The evolving early Cambrian Ocean of the Nanhua Basin, South China

Abstract

The early Cambrian was an important period of environmental change and biological evolution, however, redox states and chemical features of the coeval ocean are still matters of debate. The Nanhua Basin in South China was connected to an open sea during this period, sedimentary successions deposited in the basin are thus ideal for paleoclimatic and paleoecologic reconstruction. In the present study, early Cambrian phosphorites from the Meishucun and Gezhongwu sections in SW China, and black shales from the Zhajin section in SE China, were investigated to decipher the circulation patterns of both the shallow and deep oceans. The phosphorite successions are divided into lower and upper units by a series of ~536 Ma tuff layers. In the Meishucun section (nearshore), rocks from the lower unit have abundant cyanobacterial-like microfossils, radial francolite aggregates and kerogen-like REE patterns, suggesting the early Cambrian phosphogenesis was intimately linked to proliferation of primary producers. Rocks from the upper unit of the Meishucun section have distinctly higher Zn, Cd and Pb concentrations than those in the lower unit, but are similar to those of the offshore phosphorites in the Gezhongwu section. Negative δ13Ccar excursions present in the upper phosphorite unit indicates extensive upwelling aftermath the ~536 Ma volcanisms was attributed to the chemical changes of the shallow ocean. Black shales in the Zhajin section deposited in the deep basin and have extremely high V concentrations and low δ98Mo values. An Fe-oxide shuttle may have developed in the depositional site to accumulate light Mo isotope. Substantial V enrichment in early Cambrian black shales elsewhere in the world indicates the presence of large marine vanadium reservoirs within the well- oxygenated ocean. Negative correlation between V concentrations and δ98Mo values reflects a significant burial of Fe-oxide during the early Cambrian which may have resulted in the termination of the ferruginous ocean existed since the early Archean. Authigenic anatases in organic-rich Zhajin black shales, indicates that Ti was migrated by hydrocarbon-rich liquids in organic rich shales during diagenesis. Methane generated from black shales may have consumed sulfates in the deep ocean during the Middle Cambrian, and caused global warming and mass extinction. Low δ34S values of carbonate-associated sulfates (CAS) but high δ34S values of disseminated pyrites from carbonaceous carbonates in the Zhajin area suggest sulfur stratification of the Nanhua Basin, in which the surface water had low δ34S but high SO42- and the bottom water had high δ34S but low SO42-. In general, during the early Cambrian, the shallow ocean was of high bio-productivity and was chemically modified by ocean upwelling after ~ 536 Ma. Proliferation of primary producers and intensified ocean circulation contributed to progressively oxygenation of the deep ocean. Post-depositional diagenesis of early Cambrian black shales released abundant methane to the ocean during the middle Cambrian, leading to oceanic S-stratification and global warming.