Early Permian chemical weathering indices and paleoclimate transition linked to the end of the coal-forming episode, Ordos Basin, North China Craton

Abstract

The Early Permian is recognized as the peak of the Late Paleozoic Ice Age (LPIA), and ice sheet covered most of Gondwana. During this time interval, the North China Craton (NCC) developed abundant economic coal seams within the Taiyuan Formation and the Lower Shanxi Formation. However, coal seams are absent in the Upper Shanxi Formation. In order to explore whether a change in paleoclimatic conditions may have been a factor in this coal-forming episode termination, we conducted a chemical weathering study on the Shanxi and Xiashihezi formations in the Ordos Basin of the NCC. Multiple chemical weathering indices (CIA, CIW, CIX, PIA and WIP) calculated from geochemical ratios in mudstones and derived paleotemperature estimates indicate that the chemical weathering intensity underwent a rapid decrease near the base of the Sakmarian Stage, remained low through the lower Sakmarian, then fluctuated at relatively high levels through the upper Sakmarian. A second drop in weathering intensity occurred near the base of the Artinskian Stage. This pattern was produced by cool climates during the Early Sakmarian associated with the P1 glacial interval (the first stage of the Permian glaciation), by a high temperature climate corresponding to the interglacial interval during Late Sakmarian, and then by another cooling into the P2 glacial at the beginning of the Artinskian. Similar changes in chemical weathering indices occur in the Paraná Basin of South America in southern high latitudes and in the mid-latitude Junggar Basin that is now in Northwest China. This chemical weathering trend mirrors reconstructed paleotemperature from global sea-level changes, from fusulinids diversity on North American shelves, and from atmospheric CO2 concentrations. A sharp decline of coal seams on the NCC is related to the mass extinction of wetland plants. The Late Sakmarian high-temperature event played an important role, and the continuous drift of the NCC toward higher latitudes and the regression associated with orographic uplift also exacerbated regional aridification.