達爾文對於珊瑚環礁如何形成的理論有致命的缺陷

 原文網址：http://news.rice.edu/2020/10/12/study-darwins-theory-about-coral-reef-atolls-is-fatally-flawed-2/

達爾文對於珊瑚環礁如何形成的理論有致命的缺陷

科學家彙整了環礁是由海平面高度的週期變化所形成的證據

By Jade Boyd

身為海洋地質學家與海洋學家的André Droxler研究珊瑚礁已經超過了40年。他知道達爾文提出的環礁形成理論並不正確，不過他也理解為什麼達爾文的模型可以歷久不衰，仍然存在於教科書、大學講堂、自然科學博物館以及維基百科的條目當中。

比較現在Google地球的衛星影像(右)和1842年查爾斯．達爾文繪製的地圖(左)，這些地方分別為：所羅門群島的瓦尼科羅群礁(上)、密克羅尼西亞的楚克堡礁(中)、印度洋查戈斯群島的佩魯斯巴紐斯環礁(下)。圖片來源：A. Droxler

「這項理論非常簡潔優美而且賞心悅目，所以大家還是把它當作教材，」最近剛從萊斯大學退休的Droxler表示。「所有你能找到的地球科學和海洋科學的入門書籍當中，都還可以看到達爾文的模型。如果海洋科學導論要挑一個有關珊瑚礁或碳酸鹽的知識來教，勢必會選擇達爾文的模型。」

Droxler教授任職於萊斯大學的地球、海洋和行星科學系已經33年。為了傳達正確觀念，本月他在《海洋科學年度評論》(Annual Review of Marine Science)發表了一篇37頁有關環礁起源的力作。共同作者為Droxler的長期合作夥伴，任職於法國海洋開發研究院的海洋地質學家暨海洋學家Stéphan Jorry。

達爾文在1842年發表了環礁形成的理論。在六年前，他剛結束在英國測量船小獵犬號上那傳奇性的旅程。身為一名訓練有素的地質學家，達爾文對他在小獵犬號上的五年裡遇見的岩石與地形深感興趣。這艘船艦的主要任務是調查海岸線以及危及航行的事物，其中一項指令是詳細觀測一座珊瑚環礁附近的潮汐和海水深度。

Droxler表示英國皇家海軍「花了許多時間繪製珊瑚礁的位置，因為它們對於船隻來說相當危險。」其中環礁是特別有趣卻也相當危險的類型。有些環礁的頂部是地勢低平的島嶼，但許多環礁是被珊瑚礁覆蓋、斷斷續續的環狀岩石，它們潛伏在海面不遠處，隨時等著來往船隻失去戒心時把船底撕裂開來。「環礁會從深海平原直接竄升到相當靠近海面的地方，」Droxler 說。「所以海軍必須知道它們的精確位置。」

小獵犬號和每艘英國皇家海軍的船艦一樣，都配有標示所有已知礁體位置的海圖。而達爾文也把這份圖表放入他1842年的文章當中。

「他發表的地圖相當漂亮，集合了當時地球上所有已知的礁體，」Droxler 表示。「這真的相當驚人。如果你把他的地圖和現在礁體的衛星影像互相比較，兩者幾乎是一模一樣。可以畫得這麼精確真是不可思議。」

但是達爾文不像海軍只是把礁體標記成對船隻的潛在危害，他還有系統地把礁體分成三類，每一座都屬於其中一類。達爾文把那些跟陸地相連的礁體稱為裙礁；跟陸地分離，中間隔著潟湖的則是堡礁；第三種類型叫做環礁，這種環狀的礁體中間會圍著一座潟湖，但裡面沒有陸地。

當時的地質學家相信陸地會穩定地從地表抬升，而海洋則是持續下沉。不過達爾文注意到裙礁和堡礁常常出現在火山島周圍，因此他推論礁體原本是在火山島的邊緣形成。接著火山死亡之後慢慢沉回海底，此時礁體仍會持續生長，最初先變成和陸地分開的堡礁，到最後火山完全沉沒，礁體就成為了環礁。

Droxler表示達爾文能夠綜合當時有關珊瑚礁的所有知識，推導出如此淺顯易懂、面面俱到又有說服力的理論模型，令他感到相當敬佩。但Droxler 表示撇除這個理論的漂亮之處，達爾文並無法準確地預測環礁如何形成，原因是他缺乏了某些關鍵資訊。

「海平面的周期性變化會讓環礁形成，」Droxler表示。「達爾文沒有海平面能夠上升下降的概念，原因是在1860年代以前冰河期還沒有成為家喻戶曉的知識。」

Droxler說達爾文的分類系統和理論是如此簡單，也許是其吸引力可以持續不輟的關鍵。雖然在1930年代，就已經有別的理論可以更加精確地描述環礁如何形成，但相較於達爾文的實在複雜太多；此外，大部分支持該理論的證據也是最近才得到的，來自於過去四十年學術人員和石油業者數十次海上鑽探的成果，以及彙整地球氣候和海平面的歷史紀錄得到的資訊。

Droxler表示過去五十萬年，海平面以十萬年為間隔發生了五次劇烈的動盪，使得環礁得以形成。在每個循環當中，海平面的上升下降幅度可以到達120公尺甚至更多。不過，他接著表示如果想要徹底瞭解海平面變化如何創造出環礁，將時間拉回到更久更久以前開始說起會比較容易。

「最近五百萬年地球氣候變化得十分劇烈，」他說。「大約五百萬年前到兩百五十萬年前，氣候相當溫暖且變化不大，海平面的高度也相對穩定。當時地球在今日環礁所在的地方形成了頂部平緩的沙洲。」

當時在地勢平緩的淺海生態系，會有珊瑚和其他海洋生物在此生活，然後死亡。無數個世代下來，牠們的骨頭持續堆積在海床，變成了礦物，形成了之後會孕育出環礁、頂部平緩的沙洲。此時海平面雖持續上升但速度不快，因此這些頂部平緩的沙洲可以跟隨海平面的上升步調穩定成長――直到這段溫暖的時期在兩百五十萬年前結束。

「接著氣候開始發生週期性的震盪，」Droxler說。「此時溫暖與寒冷的時期交替出現，但地球氣候的整體趨勢是變冷、變冷、再變冷，中間夾雜著暖期造成的短暫波動。」

冰冠在這段時期不斷加厚。當冰層持續累積，海平面便會下降，使這些平坦的碳酸鹽沙洲露出頂端，看起來就像是從海面升起、頂部平坦的白色巨石。雨水落在露出的沙洲上方會慢慢的把碳酸鹽溶解掉，而水體無法流動又會讓過程加速，使得水漥和水塘逐漸刻出碗狀的凹洞。在二百萬年當中這種作用不斷進行著，造成白色的方山變成中間下凹、佈滿坑洞的環狀高地。

「地球冰層的體積大約在五十萬年到六十萬年前首次達到高峰。此時覆蓋在北美的冰層面積跟現在的南極差不多大，芝加哥遭到一英里厚的冰塊覆蓋，而紐約則在冰層的南緣。」Droxler說。「由於北美洲、蘇格蘭、斯堪地那維亞和西伯利亞堆積了極為大量的冰雪，使得海平面的高度比現在低了120到130公尺。」

過去五百萬年內海平面發生了五次劇烈的漲落。原因是地球雖然可以在北方形成廣大的冰層，但它們對於輕微的氣候變化也相當敏感，像是地球繞日軌道與自轉軸傾角的些微變化，會讓北半球最北方接收到的日照量跟著變動。

「這些在北美洲形成的廣大冰層並非位於北極，而是集中在緯度較低的北緯65度，所以它們能累積相當大量的冰塊，使得海平面可以下降超過120公尺，」他說。「但是它們的融化速度也可以很快，因此我們才能觀測到當時的海平面反覆地大幅震盪。第一次的大幅漲落大概發生在四十五萬年前，海平面從比現在低135公尺升高到比現在高10公尺。」

在每一次的循環期間，當地球處在和現在一樣的暖期，海平面升高至將近最高處時，頂部平坦、佈滿坑洞的沙洲就會再次被淹沒。此時珊瑚便會重新在這些遭到侵蝕的沙洲上面定居，特別是在地勢較高的外環地帶。

「所以說，在它們高起的環狀邊緣有更多碳酸鹽堆積上去，使得今日所見的環礁開始成形，」Droxler說，「不過海水高漲的時間最多只能維持一萬到一萬兩千年，接著又會再次下降。這時沙洲的邊緣累積了一些礁體，但中央的潟湖又在海水下降時繼續遭到溶解。結果便是每經過一次循環，邊緣就會往上長，中間則會往下溶解，使得環狀的樣子變得越來越明顯。」

這篇Droxler和Jorry的深入研究包含了數十張圖片，其中幾張比較了達爾文繪製的原圖跟現今的地圖與衛星影像。論文採納的海洋地質資料是彙整了幾十年來數十次的考察成果，包括最近由兩位共同作者在印度洋取得的資料集。Droxler說他和Jorry已經討論要寫這種文章好幾年了，如果2020年沒有發生疫情的話，他們大概也不會付諸行動。

Droxler退休之後搬到德州中部一座偏遠的農場。三月由於疫情而發布旅行禁令之後，他便面臨了要待在那兒一段時間的處境。

「Stéphan好幾年來一直建議我說我們應該發表這樣的文章，但你知道，人總是會一直找別的事來做。」Droxler 說，「Stéphan三月底打給我說：『我被關在法國的家裡，而你則被關在梅森郡的鄉間某處。那我們乾脆就來寫這篇文章吧。』所以我們就這麼辦了。」

研究經費來自美國國家科學基金會、荷蘭皇家殼牌石油公司、道達爾石油公司、馬爾地夫環境保護局、南佛羅里達大學、法國海洋開發研究院的被動大陸邊緣探勘實驗室計畫、法國國家科學研究中心、雷恩第一大學、西布列塔尼大學、皮耶和瑪麗．居禮大學、法國石油研究所。

 

Darwin’s theory about coral reef atolls is fatally flawed

Scientists compile new evidence that atolls are formed by cyclic changes in sea level

Marine geologist and oceanographer André Droxler knows Charles Darwin’s theory about atolls is incorrect. But Droxler, who’s studied coral reefs for more than 40 years, understands why Darwin’s model persists in textbooks, university lecture halls, natural science museums and Wikipedia entries.

“It’s so beautiful, so simple and pleasing that everybody still teaches it,” said Droxler, who recently retired from Rice University. “Every introductory book you can find in Earth science and marine science still has Darwin’s model. If they teach one thing about reefs or carbonates in marine science 101, they teach that model.”

Droxler, a professor of Earth, environmental and planetary sciences at Rice for 33 years, is hoping to set the record straight with a 37-page, tour de force paper about the origins of atolls. Published this month in the Annual Review of Marine Science, the paper was co-authored by Droxler and longtime collaborator Stéphan Jorry, a marine geologist and oceanographer at the French Research Institute for Exploitation of the Sea (IFREMER).

Darwin’s theory about the formation of atolls was published in 1842, six years after his legendary voyage aboard the British survey ship HMS Beagle. A geologist by training, Darwin was keenly interested in the rocks and landforms he encountered in his five years aboard the Beagle. The ship’s primary mission was surveying coastlines and hazards to navigation, and the ship’s orders included collecting detailed observations of the tides and ocean depths around a coral atoll.

“They spent a lot of time mapping reefs because they were such hazards to shipping,” Droxler said of the Royal Navy. Atolls were particularly interesting and dangerous. Some were topped with low-lying islands but many were jagged rings of coral-topped rock that sat just below the water’s surface, ready to rip the bottom out of unwary passing ships. “They come out of the abyssal plain of the ocean to almost no depth,” Droxler said. “So they needed to know exactly where they were located.”

The Beagle, like every Royal Navy vessel, carried charts with the marked location of every known reef, and Darwin put these to use in his 1842 paper.

“He published a beautiful map that compiled all the known reefs on Earth,” Droxler said. “It’s amazing, when you compare satellite images of reefs today versus his map. It’s almost the same. It’s unbelievably accurate.”

But unlike navy maps that simply marked reefs as hazards to navigation, Darwin systematically classified each into one of three categories. Those attached to land, he called fringing reefs. Those detached from land and separated by a lagoon were barrier reefs. The third category was atolls, ring reefs that enclosed a central lagoon but no land.

At the time, geologists believed continents were steadily rising out of the Earth and oceans were steadily sinking. Darwin noticed that both fringing and barrier reefs tended to surround volcanic islands, and he reasoned reefs initially formed on the fringe of volcanic islands. When the volcano died and slowly sank back into the ocean, the reef remained, first becoming a separated barrier reef and eventually, after the volcano sank entirely, an atoll.

Droxler said he’s awed by Darwin’s ability to synthesize all that was known about reefs in his day and come up with such a simple, comprehensive and compelling theoretical model. But beauty aside, Darwin could not have accurately predicted how atolls form because he lacked the key piece of information, Droxler said.

“Cyclic changes in sea level drive atoll formation,” Droxler said. “Darwin had no concept that sea level could go up and down, because glaciation didn’t become common knowledge until the 1860s.”

Droxler said the simplicity of Darwin’s classification system and theory could play a role in its continued appeal. A more accurate description of atoll formation has been around since the 1930s, but it is considerably more complicated and much of the evidence to support it is more recent, coming in the past 40 years from dozens of scientific and oil industry drilling expeditions as well as from the compiled record of Earth’s climate and sea level history.

Today’s atolls formed in the past 500,000 years, Droxler said, driven by five wild swings in sea level that occurred every 100,000 years. In each cycle, sea level rose and fell by 120 meters or more. But to fully appreciate how changing sea level created atolls, it helps to start much earlier, Droxler said.

“The Earth’s climate has changed quite dramatically in the last 5 million years,” he said. “From about 5 million years ago to 2.5 million years ago, we had a rather warm climate that did not change very much, and sea level remained relatively constant. At that time, the Earth was producing flat-topped banks where today we have atolls.”

Sea level rose steadily but slowly, and the flat-topped banks that would give birth to atolls were created by countless generations of corals and other sea creatures that lived and died, adding their skeletons to the mineralized floor of the shallow, flat-topped marine ecosystem. The flat-top banks grew steadily, keeping pace with sea level until the warm period ended about 2.5 million years ago.

“The climate began to fluctuate into cycles,” Droxler said. “There were alternate periods of warm and cold, but overall, the trend was that the Earth’s climate got colder and colder and colder, interrupted by short warm swings.”

Throughout this period, ice caps were thickening. As ice accumulated, sea level fell, exposing the tops of the flat carbonate banks, which rose out of the ocean like bleached stone mesas. When rain fell atop the exposed banks, it slowly dissolved the carbonate, and standing water sped up the process. Puddles and ponds gradually carved bowl-like depressions. And over 2 million years, this process turned white mesas into ring-like towers with hollowed-out central depressions.

“Ice volume on Earth reached its initial maximum around 500,000-600,000 years ago, when a mile of ice covered Chicago, and New York was at the southern edge of a massive ice sheet on North America, comparable to modern Antarctica,” Droxler said “There was so much ice in North America, Scotland, Scandinavia and Siberia that sea level was reaching 120 meters to 130 meters below what it is today.”

The five dramatic swings in sea level that occurred in the past half-million years were driven by Earth’s ability to form large northern ice sheets, and by the sensitivity of those ice sheets to slight climatic changes, like fluctuations in the amount of sunlight landing in the far Northern Hemisphere due to slight changes in the planet’s orbit and tilt.

“Because these big ice sheets formed in North America, not on the pole, but centered in relatively low latitude about 65 degrees north, they accumulated huge ice volumes that lowered sea level by more than 120 meters,” he said. “But also, they were able to melt very quickly. And so we observe, now, these cyclic high-amplitude swings of sea level. And the first big swing was about at 450,000 years ago, when it went from minus-135 meters to plus-10 meters of what we have today.”

During each swing, the hollowed out flat-topped banks were resubmerged during warm periods, when sea level rose near its highest level. During those periods, as today, coral recolonized the highest parts of the eroded banks, in particular along their raised outer rims.

“Now you’re adding carbonate on to their raised rim, and you’re creating the modern atolls,” Droxler said. “But this period of high sea level doesn’t last for more than 10,000-12,000 years, and it goes down again. So now you are adding some reef on the rim, but then dissolving the lagoon again when sea level retreats. So you are enhancing the morphology even more with each cycle now, growing on the margin and dissolving in the middle.”

Droxler’s and Jorry’s in-depth study contains dozens of illustrations, including some comparisons of Darwin’s original drawings with contemporary maps and satellite imagery. And the paper draws on decades of marine geological data compiled from dozens of expeditions, including recent datasets acquired by both co-authors in the Indian Ocean. Droxler said he and Jorry had discussed such a paper for years, but it might not have happened without the 2020 pandemic.

Droxler moved to a remote ranch in Central Texas after retiring, and when the pandemic shut down travel in March, he was faced with the prospect of being there for some time.

“For years, Stéphan had pushed me, saying we should publish this, but you know, you always kind of do something else and something else,” Droxler said. “At the end of March, Stéphan called me and said, ‘I’m stuck in France, at home, and you’re stuck in the middle of the countryside in Mason County. Let’s write this paper.’ And so that’s what we did.”

The research was supported by the National Science Foundation, Royal Dutch Shell, Total, the Environmental Protection Agency of the Maldives, the University of South Florida and the Passive Margins Exploration Laboratories research program of IFREMER, Total, the French National Center for Scientific Research, the University of Rennes, the University of Western Brittany, Pierre and Marie Curie University and the French Institute of Petroleum.

原始論文：André W. Droxler, Stéphan J. Jorry. The Origin of Modern Atolls: Challenging Darwin's Deeply Ingrained Theory. Annual Review of Marine Science, 2015; 13 (1) DOI: 10.1146/annurev-marine-122414-034137

引用自：Rice University. “Study: Darwin’s theory about coral reef atolls is fatally flawed”