Climate history of the Southern Hemisphere Westerlies belt during the last glacial-interglacial transition revealed from lake water oxygen isotope reconstruction of Laguna Potrok Aike (52° S, Argentina)

摘要

The Southern Hemisphere westerly winds (SHW) play a crucial role in the large-scale ocean circulation and global carbon cycling. Accordingly, the reconstruction of its latitudinal position and intensity is essential for understanding global climatic fluctuations during the last glacial cycle. The southernmost part of the South American continent is of great importance for paleoclimate studies as the only continental mass intersecting a large part of the SHW belt. However, continuous proxy records back to the last Glacial are rare in southern Patagonia, owing to the Patagonian Ice Sheets expanding from the Andean area and the scarcity of continuous paleoclimate archives in extra-Andean Patagonia. Here, we present an oxygen isotope record from cellulose and purified bulk organic matter of aquatic moss shoots from the last glacial-interglacial transition preserved in the sediments of Laguna Potrok Aike (52° S, 70° W), a deep maar lake located in semi-arid, extra-Andean Patagonia. The highly significant correlation between oxygen isotope values of aquatic mosses and their host waters and the abundant well-preserved moss remains allow a high-resolution oxygen isotope reconstruction of lake water (δ18O_lw) for this lake. Long-term δ18O_lw variations are mainly determined by δ18O changes of the source water of lake, surface air temperature and evaporative 18O enrichment. Under permafrost conditions during the Glacial, the groundwater may not be recharged by regional precipitation. The isolated groundwater could have had much less negative δ18O values than glacial precipitation. The less 18O depleted source water and prolonged lake water residence time caused by reduced interchange between in- and outflows could have resulted in the reconstructed glacial δ18O_lw that was only ca. 3‰ lower than modern values. The significant two-step rise in reconstructed δ18O_lw during the last deglaciation demonstrated the response of isotope composition of lake water to fundamental climatic shifts. 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 transition period of receding values by up to 0.7‰, the reconstructed δ18O_lw 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 SHW exerted its full influence on the lake water balance, reflected by reconstructed δ18O_lw approaching modern values, indicating a strongly evaporative steppe climate in the Laguna Potrok Aike region.