Density-driven bottom currents control development of muddy basins in the southwestern Baltic Sea

Abstract

The development of two Holocene muddy depocenters in the southwestern Baltic Sea is investigated using sediment budget analysis and numerical modeling. Material derived from the erosion of coastal cliffs surrounding the study area is shown to dominate the supply of fine-grained sediment to the depocenters, while the riverine contribution is an order of magnitude smaller. Comparison with the sink terms, compiled from published geological data, reveals that a substantial additional source of at least 900 kt/yr is required to close the budget, and high-salinity dense inflows from the North Sea carrying suspended sediment are proposed as an additional source mechanism. Seismo-acoustic data show the long-term impact of strong bottom currents, likely linked to dense-water inflows, which produce contouritic deposits in flow-confining channels. We reproduce two distinct inflow events using a coupled hydrodynamic-sediment transport coastal ocean model. The simulations confirm that major inflows are capable of advecting a significant amount of fine-grained sediment into the study area. A scaling relationship based on the simulated fluxes estimates the average amount of sediment imported in this way to the order of 100–900 kt/yr, which is in agreement with the lower limit of the gap in the budget. The amount of sediment advected seems to scale non-linearly with the intensity of the inflow. More field data points are needed in order to improve the accuracy of modeled fluxes and the precision of the scaling relationship. This study shows how the relative contributions of episodic sedimentation events on the longer-term morphology may be quantified.