Mastogloia Sea

The Mastogloia Sea is one of the prehistoric stages of the Baltic Sea in its development after the last ice age. This took place ca. 8000 years ago following the Ancylus Lake stage and preceding the Littorina Sea stage.

Note: The dates used in this article are expressed in radiocarbon years before present (‘present’ in the radiocarbon context meaning, for historical reasons, the year 1950 AD). Expressed in calendar years before present, all dates would be several hundred years older.

Towards the end of its history the level of the Ancylus Lake was falling following the formation of a new outlet at the Great Belt. The Ancylus Lake reached the level of the sea ca. 8500 years ago, marking the beginning of the Mastogloia Sea (Björck 1995, Donner 1995).

At this time the global sea level was rising rapidly as the melting of the last remnants of the great ice age ice sheets still continued (Fleming et al. 1998). As a result, some amounts of salt water started to penetrate into the Baltic basin through the Danish straits, mixing into the vast freshwater body. This resulted in slightly brackish conditions in the Baltic. The Mastogloia Sea bears the name of the Mastogloia genus of brackish water diatoms, the species of which are considered characteristic of the geological deposits of this stage (Donner 1995, Eronen 1974).

Continuing sea level rise during the Mastogloia Sea stage had the effect of deepening the straits connecting the Baltic with the ocean, thus increasing the influx of salt water into the Baltic. A significant hydrographic shift occurred 8500 years ago, which corresponds to shifts in currents in the Skagerrak, Kattegat and the Norwegian Channel, as they transition to the modern circulation system in the eastern North Sea. This is a consequence of the opening of the English Channel and the Danish straits and increased Atlantic water inflow, and the subsequent development of the South Jutland Current.

Between 8000 and 7000 years ago the Baltic became distinctly brackish, starting from the southern parts closest to the ocean and spreading therefrom into central Baltic and finally the Gulf of Finland and the Gulf of Bothnia. The arrival of markedly brackish conditions marks the beginning of the Littorina Sea stage (Miettinen 2004). The Mastogloia Sea stage thus constitutes a transitional phase between the freshwater Ancylus Lake stage and the Littorina Sea stage during which the Baltic was clearly brackish (Donner 1995, Hyvärinen et al. 1988)

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