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For decades, the Canadian geologist Martin Gibling has been intrigued by the tough-to-prove hypothesis that land plants created the shape of modern rivers hundreds of millions of years ago.
Plant roots reinforced the ground, the thinking goes, creating stable banks that funneled what once were wide, shallow water flows into narrower and deeper channels. By extension, that set the stage for lots of significant Earth history events, including the rise of human civilizations in modern river basins so many millennia later.
Now Gibling and postdoctoral scientist Neil Davies, both at Dalhousie University, have strengthened this case. When the pair compared a much-improved plant fossil record with evidence of how rivers changed very long ago, the transitions matched up.
”As soon as the plants got a foothold on land and rooted vegetation started, that changed the landscape. Basically plants engineered that landscape as they evolved,” says Davies. He and Gibling have published the findings in both Geology and Earth-Science Reviews.
几十年来，加拿大地质学家马丁·吉布林（Martin Gibling）沉迷于研究数十亿年前陆地植物形成现代河流的难以证实的假设
有思想认为，植物根系增强了地面，创造了稳定的河岸，这些河岸曾经是宽阔的，浅水流入更窄更深的河道。通过扩展，这为许多重大的地球历史事件奠定了基础，包括数千年后现代河流流域人类文明的兴起。
现在，Dalhousie大学的Gibling和博士后科学家Neil Davies强化证明了这个case。当两人将大大改善过的植物化石记录与很久以前河流如何变化的证据进行比较时，这些转变相互匹配。
“一旦植物在陆地上立足并开始植根植被，景观就会改变。戴维斯说，基本上植物在进化过程中设计了景观。他和Gibling在G和E中发表了这些发现。
Back in the Cambrian period, which ended some 500 million years ago, the geologic record indicates that rivers were very shallow but wide things, almost floods that allowed rainwater to wash from largely barren solid ground to sea. Deposits left behind were preserved as sheets of coarse grains, some of which suggest these rivers were 1,000 or more times as wide as they were deep.
”There is probably nowhere on Earth where rivers form the way they did before vegetation,” Gibling says.
回到大约5亿年前结束的寒武纪时期，地质记录显示河流是非常浅但是很宽阔的东西，几乎可以让雨水从大部分贫瘠的坚实地面冲刷到大海。尽管留下的沉积物被保存为粗粒？？？，其中一些表明这些河流的宽度是他们深度的1000倍或更多。
“在地球上可能没有什么地方，河流可以形成他们在植被覆盖之前的方式，”吉布林说。
But by the time of the Silurian-Devonian boundary, some 420 million years ago, the picture found in preserved sedimentary rock changes. The blankets of unconsolidated sediment found in earlier river deposits appear less frequently. It happens just as evidence of land vegetation with root systems also expands in the rock record.
In addition, more complex and diverse river remains emerge, including more traces of mud, probably due to the enhanced chemical weathering that plants assist; smaller-sized sand grains; and samples of organic remains. Significantly, shapes shift too.
Organized deposits become visible in the remains of highly sinuous, single-thread channels. Evidence of lateral accretion—the digging away of material at the outer bends of a river and the simultaneous deposition of material at the inner bends—is more abundant.
但到了大约4.2亿年前的志留纪 - 泥盆纪边界时，在保存的沉积岩中发现的图像发生了变化。在早期河流沉积物中发现的疏松沉积物的毯子???似乎不那么频繁。它发生的同时，根系统的土地植被的证据也在岩石记录中扩展。
此外，还会出现更复杂多样的河流，包括更多的泥浆痕迹，这可能是由于植物协助增强的化学风化;较小的砂粒;和有机残留物样本。重要的是，形状也发生了变化。
有组织的沉积物在高度蜿蜒的单线程通道中变得清晰可见。侧向增生的证据 - 在河流外弯处挖掘材料以及在内弯处同时沉积材料 - 更为丰富。
There is also variation that appears to be related to the local climate during the times that the rivers flowed. ”Before plants evolved, it didn’t matter if a river was in a polar region, a temperate region or an arid region, the rivers looked the same. Later you find differences,” says Davies, who devoted two and a half years to this project.
Edward Cotter, a geologist long on the faculty of Bucknell University, was among the people arguing 30 years ago that rivers went through a big transition during the same period that Gibling and Davies emphasize. He observed it in sedimentary rocks in the central Appalachian Mountains dating from 450 to 250 million years ago. But unlike Gibling and Davies, he had limited evidence with which to extrapolate globally from his observations.
在河流流淌的时期，似乎也存在与当地气候有关的变化。 “在植物进化之前，无论河流是在极地地区，温带地区还是干旱地区，河流看起来都是一样的。后来你发现了不同之处，“戴维斯说，他为这个项目投入了两年半的时间。
长期以来在巴克内尔大学（Bucknell University）教授的地质学家爱德华•科特（Edward Cotter）是30年前人们争论说，在吉布林和戴维斯强调的同一时期，河流经历了一次大转变。他在公元前4.5亿至2.5亿年前的阿巴拉契亚山脉中部的沉积岩中观察到它。但与Gibling和Davies不同，他拥有有限的可以从他的观察中进行全球推断的证据。
”They had a much richer database. They have a much healthier under- standing of how rivers run. They went around to different parts of the world and looked with their own eyes,” says Cotter, whose research Gibling and Da- vies cite in their publications.
Using funding from the Canadian government, Davies and Gibling reviewed 144 published reports describing river sediment preserved in the rock record, dating from the Cambrian to the Devonian, to build their case. They Visited 34 spots themselves in North America and Europe. And they scrutinized experimental results.
One laboratory finding that impressed Gibling was achieved at St. Anthony Falls Laboratory at the University of Minnesota. Working in a tank, researchers there described how vegetation—in this case alfalfa sprouts that were allowed to germinate on banks—transformed a channel that flowed between multiple sandbars into one that self-organized into a single-thread channel.
“他们有一个更丰富的数据库。他们对河流如何更加健康。他们走遍了世界的不同地方，亲眼目睹，“科特说，他的研究表明，吉布林和戴维斯在他们的出版物中引用了这些研究。
利用加拿大政府提供的资金，戴维斯和吉布林审查了144份报告，这些报告描述了从寒武纪到泥盆纪的岩石记录中保存的河流沉积物，以建立他们的案例。他们在北美和欧洲访问过34个景点。他们仔细检查了实验结果。
明尼苏达大学的圣安东尼Falls实验室取得的一个发现给Gibling留下深刻印象。在那里的研究人员中，研究人员描述了植被 - 在这种情况下，苜蓿芽是如何允许在河岸上发芽 - 将多个沙洲之间的通道转变成一个自组织成单线通道的通道。
”The strength of the roots of alfalfa was enough to completely change the whole pattern. That generated a meandering river with banks that migrate and are erosion resistant,” Gibling says.
Dov Corenblit, an associate professor at the University of Paris who describes himself as a biogeomorphologist, says Davies and Gibling have delivered more than just insight into the history of rivers. They have expanded evidence that the biotic and abiotic features of this planet influence one another.
Their findings ”may be considered significant progress in the comprehension of one of the most critical phases in the coupling between physical and biological processes on Earth,” Corenblit says.
The Dalhousie University geologists aren’t done. They want to explore whether any of the periodic mass extinctions experienced on Earth might have affected the shapes of rivers as well. They are scouring the literature for changes preserved from the end of the Permian, when a lot of plant life was wiped out.
”We’ll look to see if rivers reverted to the older form,” Davies says.
“紫花苜蓿根系的强度足以彻底改变整个模式。这产生了一条蜿蜒的河流，河岸迁移并且具有抗侵蚀性，“吉布林说。
巴黎大学副教授Dov Corenblit称自己是一名生物地貌学家，他说戴维斯和吉布林提供的不仅仅是洞察河流的历史。他们扩大了证据证明这个星球的生物和非生物特征相互影响。
他们的研究结果“可能被认为是在理解地球上物理和生物过程之间最重要阶段之一的重要进展，”Corenblit说。
达尔豪斯大学的地质学家还没有完成。他们想要探索地球上经历的任何周期性大规模物种灭绝是否也会影响河流的形状。他们正在搜寻文献，以了解二叠纪末期保存的变化，当时大量的植物生命被消灭。
戴维斯说：“我们将看看河流是否会恢复到旧形态。”
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发表于 2018-11-4 16:35:31
