DOES THE PEB INDEX RESPOND ONLY TO HYPOXIA IN THE MISSISSIPPI DELTA, GULF OF MEXICO?

Abstract

The frequency of anthropogenic seasonal hypoxia on the continental shelf west of the Mississippi Delta (the Louisiana Bight) has increased since the middle of the 20 th century. This study applies the PEB index, a proxy for hypoxia, to four ~2 m kasten cores taken southwest of Southwest Pass of the Mississippi Delta. The PEB index is defined as the cumulative percent of Protononion atlanticum (= Nonionella atlantica of this study), Nonionella opima, Epistominella vitrea and Buliminella morgani . The PEB index varies little in the shallowest core, KC4 (59 m water depth). Assemblages were strongly dominated by PEB taxa (average 95%), due mainly to the dominance of E. vitrea (average 76%). In core KC3 (75 m), an average PEB value of 84% is also due mainly to E. vitrea (61%). The PEB index of KC3 was consistently higher above 90 cm, a level dated at 1955–1960 ( 210 Pb age estimate), and could reflect an increased influence of hypoxia or an increased rate of sediment accumulation associated with delta progradation. A similar trend characterizes core KC2 (87 m). The PEB index is 18% from 240 cm to 140 cm, increasing up core as the abundance of E. vitrea increases, likely due to delta progradation. Other PEB taxa, B. morgani and N. opima , increase in the top 30 cm of the core, probably due to increased hypoxia between 1952 and 1968 ( 210 Pb age estimate). Core KC1 (473 m) differed from shallower cores in its higher species diversity. Bolivina lowmani, Cassidulina neocarinata , and Bolivina ordinaria each comprise ~15% of assemblages and PEB taxa are rare (average 5%) but increase to 19% in a 30 cm interval attributed to off-shelf transport. We conclude that the PEB index is a useful indicator of anthropogenic hypoxia but dominance of E. vitrea may also be attributable to sedimentological factors related to delta progradation.