Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Polycyclic aromatic hydrocarbons (PAHs) are introduced into the marine environment via oil seeps/spills, riverine discharges, continental runoff, coastal erosion, and atmospheric deposition. An estimated 2.1 x 1010g of PAHs entered into the northern Gulf of Mexico (GOM) during the Deepwater Horizon (DWH) oil spill in 2010. It became evident following the oil spill that accurate quantification of ultimate fate of these potentially carcinogenic and/or mutagenic organic pollutants is extremely challenging. In general, very little is known about PAHs fate, distribution and accumulation in the open ocean ecosystems. This study determines the upper ocean vertical fluxes and sedimentary PAHs accumulation rates in the GOM. The concentrations of particulate and dissolved ΣPAH43 varied between 0.2-1.3 ng/L, and 24-58 ng/L, respectively during this study period in April 2012 and 2013. Dissolved ΣPAH43 were found to be orders of magnitude lower than the values reported during DWH oil spill. Sediment trap-based vertical fluxes of particulate ΣPAH43 varied between 2-8 μg m-2 d-1. The vertical fluxes are found to be an important loss term for PAHs in the upper ocean with 3-7% of total particulate PAHs inventory in the euphotic zone being lost daily via this pathway. The trap-independent PAHs fluxes estimated via 238U-234Th disequilibria are similar to the trap-derived fluxes, within the factor of three. The 234Th-based method provides a larger spatial coverage in relatively shorter time and thus can be more appropriate for upper ocean PAHs flux estimation in high traffic areas like the GOM. The concentrations and sediment ΣPAH43 accumulation rates in sediment cores collected one to three years after DWH varied between 26-160 ng/g and 1.4-63 ng cm-2y-1, respectively. Observed ΣPAH43 concentrations are similar to the background pre-spill values, indicating long term impacts of the oil spill on sediment PAHs to be minimal. The source diagnostic analyses suggest a noticeable change in PAHs composition in the last few years, towards lower molecular weight dominancy in PAHs profiles, attributed to the deposition of higher molecular weight PAHs byproducts by control combustion during the DWH oil spill. This study can serve as post-spill baseline PAHs concentrations and their accumulation rates in the GOM.
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Adhikari, Puspa Lal, "Polycyclic Aromatic Hydrocarbons Dynamics in the Northern Gulf of Mexico" (2015). LSU Doctoral Dissertations. 3028.