The thermal history of thousands of rock samples convincingly confirms the idea that climate cooling accelerates the rate of erosion at Earth's surface — and implicates glaciers in particular. See Letter p.423
On page 423 of this issue, Herman et al.1 report an analysis of mountain-range erosion rates which shows that global cooling in the past 6 million years has accelerated the destruction of mountains. This result reignites a long-lived debate about links between climate, topography and plate tectonics.
The high topography of Earth's mountain ranges is made when plate tectonics force the continental plates to slowly collide. However, erosion by rivers, glaciers and landslides constantly counteracts this mountain-building process by breaking down bedrock and moving the resulting sediment to lower elevations, where it accumulates in sedimentary basins or oceans. The structure of mountain ranges therefore reflects a complex balance between constructive and destructive forces.
Although we can measure current changes in topography using the Global Positioning System, finding out what happened in the past and obtaining data that span the extremely long timescales of these processes are major challenges. An intriguing question relates to how climate influences the erosion processes, and thereby the elevation and morphology of mountain ranges2,3.
About 6 million years ago, Earth's global climate started a strong cooling trend that led to glaciations in high mountain ranges and at high latitudes4. Extensive ice masses then developed at the beginning of the Quaternary period (the most recent 2.6 million years). It has long been recognized that the volume of sediment that has accumulated in the oceans within the past few million years far exceeds that measured for any other period of a similar length5, which points to increased erosion rates during the Quaternary6. But some have questioned whether this observation is biased by the difficulty of measuring the correct volume of older sediments and, therefore, whether surface processes truly led to faster erosion when the global climate cooled and started to fluctuate7. Herman et al. address this question by analysing new information: the thermal history of rocks.
Using a technique known as thermochronology, the thermal history of a rock sample can be reconstructed from the relative concentrations of certain noble gases within it or from the distribution of damage trails produced by radioactive decay8. Specifically, thermochronology allows the dating of the time when a rock cooled to a 'closure' temperature — the temperature below which gaseous isotopes no longer diffuse out of the rock and/or when damage trails stop annealing. This, in turn, provides an estimate of how fast erosion brought the rock closer to the surface, because temperature decreases with distance from the centre of the Earth. Closure temperatures vary from 70 to 250 °C, depending on the specific thermochronometer used.
By combining several thermochronometers that had different closure temperatures, Herman and colleagues implemented a clever approach to determine past changes in erosion rate. They compiled a global set of nearly 18,000 thermochronology data points and then used an algorithm to reconstruct patterns of erosion rate for several time intervals. The results reveal that the erosion of Earth's mountain ranges did indeed accelerate globally as the climate cooled, confirming the information provided by sediment volumes.
The progressive increase in erosion rate is most pronounced at intermediate latitudes (30° to 50°) within the past 2 million years. Herman and co-workers therefore propose that glaciers are the main driver of the accelerated erosion because, at these latitudes, many high-elevation landscapes were glaciated for the first time. Their conclusion is supported by the fact that glaciers are known to have left their distinctive imprint on the morphology of landscapes within a fairly short period. For example, the substantial landscape modifications that formed the spectacular glacial fjord systems of Norway, Greenland, western North America, Chile and New Zealand (Fig. 1) must have occurred within a few million years. Glaciers are efficient agents of erosion because they can abrade and quarry bedrock as they slide down through steep topography.
The hypothesis that climate change was the main driver of recent increased mountain-range erosion has provoked intense debates9,10. Sudden pulses of erosion have conventionally been attributed to changes in tectonic activity rather than climate. Many geologists have therefore interpreted the increased erosion of the recent past as a product of tectonic uplift — even in places where no direct evidence of tectonic plate movements exists.
Unfortunately, Herman and colleagues' analysis cannot resolve what happened in these controversial regions. The reason is that erosion rates in these areas were generally low before they started to increase less than 10 million years ago, but the authors' thermochronological method requires total erosion to be high enough to uncover rocks from depths that are associated with closure temperatures. This amounts to kilometre-scale erosion, even for the thermochronometers that have the lowest closure temperatures. Such levels of erosion are generally reached only in areas where tectonic uplift has maintained high erosion rates for a long period, which is why the researchers' analysis is limited to areas where substantial tectonic activity happens today or occurred at about the time of global cooling.
Even with this limitation, Herman et al. convincingly demonstrate the global scale of the recent erosion phenomenon. Their results suggest that climate drives erosion, because, unlike tectonic activity, climate can change synchronously on a global scale.
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