STABLE WATER ISOTOPES AND PALEOALTIMETRY IN A WARM CLIMATE

The modern relationship between stable water isotopes and orography has been exploited using isotopic records preserved in geological archives to decipher past changes in Earth’s surface elevation. Previous studies have demonstrated that climate change associated with surface uplift can influence past stable isotopic lapse rates and complicate or even compromise paleoaltimetry estimates. The influence of global climate change on stable isotope paleoaltimetry in the absence of surface uplift has received less attention. It has recently been hypothesized that in a warmer climate an intensified hydrological cycle would enhance precipitation and isotopic fractionation through Rayleigh distillation, increasing isotopic gradients in orogenic settings. As a consequence, under warm conditions, isotopic gradients similar to modern would represent higher relief. This hypothesis has been used to argue that stable isotope paleoaltimetry likely underestimates Cenozoic surface uplift of the North America Cordillera.

In this contribution we examine the influence of climate warming on stable isotopic compositions in North America. Using theoretical arguments, we suggest that global warming likely had only a small influence on past precipitation amounts and isotopic fractionation through rainout in western North America. We also present results from an isotope-tracking general circulation model for a pre-industrial (280 ppmv CO₂) and warm world (1120 ppmv CO₂). The model response to elevated CO₂ is an amplification of mid-troposphere temperatures and a reduction in the surface to upper troposphere temperature gradient. Due to the temperature dependence of isotopic fractionation, the isotopic composition of vapor follows suit leading to a reduction in the surface to upper troposphere isotopic gradient. The climate model also predicts enhanced subsidence and mixing of isotopically enriched vapor on the crest and leeward side of the North American Cordillera, resulting in further reduction of the isotopic gradients across the orogen. As a consequence of these effects, precipitation at high elevations is isotopically enriched relative to that at low-elevations. Unless taken into consideration, this climatic effect on stable isotopes would lead to an overestimate of Cenozoic surface uplift.