The terrestrial Frasnian/Famennian mass extinction event?

我们知道，地球上的生命不断进化，不断地灭绝，并被新的物种取代。把它想象成一个足球俱乐部。为了保持成功，球队必须不断改变，招募新星，并因年龄、缺乏成功或受伤而将失败的球员调离。然而，有时这种持续不断的灭绝和进化更替会被物种总数的灾难性崩溃所打断。这些都是大规模灭绝。在这些时候，整个动植物种群在地质上很短的时间内就会消失。白垩纪末期的恐龙和二叠纪末期的三叶虫就是很好的例子，它们都是最大的大规模灭绝。在过去的5亿年里，有5次这样的大灭绝。一些我们知道的更好地理解，例如白垩纪/第三纪边界的陨石撞击(6500万年前)，以及奥陶纪末期相对较短和严重的冰期(4.4亿年前)。一次我们仍不了解的大灭绝事件发生在约3.765亿年前的泥盆纪晚期。这是一次弗拉斯尼亚/法门或F/F事件，比杀死恐龙的事件更大的灭绝。古生物学家提出了F/F事件的多种原因，包括陨石撞击、大规模火山喷发、全球变暖和全球变冷。地球上的生命甚至被认为是巨大的珊瑚礁和陆地植物共同作用的结果，它们锁住了二氧化碳，导致了全球变冷和冰河时代。当时，陆地植物对地球来说是相对较新的，它们都在陆地表面迅速蔓延，体型不断增大，成为重要的树木，因此有可能锁定大气中的二氧化碳。对于大多数其他大规模灭绝，我们现在能够更好地了解其原因，因为我们能够看到陆地和海洋对动植物灭绝的综合影响。然而，对于F/F事件，我们只有一个非常糟糕的记录，直到物种灭绝，特别是不知道是否有任何物种灭绝发生在与导致海洋生物死亡的物种灭绝完全相同的时间。幸运的是，我们在俄罗斯北部发现了一个新的岩石单元，它沉积在海边，包含了灭绝前陆地植物的良好记录。我们之所以能做到这一点，是因为我们正在研究植物中的微小孢子(就像花粉一样)。这些孢子具有很强的抵抗力，即使在母植物没有石化的情况下，也可以从岩石中找到并提取出来。不幸的是，这个新的岩石单元大多埋在河岸下。但是我们可以通过使用钻机获得坚固的岩心。然后，我们可以使用这个岩心来提取化石孢子，并通过岩石单元在紧密间隔的样本中对它们进行计数。这将显示植物何时灭绝，例如，随着时间的推移，没有新的植物进化，是突然随着许多植物一起灭绝，还是作为一系列较短持续时间的连续危机，每次都有少数植物灭绝。然后我们就可以将这一记录与海洋中的物种灭绝相匹配。我们已经研究了东格陵兰岛的岩石，那里在泥盆纪时位于南半球干旱地带。这种对陆地气候的直接记录显示了严重干旱的时期，当孢子变得简单和稀有时。利用这些孢子，我们可以将这种直接的气候记录与来自俄罗斯的钻孔以及海洋物种灭绝进行比较，试图证明凉爽的干旱是F/F大灭绝的原因之一。

We know life on Earth constantly evolves with continual extinctions and their replacement by new species. Think of it as a football club. To remain successful the team has to constantly change by recruiting new stars and transferring out failing players because of age, lack of success or injury. However, there are times when this constant churn of extinction and evolutionary replacement is interrupted by catastrophic collapses in total species numbers. These are mass extinctions. At these times entire groups of plants and animals disappear during a geologically short time. Good examples are the dinosaurs at the end of the Cretaceous and the trilobites at the end of the Permian, the greatest mass extinction of them all. There are 5 of these big mass extinctions during the last 500 million years. Some we know understand better, such as the meteorite impact at the Cretaceous/Tertiary boundary (65 million years ago) and the relatively short and severe ice age at the end of the Ordovician Period (440 million years ago). One mass extinction event that we still don't understand occurred ~376.5 million years ago in the later part of the Devonian Period. This, was the Frasnian/Famennian or F/F event and was a greater extinction than that which killed the dinosaurs. Palaeontologists have suggested many causes for the F/F event ranging from meteorite impacts, massive volcanic eruptions, global warming and global cooling. Life on Earth has even been suggested as the cause with the combined effects of giant reefs and land plants both locking up carbon dioxide and causing global cooling and an ice age. Land plants were then relatively new to the Earth and were both spreading rapidly across the land surface and increasing in size to become significant trees and were therefore potentially locking up atmospheric carbon dioxide. For most of the other mass extinctions we now better understand the causes by being able to see the combined effects of extinction on plants and animals from land and sea. However, for the F/F event we only have a very poor record of plant numbers through the time of the extinction and in particularly do not know whether any extinctions occur at exactly the same times as the extinctions that killed the sea life. Fortunately we have discovered a new rock unit in northern Russia that was deposited on the edge of the sea and contains a good record of land plants through the time of extinction. We can do this because we are studying the microscopic spores (just like pollen) from the plants. These spores are very resistant and can be found and extracted from rocks even when the parent plants are not fossilized. Unfortunately this new rock unit is mostly buried in the bank of a river. But we can get a solid core of rock by using a drilling rig. We can then use this core to extract the fossil spores and count them in closely spaced samples through the rock unit. This will show when the plants became extinct, for example was is slowly through time with no new plants evolving, was it suddenly with many of the the plants becoming extinct together or as a series of shorter duration successive crises with a few becoming extinct each time. We can then match this record to the extinctions in the sea. We have already studied rocks in East Greenland which during the Devonian was in the southern hemisphere arid zone. This direct record of land climate shows times of severe aridity when the spores became simple and rare. Using these spores we can compare this direct climate record to the borehole from Russia together with the marine extinctions to try and prove that cool aridity was a cause of the F/F mass extinction.