Sedimentation Pulse in the NE Gulf of Mexico following the 2010 DWH Blowout

Abstract

The objective of this study was to investigate the impacts of the Deepwater Horizon (DWH) oil discharge at the seafloor as recorded in bottom sediments of the DeSoto Canyon region in the northeastern Gulf of Mexico. Through a close coupling of sedimentological, geochemical, and biological approaches, multiple independent lines of evidence from 11 sites sampled in November/December 2010 revealed that the upper ~1 cm depth interval is distinct from underlying sediments and results indicate that particles originated at the sea surface. Consistent dissimilarities in grain size over the surficial ~1 cm of sediments correspond to excess (234)Th depths, which indicates a lack of vertical mixing (bioturbation), suggesting the entire layer was deposited within a 4-5 month period. Further, a time series from four deep-sea sites sampled up to three additional times over the following two years revealed that excess (234)Th depths, accumulation rates, and (234)Th inventories decreased rapidly, within a few to several months after initial coring. The interpretation of a rapid sedimentation pulse is corroborated by stratification in solid phase Mn, which is linked to diagenesis and redox change, and the dramatic decrease in benthic formanifera density that was recorded in surficial sediments. Results are consistent with a brief depositional pulse that was also reported in previous studies of sediments, and marine snow formation in surface waters closer to the wellhead during the summer and fall of 2010. Although sediment input from the Mississippi River and advective transport may influence sedimentation on the seafloor in the DeSoto Canyon region, we conclude based on multidisciplinary evidence that the sedimentation pulse in late 2010 is the product of marine snow formation and is likely linked to the DWH discharge.

Fig 1. Study area map.

Location map of the northeastern Gulf of Mexico showing core sites discussed here in proximity to the DWH wellhead, Desoto Canyon, the Mississippi River, and the extent of the sea surface oil slick (gray shading) mapped by Garcia-Pineda [11].

Fig 2. Core P-06 description.

Description of core P–06 collected in December 2010 showing a surficial brown layer containing multiple dark brown-black bands corresponding to Mn spikes, and distinct sediment texture/composition, benthic foraminifera density, natural abundance radiocarbon (∆¹⁴C), and biomarkers over the surficial ~1 cm (see Fig 1 for location).

Fig 3. Core D-08 description.

Description of core D–08 collected in December 2010 showing a surficial brown layer containing dark brown-black bands corresponding to Mn spikes, and distinct sediment texture/composition, benthic foraminifera density, natural abundance radiocarbon (∆¹⁴C), and biomarkers over the surficial ~1 cm (see Fig 1 for location).

Fig 4. Core D-10 description.

Description of core D–10 collected in December 2010 showing a surficial brown layer containing dark brown-black bands corresponding to Mn spikes, and a distinct sediment texture/composition, phytoplankton-affiliated gene sequences, natural abundance radiocarbon (∆¹⁴C), and biomarkers over the surficial ~1 cm (see Fig 1 for location).

Fig 5. Excess ²¹⁰Pb and ²³⁴Th profiles for time series sites.

Excess ²¹⁰Pb and excess ²³⁴Th profiles for time series cores collected at site M-04 in November 2010 and October 2012, and sites P–06, D–08, and D–10 collected in December 2010, February 2011, September 2011 and August 2012. Profiles are expanded to show the decrease in decay-corrected excess ²³⁴Th activities and excess ²³⁴Th depths following initial coring in December 2010 (see Fig 1 for core site locations).

Fig 6. Mass accumulation rates (MAR) and ²³⁴Th inventories for time series sites.

Graphs showing (A) average MAR over the past ~100 years calculated using excess ²¹⁰Pb, (B) MAR of the four time series sites from November 2010 to October 2012 calculated using excess ²³⁴Th, (C) excess ²³⁴Th inventories of the four time series sites from November 2010 to October 2012.