A paleolimnological reconstruction of Holocene climate change in southern Patagonia

To fully understand the Holocene climatic variability in the sub-polar latitudes of the Southern Hemisphere and its driving mechanisms, like the Southern Hemisphere westerly winds, we undertook a multi-proxy analysis of a lake sediment core from Lago Pato (51°18.020’S, 72°40.716’W; Chile). The bottom sediments of the core are of glaciogenic origin and gave bulk ages of 30 000 - 22 000 cal. yr. B.P. These are overlain by brown, organic rich sediments which are 9500 cal. yr. B.P. old. The hiatus between both stratigraphic units is possibly related to sediment erosion as a result of the outflow of a large lake after regional deglaciation of the Patagonian Ice Sheet. The pollen record and a diverse benthic diatom community in the Holocene sediments point to dry conditions between ca. 9500 and 6035 cal. yr. B.P. This is coincident with the Early Holocene climate optimum recorded in for example the North Atlantic and Antarctica. From 6035 cal. yr. B.P. until the most recent period a higher biological production compared with the Early Holocene can be inferred from all proxies, probably resulting from wetter and/or warmer conditions. Apart from the diatoms, changes in the proxies are small since the Mid Holocene. The establishment of a planktonic diatom flora between 6035 and 3780 cal. yr. B.P. with the presence of the small diatom Discostella stelligera s.l. as well as the larger species Aulacoseira ambigua, A. granulata s.l. and Cyclostephanos cf. patagonicus possibly points to a period with sufficient mixing during autumn, winter or spring as well as enhanced thermal stratification during summer. Between ca. 3780 and 2080 the diatom community is dominated by Discostella stelligera which suggest a reduced water column mixing and a more stable lake stratification. From ca. 2080 cal. yr. B.P. till present an abrupt shift to and dominance of small chain-forming benthic fragilariod diatoms took place. This period is coincident with the Neoglacial cooling and could be the result of a change in lake-level, longer ice-cover, a turbid/low-nutrient environment, or a more alkaline environment. In the most recent sediments also an increase of Cyclotella cf. meneghiniana could be noticed.

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