Mo-isotope constraints on the mid-Proterozoic ocean oxidation: Insights from the black shales of the ∼1.64 Ga Chuanlinggou Formation, North China Craton

Oxygen is a critical driver in the evolution of life on Earth. Current views suggest a persistently low oxygen level during the mid-Proterozoic, while more evidence points to episodic oxygenation events in this period, matching with remarkable biological evolution events. The ∼1.64 Ga Chuanlinggou Formation in the eastern North China Craton (NCC) offers a well-preserved fossil record of the earliest multicellular eukaryotes, and the first large-scale black shale occurrence since the late Statherian. This is correlated with contemporaneous black shales widely distributed in different cratons, reflecting a global geological event linked to the rifting of Columbia supercontinent. However, the redox conditions of the ocean system at that time remain unclear. In this study, we mostly present a multi-proxy geochemical dataset of the Chuanlinggou black shales, including molybdenum (Mo) concentration, Mo isotopic composition (δ^98/95Mo), and redox-sensitive trace elements (RSE, e.g., Mo, U, V), to quantify the redox state of the seawater during sedimentation. The whole-rock Mo concentrations of samples range from 0.19 ppm to 1.47 ppm and δ^98/95Mo from −0.01 ‰ to +1.42 ‰. An anomalous high Mo concentration (4.09 ppm) and high δ^98/95Mo (1.01 ‰) values were observed in the upper part of this sequence, corresponding with the high total organic carbon contents (TOC = 2.17 %) and enrichment factors of high Mo_EF (3.91), V_EF (0.88) and U_EF (1.35). This represents a redox condition closest to the euxinic state, which may imply an enhanced weathering effect. The Mo isotope mass balance modeling suggests a minimum seafloor oxidation area of ∼38 %. Alternatively, the maximum euxinic system could be limited to ∼45 %. We infer a short-lived oxygenation event during the early mid-Proterozoic, driven by increased nutrient fluxes resulting from the comprehensive effects of intensified rift volcanism, enhanced oxidative weathering and expanded accommodation space. It may have provided favorable environmental conditions for the landmark evolution of eukaryotes during that time.