Project: Hydrothermal and microbial controls on organic carbon mobilization from Guaymas Basin sediments

NSF abstract:
Organic carbon stored in marine sediments and sedimentary rocks are a critical component of the global carbon cycle. Buried organic carbon balances atmospheric CO2 concentrations over long timescales and is linked to the regulation of Earth’s climate. Microbial activity is a key driver of processes that break down and release carbon from sediments. This project investigates organic carbon in marine sediments and examines its bioavailability to microbes. The setting for the project is the Guaymas Basin of the Gulf of California which receives organic matter inputs from land during seasonal rains and from marine phytoplankton at the sea surface. Guaymas Basin is also a site of active seafloor spreading and hydrothermal heating (i.e., the transfer of heat from magma to sediments and ocean water), which will allow the effects of heating on the bioavailable portion of organic carbon to be investigated. Results from this project will improve our understanding of how microbes reintroduce this buried organic carbon back into the “active” carbon cycle, as well as how this microbial activity is influenced by hydrothermal heating and organic carbon composition. The project also incorporates an international collaboration and an educational cross-border exchange of students between the US and Canada. Results will be shared broadly through videos produced by the Delaware Sea Grant and a content created by the Canadian podcast, Broad Science.

Using the gradients available in the context of Guaymas Basin sediments, we will investigate the effects of hydrothermal heating, burial depth (physical factors), sedimentary organic carbon composition and oxygen availability (geochemical factors) and microbial community (biological factors) on the bioavailability and mobilization/utilization of sedimentary organic carbon. Guaymas Basin features several advantages that make it uniquely suited for our proposed studies including the fact that 150 meters of sediment thickness are within the analytical window of radiocarbon dating which will allow us to take advantage of 14C (radiocarbon) as a natural tracer of marine carbon sources. Large gradients of temperature and geochemistry are also created by widespread emplacement of magmatic sills at varied depths in the sediment column. Such hydrothermal heating transforms sedimentary organic carbon in some locations from freshly synthesized photosynthetic biomass to petroleum hydrocarbons on a compressed, 14C-friendly timescale. Analyses will include: (1) carbon isotopic analysis of biomarkers representative of marine-derived, terrestrial and hydrothermal organic carbon reservoirs within sediments, (2) carbon isotopic measurements on biomolecules representative of natural microbial populations, (3) aerobic bioreactor incubations of sediments with their natural microbial population and analysis of the isotopic composition of CO2 released by respiration, and (4) aerobic bioreactor incubations of sediments that have been pre-heated to promote organic carbon transformations and isotopic analysis of CO2 respired by a bacterial isolate. Our results will provide an exceptionally detailed view of the dependence of organic carbon bioavailability on burial depth, heating history, in situ temperature, oxygen availability, organic carbon composition, and microbial functional capabilities that, to our knowledge, will be unprecedented.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.