Isotopic Composition Of Sinking Particles

The extensive release of oil during the 2010 Deepwater Horizon spill in the northern Gulf of Mexico perturbed the pelagic ecosystem and associated sinking material. To gauge the recovery and post-spill baseline sources, we measured Delta C-14, delta C-13 and delta S-34 of sinking particles near the spill site and at a reference site and natural seep site. Particulates were collected August 2010-April 2016 in sediment traps moored at sites with depths of 1160-1660 m. Near the spill site, changes in Delta C-14 indicated a 3-year recovery period, while delta S-34 indicated 1-2 years, which agreed with estimates of 1-2 years based on hydrocarbon composition. Under post-spill baseline conditions, carbon inputs to sinking particulates in the northern Gulf were dominated by surface marine production (80-85%) and riverine inputs (15-20%). Near the spill site, Delta C-14 values were depleted in October 2010 (-140 to -80 parts per thousand), increasing systematically by 0.07 +/- 0.02 parts per thousand day(-1) until July 2013 when values reached -3.2 +/- 31.0 parts per thousand. This Delta C-14 baseline was similar to particulates at the reference site (3.8 +/- 31.1 parts per thousand). At both sites, delta C-13 values stayed constant throughout the study period (-21.9 +/- 0.5 parts per thousand and -21.9 +/- 0.9 parts per thousand, respectively). delta S-34 near the spill site was depleted (7.4 +/- 3.1 parts per thousand) during October 2010-September 2011, but enriched (16.9 +/- 2.0 parts per thousand) and similar to the reference site (16.2 +/- 3.1 parts per thousand) during November 2012-April 2015. At the seep site,Delta C-14 values were -21.7 +/- 45.7 parts per thousand except during August 2012-January 2013 when a significant Delta C-14 depletion of -109.0 +/- 29.1 parts per thousand was observed. We interpret this depletion period, also observed in delta C-13 data, as caused by the incorporation of naturally seeped oil into sinking particles. Determination of post-spill baselines for these isotopic signatures allows for evaluation of anthropogenic inputs in future.