早期地球是個「水世界」？地質學家研究露出地表的海洋地殼之後如此認為

原文網址：https://www.news.iastate.edu/news/2020/02/27/waterworld

早期地球是個「水世界」？地質學家研究露出地表的海洋地殼之後如此認為

現在的澳洲有露出地表的遠古海洋地殼。地質學家分析它們的氧同位素數據之後，認為32億年前的地球是個陸地沉在水中的「水世界」。

愛荷華州立大學的Benjamin Johnson正在偏遠的澳洲西部調查一座露頭，此處經地質學家研究為32億年前的海洋地殼。圖片來源：由Jana Meixnerova攝影並由Benjamin Johnson提供

這對於生命起源來說具有重大意義。

在甫刊登於期刊《自然―地球科學》(Nature Geoscience)線上版的論文中，地質學家Benjamin Johnson和Boswell Wing寫道：「早期地球可能沒有陸地浮出海洋而類似於『水世界』，對於地球生命的起源以及演化來說是相當重要的環境限制，就其他有生命存在的世界來說可能也是如此。」

Johnson最近剛結束在科羅拉多大學波德分校的博士後研究，現於愛荷華州立大學擔任地質與大氣科學的助理教授；Wing則是科學拉多大學的地質科學副教授。這項研究的經費來自美國國家科學基金會；Johnson在澳洲的野外調查費用則來自於美國哲學會的路易斯與克拉克獎學金。

Johnson說他們這項計畫的起源為他和Wing在一場會議中的談話，從中他得知澳洲西部某個偏遠地區具有32億年前太古宙(40億至25億年前)的海洋地殼，而且保存狀況十分良好。先前的研究已經顯示該地就像是一座大型圖書館，含有許多地球化學的資料。

Johnson加入Wing的研究團隊並在2018年親自去調查這塊海洋地殼。這趟旅程得先飛往伯斯，然後再往北開17個小時的車到黑德蘭港附近的海岸地帶。

Johnson取得自己的岩石樣品並且探究現有數據的文獻之後，製作了一幅剖面網格顯示此岩石的氧同位素值及溫度如何分布。

(同位素是屬於同一種化學元素的原子，它們的原子核內質子數相同但中子數卻不同。在本研究中，古代岩石保存的氧同位素差異可以告訴我們數十億年前岩石和水之間的交互作用如何進行。)

在以全岩數據得出二維網格之後，Johnson建立反演模型來估計古代海洋裡的氧同位素。結果為古代海水裡氧的重同位素(有八個原子和十個中子的氧原子，寫作¹⁸O)比現在沒有冰的海水中還高出大約千分之四。

隨著時間經過重同位素越來越少的現象該如何解釋？

Johnson和Wing提出了兩種可能方法：首先是跟海水現今在海洋地殼中循環的方式不同，古時候的循環過程可能涉及更多的高溫作用，造成海洋裡富含氧的重同位素。或者是水流經陸地的岩石之後會讓海水裡重同位素的比例降低。

「我們偏好的理論，而且在某種程度上來說也是最簡單的，是大約在32億年前陸地上的風化作用開始進行，造成海水裡的重同位素跟著減少。」

海水在海洋地殼裡的循環方式跟現在有不同之處，造成同位素出現差異的說法「無法受到岩石證據支持」。Johnson說：「我們研究的海洋地殼在32億年前的段落看起來和年輕許多的海洋地殼非常相似。」

Johnson表示這項研究呈現出地質學家可以透過建立模型，找出新的定量化方法來解決問題——即使這項問題是和32億年前，我們永遠無法看見或採集樣品的海水有關。

Johnson接著表示模型也告訴我們生物起源並且演化時的環境資訊：「沒有大陸或者陸地高出海平面的情況下，能讓最初的生態系演化出來的場所勢必只有海洋而已。」

Geologists determine early Earth was a ‘water world’ by studying exposed ocean crust

The Earth of 3.2 billion years ago was a “water world” of submerged continents, geologists say after analyzing oxygen isotope data from ancient ocean crust that’s now exposed on land in Australia.

And that could have major implications on the origin of life.

“An early Earth without emergent continents may have resembled a ‘water world,’ providing an important environmental constraint on the origin and evolution of life on Earth as well as its possible existence elsewhere,” geologists Benjamin Johnson and Boswell Wing wrote in a paper just published online by the journal Nature Geoscience.

Johnson is an assistant professor of geological and atmospheric sciences at Iowa State University and a recent postdoctoral research associate at the University of Colorado Boulder. Wing is an associate professor of geological sciences at Colorado. Grants from the National Science Foundation supported their study and a Lewis and Clark Grant from the American Philosophical Society supported Johnson’s fieldwork in Australia.

Johnson said his work on the project started when he talked with Wing at conferences and learned about the well-preserved, 3.2-billion-year-old ocean crust from the Archaean eon (4 billion to 2.5 billion years ago) in a remote part of the state of Western Australia. Previous studies meant there was already a big library of geochemical data from the site.

Johnson joined Wing’s research group and went to see ocean crust for himself – a 2018 trip involving a flight to Perth and a 17-hour drive north to the coastal region near Port Hedland.

After taking his own rock samples and digging into the library of existing data, Johnson created a cross-section grid of the oxygen isotope and temperature values found in the rock.

(Isotopes are atoms of a chemical element with the same number of protons within the nucleus, but differing numbers of neutrons. In this case, differences in oxygen isotopes preserved with the ancient rock provide clues about the interaction of rock and water billions of years ago.)

Once he had two-dimensional grids based on whole-rock data, Johnson created an inverse model to come up with estimates of the oxygen isotopes within the ancient oceans. The result: Ancient seawater was enriched with about 4 parts per thousand more of a heavy isotope of oxygen (oxygen with eight protons and 10 neutrons, written as ¹⁸O) than an ice-free ocean of today.

How to explain that decrease in heavy isotopes over time?

Johnson and Wing suggest two possible ways: Water cycling through the ancient ocean crust was different than today’s seawater with a lot more high-temperature interactions that could have enriched the ocean with the heavy isotopes of oxygen. Or, water cycling from continental rock could have reduced the percentage of heavy isotopes in ocean water.

“Our preferred hypothesis – and in some ways the simplest – is that continental weathering from land began sometime after 3.2 billion years ago and began to draw down the amount of heavy isotopes in the ocean,” Johnson said.

The idea that water cycling through ocean crust in a way distinct from how it happens today, causing the difference in isotope composition “is not supported by the rocks,” Johnson said. “The 3.2-billion-year-old section of ocean crust we studied looks exactly like much, much younger ocean crust.”

Johnson said the study demonstrates that geologists can build models and find new, quantitative ways to solve a problem – even when that problem involves seawater from 3.2 billion years ago that they’ll never see or sample.

And, Johnson said these models inform us about the environment where life originated and evolved: “Without continents and land above sea level, the only place for the very first ecosystems to evolve would have been in the ocean.”

原始論文：Benjamin W. Johnson & Boswell A. Wing. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, 2020 DOI: 10.1038/s41561-020-0538-9

引用自：Iowa State University. "Geologists determine early Earth was a 'water world' by studying exposed ocean crust."