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HYPOXIA HOTSPOTS IN THE MISSISSIPPI BIGHT

¹ Department of Marine Science, The University of Southern Mississippi, Stennis Space Center, MS 39529, USA. E-mail: charlotte.brunner{at}usm.edu
² Naval Research Laboratory, Stennis Space Center, MS 39529, and Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, USA. Current address: Coastal Management Division, Louisiana Department of Natural Resources, P.O. Box 44487, Baton Rouge, LA 70804-4487, USA. E-mail: jenniferb{at}dnr.state.la.us
³ Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA. E-mail: sjb{at}lsu.edu
⁴ Naval Research Laboratory, Stennis Space Center, MS 39529, USA. E-mail: yfurukawa{at}nrlssc.navy.mil

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 ²¹⁰Pb 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 à 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 mid-shelf depths >30 m. The PEB index, size distributions of framboidal pyrite, and other indicators in a gravity core 44 km northeast of Pass à Loutre indicate no clear change in conditions over the past century, constraining the seaward extent of the hotspot near the Balize delta.