DID METHANE FROM MARINE METHANE HYDRATE DECREASE THE ATMOSPHERIC pO2 LEVEL TO CAUSE THE P-T EXTINCTION?

Researchers have suggested that the release of methane (CH₄) from the marine methane hydrate zone (MHZ) has caused the global warming and mass extinctions during the geologic history. To quantitatively assess the change in CH₄ flux from the MHZ due to a climatic or a global sea-level change, we have derived the equations relating the world’s CH₄ inventory as a function of the ocean water depths and the bottom-water temperature.

The results suggest that the change in CH₄ flux from the global MHZ will be ~0.33 Gt/yr due to the bottom-water temperature rise at <1°C/1,000yrs, and ~0.03 Gt/yr due to a sea-level drop at <100m/10,000yrs. These values are comparable to or less than the C fluxes of ~0.5Gt/yr by anthropogenic CH₄ and ~0.3 Gt/yr by biogenic CH₄ (e.g., by methanogenic bacteria and termite). The amount of CH₄ released over a 100,000 year period would not have affected the climate or directly reduced the atmospheric O₂ contents. However, the atmospheric O₂ level could have dropped to <80% of the present level to cause the mass extinction of the land-based plants and animals during the Permian-Triassic (P-T) transition through the following chain of events: (1) an increase in the bottom water-temperature of ~6-12°C or a sea level drop of 100 m during a ~20 Ma period; (2) creation of anoxic seas (e.g., Tethys) by scavenging of dissolved O₂ by rising methane bubbles from the MHZ; (3) development of euxinic seas with increased activity of sulfate reducing bacteria; (4) a rise of sulfidic water mass and release of H2S to the surface zone to kill the majority of photoautotrophs; (5) a decrease in the primary productivity to <1/7 of the present value to lower the long-term O₂ production flux to below the present level; (6) an increased O₂ consumption flux by soil due to an increase in surface temperature; (7) a combined effect of (5) and (6) to decrease the atmospheric pO₂ to <80% of the present atmospheric level. The suggested sequence of events is supported by various geological, sedimentological, and geochemical records during the P-T extinctions, such as ^87/86Sr, ¹³C, and ³⁴S isotopes or coal gaps.