Controls on the distributions of GDGTs and n-alkane isotopic compositions in sediments of the Amazon River Basin

Author(s)

Bertassoli, D. J., Jr. ; Haeggi, C. ; Chiessi, C. M. ; Schefuss, E. ; Hefter, J. ; Akabane, T. K. ; Sawakuchi, A. O.

• Document type: Journal article
• Source: CHEMICAL GEOLOGY; v. 594, p. 12-pg., 2022-04-05.

Abstract

Lipid biomarker proxies from terrigenous sediments have been extensively used to understand variations in paleoenvironmental conditions, but many of the mechanisms affecting these proxies during riverine transport are still poorly understood. Here, we analyze glycerol dialkyl glycerol tetraether (GDGT) distributions and n-alkane isotopic compositions of soils and sediments from the Amazon River Basin. Our dataset includes suspended sediments of the Amazon River and its main tributaries, as well as soils and sediments of the Xingu River, a large clearwater tributary draining the easternmost part of the Amazon River Basin. Our sampling design aimed at understanding the processes behind spatially distinct GDGT distributions and n-alkane isotopic signatures across lowland Amazonia. Gradual changes in the fractional abundances of isoprenoid GDGTs and in 5- and 6- methyl branched GDGT ratios in suspended sediments of the Amazon River towards its mouth suggest that riverine production is an increasingly important control on the distribution of GDGTs in the lower parts of the system, while values from the western parts are more in line with a dominant soil sourcing. In the Xingu River, indices based on the fractional abundances of branched GDGTs and long-chain n-alkanes demonstrate a strong contribution of terrestrial organic material during the high-water season and an important aquatic component during low-water season. Meanwhile, average stable carbon (delta C-13) and hydrogen (delta D) isotopic signatures of long-chain n-alkanes in soils, riverbed and suspended sediments of the Xingu River are similar and reinforce the relatively conservative behaviour of these proxies within large river systems. The average compound-specific delta C-13 signatures of sediments in the Xingu River are within the expected range for C-3 vegetation and do not seem to capture the signals from the nearby deforested areas. n-Alkanes delta D signals in the Xingu Basin are similar to values obtained in the Amazon River mouth and indicate that n-alkanes sourced from easternmost Amazonian lowlands may predominate over signals from western areas. (AU)

• FAPESP's process: 16/02656-9 - The response of sedimentary dynamics of the Xingu and Tapajós rivers to climate changes and hydropower dams: risks for biodiversity conservation and energy production in Amazonia
• Grantee: André Oliveira Sawakuchi
• Support Opportunities: Research Program on Global Climate Change - Regular Grants
• FAPESP's process: 19/24349-9 - Assessing the effects of past and future climate change on Amazonian biodiversity (CLAMBIO)
• Grantee: Cristiano Mazur Chiessi
• Support Opportunities: BIOTA-FAPESP Program - Regular Research Grants
• FAPESP's process: 19/24977-0 - Environmental signals recorded in modern sediments of tropical South American rivers
• Grantee: Dailson José Bertassoli Junior
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• 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: 19/19948-0 - Influence of Late Pleistocene climate changes on the Amazon vegetation
• Grantee: Thomas Kenji Akabane
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 18/15123-4 - Past perspectives on tipping elements of the climate system: the Amazon Rainforest and the Atlantic Meridional Overturning Circulation (PPTEAM)
• Grantee: Cristiano Mazur Chiessi
• Support Opportunities: Research Grants - Research Program on Global Climate Change - Young Investigators - Phase 2
• FAPESP's process: 18/23899-2 - Trans-Amazon Drilling Project: origin and evolution of the forests, climate, and hydrology of the South American tropics
• Grantee: André Oliveira Sawakuchi
• Support Opportunities: Research Program on Global Climate Change - Thematic Grants
• FAPESP's process: 17/25735-4 - Organic geochemistry analysis to reconstruct hydroclimate changes across eastern and western Amazon during the last 4,000 years
• Grantee: Dailson José Bertassoli Junior
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate