經常用來追溯地球氧含量歷史演變的工具可能會給出偽陽性結果
對於追尋地球大氣中的氧氣最初如何演變的研究人員來說，一項新研究可能會讓他們失望地大喊「真的嗎？」。根據此研究，一種檢驗古代岩層以得到氧氣含量的現行工具會產生偽陽性結果，而這種隨機性可能會讓科學家誤以為他們得到的發現十分驚人。
研究人員時常利用一種稱作鉻同位素系統的化學示蹤劑來檢驗沉積岩層，以得到岩石形成時大氣氧含量的相關線索，但一種稱作配位基的常見分子卻會使結果產生偏差。喬治亞理工學院的研究人員在實驗室證明許多種配位基產生的訊號跟氧分子產生的十分相似。
研究主要作者之一Chris Reinhard表示：「在某些地理位置和古代環境所測得的訊號值，產生原因可能跟周遭的氧氣多寡沒有任何關係。」雖然近期某些發現所用的估算方式可能會受此新研究影響，但不表示這項工具變得全然無用。
岩石的紀錄工具
「我們並非在嘗試徹底顛覆人們對這項工具的評價。」同為研究主要作者的Yuanzhi Tang表示。「我們是在瞭解它的潛在限制而讓它在特定情況下可以當作具有鑑別力的工具。」
Tang和Reinhard皆為喬治亞理工大學地球和大氣科學院的生物地球化學助理教授，他們團隊的研究成果刊登在2017年11月17日期刊《自然通訊》(Nature
Communications)的論文之中。資助他們研究的單位包括NASA天體生物學研究所、NASA地外生物學計畫以及艾古隆研究所。
「從全球層級來看，鉻同位素系統仍然是指示各個年代大氣氧含量的良好指標。」Tang表示。「而我們在實驗室呈現的議題則攸關局部地區的個別樣本，特別是在大氣氧含量還很低的年代所形成的岩石。」
活躍的配位基
研究人員從和鉻有關的化學反應中證實若環境中的氧氣不多，配位基可能會取代氧氣成為相當活躍的反應物。因為配位基這種化學族的特徵跟氧氣類似，非常容易吸引電子對。
就像跟氧氣發生反應一樣，類似鉻的金屬與配位基發生反應之後可以更容易地遷移到世界各處。研究人員在此研究中的關注重點是有機配位基，也就是含有碳的配位基。
鉻的遷移能力會在沉積岩中留下訊號，成為今日科學家探討古代大氣氧含量時用的指標。而此篇研究的作者即著重於比較氧氣和有機配位基對鉻遷移能力的影響。
接著是鉻同位素系統運作方法的大略介紹，然後是有機配位基如何產生偽陽性結果。
載運鉻的高速列車
在地球這座巨大的化學實驗室中進行的化學反應涵蓋了各種環境條件，從極地的嚴寒至火山的熾熱，從海洋深處的極大壓力至大氣上層的毫無壓力。風吹水流像是繁忙而紊亂的輸送帶將不同物質帶到世界各處，其中有些會落腳在沉積物當中並在之後轉變成岩石。
鉻要搭上通往沉積物的高速列車通常需要氧化劑這張票卷，它可以提高鉻的溶解程度使得鉻更容易跟著水流移動，而大氣中的氧氣即為理想氧化劑。此研究提及的反應有點像是替含鉻化合物裝上浮筒一樣，而可以把氧轉手給鉻的錳氧化物也參與其中。
地球最初數十億年大氣處於幾乎無氧的狀態，但氧氣開始增加之後氧氣就變成了主要的氧化劑，尤其是在最近8億年。因此，古代岩層中含鉻沉積物的性質也成為當時大氣含有多少氧氣的重要指標。
今日的研究人員測量遠古岩層樣品中兩種鉻同位素的關係，以參透氧氣在整部地質歷史中的蹤跡。這兩種同位素分別是52Cr(目前世上最多的鉻同位素)和53Cr。
「將岩石磨碎後用酸溶解，接著就能用質譜儀測量樣品中52Cr和53Cr的比例。」Reinhard表示。「我們關注的便是它們之間的比例關係，此數值會受控於許多複雜的作用；但整體來說，海洋沉積岩中的53Cr含量升高通常意謂大氣出現了氧氣。」
順道一提，這些鉻同位素為穩定同位素，不會發生放射性衰變。故此系統的運作方式和依據碳14衰變的放射性碳定年法不同。
化學裡的騙徒
Tang的團隊在實驗室利用少數幾種類型的有機配位基，顯示鉻和配位基反應過後產生的53Cr/52Cr訊號跟源自於氧氣―鉻化學反應的訊號十分相像。
Tang表示：「配位基同樣具有讓鉻遷移的能力。事實上，在某些岩石紀錄中，配位基或許會是操控鉻同位素訊號的重要因子。」
在地球大氣充滿氧氣許久之前，有機配位基可能就已經出現在世上許多地方了。而在化學反應發生過後數億年的今天，我們基本上是無法得知當時作用的究竟是氧氣還是配位基。
毫微之差
如果沒有考慮到配位基參與的反應，可能會因此誤判岩石紀錄中有關大氣氧含量的微小細節。事實上，這可能已經發生了。
就像分門別類古代動物骸骨和其他化石的古生物學家，地質學家也保有數量龐大且數位化的岩石檔案庫，並研究它們以更加瞭解地球古老的地質歷史。大約2009年開始，科學家開始以鉻同位素系統檢驗這些岩石的原始樣品，並將結果納入文獻紀錄當中。
「之後，某些歧異開始浮現出來。」Reinhard表示。「在氧氣還不該出現的時候，古代的土壤層卻呈現出氧氣存在的證據，而同一時期的其他樣品卻沒有出現該訊號。」
但面對這類特殊鉻訊號的某些研究人員卻認為他們可能偶然之間得到了基礎層面的發現，並推論出一套解釋認為當氧分子在全球各處還很稀少的時候，形成這些特殊岩層的局部點位可能具有含量十分驚人的氧氣。另外一些人則思索在遍及全球的證據出現許久之前，大氣氧濃度或許就已經開始攀升。
Reinhard表示：「這些證據記下的可能是其他化學作用，而非氧氣造成的反應。」
此研究或許警惕了我們要審慎看待鉻同位素的紀錄，特別是當它們看起來格外誘人的時候。
A popular tool to trace Earth’s oxygen history can
give false positives
For researchers pursuing the primordial history of oxygen in
Earth’s atmosphere, a new study might sour some “Eureka!” moments. A
contemporary tool used to trace oxygen by examining ancient rock
strata can produce false positives, according to the study, and the
wayward results can mask as exhilarating discoveries.
Common molecules
called ligands can bias the results of a popular chemical tracer
called the chromium (Cr) isotope system, which is used to test sedimentary rock
layers for clues about atmospheric oxygen levels during the epoch when the rock
formed. Researchers at the Georgia Institute of Technology have demonstrated in
the lab that many ligands could have created a signal very similar to that of
molecular oxygen.
“There are some
geographical locations and ancient situations where measurable signals could
have been generated that had nothing to do with how much oxygen was around,”
said Chris Reinhard, one of the study’s lead authors. Though the new
research may impact how some recent findings are assessed, that doesn’t mean the
tool isn’t useful overall.
Rock record tool
“We’re not trying to
revolutionize the way the tool is viewed,” said Yuanzhi Tang, who co-led
the study. “This is about understanding its possible limitations to make
discerning use of it in particular cases.”
Tang and Reinhard, both
assistant professors of biogeochemistry in Georgia Tech’s School of Earth and
Atmospheric Sciences, published their team’s results in a study on
November 17, 2017, in the journal Nature
Communications. Their
work was funded by the NASA Astrobiology Institute, the NASA Exobiology
program, and the Agouron Institute.
“On a global level, the
chromium isotope system is still a great indicator of atmospheric oxygen levels
through the ages,” Tang said. “The issue we exposed in the lab is more local
with isolated samples, especially during eras when there wasn’t much
atmospheric oxygen.”
Leaping ligands
Without a dominant oxygen
presence, ligands likely made a great reactive substitute, as the
researchers demonstrated in reactions with chromium. Like oxygen, ligands
strongly attract electron pairs, which is what characterizes them as a chemical
group.
And like reactions with
oxygen, reactions with ligands enable metals like chromium to move around more
easily in the world. In this case, the researchers were interested in organic
ligands, ligands that contain carbon.
They were more apt to match
oxygen’s mobility effect on chromium that made it end up as the signals in
sedimentary rock that scientists, today, look for as a sign of ancient
atmospheric oxygen.
Here’s roughly how the
chromium isotope system works, followed by how organic ligands could make for
false positives.
Chromium rollercoaster
The Earth is an enormous
chemical laboratory performing reactions in conditions varying from arctic cold
to volcanic heat, and from crushing ocean depths to no-pressure upper
atmospheres. Winds and waves sweep around materials like turbulent conveyor
belts, depositing some in sediments that later turn to stone.
Chromium’s ticket for the
rollercoaster ride into sedimentary rock was usually an oxidizing agent that
made it more soluble and better able to float, and atmospheric oxygen was
an ideal oxidizer. The chemical reaction, which can be found in the
study and involved manganese oxide handing off oxygens to chromium, would
be a little like adding pontoons to chromium compounds.
For billions of years,
Earth’s atmosphere was nearly devoid of O, but after oxygen began increasing,
especially in the last 800 million years, it became the domineering
oxidizer. And characteristics of chromium deposits in ancient layers of rock
became a great indicator of how much O2 was in the atmosphere.
Today, researchers test
deep rock layer samples for the relation between two chromium isotopes, 52Cr,
by far the most common Cr isotope, and 53Cr, to get a read on
oxygen presence across geological eras.
“You powder the rock up;
you dissolve it with acid, and then you measure the ratio of 53Cr
to 52Cr in the material by using mass spectrometry,” Reinhard
said. “It’s the ratio that matters, and it will be controlled by a range of
complex processes, but generally speaking, elevated 53Cr in
ocean sediment rock tends to indicate oxygen in the atmosphere.”
By the way, these Cr
isotopes are stable and don’t undergo radioactive decay, thus the system does
not work the way radiocarbon dating does, which relies on the decay of
carbon 14.
Chemical imposter
In the lab, with a small
assortment of organic ligands, Tang’s group showed that reactions of chromium
with ligands led to 53Cr/52Cr signals that closely
mimicked those stemming from oxygen-chromium reactions.
“Ligands have the
capability to mobilize chromium as well,” Tang said. “In fact, ligands might be
a significant factor in controlling chromium isotope signals in certain rock
records.”
Organic ligands were
probably around long before Earth’s atmosphere filled up with O2.
And today, hundreds of millions of years after the reactions occurred, it’s
basically impossible to find out if oxygen or ligands were at work.
Little discrepancies
If not accounted for,
ligand reactions can distort small details in rock records about atmospheric
oxygen, and they may have already.
Like paleontologists, who
catalog ancient animal bones and other fossils, geologists keep massive,
digitized archives of rock that they study to learn more about Earth’s ancient
geological history. Scientists began testing physical samples of them with the
Cr isotope system around 2009 and adding the results to the records.
“Since then, some
discrepancies have turned up,” Reinhard said. “Ancient soil layers were showing
evidence of oxygen when it probably shouldn’t have been there. Other samples
from the same period weren’t showing that signal.”
But some researchers
confronted with odd Cr signals have thought they had perhaps stumbled upon a
radical find, and they developed explanations for how O2 may have been
surprisingly bountiful on the lonesome spot where a particular rock layer
formed, while molecular oxygen was scant on the rest of the globe. Others
puzzled that atmospheric O2 levels may have risen much earlier
than overwhelmingly broad evidence has indicated.
“A lot of that could be
chalked up to other chemical processes and not to interactions with oxygen,”
Reinhard said.
The study may serve as a
cautionary tale about how to view Cr isotope data, especially when they leap
off the page.
原始論文：Emily M. Saad,
Xiangli Wang, Noah J. Planavsky, Christopher T. Reinhard, Yuanzhi Tang. Redox-independent
chromium isotope fractionation induced by ligand-promoted dissolution. Nature
Communications, 2017; 8 (1) DOI: 10.1038/s41467-017-01694-y
引用自：Georgia Institute of Technology. "A
popular tool to trace Earth's oxygen history can give false
positives."
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