RECONSTRUCTING AN 180-YR RECORD OF NATURAL AND ANTHROPOGENIC INDUCED HYPOXIA FROM THE SEDIMENTS OF THE LOUISIANA CONTINENTAL SHELF

Recent hypoxia (low oxygen levels) in Louisiana (LA) continental shelf bottom water has been tied to nutrient loading and freshwater stratification from the Mississippi River (MR). We investigate evidence for past episodes of hypoxia in five cores collected on the LA continental shelf with geochronologies derived from modeling excess ²¹⁰Pb activities. Variations in the relative abundance of low-oxygen tolerant benthic foraminifers provide a reliable record of oxygen depleted bottom water. The foraminifer data are consistent with previous studies indicating that the intensity of hypoxic events has increased over the last 50 years due to increased nutrient loading associated with the use of commercial fertilizer. However, the foraminifer data also reveal several hypoxia events between 1817 and 1910 AD, prior to the widespread use of fertilizer. The low-oxygen foraminifer data are compared to the MR flow record at Vicksburg MS. Pre-1910 hypoxia events coincide with high discharge rates of the MR, indicating that past episodes of hypoxia correspond to natural variations in MR drainage through the enhanced transport of nutrients and freshwater onto the continental shelf. Our data show that hypoxia events of the last few decades were more extreme than any hypoxia event that occurred in the previous ~180 years, and support the interpretation that the increased use of commercial fertilizer has amplified an otherwise naturally occurring process.