從十幾億年前的湖床沉積物中取得地球古代生物圈的線索
這項發現有助於指引天文學家如何尋找太陽系以外的生命
在加拿大安大略省，科學家從14億年前湖泊蒸發出來的沉積物中揀選出一具含有古代氧氣的樣品。從裡面得到的最新證據顯示動物即將出現的那段時期中，地球的大氣層和生物圈呈現何種樣貌。
這項發表在期刊《自然》(Nature)的最新發現比先前年代最早的大氣氧同位素測量結果還早了將近十億年。結果支持過往研究提出的說法，地球歷史上該時期因為生物圈的生產力遠不如今日，造成大氣氧濃度相比起來也低落許多。
「科學家數十年來皆認為大氣的成分會隨著時間而有很大的變化。」領導這項研究的麥基爾大學博士生Peter
Crockford表示，「我們提出了十分明確的證據顯示14億年前的大氣確實和現在有很大的差異。」
這項研究對地球科學家所謂的「初級生產力」(primary
production)提出了迄今年代最早的測量記錄。初級生產力是食物鏈最底層的生物，像是藻類、藍綠菌之類的生物利用二氧化碳製造有機物，同時排放氧氣至空氣當中的量有多少。
規模較小的生物圈
共同資深作者Boswell
Wing表示：「研究顯示14億年前的初級生產力跟現今相比低落許多。」Wing在麥基爾大學時協助指導Crockford進行研究，現在則為科羅拉多大學波德分校的地質科學副教授，「這意味著全球生物圈的規模必定比今日更小，而且可能無法產生足夠的食物，也就是有機碳，來餵養大量肉眼可見的複雜生物。」
研究過程中，Crockford和耶魯大學、加州大學河濱分校、安大略省雷灣的湖首大學的研究人員合作，從蘇必略湖北邊的沉積岩層中取得了古代鹽類(硫酸鹽)的完整樣品。接著他把樣品帶到路易斯安那州立大學並和該大學的共同作者Huiming
Bao、Justin
Hayles、Yongbo
Peng密切合作來研究這些樣品。Yongbo
Peng的實驗室是世上少數幾個具有專門的質譜儀技術，能夠偵測硫酸鹽中的稀有氧同位素。
這項成果也讓我們對地球歷史上一段漫長的時期――稱為「枯燥的十億年」(boring
billion)有更新的見解，這段時期因為生物與環境沒有出現什麼顯著變化而得其名。
「長久以來研究指出初級生產力大約從20億年前至8億年的元古代中期都處於減弱的狀態，卻一直沒有人能提出硬數據(hard
data，量化的客觀數據)來強力支持此說法。」研究共同作者，麥基爾大學地球與行星科學的副教授Galen
Halverson如此強調，「這使得有其他的可能性可以解釋元古代中期的海洋，在有機碳的生成與沉積這方面如此一成不變的原因。」而Crockford的數據「提出了直接證據指出單調的碳循環是由初級生產力低落造成。」
對地外行星的意義
這些發現也有助於指引天文學家如何尋找太陽系以外的生命。
「地球歷史上大部分時間只有微生物居住，而放眼未來，在我們消逝之後微生物大概會繼續接管地球很長一段時間。」Crockford表示。他現在於普林斯頓大學和以色列魏茨曼科學研究學院擔任博士後研究員。「瞭解微生物塑造出來的環境樣貌，不僅可以告訴我們自身的過往以及我們一路走來的歷程，也能提供線索來讓我們瞭解如果找到適合居住的地外行星時，它可能會是什麼樣子。」
Billion-year-old lake deposit yields
clues to Earth’s ancient biosphere
Finding could help inform astronomers’ search for life outside our solar
system
A sample of
ancient oxygen, teased out of a 1.4 billion-year-old evaporative lake deposit
in Ontario, provides fresh evidence of what the Earth’s atmosphere and
biosphere were like during the interval leading up to the emergence of animal
life.
The findings, published in the journal Nature, represent the oldest measurement
of atmospheric oxygen isotopes by nearly a billion years. The results support
previous research suggesting that oxygen levels in the air during this time in
Earth history were a tiny fraction of what they are today due to a much less
productive biosphere.
“It has been suggested for many decades now that the
composition of the atmosphere has significantly varied through time,” says
Peter Crockford, who led the study as a PhD student at McGill University. “We
provide unambiguous evidence that it was indeed much different 1.4 billion
years ago.”
The study provides the oldest gauge yet of what earth
scientists refer to as “primary production,” in which micro-organisms at the
base of the food chain – algae, cyanobacteria, and the like – produce organic
matter from carbon dioxide and pour oxygen into the air.
A smaller biosphere
“This study shows that primary production 1.4 billion
years ago was much less than today,” says senior co-author Boswell Wing, who
helped supervise Crockford’s work at McGill. “This means that the size of the
global biosphere had to be smaller, and likely just didn’t yield enough food –
organic carbon – to support a lot of complex macroscopic life,” says Wing, now
an associate professor of geological sciences at University of Colorado at
Boulder.
To come up with these findings, Crockford teamed up
with colleagues from Yale University, University of California Riverside, and
Lakehead University in Thunder Bay, Ontario, who had collected pristine samples
of ancient salts, known as sulfates, found in a sedimentary rock formation
north of Lake Superior. Crockford shuttled the samples to Louisiana State
University, where he worked closely with co-authors Huiming Bao, Justin Hayles,
and Yongbo Peng, whose lab is one of a handful in the world using a specialized
mass-spectrometry technique capable of probing such materials for rare oxygen
isotopes within sulfates.
The work also sheds new light on a stretch of Earth’s
history known as the “boring billion” because it yielded little apparent
biological or environmental change.
“Subdued primary productivity during the
mid-Proterozoic era – roughly 2 billion to 800 million years ago – has long
been implied, but no hard data had been generated to lend strong support to
this idea,” notes Galen Halverson, a co-author of the study and associate
professor of earth and planetary sciences at McGill. “That left open the
possibility that there was another explanation for why the middle Proterozoic
ocean was so uninteresting, in terms of the production and deposit of organic
carbon.” Crockford’s data “provide the direct evidence that this boring carbon
cycle was due to low primary productivity.”
Exoplanet clues
The findings could also help inform astronomers’
search for life outside our own solar system.
“For most of Earth history our planet was populated
with microbes, and projecting into the future they will likely be the stewards of
the planet long after we are gone,” says Crockford, now a postdoctoral
researcher at Princeton University and Israel’s Weizmann Institute of Science.
“Understanding the environments they shape not only informs us of our own past
and how we got here, but also provides clues to what we might find if we
discover an inhabited exoplanet.”
原始論文：Peter W.
Crockford, Justin A. Hayles, Huiming Bao, Noah J. Planavsky, Andrey Bekker,
Philip W. Fralick, Galen P. Halverson, Thi Hao Bui, Yongbo Peng, Boswell A.
Wing. Triple oxygen isotope evidence for limited mid-Proterozoic
primary productivity. Nature, 2018; DOI: 10.1038/s41586-018-0349-y
引用自：McGill University. "Billion-year-old lake
deposit yields clues to Earth's ancient biosphere”
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