Physical and biological influences on the formation of sedimentary fabric in an oxygen-restricted depositional environment: Eckernforde Bay, Southwestern Baltic Sea

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Sedimentological and radiochemical investigations of sediments in Eckernforde Bay were coordinated with observations of benthic biology and benthic-boundary-layer dynamics to identify processes influencing the formation of preserved sedimentary fabric. The bay is a trap for fine sediments because of its fjord-like basin geometry and hydrodynamic regime. Sediments are derived from both local sources (transported by winter storms) and sources in adjacent Kiel Bight (fair-weather transport of suspended sediment by internal waves). In the central basin of the bay, strata consist of pelletized, clay-rich beds and silty laminations, and reflect the interaction of bioturbation with alternating fair-weather and storm-associated sediment transport and deposition. Sediment-accumulation rates in the central basin of the bay are ~0.39 cm/yr (from 210Pb geochronology). Results from a numerical model, as well as study of X-radiographs and accumulation rates, indicate that fair-weather processes contribute 0.15 cm/yr of time-averaged sediment accumulation, and the remaining 0.24 cm/yr is contributed by storm-generated deposition. Seasonal oxygen depletion restricts the benthic community to a pioneer assemblage of small-bodied, deposit-feeding polychaetes and bivalves. Excess 234Th analyses indicate that bioturbation is restricted to the upper 1 cm of the seabed. Macrofaunal feeding activity produces abundant ovoid fecal pellets and biogenic microfabric in beds deposited under fair-weather conditions. In contrast, primary depositional fabric is preserved as laminations when storm deposits thicker than the depth of bioturbation disrupt biological communities.

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