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A speleothem collected from Palco Cave (Puerto Rico) spans the 8.2 ka event, a time interval associated with fluctuations of Atlantic Ocean circulation and possible drying in the Caribbean region. While stalagmite δ18O, δ13C, and Mg/Ca data do not show a sustained change in mean state over the 8.2 ka event, the proxies provide robust evidence for three abrupt fluctuations toward drier conditions in rapid succession, each lasting less than two decades, occurring at 8.20, 8.14, and 8.02 ka BP. A cave monitoring program at Palco Cave supports the interpretation of the speleothem proxy records. Because changes in the position of the Intertropical Convergence Zone (ITCZ) are directly coupled to sea-surface temperature variations in the North Atlantic, we hypothesize that cold events in the North Atlantic temporarily limited the northward migration of the ITCZ and tropical rain belt in boreal summer during these abrupt drying periods. The speleothem record suggests that the 8.2 ka event was associated with rapid rainfall fluctuations in the northern Caribbean followed by a comparably warm and wet phase after the 8.2 ka event. This enhanced variability during the transitional period of the deglaciation appears to be linked to a fast coupling between interacting oceanic and atmospheric processes. This involves, in particular, the Atlantic Meridional Overturning Circulation in modulating interhemispheric heat transport. [ABSTRACT FROM AUTHOR]

We present speleothem fluid inclusion δ18 O f and δ2 H f values from Larga Cave, Puerto Rico, that cover the interval between 46.2 and 15.3 ka on the millennial scale, including the Last Glacial Maximum (LGM) and several stadial and interstadial cycles. The data set can be divided in two main clusters of stable isotope compositions of the fluid inclusion water with respect to the global meteoric water line (GMWL), which coincide with strong variations in the water content of the stalagmite. In particular, this clustering is found to be climate related, where one cluster comprises samples from cold and dry periods, such as the Heinrich and Greenland stadials (HSs and GSs), as well as parts of the LGM, which exhibit very high δ18 O f and δ2 H f values. We interpret this enrichment as being caused by evaporation inside the cave due to enhanced cave ventilation during these colder and drier times. In contrast, in most samples corresponding to warmer and wetter Greenland interstadials (GIs), but also for some from HS 2 and 3, the δ18 O f and δ2 H f values plot on the meteoric water line and modification of fluid inclusion water due to "in-cave" evaporation are found to be negligible. Consequently, variations of recent glacial hydro-climate and temperatures in the western tropical Atlantic can be constrained. In general, δ18 O f values from fluid inclusions are up to 3 ‰ higher than those of modern drip water, which is interpreted as a weaker atmospheric convective activity during the last glacial period. In addition, reconstructed temperatures suggest an average cooling of 2–3 ∘ C during the LGM compared to modern cave temperatures. Reconstructed cave temperatures yield an average cooling of - 1.4 ± 2.8 ∘ C for HS 2 and - 3.6 ± 2.2 ∘ C for HS 3. Higher δ18 O f values of these samples further suggest that the drip water was dominated by orographic rainfall and/or cold fronts, along with weak or even absent convective activity. In contrast, during interstadial phases, reconstructed temperatures reached nearly modern values, and convective activity was comparable to or only slightly weaker than today. [ABSTRACT FROM AUTHOR]

The sensitivity of tropical Atlantic precipitation patterns to the mean position of the Intertropical Convergence Zone (ITCZ) at different time scales is well‐known. However, recent research suggests a more complex behavior of the northern hemispheric tropical rain belt related to the ITCZ in the western tropical Atlantic. Here we present a precisely dated speleothem multi‐proxy record from a well‐monitored cave in Puerto Rico, covering the period between 46.2 and 15.3 ka. The stable isotope and trace element records document a pronounced response of regional rainfall to abrupt climatic excursions in the North Atlantic across the Last Glacial such as Heinrich stadials and Dansgaard/Oeschger events. The annual to multidecadal resolution of the proxy time series allows substructural investigations of the recorded events. Spectral analysis suggests that multidecadal to centennial variability persisted in the regional hydroclimate mainly during interstadial conditions but also during the Last Glacial Maximum. In particular, we observe a strong agreement between the speleothem proxy data and the strength of the Atlantic meridional overturning circulation, supporting a persistent link of oceanic forcing to regional precipitation. Comparison to other paleo‐precipitation records enables the reconstruction of past changes in position, strength, and extent of the ITCZ in the western tropical Atlantic in response to millennial‐ and orbital‐scale global climate change. Plain Language Summary: It is important to understand the climatic circumstances of how rainfall in the western tropical Atlantic varies under a changing climate to better manage the water supply for millions of people. However, it is not well understood how rainfall varied in the past, especially during the Last Glacial period, a time of strong climate variability and abrupt climate changes. Here, we use a stalagmite from Puerto Rico to create a new record of past changes in rainfall in this region. For this purpose, we analyzed proxy data that reveal a series of wet and dry periods during the Last Glacial corresponding to rapid global climate shifts. Our rainfall‐sensitive stalagmite record captured changes of the tropical rain belt on various timescales and shows that this variability in rainfall is closely connected to changes in the strength of the ocean circulation. This suggests that the link between the ocean and the atmosphere is more robust than previously assumed. The comparison of our record with other rainfall‐sensitive records from Central America and the northern Caribbean allows for a detailed reconstruction of the spatial and temporal changes of the western tropical Atlantic rain belt. Key Points: Puerto Rican speleothem record documents multidecadal to millennial‐scale precipitation variability between 46.2 and 15.3 kaClimate proxies show a distinct rainfall response to abrupt North Atlantic climate change including Heinrich and Dansgaard/Oeschger eventsCompilation of regional precipitation records allows to reconstruct past changes in ITCZ patterns in the western tropical Atlantic [ABSTRACT FROM AUTHOR]

We present a new speleothem trace element and stable isotope record, which extends previous paleoclimate evidence from Cuban speleothems to the last 96 ka. Multiple proxies were used to reconstruct Caribbean hydroclimate and vegetation variability on orbital to millennial timescales. In particular, our proxies indicate a shift to more abundant C4 (compared to C3) vegetation and/or reduced soil activity during the Last Glacial Maximum. Our results confirm the close link of Caribbean precipitation to North Atlantic climate variability, insolation and the strength of the AMOC. The associated variations in Caribbean SSTs and the shift of the ITCZ led to substantial changes of the hydrological cycle. During Heinrich stadials and the Younger Dryas, climatic conditions in Cuba were comparably cool and/or dry. In contrast, warm Dansgaard/Oeschger cycles were accompanied by wetter conditions. This is in agreement with previous evidence for the Holocene, when North Atlantic cold (warm) events coincided with drier (wetter) conditions in Cuba. However, our record suggests that this connection was less dominant during MIS 4 to 2, displayed by a weaker response of Cuban precipitation proxies to North Atlantic climatic shifts than during MIS 5 in particular. We hypothesize that this weaker connection was mediated by the southerly shifted ITCZ inducing a weakening of the northern branch of the Caribbean Low Level Jet which usually transports moisture into the northern Caribbean. This is strikingly evident during the deglacial, when driest and/or coolest conditions prevailed in western Cuba from HS 1 into the Younger Dryas, including the Bølling/Allerød interstadial. • We present a speleothem stable isotope and trace element record from Cuba covering the last 96 ka. • Our results confirm the link of Caribbean precipitation to NA climate, insolation and the AMOC. • Last glacial NA cold (warm) (inter)-stadials coincided with drier (wetter) conditions in Cuba. • During the LGM, our data suggest a shift to more abundant C4 vegetation and/or reduced soil activity. • We observe a weaker sensitivity to NA climate during MIS 4 to 2 compared to MIS5 and the Holocene. [ABSTRACT FROM AUTHOR]

Abstract Here we present novel cave-analogue experiments directly investigating stable carbon and oxygen isotope fractionation between the major involved species of the carbonate system (HCO 3 −, CO 2 , CaCO 3 and H 2 O). In these experiments, which were performed under controlled conditions inside a climate box, a thin film of solution flew down an inclined marble or glass plate. After different distances of flow and, thus, residence times on the plate, pH, electrical conductivity, supersaturation with respect to calcite, precipitation rate as well as the δ18O and δ13C values of the dissolved inorganic carbon (DIC) and the precipitated CaCO 3 were obtained. Progressive precipitation of CaCO 3 along the plate is accompanied by degassing of CO 2 and stable isotope fractionation, and the system is driven out of isotope equilibrium. We observe a strong enrichment of the δ13C values with increasing residence time and a smaller enrichment in δ18O. The temporal evolution of the δ18O and δ13C values of both the DIC and the precipitated CaCO 3 can be explained by a Rayleigh fractionation model, but the observed enrichment in δ13C values is much larger than expected based on isotope equilibrium fractionation factors. Our setup enables to determine the fractionation between CaCO 3 and HCO 3 −, i.e., ε CaCO3/HCO3−. Carbon isotope fractionation, 13ε CaCO3/HCO3− , is strongly negative for all experiments and much lower than equilibrium isotope fractionation (0–1‰). In addition, 13ε CaCO3/HCO3− decreases with increasing residence time on the plate, and thus decreasing supersaturation with respect to calcite. Thus, isotope fractionation depends on precipitation rate and consequently occurs under kinetic conditions. This is in contrast to previous studies, which found no rate-dependence and no or even a positive carbon isotope fractionation between CaCO 3 and HCO 3 −. Oxygen isotope fractionation, 18ε CaCO3/HCO3− , is also negative and dependent on precipitation rate. Since no literature values for 18ε CaCO3/HCO3− are available, we calculated 18ε CaCO3/HCO3− using equilibrium oxygen isotope fractionation factors between water and calcite and water and HCO 3 −, respectively. At the beginning of the plate, the fractionation is in agreement with the fractionation calculated using fractionation factors determined in cave systems. The observed fractionation between CaCO 3 and water, 1000ln18α, is also in good agreement with the values determined in cave systems and shows a very similar temperature dependence 1000 ln 18 α = 16.516 ± 1.267 ∗ 10 3 T − 26.141 ± 4.356. However, with progressive precipitation of CaCO 3 along the plate, the system is forced out of isotope equilibrium with the water, and 1000ln18α increases. The large, negative, rate-dependent isotope fractionations observed in this study suggest that precipitation of speleothem calcite is strongly kinetically controlled and may, thus, have a large effect on speleothem δ18O and δ13C values. Since these values may erroneously be interpreted as reflecting changes in past temperature, precipitation and/or vegetation density, these results have important implications for paleoclimate reconstructions from speleothems. [ABSTRACT FROM AUTHOR]

Stable oxygen and carbon isotope ratios (d13C and d18O) are the most applied climate and environmental proxies in speleothems allowing to infer past changes in cave drip water d13C and d18O related to climate and environmental variations from above the cave. However, disequilibrium isotope fractionation processes can modify d13C and d18O values in speleothems, which is in most cases difficult to estimate due to inter-dependencies on various cave specific parameter. To better understand the effect of these disequilibrium isotope fractionation processes proxy system models were developed in recent years, such as the ISOLUTION model. Here the code of the ISOLUTION model is made available for the public and the speleothem community to be applied to research questions that arise from e.g. monitoring programs that investigate d13C and d18O values of in situ calcite precipitates on watch glasses or modern speleothem calcite, respectively. Another application of the ISOLUTION model is to investigate the dependence of calcite d13C and d18O on the variation of one or multiple cave specific parameter, such as cave air temperature, drip interval, cave air pCO2, Ca2+ concentration of the drip water as well as on relative humidity and wind velocity. This allows to quantitatively estimate the effect of disequilibrium isotope fractionation processes in individual caves and drip sites on speleothem d13C and d18O values for modern and past climates and may help to further elucidate the complex interplay of kinetic and disequilibrium isotope fractionation. [ABSTRACT FROM AUTHOR]

Here, we present high-resolution trace element and stable isotope records from three coeval Holocene stalagmites from the Herbstlabyrinth cave system, Central Germany. All stalagmites were precisely dated using MC-ICPMS 230Th/U-dating. One stalagmite started to grow at 13.62 ± 0.13 ka BP, covering the late Glacial; the other two speleothems started to grow at 11.13 ± 0.08 and 10.26 ± 0.08 ka BP, respectively. The combined record covers the entire Holocene. The interpretation of the different climate proxies is supported by data from a detailed cave monitoring programme. Cold conditions during the Younger Dryas are reflected by intermittent stalagmite growth at the Herbstlabyrinth. The δ18O records are in general agreement with the NGRIP δ18O record on millennial time scales indicating that speleothem δ18O values at the Herbstlabyrinth reflect large-scale climate variability in the North Atlantic area. The 8.2 ka event is clearly visible as a pronounced negative excursion in the δ18O records. In all other proxies, it is not reflected as a major excursion. Correlation and principal component analysis enable us to disentangle the various processes affecting the stable isotope and trace element signals. Phases with higher P, Ba and U concentrations and more negative δ13C values are interpreted as reflecting more productive vegetation above the cave. The negative correlation of Mg with P, Ba and U and the positive correlation with δ13C indicate more recharge during phases of more productive vegetation, probably because of increased rainfall. The majority of the observed phases of reduced vegetation productivity and drier climate coincide with cooler periods in the polar North Atlantic as reflected by a higher abundance of hematite-stained grains (i.e. the Bond events), suggesting a close relationship between terrestrial climate in Central Europe and the polar North Atlantic. [ABSTRACT FROM AUTHOR]

We present cave monitoring data and two drip water δO models for the Herbstlabyrinth-Adventhöhle (HL) cave system, Germany. Winter climate of the cave region is influenced by the North Atlantic Oscillation (NAO), as documented by the positive correlations between winter temperature, winter rainfall δO values and the winter NAO index. Cave monitoring data show that recharge of the aquifer at the HL cave system mainly occurs during winter months, confirming our drip water model, which calculates recharge as the difference between precipitation and potential evapo-transpiration. Cave drip water from the HL cave system should, thus, be a sensitive proxy for reconstruction of the NAO. The infiltration models accounting for mixing effects occurring in the karst aquifer show a very good agreement with the monitoring data. Comparison of the modelled and the monitored drip water δO values suggests a mean residence time of the water in the aquifer of 12 months and a transmission time of 10 months. Using these parameters, a long-term dataset from the nearby meteorological station Frankfurt am Main and the rainfall δO data from our monitoring program, we model the δO values of the cave drip water for the last 144 years. Despite of the favourable conditions of the HL cave system for NAO reconstruction, the modelled drip water δO time series only shows a moderate correlation with the winter NAO index (r = 0.33). The major reason for the relatively low correlation is the substantial contribution of precipitation from the remaining seasons to the drip water, for instance, due to heavy precipitation events occurring during summer. These rainfall events bias the seasonality of infiltration and weaken the winter NAO signal recorded in the drip water δO values. Reconstruction of the NAO from speleothems is, thus, challenging, in particular considering further complications arising from chronological uncertainties and calcite precipitation under conditions deviating from isotopic equilibrium. These problems may, at least in part, be avoided by multi-proxy studies of annually laminated stalagmites. [ABSTRACT FROM AUTHOR]

Abstract: We present a speleothem stable oxygen isotope record for the last 12.5ka based on two stalagmites from western Cuba. The δ 18O signal is interpreted to represent past precipitation variability. Both stalagmites show a pronounced transition from higher δ 18O values (indicating drier conditions) to more negative δ 18O values (suggesting wetter conditions) between 10 and 6ka. This transition is also visible in a planktonic δ 18O record off Haiti. On orbital timescales, the δ 18O value of Caribbean precipitation, thus, strongly resembles the oxygen isotope composition of Caribbean surface water. On millenial timescales, the speleothem δ 18O record shows a high correlation to a North Atlantic sea surface temperature (SST) record off West Africa as well as a similarity with the Bond events. Periods of lower North Atlantic SST correspond to less precipitation in the Caribbean and vice versa. The potential teleconnection to the Caribbean may reflect the position of the Intertropical Convergence Zone because a further southward position of the ITCZ leads to reduced precipitation in the northern Caribbean. [Copyright &y& Elsevier]

Here we present the first high-resolution δ18O record of a stalagmite from western Cuba. The record reflects precipitation variability in the northwestern Caribbean during the last 1.3 ka and exhibits a correlation to the Atlantic Multidecadal Oscillation (AMO). This suggests a relationship between Caribbean rainfall intensity and North Atlantic sea-surface temperature (SST) anomalies. A potential mechanism for this relationship may be the strength of the Thermohaline Circulation (THC). For a weaker THC, lower SSTs in the North Atlantic possibly lead to a southward shift of the Intertropical Convergence Zone and drier conditions in Cuba. Thus, this Cuban stalagmite records drier conditions during cold phases in the North Atlantic such as the ‘Little Ice Age’. This study contributes to the understanding of teleconnections between North Atlantic SSTs and northern Caribbean climate variability during the past 1.3 ka. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the relationship between stable isotope signals recorded in speleothems (δ13C and δ18O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO2. The water entering the cave flows as a thin film towards the drip site. CO2 degasses from this film within approx. 10s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scale of several 10–100s. The δ13C value of the drip water is mainly determined by the isotopic composition of soil CO2. The evolution of the δ18O value of the carbonate species is determined by the long exchange time T ex, between oxygen in carbonate and water of several 10,000s. Even if the oxygen of the CO2 in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ18O value of the carbonate species. Consequently, the δ18O value of the rainwater will only be reflected in the drip water if it has stayed in the rock for a sufficiently long time. After the water has entered the cave, the carbon and oxygen isotope composition of the drip water may be altered by CO2-exchange with the cave air. Exchange times, , of about 3000s are derived. Thus, only drip water, which drips in less than 3000s onto the stalagmite surface, is suitable to imprint climatic signals into speleothem calcite deposited from it. Precipitation of calcite proceeds with time constants, τ p, of several 100s. Different rate constants and equilibrium concentrations for the heavy and light isotopes, respectively, result in isotope fractionation during calcite precipitation. Since T ex ≫ τ p, exchange with the oxygen in the water can be neglected, and the isotopic evolution of carbon and oxygen proceed analogously. For drip intervals T d <0.1τ p the isotopic compositions of both carbon and oxygen in the solution evolve linearly in time. The calcite precipitated at the apex of the stalagmite reflects the isotopic signal of the drip water. For long drip intervals, when calcite is deposited from a stagnant water film, long drip intervals may have a significant effect on the isotopic composition of the DIC. In this case, the isotopic composition of the calcite deposited at the apex must be determined by averaging over the drip interval. Such processes must be considered when speleothems are used as proxies of past climate variability. [Copyright &y& Elsevier]

Abstract: In recent years, stalagmites have become important archives for paleoclimate. Several studies applying carbon and oxygen isotopes of stalagmites show a simultaneous increase of δ 13C and δ 18O along individual growth layers, which is interpreted as being indicative of isotope fractionation under disequilibrium conditions. In order to obtain robust paleoclimatic information from calcite precipitated under these non-equilibrium conditions it is important to improve the quantitative understanding of the corresponding isotope fractionation processes. Here we present laboratory experiments simulating calcite precipitation under cave-analogue conditions. The major focus was the investigation of the temporal evolution of the δ 13C and δ 18O values of the precipitated calcite for varying temperature, drip interval and initial SI CaCO 3 . All experiments show an isotopic enrichment of both δ 13C and δ 18O with increasing distance from the point of drip water impinge. Longer drip intervals and higher temperatures result in a larger enrichment. In addition, the slope between δ 18O and δ 13C is lower for higher temperatures indicating faster oxygen isotope exchange between the water reservoir and the bicarbonate in the solution. In case of δ 13C, the fractionation factor between the precipitated calcite and the bicarbonate in the solution, 13 α CaCO3 −HCO3 − , shows a larger increase with higher temperatures in comparison to previous studies. This possibly indicates an increasing contribution of disequilibrium isotope fractionation processes for increasing temperatures, which are not accounted for by the equilibrium isotope fractionation factors. Furthermore, a quantitative determination of the calcite precipitation time, τ p, and the oxygen isotope exchange time between the bicarbonate and the water, τ b, yields faster reaction rates in comparison to previously published time constants, particularly at higher temperature (23°C). [Copyright &y& Elsevier]

Abstract: High resolution δ 13C and δ 18O profiles recorded in precisely dated speleothems are widely used proxies for the climate of the past. Both δ 13C and δ 18O depend on several climate related effects including meteorological processes, processes occurring in the soil zone above the cave and isotope fractionation processes occurring in the solution layer on the stalagmite surface. Here we model the latter using a stalagmite isotope and growth model and determine the relationship between the stable isotope values in speleothem calcite and cave parameters, such as temperature, drip interval, water and a mixing coefficient describing mixing processes between the solution layer and the impinging drop. The evolution of δ 13C values is modelled as a Rayleigh distillation process and shows a pronounced dependence on the residence time of the solution on the stalagmite surface and the drip interval, respectively. The evolution of δ 18O values, in contrast, is also influenced by buffering reactions between the bicarbonate in the solution and the drip water driving the δ 18O value of the bicarbonate towards the value expected for equilibrium isotope fractionation between drip water and calcite. This attenuates the dependence of the δ 18O values on drip interval. The temperature dependence of δ 18O, however, is more pronounced than for δ 13C and in a similar range as expected for fractionation under equilibrium conditions. We also investigate the isotopic enrichment of the δ 13C and δ 18O values along individual growth layers and, thus, the slopes expected for Hendy tests. The results show that a positive Hendy test is only possible if isotope fractionation occurred under disequilibrium conditions. However, a negative Hendy test does not exclude that isotope fractionation occurred under disequilibrium conditions. A more reliable indicator for disequilibrium fractionation is the enrichment of the δ 13C values along an individual growth layer. [Copyright &y& Elsevier]

Abstract: Here we present the first U-series dated stable oxygen isotope record in continental Europe that covers the entire tripartite Penultimate Interglacial, i.e. Marine Isotope Stages 7.5 to 7.1. This record was obtained from a U-rich stalagmite from Spannagel Cave in the Austrian Alps which is largely devoid of kinetic isotope fractionation effects which comprised previously studied flowstone from this site. Composed of dense, columnar calcite this speleothem apparently grew without significant interruption for ca. 50 ka, albeit at a very slow rate. The age model was established based on 23 individual U/Th TIMS dates using a combination of a Bayesian approach, which includes additional information using stratigraphic constraints, and the mixed-effect regression model. The resulting record shows three prominent maxima in δ18O (MIS 7.5, 7.3 and 7.1) separated by a long earlier and a shorter later cold period (MIS 7.4 and 7.2, respectively). The mid points of the transitions into the three warm phases occurred at 240±3, 215±2 and 200±3 ka and the MIS 7/6 transition was dated to 190±3 ka. The timing of these climate transitions is supported by other samples from the same cave and agrees within analytical uncertainties with U/Th-dated evidence from corals, marine aragonitic sediments and submerged stalagmites, as well as with orbitally tuned deep-sea records. Good agreement was also found with new speleothem data from China as far as MIS 7.1 and 7.5 are concerned, whereas MIS 7.3 apparently started earlier and MIS 7.2 lasted longer than in the Alps. Comparing the European speleothem data with Antarctic ice cores reveals obvious differences in age (the latter suggesting an earlier start and a generally shorter duration of the three warm episodes). In essence, this study underscores the great potential of speleothems as accurate and precise chronometers of Pleistocene climate change and emphasizes the need for similar studies in other continents (including submerged caves) in order to improve the imperfect chronological framework of MIS 7. [Copyright &y& Elsevier]

Abstract: Growth and stable isotope composition of stalagmites are affected by climate changes. To understand the underlying mechanisms, we developed a time dependent multi-box model that describes stalagmite growth and stable isotope fractionation of carbon under disequilibrium conditions. The model takes variations of the drip interval, temperature and the amount of mixing between the impinging drops and the solution on top of the stalagmite into account, which allows to quantify the influence of these parameters. To calculate the variations of δ13C, the multi-box model was calibrated by comparison with an existing growth model. The results show that drip interval, temperature and the mixing coefficient do have a significant influence on δ13C. However, considering the higher natural variability of the drip interval, this parameter might have the largest influence. [Copyright &y& Elsevier]

The most pronounced climate anomaly of the Holocene was the 8.2 ka cooling event. We present new 230Th/U-ages as well as high-resolution stable isotope and trace element data from three stalagmites from two different cave systems in Germany, which provide important information about the structure and climate variability of the 8.2 ka event in central Europe. In all three speleothems, the 8.2 ka event is clearly recorded as a pronounced negative excursion of the δ18O values and can be divided into a 'whole event' and a 'central event'. All stalagmites show a similar structure of the event with a short negative excursion prior to the 'central event', which marks the beginning of the 'whole event'. The timing and duration of the 8.2 ka event are different for the individual records, which may, however, be related to dating uncertainties. Whereas stalagmite Bu4 from Bunker Cave also shows a negative anomaly in the δ13C values and Mg content during the event, the two speleothems from the Herbstlabyrinth cave system do not show distinct peaks in the other proxies. This may suggest that the speleothem δ18O values recorded in the three stalagmites do not primarily reflect climate change at the cave site, but rather large-scale changes in the North Atlantic. This is supported by comparison with climate modelling data, which suggest that the negative peak in the speleothem δ18O values is mainly due to lower δ18O values of precipitation above the cave and that temperature only played a minor role. Alternatively, the other proxies may not be as sensitive as δ18O values to record this centennial-scale cooling event. This may particularly be the case for speleothem δ13C values as suggested by comparison with a climate modelling study simulating vegetation changes in Europe during the 8.2 ka event. Based on our records, it is not possible to resolve which of these hypotheses is most appropriate, but our multi-proxy dataset shows that regional climate evolution during the event was probably complex, although all δ18O records show a clear negative anomaly. • Three speleothems show pronounced negative peaks in δ18O values during 8.2 ka event. • All records can be divided into a 'whole event' and a 'central event'. • Other proxies show no distinct consistent peaks during the 8.2 ka event. • Magnitude of δ18O peaks can be explained by change in δ18O values of precipitation. • Speleothem δ18O values reflect large-scale changes in the North Atlantic. [ABSTRACT FROM AUTHOR]

Mainland Southeast Asia experiences complex and variable hydroclimatic conditions, mainly due to its location at the intersection of Asian monsoon subsystems. Predicting future changes requires an in-depth understanding of paleoclimatic conditions that is currently hindered by a paucity of records in some regions. In this paper, we present the first speleothem stable isotope records from western Thailand detailing the Bølling-Allerød interstadial, Younger Dryas termination, and early- to mid-Holocene period. We find evidence of higher precipitation during the Bølling-Allerød (14,321–12,824 years before present (1950: BP)) compared to a Younger Dryas termination that starts 11,702–11,674 BP, has a rapid shift centered on 11,660–11,641 BP, and ends 11,603–11,589 BP. In addition, our records show Holocene monsoon intensity peaking at 8250 BP or before, a multi-millennia delay from the Northern Hemisphere summer insolation peak, followed by a trend to drier conditions until at least 750 BP. Assessment of the timing of the Younger Dryas termination in paleoclimate records across Southeast Asia reveals an earlier shift of the Indian Summer Monsoon to global climate shifts when compared to East Asian Summer Monsoon records. The causes of this are currently unknown. Some potentially important aspects include: an Indian Summer Monsoon influence on East Asian Summer Monsoon strength via the Indian Ocean Dipole climate pattern, the role of the Tibetan Plateau in monsoon dynamics, and exposure of the Sundaland shoreline. More high-resolution paleoclimate records, especially on the pathway of Indian Summer Monsoon to East Asian Summer Monsoon, are required for further discussion on the mechanisms controlling the differences between climate regimes. • We present two new speleothem stable isotope records from western Thailand. • Conditions trend wetter during the Bølling-Allerød. • Early and abrupt termination of the Younger Dryas (11,675–11,600 BP, ±73 years). • Change Point Analysis of records finds earlier rapid shift in Indian Summer Monsoon. • Distinct trend of increasing δ18O and δ13C values throughout Holocene. [ABSTRACT FROM AUTHOR]

We present a theoretical derivation of the exchange time, τ ex , needed to establish isotopic equilibrium between atmospheric CO 2 in a cave and HCO 3 − dissolved in a thin water film covering the surface of a speleothem. The result is τ ex = τ red ex · [ HCO 3 - ] K H · p CO 2 cave , where τ red ex depends on the depth, a , of the water film and on temperature. [ HCO 3 - ] is the concentration of bicarbonate, p CO 2 cave the partial pressure of CO 2 , and K H is Henry’s constant. To test the theory we prepared stagnant or flowing thin films of a NaHCO 3 solution and exposed them at 20 °C to an CO 2 containing atmosphere of p CO 2 500, 12,500, or 25,000 ppmV and defined isotope composition. The δ 13 C and δ 18 O values of the DIC in the solution were measured as a function of the exposure time. For stagnant films with depths between 0.06 and 0.2 cm the δ 13 C values exhibit an exponential approach towards isotope equilibrium with the atmospheric CO 2 with exchange time, τ ex . The δ 18 O values first evolve towards isotopic equilibrium with atmospheric CO 2 , reach a minimum value and then drift away from the isotopic equilibrium with atmospheric CO 2 approaching a steady state caused by isotopic exchange of oxygen with water. The experimental findings are in satisfactory agreement with the theoretical predictions. To further investigate isotope evolution in cave analogue conditions, a water film containing 5 mmol/L of NaHCO 3 with a depth of 0.013 cm flowing down an inclined borosilicate glass plate was exposed to an atmosphere with p CO 2 = 500 ppmV at a temperature of 20 °C. The δ 13 C and δ 18 O values were measured as a function of flow (exposure) time, t . The isotope compositions in the DIC of the water film decrease linear in time by δ DIC ( t ) = δ DIC ( 0 ) - ( δ DIC ( 0 ) - δ DIC ( ∞ ) ) · t / τ ex where δ DIC ( 0 ) is the initial isotope composition of dissolved inorganic carbon (DIC) in the water film and δ DIC ( ∞ ) its final value. From these data an exchange time τ ex of ca. 7000 s was obtained, in satisfactory agreement with the theoretical predictions. The exchange times can be calculated by τ ex = τ red ex · [ HCO 3 - ] K H · p CO 2 cave , where τ red ex is given by the theory as function of temperature and the depth, a , of the water film. This way it is possible to obtain exchange times for various conditions of stalagmite growth as they occur in caves. [ABSTRACT FROM AUTHOR]

Abstract: Rapid degassing of CO2 from a thin film of drip water on the surface of stalagmites is often considered to have a large effect on both speleothem growth and stable isotope values and is offered as an explanation for higher δ13C and δ18O values than expected under conditions of stable isotope equilibrium. However, the time constant for degassing of CO2 from the solution only depends on film thickness and the coefficient of molecular diffusion for CO2. Thus, for thin films, the time for degassing of CO2 is much shorter than the time for subsequent equilibration of the dissolved carbon species and precipitation of CaCO3. In this context, degassing of CO2 is always fast. Here we present three experiments that enable the determination of the time constants for degassing of CO2, τ deg, subsequent equilibration to a new pCO2, τ eq, and precipitation of CaCO3, τ pr, in a thin film of an H2O–CO2–CaCO3 solution flowing on a calcite surface. The experiments are performed under cave-analogue conditions. At a temperature of 20°C and for a film thickness of δ ≈0.01cm, τ deg ≈2s. τ eq ≈13s and, thus, one order of magnitude larger. Finally, τ pr ≈400s for δ ≈0.01cm, again one order of magnitude larger. The experimentally determined values for τ deg, τ eq τ pr are in good agreement with the theoretical predictions. Our results confirm that the chemical evolution of the drip water proceeds in three subsequent major steps. During the first step of degassing of CO2, pH and Ca2+ concentration remain almost constant. During equilibration to the lower pCO2 of the solution, pH increases to about 8 whereas the Ca2+ concentration still remains constant. Finally, during precipitation of calcite, pCO2 remains at its low level and pH decreases slightly. These results suggest that the drip rate may have an important influence on the stable isotope signals recorded in speleothems. Stalagmites growing beneath drip sites with stable, intermediate drip rates (i.e., in the range of τ eq) may be best suited for palaeoclimate reconstruction. [Copyright &y& Elsevier]

Summary: Monitoring cave environments is important to understand processes in karst systems. If stalagmites from a specific cave are used as archives of past climate variability, a quantitative understanding of the soil–karst–speleothem system is crucial. The monitoring program performed in Bunker Cave (NW Germany), which includes monthly collection of climatological data as well as air and water samples from the cave and the overlying soil since 2006, is a prerequisite for the interpretation of speleothem data from the cave in terms of climate variability. The results show that Bunker Cave is a homogeneously ventilated cave with rather low pCO2 values of 580–1200ppmv, which lacks strong seasonal variations. The δ 18O value of cave drip water reflects the mean annual composition of the rain water, and the seasonal variability is strongly attenuated indicating a well-mixed karst aquifer. Hence, stalagmites from Bunker Cave should be well suited to record interannual/decadal climate trends. Seven drip sites in two cave chambers show three different types of discharge behaviour including slow seasonal drip, fast seasonal drip, and seepage-flow. The drip-rate patterns of most drip sites are consistent with the main infiltration events, taking into account a delay of several months. An instantaneous response to precipitation (i.e., piston-flow) is not observed for any drip site indicating a specific water capacity threshold in the soil/karst aquifer. Prior Calcite Precipitation (PCP) was also identified especially during times of low discharge. Though PCP is not as pronounced at Bunker Cave as in other caves, it probably has an influence on the Mg/Ca ratios of the stalagmites. [Copyright &y& Elsevier]

Carbonate deposits from a Roman aqueduct in Béziers, southern France, record environmental conditions during the late first century C.E. These deposits formed in a steep section of the aqueduct with a high flow velocity, which caused rapid deposition of up to 11 mm of calcite per year over a period of 22–24 years. The microstructure, trace element and stable isotope composition show that regular deposition was interrupted by high-discharge events, probably in response to heavy rainfall during autumn and winter, transporting colloidally- and particle-bound elements and depositing calcite with elevated δ 18 O values. Individual autumn high-discharge events coincide with abrupt decreases in δ 13 C from − 8 to − 12‰ giving rise to a saw-tooth profile. In some years, several high flow events persisted throughout the winter, suppressing this profile. Event horizons of micrite capping sparite growth surfaces, enriched in Mg, may represent anomalously low water levels or periods when the channel fell dry. In comparison to carbonate deposits from Roman aqueducts in the Eastern Mediterranean, visible layering is less regular and pronounced in Béziers, reflecting a more complex precipitation pattern. [ABSTRACT FROM AUTHOR]

Stable hydrogen isotope ratios of lignin methoxyl groups (δ2H LM) of wood have been shown to reflect climate-sensitive δ2H values of precipitation (δ2H precip). However, a detailed calibration study between high-resolution δ2H LM and δ2H precip data has not been performed yet. Here, we present annually resolved δ2H LM values from nine tree-ring series (derived from four Fagus sylvatica L. trees) collected near a station of the Global Isotope Network of Isotopes in Precipitation (GNIP) and the meteorological observatory at Hohenpeißenberg (Germany; ~48°N, 11°E). The nine δ2H LM tree-ring series show highly significant inter-series correlations (Rbar = 0.52) over the common period of overlap 1916–2015 and no sign of a major juvenile trend affecting the first decades of growth. When compared with local instrumental data, the combined δ2H LM chronology is most indicative for annually averaged δ2H precip values. In line with this finding, the δ2H LM chronology shows similar climatic imprints as commonly observed for European stable water isotopes. We find the highest correlation (r = 0.72, p < 0.001) with temperature changes averaged over large-scale areas (range: 15°W–20°E, 25–75°N) including marine and continental Western Europe. This large-scale temperature signal is not entirely stable over time, but weakens from 1948 to 1982 where 31-year running correlations drop systematically. The temporal signal strength coincides with a decline in Rbar values but also with known anomalous atmospheric circulation events (e.g., winter of 1956). We conclude two main findings. First, the temporal robustness of the transfer model is mainly controlled by the internal coherency among the tree-ring δ2H LM series. An improved understanding of such temporal discrepancies among the proxy data may therefore optimize reconstruction models. Second, this study highlights the need to critically assess the impact of changing atmospheric circulation patterns on the relationship between climate parameters and stable water isotopes. • δ2H lignin methoxyl was analyzed for nine tree-ring cores (covering 1916–2015). • The δ2H proxy reflects primarily annually averaged δ2H values of precipitation. • Tree-ring δ2H indicates strongest sensitivity for large-scale temperature changes. • The inferred temperature sensitivity co-varies with the internal δ2H coherency. [ABSTRACT FROM AUTHOR]

Speleothem records have been successfully calibrated against instrumental data, e.g., using stable oxygen isotope or annual growth lamina data considered as reliable recorders of air temperature or precipitation amount. We present the first high-resolution trace element (Mg/Ca, Sr/Ca, Ba/Ca) record providing the potential for quantitative climate reconstruction, deduced from stalagmites from Clo¸sani Cave, SW Romania.The calibration approach relies on a precise age control - in particular for the last 250 years, where the chronology is based on three independent dating methods. Chemical lamina counting is combined with the identification of the 20th century 14C bomb peak and 230Th/U dating. Long-term cave monitoring and model simulations of drip water and speleothem elemental variability support the inference that precipitation-relatedprocesses are the main drivers of speleothem Mg/Ca ratios. Calibration against instrumental climate data shows a significant anti-correlation of speleothem Mg/Ca ratios with autumn/winter (October to March) precipitation (r = -0.61, p < 0.01), which is statistically robust whenconsidering age uncertainties and auto-correlation. This relationship is then used to develop a quantitative reconstruction of Holocene autumn/winter precipitation.During the last 3.6 ka, our data suggest a heterogeneous pattern of past regional winter hydroclimate in the Carpathian/Balkan realm, along with intermittent weakening of the dominant influence of North Atlantic forcing. In agreement with other regional paleo-hydrological reconstructions, the observed variability reveals periodically occurring strong NW-SE hydro-climate gradients. We hypothesize, that this pattern is caused by shifts of theeastern edge of the area of influence of the NAO across central-eastern Europe due to the confluence of North Atlantic forcing, and other climatic features such as the East Atlantic/Western Russia (EAWR) pattern. [ABSTRACT FROM AUTHOR]

The SISAL database v1 [1] is a comprehensive quality-controlled global compilation of oxygen and carbon isotope records from speleothems spanning large parts of the late Pleistocene compiled as a community effort supported by Past Global Changes (PAGES). The stable isotope records provide information on past changes in climate and the environment, both for reconstruction, as well as for comparison to climate model output.\\The chronology of the majority of the records is based on precise $^\text{230}$Th/U-dating, and speleothems are palaeoclimate archives with generally good chronological control. However, various types of age-depth modelling with different degrees of complexity (e.g., linear interpolation, etc.) have been used to construct the chronologies, and most of the records provide no estimates of age uncertainty. Some data sets have no age model at all. This limits the suitability for assessing abrupt transitions as well as the synchronicity of events across space. \\Here we apply six different age-depth modeling approaches to a representative subset from the SISAL database, to compare and evaluate their suitability as final SISAL chronologies for the next version of the database. The methods include linear models (linear regression, linear interpolation), fitting approaches (StalAge), ensemble-based methods (COPRA), as well as Bayesian models (OxCal, Bacon, Bchron). The subset of records from the SISAL database is representative in a sense that they include speleothems with ''well-behaved'' constant and continuous growth, variable growth rates, age outliers as well as growth interruptions.\\We find that for many speleothems, age models can be established, and that there is an overall consensus on the median, or most likely, age-depth trajectory. Not all methods are applicable in all situations, however, and the resulting confidence intervals differ. We compare and evaluate the methods also by applying them to virtually grown speleothems.\\We discuss challenges that arise (data inconsistencies, missing hiatuses, duplicated sample depths, multi-model integration) and the format we intend for the final chronologies. The new chronologies will allow to consider the impact of age uncertainty on analyses in the SISAL working group consistently across the database.\\[1] Atsawawaranunt \textit{et al}., The SISAL database: a global resource to document oxygen and carbon isotope records from speleothems, Earth Syst. Sci. Data, 10, 1687-1713, https://doi.org/10.5194/essd-10-1687-2018, 2018. [ABSTRACT FROM AUTHOR]

Climate variability in Central Europe during the Holocene was characterised by changes on a millennial to decadal time-scale. Speleothems provide the opportunity to study past climate variability using high temporal resolution proxy records (e.g. stable isotopes and trace elements [1, 2]), and offer precise age models by U-series dating [3]. However, the climatic significance of an individual speleothem record is still a matter of debate, and limitations in sample availability often hamper the possibility to reproduce climate patterns observed at a specific cave site or to resolve spatial climate patterns. Here we present a palaeoclimate record based on three speleothem samples from the Hüttenbläserschachthöhle (HBSH), western Germany, which were sampled in close proximity. Two samples cover almost the entire Holocene, except for a short time interval (speleothem growth stop) around 8 ka. A third sample grew continuously between 10 and 8 ka. Trace element and stable isotope data were analysed at high resolution and allow us to investigate climatic patterns during the Holocene. The aim of this study is to evaluate the consistency of the proxy data recorded in the individual stalagmites and potentially to create a stack from the three HBSH speleothems. Correlations between the three speleothems will be investigated on different time-scales to identify the underlying mechanisms influencing speleothem growth and proxy signals in the HBSH system. Due to the close proximity of HBSH (<1 km) to the intensively investigated Bunker Cave system [4], this data set also provides the unique opportunity to compare the Holocene record with a time-series from different cave systems, which should have recorded the same climate signal. 1. F. McDermott, Quaternary Science Reviews, 2004, 23, 901-918.2. I. J. Fairchild and P. C. Treble, Quaternary Science Reviews, 2009, 28, 449-468.3. D. A. Richards and J. A. Dorale, Reviews in Mineralogy and Geochemistry, 2003, 52, 407.4. J. Fohlmeister, A. Schröder-Ritzrau, D. Scholz, C. Spötl, D. F. C. Riechelmann, M. Mudelsee, A. Wackerbarth, A. Gerdes, S. Riechelmann, A. Immenhauser, D. K. Richter and A. Mangini, Clim. Past, 2012, 8, 1751-1764. [ABSTRACT FROM AUTHOR]