Early Cambrian ocean mixing recorded by phosphorite successions in the Nanhua Basin, South China

Abstract

Phosphorite successions in the nearshore Meishucun section and the offshore Gezhongwu section of the Nanhua Basin, South China, were investigated to decipher the circulation pattern of the early Cambrian ocean. The 539–535 Ma phosphorites from the Meishucun section contain authigenic phosphorus-rich mineral aggregates (collophanite grains) with much lower bio-limiting elements, Zn, Cd and Ni, than those from the Gezhongwu section. In contrast, collophanite grains in the 535–526 Ma phosphorites from both sections have similar concentrations of these elements. This change indicates that the ocean likely transformed from a chemically stratified state to a homogeneous state at ~535 Ma. Phosphorites from both sections have constant 87Sr/86Sr ratios and εNd(t) values, supporting the interpretation of a relatively constant terrestrial input into the Nanhua Basin between 539 Ma and 526 Ma. Instead, negative δ13C excursions and similar 208Pb/204Pb ratios of the 535–526 Ma phosphorites are indicative of an extensive upwelling of the deep oceanic water. In combination with previous studies of deep ocean oxygenation, our work provides evidence for the vertical mixing of the ocean between 535 Ma and 526 Ma, which introduced O2-rich water at the ocean bottom to promote early Cambrian biodiversity.