Molybdenum Biogeochemical Cycling In The Neoproterozoic-to-Cambrian Redox-stratified Oceans

The period of Neoproterozoic to Cambrian is a key stage in the Earth’s history when significant oceanic oxygenation and biological innovations occurred. Molybdenum (Mo) isotopes can serve as a powerful proxy to track the local and global redox states of early oceans (>520 Ma). Although widespread redox-stratification of the Neoproterozoic-to-Cambrian oceans has been detected, the marine Mo biogeochemical cycling in the context of the stratified waters remains largely unknown, preventing our better application of Mo isotopes in marine redox reconstruction. For this reason, we carried out a comprehensive research on the marine Mo biogeochemical cycles in the Neoproterozoic to Cambrian oceans. By doing this, we try to figure out the effects on the Mo cycles and isotopic fractionations at local and global scales, further constrain the oceanic redox state during this period and its relationship with the known biological innovation.Mo cycling in a stratified ocean is controlled by the Fe-Mn shuttle and H2S. By studying the Mo isotopes at Cambrian Yangjiaping section (Northwest Hunan province), we found that the former not only resulted in higher Mo enrichment relative to U, but also led to extremely light Mo isotope in the sediments. In euxinic/sulfidic environments, the Mo-isotopic compositions of the sediments were significantly influenced by the H2S gradient in the waters. These two processes could lead to a specific distribution of sedimentary Mo isotopes in space. Therefore, investigating the spatial pattern of sedimentary Mo isotopes from equivalent strata helps to eliminate the effects caused by the local redox variation and to accurately obtain the contemporaneous seawater composition.The conversion of Mo to MoS42- under the effect of H2S needs a persistently euxinic environment, which provides a possibility to track the transient oxygenation in ancient oceans. Mo isotopes from the sponge-fossil-bearing Cambrian Lantian section (Anhui province) show several negative excursions in the lower part of the section, accompanying with higher RSTE/TOC (Redox Sensitive Trace Element/Total Organic Carbon), supporting inflows of oxic waters into the deep Nanhua Basin. The oxic conditions of the basinal waters might only sustain for limited time, as they were not recorded by the iron speciation—a common redox proxy, but enough for the short flourishing of sponges given their high tolerance of low oxygen level. This work demonstrates the great potential of Mo-isotope in tracking quick variations of redox conditions in ancient oceans..The Ediacaran Lantian Biota demonstrates higher diversity than before, the role of oxygen rise in coeval ocean remians not clear. New data from the Ediacaran Zhongling section (northwestern Hunan province)and Yuanjia section (western Hunan province) of this period indicate that the Mo isotope of contemporaneous seawater at this stage was as low as +1.2%o, which is not significantly different from those of earlier oceans. Our finding suggests that the oxygenation states of the Ediacaran oceans in the wake of the Marinoan Snowball Earth might be low, and oceanic oxygen rise might not serve as a key trigger for this biological event.Redox evolution of the Neoproterozoic to Cambrian ocean has received tons of attentions, but it was poorly constrained at present. We have further reconstructed the Mo isotopic curve of contemporaneous oceans for this period. Our reconstruction suggests persistently low values around +1.1‰ from ～750 Ma to ～560 Ma, but an abrupt increase to +2.0%o between ～560 Ma and 551 Ma, corresponding to the emergence of Kimberella which, as the earliest triploblastic animal, needs more oxygen than older animals. Therefore, the secular pattern of oceanic Mo-isotopic composition reconstructed in our study supports the a key role of oceanic oxygen level in the diversification of early animals.