Alkalinity generation from rock weathering modulates Earth’s climate at geological time scales. Although lithology is thought to dominantly control alkalinity generation globally, the role of other first-order controls appears elusive. Particularly challenging remains the discrimination of climatic and erosional influences. Based on global observations, here we uncover the role of erosion rate in governing riverine alkalinity, accompanied by areal proportion of carbonate, mean annual temperature, catchment area, and soil regolith thickness. We show that the weathering flux to the ocean will be significantly altered by climate warming as early as 2100, by up to 68% depending on the environmental conditions, constituting a sudden feedback of ocean CO2 sequestration to climate. Interestingly, warming under a low-emissions scenario will reduce terrestrial alkalinity flux from mid-latitudes (–1.6 t(bicarbonate) a−1 km−2) until the end of the century, resulting in a reduction in CO2 sequestration, but an increase (+0.5 t(bicarbonate) a−1 km−2) from mid-latitudes is likely under a high-emissions scenario, yielding an additional CO2 sink.

<p>Alkalinity generation from rock weathering is thought to modulate the Earth’s climate at geological time scales. Here, we use global alkalinity data paired with consistent measurements of erosion rates to develop an empirically-based model for riverine alkalinity concentration, demonstrating the impact of both erosion (i.e. erosion rate) and climate (i.e. temperature) on alkalinity generation, globally. We show that alkalinity generation from carbonate rocks is very responsive to temperature and that the weathering flux to the ocean will be significantly altered by climate warming as early as the end of this century, constituting a sudden feedback of ocean CO<sub>2</sub> sequestration to climate. While we anticipate that climate warming under a low emissions scenario will induce a reduction in terrestrial alkalinity flux for mid-latitudes (-1.3 t(bicarbonate) a<sup>-1</sup> km<sup>-2</sup>) until the end of the century, resulting in a temporary reduction in CO<sub>2</sub> sequestration, we expect an increase (+1.6 t(bicarbonate) a<sup>-1</sup> km<sup>-2</sup>) under a high emissions scenario, causing an additional short-term CO<sub>2</sub> sink at decadal timescales.</p>

Abstract. The weathering rate of carbonate minerals is several orders of magnitude higher than for silicate minerals. Therefore, small amounts of carbonate minerals have the potential to control the dissolved weathering loads in silicate-dominated catchments. Both weathering processes produce alkalinity under the consumption of CO2. Given that only alkalinity generation from silicate weathering is thought to be a long-term sink for CO2, a misattributed weathering source could lead to incorrect conclusions about long- and short-term CO2 fixation. In this study, we aimed to identify the weathering sources responsible for alkalinity generation and CO2 fixation across watershed scales in a degrading permafrost landscape in northern Norway, 68.7–70.5° N, and on a temporal scale, in a subarctic headwater catchment on the mountainside of Iskorasfjellet, characterized by sporadic permafrost and underlain mainly by silicates as the alkalinity-bearing lithology. By analysing total alkalinity (AT) and dissolved inorganic carbon (DIC) concentrations, as well as the stable isotope signature of the latter (δ13C-DIC) in conjunction with dissolved cation and anion loads, we found that AT was almost entirely derived from weathering of the sparse carbonate minerals. We propose that in the headwater catchment, the riparian zone is a hotspot area of AT generation and release due to its enhanced hydrological connectivity, and that the weathering load contribution from the uphill catchment is limited by insufficient contact time of weathering agent and weatherable material. By using stable water isotopes, it was possible to explain temporal variations in AT concentrations following a precipitation event due to surface runoff. In addition to carbonic acid, sulphuric acid, probably originating from pyrite oxidation, is shown to be a potential corrosive reactant. An increased proportion of sulphuric acid as a potential weathering agent may have resulted in a decrease in AT. Therefore, carbonate weathering in the studied area should be considered not only as a short-term CO2 sink, but also as a potential CO2 source. Finally, we found that AT increased with decreasing permafrost probability, and attributed this relation to an increased water storage capacity associated with increasing contact of weathering agent and rock surfaces, and enhanced microbial activity. As both soil respiration and permafrost thaw are expected to increase with climate change, increasing the availability of weathering agent in the form of CO2 and water storage capacity, respectively, we suggest that future weathering rates and alkalinity generation will increase concomitantly in the study area.