LINKING DEPOSITIONAL PROCESSES TO SEDIMENT ACCUMULATION on A SUB-ANNUAL SCALE: ST. JOHN, US VIRGIN ISLANDS

Beryllium-7 and elemental analyses were utilized on mm-scale laminated sediment records from coastal salt ponds in the US Virgin Islands (USVI) to determine terrigenous sediment accumulation during the annual dry/wet seasons. This provides a framework for comparing natural terrigenous sediment accumulation and how and to what magnitude anthropogenic activities in upslope watersheds may alter terrigenous sediment accumulation in coastal environments. Sediment cores collected in salt ponds, along the coast of St. John, consisted of well-preserved mm to cm-scale laminations and provide a high-resolution record of variations in sediment sources (terrigenous, marine, atmospheric) and depositional processes (rainfall/runoff, marine overwash).

Terrigenous sedimentation has been linked to intense rainfall events, which occur at greater frequency during the wet season. Historical daily rainfall and elemental scanning show that terrigenous sedimentation is greater during periods when there are more frequent intense rainfall events that exceed a rainfall threshold (minimum rainfall required for sediment to erode and be transported downslope). Analysis of ⁷Be to calculate Mass Accumulation Rates (MAR) also shows lower MAR during the dry season and an increase in MAR as rainfall/runoff increased during the wet season (May/June and Sept./Oct./Nov). High-resolution scanning XRF and scanning LA-ICP-MS provide elemental composition of sediments reflecting an increased terrigenous sediment accumulation (laminae with higher Al, Ti, Fe, Cu, and Si) associated with heavy rainfall events during the wet season.

These sediment records provide information about terrigenous sediment runoff on seasonal/annual scales, which can be used to help correlate the amount of sediment runoff with rainfall. This can provide crucial information about rainfall patterns that are important for human populations and runoff patterns that are important for coastal ecosystems on pertinent time-scales (annual and decadal) as well as how anthropogenic activities may have altered these patterns.