Graduation Year

2008

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Marine Science

Major Professor

David J. Hollander, Ph.D.

Keywords

Hypoxia, Isotopes, Sediment, Nitrogen, History

Abstract

The Louisiana (LA) shelf is chronically affected by seasonal hypoxia that has been shown to be spatially expanding and growing progressively more severe. Hypoxic conditions on the shelf have been closely linked to the large quantities of nutrients delivered to the Gulf of Mexico via the Mississippi River. Multiple geochemical proxies on a suite of sediment cores from the LA shelf provide a record of environmental change that parallels the onset of hypoxic conditions over the last century and prior to anthropogenic influences. The sedimentary record for the last century shows a shift from terrestrial to algal sources to the sediments, as well as a trend of increasingly enriched nitrogen isotopes, which is probably caused by a combination of large amounts of denitrification, either within the Mississippi watershed or on the continental shelf, increased primary productivity, compounded with relatively minor increases of enriched nitrogen source inputs. The current chronic hypoxia, especially since the early 1970's, is exacerbated by anthropogenic nutrient loading from the Mississippi River basin, but other processes must be responsible in the past. In the historic record, several episodic low-oxygen events are defined by forminiferal assemblages and shifts in the geochemical records. Geochemistry of three sediment cores from the Louisiana (LA) shelf indicates that these historic events are likely caused by increased inputs of terrestrially-derived organic matter during peaks in Mississippi River discharge. These results suggest that low-oxygen conditions may be a natural, aperiodic phenomenon on the LA shelf, and that the current seasonally severe hypoxia resulted from the excess anthropogenic nutrient input.

Share

COinS