生命在陸地扎根的時間比過往認為的還要早了3億年

原文網址：www.sciencedaily.com/releases/2016/11/161108090052.htm

生命在陸地扎根的時間比過往認為的還要早了3億年

德國柏林、波茨坦和耶拿的科學家進行的研究認為生命早在32億年前就已經在陸地站穩腳跟了。由柏林自由大學的Sami Nabhan領導的團隊對南非巴柏頓綠岩帶的遠古岩層進行研究而得到了上述結論。

這些形成年代可追溯至35億年前的岩石是地球已知最古老岩石中的一員。科學家於其中定年為32.2億年的岩層裡，從硫化鐵礦物―黃鐵礦的微小顆粒中，發現了微生物活動的明顯痕跡。

這些痕跡同時紀錄在黃鐵礦的稀有元素分布以及硫同位素³⁴S和³²S的比例當中。

利用波茨坦大學2013年設置的儀器，科學家發現某些晶體核心部位³⁴S的比例跟周圍環狀部分有顯著差異，代表微生物代謝硫時該顆粒的外圍有參與其中，此作用稱為「生物成因分化作用」(biogenic fractionation)。GFZ(德國地質科學研究中心)二次離子質譜儀實驗室的Michael Wiedenbeck利用該機構的儀器，可以憑藉小於10億分之一公克重的樣品即能測出³⁴S和³²S的比例。

岩石成分、晶體形狀以及在野外可見的層狀構造皆指出他們研究的岩石序列源自於古代土壤剖面，此「古土壤」(paleosol)形成於32.2億年前一處河流氾濫平原。

在他們的解釋中，此研究蒐集到的野外調查數據顯示當時有條辮狀河系攜帶的沉積物含有硫化鐵晶體。接著，在位置處於乾溼環境交替出現的土壤中生長的微生物，會讓這些黃鐵礦上增生一圈新的晶體。

根據這項證據，刊登於期刊《地質》(Geology )的論文中科學家總結說他們找到了十分早期陸地上便有生物活動的鐵證。相較於前人文獻，此篇研究將陸地生命出現的最早證據往前推進了約3億年。

Life took hold on land 300 million years earlier than thought

Life took hold on land at least as early as 3.2 billion years ago, suggests a study by scientists from Berlin, Potsdam and Jena (Germany). The team led by Sami Nabhan of the Freie Universität Berlin studied ancient rock formations from South Africa's Barberton greenstone belt.

These rocks are some of the oldest known on Earth, with their formation dating back to 3.5 billion years. In a layer that has been dated at 3.22 billion years old, tiny grains of the iron sulfide mineral pyrite were discovered that show telltale signs of microbial activity.

These signs are recorded both in trace element distributions as well as in the ratio between the sulfur isotopes 34S and 32S in the pyrite.

Using instrumentation installed in Potsdam in 2013, the scientists showed that the fraction of 34S in the core of some crystals differ characteristically from that of the same crystal's rim, indicating that the exterior of the grain involved a processing of sulfur by microbes, so-called biogenic fractionation. The determination of the 34S/32S ratio, using sample masses less than one billionth of a gram, was carried out at the GFZ German Research Centre for Geosciences by Michael Wiedenbeck of the GFZ's secondary ion mass spectrometry (SIMS) lab.

The composition of the rock, the shape of the crystals, and the layering visible in the field all indicate that the studied rock sequence was derived from an ancient soil profile; this so-called paleosol developed on a river flood plain 3.22 billion years ago.

Field data collected during this study imply that a braided river system transported the sediment containing the iron sulfide crystals. It is interpreted that microbes living in the soil, at a level that was continually shifting between wet and dry conditions, subsequently produced the rim overgrowths on the pyrite crystals.

Based on this evidence, the scientists conclude in their publication in the journal Geology that they found evidence for biological activity on land at this very early date. Their research pushes back the date for the oldest evidence of life on land to some 300 million years earlier than previously documented.

原始論文：Sami Nabhan, Michael Wiedenbeck, Ralf Milke, Christoph Heubeck. Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols. Geology, 2016; 44 (9): 763 DOI: 10.1130/G38090.1

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "Life took hold on land 300 million years earlier than thought." ScienceDaily. ScienceDaily, 8 November 2016.