Evolution of a Holocene Reef-Lagoon System: St. Croix, U.S. Virgin Islands

The reef system on the north shore of St. Croix, US. Virgin Islands has been studied extensively over the past 35 years. However, the subsurface history of the lagoon has not been investigated. In the summer of 2007, thirteen three-inch vibrocores were taken across the lagoon through three primary environments: 1) extensively bioturbated Callianassid shrimp zones, 2) seagrass areas, and 3) bare sand. In addition, the molluscan fauna (combined living and death assemblage) were sampled using a suction dredge. Cores were split, photographed and sampled. Grain size and constituent analyses were performed on the first 8cm-section of each 20-cm interval. The remaining 12cm were sieved to extract the molluscan fauna. Molluscs were identified and assigned a taphonomic grade.

The sediment package reaches 1.5- >3.5m thickness over the antecedent Pleistocene topography ranging 3-10 meters below present sea level. Based on Caribbean sea-level history, this lagoon initially flooded between 8,000 and 7,000ybp. The base of all cores intersecting the Pleistocene surface had a coarse lag of shell, Halimeda, and/or terrigenous pebbles. Bulk molluscan radiocarbon dates from three shelly lags are ~3,200ypb, which suggests an average accumulation over the lagoon's 7000-year history. Above the lag, shell content and grain size decline until half-way up the core where it changes to fine, featureless sand with few shells. Molluscs within the cores are generally small, thin-shelled, shallow infaunal bivalves and gastropods and nearly all shells are in good taphonomic condition. In contrast, the present day surface assemblages in most locations reflect the presence of seagrass (epifaunal bivalves and grazing gastropods with greater taphonomic damage). The results suggest changing energy during lagoon flooding and a long interval of Callianassid shrimp activity that mixed and sequestered infaunal shells. This was followed by stabilization of the lagoon and a change to present conditions.