Ocean Drilling Program Leg 155 Site 942 on the Amazon Fan is an ideal location for monitoring palaeoclimatic changes within a significant proportion of the Amazon Basin. We present n-alkane d13C and taraxerol and laevoglucosan concentration records from this site covering the last 38?ka. The entire n-alkane d13C record is constrained between -31 parts per thousand and -34 parts per thousand, which is well within the isotopic range occupied by C3 vegetation. The concentration and relative abundance of taraxerol, a mangrove indicator, varies by over an order of magnitude, but seems to have had no effect on the n-alkane d13C record. The laevoglucosan concentrations are extremely low during the last glacial period, suggesting a relatively low occurrence of forest fires. Laevoglucosan concentrations are highest between 13.5 and 12.5?ka, suggesting an increased incidence of Amazon forest fires at the very end of the Younger Dryas. These records, combined with previously published pollen records from Site 932, reveal no evidence for massive incursions of grasslands into Amazonia during the last glacial period, despite evidence of reduced outflow of the Amazon River indicating more arid conditions. We therefore suggest that savannah encroachment, as proposed by the Pleistocene refuge hypothesis, can be refuted as an explanation for high species endemism within the Amazon Basin, and alternative explanations are required. Copyright (c) 2012 John Wiley & Sons, Ltd.

The Amazon basin is a major component of the global carbon and hydrological cycles, a significant natural source of methane, and home to remarkable biodiversity and endemism. Reconstructing past climate changes in the Amazon basin is important for a better understanding of the effect of such changes on these critical functions of the basin. Using a novel biomarker proxy, based on the membrane lipids of soil bacteria with a new regional calibration, we present a reconstruction of changes in mean annual air temperatures for the Amazon catchment during the last 37 kyr B.P. Biomarkers were extracted from Ocean Drilling Program sediment core ODP942 recovered from the Amazon fan. The Amazon fan is a major depository for terrestrial sediments, with the advantage that the terrestrial material captured reflects a regional integration of the whole river catchment. The reconstructed tropical Amazonian temperatures were ∼5°C cooler at the Last Glacial Maximum (∼21°C) compared to modern values (∼26°C). This is in agreement with previous estimates of tropical continental temperatures in the tropical Amazon basin and tropical Africa during the Last Glacial Maximum. Moreover, we also illustrate how the soil bacterial membrane lipid record reveals major changes in basin dynamics and sediment provenance during the glacial‐Holocene transition, impacting the biomarker reconstructions from ∼11 kyr onward.

The intensification of the Northern Hemisphere Glaciation (INHG) was a major event in the development of the current climate state, and as one of the most productive regions in the world's oceans, the behaviour of the Benguela Upwelling System (BUS) following the INHG is of wide interest. To investigate post-INHG changes in productivity and organic matter accumulation, total organic carbon and biomarker accumulation rates were determined for sediments from COP Site 1083 and compared to alkenone-derived sea surface temperatures and nitrogen isotopic compositions. These data indicate that the interval between 2.6 and 2.4 Ma was characterized by dramatic changes in upwelling intensity and organic carbon export on the northern edge of the modern BUS. The upwelling is reflected by significant changes in alkenone-derived SST estimates between glacial and interglacial intervals, with a total variability of 16 degrees C. The studied interval is also characterized by large changes in organic matter export as reflected by changes in TOC and biomarker accumulation rates, which show maxima during OIS 98 and during the transition from OIS 97 to 96. Intervals of elevated TOC are also characterized by elevated concentrations of sedimentary microbial biomarkers and lower %CaCO3, suggesting that enhanced delivery of labile organic matter to the seafloor resulted in enhanced remineralisation with released CO2 being consumed by CaCO3 dissolution. However, in apparent contrast to recent Pleistocene sediments at the same site, organic matter export after the INHG was not solely driven by upwelling intensity. Of the three Pliocene glacial-interglacial cycles examined (OIS 101 to 96). each is unique with respect to the timing and magnitude of changes in organic matter accumulation. Each is also characterized by different algal assemblages as inferred from biomarker distributions, with OIS 97 and 96 particularly dominated by diatoms. We suggest that these differences reflect the important but evolving role of Southern Ocean waters in the Pliocene BUS: nutrient depletion of SO waters occurred during parts of Pliocene glacial intervals such that even intense upwelling did not persistently result in enhanced organic matter accumulation rates. (C) 2009 Elsevier B.V. All rights reserved.