Origin, transport and deposition of leaf-wax biomarkers in the Amazon Basin and the adjacent Atlantic

Author(s)

Haeggi, Christoph ; Sawakuchi, Andre O. ; Chiessi, Cristiano M. ; Mulitza, Stefan ; Mollenhauer, Gesine ; Sawakuchi, Henrique O. ; Baker, Paul A. ; Zabel, Matthias ; Schefuss, Enno

• Document type: Journal article
• Source: GEOCHIMICA ET COSMOCHIMICA ACTA; v. 192, p. 149-165, NOV 2016.
• Web of Science Citations: 15

Abstract

Paleoenvironmental studies based on terrigenous biomarker proxies from sediment cores collected close to the mouth of large river systems rely on a proper understanding of the processes controlling origin, transport and deposition of biomarkers. Here, we contribute to the understanding of these processes by analyzing long-chain n-alkanes from the Amazon River system. We use the delta D composition of long-chain n-alkanes from river bed sediments from the Amazon River and its major tributaries, as well as marine core-top samples collected off northeastern South America as tracers for different source areas. The delta C-13 composition of the same compounds is used to differentiate between long-chain n-alkanes from modern forest vegetation and petrogenic organic matter. Our delta C-13 results show depleted delta C-13 values (-33 to -36%) in most samples, indicating a modern forest source for most of the samples. Enriched values (-31 to -33%) are only found in a few samples poor in organic carbon indicating minor contributions from a fossil petrogenic source. Long-chain n-alkane delta D analyses show more depleted values for the western tributaries, the Madeira and Solimoes Rivers (-152 to -168%), while n-alkanes from the lowland tributaries, the Negro, Xingu and Tocantins Rivers (-142 to -154%), yield more enriched values. The n-alkane delta D values thus reflect the mean annual isotopic composition of precipitation, which is most deuterium-depleted in the western Amazon Basin and more enriched in the eastern sector of the basin. Samples from the Amazon estuary show a mixed long-chain n-alkane delta D signal from both eastern lowland and western tributaries. Marine core-top samples underlying the Amazon freshwater plume yield delta D values similar to those from the Amazon estuary, while core-top samples from outside the plume showed more enriched values. Although the variability in the river bed data precludes quantitative assessment of relative contributions, our results indicate that long-chain n-alkanes from the Amazon estuary and plume represent an integrated signal of different regions of the onshore basin. Our results also imply that n-alkanes are not extensively remineralized during transport and that the signal at the Amazon estuary and plume includes refractory compounds derived from the western sector of the Basin. These findings will aid in the interpretation of plant wax-based records of marine sediment cores collected from the adjacent ocean. (C) 2016 Elsevier Ltd. All rights reserved. (AU)

• FAPESP's process: 12/17517-3 - Response of the Western Atlantic Ocean to changes in the Atlantic meridional overturning circulation: from millennial to seasonal variability
• Grantee: Cristiano Mazur Chiessi
• Support Opportunities: Research Program on Global Climate Change - Young Investigators
• FAPESP's process: 11/14502-2 - Regulating factors of methane (CH4) emission in depositional environments of the Rivers Negro, Tapajos, Xingu and Amazonas
• Grantee: Henrique Oliveira Sawakuchi
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 11/06609-1 - Provenance, transport and storage of sediments in Amazon rivers
• Grantee: André Oliveira Sawakuchi
• Support Opportunities: Regular Research Grants