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Recent studies increasingly recognize the importance of critical-zone weathering during mountain building for long-term CO₂drawdown and release. However, the focus on near-surface weathering reactions commonly does not account for CO₂emissions from the crust, which could outstrip CO₂drawdown where carbonates melt and decarbonize during subduction and metamorphism. We analyse water chemistry from streams in Italy’s central Apennines that cross a gradient in heat flow and crustal thickness with relatively constant climatic conditions. We quantify the balance of inorganic carbon fluxes from near-surface weathering processes, metamorphism and the melting of carbonates. We find that, at the regional scale, carbon emissions from crustal sources outpace near-surface fluxes by two orders of magnitude above a tear in the subducting slab characterized by heat flow greater than 150 mW m^–2and crustal thickness of less than 25 km. By contrast, weathering processes dominate the carbon budget where crustal thickness exceeds 40 km and heat flow is lower than 30 mW m^–2. The observed variation in metamorphic fluxes is one to two orders of magnitude larger than that of weathering fluxes. We therefore suggest that geodynamic modulations of metamorphic melting and decarbonation reactions are an efficient process by which tectonics can regulate the inorganic carbon cycle.

 

Arctic rivers provide an integrated signature of the changing landscape and transmit signals of change to the ocean. Here, we use a decade of particulate organic matter (POM) compositional data to deconvolute multiple allochthonous and autochthonous pan-Arctic and watershed-specific sources. Constraints from carbon-to-nitrogen ratios (C:N), δ 13 C, and Δ 14 C signatures reveal a large, hitherto overlooked contribution from aquatic biomass. Separation in Δ 14 C age is enhanced by splitting soil sources into shallow and deep pools (mean ± SD: −228 ± 211 vs. −492 ± 173‰) rather than traditional active layer and permafrost pools (−300 ± 236 vs. −441 ± 215‰) that do not represent permafrost-free Arctic regions. We estimate that 39 to 60% (5 to 95% credible interval) of the annual pan-Arctic POM flux (averaging 4,391 Gg/y particulate organic carbon from 2012 to 2019) comes from aquatic biomass. The remainder is sourced from yedoma, deep soils, shallow soils, petrogenic inputs, and fresh terrestrial production. Climate change-induced warming and increasing CO 2 concentrations may enhance both soil destabilization and Arctic river aquatic biomass production, increasing fluxes of POM to the ocean. Younger, autochthonous, and older soil-derived POM likely have different destinies (preferential microbial uptake and processing vs. significant sediment burial, respectively). A small (~7%) increase in aquatic biomass POM flux with warming would be equivalent to a ~30% increase in deep soil POM flux. There is a clear need to better quantify how the balance of endmember fluxes may shift with different ramifications for different endmembers and how this will impact the Arctic system. 

Abstract The rate and consequences of future high latitude ice sheet retreat remain a major concern given ongoing anthropogenic warming. Here, new precisely dated stalagmite data from NW Iberia provide the first direct, high-resolution records of periods of rapid melting of Northern Hemisphere ice sheets during the penultimate deglaciation. These records reveal the penultimate deglaciation initiated with rapid century-scale meltwater pulses which subsequently trigger abrupt coolings of air temperature in NW Iberia consistent with freshwater-induced AMOC slowdowns. The first of these AMOC slowdowns, 600-year duration, was shorter than Heinrich 1 of the last deglaciation. Although similar insolation forcing initiated the last two deglaciations, the more rapid and sustained rate of freshening in the eastern North Atlantic penultimate deglaciation likely reflects a larger volume of ice stored in the marine-based Eurasian Ice sheet during the penultimate glacial in contrast to the land-based ice sheet on North America as during the last glacial. 

Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon ( 14 C) ages on two groups of molecular tracers of plant-derived carbon—leaf-wax lipids and lignin phenols—from a globally distributed suite of rivers. We find significant negative relationships between the 14 C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14 C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change–induced perturbations of soil OC turnover and stocks. 

Abstract. Biogeochemical cycling in the semi-enclosed Arctic Ocean is stronglyinfluenced by land–ocean transport of carbon and other elements and isvulnerable to environmental and climate changes. Sediments of the ArcticOcean are an important part of biogeochemical cycling in the Arctic andprovide the opportunity to study present and historical input and the fate oforganic matter (e.g., through permafrost thawing). Comprehensive sedimentary records are required to compare differencesbetween the Arctic regions and to study Arctic biogeochemical budgets. Tothis end, the Circum-Arctic Sediment CArbon DatabasE (CASCADE) wasestablished to curate data primarily on concentrations of organic carbon(OC) and OC isotopes (δ13C, Δ14C) yet also ontotal N (TN) as well as terrigenous biomarkers and other sedimentgeochemical and physical properties. This new database builds on thepublished literature and earlier unpublished records through an extensiveinternational community collaboration. This paper describes the establishment, structure and current status ofCASCADE. The first public version includes OC concentrations in surfacesediments at 4244 oceanographic stations including 2317 with TNconcentrations, 1555 with δ13C-OC values and 268 with Δ14C-OC values and 653 records with quantified terrigenous biomarkers(high-molecular-weight n-alkanes, n-alkanoic acids and lignin phenols).CASCADE also includes data from 326 sediment cores, retrieved by shallowbox or multi-coring, deep gravity/piston coring, or sea-bottom drilling.The comprehensive dataset reveals large-scale features of both OC contentand OC sources between the shelf sea recipients. This offers insight intorelease of pre-aged terrigenous OC to the East Siberian Arctic shelf andyounger terrigenous OC to the Kara Sea. Circum-Arctic sediments therebyreveal patterns of terrestrial OC remobilization and provide clues about thawing of permafrost. CASCADE enables synoptic analysis of OC in Arctic Ocean sediments andfacilitates a wide array of future empirical and modeling studies of theArctic carbon cycle. The database is openly and freely available online(https://doi.org/10.17043/cascade; Martens et al., 2021), is provided in variousmachine-readable data formats (data tables, GIS shapefile, GIS raster), andalso provides ways for contributing data for future CASCADE versions. Wewill continuously update CASCADE with newly published and contributed dataover the foreseeable future as part of the database management of the BolinCentre for Climate Research at Stockholm University.