Stratigrafin i Hanöbukten under senaste glaciationen : en studie av borrkärnor från IODP’s expedition nr 347

Sammanfattning: The development of Hanö Bay during the last ice age has been investigated by analyzing cores from IODP’s Balitc Sea Palaeoenvironment expedition in the Baltic Sea. By analyzing the presence of Ice Rafted Debris, correlating one of the varved clay units to Ringbergs (1991)regional varve chronology from southeastern Sweden, analyzing pollen grains and measuring loss on ignition a few gaps in the history of Hanö Bay has been uncovered. Four common units are found in the cores; a diamict unit, two varved clay units and an organic layer that separates the two varved clay units. The results from the analyzes, combined with the results from similar studies, show that the three lower units have been deposited after the Middle Weichselian Ristinge Ice Advance at approximately 60 ka BP. This is based partly on a former dating of unit 3, the organic layer, which showed an age of more than 38 ka. The varved clays are presumed to be deposited during glacial/interstadial conditions with a proximal ice margin while the organic layer, unit 3, is supposed to have been deposited further away from the ice margin in an environment with some vegetation including herbs, grasses, shrubs and some trees. Unit 4 was correlated to Ringberg (1991) and was therefore dated to between 14 550 cal yr BP and 14 750 cal yr BP placing the unit during the Bølling interstadial. From this it was concluded that 25 ka is missing between units 3 and 4. This hiatus can be explained in different ways. It is, for example, possible that the Scandinavian ice sheet has eroded the unit during the Last Glacial Maximum, but there is also a possibility that no deposition took place during this time. It was finally concluded that the younger geologic development of the Hanö Bay still is still not fully understood but that at least some of the earlier questions have been solved by these new results.