Heavy erosion caused by the Ganga at Jalangi in Murshidabad district

The muddy rivers of the Himalayas may sometimes be a bane for locals living in the hills. A source of continuous land erosion, they often lead to landslides and other natural disasters. But rivers like the Ganga and the Brahmaputra — which deposit 1 to 2 billion tonnes of sediment in the Bay of Bengal every year — also do a world of good to the global environment. By burying humongous quantities of organic carbon, these rivers help to reduce the quantity of heat-trapping carbon dioxide that is released in the atmosphere.

The work, by a team of European scientists, is significant because it overturns an earlier theory of the role played by rivers in carbon sequestration. Similar studies in the past conducted on the Amazon and the Mississippi have given a completely contrasting picture, with scientists concluding that up to 70 per cent of the organic carbon from these rivers escape into the atmosphere as carbon dioxide, thanks to a chemical reaction with minerals, before they reach the ocean. The latest findings have been reported in Nature Geoscience.

The researchers — led by Fabien Palhol, a geologist with the French national research system (CNRS) — found that in the Ganga-Brahmaputra system, nearly 70 to 85 per cent of the organic carbon in the sediments gets trapped in the ocean bed. The study is also significant from the point of view of global climate change as the Bay of Bengal accounts for 10 to 20 per cent of the total organic carbon trapped by all the oceans in the world.

The Himalayas, as is known, is the youngest of all the mountain ranges in the world. The high erosion produces an enormous flux of sediments, which greatly increases the rate of sedimentation in the Bengal Fan (or Bay of Bengal). This very intense physical erosion finally promotes the burial efficiency of organic carbon, the researchers say. And this is true of all the young ranges of the tropical region.

The process also directly helps the atmosphere to retain a positive balance between the amount of oxygen produced by plants through photosynthesis and that consumed during respiration. This is because when organic matter is buried in ocean sediments, and ceases to be decomposed, atmospheric oxygen accumulates. The exact rate of organic matter burial, therefore, is a significant determinant of atmospheric composition — and thus of global climate over geological time scales, the study says.

“It is crucial to understand that the process we are documenting is effective only on geological time scales. The burial of photosynthetic organic carbon in marine sediments constitutes a net sink of atmospheric carbon dioxide and a net source of oxygen over such a large period of time. This way it counterbalances the injection of carbon dioxide by volcanism and metamorphism,” says Dr Valier Galy, a co-author of the study and a post doctoral scholar at the Woods Hole Oceanographic Institution in Massachusetts, the US. “Over the geologic history of the earth, the process has allowed the oxygen to accumulate, also helping to maintain a moderate level of carbon dioxide in the atmosphere. This, in turn, has resulted in reasonable global temperatures,” he explains.

So does that mean that if such river deposition continues to occur, it may help combat the problem of global warming? Well, it’s not so simple, says Galy. “The process we describe is not effective on short time scales and will, therefore, not help to counterbalance the anthropogenic (resulting from our daily activities) emissions of carbon dioxide. However, assuming that we stop our carbon dioxide emissions, this natural process (along with others) will tend to consume the excess carbon dioxide that we have been rejecting. But this will take hundreds of thousands of years,” he says.

The study also explains how the Ganga-Brahmaputra system scores over some of the other major river systems such as the Amazon rivers. “In the case of the Himalayan deposition, 100 per cent of the organic matter transported by the rivers is actually buried in marine sediments. This contrasts with the Amazon system where only 30 per cent of the organic mater is buried. The remaining 70 per cent is oxidised, resulting in the release of carbon dioxide,” explains Galy.

So in addition to quantifying the processes at work, Galy’s work also helps to get a better picture of the might of the Himalayan rivers. “The findings illustrate the role of erosion in the carbon cycle and long-term climate regulation. The erosion brought about by mountainous rivers does play a role in cooling the climate, but over tens of millions of years,” sums up Galy.