因火山而滅亡？

原文網址：www.sciencedaily.com/releases/2017/05/170530082345.htm

因火山而滅亡？

有個想法認為人類或許可以嘗試往大氣注入大量硫酸氣膠來對抗全球暖化。任何關注這個想法的人都應該要看看發表在2017年5月1日刊登在當期《地質》(Geology)上的一篇文章。

在此文章中，一名華盛頓大學的科學家和其同事描述了在4億4000萬年以前的奧陶紀末期，當大氣中有數次大量二氧化碳和硫酸氣膠互相混和的事件發生時，地球產生了何種變故。

對應到天空的一片混亂是海洋的一場浩劫。當時地球熱帶以北幾乎是被一整片海洋覆蓋，複雜的多細胞生物絕大多數生活在這片海洋當中。在此浩劫中，有85%的海洋生物從此消失，造成此事件被稱作奧陶紀大滅絕，是地球歷史上發生的五次最大滅絕事件的其中之一。

雖然這些氣體是由巨型火山噴發事件灌入大氣層中，而非源於大量燃燒化石燃料，且完全同樣的情況在未來也不會再次發生，但它的確是一件令人憂心的歷史案例，顯示氣候系統在行星尺度上具有潛在不穩定性。

此篇研究的共同作者，華盛頓大學文理學院的地球和行星科學副教授David Fike表示，想要找出奧陶紀大滅絕，或者地球歷史上其他大滅絕事件的成因，是出了名的困難。

由於古代大氣和海洋早已歷經許久的變化而無從辨認，科學家必須要利用其它代用指標，像是古代岩石中的氧同位素變化，來了解久遠以前的氣候狀況。Fike是分析岩石紀錄中的化學訊號以解讀過往生物和地質活動的專家，他說岩石中多數元素的變化牽涉到許多種化學反應，使得單一訊號通常可以用一種以上的面向來解讀，造成大部分的代用指標使用上有其困難之處。

但由安默斯特學院的地球科學家David Jones領導的研究團隊卻能迴避此問題。他們的方法是測量汞在岩石中的含量多寡。今日汞的主要來源是燃燒煤炭的火力發電廠以及其他人類活動，然而在奧陶紀時，汞的主要來源是火山活動。

Fike表示火山活動的發生頻率和大滅絕之間有令人疑惑的關係。他所說的並非是單獨一座火山，而是超大型的火山爆發事件，產生的厚層熔岩流足以覆蓋數千平方公里，冷卻後會形成大型火成岩區域(large igneous province, LIP)。在美國境內最著名的LIP是哥倫比亞河玄武岩區域，其涵蓋了華盛頓州的東南方，並延伸至太平洋以及奧勒岡州 (最靠近台灣的為中國峨嵋山玄武岩) 。

我們可以合理認為火山是驅動氣候的因子，或者是變化要素。因為它排出的二氧化碳可以造成長期溫室效應使氣候暖化，但它同時釋放出的二氧化硫則能反射陽光而造成短期冷化。此外，岩漿冷卻後新生的大片裸露岩石會吸收二氧化碳，並將其以石灰岩礦物的形式埋藏在海洋當中，這也可以讓氣候冷卻下來。

當Jones分析中國南部和內華達Monitor山脈的奧陶紀岩石樣品時，他發現樣品汞含量高得十分異常，其中有些樣品的汞濃度是背景值的500倍以上。汞分成三道高峰出現，在大滅絕之前和期間都有。

到底發生了什麼事情？由於此次大滅絕事件跟冰河期同時發生且具有(與眾不同的)兩次高峰，使得火山事件的序列顯得相當特殊。

隨著科學家將故事情節逐漸拼湊出來，他們開始猜測第一波噴發可能並未把地球的氣候推往十分脆弱的狀態，但卻為接下來的火山爆發引起的氣候浩劫埋下了火種。

第一波噴發產生的LIP被風化後吸收了大氣的二氧化碳，使得氣候冷卻並讓當時位處南半球的岡瓦那超大陸上方形成了冰河。

對流層頂(tropopause)分開了大氣中擁有不同溫度梯度的兩個分層。第一波火山噴化造成的氣候冷化可能讓對流層頂的所在高度下降。之後第二波火山噴發往對流層頂的上方注入了極為大量的二氧化硫，使得地球的反照率(albedo)，也就是陽光被反射回太空的比率急遽增加。

這造成了第一波滅絕事件，同時也是最嚴重的一波。隨著冰層擴張，海平面會跟著下降且海水溫度也會變冷，使得許多生物因而罹難。

在下一波火山活動期間，由二氧化碳造成的溫室效應會逐漸超越由二氧化硫造成的冷化效應，使得氣候暖化，讓冰層融化以及海平面上漲。接踵而來的是較暖且缺乏氧氣的水淹沒了生物棲地，造成許多在第一波滅絕事件中倖存下來的生物仍難逃一死。

Fike說重點是，能對地球氣候造成影響的因子會以意想不到的方式共同作用；而本身看起來或許沒有那麼嚴重的事件，卻有可能會把地球氣候系統推向十分危險的狀態，讓新加入的擾動可以造成毀滅性的結果。

Fike表示：「這是當我們打算利用地球工程方案來減緩全球暖化時，必須謹記在心的事情。」他教授的課程要求學生檢視這些方案後，評估自身有多大意願來執行這些方案。

Death by volcano?

Anyone concerned by the idea that people might try to combat global warming by injecting tons of sulfate aerosols into Earth's atmosphere may want to read an article in the May 1, 2017 issue of the journal Geology.

In the article, a Washington University scientist and his colleagues describe what happened when pulses of atmospheric carbon dioxide and sulfate aerosols were intermixed at the end of the Ordivician geological period more than 440 million years ago.

The counterpart of the tumult in the skies was death in the seas. At a time when most of the planet north of the tropics was covered by an ocean and most complex multicellular organisms lived in the sea, 85 percent of marine animal species disappeared forever. The end Ordivician extinction, as this event was called, was one of the five largest mass extinctions in Earth's history.

Although the gases were injected into the atmosphere by massive volcanism rather than prodigious burning of fossil fuels and under circumstances that will never be exactly repeated, they provide a worrying case history that reveals the potential instability of planetary-scale climate dynamics.

Figuring out what caused the end Ordivician extinction or any of the other mass extinctions in Earth's history is notoriously difficult, said David Fike, associate professor of earth and planetary sciences in Arts & Sciences and a co-author on the paper.

Because the ancient atmospheres and oceans have long since been altered beyond recognition, scientists have to work from proxies, such as variations in oxygen isotopes in ancient rock, to learn about climates long past. The trouble with most proxies, said Fike, who specializes in interpreting the chemical signatures of biological and geological activity in the rock record, is that most elements in rock participate in so many chemical reactions that a signal can often be interpreted in more than one way.

But a team led by David Jones, an earth scientist at Amherst College, was able to bypass this problem by measuring the abundance of mercury. Today, the primary sources of mercury are coal-burning power plants and other anthropocentric activities; during the Ordivician, however, the main source was volcanism.

Volcanism coincides with mass extinctions with suspicious frequency, Fike said. He is speaking not about an isolated volcano but rather about massive eruptions that covered thousands of square kilometers with thick lava flows, creating large igneous provinces (LIPs). The most famous U.S. example of a LIP is the Columbia River Basalt province, which covers most of the southeastern part of the state of Washington and extends to the Pacific and into Oregon.

Volcanoes are plausible climate forcers, or change agents, because they release both carbon dioxide that can produce long-term greenhouse warming and sulfur dioxide that can cause short-term reflective cooling. In addition, the weathering of vast plains of newly exposed rock can draw down atmospheric carbon dioxide and bury it as limestone minerals in the oceans, also causing cooling.

When Jones analyzed samples of rock of Ordivician age from south China and the Monitor Range in Nevada, he found anomalously high mercury concentrations. Some samples held 500 times more mercury than the background concentration. The mercury arrived in three pulses, before and during the mass extinction.

But what happened? It had to have been an unusual sequence of events because the extinction (atypically) coincided with glaciation and also happened in two pulses.

As the scientists began to piece together the story, they began to wonder if the first wave of eruptions didn't push Earth's climate into a particularly vulnerable state, setting it up for a climate catastrophe triggered by later eruptions.

The first wave of eruptions laid down a LIP whose weathering then drew down atmospheric carbon dioxide. The climate cooled and glaciers formed on the supercontinent of Gondwana, which was then located in the southern hemisphere.

The cooling might have lowered the tropopause, the boundary between two layers of the atmosphere with different temperature gradients. The second wave of volcanic eruptions then injected prodigious amounts of sulfur dioxide above the tropopause, abruptly increasing Earth's albedo, or the amount of sunlight it reflected.

This led to the first and largest pulse of extinctions. As ice sheets grew, sea level dropped and the seas became colder, causing many species to perish.

During the second wave of volcanism, the greenhouse warming from carbon dioxide overtook the cooling caused by sulfur dioxide and the climate warmed, the ice melted and sea levels rose. Many of the survivors of the first pulse of extinctions died in the ensuing flooding of habitat with warmer, oxygen-poor waters.

The take-home, said Fike, is that the different factors that affect Earth's climate can interact in unanticipated ways and it is possible that events that might not seem extreme in themselves can put the climate system into a precarious state where additional perturbations have catastrophic consequences.

"It's something to keep in mind when we contemplate geoengineering schemes to mitigate global warming," said Fike, who teaches a course where students examine such schemes and then evaluate their willingness to deploy them.

原始論文：David S. Jones, Anna M. Martini, David A. Fike, Kunio Kaiho. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, 2017; G38940.1 DOI: 10.1130/G38940.1

引用自：Washington University in St. Louis. "Death by volcano?." ScienceDaily. ScienceDaily, 30 May 2017.