A SEVERAL-CENTURY RECORD OF LOW-OXYGEN CONDITIONS FROM TWO GRAVITY CORES FROM THE LOUISIANA CONTINENTAL SHELF

The relative abundance of three low-oxygen-tolerant benthic foraminifers (Pseudononion atlanticum, Epistominella vitrea, and Buliminella morgani) is used as a proxy for present hypoxic (dissolved oxygen concentrations <2 mg L^-1 ) conditions on the Louisiana Shelf. An up-core increase in this proxy (the PEB index) in sediment cores from ~1950 to present coincides with the well-known increased occurrence of hypoxic bottom-water conditions on the Louisiana Shelf. This seasonal hypoxic condition on the Louisiana Shelf has been linked to increased nutrient influx from the Mississippi River drainage basin.

Using a sedimentation rate extrapolated from excess ²¹⁰Pb activities, down-core changes in the PEB index in cores PE0305-GC1 (120 cm) and MRD05-04 (228 cm) suggest that periodic low-oxygen events pre-date the start of extensive commercial fertilizer use within the Mississippi Basin (~1910). Additionally, high concentrations of terrestrial (beta-sitosterol) biomarkers co-vary with the PEB index. Together these data support the hypothesis that the earlier low-oxygen conditions are likely associated with freshwater and nutrients delivered to the continental shelf by Mississippi River flood events. Multiple pre-1900 low-oxygen events are well developed in both cores suggesting that significant widespread fluctuations of bottom-water oxygen has occurred naturally on the Louisiana Shelf for the last few centuries. Our results thus indicate that the development of low-oxygen bottom water on the Louisiana Shelf is a complex natural process that has been enhanced by human activities.