Erosion can turn rivers from carbon sinks into carbon sources, Lancang study shows

For decades, one of the most influential ideas in earth science has been that mountain building cools the planet. The uplift-weathering hypothesis holds that tectonic uplift steepens landscapes, accelerates erosion, and exposes fresh silicate minerals to rain and soil water. As those minerals dissolve, they pull carbon dioxide out of the atmosphere and lock it away in dissolved form that eventually reaches the ocean, where it is stored in carbonate shells and sediments. This mechanism has frequently been invoked to explain long-term global cooling episodes, including the climatic transition associated with the rise of high mountain ranges such as the Himalaya and the Tibetan Plateau. But a new study of one of Asia’s great rivers suggests the story is considerably more complicated, and that in some landscapes, more erosion may actually push the carbon balance in the opposite direction.

Dr. Jinke Liu and Prof. Guilin Han of China University of Geosciences in Beijing turned their attention to the Lancang River basin, which drains the southeastern margin of the Tibetan Plateau and forms the upper reaches of the Mekong. The Lancang is an ideal natural laboratory for testing how erosion shapes weathering, because it cuts through steep terrain with rapid uplift, varied rock types, and strong gradients in erosion rate from headwaters to lowland plains. The researchers assembled an unusually comprehensive geochemical dataset, combining river water chemistry, the geochemistry of suspended sediment, stable isotope measurements, channel steepness indices derived from topographic analysis, and independent records of erosion rate across the basin.

A central challenge in studies of this kind is that river water is a mixture of solutes derived from many different sources. Rainfall, evaporite deposits, carbonate rocks, silicate rocks, and sulfide minerals all contribute dissolved ions, and disentangling their relative contributions requires careful quantitative treatment. The team applied an inverse mixing model based on convex optimization, a mathematical framework that allows the contributions of each source to the dissolved load to be estimated while respecting the physical constraints of the system. This approach enabled them to separate, for the first time in this basin, the specific weathering fluxes associated with silicate minerals, carbonate minerals, and sulfide oxidation, and to track how each one responds as erosion intensifies.

The results reveal a striking divergence among the three weathering pathways. Dissolved solutes in the Lancang River are dominated by carbonate and evaporite weathering, while the sulfate carried by the river derives mainly from the oxidation of sulfide minerals. Once the confounding influence of runoff was statistically removed, the contrasting behavior of the different weathering processes became clear. As erosion rates increase, silicate weathering rates actually decrease, whereas carbonate weathering and sulfide oxidation rates rise. This counterintuitive pattern strikes at the heart of the uplift-weathering hypothesis, because it means that faster erosion does not necessarily translate into greater consumption of atmospheric carbon dioxide.

The reason lies in the chemistry of the rocks being ground up. Silicate weathering is the only weathering pathway that consumes carbon dioxide over geological time scales, converting atmospheric carbon into dissolved bicarbonate that is ultimately stored in the ocean. Carbonate weathering, by contrast, is roughly carbon neutral on long time scales, because the carbon it releases when the dissolved products precipitate in the sea balances the carbon consumed during dissolution. Sulfide oxidation is actively harmful to the carbon budget: when sulfide minerals such as pyrite are exposed to oxygen and water, they generate sulfuric acid, and that acid dissolves carbonate rocks in a reaction that releases carbon dioxide to the atmosphere. In a landscape where erosion increasingly exposes sulfides and carbonates but silicate weathering slows, the net effect of weathering flips from a carbon sink to a carbon source.

That is precisely what the study documents in the Lancang basin. On geological time scales, the carbon dioxide released by sulfide oxidation coupled to carbonate dissolution exceeds the carbon dioxide consumed by silicate weathering, meaning that weathering in the basin as a whole acts as a net source of carbon to the atmosphere. The finding is particularly significant because the Lancang is a large, tectonically active river system, the very kind of landscape in which enhanced weathering has been assumed to draw down carbon. The study suggests that lithology, the specific assemblage of rock types available for weathering, can override the simple expectation that rapid erosion always means strong carbon sequestration.

The researchers also uncovered several subtle processes that complicate the erosion-weathering relationship further. Evidence from calcium isotopes in suspended sediment and river water points to secondary calcite precipitation within the river system, a process that can alter the apparent carbon balance recorded by water chemistry. Changes in the chemical composition of suspended sediment downstream indicate that silicate minerals may continue to dissolve during transport, meaning that some weathering occurs well after sediment leaves the mountain front. In addition, lithium isotopes, hydrogen and oxygen isotopes, and water chemistry all indicate that non-geothermal groundwater contributes substantial quantities of dissolved solutes to the river. Groundwater can bypass the shallow, soil-dominated weathering zone where silicate dissolution is most active, delivering deep-sourced solutes that do not reflect surface erosion rates in a straightforward way.

These processes matter because they can modify the link between erosion rate and chemical weathering at the scale of an entire basin, and they may help explain why different river basins around the world show such different sensitivities of weathering to erosion. The study highlights that a basin’s carbon budget cannot be predicted from erosion rate alone. Instead, it depends on the interplay of lithology, the contribution of suspended sediment weathering during downstream transport, and the input of groundwater carrying solutes from depth. Ignoring these factors risks misattributing changes in river chemistry to erosion when they actually reflect other parts of the earth system.

The spatial dimension of the findings is equally compelling. Downstream, in the lower Mekong River plain, erosion rates are lower and the underlying lithology changes, and there the carbon effect of weathering shifts from a carbon dioxide source to a carbon dioxide sink. When the authors compared the Lancang basin with small, silicate-dominated catchments in orogenic belts, they found that the Lancang has only a moderate erosion rate yet still functions as a carbon source, a combination that only makes sense when lithological differences are taken into account. The comparison complements and deepens previous understanding of how erosion rate and carbon effects are related, showing that the same tectonic forcing can produce opposite carbon outcomes depending on what rocks the rivers are eroding.

The implications extend beyond the Tibetan Plateau. As scientists refine models of the long-term carbon cycle and attempt to reconstruct how tectonics, weathering, and climate have interacted over millions of years, the Lancang study provides new geochemical evidence that lithology, suspended sediment weathering, and groundwater input are first-order controls on chemical weathering and its carbon consequences in large river basins. It also refines how researchers evaluate the relationship among tectonic erosion, chemical weathering, and climate change, cautioning against treating erosion as a universal proxy for carbon drawdown. The research, published in Science China Earth Sciences, adds an important nuance to one of geology’s grand narratives: mountains may indeed shape climate, but whether they cool or warm the planet depends on the rocks they are made of.

Subject of Research: Differential responses of silicate, carbonate, and sulfide weathering to erosion in the Lancang River basin and their net carbon effects

Article Title: Rock weathering tells a new story: more erosion does not always mean more carbon consumption

Article References: Rock weathering tells a new story: more erosion does not always mean more carbon consumption. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: rock weathering, erosion, carbon cycle, silicate weathering, sulfide oxidation, carbonate weathering, Lancang River, Tibetan Plateau, uplift-weathering hypothesis, geochemistry, carbon dioxide, river basin