Nature Geoscience：科学家否定物种“大灭绝”成因假说
英国科学家日前否定了一种解释地球上最大规模的物种灭绝事件的重要假说。在那次灭绝事件中，９５％的海洋物种以及７０％的陆地物种被毁灭。那次灾难被称为“大灭绝”，发生于约２．５亿年前的二叠纪末期。在历史长河中，该事件也许已经逐步为人们所了解，有越来越多的线索证明了其严重程度，包括在全球范围发现的难以解释的、已石化的、产生了突变的植物种子。尽管如此，是什么原因导致了该事件尚不清楚。
据法新社报道，被英国研究人员否定的理论认为，海洋变得缺氧，富含硫化物，致使海洋生物灭绝。
硫化物烟雾从海面升腾而起，在频繁的火山活动释放的副产品甲烷的协助下，破坏了臭氧层。臭氧层相当于大气平流层中的盾牌，可以过滤掉阳光中紫外线Ｂ的辐射。
这种理论认为，在地面上生物遭到了彻底毁灭。生物因硫化物累积到有毒程度而中毒，其ＤＮＡ则因阳光辐射而被完全破坏。
由英格兰北部设菲尔德大学的戴维·比尔林带领的研究小组创建了一种大气化学研究二维计算机模型，来检验这种观点。
据他们的计算结果，赤道地区的低气压会起到氧化缓冲作用，阻止硫化物严重破坏臭氧层。他们在《自然地球科学》杂志上报告说：“这些毒气似乎不太可能是偶然发生的地球物种灭绝的原因。”
关于“大灭绝”还有其他多种理论，包括小行星造成的一次或是一系列影响。这与人们认为的６２００万年前造成恐龙灭绝的事件相似。
（Nature Geoscience），doi:10.1038/ngeo154，Michael B. Harfoot, John A. Pyle & David J. Beerling）
End-Permian ozone shield unaffected by oceanic hydrogen sulphide and methane releases
Michael B. Harfoot1, John A. Pyle1,2 & David J. Beerling3
Earth experienced repeated episodes of widespread surface and deep-ocean anoxia with a significant accumulation of sulphide-rich waters over the past two billion years1, 2. The resulting massive releases of hydrogen sulphide from the oceans, together with methane from the geosphere, have been suggested as a cause for mass extinctions through destruction of the ozone shield and a lethal accumulation of hydrogen sulphide at the surface1, 2, 3, 4. Here, we use a two-dimensional atmospheric chemistry-transport model5, 6, 7 with representative climate8 and atmospheric composition9 to simulate the effect of large hydrogen sulphide and methane releases at the time of the end-Permian mass extinction 251 million years ago. In our simulations, the integrity of the stratospheric ozone shield is maintained for oceanic hydrogen sulphide releases up to 15,000 Tg S yr-1, a limit far exceeding the threshold for ozone collapse identified previously1 (2,000–4,000 Tg S yr-1). Scenarios of simultaneous hydrogen sulphide and methane injections also failed to significantly deplete the Earth's ozone shield, and generated non-lethal hydrogen sulphide concentrations (1–2 p.p.m.) at the surface. In our two-dimensional model simulations, the high photolysis environment in the tropics maintains the oxidizing capacity of the tropical troposphere, with high local hydroxyl radical concentrations, and greatly diminishes hydrogen sulphide entry into the stratosphere. We suggest that given current constraints on possible hydrogen sulphide and methane releases from anoxic oceans, and the geosphere, over the past 0.5 billion years, these gases seem unlikely to be the cause of coincident terrestrial biotic mass extinctions.
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