Researchers determined weathering rates over the past 2 million years most likely did not vary between glacial and interglacial periods, contrary to expectation.
Chemical weathering processes dissolve rock into molecules that can be carried out into the sea, wearing down mountains and even changing the shape of continents Stanford University reported. These processes can have important implications for Earth's carbon cycle which moves carbon dioxide between the land and sea and has an impact on global air temperatures. Scientists had expected weathering rates to slow during Earth's ice age due to lower air temperatures, causing more rainwater to be frozen into glaciers in Europe and North America.
"If you look at how these attributes of climate control weathering rates today, you would expect that weathering and sedimentation rates can vary widely between glacial to interglacial times," said study author Friedhelm von Blanckenburg, a geochemist at the German Research Centre for Geosciences GFZ Potsdam.
The researchers employed a geochemical technique that allowed them to compare the concentration of the isotopes 9Be (found in silicate rock) and 10Be (caused by the collision of cosmic rays with nitrogen and oxygen molecules in the atmosphere) of the element beryllium (Be).
"Because 10Be rains down onto Earth's continents and oceans at more or less a constant rate, it's like a clock that can be used to time processes," von Blanckenburg said. "9Be, on the other hand, can be used to calculate how much dissolved rock has washed into the oceans from rivers."
These isotopes allowed the scientists to reconstruct the weathering flux for nearly the entire Quaternary Period, which lasted about 2.6 million years. The team was surprised to find very little weathering variation between glacial and interglacial periods. The team then performed computer simulations to determine how the flow of water could control weathering. The models came to the same conclusion, as the isotope analysis.
"Our results suggested that globally the aggregate change in discharge from all the rivers was effectively zero between the glacial and interglacial times. That was surprising," said Kate Maher, an assistant professor of geological sciences.
The models showed that while changes in water discharge for rivers at high latitudes in the northern hemisphere could vary between glacial and interglacial periods, the flux for rivers in the tropic did not change by a significant amount.
"The tropics account for more than half of the river runoff globally, so they strongly moderate chemical weathering fluxes during global shifts in climate," Ibarra said. "Because weathering helps balance the global carbon cycle, that means the tropical weathering is a primary driver of atmospheric CO2 levels over very long time scales."
The findings were published in a recent edition of the journal Nature Geoscience.
