QUANTIFYING VARIATIONS IN CLIMATE OF THE JURASSIC MORRISON FORMATION FROM OXYGEN ISOTOPES FROM AQUATIC VERTEBRATES AND CARBONATES

The isotopic composition of meteoric water is often preserved by materials such as soil carbonates and biogenic apatites. Freshwater aquatic 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 used oxygen isotopes from crocodile teeth and turtle scutes from the Brushy Basin Member as a proxy for the isotopic composition of meteoric water in order to infer information about the hydrologic cycle during the Late Jurassic at different latitudes. Fossils were gathered from localities ranging from Northern Wyoming to Oklahoma to document differences in δ¹⁸O of meteoric water throughout the Morrison Formation. In addition palustrine carbonates were collected from the Cleveland-Lloyd Dinosaur Quarry, in east-central Utah. The combined use of biogenic apatites and carbonates allow for a paleotemperature estimation.

The water values derived from the δ¹⁸O_Phosphates indicate all Wyoming and Oklahoma samples to be uniform with the Wyoming δ¹⁸O_Water values of -8.34 ± 0.97 (vs. V-SMOW) (n=176) and the Oklahoma δ¹⁸O_Water values of -7.88 ± 2.11 (vs. V-SMOW) (n=155). Those localities are significantly different from Colorado samples with δ¹⁸O_Water values of -12.28 ± 0.61(vs. V-SMOW) (n=38). The δ¹⁸O_carbonatevalues averaged -10.03 ± 0.65 (vs. V-PDB) (n=30). Using water values derived from the bioapatites, provides an estimated temperature of 6.8°C ± 2.3°C.

The isotopic analysis suggests that climate during Morrison deposition was equable, but that variation existed, likely due to topographic effects. The highlands to the west seems to have played a larger role than latitude in affecting the variations in meteoric δ¹⁸O. The large variations seen in the Oklahoma samples may suggest there was a greater seasonal influence in the southern portion of North America. More than one source of moisture affected the southern region, likely a source from the Pacific Ocean and a source from the newly formed Gulf of Mexico. The temperature is a lower than existing paleotemperature estimates for the Late Jurassic. These results may be explained by colder (and lower δ¹⁸O) spring waters recharging from higher altitudes. Another possibility is that the carbonates have be altered by burial diagenesis and provide inaccurate temperature estimates.