Paleogeographic numerical modeling of marginal seas for the Holocene – an exemplary study of the Baltic Sea

Abstract. The sustainable management of marginal seas is based on a thorough understanding of their evolutionary trends in the past. The paleogeographic evolution of marginal seas is controlled not only by global and regional driving forces (eustatic sea-level change and isostatic/tectonic movements) but also by sediment erosion, transport, and deposition at smaller scales. Consistent paleogeographic reconstructions at a marginal sea scale considering the global, regional, and local processes are yet to be derived, and this study presents an effort towards this goal. We present a high-resolution (0.01°×0.01°) paleogeographic reconstruction of the entire Baltic Sea and its coast for the Holocene period by combining eustatic sea-level change, glacio-isostatic movement, and sediment deposition. Our results are validated by comparison with field-based reconstructions of relative sea level (RSL) and successfully reproduce the connection/disconnection between the Baltic Sea and the North Sea during the transitions between lake and sea phases. A consistent map of Holocene sediment thickness (SED) in the Baltic Sea has been generated, which shows that relatively thick Holocene sediment deposits (up to 36 m) are located in the southern and central parts of the Baltic Sea, corresponding to depressions of sub-basins, including the Arkona Basin, the Bornholm Basin, and the Eastern and Western Gotland Basin. In addition, some shallower coastal areas in the southern Baltic Sea also host locally confined deposits with thicknesses larger than 20 m and are mostly associated with alongshore sediment transport and the formation of barrier islands and spits. In contrast to the southern Baltic Sea, the Holocene sediment thickness in the northern Baltic Sea is relatively thin and mostly less than 6 m. The morphological evolution of the Baltic Sea and its coastline is featured by two distinct patterns. In the northeastern part, the change in the coastline and offshore morphology is dominated by regression caused by post-glacial rebound that outpaces the eustatic sea-level rise, and the influence of sediment transport is very minor, whereas a transgression, together with active sediment erosion, transport, and deposition, has constantly shaped the coastline and the offshore morphology in the southeastern part, leading to the formation of a wide variety of coastal landscapes such as barrier islands, spits, and lagoons.