Mastogloia Sea
The Mastogloia Sea is one of the prehistoric stages of the Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

 in its development after the last ice age
Wisconsin glaciation
The last glacial period was the most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago....

. This took place ca. 8000 years ago following the Ancylus Lake
Ancylus Lake
Ancylus lake is a name given by geologists to the body of fresh water that replaced the Yoldia Sea after the latter had been severed from its saline intake across central Sweden by the isostatic rise of south Scandinavian landforms. The dates are approximately 9500-8000 BP calibrated, during the...

 stage and preceding the Littorina Sea
Littorina Sea
Littorina Sea is a geological brackish-water stage of the Baltic Sea, which existed around 7500–4000 BP and followed the Mastogloia Sea, transitional stage of the Ancylus Lake...


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
Great Belt
The Great Belt is a strait between the main Danish islands of Zealand and Funen . Effectively dividing Denmark in two, the Belt was served by the Great Belt ferries from the late 19th century until the islands were connected by the Great Belt Fixed Link in 1997–98.-Geography:The Great Belt is the...

. 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
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 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
Danish straits
The Danish straits are the three channels connecting the Baltic Sea to the North Sea through the Kattegat and Skagerrak. They transect Denmark, and are not to be confused with the Denmark Strait between Greenland and Iceland...

, mixing into the vast freshwater body. This resulted in slightly brackish
Brackish water
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...

 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 at 8.5k BP, 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
Gulf of Finland
The Gulf of Finland is the easternmost arm of the Baltic Sea. It extends between Finland and Estonia all the way to Saint Petersburg in Russia, where the river Neva drains into it. Other major cities around the gulf include Helsinki and Tallinn...

 and the Gulf of Bothnia
Gulf of Bothnia
The Gulf of Bothnia is the northernmost arm of the Baltic Sea. It is situated between Finland's west coast and Sweden's east coast. In the south of the gulf lie the Åland Islands, between the Sea of Åland and the Archipelago Sea.-Name:...

. 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)

Disputed status

Many researchers have been unwilling to recognize the Mastogloia Sea as a separate stage in the development of the Baltic Sea, favouring including it in either the Ancylus Lake stage or the Littorina Sea stage (Hyvärinen et al. 1988, Miettinen 2002).

In the stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

 of Baltic sediments the Mastogloia stage is difficult to detect, its sediments being visibly identical to those of the Ancylus Lake (Donner 1995, Eronen 1983). Even the fossil diatom content of the Mastogloia sediments – employed by researchers as the key method of distinguishing deposits of different Baltic stages – is ambiguous, in many locations showing no difference from that of Ancylus deposits, and at best including an admixture of the aforementioned Mastogloia diatoms in an otherwise typical Ancylus flora (Eronen 1974). Deposits of the Littorina Sea, on the other hand, show a drastic change both in the visible characteristics of the sediment and its diatom content (Donner 1995, Eronen 1974) One could thus argue for the inclusion of the Mastogloia Sea stage in the Ancylus Lake stage. Others prefer to include it in the Littorina Sea stage as a transitional phase after the establishment of the marine connection (Hyvärinen et al. 1988).

In spite of these objections, though, the concept of a Mastogloia Sea persists in literature concerning the development of the Baltic Sea. It has been noted that a separate Mastogloia stage is useful in maintaining the clarity of the system, delimiting the period with undeniable if slight marine influence following the fall of the Ancylus Lake to sea level that predates the profound changes at the beginning of the Littorina Sea stage (Eronen 1983).
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