Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ¹³C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ¹³C_DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO₂ fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO₂ and precipitated CaCO₃(1000lnα_{[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2}) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of ¹⁴C mass balance and paired ¹⁴C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from ¹⁴C-dead dolomite cap while the remainder of ∼77% is derived from ¹⁴C-live biomass. δ¹³C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ¹³C values calculated using the shallow pool summer/early fall data thus confirming the δ¹³C seasonality in both drips and coeval aragonite. δ¹³C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ¹³C records of stalagmites at DeSoto Caverns with resolving power >10⁻¹ year will likely carry the imprints of drip annual means that record climate-driven δ¹³C seasonal biases.     

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.     

The significance of chemical,isotopic, and detrital components in three coeval stalagmites from the superhumid southernmost Andes (53°S) as high-resolution palaeo-climate proxies

Stalagmites are important palaeo-climatic archives since their chemical and isotopic signatures have the potential to record high-resolution changes in temperature and precipitation over thousands of years. We present three U/Th-dated records of stalagmites (MA1–MA3) in the superhumid southern Andes, Chile (53°S). They grew simultaneously during the last five thousand years (ka BP) in a cave that developed in schist and granodiorite. Major and trace elements as well as the C and O isotope compositions of the stalagmites were analysed at high spatial and temporal resolution as proxies for palaeo-temperature and palaeo-precipitation. Calibrations are based on data from five years of monitoring the climate and hydrology inside and outside the cave and on data from 100 years of regional weather station records.Water-insoluble elements such as Y and HREE in the stalagmites indicate the amount of incorporated siliciclastic detritus. Monitoring shows that the quantity of detritus is controlled by the drip water rate once a threshold level has been exceeded. In general, drip rate variations of the stalagmites depend on the amount of rainfall. However, different drip-water pathways above each drip location gave rise to individual drip rate levels. Only one of the three stalagmites (MA1) had sufficiently high drip rates to record detrital proxies over its complete length. Carbonate-compatible element contents (e.g. U, Sr, Mg), which were measured up to sub-annual resolution, document changes in meteoric precipitation and related drip-water dilution. In addition, these soluble elements are controlled by leaching during weathering of the host rock and soils depending on the pH of acidic pore waters in the peaty soils of the cave’s catchment area. In general, higher rainfall resulted in a lower concentration of these elements and vice versa. The Mg/Ca record of stalagmite MA1 was calibrated against meteoric precipitation records for the last 100 years from two regional weather stations. Carbonate-compatible soluble elements show similar patterns in the three stalagmites with generally high values when drip rates and detrital tracers were low and vice versa. δ¹³C and δ¹⁸O values are highly correlated in each stalagmite suggesting a predominantly drip rate dependent kinetic control by evaporation and/or outgassing. Only C and O isotopes from stalagmite MA1 that received the highest drip rates show a good correlation between detrital proxy elements and carbonate-compatible elements. A temperature-related change in rainwater isotope values modified the MA1 record during the Little Ice Age (～0.7–0.1 ka BP) that was ～1.5 °C colder than today. The isotopic composition of the stalagmites MA2 and MA3 that formed at lower drip rates shows a poor correlation with stalagmite MA1 and all other chemical proxies of MA1. ‘Hendy tests’ indicate that the degassing-controlled isotope fractionation of MA2 and MA3 had already started at the cave roof, especially when drip rates were low. Changing pathways and residence times of the seepage water caused a non-climatically controlled isotope fractionation, which may be generally important in ventilated caves during phases of low drip rates. Our proxies indicate that the Neoglacial cold phases from ～3.5 to 2.5 and from ～0.7 to 0.1 ka BP were characterised by 30% lower precipitation compared with the Medieval Warm Period from 1.2 to 0.8 ka BP, which was extremely humid in this region.     

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.     

Sea-rain-lake relation in the Last Glacial East Mediterranean revealed by δO-δC in Lake Lisan aragonites

We investigated the Sea-Rain-Lake relation during the Last Glacial-Holocene in the East Mediterranean region by comparing the δ¹⁸O and δ¹³C records of authigenic aragonite deposited in Lake Lisan, the Dead Sea, Mediterranean foraminifera, and speleothems. The Lisan Formation data display long- and short-term variations of δ¹⁸O, representing steady-state conditions of the lake (e.g., 5.6‰ ± 0.5‰ and 4.5‰ ± 1‰ in the Upper and Lower Members of the Lisan Formation, respectively), and short-term excursions reflecting large floods and droughts. The long-term (steady-state) δ¹⁸O values of the Lisan aragonites show similarity to the corresponding time-equivalent records of the Eastern Mediterranean foraminifera and Judea Mountain speleothems: The Last Glacial deposits are in all of them 2‰-3‰ heavier than the Holocene ones. We interpret this similarity as reflecting the significance of the source effect on the long-term behavior of isotopic reservoirs: Speleothem δ¹⁸O is strongly influenced by the marine reservoir that contributes its vapor to rain formation; the lake δ¹⁸O is dominated by the composition of the inflowing water. Short-term variations in the isotopic composition of rainfall are dominated by the amount effect and the temperature and those of the Lake’s upper water mass by the lake’s water balance.δ¹³C values are more variable than δ¹⁸O in the same Lisan sequences (e.g., δ¹³C in the Lower Member is 1.0‰ ± 1.7‰, whereas δ¹⁸O is 4.6‰ ± 0.7‰) and are 1‰ to 1.5‰ higher in the Upper Member than in the Lower and Middle Members of the Lisan Formation. These variations reflect significant increase in primary productivity of the lake and algal bloom activity. It appears that the hypersaline-saline lakes were not as “dead” as the Dead Sea is and that algal activity had an important impact upon the geochemistry of Lake Lisan.The δ¹⁸O data combined with independent geochemical and limnologic information (e.g., level fluctuations) indicate that Lisan time was characterized by high precipitation-high lake stands-high atmospheric humidity, whereas the Holocene Dead Sea shows the opposite behavior. This paleoclimatic reconstruction is consistent with independent evidence for significantly wetter conditions in the East Mediterranean region during the Last Glacial period.     

Analysis of the climate signal contained within δO and growth rate parameters in two Ethiopian stalagmites

We combine surface and cave climate monitoring with multiple stalagmite parameters to help understand and calibrate the climate records contained within stalagmites from a region with strong rainfall seasonality. Two actively growing stalagmites from Ethiopia were analysed in order to investigate the climate signal contained within δ¹⁸O and growth rate parameters. The δ¹⁸O and growth rate of the two stalagmites give different responses to surface climate due to variations in the climate signal transfer. Both stalagmites (Merc-1 and Asfa-3) have a climate response that is seasonal; however this signal is subsequently smoothed by the mixing of event and storage water within the aquifer. Merc-1 responds more to high frequency (‘event’) climate, due to a greater ratio of event to storage water in this sample, whereas Asfa-3 responds more to low frequency (‘storage’) climate. In addition, different parameters respond to different seasons. For example, stalagmite Asfa-3, from greater depth from the surface and with a slow drip rate, has a growth rate that responds to the amount of summer rain. In contrast, Merc-1, closer to the surface and with a faster drip rate, exhibits no clear response to surface climate, probably due to a more complex climate signal transfer. δ¹⁸O response varies with stalagmite due to the interplay between rainfall forcing factors (amount, seasonality) and disequilibrium kinetics, with opposing correlations between seasonal rainfall and δ¹⁸O between the samples. Our results demonstrate that analysis of seasonal climate forcing, and transfer functions reflecting the mixing of event and storage water, may be the most appropriate approach to develop of transfer functions appropriate for high-resolution, stalagmite climate reconstruction.     

Sea-land oxygen isotopic relationships from planktonic foraminifera and speleothems in the Eastern Mediterranean region and their implication for paleorainfall during interglacial intervals

The oxygen and carbon stable isotope compositions of cave speleothems provide a powerful method for understanding continental climate change. Here, we examine the question of the regionality of this isotopic record and its linkage with the marine isotopic record in the Eastern Mediterranean (EM) region. The study presents a new, accurately dated 250-kyr δ¹⁸O and δ¹³C record determined from speleothems of the Peqiin Cave, Northern Israel. Its comparison with the continuous 185-kyr isotopic record of the Soreq Cave speleothems from Central Israel reveals striking similarities. Thus, a strong regional climatic signal, brought about by variations in temperature and rainfall amount, is reflected in both cave records. Low δ¹⁸O minima in the Peqiin profile for the last 250- to 185-kyr period (interglacial marine isotopic stage 7) match the timing of sapropels 9 to 7 and are indicative of high rainfall in the EM region at these times. The combined Soreq and Peqiin δ¹⁸O record for the last 250 kyr excellently matches the published Globigerinoides ruber (G. ruber) marine δ¹⁸O record for the EM Sea, with the isotopic compositional difference Δ_{G.ruber-speleothems} remaining relatively constant at −5.6 ± 0.7‰, thus establishing for the first time a robust, exploitable link between the land and the marine isotopic records. The correspondence of low δ¹⁸O speleothem values and high cave water stands with low G. ruber δ¹⁸O values during interglacial sapropel events indicates that these periods were characterized by enhanced rainfall in the EM land and sea regions. By use of sea surface temperatures derived from alkenone data as a proxy for land temperatures at the Soreq Cave, we calculate the paleorainfall δ¹⁸O values and its amounts. Maximum rainfall and lowest temperature conditions occurred at the beginning of the sapropel events and were followed by decrease in rainfall and increase in temperatures, leading to arid conditions. The record for the last 7000 yr shows a trend toward increasing aridity and agrees well with climatic and archeological data from North Africa and the Middle East.     

A late Holocene climate record in stalagmites from Modrič Cave (Croatia)

Few terrestrial Holocene climate records exist from south‐eastern Europe despite its important geographical position as a transitional climatic zone between the Mediterranean and mainland continental Europe. Here we present new petrographic and stable isotope data for two Holocene speleothems from Modri? Cave, Croatia (44°15′N, 15°32′E), a coastal Adriatic site (120 m inland). Modern meteorological and cave conditions have been monitored for 2 years to understand the links between climate variability and stable isotope time‐series records in speleothems. Typical of a Mediterranean‐type climate, a negative water balance exists between April and September, so that recharge of the aquifer is restricted to the winter months. The weighted mean δ¹⁸O of the rainfall is ?5.96‰ (2σ = 2.83), and the weighted mean D/H rainfall value is ?36.83‰ (2σ = 19.95), slightly above the Global Meteoric Water Line, but well below the Mediterranean Meteoric Water Line. Modern calcite from the tops of each stalagmite exhibits δ¹⁸O values that are close to isotopic equilibrium with their respective drip water values. Unfortunately, the relatively young ages and low uranium contents (ca. 50 p.p.b.) of both stalagmites hamper the use of U‐series dating. Radiocarbon dates have been used instead to constrain their chronology using a dead carbon correction. Apart from some Isotope Stage 3 material (ca. 55 ka), both stalagmites were deposited during the late Holocene. Climatic conditions during the late Holocene are inferred to have been sufficiently wet to maintain stalagmite growth and any hiatuses appear to be relatively short lived. Inferred changes in the stalagmite diameters during deposition are linked to δ¹³C and δ¹⁸O variations, indicating alternating periods of drier and wetter conditions. Drier conditions are inferred for the late Roman Ages warm period and the mid‐Medieval Warm Period. Wetter conditions are associated with the Little Ice Age. Copyright © 2012 John Wiley & Sons, Ltd.     

Carbon mass-balance modelling and carbon isotope exchange processes in dynamic caves

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 δ¹³C_DIC 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.     

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.     

Seasonal variability of oxygen and hydrogen stable isotopes in precipitation and cave drip water at Guilin,southwest China

The interpretation of climatic information from stalagmites has traditionally been a complex research problem, with oxygen isotopes playing a particularly important role in global climate change studies. This study investigates the relationship between oxygen isotope composition of the atmospheric in precipitation and cave drip water at Panlong cave in southwest China on seasonal timescales of variability. Time series seasonal variability was derived from Panlong cave in Guilin by collecting daily precipitation samples for stable isotope analysis during 2012. Results indicate that δ¹⁸O of precipitation contains a clear seasonal variation whereby higher values are mainly distributed during winter and lower values during summer. Seasonal variations in water sources affect the precipitation δ¹⁸O values. Drip water δ¹⁸O also displayed a seasonal cycle which is attenuated relative to δ¹⁸O of precipitation. Drip water time series display seasonal cycle ranges from 1.5 to 3.5 ‰ relative to Vienna Standard Mean Ocean Water, which mainly follow the precipitation δ¹⁸O seasonal cycle. Seasonal variation in drip water δ¹⁸O supports interpretations of the stalagmite δ¹⁸O record as a paleoclimate proxy sensitive to the local environment. This monitoring experiment revealed that drip water must be transported through the epikarst in approximately 1.5 months during cold periods, and <0.5 months during warm periods. Different residence time percolation is mainly affected by the atmospheric precipitation amount, depending on whether soil moisture reaches saturation.     

A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave,Sauerland, Germany

Petrographical and geochemical parameters of stalagmites from the B7 cave in Iserlohn–Letmathe (Northern Rhenish Massif, NW Germany) record Late- and postglacial climate changes (temperature and/or precipitation). Fabrics and microfacies of the stalagmite profiles lead to a differentiation of four hierarchies of rhythms. Clastic layers in the stalagmites are caused by flooding events and are time markers. Twenty-four TIMS Th/U-age-dates provide a time calibration of stalagmite growth phases. One stalagmite reveals an early growth period between 17.6 and 16.7 ka BP. Between 9.6 and 5.5 ka BP (Atlantic episode of the Holocene) the growth rate of the stalagmites was higher than before and after this time, with dominant light-porous microfacial laminae and high δ¹⁸O and δ¹³C values representing partly kinetic fractionation effects. This part of the Holocene is interpreted as a mainly warm episode with frequent interruptions of dripping. Within the past 4 ka the profiles with predominant dark compact facies reveal low isotopic values which may be interpreted as a temperature proxy record. The stalagmite records resemble records from an Irish stalagmite. Correlation with the Δ¹⁴C record from European tree rings suggests that colder periods in the North Atlantic were accompanied by drier winters in central Europe.     

文石石笋发生矿物重结晶的影响因素及其古气候意义

洞穴沉积物—石笋已成为研究岩溶区环境气候变化历史的重要载体。在我国湘西地区,某些洞穴石笋原始沉积多为不稳定的文石矿物,极易发生重结晶,可能使石笋中相关化学元素含量最终偏离原生矿物的特征,限制了文石石笋某些代用指标在古气候研究中的应用。文章以前人研究成果为基础,总结梳理了文石石笋发生重结晶的影响因素及其对石笋记录古气候的影响:（1）石笋剖面特征、XRD结果、显微镜观察和地球化学元素特征等可作为石笋发生重结晶的判别依据;（2）洞穴滴水和石笋孔隙水饱和度、文石晶体缺陷和晶体之间的方解石胶结物以及岩溶水体中Mg2+浓度等均会影响文石石笋的矿物转变;（3）在文石向方解石转变过程中,石笋铀含量会有一定程度的流失,可导致放射性铀系定年的异常或年代倒序;（4）矿物重结晶可导致δ18O、δ13C及石笋微量元素浓度（或比值）等指标发生改变,其变化特征因洞穴而异,从而影响其作为环境指示器的可靠性;（5）湖南龙山惹迷洞石笋（RM2）发生了不均一的矿物重结晶,自顶部至20.3 cm以放射状为主,20.3 cm至底部主要为糖粒状,并结合年代结果发现文石重结晶对石笋铀系定年产生了影响,而重结晶作用对该石笋其他指标的影响还有待进一步研究。      

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.     

Modelling δC and δO in the solution layer on stalagmite surfaces

We derive equations describing the evolution of the carbon and oxygen isotope composition of the bicarbonate in a calcite precipitating solution on the surface of a stalagmite using a classical Rayleigh approach. The combined effects of calcite precipitation, degassing of CO₂ and the buffering effect of the water reservoir are taken into account. Whereas δ¹³C shows a progressive increase to a final constant value, δ¹⁸O shows an initial isotopic enrichment, which exponentially decays due to the buffering effect of the water reservoir. The calculated evolution is significantly different for both carbon and oxygen isotopes than derived in a recent paper [Dreybrodt W. (2008) Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H₂O-CO₂-CaCO₃ solution and the related isotopic composition of calcite in stalagmites. Geochim. Cosmochim. Acta72, 4712-4724.].Furthermore, we discuss the isotopic evolution of the bicarbonate in the solution for long residence times on the stalagmite surface, i.e., for t→∞. The equilibrium isotope ratio of the bicarbonate is then determined by isotopic exchange between the cave atmosphere and the bicarbonate in the solution and can be calculated by equilibrium isotope fractionation. For strongly ventilated caves exchange with the cave atmosphere will result in higher δ¹³C and δ¹⁸O values than those observed in a pure Rayleigh distillation scenario, for sparsely ventilated caves it will result in lower δ¹³C and δ¹⁸O values.     

重庆丰都雪玉洞群洞穴现代监测与古环境研究回顾和展望

重庆丰都雪玉洞群包括羊子洞、雪玉洞和水鸣洞,西南大学的研究团队从2008年开始对雪玉洞群开展了系统的现代过程监测,以了解洞穴系统气候和环境信息的传输、转化和记录过程。通过对雪玉洞洞穴内外的大气、植被、土壤、基岩、滴水和洞穴沉积物等的动态监测,研究了碳酸盐沉积过程的水化学指标变化,揭示了现代洞穴滴水的影响因素和变化过程,以及碳酸盐沉积物对现代气候环境变化的响应,也为古气候的定量化研究提供了基础支撑。现代过程监测记录表明:雪玉洞CO₂主要来源于上覆土壤,其季节变化受降水的影响较大;在短时间尺度上受到游客旅游活动的影响明显,但幅度远远小于自然过程引起的变化。雪玉洞内次生沉积物的沉积速率具有明显的旱季、雨季特征,不同滴水点下方沉积物的沉积速率变化较大。雪玉洞群三个洞穴的石笋古环境记录研究表明,本区石笋的230Th/232Th比值较适合高精度铀系测年;部分石笋沉积速率较快,平均沉积速率达到0.25 mm·a?1,可以开展高分辨率的气候和环境变化研究。羊子洞YZ1石笋的年龄范围在116~3 ka B.P.之间（平均测年精度2σ,269年）,覆盖了整个末次冰期,δ18O和δ13C的变化曲线和东亚季风区的其他记录具有明显的一致性。同位素测试的时间分辨率平均为88年,成功记录了一些百年－千年尺度的气候突变事件,如Heinrich事件、7.2 ka事件、小冰期等。在精确年代学的基础上,雪玉洞群石笋具有重建高分辨率气候环境变化的潜力。      

Episodes of late Holocene aridity recorded by stalagmites from Devil's Icebox Cave, central Missouri, USA

Two stalagmites from Devil's Icebox Cave, central Missouri, display similar δ¹³C and δ¹⁸O values and trends during the late Holocene. Positive δ¹³C excursions at 3.5-2.6 ka and 1.2-0.9 ka are interpreted to reflect drier conditions. These elevated stalagmite δ¹³C values could have plausibly been driven by increasing C₄ plant abundances over the cave or an increased contribution of bedrock carbon, both of which could reflect decreased effective moisture. A lack of corresponding oxygen isotopic anomalies during these intervals suggests that neither mean annual temperature nor the seasonality of precipitation changed concomitantly with dryness. Both of the δ¹³C excursions identified in our stalagmite record are roughly coincident with dry intervals from a number of sites located across the Great Plains.     

Intra‐annual variation of the calcite deposition rate of drip water in Shihua Cave,Beijing, China and its implications for palaeoclimatic reconstructions

Cai, B., Zhu, J., Ban, F. & Tan, M. 2011: Intra‐annual variation of the calcite deposition rate of drip water in Shihua Cave, Beijing, China and its implications for palaeoclimatic reconstructions. Boreas, Vol. 40, pp. 525–535. 10.1111/j.1502‐3885.2010.00201.x. ISSN 0300‐9483. Monthly in situ monitoring of the calcite deposition rate, drip‐water chemistry and surrounding cave environment was carried out at Shihua Cave, Beijing, China, through two hydrological years (from January 2006 to February 2008) to determine the seasonal variability and mechanisms of stalagmite growth in Shihua Cave. Calcite deposition rates exhibit significant intra‐annual variation, with the lowest values during the summer monsoonal rainy season (July–August) and peak values from autumn to spring. The temporal change in the calcite deposition rate is negatively correlated with the drip rate, cave‐air PCO₂ (CO₂ partial pressure) and Ca concentration, and positively correlated with the pH of the feeding drip water. The seasonal recharge regime of drip water is likely to be the primary control on the drip‐water quality and quantity, which, in turn, control the calcite deposition rate in Shihua Cave. During the summer rainy season, periodic and intense rainstorms increase the drip rate and cave‐air PCO₂, leading to drip water with a lower pH and saturation index of calcite, thereby reducing the calcite precipitation. It seems that the high cave‐air PCO₂ is the dominant control on the calcite deposition rate during the rainy season. Our previous study on the dissolved organic carbon of drip water concluded that the thin luminescent bands in stalagmite laminae from Shihua Cave form during the rainy season. The lower calcite deposition rate during the rainy season further supports this suggestion. The significant intra‐seasonal variability of the calcite deposition rate implies that the seasonal bias of δ¹⁸O of stalagmites should be considered when stalagmite δ¹⁸O is used as a high‐resolution palaeoclimatic archive.     

新生碳酸钙沉积矿物形态的影响因素分析

石笋是古气候重建的重要地质载体,文石与方解石是石笋碳酸钙晶体的常见矿物形态。根据现代洞穴监测数据分析洞穴新生碳酸钙沉积物 (Active Speleothem: AS)的矿物形态的研究较少。本文在重庆武隆芙蓉洞三个滴水点 (MP2、MP5、MP9)下放置玻璃片,收集新生碳酸钙沉积物和滴水样品,监测新生碳酸钙沉积物矿物形态、滴水的Mg/Ca比值、pH、滴率和洞穴环境等指标,分析玻璃片正面和反面新生碳酸钙沉积物的δ18O、δ13C和Mg/Ca比值。研究发现:(1) MP2滴水点下的玻璃片正反面新生碳酸钙沉积物的矿物形态均为方解石;MP5和MP9滴水点的正面沉积方解石和文石-方解石混合两种情况,反面沉积文石-方解石,且反面文石多于正面。 (2) MP2滴水Mg/Ca比值小于MP5和MP9,说明滴水Mg/Ca比值是影响新生碳酸钙沉积物矿物形态的重要因素;而滴水pH值对AS矿物形态的影响在不同滴水点有差异。(3) 不论是玻璃片正面还是反面,文石-方解石混合的新生碳酸钙沉积物δ18O和δ13C比以方解石为主的沉积物偏正,说明AS矿物形态的变化会导致δ18O和δ13C发生变化。通过在芙蓉洞的系统监测和分析,发现新生碳酸钙沉积物的矿物形态与地表环境、洞穴上部岩溶水文地质条件密切相关,并验证了洞穴新生碳酸钙沉积物的矿物形态对石笋δ18O和δ13C具有重要影响。      

Controls, variation, and a record of climate change in detailed stable isotope record in a single bryozoan skeleton

The long-lived (about 20 yr) bryozoan Adeonellopsis sp. from Doubtful Sound, New Zealand, precipitates aragonite in isotopic equilibrium with seawater, exerting no metabolic or kinetic effects. Oxygen isotope ratios (δ¹⁸O) in 61 subsamples (along three branches of a single unaltered colony) range from −0.09 to +0.68‰ PDB (mean = +0.36‰ PDB). Carbon isotope ratios (δ¹³C) range from +0.84 to +2.18‰ PDB (mean = +1.69‰ PDB). Typical of cool-water carbonates, δ¹⁸O-derived water temperatures range from 14.2 to 17.5 °C. Adeonellopsis has a minimum temperature growth threshold of 14 °C, recording only a partial record of environmental variation. By correlating seawater temperatures derived from δ¹⁸O with the Southern Oscillation Index, however, we were able to detect major events such as the 1983 El Niño. Interannual climatic variation can be recorded in skeletal carbonate isotopes. The range of within-colony isotopic variability found in this study (0.77‰ in δ¹⁸O and 1.34 in δ¹³C) means that among-colony variation must be treated cautiously. Temperate bryozoan isotopes have been tested in less than 2% of described extant species — this highly variable phylum is not yet fully understood.