A feast on Earth’s topsoil left crumbs in the geologic record — big
ones. And while each piece offers scientists pleasing examples of erosional
events, it was uncertain how the events were related. A new study amassing
the banquet of data shows both sedimentation rates and grain sizes increased
globally some time beginning 2 million to 4 million years ago.
Before this study, “people had no compiled record that sedimentation rates picked up in a global way,” says Kelin Whipple of the Massachusetts Institute of Technology, whose own work on erosion took him to northern Pakistan in 1997. “The data existed but no one had brought it all together.”
That is until Zhang Peizhen of the State Seismology Bureau in Beijing, China, Peter Molnar of the Massachusetts Institute of Technology and William Downs of Northern Arizona University in Flagstaff, reported their analysis in the April 19 Nature. But, as with any global phenomena, a global jump in sedimentation during a specific time in Earth’s history demands an explanation. After ruling out globally synchronous changes in tectonic-plate motions (the mountains weren’t growing in unison), Peizhen, Molnar and Downs suggested that climate should be held responsible.
The researchers compared sediment accumulation from variable geologic conditions, from active to inactive tectonic regions, and from off-shore and onshore basins. The oldest sediments, dating back to 65 million years, were recorded from the Williston Basin of North Dakota, the Mississippi River drainage basin, the Vøring Plateau off Norway, the northwestern Sumatra Basin and the Mergui Basin to its north, the North Sea, the Apennine Foredeep in the Adriatic Sea, the northern margin of the Tibetan Plateau, the northern mountain margins of Tien Shan, China, and the Valley of Lakes, Mongolia.
Between 65 million and 5 million years ago, sedimentation rates appeared fairly constant or increased only gradually. This rate spiked around 5 million years ago almost everywhere the researchers looked, with few exceptions. The authors say this global sediment spike is evidence for a global increase in erosion.
Whipple applauds the authors for the comprehensive analysis, but he disagrees with their using increased sedimentation rates along continental margins as examples of primary erosion. In the report, Peizhen and colleagues site an earlier theory that episodes of continental ice sheets lowered sea level and exposed continental margins to fluvial attack. The Mississippi delta is one such example where rivers chowed down and left terrigenous deposits offshore. But, Whipple argues, “you’re looking at a redistribution of already eroded sediment. It is not telling you anything about the increased primary erosion. This is simply shifting from one temporary storage location to the next.”
Had the authors thrown out all of their samples from offshore basins, they would still have a solid argument for a global rise in sedimentation rates and hence erosion “because most of their sites were not in that category,” Whipple says. Most of their pickings came from inland Asia, Australia, Europe and the United States.
“Sedimentation rates are determined by measuring the thickness of material accumulated in a finite time, summing it over the area of deposition and dividing by the time interval,” Molnar says. Working on a million-year time scale smoothes out short term variations, such as floods and ice ages, and ensures that the erosion they investigated is not an artifact of the latest dirt in the geologic record. “Dirt lying around that long would become rock,” he says.
The steepness of a mountain determines erosion. The higher a mountain grows, the more gravity will act on rivers and glaciers moving sediment downhill. But there was no growth hormone acting on the mountain belts close to 5 million years ago. This leaves climate as the guilty culprit, the team says.
In a steady climate, erosion rates tend to come into balance with the growth rate of the mountains, the researchers say. But as Earth’s climate cooled and then continued to vary with on-and-off again glaciers, protective vegetation grew sporadically and erosive agents such as chemical weathering, glacial fracturing and rivers went wild, they say. “The inescapable conclusion that sedimentation, and therefore presumably erosion, increased beginning at 2 million to 4 million years ago requires an explanation, and if there is a simple one, that is the simplest we can find,” Molnar says.