原文網址：https://www.geosociety.org/GSA/News/pr/2024/24-05.aspx
疊層石是地球生命留下的最早地質紀錄。這些奇特的生物結構是由朝向陽光生長的藻毯跟碳酸鹽沉澱物組成。在距今34億8000萬年前初次出現之後，將近30億年的時間疊層石都是唯一的活體碳酸鹽工廠而主宰了地球。
疊層石也是大氧化事件的推手之一，這起事件將大量氧氣引入了大氣層，使其成分發生了劇變。一開始氧氣把疊層石的競爭對手給驅逐殆盡，造成疊層石在太古代和早元古代的環境中處於主流地位。然而，隨著越來越多生命形式的代謝作用適應了含氧的大氣，疊層石的數目也開始減少，只有在大滅絕過後或者嚴峻的環境下形成的地質紀錄中偶爾出現。
「這些細菌一直都在，但通常沒什麼機會形成疊層石，」Volker
Vahrenkamp表示。他是這篇發表在《地質》(Geology)新研究的作者。「它們大部分的時間都競爭不過珊瑚。」
現代的疊層石已經退居至極端環境當中，像是高鹽度的海洋環境(如：澳洲鯊魚灣)與鹽湖。而大部分的元古代疊層石發育在生物多樣性高的開闊淺海環境，直到最近，人們所知現代可以做為類比的地點，就只有巴哈馬的埃克蘇馬島(Exuma
Islands)。
然而，Vahrenkamp最近在沙烏地阿拉伯，紅海東北方陸棚的Sheybarah島發現了現生的疊層石。當時他正在研究帳篷狀(tepee)構造——一種外層為岩鹽，可以從太空看到的圓丘狀構造。不過Vahrenkamp偶然注意到有塊不顯眼的區域長有許多疊層石。這項令人驚奇的發現帶有運氣成分，因為Vahrenkamp是少數有在巴哈馬親眼看過疊層石的人之一。
「我一踩上去就知道它們是什麼東西，」Vahrenkamp解釋。「這條兩千公里長的海岸線是由碳酸鹽平台組成，所以理論上是個尋找疊層石的理想地點……然而就跟巴哈馬一樣，只有在這個很小的區域發現它們的蹤影。」
Sheybarah的環境為潮間帶至亞潮帶淺層，特徵為規律的乾溼交替、極端的溫度變化(8°C至48以上)以及貧養的狀態——跟巴哈馬非常相似。既然Al
Wajh碳酸鹽平台很多地方都有著類似的環境條件，或許還有其他區域會有疊層石生長。Vahrenkamp和團隊已經開始進行搜尋，但是疊層石的寬度大概只有15公分，因此不到非常靠近的地方是很難辨識出來的。
在Sheybarah島的該處區域中有數百顆疊層石。有些發育得相當漂亮，堪稱完美的教科書案例；其他的則高度較低，像是毯子一樣。「也許它們是青少年時期的疊層石，」Vahrenkamp猜測。「但我們不知道疊層石寶寶會是什麼樣子。它們形成時肯定很小，除此之外我們毫無頭緒。」
一部分的問題在於我們不曉得疊層石的生長速度。要定年它們相當困難，因為疊層石的碳酸鹽組成有兩種，但幾乎不可能將其分離：一種是微生物沉澱出來的新生碳酸鹽，這是研究人員想瞭解的；另一種則是環境中的碳酸鹽砂礫，而它們會誤導定年結果。目前Vahrenkamp的團隊每個月會監測這些疊層石來記錄任何可見的變化。他們不久之後可能會試著把一些Sheybarah島的疊層石移植到水族箱並加以培養——這項實驗的前景令人相當興奮。
Vahrenkamp的發現讓我們有機會更加瞭解疊層石的形成與生長過程。這可以為早期生命與海洋的演化過程帶來新的觀點，甚至可以幫助我們在其他星球(像是火星)上面搜尋生命。火星的生命會是什麼樣子？我們又該如何認出它們？看看疊層石吧——這些在地球大氣還沒有氧氣之前就已經出現的第一批生命型態，就是解開上述問題最有潛力的途徑。
New vestiges of
the first life on Earth discovered in Saudi Arabia
Stromatolites are the earliest geological
record of life on Earth. These curious biotic structures are made of algae
carpets growing toward the light and precipitating carbonates. After their
first appearance 3.48 Ga ago, stromatolites dominated the planet as the sole
living carbonate factory for almost three billion years.
Stromatolites are also partially responsible for the
Great Oxygenation Event, which drastically changed the composition of our
atmosphere by introducing oxygen. That oxygen initially wiped out
stromatolites’ competition, enabling their prominence in the Archean and early
Proterozoic environment. However, as more life forms adapted their metabolism
to an oxygenated atmosphere, stromatolites started to decline, popping up in
the geologic record only after mass extinctions or in difficult environments.
“The bacteria are always around, but they don’t
usually get the chance to make stromatolites,” explains Volker Vahrenkamp, the
author of a new study in Geology.
“They are largely outcompeted by corals.”
In modern times, stromatolites are relegated to niche
extreme environments, such as marine settings (e.g., Shark Bay, Australia) and
alkaline lakes. Until recently, the only known modern analogue to the
biologically diverse, open shallow marine settings where most Proterozoic
stromatolites developed was the Exuma Islands in the Bahamas.
That is, until Vahrenkamp discovered living
stromatolites on Sheybarah Island, on the northeastern shelf of the Red Sea in
Saudi Arabia. Vahrenkamp was studying tepee structures—salt crust domes that
can be seen from space—when he happened upon the unassuming stromatolite field.
The discovery was surprising, but luckily, Vahrenkamp is one of the few people
to have previously seen stromatolites in the Bahamas.
“When I stepped on them, I knew what they were,”
explains Vahrenkamp. “It is 2000 km of carbonate platform coastline, so in
principle it is a desirable area to look for stromatolites . . . but then, it
is the same in the Bahamas, and yet there is only one small area where you find
them.”
Sheybarah Island is an intertidal-to-shallow subtidal
setting, with regularly alternating wetting and drying conditions, extreme
temperature swings between 8 °C and >48 °C, and oligotrophic conditions—much
like the Bahamas. Since similar environmental conditions are widespread across
the Al Wajh carbonate platform, there might be other stromatolite fields
nearby. Vahrenkamp and his team have started this exploration work, but
stromatolites are small, about 15 cm across, and thus are difficult to spot
until one gets very close.
There are several hundred stromatolites in the
Sheybarah Island field. Some are well-developed, perfect textbook examples.
Others are more sheet-like, with a low relief. “Perhaps they could be
juvenile,” hypothesizes Vahrenkamp, “but we don’t know what a baby stromatolite
looks like. They must start small, but we don’t know.”
Part of the issue is that we don’t know how fast
stromatolites grow. Dating them is very hard, because they contain two
different carbonate components that are virtually impossible to separate: the newly
microbe-precipitated one, which is of interest, and carbonate sand present in
the environment, which is misleading. Currently, Vahrenkamp’s team monitors the
field monthly to record any visual changes. Soon, there might be an attempt to
transfer some Sheybarah Island stromatolites to an aquarium and grow them
there—an exciting experimental prospect.
Vahrenkamp’s discovery affords us the opportunity to
better understand the formation and growth of stromatolites. This will provide
insights into early life and ocean evolution on Earth and may even assist us in
the search for life on other planets such as Mars. What would life look like on
Mars, and how would we recognize it? Looking at stromatolites, which were the
first life forms on Earth, before our planet even had an oxygenated atmosphere,
is a most promising avenue.
原始論文：Volker
Vahrenkamp, Viswasanthi Chandra, Elisa Garuglieri, Ramona Marasco, Kai
Hachmann, Pankaj Khanna, Daniele Daffonchio, Alexander Petrovic. Discovery of modern living intertidal
stromatolites on Sheybarah Island, Red Sea, Saudi Arabia. Geology, 2024; 52 (5)
DOI: 10.1130/G51793.1
引用自：The
Geological Society of America. “New Vestiges of the First Life on Earth
Discovered in Saudi Arabia.”
沒有留言:
張貼留言