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We present the first high-resolution trace element (Mg/Ca, Sr/Ca, Ba/Ca) record from a stalagmite in southwestern Romania covering the last 3.6 ka, which provides the potential for quantitative climate reconstruction. Precise age control is based on three independent dating methods, in particular for the last 250 yr, where chemical lamina counting is combined with the identification of the 20th century radiocarbon bomb peak and Th-230/U dating. Long-term cave monitoring and model simulations of drip water and speleothem elemental variability indicate that precipitation-related processes 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 when considering age uncertainties and auto-correlation. This relationship is used to develop a quantitative reconstruction of autumn/winter precipitation. During the late Holocene, 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 the eastern 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. (C) 2018 Elsevier B.V. All rights reserved.
Marine Isotope Stage 3 (MIS 3, 57-27 ka) was characterised by numerous rapid climate oscillations (i.e., Dansgaard-Oeschger (D/O-) events), which are reflected in various climate archives. So far, MIS 3 speleothem records from central Europe have mainly been restricted to caves located beneath temperate Alpine glaciers or close to the Atlantic Ocean. Thus, MIS 3 seemed to be too cold and dry to enable speleothem growth north of the Alps in central Europe. Here we present a new speleothem record from Bunker Cave, Germany, which shows two distinct growth phases from 52.0 (+0.8, -0.5) to 50.9 (+0.6, -1.3) ka and 473 (+1.0, -0.6) to 42.8 (+/- 0.9) ka, rejecting this hypothesis. These two growth phases potentially correspond to the two warmest and most humid phases in central Europe during MIS 3, which is confirmed by pollen data from the nearby Eifel. The hiatus separating the two phases is associated with Heinrich stadial 5 (HS 5), although the growth stop precedes the onset of HS 5. The first growth phase is characterised by a fast growth rate, and Mg concentrations and Sr isotope data suggest high infiltration and the presence of soil cover above the cave. The second growth phase was characterised by drier, but still favourable conditions for speleothem growth. During this phase, the delta C-13 values show a significant decrease associated with D/O-event 12. The timing of this shift is in agreement with other MIS 3 speleothem data from Europe and Greenland ice core data. (C) 2018 Elsevier Ltd. All rights reserved.
Preceramic human skeletal remains preserved in submerged caves near Tulum in the Mexican state of Quintana Roo, Mexico, reveal conflicting results regarding C-14 dating. Here we use U-series techniques for dating a stalagmite overgrowing the pelvis of a human skeleton discovered in the submerged Chan Hol cave. The oldest closed system U/Th age comes from around 21 mm above the pelvis defining the terminus ante quem for the pelvis to 11311 +/- 370 y BP. However, the skeleton might be considerable older, probably as old as 13 ky BP as indicated by the speleothem stable isotope data. The Chan Hol individual confirms a late Pleistocene settling of Mesoamerica and represents one of the oldest human osteological remains in America.
The ‘bomb-pulse’ method is a chronological approach to further constrain the age of speleothems that grew between 1950 CE – present. Establishing dependable chronological constraints is crucial for modern calibration studies of speleothems to instrumental climate records, which provides the basis for paleoclimate interpretations. However, a large unknown is how 14C is transferred from the atmosphere to any individual speleothem owing to the site-specific residence times of organic matter above cave systems. Here, we employ the bomb-pulse method to build chronologies from 14C measurements in combination with a new unsaturated zone C model which considers C decomposition as a continuum, to better understand unsaturated zone 14C dynamics. The bomb-pulse curves of eight speleothems from southern Australia in three contrasting climatic regions; the semi-arid Wellington Caves site, the mediterranean Golgotha Cave site and the montane Yarrangobilly Caves site, are investigated. Overall, the modelled 14C bomb-pulse curves produce excellent fits with measured 14C speleothem data (r2 = 0.82–0.99). The C modelling reveals that unsaturated zone C is predominately young at the semi-arid site, with a weighted-mean residence time of 32 years and that tree root respiration is likely an important source of vadose CO2. At the montane site, ∼39% of C is young (<1 years), but the weighted-mean C ages are older (145–220 years). The mediterranean site has very little contribution from young C (<12%: 0–1 years), with weighted-mean ages between 157 and 245 years, likely due to greater adsorption of organic matter in the upper vadose zone during matrix flow, and remobilisation of C from young syngenetic karst. New end members for low speleothem Dead Carbon Proportion (DCP) are identified (2.19% and 1.65%, respectively) for Australian montane and semi-arid zone speleothems, where oversupply of modern CO2 in the vadose zone leads to lower DCP. It is also demonstrated that DCP can be quite variable over small time scales, that processes may be difficult to untangle and a constant DCP assumption is likely invalid. DCP variability over time is mainly controlled by the changes vadose zone CO2, where vegetation regeneration, wild-fires and karst hydrology play an important role.
We report the chronological construction for the top portion of a speleothem, PC1, from southern Cambodia with the aim of reconstructing a continuous high-resolution climate record covering the fluorescence and decline of the medieval Khmer kingdom and its capital at Angkor (similar to 9th-15th centuries AD). Earlier attempts to date PC1 by the standard U-Th method proved unsuccessful. We have therefore dated this speleothem using radiocarbon. Fifty carbonate samples along the growth axis of PC1 were collected for accelerator mass spectrometry (AMS) analysis. Chronological reconstruction for PC1 was achieved using two different approaches described by Hua et al. (2012a) and Lechleitner et al. (2016a). Excellent concordance between the two age-depth models indicates that the top similar to 47 mm of PC1 grew during the last millennium with a growth hiatus during similar to 1250-1650 AD, resulting from a large change in measured C-14 values at 34.4-35.2 mm depth. The timing of the growth hiatus covers the period of decades-long droughts during the 14th-16th centuries AD indicated in regional climate records.
Here we present high-resolution delta O-18 records obtained from speleothems collected in the eastern Bolivian Andes. The stable isotope records are related to the regional- to large-scale atmospheric circulation over South America and allow interpreting changes in delta O-18 during the last 1400 yr as a function of changes in precipitation regimes over the southern tropical Andes. Two distinct phases with more negative delta O-18 values, interpreted as periods of increased convective activity over the eastern Andean Cordillera in Bolivia are observed concomitantly with periods of global climate anomalies during the last millennium, such as the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) respectively. Changes in the Bolivian delta O-18 record during the LIA are apparently related to a southward displacement of the Intertropical Convergence Zone (ITCZ), which acts as a main moisture driver to intensify convection over the tropical continent. During the MCA, however, the increased convective activity observed in the Bolivian record is likely the result of a different mechanism, which implies moisture sourced mainly from the southern tropical Atlantic. This interpretation is consistent with paleoclimate records further to the north in the tropical Andes that show progressively drier conditions during this time period, indicating a more northerly position of the ITCZ. The transition period between the MCA and the LIA shows a slight tendency toward increased delta O-18 values, indicating weakened convective activity. Our results also reveal a non-stationary anti-phased behavior between the delta O-18 reconstructions from Bolivia and northeastern Brazil that confirms a continental-scale east-west teleconnection across South America during the LIA.
Speleothems, secondary cave carbonates, are important tools for climate reconstruction, especially as they often can be very precisely dated with the UTh method. If the U-Th method fails, dating becomes difficult, and often results in abandonment of samples and study sites. Radiocarbon dating is the only other radiometric dating technique applicable to the last similar to 50 ka, but presents complexities related to temporal variability of the reservoir effect in speleothems. Thus, radiocarbon dating of speleothems is not straightforward, and there are currently no publicly available tools to define proper age-depth relationships with this method. Here, we present an improved version of a previously published radiocarbon based age-depth modelling approach (star, Lechleitner et al., 2016b), which is now made freely available. The software is easy to use and provides the possibility to obtain reliable age-depth relationships, without prior knowledge of reservoir effects and their variability. In addition, star is able to detect and handle growth stops and phases with different growth rates. We test star on artificially constructed data sets and illustrate steps to improve the model performance. Furthermore, we apply the new approach to published radiocarbon data of U-Th dated stalagmites. This offers the possibility to investigate the strengths and weaknesses of the new approach with respect to potentially significant long term trends in the radiocarbon reservoir effect, which might otherwise remain undetected. In summary, we have produced a valuable software, which easily enables to construct age-depth relationships on the basis of reservoir effect disturbed radiocarbon measurements.
Three small stalagmites from Zoolithencave (southern Germany) show visible laminae, which consist of a clear and a brownish, pigmented layer pair. This potentially provides the opportunity to construct precise chronologies by counting annual laminae. The growth period of the three stalagmites was constrained by the C-14 bomb peak in the youngest part of all three stalagmites and C-14-dating of a piece of charcoal in the consolidated base part of stalagmite Zoo-rez-2. These data suggest an age of AD 1970 for the top laminae and a lower age limit of AD 1973-1682 or AD 1735-1778. Laminae were counted and their thickness determined on scanned thin sections of all stalagmites. On stalagmites Zoo-rez-1 and -2, three tracks were measured near the growth axes, each separated into three sections at prominent anchor laminae (I, II, III). Each section was replicated three times (a, b, c). For Zoo-rez-3, only one track was measured. The total number of laminae counted for Zoo-rez-1 ranges from 138 to 177, for Zoo-rez-2 from 119 to 145, and for Zoo-rez-3 from 159 to 166. The numbers agree well with the range constrained by the bomb peak and the age of the charcoal, which supports the annual origin of the laminae. The replicated measurements of the different tracks as well as the three different tracks on the stalagmites Zoo-rez-1 and-2 were cross-dated using the TSAP-Win (R) tree-ring software. This software is very useful for cross-dating because it enables to insert or delete missing or false laminae as well as identifying common pattern by shifting the series back and forth in time. However, visual inspection of the thin sections was necessary to confirm detection of missing or false laminae by TSAP-Win (R). For all three Zoo-rez speleothems, crossdating of the mean lamina thickness series was not possible due to a missing common pattern. The cross-dating procedure results in three refined chronologies for the three Zoo-rez stalagmites of ranging from AD 1821-1970 (Zoo-rez-1), AD 1835-1970 (Zoo-rez-2), and AD 1808-1970 (Zoo-rez-3).
The strength of the North Atlantic Oscillation (NAO) is considered to be the main driver of climate changes over the European and western Asian continents throughout the last millennium. For example, the predominantly warm Medieval Climate Anomaly (MCA) and the following cold period of the Little Ice Age (LIA) over Europe have been associated with long-lasting phases with a positive and negative NAO index. Its climatic imprint is especially pronounced in European winter seasons. However, little is known about the influence of NAO with respect to its eastern extent over the Eurasian continent. Here we present speleothem records (PC, 8180 and Sr/Ca) from the southern rim of Fergana Basin (Central Asia) revealing annually resolved past climate variations during the last millennium. The age control of the stalagmite relies on radiocarbon dating as large amounts of detrital material inhibit accurate 230Th dating. Present-day calcification of the stalagmite is most effective during spring when the cave atmosphere and elevated water supply by snow melting and high amount of spring precipitation provide optimal conditions. Seasonal precipitation variations cause changes of the stable isotope and Sr/ Ca compositions. The simultaneous changes in these geochemical proxies, however, give also evidence for fractionation processes in the cave. By disentangling both processes, we demonstrate that the amount of winter precipitation during the MCA was generally higher than during the LIA, which is in line with climatic changes linked to the NAO index but opposite to the higher mountain records of Central Asia. Several events of strongly reduced winter precipitation are observed during the LIA in Central Asia. These dry winter events can be related to phases of a strong negative NAO index and all results reveal that winter precipitation over the central Eurasian continent is tightly linked to atmospheric NAO modes by the westerly wind systems. (C) 2017 Elsevier Ltd. All rights reserved.
The precise determination of the stable C and O isotope fractionation between water and calcite (CC) and water and aragonite (AR) is of special interest for climate reconstructions, e.g. paleotemperatures. Previous studies reported results from both laboratory and field experiments, but their results are only partly consistent. Here we present C and O isotope data of a stalagmite from the Swiss Alps, which shows CC-AR transitions along individual growth layers. Using detailed analyses both laterally and perpendicular to such layers we examined the difference in the C and O isotope fractionation factor of the HCO3- - CC and the HCO3- - AR system. For O this difference is similar to the water-CC and water-AR offset provided in experimental studies. The O isotope fractionation difference in the water-CC and water-AR system is comparable to those determined in laboratory studies but shows a statistically significant correlation with the CaCO3 precipitation rate. For C we found a fractionation difference, which is independent of CaCO3 precipitation rate and with slightly smaller values for the fractionation offset between HCO3- - CC and HCO3- - AR compared to literature values. However, we also found an unexpected decrease in delta C-13 along growth layers, which contradicts the widely used concept of Rayleigh fractionation during CO2 degassing and CaCO3 precipitation. The results of this study can be used e.g., to correct stable isotope time series of stalagmites showing CC-AR transitions along their growth axes. (C) 2018 Elsevier Ltd. All rights reserved.