A FOUR THOUSAND YEAR TEX86 TEMPERATURE RECORD FOR LONG ISLAND SOUND

Long Island Sound (LIS) has experienced major environmental changes throughout European colonization and industrialization. Low oxygen conditions in the Sound are considered one of the most severe stressors for biota, including valuable fisheries species. Low oxygen conditions develop when oxygen consumption from the oxidation of organic matter exceeds supply from the atmosphere and photosynthesis. Annual hypoxic/anoxic events have been recognized since the 1970s, and are most extreme in westernmost LIS. Their extent, duration and severity are influenced by the stratification of the water column, measured as density contrast in surface and bottom waters due to differences in temperature and salinity. Temperature variability in LIS over the last few thousand years has not been documented, as paleotemperature proxies are commonly difficult to use in estuarine settings. Oxygen isotopes of carbonate microfossils are influenced by salinity, and their Mg/Ca values may be influenced by the carbonate saturation state of the water. The tetraether index (TEX₈₆) temperature proxy is based on the relationship between water temperature and the number of cyclopentane rings in the glycerol dialkyl glycerol tetraethers (GDGTs) which serve as the membrane lipids of marine Thaumarchaeota, a widely distributed, abundant and ecologically diverse group of non-thermophilic Archaea. This proxy can be used in organic-rich sediments deposited in a variety of settings, including estuaries. We analyzed samples from three cores representing different hypoxic regimes in western and central LIS, dated using Hg-pollution profiles, ²¹⁰Pb - ¹³⁷Cs, and ¹⁴C, to generate a 4,000 year LIS surface temperature record. We compared this TEX₈₆ record with a Mg/Ca record based on benthic foraminifera that reflects LIS bottom water temperatures, and an oxygen isotope record based on benthic foraminifera corrected for salinity changes by using the Mg/Ca record. The Mg/Ca record shows evidence for the Little Ice Age and Medieval Warm Period. Our TEX₈₆ records indicate that temperatures of LIS surface waters, as averaged over the part of the year that the proxy organisms are biologically active, generally fluctuated between 12-14^oC. Comparison of the records shows that vertical temperature gradients may have experienced variability.