原文網址：https://news.climate.columbia.edu/2021/10/24/tackling-a-40-million-year-old-conundrum/
By Shilei Li,
Steven L. Goldstein and Maureen E. Raymo
矽酸鹽礦物是多數岩石的主要成分。當這些礦物在地表跟水接觸，會和水中的二氧化碳反應而溶解一部份。這些溶掉的礦物接著被河流帶到海洋，最後由生物利用而在海中形成石灰岩之類的碳酸鹽。大氣裡的二氧化碳可以經由此過程移除並轉化成岩石――這道稱為矽酸鹽風化的作用可以說是自然界的碳封存範例。近代的矽酸鹽風化會隨著侵蝕速率而加快，也就是岩石破碎成更加細小的碎片的作用，像是溪水流動、凍融循環或是山崩。
新研究支持了數千萬年前安地斯山與喜馬拉雅山抬升，協助促成了隨後發生的冰河期的理論。圖中為尼泊爾的高山。圖片來源：Bisesh Gurung/unsplash
我們之中的一員，哥倫比亞大學拉蒙特―多爾蒂地球觀測所的Maureen
Raymo在三十年前與他的同僚William
F. Ruddiman和Philip
N. Froelich，提出喜瑪拉雅山和安地斯山的抬升造成侵蝕作用與岩石風化速率提高，使得大氣中的二氧化碳減少。這導致過去4000萬年全球都可以觀察到氣溫逐漸降低的趨勢，最後還造成了反覆發生的冰河期。此概念被稱為「抬升―風化假說」(Uplift-Weathering Hypothesis)。
許多地質與地球化學紀錄也支持此概念，這些紀錄包括了海洋溶解的二氧化碳量的變化、特定岩石冷卻與凝固的時間點、海床的沉積速率……等。
鍶、鋨、鋰……等元素的同位素，也一致地呈現出這段時期全球陸地的侵蝕與風化速率皆有所提升。因此它們也支持了「抬升―風化假說」。
然而，有個特別顯眼的重要例外是鈹同位素。岩石裡面只有鈹-9(含有4個質子及5個中子)；而在地球的高層大氣，太陽來的高能宇宙粒子會和氧與氮撞擊產生鈹-10(含有4個質子及6個中子)，接著落到地球表面。如果侵蝕和風化速率增加，就會有更多鈹-9流入河水；於此同時，由於鈹-10的生成速率是固定的，因此一般預測海洋裡鈹-9/鈹-10的比率會跟著增加。既然我們沒有看到這種現象，某些科學家便將此解讀為過去1200萬年陸地的侵蝕與風化速率變化不大，進而否決了抬升―風化假說。
在最近發表於《美國國家科學院院刊》(Proceedings
of the National Academy of Sciences)的研究當中，我們建構出一種新的鈹循環模型來重新詮釋過去的鈹同位素紀錄。我們的模型呈現出當陸地的侵蝕與風化速率增加，從河川流到海洋的鈹-9會在海岸附近被化學作用消耗，使得多餘的鈹-9被移除而遭到抵銷。模型預測海水中鈹-9/鈹-10的同位素比率會保持恆定――正如我們觀察到的。這可以解釋從大約3400萬年前的新生代晚期開始，為什麼大陸地殼的侵蝕與風化速率增加的同時，鈹同位素紀錄卻仍然維持不變；此外，這項結果也解決了鈹同位素紀錄和其他風化代用指標之間的矛盾。
研究也提供了重要資訊讓我們了解地球的碳循環如何運作。有鑑於許多線索都指出過去1200萬年的侵蝕與風化速率變得更快，我們可以確定還有其他不同於矽酸鹽風化的地質作用也在運作，才能讓地球的氣溫不會一路跌到谷底。之前已經提出了一些機制能夠補償侵蝕和風化作用移除的二氧化碳，使得地球可以一直維持其適居性。
身為地球科學家，我們面臨的挑戰是要更加深入地瞭解地球如此複雜的碳循環，而地球海洋地殼與海水儲存碳的地方之間是如何進行交換，更是需要進行廣泛地探討。
Tackling a 40 million-year-old
conundrum
Silicate minerals are the major
components of most rocks. When these minerals on the Earth’s surface come into
contact with water, they partially dissolve as they react with carbon dioxide
in the water. This dissolved material is then transported via rivers to the
ocean, where it is ultimately used by organisms to make marine carbonates such
as limestone. In this way, carbon dioxide is removed from the atmosphere and
turned into rock. The process is called silicate weathering—a natural example
of carbon sequestration. In the modern world, the rate of this process
increases along with the rate of erosion, the breaking down of rock into ever
smaller pieces by processes such as water flow by streams and rivers,
freeze-thaw cycles or landslides.
Three decades ago at Columbia University’s
Lamont-Doherty Earth Observatory, one of us, Maureen Raymo, together with
colleagues William F. Ruddiman and Philip N. Froelich, proposed that enhanced
erosion and weathering of rocks, driven by the uplift of the Himalayas and
Andes, caused a decline in atmospheric carbon dioxide. This resulted in the
global cooling observed over the past 40 million years, and ultimately, to
repeated ice ages. This idea is known as the Uplift-Weathering Hypothesis.
Multiple geological and geochemical records support
this idea. These include records tracing changes in carbon dioxide dissolved in
the oceans; the timing of when certain rocks cooled and solidified; and rates
of sedimentation on the ocean floor.
Isotopes of several elements including strontium,
osmium and lithium are also consistent with a global acceleration of
continental erosion and weathering during this time. They therefore provide
support for the Uplift-Weathering Hypothesis.
However, isotopes of the element beryllium have stood
out as an important exception. Rocks include only beryllium-9 (it has 4 protons
and 5 neutrons). In the Earth’s upper atmosphere, high-energy cosmic particles
from the Sun collide with oxygen and nitrogen to create beryllium-10 (with 4
protons and 6 neutrons), which falls to the Earth’s surface. If erosion and
weathering increases, more beryllium-9 would come down rivers, while the rate
of beryllium-10 creation would stay the same, so we might expect the ratio of
beryllium-9/beryllium-10 in the oceans to increase. Since it did not, some
scientists have interpreted this to indicate that continental
erosion-weathering rates were stable over the last 12 million years, which
would falsify the Uplift-Weathering Hypothesis.
In a study just published in the Proceedings of the National Academy of Sciences, we have developed
a new model of the beryllium cycle that reinterprets the isotope record of the
past. The model demonstrates that under increasing continental erosion and
weathering, increases in beryllium-9 flowing through rivers to the ocean would
be counterbalanced by chemical scavenging processes along ocean coasts that
would remove the extra beryllium-9. The predicted result would be a constant
beryllium-9/beryllium-10 isotope ratio in seawater, as observed. Similar
processes do not take place with the other isotopes, such as strontium and
lithium. This explains why beryllium isotope records are consistent with
increased erosion and weathering of the continental crust during the late
Cenozoic, starting some 34 million years ago, and resolves the contradiction
between the beryllium records and the other weathering proxies.
This study also provides important information on how
our planet’s carbon cycle works. Given the many lines of evidence indicating
that increased erosion and weathering occurred over the last 12 million years,
one must conclude that geologic processes other than silicate weathering must
have prevented runaway cooling of the planet. A number of mechanisms have been
proposed to compensate for the removal of carbon dioxide by erosion and
weathering, and thus maintaining our planet’s habitability.
Our challenge as geoscientists is to reach a deeper
understanding of the complexities of the Earth’s carbon cycle, especially the
vast understudied reservoirs of carbon that are exchanged between the planet’s
oceanic crust and seawater.
原始論文：Shilei
Li, Steven L. Goldstein & Maureen E. Raymo. Neogene
continental denudation and the beryllium conundrum.
Proceedings of the National Academy of
Sciences, 2021; DOI: 10.1073/pnas.2026456118
引用自：Columbia
Climate School. “Tackling a 40 Million-Year-Old Conundrum.”
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