Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction

Increasing evidence shows that the recovery of marine ecosystems after the Permian/Triassic mass extinction (PTME) was coupled with environmental amelioration. Recently, abundant insects and complete trophic structures were unraveled from the Ladinian (late Middle Triassic) in the Ordos basin, central China, deciphering recovery of lacustrine ecosystem from the PTME. Coeval environmental settings, however, remain unclear. In the Ordos basin, the Ladinian succession is dominated by black organic-matter-rich shales of the Chang 7 Member of the Yanchang Formation, and their depositional environments and palaeo-productivity are reconstructed based on major and trace element concentrations, pyrite framboid size and morphology, and total organic carbon (TOC) contents. High values of TOC and phosphorous (P) concentrations, as well as more enrichment of trace-metal elements such as Copper (Cu) and zinc (Zn) all indicate a rather high primary productivity. Such a high productivity during the deposition of the Chang 73 sub-member is also reinforced by and the presence of abundant organic-matter-enrich laminae in black shales and the diverse chrysophytes, as well as intensive occurrence of tuff layers and possible hydrothermal fluids. Analyses of framboidal pyrite size and morphology as well as C-org/P ratio and trace-metal elements such as molybdenum (Mo) and uranium (U), indicate the dysoxic to oxic conditions in water columns of the Ordos lake during the late Middle Triassic. The Ladinian-Carnian cooling likely promoted the occurrence of dissolved oxygens in the lake. As a result, although the massive black shales of the Chang 73 sub-member were deposited and suggest a rather high primary productivity, they also indicate slightly oxygen-poor to oxygenated conditions of water columns, which are inductive for lacustrine biota to inhabit, and also offer hospitable habitats for freshwater lake ecosystems to recover from the PTME.