Early Holocene deglaciation chronology in Västergötland and Närke, southern Sweden — biostratigraphy, clay varve, 14C and calendar year chronology

Abstract

Early Holocene varved clay sequences from Västergötland and Närke, southern Sweden have been analysed for pollen and marine calcareous fossils. A chronology was derived from clay varve measurements and AMS 14C dating and pollen stratigraphy. The results suggest that the varve sequences were deposited during the early Holocene between c. 11,500 and 11,200 calendar years BP which corresponds to the deglaciation of the area. The corresponding varve ages in the Swedish Time Scale is c. 700–800 years younger which is broadly in accordance with the earlier reported error in the Swedish Time Scale. The discovery of the marine bivalve Portlandia arctica in clay varve sequences from northern Lake Vättern is the first reported evidence of a marine ingression into the Baltic basin prior to the Yoldia Sea brackish phase. The fossil and lithological evidence indicates that the period of marine influence lasted 10–50 years (min age) and occurred c. 140 years before the deglaciation of the Närke Strait and the subsequent brackish water phase of the Baltic Yoldia Sea stage. Comparative AMS 14C datings on terrestrial macrofossils and the shell of the marine bivalve P. arctica suggest a reservoir effect of more than 1000 year. However, dates on periostracum from P. arctica gave ages closer to the terrestrial AMS 14C ages.