A simplified isotope dilution approach for the U–Pb dating of speleogenic and other low-232Th carbonates by multi-collector ICP-MS

We describe a new method for the measurement of ratios by isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the dating of geologically young clean carbonates, particularly speleothems. The method is intended for materials containing little or no initial 232Th. We illustrate and validate the method with four examples ranging from 0.57 to 20 Ma. The new method is capable of applying the 235U–207Pb and 238U–234U–206Pb chronometers, common Pb and quantifiable residual disequilibrium permitting. These provide an alternative to the more widely used 238U–206Pb chronometer, which can be highly inaccurate for samples that are < ca. 20 million years old, owing to uncertainties in the excess initial 234U (hence, excess radiogenic 206Pb) commonly observed in speleothems

Permafrost-related hiatuses in stalagmites: Evaluating the potential for reconstruction of carbon cycle dynamics

Permafrost is widely present throughout the Northern Hemisphere high latitudes, and stores large amounts of carbon in the form of frozen soil organic matter. The response of permafrost regions to anthropogenic climate change remains uncertain, in part because of a lack of information on their response to past changes in global climate. Here we test the use of stalagmites from two caves in Siberia as a novel, precisely dated, and highly localised archive of past permafrost carbon cycle dynamics. Stalagmite growth at these sites is controlled by the presence/absence of permafrost above the cave over glacial-interglacial time scales. We target the transition layer between two subsequent growth phases (interglacials) and the interval directly following growth resumption after the last glacial in three stalagmites, as this is where a geochemical imprint of thaw-related processes in the frozen zone between surface and cave would be recorded. We apply a multi-proxy approach including carbon isotopes (δ13C and 14C) and trace element concentrations, combined with petrographic analyses and high-resolution U-Th chronology. Our dataset indicates complex growth patterns and possible intervals of microbial colonisation of the stalagmite surface in the transition layers. High-resolution U-Th ages confirm that the transition layer is not a single, long growth hiatus, but rather a period of extremely slow or episodic growth phases, possibly during “skipped” interglacials. However, we find no conclusive evidence for a geochemical signature related to permafrost degradation and related local carbon cycle dynamics, which might be related to insufficient sensitivity of the archive for high-frequency processes and/or insufficient measurement resolution

New Data from Shovakh Cave and Its Implications for Reconstructing Middle Paleolithic Settlement Patterns in the Amud Drainage, Israel

Abstract: This study presents the geoarchaeological and geochronological aspects of Shovakh Cave and the first comparative context to the nearby Amud Cave (~ 500 m downstream), providing an exceptional opportunity to explore the range of human behaviours within a small geographic area. Sediment samples from two newly excavated areas at the rear and entrance of the cave were analysed using infrared spectroscopy, micromorphology and phytolith analysis and dated through uranium-thorium and luminescence techniques. The rear of the cave shows carnivore activity and low artefact concentrations. It also exhibits a shift in sedimentation from wind-blown deposits to colluviation of terra rossa. Direct dating of the deposits associated with the Middle Palaeolithic (MP) occupation at this area could not be obtained due to the breccia forming at the lower part of the excavation area. However, the later phases of the Middle Palaeolithic occupation at this area gave an age estimate of 45.5 ± 3.7 ka. At the entrance of the cave, there are relatively more residues associated with human use of fire. Post-depositional processes in this area include decalcification of the upper layer, cementation of the lower layer and phosphatisation due to guano decomposition, which indicates that this area was probably roofed. The ages obtained in this area range between 67.5 ± 5.5 to 56.2 ± 5.9 ka, overlapping with the occupation time of Amud cave. The evidence from Shovakh Cave presents lower intensity of occupation compared to Amud, indicating variable modes of site use by humans in the Amud drainage during the Late Middle Palaeolithic

The SISAL database: a global resource to document oxygen and carbon isotope records from speleothems

Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide “out-of-sample” evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ 18O, δ 13C) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at https://doi.org/10.17864/1947.147

Evaluating model outputs using integrated global speleothem records of climate change since the last glacial

Although quantitative isotopic data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to use the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally-distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotopic values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model’s ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotopic data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on 18O values, the optimum period for the modern observational baseline, and the selection of an appropriate time-window for creating means of the isotope data for palaeo time slices

Quaternary paleoclimate of the north-eastern boundary of the Saharan Desert: reconstruction from speleothems of Negev Desert, Israel

Carbonate cave deposits (speleothems) from caves of the Israeli deserts were used to reconstruct the paleoclimate conditions of the northern boundary of SaharanArabian Desert. The deserts studied in this work are of two types: the Negev Desert in southern Israel, which is part of the Saharan-Arabian desert belt; and the rain shadow desert located east of Central Mountain Ridge (CMR) in the rift valleys of the Jordan River and Dead-Sea (Judean Desert). The presence of speleothems in numerous caves in these present-day arid regions indicates that humid climatic conditions (i.e., periods with positive effective precipitation/infiltration index) occurred in the past. In this study, the timing of the speleothem growth, as an indicator of periods of increased effective precipitation, was obtained by precise U-Th dating; the origin of rainfall, its amounts and the types of vegetation were examined from studies of the speleothem δ18O and δ13C values and the δD values of their fluid inclusions, and the sources of dust were studied through the Sr isotopic composition of the speleothems. The minimum precipitation amounts necessary to deposit speleothems are estimated to be: 200-275 mm/year during glacial periods and 300-350 mm/year during interglacial periods. Speleothem deposition in the Jordan Valley mainly occurred during the three last glacial periods, with minor deposition occurring during Termination II (~135 ka) and MIS-7 interglacial (225-205 ka), and no speleothem deposition taking place during the Holocene, the peak of last glacial maximum (~19 ka), and for most of the previous interglacial MIS-5 (130-75 ka). The δ18O and δ13C profiles of speleothems deposited between 67 ka and 25 ka in the Jordan Valley match the general isotopic trends of previously studied speleothems from central and northern Israel; suggesting a similar Eastern Mediterranean (EM) Sea source for the precipitation and similar climatic conditions. Decrease in temperature and evaporation, and the consequent increase in effective precipitation

Quaternary paleoclimate of the north-eastern boundary of the Saharan Desert: reconstruction from speleothems of Negev Desert, Israel

Carbonate cave deposits (speleothems) from caves of the Israeli deserts were used to reconstruct the paleoclimate conditions of the northern boundary of SaharanArabian Desert. The deserts studied in this work are of two types: the Negev Desert in southern Israel, which is part of the Saharan-Arabian desert belt; and the rain shadow desert located east of Central Mountain Ridge (CMR) in the rift valleys of the Jordan River and Dead-Sea (Judean Desert). The presence of speleothems in numerous caves in these present-day arid regions indicates that humid climatic conditions (i.e., periods with positive effective precipitation/infiltration index) occurred in the past. In this study, the timing of the speleothem growth, as an indicator of periods of increased effective precipitation, was obtained by precise U-Th dating; the origin of rainfall, its amounts and the types of vegetation were examined from studies of the speleothem δ18O and δ13C values and the δD values of their fluid inclusions, and the sources of dust were studied through the Sr isotopic composition of the speleothems. The minimum precipitation amounts necessary to deposit speleothems are estimated to be: 200-275 mm/year during glacial periods and 300-350 mm/year during interglacial periods. Speleothem deposition in the Jordan Valley mainly occurred during the three last glacial periods, with minor deposition occurring during Termination II (~135 ka) and MIS-7 interglacial (225-205 ka), and no speleothem deposition taking place during the Holocene, the peak of last glacial maximum (~19 ka), and for most of the previous interglacial MIS-5 (130-75 ka). The δ18O and δ13C profiles of speleothems deposited between 67 ka and 25 ka in the Jordan Valley match the general isotopic trends of previously studied speleothems from central and northern Israel; suggesting a similar Eastern Mediterranean (EM) Sea source for the precipitation and similar climatic conditions. Decrease in temperature and evaporation, and the consequent increase in effective precipitation

Quaternary paleoclimate of the north-eastern boundary of the Saharan Desert: reconstruction from speleothems of Negev Desert, Israel

Carbonate cave deposits (speleothems) from caves of the Israeli deserts were used to reconstruct the paleoclimate conditions of the northern boundary of SaharanArabian Desert. The deserts studied in this work are of two types: the Negev Desert in southern Israel, which is part of the Saharan-Arabian desert belt; and the rain shadow desert located east of Central Mountain Ridge (CMR) in the rift valleys of the Jordan River and Dead-Sea (Judean Desert). The presence of speleothems in numerous caves in these present-day arid regions indicates that humid climatic conditions (i.e., periods with positive effective precipitation/infiltration index) occurred in the past. In this study, the timing of the speleothem growth, as an indicator of periods of increased effective precipitation, was obtained by precise U-Th dating; the origin of rainfall, its amounts and the types of vegetation were examined from studies of the speleothem δ18O and δ13C values and the δD values of their fluid inclusions, and the sources of dust were studied through the Sr isotopic composition of the speleothems. The minimum precipitation amounts necessary to deposit speleothems are estimated to be: 200-275 mm/year during glacial periods and 300-350 mm/year during interglacial periods. Speleothem deposition in the Jordan Valley mainly occurred during the three last glacial periods, with minor deposition occurring during Termination II (~135 ka) and MIS-7 interglacial (225-205 ka), and no speleothem deposition taking place during the Holocene, the peak of last glacial maximum (~19 ka), and for most of the previous interglacial MIS-5 (130-75 ka). The δ18O and δ13C profiles of speleothems deposited between 67 ka and 25 ka in the Jordan Valley match the general isotopic trends of previously studied speleothems from central and northern Israel; suggesting a similar Eastern Mediterranean (EM) Sea source for the precipitation and similar climatic conditions. Decrease in temperature and evaporation, and the consequent increase in effective precipitation

An acetic-acid-based extraction protocol for the recovery of U, Th and Pb from calcium-carbonates for U-(Th)-Pb geochronology

A new method for the simultaneous recovery of U, Th and Pb from c. 0.5 g calcium carbonate samples for the purpose of U-(Th)-Pb geochronometry is presented. The protocol employs ion-exchange chromatography. Standard anion-exchange resin (AG 1-X8 100-200 mesh) is used as the static phase and 90% acetic acid is used as the mobile phase to elute the unwanted matrix components; dilute nitric acid is used to elute the U, Th, and Pb. Blanks of 1.8 pg Th, 6.4 pg Pb and 8.4 pg U have been obtained. The protocol has been evaluated by analysing the isotopic composition of U-Th-Pb separates obtained from an in-house standard (prepared from a natural speleothem) by MC-ICP-MS. An independently dated speleothem has also been reanalysed. Based on these tests, the extraction protocol has an acceptable blank and produces a Pb-separate sufficiently free of matrix-induced instrumental biases to be appropriate for U-Th-Pb chronology.</p