Little is known of modern carbon cycling in East African rift lakes. These large tropical systems with anoxic bottom waters can act as test cases for carbon cycling under future climate change scenarios as well as key sediment repositories for paleolimnology studies and fossil fuel generation. This study is the first to combine stable and radiocarbon isotopic information on bulk sediment and biomarkers (n-alcohols) in an African rift lake by investigating surface sediments collected on a nearshore to offshore transect in northern Lake Malawi. This study is also the first to report radiocarbon content in water column particulate organic, dissolved organic and inorganic carbon from an African rift lake. General surface-sediment trends with increasing water depth were an increase in %TOC, and decreases in grain size, δ13C and C/N. Carbon radioisotope analyses showed that bulk sedimentary organic matter was modern across the transect. Biomarker data indicated a primarily aquatic origin for sedimentary organic matter at offshore sites; terrestrial material dominated the n-alcohol signature closest to the shore. Protokerogen stable isotope signatures were similar, and radiocarbon was somewhat depleted relative to bulk sediment organic matter. As compared to bulk organic matter, lipids were 13C-depleted throughout the sampling transect and 14C-depleted at the most nearshore location. This indicates an aged lipid carbon source and likely significant diagenetic alteration of modern sources prior to deposition in nearshore sediments, perhaps a combination of soil erosion, in-river processing, sediment resuspension and mobilization, and preferential degradation of younger organic matter in the oxic nearshore region.
|Original language||English (US)|
|State||Published - Jan 2023|
Bibliographical noteFunding Information:
This work was supported by the American Chemical Society Petroleum Research Fund (ACS-PRF #48595-AC2, to ECM), funding from the University of Minnesota, Office of International Programs (to ECM and JPW), and a NOSAMS graduate internship (to BRK). The authors thank Dr. Thomas Johnson from the Large Lakes Observatory, University of Minnesota Duluth for leading the coring work in Malawi and for comments on early versions of this manuscript, Erik Brown for assistance sampling the Lake Malawi water column, Julia Halbur for data acquisition, Ehen Inkel for grain size analyses, and Evan Collins and Molly O’Beirne for assistance with drafting figures. The authors also thank the reviewers, Associate Editor Dr Julian Sachs, and Co-Editor-in-Chief Dr John Volkman for thoughtful comments that significantly improved the manuscript.
© 2022 Elsevier Ltd
- C/N ratio
- Carbon isotopes
- Lake Malawi
- Sedimentary organic carbon