THE EVOLUTION OF GREAT SALT POND (BLOCK ISLAND, RI) OVER THE LAST 3500 YEARS

Great Salt Pond (GSP; max. depth 16m) in the center of Block Island (RI), ~12 miles S of the RI shoreline, is bounded by morainal hills connected by tombolos, and open to the ocean through a man-made breach (1895 AD) in the western tombolo. Cores taken in GSP were dated with ¹⁴C and through chemostratigraphy with ²¹⁰Pb ages, creating a 3.5 kyr environmental record. Deeper core sections contain common, diverse bivalves and fairly diverse benthic foraminifera (Elphidium spp., Haynesina germanica, Ammonia spp., Quinqueloculina spp.), whereas core tops have rare bivalves and abundant slipper limpets, with benthic foraminifera dominated by Elphidium excavatum. The recent decrease in diversity of mollusca and foraminifera may indicate eutrophication, possibly by sewage influx. A distinct sand layer in a core from northern GSP was dated at ~1940 by chemo-stratigraphy (Hg pollution profiling), and contains marine gastropods, bryozoans, and the shelf foraminifer Poroeponides lateralis. This layer was probably deposited by the 1938 hurricane. Benthic foraminiferal oxygen isotope data combined with an external paleo-temperature record provide a salinity record, showing strongly reduced ventilation (no foraminifera) at ~2800-3000BP. High-frequency oscillations are superimposed on a long-term decline in salinity. The high frequency component may be driven by opening/closing of the breach to the ocean, the long-term trend by changes in sea level and climate. The mean rate of relative sea level rise over the last 3 kr was ~1 mm/yr (based on ¹⁴C dated oyster shells), whereas a rate of ~3mm/yr was calculated for sediments deposited since ~1940. Organic matter in the cores consists of a mixture of C3 land-plants and marine algae. We used stable isotope ratios and elemental composition of bulk organic matter (δ¹⁵N , δ¹³C, C/N) to distinguish between these sources. Abundant land-plant debris accumulated prior to the period of reduced ventilation (2800-3000BP), and apparent peaks in marine productivity occurred at 3300BP, 2700BP and 1700BP. The GSP environments thus were affected by rising sea-levels, hurricanes, and changes in connection to the ocean (both natural and man-made), as well as changes in land-cover and Recent eutrophication .