HIGH-RESOLUTION ELEMENTAL RECORD OF TERRIGENOUS SEDIMENT RUNOFF ASSOCIATED WITH RAINFALL EVENTS: US VIRGIN ISLANDS

A multi-proxy approach utilized on mm- to cm-scale laminated sediment records in coastal salt ponds on St. John, U.S. Virgin Islands is used to characterize sediments, and identify their sources and depositional processes/events (heavy rainfall, tropical cyclones, tsunamis). Historical records, combined with high-resolution geochronology (short-lived radioisotopes, ²¹⁰Pb, ¹³⁷Cs, ⁷Be) and scanning elemental techniques (XRF and LA-ICP-MS), provide a link between depositional events to their sedimentary signature in the geologic record. St. John is a volcanic island with sediments of terrigenous source composed dominantly of Al, Fe, Ti, Co, and Si (terrigenous indicator elements). A threshold value of >2.0 mm per day (minimum required to mobilize and transport sediment downslope) of rainfall has been determined for the study area. The frequency of heavy rainfall events that exceed the threshold of >2.0 mm per day is significantly correlated to increased accumulation of Aluminum (terrigenous indictor element) in salt pond sediment records. Variability in the sedimentary record of terrigenous indicator elements reflects changes between periods of “wet” (increased frequency of heavy rainfall events) and “dry” (decreased frequency of heavy rainfall events). Tropical cyclone and tsunami events can cause marine overwash into salt ponds leading to deposition of marine sediments. Elemental scans of Ca and Sr as overwash indicator elements are complicated by grain size effects of measurement techniques, as well the difficulty in differentiating between tropical cyclone overwash and tsunami deposits.

By defining the sedimentary signature for depositional events, geologic records can be interpreted to provide insight into the natural variability of these processes throughout geologic time for comparisons to the more recent anthropogenic time period. This study provides a framework that can be applied to other coastal environments on high-relief tropical islands, to compare local records, and provide information on regional processes controlling rainfall variability in tropical latitudes.