NATURAL AND HUMAN IMPACTS ON THE EVOLUTION OF BLOCK ISLAND (RI)

Block Island (RI), ~12 miles S of the RI shoreline, consists of two morainal hills connected by two tombolos, enclosing Great Salt Pond (GSP; max. depth 16m). The GSP currently connects with the ocean through a breach in the western tombolo. Cores dated with ¹⁴C and ²¹⁰Pb provide a 3.500 kyr record of the interplay between sea level rise, hurricanes, and human activities. The deeper sections of the GSP cores contain oysters, clams, mussels, quahog, and Pecten, but the core tops have abundant Crepidula fornicata (slipper limpets) with rare bivalves only. These core tops have benthic foraminiferal assemblages dominated by various Elphidium species with rare Haynesina germanica, Ammonia tepida, and Quinqueloculina spp, whereas lower sections have more even species distributions. Benthic foraminiferal stable isotope data combined with an external paleo-temperature record provide a salinity record, with high-frequency oscillations, strongly reduced ventilation at ~2800-3000BP, and high calculated salinities in the deepest core section. The high frequency component may be driven by the opening/closing of the connection to the ocean, whereas the long-term trend may be driven by changes in sea level and climate. Mean sea level rise over the last 3000 years is ~1 mm/yr, but ~ 3 mm/yr since 1938. Organic matter in GSP sediment is a mixture of C3 land-plants and marine algae, with abundant C3 plant debris deposited just prior to the period of reduced ventilation. Major peaks in marine productivity occurred at 3300BP, 2700BP and 1700BP, as shown by stable isotope ratios and elemental composition (δ¹⁵N, δ¹³C, C/N). Cores from salt- and fresh water ponds show signs of recent eutrophication, possibly caused by runoff or sewage influx. The modern GSP salt marshes developed on a thick sand layer that was probably deposited by the 1938 hurricane. The hurricane deposited a distinct sand layer in the N-section of the Pond with marine gastropods, bryozoans, and the shelf foraminifer Poroeponides lateralis. The hurricane also opened Sachem Pond to the ocean, and deposited organic matter dominated by C4 salt marsh grasses, possibly followed by bacterial blooms. Block Island environments thus were affected by rising sea-levels, hurricanes, and man-made connections to the ocean, as well as changes in land-cover and Recent eutrophication.