Amazon Fan biomarker evidence against the Pleistocene rainforest refuge hypothesis?

Abstract

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 δ 13 C and taraxerol and laevoglucosan concentration records from this site covering the last 38 ka. The entire n ‐alkane δ 13 C record is constrained between −31‰ and −34‰, which is well within the isotopic range occupied by C 3 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 δ 13 C 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 © 2012 John Wiley & Sons, Ltd.

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