RE-EVALUATION OF OXYGEN ISOTOPIC GRADIENT AND OXYGEN ISOSCAPE OF THE 'MIDDLE' CRETACEOUS OF NORTH AMERICA

Greenhouse climates of the Cretaceous Period promote an intensification of the hydrologic cycle since a warmer atmosphere holds more water in the form of water vapor. Previous research suggests this process would produce a steepened oxygen isotopic gradient in meteoric water due to intense Rayleigh distillation, however recent advancements in geospatial technology and continental plate reconstruction, a different interpretation is explored here. In this study, reanalysis and addition of new and repurposed of oxygen isotopic composition of pedogenic sphaerosiderite, calcium carbonate nodules, and turtle shell and crocodile tooth phosphate in North America during the ‘Mid’ Cretaceous (113Ma to 94 Ma) was conducted to fill gaps within the spatial distribution. Proxy data was put into a new paleogeographic context based on calculated latitude and longitude using the program GPlates. With a readjustment to paleolatitude, a shallow temperature gradient from the equator to the poles is matched with a shallow calculated O-isotopic composition of meteoric water during the Albian to Cenomanian in the ‘Mid’ Cretaceous. The results of this recalculation have had a significant impact, and the calculated latitudinal effect for the Albian-Cenomanian is now, coincidentally, more in line with a modern oxygen isotopic gradient:

δ¹⁸O_w = -0.0032(°latitude)² - 0.0108(°latitude) - 1.223

An accompanying isoscape has also been created for the Cretaceous Western Interior Basin. The isoscape was produced by interpolation (kriging) of meteoric oxygen isotope values from the above compilation of oxygen isotope proxies. The subsequent terrestrial isoscape for meteoric water-derived oxygen isotope values of North America in the Cretaceous appears to have a distribution that is dominated by a latitudinally-induced distribution. This isoscape also features a strong heavy oxygen isotopic bias in the south-east quadrant of the Western Interior Basin likely from vapor in airmasses originating from the ancient Gulf of Mexico. A lack of any altitudinal effect is a distinguishing feature of this isoscape as compared to the effect the North American Cordillera has today. This suggests that any effects from increased elevations on meteoric water were 1) not sufficient to affect proxy data, 2) not widespread enough to affect proxy data, and/or 3) cloaked by increased global humidity.