DOCUMENTING HISTORICAL LOW-OXYGEN BOTTOM WATER ON THE LOUISIANA AND TEXAS COASTS, GULF OF MEXICO

Locations of coastal eutrophication and oxygen-depleted bottom waters have doubled worldwide since 1990. Since systematic measurements of Louisiana continental-shelf waters were initiated in 1985, summer-time seasonal hypoxia (oxygen content < 2 mg L^-1) has increased considerably in an area termed the dead zone. Although worldwide the causes vary, monitoring and modeling studies have concluded that the expansion of the Louisiana shelf dead zone is related to increased anthropogenically derived nutrient delivery (nitrogen and phosphorous) from the Mississippi River drainage basin. On the Louisiana shelf, these nutrients stimulate blooms of algae. Upon death, the algae sink to the seafloor where their decay consumes oxygen and results in hypoxia and even anoxia (oxygen content = 0 mg L^-1). In order to track the development and expansion of seasonal low-oxygen conditions on the northern Gulf of Mexico shelf prior to 1985, we used a specific low-oxygen benthic foraminiferal proxy, the PEB index, that statistically discriminates between hypoxic and normal conditions on the modern Louisiana shelf. We constructed an expansive network of 15 PEB records with excess ²¹⁰Pb-derived chronologies in Gulf of Mexico shelf cores to determine the 100-year history of low-oxygen and hypoxic conditions. Our reconstruction indicates that areas of low-oxygen bottom water occurred in the early 1910s as isolated hotspots near the Mississippi River Delta and expanded spatially across the entire Louisiana shelf beginning in the 1950s. By 1960, low-oxygen bottom-water conditions were occurring seasonally over a large part of the geographic area now known as the dead zone. Long-term trends in the PEB index are consistent with 20^th century observational and proxy data for low-oxygen and hypoxia in the northern Gulf of Mexico.