Chemical rock weathering in North America as source of dissolved silica and sink of atmospheric CO2

The first part of the work is the development of a high resolution lithological map of North America to be used as a tool for regional scale modeling. 17 geological maps were assembled and combined with 43 references for lithology information. The new lithological map consists of 262 111 polygons, with an average size of 75 km². The features are grouped into lithological classes of three independent levels. 19 major lithological classes are defined, further detailed by 8+11 minor classes (Table 2). Comparison with existing maps highlights large differences in the distribution of lithological classes in North America. The comparison suggests that lithological classes from different maps comprise different rocks, even if they have the same name. Thus, mixing inconsistent lithological information sources into one model, e.g., by calibrating on one map and applying the model to another, shout be treated with great care. The second part consists of the calibration and development of a large scale model for dissolved silica (DSi) mobilization by chemical rock weathering in the conterminous USA. The DSi model was calibrated on 142 river catchments applying the new map and using a multilithological approach originally introduced and tested for the Japanese Archipelago (Hartmann et al., 2010). The calibrating catchments were chosen for small anthropogenic and water body impacts. The model predicts the observed DSi yields in the calibration catchments well (r=0.94) by using the predictors runoff and lithology only. Apart from reconfirming the importance of basic igneous rocks (PB+, VB+) for DSi mobilization known from previous studies (Bluth and Kump, 1994; Hartmann et al., 2010), the DSi model found carbonate-rich lithological classes (SC and SM) to be important contributors for DSi yields. Differences to exsiting models are in part attributed to regional variations in geological settings and suggests the importance of regional calibration of large scale models. Apart from runoff and lithology the DSi model shows significant trends of relative model residuals with land cover and temperature. Although those trends suggest a control of these factors of DSi mobilization by chemical weathering, they could not be included into the model equations as significant predictors. In the third part, a model for consumption of atmospheric/soil CO2 by chemical weathering is presented. It uses the same techniques as the DSi model for predicting fluvial HCO3- fluxes and is calibrated on 338 catchments with low anthropogenic impact. Correction of HCO3- fluxes for carbonate contribution is necessary to quantify CO2 consumption. In addition to contributions of carbonate-rich lithological classes (classes SM and SC), carbonate contributions from silicate dominated lithological classes were corrected, using Ca/Na elemental ratios as introduced by Hartmann (2009). Calculation of those ratios is complicated by a number of possible Ca or Na sources (e.g. street salt). Two models for HCO3- are developed that are later corrected for carbonate contributions. One model uses runoff and lithology, the second adds land cover, as predictors for HCO3- flux and thus CO2 consumption. Carbonate-rich lithological classes consume most CO2 per runoff due to their low resistance to weathering. However, basic plutonic rocks are also predicted to exhibit a large CO2 consumption per runoff. The model incorporating land cover classes as predictors shows HCO3- fluxes to increase with urban areas, shrublands, grasslands and managed lands. For the first time, land cover is included as a predictor into a multilithological forward model for CO2 consumption by chemical weathering. Although prediction quality of the models for HCO3- flux is high, correction for carbonate contribution from predominantly silicate lithological classes contains significant uncertainties. A comparison of different reasonable Ca/Na ratios for correction shows a variation of CO2/HCO3- ratios of up to 10 % for individual lithological classes. The extrapolation of the models results in a total CO2 consumption of 50.0 Mt C a-1 for the model using runoff and lithology as predictors and 51.4 Mt C a-1 for the model additionally using land cover. The models show the importance of the northern Rocky Mountains for CO2 consumption and that of small, highly active areas in general. Depending on the applied model, 8.4 % to 11.8 % of the area of North America are responsible for 50 % of the total CO2 consumption.