A Novel Tetraether Membrane Lipid Record from the Amazon Fan: The Influence of Changes in Sediment Supply on Estimates of Palaeo-Temperature, pH and Wetland Extent

Reconstructing Amazon basin paleoclimatic change is important because the region is a major component of the global carbon and hydrological cycles. Moreover, the region's tropical wetlands represent a major source of atmospheric methane. In order to provide a multiproxy 35 kyr record of terrestrial paleoclimatic change from the Amazon basin, analysis of glycerol dialkyl glycerol tetraethers (GDGTs) was carried out on sediment recovered from Ocean Drilling Program (ODP) Site 942 (Amazon Fan). The GDGTs include those with methyl-branched alkyl components, derived from unknown terrestrial bacteria (present worldwide in peat bogs and soils) which have been fluvially transported from the Amazon basin and sequestered in the Amazon fan. The catchment area supplying sediment to the Amazon fan covers a large portion of South America, including vast areas of tropical forests and wetlands; hence proxy records from ODP 942 potentially provide a spatially averaged terrestrial signal of major significance. We use recently developed indices that predict changes in: the relative fluvial input of terrestrial organic material; annual mean air temperature (MAT) and soil pH. We also use the ratios of certain alcohols as qualitative proxies for the relative inputs of wetland material. The Amazon basin shows a glacial to interglacial temperature increase of around 5°C; consistent with pollen and other geochemical records. However, this is followed by an apparent drop of 11°C at the start of the Holocene. We discuss how this seemingly counter intuitive result may be explained by changes in sediment supply from the Andean region relative to the warmer eastern Amazonia. In the glacial the soil pH increases dramatically from 13 to 11 kyr; possibly related to an (inferred) ~40 percent decrease in effective moisture in the Amazon basin. A post-glacial increase in wetland biomarkers suggests that the propagation of tropical wetlands, on continental shelves, may have been a key source of the post-glacial increase in global atmospheric methane.