Stable and radiocarbon isotopic composition of dissolved organic matter in the Gulf of Mexico

Dissolved organic carbon (DOC) is of primary importance to marine ecosystems and the global carbon cycle. Stable carbon (C-13) and radiocarbon (C-14) isotopic measurements are powerful tools for evaluating DOC sources and cycling. However, the isotopic signature of DOC in the Gulf of Mexico (GOM) remains almost completely unknown. Here we present the first DOC C-14 and C-13 depth profiles from the GOM. Our results suggest the Mississippi River exports large amounts of DOC with an anthropogenic bomb C-14 signature. Riverine DOC is removed and recycled offshore, and some marine production of DOC is observed in the river plume. Offshore profiles show that DOC has higher C-14 than its Caribbean feed waters, indicative of a modern deep DOC source in the GOM basin. Finally, high DOC with negative C-13 and C-14 values were observed near the Macondo Wellhead, suggesting a transformation of Deepwater Horizon hydrocarbons into a persistent population of DOC. Plain Language Summary The Deepwater Horizon (DWH) spill released 4.1-4.6 million barrels of oil over 87days to the deep Gulf of Mexico (GOM) and is considered one of humankind's largest environmental disasters. Now, 7years after DWH, a significant portion of this oil (10-25%) remains unaccounted for. We estimate that 10-16% of DWH hydrocarbon has been incorporated into marine dissolved organic carbon (DOC)the largest organic C reservoir in the ocean. Our results suggest that natural GOM bacteria transformed much of the spilled petroleum and methane into natural DOC molecules, effectively dissolving this C into deep ocean water. Our results also highlight the dynamic cycling of DOC in the GOM basinincluding the rapid removal of modern Mississippi River C as its plume moves offshore and a modern C contribution to the deep GOM DOC reservoir.