LATE HOLOCENE RECORD OF HYPOXIA FROM THE LOUISIANA SHELF, GULF OF MEXICO

Hypoxia (low-oxygen water column conditions) on the Louisiana Shelf has been linked to increased nitrogen-rich fertilizer inputs in the Mississippi River drainage basin. We have identified an assemblage of benthic foraminifers from the Louisiana Shelf that occur as the dominant living species in samples collected from within the chronically hypoxic zone. The species have similar morphologies and are identical to other low-oxygen faunas worldwide. It is believed that the three low-oxygen (<2 mg/l) tolerant species, Pseudononion atlanticum, Epistominella vitrea, and Buliminella morgani, respond to the increased organic detritus and lack of competition occurring in the hypoxia zone. This hypoxia-tolerant assemblage can be used as a paleo-hypoxia index (PEB index). Preliminary analyses of five Louisiana Shelf cores dated using ²¹⁰Pb provide a record of the variability of hypoxia in the northern Gulf of Mexico over the last few centuries. This PEB index records a steady increase in hypoxia from the 1900’s to the present. However, episodic pulses of the PEB index in sediments older than 100 years suggest that natural variability may have driven hypoxia during times prior to the recent increase in fertilizer use. It is believed that earlier episodes of hypoxia may be related to El Nino/La Nina flooding cycles. Large floods in 1979, 1983, 1993, and 1998, compounded with the use of fertilizer, also appear responsible for the recent (post 1980) dramatic increase of hypoxia off the Mississippi Delta.