THE HISTORY OF HYPOXIA IN LONG ISLAND SOUND

Long Island Sound (LIS) suffers from seasonal hypoxia, especially in its western ‘Narrows’ section, near New York City. The prime driver for deoxygenation probably is excess algal productivity due to nitrogen inputs from the watershed, combined with input from waste water treatment plants in NY and CT, thus the EPA mandated a reduction of nitrogen input of >50% relative to 1990 input by 2017. The joint NY-CT N release was close to that benchmark, and there has been some improvement in water quality, as shown by relatively small areas and short duration of hypoxia over the last few years. An outstanding question remains: when did LIS start to suffer from seasonal hypoxic water column conditions resulting in strongly reduced sediments? We developed proxies for paleo-oxygenation of LIS, including the δ¹³C signal in benthic foraminifera, and used Mo-concentrations and isotope ratios in sediment. Pore waters in sulphide-rich sediment scavenge dissolved molybdate from seawater, leading to Mo-enrichment of sediment during hypoxia, and δ⁹⁸Mo values in the marine Mo sediment fraction tend to be lower than values in continental debris or seawater. We dated sediment cores (¹⁴C, Hg pollutant stratigraphy, ²¹⁰Pb), and analyzed them for C, N, biogenic silica (diatoms), foraminifera, δ¹³C and δ¹⁵N of bulk sediment, δ¹³C-δ¹⁸O and Mg/Ca of the foraminifer Elphidium excavatum, and membrane lipids of Thaumarchaeota (Tex₈₆). Paleotemperature records were derived from Mg/Ca and TEX₈₆, and we reconstructed mass accumulation rates of foraminifera, C_(org) and N as indicators of eutrophication. We used the abundance of the bacterial spore Clostridium perfringens as an indicator for wastewater N inputs. Eutrophication and hypoxia started in the middle 1800s with the strong population growth in the region, at which time the δ¹⁵N record shows a discrete shift to heavier values due to sewage release. Sedimentary organic matter in the colonial era was mainly sourced from the marine realm, but by the start of the 20^th century terrestrial carbon became dominant. The ratio of marine versus terrestrial carbon inputs shows wide swings, reflecting deforestation and changes in agricultural land use and reforestation in the 20^th century. Here we will present new Mo and sulfur data, and an extended data base of carbon and nitrogen mass accumulation rates over time.