Abstract

Paleoaltimetry is the quantitative estimate of the surface height above mean sea level of ancient landforms. Atmospheric thermodynamic modeling of the behavior of 18O relative to 16O during condensation from water vapor establish the systematic relationship that exists between Δ(δ 18Op) and elevation, where Δ (δ18Op) is the difference between a low altitude, preferably sea level δ18Op and a potentially high elevation sample. Comparison of model predictions with observations suggests that the model captures the first-order behavior of δ18Op during condensation and precipitation in orographic settings. The actual relationship between Δ(δ 18Op) and elevation depends sensitively on climate, and specifically starting temperature and to a lesser degree relative humidity. Thermodynamic modeling allows the Δ(δ18Op) and elevation relationship to be explicitly calculated for any given starting climate state. This makes the theoretical approach significantly more appropriate than empirically calibrated approaches based typically on quite limited samples presently available in most orographic settings today.

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 should reflect the precipitation amount weighted hypsometric mean elevation of the (paleo)-drainage basin from which they derive their waters. Analysis of modern data from the Himalayan region supports this expectation. Foreland basin rivers are such integrators and it is shown, using an example from the Siwaliks that the rivers draining the front of the Himalayas in the past had isotopic compositions comparable with modern rivers draining the Himalayan front suggesting little net change in Himalayan hypsometry over the past 11 million years.

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