Geochemical characteristics and geological significance of humid climate events in the Middle-Late Triassic (Ladinian-Carnian) of the Ordos Basin, central China

Many Middle-Late Triassic (Ladinian-Carnian) global and local humid climate events that impacted on global palaeontological evolution and mineral distribution have been recorded in the rocks. The lacustrine sedimentary sequence of the Yanchang Formation in the Ordos Basin in the northeastern Tethys region may be an important example of control by global humid climate events. As revealed by an established chronostratigraphic framework and the study of organic carbon isotopes, elemental geochemistry and clay minerals, the Chang 7 Member of the Yanchang Formation was deposited in the Ladinian-Carnian, and the high total organic carbon (TOC) and total nitrogen (TN) contents indicate the high productivity of the ancient lake. The observed elemental ratios (Sr/Cu, Rb/Sr and Al2O3/MgO) and high contents of mixed-layer smectite indicate humid climate events in the Chang 73 and Chang 71 members. The increases in weathering degree, surface temperature and terrigenous clastic material input indicate enhancement of the hydrological cycle driven by a warm-humid climate. The arid-wet transitions, the negative excursions of the studied organic carbon isotope and the temperature fluctuations during the deposition of the Chang 73 and Chang 71 members of the Yanchang Formation are consistent with the Carnian humid event (CHE) and Ladinian-Carnian humid interval (L-CHI). The palaeontological diversity and the development of organic shales, turbidity currents and sandy debris flows in the Chang 7 Member may represent the continental sedimentary responses to coeval global humid climate events. The isochronous eruptions of volcanoes around the Ordos Basin may have intensified the global greenhouse effect and become an important factor driving the multi-stage humid climate events in the Ordos Basin.