Baltic Ice Lake

About

The Baltic Ice Lake is a name given by geologists to a freshwater lake that evolved in the Baltic Sea basin as glaciers retreated from that region at the end of the last ice age. The lake's existence was first understood in 1894. The lake existed between about 16,000 and 11,700 years ago with well defined evidence from the warming of the Bølling–Allerød Interstadial to the period of cooling called the Younger Dryas before the Holocene, the onset of which is close in time to the end of the ice lake. The lake drained into the raising world ocean on two occasions and when water levels became the same on the second, with a sea level passage in the Billingen region of southern Sweden, it became the Yoldia Sea.

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. Currently the Baltic Sea has different salinity in layers, seasons and distance from its North Sea connection, as well as mixing events separated by decades. The saline gradients across the area can be assumed to have been greater at times of the massive ice cap melting with earlier seas, but even now there is a marked gradient west to northeast. Seasonal ice cover also is relevant, and forms easier the lower the salt content of the water. Surface waters will tend to have lower salinity than deeper waters and high organic content with oxygenation is more likely closer to the connection to the world seas, if one exists. The main factors relevant were the advance or recession of the Weichselian glaciation responsible for the Fenno-Scandian ice sheet and the isostatic sinking of the landforms due to the weight of ice or rebound when it melts (springing back, post-glacial rebound, glacial isostatic adjustment), and this was known by geologists to be relevant to the Baltic area by the end of the 19th century, with a flurry of consolidation work in the early 20th century. These processes happen at different rates, often over periods of tens to thousands of years. Timing of such events can have uncertainty and for example the onset of the Younger Dryas is apparently 180 years later in Northern Europe than Greenland. Melting of the ice cap provides a massive source of fresh water. This was associated with the formation of various glacial lakes and influenced sea levels worldwide, which have risen since 22,000 years ago about 120 m (390 ft). Locally salt water entered from the North Sea through straits when the sea level was 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 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, as occurred after the lake reached sea level, causes greater deposition of iron sulfide, which appears as a black varve. This has been demonstrated well in sediment cores from the depths of the Baltic Sea.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.