The brackish Baltic Sea Yoldia Stage – palaeoenvironmental implications from marine benthic fauna and stable oxygen isotopes

Abstract

Eighteen sites in middle Sweden with sediments deposited during the brackish phase of the Yoldia Sea have been investigated with respect to the marine calcareous benthic fauna (i.e. foraminifera, ostracods and molluscs) and stable oxygen isotopes. At most sites the sediments consist of varved clay sequences which are correlated to the Swedish Time Scale and dated to the Early Preboreal. A marine benthic fauna has been recorded at 11 sites. The fauna occurs during c. 100 clay‐varve years and the total length of the brackish phase is estimated to be 110–190 years. Near the inlet of saline water the fauna was dominated by the foraminifera Elphidium excavatum (Terquem) and further to the east the ostracod Cytheropteron montrosiense (Brady, Crosskey and Robertson) dominated. At shallower depths the fauna occurred during a shorter period owing to the formation of a stratified water column with a low‐saline surface water. Very low δ18Oc values were recorded, i.e. between ‐9.9% and ‐4.7% because of the influence of meltwater from the ice sheet. A different isotope fractionation between the foraminifera E. excavatum and the ostracod C. montrosiense was recorded.