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

Abstract

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.

Changes in the inflow of saline water into the Bornholm Basin (SW Baltic Sea) during the past 7100 years – evidence from benthic foraminifera record

“…; Häusler et al . ). According to Jilbert & Slomp (), multiple hypoxic events occurred in the Gotland Basin during the HTM ( c. 7.5–4.0 ka BP), as defined by variation in bulk sediment Mo/Al ratio.…”

“…Variability of sediment geochemistry TOC from core M86‐1/24/3 (black curve) compared to TOC from core 303770 (green curve) located in the Bornholm Basin and LOI from core 372540 located in the Gotland Basin (Häusler et al . ) together with high‐resolution foraminiferal data and other palaeoproxies (ostracods and cladocerans) from core M86‐1/24/3 (see Fig. ).…”

“…; Häusler et al . ) and, perhaps, freshwater input was limited (Gustafsson & Westman ). The sea floor was still supplied by a dense bottom water‐mass from the North Sea, creating favourable conditions for marine benthic organisms and deposition of homogenous sediments.…”

“…() and Häusler et al . () looked at long‐term foraminiferal changes in sediment cores. A low number of palaeo‐studies is, perhaps, explained by the problem of carbonate dissolution emphasized by Wefer () and Wefer & Lutze () for the shallow areas of the western Baltic Sea.…”

The Baltic Sea: geology and palaeoenvironmental evolution – introduction

“…This region represents one important gateway between the Baltic Sea and the North Sea. A palaeoceanographical update on the entire Littorina Sea stage palaeoenvironmental Baltic Sea development is provided by Ning et al (2017) andH€ ausler et al (2017). Here, lagoonal marls were deposited along the developing Curonian Spit that were subsequently overridden by massive sand dunes and partly extruded by 'dune tectonic' processes.…”