從陸地侵蝕而來的物質使古代海洋中的生命得以生存

原文網址：www.sciencedaily.com/releases/2016/09/160926221617.htm

從陸地侵蝕而來的物質使古代海洋中的生命得以生存

隨著科學家陸續找到30億年前海洋已經有生命存在的證據，這些遠古化石卻也造成了一種矛盾。包括於如此早期的海洋中生活的單細胞細菌在內，生物都需要穩定供給磷才能生存，但是「除非有從陸地上侵蝕而來的磷，否則很難說明它們的來源。」威斯康辛大學麥迪遜分校地質科學系的Aaron Satkoski表示。「因此要解釋我們在如此久遠的時代中找到的化石是相當困難的。」

Satkoski是對此遙遠年代的海洋化學進行研究的第一作者。他說地質科學界一般預期當時的地球是顆海洋行星，僅有少數陸地矗立於海浪之上，甚至根本就沒有。「回到1960年代，這時人們基於諸多因素而宣稱當時地球幾乎沒有任何陸地，因此風化作用不足以影響海洋化學。然而，並沒有多少顯示30億年前情況如何的實際數據來支持上述說法。」

Satkoski說發現超過30億年前由細菌遺骸形成的化石改變了這番情節。「但如果當時海洋中已有生命存在，就得發生某種程度的陸地風化作用，才能供應支持生物存活所需的磷。」

當今海洋化學成分的主要影響因子為來自熱泉的流體(循環至地殼而產生的熱水)，以及地表風化作用(從陸地侵蝕下來的物質經由河水運輸至海洋)。

為了估計32.6億年前兩種因素個別影響多少，地球化學教授Clark Johnson和Satkoski從南非採集形成於當時的樣品，並且比較兩種不同形式的重晶石礦物中同位素的差異。其中膠結在一起的顆粒是在水中分別形成，之後沉澱至海床時才彼此黏合。

而立方體或者片狀的重晶石則是直接於海床形成。Johnson、Satkoski及其同僚Brian Beard推測顆粒狀重晶石反映了海水化學成分，因此也可看出是否有任何從陸地侵蝕而來的物質。另一方面，片狀重晶石則代表了海水化學跟熱泉流體混和後的結果。這篇研究的關鍵為精確測量兩種重晶石的同位素組成。所謂同位素是指化學性質相同但質量不同的原子。

著極其微小―但仍相當顯著的差異，意味結果顯示它們的同位素比例之間有著粒狀重晶石確實是由陸地侵蝕而來的物質生成。也就是說，32.6億年前就已經有一定程度的侵蝕作用發生了。

這項甫發表於《地球和行星科學通訊》(Earth and Planetary Science Letters)的論文將確實有大規模陸地侵蝕作用發生的時間點提前了4億年。

「雖然對於當時地球表面有多少比例是陸地不過是猜測，但我們認為可能有高達今日陸地面積的三分之二。」Johnson表示。他是威斯康辛大學NASA天體生物學研究所的負責人。「過往有些研究估計當時完全沒有一塊陸地存在。」

「科學家在思考海洋化學時總是會把焦點放在熱泉流體上，但卻很少有數據可以用來研究它的影響，」Johnson說。「我們正嘗試將某些數據納入整個過程當中。」

這些關於陸地的研究跟來自火成岩的證據一致。從高熱融化狀態岩石中得到的證據指出，那段時期地表已經足夠堅硬而足以支撐山脈形成，使得侵蝕作用可以發生。「既然我們對當時情況擁有更趨完整的理解，整個故事也會更加完備。」Satkoski說。

這項結果也跟氣候方面的數據互相吻合，因為大氣中二氧化碳濃度上升會導致陸地風化作用更加強烈。Satkoski說雖然太陽當時相對來說溫度較低，但海洋卻未凍結。「這代表當時大氣中有更多溫室氣體。因為二氧化碳會形成碳酸並產生酸雨而加速化學風化，造成當時氣候較溫暖且風化作用更為劇烈。」

大陸出現也代表在如此遙遠的年代就已經開始有大範圍板塊構造運動緩緩發生。「普遍認為當時形成陸地的板塊構造運動尚未發生，使得地球上沒有多少陸地。」Satkoski表示，「我們呈現的證據卻持有相反意見。」Johnson說總括而言，他們的成果將各方面的證據充分統整起來。「我們正慢慢得出一個理論可以同時解釋生命源起、海洋營養源來自何方以及地球為何並未結凍。雖然這些現象彼此之間狀似互相切合，但僅僅20年前我們對早期地球的理解卻跟此天差地遠。」

Life in ancient oceans enabled by erosion from land

As scientists continue finding evidence for life in the ocean more than 3 billion years ago, those ancient fossils pose a paradox. Organisms, including the single-celled bacteria living in the ocean at that early date, need a steady supply of phosphorus, but "it's very hard to account for this phosphorus unless it is eroding from the continents," says Aaron Satkoski, a scientist in the geoscience department at the University of Wisconsin-Madison. "So that makes it really hard to explain the fossils we see at this early era."

Satkoski, who is first author of a new report on ocean chemistry from this remote period, says the conventional wisdom of geology has envisioned an oceanic planet, with little or no land above the waves. "Starting back in the 1960s, for various reasons people claimed there was very little continental mass, and so there wasn't enough weathering to affect the chemistry of the ocean. But there wasn't much real data from more than 3 billion years ago to support that."

Discoveries of fossil remains of bacteria from over 3 billion years ago have changed that picture, says Satkoski. "But if there was life in the ocean, you need some amount of continental weathering taking place to deliver phosphorus so the organisms can live."

The major influences on ocean chemistry today are hydrothermal flow (hot water that has circulated through the crust) and surface weathering (the river transport of material eroded from land into the ocean).

To evaluate each influence 3.26 billion years ago, geoscience Professor Clark Johnson and Satkoski collected samples from South Africa and compared isotopes in two forms of a rock called barite. The cemented granules had formed in the water, then fused after dropping to the ocean floor.

A solid, or bladed, type of barite had formed at the ocean floor. Johnson, Satkoski and colleague Brian Beard assumed that the granular rock would reflect ocean water chemistry, and therefore any eroded, continental material. The bladed barite would represent a mix of ocean chemistry and hydrothermal flow. The study hinged on precise measurements of isotopes -- atoms that are chemically identical but that have different masses.

The result was a nearly infinitesimal -- but still significant -- difference in the isotope ratios, signifying that the granular barite indeed was derived from sediment eroded from land. In other words, a significant amount of erosion was taking place 3.26 billion years ago.

Their report, just published online by Earth and Planetary Science Letters, pushes back the first solid date for large-scale continental erosion by 400 million years.

"It's a guess how much of the planet's surface was land, but it could be as high as two-thirds of the area of today's continents," says Johnson, who leads the NASA Astrobiology Institute at the University of Wisconsin. "Some previous estimates had no continents at all."

"When people were thinking about ocean chemistry, it was always centered on hydrothermal flow, but there was little data," Johnson says. "We are trying to put some data into the equation."

The finding about continents jibes with evidence from igneous rocks -- those sourced in hot, molten rock -- which indicated that the surface became rigid enough to support mountain belts, which would have eroded, during this period. "Now that we have a more complete picture, the story becomes more coherent," Satkoski says.

The result also meshes with climate data, as intense continental weathering could result from an increase in carbon dioxide in the atmosphere. Although the sun was relatively cold at that time, the oceans were not frozen, Satkoski says. "That suggests there was more greenhouse gas in the atmosphere, which would produce a warmer climate combined with increased weathering, because carbon dioxide creates carbonic acid and acid rain, which speeds chemical weathering."

The presence of continents also indicates that the broad, slow movements of plate tectonics had started at this distant time. "Conventional wisdom says Earth had few continents because it did not have plate tectonics, which is how continents are made," Satkoski says. "Our evidence says the opposite." Overall, the result provides a satisfying unification of diverse streams of evidence, Johnson says. "We are moving toward an explanation for the presence of life, and the nutrients in the ocean, and why Earth was not frozen. They seem to fit together, but this is a very different picture of the early Earth than we had just 20 years ago.

原始論文：Aaron M. Satkoski, Donald R. Lowe, Brian L. Beard, Max L. Coleman, Clark M. Johnson. A high continental weathering flux into Paleoarchean seawater revealed by strontium isotope analysis of 3.26 Ga barite. Earth and Planetary Science Letters, 2016; 454: 28 DOI:10.1016/j.epsl.2016.08.032

引用自：University of Wisconsin-Madison. "Life in ancient oceans enabled by erosion from land." ScienceDaily. ScienceDaily, 26 September 2016.