THE INFLUENCE OF WESTERN INTERIOR SEAWAY ISOTOPIC COMPOSITION ON PALEOELEVATION ESTIMATES FOR CAMPANIAN WESTERN NORTH AMERICA

Geological evidence indicates that Cretaceous subduction of the Farallon plate under North America drove broad regional folding and thrusting and resulted in extensive highlands in western North America. While the regional extent of the fold and thrust belt is relatively well constrained, the elevation of the resulting Campanian-age highlands is not. Previous research has used an isotope-tracer-enabled atmospheric general circulation model (AGCM) and δ¹⁸O from terrestrial bivalves and soil carbonate to constrain Campanian-age highland elevations in western North America as between 3500 and 4000 m. However, simulated δ¹⁸O_pt over the highlands may be influenced by the starting isotopic composition of the moisture source (in this case, the Western Interior Seaway [WIS] of North America). Geochemical and paleoceanograpical research has suggested that the oxygen isotopic composition of the WIS might be as much as 2.5‰ lower than the ice-free, global averaged value of -1‰ and, consequently, previous estimates of highland elevation based on a starting δ¹⁸O of -1‰ might overestimate Campanian paleoelevation in western North America. Here we test this hypothesis by comparing two isotope-tracer-enabled AGCM experiments with specified highland elevations of 2800m and specified WIS δ¹⁸O of -1‰ and -3‰ respectively. We find that decreases in δ¹⁸O of the WIS by 2‰ decrease δ¹⁸O_pt over the WIS and the immediate coastal plain by up to 1.5‰ but have a negligible effect in more inland locations. Based on these results, we conclude that reasonable changes in the isotopic composition of the WIS have little effect on previous elevation estimates for the Campanian highlands in western North America and those highlands were, on average, 3500 – 4000m high.