GoKawiilGlobal CO 2 uptake from silicate and basalt rock weathering
Global silicate weathering is estimated from river chemical fluxes with values from 0.525 to 0.612 Gt CO 2 year−1 (refs. 12,14,54). An estimate that sought to balance weathering fluxes with marine carbonate deposition fluxes is near the lower end of this range55. Estimating the fraction of silicate weathering contributed by basaltic rocks is complicated by varied rock classification and insufficient data on both river and groundwater fluxes in volcanic terranes13,25,30,56. Estimates vary from 15 to 35% (refs. 13,14,24), with overall basalt weathering fluxes in the range 0.084–0.180 Gt CO 2 year−1 depending on assumptions, or about 0.3% of anthropogenic emissions57.
Alkalinity fluxes from basaltic watersheds
River solute data were compiled from several published sources (Supplementary Information). When available, we used titration alkalinity, otherwise alkalinity was calculated by charge balance: Alk = 2[Ca++] + 2[Mg++] + [Na+] + [K+] − [Cl−] − 2[SO 4 =] − [NO 3 −] ≅ [HCO 3 −] + 2[CO 3 =]. Where time series data were available, we established a discharge-weighted concentration and used the geometric mean discharge (Q geo ) to compute the watershed flux, as Q is often log-normally distributed in smaller watersheds. In other cases, we used mean annual discharge estimates. The accuracy and bias of watershed flux calculations are almost always affected by lack of concentration and discharge (C–Q) data at sufficiently high frequency58.
Before calculation of alkalinity, flux stream data were corrected for atmospheric input using either local (if available) or seawater X/Cl− ratios. For example, Na* = [Na+] river − (Na/Cl) precip × [Cl−] river . Where available, we used strontium isotope mass balances as a check for correcting atmospheric inputs or sedimentary carbonate inputs. In active volcanic zones, samples with high [SO 4 =] or SO 4 =/Cl− were screened out as having hydrothermal contributions. Some samples that do not carry anomalous sulfate still seem to be anomalously high in dissolved solutes, as sulfate may have been partially consumed by other reactions. If rocks in the watershed were previously altered by temperature hydrothermal fluids, they may contain alteration minerals such as zeolites and carbonates that can weather readily, thus contributing a ‘relict’ hydrothermal signal to the stream solute load. This signal can be difficult to quantify and correct for26. Consequently, it is likely that some values from active volcanic regions overestimate the true low-temperature weathering signal.
Cation retention estimates
One estimate of the fraction of a partially soluble element j that is retained in the critical zone is given by
$${f}_{{\rm{retained}}}^{j}=1-\frac{{(j/{\rm{Na}})}_{{\rm{discharge}}}}{{(j/{\rm{Na}})}_{{\rm{bedrock}}}}$$ (7)
This assumes that the dissolution step of volcanic rock weathering is congruent and that sodium is not notably lost to adsorption or precipitation, that is, is conservative. Generally, Si/Na, Ca/Na and Mg/Na in stream fluxes are low relative to the source rock, implying sequestration or retention of silica and base cations in the weathering8 (Extended Data Table 2). Similarly, we can estimate cation losses in river sediment clay mineral fraction by comparing the river clay fraction compositions to the bedrock composite. We computed mass transfer coefficients as:
$${\tau }_{i,j}=\frac{{({C}_{j}/{C}_{i})}_{{\rm{wea}}}}{{({C}_{j}/{C}_{i})}_{{\rm{br}}}}-1$$ (8)
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