Glacial to Holocene climate changes in the SE Pacific. The Raraku Lake sedimentary record (Easter Island, 27ºS)

摘要

Easter Island (SE Pacific, 27ºS) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Climates in the mid- to lowlatitude region of the eastern South Pacific Ocean are controlled by fluctuations in the Westerlies winds, the South Pacific Convergence Zone and the South Pacific Anticyclone. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34000 years based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 to 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest in the watershed during the Glacial period and a cold and relatively humid climate. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major floods due to the erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca. 9.5 cal kyr BP) and peatbog and shallow lake conditions dominated till mid Holocene, partially favored by the colmatation of the lacustrine basin. During the mid to late Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The human colonization of the island coincides with a new humid episode that started 800 yrs ago. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2 to 0.8 cal kyr BP sedimentary gap. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake.