Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling in Cryogenian Seawater
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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.Keywords:
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Stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography, and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic (in-situ formed) minerals. To illustrate the usefulness as well as potential limitations of this method, we review (1) processes controlling the isotopic composition of modern precipitation, (2) stable isotope data from modern precipitation across regions of high topography, and (3) stable isotope data from authigenic minerals that have been used to infer paleotopography. From this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography, with uncertainties that critically depend on a detailed understanding of local- to global-scale paleoclimate during the time interval of interest.
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This study aims to unravel the main diagenetic events featuring the eo-, meso- , and telo-diagenetic stages of the turbidite sandstones of the "San Mauro formation", "Cilento group", Southern Apennines Foreland Basin, Italy. Several diagenetic processes, such as compaction, dissolution, and cementation occurred. Framework grains are compacted, showing common deformation of micas, matrix, and ductile grains. Authigenic minerals occur as both pore-filling cements and as replacement. The main diagenetic minerals are carbonates, mainly as replacement and less as pore-filling cements; small amount of authigenic quartz, as little and discontinuous overgrowths; phyllosilicate cement and epimatrix, mainly of kaolinite and illite; Fe-oxides as interstitial cements or single crystals. The diagenetic events of the "San Mauro formation" are closely related to the detrital composition and the subsidence history characterizing the "Cilento group" within the early to middle Miocene wedge-top basin of the Southern Apennines.
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Abstract Measurements of natural cosmogenic 32 Si ( t 1/2 ~ 140 years) in tropical deltaic sediments demonstrate for the first time that most 32 Si is present in rapidly formed authigenic clays and not biogenic opaline silica (bSi). The burial of bSi in deltaic and continental margin sediments has likely been greatly underestimated because of diagenetic alteration of bSi to clay, little of which dissolves in the classically used operational bSi leach. Rapid reverse weathering reactions during early diagenesis must be considered as a significant pathway of reactive Si storage in deltaic deposits. Based on 32 Si, actual storage may be 2–3 times the best recent estimates extrapolated from diagenetic models or attempts to modify operational bSi methods to include authigenic clay (~900 µmol/g versus ~250 µmol/g). Measurements of natural 32 Si inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.
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The effect of diagenesis upon natural remanent magnetization (NRM) has been studied at two sites within a single bed of dolomite from the Monterey Formation in Santa Barbara County, California. Paleomagnetic direction demonstrates that the NRM is primary at one site and remagnetized at the other site. Magnetic separates from samples carrying a primary NRM yielded detrital magnetite. Separates from samples carrying secondary NRM yielded an unusual form of authigenic magnetite. Evidence suggests that the formation of the authigenic magnetite accompanied late stage dolomite diagenesis. Correlating the remagnetization direction to the paleomagnetic history of the region gives a probable age of 0.72–2.47 m.y.b.p. for both of these diagenetic events.
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Paleoecological interactions among fossil microorganisms have garnered significant interest within the paleobotanical community; however, an understanding of the early diagenesis of associated plant material is of critical importance when assessing putative body fossils of fungi and bacteria.Structures preserved within permineralized petioles of the Carboniferous fern Botryopteris tridentata Felix (Scott) have been interpreted as the earliest remains of Actinobacteria found in association with vascular plants, but re-examination of the specimens indicates instead that these biomimetic structures (BMS) are authigenic carbonate minerals.Using spinning disk confocal microscopy, we generated monochromatic luminescence maps of BMS found within the phloem cells of Botryopteris.Luminescence was captured at wavelengths of 665 nm, consistent with an interpretation of these structures as disordered dolomites, an inference subsequently corroborated with energy-dispersive X-ray spectrometry (SEM-EDS).The presence of high-magnesium carbonates within Botryopteris is suggestive of an early anaerobic stage of plant tissue degradation characterized by metabolic activities of sulfate-reducing bacteria.Anaerobic biodegradation may also have been performed by chytridiomycetes, and we interpret larger (5-8 mm) unicells found within the specimens as fossils of chytrid zoosporangia.Understanding microbial contribution to the early diagenesis of plants preserved within calcium carbonate concretions (coal balls) is dependent upon both characterizing diversity of microbial communities within fossil plants, and elucidating the geomicrobiological parameters of mineralization.As such, this study underscores the necessity of integrating geomicrobiology with plant taphonomy in investigations of the microbial component of ancient ecosystems.
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