Assessing latitudinal variations in climate of the Jurassic Morrison Formation from oxygen isotopes from aquatic vertebrates

The depositional environment of the continental Morrison Formation is composed of fluvial and floodplain environments generally thought to be deposited in a semi-arid to sub-humid climate. It is unknown if conditions were uniform across the entire extent of the formation. This study used oxygen isotopes from aquatic crocodile teeth and aquatic turtle scutes from the Brushy Basin Member as a proxy for the isotopic composition of meteoric water in order to infer information about the hydrological cycle during the Late Jurassic at different latitudes.

The isotopic composition of meteoric water is often preserved by materials such as soil carbonates and biogenic apatites. Aquatic terrestrial vertebrates spend most of their life in water, so the bioapatites should reflect the isotopic composition of the water in which they lived in. Samples were gathered from localities ranging from Northern Wyoming to Oklahoma to document differences in paleolatitudes throughout the Morrison Formation.

Results showed all the Wyoming and Oklahoma samples to be fairly uniform with d18O phosphate values of 13.86 ± 1.72 (n=331) which are significantly different from the Colorado samples with d18O phosphate values of 9.93 ± 0.61 (n=38). This pattern might be due to different sources of water and the effect of different paleogeography or paleotopography for each site. This suggests that climate during the deposition of the Morrison Formation was equable, and the existing variation was likely due to topographic effects.