Researchers Unlock 7000-Year Environmental Archive from a Baltic Sea Concretion
In a breakthrough published in Communications Earth & Environment, researchers from the Geological Survey of Finland GTK have shown that an iron-manganese concretion collected off the coast of Espoo records detailed environmental and human-induced changes over the past 7000 years. By combining several dating methods, the team was able to determine the concretion’s growth rate and identify pollution signals. The sample was collected by GTK’s research vessel Geomari from the Gulf of Finland.
A research team from GTK has revealed that iron-manganese concretions found off the northern coast in the Gulf of Finland serve as detailed archives of environmental change and human impact spanning the last 7000 years.
These concretions, which form in non-depositional coastal zones, have recorded shifts in oxygen levels, climate variability, and pollution events with remarkable precision.
“This is the most detailed age model ever constructed for a shallow-water iron-manganese concretion”, says Joonas Wasiljeff from GTK, the lead author of the study. “It allows us to trace both natural and anthropogenic changes in the Baltic Sea ecosystem over millennia.”
The team developed a high-resolution probabilistic age model by integrating radiocarbon dating, cobalt chronometry, and anthropogenic lead accumulation. These methods enabled them to pinpoint the onset of concretion growth around 7500 years ago, coinciding with the marine flooding that established the modern brackish Baltic Sea.
Environmental changes are recorded in the layers of the concretion
Microstructural, magnetic, and geochemical analyses revealed two distinct growth phases in the concretion. The first, dominated by manganese-rich layers, reflected calm and oxygen-deficient conditions.
The second phase, marked by alternating iron and manganese bands, indicated more turbulent environments and generally improved, but fluctuating oxygenation due to combined effects of climatic shifts, glacio-isostatic uplift and increased hydrodynamic energy.
The researchers also identified clear signals of human activity, including lead pollution peaks linked to Roman metallurgy, medieval mining, and later industrialization. These findings align with known historical events and demonstrate the concretion’s ability to record anthropogenic influence with high fidelity.
“The layered structure of a concretion resembles tree rings”, says co-author and research group leader Joonas Virtasalo from GTK. “It records how coastal environments respond to both climate shifts and human pressures, and it underscores the importance of preserving these natural archives.”
The study not only enhances our understanding of Baltic Sea history but also provides an example of how similar deposits can be used in other coastal regions around the world to study environmental change and the impacts of human activity. It opens new perspectives on coastal resilience and the influence of human actions on marine ecosystems.
Further information
Article on Communications Earth & Environment: Centennial to millennial-scale paleoenvironmental record from a coastal iron-manganese concretion
Joonas Wasiljeff, Research Scientist
Geological Survey of Finland GTK
joonas.wasiljeff@gtk.fi
