LINKING SOURCE AND SINK: WATERSHED EVALUATION AND MINERALOGICAL DISTRIBUTION OF SEDIMENTS IN EASTERN ST. JOHN, US VIRGIN ISLANDS

Tropical islands such as St. John in the U.S. Virgin Islands are naturally susceptible to terrigenous (land-based) sediment erosion due to their high-relief slopes, fast weathering rates, and intense precipitation events. Nearshore ecosystems that exist near these islands thrive in static conditions, and are especially stressed by increases in terrigenous input. In the last few decades, island development and population have increased dramatically in some areas of St. John. We conduct a detailed characterization of watersheds and their sediments from ‘source to sink’ in eastern St. John. We combine field observations and various forms of sampling with a digital elevation model to complete this phase of characterization. We utilize sediment core and grab samples from 2002-2015 to show mineralogic distribution of two main embayments in Coral Bay, St. John (western & northern). Upslope of these embayments we profile each watershed by identifying mineralogic distribution, human influence, sediment transport pathways, area, volume, vegetation, and slope. Beyond this basic characterization our research is focused on several, small, morphologically similar embayments in Coral Bay; three impacted by anthropogenic development (Coral Harbor, Johnson Bay, and Sanders Bay) and an adjacent, virtually undeveloped bay within the Virgin Islands National Park and Virgin Islands Coral Reef National Monument (Otter Creek). We find a large disparity in upslope watershed size between Otter Creek and Coral Harbor: Otter Creek (0.09 km²) is ~73x smaller than Coral Harbor (6.54 km²). As expected, watersheds transport terrigenous volcaniclastic sediments directly to the marine environment where shallow-water marine carbonates are precipitated. Terrigenous volcaniclastic sediments persist furthest from the source in the basin of the largest watershed with the most development (Coral Harbor), and decay closest to the source in the basin of the smallest watershed with the least development (Otter Creek). Due to large disparities in watershed size, further research is required in order to determine the relative contribution of development on the distribution of terrigenous sediments.