Rates of Holocene isostatic uplift and relative sea-level lowering of the Baltic in SW Finland based on studies of isolation contacts

Southwestern Finland was covered by the Weichselian ice sheet and experienc ed rapid glacio-isostatic rebound after early Holocene deglaciation. The pr esent mean overall apparent uplift rate is of the order of 4-5 mm/yr, but i mmediately after deglaciation the rate of crustal rebound was several times higher. Concurrently with land uplift, relative sea level in the Baltic ba sin during the past more than 8000 years was also strongly affected by the eustatic changes in sea level. There is ample evidence from earlier studies that during the early Litorina Sea stage on the southeastern coast of Finl and around 7000 yr BP (7800 cal. yr BP), the rise in sea level exceeded the rate of land uplift, resulting in a short-lived transgression. Because of a higher rate of uplift, the transgression was even more short-lived or of negligible magnitude in the southwestern part of coastal Finland, but even in this latter case a slowing down in the rate of regression can still be d etected. We used evidence from isolation basins to obtain a set of 71 C-14 dates, and over 30 new sea-level index points. The age-elevation data, obta ined from lakes in two different areas and located between c. 64 m and 1.5 m above present sea level, display a high degree of internal consistency. T his suggests that the dates are reliable, even though most of them were bas ed on bulk sediment samples. The two relative sea-level curves confirm the established model of relatively gradually decreasing rates of relative sea- level lowering since c. 6100 yr BP (7000 cal. yr BP) and clearly indicate t hat the more northerly of the two study areas experienced the higher rate o f glacio-isostatic recovery. In the southerly study area, changes in diatom assemblages and lithostratigraphy suggest that during the early Litorina S ea stage (8300-7600 cal. yr BP) eustatic sea-level rise exceeded land uplif t for hundreds of years. Evidence for this transgression was discovered in a lake with a basin threshold at an elevation of 41 m above sea level, whic h is markedly higher than any previously known site with evidence for the L itorina transgression in Finland. We also discuss evidence for subsequent s hort-term fluctuations superimposed on the main trends of relative sea-leve l changes.