Linking Land & Sea: Watershed Evaluation and Mineralogical Distribution of Sediments in Eastern St. John, USVI

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Abstract
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 tend to 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 conducted a detailed characterization of watersheds and their sediments from ‘source to sink’ in eastern St. John. To accomplish this we combined field observations and sampling with a digital elevation model. Our research was focused on several morphologically similar embayments in eastern St. John; 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 found a large disparity in upslope watershed size between Otter Creek and Coral Harbor: Otter Creek (0.09 km2) is ∼73× smaller than Coral Harbor (6.54 km2). As expected, watersheds transport terrigenous volcaniclastic sediments directly to the marine environment where shallow-water marine carbonates precipitate. 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.

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