Abstract
AbstractLoess sediments are windblown silt deposits with, in general, a carbonate grain content of up to 30%. While regionally, loess was reported to increase weathering fluxes substantially, the influence on global weathering fluxes remains unknown. Especially on glacial‐interglacial time scales, loess weathering fluxes might have contributed to land‐ocean alkalinity flux variability since the loess areal extent during glacial epochs was larger. To quantify loess weathering fluxes, global maps representing the loess distribution were compiled. Water chemistry of rivers draining recent loess deposits suggests that loess contributes over‐proportionally to alkalinity concentrations if compared to the mean of alkalinity concentrations of global rivers (~4,110 µeq L−1 for rivers draining loess deposits and ~1,850 µeq L−1 for the total of global rivers), showing comparable alkalinity concentration patterns in rivers as found for carbonate sedimentary rocks. Loess deposits, covering ~4% of the ice‐ and water‐free land area, increase calculated global alkalinity fluxes to the coastal zone by 16%. The new calculations lead to estimating a 4% higher global alkalinity flux during the Last Glacial Maximum (LGM) compared to present fluxes. The effect of loess on that comparison is high. Alkalinity fluxes from silicate‐dominated lithological classes were ~28% and ~30% lower during the LGM than recent (with loess and without loess, respectively), and elevated alkalinity fluxes from loess deposits compensated for this. Enhanced loess weathering dampens due to a legacy effect changes in silicate‐dominated lithologies over the glacial‐interglacial time scale.
Highlights
Loess sediments cover extensive areas on the Earth's surface (Muhs et al, 2014)
The new calculations lead to estimating a 4% higher global alkalinity flux during the Last Glacial Maximum (LGM) compared to present fluxes
In order to get a global doubling of the loess extent for LGM times as first‐order approximation, the value of 10 km was chosen for the extrapolation method
Summary
Loess sediments cover extensive areas on the Earth's surface (Muhs et al, 2014). They can provide insights into dynamical sedimentation processes of the past and serve as terrestrial archives for studying dust deposition and atmospheric circulation (Muhs & Bettis, 2000; Muhs et al, 2014; Schaetzl et al, 2018). Loess deposits are defined as windblown silt deposits which typically contain quartz, feldspar, mica, and clay minerals, and carbonate minerals (Muhs et al, 2014; Pye, 1984; Smalley et al, 2011). Besides their significance to climate‐related studies, loess sediments are assumed to be important regarding chemical weathering fluxes, and their weathering behavior, including, for example, weathering proxies, has to be understood (e.g., Buggle et al, 2011).
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