火山爆發開啟了恐龍時代的序幕

原文網址：www.sciencedaily.com/releases/2017/06/170619151530.htm

火山爆發開啟了恐龍時代的序幕

三疊紀末期的滅絕事件為恐龍時代的興起奠下了基礎。根據牛津大學進行的新研究，一波波劇烈的火山活動可能在其成因中扮演了關鍵性腳色。

發生在恐龍時代之前的三疊紀滅絕事件約發生在2億年前，其為紀錄中規模最大的動物滅絕事件之一，受害者名單包括了類似鱷魚的巨大爬蟲類以及好幾種海洋無脊椎動物。此次事件也造成了陸地植被的劇烈變化，雖然至今仍然不曉得恐龍為何能倖存下來，但牠們確實跟原始哺乳類和兩棲類一同填補了其他野生動物滅亡後，空出來的生態棲位。長久以來科學家將此次大滅絕的成因連結至大量二氧化碳突然排放到大氣當中，不過二氧化碳的確切來源卻仍然尚未知曉。

由於有發現跟滅絕事件同年代的火山岩，因此科學家之前提出火山排放的二氧化碳是滅絕事件的重要推手之一。前人研究顯示火山活動可能是分成好幾波發生，但是這些火山事件對全世界的影響程度及可能造成的效應仍然無法確定。它們形成的火山岩覆蓋區域之大橫跨了四座大陸，稱為中大西洋火成岩區域(CAMP)。

牛津大學地球科學系的研究人員跟艾克斯特大學和南安普敦大學的學者合作，追查這些巨量火山氣體對全球的影響為何，以及跟三疊紀末期的事件有什麼樣的關聯。他們發現火山活動可以連結至先前觀察到的數波大量二氧化碳排放事件。這些事件對全球氣候造成深遠影響而促成了三疊紀末的大滅絕事件，並延緩了之後動物復甦的腳步。

經由探討在滅絕事件發生期間形成的沉積岩中的汞含量，研究清楚顯示CAMP火山活動的發生時間和三疊紀末的滅絕事件有明顯關聯。火山活動會排出含汞氣體，並經由大氣循環而散佈到全球各地，最終沉降至沉積物裡。因此，科學家預期在大型火山事件發生期間形成的所有沉積物，都會含有異常高含量的汞。

研究團隊取得的沉積物樣品分別來自英國、奧地利、阿根廷、格陵蘭、加拿大和摩洛哥，接著他們分析了樣品中的汞含量。六筆紀錄中的五筆顯示在三疊紀末滅絕事件的層位，汞含量開始有大幅上升的現象。其他高峰則出現在滅絕事件的層位和三疊紀―侏儸紀交界之間，大約是開始大幅上升的20萬年之後。

汞排放量的上升也跟先前已確立的大氣二氧化碳濃度增加同步發生，代表二氧化碳是來自於火山活動排放的氣體。

主要作者，牛津大學地球化學的研究生Lawrence Percival表示：「這些結果強烈支持了在三疊紀末曾經有數波火山活動事件接連發生，且它們起始於三疊紀末滅絕事件的發生期間。」

「這項研究大幅強化了三疊紀大滅絕事件跟火山排放二氧化碳之間的關聯性，也進一步證實了火山數度排放二氧化碳到大氣當中可能是滅絕事件的成因。這讓我們對此事件有更深的認識，或許也能幫助我們瞭解地球歷史上的其他氣候變遷事件。」

Volcanic eruptions triggered dawn of the dinosaurs

Huge pulses of volcanic activity are likely to have played a key role in triggering the end Triassic mass extinction, which set the scene for the rise and age of the dinosaurs, new Oxford University research has found.

The Triassic extinction took place approximately 200 million years ago, and was proceeded by the dinosaur era. One of the largest mass extinctions of animal life on record, the casualty list includes large crocodile-like reptiles and several marine invertebrates. The event also caused huge changes in land vegetation, and while it remains a mystery why the dinosaurs survived this event, they went on to fill the vacancies left by the now extinct wildlife species, alongside early mammals and amphibians. This mass extinction has long been linked to a large and abrupt release of carbon dioxide into the atmosphere, but the exact source of this emission has been unknown.

Following the discovery of volcanic rocks of the same age as the extinction, volcanic carbon dioxide (CO₂) emissions had previously been suggested as an important contributor to this extinction event. Previous studies have also shown that this volcanism might have occurred in pulses, but the global extent and potential impact of these volcanic episodes has remained unknown. These volcanic rocks covered a huge area, across four continents, representing the Central Atlantic Magmatic Province (CAMP).

Researchers from the Oxford University Department of Earth Science worked in collaboration with the Universities of Exeter and Southampton to trace the global impact of major volcanic gas emissions and their link to the end of the Triassic period. The findings link volcanism to the previously observed repeated large emissions of carbon dioxide that had a profound impact on the global climate, causing the mass extinction at the end of the Triassic Period, as well as slowing the recovery of animal life afterwards.

By investigating the mercury content of sedimentary rocks deposited during the extinction, the study findings revealed clear links in the timing of CAMP volcanism and the end-Triassic extinction. Volcanoes give off mercury gas emissions, which spread globally through the atmosphere, before being deposited in sediments. Any sediments left during a large volcanic event would therefore be expected to have unusually high mercury content.

The team sourced six sediment deposits were sourced from the UK, Austria, Argentina, Greenland, Canada and Morocco, and their mercury levels analysed. Five of the six records showed a large increase in mercury content beginning at the end-Triassic extinction horizon, with other peaks observed between the extinction horizon and the Triassic-Jurassic boundary, which occurred approximately 200 thousand years later.

Elevated mercury emissions also coincided with previously established increases in atmospheric CO₂ concentrations, indicating CO₂ release from volcanic degassing.

Lawrence Percival, Lead author and Geochemistry Graduate student at Oxford University, said: 'These results strongly support repeated episodes of volcanic activity at the end of the Triassic, with the onset of volcanism during the end-Triassic extinction.

'This research greatly strengthens the link between the Triassic mass extinction and volcanic emissions of CO₂. This further evidence of episodic emissions of volcanic CO₂ as the likely driver of the extinction enhances our understanding of this event, and potentially of other climate change episodes in Earth's history.'

原始論文：Lawrence M. E. Percival, Micha Ruhl, Stephen P. Hesselbo, Hugh C. Jenkyns, Tamsin A. Mather, and Jessica H. Whiteside. Mercury evidence for pulsed volcanism during the end-Triassic mass extinction. PNAS, June 2017 DOI: 10.1073/pnas.1705378114

引用自：University of Oxford. "Volcanic eruptions triggered dawn of the dinosaurs." ScienceDaily. ScienceDaily, 19 June 2017.