化學風化控制了河川的侵蝕速率

原始網址：www.sciencedaily.com/releases/2016/04/160414095555.htm

Chemical weathering controls erosion rates in rivers

化學風化控制了河川的侵蝕速率

Chemical weathering can control how susceptible bedrock in river beds is to erosion, according to new research. In addition to explaining how climate can influence landscape erosion rates, the results also may improve scientists' ability to interpret and predict feedbacks between erosion, plate tectonics and Earth's climate.

根據新的研究顯示，化學風化可以控制河床的岩石有多容易受到侵蝕作用影響。除了解釋氣候會如何影響地表的侵蝕速率之外，研究結果或許也能讓科學家對於侵蝕作用、板塊構造運動和地球氣候彼此之間的交互作用做出更準確的分析和預測。

The research, led by The University of Texas at Austin, was published in Nature on April 14, 2016.

這項由德州大學奧斯汀分校領導的研究成果刊登於2016年4月14日的《自然》(Nature)期刊上。

"Our research presents a specific, process-based mechanism to explain how and why river erosion depends on climate, and also perhaps why previous studies have found conflicting sensitivities to climate in different landscapes," said Brendan Murphy, a Ph.D. student at The University of Texas Jackson School of Geosciences who led the research.

「我們在研究成果中呈現的精確流程化機制，不僅可以用來解釋氣候如何影響河川侵蝕作用，以及兩者之間的因果關係，或許也能告訴我們為何在之前的研究中，河川侵蝕作用受氣候的影響程度在不同地景中並不一致。」德州大學奧斯汀分校地球科學院，主持此研究的博士生Brendan Murphy說。

Murphy conducted the research with Joel Johnson, a professor in the Jackson School's Department of Geological Sciences, Nicole Gasparini of Tulane University and Leonard Sklar of San Francisco State University.

Murphy偕同德州大學奧斯汀分校地球科學院的教授Joel Johnson、杜蘭大學的Nicole Gasparini以及舊金山州立大學的Leonard Sklar進行了此篇研究。

Chemical weathering occurs when minerals in rock react with water. These chemical reactions physically weaken rock by altering its structure. Rocks in streambeds then become more susceptible to erosion by physical processes, such as impacts by sediment carried in flowing water.

化學風化發生在當礦物與水接觸而發生反應時，這些化學作用會改變岩石的結構而弱化其物理強度。河床上的岩石因此變得更容易遭受物理過程所導致的侵蝕作用，像是流水中攜帶的沉積物顆粒造成的衝擊。

It has been established that chemical weathering influences rock strength, Murphy said. But scientists have lacked data on the extent to which chemical weathering influences river erosion. To explore the issue, the team travelled to the Big Island of Hawaii, where the bedrock is made entirely of volcanic basalt, to collect data on chemical weathering, rock strength, and erosion rates in streams across wet and dry regions of the island.

Murphy說科學家早已確認化學風化會影響岩石強度，但他們在化學風化對河川侵蝕的影響程度究竟有多深這方面並未擁有足夠的資料。為了深究這項議題，研究團隊前往夏威夷的大島(Big Island)。此地的岩石幾乎都是由火山噴發出來的玄武岩構成。研究團隊蒐集了這座島上乾燥與濕潤區域中各溪流流域內的化學風化、岩石強度以及侵蝕速率的資料。

"Hawaii is a simple, natural laboratory for studying how climate controls river erosion because it has uniform lithology and a very extreme precipitation gradient," Murphy said. "We went to investigate if the local precipitation rate was changing the rock strength in the rivers and then looked for a mechanism to explain it."

「在研究氣候如何控制河川侵蝕作用時，夏威夷可視為一座相當單純的天然實驗室。這是因為這座島上各地的岩石組成相當一致，然而降水梯度(precipitation gradient)則相當極端。」Murphy說。「我們前去調查當地的降雨量是否會影響河川中岩石的強度，接著再探討可以解釋其箇中緣由的機制。」

They measured the strength of the rock using a Schmidt hammer, a device that measures surface hardness in the field, and also analyzed the chemistry and density of rock samples back in the lab to determine the influence of chemical weathering.

他們利用施密特錘(Schmidt hammer)來測量岩石的強度，這是一種可以在野外量測岩石表面硬度的裝置；並且將岩石樣本帶回實驗室分析它們的化學性質和密度，以得知化學風化的影響程度。

Consistent with their hypothesis, they found that bedrock was more chemically weathered and physically weaker where local precipitation rates were greater. More significant, Murphy said, was their finding that locations of high precipitation could maintain high erosion rates despite continuously exposing "fresh rock" -- rock that was previously below the eroded surface and is not chemically altered.

他們發現在降雨量較高的地區，河床岩石的化學風化程度就越高且物理強度越脆弱，這點與他們的假設相符。Murphy說更重要的是，他們發現在降雨量高的地區，儘管「新鮮岩石」—也就是之前位於受侵蝕的表面之下，化學性質尚未改變的岩石—會不斷地出露在地表，侵蝕速率卻仍能維持在高檔。

Fresh bedrock weathers rapidly when exposed at the surface, which weakens rock and allows it to be efficiently eroded by the river, the researchers found.

研究人員發現新鮮的底岩一旦暴露於地表就會迅速地遭受風化，這使得岩石變得脆弱而讓河川能夠進行高效率的侵蝕作用。

"This presents a positive feedback allowing river streambeds to maintain high weathering rates, weaker rock, and high erosion rates," Murphy said.

「在此呈現出的正回饋效應使得河床可以維持在高風化速率、岩石強度弱化以及高侵蝕速率的狀態。」Murphy說。

Based on their findings, the researchers modified a numerical model that describes how rivers cut into a landscape, Johnson said, finding that chemical weathering data drastically improved their ability to predict patterns of river incision.

Johnson說研究人員根據他們的成果，修改了之前用來描述河川如何切割地表的數值模型，而他們發現這些關於化學風化的數據可以大幅精進模型預測河川侵蝕模式的能力。

"Once we included the climate effect demonstrating that the chemical weathering is weakening the bedrock and making it more erodible, we can do a much better job of matching the pattern and rates of incision that occur across this landscape." Johnson said.

「我們一將化學風化作用使岩石弱化以及更易侵蝕的氣候效應囊括進模型當中，就能使模擬結果更符合研究中地貌的侵蝕速率與模式。」Johnson說。

Even though researchers examined only a single rock type, Murphy said that the mechanism linking chemical weathering to rock strength and erosion should apply to all types of rock. Understanding the relationship between erosion and chemical weathering can help tease out the role climate has on sculpting landscapes and influencing global cycles, Murphy said.

即便研究人員只探討了單一岩石種類組成下的情況，Murphy說化學風化跟岩石強度和侵蝕作用之間的關聯應該可以適用於所有的岩石類別。他也表示了解侵蝕作用和化學風化之間的關係，有助於我們釐清氣候在形塑地表的作用中扮演了何種腳色，以及氣候如何影響全球物質的循環。

"The ability to better understand how landscapes erode is important, because bedrock erosion affects chemical weathering, which is a major component of the global carbon cycle and can influence global climate by the removal of carbon dioxide from the atmosphere," Murphy said. "The ability to model landscape evolution and how climate plays into it is important for tying these larger global cycles together."

「更加了解地表如何遭受侵蝕是件相當重要的事，這是因為岩石的侵蝕作用會影響化學風化，而化學風化在全球碳循環中位處至關重要的一環，它會將二氧化碳從大氣中移除進而影響全球氣候。」Murphy說。「若我們要嘗試將不同的大型全球循環系統整合在一起，那麼可以更精確地模擬地貌演變及氣候在其中所占地位的能力便相當重要。」

The research was funded by the National Science Foundation and a Tulane Research Enhancement grant.

這項研究由國家科學基金會以及杜蘭大學研究發展津貼贊助。

引用自：University of Texas at Austin. "Chemical weathering controls erosion rates in rivers." ScienceDaily. ScienceDaily, 14 April 2016.