HR: 12:05h
AN: B51F-08    [PDF]
TI: Carbon Cycling in Long Island Sound Over the Last 1000 Years
AU: Lugolobi, F
EM: flugolobi@wesleyan.edu
AF: Boston University, Earth Sciences, Boston, MA 02215 United States
AU: * Varekamp, J C
EM: jvarekamp@wesleyan.edu
AF: Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459 United States
AU: Thomas, E
EM: ethomas@wesleyan.edu
AF: Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459 United States
AU: Mecray, E
EM: emecray@usgs.gov
AF: USGS, Coastal and Marine Geology, Woods Hole, MA 02543 United States
AB: Long Island Sound is a eutrophied urban estuary that suffers from anoxia during the late summer months. Is the narrow western part of the estuary prone to anoxia as a result of poor circulation during warm periods or is the anthropogenic eutrophication the main driver for the seasonal anoxia? We have reconstructed the history of bottom water oxygenation through the use of stable isotope studies of foraminifera in sediment core samples. The paleo water temperature was approximated from Mg/Ca measurements in shells of the foraminifer Elphidium excavatum and the paleo salinity was calculated from the foram shell's delta 18O values and the temperature estimates. The measured delta 13C values of the shells were corrected for salinity, using our river-seawater mixing model, and the excess delta 13C values were recast into additions of milligrams of algal carbon per liter of water through an isotopic mass balance statement. The equivalent oxygen consumption was then used to calculate the water oxygen contents, which was divided by the saturated oxygen contents of the waters at the calculated temperature and salinity. The resulting percent oxygen saturation values for 5-10 year average values (time in a sediment slice) were plotted as a function of time (cores were dated with a variety of isotopic and pollen techniques). The results show that from 1000 AD to about 1850 AD, mean levels of oxygen saturation were about 70-80 percent; these values started dropping in the mid 19 and 20 century, reaching 30-40 percent saturation values over the last 50 years. The temperature record shows evidence for the warm Mediaeval period, but there is no evidence that the Sound suffered from anoxia at that time. Organic carbon contents of the sediment increased over the last 250 years and organic carbon accumulation rates increased almost 5 fold. Both the amount of sediment-stored organic carbon and the oxidized organic carbon fraction of the total primary productivity increased during the gradual eutrophication of the Sound. We conclude that the main driver of the seasonal anoxia in western Long Island Sound is the release of nutrients (mainly nitrogen compounds) from sewage and run-off, which leads to eutrophication and excess algal carbon production. Modern global warming may provide an additional effect by enhancing water column stratification.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1055 Organic geochemistry
DE: 1065 Trace elements (3670)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting