Paleoenvironmental changes in Southern Patagonia during late Quaternary inferred from lake sediments of Laguna Potrok Aike, Argentina

摘要

Southern Patagonia possesses a unique value for the assessment of the global paleoclimatic system, because it is the only continental mass intersecting the core of the Southern Hemisphere Westerlies (SHW), which could have played a crucial role in driving the global deglacial warming during the last glacial termination. Paleoclimatic archives in southern Patagonia provide also terrestrial linkage to the sediment records from the Southern Ocean and the ice cores from Antarctica. Despite the great importance, continuous paleoclimatic records in southern Patagonia are only available for the period from the last deglaciation onwards, because most study sites are located in the Andean region, which was covered by the immense Patagonian Ice Sheet during the last Glacial. This research gap is being bridged by interdisciplinary multiproxy investigations within the framework of the Potrok Aike maar lake sediment archive drilling project (PASADO). The project provides a lake sediment record reaching back more than 50,000 years taken from Laguna Potrok Aike, a deep maar lake located in semi-arid southern extra-Andean Patagonia (51°58' S, 70°23' W, 113 m a.s.l.). By using stable isotope analysis of sedimentary organic material, this thesis has investigated the PASADO sediment section ranging from 26,000 to 8500 cal. BP, covering the last glacial-interglacial transition, to provide paleoenvironmental evidences from southern extra-Andean Patagonia for a time period with dramatic climatic changes. The work presented in Chapter 2, employs stable isotope (δ13CTOC, δ15NTN) and elemental (TOC, TN) analyses of fine-grained (200µm) bulk organic matter preserved in pelagic sediments to reconstruct the lacustrine primary productivity and nutrient supply. Furthermore, a unique proxy approach based on cellulose and bulk organic matter of handpicked aquatic moss remains is established to infer the past lake water δ18O (δ18Olw) (Chapter 3). The validity of this approach is determined by the highly significant correlation between δ18O of submerged aquatic mosses and their host waters. Using this proxy approach, a high-resolution δ18Olw record is generated for the investigated period and interpreted in the context of the fundamental climatic shifts during the last glacial-interglacial transition to assess the evolution of the SHW (Chapter 4). Between 26,000 and 17,600 cal. BP, lacustrine phytoplankton was presumably the predominant organic matter source in an aquatic environment with low primary productivity. Meanwhile, the reconstructed glacial δ18Olw had a mean value of around -6.5‰ that was only ca. 3‰ lower than the modern values. This is less negative than expected in consideration of probably large 18O depletion of meteoric water during the full Glacial. It is argued that the isolated groundwater resulted from permafrost conditions could have had much less negative δ18O values than glacial precipitation and prolonged lake water residence time could have further caused observed less negative glacial δ18Olw. At the onset of the last deglaciation, abrupt and distinct shifts of C and N isotopic and elemental values indicate that the lacustrine system underwent a rapid reorganization, synchronous with the rapid glacier retreat in southern Patagonia, the Antarctic warming and the elevated atmospheric CO2 concentrations. Over the course of the last deglaciation, lacustrine primary productivity of both phytoplankton and aquatic macrophytes showed high levels despite large variations. The increased productivity can be attributed to improved growing conditions for primary producers because of deglacial warming in combination with sufficient nutrient availability and calm wind conditions. The reconstructed δ18Olw responded to the deglacial climatic shift with a significant two-step rise. Rapid deglacial warming is supposed to cause the 18O enrichment of lake water by ca. 2‰ during the first rise between 17,600 and 15,600 cal. BP by increasing temperature-induced evaporation and more 18O enriched precipitation. After a millennial period of receding values by up to 0.7‰, the reconstructed δ18Olw resumed pronounced increase since 14,600 cal. BP. This cumulative enrichment in 18O of lake water could be interpreted as a response to the strengthened wind-driven evaporation, implying the intensification and establishment of the SHW at the latitude of Laguna Potrok Aike (52°S). During the early Holocene the lake approached a new state with reduced primary productivity probably induced by unfavorable growing conditions for primary producers in the lake like nutrient shortage after the preceding productive phase with high consumption as well as strengthened westerly winds. Since around 13,000 cal. BP, the SHW exerted its dominant influence on the lake water balance, reflected by reconstructed δ18Olw values close to the modern ones, indicating the establishment of a strongly evaporative steppe climate in the Laguna Potrok Aike region.