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Trace element and mineral composition of Ediacaran-early Cambrian strata in South China and their geochemical implications

Ediacaran-early Cambrian is a critical interval in the evolution of atmosphere-ocean environment and biodiversity. The continuous Ediacaran-Cambrian strata in South China provide a good opportunity for investigating the associated environment with its ecological evolution. In this study, outcrop sedimentary rock samples collected from the Dongkanshang and Fengtan sections on the Yangtze Block were analyzed for their mineral and trace element compositions. The results showed some heterogeneity in the geochemistry of the Ediacaran-early Cambrian ocean in South China. Based on the U/Th, V/(V+Ni), V/Cr, and Ni/Co ratios, the middle-deep water was predominated by anoxic or euxinic condition with interferences from several transient oxidation events. The basinal Fengtan section indicates a more reduced depositional environment when compared with the slope Dongkanshang section. This shows that the seawater oxygenation was a gradual process that expanded from shallow to deep water.


Introduction
Throughout geological history, the Ediacaran-Cambrian transition is the most critical interval, during which dramatic changes occurred in seawater chemistry and biological evolution. Abundant geochemical studies on Ediacaran-Cambrian strata have been conducted to discuss the co-evolution between life and the environment. For instance, a stratified redox model [1] and wedge-shaped oxygen minimum zones [2] have been proposed as two models for characterizing the structure of the Ediacaran-Cambrian oceans. However, more high-resolution geochemical work is still required to study the temporal and spatial evolution of redox condition during Ediacaran-Cambrian, which will be helpful in understanding the marine environment at the dawn of complex life. In this study, mineral composition and trace elements geochemistry of two sections having different sedimentary settings on Yangtze Block were evaluated in order to provide more insight into the reconstruction of redox dynamics at the basinal scale during Ediacaran-early Cambrian.

Geological setting and analytical methods
The Yangtze Block evolved from a rifted basin to a passive continental margin during the Ediacaran-Cambrian transition [3]. The paleogeographic study suggests a shallow-water platform to the northwest, a deep-water basin to the southeast, and a transitional slope in between [4,5].  The mineral composition was determined using OLYMPUS Innova-X BTX X-ray diffractometer. Trace elements analyses were carried out on an Agilent 7700x ICP-MS after the digestion with HNO3-HClO4-HF-HCl mixture.

Mineral composition
The mineral composition of sedimentary rocks from the two sections is displayed in figure 2. The chief minerals of the Doushantuo Formation in the Dongkanshang section are dolomite and quartz, whose relative content is greater than 80%. Other minerals include feldspar, calcite, pyrite, illite, and a small amount of zeolite. At the same stratigraphic unit of the Fengtan section, there are additional  Compared with the Fengtan section, there are more feldspars in the sediments at the Dongkanshang section probably due to the more susceptibility of the slope to receiving terrigenous debris. Besides, phosphorous deposit in the middle of the Doushantuo Formation in the Fengtan section was identified, with relative apatite content up to 50% ( figure 2). Similarly, the massive phosphorous deposit has been found in the Doushantuo Formation at Hubei and Guizhou Provinces which is currently under exploitation [6]. During the early Cambrian, there was an occurrence of large-scale transgression, leading to the widespread deposition of the Niutitang black shale [7].

Trace element ratios
Redox-sensitive trace element ratios are valid proxies for assessing the paleo-redox condition. For example, U/Th>1.25, V/(V+Ni)>0.54, V/Cr>4.25, and Ni/Co>7 could serve as indicators of anoxic condition, whereas these ratios decrease with the increased oxidation of the depositional environment [8,9]. Moreover, euxinic condition (presence of H2S) can be distinguished from anoxic condition (without H2S) when V/(V+Ni)>0.84. The U/Th, V/(V+Ni), V/Cr, and Ni/Co proxies display overall strong variations throughout the two studied sections. However, their patterns of variation are not the same as they provide different hints on the paleo-redox condition at some particular beds. For example, at the lower Niutitang Formation from the Dongkanshang section, the U/Th ratio indicates a suboxic state while the V/(V+Ni) ratio indicates a euxinic state, and both V/Cr and Ni/Co ratios indicate an anoxic condition. The conflicting redox interpretations are likely to be, in part, the consequence of the differing sensitivities of the employed proxies to high-frequency redox variation in the Ediacaran-early Cambrian oceans [10]. On the other hand, these proxies have multiple controlling factors besides benthic redox condition, such as organic matter accumulation, sedimentation rate, and diagenetic process [11,12].
Nevertheless, the comprehensive evaluation of paleo-redox conditions of these two sections is displayed in figure 3. The results show a significant spatial and temporal variation in redox condition from the slope to basin area across the Yangtze Block. The Ediacaran-early Cambrian oceans were