Paper No. 6
Presentation Time: 9:30 AM


THOMAS, Ellen, Geology and Geophysics and Department of Earth and Environmental Sciences, Yale University and Wesleyan University, P O Box 208109, New Haven, CT 06520-8109 and VAREKAMP, J.C., Department of Earth & Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, CT 06459,

Block Island (RI) is about 21 km South of the Connecticut - Rhode Island coast, east of Long Island. Its northern and southern areas are parts of the terminal and recessional Wisconsinan moraines, and connected by two sand spits enclosing Great Salt Pond. The western spit is breached by dredging so that ocean waters enter the Pond. Block Island does not have local pollution sources for mercury (Hg), and its deposits thus provide a locale to determine the 'pure' atmospheric deposition component of Hg contamination in southern New England. We studied cores from from different environments on Block Island for evidence of the effects of the Great New England Hurricane (1938) with its storm surge of up to 1.5 m. Cores from Great Salt Pond typically contain fine sands and silts with estuarine, low diversity benthic foraminiferal assemblages dominated by Ammonia tepida, Buccella frigida, Elphidium species, and Haynesina germanica. In a core in western Great Salt Pond, however, there is a coarse sand layer dated to about 1940 through its Hg pollution profile, with marine snails, bryozoans, serpulid worms and the foraminifer Poroeponides lateralis, described by Cushman (1994) from waters along the New England coastal shelf. Extensive coring in salt marshes along Great Salt Pond shows that marshes date back to around 1940, without any older marsh peats. Typical New England salt marsh foraminifera (e.g., Trochammina macrescens, Trochammina inflata, Miliammina fusca) are zoned according to fractional exposure time in the intertidal zone, and the local rate of relative sea level rise was estimated at 3.0-3.6 mm/yr. In Sachem Pond, bulk organic carbon isotope values typically are around -21 ‰, nitrogen isotope values 4.5 to 5.0 ‰. In contrast, sediment deposited around 1940 shows an increase in carbon isotope values to about -16‰, a decrease in N-isotope values to 2.5‰, reflecting increased input of organic matter from C4 salt-marsh grass. We thus show that the storm surge of the 1938 hurricane deposited a large amount of coarse sand derived from the shelf into Great Salt Pond, and destroyed the coastal salt marshes around the Great Salt Pond, depositing abundant marsh plant debris in nearby ponds. The Block Island salt marshes thus are able to keep up with the present rate of relative sea level rise, but are extremely vulnerable to severe hurricanes.