Molybdenum isotope-based redox deviation driven by continental margin euxinia during the early Cambrian

Mass Extinction Coincided With Expanded Continental Margin Euxinia During the Cambrian Age 4

“…Some researchers assumed that the marine Mo cycling mode during the early Cambrian resembled that of the modern ocean, and suggested that the δ 98 Mo signature was indicative of modern‐level oceanic oxygenation (e.g., Chen et al., 2015). In contrast, other researchers emphasized a more significant role of Fe‒Mn shuttles in the less oxygenated early Cambrian ocean, and suggested that the elevated seawater δ 98 Mo actually resulted from expanded continental margin euxinia and concomitant intensification of marine Mo deposition as well as isotopic fractionation via Fe‒Mn shuttling (Cheng et al., 2016; Qin et al., 2022).…”

“…As discussed above, the Mo-U covariation pattern reveals prevalence of Fe-Mn shuttles on the Yangtze Block during the Cambrian Age 4 (Figure 2a). Moreover, the oceanic Mo reservoir during the Cambrian Age 4 was likely smaller than that of the modern ocean (Section 5.2), disagreeing with a comparable global marine redox state (Cheng et al, 2016(Cheng et al, , 2017Qin et al, 2022). Therefore, the modern-level marine δ 98 Mo signals can be most parsimoniously explained by expanded continental margin euxinia and intensified oceanic Mo deposition via Fe-Mn shuttles at the lower and upper boundaries of the Cambrian Stage 4.…”

Geochemical Characteristics of Trace Elements and Mineralization Model of the Ediacaran-Early Cambrian Phosphorites, South China

“…In recent years, with the development of metal stable isotope analysis technology, based on C, S isotopes, stable isotopes such as U, Mo and Fe have been gradually applied to paleo-marine environment research, becoming effective indicators for tracing the redox state of marine water. ,,, Many studies based on various geochemical indicators such as inorganic carbon isotope, molybdenum isotope and sulfur isotope show that there is a highly stratified redox structure in the Ediacaran to Early Cambrian oceans in the Yangtze region of South China. − At the Doushantuo Formation, Dengying Formation and Niutitang Formation in Ediacaran to Early Cambrian in South China, synthesizing many discrimination indexes of trace elements and rare earth elements, the average V/(V + Ni) value is 0.70, 0.79, and 0.86 respectively, and the average δU value is 1.83, 1.92, and 1.87, respectively (Table ); δCe always shows negative anomaly (δCe value is 2.70, 1.74, and 1.57 respectively, Table ), and the negative anomaly trend gradually weakens with time. It is considered that the Ediacaran to Early Cambrian phosphorus mineralization period in South China is generally a reducing environment, but with the end of the ice age, climate warming promotes the enhancement of continental chemical weathering and promotes the increase of oxygen content, and there are local oxidation events.…”

“…The transitional period of the Ediacaran-early Cambrian witnessed dramatic changes in the marine environment, significant negative carbon isotope excursion of seawater and radiation evolution of metazoan in the early Cambrian, − leaving abundant geological evidence in several sedimentary sequences. At the Precambrian-Cambrian boundary, it is accompanied by frequent dynamic changes in marine redox conditions, − the rapid fluctuation of δ 13 C carb , long-term positive δ 34 S sulfate excursion and instantaneous anomaly of δ 95/97 Mo all record the frequent changes of marine environment, , in response to the change of atmospheric-ocean oxygenation after the Snowball Earth event .…”

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

“…Furthermore, the δ 98 Mo value shifts toward more negative at the SMTZ and the hydrothermally altered zone compared to the upper zone (Figure S5 in Supporting Information S1). Previous studies on the early Cambrian black shales in South China have attributed the untypically low δ 98 Mo to a more stratified ocean at continental margin settings, where the Fe-Mn (oxyhydr)oxides shuttle at shallow water depth preferentially releases isotopically depleted Mo to weakly euxinic bottom water and results in the δ 98 Mo lower than −1.5‰ (e.g., Ostrander et al, 2019;Qin et al, 2022). However, the early Cambrian hydrothermal exhalative ore layers are present around those areas (Derkowski et al, 2013;Steiner et al, 2001), implying potential impacts from hydrothermal fluids and the associated alteration.…”