UNDERSTANDING NATURAL COASTAL SEDIMENTATION TO DETERMINE ANTHROPOGENIC IMPACTS: VIRGIN ISLANDS NATIONAL PARK

A suite of sediment cores from coastal environments surrounding St. John, US Virgin Islands, provides insight into the “natural” sedimentation patterns, which is necessary in order to determine potential sedimentary impacts due to anthropogenic activities. Understanding terrestrial sediment dynamics in high-relief, tropical island settings, such as the USVI, has become a critical issue, as sediments are a potential threat to the health of down-slope environments, particularly coral reefs. Due to the high vulnerability to erosion of high-relief tropical volcanic islands, large amounts of sediment are weathered, eroded, and quickly transported down-slope. The ultimate fate of these sediments include; 1) deposition within the watershed (rare); 2) coastal buffer zones such as salt ponds, which trap sediments and prevent input into the marine environment; 3) nearshore marine environments (coral reefs, seagrasses, algal flats), many of which are adversely affected by terrigenous sedimentation; and, 4) offshore marine environments, (i.e., exiting the coastal system). Sediment studies within the VI National Park provide a critical understanding of the controls, patterns and variability in terrigenous sediment runoff/accumulation in coastal environments where there is little to no anthropogenic development in the upslope watershed. The dominant, “natural”, controls on sediment production/delivery from the watershed include 1) basin geology; 2) vegetation; 3) climate/wet vs dry; and 4) slope. In general, locations within the National Park show little recent (past ~100 yrs) changes in sedimentation patterns as indicated by sediment texture, composition, and accumulation rate (⁷Be, ²¹⁰Pb, ¹³⁷Cs, ¹⁴C). This “natural” base-line is utilized to compare to watersheds/coastlines that have undergone various degrees of alteration by anthropogenic activities (roads, buildings, etc.) to determine recent deviations in sedimentation. Locations that have experienced significant anthropogenic development show increases in terrigenous input/sedimentation, and up to 10 times the “natural” sediment accumulation rates.