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Broader Impacts

Existing Approaches

There are various paleoelevation measurement techniques that can already be identified, and additional approaches may come to light in the process of publicizing and convening the workshop. A preliminary list includes:

bulletFission tracks & unroofing
bulletCosmogenic nuclides and exposure ages
bulletStratigraphy in destination basins
bulletO-isotopes of authigenic minerals
bulletVesicular basalts

For example, some of these techniques have been applied extensively (yet to contrasting conclusions) to the Colorado Plateau, for instance. The presently high-standing Cretaceous marine sediments (McDonough and Cross, 1991; Sahagian, 1987) may have been uplifted anytime since the Cretaceous. Many investigators have suggested that uplift occurred in the Neogene based on tectonic, geomorphologic and sedimentary lines of evidence (Burchfiel et al., 1992; Eaton, 1986; Hay et al., 1989; Izett, 1975; Love, 1970; McKee and McKee, 1972; Ruddiman and Kutzbach, 1989; Scott, 1975; Small and Anderson, 1998; Steven et al., 1995; Trimble, 1980; Tweto, 1975; Wahlstrom, 1947). Paleofloral analyses have also led to interpretations of a Neogene uplift phase (Axelrod and Bailey, 1976; MacGinitie, 1953). Authigenic minerals formed in the presence of meteoric water support a recent rapid uplift scenario (Chamberlain and Poage, 2000; Chamberlain et al., 1999) as does deuterium in Holocene wood fragments (Epstein and Xu, 1999). Releveling along the Rio Grande across the margin of the Colorado Plateau has suggested an average uplift rate in the mid-20th century of 5 mm/yr. (Reilinger and Oliver, 1976).
These approaches have led to a general acceptance of relatively late uplift of the Colorado Plateau. However, some recent studies have challenged this "traditional" view. For example, an argument suggesting recent uplift based on marine deposition of the Late Miocene Bouse formation (Buising, 1990; Lucchitta, 1979; Lucchitta and Morgan, 1998) may be weakened by Sr-isotopic interpretations for lacustrine origin (Spencer and Patchett, 1997). In addition, new approaches to paleofloristics involving physiognomy rather than nearest living relatives, and enthalpic calculations rather than assumed lapse rates suggest that high elevations were already established in the Rockies by the Eocene (Forest et al., 1995; Forest et al., 1999), and on the Colorado Plateau and westward by the Miocene (Wolfe et al., 1997) or Late Oligocene (Wolfe et al., 1998). Oxygen isotopic evidence also suggests high elevations in the Rockies by the Eocene (Norris et al., 1996). Fission track arguments determine the time that a mineral cooled to the isotopic closure point (corresponding to 3-17 km depth, depending on the mineral) (Bryant and Naeser, 1980; Coughlin et al., 1998; House et al., 1998; Kelley and Duncan, 1986; Naeser et al., 1983). The time of uplift beyond that is unconstrained.

Each of the above approaches uses a proxy for paleoelevation that also depends on other factors. For instance, sedimentation rates also depend on erosion rates, which are greatly increased in glacial times. Geomorphologic approaches involving river downcutting also depend on changes in base level. Paleoflora reflect temperature that depends on climate changes, and enthalpic calculations also depend on longitudinal variations in enthalpy (but the errors have been estimated to be relatively small (Wolfe et al., 1998)). Further, no method based on flora can be used at high elevations (above tree line) or where the necessary flora are simply absent. For instance, there are no appropriate flora across the breadth of the Colorado Plateau to help constrain uplift history (Chase et al., 1998). A lack of sedimentary deposits would in fact be expected in an uplifting area such as an elevated plateau.

When there is more than one variable being recorded by a proxy, it can lead to ambiguous interpretations and considerable debate (Gregory and Chase, 1994; Lucchitta, 1979; Lucchitta, 1989; Morgan and Swanberg, 1985; Parsons and McCarthy, 1995; Spencer and Patchett, 1997). This problem with proxies highlights the importance of combining our efforts and bringing as many proxies as possible to bear on the problem of paleoelevation. In simple mathematical terms, we need as many "equations" as we have "variables", and more is better (no problem with overdetermination in this case). In fact, with multiple proxies, it will be possible to compare the results (for the same time and place) and explore the reasons for the differences, thus enabling each part of the research community to better understand the processes responsible for creating the individual proxies.


Send mail to dork.sahagian@lehigh.edu about the Workshop or to alex.proussevitch@unh.edu about this web site.
Last modified: 04/27/05