全球氣候跟侵蝕速率的關係？

原文網址：https://www.gfz-potsdam.de/en/media-and-communication/news/details/article/what-does-global-climate-have-to-do-with-erosion-rates/

全球氣候跟侵蝕速率的關係？

Josef Zens

過去數十年，地質科學家一直對地球表面的侵蝕速率和全球氣候變化之間的潛在關聯深感興趣，不過其中的成因和作用仍然不明。然而，一項新研究則直接質疑關聯本身的存在與否。由德國波茨坦地質科學研究中心、波茨坦大學、格勒諾布爾大學和愛丁堡大學組成的研究團隊重新檢視了過往研究中指出的，在數百萬年前的冰期―間冰期循環開始之後侵蝕作用也加速的30個地點。結果近乎在所有地點中提出的侵蝕和全球氣候的潛在關聯都無法被確實證明。他們的研究成果刊登於期刊《自然》(Nature)。

西阿爾卑斯山的Pilatte冰河(圖片來源：Taylor Schildgen)

這項基礎概念聽起來很有說服力：侵蝕速率加快會造成矽酸鹽的風化速度加快，並讓沉積盆地中埋藏更多有機碳，兩者都可以移除大氣中的二氧化碳而引發全球冷化。另一方面，根據過去數百萬年以來全世界海洋的沉積速率都有加快的現象，科學家提出這段期間的全球侵蝕速率加快和冰期―間冰期循環有關。冰河刮過地表之後，隨後的暖化會讓融水運走更多沉積物到海洋，這或許能解釋為何沉積物累積的速率會加快。

但其他研究卻指出全球侵蝕速率在這段期間可能處在穩定狀態，而我們表面上看到的沉積物累積速率增加，其實是由於沉積物堆積時在空間和時間上的不規則導致；另外，年代較老的沉積物相較於較年輕的，也更有可能因為侵蝕而消失。

熱定年法(thermochronology)可以追溯出岩石往地表移動時其溫度如何下降的歷史。不久之前，彙整全球熱定年的數據顯示過去數百萬年以來，山脈地區的地表侵蝕速率增加了近乎兩倍。因此，冰期―間冰期循環和侵蝕速率加快之間的連結似乎已經罪證確鑿――直到這篇研究發表之前。由GFZ、波茨坦大學、格勒諾布爾大學和愛丁堡大學組成的研究團隊在Taylor Schildgen的領導下，重新檢驗了之前發表的研究中，根據熱定年法而被認為侵蝕速率有加快的30個地點。

他們的分析顯示在其中23個地點所提出的侵蝕加速，其實是由他們稱為的「空間相關性誤差」(spatial correlation bias)造成。也就是結合許多數據時若它們代表的降溫歷史並非全然一致，則此過程就會將沉積速率的空間變化轉譯成隨時間經過而增加。在多數案例中，研究結合的數據會有各自不同的降溫歷史是因為這些數據的點位跨過了大型的構造界線(如：斷層)。而在另外四個地點，侵蝕加速則可以用構造的變形速度增加來解釋(也就是造山運動的過程加快)，而非氣候變遷。

總結來說，過往研究中提出侵蝕速率加快和氣候有關的30個地點，其中27個是有誤的。這可以用在先前的分析中忽視了數據含有的區域性性質來解釋――此為潛藏在大數據分析中的重大陷阱。只有三個案例記下了氣候引起的侵蝕加速，成因為冰河在當地往下切出谷地而導致。

研究團隊的發現顯示在評估過去數百萬來的氣候變遷於全球尺度上是否有影響到侵蝕速率時，熱定年法的數據目前還尚未具有足夠的解析度。他們的結論認為目前為止還沒有任何數據能明確支持侵蝕速率加快和全球冷化有所關連的理論。不過，在結合了含有區域特有資訊的地區性發現之後，或許科學家能更深入地探討全球冷化和侵蝕速率的驅動因素。

What does global climate have to do with erosion rates?

For the last several decades, Geoscientists have been intrigued by a potential link between erosion rates at the Earth’s surface and changes in global climate. What was the cause and what the effect remained unclear. However, a new study now calls into question the link itself. A team of researchers from the GFZ German Research Centre for Geosciences in Potsdam, the University Potsdam, University of Grenoble, and University of Edinburgh re-examined 30 locations with reported accelerated erosion after the onset of glacial-interglacial cycles a few million years ago. In nearly all of the locations, the proposed link between erosion and global climate could not be confirmed. Their study is published in Nature.

The basic ideas sound convincing: Faster erosion rates can lead to faster silicate weathering and efficient burial of organic carbon in sedimentary basins, both of which can induce global cooling by removing CO2 from the atmosphere. On the other hand, a global increase in erosion rates over the last several million years was associated with glacial-interglacial cycles. This was proposed on the basis of accelerated worldwide sedimentation rates in the oceans. Glaciers scraping off landscapes and subsequent warmings that lead to meltwater transporting sediment into the sea are plausible causes for increased sediment-accumulation rates.

Yet, other studies have indicated that global erosion rates may have remained steady over this time period, and that the apparent increased sediment-accumulation rates are due to the irregularities in how sediments are deposited in space and time, and because older deposits are more likely to be lost by erosion compared to younger deposits.

More recently, a global compilation of thermochronology data, which tracks the cooling history of rocks as they move toward the surface, has been used to infer a nearly two-fold erosion-rate increase from mountainous landscapes over the last several million years. So the link between glacial-interglacial cycles and faster erosion seemed to be confirmed – until a team of researchers from the GFZ, led by Taylor Schildgen, and from the Universities of Potsdam, Grenoble, and Edinburgh re-examined the 30 locations with reported accelerated erosion based on thermochronology.

Their analysis shows that in 23 of these locations, the reported increases are a result of what they term a 'spatial correlation bias'; i.e., combining data with disparate cooling histories, a process that converts spatial variations in erosion rates into temporal increases. In most cases, the disparate cooling histories result because data points were combined across major tectonic boundaries (faults). In four other locations, the increases can be explained by accelerated tectonic deformation (i.e., faster mountain-building processes), rather than climatic changes.

Together, these 27 erroneous out of 30 proposed links between faster erosion and climate can be explained by neglecting the local context of the data in the earlier analysis, a dangerous potential pitfall in big-data analysis. In only three cases, climatically induced accelerations are recorded, driven by localized glacial-valley incision.

The team’s findings suggest that thermochronology data currently have insufficient resolution to assess if climate change over the last several millions years affected erosion rates on a global scale. They conclude that currently, no data provide clear support for the hypothesized link between faster erosion and global cooling. Nonetheless, a synthesis of local findings that include location-specific information may help to further investigate drivers of global cooling and erosion rates.

原始論文：Taylor F. Schildgen, Pieter A. van der Beek, Hugh D. Sinclair, Rasmus C. Thiede. Spatial correlation bias in late-Cenozoic erosion histories derived from thermochronology. Nature, 2018; 559 (7712): 89 DOI: 10.1038/s41586-018-0260-6

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "What does global climate have to do with erosion rates?”