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

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 similar to 0.39 cm/yr (from Pb-210 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 Th-234 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.