GRGC: Examining Isotopic Signals in the Geologic Record Through a General Circulation Model Approach

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9316691 Charles An impressive network of oxygen and hydrogen isotopic records from ice cores, lake cores, deep sea cores and tree rings can now be compiled for a variety of time periods. These records potentially offer quantitative insight to past changes in climate processes, but their true climatic significance cannot be fully realized without knowledge of the fundamental driving forces for isotopes in precipitation. Simple air parcel models cannot account for the range of processes which control regional isotope distributions, and therefore the coupling of isotope tracers to a general circulation model (GCM) represents a necessary first step towards a more complete understanding of the isotope signals recorded in the geologic record. We propose to investigate these isotopic records by means of the Goddard Institute for Space Studies General Circulation Model (GISS GCM). The major issue we will pursue is how the changes in sourcewater characteristics, air mass trajectories, and air mass mixing-- all factors which must have varied over climate cycles with the major components of the climate system--affect regional isotopic distributions. Over the next two years, we will conduct a number of paleoclimatic simulations to determine the sensitivity of the model moisture source contributions and isotope fields to specific changes in boundary conditions (ice sheets, sea ice, sea surface temperature, etc.) Results of the paleoclimatic simulations will be compared to the observations, which we will compile. This data-model comparison will help identify the components of Northern Hemisphere climate system that are most important in shaping the isotope records. The comparison will also provide a test for the reliability and sensitivity of the GCM's hydrological cycle.