Rock weathering can counteract river CO₂ emissions induced by permafrost thaw

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River networks perfuse and integrate landscapes 6 , receiving organic carbon (OC) from terrestrial primary production and dissolved inorganic carbon (DIC) \(({\rm{D}}{\rm{I}}{\rm{C}}=\sum {{\rm{C}}{\rm{O}}}_{2({\rm{a}}{\rm{q}})}+{{{\rm{HCO}}}_{3}}^{-}+{{{\rm{CO}}}_{3}}^{2-})\) from soil respiration and chemical rock weathering. This OC is metabolized in rivers and returned to the atmosphere as carbon dioxide (CO 2 ) or methane (CH 4 ), buried in sediments, or transported to coastal oceans. Similarly, DIC can precipitate, outgas or travel downstream 7 , 8 . Global carbon inputs to rivers (about 3.2 petagrams of carbon (PgC) yr −1 ), the resulting riverine carbon emissions (around 2.3 PgC yr −1 ) and carbon export to the ocean (approximately 0.8 PgC yr −1 ) 8 are on par with the net terrestrial carbon sink (that is, the net primary productivity minus respiration without considering land-use changes; about 3.2 PgC yr −1 ) 9 . Consequently, rivers forge important connections between terrestrial, marine and atmospheric carbon pools that modulate the global carbon cycle 8 . Despite major advances in understanding the role of rivers in the carbon cycle, key knowledge gaps persist regarding the interaction between biological and geological carbon sources and sinks 5 , as well as their responses to climate change.

Of primary concern is the impact of climate warming on the vast OC stocks (approximately 1,014 PgC) stored in the Northern Hemisphere permafrost soils 10 . Permafrost thaw is mobilizing a portion of this large, near-surface OC reservoir on Earth 11 . The subsequent breakdown of this ancient, biolabile OC in rivers fuels in situ respiration and methanogenesis, thus amplifying emissions of aged CO 2 and CH 4 , and potentially perturbing the contemporary carbon cycle 12 , 13 , 14 , 15 , 16 , 17 , 18 . However, permafrost thaw has effects beyond mobilizing ancient OC. In particular, recent studies have demonstrated that multi-decadal increases in DIC fluxes and associated dissolved solutes in permafrost rivers could reflect intensified chemical weathering 2 , 3 , 4 , 19 , 20 . Given that landscape-scale weathering fluxes are controlled by a combination of lithology 21 , hydrology 22 , 23 , 24 , temperature 25 , 26 and exposure of reactive surfaces 27 , 28 , climate warming and permafrost thaw could have multifaceted impacts on these fluxes 2 , 3 , 4 . Whereas climate warming can directly increase dissolution kinetics 26 , the associated permafrost thaw may alter weathering reactions by exposing minerals or by modulating the hydrology and promoting acidic redox conditions in sub-surface environments 4 . Importantly, weathering reactions in the critical zone affect biogeochemistry through the hydrosphere because groundwater transports weathering products—including carbon—to fluvial ecosystems.

The role of weathering reactions for the carbon cycle depends on the nature of the exposed minerals 21 , 29 . Weathering of silicate and carbonate minerals by carbonic acid produces alkalinity and draws down CO 2 (refs.  30 , 31 ). By contrast, weathering of sulfide minerals—such as pyrite—produces sulfuric acid and releases CO 2 , either by interacting with the alkalinity pool or by dissolving carbonate minerals 32 , 33 . In reality, biological and geological CO 2 fluxes are intertwined and co-vary across permafrost rivers. For example, acidity produced by respiration of OC could be buffered by alkalinity production from chemical weathering 28 . These interactions may be particularly important in mountainous landscapes with widespread permafrost coverage that are strongly exposed to climate change and characterized by rapid weathering rates 34 , 35 . Efforts to isolate the impact of permafrost thaw on chemical weathering are rare, and no studies have quantified the balance between biological and geological CO 2 sources and sinks in rivers draining permafrost landscapes subject to climate warming. These knowledge gaps introduce substantial uncertainties in understanding the contemporary and future role of permafrost landscapes and their rivers within the global carbon cycle 8 .

Here, we examine the balance of riverine CO 2 fluxes resulting from OC mineralization and chemical weathering spanning a permafrost gradient on the Qinghai–Tibet Plateau (QTP; Fig. 1a ). The QTP is the largest contiguous cryosphere outside the Arctic and Antarctic 36 , with an estimated OC storage of 50.4 Pg in its permafrost soils 37 , partly dating back to the late Pleistocene 38 . The permafrost on the QTP is, on average, warmer and has a higher mean active-layer thickness (about 0.21 m) compared with Arctic permafrost (about 0.10 m), with 44% of permafrost at a mean ground temperature above −0.5 °C, which makes it particularly vulnerable to climate warming 39 . From the cold plateau in the north and west of the study area, air temperatures increase eastward and southward, and permafrost extent decreases from continuous to discontinuous, spo…