Methods and Technical Developments in Speleothem Science & Geochemical Modelling and Laboratory Experiments #2
Speleothems have been used as climate and environmental archives over many decades, but their full potential as multi-proxy, quantitative archives has not yet been explored. As the field expands, however, an exciting array of new approaches is becoming available, such as biomarker or DNA studies, ultra-high resolution methods, and novel isotope systems. In addition, continuing improvements in U-Th and U-Pb dating are allowing increasingly precise and accurate age control. With these techniques, new insights can be gained, including the re-assessment of more traditional methods, steps towards quantification of speleothem palaeoclimate records, and the reconstruction of previously elusive environmental processes with more confidence. This session welcomes all contributions that extend our toolbox for deciphering climate and environmental changes from speleothems, by developing and/or testing novel methods, improving existing methods, or re-assessing more traditional approaches.
The increasing number of high-resolution speleothem proxy records and detailed cave monitoring programs highlight the complexity of the processes affecting speleothem proxy signals. In particular, processes occurring in the soil and karst above the cave as well as inside the cave during precipitation of speleothem CaCO3, have for a long time been considered as generating “noise” superimposing the climate signal contained in the δ 18O values of the rainfall and drip water. In the recent decade, however, major progress has been achieved in quantitative modeling of the processes affecting speleothem proxy signals both in the karst and the cave. These models are not only useful to improve the understanding of a particular cave system, but also to test the general potential and limitations of speleothems for reconstruction of specific climate phenomena. In some cases, they may even allow us to relate karst and in-cave processes to surface climate and eventually utilize them for palaeoclimate reconstruction. All models require isotope fractionation factors and element distribution coefficients specifically determined for speleothems, which do not necessarily grow under conditions of stable isotope equilibrium. For this session, we welcome contributions on: quantitative descriptions of processes occurring in the soil and karst above the cave as well as during precipitation of CaCO3, and; laboratory experiments aiming to determine isotope fractionation factors and element distribution coefficients.
8:30am - 9:00am
17Oexcess in CaCO3 as a paleo-hydrology proxy
1Institute of Earth Sciences, Hebrew University, Israel; 2Geological Survey of Israel, Jerusalem, Israel
Carbonates are common paleoclimate archives in a variety of environments, with δ18O in land carbonates recording both temperatures and hydrological conditions. Whereas δ18O has long been used for that purpose, triple oxygen isotopes in CaCO3 have only recently been analysed. The parameter 17Oexcess is defined as a deviation of δ17O from a mass dependent relationship with δ18O, following a reference slope of 0.528 [ 17Oexcess = 106 [ln(δ17O/1000 + 1) – 0.528 ln(δ18O/1000 + 1)]. Carbonate 17Oexcess is expected to record the 17Oexcess of its parent water, which in turn may provide paleo-hydrological information related to relative humidity, in analogy to d-excess.
The reconstruction of past water 17Oexcess values from carbonate records requires characterization of the 17O fractionation between water and CaCO3. We derive this fractionation from a combination of laboratory precipitation experiments and analysis of freshwater mollusks, resulting in a fractionation slope θ (=ln17α/ln18α) of 0.5231±0.0003, with no discernible temperature dependence. Further, we test the relevance of this fractionation to speleothems, by characterizing modern rain and cave water together with speleothem carbonates in Soreq cave.
17Oexcess values of daily rainfall near Soreq cave vary over a wide range, but the mean of major rain storms is 54±13 permeg, similar to values observed in local spring water. This value is consistent with a Mediterranean source of moisture, where evaporation occurs under low relative humidity. Cave water have a slightly lower 17Oexcess of 38±12 permeg. Comparing cave water with modern and late Holocene speleothem carbonates results in θ of 0.5229, consistent with that observed in biogenic carbonates and laboratory precipitation, suggesting no significant dis-equilibrium effect in 17Oexcess. This fractionation slope, together with a speleothem specific 18α, is applied to ancient Soreq cave samples to reconstruct paleo-rainfall 17Oexcess.
9:00am - 9:15am
Refining the TEX86 paleothermometer in speleothems
1Max Plank Institute for Chemistry, Mainz, Germany; 2Northumbria University, Newcastle upon Tyne, UK; 3Universidade de São Paulo, São Paulo, Brazil; 4University of Bergen, Bergen, Norway; 5Université de Rouen, Mont-Saint-Aignan, France; 6University of Tübingen, Tübingen, Germany; 7Johannes Gutenberg-Universität, Mainz, Germany
The TEX86 index has shown great promise as paleotemperature proxy in different environmental settings, including speleothems. TEX86 measurements in modern speleothems from caves located in different climate settings show a strong correlation with mean annual cave and air temperatures. However, so far, the application of the TEX86 index to reconstruct past temperatures has been limited. One of the main limitations for a widespread application of this technique has been the large sample size used in previous studies (approximately 10 g of calcite) due to the low concentration of these organic compounds in speleothems. Here, we show that, in most speleothem samples, accurate TEX86 measurements can be obtained using around 1 g of speleothem calcite. We report new measurements of modern speleothems that extend the geographical coverage, as well as, the temperature range of existing calibrations. Our new measurements confirm the robustness of the speleothem TEX86 paleothermometer, and reduce the uncertainty of previous calibrations. In addition, we show that glacial-interglacial paleotemperature estimates obtained with this method agree well with those obtained with other techniques (e.g., clumped isotopes).
9:15am - 9:30am
Multi-proxy, localised reconstructions of climate and weathering from cave speleothem samples
1University of Oxford, United Kingdom; 2University of Mainz, Germany
Speleothems (secondary calcium carbonate formations) offer significant potential for recording environmental processes above caves, an area increasingly referred to as the Critical Zone. Speleothems grow for hundreds to millions of years, with absolute chronology from U-Th and U-Pb chronometers. Rainwater infiltrating the soil and flowing down through the cave responds to environmental processes. These complex environmental signals can be preserved within speleothem carbonates. Recent efforts to calibrate, model and interpret this complex geochemistry has progressed along multiple paths. Here we bring together recent examples, including: i) calibrating and using Li isotopes for reconstructing weathering intensity [1,2]; ii) the use of Ca isotopes for reconstructing changes in rainfall amount ; iii) additional information from laboratory experiments regarding growth-rate controls and aragonite partition coefficients. In future years, combining these proxies provides the potential of regional-scale input into climate, weathering and the chemical cycling of elements, on timescales from thousands to millions of years.
 C.C. Day et al. 2021. Lithium isotopes and partition coefficients in inorganic carbonates: proxy calibration for weathering reconstruction. Geochimica et Cosmochimica Acta. 305, 243-262  P.A.E. Pogge von Strandmann et al. 2017. Lithium isotopes in speleothems: Temperature-controlled variation in silicate weathering during glacial cycles. Earth and Planetary Science Letters. 469, 64–74.  R.A. Owen et al. 2016. Calcium isotopes in caves as a proxy for aridity: Modern calibration and application to the 8.2 kyr event. Earth and Planetary Science Letters, 443, 129–138.
9:30am - 9:45am
Evolution of tropical land temperature across the last glacial termination
1Department of Earth Sciences, University of Bergen, Norway; 2Bjerknes Centre for Climate Research, University of Bergen, Norway; 3Andalusian Institute of Earth Sciences, University of Granada, Spain; 4Department of Earth Sciences, University of Oxford, UK; 5Department of Earth Sciences, University of Cambridge, UK; 6Geophysical Institute, University of Bergen, Norway; 7Division of Geological and Planetary Sciences, California Institute of Technology, USA; 8Department of Earth and Atmospheric Sciences, Georgia Institute of Technology, USA
The tropical West Pacific hosts the warmest part of the surface oceans and has a considerable impact on the global climate system. Reconstructions of past temperatures in this region can elucidate climate connections between the tropics and poles and the sensitivity of tropical temperature to greenhouse forcing. However, influences on proxy signals lead to discrepancies between different types of proxy data, and reliable information from terrestrial archives is particularly sparse, causing ambiguity in deglacial tropical climate evolution.
In this study we constrain the magnitude and timing of land temperature change in the tropical West Pacific across the last deglaciation using a physical method based on speleothem fluid inclusions applied to a well-dated speleothem from Secret Cave (Gunung Mulu, Northern Borneo).
We show that the cave temperature increased by 4.4 ± 0.2 °C (2 SEM) from the Last Glacial Maximum to the Holocene, amounting to 3.6 ± 0.3 °C (2 SEM) when correcting for sea-level induced cave altitude change. The warming closely follows atmospheric CO2 and Southern Hemisphere warming. This contrasts with hydroclimate, as reflected by drip water δ18O, which responds to Northern Hemisphere cooling events in the form of prominent drying, while the temperature was rising. Our results thus show a close response of tropical temperature to greenhouse forcing, independent of shifts in the tropical circulation patterns.
9:45am - 10:00am
Was the Last Glacial Maximum dry? New insights from speleothem palynology
1School of Geography, Earth and Atmospheric Sciences, University of Melbourne, Australia; 2Department of Geological Sciences, University of Cape Town, Cape Town, South Africa
For the past ~half century, the climate of the last glacial maximum (LGM, ~21 ka) has usually been interpreted as substantially drier than today, in most low- to mid-latitude regions. This interpretation has been based largely on evidence from fossil pollen indicating widespread treelessness, along with evidence of increased dust deposition rates, and, in some regions, mobilisation of sand dunes that are currently stabilised by vegetation. However, there is an increasing awareness that the ‘arid’ character of glacial vegetation, and of LGM vegetation in particular, is partly an artefact of C3 plants’ reduced water-use efficiency under low atmospheric CO2. Nevertheless, LGM vegetation, and dust-based indicators, continue to be widely interpreted as self-evidently indicating arid LGM climates.
Speleothems don’t strongly support this glacial aridity paradigm, and may contradict it, in some regions where they grew through the LGM. Nevertheless, evidence of speleothem growth during the Late Pleistocene has so far had little influence on the glacial aridity paradigm. U-Th-dated speleothem palynology – the analysis of fossil pollen preserved within speleothems – has the potential to provide new insights into LGM hydroclimate, especially in seasonally dry, low- to mid-latitudes in which low organic content of Late Pleistocene wetland sediments has hindered the application of radiocarbon chronologies. Speleothem palynology can permit the development of LGM vegetation and climate records supported by firm radiometric age models, and can thus allow detailed interrogation of LGM hydroclimate even in regions where ‘conventional’ wetland-based palynology has failed for lack of suitable sediments, or because LGM pollen records are dominated by a small number of widespread, climatically uninformative plant taxa. In addition, the multivariate nature of pollen records may complement the climatic information gained from the C and O stable isotope records that dominate most speleothem palaeoclimatology.
Here, I will present new pollen records recovered from stalagmites that grew through the ‘extended’ Southern Hemisphere LGM (28-18 ka), from Naracoorte (South Australia) and Weelawadji and Mammoth caves (Western Australia), regions which today experience moderately dry mediterranean-type climates. The records show substantial changes in vegetation structure with reduced tree-cover, but pollen of climatically sensitive, moisture-demanding taxa demonstrates the persistence of current biomes, rather than replacement by more arid-adapted biomes. Quantitative reconstructions indicate that climatic moisture availability was at least as high as today, inconsistent with the idea of increased aridity at the LGM. These reconstructions are consistent with the most recent PMIP ensemble results, which indicate decreased LGM precipitation over Australia, but also decreased evaporation, resulting in higher precipitation minus evaporation at the LGM, over most of the continent