In both the marine Sr and Os isotope records Cenozoic shifts toward more radiogenic isotope ratios are suggestive of intensified continental weathering. Unlike Sr, Os may be more directly related to the weathering of the silicate fraction of the continental crust than Sr, because it is not buffered by carbonate weathering. To evaluate the significance of the silicate portion of the continental crust for the isotopic composition of the continental runoff we have analyzed soil leaches and digests from profiles developed on a set of glacial tills of different ages (0.4-133ka) from a restricted catchment area in the Wind River Range in Wyoming (Blum and Erel, 1995). This chronosequence, with lithologically homogeneous ~2Ga old felsic gneiss parent materials (Taylor and Blum, 1995) allows us to investigate the effect that the age of soils has on the isotopic composition released by mineral weathering.
Blum and Erel (1995) reported a decrease in bulk till 87Sr/86Sr and an even more pronounced decrease in the easily exchangeable 87Sr/86Sr with the age of the tills. This suggests that periods of glaciations affect the 87Sr/86Sr delivered to the oceans because radiogenic Sr is lost in the early stages
of weathering and soil formation. The global effect of
this process on the marine Sr record, although significant,
is greatly damped by the buffering effect of carbonate
weathering. We have analyzed the same samples for their bulk 187Os/186Os, and find values of 45-50 for the unweathered gneisses and the youngest till (~0.4ka), but less radiogenic 187Os/186Os of 10-17 for tills older than ~2ka. Leaching experiments (dilute HCl) on the youngest till yield even more radiogenic values (63-73) than the bulk till, with decreasing 187Os/186Os as the pH of the leaching solution decreases. Additional Re-Os analyses are in progress to investigate the mineralogical processes that lead to changes in the Os isotopic composition with increased intensity of weathering.
Based on these data we suggest that (1) a radiogenic Os component in easily lost from freshly exposed rock surfaces during the early stages of weathering; (2) the mean 187Os/186Os of the continental crust may be more radiogenic than previously suggested (10-11, Esser and Turekian, 1993), because this estimate was based on analyses of loess deposits and estuarine sediments which may have already lost their radiogenic Os fraction; (3) the mean 187Os/186Os of the continental runoff may be more radiogenic than previously suggested (~16, Pegram et al., 1994) because contributions from Precambrian terrains were not considered in the previous estimate, and (4) that the shorter oceanic residence time of Os compared to Sr increases the plausibility of glacial-interglacial variations in the marine Os isotope record. We expect the marine Os isotope record to be a more sensitive tracer of changes in the rate of silicate weathering than the marine Sr isotope record.
Blum, J.D. & Erel, Y., Nature 373, 415-418 (1995).
Esser, B.K. & Turekian, K.K., Geochim. Cosmochim. Acta 57, 3093-3104 (1993).
Pegram, W.J. et al., Earth Planet. Sci. Letters 128, 591-599 (1994).
Taylor, A. & Blum, J.D., Geology 23, 979-982 (1995).