Can atmospheric dust deposition account for Earth's riverine Si discharge?
Abstract
Silicate (Si) weathering is Earth's primary sink for atmospheric CO 2 . Though riverine Si discharge is usually attributed primarily to rock weathering, biogeochemists and geomorphologists have noted that dust may be a significant contributor to weathering in many areas due to its high reactive surface area, which should in principle make it the most readily weatherable fraction. However, disentangling the contribution of dust from upper continental crust rock weathering is confounded because their average compositions are roughly the same.
In this contribution, we test the hypothesis that weathering of dust could possibly account for the magnitudes and spatial variation in observed riverine Si discharge. We combined atmospheric GCM-estimates of dust fluxes with analysis of digital topography to place maximum bounds on the dust-derived (DD) contribution to weathering products. We partitioned a global 0.5° x 0.5° dust flux grid with global hydrography from HYDRO1k data. Next, we calculated upper bounds on DD ion fluxes in sampled watersheds by assuming similar dust and continental crust composition. Lastly, we calculated the proportion of these DD discharges to measured riverine dissolved loads to determine maximum DD input. We found Si discharge in almost all sampled rivers could be entirely or almost entirely explained by upstream-derived dust inputs. Additionally, OSL and zircon geometry revealed significant differences in cooling histories and luminescence sensitivity in polymineral fractions of silt-size grains, and increased euhedral to subrounded morphologies of soil-derived grains compared to underlying saprolite, consistent with a high amount of wind-blown material in the soils. These prevent us from rejecting the hypothesis that dust is the primary contributor to observed riverine Si discharges. DD K contributed on average up to <35% of the riverine dissolved load, while Ca, Mg, and Na contributed <10%. DD Cl inputs were negligible. These trends are consistent with the derivation of Si from the most recalcitrant weathering phases with rock inputs required to explain ions derived from increasingly weatherable phases. Our results highlight the need to better understand the dynamics of dust deposition and subsequent weathering when assessing weathering impacts on global biogeochemical cycling.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP41A..05P
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 1815 Erosion;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY