科研之友
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摘要
The Earth experienced a severe mass extinction at the Permian-Triassic boundary (PTB) about 252 million years ago. This biological catastrophe was accompanied by major changes in geochemical composition of the atmosphere and ocean and the appearance of sedimentary features which had not occurred since the Precambrian time. The eruption of the largest continental flood basalt, the Siberian Traps, overlapped this mass killing. Many hypotheses have been proposed but no definitive conclusion currently exits. Here we present characteristics of three sections from Iran and China and propose that an active mantle plume initiated a series of processes which led to the mass mortality and produced major sedimentological, mineralogical, and geochemical changes observed in the transition from the Paleozoic to the Mesozoic.
The injection of mantle plume-related igneous dike swarms into the continental margin facilitated the release of massive amounts of CH4 primarily from the dissociation of marine gas hydrates and secondarily from the maturation of organic-rich sediments and fracturing of hydrocarbon reservoirs. The bulk of the CH4 was aerobically oxidized in the water column producing dissolved CO2 with low delta C-13 values. This CO2-saturated seawater became acidic to the point of dissolution of shelf carbonates promoting precipitation of siliciclasticrich strata in the transition from the Permian to the Triassic. Methane-derived CO2 also lowered carbon isotopic composition of seawater leading to the observed decline in delta C-13 composition of organic and inorganic marine carbon at the PTB.
Gas-charged oceans released large volumes of CO2 and CH4 into the atmosphere which created a severe global warming (the end-Permian inferno) causing the release of additional CH4 from the dissociation of polar gas hydrates. These events lowered delta C-13 Compositions of terrestrial carbon. Simultaneously, feeder dikes from the mantle plume formed the Siberian Traps flood basalt.
Marine mass extinction was the result of a change in seawater composition due to the injection and oxidation of CH4 in the water column causing low pH, high concentrations Of CO2, Ca2+ and HCO3-, and low CO32- U S. Combined with a hot seawater temperature, these changes made calcification of marine organisms difficult and produced major physiological crisis including reduced metabolic rates, high sensitivity to environmental stress, and hampered growth and reproduction. Terrestrial mass extinction can be attributed to severe global warming and soil acidification produced by increased atmospheric CO2, acid rain that was generated by SO2 derived from the Siberian trap eruption, and loss of habitat.
Cessation of the plume activity during Early Triassic stopped the release of CH4 into the ocean and terminated continental flood basalt eruption ending the environment of death on land and in sea. The Cut off of CO2 production in the ocean instantly increased carbonate saturation of seawater resulting in extensive seafloor cementation. It also resulted in the deposition of marine carbonates by microbial activities in the hostile postextinction environment. From the trigger to recovery, the perturbation which included the end-Permian mass mortality could have lasted for at least 2 Myr.
Several major mass extinctions of the Phanerozoic are temporally accompanied by flood basalt eruptions. So far, these two events have been interpreted in a cause-and-effect relation: flood basalt eruption causes mass extinction. This study proposes that flood basalts and their time correlative biological crises are themselves the consequence of a complex perturbation caused by mantle plume activities. If so, major disturbances in the near surface of the Earth are ultimately controlled by changes in the mantle.
The physical sedimentological observations presented here combined with previous paleontological evidence cast doubt on other interpretations which use geochemical variables, numerical modeling, biomarkers, and Ce anomaly to suggest that Late Permian ocean was anoxic all the way to the photic zone.
- 出版日期2008-7-7
