Date of Award

Spring 5-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Marine Science

Committee Chair

Dr. Alan Shiller

Committee Chair Department

Marine Science

Committee Member 2

Dr. Laodong Guo

Committee Member 3

Dr. Karen Johannesson

Committee Member 4

Dr. Jerry Wiggert

Committee Member 4 Department

Marine Science

Committee Member 5

Dr. Scott Milroy

Committee Member 5 Department

Marine Science

Abstract

Selected trace elements (TEs), dissolved organic carbon, and nutrients were studied in Louisiana Shelf waters including the Mississippi (MR) and Atchafalaya (AR) River plumes during periods of high, intermediate, and low river discharges. Seasonal variations in TEs were observed at low salinity, reflecting seasonal changes in the river water endmembers. Shelf surface water dissolved Mo, Cs, U, Ni, and Cu showed conservative behavior with minor scattering in some high salinity waters. Based on associated mixing experiments, nutrient and chlorophyll distributions, as well as surface-bottom concentration contrasts, the non-conservative behavior of TEs was variously related to colloidal flocculation (Fe, Cr), biological activity (Fe, Mn), desorption (Ba, Co, Mn), photochemical reaction (Cr) and benthic mobilization (Ba, Co, Cu, Ni, Mn). These processes resulted in seasonal variation of the Ba-salinity relationship in the shelf surface waters, which may lead to considerable uncertainty in paleo-freshwater input estimations. In bottom waters, TEs were either negatively or positively correlated with dissolved oxygen, suggestive of sedimentary diffusion, particle dissolution, or adsorptive removal onto particles under reducing conditions. During bottom water hypoxia, the eberincreases of dissolved Co, Fe and Mn in some high salinity surface waters were observed and were due to episodic vertical mixing. Different distributions of the studied TEs were observed in the mixing zones of the MR and AR plumes, probably due to the different biogeochemical characteristics of the two river plumes. Additional inputs from the Red River and wetland waters in the AR Basin resulted in different river concentrations and consequently led to a considerable AR contribution for some TEs to the shelf, exceeding the AR hydrological contribution of the shelf. The AR plays a critical role in TE distributions of the Louisiana Shelf waters because it can be the dominant freshwater source to the shelf during summertime.

In addition to the Louisiana Shelf work, the impact of the Deepwater Horizon oil spill on trace element distributions was investigated. An examination of profiles, ancillary data, and oil/dispersant leaching experiments suggests that subsurface concentration changes were related to inputs from crude oil (Co), drilling mud (Ba), and bottom sediment resuspension (Fe). Biological removal of Fe during oil/gas degradation may have been a factor, as well.

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