Drivers of Oxygen Consumption in the Northern Gulf of Mexico Hypoxic Waters—A Stable Carbon Isotope Perspective

Abstract

We examined the stable carbon isotopic composition of remineralized organic carbon (δ13COCx) in the northern Gulf of Mexico (nGoM) using incubations (sediment and water) and a three-end-member mixing model. δ13COCx in incubating sediments was −18.1‰ ± 1.3‰, and δ13COCx in incubating near-surface and near-bottom waters varied with salinity, ranging from −30.4‰ to −16.2‰ from brackish water to full-strength Gulf water. The average δ13COCx was −18.6‰ ± 1.8‰ at salinity >23. A three-end-member mixing model based on a multiyear data set collected in previous summer hypoxia cruises (2011, 2012, 2014, 2015, and 2016) suggested that δ13COCx in near-bottom waters across the nGoM (5–50 m) was −18.1‰ ± 0.6‰. The close agreement of δ13COCx obtained from the three independent approaches, that is, incubations of water column, surface sediments, and mixing model, suggests that 13C-enriched organic matter of marine origin played the dominant role in near-bottom water and benthic oxygen consumption in the nGoM shelf in summer.