UTILIZING 234TH AND 210PB GEOCHRONOLOGIES TO DETECT INCREASED SEDIMENT MASS ACCUMULATION FOLLOWING THE BP BLOWOUT EVENT: NE GULF OF MEXICO

Multicores from >40 sites along the NE Gulf of Mexico slope indicate shifts in sediment accumulation patterns during the months following the BP Blowout event. This is manifested as a 1-6 cm thick well-defined, internally stratified medium-dark brown surface layer of sandy mud, which is finer grained and has a higher organic content than underlying sediments. It also exhibits a hydrocarbon signal much closer to the subsurface oil detected following the BP blowout event, than the land-derived signal characteristic of underlying sediments.

A number of core sites have been revisited multiple times (Aug, 2010, Dec. 2010, Feb, 2011, and Sept. 2011) following the event in order to determine if there have been changes in sedimentation patterns on monthly time scales. Due to the stratigraphic integrity of these cores, Th-234 is utilized as a geochronological tool, indicating active sedimentation during 4-6 months prior to core collection. Pb-210 is utilized to aid in defining pre-event sedimentation patterns and the natural variability over the past 100 years. Cores collected in Dec. 2010 indicated increased MAR’s (mass accumulation rates) of 0.7-1.8 g/cm²/yr (based on Th-234 dating), as compared to pre-event rates 0.06-0.1 g/cm²/yr (based on Pb-210 dating). This indicates a pulse of sediment accumulation occurred between Aug. 2010 and Dec. 2010. Re-collection of cores in Sept. 2011 at the same sites indicated that sediment MAR’s of 0.1-0.2 g/cm²/yr (based on Th-234 dating) between June and Sept. 2011 had returned to pre-event MAR’s. As this example shows, paired Th-234 and Pb-210 age dating can be a viable tool for tracking very short time scale variations in sediment accumulation patterns.