Global Bedrock Geology and River Chemistry

The lack of modern quantitative estimates of the Earth^{`</sup>s surface geology, one of the key parameters influencing river (and ocean) chemistry, is striking. Most published estimates of area-age relationships of sedimentary bedrock, for instance, were published before the 1980s, were based on less detailed geologic maps often decades older, and used techniques such as cutting and weighting age correlative map units or point counting at fairly coarse resolution (Higgs, 1949; Gilluly, 1969; Blatt and Jones, 1975; Ronov, 1980). Even more recent estimates (e.g., Berry and Wilkinson, 1994) are based on data predating the 1980s (Cook and Bally, 1975; Ronov, 1980). In an attempt to gain quantitative understanding of the link between surface geology (bedrock chemistry and weatherability) and river (and ocean) chemistry we have initiated a global assessment of the Earth<sup>`}s surface geology that is based on the latest digital geologic maps using modern geographic information system technology (Environmental Systems Research Institute^`s ArcInfo software). To date we have completed analysis of three digital data sets: 1) the geologic map of the conterminous United States of America by King and Beikman (1974; 1:2,500,000; spatial resolution ~600 km² per polygon), made available in digital format by the USGS, 2) the geologic map of Alaska by Beikman (1980; 1:2,5000,000; ~300 km² per polygon), also available in digital form through the USGS, and 3) the geologic map of Canada that is based on a revised and updated version of the geologic map by Douglas (1969; 1:5,000,000; ~780 km² per polygon), made available in digital form by the Geologic Survey of Canada. The data reveal, among others, the area-age relationship of sedimentary, volcanic, intrusive and metamorphic rocks at unprecedented temporal and spatial resolution. The data also provide quantitative estimates of the abundance of major rock types: sedimentary rocks make up 83% (US; 69.7% stratified, mostly marine and 13.5% continental), 73% (Alaska; 66.5% stratified, mostly marine and 6.3% continental), and ~52% (Canada; 43% mostly marine, 9% continental) of the bedrock. Volcanic rocks cover 9% (US), 11.6% (Alaska), and ~6% (Canada) of the surface, whereas intrusive rocks make up 5% (US), 6.6% (Alaska), and 24.5% (Canada) of the bedrock. Metamorphic rocks comprise 2.6% (US), 3.4% (Alaska), and 16.4% (Canada) of the bedrock. The maps also reveal the areal extent of minor lithologies such as ultramafic rocks that make up 0.15% (US), 0.20% (Alaska), and 0.08% (Canada) of the bedrock. In the next project phase we will combine information on the spatial distribution of bedrock in North America with digital maps of major river basins to investigate correlations between the abundance of rock types in drainage basins with river chemistry, specifically radiogenic isotope systems and macronutrients. Depending on the availability of additional digital geologic maps we will expand the assessment to other continents. Combining digital information on bedrock geology with digital maps of precipitation may allow us to use precipitation-weighted bedrock geology rather than simple area-lithology relationships. Extending this analysis to pre-Quaternary periods is beyond the current focus of our study. However, we feel such a historic perspective is necessary to fully understand and utilize reconstructions of ocean paleochemistry (e.g., the marine Os-, Hf-, Nd-, and Sr -isotope records) in models of global biogeochemical cycles (e.g., Bluth and Kump, 1991).