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, which ranged from about 15°N to 30°N paleolatitude. This study used oxygen isotopes from aquatic crocodile teeth and aquatic turtle scutes from the Brusy 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. This study focused primarily on scutes from turtles and the teeth of crocodiles to minimize the effects of diagenesis. Samples were gathered from localities ranging from Northern Wyoming to Oklahoma to document differences in paleolatitudes throughout the Morrison Formation. To analyze the oxygen isotopes, the scutes and teeth were drilled with a microdrill to produce between 400 µg and 800 µg of powder. The samples were combusted in the High Temperature-Combustion Elemental Analyzer (TCEA), creating CO gas which was analyzed by a Thermofinnegan Delta Plus XP isotope ratio mass spectrometer at the University of Texas at San Antonio.

Results show all the Wyoming samples are fairly equal with meteoric water quartile values ranging between -7.26 and -9.53 which are similar to the Oklahoma samples which range between -5.06 and -9.85. The Colorado samples are much more isotopically depleted than all of the other localities with quartile values of -11.89 and -12.73. This pattern might be due to different sources of water and the effect of different paleogeography or paleotopography for each site. Sample variability appears greater in the Oklahoma samples, which might signify seasonal differences in water source to the area. This suggests that climate during the deposition of the Morrison Formation may not have been homogeneous as previously thought.