MEAN ANNUAL PRECIPITATION AND ISOTOPIC COMPOSITION INTERPRETED FROM VERTEBRATE BIOAPATITE AND BULK ORGANIC CARBON FOR THE EARLY CRETACEOUS HEKOU GROUP, GANSU PROVINCE, CHINA

Isotopic samples of two teeth from the same jaw of the basal iguanodontids Lanzhousaurus magnidens from the Hekou Group of NW China were analyzed to interpret paleonenvironmental conditions of an early Cretaceous inter-montane basin. Lanzhousaurus has the largest herbivorous dinosaur teeth ever discovered at over 10 cm in length; offering a long time series record. Analysis was taken by sampling δ¹⁸O_p every mm and carbonate samples for δ¹³C and δ¹⁸OCO3 taken approximately every 2 mm. The δ¹⁸O_p values range between 16.8 to 22.6‰ V-SMOW, δ¹⁸OCO3 range between 26.5 to 21.8‰ V-SMOW; and the d¹³CCO3 values ranged between -10.0 and -4.9‰ V-PDB. Turtle shell and crocodile teeth were also analyzed for δ¹⁸O_p resulting in an average of 13.0 ± 0.4‰ for turtle and 12.3 ± 0.2‰ for crocodile. Using water-phosphate relationships, the isotopic composition of meteoric water recorded by turtles was δ¹⁸O_w = -9.2 V-SMOW which was consistent with values generated by crocodile phosphate-water relationships at δ¹⁸O_w = -8.9 V-SMOW. These values are depleted in ¹⁸O relative to North American early-mid Cretaceous paleo-precipitation values of ~ -4.5‰ at ~25°N suggesting influence of orographic effects. Using a revised mass balance model and estimations of humidity between 50 to 70%, ingested water δ¹⁸O consumed by Lanzhousaurus ranged between -5.7‰ for 50% humidity and -1.8‰ at 70% humidity suggesting Lanzhousaurus consumed isotopically enriched water, likely from isotopically enriched plant water.

A diet-tooth enamel fractionation of 18‰ was used to calculate isotopic composition of plants from the Hekou Group. This calculated δ¹³C_diet was then used to calculate mean annual precipitation (MAP). Lanzhousaurus δ¹³C_diet was estimated at -24.4‰, on the heavy end of the C₃ plant spectrum. The calculated MAP ranged between a maximum of 2,591.0 mm/yr to a minimum of 59.2 mm/yr and averaged 369.7 mm/yr. Maximum and minimum values are unrealistic, however average values seem consistent with sedimentologic data. These values will be compared to bulk organic C δ¹³C and charcoal fragments to determine their validity. Based on sedimentological evidence and the isotopic evidence presented here, the paleoenvironment of the Hekou Group was semi-arid to semi-humid with a significant influx of orographically depleted water from the surrounding mountains.