Marine redox variations during the Ediacaran–Cambrian transition on the Yangtze Platform, South China

Abstract

The widely developed black shales on the Yangtze Platform recorded palaeoceanographic environment information during the EdiacaranCambrian transition. This paper describes an integrated geochemical study of rare earth elements (REEs), redox-sensitive trace elements (RSTEs), and total organic carbon (TOC) contents in Ediacaran-Cambrian black shales at Daotuo, northeastern Guizhou Province, South China. Integrated RSTE data from the Daotuo area, in combination with previously published Fe speciation and Mo-based proxies from another six sections (Shatan, Jiulongwan, Zhongling, Yangjiaping, Longbizui and Wuhe), suggest three major periods of water euxinia during the Ediacaran-Cambrian transition. Under these conditions, organic matter and RSTEs experienced various levels of enrichment in the black shales, especially in the lower Jiumenchong Formation. Given the patterns of Mo-U covariations, metal-oxyhydroxide particulate shuttles may have operated during the black shale deposition of the Doushantuo Formation (Member II) at Daotuo. Conspicuously, the upper slope water was oxic-dysoxic during the earliest Cambrian, as determined by the REE, RSTE data and sedimentological characteristics of the Liuchapo Formation and the basal Jiumenchong Formation. The generally low RSTE concentrations in the Bianmachong Formation black shales (Cambrian Series 2, end of Stage 3) suggest a persistently oxic water column in upper slope settings. The coincidence between the marine oxygenation and the development of the ecosystem likely indicates the galvanizing effects of enhanced oxygen and biological element content on the fauna during the Ediacaran-Cambrian transition.

Organic-Inorganic Geochemical Characteristics of the Upper Permian Pusige Formation in a High-Saline Lake Basin, Tarim Basin: Implications for Provenance, Paleoenvironments, and Organic Matter Enrichment

“…Additionally, Mo and U accumulation may not be influenced 18 Geofluids by an active particulate shuttle, but are largely controlled by oxic to dysoxic conditions (Figure 12). In dysoxic waters close to the Fe (III)-Fe (II) transition, soluble U (VI) can be largely reduced to insoluble U (IV) and, then, preferentially incorporated into the sediments over Mo element (especially in the presence of OM) [29,81]. This is consistent with that the level of U enrichment is greater than that of Mo enrichment in most samples.…”

“…Algeo and Tribovillard [19] investigated the covariations in U and Mo enrichment factors in some modern lowoxygen marine basins and established three Mo-U covariation patterns associated with different specific redox conditions and enrichment processes like "unrestricted marine," "particulate shuttle," and "strongly restricted basin" [19,80]. These Mo EF -U EF covariation patterns have successfully been applied to reconstruct the redox environments in both ancient oceans and lakes [80,81]. The U EF values for all the studied samples are greater than 1.0, varying between 1.21 and 3.15 with an average of 1.88, revealing a relatively stable enrichment of U in these mudstones.…”

“…Figure7: Chondrite-normalized[48] REE spider plots of the M3 mudstones in Well DW1. Red and green lines represent the chondritenormalized patterns of Paleozoic granite and granodiorite for comparisons, respectively[35,38];(Zhang et al, 2016).…”

In siturare earth element analysis of a lower Cambrian phosphate nodule by LA-ICP-MS

“…The partition coefficients of the REE between apatite and melt exhibit a convex-upwards shape, indicating that apatite accommodates MREE more readily than LREE or HREE (Reynard et al 1999;Klemme & Dalpé, 2003). However, seawater-like REE distributions are known to be preserved in many modern and ancient P-enriched deposits (Toyoda & Tokonami, 1990;Jiang et al 2007;Zhu et al 2014;Xin et al 2015;Zhai et al 2016); the substitution model alone may therefore not explain all of the MREE enrichments.…”

“…During early Cambrian time, the platform setting suffered widespread drowning and it subsequently evolved into a muddy shelf (Yeasmin et al 2017). The Niutitang Formation, which overlies the Dengying/Liuchapo Formation, is dominated by high total organic carbon (TOC) (up to 15%) black shales (Zhai et al 2016). This black shale sequence also hosts discontinuous phosphate nodule, barite, Ni-Mo-PGE (platinumgroup elements)-Au sulphide and V-rich deposits (Xu et al 2011;Lehmann et al 2016).…”