Long-term coastal openness variation and its impact on sediment grain-size distribution: a case study from the Baltic Sea

We analysed the long-term variations in grain-size distribution in sediments from Gasfjarden, a fjord-like inlet in the southwestern Baltic Sea, and explored potential drivers of the recorded changes in the sediment grain-size data. Over the last 5.4 thousand years (ky) in the study region, the relative sea level decreased 17 m, which was caused by isostatic land uplift. As a consequence, Gasfjarden was transformed from an open coastal setting to a semi-closed inlet surrounded by numerous small islands on the seaward side. To quantitatively estimate the morphological changes in Gasfjarden over the investigated time period and to further link the changes to the grain-size distribution data, a digital elevation model (DEM)-based openness index was calculated. The largest values of the openness indices were found between 5.4 and 4.4 cal ka BP, which indicates relatively high bottom water energy. During the same period, the highest sand content (similar to 0.4 %) and silt/clay ratio (similar to 0.3) in the sediment sequence were also recorded. After 4.4 cal ka BP, the average sand content was halved to similar to 0.2% and the silt/clay ratio showed a significant decreasing trend over the last 4 ky. These changes were found to be associated with the gradual embayment of Gasfjarden, as represented by the openness indices. The silt/clay ratios exhibited a delayed and relatively slower change compared with the sand content, which indicates different grain-size sediment responses to the changes in hydrodynamic energy. Our DEM-based coastal openness indices have proved to be a useful tool for interpreting the temporal dynamics of sedimentary grain size.