Paleoelevation of the Sierra Nevada

One approach to finding paleoelevation is to examine oxygen isotope composition on the lee side of a mountain range, and infer an increase in relief when a decrease in d¹⁸O appears in the isotopic signal. Chamberlain and Poage (2000) and Poage and Chamberlain (2002) performed isotope analysis on authigenic smectite in the western Basin and Range. They postulated that a decrease in d¹⁸O with time would indicate developing relief of the Sierra.

Measurements of present-day changes in isotopic composition (Dd¹⁸O) of surface waters along a change in relief, mostly collected in North America, South America, and Europe, are well described with an isotopic lapse rate of about 0.21‰/100 m (Chamberlain and Poage, 2000) (Fig. 1). This relationship can be used as a proxy for paleoelevation from isotope data, assuming that isotopic fractionation processes of the past are similar to those of present day.

Figure 1: Surface water Dd¹⁸O as a function of net elevation. Samples were collected from the Americas, Asia, and Europe. Note the decrease of Dd¹⁸O with increasing elevation. Best fit line is solid, 95% confidence interval dashed. Figure from Chamberlain and Poage (2000).

Chamberlain et al. (1999) showed that authigenic clay minerals represent the isotopic composition of surface waters at the time of clay formation. Thus, a change in isotopic composition with time recorded in these clays should indicate a change in source and/or amount of precipitation. Using isotopic compositions of smectites and geochronology of interbedded ash layers, Poage and Chamberlain (2002) inferred the age of the present relief of the Sierra Nevada. To limit variation in isotopic composition due to changes in precipitation source, they collected samples in regions that receive the majority of their precipitation in the winter, when storms track from the Pacific over the Sierra Nevada (Fig. 2).

Figure 2: Sample location map (left), and atmospheric circulation patterns (right) from Poage and Chamberlain (2002). Note that all samples except EPB were composited into a central Sierra sample. EPB served as the southern Sierra sample. Also note storm tracks to the study area generally originate in the Pacific and cross the Sierra.

If the development of topography occurred during the time span observed, one would expect to see a marked decrease in d¹⁸O for two reasons. First, storms carrying moisture must travel further to reach the area (concenration effect), and second, storms may cool as they travel over high topography (temperature effect). However, isotopic data shows a slight increase of about 0.25 ‰/Ma in d¹⁸O east of the central Sierra Nevada (Fig. 3), and a larger increase of about 0.5 ‰/Ma in d¹⁸O east of the southern Sierra Nevada (Fig. 3). These data indicate that the Sierra Nevada have been at or above their present elevation for at least 16 Ma in the central part of the range, and 14 Ma in the southern part of the range.

Figure 3: d¹⁸O results for the central Sierra (left) and southern Sierra (right). Increase in d¹⁸O with time indicates possible weakening of rain shadow and elevation loss (Poage and Chamberlain, 2002).

Factors affecting isotopic composition include changes in surface temperature, changes in the location and isotopic composition of the precipitation source, seasonality of mineral formation, and the decrease in temperature along an elevation gradient (Chamberlain and Poage, 2000). However, Chamberlain and Poage (2000) argue that changes in isotopic composition across an elevation gradient are much larger than changes due to the other factors listed above. In order to rule out the possibility of other variation as the cause of the differences in isotopic composition, one may calculate the expected d¹⁸O for a given scenario, and compare it to the data. Poage and Chamberlain (2002) present expected Dd¹⁸O for increased elevation of the Sierra Nevada over various amounts of time, and one calculation for climate change. None of these calculations show an increase in Dd¹⁸O with time, and, perhaps most importantly, the climate change calculation can not explain the variation in their data. Thus, it appears that the data show a rain-shadow effect over the past 14-16 Ma, and do not support late Cenozoic surface uplift of the Sierra Nevada.

One complication Poage and Chamberlain (2002) do not address is the development of a rain shadow on the lee side of a mountain range. When relief increases, the amount of precipitation reaching the lee side of the range decreases, and d¹⁸O becomes enriched (Dansgaard, 1964). Thus, the increase in d¹⁸O may indicate a developing rain shadow, rather than a decrease of relief.

Paleoelevation of the Rocky Mountains

Summary

References

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