Examining the biogeochemistry behind the organic rich sediments of Guaymas Basin, Gulf of California Rebecca S. Robinson and Julie Fliegler, Graduate School of Oceanography, University of Rhode Island, Narragansett, RI Nitrogen (N) is an essential nutrient. High productivity systems lose N from their bottom waters through denitrification in response to the depletion of oxygen associated with the oxidation of sinking organic matter. It has been hypothesized that N losses are countered through nitrogen fixation, the formation of bioavailable-N from atmospheric N2 by cyanobacteria, within the Gulf of California. Our results suggest that export productivity may also be supported by organisms tapping into subsurface nutrient-N pools in addition to or instead of N fixation. Sources of nutrient-N Our results indicate that annual productivity is supported primarily by the upwelling of nutrients during the spring, where nutrient-N is incompletely consumed. During the summer months, when N becomes scares, larger diatoms (examples below), who may be capable of vertical migration or living at depth under low light conditions, are responsible for a smaller, although significant, fraction of the total export. We cannot discount nitrogen fixation as a third source of nutrient-N, however we do not have strong evidence in support of it either. Approach Six sections of sediment core, spanning ~30-40 years, were sampled on cm and mm scale resolution. All samples were analyzed for %wt opal, %Corg, bulk sedimentary δ15N and diatom bound δ15N from two size fractions, greater and less than 63μm. Data from a representative section, from 247-254 cm below seafloor, is shown to the right. There is relatively little variation in the bulk N isotope profile while the diatom-bound fractions show significant changes. Most of the variability is attributed to seasonal and longer timescale changes export productivity.