Chemical weathering: Theoretical models, field study in the Yellow River basin, and laboratory study of microbially-mediated basalt and granite weathering

Consumption of CO2 by chemical weathering is one of the major fluxes in the global carbon cycle that drives long-term climate. In an effort to understand silicate weathering, which is a net sink of CO2 over geological time scales, as a function of environment (e.g., exposure of parent rocks, climate, and tectonics), the fluvial geochemistry of the Upper Yellow River was examined to determine natural chemical weathering rates on the northeastern Qinghai-Tibet Plateau. An inversion model was used to apportion total dissolved cations to atmopsheric, evaporite, carbonate and silicate sources, and it was extended to literature data for rivers draining orogenic zones worldwide. In these orogenic zones, silicate weathering rates are only weakly coupled with temperature and become independent of runoff above ∼800 mm/yr.;The phosphorus (P) cycle is closely coupled to the carbon cycle. In order to understand controls on riverine P yields, I used a stepwise regression of various parameters, such as precipitation, temperature, runoff, population density and relief. The results indicate that riverine P yields are not affected by any single factor, but precipitation and population density can explain up to 44% of the variability in the DRP yield of the East Asian rivers.;The final part of the study was devoted to understanding microbial influences on basalt and granite weathering on the continents. Batch reactors were used to characterize the mechanisms and rates of elemental release during the interaction of Burkholderia fungorum with basalt and granite at T = 28°C for 36 days. Reactors containing viable bacteria yielded the highest elemental release rates. Large pH decreases occurred when using NH4+. No pH changes occurred when using NO 3-. pH lowering resulted from organic acid production and H+ extrusion during NH4+ uptake. Elemental release rates inversely correlated with pH, suggesting that proton-promoted dissolution was the dominant reaction mechanism. Within the context of these experiments, bacteria appear to elevate the rate of long-term atmospheric CO2 consumption by Ca-Mg silicate weathering between factors of 2 and 5 over corresponding inorganic rates. The effect is greater for basalt than granite, where trace calcite, apatite, and fluorite release most of the dissolved Ca.