苔蘚造成的岩石風化作用或許可以解釋發生於奧陶紀晚期的氣候現象

原文網址：www.sciencedaily.com/releases/2016/07/160707101029.htm

Weathering of rocks by mosses may explain climate effects during the Late Ordovician

苔蘚造成的岩石風化作用或許可以解釋發生於奧陶紀晚期的氣候現象

During the Ordovician period, the concentration of CO2 in Earth's atmosphere was about eight times higher than today. It has been hard to explain why the climate cooled and why the Ordovician glaciations took place. A new study, published in Nature Communications, shows that the weathering of rock caused by early non-vascular plants had the potential to cause such a global cooling effect.

奧陶紀時地球大氣二氧化碳的濃度是現在的八倍左右。因此很難解釋為何當時氣候會逐漸變冷以及奧陶紀冰河期的發生。刊登於期刊《自然通訊》(Nature Communications)的新研究顯示早期非維管束植物造成的岩石風化作用可能是全球冷化效應的成因。

"When we can better understand the carbon cycle in the past, we can better predict what happens with the climate in the future," says Philipp Porada of Stockholm University, one of the authors of the study.

「當我們對過去的碳循環有更加透徹的了解，我們就能更加準確地預測未來氣候會發生的變化。」本篇研究作者之一，斯德哥爾摩大學的 Philipp Porada說。

Non-vascular plants, such as mosses, hornworts and liverworts, probably evolved during the Ordovician period, around 450 million years ago. They are older than vascular plants, such as trees and grasses, and together with lichens, which are a symbiosis of fungi and algae, they formed the earliest terrestrial vegetation. Today's successors of these organisms are distributed worldwide and are characterised by their ability to survive in environments in which the supply of both water and nutrients is scarce. They are found in both cold and warm desert regions and are able to grow on rock surfaces and the bark of trees. Although they do not have real roots, they affect the surfaces on which they grow: the release of various organic acids dissolves underlying rock minerals.

非維管束植物，像是苔蘚、角苔和地錢可能是在4億5000萬年前左右的奧陶紀演化出來。其歷史早於樹木與花草這類的維管束植物許久，並且跟地衣(真菌和藻類的共生體)一起組成了最原始的陸上植被。今日在世界各地都有這些生物後代的蹤跡，並以能夠生活在水分及養分供應皆相當匱乏的環境聞名。它們可以存活於寒冷和炎熱的荒漠地區，也能生長在岩石表面及樹皮表層。雖然缺乏真正的根系，它們卻有方法影響它們生存的表面。它們可以分泌出各種有機酸來溶解下方岩石含有的礦物。

This process of dissolution and chemical transformation of rock minerals is called chemical weathering. Non-vascular plants and lichens may considerably increase weathering rates of the rock surfaces on which they grow. This has important implications for the climate system, since chemical weathering of silicate rocks such as granite results in a drawdown of atmospheric CO2 and may therefore lead to global cooling. During the weathering process CO2 dissolves in water as acid, and is then transported to the ocean where the carbon is buried as carbonate rock. Consequently, it has been hypothesised that early non-vascular vegetation caused an interval of glaciations at the end of the Ordovician period, when they became globally abundant. Without the drawdown of atmospheric CO2 caused by the enhancement of weathering rates, the Ordovician glaciations are hard to explain, since they started under conditions of eight times higher atmospheric CO2 than today.

岩石含有的礦物溶解且化學成分改變的過程稱作化學風化。非維管束植物和地衣或許能夠大幅增加它們生長的岩石表面的風化速率。這對氣候系統來說具有相當重大的意義，因為像花崗岩這類的矽酸岩發生化學風化時會減少大氣中的二氧化碳，而可能造成全球冷化。在風化過程中二氧化碳會以酸的形式溶解在水裡，接著被運輸至海洋，在此碳會形成碳酸鹽並埋藏在海底。因此，有項假說認為奧陶紀末期出現的冰河期，便是發生於當這些早期非維管束植物在全球數量大為增加之後。由於奧陶紀開始時大氣二氧化碳濃度是當今的八倍左右，因此若沒有風化速率增加導致的大氣二氧化碳含量下降，奧陶紀冰河期的成因便相當難以解釋。

"I believe that the most interesting thing about the study is that tiny plants such as mosses and lichens can influence global climate in the long run," says Philipp Porada.

「我認為這項研究中最有趣的部分是像苔蘚和地衣這般矮小的植物，長期而言卻能夠影響全球氣候。」 Philipp Porada說。

"However, it is difficult to extrapolate today's weathering rates by non-vascular plants and lichens measured in the field to a global effect on chemical weathering in the Ordovician. In our study we therefore use a process-based numerical model of non-vascular vegetation to simulate weathering by these organisms in the Late Ordovician. We find a high potential for weathering, which means that the emergence of early non-vascular plants and lichens indeed may have been the reason for the Late Ordovician glaciations."

「然而，要將今日在野外測得由非維管束植物和地衣導致的風化速率，外推至在奧陶紀時此效應對全球化學風化的影響是相當困難的。因此，在我們的研究當中，利用了一種基於過程的非維管束植物數值模型來模擬奧陶紀晚期時這些生物造成的風化作用。我們發現它們對風化作用可能具有相當大的影響，代表早期維管束植物跟地衣的出現確實有可能是晚奧陶紀冰河期的成因。」

引用自：Stockholm University. "Weathering of rocks by mosses may explain climate effects during the Late Ordovician." ScienceDaily. ScienceDaily, 7 July 2016.