QUANTIFYING VARIATIONS IN CLIMATE OF THE JURASSIC MORRISON FORMATION FROM OXYGEN ISOTOPES FROM AQUATIC VERTEBRATES AND CARBONATES
The water values derived from the δ18OPhosphates indicate all Wyoming and Oklahoma samples to be uniform with the Wyoming δ18OWater values of -8.34 ± 0.97 (vs. V-SMOW) (n=176) and the Oklahoma δ18OWater values of -7.88 ± 2.11 (vs. V-SMOW) (n=155). Those localities are significantly different from Colorado samples with δ18OWater values of -12.28 ± 0.61(vs. V-SMOW) (n=38). The δ18Ocarbonatevalues 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.