Study finds rapid Himalayan erosion releases CO2 through pyrite oxidation
Mountains are said to pull carbon out of the air as rocks weather. In the upper Indus, the balance runs the other way.
Published 13 September 2026. Written by Pratidin from the reports linked at the end; every fact checked by a separate review before publishing. How we work
Scientists from IISER Pune, the Wadia Institute of Himalayan Geology and IIT Roorkee report that fast erosion in the upper Indus basin releases carbon dioxide (CO2) through a hidden chemical route, according to a study in the journal Chemical Geology reported by The Indian Express. When erosion and glaciers grind rock and expose fresh pyrite, an iron sulphide mineral, the pyrite reacts with oxygen and water to form sulphuric acid. That acid dissolves carbonate rocks such as limestone, and the reaction gives off CO2. The team found that CO2 released through this route was about three times the CO2 taken up by silicate weathering, the process usually credited with drawing CO2 out of the atmosphere in mountains.
To separate the sources of sulphate dissolved in the river, the researchers used the ratios of sulphur and oxygen isotopes (forms of an element with different masses) in the Indus river system. These signatures let them tell sulphate formed by pyrite oxidation apart from sulphate from other sources. On this basis they conclude that the steep, rapidly eroding mountain stretches act as a net source of CO2, while the floodplains downstream act as sinks. Glaciers speed up the source side because they crush rock efficiently and keep exposing new pyrite.
The finding matters for a long-running idea in earth science. Silicate weathering, in which rainwater carrying dissolved CO2 breaks down silicate rocks, removes CO2 over very long periods and helps regulate the planet's temperature. The uplift of the Himalayas has been linked to increased silicate weathering and a draw-down of atmospheric CO2 that may have helped start the late Cenozoic ice ages. The new study suggests that in fast-eroding terrain, sulphuric acid weathering of carbonates can outweigh this sink. The effects play out over geological timescales, not the decades that matter for today's warming, but they change how the carbon budget of mountain belts is calculated and are relevant to proposals that use rock weathering to remove carbon.
Prelims facts
- A study in Chemical Geology by IISER Pune, the Wadia Institute of Himalayan Geology and IIT Roorkee focused on the upper Indus basin.
- Oxidation of pyrite exposed by erosion forms sulphuric acid, which dissolves carbonate rocks and releases CO2.
- In the study area, CO2 released by this route was about three times the CO2 taken up by silicate weathering.
- Sulphur and oxygen isotope ratios of river sulphate were used to tell pyrite-derived sulphate from other sources.
- Steep, fast-eroding mountain stretches acted as net CO2 sources, while downstream floodplains acted as sinks.
Quick recall
- Which basin did the 2026 Chemical Geology study on pyrite oxidation focus on?
- The upper Indus basin.
- Which institutions carried out the study?
- IISER Pune, the Wadia Institute of Himalayan Geology and IIT Roorkee.
- What is pyrite?
- An iron sulphide mineral, often called fool's gold.
- What does pyrite form when it oxidises?
- Sulphuric acid (and iron oxides).
- How does sulphuric acid release CO2 from rock?
- It dissolves carbonate rocks such as limestone, setting free the carbon locked in them.
- Key ratio found in the upper Indus?
- CO2 released by pyrite oxidation was about three times the CO2 taken up by silicate weathering.
- Which tools separated sources of river sulphate?
- Sulphur and oxygen isotope ratios.
- What is enhanced rock weathering?
- Spreading crushed silicate rock such as basalt on land to speed up CO2 capture.
Prelims practice question
Consider the following processes:
1. Weathering of silicate rocks by carbonic acid in rainwater
2. Dissolution of carbonate rocks by sulphuric acid formed from pyrite oxidation
3. Burial of organic carbon in ocean sediments
Which of the above tend to release carbon dioxide to the atmosphere over geological time?
- 1 only
- 2 only
- 1 and 3 only
- 2 and 3 only
Show answer
Answer: (b) 2 only. Only 2 releases CO2: sulphuric acid dissolving carbonate frees carbon that was locked in rock. Silicate weathering (1) and burial of organic carbon (3) remove CO2 from the atmosphere over long timescales.
Asked before in UPSC
Recurring theme: Weathering, the carbon cycle and carbon removal
Himalayan processes and climate are linked both ways; this study adds that erosion in the Himalayas can itself be a source of CO2 over long timescales.
Spreading basalt on farmland is enhanced weathering; this story explains the chemistry and why rock choice matters, since sulphide-bearing rock can release CO2.
Mains practice question
Mountain building is often linked to cooling of the Earth through rock weathering. How does the recent finding on pyrite oxidation in the upper Indus basin complicate this view? (150 words)
Model answer
Rock weathering is part of Earth's long-term carbon cycle, and the uplift of the Himalayas has been linked to faster silicate weathering and lower atmospheric CO2.
The established view
- Rainwater carrying CO2 breaks down silicate rocks; the carbon ends up as bicarbonate and, finally, ocean carbonate.
- More uplift and erosion mean more fresh rock and more CO2 drawn down, a link proposed for late Cenozoic cooling.
What the new study adds
- In the upper Indus basin, erosion and glaciers expose pyrite, which oxidises to sulphuric acid.
- This acid dissolves carbonate rocks and releases CO2.
- CO2 released this way was about three times the CO2 taken up by silicate weathering.
- Sulphur and oxygen isotopes showed steep mountains as net sources and floodplains as sinks.
Implications
- Mountain carbon budgets must count sulphide oxidation, not only silicate weathering.
- Weathering-based carbon removal needs careful rock choice.
The Himalayas may warm as well as cool the planet, depending on which rocks erosion exposes.
The basics
Why this matters
Carbon moves between air, rocks and oceans on timescales from years to millions of years. UPSC tests the basics of this carbon cycle, weathering and carbon removal. A new study from the upper Indus basin shows that the same mountain can pull CO2 out of the air in one way and put it back in another.
Weathering in one paragraph
Chemical weathering is the slow breakdown of rock by water and the acids dissolved in it. Rain absorbs CO2 and becomes weak carbonic acid. When that acid attacks silicate rocks such as granite or basalt, the carbon ends up as bicarbonate in rivers and, in time, as carbonate on the sea floor. That is a long-term sink.
- Acid comes from CO2 in rain
- Carbon is carried to the sea as bicarbonate
- Net result: CO2 removed over long periods
- Acid comes from oxidised pyrite
- Carbon locked in limestone is set free
- Net result: CO2 released
The hidden source
Pyrite oxidation is the twist. Pyrite, an iron sulphide often called fool's gold, is common in sedimentary rocks. Fresh surfaces exposed by landslides, rivers and glaciers react with oxygen and water to form sulphuric acid. Where that acid meets carbonate rock, CO2 escapes.
- 1Erosion and glaciers grind rockFresh pyrite is exposed at the surface
- 2Pyrite reacts with oxygen and waterSulphuric acid forms
- 3Acid dissolves carbonate rockCarbon locked in limestone is released
- 4CO2 enters the airSteep, fast-eroding slopes become net carbon sources
The bigger picture
The Carbonate-silicate cycle keeps Earth's climate in check over millions of years: warmer, wetter conditions speed up silicate weathering, which draws down CO2. The rise of the Himalayas has been linked to such a draw-down and to the cooling of the late Cenozoic. The new study says that in fast-eroding areas the sulphuric acid route can outweigh the sink. It also matters for Enhanced rock weathering, a proposed way to remove CO2 by spreading crushed rock on land.
Go deeper
In one line: In the fast-eroding upper Indus basin, oxidation of freshly exposed pyrite releases more CO2 than silicate weathering removes, turning steep mountain stretches into net carbon sources.
Why it matters for UPSC
Weathering, the carbon cycle and carbon removal methods are GS1 physical geography and Prelims environment topics. UPSC asked in 2023 about spreading basalt on farmland as a carbon capture method, which rests on the same chemistry.
The core idea
Two kinds of Chemical weathering pull in opposite directions. Carbonic acid attacking silicate rocks stores carbon, the basis of the Carbonate-silicate cycle. Sulphuric acid from Pyrite oxidation attacking carbonate rocks releases it. Which one wins depends on how fast rock is exposed and what it is made of. In the upper Indus the release was about three times the uptake, which also carries a caution for Enhanced rock weathering.
Numbers and dates to remember
- About 3 times: CO2 from pyrite oxidation compared with silicate weathering uptake.
- Institutions: IISER Pune, Wadia Institute of Himalayan Geology, IIT Roorkee.
- Journal: Chemical Geology; reported 13 September 2026.
- Method: sulphur and oxygen isotope ratios of river sulphate.
- Pattern: steep mountains are sources, floodplains are sinks.
Where to go next
- Chemical weathering: how water and acids break rock down.
- Pyrite oxidation: the reaction behind the new CO2 source.
- Carbonate-silicate cycle: Earth's long-term thermostat.
- Enhanced rock weathering: using weathering to remove CO2 on purpose.
Go deeper: are young mountains coolers or heaters?
The uplift-cooling view. Many geologists have linked the rise of the Himalaya and Tibetan Plateau to global cooling. Uplift creates steep slopes, heavy monsoon rain and constant erosion, which keep exposing fresh silicate rock. Faster silicate weathering draws down CO2, and the Carbonate-silicate cycle stores that carbon in ocean carbonates. This is one explanation for the start of the late Cenozoic ice ages.
The sulphide challenge. Erosion does not only expose silicates. Pyrite oxidation creates sulphuric acid, and when that acid dissolves carbonate rock it releases CO2 instead of storing it. The new upper Indus study puts numbers on this: CO2 released by this route was about three times the CO2 removed by silicate weathering. The same erosion that was thought to cool the planet can, in some settings, warm it.
Where the balance falls. The study finds a split along the river: steep headwaters are net sources, while floodplains downstream act as sinks. So the net effect of a mountain range depends on its rock types, erosion rate and the fate of sediment downstream.
Why policy cares. Enhanced rock weathering proposes to speed up Chemical weathering of basalt on farmland to capture CO2. The Indus result is a reminder that crushed rock containing sulphides or carbonates could release CO2 rather than store it, so rock choice and careful measurement matter. These are geological processes, and they do not change the need to cut emissions from fossil fuels now.
Chemical weathering
The basic process behind both the sink and the source.
In one line: Chemical weathering is the breakdown of rocks by chemical reactions with water, oxygen and acids, changing their minerals rather than just breaking them into pieces.
Main types
- Carbonation: CO2 dissolved in rain forms weak carbonic acid, which dissolves limestone and alters silicate minerals.
- Oxidation: minerals containing iron or sulphur react with oxygen; rusting of iron minerals and oxidation of pyrite are examples.
- Hydration and hydrolysis: water combines with or splits minerals, turning feldspar into clay, for example.
Warm, wet climates and freshly exposed rock speed up chemical weathering. Physical weathering (frost, grinding by glaciers) helps by creating fresh surfaces.
Why it is in the news
In the upper Indus basin, glaciers and erosion grind rock and expose pyrite, and oxidation then produces sulphuric acid that attacks carbonate rock.
Where to go next
Pyrite oxidation
The reaction that makes eroding mountains a CO2 source.
In one line: Pyrite oxidation is the reaction of the iron sulphide mineral pyrite with oxygen and water, which produces sulphuric acid and iron oxides.
Where pyrite is found
Pyrite is common in shales and other sedimentary rocks and in coal. Its brassy shine gives it the name "fool's gold". Buried deep, it is stable; exposed at the surface, it oxidises.
Two consequences
- Acid mine drainage: in mining areas, pyrite oxidation makes water acidic and releases metals, a known pollution problem.
- Carbon release: when the sulphuric acid formed meets carbonate rock such as limestone, it dissolves it and releases CO2.
Why it is in the news
The upper Indus study traced sulphate in river water back to pyrite using sulphur and oxygen isotope ratios, and found that this route released about three times as much CO2 as silicate weathering took up.
Where to go next
Carbonate-silicate cycle
Earth's slow thermostat, which this study complicates.
In one line: The carbonate-silicate cycle is the slow exchange of carbon between the atmosphere, rocks and oceans through silicate weathering, carbonate formation and volcanic release, which regulates Earth's climate over millions of years.
How it works
- Carbonic acid in rain weathers silicate rocks; calcium and bicarbonate ions flow to the sea.
- Marine organisms and chemical processes turn them into calcium carbonate, which is buried in sediments.
- Over millions of years, tectonics carries carbonates into the Earth, and volcanoes release CO2 again.
The feedback
If CO2 rises, the planet warms, rainfall increases and silicate weathering speeds up, removing more CO2. If CO2 falls, weathering slows. This negative feedback keeps the climate within limits over geological time. The uplift of the Himalayas and Andes is thought to have increased silicate weathering and helped begin the late Cenozoic ice age.
Where to go next
Enhanced rock weathering
A carbon removal idea that rests on the same chemistry.
In one line: Enhanced rock weathering is a proposed way to remove CO2 from the air by spreading finely crushed silicate rock, such as basalt, on farmland so that it weathers faster.
The idea
Grinding rock increases its surface area, so carbonation reactions that normally take thousands of years happen much faster. The captured carbon is carried away as bicarbonate in soil water and rivers. Basalt dust can also add nutrients to soils. UPSC's Prelims in 2023 listed spreading basalt on farmland as one of the activities discussed for carbon capture and sequestration.
The caution from the Indus study
The choice of rock matters. Rock that contains pyrite can produce sulphuric acid, and if that acid dissolves carbonates the process releases CO2 instead of storing it. Measuring the net effect in the field is therefore essential.
Where to go next
Take the 13 September 2026 quiz: 30 Prelims-style questions with answers