Evidence for the Recent Development of Low-Oxygen Conditions on the Texas Shelf from Benthic Foraminiferal Fossil Assemblages

Since the 1950s, low-oxygen (hypoxic) conditions have developed in the northern Gulf of Mexico (GoM) due to increased nutrient flux from the Mississippi River. Low-oxygen conditions on the modern Texas Shelf have been attributed to the migration of the Mississippi/Atchafalaya plume and to local flooding events that increase freshwater runoff to the coast. Here, we use variation in benthic foraminiferal assemblages in two 210Pb- and 137Cs-dated sediment cores to access recent changes in the oxygenation of near Port Aransas, TX. Benthic foraminifera are sensitive to changes in oxygenation and we reconstructed oxygenation records downcore using three benthic foraminiferal faunal indices commonly used in the northern GoM: the PEB index based on the relative abundance of three hypoxia-tolerant taxa (Pseudononion atlanticum, Epistominella vitrea, and Buliminella morgani), the PEBn index (PEB taxa + Nonionella opima), and the A-E index, which is based on the relative abundance of Ammonia to Elphidium species. Faunal indices in both cores suggest oxygenation on the Texas Shelf near Port Aransas, TX declined after ~1950. Further, multivariate analyses of the total fauna reveal increases in the relative abundance of other low-oxygen tolerant species and the decline in oxygenation is paralleled by a decrease in benthic foraminiferal diversity. Intersite faunal differences reflect different degrees of freshwater influence at each site, however d18O analyses on Elphidium excavatum (a proxy for freshwater input) are not well correlated with changes in oxygenation. This investigation demonstrates that modern hypoxia on the Texas Shelf is a recent development and may be more related to the migration of the Mississippi/Atchafalaya plume than local runoff near Port Aransas, TX. Understanding the drivers of declining oxygen on the Texas Shelf is important for protecting the ecosystems and economically important fisheries of the Northern Gulf of Mexico.