Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling in Cryogenian Seawater

Simon V Hohl,Shao-Yong Jiang,Stephen J.G Galer,Hai-Zhen Wei,Qian Liu,Wei Wei,Sebastian Viehmann

doi:10.3390/geosciences10010036

Simon V Hohl, Shao-Yong Jiang + Show 5 more

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https://doi.org/10.3390/geosciences10010036

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Abstract

The behaviour of bioavailable trace metals and their stable isotopes in the modern oceans is controlled by uptake into phototrophic organisms and adsorption on and incorporation into marine authigenic minerals. Among other bioessential metals, Cd and its stable isotopes have recently been used in carbonate lithologies as novel tracer for changes in the paleo primary productivity and (bio)geochemical cycling. However, many marine sediments that were deposited during geologically highly relevant episodes and which, thus, urgently require study for a better understanding of the paleo environment are rather composed of a mixture of organic matter (OM), and detrital and authigenic minerals. In this study, we present Cd concentrations and their isotopic compositions as well as trace metal concentrations from sequential leachates of OM-rich shales of the Cryogenian basal Datangpo Formation, Yangtze Platform (South China). Our study shows variable distribution of conservative and bioavailable trace metals as well as Cd isotope compositions between sequential leachates of carbonate, OM, sulphide, and silicate phases. We show that the Cd isotope compositions obtained from OM leachates can be used to calculate the ambient Cryogenian surface seawater of the restricted Nanhua Basin by applying mass balance calculations. By contrast, early diagenetic Mn carbonates and sulphides incorporated the residual Cd from dissolved organic matter that was in isotopic equilibrium with deep/pore waters of the Nanhua Basin. Our model suggests that the Cd isotopic composition of surface seawater at that time reached values of modern oxygenated surface oceans. However, the deep water Cd isotope composition was substantially heavier than that of modern fully oxygenated oceans and rather resembles deep waters with abundant sulphide precipitation typical for modern oxygen minimum zones. This argues for incomplete recycling of Cd and other bioavailable metals shortly after the Sturtian glaciation in the redox stratified Cryogenian Nanhua Basin. Our study highlights the importance of sequential leaching procedures when dealing with impure authigenic sediments such as OM-rich carbonates, mudstones, or shales to achieve reliable trace metal concentrations and Cd isotope compositions as proxies for (bio)geochemical metal cycling in past aquatic systems.

Highlights

We here report the absolute obtained concentrations of trace metals that were considered important in supporting phototrophic life: Cd, Co, Cu, Ni, Zn, Ba, Mo, Ba, and minerals such as (Mn) [28] (Figure 3A)
Our results showed that relative depletion of Ba, Zn, and Mo together with a more than 10-fold relative enrichment of Mn into carbonate phases argues for a reducing depositional environment in which Mn-oxides dissolved, releasing excess Mn into the ambient seawater
Applying the idea of Hohl et al [90], the dissolved organic carbon (DOC)‘s Cd isotope composition and that of presumable ambient seawater was likely the result of we suggest that the decoupling of the DOC‘s Cd isotope composition and that of presumable ambient fluctuating and progressively increasing deep water oxygenation in the early Cambrian that seawater was likely the result of fluctuating and progressively increasing deep water oxygenation in led to ε112/110 Cd values in the deep and surface waters that are in the range of modern oceans

Summary

Introduction

Biomarkers from Cryogenian strata indicate that demosponges [15] and algae [16] became important in Earth’s oceans by that time These two organisms presumably led to subsequent oxygenation of the deep oceans by filtration and recycling of sinking organic matter (OM) as well as by photosynthesis, and, in the case of algae, as efficient nutrient recyclers and main food source for the emerging animal classes in the late Neoproterozoic following the Marinoan glaciation [17]. The research community still lacks a general understanding of if nutrient conditions in the Cryogenian ocean were generally unfavourable for life and which role primary producers played during interglacial periods as these processes may have led the foundation for the following ‘explosion’ of life

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