Revisiting paleoenvironmental changes on the Upper Yangtze Block during the Ordovician-Silurian transition: New insights from elemental geochemistry

Temporal and spatial patterns of paleoenvironmental change on the Upper Yangtze Block during the Ordovician-Silurian transition remain debatable and somewhat self-contradictory. The present study is a reassessment of how regional and global events may have influenced the paleoenvironmental evolution of the Upper Yangtze Block. Abundances of redox-sensitive trace elements, including Mo and U, suggest a general ascending stratigraphic pattern from the oxic to anoxic (euxinic) conditions during Late Katian time. High-resolution fluctuations of redox conditions may reflect frequent eustatic sea-level changes. The Kuanyinchiao Bed accumulated under less reducing conditions during the climax of Hirnantian glaciation. Post-glacial transgression during the latest Hirnantian to Early Rhuddanian was accompanied by the re-establishment of euxinic bottom-water conditions. Authigenic MoU covariation and Mo/TOC values of the studied stratigraphic successions suggest that the Upper Yangtze Shelf Sea was weakly to moderately restricted. Patterns of Cd/Mo and Co (ppm) × Mn (wt%) values suggest that the study area of the basin did not experience persistent upwelling. Instead, only seasonal upwelling occurred on the outer shelf, which is somewhat analogous with the modern Cariaco Basin. Terrestrial inputs, as reflected by Si/Al values, correspond with periods of diminished sedimentation rates and the establishment of anoxic conditions. Uplift associated with the Kwangsian Orogeny during latest Hirnantian to the early Rhuddanian time caused an increase in sedimentation rates in the eastern region of the study area. Strong covariance of Baxs and Eu/Eu* suggests that paleoproductivity was enhanced by regional hydrothermal events. Our results suggest that redox conditions and clastic inputs during deposition of the Ordovician-Silurian transition succession of the Upper Yangtze shelf were linked to climatically-induced sea-level fluctuations whereas the spatial gradient of redox conditions and clastic inputs from inner shelf to outer shelf was strongly controlled by paleogeography and orogenic movements.