Global chemical weathering and associated P-release — The role of lithology, temperature and soil properties

Abstract

Because there remains a lack of knowledge about the spatially explicit distribution of chemical weathering rates at the global scale, a model that considers prominent first-order factors is compiled step by step and the implied spatial variability in weathering is explored. The goal is to fuel the discussion about the development of an “Earth System” weathering function. We use as a starting point an established model of the dependence of chemical weathering on lithology and runoff, calibrated for an island arc setting, which features very high chemical weathering rates and a strong dependence on lithology and runoff. The model is enhanced stepwise with further factors accounting for soil shielding and temperature, and the observed variation of fluxes is discussed in context of observed data from large rivers globally. Results suggest that the global soil shielding reduces chemical weathering (CW) fluxes by about 44%, compared to an Earth surface with no deeply weathered soils but relatively young rock surfaces (e.g. as in volcanic arc and other tectonically active areas). About 70% of the weathering fluxes globally derive from 10% of the land area, with Southeast Asia being a primary “hot spot” of chemical weathering. In contrast, only 50% of runoff is attributed to 10% of the land area; thus the global chemical weathering curve is to some extent disconnected from the global runoff curve due to the spatially heterogeneous climate as well as rock and soil properties. The analysis of carbonate dissolution reveals that about half of the flux is not delivered from labeled carbonate sedimentary rocks, but from trace carbonates in igneous rocks as well as from siliciclastic sediment areas containing matrix carbonate. In addition to total chemical weathering fluxes, the release of P, a nutrient that controls biological productivity at large spatial scales, is affected by the spatial correlation between runoff, lithology, temperature and soil properties. The areal abundance of deeply weathered soils in Earth's past may have influenced weathering fluxes and P-fuelled biological productivity significantly, specifically in the case of larger climate shifts when high runoff fields shift to areas with thinner soils or areas with more weatherable rocks and relatively increased P-content. This observation may be particularly important for spatially resolved Earth system models targeting geological time scales. The model is discussed against current process knowledge and geodata with focus on improving future global chemical weathering model attempts. Identified key processes and geodata demanding further research are a) the representation of flowpaths to distinguish surface runoff, interflow and baseflow contributions to CW-fluxes, b) freeze-thaw effects on chemical weathering, specifically for the northern latitudes, c) a more detailed analysis to identify to what extent the spatially heterogeneous distribution of Earth surface properties causes a decoupling of the Earth system rating functions between CW-fluxes and global runoff, as well as d) an improved understanding of where and to what extent trace or matrix carbonates in silicate-dominated rocks and sediments contribute to carbonate weathering. The latter demands e) an improved representation of carbonate content in lithological classes in the lithological representation of the Earth surface. Further improvement of the lithological database is needed for f) the age of rocks and g) the geochemistry of sediments with focus on unconsolidated sediments in the large basins. And clearly h) an improved global soil database is needed for future improvements with reliable soil depth, mineralogical composition as well as physical properties.