Winter-time deep-water formation / convection in the Baltic Sea – affecting seabed dynamics and ventilation changes over the past

New geophysical, oceanographic data and studies on sediment core material strengthen the hypothesis that winter-time deep-water formation / convection played a critical role in seabed dynamics and bottom water ventilation in the Baltic Sea during the last 7000 years. Moreover, we suggest that this process was also critical for marked environmental changes observed and reconstructed for the last 150 years, the transition from the cold Little Ice Age towards the Modern Warm Period.

Our inferences are based on a comparison of instrumental data comprising oceanographic measurements (temperature, salinity, oxygen), air temperature data from Stockholm and sediment proxy records. A solid proxy for bottom water salinity changes are benthic foraminiferal counting data. X-ray fluorescence (XRF) scanning data of short sediment cores together with organic carbon content data are used to evaluate the input of terrigenous particles at the respective sites. XRF data are also used to identify manganese-carbonate layers that form at the sediment surface when anoxic bottom waters are ventilated. A sound chronostratigraphy for the last c. 150 years critical for a comparison between proxy and instrumental data is achieved by combining radionuclide, inorganic and organic pollutant downcore data.

Our proxy records and the instrumental data indicate that during climate warming since AD 1850 the winter-time deep-water formation / convection decreased steadily as evident from deep basin study sites (e.g., in the Landsort Deep). A marked environmental change occurred in the late 1950s when the input of re-worked terrigenous material stopped suddenly and the bottom water conditions switched from oxic to hypoxic at different water depths at all sites (deeper than 160 m) in the Baltic Proper. This switch occurred shortly after the strong inflow of saline waters in the early AD 1950s when highest salinities were observed (instrumental data) and reconstructed (benthic foraminifera) in all sub-basins of the Baltic Proper. We argue that stratification strengthened markedly resulting from this pronounced increase in bottom water salinity, and the already rather weak winter-time deep-water formation / convection collapsed. The collapse led to a stop of fine-grained material input and bottom water ventilation decreased.

During the late 1980s, however, bottom water salinities decreased and therefore stratification weakened cause of a lack of significant inflows. This together with colder winter air temperatures lead to a bottom water ventilation of sub-basins located north of a marked topographic seabed feature, the Baltic Sea Klint, due to the onset of the postulated winter-time deep-water formation / ventilation process.

After the early 1990s only the major saline water inflow of 2003 was able to effectively ventilate the bottom waters of the sub-basins north of the Baltic Sea Klint, which is evident from instrumental and foraminiferal proxy data.

There is an urgent need to include the winter-time deep-water formation process in ecosystem modelling approaches as this may help to improve the relatively poor model performance for ventilation changes in sub-basins located North of the Baltic Sea Klint.