# Yoldia Sea
The **Yoldia Sea** was a brackish-water stage in the postglacial history of the [Baltic Sea](https://www.edgepedia.org/baltic-sea) basin, lasting from about 11.7 to 10.7 thousand years before present, between the freshwater [Baltic Ice Lake](https://www.edgepedia.org/baltic-ice-lake) and the freshwater [Ancylus Lake](https://www.edgepedia.org/ancylus-lake).[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) It takes its name from marine macrofossils of the bivalve *Portlandia* (*Yoldia*) *arctica* found in land exposures in central Sweden.[2](https://www.researchgate.net/publication/291197113_The_Post-Glacial_Evolution_of_the_Baltic_Sea) The stage is one of the four classical stages of the basin's postglacial evolution, together with the Baltic Ice Lake, the Ancylus Lake, and the Littorina Sea.[3](https://www.sciencedirect.com/science/article/pii/104061829400065D)
| Key fact | Detail |
|---|---|
| Age | About 11.7–10.7 ka BP (calibrated years), the first stage of the Holocene in the Baltic basin[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) |
| Salinity sequence | Freshwater, then weakly brackish water, then freshwater again[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) |
| Connection to the ocean | Inlet and outlet through south central Sweden, via the Närke Strait[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) • [4](https://www.sciencedirect.com/science/article/pii/104061829400069H) |
| Beginning | Final drainage of the Baltic Ice Lake lowered the basin water level by about 25 m[4](https://www.sciencedirect.com/science/article/pii/104061829400069H) |
| Brackish phase | A short saline ingression at ca. 9.9 ka BP lasting 100–200 years, recorded from Stockholm to the southernmost parts of the sea[4](https://www.sciencedirect.com/science/article/pii/104061829400069H) |
| Sediments | Clay, weakly varved or homogeneous, in the Baltic proper[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) |
| End of the stage | Uplift closed the connection, and the freshwater Ancylus Lake followed from about 10.7 ka BP[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) |
## Position in Baltic Sea history
During the last deglaciation, the Baltic basin held the Baltic Ice Lake, a pro-glacial freshwater lake in front of the retreating Scandinavian Ice Sheet. As the balance between global sea-level rise and the isostatic rebound of the Scandinavian Shield shifted, gateways between the basin and the [North Sea](https://www.edgepedia.org/north-sea) opened and closed, producing alternating freshwater and brackish-marine conditions.[5](https://enviromicro-journals.onlinelibrary.wiley.com/doi/10.1111/1462-2920.14920) The transient brackish Yoldia Sea and freshwater Ancylus Lake periods together span 11,700–9,800 years before present, before the fully brackish-marine Littorina Sea was established about 8,500 years ago.[5](https://enviromicro-journals.onlinelibrary.wiley.com/doi/10.1111/1462-2920.14920)
**The transition from the Baltic Ice Lake.** At about 10.5 ka BP a gradual ice recession began, and roughly 200 years later the receding ice sheet could no longer hold back the up-dammed water. The final drainage of the Baltic Ice Lake rapidly lowered the water level in the basin by about 25 m and ended that stage.[4](https://www.sciencedirect.com/science/article/pii/104061829400069H) A related account gives the drop as 25–27 m, after which the Preboreal Yoldia Sea came into being as the basin connected with the world ocean.[6](https://www.sciencedirect.com/topics/earth-and-planetary-sciences/yoldia-sea) Glacier retreat and global sea-level rise re-opened the gate between the Baltic basin and the North Sea through the central Swedish lowland, initiating the brackish Yoldia Sea stage at about 11.7 kyr BP.[7](https://esd.copernicus.org/articles/16/585/2025/)
Nils-Axel Mörner, a [Quaternary](https://www.edgepedia.org/quaternary) geologist who worked on Baltic shore-level history, argued that a separate lake stage must be inserted between the Baltic Ice Lake and the Yoldia Sea: an entirely lacustrine phase of 300 years after the drainage and before the salt-water ingression that, by definition, marks the onset of the Yoldia Sea. During this phase a fully lacustrine "Ancylus fauna" lived along the coasts of Gotland and Öland.[3](https://www.sciencedirect.com/science/article/pii/104061829400065D)
## The brackish phase
The Yoldia Sea's defining brackish interval was brief. In sediment cores from the north-western Baltic Proper, the first ingression of saline water is recorded in a unit containing brackish-water ostracods and foraminifera, which represents the only saline phase of the stage. At about 9.9 ka BP a short ingression, lasting 100–200 years, appears in the sediment records from Stockholm in the north to the southernmost parts of the Yoldia Sea.[4](https://www.sciencedirect.com/science/article/pii/104061829400069H)
The stage's water level was set by the world ocean, unlike the preceding and following lake stages.[8](https://balticsea2020.org/english/images/Bilagor/coley%20ecoengi%20final%20report.pdf) Relative sea level changed in a complex pattern with large differences within the basin: the sea was very restricted in the south, leaving an extensive landbridge there, while in the south and east, where eustatic sea-level rise exceeded isostatic uplift (land rising as glacial ice weight is removed), the shoreline experienced a transgression.[4](https://www.sciencedirect.com/science/article/pii/104061829400069H) • [2](https://www.researchgate.net/publication/291197113_The_Post-Glacial_Evolution_of_the_Baltic_Sea) In the Tallinn area of northern Estonia, sites at Yoldia water depths of about 40 m at Pirita and 60 m at Harku registered pronounced brackish conditions in their diatom assemblages, characteristic of the littoral to sublittoral environment of the sea; the litho- and biostratigraphic data there suggest a regressive rather than transgressive shoreline development, and the abundance of saline-water diatoms increased with water depth.[9](https://www.sciencedirect.com/science/article/pii/104061829400064C)
## End of the stage
The brackish connection closed as land uplift outpaced sea-level rise. The Närke Strait, which lay across south central Sweden, gradually grew shallower and cut off the saltwater input; some 300–400 years afterward, the Göta Älv and the Otteid-Steinselva, the last two passages remaining west of [Lake Vänern](https://www.edgepedia.org/vanern), had silted up so much that water draining from the Yoldia Sea could no longer flow through them.[4](https://www.sciencedirect.com/science/article/pii/104061829400069H) At about 10.7 kyr BP isostatic uplift re-closed the gate of the Swedish Depression, and the basin turned back into a freshwater lake, the Ancylus Lake.[7](https://esd.copernicus.org/articles/16/585/2025/) About 10,700 years ago the connection to the [Skagerrak](https://www.edgepedia.org/skagerrak) had almost completely closed, forming a freshwater lake that at times covered a larger area than today's Baltic Sea.[10](https://www.geomar.de/en/discover/the-origin-of-the-baltic-sea?cHash=5ef61d45090f565793baf32ca9bb0796&mdrv=www.geomar.de)
## Sedimentary record
The dominant deposits of the Yoldia Sea stage are organic-poor, often varved silty clays north of the [Gotland Basin](https://www.edgepedia.org/gotland-basin).[8](https://balticsea2020.org/english/images/Bilagor/coley%20ecoengi%20final%20report.pdf) In the IODP Expedition 347 classification of Baltic Sea stages, the stage's sediment in the Baltic proper is described as clay, weakly varved or homogeneous.[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) In the Landsort Deep, clay from the Ancylus Lake and Yoldia Sea together forms an approximately 20-m-thick unit between the Holocene brackish-marine mud above and the varved Baltic Ice Lake clay below.[5](https://enviromicro-journals.onlinelibrary.wiley.com/doi/10.1111/1462-2920.14920)
The stage's brevity complicates its record. In the eastern Baltic, the proposition that the water was fresh or of very low salinity, based on freshwater diatoms found at 35 m depth, has been questioned as possibly incorrect, since the short existence of the stage makes salinity reconstructions sensitive to depositional environment.[11](https://rgs-ibg.onlinelibrary.wiley.com/doi/10.1002/geo2.87) Dating schemes also differ among sources: the IODP table gives 11.7–10.7 ka,[1](https://publications.iodp.org/preliminary_report/347/347_t2.htm) an Estonian popular-science account places the stage at about 11,600–10,800 years ago,[12](https://loodusveeb.ee/en/themes/baltic-sea/history-baltic-sea) and a Finnish source describes the brackish sea as forming around 11,000 years ago and ending around 10,000 years ago.[13](https://itameri.fi/en/learn-and-study/characteristics-of-the-baltic-sea/origin-of-the-baltic-sea/) These differences reflect the precision limits of dating a roughly thousand-year stage at the Pleistocene–Holocene boundary.
## References
1. [Table T2, Summary of the late glacial and Holocene Baltic Sea stages, IODP Expedition 347 Preliminary Report](https://publications.iodp.org/preliminary_report/347/347_t2.htm)
2. [The Post-Glacial Evolution of the Baltic Sea (ResearchGate)](https://www.researchgate.net/publication/291197113_The_Post-Glacial_Evolution_of_the_Baltic_Sea)
3. [Nils-Axel Mörner (1995). The Baltic Ice Lake–Yoldia Sea transition. Quaternary International 27, 95–98.](https://www.sciencedirect.com/science/article/pii/104061829400065D)
4. [Different phases of the Yoldia Sea in the north-western Baltic Proper. Quaternary International 27, 121–129 (1995).](https://www.sciencedirect.com/science/article/pii/104061829400069H)
5. [Bo Barker Jørgensen, Thomas Andrén, Ian P. G. Marshall (2020). Sub-seafloor biogeochemical processes and microbial life in the Baltic Sea. Environmental Microbiology 22(5).](https://enviromicro-journals.onlinelibrary.wiley.com/doi/10.1111/1462-2920.14920)
6. [Yoldia Sea – an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/earth-and-planetary-sciences/yoldia-sea)
7. [Miluch et al. (2025). Paleogeographic numerical modeling of marginal seas for the Holocene – an exemplary study of the Baltic Sea. Earth System Dynamics 16, 585–605.](https://esd.copernicus.org/articles/16/585/2025/)
8. [The postglacial history of the Baltic Sea (Baltic Sea 2020 report)](https://balticsea2020.org/english/images/Bilagor/coley%20ecoengi%20final%20report.pdf)
9. [Palaeoenvironmental conditions during the Baltic Yoldia stage in the Tallinn area, northern Estonia. Quaternary International 27, 83–94 (1995).](https://www.sciencedirect.com/science/article/pii/104061829400064C)
10. [The origin of the Baltic Sea, GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel](https://www.geomar.de/en/discover/the-origin-of-the-baltic-sea?cHash=5ef61d45090f565793baf32ca9bb0796&mdrv=www.geomar.de)
11. [The eastern shores of the Baltic Sea in the Early Holocene according to natural and cultural relict data. GEO: Geography and Environment 7(1) (2020).](https://rgs-ibg.onlinelibrary.wiley.com/doi/10.1002/geo2.87)
12. [The history of the Baltic Sea, Loodusveeb](https://loodusveeb.ee/en/themes/baltic-sea/history-baltic-sea)
13. [Origin of the Baltic Sea, Itämeri.fi](https://itameri.fi/en/learn-and-study/characteristics-of-the-baltic-sea/origin-of-the-baltic-sea/)
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*Topic: Encyclopedia › Places and geography › Waters and hydrographic features › Seas, oceans, and coastal waters › Oceans › Historical and palaeo-oceans › Glacial and postglacial seas*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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