SUCCESSION OF LIVE FORAMINIFERS IN CONTRASTING LITHOFACIES OF A DYNAMIC CONTINENTAL SHELF

The succession of live foraminifers is examined in two contrasting depositional environments at 20 m depth on the Mississippi-Alabama Continental Shelf. One site currently lies in a sandy, fossil-rich facies south of Petit Bois Pass and the other site lies to the west in a muddy, fossil-poor facies south of Horn Island, where a dramatic change in lithofacies impacted the fauna. In a 1999 survey of texture, the site south of Horn Island was characterized as a sand, but during 2004 the site became hypoxic and underwent rapid deposition of mud at least one meter deep, based on anecdotal reports. The study captures recolonization of the site in the aftermath of this event.

Two replicate diver cores 80 cm² in cross-sectional area were taken at both the muddy and sandy sites in June, August, and October of 2005 along with water column temperature, salinity, oxygen, current, and turbidity profiles. The period captures development and turnover of the seasonal pycnocline. The cores were sliced into 1-cm slabs from 0-10 cm and placed in a solution of rose Bengal and seawater for two days, then preserved by addition isopropyl alcohol. The pH of the samples was monitored throughout and maintained at >8.1 by addition of sodium borate, which was needed, in particular, in samples from the muddy locale. Samples were washed on nested screens of 45 and 63 micrometer openings, and the sand-size fraction was split to a convenient amount using a settling type splitter. The residue was suspended in tap water (pH = 8) in a gridded petri dish, from which deeply stained specimens were pipetted and mounted onto a gridded paleontological slide for identification and enumeration.

Results show that the assemblage of living foraminifers in sands south of Petit Bois Pass is diverse with common (25-15%) Brizalina lowmani, Nonionella atlantica, and Rosalina spp., low frequencies (6-4%) of Elphidium, Ammonia tepida, Buliminella morgani, Fursenkoina pontoni, Hanzawaia strattoni, and agglutinated and miliolid taxa, and trace amounts of 11 other rotalids. In contrast, the muddy site south of Horn Island is dominated by Nonionella basiloba (75%) with low to trace frequencies of ten other taxa. Such high frequencies of Nonionella basiloba are associated with high levels of relatively refractory organic carbon and low pore water oxygen on the neighboring Louisiana shelf.