Rivers release ancient carbon into the atmosphere on a global scale

Most of the carbon dioxide released by rivers and streams around the world comes from very old carbon stores. Scientists' analysis shows that 59% of river CO2 emissions result from the release of ancient or even petrogenic carbon, which has been stored for more than 55 years.

Until now, it was believed that the CO2 emitted by rivers came mainly from the respiration of fresh organic matter, captured by photosynthesis on time scales of less than ten years. This assumption underpinned the main models of the terrestrial carbon cycle.

The new study, led by scientists at the University of Bristol, suggests that river carbon, by contrast, carries the signature of much older sources, implying a real loss of carbon stored in soils, sediments and rocks.

Global database reveals age

The research used a global database of 1.195 measurements of radiocarbon (F14C) content in dissolved inorganic carbon (DIC), CO2 and methane (CH4) from rivers on all continents.

Both previously published measurements and new collections from rivers in China, the United Kingdom, Cambodia, Taiwan and Antarctica were considered. The analysis showed that the average age of river CO2 ranges from 320 to 722 radiocarbon years, with some samples indicating much older ages.

To explain these data, the researchers modeled the proportion of carbon sources. The results indicate that about 1,2 ± 0,3 petagrams of carbon per year (Pg C/yr) emitted by rivers comes from “old” carbon – ancient or petrogenic. This is equivalent to more than half of all CO2 emissions from rivers, estimated at 2,0 Pg C/yr. In comparison, carbon sequestration by terrestrial plants is around 2,9 Pg C/yr.

Stored carbon conduits

The discovery forces a rethink of the role of rivers in the carbon cycle. Rather than simply recycling short-term carbon, these waterways act as conduits that release carbon stored over thousands of years. This means that some of the carbon previously thought to be stable in soils and rocks is actually being released into the atmosphere.

The origin of this older carbon is associated with two main mechanisms.

The first is the chemical weathering of sedimentary rocks, especially those rich in carbonates and fossil organic matter.

The second involves the degradation of organic carbon deep in soils, released by subsurface hydrological flows and erosion.

Both processes were not properly accounted for in current carbon cycle models.

Lithology, biome and climate

The analysis also showed that terrain characteristics strongly influence the age of the carbon emitted. Watersheds over sedimentary lithologies, such as limestone, presented the lowest F14C values, indicating the presence of very old carbon. Larger basins, with higher altitudes and less forest cover, also tend to release more ancient carbon.

Other factors such as precipitation, temperature, carbon content and soil sand influence the type of carbon released. Basins with higher average annual temperatures and heavy rainfall tend to release younger carbon, but up to a certain limit. In extremely humid or arid regions, the release of older carbon increases.

Models need to be revised

The implications are relevant for understanding climate change. Some of the CO₂ emissions attributed to the respiration of recent ecosystems actually come from the loss of ancient stocks.

Furthermore, contrary to what was previously thought, the size of the river does not reduce the influence of ancient carbon. Even large rivers, such as the Mekong or the Amazon, release significant amounts of ancient CO2.

By reassessing the global carbon budget, the authors show that the contribution of rivers to past carbon emissions directly affects the estimate of how much anthropogenic carbon the terrestrial biosphere is capable of storing. If part of the river emissions does not correspond to recent carbon, then the sequestration capacity of vegetation may be underestimated.

More information at doi.org/10.1038/s41586-025-09023-w