Sources and Cycling of Dissolved Organic Carbon in the Gulf of Mexico: Insights from Stable (δ13C) and Radiocarbon (Δ14C) Signatures

Understanding the production and remineralization of marine dissolved organic carbon (DOC; 662 GtC) is of primary importance to the global carbon cycle. Together, DOC concentrations, stable (δ¹³C) and radiocarbon (Δ¹⁴C) isotopic measurements provide a powerful toolset for evaluating DOC sources and cycling in aquatic environments. However, to date the Δ¹⁴C and δ¹³C composition of total DOC in both the Mississippi River and the Gulf of Mexico (GOM) basin remains largely unconstrained. This has precluded our basic understanding of DOC biogeochemistry, its persistence and contribution to the base of the marine food web in an economically important U.S. ocean region. The Deepwater Horizon (DWH) spill event in 2010 further exemplified the need for understanding the baseline biogeochemistry of DOM across the terrestrial-marine interface in the Northern GOM. In particular, the relative persistence (e.g. biodegradation) and contribution of DWH oil to the DOC reservoir remains largely unknown. Here we present the first DOC Δ¹⁴C and δ¹³C depth profiles taken from five stations in the Northern GOM: 1) the Mississippi River mouth, 2) the shelf bound, aged river plume, 3) the shelf/slope near the Macondo Well site, 4) offshore in the Loop Current and 5) a nearshore mesoscale eddy. We will discuss these DOC Δ¹⁴C and δ¹³C data with three goals in mind. First, we will attempt to disentangle the complex interplay between riverine, coastal, open and deep ocean DOC cycling. Second we will compare these offshore data to a recently measured DOC Δ¹⁴C profile from waters feeding the GOM from the Caribbean in order to evaluate DOC cycling and residence time in the deep GOM basin. Finally, we will discuss results suggesting ~10-16% of DWH oil has been incorporated into the marine DOC reservoir.