Fig. 7: Schematic model showing the biogeochemical cycles during the termination of Marinoan snowball Earth in the Yangtze Block.

The recovery of primary productivity provided abundant organic matter in the euphotic zone by photosynthesis (PS), fueling microbial sulfate reduction (MSR) in the water column. Seawater MSR sustained sulfidic condition.⁷ Active methanogenesis (MG) might be fueled by methyl sulfide that was produced by H₂S-methylation in sulfidic seawater. Since methanogens preferentially absorb light Ni and heavy rare earth element (HREE), seawater was characterized by higher δ⁶⁰Ni and displayed a HREE-depleted pattern. Sinking of particulate biomass of methanogens into sediments sustained microbial iron reduction (MIR), which converted ferric Fe (Fe³⁺) to Fe²⁺, and released absorbed light Ni and HREE in porewater. Syndepositional pyrite precipitation in sediment porewater would incorporate both seawater and porewater signals, and various degree of degradation of methanogen-derived organic matter led to a negative correlation between Ni concentration and δ⁶⁰Ni.