SR-87/SR-86 RATIOS OF SIERRA-NEVADA STREAM WATERS - IMPLICATIONS FOR RELATIVE MINERAL WEATHERING RATES

The Sr-87/Sr-86 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 double right arrow kaolinite and biotite double right arrow vermiculite. Stream drainages that were glaciated similar to 10 kyr ago (exposing fresh bedrock surfaces) have Sr-87/Sr-86 ratios 0.00036 to 0.00105 higher than drainages that were not glaciated in the past similar to 100 kyr. We interpret the Sr-87/Sr-86 ratios of the stream water to indicate that biotite is weathering similar to 6 times and similar to 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 Sr-87/Sr-86 ratios in regions draining silicate bedrock for periods of similar to 100 kyr.