Meishucun phosphorite succession (SW China) records redox changes of the early Cambrian ocean

Two early Cambrian phosphorite units from the Meishucun section, SW China, were investigated to decipher the mechanism of phosphogenesis and to understand the redox states of the early Cambrian ocean. Rocks from the lower unit contain abundant -cyanobacterial-like microfossils and radial francolite aggregates, and they have oolitic textures, a negative delta C-13(carb) shift, and kerogen-like rare earth element (REE) patterns. These rocks were derived from degradation of organisms and were subsequently reworked by high-energy water. In contrast, rocks from the upper unit are dominated by stumpy francolite and have high Zn (40185 ppm) and Pb (15-845 ppm) contents and seawater-like REE patterns, indicating that P adsorption and regeneration were mediated by Fe-(oxyhydr) oxides. Phosphorites in this unit were episodically exposed above the wave base and are characterized by bedding structures. Rocks from the lower unit have low V (3-18 ppm) and Cr (7-20 ppm) contents and negative delta Mo-97/95 values, clearly suggesting that the ambient water was oxygenated due to proliferation of cyanobacteria. The upper unit must have formed near a Fe-redox boundary in sediments that allowed the transformation between Fe3+ and Fe2+, consistent with high V (18-55 ppm) and Cr (20-62 ppm) contents, and positive delta Mo-97/95 values in the unit. The oceanic reduction may be attributed to the ca. 535 Ma volcanism and hydrothermal venting in the basin. Similar compositions of phosphorites from the lower unit and equivalents elsewhere indicate that the early Cambrian phosphogenesis was controlled mainly by proliferation of primary producers. Given that shallow water above the wave base can be well mixed and in equilibrium with atmospheric oxygen, the subsequent reducing environment in the ocean recorded in the upper unit may have been a global phenomenon.