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Abstract To date, the most complete paleolake‐level reconstructions for the late Pleistocene water bodies that once occupied the Dead Sea depression have been based on the combination of dating of lake sediments and terrestrial materials. However, despite these major accomplishments, there is still limited spatial control regarding the water levels, suggesting some degree of uncertainty concerning the magnitude and rate of lake‐level changes. Here, we re‐examine the late Pleistocene lake‐level changes in the Dead Sea during the transition from paleolake Lisan to the present‐day Dead Sea. We rely on systematic dating of fossil stromatolites including 84 radiocarbon and 15 U‐series ages, stable‐isotope measurements, paleobiology, high‐resolution topography, and numerical modeling to assess lake‐level changes. Our results indicate that the highstand of paleolake Lisan was of shorter duration and the transition between Lake Lisan and the Dead Sea occurred at least 5 Kyrs earlier than previously indicated. By refining the timeline and accuracy of lake‐level positions during the transition paleolake Lisan—Dead Sea, our study offers new insights into the regional and local paleo‐climatic conditions during the last glacial period in this region.

Abstract Mountain glaciers are sensitive recorders of natural and human-induced climate change. Therefore, it is imperative to obtain a comprehensive understanding of the interplay between climate and glacier response on both short and long timescales. Here we present an analysis of oxygen and carbon isotope data from speleothems formed mainly below a glacier-covered catchment in the Alps 300,000 to 200,000 years ago. Isotope-enabled climate model simulations reveal that δ18O of precipitation in the Alps was higher by approximately 1 ‰ during interstadials compared to stadials. This agrees with interstadial-stadial amplitudes of our new speleothem-based estimate after correcting for cave-internal effects. We propose that the variability of these cave-internal effects offers a unique tool for reconstructing long-term dynamics of warm-based Alpine palaeoglaciers. Our data thereby suggests a close link between North Atlantic interstadial-stadial variability and the meltwater dynamics of Alpine glaciers during Marine Isotope Stage 8 and 7d.

Abstract Variations of atmospheric CO2 during the Pleistocene ice-ages have been associated with changes in the drawdown of carbon into the deep-sea. Modelling studies suggest that about one third of the glacial carbon drawdown may not be associated to the deep ocean, but to the thermocline or intermediate ocean. However, the carbon storage capacity of thermocline waters is still poorly constrained. Here we present paired 230Th/U and 14C measurements on scleractinian cold-water corals retrieved from ~ 450 m water depth off the Maldives in the Indian Ocean. Based on these measurements we calculate ∆14C, ∆∆14C and Benthic-Atmosphere (Batm) ages in order to understand the ventilation dynamics of the equatorial Indian Ocean thermocline during the Last Glacial Maximum (LGM). Our results demonstrate a radiocarbon depleted thermocline as low as -250 to -345‰ (∆∆14C), corresponding to ~ 500–2100 years (Batm) old waters at the LGM compared to ~ 380 years today. More broadly, we show that thermocline ventilation ages are one order of magnitude more variable than previously thought. Such a radiocarbon depleted thermocline can at least partly be explained by variable abyssal upwelling of deep-water masses with elevated respired carbon concentrations. Our results therefore have implications for radiocarbon-only based age models and imply that upper thermocline waters as shallow as 400 m depth can also contribute to some of the glacial carbon drawdown.

Abstract Previous glacial intervals were punctuated by abrupt climate transitions between cold (stadial) and warm (interstadial) conditions. Many mechanisms leading to stadial-interstadial variability have been hypothesized with ice volume being a commonly involved element. Here, we test to which extent insolation modulated stadial-interstadial oscillations occurred during the penultimate glacial. We present a replicated and precisely dated speleothem record covering the period between 200 and 130 ka before present from caves located in the European Alps known to be sensitive to millennial-scale variability. We show that the widely proposed relationship between sea level change and stadial-interstadial variability was additionally modulated by solar insolation during this time interval. We find that interstadials occurred preferentially near maxima of Northern Hemisphere summer insolation, even when sea level remained close to its minimum during peak glacial periods. We confirm these observations with model simulations that accurately reproduce the frequency and duration of interstadials for given sea-level and insolation forcing. Our results imply that summer insolation played an important role in modulating the occurrence of stadial-interstadial oscillations and highlight the relevance of insolation in triggering abrupt climate changes.

Abstract Proxy reconstructions suggest that mid-Holocene East African temperatures were warmer than today between 8 and 5 ka BP, but climate models cannot replicate this warming. Precessional forcing caused a shift of maximum insolation from boreal spring to fall in the mid-Holocene, which may have favored intense warming at the start of the warm season. Here, we use three Porites corals from Kenya that represent time windows from 6.55 to 5.87 ka BP to reconstruct past sea surface temperature (SST) seasonality from coral Sr/Ca ratios in the western Indian Ocean during the mid-Holocene. Although the Indian monsoon was reportedly stronger in the mid-Holocene, which should have amplified the seasonal cycle of SST in the western Indian Ocean, the corals suggest reduced seasonality (mean 3.2 °C) compared to the modern record (mean 4.3 °C). Warming in austral spring is followed by a prolonged period of warm SSTs, suggesting that an upper limit of tropical SSTs under mid-Holocene conditions was reached at the start of the warm season, and SSTs then remained stable. Similar changes are seen at the Seychelles. Bootstrap estimates suggest a reduction in SST seasonality of 1.3 ± 0.22 °C at Kenya and 1.7 ± 0.32 °C at the Seychelles. SST seasonality at Kenya corresponds to present-day SST seasonality at 55° E–60° E, while SST seasonality at the Seychelles corresponds to present day SST seasonality at ~ 65° E. This implies a significant westward expansion of the Indian Ocean warm pool. Furthermore, the coral data suggests that SST seasonality deviates from seasonal changes in orbital insolation due to ocean–atmosphere interactions.

Abstract Hydrocarbon seeps are common manifestations of gas leakage from the seafloor. However, the fate of methane seepage within the gas hydrate stability zone at active margins is poorly constrained. This study presents a 40‐thousand‐year record of hydrocarbon seepage archived by a ∼5‐m long core composed of authigenic carbonate from the Yam Seep area, Four‐Way Closure Ridge off SW Taiwan. Different carbonate microfacies could be distinguished: Consolidated microcrystalline aragonite representing lithified host sediments intercalated by pure aragonite present in 10–50 cm thick intervals in the core. These aragonite intervals are interpreted as having precipitated within former fractures in the host rock. High resolution U‐Th dating of these aragonites is interpreted to record the minimum age of the opening of these fractures. The chronology of aragonite precipitation throughout the core suggests a record of continuous seepage from ∼41 to 2 ka that fluctuated in intensity over this time period. The chronology of putative fracturing events and observed carbonate precipitation suggest (a) an active period of fracturing and seepage from ∼37 to 27 ka, (b) a more quiescent period from ∼27 to 16 ka, followed by (c) another active period from ∼16 to 12 ka. A schematic model illustrates the evolution of carbonate formation within the core influenced by faulting, fracturing, erosion, gas hydrate accumulation, and aragonite precipitation and provides a unique 40,000‐year‐old record of methane seepage and crucial insights into the dynamics of long‐term seepage systems at active margins.

Abstract Spring systems are efficient and fast precipitating non‐marine carbonate factories, but they are also prone to (early) diagenesis. Evidence of diagenesis in travertines remains controversial as the resulting textures have been considered primary in other deposits and the impact on geochemical/palaeo‐environmental signals is often poorly addressed. This study revisits the Y‐10 core, taken in 1967 at one of the upper terraces of Mammoth Hot Springs in Yellowstone National Park (USA). The travertine depositional facies in the upper 50 m of the core show a general distal to proximal facies trend upwards. To unravel the nature, impact and timing of early diagenesis, fabrics and geochemical signatures are also compared to modern hot spring carbonates. Secondary ion mass spectrometry and detailed microscopy proved powerful in determining the stable isotope signature of different cement generations, thereby detangling the fluid history. Diagenesis starts during spring activity, but these warm waters not only precipitate CaCO3 at the top, they also seem to have circulated through the porous spring fabrics below. It resulted in coarsening and homogenization of the primary textures through neomorphism of aragonite and calcite cementation, often resulting in new intracrystalline microporosity within the calcite crystals. Subsequent circulation of meteoric fluids led to dissolution and cementation, particularly near the top of the core. Careful interpretation of U/Th dates suggests that travertine deposition started at the beginning of the Holocene/end Pleistocene. Calculated depositional (precipitation) rates vary between 2 and 20 mm/year, assuming a constant rate between samples at different depths. Aragonite deposits are completely transformed to calcite below depths of 5–10 m, corresponding to a time span of 4,000 years. Core and petrography observations, together with dating, suggest that travertine strata below marine sandstones in the lower part of the core may relate to fault displacement after the onset of travertine deposition.

To date, the most complete paleolake‐level reconstructions for the late Pleistocene water bodies that once occupied the Dead Sea depression have been based on the combination of dating of lake sediments and terrestrial materials. However, despite these major accomplishments, there is still limited spatial control regarding the water levels, suggesting some degree of uncertainty concerning the magnitude and rate of lake‐level changes. Here, we re‐examine the late Pleistocene lake‐level changes in the Dead Sea during the transition from paleolake Lisan to the present‐day Dead Sea. We rely on systematic dating of fossil stromatolites including 84 radiocarbon and 15 U‐series ages, stable‐isotope measurements, paleobiology, high‐resolution topography, and numerical modeling to assess lake‐level changes. Our results indicate that the highstand of paleolake Lisan was of shorter duration and the transition between Lake Lisan and the Dead Sea occurred at least 5 Kyrs earlier than previously indicated. By refining the timeline and accuracy of lake‐level positions during the transition paleolake Lisan—Dead Sea, our study offers new insights into the regional and local paleo‐climatic conditions during the last glacial period in this region. Plain Language Summary: Index points, reflecting the unique position of the water levels in space and time, have been broadly used in marine realms to reconstruct past sea‐level changes. Here we study lake‐level variations using index points based on geomorphic and biological markers collected along the western and eastern coasts of the Dead Sea. Previous studies indicate that during the last glacial period (20–30 ka), the Dead Sea depression was filled by paleolake Lisan, which lasted 10 Kyrs. As it dried, dozens of paleo‐shorelines remained etched in the landscape. Lake level index points estimated from 99 radiometric ages in these paleo‐shorelines suggest that Lisan's highstand was shorter and the transition to the Dead Sea occurred 5 Kyrs earlier than previously thought. These novel results suggest an initial and wet period followed by drier conditions, altering the regional paleo‐climatic narrative during the last glacial period. Key Points: Lake paleo‐shorelines and fossil stromatolites are used to reconstruct lake‐level changes during the transition paleolake Lisan‐Dead SeaFossil stromatolites were dated using radiocarbon and U‐series applying temporal and vertical correctionsThe transition between paleolake Lisan and the Dead Sea occurred ∼5 Kyrs earlier than estimated by previous reconstructions [ABSTRACT FROM AUTHOR]

Abstract The Erdmannshöhle is located at the NE margin of the Dinkelberg plateau in SW Germany. With a length of 2315 m, it is the longest cave in the deep open karst area near the village of Hasel. Three main cave levels developed in moderately SW-dipping, thinly bedded and fractured limestones of the Upper Muschelkalk (Triassic). The youngest cave level containing the cave stream is still active. Eighteen samples for U/Th dating were taken from the oldest and the intermediate cave levels. At the oldest cave level, the ages range from 162 to 110 ka, indicating speleothem growth starting in the middle Beringen Glaciation and terminating at the end of the Eem Interglacial. At the intermediate cave level, the ages range from 100 to 12 ka, i.e. early Birrfeld Glaciation to Younger Dryas Stadial. The age dating shows that speleothem growth did not cease completely during long periods of harsh climate conditions during the Beringen and Birrfeld glaciations and that permafrost terminating speleogenesis and speleothem growth was thus probably established only temporarily over relatively short periods. A conceptual model for the Middle Pleistocene to Holocene development of the Erdmannshöhle is presented within the framework of modern Quaternary lithostratigraphy. This model facilitates a first correlation of the cave evolution with the Middle to Late Pleistocene depositional record in the Möhlinerfeld, which is a key area for modern Quaternary lithostratigraphy in Switzerland and Southern Germany. Thus, speleological and Quaternary research, both based on a regional scale data compilation, complement one another with respect to the timing of events and the landscape evolution during the Quaternary and Late Neogene.

In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (14C) dates. The data reveal an extreme case of very high and also climate-dependent DCF. For M1-5, an average DCF of 56.2±3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed-system carbonate dissolution conditions. Towards the end of the last glacial period, decreasing Mg∕Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and DCF as seen in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net infiltration after the Last Glacial Maximum (LGM) led to dense vegetation and thus lowered the DCF by increasing 14CO2 input into the soil zone. At the onset of the Younger Dryas (YD) a sudden change in DCF towards much higher, and extremely variable, values is observed. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite DCF on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular relevance for speleothem studies that aim to reconstruct past atmospheric 14C (e.g., for the purposes of 14C calibration), as these would rely on largely climate-independent soil carbon cycling above the cave.

The Late Holocene was characterized by several centennial-scale climate oscillations including the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period and the Little Ice Age. The detection and investigation of such climate anomalies requires paleoclimate archives with an accurate chronology as well as a high temporal resolution. Here, we present 230Th/U-dated high-resolution multi-proxy records (δ13C, δ18O and trace elements) for the last 2500 years of four speleothems from Bunker Cave and the Herbstlabyrinth cave system in Germany. The multi-proxy data of all four speleothems show evidence of two warm and two cold phases during the last 2500 years, which coincide with the Roman Warm Period and the Medieval Warm Period, as well as the Dark Ages Cold Period and the Little Ice Age, respectively. During these four cold and warm periods, the δ18O and δ13C records of all four speleothems and the Mg concentration of the speleothems Bu4 (Bunker Cave) and TV1 (Herbstlabyrinth cave system) show common features and are thus interpreted to be related to past climate variability. Comparison with other paleoclimate records suggests a strong influence of the North Atlantic Oscillation at the two caves sites, which is reflected by warm and humid conditions during the Roman Warm Period and the Medieval Warm Period, and cold and dry climate during the Dark Ages Cold period and the Little Ice Age. The Mg records of speleothems Bu1 (Bunker Cave) and NG01 (Herbstlabyrinth) as well as the inconsistent patterns of Sr, Ba and P suggests that the processes controlling the abundance of these trace elements are dominated by site-specific effects rather than being related to supra-regional climate variability.

Two years of cave monitoring investigate ventilation processes in Cueva Larga, a tropical cave in Puerto Rico. The cave is 1,440 m long with a large main passage (about 120,000 m3). Cave air pCO2 in the main passage varies seasonally, between 600 ppm in winter and 1,800 ppm in summer. The seasonal variability in cave pCO2 permits the estimation of a cave air exchange time of 36 ± 5 days and a winter ventilation rate of 3,300 ± 1,000 m3/day for the main cave passage. Calculations of virtual temperature and differences between cave and surface temperature indicate that the seasonal temperature cycle is the main driver of the alternation between a well-ventilated winter mode and a near-stagnant summer mode. The winter mode is characterized by a positive buoyancy contrast at night leading to maximal cave ventilation, while cave ventilation is at a minimum during summer. Between winter and summer, a transitional mode of partial cave ventilation is observed. On shorter time scales (diurnal to weekly), cave pCO2 is also influenced by atmospheric pressure but this variation is one order of magnitude lower than the seasonal pCO2 change. The cave morphology of Cueva Larga including its large volume, tubular shape and the obstructed cave entrance geometry are important boundary conditions for the observed ventilation patterns. Our findings emphasize that cave systems with varying morphology have to be studied individually in order to correctly describe ventilation processes.

Continental records with absolute dates of the timing and progression of climatic conditions during the Last Interglacial (LIG) from northern Europe are rare. Speleothems from northern Europe have a large potential as archives for LIG environmental conditions since they were formed in sheltered environments and may be preserved beneath ice sheets. Here, we present δ13C and δ18O values from speleothem Kf-21, from Korallgrottan in Jämtland (northwest Sweden). Kf-21 is dated with five MC-ICPMS U-Th dates with errors smaller than ~1 ka. Kf-21 started forming at ~130.2 ka and the main growth phase with relatively constant growth rates lasted from 127.3 ka to 124.4 ka, after which calcite formation ceased. Both δ13C and δ18O show rapid shifts but also trends, with a range of values within their Holocene counterparts from Korallgrottan. Our results indicate an early onset of the LIG in northern Europe with ice-free conditions at ~130 ka. Higher growth rates combined with more negative δ18O values between ~127.3 and 126.8 ka, interpreted here as warmer and more humid conditions, as well as indications of a millennial-scale cold spell centered at 126.2 ka, resemble findings from speleothem records from other parts of Europe, highlighting that these were regional scale climatic patterns.