Phenomena Related To Ice Lake and Sea Formation
The term lake is used to mean a body of primarily fresh water. A sea is filled with brackish or salt water. In the history of the Baltic Sea, the distinction is not always clear. Salinity has varied with location, depth and time.
The main factors are the advance or recession of the Scandinavian glacier and the isostatic sinking of the landforms due to the weight of ice or isostatic rebound (springing back) when relieved of it. The glacier provides a massive flow of fresh water. Salt water enters from the North Sea through straits when the sea level is high enough to allow reverse flow over the sill. When the straits are above sea level or close to sea level, fresh water will accumulate and a lake forms. Fresh water will accumulate to levels substantially higher than sea level when the sills are substantially above sea level. The release of fresh water from the glaciers depends on climate; the presence or absence of entrances to the Ocean depends on land rise and oceanic water level; the latter of course is also affected by the amount of ice held in glaciers worldwide.
Several methods are used to determine the quality (temperature, salinity, solids content) of ancient sea water. The main one is the type of diatoms found in the sediment. Some species require salt water, while others require fresh. Other invertebrates serve as marker species as well. Also, periods of maximum supply from melt water are marked by low organic carbon in the sediment. Higher carbon content causes greater deposition of iron sulfide, which appears as a black varve.
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