FORAMINIFERAL EVIDENCE FOR CA. 200 YEARS OF ENVIRONMENTAL CHANGE ON THE MISSISSIPPI DELTA SHELF

Four kasten cores approximately 2 m long were taken in 2007 along a transect of increasing depth (59m, 75m, 87m and 473m) off Southwest Pass, Mississippi Delta. The cores were sampled at 10 cm intervals to investigate whether the effects of hurricanes on foraminiferal assemblages are preserved over a time-scale (ca. 200 years) that would allow the history of hurricane strikes to be determined. At 59m water depth Epistominella vitrea dominates assemblages, averaging 77% of each sample, and is accompanied by Buliminella morgani and Nonionella opima. At 75m water depth Epistominella vitrea still dominates but is less abundant, averaging 61%: Uvigerina peregrina and Bolivina lowmani are more abundant at this depth. At 87m water depth assemblages have higher diversity with increased abundance of, for example, Bolivina barbata and Bulimina marginata: Epistominella vitrea is still the most common species, averaging 31% of assemblages. At 473m water depth Bolivina simplex, Islandiella norcrossi australis, and Cassidulina neocarinata are the most abundant species. Each assemblage was compared to known assemblages of hurricane units (determined in box cores) deposited by Hurricanes Ivan, Katrina and Rita. Hurricane units have low densities of foraminifera and increased relative abundances of Textularia earlandi and Ammonia tepida . No samples in the kasten cores yielded assemblages comparable to those of known hurricane units, suggesting that the foraminiferal signatures of hurricanes are lost to bioturbation and/or to dilution by in situ test input. Even so, environmental change is recorded in the cores. For example, cores from 75m and 87m water depth show increasing abundances of Epistominella vitrea, Buliminella morgani, and Nonionella spp. up-core. These species are associated with hypoxia and the cores thus suggest an increase in hypoxic conditions over the past ca. 200 years.