太古宙的氧氣綠洲
Maya L. Gomes
在大氣氧濃度呈兩階段上升的過程中，第一階段發生在太古宙結束之際。分析黃鐵礦的硫和鐵同位素，顯示太古宙中期的沿岸環境有個地方處於局部氧化的狀態。
大約在25億年前，地球大氣的氧濃度突然開始飆升。此次「大氧化事件」(Great Oxidation Event)造成地表的生態系從無氧為主轉變成有氧狀態，最終使地球演化出複雜的生命形式。藍綠菌是此次變化的推手之一，這種細菌演化出的能力可以把水當作電子供應者來獲得能量，並釋放出副產物――氧氣。但是科學家仍不清楚大氧化事件是在產氧光合作用演化出來之後立刻發生，或者是一段時間之後才因為其他演化革新或是環境因素而引發。Eickmann等人在《自然―地質科學》(Nature
Geoscience)撰寫的論文中，呈現的證據指出大約在30億年前的太古宙中期，海洋近岸地區有座氧氣綠洲。意謂藍綠菌演化出產氧光合作用之後過一段時間大氣才開始出現大規模的氧化現象。
由於太古宙中期(32億至28億年前)位於大氧化事件發生之前，所以一般認為當時的大氣幾乎是處在無氧狀態。在無氧大氣下，光化學反應可以造成跟質量無關的硫同位素分餾。因此，老於24.5億年前的岩石中保有跟質量無關的硫同位素訊號而之後的岩石卻沒有，通常被視為大氧化事件時氧氣濃度上升的決定性證據。然而，跟質量無關的硫訊號卻在太古宙中期沉寂下來而跟前述現象形成矛盾。這可能是因為當時大氣光化學作用有所改變或者是被跟質量相關的硫同位素訊號淡化所導致。造成後者的可能嫌犯之一是微生物進行的硫酸鹽還原作用，它產生的硫會帶有跟質量相關的訊號。因此，有人認為大氣氧濃度上升加上海洋硫酸鹽增加，造成微生物硫酸鹽還原作用產生的硫增加，導致跟質量無關的硫同位素訊號在當時減弱。但是，以往缺乏證據可以指出太古宙中期有發生跟質量相關的分餾作用，使得這項解釋的說服力不足。
Eickmann和其同事決心要解決此矛盾。他們呈現的硫同位素數據來自南非沉積岩中的黃鐵礦，其形成於30億年前卡普瓦克拉通(Kaapvaal Craton)上受到潮汐影響的淺海環境。數據中的黃鐵礦有兩種不同類型：全岩樣品磨碎後取出的浸染狀黃鐵礦細粒，以及用微鑽孔得到的黃鐵礦結核樣品。浸染狀黃鐵礦細粒跟過往研究一致，並沒有確切證據顯示曾經發生過跟質量相關的硫同位素分餾作用。相較之下，黃鐵礦結核的硫同位素訊號卻指示了在富含硫酸鹽的環境下，由微生物進行的跟質量相關的硫同位素分餾作用。相當重要的一點是，兩種類型的黃鐵礦都含有跟質量無關的硫同位素訊號，表示它們是在幾乎無氧的大氣中沉積而成。因此，當時必定有一座維持高硫酸鹽濃度的氧氣綠洲，使得黃鐵礦結核擁有跟質量相關的硫同位素訊號。
為了進一步探討太古宙中期淺海環境氧化還原條件的不均情形以及生物地球化學循環，Eickmann和其同事也分析了鐵同位素。他們指出相較於元古宙中期從熱泉進入深海的鐵，黃鐵礦結核的鐵缺乏重同位素。來自熱泉的鐵在上湧過程中重同位素會逐漸減少，這是因為氧化反應以及後續的沉澱作用所造成(經由微生物參與的作用或者有自由氧的情況下發生的非生物作用)。Eickmann等人假設剩餘的鐵到達近岸環境時都已經完全氧化，因此含有較輕的鐵同位素訊號。而這些鐵之後在沉積物內部被還原時，就成為了黃鐵礦結核的原料。
要把硫和鐵同位素訊號解釋成局部地區處於氧化環境，所仰賴的前提是黃鐵礦形成於成岩作用的早期，因此仍保有當時近岸環境的資訊。作者提出地球化學和組構證據來佐證這些黃鐵礦結核跟其他太古宙的黃鐵礦結核研究類似，都顯示了原生環境的訊號。如果此篇研究中的黃鐵礦結核是由沉積物裡的浸染狀黃鐵礦在成岩階段早期溶解而形成，解開兩者之間的硫同位素訊號為何不一樣，或許可以提供額外資訊讓科學家瞭解元古宙中期海洋氧化還原條件不均的情形。
Eickmann和其同事雖然讓我們更加瞭解太古宙中期為何跟質量無關的硫同位素訊號會沉寂下來，但是氧氣綠洲的解釋本質上來說仍然是出現在局部地區的現象。從這4億年之間堆積的沉積物中得到的數據，都可以發現硫同位素訊號有沉寂下來的情形。因此，它和氧氣綠洲這種局部現象之間的關連仍然是有待解決的難題。此外，在太古宙中期之後的太古宙晚期，如果陸上黃鐵礦持續跟氧氣作用而風化，使得海洋繼續累積硫酸鹽，那麼跟質量無關的硫同位素訊號應該會越來越淡。但實際上，太古宙晚期跟質量無關的硫同位素訊號卻比較強。這或許暗示了在大氧化事件發生之前，太古宙晚期的氧氣濃度曾一度下降過。或者是硫循環內部還有別種未知作用，能改變大氣的光化學反應而強化跟質量無關的硫同位素訊號。
太古宙結束之際為何大氣氧濃度會快速上升仍然是個謎團。已經出現的藍綠菌也許有什麼生物學上的革新，使得它們可以進行更大規模的產氧光合作用，造成大氣中的自由氧快速累積。或者是環境變遷，比方說全球被冰河覆蓋，也有可能改變生物地球化學循環的狀態，使得平衡往大氣氧濃度較高的那側傾斜。不管是透過哪種方式，這些理論都還需要岩石紀錄來驗證。而Eickmann和其同事提供了相當重要的一片線索，在未來解決大氧化事件的謎題時勢必能派上用場。
太古宙硫同位素的紀錄有許多細微差異。Eickmann和其同事提出的地球化學證據雖然並未完全剔除藍綠菌對大氧化事件的重要性，但確實進一步指出在大氧化事件發生許久之前，會釋放氧氣的藍綠菌就已經出現在地球且相當活躍了。
An Archaean oxygen oasis
The first of two
stepwise increases in atmospheric oxygen occurred at the end of the Archaean
eon. Analyses of sulfur and iron isotopes in pyrite reveal a near-shore environment
that hosted locally oxygenated conditions in the Mesoarchaean era.
About
2.5 billion years ago, oxygen concentrations in the Earth’s atmosphere rose
sharply. This Great Oxidation Event1,2 caused a transition from dominantly anaerobic to
aerobic surface ecosystems, and ultimately led to the evolution of complex life
on Earth. The agents of change were cyanobacteria, a type of bacteria that
evolved the ability to gain energy using water as an electron donor, and
release oxygen as a by-product. But it is not clear whether the Great Oxidation
Event immediately followed the evolution of oxygenic photosynthesis or was
initiated later by another evolutionary innovation or environmental driver.
Writing in Nature Geoscience, Eickmann et al.3 present geochemical evidence for the
existence of a near-shore marine oxygen oasis about 3 billion years ago, in the
Mesoarchaean era. This suggests a delay between the evolution of oxygenic
photosynthesis by cyanobacteria and the later widespread oxygenation of the
atmosphere.
As the
Mesoarchaean (3.2–2.8 billion years ago) falls before the Great Oxidation
Event, atmospheric conditions are thought to have been mostly anoxic. Under an
anoxic atmosphere, photochemical reactions can fractionate sulfur isotopes
independent of mass. Thus, the change from mass-independent sulfur isotopic
signatures preserved in older rocks to their absence in rocks younger than 2.45
billion years old is considered to be conclusive proof of the rise of oxygen at
the Great Oxidation Event1. However, the mass-independent sulfur
signal is muted during the Mesoarchaean, presenting a conundrum. This muted
signal could be due to differences in atmospheric photochemistry4or dilution of the signal by sulfur with a mass-dependent
signal. A likely culprit for the latter is the contribution of sulfur with a
mass-dependent signal from microbial sulfate reduction. This requires high
sulfate levels generated by enhanced oxidative weathering on land under an
oxygenated atmosphere. But lack of evidence for mass-dependent fractionations
in the Mesoarchaean has previously hampered the interpretation that
mass-independent signatures were dampened by inputs from microbial sulfate
reduction under elevated atmospheric oxygen and marine sulfate levels.
Eickmann
and colleagues3 aim to resolve this conundrum. They
present sulfur isotope data from pyrites in sediments from South Africa that
were deposited 3 billion years ago in a tide-influenced, shallow-marine
environment on the Kaapvaal Craton. The data come from two distinct types of
pyrites: finely disseminated pyrite from powdered whole-rock samples and pyrite
nodules that were sampled by micro-drilling. The finely disseminated pyrites
show no conclusive evidence of mass-dependent sulfur isotope fractionation, in
line with previous studies. In contrast, sulfur isotope signatures from the
pyrite nodules are indicative of mass-dependent sulfur fractionation by
microbial sulfate reduction in a sulfate-rich environment. Importantly, both
types of pyrite carry mass-independent sulfur isotope signals indicating that
they were deposited under a largely anoxic atmosphere. Thus, a localized oxygen
oasis must have maintained sufficiently high sulfate levels to impart the
mass-dependent sulfur isotope signals in the pyrite nodules.
To
further explore redox heterogeneity and biogeochemical cycling in this
Mesoarchaean shallow-marine environment, Eickmann and colleagues also analyse
iron isotopes. They show that the pyrite nodules are depleted in the heavy iron
isotope relative to iron that entered the Mesoarchaean deep sea from
hydrothermal vents. The hydrothermally sourced iron became progressively
depleted during upwelling owing to oxidation and subsequent precipitation,
either by microbially mediated reactions or abiotic reactions in the presence
of free oxygen. Eickmann et al. hypothesize that the residual iron reaching the
near-shore environment was completely oxidized, inheriting the light isotope
signal, and was the source of iron for pyrite nodule formation after subsequent
reduction in the sediment.
The
interpretation that the sulfur and iron isotopes indicate locally oxidized
conditions relies on the assumption that the pyrite nodules were formed during
early diagenesis and thus preserve information about the near-shore
environment. The authors present geochemical and textural evidence in support
of a primary environmental signal, similar to other Archaean pyrite nodule
studies5. Understanding why the nodule pyrites in
this study have different sulfur isotope signals from the disseminated pyrites
if they formed from dissolution of disseminated pyrite in the sediments during
early diagenesis6 may provide additional information
about redox heterogeneity in Mesoarchaean oceans.
Although
Eickmann and colleagues advance our understanding of the muted mass-independent
signal in the Mesoarchaean, the oxygen oasis interpretation is, by nature, a
localized condition. Thus, a challenge remains to explain how these local
conditions might be related to the muted sulfur signature that is found in all
the data generated from sediments deposited over approximately 400 million
years. Further, after the Mesoarchaean — in the Neoarchaean — the
mass-independent sulfur signal should continue to be increasingly diluted as
sulfate accumulates in the ocean through oxidative weathering of terrestrial
pyrite. But, in fact, the mass-independent signal is stronger in the
Neoarchaean4. This could imply that the oxygen levels decreased in
the Neoarchaean before the Great Oxidation Event. Alternatively, there may be
additional processes, not yet recognized, of sulfur cycling associated with
changing atmospheric photochemistry that strengthens the mass-independent
signal.
The
cause of the rapid rise of atmospheric oxygen at the end of the Archaean
remains enigmatic. Biological innovations by existing cyanobacteria could have
led to an expansion of their oxygenic photosynthetic activity and the
accumulation of free oxygen in the atmosphere7. Or environmental changes, such as a
global glaciation, could have led to a state change in biogeochemical cycling
that tipped the scale towards higher atmospheric oxygen levels8. Either way, these hypotheses need to be
tested in the rock record, and Eickmann and colleagues contribute an important
piece of evidence that will be used to solve the mystery of the Great Oxidation
Event.
There
are many nuances in the sulfur isotope record of the Archaean. Although not
necessarily absolving the cyanobacteria of responsibility, Eickmann and
colleagues3provide further geochemical evidence to
suggest that oxygen releasing cyanobacteria were present and active long before
the Great Oxidation Event.
原始論文：Benjamin Eickmann, Axel Hofmann, Martin
Wille, Thi Hao Bui, Boswell A. Wing & Ronny Schoenberg. Isotopic evidence for oxygenated Mesoarchaean shallow oceans. Nature Geoscience, 2018; Doi:10.1038/s41561-017-0036-x
引用自：Maya L. Gomes. An Archaean oxygen oasis. Nature
Geoscience, 2018.
