The Mastogloia stage in the Baltic Sea history: diatom evidence from southern Finland

Abstract

Studies of ancient Baltic sediments obtained from isostatically uplifted lake basins near Helsinki, on the south coast of Finland, have yielded diatom sequences across the transition from Ancylus Lake to Litorina Sea strata. Different facies within the general near-shore environment, from shallow littoral to moderately deep (10‒20 m) open bay, are represented. All the sediments deposited before 8000 B.P. contain oligohalobous diatom floras normally associated with the Ancylus Lake stage. Between 8000 and 7500 B.P., an interval often singled out as the Mastogloia stage, weakly brackish taxa are regularly introduced into the strictly littoral floras, whereas the floras deposited at the same time in deeper water remain unchanged and consist of ordinary Ancylus taxa. After 7500 B.P. rich brackish floras characteristic of the Litorina stage proper appear at all sites that retained their contact with the Baltic. These results confirm the earlier conclusions based on off-shore data that the Mastogloia stage can only be demonstrated in the littoral facies. Nevertheless, in view of its consistent and non-local occurrence after but not before a certain time point, this stage is considered to reflect a general change in conditions in the Baltic Sea history; the apparent discrepancy between the littoral and off-shore records may be due to differential response to the change by the littoral and off-shore diatom assemblages.

Keywords

• Baltic Sea history
• diatom stratigraphy
• Mastogloia stage
• Litorina Sea
• Ancylus Lake
• Clypeus limit
• sea-level changes
• Flandrian
• Finland