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The Bohai Bay Basin straddled the Eocene boundary between arid and wet climates in eastern China. The basin was simultaneously affected by both arid and wet climates, and contemporaneous sediments developed under both extremely arid and extremely wet conditions. Data from every well drilled in the oil fields in the Bohai Bay Basin have been used to study the lower fourth member and the upper third member of the Shahejie Formation. The spatial distribution of climate indicators in the lower fourth member of the Shahejie Formation shows that the climate boundary extended from the northern Jizhong Depression to the Bozhong Depression. The portion of the basin to the north of this boundary mostly developed coalbeds, dark mudstone, and oil shale (indicative of a warm and wet climate), whereas the portion of the basin to the south of this boundary mostly developed gypsum, rock salt, and magenta dolomitic mudstone (indicative of a hot and arid climate). During the formation of the upper third member of the Shahejie Formation, the climate boundary shifted southward to the region between the southern part of the Jizhong Depression and the Jiyang Depression. The wet climate zone in the north expanded southward, and the arid climate zone in the south shrank. This shift is evident in the vertical lithologic characteristics of the members that formed before and after this shift in the climate boundary. The lithologic succession in the central part of the basin exhibits significant changes, whereas the lithologic successions in the southern and northern parts of the basin remain homogeneous and stable through time. Therefore, between the formation of the third and fourth members of the Eocene Shahejie Formation, the Bohai Bay Basin experienced a significant climate change. This change directly affected the pattern of the Eocene climate zones in eastern China and is indicative of changes in the paleoclimate zones near the Tropic of Cancer.

Quantitative reconstructions of atmospheric CO 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 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 2 reduction from the early to late Santonian (~661 and ~565 ppm, respectively). The relatively high CO 2 levels provide additional evidence for paleoclimatic warmth in this interval. Moreover, available paleobotanical data illustrate a decline trend of CO 2 contents throughout the Late Cretaceous, punctuated by several fluctuations in particular episodes with different magnitudes. The CO 2 contents shifted notably in the late Cenomanian, Turonian, early Santonian, late Campanian, and probably latest Maastrichtian. Furthermore, a comprehensive study based on CO 2 data shows that the global mean land surface temperature (GMLST) fluctuated several times accordingly. The change ratios of GMLST (ΔT) increased from ~3°C in late Cenomanian to ~4.7°C in mid Turonian, and then dramatically reduced to ~2.2°C in mid Coniacian. From the Santonian onward, it appears that the temperature gradually decreased with a few minor fluctuations. Santonian, Late Cretaceous, global mean land surface temperature, stomatal index, Yong'ancun Formation, JiayinCitation:Wan C B, Wang D H, Zhu Z P, et al. Trend of Santonian (Late Cretaceous) atmospheric CO 2 and global mean land surface temperature: Evidence from plant fossils.