Accumulation and intense bioturbation of bioclastic muds along a carbonate-platform margin: Dry Tortugas, Florida

Abstract

A sedimentological study was undertaken in the Dry Tortugas, lowermost Florida Keys, to examine processes influencing the accumulation and sedimentary-fabric characteristics of carbonate muds along the shallow margin of a carbonate platform. Sedimentological studies (physical properties, 234Th and 210Pb geochronology) were coordinated with observations of benthic biology and benthic-boundary-layer dynamics. Sampling was concentrated around a mud depocenter in Southeast Channel, a reentrant 20–30m deep in the southern edge of the Dry Tortugas platform.In the depocenter of Southeast Channel, accumulation rates of 0.10–0.32cmy−1 are estimated from 210Pb profiles. Likely sources of sediment are in shallower water, and include reefs and beds of calcareous green algae. Fine sediment (silt- and clay-sized) is advected to the seabed of Southeast Channel by a combination of tidal and storm-driven currents. Flows from shallow water decelerate upon entering deeper water in Southeast Channel, producing conditions favorable to long-term accumulation.Once deposited, sediment in Southeast Channel is subjected to bioturbation by a two-tiered benthic community. The surface zone (0–5cm) is mixed biodiffusively by a surface community of small bivalves and polychaetes (Db≈9cm2yr−1) over periods of weeks to months, and is characterized by mm-scale mottling. Other near-surface structures include biodeposited fecal mounds of deep‐deposit feeders and ephemeral physical stratification as thin (1–5cm) layers of fine sediment. Below ~5cm, the dominant post-depositional process is bioadvective mixing by a deep-deposit-feeding community of callianassid shrimp and notomastid polychaetes. The upper 10–15cm is mixed completely every 6–21years, and bioturbation below this depth is slower.Even though the study area has been hit by >10 major hurricanes in the last 120years (winds >190kmh−1), sediments preserved (i.e., below 5cm depth) in Southeast Channel during this period reveal wholly biogenic sedimentary fabric produced by the deep callianassid/notomastid community. Bioturbation is thus capable of erasing the stratigraphic signature of the most extreme sediment-transport events over decadal time scales.