Rivers and streams play a significant role in the global carbon cycle, serving as an important pathway through which carbon dioxide (CO2) and methane (CH4) are released from water surfaces into the atmosphere. While it was previously thoughtWhile these emissions come largely from recent (up to ten years old) biomass production, new research using radiocarbon dating reveals a significant and previously unrecognized flux old carbon (thousands of years old or older) from land to the atmosphere via global river systemsThis ancient carbon comes from long-term carbon stores in soil, sediments and geological formations.

According to resources We can divide the carbon present in rivers into three age categories:

• Decadal: Carbon fixed by the biosphere through photosynthesis since 1955. Represents the rapid cycle of carbon in ecosystems.
• Millennial: Biosphere carbon, hundreds to thousands of years old. Often comes from deeper soil profiles or older dissolved/particulate organic matter in rivers.
• Petrogenic: Carbon older than about 55,000 years. Found in carbonate minerals and organic matter in rocks. Mobilized by rock weathering and erosion.

Global riverine CO2 emissions are estimated at 2.0 (1.6-2.2) Pg C per year. Radiocarbon analysis of dissolved inorganic carbon (DIC), CO2 and CH4 in rivers found that 59 ± 17% of global riverine CO2 emissions come from ancient carbon (millennium or older). This corresponds to a flux of 1.2 ± 0.3 Pg C per year. This flux is similar to the net carbon exchange in terrestrial ecosystems.

The release of this old carbon is linked to the lithology (geological composition) and biome (type of ecosystem) of the basin. Basins dominated by sedimentary rocks tend to release older carbon (lower F14Catm values) compared to igneous and metamorphic rocks. Weathering of carbonate minerals and organic carbon in sediments contributes petrogenic carbon to rivers.

Climatic factors and anthropogenic influences are key to understanding this ancient carbon flux. Analysis of the data suggested that average annual precipitation and temperature were generally positively correlated with younger carbon (higher F14Catm) in large basins. However, at extreme values (above 2000 mm of precipitation and above 20 °C) this correlation weakened, which may indicate that very warm and humid or dry areas can potentially release older carbon. Higher altitude was associated with the release of older carbon, which may be related to erosion processes in mountainous areas.

There is a possibility that anthropogenic disturbances, such as climate and land use change, may have increased the release of old carbon into the atmosphere via rivers. A trend of aging carbon in rivers (decreasing F14Catm values) was observed in the time series of observations (1991–2023). This could indicate increased emissions of ancient carbon due to destabilization of global soil carbon stocks and changes in rates of weathering, erosion and oxidation of rocks resulting from climatic and anthropogenic influences.

Anthropogenic climate change may increase CO2 delivery to rivers due to warming soils and/or increasing soil moisture, which increases microbial respiration. The supply of DIC and CO2 from rock weathering may also increase with global warming. Riverine CO2 emissions are thus sensitive to inputs from ancient carbon sources and could increase due to direct anthropogenic disturbances (such as drainage, grubbing, burning and agricultural cultivation of the land), as well as due to anthropogenic climate change.

The discovery of extensive ancient carbon flux through rivers requires a reassessment of current models of the carbon cycle. It means that only a fraction of riverine CO2 emissions (estimated at 41 ± 16% or 0.9 ± 0.3 Pg C per year) could contain recent carbon from anthropogenic emissions. The rest comes from ancient stocks that existed before widespread fossil fuel combustion. This changes our understanding of where anthropogenic carbon is stored in Earth's major reservoirs.

Although the precise distribution of the impact of anthropogenic climate change and other factors on the observed trend of aging carbon in rivers is not yet fully clear, the analysis provides evidence for a previously unrecognized, planetary release of ancient carbon from land to the atmosphere. River emissions are vulnerable to perturbations to carbon cycles (short-term, millennial, geological), which can channel carbon from watersheds to the atmosphere via river levels. Spring

The report was published in the journal nature.com

Glossary of key terms:

• River systems (Rivers and streams): Watercourses of varying sizes that carry water and materials (including carbon) from land.
• Global carbon cycle: The natural cycle of carbon between the Earth's atmosphere, hydrosphere, biosphere and lithosphere.
• Carbon dioxide (CO2): A greenhouse gas that is emitted from river systems into the atmosphere.
• Methane (CH4): A greenhouse gas that is also emitted from river systems into the atmosphere.
• Dissolved inorganic carbon (DIC): The sum of all inorganic forms of carbon dissolved in water, including CO2(aq), carbonic acid (H2CO3), bicarbonate (HCO3-), and carbonate (CO32-).
• Radiocarbon (14C): A radioactive isotope of carbon with a half-life of approximately 5,730 years. Its concentration in a sample is used to determine the age of carbon.
• Modern carbon fraction (F14C): The ratio of the 14C concentration in a sample to the 14C concentration in a reference modern carbon (usually from 1950). F14C = 1.0 for 1950. Values > 1.0 mean younger carbon (impact of nuclear tests), < 1.0 mean older carbon.
• Modern carbon fraction normalized to the atmosphere (F14Catm): The F14C value of a sample normalized to the F14C value of atmospheric CO2 in the year of sampling. Used to better compare the age of carbon independent of historical fluctuations in atmospheric 14CO2. F14Catm = 1.0 means that carbon is in equilibrium with the atmosphere in that year.
• Decadal carbon: Carbon fixed into the biosphere by photosynthesis since 1955; represents young, rapidly cycling carbon.
• Millennial carbon: Biosphere carbon is hundreds to thousands of years old; it comes from longer-term reservoirs, mainly in soil.
• Petrogenic carbon: Carbon derived from rocks (minerals or organic matter of rocks) older than approximately 55,000 years.
• Lithology: Physical and chemical properties of rocks in the basin.
• Biome: A major regional or global biological community, such as forests, grasslands, or tundra, characterized by prevailing climatic conditions and vegetation types.
• Catchment/Watershed: The area from which water drains to a specific watercourse or point.
• Weathering: The processes of decomposition of rocks and minerals on the Earth's surface (mechanical, chemical, biological). Chemical weathering of carbonates and rock organic matter can release petrogenic carbon.
• Terrestrial ecosystems (Terrestrial ecosystems): Terrestrial ecosystems (forests, grasslands, soil, etc.).
• Net ecosystem exchange (NEE): The balance of carbon intake (photosynthesis) and loss (respiration) by an ecosystem.
• Global carbon budget: A quantitative description of carbon flows and stocks between Earth's major reservoirs.
• Isotope mixing model: A mathematical model that uses isotopic ratios (in this case, 14C) to estimate the proportional contributions of different sources to the mixture (in this case, river carbon).
• Monte Carlo simulation: A computer technique that uses random samples to model the probability distribution of possible outcomes.
• Bayesian isotope mixing model: A statistical approach to isotope mixing models that uses Bayesian inference to estimate the probability distribution of source contributions.