Paleoclimate and Ecology of Cretaceous Continental Ecosystems of Japan Inferred from the Stable Oxygen and Carbon Isotope Compositions of Vertebrate Bioapatite

We investigated the environmental conditions that prevailed in continental ecosystems recorded in sedimentary deposits of Japan during the Cretaceous through the analysis of oxygen and carbon isotope compositions phosphate (delta O-18(p)) and apatite-bound carbonate (delta O-18(c) and delta C-13(c)) of vertebrate teeth and bones. Local surface water delta O-18(w) values were calculated using known phosphate-water isotope fractionation equations. Anomalously low delta O-18(w) values of local waters strongly suggest a significant contribution of high-altitude precipitation from nearby mountains to local surface waters. Mean air temperatures were estimated using a global meteoric water delta O-18(mw) value - Mean Annual Air Temperature relationship, and compared to surface water temperatures estimated from fish apatite delta O-18(p) values. Local mean annual precipitations (MAP) were estimated using the known relationship existing between MAP and C3 plant delta C-13(p) value, the latter being calculated using apatite-diet C-13 enrichment applied to plant-eating sauropod and ornithopod dinosaur delta C-13(c) values. Reconstructed environmental conditions suggest that climate changed from cool temperate to warm temperate, being relatively cold and dry during the Late Hauterivian and Barremian to warmer and seasonally more humid during the Aptian and Albian, and even warmer during the Cenomanian-Coniacian. Proposed thermal evolution during the Early Cretaceous is compatible with the absence of thermophilic taxa such as crocodylomorphs before the Aptian in the fossil record of Japan.