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Title
Abstract
Experimental and Analytical Methods
Experimental Setup
Discussion
Results
Introduction
Results and Discussion
Conclusions
References
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Earth Sciences
Geology

Isotopes in Speleothems: Methods and Application

Profile image of mohammed el shenawymohammed el shenawy

2017

Abstract

Speleothems (cave carbonate deposits) have been recognized as a multi-proxy paleoclimate archive. Variations in carbon and oxygen isotopes in speleothems can record past climate changes (e.g., temperature, rainfall and vegetation) under isotopic equilibrium conditions. However, non-climatic noises caused by in-cave processes may affect these stable isotope records under non-equilibrium isotopic conditions. The Ph.D. Thesis-M. EL-Shenawy; McMaster University-School of Geography and Earth Sciences viii Table of Contents Abstract………………………………………………………………….……………….iii Acknowledgements……………………………………………………………………....vi Table of Contents……………………………………………………….……………...viii List of Figures……………………………………………………………….…………..xii List of Tables……………………………………………………...…………………….xiv List of Equations…………………………………………………………..….…………xv Contribution of Authors……………………………………………………….………xvi Chapter 1-Introduction……………………………………………………...………….1 Chapter 2-Carbon and oxygen isotope systematics in cave environments: Lessons from an artificial cave………………………………………………………..…………18 2.1 Abstract………………………………………………………………………………19 2.2 Introduction…………………………………………………………………..………21 2.3 Experimental and analytical Methods……………………………………….……….26 2.3.1 Experimental Setup………………………………………………………..……26 2.3.2 Preparation of solutions……………………………………..…………………29 2.3.3 Stable isotope and other analyses…………………………………………..….30 2.4 Results……………………………………………………………………..…………32 2.4.1 Mineralogy and weight of CaCO 3 precipitates ………………………………..32 2.4.2 DIC concentration, pH and isotopic compositions of drip water and cave CO 2 …………………………………………………………………………………...33 2.4.3 Carbon and oxygen isotope compositions in the precipitated calcite………….35 2.4.3.1 Carbon and oxygen isotope compositions of calcite precipitated on the upper watch glasses (stalagmite-like settings)……………………...……….35 2.4.3.2 Carbon and oxygen isotope compositions of calcite precipitated on the lower watch glasses (pool-like settings)………………………………..……36 2.5 Discussion……………………………………………………………………………37 2.5.1 Temperature and flow rate effects on stalagmite growth rate…………….…..39 Ph.D. Thesis-M. EL-Shenawy; McMaster University-School of Geography and Earth Sciences ix 2.5.2 Chemical and isotopic evolution of drip water…………………………….….39 2.5.2.1 Chemical evolution in drip water………………………………………….39 2.5.2.2 Carbon isotope evolution in drip water……………………………………40 2.5.2.3 Oxygen isotope evolution in drip water……………………………..……..42 2.5.3 Equilibrium carbon and oxygen isotope fractionations during slow calcite precipitation in poollike settings……………………………………...……….44 2.5.3.1 Equilibrium carbon isotope fractionation between calcite and DIC in lower watch glasses…………………………………………………….……44 2.5.3.2 Equilibrium oxygen isotope fractionation between calcite and H 2 O in lower watch glasses…………………………………………………….……46 2.5.4 Non-equilibrium carbon and oxygen isotope fractionations during fast calcite precipitation in stalagmitelike settings………………………………….49 2.5.4.1 Flow rate and temperature effects on carbon isotope compositions of the calcite precipitated on the upper watch glasses…………………………..…50 2.5.4.2 Flow rate and temperature effects on oxygen isotope compositions of the calcite precipitated on the upper watch glasses……………………………..55 2.5.5 Insights into the debate of equilibrium oxygen isotope fractionation between calcite and water in cave environments……………………………………………..58

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A multiple cave deposit assessment of suitability of speleothem isotopes for reconstructing palaeo-vegetation and palaeo-temperature
Steven G Driese

Sedimentology, 2014

The suitability of speleothems for interpreting palaeoclimate is typically determined by using either the Hendy Test, overlapping analysis or longterm 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 (δ 13 C: À10Á3& to À2Á2&) over a relatively short distance (ca 260 m). Drip water δ 18 O 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 CO 2 : (i) CO 2 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) δ 13 C (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 δ 18 O 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 δ 13 C 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.

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Stable isotopes in caves over altitudinal gradients: fractionation behaviour and inferences for speleothem sensitivity to climate change
Vanessa E Johnston, Andrea Borsato

Climate of the Past, 9, 99-118 (2013), 2013

""The interpretation of stable isotope ratios in speleothem calcite is complex, and only in a few cases, unequivocal relationships with palaeoclimate parameters have been attained. A major issue is temperature, which has an effect on both the isotope incorporation into calcite and on environmental processes. Here, a field approach is taken, by studying the isotopic composition of calcites from monitored caves located in steep altitudinal topography in the northern Italian Alps. These create a thermal gradient (3–12 °C) apt to study the effects of temperature on the speleothem isotope record. Our data indicate that the magnitude of oxygen isotope disequilibrium effects, calculated as an offset from the experimentally determined equilibrium, decreases with increased elevation (cooler temperatures) and faster drip rate. Carbon isotope values exhibit 13C enrichment at high altitudes (colder temperatures) and slow drip rates. The results obtained support modelling and laboratory cave analogue experiments that indicate temperature, drip rate, pCO2 and supersaturation are important factors controlling stable isotope fractionation, but also stress the significance of ventilation and evaporation in the cave environment. It is proposed that the effects on stable isotope ratios observed along the altitudinal gradient can be analogues for glacial to interglacial temperature changes in regions which were extensively glaciated in the past.

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Modern C, O, and H isotope composition of speleothem and dripwater from Modrič Cave, eastern Adriatic coast (Croatia)
Maša Surić

International Journal of Speleology, 2010

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Climatic and environmental controls on speleothem oxygen-isotope values
Matthew Lachniet

Quaternary Science Reviews, 2009

Variations in speleothem oxygen-isotope values (d 18 O) result from a complicated interplay of environmental controls and processes in the ocean, atmosphere, soil zone, epikarst, and cave system. As such, the controls on speleothem d 18 O values are extremely complex. An understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem d 18 O as paleoclimate and paleoenvironmental proxies, and is best complemented by study of site-specific cave processes such as infiltration, flow routing, drip seasonality and saturation state, and cave microclimate, among others. This review is a process-based summary of the multiple controls on d 18 O in the atmosphere, soil, epikarst, and speleothem calcite, illustrated with case studies. Primary controls of d 18 O in the atmosphere include temperature and relative humidity through their role in the multiple isotope ''effects''. Variability and modifications of water d 18 O values in the soil and epikarst zones are dominated by evaporation, mixing, and infiltration of source waters. The isotopically effective recharge into a cave system consists of those waters that participate in precipitation of CaCO 3 , resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state. Recent modeling, experimental, and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid CaCO 3 , and have implications for the 'Hendy' test. To assist interpretation of speleothem d 18 O time series, quantitative and semi-quantitative d 18 O-climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records. Finally, several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of d 18 O to reconstruct past climate changes in regions that have been typically poorly represented in paleoclimate records, such as tropical and subtropical terrestrial locations. The new approach to speleothem paleoclimatology emphasizes climate teleconnections between regions and attribution of forcing mechanisms. Such investigations allow paleoclimatologists to infer regional to global-scale climate dynamics.

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Simulated oxygen isotopes in cave drip water and speleothem calcite in European caves
Martin Werner, A. Mangini, Andrea Borsato

Interpreting stable oxygen isotope (18O) 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 18O 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 18O values of drip water and speleothem calcite to which we compare our simulated isotope values. For six of the seven caves modeled 18O 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 18O 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 18O variations on a spatially large (regional to global) scale. Modelled as well as measured European 18O 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.

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