古代的全球冷化事件造就了當代生態系

原文網址：www.sciencedaily.com/releases/2016/09/160930144424.htm

古代的全球冷化事件造就了當代生態系

大約7百萬年前，全球各處的地貌及生態系統開始發生了劇烈變化。非洲的副熱帶地區越趨乾燥，導致撒哈拉沙漠形成。雨林在南北美、非洲和亞洲的面積減少，直到今日仍被廣大的莽原和草原取代。

目前為止，通常以互不相關的構造運動，像是山脈隆起或海盆變化，各自造成局部地區的氣候變化來解釋上述事件。但在一項新研究中，研究人員發現這些環境變遷跟一起先前無文獻紀載的全球冷化事件正好同期。它的成因可能是因大氣二氧化碳含量銳減而造成。

由布朗大學的地質學家領導，發表在期刊《自然―地質科學》(Nature Geoscience)的研究，主要立論於一段新建立起來的全球海溫紀錄，其區間橫跨了過去1200萬年。紀錄顯示在700萬至540萬年前的中新世(Miocene)末，有段時期海洋表面溫度特別的低。中新世全球氣候據信是比現在還要溫暖許多，但在這篇研究中發現的寒冷時期，海洋表面溫度卻驟降到近似於現在的數值。

「這是第一次有全球海洋表層溫度記錄將中新世晚期納入其中，而我們從中發現的溫度下降幅度連我們自己也感到相當驚訝。」領導此研究的布朗大學地球、環境與行星科學系的教授Timothy Herbert表示。「有了這段溫度變化，於此時開始發生的古生物學現象便開始顯得合理許多。」

這項新的海洋表面溫度紀錄源自於從世界各地17個不同地點採集的海洋沉積物樣品。當中含有一種浮游生物的遺骸，其細胞化學物質會依據溫度高低而產生變化。科學家測量這種對溫度敏感的分子含量多寡，便能重新建構溫度如何隨時間改變。

從南北半球各個海盆採取的樣品中都能偵測到在中新世晚期有段寒冷時期。冷化幅度越往兩極就越強烈，而往赤道則較緩和。Herbert說這種模式代表氣溫下降是由全球大氣發生變化所導致，最有可能的嫌疑犯便是二氧化碳(CO₂)。

「這起冷化事件具有兩半球對稱且在高緯度較為劇烈的特性―這些都是跟二氧化碳相關的溫度變化會留下的特徵。」Herbert說。「由於我們無法直接測量二氧化碳，因此不能證明其濃度有下降，但我們的確找到了二氧化碳濃度降低造成的間接證據。」

研究人員說氣溫降低在中新世末發生的副熱帶越趨乾燥現象中可能具有重大影響。「當地球變得較冷，尤其是海洋溫度降低，會造成大氣中的水氣減少。」Herbert表示，「隨著地球冷化，水循環的速率基本上也會減慢。」

如果大氣二氧化碳含量降低確實驅使了冷化現象發生，那麼這就能解釋為何在中新世晚期全球植被組成產生了劇烈改變，也就是南北美、亞洲和非洲副熱帶地區的森林轉變成草原或莽原。時至今日這些地區仍以此種生態系為主。在非洲，草原和莽原棲地出現可能跟早期人類祖先的演化有所關聯。

在這段時期開始佔據主流地位的草本植物為「四碳」植物(C4 plant)。它們行光合作用的路徑跟樹木和其他植物有些微不同，造成四碳路徑在低二氧化碳的環境條件中可以行使得更有效率。「如果當時二氧化碳濃度變得較低，就比較利於這些四碳植物生存。」Herbert說，「因此我們可以將植被轉變的原因和造成中新世末期冷化事件的可能成因連結至同一現象。」

Herbert表示現在還不清楚是什麼導致了二氧化碳濃度降低。有可能是當時發生了大規模地質現象而對碳循環產生重大影響。Herbert的實驗室目前正在深入探討這種可能性。

但可以確定的是在中新世末期全球氣溫發生了重大變化。

「現行觀點認為當時的全球氣候並無特別可看之處。」Herbert說，「但從研究結果看來，它其實比人們以往認為的還要有趣許多。」

Ancient global cooling gave rise to modern ecosystems

Around 7 million years ago, landscapes and ecosystems across the world began changing dramatically. Subtropical regions dried out and the Sahara Desert formed in Africa. Rain forests receded and were replaced by the vast savannas and grasslands that persist today in North and South America, Africa and Asia.

Up to now, these events have generally been explained by separate tectonic events -- the uplift of mountain ranges or the alteration of ocean basins -- causing discrete and local changes in climate. But in a new study, a team of researchers has shown that these environmental changes coincided with a previously undocumented period of global cooling, which was likely driven by a sharp reduction in atmospheric carbon dioxide.

The research, led by a Brown University geologist and published in Nature Geoscience, is based on a newly developed record of global sea surface temperatures spanning the past 12 million years. The record reveals a distinct period of cooler sea surface temperatures spanning 7 million to 5.4 million years ago, the end of the Miocene epoch. The global climate during the Miocene is known to have been much warmer than the present. During the cool period detected in this study, sea surface temperatures dropped to near modern levels.

"This is the first time the late Miocene has been put in a context of global sea surface temperatures, and we were surprised to see the amount of cooling we found," said Timothy Herbert, professor in the Department of Earth, Environmental and Planetary Sciences at Brown, who led the study. "In light of this temperature change, the paleobiological observations from this period start to make a lot more sense."

The new record of sea surface temperatures was derived from ocean sediment sampled at 17 different sites around the world. The sediment preserves the remains of a plankton species that varies cellular chemistry with temperature. By measuring amounts of those temperature-sensitive molecules, scientists can recreate temperature through time.

The late Miocene cool period was detected at every site sampled, in both hemispheres and in every ocean basin on the planet. The cooling was strongest toward the poles and more moderate toward the equator. That pattern, Herbert says, suggests a global atmospheric cause for the temperature decline. The most likely suspect is carbon dioxide (CO₂).

"The hemispheric symmetry and the fact that cooling is much greater at the high latitudes -- these are the fingerprints of CO₂-related temperature change," Herbert said. "We haven't proven that it was a decline in CO₂ because we're not measuring it directly, but we're making a circumstantial case for a reduction in CO₂."

The cooler temperatures would likely have played a role in the drying of the subtropics in the late Miocene, the researchers say. "A cooler world -- particularly a cooler ocean -- would have decreased moisture in the atmosphere," Herbert said. "The hydrological cycle basically slows down with cooling."

And if the cooling was indeed driven by a reduction in atmospheric CO₂, it could explain a critical shift in global vegetation that occurred during the late Miocene: the transition from forests to grassland and savanna in the subtropical regions of North and South America, Asia and Africa. These ecosystems are still present today. In Africa, these are the habitats associated with the evolution of our early human ancestors.

Many of the grassy plant species that began thriving during this period are "C4" plants. These species use a slightly different photosynthetic pathway than trees and other plants. The C4 pathway is more efficient in low CO₂ environments. "It could be that that if CO₂ declined, these C4 species were favored," Herbert said. "So we can associate that shift in vegetation with the same thing that we suspect was driving the late Miocene cooling."

It isn't clear at this point what might have driven a reduction in CO₂ during this period, Herbert says. It could be that there were large-scale geological changes occurring at this time that affected the carbon cycle. Herbert's lab is looking into that possibility now.

But what is clear is that there was a significant global shift in global temperatures during the late Miocene.

"The prevailing view was that this wasn't a particularly exciting time in terms of global climate," Herbert said. "It turns out to be more interesting than people thought."

原始論文：Timothy D. Herbert, Kira T. Lawrence, Alexandrina Tzanova, Laura Cleaveland Peterson, Rocio Caballero-Gill, Christopher S. Kelly. Late Miocene global cooling and the rise of modern ecosystems. Nature Geoscience, 2016; DOI:10.1038/ngeo2813

引用自：Brown University. "Ancient global cooling gave rise to modern ecosystems." ScienceDaily. ScienceDaily, 30 September 2016.