Award: OCE-2023656

This project examined how transformation and bioavailability of organic carbon in sediments are impacted by hydrothermal heating, burial depth, organic carbon sources and composition, and microbial community with the goal of better understanding the role of hydrothermal systems in mobilizing buried sedimentary organic carbon into the active carbon cycle. We focused on drill core sediments recovered on IODP Expedition 385 from unheated, previously-heated and actively heated sites in Guaymas Basin, a tectonically-active basin in the Gulf of California. We employed an integrated, multidisciplinary approach using microbiological incubations and lipid biomarker analysis along with radiocarbon and stable carbon isotopic analyses. 

Our geochemical results confirm that active hydrothermal heating mobilizes buried organic matter, and further show that marine-derived organic matter is most vulnerable to thermal alteration. The shallowest sediment depths at the actively-heated site hosted the highest concentration of PAHs and thermally-cracked biomass biomarkers suggesting the presence of thermally-degraded organic matter. Terrestrial organic matter, on the other hand, was preferentially associated with minerals and relatively protected from thermal degradation. Hydrothermal heating appears to also have promoted mineral association as higher concentrations of bound leaf wax fatty acids were found in heated cores and released by acid and base hydrolysis. Mineral-bound biomarkers also had a more muted attenuation with depth compared to non-bound biomarkers. Our microbiology results from incubation experiments, in which the time-resolved isotopic composition of respired CO2 was measured, shed additional light on the mineral-bound fraction. With increasing exposure to heating, the bioavailable organic matter changed little in concentration or 13C content, but we discovered that it was more depleted in 14C, leading us to propose an “onion” model of organic matter association with minerals. In this model, layers of organic material with younger, more 14C-enriched organic matter on the outside, are progressively released by disruption of electrostatic interactions upon exposure to heating. The inner, more “aged” layers are exposed by heating and vulnerable to respiration by microbial populations, but the composition of organic matter is similar for all layers. Through these findings we also develop bioavailability as a quantitative proxy of sedimentary organic matter composition that could be used jointly with other measures of composition such as isotopic or biomarker composition.

In the actively-heated core, we also find that sediments retain their ability to generate small molecules (e.g., acetate) upon heating, and these are likely to support intensified microbial activity relative to background or previously-heated sites. Shallow sediments at the actively-heated site host the highest concentration of biomarkers for living microbial biomass. These biomass biomarker concentrations fall off rapidly with depth and increased temperature. We hypothesize that the shallow peak in microbial populations indicates they are supported by upward migration of thermally-altered organic matter generated in higher-temperature sediments below. We also find that microbial cooperation is a key contributor to the respiration of higher amounts of organic matter as well as organic matter with a more “aged” radiocarbon signature.

Two PhD students were trained and supported through this project, one at the University of Delaware and a second through an international collaboration with McGill University in Canada. These students received interdisciplinary training and opportunities to engage in public outreach. As part of the project, multiple videos explaining the fundamental science of this project were created as part of Delaware Sea Grant’s “15-Second Science” series on YouTube.

Last Modified: 06/26/2026
Modified by: Sunita R Shah Walter