸O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ¹⁸O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.     

Climatic dependence of stable carbon and oxygen isotope signals recorded in speleothems: From soil water to speleothem calcite

Understanding the relationship between stable isotope signals recorded in speleothems (δ¹³C and δ¹⁸O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO₂.The water entering the cave flows as a thin film towards the drip site. CO₂ degasses from this film within approx. 10 s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scale of several 10-100 s. The δ¹³C value of the drip water is mainly determined by the isotopic composition of soil CO₂. The evolution of the δ¹⁸O value of the carbonate species is determined by the long exchange time T_ex, between oxygen in carbonate and water of several 10,000 s. Even if the oxygen of the CO₂ in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ¹⁸O value of the carbonate species. Consequently, the δ¹⁸O value of the rainwater will only be reflected in the drip water if it has stayed in the rock for a sufficiently long time.After the water has entered the cave, the carbon and oxygen isotope composition of the drip water may be altered by CO₂-exchange with the cave air. Exchange times, , of about 3000 s are derived. Thus, only drip water, which drips in less than 3000 s onto the stalagmite surface, is suitable to imprint climatic signals into speleothem calcite deposited from it.Precipitation of calcite proceeds with time constants, τ_p, of several 100 s. Different rate constants and equilibrium concentrations for the heavy and light isotopes, respectively, result in isotope fractionation during calcite precipitation. Since T_ex ? τ_p, exchange with the oxygen in the water can be neglected, and the isotopic evolution of carbon and oxygen proceed analogously. For drip intervals T_d < 0.1τ_p the isotopic compositions of both carbon and oxygen in the solution evolve linearly in time. The calcite precipitated at the apex of the stalagmite reflects the isotopic signal of the drip water.For long drip intervals, when calcite is deposited from a stagnant water film, long drip intervals may have a significant effect on the isotopic composition of the DIC. In this case, the isotopic composition of the calcite deposited at the apex must be determined by averaging over the drip interval. Such processes must be considered when speleothems are used as proxies of past climate variability.     

Stable isotope studies of fluid inclusions in speleothems and their paleoclimatic significance

Fluid inclusions found trapped in speleothems (cave deposited travertine) are interpreted as samples of seepage water from which enclosing calcium carbonate was deposited. The inclusions are assumed to have preserved their D/H ratios since the time of deposition. Initial ¹⁸O/¹⁶O ratios can be inferred from δD because rain- and snow-derived seepage waters fall on the meteoric water line (δD = 8δ¹⁸O + 10). Estimates of temperature of deposition of the carbonate can be calculated from inclusion D/H ratios and δ¹⁸O of enclosing calcite in Pleistocene speleothems. For most speleothems investigated (0–200,000 yr old) δ¹⁸O of calcite appears to have decreased with increasing temperature of deposition indicating that the dominant cause of climate-dependent change in δ¹⁸O of calcite was the change in K_cw, the isotope fractionation equilibrium constant, with temperature; δ¹⁸O of meteoric precipitation generally increased with increasing temperature, but not sufficiently to compensate for the decrease in K_cw.     

Modeling speleothem δC variability in a central Sierra Nevada cave using C and Sr/Sr

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.We use geochemical and isotope mixing models to estimate the host-carbonate contribution throughout the δ¹³C time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite. The degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators, with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals. Modeled δ¹³C values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite δ¹³C values, even during relatively wet intervals. These results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem δ¹³C records, particularly those from caves that formed in seasonal semi-arid to arid environments.     

Water release patterns of heated speleothem calcite and hydrogen isotope composition of fluid inclusions

Speleothem fluid inclusions are a potential paleo-precipitation proxy to reconstruct past rainwater isotopic composition (δ¹⁸O, δD). To get a better insight in the extraction of inclusion water from heated speleothem calcite, we monitored the water released from crushed and uncrushed speleothem calcite, heated to 900 °C at a rate of 300 °C/h, with a quadrupole mass spectrometer. Crushed calcite released water in three not well individualised peaks between 25 and 360 °C, 360 and 650 °C and between 650 and 800 °C while uncrushed calcite released water in two distinct temperature intervals: between 25 and 550 °C and between 550 and 900 °C.Water from two speleothems from the Han-sur-Lesse cave was recovered using three different techniques: i) the crushing and heating to 360 °C technique, ii) the decrepitation by heating to 550 °C and iii) the decomposition by heating to 900 °C technique. Measurements of the δD of water recovered by the decomposition of Han-sur-Lesse calcite heated to 900 °C did not show a 20 to 30‰ offset as found by previous authors. However a difference of 7‰ was observed between water released before and after decomposition of the calcite. Water recovery from the Han-sur-Lesse samples suggests that a simple heating technique (up to 550 °C) without crushing could both (a) recover water with δD representative of that of the drip water and (b) double the water yield as compared to the crushing and heating method.Our study warns for possible contamination of the recovered inclusion water with hydration water of lime, responsible for the recovery of water with very negative δD values.     

Stable isotopic compositions of waters in the karst environments of China: Climatic implications

A total of 117 water samples, including cave water, ground water, spring water and river water, collected from the monsoonal area of China have been analyzed for their H- and O-isotope composition. Overall, a δ¹⁸O–δD correlation is observed of δD = −4.45 + 6.6δ¹⁸O (R² = 0.90) and a significant evaporation effect observed for the southern sites. Average δ¹⁸O and δD site values generally correspond to those of precipitation in nearby cities, with correlations of δD = 2.18 + 7.23δ¹⁸O (R² = 0.95) for the sample sites and δD = 11.05 + 7.95δ¹⁸O (R² = 0.95) for the cities. The effects of rainfall amount and temperature on precipitation δ¹⁸O were calculated using a simplified theoretical model derived from the Rayleigh distillation equation, which demonstrated that the sign of δ¹⁸O_pvs. T correlation is dependent on precipitation intensity. The mean δ¹⁸O value of cave waters exhibit decreasing trends with increasing latitude and reveal a spatial pattern of positive correlation with annual mean temperature and precipitation, mainly reflecting isotopic fractionations in the moisture source traveling from the ocean side to the inland continent. This spatial pattern implies that the δ¹⁸O values recorded in the proxy climate records derived from speleothems might be influenced by shifts in monsoon boundary during the past, especially between glacial and interglacial intervals.     

Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave - Field study and experiments

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ²⁶Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ²⁶Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000lnα_Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000lnα_Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, ²⁶Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg²⁺ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.     

A speleothem record of Younger Dryas cooling, Klamath Mountains, Oregon, USA

A well-dated δ¹⁸O record in a stalagmite from a cave in the Klamath Mountains, Oregon, with a sampling interval of 50 yr, indicates that the climate of this region cooled essentially synchronously with Younger Dryas climate change elsewhere in the Northern Hemisphere. The δ¹⁸O record also indicates significant century-scale temperature variability during the early Holocene. The δ¹³C record suggests increasing biomass over the cave through the last deglaciation, with century-scale variability but with little detectable response of vegetation to Younger Dryas cooling.     

东亚季风区石笋δ13C是否记录植被变化:从川东北石笋记录的Heinrich事件说起

以往的研究中一般认为石笋δ13C变化的影响因素复杂,但在大多数报道中地表植被仍被认为是影响石笋δ13C变化的主要因素之一.本文综合国内外已发表的模拟试验研究结果及川东北地区已发表和未发表的石笋δ18O-δ13C数据,特别是Heinrich事件中石笋δ18O-δ13C的表现,指出地表植被不一定是影响石笋δ13C变化的主要因素,洞穴水文(如地下水流速、滴水速率、水文化学,等等)的变化可以解释通常观察到的石笋δ13C变化.植被变化的效应可以叠加在洞穴水文变化的效应上.洞穴系统的水文变化复杂性可能是造成石笋δ13C变化呈现较复杂特征的主要原因.     

A multiple cave deposit assessment of suitability of speleothem isotopes for reconstructing palaeo‐vegetation and palaeo‐temperature

The suitability of speleothems for interpreting palaeoclimate is typically determined by using either the Hendy Test, overlapping analysis or long‐term cave environment monitoring. However, in many cases, these methods are not applicable, because a speleothem lacks clearly traceable layers for the Hendy Test, it is difficult to obtain an overlapping speleothem nearby, or long‐term cave monitoring is impractical. The authors propose a multiple cave deposit approach to assess the suitability of speleothems for palaeoclimate study. Speleothems collected from two sites within Raccoon Mountain Cave, Tennessee (USA) exhibit remarkable spatial variation (δ¹³C: ?10·3‰ to ?2·2‰) over a relatively short distance (ca 260 m). Drip water δ¹⁸O values exhibit a seasonal precipitation signal at Site 1 and an annual signal at Site 2. Combining field observations, water isotope analysis and trace‐element data, the authors propose that the speleothem formation at Site 1 and Site 2 tapped distinct sources of CO₂: (i) CO₂ derived from overlying soils for Site 1; and (ii) limestone dissolved inorganic carbon induced by ground water dissolution for Site 2. Using fresh cave deposits (modern speleothem) δ¹³C (100% C3 vegetation) as an analogue, a simple model was developed to estimate land surface vegetation for speleothems. The speleothem formation temperature estimated using fresh cave deposit δ¹⁸O values generally reflects the mean annual temperature in this region. This study indicates that spatial variations in carbon isotopes could be caused by different carbon sources dominating in different parts of the cave, which should be taken into consideration by researchers when using speleothem δ¹³C values to reconstruct temporal palaeo‐vegetation changes. This study demonstrates a practical sampling strategy for verifying suitability of speleothems for palaeo‐vegetation and palaeo‐temperature reconstructions by analysing multiple cave deposits, especially for cases in which the Hendy Test, parallel sampling and long‐term monitoring of cave environment are not feasible.     

Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry

‘Clumped isotope’ thermometry is based on analyzing mass 47 in CO₂ extracted from carbonates and uses the tracer mass 47 anomaly (Δ₄₇). Δ₄₇ is defined as the deviation of R⁴⁷ from that expected for a random distribution of isotopologues and reflects a temperature dependent preference of ¹³C and ¹⁸O to create a bond with each other in CO₂ or in the carbonate lattice. Being an internal characteristic of the carbonate mineral, it is independent of the isotopic composition of the water in which equilibrium precipitation of the carbonate occurs and can therefore be used to independently determine carbonate growth temperatures. This work provides a first examination of the applicability of ‘clumped isotopes’ thermometry to reconstructing the growth temperatures of speleothems, by examining the glacial/interglacial variations of the Δ₄₇ values of speleothem carbonates from Soreq cave, Israel. The results indicate that the last glacial maximum temperatures were 6-7 °C colder than modern day temperature and a sample at 56 Ky BP was 3 °C colder than the modern. Early Holocene temperatures were slightly above modern day, and late Holocene temperatures were slightly below modern day. These temperature variations are similar to those previously estimated for Eastern Mediterranean sea surface water. Cave water was ¹⁸O depleted in the Holocene compared to modern day (by 0.6-1‰) and 1.1‰ more enriched in the last glacial maximum. Comparison of these cave water δ¹⁸O values with fluid inclusion δD values indicated a late Holocene d-excess value within the range of modern rainfall, implying ∼45% relative humidity. Last glacial maximum and early Holocene d-excess values were significantly lower, suggesting relative humidity of ∼60% and ∼70%, respectively. The temperatures reported in this study were empirically corrected for a non-equilibrium artifact observed in a modern speleothem. The similarity of the temperature variations obtained here to other, independent, records in the region suggests that the Δ₄₇-temperature calibration slope observed in inorganic synthetic calcite and marine organisms may also be applied in speleothems. But the offset observed in modern temperature suggests that the intercept is different so that a separate calibration is needed for accurate absolute temperature reconstruction using speleothem ‘clumped isotopes’. Similar examination of additional caves would be necessary to determine whether such empirical correction can be generally applied or is it a unique characteristic of Soreq cave.