An integrated carbon isotope record of an end-Permian crater lake above a phreatomagmatic pipe of the Siberian Traps

• We investigate the carbon isotope record of a Siberian Trap volcanic crater lake.
• Carbonate and organic matter reflect 12C-enrichment early in lake history.
• The pattern of 12C-enrichment is consistent with a 12C-enriched carbon source.
• Carbon isotope values are consistent with the thermogenic gas venting hypothesis.

The largest mass extinction in Earth history occurred at the end-Permian (~ 252 million years ago) and is marked by a global negative carbon isotope excursion and the onset of Siberian Trap volcanism, prompting diverse hypotheses on the link between flood basalt volcanism, carbon cycle perturbations, and mass extinction. Phreatomagmatic pipes associated with Siberian Trap volcanism have been proposed as conduits for the release of 12C-enriched carbon gases from thermogenic and/or magmatic sources to the end-Permian atmosphere. Some of the pipes have preserved crater-lake sediments of volcaniclastic origin. This study examined the preserved evidence for 12C-enriched carbon release into the Western Oktyabrsk crater in east Siberia from the underlying volcanic basin. We find that the 13C/12C ratio of the carbonate cement, organic matter, and long-chain n-alkanes in the lacustrine crater sediments support the hypothesis that 12C-enriched carbon infiltrated the basal crater sediments and lake water immediately after crater formation. The values and trends of δ13CCarb, δ13CTOC, and δ13Cn-alkanes in the crater sediments are consistent with 12C-enriched carbon with isotopic values similar to that of carbon sourced from thermogenic and/or 12C-enriched magmatic sources. This implies that carbon release through the pipes in the Tunguska Basin may explain the source of the global negative carbon isotope perturbations, and their coincidence with Siberian Trap volcanism, at the end-Permian.