Modern and late Pleistocene particulate organic carbon transport by the Amazon River: Insights from long-chain alkyl diols

Author(s)

Haggi, Christoph ^{[1, 2, 3]} ; Schefuss, Enno ^[1] ; Sawakuchi, Andre O. ^[4] ; Chiessi, Cristiano M. ^[5] ; Mulitza, Stefan ^[1] ; Bertassoli Jr, Dailson J. ; Hefter, Jens ^[6] ; Zabel, Matthias ^[1] ; Baker, Paul A. ^[7] ; Schouten, Stefan ^{[2, 3, 8]}

Affiliation

^[1] Univ Bremen, MARUM Ctr Marine Environm Sci, Leobener Str 8, D-28359 Bremen - Germany

^[2] Royal Netherlands Inst Sea Res, NIOZ, Dept Marine Microbiol & Biogeochem MMB, POB 59, NL-1790 AB Den Burg - Netherlands

^[3] Univ Utrecht, POB 59, NL-1790 AB Den Burg - Netherlands

^[4] Univ Sao Paulo, Inst Geosci, Rua Lago 562, BR-05508080 Sao Paulo, SP - Brazil

^[5] Univ Sao Paulo, Sch Arts Sci & Humanities, Av Arlindo Bettio 1000, BR-03828000 Sao Paulo, SP - Brazil

^[6] Helmholtz Ctr Polar & Marine Res, Alfred Wegener Inst, Handelshafen 12, D-27570 Bremerhaven - Germany

^[7] Duke Univ, Div Earth & Ocean Sci, Durham, NC 27708 - USA

^[8] Univ Utrecht, Fac Geosci, Dept Earth Sci, Budapestlaan 4, NL-3584 CD Utrecht - Netherlands

• Document type: Journal article
• Source: GEOCHIMICA ET COSMOCHIMICA ACTA; v. 262, p. 1-19, OCT 1 2019.
• Web of Science Citations: 0

Abstract

The relative abundance of the C-32 1,15 long-chain alkyl diol (LCD) is an emerging proxy for the input of riverine aquatic particulate organic carbon (POC) into coastal oceans. This compound has the potential to complement other established proxies reflecting riverine terrestrial POC input and allows for a more nuanced assessment of riverine POC export to coastal seas. The current understanding of this proxy is, however, limited. In this study, we compare different indices for riverine sediment input to coastal marine waters (i.e. C-32 1,15-LCD, BIT index and Fe/Ca ratio) in a source-to-sink assessment in the Amazon River drainage system and the northeast South American continental margin, and we test their down-core applicability in a marine gravity core containing late Pleistocene fluvial Amazonian sediments. We show that the relative abundance of the C-32 1,15-LCD is highest in water bodies with low flow velocity and low turbidity such as the downstream portion of lowland tributaries and floodplain lakes. Relative C-32 1,15-LCD abundance is lowest in Andean white water tributaries where autotrophic productivity is hindered by high turbidity and high flow velocity. We also find that suspended particulate matter from all major tributaries during the extreme 2015 dry season has a similar LCD distribution to that of floodplain lakes. This indicates that the chemical composition of the tributaries is less relevant for the LCD distribution than their physical properties such as flow velocity and turbidity. Results from marine surface sediments offshore the Amazon River estuary show significant positive correlations between all three studied proxies. In contrast, we find that the relative C-32 1,15-LCD abundance in the down-core record is anti-correlated to the BIT index and Fe/Ca ratio. While BIT index and Fe/Ca ratio show high (low) values during Heinrich stadials (Dansgaard-Oeschger interstadials), the C-32 1,15-LCD proxy shows the opposite signal. BIT values are also higher during Marine Isotope Stage (MIS) 2 than during MIS 3, in contrast to trends in the C-32 1,15-LCD proxy. We posit that this pattern arises from a reduction in relative C-32 1,15-LCD abundance and total LCD productivity in the Amazon River during MIS 2 when less-humid conditions and lower sea level led to reduced area of floodplains. During Heinrich stadials, Andean sediment input increased and led to higher turbidity that resulted in lower C-32 1,15-LCD production. Our study shows that major changes in water discharge, sediment transport and river morphology can lead to discrepancies between the BIT index and the relative abundance of the C-32 1,15-LCD. Thus, we suggest that Amazonian aquatic and terrestrial POC pools had contrasting responses to changes related to both climate (e.g. increased Andean precipitation) and river morphology (e.g. steeper along-channel slope due to falling and low stand sea level). (C) 2019 Elsevier Ltd. All rights reserved. (AU)

• FAPESP's process: 16/11141-2 - Hydrologic variability and sediment supply of the Xingu and Tapajós rivers: climate change and anthropogenic impacts in eastern Amazon rivers during Holocene
• Grantee: Dailson José Bertassoli Junior
• Support Opportunities: Scholarships in Brazil - Doctorate
• 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: 12/50260-6 - Structure and evolution of the Amazonian biota and its environment: an integrative approach
• Grantee: Lúcia Garcez Lohmann
• Support Opportunities: BIOTA-FAPESP Program - Thematic Grants