Genesis of iron and manganese sediments in Zoloushka Cave (Ukraine/Moldova) as revealed by δ13C organic carbon

Zoloushka Cave is one of the largest maze gypsum caves in the world. Mining of the gypsum bedrock and lowering of the water level due to the pumping of groundwater led to exposure of the cave passages to vadose conditions and changed the hydrochemistry of the karst water. As a result, large quantities of Fe and Mn hydroxides were deposited in the passages. It was found that at least two groups of various organisms were involved in depositing ferrous and manganese sediments. In order to establish the mechanism of deposition, we conducted chemical analyses of the sediments and isotopic analyses of organic carbon. This enabled us to formulate a hypothetical model of the biogenic deposition of Fe and Mn compounds. According to the model, autotrophic iron bacteria precipitated Fe hydroxides. Organisms of this type assimilate CO2 from cave atmospheres and, as a result of isotopic fractionation during that process, organic matter in 13C is depleted by 7.3‰ relative to CO2. Heterotrophic bacteria (responsible for depositing manganese oxides) parasitise on autotrophic bacteria without changing the carbon isotope composition of organic matter. Fungal organisms living in ferrous sediments separate carbon from organic matter, resulting in enrichment with the heavier carbon isotope by 2‰