原文網址：http://news.rice.edu/2017/07/31/glaciers-may-have-helped-warm-earth-2/
冰河或許有助地球暖化
雖然看似違反直覺，但根據萊斯大學教授的研究，冰河經過上萬年之後或許會讓地球變得更加溫暖。
於七月成為萊斯大學教職員的Torres是這篇刊登於《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)論文的主要作者。他想要知道陸上風化作用釋放的化學物質到達海洋和大氣的方式以及時間，以及它們造成的影響。
研究顯示透過使侵蝕作用變得更加劇烈，冰河作用或許能增加二氧化碳釋放到環境當中的速率。
研究人員得出有更多黃鐵礦(pyrite)――又稱為愚人金的硫化鐵――受到氧化作用很可能會增加海洋的酸度，而讓更多二氧化碳進入海洋並影響碳循環。冰河在數萬年間的擺盪對大氣二氧化碳濃度的改變或許可以達到25 ppm(百萬分之一)或更多。雖然對於近幾個月測量到的400 ppm來說此數值佔的比例相當大，不過現今人為排放的二氧化碳正以比冰河作用自然排放高出許多的速率發生。
他們發現從長時間尺度來看，冰河對二氧化碳排放的貢獻或許形成了一種負回饋迴圈，也許能抑制冰河無止盡地擴張。
Torres表示：「海洋儲存了十分大量的碳。如果海洋的化學成分發生改變，儲存其中的碳有部分就會以二氧化碳的形式釋放到大氣當中。由於溫室效應的作用，這些釋放出來的二氧化碳便能影響地球氣候。」
冰河產生的流水對二氧化碳濃度的影響似乎超出溫暖氣候的河流許多。Torres直到最近都還在加州理工學院擔任博士後研究員，他研究以冰河為源頭的河流並利用現有資料庫，比較它們跟世界各地其他數千條河流的化學性質。目標是計算和冰河風化作用有關的主要化學反應，並探討其長期影響。
他說：「我們主要是在考量冰河和冰河作用如何影響地球的運作方式。我們特別關注流域中地表有被冰河覆蓋的河流，並探討那些河流的化學成分是否存在任何差異。」
研究人員表明由於冰河也會分解岩石中的矽酸鹽，所以它們對風化作用的另一面也具有影響。這是因為矽酸鹽釋放出的鹼性物質可以移除大氣中的二氧化碳。不過他們仍然認為冰河作用造成的總體效應會提供大氣二氧化碳而非減少。
這項結果也支持了其他一些有趣的理論。其中之一認為地球在數十億年前的元古宙和太古宙大氣氧含量不多的時候，由於冰河地區氧化造成的風化作用並不活躍連帶使放出的碳變少，可能造成整個地球變成一顆「大雪球」。
另外一項則認為地殼儲存的硫化物隨著時間越來越多或許有助於讓氣候穩定，這對地球經過各個地質時代仍可以一直維持其適居程度來說相當重要。
It seems
counterintuitive, but over the eons, glaciers may have made Earth warmer,
according to a Rice University professor.
Mark Torres, an assistant professor of Earth, environmental and planetary
sciences, took a data-driven dive into the mechanics of weathering by
glaciation over millions of years to see how glacial cycles affected the oceans
and atmosphere and continue to do so.
Torres, who
joined the Rice faculty in July, is lead author of a paper in the Proceedings
of the National Academy of Sciences. He wanted to
know how and when chemicals released by weathering of the land reached the
atmosphere and ocean, and what effect they have had.
The study
shows that glaciation, through enhanced erosion, probably increased the rate of
carbon dioxide released to the environment.
The
researchers determined enhanced oxidation of pyrite, an iron sulfide also
known as fool’s gold, most likely generated acidity that fed carbon dioxide
into the oceans and altered the carbon cycle. The oscillation of glaciers over
10,000 years could have changed atmospheric carbon dioxide by 25 parts per
million or more. While this is a significant percentage of the 400 parts per million measured in recent months, present
anthropogenic carbon dioxide release is occurring at a much faster rate than it
is naturally released by glaciation.
Over long
timescales, they found, glaciers’ contribution to the release of carbon dioxide
could have acted as a negative feedback loop that may have inhibited runaway
glaciation.
“The ocean
stores a lot of carbon,” Torres said. “If you change the chemistry of the
ocean, you can release some of that stored carbon into the atmosphere as carbon
dioxide. This release of carbon dioxide affects Earth’s climate, due to the
greenhouse effect.”
Glacial runoff
appeared to have an outsize effect on carbon dioxide levels compared with that
of rivers in warmer climes. Torres, until recently a postdoctoral researcher at
the California Institute of Technology, studied glacier-fed rivers and used
existing databases to compare their chemical contents with that of thousands of
rivers around the world. The goal was to evaluate the dominant chemical
reactions associated with glacial weathering and explore the long-term
implications.
“Mainly, we’re
thinking about the effect of glaciers and glaciation on the way our planet
works,” he said. “In particular, we’re looking at rivers that drain areas of
land surface that are covered by glaciers, and whether or not there are any
differences in the chemical composition of those rivers.”
The
researchers acknowledged that glaciers are equal-opportunity weathering agents,
as they also break down silicates in rocks. Silicates release
alkalinity that removes carbon from the atmosphere. Still, they believe the net
effect of glaciation could be to supply carbon dioxide to the atmosphere rather
than to remove it.
The results
support a couple of interesting additional theories. One is that billions of
years ago in the Archean eon and Paleoproterozoic era,
when the atmosphere contained little oxygen, Earth may indeed have been a “snowball” as oxidative
weathering in glaciated regions and the subsequent release of carbon would have
been less active.
Another is
that the growth of a sulfide reservoir in Earth’s crust over time may have
helped to stabilize the climate, which is important for maintaining Earth’s
habitability over geologic timescales.
原始論文：Mark A. Torres
et al. Glacial weathering, sulfide oxidation, and global carbon cycle
feedbacks. PNAS, 2017 DOI: 10.1073/pnas.1702953114
引用自：Rice University. "Glaciers may have helped
warm Earth: Study details effect of glacial versus nonglacial weathering on
carbon cycle."
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