High-Resolution Event Stratigraphy of mm-Scale Laminated Sediments from Coastal Salt Ponds: St. John, US Virgin Islands .

摘要

A multi-proxy approach is utilized on mm- to cm-scale laminated sediment records in coastal salt ponds on St. John, U.S. Virgin Islands to characterize the sediments, identify their sources and depositional processes/events (heavy rainfall, tropical cyclones, tsunamis). Historical records are combined with high-resolution geochronology (short-lived radioisotopes, 210Pb, 137Cs, 7Be) and scanning elemental techniques (XRF and LA-ICP-MS) to link depositional events to how they are manifested in the sedimentary record. Volcanic rocks are the terrigenous sediment source and the sedimentary signature of terrigenous sediment in the geologic record consists of higher amounts of Al, Fe, Ti, Co, and Si, and is associated with terrigenous runoff due to rainfall events. A minimum threshold value of >2.0 mm per day (minimum to erode and transport terrigenous 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 the amount of terrigenous sediment accumulation of the terrigenous indictor element Al measured by scanning LA-ICP-MS. There is a robust sedimentary record of terrigenous sediment runoff that is a function of the frequency of heavy rainfall events (exceed threshold). Variability in the sedimentary record reflects changes between periods of "wet" increased frequency of heavy rainfall events and "dry" decreased frequency of heavy rainfall events. Tropical cyclones and tsunamis can cause marine overwash into salt ponds leading to deposition of marine sediments. Elemental scans for Ca and Sr and overwash indicator elements are complicated by grain size effects of LA-ICP-MS techniques, as well the difficulty in differentiating between tropical cyclone overwash deposits 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.