Using Carbon Isotope Equilibrium to Screen Pedogenic Carbonate Oxygen Isotopes: Implications for Paleoaltimetry and Paleotectonic Studies

Figure 9

Montana paleoelevation revisited. (a) Plot of δ¹⁸O_carb versus Δ¹³C for the past 75 Ma in southwestern Montana (data from [22]). Based upon the theoretical and empirical constraints from Figures 3 and 5, the data set has been literally decimated, with only 29 of 308 (9.4%) pedogenic carbonates were formed in isotopic equilibrium and most of the data are extremely out of equilibrium (note the scale change versus Figure 8). (b) Plot of the remaining pedogenic carbonates that formed in isotopic equilibrium (black circles). Almost all of the structure in the dataset present in Figure 1 is gone. If one allows for evaporative due to water stress on the plants producing the organic matter of +1.5‰ (see text for discussion), the white circles would also represent equilibrium conditions. However, in both cases, little or no change in elevation is indicated by δ¹⁸O_carb values, and there are essentially no pre-Miocene data points that can be used to assess the previously identified elevation change between 50 and 30 Ma ago identified by Chamberlain et al. [22].

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