Mesoproterozoic oxygenated deep seawater recorded by early diagenetic carbonate concretions from the Member IV of the Xiamaling Formation, North China

摘要

Oxygen availability is crucial for the evolution of eukaryotes in geological history. However, the evolution of mid-Proterozoic oceanic-atmospheric redox conditions remains heavily debated, e.g., [O-2] < 0.1-1% PAL vs. > 4-8% PAL (present atmospheric level). In order to further constrain the surface oxygen levels during Mesoproterozoic, an investigation on the I/(Ca + Mg) ratios of deep-water (> 100 m) early diagenetic carbonate concretions from the shale-predominated Member IV of the Xiamaling Formation (similar to 1.4 Ga) in three sections of the North China Platform was conducted. The results show that more than half (36/47) of the I/(Ca + Mg) values obtained from the concretions are higher than 0.5 mu mol/mol (avg. 0.68), indicating non-negligible iodate retained in the porewaters where the concretions were formed. Compared with the iodine redox cycle and oxygen transportation pathway from bottom seawater to surface sediments in modern oceans, the oxygen concentration of the bottom seawater was estimated to be higher than 16-22 mu M, and the minimal atmospheric oxygen level should be higher than 6-9% PAL. This result differs from the previous estimation that the atmospheric oxygen level is lower than 0.1-1% PAL, but is consistent with the estimation of > 4-8% PAL at similar to 1.4 Ga.