Mechanisms of terrestrial organic carbon export and preservation in the marine environment

Abstract

Export of terrestrial carbon from land to sea is a globally important carbon flux that is poorly constrained and has implication for atmospheric carbon levels over modern and geologic timescales. Many factors control the fate of exported carbon and the subsequent impact on carbon budgets, including the timescales of export, the composition of organic matter, and degradation processes. This thesis uses biomarkers, bulk geochemical tools, and incubation studies to interrogate the factors controlling terrestrial carbon export and preservation in the marine environment. The thesis focuses on two globally important river systems that collectively deliver 25% of the total terrestrial carbon flux to the ocean, the Ganges-Brahmaputra (G-B) Rivers and the Amazon River. The first two chapters focus on the G-B Rivers, utilizing compound specific biomarker analysis within a high sedimentation rate (30 cm/yr) terrestrial archive in the Bay of Bengal, we interrogate (i) timescales of organic carbon export from land to sea, and (ii) basin-scale geochemical responses to rice agriculture expansion. These analyses utilize the radiocarbon ages and stable carbon-13 isotopic composition of lipids produced by Archaea and Bacteria. We identify that ca. 75% of these biomarkers experience millennial scale storage in the G-B basin, in agreement with previously assessed plant-derived compounds, highlighting that an overarching soil stabilization mechanism controls the age of exported terrestrial organic matter. Individual biomarkers and bulk geochemical analysis chronicle the change in methane-derived soil carbon within the basin due to rice paddy expansion, highlighting that 49% of Bangladesh’s methane emissions from 1990-2008 have been abated by soil storage. The last two chapters focus on the Amazon River, to examine the fate of terrestrial organic carbon in the marine environment, (iii) utilizing geochemical analysis of historical sediments and sediments from a field campaign in 2023, and (iv) utilizing terrestrial and marine endmembers in incubation experiments simulating the dynamic coastal environment. Sediment geochemical and biomarker analyses highlight the preservation of an isotopically distinct terrestrial endmember in the coastal sediments, which has led to at least 50% underestimation of the burial efficiency. Quantitative stable isotope probing incubations using 13C-lignin indicate the dual role of microbially-mediated and photo-degradation, and highlight that the microbial communities primarily responsible for lignin degradation in the marine environment are of terrestrial origin, and identify a new ecological role for Bathyarchaeota. This thesis integrates diverse biogeochemical techniques across the terrestrial-marine interface to examine important open questions in globally important carbon budgets, merging isotope geochemistry, microbiology and earth science. The findings contribute to our understanding of the modern carbon cycle and the impact of anthropogenic perturbations of the last decades and into the future.