Stromatolites as geochemical archives to reconstruct microbial habitats through deep time: Potential and pitfalls of novel radiogenic and stable isotope systems
Understanding the origin and evolution of life on Earth and potentially other planets in our solar system is of fundamental interest for humanity. For the longest time in Earth's history, life evolved in microbial communities. It is, however, still incompletely understood how, when and where such habitable environments formed and how microbial communities adopted to the drastic changes of the atmosphere-hydrosphere-lithosphere systems through deep time.
Exceptionally preserved early Cambrian bilaterian developmental stages from Mongolia
Fossilized invertebrate embryonic and later developmental stages are rare and restricted largely to the Ediacaran-Cambrian, providing direct insight into development during the emergence of animal bodyplans. Here we report a new assemblage of eggs, embryos and bilaterian post-embryonic developmental stages from the early Cambrian Salanygol Formation of Dzhabkan Microcontinent of Mongolia. The post-embryonic developmental stages of the bilaterian are preserved with cellular fidelity, possessing a series of bilaterally arranged ridges that compare to co-occurring camenellan sclerites in which the initial growth stages retain the cellular morphology of modified juveniles. In this work we identify these fossils as early post-embryonic developmental stages of camenellans, an early clade of stem-brachiopods, known previously only from isolated sclerites. This interpretation corroborates previous reconstructions of camenellan scleritomes with sclerites arranged in medial and peripheral concentric zones. It further supports the conjecture that molluscs and brachiopods are descended from an ancestral vermiform and slug-like bodyplan.
Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling
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.
Cadmium isotope variations in Neoproterozoic carbonates - A tracer of biologic production?
Cadmium concentrations and stable isotopic compositions in seawater are important tools for studying the biogeochemical cycling of Cd in the modern oceans and as a proxy for micronutrient utilisation by phytoplankton. It is now well established that Cd isotopes become “heavier” as the primary production in the surface ocean increases, even though the mechanism driving the isotopic fractionation is still debated. Here, we use this property of Cd isotopes to examine changes that took place in the oceans during the emergence of multicellular life in the Neoproterozoic. Isotopic compositions and concentrations of Cd, N and C are reported in shallow-water carbonates of Ediacaran age from the Xiaofenghe section on the Yangtze Platform, South China. The Cd isotope data - reported as ε112/110Cd - show positive excursions in the cap dolomites, while significantly lighter Cd is found in the overlying strata. After correction for salinity-controlled fractionation into inorganic calcite, calculated palaeo-seawater ε112/110Cdsw range from -2 to +1.5, overlapping values of modern surface seawater. Importantly, ε112/110Cdsw and δ13C show a general positive correlation, as would be expected in bio-productive environments. However, the trend to lighter ε112/110Cd up-section is not that explicitly expected for an “explosion of life” at the end of the Ediacaran. The upper Doushantuo also displays substantial fluctuations in REE abundances, δ15N and δ13C, which may be due to estuarine mixing. Our data suggest that the variations in ε112/110Cd are a result of biologically-induced fractionation in at least some of the Ediacaran carbonates at Xiaofenghe. Further Cd isotope fractionation processes are clearly playing a role as well, such as precipitation of sulphides under anoxic pore-water conditions and fractionation into inorganic carbonates under variable salinity conditions. These effects have to be evaluated carefully when using Cd isotope systematics in ancient marine carbonates to look for palaeo-productivity signals.
Geochemistry of Ediacaran cap dolostones across the Yangtze Platform, South China: implications for diagenetic modification and seawater chemistry in the aftermath of the Marinoan glaciation
Cap dolostones of the Ediacaran Doushantuo Formation (Yangtze Platform, South China) from various palaeo-water depths were studied to evaluate the extent of their diagenetic alteration and to assess temporal and spatial variations of seawater chemistry in the aftermath of the Marinoan glaciation. Diagenetic fluid overprint is common in cap dolostone lithologies. However, the mobilization of trace elements and the modification of Sr and O isotopic compositions are variable and were controlled by multiple stages of fluid overprint. The highest 87Sr/86Sr ratios (up to 0.7246) occur in cap dolostones from the palaeo-slope environment at the Huanglianba section, whereas cap dolostones deposited in platform settings and in one of the distal basinal settings reveal 87Sr/86Sr ratios close to the proposed late Neoproterozoic seawater composition (0.7077). Shale-normalized REE + Y patterns of carbonate leachates display enrichments of heavy over light REE and superchondritic Y/Ho ratios, typical of seawater. However, Y/Ho ratios in the cap dolostones are always lower than modern seawater values, which is interpreted to reflect dilution of the seawater signal by continent-derived meltwater influx during deglaciation. Negative Ce anomalies in carbonate leachates from platform and slope sections suggest that oxidized conditions existed in shallow marine environments shortly after the Marinoan glaciation, whereas positive or no resolvable Ce anomalies in basin settings indicate that the latter remained anoxic. Redox stratification of the Yangtze margin at the beginning of the Ediacaran is further supported by the relative enrichment of redox-sensitive trace metals in basinal sections. These data may indicate moderately anoxic (less than 10µM dissolved O2) and presumably manganous conditions during the deposition of cap dolostones in the deeper realms of the Yangtze basin.