Vegetation response to climatic evolution and environmental change during the Oligocene in the Bohai Bay Basin (eastern China): Implications of palynological and organic geochemical data

Abstract

The Eocene-Oligocene transition was marked by a notable shift in global climate patterns, from greenhouse to icehouse conditions and continental aridification, consistent with a change in vegetation cover. Tectonic uplift and the collision between the Indian and Asian plates, as well as sea regression, were of the potential derivers of global climate changes. In the lacustrine Bohai Bay Basin, thick sequences of the siliciclastic Shahejie and Dongying formations were deposited during the Eocene-Oligocene. This study examines the Oligocene First Member (Es₁) of the Shahejie Formation and the Third Member (Ed₃) of the Dongying Formation from the NP3-20 well in the Nanpu Depression based on palynology, organic geochemistry, and mineralogical composition to reconstruct the paleoclimatic and paleoenvironmental conditions. Palynomorph composition reveals that the Es₁ and Ed₃ members are dominated by moderate to high abundances of gymnosperm and angiosperm pollen grains. Gymnosperms include mainly Pinaceae (Pinuspollenites and Piceaepollenites) and Podocarpaceae conifers (Podocarpidites and Paleoconiferus), which indicate cold-temperate conifer forests. Angiosperms, represented by Fagaceae (Quercoidites), Betulaceae (Betulaepollenites, Alnipollenites, and Momipites), Juglandaceae (Juglanspollenites and Caryapollenites), and Ulmaceae (Ulmipollenites and Ulmoideipites), suggest a broadleaved deciduous forest. These angiosperms together with fern spores reflect warm-temperate and humid climates typical of the lower vegetation belts in temperate and subtropical mountain ranges. Thus, the palynomorph content indicates that a mixed conifer and broadleaved deciduous forest was prevalent during deposition of the Oligocene Es₁ and Ed₃ members in the Nanpu Depression. This mixed forest is characterized by average annual temperatures of 2–8 °C and precipitation levels of 500–1000 mm, reflecting a temperate-warm and humid climate. Moderate acritarchs and rare dinoflagellate cysts suggest successive phases of marine incursions and increased hypersaline conditions during deposition of the Es₁ and Ed₃ members. Organic geochemical proxies, such as Pr/Ph, Pr/nC₁₇, and Ph/nC₁₈, provide further evidence of enhanced anoxia and increased marine organic matter input. The MPI and MPRs aromatic maturity parameters revealed that the Es₁ and Ed₃ members are thermally mature and reached the peak to late stages of the oil window. SARA analysis shows that most samples of the Es₁ and Ed₃ members rich in saturates, and the oils are normal paraffinic oils.