|Evolution of the Baltic Sea|
|Eemian Sea (130,000–115,000 BP)
Ice sheets and seas (115,000–12,600 BP)
|Baltic Ice Lake (12,600–10,300 BP)
Yoldia Sea (10,300–9500 BP)
Ancylus Lake (9,500–8,000 BP)
Mastogloia Sea (8,000–7,500 BP)
Littorina Sea (7,500–4,000 BP)
Modern Baltic Sea (4,000 BP–present)
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 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 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)
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 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).
- Björck, Svante (1995). "A Review of the History of the Baltic Sea, 13.0-8.0 ka BP". Quaternary International (Elsevier) 27: 19–40. Bibcode:1995QuInt..27...19B. doi:10.1016/1040-6182(94)00057-C.
- Donner, J. (1995) The Quaternary History of Scandinavia. Cambridge University Press, 210 pp. ISBN 9780521417303
- Eronen, M. (1974) The history of the Litorina Sea and associated Holocene events. Societas Scientarum Fennicae, Commentationes Physico-Mathematicae 44, 79-195.
- Eronen, M. (1983) Late Weichselian and Holocene shore displacement in Finland. In Smith, D. E. and Dawson, A. G. (eds.) Shorelines and Isostasy, Academic Press, London, 183-207.
- Fleming, K., Johnston, P., Zwartz, D., Yokoyama, Y., Lambeck, K., and Chappell, J.(1998) Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites. Earth and Planetary Science Letters 163, 327-342.
- Hyvärinen, H., Donner, J., Kessel, H., and Raukas, A. (1988) The Litorina Sea and Limnaea Sea in the Northern and Central Baltic.. In Donner, J. and Raukas, A. (eds.) Problems of the Baltic Sea History, Annales Academiae Scientarum Fennicae A III 148, 13-23.
- Miettinen, A. (2002) Relative sea level changes in the eastern part of the Gulf of Finland during the last 8000 years. Annales Academiae Scientiarum Fennicae, Geologica-Geographica 162, 100 pp.
- Miettinen, A. (2004) Holocene sea-level changes and glacio-isostasy in the Gulf of Finland, Baltic Sea. Quaternary International 120, 91-104,  doi:10.1016/j.quaint.2004.01.009