Sediment accumulation and radionuclide inventories (239,240Pu, 210Pb and 234Th) in the northern Gulf of Mexico, as influenced by organic matter and macrofaunal density

Six cores were collected from the Northern Gulf of Mexico (GOM) as part of the “Deepwater Program: Northern Gulf of Mexico Continental Slope Habitats and Benthic Ecology” (DGoMB). These cores were collected from water depths ranging from ≈700 to 3500 m and processed for radiochemical assays to determine particle reworking (bioturbation) and sedimentation rates in these sediments, pursuant to the research objective to investigate biological, chemical, and physical processes that control the environmental setting for GOM benthic fauna. Bioturbation rates were quantitatively derived from 234Thxs profiles, with 234Thxs penetration depths ranging between 0.5 and 4 cm, and bioturbation coefficient values (Db), ranging from ≈2 to 30 cm2/year. 234Thxs data were also used to determine the shallow depth limit for particle mixing. Significant values of 239,240Pu were found over the 3–15 cm depth range, without any pronounced peak activities suitable for sediment dating, indicating periodic and nonlocal mixing events. Sedimentation rates (S) were therefore calculated from 210Pbxs profiles, assuming steady state conditions, and using the constant flux-constant sedimentation (CF-CS) model. However, only 210Pbxs data below the 239,240Pu penetration depth were used for the purpose of determining an upper limit of S. The range of apparent sedimentation rates determined by this approach for these stations is 0.04–0.44 cm/year, after approximately 1000 m depth, sedimentation becomes essentially constant at ≈0.08 cm/year. To the best of our knowledge, there are no other published sedimentation rates for the GOM outside the immediate area of the Mississippi River Delta region. However, estimates of sedimentation rates presented here fall in line with rates from similar continental margin marine settings. 239,240Pu and 210Pbxs inventories are positively correlated (r=0.91, p=0.01; r=0.80, p=0.05, respectively) with macrofauna density, which itself correlates well (r=0.94, p=0.005) to particulate organic carbon (POC) inventories over the top 5 cm, the approximate average particle mixing (bioturbation) depths of 234Thxs at these GOM stations. However, these inventories did not correlate to benthic mixing intensities and depths. High macrofaunal densities and bioturbation intensities are a consequence of elevated rain rates of organic carbon, which is efficiently degraded at the sediment–water interface before incorporation into the sedimentary record. We hypothesize that relatively high densities of macrofauna in a given location on the sea floor, provide for a well mixed near surface sediment layer, wherein fallout radionuclides are more efficiently incorporated via adsorption and mixing into the sediment profile as compared to locations where macrofaunal densities are less, even when radionuclide fluxes might be similar.