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