透過分析硫而證實氧氣出現的時間
萊斯大學的科學家利用河水證實大氣中的氧含量在27億年前開始上升
Mike Williams
科學家長久以來認為氧氣在27億年前開始出現在地球的低層大氣中，使得我們所知的許多生命得以誕生。而一位萊斯大學的研究人員得到了新的證據來支持這個數字。
此為加拿大安大略省沃瓦鎮的高山瀑布。這些河水流過並侵蝕了蘇必略穩定地塊露出的古老岩石，其中蘊含了地球大氣在27億年前如何演變的線索。一項由萊斯大學進行的研究，顯示這些岩石中由硫寫下的紀錄標示出地球大氣當時出現了劇烈變化，進而使複雜的生命得以誕生。圖片來源：Tom
Samworth/www.itsabouttravelling.com
雖然古代岩石中由硫寫下的紀錄標註了地球大氣出現劇烈變化，進而使複雜生命得以誕生的時間，但岩石卻只能指示某個地區的狀況。為了得到更加全面的圖像，萊斯大學的生物地球化學家Mark
Torres利用在地表流動並侵蝕岩石的水來當作指標。
Torres是萊斯大學地球、環境與行星科學系的助理教授。他和同事發表在《自然―地質科學》(Nature Geoscience)的文章指出在太古宙岩石中，硫同位素異常――也就是「大氧化事件」的標記――回復平衡的現象，也能在侵蝕這些岩石的河川中辨識並測量出來。
地球上只有少數幾個地方有出露大量的太古宙岩石，研究人員在其中兩個地點採集當地的水樣：分別是加拿大的蘇必略穩定地塊和南非。他們得出雖然個別的岩石樣品也許還能測出硫同位素的不平衡(也就是異常)，仔細分析把硫從岩石中溶出並運送數千公里至海洋的河水，卻顯示最終它們含有的硫還是會跟地球整體的硫同位素訊號一致。
「化學訊號的改變可以讓你知道環境中的某些資訊，經由岩石你能瞭解在某個特定的時刻，環境中是否含有氧氣。」Torres表示，「在地球歷史的早期，到處都能看見硫同位素的異常。接著，在大約27億年前，這些異常訊號卻全數消失而且再也沒有出現。」
硫有四種穩定同位素，質量數分別是32、33、34和36，由於它們在大氣中的行為都各自不同，使得硫同位素可以當作一種記號。Torres說：「大部分的硫的質量為32，但還存在著少量質量不同的硫。」
陽光裡的紫外線會跟含硫氣體反應，使其分裂成同位素含量輕重不同的化合物。最後，這些化合物會沉積下來而被保存在當時形成的岩石中。
「然而奇怪的是，我們發現在相當古老的岩石中硫33的含量，比利用它們之間的相對質量得出的預期結果還要高。」Torres表示，「由於硫33的質量比硫32還多1，因此利用物理化學我們可以輕易預測出它們之間的相對豐度。不過，我們卻發現硫33的含量比預期中還要多，所以才稱其為異常值。」
氧氣出現時，它會吸收紫外光而減緩含硫氣體的反應，就像我們在岩石中所看到的。Torres表示這項理論固然不錯，但卻無法解釋為何太古宙之後的岩石沒有出現硫異常。因為從太古宙岩石中持續釋放出來至地表水中的硫異常，應該會運送至海洋然後封存到新形成的岩石當中。
他說：「這種使古代岩石重新循環的作用能讓硫的異常訊號留存許久，即使是在氧濃度提高之後。」之前的研究人員認為異常訊號的延長可以混淆我們認為氧氣開始上升的時間點，幅度可以多達1億年。
他們發現不存在這種作用，但過程並不簡單。此研究團隊包括加州理工學院以及法國南錫岩相學與地球化學研究中心的研究人員。他們從加拿大的研究地點採集大量樣品，並和本來就擁有的南非樣品一起測定它們的硫訊號。過程中他們去除了酸雨中的硫酸、用作融化道路積雪的鹽類、以及當地採礦作業產生的沙塵等作用造成的汙染。最後他們的計算結果顯示河川逕流在流經廣大地區之後，其中匯集的硫33會完全達到平衡。
「我們的成果讓我們可以很有自信地說我們確實得知了大氧化事件的發生時間，因此我們現在可以著手探討其中的機制了。」Torres表示，「如果你從地球歷史的角度來思考，一億年其實相當短暫；但在生物演化的歷程上，一億年可謂舉足輕重。」
Sulfur analysis supports timing of
oxygen’s appearance
River water helps Rice U. scientist support rise of atmospheric oxygen 2.7
billion years ago
Scientists
have long thought oxygen appeared in Earth’s lower atmosphere 2.7 billion years
ago, making life as we know it possible. A Rice University researcher has added
evidence to support that number.
The sulfur record held by ancient rock marks the
dramatic change in the planet’s atmosphere that gave rise to complex life, but
rocks are local indicators. For the big picture, Rice biogeochemist Mark Torres
used water that flows over and erodes the rocks as a proxy.
Torres, a Rice assistant professor of Earth,
environmental and planetary sciences, and his colleagues report in Nature Geoscience that the balance of
sulfur isotope anomalies in Archean rock, a marker of the “great oxygenation
event,” can also be recognized and measured in the rivers that erode it.
The researchers sampled water from two of the few
places on Earth where Archean rock is exposed in abundance: at the Superior
Craton in Canada and in South Africa. They determined that while individual
samples of rock may still show an imbalance (the anomalies) of sulfur isotopes,
careful analysis of the water that diffuses and transports sulfur from
thousands of miles of rock to the ocean shows that the contents are ultimately
in alignment with bulk Earth’s sulfur signature.
“Changes in chemistry can tell you something about
the environment, and rocks can tell you whether there was oxygen at a
particular time,” Torres said. “Early in our history, sulfur isotope anomalies
are all over the place. Then, roughly 2.7 billion years ago, they disappear and
they never come back.”
Sulfur is a marker because four stable isotopes,
known by their molecular masses of 32, 33, 34 and 36, can show different
behaviors when present in the atmosphere. “Most sulfur is mass 32, but there
are small amounts of the other masses,” Torres said.
Ultraviolet light from the sun reacted with sulfur
gas and split it into separate compounds with heavier and lighter isotopes.
Eventually, these compounds sunk into and remain in rock that formed at the
time.
“But there’s this weird thing: Really old rocks have
more 33-sulfur in them than we would expect, based on the relative masses,”
Torres said. “Because 33 is one heavier than 32, we should easily be able to
predict their relative abundances using physical chemistry. But, we find that
33 is way more abundant than expected. That’s why we call it an anomaly.”
When oxygen appeared, it absorbed ultraviolet light
and quenched the sulfur reaction, as seen in the rock. That’s all well and
good, Torres said, but the theory doesn’t account for anomalous sulfur that
continued to leach from Archean rock into surface water, be carried to the
ocean and then condense into new rock that would also have the anomaly.
“This recycling of ancient rock was a way to
perpetuate the anomaly even after oxygen had arisen,” he said. The researchers
suspected persistence of the anomaly could blur understanding of the timing of
oxygen’s rise by as much as 100 million years.
It didn’t, they discovered, but it wasn’t easy. The
team included researchers from the California Institute of Technology and the
Center for Petrographic and Geochemical Research in Nancy, France. Members
collected scores of samples from the Canadian sites to go along with South
African samples they already had and checked their sulfur signature after
eliminating the effects of contaminants from sulfurous acid rain, ice-melting
road salt and dust from local mining operations. But their final calculations
showed a robust balance in 33-sulfur collected by river runoff over a wide
area.
“Our effort allows us to be confident we’ve got the
timing for this great oxidation event, so now we can start to understand the
mechanisms,” Torres said. “If you think about the whole scope of Earth’s
history, 100 million years is small, but on the evolutionary timeline of
organisms, it matters.”
原始論文：Mark
A. Torres, Guillaume Paris, Jess F. Adkins, Woodward W. Fischer. Riverine evidence for isotopic mass balance
in the Earth’s early sulfur cycle. Nature
Geoscience, 2018; DOI: 10.1038/s41561-018-0184-7
引用自：Rice
University. “Sulfur analysis supports timing of oxygen’s appearance”
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