Terrestrial organic carbon (OC) plays an important role in the carbon cycle, but questions remain regarding the controls and timescale(s) over which atmospheric CO 2 remains sequestered as particulate OC (POC). Motivated by observations that terrestrial POC is physically stored within soils and other shallow sedimentary deposits, we examined the role that sediment storage plays in the terrestrial OC cycle. Specifically, we tested the hypothesis that sediment storage impacts the age of terrestrial POC. We focused on the Efri Haukadalsá River catchment in Iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon-based determinations of POC storage duration by supplying pre-aged "petrogenic" POC.Our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial-scale storage times. Comparison between the sample types (suspended and deposits) suggested an age offset between transported (suspended sediments) and stored (deposits) POC at the time of sampling, which is predicted by theory for the sediment age distribution in floodplains. We also observed that POC in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data, which suggested an additional role for OC cycling. Consistent with this, we observed interparticle heterogeneity in the composition of POC by imaging our samples at the microscale using X-ray absorption spectroscopy. Specifically, we found that particles within individual samples differed in their sulfur oxidation state, which is indicative of multiple origins and/or diagenetic histories. Altogether, our results support recent coupled sediment storage and OC cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling.

Silicate mineral dissolution promotes the removal of CO 2 from the ocean-atmosphere system by providing the cations and alkalinity necessary for carbonate mineral burial, which is the primary geologic sink for CO 2 released from solid Earth degassing and sedimentary recycling (Berner et al., 1983). Though the dissolution of carbonate minerals also supplies alkalinity to the ocean, carbonate weathering alone cannot drive net carbonate burial and thus influences CO 2 consumption over shorter timescales compared to silicate weathering (Broecker & Sanyal, 1998). Therefore, quantifying the fractional contribution of cations derived from silicate versus carbonate mineral dissolution during chemical weathering is critical to understanding the geologic C cycle.The main factors that modulate silicate weathering fluxes are thought to include climate, lithology, and uplift/ erosion rates. These mechanisms have been explored through numerous field studies of riverine solute fluxes (e.g.

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Basaltic watersheds such as those found in Iceland are thought to be important sites of CO₂ sequestration via silicate weathering. However, determining the magnitude of CO₂ uptake depends on accurately interpreting river chemistry. Here, we compile geochemical data from Iceland and use them to constrain weathering processes. Specifically, we use a newly developed inverse model to quantify solute supply from rain and hydrothermal fluids as well as allow for different mineral phases within basalts to react at different rates, solutes to be removed via clay formation, and some Ca to be sourced from carbonate dissolution. While some of these processes have been considered previously, they have not been considered together allowing us to newly determine their relative contributions.We find that weathering in Iceland is incongruent in two ways. Firstly, solute release from primary silicates is characterized by a higher proportion of Na than would be expected from bulk basalts, which may reflect preferential weathering or some contribution from rhyolites. This Na enrichment is further enhanced by preferential Mg and K uptake by clays. No samples in our dataset (n=537) require carbonate dissolution even if isotopic data (δ26Mg, δ30Si, δ44Ca, and/or 87Sr/86Sr) are included. While some carbonate weathering is allowable, silicate weathering likely dominates. The complexity we observe in Iceland underscores the need for inverse models to account for a wide range of processes and end-members. Given that riverine fluxes from Iceland are more Na-rich than expected for congruent basalt weathering, the characteristic timescale of CO₂ drawdown is likely affected.