Stable Carbon Isotope Composition of Remineralizing Organic Matter in the Northern Gulf of Mexico Continental Shelf

Despite years of study, whether water column or benthic respiration controls oxygen consumption in the seasonally hypoxic Northern Gulf of Mexico (nGOM) waters remains contentious. Elucidating this control is essential for long-term nutrient management purposes. In this study, we examined stable carbon isotope composition (δ¹³C) of organic matter that was remineralized in subsurface water of the nGOM continental shelf using both shipboard incubations (water and sediment) and a three-endmember mixing model. Based on our 2014 data, sediment and water incubations yielded disparate δ¹³C signatures in the respiration produced CO₂, with the sediment incubation generating substantially more ¹³C-enriched CO₂ (-16~-21‰ in sediment vs. -27~-29‰ in water), though water column bulk particulate organic matter (POM) had δ¹³C ranging from -23‰ to -25‰. However, from the three-endmember mixing model, our calculated CO₂-δ¹³C due to respiration in the entire surveyed nGOM shelf was -18.5‰. This value was consistent with the results obtained in previous annual shelfwide cruises (-17.2~-19.5‰). The close agreement between respirational CO₂-δ¹³C from the mixing model and that from sediment incubation suggests that benthic process likely played a dominant role in subsurface respiration in the nGOM shelf. This result also indicates that hydrocarbon remineralization was likely insignificant on the ecosystem level after the 2010 Deepwater Horizon oil spill.