<p>Investigating palaeoclimate records is of major importance for evaluating the impact of past forcing factors on the evolution of ecosystems, megafauna and human dispersal, especially in Southern Patagonia where few records are available. We report on a 40 cm long flowstone core S6, and fragments of flowstone and a stalagmite from Cueva Chica. The samples were radiometrically dated (U-Th & <sup>14</sup>C) to construct age-depth models for the proxy profiles (&#948;<sup>13</sup>C, &#948;<sup>18</sup>O, and chemical composition). The speleothem proxy data are further informed by both petrographic analysis of the flowstone, and monitoring data. The main objectives of this work are to: i) reconstruct past climate variations using geochemical analyses conducted on the speleothems, and ii) assess the palaeoclimatic context of megafauna extinction in the area. The flowstone core S6 grew discontinuously from ~13 ka to ~1 ka with several possible hiatuses at ~10 ka BP, from 5.7 to 3.0 ka BP, and 2.5 to 1.8 ka BP (interpolated ages). Sample S8 grew from 6.8 to 5.8 ka BP and after 1.2 ka BP.&#160;Stable isotopes analyzed at sub-centennial resolution show a 3&#8240; range for &#948;<sup>18</sup>O, and more than 14&#8240; for &#948;<sup>13</sup>C, and the isotope ratios covary along the entire record. These changes are likely caused by kinetic fractionation and prior calcite precipitation (PCP), controlled mostly by changes in moisture availability. The sensitivity of the proxies to hydrological changes and PCP is further tested with indicators using &#956;XRF element data.&#160;The multiproxy record from Cueva Chica suggests a wet phase from ~13 to 9 ka BP, likely related to strong westerlies in the Southern Hemisphere, preceded by a short dry/cold spell at ~13 ka BP. This early Holocene wet phase was followed by a colder/drier period from 8.5 to 5.8 ka BP, likely related to weaker westerlies, especially during the mid-Holocene. High precipitation and strong westerlies prevailed from 3.0 to 2.5 ka BP and in Medieval times.&#160;Our paleoclimate record implies that the presence of extensive megafauna, the development of Nothofagus forest and human arrival, all occurred during a climatically favorable wet/warm period ca. 13 to 9 ka BP, after the Antarctic Cold Reversal. However, the deterioration of the vegetation cover at the Cerro Benitez coinciding with high &#948;<sup>13</sup>C values excursions was initiated after ca. 11 ka BP. Previous studies suggest an extinction of major megafauna species (e.g., Mylodon, Smilodon, Panthera onca mesembrina) during this wet/warm period. Such climate-driven changes likely reduced the open ecosystem environment and may have led to local decline of herbivore populations. Later cooling/drying after ca. 9 ka may have contributed to the disappearance of megafauna and other large mamals (e.g., Hippidion Saldiasi).</p>
Stalagmites are well established as accurate archives for paleoclimate and paleoenvironmental records, with inorganic proxies being widely used. Organic proxies are a growing field, providing access to information stored in a separate carbon pool, and relating to microbial activity, vegetation cover, and temperature. However, the key factors controlling these proxies are not yet well defined. δ13C analysis of speleothem calcite (effectively the dissolved CO2 pool) is a routinely used technique, but the controls are not as well defined as for δ18O, with the proxy record potentially affected by vegetation change, soil conditions or microbial activity. Parallel measurement of δ13C of the preserved organic matter has been suggested as a method of resolving this signal, with an inverse relationship between the two records previously being suggested as a proxy for microbial control. However, this technique is in its infancy in speleothems, and requires testing in a greater range of contexts. This study uses a multi-proxy approach, combining δ13C analysis of calcite and the preserved organic matter, with a highly novel laser fluorescence technique to investigate climatic and land-use driven environmental variations during the Holocene period. Four stalagmites were sampled in the same room of the Garde-Cavale karst system (1300m a.s.l.), in the Bauges massif, in the northern Prealps. These samples cover the last 12 ka. δ13C analysis of soluble acid bulk organic matter was performed via LC-IRMS at La Trobe University and the University of Orleans. δ13C of the calcite was performed at University of Melbourne, Australia. Organic matter fluorescence was measured at the University of Savoie, France. This fluorescence technique provides a high resolution record (100 µm step along the growth axis), and is a non-destructive method. The three proxies show different trends during the Holocene period, although major environmental events are marked by changes in all records. Further work is needed to establish whether the differences in the proxy records are due to differing environmental controls, or whether there are methodological factors affecting the record. In particular, the organic δ13C proxy is currently under further investigation to test for sensitivity to fluctuations in instrumental parameters and matrix effects. However, in the long term, this multi-proxy approach, combining organic and inorganic techniques has great promise in resolving our understanding of environmental variations.
Abstract. The 8.2 ka event is regarded as the most prominent climate anomaly of the Holocene, and is thought to have been triggered by a meltwater release to the North Atlantic that was of sufficient magnitude to disrupt the Atlantic Meridional Overturning Circulation (AMOC). It is most clearly captured in Greenland ice-core records, where it is reported as a cold and dry anomaly lasting ~160 years, from 8.25 ± 0.05 ka BP until 8.09 ± 0.05 ka BP (Thomas et al., 2007). It is also recorded in several archives in the North Atlantic region, however its interpreted timing, evolution and impacts vary significantly. This inconsistency is commonly attributed to poorly constrained chronologies and/or inadequately resolved time series. Here we present a high-resolution speleothem record of early Holocene palaeoclimate from El Soplao Cave in northern Spain, a region pertinent to studying the impacts of AMOC perturbations on south-western Europe. We explore the timing and impact of the 8.2 ka event on a decadal scale by coupling speleothem stable carbon and oxygen isotopic ratios, trace element ratios (Mg / Ca and Sr / Ca) and growth rate. Throughout the entire speleothem record, δ18O variability is related to changes in effective recharge. This is supported by the pattern of changes in δ13C, Mg / Ca and growth rate. The 8.2 ka event is marked as a centennial-scale negative excursion in El Soplao δ18O, starting at 8.19 ± 0.06 ka BP and lasting until 8.05 ± 0.05 ka BP, suggesting increased recharge at the time. Although this is supported by the other proxies, the amplitude of the changes is minor and largely within the realm of variability over the preceding 1000 years. Further, the shift to lower δ18O leads the other proxies, which we interpret as the imprint of the change in the isotopic composition of the moisture source, associated with the meltwater flux to the North Atlantic. A comparison with other well-dated records from south-western Europe reveals that the timing of the 8.2 ka event was synchronous, with an error-weighted mean age for the onset of 8.23 ± 0.03 ka BP and 8.10 ± 0.05 ka BP for the end of the event. This compares favourably with the NGRIP record. The comparison also reveals that the El Soplao δ18O is structurally similar to the other archives in south-western Europe, and the NGRIP ice-core record.
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