Elemental signature of terrigenous sediment runoff as recorded in coastal salt ponds: US Virgin Islands

Abstract

A high-resolution, multi-proxy approach is utilized on mm- to cm-scale laminated coastal salt pond sediments from St. John, U.S. Virgin Islands, to determine: (1) the sedimentological signature of depositional events/processes, (2) link this sedimentological signature with known depositional events/processes in the historical (past ~100years) record; and, (3) project back into the recent geologic past (past ~1400years) to investigate the natural variability of depositional events/processes. High-resolution, short-lived radioisotope geochronology (210Pb, 137Cs, 7Be) combined with high-resolution elemental scanning techniques (scanning XRF and scanning LA-ICP-MS) allows for the direct comparison of well-preserved salt pond deposits to historical records of depositional events (e.g., runoff/rainfall, tropical cyclones, tsunamis) to identify the sedimentary signature of each type of event. There is a robust sedimentary record of terrigenous sediment runoff linked to the frequency of rainfall events that exceed a threshold of ~12mm/day (minimum to mobilize and transport sediment) for study sites. This is manifested in the sedimentary record as increases in terrigenous indicator elements (%Al, %Fe, %Ti, %Si), which agree well with rainfall records over the past ~50years. Variability in the sedimentary record over the past ~100years reflects decadal-scale fluctuations between periods of increased frequency of rainfall events, and decreased frequency of rainfall events. Dm-scale variability in terrigenous indicator elements over the past ~1400years represents the natural system variability on a decadal-centennial scale, and provides a high-resolution, long-term baseline of natural variability of rainfall/runoff events. A period of increased terrigenous sediment delivery during the 1700s and 1800s likely indicates increased erosion in response to anthropogenic activities associated with the island's plantation era, and perhaps increased frequency of rainfall events.