反應更加活躍的地表使地球冷卻下來
漫長的低溫時期在地球歷史上可謂屢見不鮮。最近一次冰河期始於250萬年前左右，當時巨大的冰層和冰川覆蓋了北半球。不過，在此事件發生的一千多萬年前，氣溫就已經開始下降了。二十多年來，地質科學上有個廣為接受的學說認為，大型山脈形成是溫度降低的原因，像是安地斯山、喜馬拉雅山、阿爾卑斯山.......等。這些山脈隆起之後讓更多岩石遭受風化作用，減少大氣中的二氧化碳，進而減弱「溫室效應」，使得全球氣溫下降。此種現象和其他作用加總起來，最終讓地球進入了「冰河期」。
智利沿海山脈裡的土壤層。雖然土壤已經高度風化，但殘留其中的花崗岩塊仍然完整而能夠發生化學反應――這類土壤具有相當高的「反應性」。圖片來源：F.
von Blanckenburg, GFZ
瑞士蘇黎世聯邦理工學院的Jeremy
Caves-Rugenstein、美國史丹佛大學的Dan
Ibarra、德國波茨坦地球科學研究中心(GFZ)的Friedhelm
von Blanckenburg在最近的一項研究中，指出此學說可能有誤。論文指出風化速率在這段區間都保持固定；反之，是地表的「反應性」(reactivity)變得更加活躍，造成大氣中的二氧化碳減少，全球氣溫下降。研究結果發表在期刊《自然》(Nature)。
重新審視同位素分析結果
在數十億年的時間尺度下，岩石的風化過程可以控制地球氣候，特別是岩石跟碳酸反應而造成的化學風化。由於碳酸是二氧化碳溶於雨水時形成的，所以風化作用可以移除大氣中的二氧化碳，其規模等同於火山氣體供應給大氣的二氧化碳。目前廣為接受的學說認為最近1500萬年大型山脈的形成強化了侵蝕作用，造成岩石和二氧化碳結合而成的風化作用也跟著增強。測量海洋沉積物地球化學性質的結果，也確實指出這段期間二氧化碳在大氣裡的比例有所下降。
「不過該理論卻有個相當大的陷阱，」GFZ的Friedhelm
von Blanckenburg加以解釋，「如果大氣減少的二氧化碳，確實等同於侵蝕加劇造成的風化作用消耗掉的量，那麼不到一百萬年大氣裡的二氧化碳就會所剩無幾。地球上所有的水分都會結成冰塊，使生物難以生存――但實情顯然並非如此。」
Von Blanckenburg 和他的同事Jane
Willenbring 在2010年同樣發表在《自然》的研究中，已經證明了這樣的懷疑確實有其道理。Friedhelm
von Blanckenburg表示：「鈹-10是宇宙射線在地球大氣層產生的稀有同位素，而鈹-9則是穩定同位素，我們測量了海洋沉積物裡兩者之間的比例，結果指出這段期間地表的風化速率完全沒有增加。」
地表變得更容易「反應」
最近發表的這項研究中，Caves-Rugenstein、Ibarra和von
Blanckenburg還加入另外一種可以指示風化作用的數據：海洋沉積物裡的穩定鋰同位素。他們想要了解為什麼岩石風化的速率不變，大氣中的二氧化碳卻會逐漸降低。因此，他們將數據輸入模擬全球碳循環的電腦模型之中。
模擬結果明確顯示風化的速率沒有增強，而是地表變得更容易受到風化作用。研究人員將地表受到風化作用的難易度稱為「反應性」。Friedhelm
von Blanckenburg解釋：「反應性代表了化合物和元素有多容易參與化學反應。」如果地表未受風化的岩石，也就是反應性越高的岩石越多，則它們跟大氣中少量二氧化碳發生化學反應的總量，跟嚴重風化的岩石和大量二氧化碳發生化學反應的總量會是一樣的。因此不需要加速風化作用，就可以解釋大氣中的二氧化碳為何減少，以及連帶造成的溫度降低。
「不過，要讓地表回春而提高『反應性』，仍然需要某種地質作用的協助。」Friedhelm
von Blanckenburg表示，「並非只有形成山脈才能達到這種效果。舉凡構造活動造成地表破裂、些微提升侵蝕速率、出露其他類型的岩石......等地質作用都有可能讓地表出現更多容易風化的物質。無論如何，我們的新假說勢必能激起地質學家重新思考末次冰河期之前的溫度下降，是由什麼原因造成的。」
More "reactive" land
surfaces cooled the Earth
From time to time, there have been long
periods of cooling in Earth's history. Temperatures had already fallen for more
than ten million years before the last ice age began about 2.5 million years
ago. At that time the northern hemisphere was covered with massive ice masses
and glaciers. A geoscientific paradigm, widespread for over twenty years,
explains this cooling with the formation of the large mountain ranges such as
the Andes, the Himalayas and the Alps. As a result, more rock weathering has
taken place, the paradigm suggests. This in turn removed more carbon dioxide
(CO2) from the atmosphere, so that the ‘greenhouse effect’ decreased
and the atmosphere cooled. This and other processes eventually led to the ‘ice
Age’.
In a new study, Jeremy Caves-Rugenstein from ETH
Zurich, Dan Ibarra from Stanford University and Friedhelm von Blanckenburg from
the GFZ German Research Centre for Geosciences in Potsdam were able to show
that this paradigm cannot be upheld. According to the paper, weathering was
constant over the period under consideration. Instead, increased ‘reactivity’
of the land surface has led to a decrease in CO2 in the atmosphere,
thus cooling the Earth. The researchers published the results in the journal Nature.
A second look
after isotope analysis
The process of rock weathering, and especially the
chemical weathering of rocks with carbonic acid, has controlled the Earth's
climate for billions of years. Carbonic acid is produced from CO2
when it dissolves in rainwater. Weathering thus removes CO2 from the
Earth's atmosphere, precisely to the extent that volcanic gases supplied the
atmosphere with it. The paradigm that has been widespread so far states that
with the formation of the large mountains ranges in the last 15 million years,
erosion processes have increased – and with them also the CO2-binding
rock weathering. Indeed, geochemical measurements in ocean sediments show that
the proportion of CO2 in the atmosphere has decreased during this
phase.
"The hypothesis, however, has a big catch,"
explains Friedhelm von Blanckenburg of GFZ. "If the atmosphere had
actually lost as much CO2 as the weathering created by erosion would
have caused, it would hardly have had any CO2 left after less than a
million years. All water would have had frozen to ice and life would have had a
hard time to survive. But that was not the case."
That these doubts are justified, was already shown by
von Blanckenburg and his colleague Jane Willenbring in a 2010 study, which
appeared in Nature likewise. "We
used measurements of the rare isotope beryllium-10 produced by cosmic radiation
in the Earth's atmosphere and its ratio to the stable isotope beryllium-9 in
ocean sediment to show that the weathering of the land surface had not
increased at all," says Friedhelm von Blanckenburg.
The land’s
surface has become more ‘reactive’
In the study published now, Caves-Rugenstein, Ibarra and
von Blanckenburg additionally used the data of stable isotopes of the element
lithium in ocean sediments as an indicator for the weathering processes. They
wanted to find out how, despite constant rock weathering, the amount of CO2
in the atmosphere could have decreased. They entered their data into a computer
model of the global carbon cycle.
Indeed, the results of the model showed that the
potential of the land surface to weather has increased, but not the speed at
which it weathered. The researchers call this potential of weathering the
‘reactivity’ of the land surface. "Reactivity describes how easily chemical
compounds or elements take part in a reaction," explains Friedhelm von
Blanckenburg. If there are more non-weathered and therefore more reactive rocks
at the surface, these can in total react as extensively chemically with little
CO2 in the atmosphere as already heavily weathered rocks would do
with a lot of CO2. The decrease in CO2 in the atmosphere,
which is responsible for the cooling, can thus be explained without an
increased speed of weathering.
"However, a geological process is needed to
rejuvenate the land surface and make it more 'reactive'," says Friedhelm
von Blanckenburg."This does not necessarily have to be the formation of
large mountains. Similarly, tectonic fractures, a small increase in erosion or
the exposure of other types of rock may have caused more material with
weathering potential to show at the surface. In any case, our new hypothesis
must trigger geological rethinking regarding the cooling before the last ice
age."
原始論文：Jeremy K.
Caves Rugenstein, Daniel E. Ibarra, Friedhelm von Blanckenburg. Neogene
cooling driven by land surface reactivity rather than increased weathering
fluxes. Nature, 2019; 571 (7763): 99 DOI: 10.1038/s41586-019-1332-y
引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz
Centre. "More 'reactive' land surfaces cooled the Earth down."
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