The spatial distribution of surface ocean primary productivity in the wake of Marinoan global glaciation

摘要

Termination of the Marinoan global glaciation (~650-635 Ma) was followed by the diversification of eukaryotes (e.g., early animals) and oxygenation of deep oceans in the early Ediacaran Period. Previous studies indicate the recovery of marine primary productivity immediately before the cap carbonate precipitation but after melting of the Marinoan global glaciation. Pyrite concretions from the topmost Nantuo Formation in the Yangtze Block, South China, decrease in abundance from shelf to basin facies, while their high positive sulfur (S) isotope values argue the development of oceanic euxinia, which require ample supply of organic matter, presumably reflecting the recovery of marine productivity. However, pyrite contents and pyrite S isotopes alone cannot uniquely constrain the extent and spatial distribution of surface ocean organic matter production. In this study, we conducted in situ analyses of pyrite iron (Fe) (856Fepy) and S isotopes (834Spy) of pyrite concretions collected from four sections of the Nantuo Formation, spanning from shelf to basin environments. Pyrite from different facies has distinct 834Spy and 856Fepy values. In general, 834Spy values display an increasing trend from shelf to basin, whereas 856Fepy values demonstrate an opposite trend with the lowest value in the basin sections. The opposite 834Spy and 856Fepy shelf-basin gradients strongly argue against the hydrothermal origin, yet verify the diagenetic precipitation of Nantuo pyrite in sediment porewater. Numerical models were developed to quantify the microbial sulfate reduction (MSR), microbial iron reduction (MIR) and pyrite precipitation processes. The modeling results provide an explanation for the shelf-basin decrease of 856Fepy values, indicating that both the fraction of MIR (fMIR) and the fraction of Fe2+ consumed by pyrite precipitation (fpy) increase from shelf to basin sections. In addition, high 834Spy values and pyrite contents in the basin sections also imply more intense MSR and higher supplies of H2S from sulfidic seawater. Both MSR and MIR were fueled by organic matter, thus suggesting that the surface ocean primary productivity displayed an unusual increasing trend toward the open ocean, which was different from the higher productivity in modern near-shore regions. We speculate that the reversed shelf-basin gradient of surface ocean primary productivity might be attributed to high P concentration in the post-glacial ocean. Terrestrial riverine P supply, on the contrary, might have diluted seawater P concentration in the nearshore regions. Therefore, our result indicates that the recovery of marine productivity in the aftermath of the Marinoan global glaciation may be not controlled by the availability of nutrients, instead, the immediate recovery of productivity might have been prohibited by, e.g., low seawater pH at a high atmospheric pCO2 level in the initial melting of the Marinoan global glaciation. The marine primary productivity did not recover until a substantive rise of seawater pH, during which deglacial intense continental weathering delivered abundant P into the ocean and accordingly elevated the seawater P concentration.