RAPID SEDIMENT INPUT/ACCUMULATION IN THE US VIRGIN ISLANDS: A DOUBLE-EDGED SWORD

Sediment input/accumulation in coastal environments surrounding the US Virgin Islands (USVI) is naturally high due to the exceptionally steep island gradients, unstable nature of the dominantly volcanic and metamorphic rocks, and the hot, humid climate. Anthropogenic activities associated with island development are perceived to increase these already high rates of sediment input/accumulation. The deleterious impacts of rapid sediment inputs to the fragile coastal environments of these islands are well established, but the specific impacts of individual (runoff) events, in terms of frequency and magnitude, have not been documented. Although rapid sediment input/accumulation may be the cause of harmful impacts, the resulting deposit provides a high-resolution record of individual events, which when coupled with appropriate geochronology, can help to link event type (tropical cyclone, rainfall, anthropogenic activity) and scale (magnitude, frequency, threshold), with an environmental response.

Over 100 sediment cores and 150 surface samples have been collected from a variety of USVI coastal environments since 2002. Selected cores (i.e., those with expected highest accumulation rates and best preserved stratigraphy) were analyzed for short-lived radioisotopes (²¹⁰Pb, ¹³⁷Cs, ⁷Be) to determine precise timing of events and accumulation rates over the past 100 years. Precise, sub-cm-scale sampling and high accumulation rates result in annual-scale geochronology. Cores were also analyzed with scanning XRF and scanning LA-ICP-MS to determine the elemental composition of sediments at the mm to sub-mm scale. Elemental compositions, along with sediment texture/composition, are used to “fingerprint” the sedimentary response to a specific event, when coupled with the high-resolution geochronology.

Results show that accumulation rates have increased relative to the degree of anthropogenic development, with heaviest developed watersheds increasing by ~10x since the early 1960s. In Coral Bay St. John, a 10x increase in accumulation rate, coupled with a change in sediment transport pathways, can be directly related to major road construction in the early 1960s. Results of these projects has helped lead to significant recent improvements in USVI watershed management policies.