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

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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 (D b≈9cm 2yr -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. © 2012 Elsevier B.V.

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Marine Geology

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