Multi-Taxa Isotopic Investigation of Paleohydrology In the Lower Cretaceous Cedar Mountain Formation, Eastern Utah, U.S.A.: Deciphering Effects Of the Nevadaplano Plateau On Regional Climate

Abstract

We investigate the regional climatic effects of the formation of the “Nevadaplano” plateau during the Sevier Orogeny in an overall warming world. Paleohydrology was reconstructed from 590 individual measurements of phosphate O isotopes in continental faunas of the Lower Cretaceous Cedar Mountain Formation, Utah, U.S.A. Semi-aquatic (turtles, crocodiles) and terrestrial (dinosaurs) taxa are compared to coeval pedogenic carbonates to interpret changing water sources over time. Samples were grouped into four stratigraphic faunas (lower Yellow Cat, upper Yellow Cat, Ruby Ranch, and Mussentuchit members). Resulting isotopic values were converted to δ18Ow values using established δ18Op–δ18Ow and δ18Oc–δ18Ow relationships. At a formation scale, turtles (δ18Op = 14.1 to 15.7‰ V-SMOW) and crocodiles (δ18Op = 15.0 to 19.2‰) document water compositions of −8.1 to −6.1‰ and −7.7 to −4.2‰, respectively, within the zonal range for formation-scale meteoric water at 34° N paleolatitude (−7.1 to −4.8‰) established by pedogenic carbonates (δ18Oc = 22.0 to 23.5‰ V-SMOW). These data suggest that, like soil carbonates, turtle and crocodile phosphate isotopes can be used as proxies for meteoric water isotopic composition. Dinosaur δ18Op (sauropods: 19.7 to 21.9‰, ornithischians: 16.6 to 21.7‰, small theropods: 16.9 to 18.2‰, and large allosauroids: 19.1 to 20.3‰) values generally exceed those of semi-aquatic taxa. Using mass-balance equations for modern terrestrial animals adjusted for size and inferred dinosaur physiology, ingested water is calculated for the above dinosaur groups. On a member scale, when meteoric-water values are compared with calculated dinosaur drinking water, values are equal to or lighter than meteoric water for most herbivorous groups (as low as −15.5‰ for ornithischians) and equal to or heavier than meteoric water for most carnivorous groups (as high as −2.0‰ for allosauroids). Changes in δ18Ometeoric water, δ18Odinosaur ingested water, faunal assemblages, and sedimentology, from member to member, correlate to thrusting events of the Sevier Orogeny. High elevations in the orogeny attenuated the influences of Pacific moisture, causing rainshadow-induced aridity on the leeward foreland basin during upper Yellow Cat time, and hosted seasonal snow accumulation by the end of Ruby Ranch time, as suggested by 18O-enriched water (e.g., up to an average of −2.0‰ from an allosauroid tooth) and extremely 18O-depleted water (e.g., −15.5‰ for ornithischians) in the Ruby Ranch Member. By Mussentuchit-time, delivery of the Western Interior Seaway–dominated moisture to the region, despite continued rise of the Sevier Mountains.