Cave Drip Water Research Articles

Tree age, site and climate controls on tree ring cellulose δ18O: A case study on oak trees from south-western France

A main concern of dendroclimatic reconstruction is to distinguish in the tree ring proxy the influence of the climate variables of interest from other controlling factors. In order to investigate age, site and climate controls on tree ring width and cellulose δ18O, measurements have been performed in nearby groups of young (145 years old) and older (310–405 years old) oak trees in south-western France, covering the period 1860–2010.Within a given site, inter-tree deviations are small, pointing to a common climatic signal. Despite a similar inter-annual variability, the average level of cellulose δ18O in the young tree group is ∼0.8‰ higher than in the old trees. Such offsets might be caused by different soil properties and differences in the fraction of the source water used by trees from different depths. The δ18O of water in the top soil layer is directly related to the current growing season precipitation, while deeper water can have a lower and more constant δ18O. Local cave drip waters at 10m depth indeed show a constant isotopic composition, which corresponds to pluri-annual mean precipitation.A 2‰ increasing trend is observed in cellulose δ18O of young trees in the first 30 years of growth, during a period when no trend is visible in older trees. This increase can be quantitatively explained by humidity gradients under the forest canopy, and a changing microclimate around the crown as trees grow higher.While relationships between tree ring width and climate appear complex, the isotopic composition of cellulose is strongly correlated with summer maximum temperature, relative humidity and evapotranspiration (r≈0.70). Weaker correlations (r≈0.40) are identified with precipitation δ18O from a 15-year long local record and from the REMOiso model output. These results imply that leaf water enrichment has a stronger control on the inter-annual variability of cellulose δ18O than the δ18O of precipitation.This study demonstrates the suitability of oak tree ring cellulose δ18O for reconstructing past summer climate variability in south-western France, provided that the sampling and pooling strategy accounts for the fact that trees from different sites and of different age can introduce non-climatic signals.

Seasonal climate signals (1990–2008) in a modern Soreq Cave stalagmite as revealed by high-resolution geochemical analysis

High-resolution isotopic and geochemical analyses in a modern (1990–2008) Soreq Cave stalagmite are compared to instrumental records of rainfall and dripwater from the cave, with the aim of determining how seasonal-resolution climate information is transmitted to speleothem geochemistry. In situ, micron-scale analysis of oxygen isotope ratios (δ18O) and trace elements by ion microprobe in combination with a continuous, linear traverse of trace element concentrations by laser-ablation ICP-MS (LA-ICP-MS) allow the definition of geochemical pathways within the cave. Fluorescent banding, imaged by confocal laser fluorescent microscopy (CLFM), as well as δ18O and trace element variations is used to define 18 annual growth increments. Reduced intensity of fluorescent banding and a change in trace element variability reflect the decrease in average rainfall from 628mm/yr (1990–1998) to 433mm/yr (1999–2008). During the wetter period before 1998, Pearson (r-value) and Spearman (ρ-value) correlation coefficients are >0.5 for ion microprobe analyses of the element pairings Sr–Y, Y–P, and Mn–Si. After the transition to the drier period in 1999, a different set of geochemical pairings have r- and ρ-values >0.5, including Mg–δ18O, Mg–Sr, and Sr–Ba. Principal component analysis of data from the adjacent LA-ICP-MS traverse identifies two primary underlying modes of trace element variability. Based on the ion microprobe correlations and principal component analyses, we suggest that a greater seasonal influx of particulate material into the cave during the wetter period (1990–1998) brought about greater P, Cu, Sr, Na, and U variability in the stalagmite. The co-variability of δ18O, Mg, Sr, and Ba is characteristic of the trace element pattern from the drier period (1999–2008) of growth when particulate transport is reduced. These findings support a two-reservoir model of Soreq Cave dripwaters. One reservoir displays a well-mixed “baseline” with a decadal residence time that supplies water to the cave year-round, probably from fine pores or grain-boundary films in the vadose zone. The second reservoir is seasonal rainwater enriched in organic acids, colloids, and small particles and is rapidly transmitted to the cave. Finally, the similar patterns of fluorescence intensity, P, and Cu concentrations support the hypothesis that fluorescent bands in Soreq Cave speleothems are caused by the influx of organic colloids.

Stable Carbon Isotope Variations in Cave Percolation Waters and their Implications in Four Caves of Guizhou, China

Abstract:Monitoring and sampling of main plants, soil CO2, soil water, bedrock, spring water, drip water and its corresponding speleothem were performed at four cave systems of Guizhou, Southwest China, from April 2003 to May 2004, in order to understand stable carbon isotope ratios variations of dissolved inorganic Carbon (DIC) in cave percolation waters (δ13CDIC) and their implications for paleoclimate. Stable carbon isotopic compositions and ions (Ca, Mg, Sr, SO4, Cl etc.) were measured for all samples. The results indicate that there are significant differences among the δ13CDIC values from inter‐cave, even inter‐drip of intra‐cave in the four caves. The δ13CDIC values from the Liangfeng Cave (LFC) is lightest among the four caves, where vegetation type overlying the cave is primary forest dominated by tall trees with lighter average δ13C value (−29.9‰). And there are remarkable differences in δ13CDIC values of different drip waters in the Qixing Cave (QXC) and Jiangjun Cave (JJC), up to 6.9‰ and 7.8‰, respectively. Further analyses show that the δ13CDIC values in cave drip waters are not only controlled by vegetation biomass overlying the cave, but also hydro‐geochemical processes. Therefore, accurate interpreting of δ13C recorded in speleothems cannot be guaranteed if these effects of the above mentioned factors are not taken into consideration.

Trace metal and carbon isotopic variations in cave dripwater and stalagmite geochemistry from northern Borneo

We investigate stalagmite trace metal ratios and carbon isotopic composition (δ13C) 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 δ13C 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 δ18O values close to the mean. We hypothesize that increased residence times translate into reduced variance in dripwater δ18O 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 δ13C 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 δ18O time series from the same stalagmites. The only shared signal among the three stalagmites' geochemical time series is a relative decrease of 1‰ in δ13C 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 δ13C exhibits some reproducible signals on glacial‐interglacial timescales.

Drip water Geochemistry of Niah Great Cave, NW Borneo, Malaysia: a base line study

A base line study was conducted to understand the geochemical variations of drip waters in Niah Great Cave, Sarawak, NW Borneo of Malaysia. Drip waters were collected at three different sites within the cave (D6: SW entrance enroute to painted Cave; D5: centre of the Great Cave, and D8: west mouth Great Cave entrance). The collected samples were analysed for temperature, pH, EC, major elements, non purgeable organic carbon, δ13C and δ18O. Mg/Ca and Sr/Ca ratios, ionic strength, partial pressure of CO2 and saturation indices of carbonate minerals have been used to dictate the geochemical signatures of drip water compositions. The data reveal a wide range of major ion chemistry at three different sites within the cave. The δ13C values in drip waters range from −10.58 to −10.09 ‰. The enriched and the depleted δ13C values are noted at the entrances of the cave due to degassing of water CO2 and warmer air results in lighter fractions of δ13C. The δ18O values range from −6.89 to –6.16 ‰. Minor δ18O deviation in the drip waters are due to the variation in drip rates as faster drip rates show depleted isotopic composition and vice versa. The spatial variability of drip water chemistry results due to water–rock interaction at various conditions of subsurface flow path. This study gives a basis for interpreting the varying controls on drip water chemistry in the cave.

Speleothem trace element signatures: A hydrologic geochemical study of modern cave dripwaters and farmed calcite

Trace element variations in ancient cave speleothems are often interpreted as indicators of changes in paleo-rainfall and hydrologic conditions. However, these records are difficult to interpret without an understanding of the physicochemical controls on stalagmite chemistry plus site-specific calibration of changes in net rainfall to variations in dripwater and speleothem chemistry. In this study we examine geochemical relationships between net rainfall (Precipitation minus Evapotranspiration; P−ET), drip rates, drip water chemistry, and contemporaneous calcite chemistry to test the hypothesis that speleothem Mg/Ca and Sr/Ca records are proxies for rainfall amount.HRC is contained within four low-magnesium limestone units capped sporadically by a remnant dolomitic limestone. Aqueous concentrations of magnesium (post evapotranspiration) decrease with increasing vertical travel distance between the soil zone and the point of in-cave drip emergence (Drip Path Length – DPL) as dissolved high-Mg solutions sourced from the dolomitic caprock are diluted with dissolved low-Mg limestone waters sourced from the host limestone. Dripwater Mg/Ca and Sr/Ca ratios covary and provide diagnostic indicators of the two dominant mechanisms controlling dripwater chemistry: (1) mixing of post-evaporative solutions derived from two geochemical endmembers (dissolution of dolomite and limestone); and (2) evolution of hydrochemistry away from dissolved bedrock compositions due to Prior Calcite Precipitation (PCP) above the drip sites. By resolving the linear mixing relationships for drip water Mg/Ca and Sr/Ca sources and the distribution coefficients for trace element transfer in the PCP dripwater-to-calcite precipitation reactions and applying these principles to our time series, we find that the extent of PCP production within the karst is directly controlled by the balance between Precipitation (P) and Evapotranspiration (ET): higher net rainfall (P−ET>1: wet conditions) reduces PCP, and lower net rainfall with increased evapotranspiration (P−ET<1) increases PCP.Farmed calcite X/Ca ratios faithfully track hydrologically-influenced seasonal variations in dripwater chemistry for X=Mg, Sr, and Na. However, the relationship between changes in net rainfall and changes in Mg/Ca and Sr/Ca ratios in modern calcite is unique at each site and differs significantly at closely-spaced drip/stalagmite locations. This suggests that in situ modern hydrochemical calibrations should be performed atop individual speleothems prior to harvesting for paleoclimate investigations, and that such calibrations may not reflect past conditions as drip paths change. We apply this understanding to published dripwater data and speleothem time series from other caves. A major implication is that in order to interpret stalagmite Sr/Ca and Mg/Ca ratios as ‘wet vs. dry’ proxies, speleothem Sr/Ca and Mg/Ca variations must be coherent and in-phase over all time periods (i.e., Sr/Mg ratios must be constant). These criteria will help to distinguish ‘rainfall amount’ versus ‘rainfall source’ in speleothem δ18O records.

Controls on trace-element partitioning in cave-analogue calcite

We report trace-element data from a series of carbonate growth experiments in cave-analogue conditions in the laboratory with the goal of better understanding environmental controls on trace-element incorporation in stalagmites. The experimental setup closely mimics natural processes (e.g. precipitation driven by CO2-degassing, low ionic strength solution, thin solution-film) but with a tight control on growth conditions (temperature, pCO2, drip rate, calcite saturation index and the composition of the initial solution). Calcite is dissolved in deionized water in a 20,000ppmV pCO2 environment, with trace-elements (Li, Na, Mg, Co, Sr, Cd, Ba, U) at appropriate concentrations to mimic natural cave drip-waters. This solution is dripped onto glass plates (coated with seed-calcite) in a lower pCO2 environment at 7, 15, 25 and 35°C and drip rates of 2, 6 and 10 drips per minute. D(Sr) was shown to be statistically invariant over the full range of temperature and growth rate studied. No relationship between Sr/Ca and growth rate is therefore expected in stalagmite samples over comparable growth rates. D(Mg) has a relationship with temperature defined by D(Mg)=0.01e0.02[±0.006]T, but temperature is not expected to be the dominant control on Mg/Ca in cave calcite due to the larger impact of calcite precipitation on Mg/Ca. Over short timescales, in conditions where temperature is well buffered, the fraction of calcium remaining in solution (f) is likely to be the dominant control on Mg/Ca and other trace-element ratios. But differences in the response of trace-elements to f and T may allow their combined use to assess past cave conditions. High Cd/Castalagmite is particularly indicative of low amounts of prior calcite precipitation and Cd/Ca would be a useful addition to trace-element studies of natural stalagmites. Significant scatter is observed in trace-element ratios during the laboratory experiments, which cannot be explained by simple Rayleigh distillation. This scatter is well explained by solution mixing and by the mixing of calcite with different fractions of calcite growth. Accounting for the effects of mixing on trace-element concentrations or ratios may help to achieve more robust interpretations of stalagmite chemistry as part of a multi-proxy approach to assessment of past environments.

Temporal and spatial variations in hydro-geochemistry of cave percolation water and their implications for four caves in Guizhou, China

Soil water and cave drip water from four cave systems in Guizhou, Southwest China, were monitored and sampled monthly from April 2003 to May 2004 to understand spatio-temporal variations in hydro-geochemistry of cave percolation water. The results indicated that among the 5 drip water sites from the Liangfeng Cave (LFC), there were no significant differences among the hydro-geochemical parameters. In the Xiniu Cave (XNC), the drip rates were variable and responded quickly to precipitation events in the 3# (No. 3) drip site with variable water head; both bulk and variation in drip rates were smaller in the 2# with constant water head. However, in the Qixing Cave (QXC) and Jiangjun Cave (JJC), drip rates, concentrations of major ions and saturation index for calcite (SIC) were smaller, and Mg/Ca ratios in Group I (1#, 2#, 6#, 7# and 8# in the QXC; 2# and 3# in the JJC) were larger than those in Group II (3#, 4#, 5# and 9# in the QXC; 1# and 4# in the JJC). These differences might be the result of different hydrogeological processes above the caves, which are divided into five categories based on hydro-geochemistry data. The formation of some proxies in speleothem, such as Mg/Ca, is likely to be affected by those processes.

Anthropogenic impact records of nature for past hundred years extracted from stalagmites in caves found in the Nanatsugama Sandstone Formation, Saikai, Southwestern Japan

In the Nanatsugama area, Saikai City, Nagasaki Prefecture, Japan, covered by Paleogene calcareous sandstone, the environmental change information for the past hundred years was extracted from growing stalagmites in two limestone caves. Their annual microbanding information was used for dating. From the Shimizu-do Cave stalagmites, the vegetation change from forest to grassland during 1500 to 1700 could be read using the carbon isotope and Mg/Ca ratios of the stalagmites. Before 1500, the stable carbon isotope ratios ranged from −9 to −10‰, which are characteristic of forest vegetation. From 1600 to 1700, the stable carbon isotope ratio increased (δ13C=−2‰), suggesting a drastic change to grassland vegetation probably for the purpose of collecting grasses for agricultural use. The increase in Mg/Ca due to the reduction in the biomass from 1500 to 1700 also showed the vegetation change from forest to grassland. A Ryuo-do Cave stalagmite recorded the change in the SO42− concentration several times during the period from 1600 to 1900. In addition to the sulfate of sea salt origin, the higher concentration and smaller δ34S of sulfate in the Ryuo-do Cave drip water compared to those in the Shimizu-do Cave drip water may be due to the oxidation of biogenic pyrite in the marine Kamashikiyama Tuff Formation, which covers the calcareous sandstone of the Nanatsugama Sandstone Formation. The oxidation of pyrite is promoted by the biological activity of sulfur-oxidizing bacteria under oxic conditions, indicating that the surface cover was repeatedly changed into non-paddy fields. The change in the SO42− concentration could then be related to the changes in the amount of the Nagasaki Prefecture coal production in the early 20th century and the China fossil fuel consumption in the late 20th century transported a long-distance by a monsoon from the China continent. Thus, the combination of annual microbanding information and the Mg2+ and SO42− concentrations and C and S stable isotope ratios of the stalagmites made it possible to extract local and/or global anthropogenic environmental changes in nature. The records extracted in this way were in good agreement with those partially extracted from ancient documents, ancient picture maps and topographical maps.

Climatic variability in Central Indian Himalaya during the last ∼1800 years: Evidence from a high resolution speleothem record

Stable isotopes from a U/Th dated aragonite stalagmite from the Central Kumaun Himalaya provide evidence of variation in climatic conditions in the last ∼1800 years. The δ18O and δ13C values vary from −4.3‰ to −7.6‰ and −3.4‰ to −9.1‰ respectively, although the stalagmite was not grown in isotopic equilibrium with cave drip water, a clear palaeoclimatic signal in stalagmite δ18O values is evident based on the regional climate data. The stalagmite showed a rapid growth rate during 830–910 AD, most likely the lower part of Medieval Warm Period (MWP), and 1600–1640 AD, the middle part of Little Ice Age (LIA). Two distinct phases of reduced precipitation are marked by a 2‰ shift in δ18O values towards the end of MWP (∼1080–1160 AD) and after its termination from ∼1210 to 1440 AD. The LIA (∼1440–1880 AD) is represented by sub-tropical climate similar to modern conditions, whereas the post-LIA was comparatively drier. The Inter Tropical Convergence Zone (ITCZ) was located over the cave location during wetter/warmer conditions. When it shifted southward, precipitation over the study area decreased. A prominent drop in δ18O and δ13C values during the post-LIA period may also have been additionally influenced by anthropogenic activity in the area.

Paleoclimate reconstruction during MIS5a based on a speleothem from Nerja Cave, M&amp;#225;laga, South Spain

Speleothems from Nerja Cave in southern Spain provide a record during interglacial period MIS5a. Period of speleothem deposition occurred from 70,000 90,000 yr ago. Oxygen (δ18O) and hydrogen (δD) isotope ratios of speleothem and fluid inclusions enable the reconstruction of climatic variability in this region of southern Spain. Fluid inclusions trapped in speleothems represent samples of drip water from which the speleothems grew. The isotopic compositions of cave dripwaters approximate average annual δ18O and δD of precipitation, therefore δ18O can be calculated from D/H of inclusion water using the MWL relationship δD = 8δ18O + 10. The measurements of the δD values of fluid-inclusion water and δ18O values from speleothems have been applied to paleoclimate reconstruction in Southern Spain indicating a colder condition than at present.

Disequilibrium carbon and oxygen isotope fractionation in recent cave calcite: Comparison of cave precipitates and model data

Speleothem proxy data provide important information in continental palaeo-climate research due to their precise chronology and wide geographic distribution. Despite a continuously growing number of field and numerical studies designed to study stable isotope fractionation effects, many aspects remain a matter of debate. Here, carbon and oxygen isotope ratios from cave drip water and calcite precipitates sampled on watch glasses in the Bunker Cave (Western Germany) are compared with the values expected for isotopic equilibrium. Furthermore, the field data are compared with the results of a numerical model simulating stalagmite growth and stable isotope ratios.Two drip sites with different drip rates were investigated. Drip site TS 1 is characterised by a high drip rate, and drip water of this site is characterised by a high saturation index with respect to calcite. TS 1 shows no evidence of prior calcite precipitation. Conversely, drip site TS 8 shows a low drip rate, and drip water from this site is characterised by a low saturation index. TS 8 shows evidence of prior calcite precipitation. Whereas the mean δ13CDIC values of the drip water are significantly different between the two drip sites, the mean δ18Odrip water values are similar. Calcite precipitation rates are higher at watch glass site U I corresponding to drip site TS 1. This is probably due to the higher SIcalcite and drip rate. The δ18O and δ13C values of the calcite precipitated on watch glasses U IV corresponding to drip site TS 8 are significantly higher than those of the calcite collected on watch glasses U I. Calcite precipitation occurs close to isotopic equilibrium at watch glass site U I, whereas the calcite precipitated at watch glass site U IV shows clear evidence of isotopic disequilibrium.The results suggest that cave calcites precipitated at lower drip rates have higher δ13C and δ18O values than predicted by equilibrium precipitation. Furthermore, our data show that stalagmites from the same cave, which are fed by drip water with similar δ18O values, may have different δ18Ocalcite values.The δ13C and δ18O values predicted by the numerical model are generally lower by 1.9‰ and 0.8‰, respectively, than those of the natural cave calcite. Nevertheless, the first-order trends observed in the cave data are well reproduced by the model. The offset between cave and model data is probably due to the fractionation factors used in the model.

Simulated oxygen isotopes in cave drip water and speleothem calcite in European caves

Abstract. Interpreting stable oxygen isotope (δ18O) records from stalagmites is still one of the complex tasks in speleothem research. Here, we present a novel model-based approach, where we force a model describing the processes and modifications of δ18O from rain water to speleothem calcite (Oxygen isotope Drip water and Stalagmite Model – ODSM) with the results of a state-of-the-art atmospheric general circulation model enhanced by explicit isotope diagnostics (ECHAM5-wiso). The approach is neither climate nor cave-specific and allows an integrated assessment of the influence of different varying climate variables, e.g. temperature and precipitation amount, on the isotopic composition of drip water and speleothem calcite. First, we apply and evaluate this new approach under present-day climate conditions using observational data from seven caves from different geographical regions in Europe. Each of these caves provides measured δ18O values of drip water and speleothem calcite to which we compare our simulated isotope values. For six of the seven caves modeled δ18O values of drip water and speleothem calcite are in good agreement with observed values. The mismatch of the remaining caves might be caused by the complexity of the cave system, beyond the parameterizations included in our cave model. We then examine the response of the cave system to mid-Holocene (6000 yr before present, 6 ka) climate conditions by forcing the ODSM with ECHAM5-wiso results from 6 ka simulations. For a set of twelve European caves, we compare the modeled mid-Holocene-to-modern difference in speleothem calcite δ18O to available measurements. We show that the general European changes are simulated well. However, local discrepancies are found, and might be explained either by a too low model resolution, complex local soil-atmosphere interactions affecting evapotranspiration or by cave specific factors such as non-equilibrium fractionation processes. The mid-Holocene experiment pronounces the potential of the presented approach to analyse δ18O variations on a spatially large (regional to global) scale. Modelled as well as measured European δ18O values of stalagmite samples suggest the presence of a strong, positive mode of the North Atlantic Oscillation at 6 ka before present, which is supported by the respective modelled climate parameters.

Identifying short-term and seasonal trends in cave drip water trace element concentrations based on a daily-scale automatically collected drip water dataset

A 13-month long, daily-scale drip water dataset collected by an automatic water sampling device placed in a small cave in Ireland provides evidence for seasonal shifts in hydrochemistry, including a pulse of colloidally-associated elements in the late summer. Ca, Sr, Mg, Na, Ba, P, Cu, Zn, Rb, Y, Cs, U, Th, and Pb concentrations of the collected water samples were determined, and flow rates were also calculated. Alkali and alkali earth metals decreased in concentration during a summer water deficit, whereas colloidally-associated element concentrations increased during the same interval and spiked dramatically in the late summer/early autumn. The observed increase in colloidally-associated element concentrations may have coincided with increased breakdown of soil organic material by microorganisms in the late summer/early autumn, which led to an increased flux of organic colloids in the drip water, or it may relate to increased rates of dry deposition from cave air. The decrease in alkali and alkali earth metal concentrations in the summer most likely resulted from the addition of dilute water linked to condensation of water vapor above the drip site. Drip water Sr and P concentrations are anticorrelated, and their variation over the study period resembles seasonal trends observed in stalagmite calcite at other sites. Because the Sr minima and P maxima are interpreted as reflecting different phenomena (increased proportion of condensation water relative to karst water, and an increased flux of organic colloids, respectively), the relative timing of these two mechanisms in the past may explain the shifting polarity of their correlation observed in some stalagmite trace element records.

A comparative study on the stable isotopes from precipitation to speleothem in four caves of Guizhou, China

Monitoring and sampling of soil waters, and cave drip waters were performed monthly at four caves of Guizhou, Southwest China, from April 2003 to May 2004, in order to comparatively study the control factors of stable isotope in drip waters and their corresponding speleothems. δD and δ18O were measured for all samples including precipitation, soil waters, drip waters and modern speleothems. These results indicate that the amplitudes of the isotope values decrease gradually from precipitation to drip water in these caves, obviously suggesting the homogenization. Under certain conditions, it is possible to use speleothems to study high resolution (seasonal, even monthly) paleoclimatic changes in Liangfeng Cave (LFC) and Qixing Cave (QXC), but only low resolutions (more than a year) could be reached in Jiangjun Cave (JJC) and Xiniu Cave (XNC). The relationships between local meteoric water line (LMWL) and isotopic data of fluid inclusions in speleothems from the four caves can estimate local paleoclimate and paleoenvironment; however, it is debatable whether d-excess values can predict them. The effects of hydrological processes on the δ18O values in drip waters cannot be ignored. Hydrochemistry may cause isotopic disequilibrium in the formation of speleothems.

Response of a modern cave system to large seasonal precipitation variability

Speleothems are capable of providing information on the response of middle and low-latitude terrestrial environments to global climate change during the Pleistocene and Holocene. Multiproxy speleothem studies, however, have demonstrated that complex interactions can occur in cave settings between processes that are directly and indirectly related to climate change. Thorough and extended monitoring of modern cave environments is necessary in order to fully understand how each cave responds to these processes on seasonal and interannual timescales, and how environmental signals are preserved in speleothem carbonate.Regular environmental monitoring began at Black Chasm Cavern in the Sierra Nevada foothills, California, during the winter of 2006–2007. Monthly measurements of cave air temperature, humidity, and pCO2 in Black Chasm demonstrate that the cave is ventilated in the winter months, when cold, dense surface air sinks into the cave. Cave drip-water flow nearly ceases during the late summer and autumn, increases substantially during the winter and spring, and responds within hours to storm events during the height of the rainy season. Rainwater and drip water δ18O and δ2H are controlled by variations in surface air temperature and moisture source. While rainfall source influences rainwater isotopes through individual storm events, it has less influence on drip water isotopic composition due to mixing of recharge waters delivered by different rainfall events in the epikarst. Variations in drip water chemistry (δ13C, Mg/Ca, and Sr/Ca) indicate that the greatest level of calcite precipitation upflow from the drip water collection site (prior calcite precipitation) occurs during the autumn (October–November) when drip rates are slow and cave air pCO2 is low. The least prior calcite precipitation occurs during the summer (July–August) when drip rates are slow but cave air pCO2 is at a maximum. While pCO2 is a primary control on prior calcite precipitation during all seasons, the predominant influence of drip rate variability on prior calcite precipitation is evident when considering only those seasons (winter, spring, and autumn) characterized by low cave air pCO2. Thus, drip rate variability, and in turn rainfall amount, should provide the primary control on trace element variations ultimately captured in speleothem calcite. The isotopic and chemical variability observed in Black Chasm drip waters supports previous interpretations of speleothem paleoclimate proxy records from a nearby cave where such monitoring is not feasible. Observations of the modern cave environment at Black Chasm provide a reference point for the interpretation of stalagmite proxy records from similar seasonal (Mediterranean) climates.

Relationship between modern rainfall variability, cave dripwater, and stalagmite geochemistry in Guam, USA

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.

From soil to cave: Transport of trace metals by natural organic matter in karst dripwaters

This paper aims to establish evidence for the widespread existence of metal binding and transport by natural organic matter (NOM) in karst dripwaters, the imprint of which in speleothems may have important climatic significance. We studied the concentration of trace metals and organic carbon (OC) in sequentially filtered dripwaters and soil leachates from three contrasting sites: Poole's Cavern (Derbyshire, UK), Lower Balls Green Mine (Gloucestershire, UK) and Grotta di Ernesto (Trentino, Italy). The size-distribution of metals in the three soils was highly similar, but distinct from that found in fractionated dripwaters: surface-reactive metals were concentrated in the coarse fraction (>100nm) of soils, but in the fine colloidal (<100nm) and nominally dissolved (<1nm) fractions of dripwaters. The concentration of Cu, Ni and Co in dripwater samples across all sites were well correlated (R2=0.84 and 0.70, Cu vs. Ni, Cu vs. Co, respectively), indicating a common association. Furthermore, metal ratios (Cu:Ni, Cu:Co) were consistent with NICA-Donnan n1 humic binding affinity ratios for these metals, consistent with a competitive hierarchy of binding affinity (Cu>Ni>Co) for sites in colloidal or dissolved NOM. Large shifts in Cu:Ni in dripwaters coincided with high fluxes of particulate OC (following peak infiltration) and showed increased similarity to ratios in soils, diagnostic of qualitative changes in NOM supply (i.e. fresh inputs of more aromatic/hydrophobic soil organic matter (SOM) with Cu outcompeting Ni for suitable binding sites). Results indicate that at high-flows (i.e. where fracture-fed flow dominates) particulates and colloids migrate at similar rates, whereas, in slow seepage-flow dripwaters, particulates (>1μm) and small colloids (1–100nm) decouple, resulting in two distinct modes of NOM–metal transport: high-flux and low-flux. At the hyperalkaline drip site PE1 (in Poole's Cavern), high-fluxes of metals (Cu, Ni, Zn, Ti, Mn, Fe) and particulate NOM occurred in rapid, short-lived pulses following peak infiltration events, whereas low-fluxes of metals (Co and V>Cu, Ni and Ti) and fluorescent NOM (< ca. 100nm) were offset from infiltration events, probably because small organic colloids (1–100nm) and solutes (<1nm) were slower to migate through the porous matrix than particulates. These results demonstrate the widespread occurrence of both colloidal and particulate NOM–metal transport in cave dripwaters and the importance of karst hydrology in affecting the breakthrough times of different species. Constraints imposed by soil processes (colloid/particle release), direct contributions of metals and NOM from rainfall, and flow-routing (colloid/particle migration) are expected to determine the strength of correlations between NOM-transported metals in speleothems and climatic signals. Changes in trace metal ratios (e.g. Cu:Ni) in speleothems may encode information on NOM composition, potentially aiding in targeting of compound-specific investigations and for the assessment of changes in the quality of soil organic matter.

Transportation characteristics of δ 13C in the plants-soil-bedrock-cave system in Chongqing karst area

Here we use an analytical method to determine δ 13C in local plants and organic matter in the soils above Furong cave, Chongqing, China. We also monitored δ 13C in dissolved inorganic carbon (DIC) of drip water, δ 13C of active deposits under the drip waters, and the concentration of air CO2 (PCO2). Based on these, we preliminarily studied the transportation characteristics of stable carbon isotope (13C) in cave system of the subtropical karst area. The average δ 13C value of 27 local plant samples, which belong to 16 families, was −32‰ and the weighted δ 13C for surface dry biomass was −33‰. We found that for 54 soil samples collected from 5 soil profiles, δ 13C of soil organic matters was −22‰, which could be attributed to the different transportation rates of stable carbon isotopes during the decomposition of plants and organic matters in soils. The relatively lighter 12C tended to transfer into gaseous CO2, which made the relatively heavier 13C concentrated in the soils. On the basis of monitoring of DIC-δ 13C in drip waters from July 2009 to June 2010, we found that values in winter months were heavier and values in summer months were lighter in general, the reason of which was that in summer months, both the temperature and the humidity were comparatively higher, resulted in more CO2 with lighter δ 13C generated from organic matters decomposition and plants respiration. The average DIC-δ 13C value was −11‰, about 11‰ heavier than the δ 13C of organic matters in soils, which proved that part of DIC in cave drip water was sourced from dissolution of inorganic carbonate (host rock, with heavier δ 13C). As for the δ 13C of active deposits at five drip water sites in Furong cave, they had almost the same variation with relatively light values. In other words, these active speleothems were deposited at equilibrium conditions for isotopic fractionation. These results suggest that the carbon isotopic information of speleothems could be used to track the evolution of local vegetation in certain situations.

Continuous analysis of δ18O and δD values of water by diffusion sampling cavity ring‐down spectrometry: a novel sampling device for unattended field monitoring of precipitation, ground and surface waters

A novel sampling device suitable for continuous, unattended field monitoring of rapid isotopic changes in environmental waters is described. The device utilises diffusion through porous PTFE tubing to deliver water vapour continuously from a liquid water source for analysis of δ¹⁸O and δD values by Cavity Ring-Down Spectrometry (CRDS). Separation of the analysed water vapour from non-volatile dissolved and particulate contaminants in the liquid sample minimises spectral interferences associated with CRDS analyses of many aqueous samples. Comparison of isotopic data for a range of water samples analysed by Diffusion Sampling-CRDS (DS-CRDS) and Isotope Ratio Mass Spectrometry (IRMS) shows significant linear correlations between the two methods allowing for accurate standardisation of DS-CRDS data. The internal precision for an integration period of 3 min (standard deviation (SD) = 0.1‰ and 0.3‰ for δ¹⁸O and δD values, respectively) is similar to analysis of water by CRDS using an autosampler to inject and evaporate discrete water samples. The isotopic effects of variable air temperature, water vapour concentration, water pumping rate and dissolved organic content were found to be either negligible or correctable by analysis of water standards. The DS-CRDS system was used to analyse the O and H isotope composition in short-lived rain events. Other applications where finely time resolved water isotope data may be of benefit include recharge/discharge in groundwater/river systems and infiltration-related changes in cave drip water.