Sources of carbon to suspended particulate organic matter in the northern Gulf of Mexico

Suspended particulate organic carbon (POCsusp) in the Gulf of Mexico is unique compared to other seas and oceans. In addition to surface primary production, isotopic analysis indicates that microbial cycling of oil and riverine inputs are primary sources of carbon to POCsusp in the Gulf. To characterize POCsusp from seep sites and non-seep north central Gulf (NCG) sites potentially affected by the Deepwater Horizon (DWH) spill, we analyzed 277 and 123 samples for delta C-13 and Delta C-14 signatures, respectively. Depth, partitioned into euphotic (<300 m) and deep (>300 m), was the main driver of spatial delta C-13 differences, with deep depths exhibiting C-13 depletion. Both deep depths and proximity to sources of natural seepage resulted in C-14 depletion. A two-endmember mixing model based on Delta C-14 indicated that sources to POCsusp were 14-29% fossil carbon at NCG sites and 19-57% at seep sites, with the balance being modern surface production. A six-component Bayesian mixing model MixSIAR, using both C-13 and C-14, suggested that riverine inputs were an important carbon source to POCsusp contributing 34-46%. The influence of seeps was localized. Below the euphotic zone at seep sites, 46 +/- 5% (n = 9) of the carbon in POCsusp was derived from environmentally degraded, transformed oil; away from seeps, transformed oil contributed 15 +/- 4% (n = 39). We hypothesized that, at NCG sites removed from hydrocarbon seep sources, isotopic signatures would be depleted following the spill and then shift towards background-like enriched values over time. At deep depths we observed decreasing Delta C-14 signatures in POCsusp from 2010 to 2012, followed by isotopic enrichment from 2012 to 2014 and a subsequent recovery rate of 159% per year, consistent with this hypothesis and with biodegraded material from DWH hydrocarbons contributing to POCsusp.