100-YEAR RECORD OF COASTAL SEDIMENT INPUT: ST. JOHN, USVI

The sedimentary record of coastal environments surrounding St. John, USVI, reveals the natural terrestrial to marine Holocene evolution, punctuated by a recent (~last 50 years) increased input of island-derived material coincident with human activities. Over 30 cores were collected from coastal marine and salt pond environments, and analyzed for sediment texture and composition. Geochronology was determined using 14C, 210Pb, 137CS, and 7Be radioisotopes. Results reveal the presence of four sedimentary facies (units) that reflect a natural transgressive evolution during the Holocene sea-level rise. The surficial facies is a cm- to dm-scale layer interpreted as “anthropogenically impacted” characterized by a recent increase in the input of island-derived sediments. This layer is further represented by an increase in sediment accumulation rate coincident with the degree of human-induced land use change in the upslope watershed. In regions with no development, there is no observable increase in sediment accumulation rate. In regions of modest development, increases in sediment accumulation rates of approximately 2.5x have been observed. In areas of major development, increases of up to 10x have been observed. This relationship rules out large-scale processes, such as rainfall patterns or other climatic fluctuations for controlling sediment input into these systems, since these large-scale processes would be expected to be evident in all environments. Sediment accumulation rates dramatically increased ~1960, coincident with a dramatic increase in population and paved road construction. Since then, sediment discharge points and distribution patterns have fluctuated in accordance with changing patterns in human activities. Results of this work are currently being used by territorial natural resource and planning departments, as well as local community groups, to help guide future island development.