The Estonian energy industry relies on local calcareous oil shale that is used to produce the majority of the country’s electricity, but the shale is also used for shale oil retorting. The solid residues remaining after combustion are upon open air deposition potentially capable of binding part of the released CO2 by carbonation of reactive Ca-phases. We studied the isotopic composition of authigenic carbonate phases in an alkaline Ca-rich oil shale ash waste deposit accumulated over nearly 50 years to reveal the carbonation mechanisms and the sources of CO2 required for carbonation. The secondary carbonate phases forming in oil shale waste deposits have low δ13C and δ18O values characterized by δ13CV-PDB values between -12‰ and -24‰ and δ18OV-PDB between -8‰ and -15‰. The negative isotopic composition of carbonate carbon seemingly points to a contribution of CO2 derived from the degradation of residual organic material, potentially present in shale retorting ashes. The low δ13C value of carbonate is rather caused by the non-equilibrium fractionation effects during diffusion and hydroxylation reactions of CO2 in hyperalkaline conditions under a limited CO2 diffusion rate.
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