Deev Jahi Model of the Permian–Triassic boundary mass extinction: a case for gas hydrates as the main cause of biological crisis on Earth

Abstract

The smoking gun revealing the secrets of the end-Permian mass mortality is a unique 1-2-m-thick layer consisting of 5–20-cm-long crystals of calcite that occurs precisely at the Permian–Triassic boundary (PTB) in Iran, Armenia, Turkey, and China. This layer is interpreted as synsedimentary, abiotic, seafloor cement indicative of precipitation from a highly carbonate supersaturated seawater. Its δ^(13)C composition (δ^(13)C=0‰ PDB) is 4‰ to 5‰ PDB lower than the typical Upper Permian values (4‰ to 5‰ PDB), suggesting the involvement of massive amounts of gas hydrate CH_4 (δ^(13)C=−60‰ PDB). The temporal coincidence of the cement layer with the PTB suggests that the process that promoted seafloor cementation was also responsible for the biological crisis. A cementation model is developed based on accumulation–dissociation cycle of gas hydrates which also explains the mass extinction at the PTB. The Upper Permian accumulation period of gas hydrates ended abruptly adjacent to the PTB and the dissociation event began releasing 3.2 to 4.7×10^(18) g CH_4 into the ocean. Oxidation of CH_4 in the water column created a seawater that was charged with CO_2 (an oceanic acid bath) and had lower than normal O_2 content (but not anoxic). This oceanic acid bath first dissolved suspended fine-grained carbonate particles and small calcareous organisms, followed by extensive dissolution of platform carbonates raising Ca^(2+) and HCO_3− concentrations of seawater. When the release of CH_4 declined, the acid-bath ocean became a soda ocean precipitating massive amount of seafloor cements observed globally at the PTB. The study suggests that prior to cement precipitation, the PTB ocean was charged with CO_2, warm, had low oxygen, high Ca^(2+), and high HCO_3− concentrations. These conditions collectively created stressful conditions causing the marine mass mortality. The leakage of CH_4 to the atmosphere produced a super-hot climate resulting in the biological devastation on land. The proposed kill mechanism is developed on the basis of the physical clue—the cement layer—left behind by the killing process—the change in ocean chemistry. The accumulation–dissociation cycles of gas hydrates also explain the δ^(13)C pattern of marine carbonates and the periodicity of mass extinction events during the Phanerozoic. Accumulation periods were long (5 to 20 My) providing favorable conditions for ecosystem development (Pardeess phase). The dissociation events were short and catastrophic (10 to 500 Ky) causing low oxygenation, super-hot climate, and biological devastation (Doozakh phase). It appears that most mass extinctions of the Phanerozoic have been related to the internal working of the Earth system. During the Phanerozoic, methane has played the role of the Deev Jahi—a female demon in Persian mythology—whose task is to attack the Earth every so often to kill life on land and in sea.