青藏高原比過去認為得還要晚抬升
研究人員徹底檢討以往慣用的數據，認為青藏高原在4000萬年前比現在矮了1.5公里
青藏高原的平均高度為海拔4500公尺，是地球最大的造山帶。目前大部分的分析結果認為大約在4000萬年前的始新世，青藏高原就已經跟現在差不多高了。
青藏高原位在喜馬拉雅山(南)和塔克拉瑪干沙漠(北)之間。圖片來源：公眾領域
德國蒂賓根大學地質科學系的Svetlana
Botsyun博士全面檢討了這項理論。在發表於最新一期《科學》(Science)的研究中，她和國際科學家團隊合作，運用許多類型的古氣候數據得出了驚人的結論：始新世青藏高原的高度並未超過3000公尺。這項新的假設有助於研究人員更加了解板塊邊界形成造山帶時，參與其中的地質作用力。
青藏高原位在歐亞板塊和印度板塊碰撞的邊界之上。板塊碰撞使得青藏高原數千萬年來不斷地抬升。為了得出青藏高原在地質歷史中的高度變化，研究人員通常會採用一種特殊的地質文獻：儲存在地下數千萬年的雨水。方法是分析水中不同的穩定氧同位素(質量不同的氧原子)之間的關係。
這項方法背後的理論是越高的地方，雨水含有的重同位素越少。因此，地質學家可以分析地下水樣本的同位素，得出採集地點過往的高度。分析青藏高原的水樣得出的數據顯示，始新世青藏高原的高度已經有4000公尺左右。Svetlana
Botsyun表示：「我們對這些結果有所疑問，因為氧同位素的分布不只可以代表海拔高度，也能反映出古氣候的影響。」
多種因素的交互作用
距今5600萬年前至3390萬年前的地質時代稱為始新世，當時大氣中二氧化碳和其他溫室氣體的濃度都比現在高出許多。此外，亞洲各地的溫度和地理條件也跟現在有很大的不同。當時歐亞板塊旁邊有座廣袤的淺海，地質學家稱為「副特提斯洋」(Paratethys)；而印度板塊所在的緯度則比現在還要往南10度。Botsyun博士說：「始新世的這些環境條件會對氧同位素的比例造成影響，所以我們進行氣候模擬時將它們納入考量。」結果呈現出跟前人研究截然不同的狀況。
「始新世青藏高原的位置比現在還南邊，造成我們的模擬結果中，雨水同位素比例跟高度的關係和理論恰好相反――在青藏高原南側，高處降下的雨水擁有更重的同位素組成。」Svetlana
Botsyun表示，「因此我們必須拋棄舊有的觀念。山脈高度和雨水的氧同位素比例，在過往的地質時間中並非一直保持著同樣的關係。」
團隊的新發現跟青藏高原當時未超過3000公尺的假說不謀而合。Botsyun博士說：「之後我們會結合氣候模型以及從其他地質材料中得到的同位素資料，希望能得到更早以前的地質時代，青藏高原有多高的可信數據。」
Tibetan plateau rose later than we thought
Researcher overhauls conventional data, says
plateau was 1.5 km lower just 40 million years ago
The Tibetan Plateau
today is on average 4,500 meters above sea level. It is the biggest
mountain-building zone on Earth. Most analyses to date indicated that, back in
the Eocene period some 40 million years ago, the plateau was about as high as
it is today.
Dr.
Svetlana Botsyun of the University of Tübingen’s Geoscience Department tested
this theory using comprehensive tools. Working with an international team of
colleagues, she made use of a wide range of palaeoclimate data and came to a
surprising conclusion: The data showed that the plateau had an elevation of no
more than 3,000 meters in the Eocene. This new scenario helps researchers to
understand the geological forces involved in the formation of mountain ranges
along the edges of tectonic plates. The study has been published in the latest
edition of the journal Science.
The
Tibetan Plateau is located on the border of the Eurasian continental plate,
which is colliding with the Indian plate. This collision has led to the uplift
of the plateau over millions of years. In order to determine the elevation of
mountains over the course of Earth’s geological history, researchers often use
a special geological archive – the water stored in the ground millions of years
ago. The method is based on the relationship between various stable oxygen
isotopes – oxygen atoms of differing mass.
The
underlying theory says that rain contains fewer heavy isotopes the higher it
falls. This means that geoscientists can draw conclusions about the previous
altitude of the location from which the sample was taken. For the Tibetan
Plateau, the samples yielded data for an altitude of about 4,000 meters in the
Eocene. “We questioned these results because the distribution of the oxygen
isotopes not only indicates the altitude above sea level, it also reflects the
influence of palaeoclimate,” Svetlana Botsyun explains.
Interplay of many
factors
In
the Eocene – the geological period from about 56 to 33.9 million years ago –
the concentration of carbon dioxide and other greenhouse gases in the
atmosphere was far higher than it is today. Asia’s temperature distribution and
geography were also very different. There was a large, shallow sea – which
geologists call the Paratethys – bordering the Eurasian Plate. And the Indian
continental plate was ten degrees latitude further south from its current
position. “All these conditions in the Eocene had an effect on the proportion
of oxygen isotopes, so we included them in our climate simulations,” Dr.
Botsyun says. That resulted in a completely different picture.
“Our
simulations showed that, due to Tibet’s more southerly position in the Eocene,
the isotope relationships in rainwater were actually reversed. On the southern
flank of Tibet, heavier water was precipitated at higher altitudes,” says Svetlana
Botsyun. “Therefore we must abandon the conventional wisdom that there was a
uniform relationship between the mountain elevation and the proportion of heavy
oxygen isotopes in rainwater during earlier geological periods.”
The
team’s new findings fit with a scenario in which the Tibetan Plateau appears to
have been no more than 3,000 meters high. “In the future we will combine
climate models with the isotopic data from the geological archives to obtain
reliable data on elevation in earlier phases of the Earth’s history,” Dr.
Botsyun explains.
原始論文：Svetlana
Botsyun, Pierre Sepulchre, Yannick Donnadieu, Camille Risi, Alexis Licht,
Jeremy K. Caves Rugenstein. Revised
paleoaltimetry data show low Tibetan Plateau elevation during the Eocene. Science, 2019 DOI:
10.1126/science.aaq1436
引用自：University
of Tübingen. “Tibetan plateau rose later than we thought.”
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