New insights into the Glacial to Holocene climatic evolution of Southern Patagonia from lacustrine lipid biomarker isotope records

Abstract:

Southern Patagonia is a key region for paleoclimatic reconstructions in the Southern Hemisphere as it is the only landmass located in the Southern Hemisphere westerly wind (SHW) belt. Within the framework of the ICDP drilling campaign PASADO ("Potrok Aike Maar Lake Sediment Archive Drilling Project"), a high resolution sediment record was recovered from Laguna Potrok Aike (LPTA, 51°58´S, 70°23´W).

In order to identify the sources of organic matter contributions to the sedimentary archive, we investigated long-chain n-alkanes as tracers for terrestrial and aquatic plants. We analysed n-alkane distributions and their compound-specific hydrogen (δD) and stable carbon (δ¹³C) isotopic composition in various sample types such as soils, dust, aquatic and terrestrial plants and lake surface sediments. Based on two different model approaches, one using the n-alkane distributions and the other the compound-specific isotope values, we traced the origin of mid- (n-C₂₃) and long- (n-C₂₉) chain n-alkanes into modern lake sediments. Both models yield similar results: around 70% of the n-C₂₃ originates from aquatic plants and more than 80% of the n-C₂₉ is delivered from dust and terrestrial plants to the sediment. These results provide the basis for a robust paleo-environmental reconstruction of the lipid biomarker isotope records from LPTA.

Compound-specific δD and δ¹³C records for the last 55,000 years from the PASADO core are interpreted in the framework of these findings. Here, δD of the n-C₂₃ alkane serves as proxy for lake water isotopic changes driven by the precipitation-evaporation balance, moisture sources and water column stratification. In contrast, we interpret changes in δD of the n-C₂₉ alkane to reflect dust source area changes and therefore, the intensity of the SHW. A 50‰ shift in the δD record of the n-C₂₃ alkane between 10.000 to 8.000 years age indicates a major hydrological change affecting the lake level while isotopic changes in the n-C₂₉ alkane were minor.