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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.
Christina Reed
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