A PRONOUNCED LATITUDINALLY-INDUCED RAINOUT EFFECT IS SHOWN IN EARLY CRETACEOUS NORTH AMERICAN WATER OXYGEN ISOSCAPE

The distribution of oxygen isotopes in terrestrial environments are heavily influenced by the water cycle and as such vary strongly with elevation, latitude, proximity to coastlines, the rate and amount of precipitation, temperature, and vital effects in organisms (the effects of physiological processes in organisms). Despite a lower temperature gradient from the equator to the poles, calculated O-isotopic composition of meteoric water shows a steeper isotopic gradient in the greenhouse world of the Cretaceous. This has been shown in analyses of oxygen isotopic composition of pedogenic siderite and carbonate nodules as well as phosphate in turtle bones present in North America during the late Early Cretaceous (Aptian to Cenomanian) (sources primarily from Suarez et al. (2021) and references therein). The explanation for this trend is that the hydrologic cycle and subsequent oxygen isotopic distribution of North America during the Cretaceous were likely profoundly influenced by increased latent heat transport toward the polar regions as a result of an intensification of the hydrologic cycle as compared to modern. This resulted in a pronounced latitudinal effect on the oxygen isotopic distribution.

This effect can be described by a polynomial equation on a latitude and oxygen isotopic value scatterplot; however, this can hide the effects of other processes such as orographic effects. Reanalysis of the findings established in Suarez et al. (2021) and previous studies is presented here using 2.5-dimensional mapping of sampling locations and oxygen isotopic values. Mapping was done using geostatistics, specifically Empirical Bayesian Kriging provided by the Geostatistical Analyst toolbox in ArcGIS Pro, to fill in the gaps between sites (kriging uses the phenomenon of spatial autocorrelation to predict the values between sites of known isotopic value). The resulting terrestrial isotopic landscape (or isoscape) for water-derived oxygen isotope values of Cretaceous North America appears to have a distribution that supports the previous finding of a pronounced latitudinal effect. This suggests that any orographic effects, such as the rise of the Sevier Mountains, on meteoric water were not sufficient to affect proxy data, or the time resolution of proxy data is not sufficient to capture orographic effects.