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In a δ13C speleothem record from Duhlata cave, Sofia region, Bulgaria we found a positive value of δ13C. It is rather unusual and only way to explain it is that all carbon in the speleothem was supplied from the bedrock above the cave during the formation of the corresponding layer in the speleothem. Such situation can happen only during total freezing of the ground above the cave during glaciations. Only in such case both organic and air carbon dioxide cannot reach the cave.doi:10.52215/rev.bgs.2021.82.3.189 fatcat:wuojhcv2pjappi22j4uulxmsia