Trend of Santonian (Late Cretaceous) atmospheric CO sub(2) and global mean land surface temperature: Evidence from plant fossils

Quantitative reconstructions of atmospheric CO sub(2) by using terrestrial and marine records are critical for understanding the so-called "greenhouse" conditions in the Cretaceous, but data from terrestrial plants for several stages of this period remain quite limited. Using the stomatal index (SI) technique, here we estimate the Santonian (Late Cretaceous) CO sub(2) contents based on a sequence of fossil cuticles of Ginkgo adiantoides (Ung.) Heer from three beds of the Yong'ancun Formation in Jiayin, Heilongjiang Province, northeastern China. By the regress function, SIs of Ginkgo fossils reveal a pronounced CO sub(2) reduction from the early to late Santonian (661 and 565 ppm, respectively). The relatively high CO sub(2) levels provide additional evidence for paleoclimatic warmth in this interval. Moreover, available paleobotanical data illustrate a decline trend of CO sub(2) contents throughout the Late Cretaceous, punctuated by several fluctuations in particular episodes with different magnitudes. The CO sub(2) contents shifted notably in the late Cenomanian, Turonian, early Santonian, late Campanian, and probably latest Maastrichtian. Furthermore, a comprehensive study based on CO sub(2) data shows that the global mean land surface temperature (GMLST) fluctuated several times accordingly. The change ratios of GMLST ( Delta T) increased from 3 degree C in late Cenomanian to 4.7 degree C in mid Turonian, and then dramatically reduced to 2.2 degree C in mid Coniacian. From the Santonian onward, it appears that the temperature gradually decreased with a few minor fluctuations.