The Littorina transgression in southeastern Sweden and its relation to mid-Holocene climate variability

Abstract: Lateglacial and Holocene shoreline displacement along the Baltic coast resulted from both the isostatic land uplift and the ice-volume-equivalent sea-level rise. Relative changes of these two components led to alternating contact/isolation of the Baltic Basin with the North Sea during the Holocene. The Littorina transgression was a significant palaeoceanographic change that took place during the mid-Holocene in southern Sweden. However, the detailed pattern of the transgression has long been debated. As yet, the rate, magnitude and cause(s) of the transgression as well as the physical link with the North Atlantic climate are poorly known. In this study, shoreline displacement and coastal palaeoecology were reconstructed on the basis of multi-disciplinary studies of sediment sequences from four basins located at an elevation range between –1 and 8 m above present sea level in southeastern Sweden. Coastal basins with well defined thresholds may provide powerful constraints on relative sea-level changes. The timing of the Littorina transgression was determined by dating the lacustrine/brackish-marine transitions in sediment sequences recovered from these basins. Following a slight rise, the transgression culminated between 8000 and 7500 cal. BP, marked by a c. 8-m relative sea-level rise at an accelerated rate of ~15 mm yr-1. This relatively rapid rise can be ascribed to the partial collapse of the Antarctic Ice Sheet. From 6500 cal. BP, the relative level of the Baltic Sea fell as a result of the deceleration of global sea-level rise and continued isostatic rebound. Superimposed on this eustatic pattern are five minor transgressions identified during the middle Holocene: L1 8500–8200, L2 7800–6900, L3 6400–5600, L4 5300–4700, and L5 4500–4100 cal. BP. Evidence from the northwestern European coasts and Greenland ice cores suggests that these episodic sea-level rises may have been related to increased storminess in the North Atlantic realm, which overprinted global ice-volume changes at millennial time scales. Superimposed on a general highstand, the Baltic Sea level exhibited significant fluctuations at centennial time scales between 8500 and 3000 cal. BP. A close correlation between sea-level proxies and Greenland ice-core sea-salt ions implies that these cyclic fluctuations of Baltic Sea level might have been causally linked to periodical variations in regional wind pattern, probably operated by solar forcing in the Suess band through a thermodynamic mechanism. Other cycles may also be a signal of long-term changes in tidal intensity, when the connection of the Baltic basin with the North Sea was wider during the middle Holocene.