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. Expressed in calendar years before present, all dates would be several hundred years older.
Overview
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. At this time the global sea level was rising rapidly as the melting of the last remnants of the great ice ageice sheets still continued. 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 brackish waterdiatom genus Mastogloia, the species of which are considered characteristic of the geological deposits of this stage. 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. 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
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. In the stratigraphy of Baltic sediments the Mastogloia stage is difficult to detect, its sediments being visibly identical to those of the Ancylus Lake. 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. 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 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. 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.
