Sediment cores reveal historic hypoxia in the Baltic Sea

Baltic Nest InstituteThe photo shows a 6,000-8,000 year old sediment core from the Bothnian Sea. The dark layers indicate anoxic periods. With courtesy of Tom Jilbert and the participants of the HYPER/COMBINE cruise of RV Aranda June 2009
Published: 2015-05-05
Sediment cores reveal historic hypoxia in the Baltic Sea
A deep dive into sediment cores from the Baltic Sea shows the recurrence of intensely hypoxic conditions several times during its 8,000 year existence. But the history of hypoxia in the different regions of the Baltic Sea varies greatly, and new research suggests that the uplift of Scandinavia since the last ice age may be the reason.
Researchers at Utrecht University, Lund University and Stockholm University have studied the chemistry of sediment cores from the Bothnian Sea and the Baltic Proper.
The sediment acts as an archive in which different substances are stored. The presence of organic carbon, molybdenum and pigments from cyanobacteria in the sediments give an overall picture of historic algal blooms and oxygen deficiency. Understanding past hypoxia allows scientists to distinguish between natural and anthropogenic effects in the modern Baltic Sea.
The Baltic Sea consists of a number of basins separated by shallower sills. The sills control how much water is exchanged between the basins – both at the surface and at depth.
Heavy saline water hinders mixing
Today the Baltic Proper is suffering from hypoxia. As well as the impact of eutrophication, this is due to a strong stratification of the water column in the Baltic Proper, maintained by inflows of saline water from the North Sea. Saltwater is heavier than the brackish surface water and sinks to the bottom. The stratification hinders vertical mixing of oxygen-rich water from the surface, which results in oxygen depletion in the saline bottom water.
In the Bothnian Sea, the situation is considerably better with oxygenated bottom water. The sills in the Åland Sea prevent northward flow of the saline and poorly oxygenated bottom waters of the Baltic Proper, resulting in weaker stratification and better ventilation of the bottom water. Moreover, the supply of nutrients to the Bothnian Sea is lower which leads to less growth of planktonic algae and therefore lower consumption of oxygen when these are decomposed.
Oxygen deficiency in the Littorina sea
But this has not always been the case. After the great ice sheets of the Pleistocene geological period began to melt 16,000 years ago, the Baltic Sea has experienced alternately freshwater or brackish conditions due to crustal uplift and rising sea levels. The current period of brackish water, Littorina, began 8,500 years ago when the sea became connected to the oceans through the Sound and the Danish belts. In connection with this, periods with oxygen-free bottoms started occurring in the Baltic Sea.
The new study by Tom Jilbert and co-authors, published in the journal Geology, shows that the sills in the Åland Sea were considerably deeper from 8,000-4,000 years ago than they are today, which led to direct exchange of deep water between the Baltic Proper and the Bothnian Sea. Because of this, a long period of anoxic bottom waters affected both basins at this time, with the most severe hypoxia found in the Bothnian Sea.
Land uplift improved oxygenation in the Bothnian Sea
Land uplift probably had fundamental importance for ending this intense period of bottom water anoxia in both basins at about 4,000 years ago. When the sills at Åland became shallower this choked off the flow of saline and oxygen deficient bottom water to the Bothnian Sea.
In connection with this the bottoms of the Bothnian Sea began to act as a sink for phosphorus and iron. Thereby phosphorus levels in the Baltic Proper were also reduced and its oxygen situation improved. Decreasing temperatures during the same period may also have contributed, along with increased rainfall and a weakened stratification of the seawater.
Hypoxia in modern Baltic Sea
Periods of anoxic bottom waters have since recurred in the Baltic Proper during the so-called Medieval Climate Anomaly, and in modern times, despite continued transport of phosphorus from the Baltic Proper to the Bothnian Sea.
Therefore, argue the authors of the study, the oxygen-free bottoms in the Baltic Proper today must be sustained by eutrophication. We simply add nutrients to the Baltic Sea in such great quantities, that the natural sedimentation processes removing nutrients are overwhelmed. The authors suggest that maintaining the efforts of HELCOM to reduce nutrient loading is the best course of action to rid the Baltic Sea of hypoxia in the future.