7Be as an Indicator of Terrigenous Sediment Input to Coastal Environments: St. John, US Virgin Islands

In high-relief, tropical island settings, sediment runoff is a threat to the health of down-slope coastal environments, including but not limited to coral reefs. A study on St. John, US Virgin Islands, utilizes ⁷Be radioisotope activities of sediments to provide insight into terrigenous sediment dynamics on monthly to seasonal time-scales. With a 53-day half-life, ⁷Be has an atmospheric source, therefore, ⁷Be can be used as an indicator for sediment input associated with rainfall/runoff. A combination of samples collected from watersheds, coastal sediment traps, and bottom sediments adjacent to traps, were analyzed for ⁷Be activity to determine variability in sediment input and accumulation. ⁷Be activities of sediments from watersheds are generally low or not detected, indicating that ⁷Be input associated with rainfall is not accumulating within the watershed, but is quickly transported to the coast. Exceptions include retention ponds and coastal salt ponds, which inherently trap sediments prior to input into coastal marine environments. Terrigenous sediment input to the coast is reflected by high ⁷Be activities measured in sediment trap samples. Bottom samples adjacent to traps often do not have detectable ⁷Be indicating that resuspension is not likely contributing to sediments accumulating in traps, and that terrigenous sediments are not accumulating in these settings. Where ⁷Be is detected in bottom sediments activities are lower than the adjacent trap and are located where sediment is more likely to accumulate including sheltered areas (e.g. Coral Harbor), deep water, and locations that indicate higher sediment input (i.e. higher trap ⁷Be activity). An increased understanding of terrigenous sediment dynamics in these coastal environments provides critical information about acute (terrigenous input with subsequent export offshore) vs chronic (terrigenous input with accumulation) influences on coastal ecosystems in both natural and anthropogenic settings.