91色情片

The culprit in a global murder mystery may finally have been nailed. Huge volcanic eruptions that covered much of Siberia with lava were probably responsible for the worst mass extinction in the last half-billion years. Atmospheric carbon dioxide rose to 10 times today鈥檚 levels, warming the surface of polar oceans and stalling global ocean circulation, which in turn starved marine life of oxygen.

The mass extinctions 251 million years ago, at the end of the Permian period, wiped out 90 to 95 per cent of marine life and some 70 per cent of terrestrial life, but they have been hard to explain. One clue comes from marine rocks which show that at the time of the extinction, the ocean contained too little oxygen to support marine animals. With no undisputed evidence of a massive asteroid impact or other catastrophe, the prime suspect has been Siberian eruptions. But past attempts to model the effects of the eruptions have not recreated the temperatures shown in the fossil record, largely because they failed to take into account the flow of heat between the air and sea.

Now that level of simulation has been achieved. Jeff Kiehl and Christine Shields of the National Center for Atmospheric Research in Boulder, Colorado, tackled the task with a supercomputer running the Community Climate System Model, the standard set-up used to study modern climate, which includes air-sea interactions. They programmed the model with the Earth鈥檚 topography at the end of the Permian, when a world-ocean called Panthalassa surrounded the single supercontinent Pangaea. On the evidence of other studies they set CO₂ concentrations for the end of the Permian at 10 times modern levels, and ran the model for a year on a supercomputer until it recreated the prevailing climate in the aftermath of the eruptions (Geology, vol 33, p 757).

鈥淎tmospheric CO₂ rose to 10 times today鈥檚 levels, warming the surface of polar oceans and starving marine life of oxygen鈥�

At that point land and ocean temperatures at high latitudes were both about 8 掳C warmer than today. But crucially, tropical waters did not heat up as much, so the temperature difference which drives ocean circulation was reduced. This weaker circulation led to less mixing of cold polar water and deep water. 鈥淪ince one of the major pathways to get oxygen to the depths is this mixing, it鈥檚 going to make it difficult to get oxygen to the bottom,鈥� Kiehl told New Scientist.

In addition, the warmer surface waters absorb less oxygen from the air. Put those factors together with relatively low atmospheric oxygen at the time and the oceans would have had very little oxygen for life. Deep-sea stagnation could also produce hydrogen sulphide, which is toxic to land animals. The resulting conditions would have led to mass extinctions on land and sea, say Kiehl and Shields.