The 87Sr/86Sr ratios of dissolved strontium in six Sierra Nevada streams at base flow are 0.00070 to 0.00175 higher than the average whole-rock value for the granodiorite bedrock underlaying the drainage basins. We suggest that the strontium isotope ratios and major cation compositions of the stream waters are controlled predominantly by the weathering reactions: plagioclase ⇒ kaolinite and biotite ⇒ vermiculite. Stream drainages that were glaciated ∼10 kyr ago (exposing fresh bedrock surfaces) have 87Sr/86Sr ratios 0.00036 to 0.00105 higher than drainages that were not glaciated in the past ∼100 kyr. We interpret the 87Sr/86Sr ratios of the stream water to indicate that biotite is weathering ∼6 times and ∼4 times more rapidly than plagioclase in the recently glaciated and nonglaciated drainages, respectively. Our results suggest that continental glaciation may have the effect of accelerating biotite weathering, significantly elevating riverine 87Sr/86Sr ratios in regions draining silicate bedrock for periods of ∼100 kyr.
Bibliographical noteFunding Information:
Acknowledgments--We thank A. Blum and R. Reynolds for helpful discussions; M. Hingston, A. Katz, P. Green, and S. Pehkonen for laboratory analyses; A. Friedland and R. Reynolds for making their laboratories available to us; J. van Wagtendonk for permission to conduct research in Yosemite National Park; and J. Drever and several anonymous referees for reviews. Funding was provided by the NSF grant EAR-9205767 to JDB and YE, a Sloan Foundation research fellowship to JDB, and a Waterhouse grant to KB.