Isotopic evidence for continental ice sheet in mid-latitude region in the supergreenhouse Early Cretaceous

Cretaceous represents one of the hottest greenhouse periods in the Earth's history, but some recent studies suggest that small ice caps might be present in non-polar regions during certain periods in the Early Cretaceous. Here we report extremely negative delta O-18 values of -18.12 parts per thousand to -13.19 parts per thousand for early Aptian hydrothermal zircon from an A-type granite at Baerzhe in northeastern China. Given that A-type granite is anhydrous and that magmatic zircon of the Baerzhe granite has delta O-18 value close to mantle values, the extremely negative delta O-18 values for hydrothermal zircon are attributed to addition of meteoric water with extremely low delta O-18, mostly likely transported by glaciers. Considering the paleoaltitude of the region, continental glaciation is suggested to occur in the early Aptian, indicating much larger temperature fluctuations than previously thought during the supergreenhouse Cretaceous. This may have impact on the evolution of major organism in the Jehol Group during this period.