Research: Paleo-Hypoxia

Figure 3. Light microscope photo showing relative size of the PEB species. Photo credit: Jessica Spear.

In this project we have documented that a specific faunal proxy, termed the PEB index (= % Protononion atlanticum + % Epistominella vitrea + % Buliminella morgani), is statistically representative of the modern seasonal Louisiana hypoxia zone (Fig. 1). The PEB index can be used as a low-oxygen proxy to detect past hypoxic (dissolved oxygen <2 mg L^-1) conditions in the Holocene-aged sediments from the Louisiana and Texas continental shelves. Records of past changes in bottom water oxygen content allow an understanding of the roles of natural variability versus human activities in the development of seasonal hypoxic conditions in the Gulf of Mexico.

Our research demonstrates that the development of low-oxygen bottom water on the Louisiana shelf is a natural process that has occurred periodically for at least the last 1,000 years in response to natural climate variations. However hypoxia became a chronic problem about 50 years ago as anthropogenic influences increased in the Mississippi Basin (Fig. 2). Our findings suggest that future population growth, and land use/ land cover changes combined with climate variability and potential climate change may cause chronic hypoxia to spread to other regions of the northern Gulf Coast.

Figure 1. Gulf of Mexico seafloor bottom sediments that experience seasonal hypoxia contain higher relative percentages of three benthic foraminiferal species. The cumulative percentage of these species forms the PEB hypoxia index, which is used to identify past occurrences of seasonal hypoxic conditions. [larger version]

Figure 2. Increasing % PEB in dated sediment cores from the Louisiana shelf hypoxic zone allow us to map the expansion of anthropogenically caused hypoxia during the 20° C. [larger version]