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Geology; April 2003; v. 31; no. 4; p. 291-294; DOI: 10.1130/0091-7613(2003)031<0291:CITVOT>2.0.CO;2
© 2003 Geological Society of America
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Constancy in the vegetation of the Amazon Basin during the late Pleistocene: Evidence from the organic matter composition of Amazon deep sea fan sediments

Thomas P. Kastner*,1 and Miguel A. Goñi*,1

1 Organic Geochemistry Laboratory, Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA

Analyses of more than 60 sediment samples from the Amazon deep sea fan show remarkably constant terrigenous biomarkers (lignin phenols and cutin acids) and stable carbon isotopic compositions of organic matter ({delta}13COM) deposited from 10 to 70 ka. Sediments from the nine Amazon deep sea fan channel-levee systems investigated in this study yielded relatively narrow ranges for diagnostic parameters such as organic carbon (OC) normalized total lignin yields ({Lambda} = 3.1 ± 1.1 mg/100 mg OC), syringyl:vanillyl phenol ratios (S/V = 0.84 ± 0.06), cinnamyl:vanillyl phenol ratios (C/V = 0.08 ± 0.02), isomeric abundances of cutin-derived dihydroxyhexadecanoic acid (f10,16-OH = 0.65 ± 0.02), and {delta}13COM (–27.6% ± 0.6 {per thousand}). Our measurements support the hypothesis that the vegetation of the Amazon Basin did not change significantly during the late Pleistocene, even during the Last Glacial Maximum. Moreover, the compositions obtained from the Amazon deep sea fan are similar to those of modern Amazon River suspended sediments. Such results strongly indicate that the current tropical rainforest vegetation has been a permanent and dominant feature of the Amazon River watershed over the past 70 k.y. Specifically, we found no evidence for the development of large savannas that had been previously postulated as indicators of increased glacial aridity in Amazonia. Climate models need to be modified to account for the uninterrupted input of moisture to the tropical Amazon region over the late Pleistocene–Holocene period.

Key Words: Amazon deep sea fan • Amazon Basin • lignin • cutin acids • 13C • paleoclimate • late Pleistocene




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