Project: Collaborative Research: What controls the marine refractory DOC reservoir?

NSF Award Abstract:
Dissolved organic carbon (DOC) in the ocean consists of free-floating organic molecules left over from the metabolisms of microscopic marine biota, analogous to the organic residues left behind within soils on land. The ocean currently holds ~660 gigatons C of this DOC, making it the largest reservoir of reduced carbon in the ocean; its size rivaling that of atmospheric CO2. Approximately 97% of this reservoir is comprised of a long-lived fraction, with a mean radiocarbon age of 4000-6000 years, termed refractory DOC (RDOC). RDOC has been traditionally considered a relatively inert and slowly-cycling C reservoir, but recent research suggests its cycling may be more dynamic than previously thought. A dynamic RDOC cycle could play a significant role in Earth’s carbon-climate system, with oxidation of only ~0.4% of the marine RDOC reservoir in one year sufficient to counterbalance the current oceanic sink for atmospheric CO2. However, the biological, chemical, and physical processes that govern this reservoir have largely eluded quantification thus far. This project will use data assimilation of marine DOC concentrations and its isotopes in a numerical modeling synthesis to test, diagnose, and quantify the environmental processes responsible for the production and removal of marine RDOC. The processes that control the marine RDOC reservoir will then be represented in a prognostic Earth System Model, using a future climate forced simulation to reveal the sign and strength of the marine RDOC carbon-climate feedback.

The standard paradigm regarding the marine RDOC reservoir is that nearly all marine DOC is internally produced in the surface ocean by primary production and that a fraction persists for 1000’s of years due to production of chemically recalcitrant organic molecules during biological utilization, a process termed the microbial carbon pump. An alternative view is that the marine RDOC pool is instead comprised of a heterogenous mixture of externally & internally produced, modern & aged molecules that have localized sources/sinks and distinct residence times. For example, recently identified localized RDOC removal processes have been identified both at the sea surface (marine aerosol formation) and near the crustal/water column interface (hydrothermal vents, crustal aquifers). Reconciling the many hypothesized DOC sources, sinks, and persistence mechanisms while synthesizing a coherent understanding of what controls the marine RDOC reservoir will require numerical modeling at global scales. This project will utilize recent advances in global ocean basin coverage of marine DOC isotope (both 14C and 13C) measurements to diagnose and quantify the sources and sinks controlling the RDOC reservoir through the implementation of a novel global inverse model of DOC cycling and its isotopes. The pertinent RDOC cycling processes and mechanisms including carbon isotopes from the results of the inverse model will be implemented within the ocean biogeochemistry component (MARBL) of the Community Earth System Model, in order to assess the sign and magnitude of the marine RDOC carbon-climate feedback with a future climate forced coupled simulation. Two graduate students will be supported by this project, one each at UNH and UCSB. The UNH team will work with high school instructors to deliver ocean carbon cycle and Earth & Ocean science curricula modules. Two undergraduate summer interns in ocean science research located at UCSB will be recruited from HBCU’s.

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.