摘要：北京时间7月25日消息，据国外媒体报道，荷兰古生态学家最新研究成果表明，海底释放的甲烷将可能会导致地球上再次出现生物大灭绝。研究人员认为，根据2亿年前生物大灭绝事件的特点，生物大灭绝似乎比预想中更容易触发。
60万年的火山活动可能会让地球的大气层充满了二氧化碳，令地球生物窒息死亡，直至全部灭绝。研究人员认为，事实上并不需要60万年如此长的时间，只需要数千年的火山活动，就足以导致海洋温度极速上升，海底泥浆中的甲烷、二氧化碳等温室气体会大量释放。在这种情况下，地球上绝大多数生物会灭绝。
荷兰乌德勒支大学古生态学家米查-鲁尔表示，“科学家们现在一直关注海底甲烷释放的问题。我们就是要对过去发生过的生物灭绝事件进行深入研究。如果再次发生此类事件，我们唯一未弄清楚的问题就是，触发灭绝事件的极限条件是什么？”
在被科学家称为“晚三叠纪生物灭绝”事件中，至少有一半的地球生物消失。这种意外大面积死亡不仅仅导致生态崩溃问题。事件发生得如此突然，以致于行星化学循环系统陷入混乱状态长达数百万年。对于这次大灭绝事件，主流的解释是由于大陆板块分裂，产生火山活动，从而导致广泛的气候变化。
不过，鲁尔此前的研究已经对此指出疑义，并提出一种不同的看法。通过计算远古沉积物中的化学成分随着地球与太阳距离变化的自然周期而变化的规律，鲁尔可以根据年代顺序详细、深入地研究“晚三叠纪生物灭绝”事件。鲁尔发现，石灰石中珊瑚和贝壳类动物的地质学残余在最初的2万年内消失了。因此，可以判断“晚三叠纪生物灭绝”事件肯定发生得比此前估计的要更突然。
在最新的研究中，鲁尔的研究团队检测了古地中海海岸死亡植物的化学遗迹。所谓的古地中海是指将劳亚古大陆与冈瓦纳古大陆分隔开来的水体的一部分。如今，这些海岸已成为了奥地利境内阿尔卑斯山脉的沉积层。研究人员对比了碳同位素的变化，他们发现，在2亿零140万年前，再缩小到发生生物灭绝事件的那2万年内，甲烷的含量急剧上升，随后二氧化碳含量也随之大幅上升。
鲁尔解释说，“火山产生少量的二氧化碳，尽管只会造成全球气候的小幅度变化，导致陆地和海洋温度提升，但是这种温度的提升又会导致海底甲烷的释放。”甲烷是人们所熟悉的一种气体，是天然气燃料的主要成分。作为一种温室气体，甲烷可能比二氧化碳的作用更大。地球上的甲烷主要包含于土壤和海床里。
科学家已经发现，全球气温升高可能会导致更多的甲烷释放到大气层中，从而又进一步提升气温，再释放越来越多的甲烷，最终形成一个恶性循环。很明显，这种恶性循环曾经发生于晚三叠纪时期。
美国布朗大学古生态学家杰茜卡-怀特塞德曾经主持了一项关于晚三叠纪时期火山活动的研究。怀特塞德认为，鲁尔的研究成果要想得到广泛的支持，必须要寻找更多的证据来提供更为合理的解释。鲁尔的研究成果反映的只是局部现象，而不是全球共性问题。
尽管怀特塞德对于甲烷在晚三叠纪时期的作用持怀疑态度，但是她也承认，可能存在“二氧化碳-甲烷”恶性循环现象，从而加速了气候变化，最终进入始新世。怀特塞德解释说，“这是一段非常酷热的时期。天气热到北极圈内出现鳄鱼、棕榈树长到北极的程度。我们预测地球下个世纪温室气体的含量将达到这种程度。”
现在的问题是，气候变暖到底精确到何种程度才会开始这种循环，究竟有多少甲烷会释放出来。鲁尔认为，“我们可能已经引起了海洋温度的小幅度提升，而这种小幅度提升又引起了甲烷的释放。但是，现在很难量化海洋到底释放了多少甲烷。”
乳齿螈和喙头龙。乳齿螈是晚三叠纪生物灭绝前最大的陆地动物之一。喙头龙也是此次灭绝的一种爬行动物。
生物探索推荐英文论文摘要：
Mass Extinction Easier to Trigger Than Thought
The cataclysmic extinctions that scoured Earth 200 million years ago might have been easier to trigger than expected, with potentially troubling contemporary implications.
Rather than 600,000 years of volcanic activity choking Earth’s atmosphere with carbon dioxide, just a few thousand years apparently sufficed to raise ocean temperatures so potent greenhouse gases trapped in seafloor mud came bubbling up.
Much of everything alive on Earth was soon wiped out. Another half-million years of vulcanism were just icing on the cake. The immediate question: What lessons, if any, can be drawn?
“Scientists have been worried about the current release of methane from seafloors. What this study shows is that it already happened in the past,” said paleoecologist Micha Ruhl of Utrecht University, whose findings are published July 21 in Science. “It could happen again. It’s only the boundary conditions that we don’t know.”
In what scientists call the end-Triassic mass extinction, at least half of all living species simply disappear from the fossil record. The die-off didn’t merely cause ecological disruption. It was so sudden and profound that planetary chemical cycles went haywire for the next several million years.
The leading explanation for the extinction invokes extended, climate-altering volcanic activity caused by splitting continental plates, but earlier research by Ruhl suggested a more nuanced and jarring narrative.
By calculating how changes in ancient sediment composition corresponded to natural cycles in Earth’s distance to the Sun, he could study the end-Triassic’s onset in fine-grained chronological detail.
Ruhl found that limestone — the geological remains of corals and shellfish — vanishes within the first 20,000 years. Similar terrestrial disruption can be inferred from changes in fossilized plant spores. The end-Triassic extinction must have occurred far more suddenly than believed.
In the latest study, Ruhl’s team examined chemical traces left by dying plants on the shores of the Tethys Sea, a body of water that separated the ancient continents of Laurasia and Gondwana. Today those shores are sedimentary layers in the Austrian Alps.
The researchers concentrated on changes to carbon isotopes, or subtly different elemental formations that betray whether carbon in plants came from carbon dioxide or methane. At 201.4 million years ago, in that narrow 20,000-year window of their updated end-Triassic cataclysm, they found a rise in CO2 followed by a tremendous spike of methane.
“A small release of CO2 from volcanoes triggered a small change in the global climate, raising land and ocean temperatures. That led to the release of methane from the seafloor,” said Ruhl.
Methane, familiar to most people as the main component of natural gas fuels, is a greenhouse gas less common than carbon dioxide in the atmosphere but far more potent. Most of Earth’s supply is contained in soils and seabeds.
Scientists have raised the possibility that rising global temperatures could release trapped methane into the atmosphere, further raising temperatures and releasing more methane in a feedback loop of warming and planetary disruption. That’s apparently what happened during the end-Triassic extinction.
According to paleobiologist Jessica Whiteside of Brown University, a leading researcher on end-Triassic vulcanism, more evidence is needed before Ruhl’s interpretations are conclusively supported. It’s possible that the new study reflects localized rather than planetary patterns.
But though Whiteside is uncertain about methane’s end-Triassic role, she said the possibility of a CO2-methane warming loop is real and likely fueled climate change leading into the Eocene, an epoch lasting from 55 to 35 million years ago.
“It was a very hot time, so hot there were crocodiles in the Arctic Circle and palm trees extending to the North Pole,” said Whiteside. “It’s the most recent time in Earth’s history that greenhouse gases were at the level where we’re predicted to be in the next century.”
Exactly how much warming would be needed to start the loop anew, and how much methane would flow forth, are open questions.
“We could potentially trigger a small increase in ocean temperatures, which triggers methane release,” said Ruhl. “But it’s difficult to quantify how much methane is in the ocean these days. Maybe we have less methane in seafloors now. Maybe we have more.”
