Sediment echo sounding survey in southern Lake Saimaa (Finland): early post-glacial low-water phase identified as sharp erosional horizons in glacial clays

摘要

Owing to isostatic land uplift in Fennoscandia, all the major lake systems in Finland have gone through major water level and outlet changes during the postglacial period (Eronen & Haila 1990). Thus, Lake Saimaa during its earliest development was a series of smaller lake basins, draining toward the northwest, and the water level at its southeast end was considerably lower than at present (Saarnisto 1970, 2000). Land tilting, or faster uplift in the northwest direction, then led to gradual flooding of the different sub-basins, which continued in the southeast until the present outlet of the lake, River Vuoksi, was formed at Imatra 5700 years ago (calibrated radiocarbon date). Raised beaches as well as isolation histories of smaller basins have enabled reconstructing and dating the high-water phases of both the Baltic Sea and the inland lakes in great detail. However, the low-water events are less well known, because their geomor-phic traces are submerged under the present lake surfaces. By sediment echosounding we have detected extensive horizontal erosion surfaces within the sediment beds of southern Lake Saimaa, apparently related to the early post-glacial low-water phase in these parts of the lake. The echograms reveal both erosional and redepositional processes reworking the glacial clays at depths 16-20 m below the present water surface. Pollen analysis of a sediment core taken through the observed erosional horizon provides rough dating for the low-water event as well as for the subsequent development of the basin.