原文網址：http://www.sciencemag.org/news/2017/05/was-amazon-once-ocean
亞馬遜河曾為海洋?
亞馬遜雨林是生物多樣性的寶庫，地球上所有物種中有10%生活在這片670萬平方公里的雨林中。它的生物多樣性為何可以達到如此境界，數十年來一直是科學家激烈討論的對象。現在一項新研究提出超過1000萬年以前，此座森林大部分區塊曾被加勒比海淹沒兩次，而形成了短暫出現的內海，其高速推動了新物種的演化過程。然而，在此議題中站在對立面的科學家，仍尚未被新證據給說服。
「難以想像有單獨一種作用可以讓如此廣大的森林被海洋淹沒。」主要作者，巴拿馬市史密森熱帶研究院的古生物學家Carlos Jaramillo表示。他同時支持雙方陣營。
研究人員大致上皆認同部分亞馬遜地區曾經被水淹沒，但他們對水的來源則莫衷一是。「河流陣營」的科學家主張從隆起的安地斯山脈流下來的淡水河川切割了下游土地，使得動植物被分隔成許多孤立集團，隨後演變為新的物種。急速增高的山脈也在不同高度形成各式各樣的微氣候，促成種化發生並讓新的動植物進入亞馬遜盆地。然而，當1990年代從亞馬遜的沉積物中發現海洋微生物時，開始有科學家假設這座森林曾被海洋淹沒，居住在森林的物種在快速適應洪水時形成了許多新物種。
但要證明兩種論點――河川論或海洋論――何者為真卻相當困難。可以確切描繪出真實圖像的岩石和化石極為稀有。因此Jaramillo和他的同僚轉而使用不同類型的數據：從森林底部鑽出的岩芯。這些圓柱形岩芯的寬度為6公分，長度為600公尺，其中含有的花粉、化石和沉積物保存了該區域過去的環境紀錄，最遠可追溯至數千萬年前。Jaramillo利用了兩根岩芯：一根為石油公司在哥倫比亞東部所鑽出；另一根則來自巴西東北部，由巴西地質調查局於1980年代取得。
Jaramillo的團隊一層一層地仔細檢查這些岩芯。其中大多數的遺骸來自於陸生生物，但在兩個薄薄的層位中，卻發現了海洋浮游生物和海生貝類。哥倫比亞的岩芯甚至擁有鯊魚牙齒和蝦蛄化石，兩者皆為海洋生物。這足以說服一度堅信河流陣營的Jaramillo，接受加勒比海曾經兩度淹至巴西亞馬遜地區的西部、厄瓜多和秘魯。他今日在期刊《科學前緣》中寫道：「1800萬年前發生過一次，1400萬年前又再度發生。」他表示：「這是一個失落的生態系統。」
這些海洋的存在時間並不久。在巴西西北部，第一次洪水持續了20萬年左右，而第二次則持續了40萬年。較接近加勒比海的哥倫比亞被淹沒的時間比較久，分別為90萬年和370萬年。Jaramillo表示這兩起洪水可能是由安地斯山脈的抬升導致。當山脈快速隆起時會將周遭的其餘陸地往下壓，使得海水湧入。但隨著淡水和沉積物從山頂流下來並重新填補盆地，很快地海水便會消失。
在地質時間中，這些洪水事件的持續長度不過是白駒過隙。Jaramillo表示：「但對樹木來說還是很長一段時間。」即使這些事件相對來說並不長久，卻能對此區域造成深遠變化。
任職於荷蘭阿姆斯特丹大學和厄瓜多特納市的Ikiam亞馬遜雨林區域大學的地質學家和孢粉學家Carina Hoorn，是最早提出海水洪氾理論的學者。她說這項新研究「使此[海水洪氾]情境更加有力，同時也讓發生的時間點更加確定」。但任職於美國北卡羅萊納州德罕的杜克大學和厄瓜多烏爾庫基的Yachay科技大學的地質學家Paul Baker，仍然是河流陣營的忠實成員。他說：「我對[哥倫比亞]曾被海水入侵沒有什麼意見。」但他表示巴西的岩芯讓他感到十分困惑，因為其中看似為海生的浮游生物，以前曾在歐洲的古代淡水湖中出現過。要提出更能說服Baker的證據得測量貝殼的氧同位素，其可以揭露它們是生活在鹹水或淡水之中。Jaramillo說他已經開始著手進行這項工作。他也想要找到更多亞馬遜雨林的生物化石，以研究在那段動盪不安的歲月中可能滅絕的物種。
目前為止，有一件事是Jaramillo、Hoorn和Baker口徑一致的：他們需要在整個地區鑽探並研究更多岩芯，才能解開亞馬遜生物多樣性的謎題。
Was the Amazon once an ocean?
The Amazon rainforest is a
treasure trove of biodiversity, containing 10% of the planet’s species in its
6.7 million square kilometers. How it got to be that way has been fiercely disputed
for decades. Now, a new study suggests that
a large section of the forest was twice flooded by the Caribbean Sea more than
10 million years ago, creating a short-lived inland sea that jump-started the
evolution of new species. But the new evidence still hasn’t convinced
scientists on the other side of the debate.
“It’s
hard to imagine a process that would cover such a large forest with an ocean,”
says lead author Carlos Jaramillo, a paleontologist at the Smithsonian Tropical
Research Institute in Panama City who has been in both camps.
Researchers
generally agree that parts of the Amazon were once under water, but they don’t
agree on where the water came from. Those in the “river camp” argue that
freshwater streaming down from the rising Andes sliced up the land below,
dividing plants and animals into isolated groups that later turned into new
species. The fast-growing mountains also created microclimates at different
elevations, sparking speciation and funneling new plants and animals into the
Amazon basin. However, when marine microorganisms were discovered in Amazonian
sediments in the 1990s, some scientists hypothesized that the forest was once
inundated by an ocean, which created new species as forest dwellers quickly
adapted to the flood.
But
proving either case—the river view or the ocean view—is tough. Rocks and
fossils that could paint a definitive picture are exceedingly rare. So Jaramillo
and his colleagues turned to a different kind of data: cores drilled into the
jungle floor. Six centimeters wide and 600 meters deep, the cylindrical cores
preserve a record of the region’s past environments in the form of pollen,
fossils, and sediments, going back tens of millions of years. Jaramillo used
two cores: one from eastern Colombia, drilled by an oil company, and one from
northeastern Brazil, taken by the Brazilian Geology Survey in the 1980s.
Jaramillo’s
team went through the cores layer by layer. Most of the remains came from
land-dwelling species. But in two thin layers, it found marine plankton and
seashells. The Colombian core even contained a fossilized shark’s tooth and a
mantis shrimp, both ocean dwellers. That was enough to convince Jaramillo, who
was once firmly in the river camp, that the Caribbean Sea had reached down into
the western Amazon of Brazil, Ecuador, and Peru twice: once 18 million years ago,
and again 14 million years ago, he writes today in Science Advances. “It’s a lost
ecosystem,” he says.
These
seas didn’t last for long. In northwest Brazil, the first flood endured some
200,000 years, while the second lasted 400,000 years. Colombia, which is closer
to the Caribbean, was inundated for a longer period, 900,000 and 3.7 million
years, respectively. Those floods could have been caused by the growing Andes,
Jaramillo says. The mountains would have pushed down the rest of the continent
as they thrust upward, letting seawater flow in. But that water would have been
quickly displaced as freshwater and sediments flowed down the peaks and rebuilt
the basin.
In
geological time, these floods lasted a mere blink of the eye, Jaramillo says,
“but it’s still a long time for a tree.” Even these relatively short events
would have transformed the region.
The
new work “makes the case [for marine flooding] much stronger, and it makes the
timing more definite,” says Carina Hoorn, a geologist and palynologist at the
University of Amsterdam and Ikiam Regional University of Amazonia in Tena,
Ecuador, who first proposed the marine flooding theory. But Paul Baker, a
geologist at Duke University in Durham, North Carolina, and Yachay Tech in
Urcuquí, Ecuador, is still a firm member of the river camp. “In [Colombia], I
don’t have any problem with there being a marine incursion,” Baker says. But
the Brazilian core troubles him, because marine-looking plankton has turned up
in other ancient freshwater lakes in Europe, he says. More convincing to Baker
would be a measurement of oxygen isotopes in the shells, which could reveal
whether they grew in salt- or freshwater. Jaramillo says he’s already working
on it. He’d also like to find more Amazonian fossils to study species that may
have gone extinct during this dynamic time.
For
now, there’s only one thing Jaramillo, Hoorn, and Baker can all agree on: They
will need to drill and study many more cores from across the region to solve
the mystery of the Amazon’s biodiversity.
原始論文：Frank P. Wesselingh et al. Miocene flooding events of
western Amazonia. Science Advances, May 2017 DOI: 10.1126/sciadv.1601693
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