LOU Fang-ju, GU Shang-yi^*

(College of Resources and Environmental Engineering, Guizhou University, Guiyan 550025, China)

Abstract: Since the Neoproterozoic Era, the evolution of oxygen in the Earth’s atmosphere has gone through two stages. However, the process of the two stages evolution is controversially debated. The REE distribution patterns of apatites in the marine sedimentary phosphorites show that they can reflect the REE composition of seawater at the time of apatite precipitation. The REDOX sensitive Ce anomaly of the seawater is controlled by oxygen concentrations in the seawater. Therefore, the evolution of atmospheric oxygen levels can be inferred from the variation of Ce anomalies of apatites in phosphorites of different geological ages. A database has been compiled based on published data of a total of 220 apatite samples from phosphorites of 9 geological periods varying from Ediacaran to Quarternary ( 570 Ma to 1 Ma) in 13 different locations and newly acquired REE contents of apatites from 18 samples of drill cores of the early Cambrian phosphorites in the Zhijin County, Guizhou Province, to assess the atmospheric oxygen evolution. In order to ensure pristine seawater Ce anomaly signals, samples that preferentially enriched with the MREE are not included in the database because the Ce anomaly is influenced by the MREE enrichment. The results show that average δCe values of apatites in phosphorites are decreased from 0.74 for samples of the Ediacaran Doushantuo Period to 0.36 for samples of the early Cambrian period, then are increased to 0.87 for samples of the Ordovician Period, indicating that the atmospheric oxygen concentration in the Cambrian Period may be higher than those in the Precambrian and Late Ordovician periods. The average δCe values of apatites in phosphorites of the Devonian, Carboniferous, and Permian samples are 0.36, 0.16, and 0.21, respectively. It indicates that the global atmospheric oxygen contents of the late Paleozoic Era from the beginning of Devonian period may reach or even exceed the modern atmospheric oxygen level. The average δCe values of apatites in phosphorites of the Mesozoic and Cenozoic eras (from Triassic to quaternary) are generally in relatively low levels varying from 0.24 to 0.54, indicating atmospheric oxygen contents of the Mesozoic and Cenozoic eras are basically maintained the modern atmospheric oxygen level. We found that the increased atmospheric oxygen contents in the Cambrian and Devonian periods are consistent with the occurrence of the Cambrian Explosion and appearance of Devonian arborescent land plants, and consistent with the marine carbonate Ce anomalies and biogeochemical modeling results. Based on the comprehensive analysis, we have suggested that the Ce anomalies of apatites in phosphorites, just like the Ce anomalies of carbonate rock, can indicate the atmospheric oxygen evolution. The evolution of atmospheric oxygen in the Earth during Phanerozoic is neither a one-way process nor a simple two-stage process, but a complex and constantly fluctuating process.

Keywords: phosphorite; apatite; the atmospheric oxygen evolution; the Ce anomaly

ACTA MINERALOGICA SINICA Vol. 39, No.4, 2019, page 412-419