A groundbreaking study has unveiled the vast and complex patterns of carbon emissions from global rivers, revealing the age and origin of the carbon that flows from land to atmosphere. By assembling an unprecedented global database of radiocarbon measurements, researchers have charted how rivers transport carbon not just from recent biological sources but also from ancient geological reservoirs, reshaping our understanding of the carbon cycle on a planetary scale.
Radiocarbon analyses of dissolved inorganic carbon (DIC), carbon dioxide (CO₂), and methane (CH₄) in rivers have historically been fragmented, focused on local or regional scales with inconsistent variables, making comparison across studies difficult. This new work synthesizes over a thousand observations, harmonizing data across continents and ecosystems, enabling an unparalleled global comparison of the radiocarbon content of riverine carbon emissions.
Central to the study’s innovation is the normalization of the radiocarbon content (expressed as F14C) of riverine carbon to atmospheric radiocarbon levels at the time of sample collection. This approach controls for temporal fluctuations in atmospheric carbon isotopic composition, especially those caused by anthropogenic influences such as nuclear testing, allowing for consistent comparison of carbon ages downriver and across regions.
The findings indicate that river CO₂ emissions are often a complex mixture of modern carbon recently cycled through ecosystems and significantly older carbon derived from soils and petrogenic sources, such as weathered rock organic matter and carbonate minerals. This "old" carbon component, which can be thousands of years in age, challenges traditional assumptions that rivers primarily release newly fixed carbon and highlights their role as conduits of fossil and stored terrestrial carbon to the atmosphere.
To unravel the contributions of these diverse carbon sources, the researchers implemented sophisticated isotope mixing models coupled with Monte Carlo simulations, anchored by global estimates of weathering fluxes. This modelling revealed that petrogenic carbon accounts for roughly 7% of the total riverine carbon flux, but the remainder includes a substantial millennial-aged carbon fraction—that is, carbon with residence times ranging from centuries to thousands of years within soils before entering the river system.
This legacy carbon, released through riverine transport and evasion to the atmosphere, underscores a major pathway by which ancient carbon stores are mobilized in the modern environment. Such insights have profound implications for carbon budgeting, as they suggest that terrestrial carbon reservoirs hold a reservoir of old carbon that is actively connected to atmospheric CO₂ levels through fluvial systems.
The study also leveraged global hydrological and environmental data extracted from HydroATLAS, a comprehensive spatial database offering consistent catchment and reach-scale attributes such as size, lithology, and biome classification. This allowed the team to link chemical signatures of carbon age with landscape characteristics, revealing that catchment size and lithology are significant controls on the age and source of river carbon emissions.
Notably, rivers draining small catchments (≤10 km²) and large catchments (>10 km²) exhibit distinct carbon age profiles, a finding supported and validated through random forest machine learning models. These models, trained on a suite of catchment and climate variables, identify the key environmental factors that correlate with variations in the radiocarbon content of riverine carbon, highlighting the non-linear interactions of geography, geology, and ecosystem processes.
New radiocarbon data included in the analysis originate from diverse locations including heavily urbanized rivers in London, pristine mountain rivers on the Qinghai–Tibet Plateau, and rivers sampled in Taiwan, Cambodia, and China. These additions offer fresh insights into how human activities and natural settings influence the transport and emission of aged carbon via rivers.
Sample collection and processing spanned advanced techniques, including super headspace equilibrations and membrane-based gas extraction methods, followed by accelerator mass spectrometry (AMS) for precise radiocarbon quantification. These rigorous methods ensure high-quality isotopic data essential for the interpretation of carbon provenance and cycling timescales.
The study also made significant progress evaluating the isotopic equilibrium between dissolved inorganic carbon and dissolved CO₂, lending confidence that DIC radiocarbon measurements can reliably represent the radiocarbon signature of riverine CO₂ emissions, a crucial step for integrating published and new datasets.
By integrating data from multiple continents and across varying catchment scales and biomes, the authors provide a global-scale perspective of how river systems transport and release carbon with a broad age spectrum. This integrative view challenges the previously held perception of rivers as vectors of primarily contemporary carbon and positions them as key participants in the mobilization of deep, stored carbon.
These findings demand a reevaluation of global carbon budgets and climate models, as the flux of aged carbon via rivers represents a significant and previously underappreciated source of atmospheric CO₂. Understanding the dynamics of this carbon pool could improve predictions of future atmospheric carbon trajectories, particularly in the context of land use change and climate-driven alterations in hydrology and soil carbon dynamics.
In sum, this comprehensive analysis of riverine carbon isotopes highlights the complex interplay between terrestrial carbon reservoirs and atmospheric emissions, revealing rivers as vital arteries through which ancient carbon is continually released back to the atmosphere. As climate change progresses, appreciating the role of these aged stores becomes critical for crafting effective carbon management and mitigation strategies.
Subject of Research: Global-scale patterns and sources of river carbon emissions, with emphasis on radiocarbon content and the contribution of old carbon to atmospheric CO₂ fluxes.
Article Title: Old carbon routed from land to the atmosphere by global river systems.
Article References:
Dean, J.F., Coxon, G., Zheng, Y. et al. Old carbon routed from land to the atmosphere by global river systems. Nature 642, 105–111 (2025). https://doi.org/10.1038/s41586-025-09023-w
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