Foraminifera as indicators of hypoxia off Southwest Pass, Mississippi Delta, Gulf of Mexico?

Abstract

The continental shelf west of the Mississippi River Delta (the Louisiana Bight) experiences seasonal hypoxia that has been increasing in frequency since the second half of the 20th century due to anthropogenic influence. To address the impact of hypoxia in the easternmost Lousiana Bight, this study looks at benthic foraminiferal assemblages from four ~2 m kasten cores taken southwest of Southwest Pass of the Mississippi River Delta. The PEB index, a proxy for hypoxia, is composed of the combined percentages of Protononion atlanticum (= Nonionella atlantica of this study), Nonionella opima, Epistominella vitrea and Buliminella morgani. Little variation in the PEB index occurred throughout the assemblages of the shallowest core, KC4, from 59 m water depth. Assemblages were strongly dominated by PEB index taxa giving an average value of 95%. This high PEB index value is due mainly to E. vitrea dominating the assemblages with a core average of 76% for this taxon. Core KC3 from 75 m was also dominated by E. vitrea, which averaged 61% and contributed to an average PEB index value of 84%. The PEB index of core KC3 changed with core depth and was consistently higher in the top 90 cm. The shift to increased PEB index occurred between 1946 and 1951 according to ²¹⁰Pb-derived age estimates, and could be due to an increased influence of hypoxia or a change in environment associated with Mississippi delta progradation. A similar trend can be seen in core KC2 from 87 m. The PEB index is approximately 18% from 240 cm, core bottom, to 140 cm, where it begins to increase due almost entirely to an increase in E. vitrea. This trend is also interpreted as reflecting an increasing influence from the Mississippi River. The top 30 cm of the core shows an increase in the other PEB taxa, B. morgani and N. opima; this shift is interpreted as reflecting increasing hypoxia that occured between 1930 and 1945 according to ²¹⁰Pb-derived age estimates. Core KC1 from within the Mississippi Canyon, at 473m, had high diversity assemblages with Bolivina lowmani, Cassidulina neocarinata, and Bolivina ordinaria each comprising ~15% of assemblages. The PEB index taxa comprised only a minor part of the assemblage except for a 30 cm interval where PEB index values increased from an average of 5% to 19%, likely representing an interval of increased off-shelf transport. In summary, PEB index values reflect increasing hypoxia in the study area over the past 60 to 80 years but are also probably affected by sedimentological factors related to Mississippi River discharge.