The effective moisture history of the Amazon Basin for the last 40,000 years, reconstructed from ODP site 942 on the Amazon Fan
The Amazon Basin is the site of the Earth's largest land-based atmospheric convection centre, and acts as a large source of latent heat release, particularly during the summer months when the South American Summer Monsoon (SASM) is most developed. Convectional rainfall over the Amazon Basin therefore plays a fundamental role in the transport of heat to the higher latitudes, and the regulation of global climate. The Pleistocene moisture history of the Amazon Basin is comparatively poorly known. However, sediments from the Amazon Fan have the potential to record a basin-wide average of past changes in the effective moisture of the Amazon Basin within single, continuous sequences, which accumulate rapidly. Oxygen isotopes (6lsO) records were measured for five planktonic foraminifera species from ODP Site 942 on the Amazon Fan. Data were constrained by an age model constructed around 36 AMS radiocarbon dates, which were converted to calibrated calendar ages. Past changes in the outflow of the Amazon River were reconstructed by attempting to isolate the relative shift in 6I80942 brought about by freshwater-driven changes in salinity in the surface water over the Site. 6I80942 records were adjusted for fractionation effects associated with changes in global ice volume, however removing the sea surface temperature (SST) effect was more problematic. A6I80942 data implied that the Amazon Basin was more arid during the glacial stage relative to the Holocene. Co-variance with November-December insolation at 10 S implied that this was associated with insolation-driven variations in the intensity of the SASM. Particularly high-resolution records were measured for the last glacial-interglacial transition. Maximum aridity was reconstructed around the onset of the Younger Dryas (YD) in the Northern Hemisphere, after which effective moisture levels exhibited an increasing trend thereafter throughout the period, correlating with a warming trend in Antarctica. A similarity between the A6I80942 data and the Vostock ice core temperature record (6D) suggests a possible Antarctic forcing over the climate of the Amazon Basin. It was hypothesised that Northern and Southern hemisphere temperature gradients exert independent control over the northerly and southerly limits of SASM convection over the Amazon Basin. An isotopic balancing model was used to attempt to semi-quantify the Amazon River outflow. Assuming a SST cooling of 2 to 3 C, the Amazon River outflow was modelled to have been reduced by up to -30-50% at the YD onset, and by up to -20-30% during the Last Glacial Maximum. However semi-quantified reconstructions are limited by the assumptions of the model.