Insights into Orogenesis Derived from Stable Isotope Paleoaltimetry
Paleoaltimetry is the quantitative estimate of the surface height above mean sea level of ancient landforms. This presentation will focus on applications of stable isotopes (δ18O, δ2H, and Δ47CO2) as paleoaltimeters.
Atmospheric thermodynamic modeling of 18O relative to 16O and 2H relative to 1H during condensation from water vapor establish the systematic relationship between Δ(δ18Op) or Δ(δ2Hp) and elevation, where Δ(δ18Op) and Δ(δ2Hp) are the differences between a low altitude, preferably sea level δ18Op or δ2Hp and potentially high elevation precipitation. Comparison of model predictions with observations suggests that the model captures the first-order behavior of δ18Op and δ2Hp of precipitation in orographic settings. The actual relationship between Δ(δ18Op) or Δ(δ2Hp) and elevation depends sensitively on climate, and specifically starting temperature and to a lesser degree relative humidity. Thermodynamic modeling allows the explicit calculation of Δ(δ18Op) and Δ(δ2Hp) with respect to elevation for any given starting climate state. This makes the theoretical approach significantly more appropriate than empirically calibrated approaches. Comparison of model output with empirical data validates the model and provides a basis for comparison between model-derived estimates of uncertainty with ranges in observed compositions. Clumped isotope paleothermometry uses Δ47CO2 , which is the ratio of simultaneously substituted 13C and 18O in CO2 evolved from dissolved carbonate relative to the isotopically abundant 12C and 18O in evolved CO2. This ratio is a function of temperature of formation of carbonate. Since surface temperature is also a function of elevation, and since Δ47CO2 and δ18Oc are measured simultaneously from the same aliquot of evolved CO2 gas, this techniques potentially provides independent estimates of elevation for a single sample.
Paleoaltimetry archives derive their isotopic compositions from surface and or ground water, and hence it is important to understand the systematic differences between these reservoirs and precipitation. Surface waters and ground waters integrate not just the change in isotopic composition with altitude, but also variations in hypsometry within the drainage basin and precipitation amount as functions of elevation. Thus these archives reflect the precipitation amount weighted hypsometric mean elevation of the (paleo)-drainage basin from which they derive their waters.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009