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Mid‐ to late Holocene environmental separation of the northern and central Baltic Sea basins in response to differential land uplift

The Littorina Sea stage (past c. 7000 years) development of the northern, and its interactions with the central Baltic Sea have been influenced by spatially different but in the north very strong glacio‐isostatic uplift. Here we investigate the impact of the isostatic readjustment on the northern Baltic Sea environment and on the water exchange with the central Baltic Sea using geochemical and microfossil records from well‐dated sediment cores of the Bothnian Sea/Bay that are compared to sedimentary records of the Gotland Basin and Landsort Deep. Benthic foraminifera and manganese carbonate layers in the sediments from northern Baltic Sea sub‐basins imply oxic marine intrusions from c. 7.0 cal. ka BP. Related water‐column stratification together with enhanced primary productivity caused oxygen‐depleted bottom waters and deposition of organic‐rich laminated sediments during the Holocene Thermal Maximum. Comparable sedimentary/geochemical records in early Littorina Sea sediments of e.g. the eastern Gotland Basin therefore imply a low impact of isostatic uplift on the northern Baltic Sea environment. A climate‐controlled decline in hypoxia induced a gradual deposition of rather homogenous sediments in the central and northern Baltic Sea during the mid‐Littorina Sea stage. Progressive isostatic uplift caused a diverging environmental development in both basins during the mid‐ to late Littorina Sea stage. The Åland sill shoaled markedly allowing only strong inflows to enter the northern Baltic Sea as late as 2.3 cal. ka BP. The restricted deep‐water exchange with the central Baltic Sea probably resulted in a pronounced freshening and oxic bottom waters. This, together with an uplift‐related sediment re‐deposition induced accumulation of rather homogenous sediments throughout the remaining Littorina Sea stage. By contrast, a climate‐controlled depositional environment in the central Baltic Sea during this time caused the formation of homogeneous sediments during cold phases and laminated sediments during warm phases.