Mn-Fe-calcite concretions from varved clays of Estonia and the Gulf of Finland: the composition and growth history

Carbonate concretions from varved ice-lake clays contain calcite with a high content of manganese and iron. The stable isotopic composition shows that the concretions grew in fresh-water conditions, during the early Holocene. Complex geological-geochemical data show that the concretion growth took no more than 500-1000 years. The Mn/Ca molar ratio in sediment water at the growth time was 0.0006-0.002. The values of δ¹⁸O (PDB) are from -14 to -6‰, those of δ¹³C (PDB) are from -18 to -3‰. The value of δ¹⁸O (SMOW) of water at the growth time of concretions was between -17 and -9‰.