Sedimentary Development of Coral Bay, St. John, USVI: A Shift From Natural to Anthropogenic Influences

Analyses of sediment cores and surface samples collected in Coral Bay, St. John, USVI, reveal four sedimentary facies. The bottom three reflect the natural transgressive evolution of the bay during the Holocene sea-level rise. The surficial facies represents a dramatic increase in terrigenous sediment input since the 1950's. This surficial layer is defined by a decrease in grain size, increase in organic content, increase in terrigenous constituents, and subtle decrease in calcium carbonate content compared to underlying sediments. Based upon ²¹⁰Pb and ¹⁴C dating, accumulation rates have increased by roughly one order-of--magnitude since the 1950's as a result of this increase in terrigenous sediment input. The surficial sediment layer likely represents an anthropogenic signal reflecting the dramatic increase in island development over the past few decades. The surficial “impacted” layer is most pronounced (i.e., exhibits the greatest deviation from underlying sediments) adjacent to the most heavily developed areas, and in protected, low energy regions. It becomes less pronounced in the more open and seaward portions of the bay. This pattern suggests proximity to input and energy level are the dominant controls governing where terrigenous sediments ultimately accumulate. Comparisons with anthropogenically-impacted estuaries along the U.S. Atlantic and Gulf coasts show that, although there are substantial differences in scale and watershed character, all have experienced recent dramatic increases in sediment accumulation. Unlike Coral Bay, however, there is no anthropogenic signal in sediment texture and composition. This may be a reflection of St. John's high-relief terrain, high erodibility of rocks/soils, and intense weathering associated with tropical volcanic island settings.