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

Dynamic boundary-monsoon intensity hypothesis : evidence from the deglacial Amazon River discharge record

Glacioeustatic- and temperature-corrected planktonic foraminiferal oxygen isotope (∆δ18O) records from ODP Site 942 on the Amazon Fan provide a means of monitoring past changes in the outflow of the Amazon River. This study focuses on the last deglaciation and reveals that during this period there were significant variations in the outflow, which implies large changes in moisture availability in the Amazon Basin. Aridity in the Amazon Basin seems to occur between 20.5 ka (calendar) to 17.0 ka and 13.6 ka to 11 ka. The second arid period correlates with the start of the Antarctic Cold Reversal and aridity continues throughout the Younger Dryas period. We find that the large-scale trends in Amazon River outflow are dissimilar to high-latitude variability in either hemisphere. Instead high-resolution variations correlate with the δ18O difference between Greenland and Antarctica ice core temperature records. This suggests a link between Hemispheric temperature gradients and moisture availability over the Amazon. Based on our results and previously published work we present a new testable ‘dynamic boundary-monsoon intensity hypothesis’, which suggests that tropical moisture is not a simple belt that moves north or south. Rather, the northern and southern boundaries of the South American Summer Monsoon (SASM) are independently dynamic and driven by temperature gradients within their individual hemispheres. The intensity of rainfall within the SASM, however, is driven by precessionally modulated insolation and the resultant convection strength. Combining these two influences produces the dynamic heterogenic changes in the moisture availability observed over tropical South America since the Last Glacial Maximum