RAPID INCREASE IN SEDIMENT ACCUMULATION IN THE NE GULF OF MEXICO FOLLOWING THE 2010 BP BLOWOUT: A NOVEL APPLICATION OF 234TH AND 210PB

Over 40 sites between 100 m and 1500 m water depths on the west Florida outer shelf/slope (Desoto Canyon region) in the northeastern Gulf of Mexico were sampled with a multicorer designed specifically to collect surface sedimentary layers with essentially no disturbance. Cores recovered a well-defined, medium-dark brown surface layer of sandy mud ranging from 1-6 cm in thickness, with a sharp basal contact and internal stratification, indicating a lack of vertical mixing.

Thorium-234 (half-life ~24.1 days) is commonly used to detect depth of mixing (e.g. bioturbation) in surface sediments. The stratigraphic integrity and well-defined basal contact of the surface layer in this study provides an excellent opportunity to test the use of ²³⁴Th as a geochronological tool for determining accumulation rates and precise timing of events on sub-annual timescales. Lead-210 (half-life ~ 22.4 years) data help constrain the timing of surface layer deposition, and provide accumulation rates over the past ~100 years for comparison.

Thorium-234 profiles display a down-core decay curve ideal for use as a geochronological tool. Data indicate that the entire 1- 6 cm-thick surface layer was deposited in less than six months at mass accumulation rates ranging from 0.6 to 20 g/cm²/yr, compared to 0.02 to 0.2 g/cm²/yr for underlying sediments determined by ²¹⁰Pb (consistent with other researchers) over the past ~100 years.

Results show that, even with the most conservative estimates, sediment accumulation rates have increased by at least one order of magnitude in the northeastern Gulf of Mexico since the 2010 BP blowout event, and that ²³⁴Th can be a viable sub-annual-scale geochronological tool where sediments are accumulating very rapidly in the absence of mixing.