Hypoxia hotspots in the Mississippi Bight

Foraminiferal proxies of hypoxia indicate apparent low oxygen to hypoxic conditions in several hotspots in the Mississippi Bight. The foraminiferal hypoxia proxies, the Ammonia to Elphidium (A/E) index and the Pseudononion-Epistominella-Buliminella (PEB) index, were tabulated from three sets of core tops collected in 1951-1956. Additionally, the oxygenation history of a site near the Balize delta was evaluated over the past one hundred years based on A/E and PEB indices and size distributions of pyrite framboids in a gravity core dated by Pb-210 geochronology. The results from the 1950's core-top collections show apparent, recurrent low-oxygen to hypoxic bottom water on the inner shelf at hotspot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the A/E index exceeds 90% on the inner shelf seaward of Horn and Dauphin islands, both of the Mississippi-Alabama barrier islands, and a center between Pass a Loutre and Main Pass of the Balize delta. In partial support of these results are reports of present-day low oxygen to hypoxic concentrations in bottom waters associated with seasonally high surface chlorophyll a and seasonal strengthening of a brackish-water cap at these locales. In contrast, the PEB index in core tops suggests good oxygenation at midshelf depths > 30 m. The PEB index, size distributions of framboidal pyrite, and other indicators in a gravity core 44 km northeast of Pass A Loutre indicate no clear change in conditions over the past century, constraining the seaward extent of the hotspot near the Balize delta.