Rapid Himalayan erosion triggers hidden source of CO2: study

On August 26, devastating flash floods swept across Nepal’s mountains, killing over 1,300 and laying waste to whole towns and villages. As a warming climate destabilises Himalayan glaciers, the glaciers themselves are influencing how much carbon dioxide (CO2) is released into the atmosphere, driving global warming.

The Indian plate continues to push into the Eurasian plate and the Himalayan mountains are getting eroded, exposing fresh silicate minerals. These minerals react with CO2 in the atmosphere and later wash into the oceans, locking the carbon in seafloor sediments.

Scientists believe this silicate weathering has helped cool the earth over the last 50 million years. However, a study published recently in Chemical Geology by researchers from IISER Pune, the Wadia Institute of Himalayan Geology, and IIT-Roorkee suggests the picture is not so simple — that, in fact, another mineral process may be more than offsetting the cooling effect.

The team studied the area around the headwaters of the Indus river. Aside from silicates, the Himalayan mountains contain many other minerals, including pyrites, a.k.a. fool’s gold. When mountain slopes are eroded rapidly, the sulphides in pyrites are exposed to air and water for the first time in millions of years, kicking off an important chemical reaction. First, the sulphides are oxidised to sulphuric acid. When it reacts with carbonate rocks — which abound in the Indus basin — CO2 is released into the air.

By measuring the ratio of isotopes of sulphur and oxygen in the Indus river system, the team was able to say if the sulphates in the water came from dissolved gypsum or from pyrite oxidation. Thus, In the upper Indus basin, it estimated the rate of CO2 oxidation to be three-times higher than CO2 uptake by silicate weathering.

The team also found while the flatter floodplains downstream were CO2 sinks, the steep mountainous regions were net CO2 sources. This is partly because water moves more slowly in the floodplains, allowing silicate weathering to dominate.

The study authors wrote that glaciers excel at crushing rocks and exposing the pyrites within, rendering erosion a double-edged sword: it provides the ‘raw materials’ to remove CO2 in the mountains even as the intensity of that erosion facilitates the more aggressive reactions of sulphides.

“Our findings demonstrate that glacier-fed river catchments are important geological sources of CO2, which may influence our long-term carbon cycle and contribute to climate warming over million-year timescales,” the team said in a release.