| Increasing evidence indicates that the evolution of early life, particularly of eukaryotes, is intimately associated with the atmosphere-ocean redox conditions. Following the Ediacaran metazoan radiation, the early Cambrian is known for the rapid radiation of bilateririans, biomineralization of animals and the buidup of major framework of modern animal phyla and marine ecosystem (i.e.,"Cambrian explosion"). In the past decades, intensive studies have been carried out on the morphological characteristics and compositions of fauna, stragraphic divisions and correlations, mechanisms of phosphate rock and Ni-Mo sulfide ores. Great progresses have been made, such as the discovery of the Chengjiang biota. Recently, more and more attentions were paid to ocean chemistry in order to explore the coevolution relationship between early animals and their living marine environments, but their relationship remains unclear.It is commonly assumed that the environmental change, participate in the oxygenation of deep oceans, causing the "Cambrian explosion". However, some researches thought that the environmental shift such as the completed ocean and/or high seawater sulfate concentrations is a result of this significant biotic events. Although the above comprehensions on the ocean chemistry and "Cambrian explosion" are fundamentally differnent, It is accepted the "Cambrian explosion" is closely with oxygenation of deep ocean. In addition, many contradictions are also existed in the early Cambrian ocean chemistry and its relationship with animal evolution:oxic or anoxic deep waters, high or low seawater sulfate concentrations, the euxinic waters and rapid evolution of animals. Recently, Fe-S-C geochemical data from shallow to deep waters suggested that a highly stratifield ocean redox structure in the Ediacaran Yangtze platform resolved the contradictions of ocean chemistry and its relationship with fossil records in South China, which can provide a new view on the above contradictions in the early Cambrian. The profiles of different sedimentary facies outcrop relatively complete across the Yangtze platform in South China. Besides, In past decades, extensive studies have been carried out on the lithostratigraphy,biostratigraphy, chronostratigraphy, and chemostratigraphy of many sections with different depositional water depths in South China, which not only provide the basis for stratigraphic division and correlation, but also make it possible to explore the spatial variability of ocean chemistry in a basin scale.In this study, to evaluate the ocean redox structure, seawater sulfate concentrations and its influences on climate change, here we present high-resolution Fe-Mo-S-C geochemical data from the Jinsha (inner shelf) and Weng’an (shelf margin) sections, and then correlate our new data with the previously reported data from Xiaotan (inner shelf), Zunyi (outer shelf), Songtao (upper slope) and Longbizui (basinal) sections by the litho-and bio-and organic carbon isotopic (δ13Corg) chemo-stratigraphy in the early Cambrian Yangtze platform, South China. In addition, by correlating our evaluated ocean chemistry to rich fossils in South China, we preliminary explore the relationship between the ocean chemistry and bioradiation in the early Cambrian.Integrated Fe speciation and Mo abundance data from shallow to deep waters indicate that the early Cambrian deep waters beneath oxygenated sufurce waters remained ferrouginous (anoxic and Fe2+rich), while a metastable euxinic (anoxic and sulfidic) waters might dynamically develop along the continental margin. This redox structrure is similar to conditions previosly suggested for the late Archean, proterozoic and Neoproterozoic marine basin, suggesting that the precambrian ocean redox conditions persisted into early Cambrian. The isotopic composition of pyrite sulfur (δ34Spy) show a vertical gradient, indicating a small size of seawater sulfate reservior in the early Cambrian, which is opposite with previosly suggested high seawater sulfate conditions. Furthermore, the low seawater sulfate concentrations would fuel the bloom of Methanogens and Methanotrophs, particularly aerobic methanotrophs, enhancing the flux of CH4release into the surface ocean and the atmosphere, which can be of a important contributor to the warming at this time. On the orther hand, from the early Cambrian Meishucnian to Qiongzhusian, a decrease and increase in δ34Spy are observed in shallow-water Jinsha (inner shelf) and deep-water Songtao and Longbizui sections, respectively, However, those δ34Spy characteristics are not shown in Weng’an section, suggesting that bacterial sulfate reduction (BSR) is dominantly controlled by terrigenous sulfate and seawater sulfate reservior in shallow and deep waters, respectively.By correlating our evaluated ocean chemistry to abundant fossils in the early Cambrian, South China, we find that complex biota dominated by arthropoda such as Chengjiang biota live in the oxygenated waters above the anoxic even euxinic deep waters, whereas the biota dominated by sponges such as Songlin biota or small shelly fauna can adapte to the anoxic but non-euxinic waters as free H2S is toxic to most eukaryotes. Furthermore, along with the stepwise enhanced bioturbation, a decline in seawater sulfate concentrantions are proposed in both Songtao (upper slope) and Longbizui (basinal) sections, indicate the bioturbation has little influence on seawater sulfate concentrations at this time. Thus, we conclude that the "Cambrian explosion" was only directly related to the local water chemistry instead of the deep-water ocean chemistry; this challenges the traditional views on the relationship between the early Cambrian ocean chemistry and "Cambrian explosion". |
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