Reconstruction of oceanic redox structures during the Ediacaran-Cambrian transition in the Yangtze Block of South China: Implications from Mo isotopes and trace elements

Abstract

To explore paleo-ocean redox variations and Mo biogeochemistry across the Ediacaran–Cambrian transition (E-C), continuous fresh core shale samples of Ediacaran to Cambrian were collected from a well drilled in the upper slope region of the Yangtze Block in South China and analyzed for δ98/95Mo and δ13Corg values, TOC, as well as the concentrations of trace and rare earth elements. The Mo–U covariations, Th/U and Re/Mo ratios as well as published iron speciation data indicate a redox stratified ocean with oxic surface waters and anoxic/euxinic deep waters prevailing across the E-C transition. During the time of Cambrian Fortunian and Stage 2, the bottom water in the upper slope became oxic due to the deepening oxycline. Afterwards, widespread anoxia and/or euxinia occurred during latest Age 2 and early Age 3. The Mo concentrations and Mo/TOC ratios suggest that the Yangtze Block was well connected with the open ocean during the late Ediacaran, and then became strongly restricted at the beginning of Cambrian. Following a large-scale transgression during Cambrian Age 2, its connection with the open ocean was enhanced again. The negative δ98/95Mo values may reflect preferential adsorption of isotopically light Mo by Mn oxides particles above the chemocline and their subsequent shuttling to euxinic sediments in the redox stratified late Ediacaran ocean; whereas the relatively low δ98/95Mo values during the Cambrian Fortunian and Age 2 indicate the decrease of δ98/95MoSeawater values in the restricted basin. The operation of local Mn particulate shuttles could also be stimulated by inflow of oxic waters into restricted basin during marine transgression, resulting in negative δ98/95Mo values. In conjunction with previous published Mo isotope data, the variations of sedimentary δ98/95Mo values from shallow shelf to deepwater slope regions draw the outline of ‘sandwich’-like model with euxinic waters (e.g., [H2S]aq > 11 μM) sandwiched by oxic surface waters and anoxic deep waters during the E-C transition. The shuttling of isotopically light Mo to euxinic sediments by Mn oxides in the redox stratified Ediacaran-Cambrian ocean may play a role in driving heterogeneous heavy seawater Mo isotopes and variable sedimentary δ98/95Mo values.