EVIDENCE OF CARBON DEGASSING THROUGH SIBERIAN TRAP PIPE STRUCTURES AT THE PERMIAN-TRIASSIC BOUNDARY

The Permian-Triassic (P-T) boundary is marked by a global negative carbon isotope excursion that is approximately coincident with the onset of Siberian Trap volcanism. Svensen et al (2009) proposed that catastrophic degassing of organic carbon gases thermally released from the aureoles of Siberian Trap sill intrusions through associated brecciated pipe structures may have caused the observed shift in global atmospheric carbon values. We present data on the carbon isotope content of lacustrine sediments preserved in one of the proposed degassing structures (Western Oktyabr’sk) within the Siberian Trap province in the Tunguska Basin to test the validity of this hypothesis. A negative carbon isotope excursion in the total organic matter, extractable n-alkanes, and bulk carbonate cement at the crater base points to degassing of organically derived, ¹³C-depleted, carbon through the sediments into the lake water early in the lake history. The pattern of carbon isotope values in the remainder of the section reflects continuous degassing of volcanic CO₂ during deposition of the entire investigated section despite metamorphic organic carbon flux ceasing early. The exact timing of pipe degassing relative to Siberian Trap volcanism or the global marine carbon excursion awaits dating results, but sedimentation rate estimates give a degassing duration on the order of decades. These findings provide the first robust evidence that pipe degassing is a contributor of ¹³C-depleted carbon to the end-Permian environment. Degassing during pipe formation and repeated short duration eruption events in the hundreds of known Tunguska pipe structures could explain both the source of ¹³C-depleted carbon and the punctuated tempo of perturbations observed in the global carbon isotope record at the P-T boundary.