Annual to millennial record of sediment delivery to US Virgin Island coastal environments

Abstract

Over 100 sediment cores were collected from US Virgin Island coastal marine and salt pond environments to document the record of sediment delivery to the coastal system on annual to millennial time scales, and the extent to which human activities have influenced sedimentation patterns. Cores were analyzed for sedimentology, geochronology (210Pb, 137Cs, 7Be and 14C), and high-resolution elemental composition (scanning x-ray fluorescence (XRF) and laser ablation inductively coupled mass spectrometry (LA–ICP–MS)). The centennial–millennial record reveals the natural transition from terrestrial to marine ~6–4 kya in response to the Holocene sea-level rise. Coastal salt pond cores record initial pond development as coral growth between island headlands cut off and isolated existing marine embayments ~2 kya. The decadal–centennial record is dominated by human activities, primarily road construction, manifested as a ~10× increase in sediment accumulation rate where development is heavy, a ~2.5× increase where development is moderate, and no detectable increase in undeveloped areas. Annual millimeter- to centimeter-scale laminae in salt pond cores represent individual depositional ‘events’. Light-colored, Ca- and Sr-rich sands represent marine overwash deposits that in some cases can be tied to tropical cyclones. Dark-colored Al-, Fe-, Ti-, Si-rich sandy muds represent island runoff when rainfall rates exceed a threshold of ~1.2 cm/day. Organic-rich layers represent microbial mat growth between depositional ‘events’. Marine overwash and rainfall/runoff layers fluctuate on an annual–decadal scale during the historical period, but show much more variability over the previous ~1400 years, suggesting sediment source(s), depositional processes, and/or driving mechanisms have not remained constant during at least the late Holocene.