Re-appearance of precipitated aragonite crystal fans as evidence for expansion of oceanic dissolved inorganic carbon reservoir in the aftermath of the Lomagundi-Jatuli Event

The initial accumulation of atmospheric oxygen is marked by the unprecedented positive δ13Ccarb excursions of the Lomagundi-Jatuli Event (LJE) and records an interval of abnormal O2 production through elevated rates of organic carbon burial. Emerging evidence suggests that the post-LJE atmosphere-ocean system might have suffered a significant deoxygenation. These dynamic perturbations in the oceanic redox state and biogeochemical cycles would have led to fundamental changes in carbonate precipitation dynamics. Here, we report the discovery of centimeter-sized crystal fans in the post-LJE Huaiyincun Formation, Hutuo Supergroup in the North China Craton. The hexagonal cross-sections and square terminations suggest that these fan-like dolomitic structures were originally aragonite crystal fans (ACF). Variations of stromatolite morphology and frequent occurrences of storm-related deposits in the Huaiyincun Formation point to repeated cycles of sea level changes. The bedding-parallel distribution of the ACF and the homogeneous δ13C values of the ACF-bearing dolostones are consistent with a primary depositional origin for the ACF. An updated compilation of published records of ACF throughout geological history highlights a clear absence of ACF from the initiation of the Paleoproterozoic Great Oxidation Event until the end of the LJE, and a global reappearance of ACF in the post-LJE late Paleoproterozoic. We propose that the reappearance of ACF is in agreement with the expansion of the oceanic dissolved inorganic carbon reservoir. At the same time, consumption of dissolved oxygen during the oxidation of organic matter might have been stimulated by ferruginous deep seawater, facilitating the formation of Huiayincun ACF

Chemically oscillating reactions during the diagenetic formation of Ediacaran Siliceous and Carbonate Botryoids

Chemically oscillating reactions are abiotic reactions that produce characteristic, periodic patterns during the oxidation of carboxylic acids. They have been proposed to occur during the early diagenesis of sediments that contain organic matter and to partly explain the patterns of some enigmatic spheroids in malachite, phosphorite, jasper chert, and stromatolitic chert from the rock record. In this work, circularly concentric self-similar patterns are shown to form in new chemically oscillating reaction experiments with variable mixtures of carboxylic acids and colloidal silica. This is carried out to best simulate in vitro the diagenetic formation of botryoidal quartz and carbonate in two Ediacaran-age geological formations deposited after the Marinoan–Nantuo snowball Earth event in South China. Experiments performed with alkaline colloidal silica (pH of 12) show that this compound directly participates in pattern formation, whereas those with humic acid particles did not. These experiments are particularly noteworthy since they show that pattern formation is not inhibited by strong pH gradients, since the classical Belousov–Zhabotinsky reaction occurs in solution with a pH around 2. Our documentation of hundreds of classical Belousov–Zhabotinsky experiments yields a number of self-similar patterns akin to those in concretionary structures after the Marinoan–Nantuo snowball Earth event. Morphological, compositional, and size dimensional comparisons are thus established between patterns from these experiments and in botryoidal quartz and carbonate from the Doushantuo and Denying formations. Selected specimens exhibit circularly concentric layers and disseminations of organic matter in quartz and carbonate, which also occurs in association with sub-micron-size pyrite and sub-millimetre iron oxides within these patterns. X-ray absorption near edge structure (XANES) analyses of organic matter extracted from dolomite concretions in slightly younger, early Cambrian Niutitang Formation reveal the presence of carboxylic and N-bearing molecular functional groups. Such mineral assemblages, patterns, and compositions collectively suggest that diagenetic redox reactions take place during the abiotic decay of biomass, and that they involve Fe, sulphate, and organic matter, similarly to the pattern-forming experiments. It is concluded that chemically oscillating reactions are at least partly responsible for the formation of diagenetic siliceous spheroids and concretionary carbonate, which can relate to various other persistent problems in Earth and planetary sciences

Evidence for high-frequency oxygenation of Ediacaran shelf seafloor during early evolution of complex life

Increasing oxygenation of the early Ediacaran Ocean is thought to have been responsible for the emergence of early animals. Although geochemical studies have suggested periods of oceanic oxygenation in the Ediacaran, direct evidence for seafloor oxygenation has been lacking. Here, we report frequent occurrences of distinctive, sub-millimetric, and early diagenetic pyrite-marcasite rosettes in phosphorites from the lower Ediacaran Doushantuo Formation (Weng’an, South China). They typically consist of a nucleus of framboidal pyrite, a cortex of radiating marcasite blades intergrown with quartz, and a rim of second-generation pyrite, recording partial oxidative dissolution of pyrite and co-precipitation of marcasite and quartz. This inference is further supported by near-zero carbon isotope values of the host dolostone, similarly low sulfur isotope values for pyrite and marcasite, and evident Fe-isotope fractionation between marcasite and pyrite. Collectively, our findings reveal intermittent bottom-water and porewater oxygenation events, providing direct evidence of high-frequency oxygenation of Ediacaran continental shelves

Organic remains in late Palaeoproterozoic granular iron formations and implications for the origin of granules

Toward the end of the Palaeoproterozoic era, over 109 billion tonnes of banded (BIF) and granular (GIF) iron formations were deposited on continental platforms. Granules in iron formations are typically sub-spherical structures 0.2 to 10 mm in size, whereas concretions are >10 mm. Both types of spheroids are preserved throughout the sedimentological record. Their formation has typically been interpreted to originate from reworked Fe-rich sediments in high-energy, wave-agitated, shallow-marine environments. New evidence from six different late Palaeoproterozoic granular iron formations (GIF), however, suggests that some granules are the result of diagenetic reactions, in addition to other features driven by microbial processes and mechanical movements. Characteristic coarse grain interiors and septarian-type cracks inside granules, akin to those features in decimetre- to meter-size concretions, are interpreted as desiccation features from hydrated diagenetic environments where sulphate and/ or ferric iron were reduced while organic matter (OM) was oxidised inside granules. Those granules derived from sulphate reduction preserve diagenetic pyrite rims, whereas those formed via ferric iron reduction preserve diagenetic magnetite along their rims. Other diagenetic minerals including apatite mixed with OM, and various carbonate phases are commonly preserved within granules. Combined with systematically 13C-depleted carbonate, these diagenetic mineral assemblages point to the oxidative decay of OM as a major process involved in the formation of granules. Spheroidal equidistant haematite laminations surround some granules and contain apatite associated with carbonate, OM, and ferric-ferrous silicates, and oxides that further suggest these structures were not shaped by wave-action along sediment-water interfaces, but rather by chemical wave fronts and biomineralisation. Our results demonstrate that the formation mechanisms of GIF also involve microbial activity and chemically-oscillating reactions. As such, granules have excellent potential to be considered as promising biosignatures for studying Precambrian biogeochemistry, as well as astrobiology

An Early Cretaceous garnet-bearing metaluminous A-type granite intrusion in the East Qinling Orogen, central China: Petrological, mineralogical and geochemical constraints

The Erlangmiao granite intrusion is located in the eastern part of the East Qinling Orogen. The granite contains almost 99 vol.% felsic minerals with accessory garnet, muscovite, biotite, zircon, and Fe-Ti oxide. Garnet is the dominant accessory mineral, shows zoned texture, and is rich in w(FeO) (14.13%–16.09%) and w(MnO) (24.21%–27.44%). The rocks have high SiO2, alkalis, FeOt/MgO, TiO2/MgO and low Al2O3, CaO with w(Na2O)/w(K2O)> 1. Their Rb, Ga, Ta, Nb, Y, and Yb contents are high and Sr, Ba, Eu, Zr, P, and Ti contents are low. These features indicate that the Erlangmiao granite is a highly evolved metaluminous A-type. Garnet crystallized at the expense of biotite from the MnO-rich evolved melt after fractionation of biotite, plagioclase, K-feldspar, zircon, apatite, and ilmenite. The relatively high initial 87Sr/86Sr ratios (0.706–0.708), low and negative εNd (120 Ma) values (−6.6 to −9.0), and old Nd model ages (1.5–1.7 Ga) suggest that the rocks were probably formed by partial melting of the Paleoproterozoic granitic gneisses from the basement, with participation of depleted mantle in an extensional setting

Automated analysis of microplastics based on vibrational spectroscopy: Are we measuring the same metrics?

The traditional manual analysis of microplastics has been criticized for its labor-intensive, inaccurate identification of very small microplastics (less than 10 µm), and the lack of uniformity between instrumentation techniques. There are already three automated analysis strategies for microplastics based on vibrational spectroscopy: laser direct infrared (LDIR)-based particle analysis, Raman-based particle analysis, and focal plane array-Fourier transform infrared (FPA-FTIR) imaging. We compared the performances of these strategies in terms of their quantification, detection limit, size measurement, and material identification accuracy and analysis speed by analyzing the same standard and environmental samples. Unfortunately, the automated analysis results are not consistent in terms of quantification and material identification. The number of particles smaller than 60 μm recognized by Raman-based particle analysis is far greater than that recognized by LDIR-based particle analysis. Raman-based particle analysis has a submicrometer detection limit but should not be used in the automated analysis of microplastics in environmental samples because of the strong fluorescence interference. LDIR-based particle analysis provides the fastest analysis speed, but we suggest using a reliable detection limit of approximately 60 μm and manually cross-check between the material identification results and the reference database used. Misidentification could occur due to the narrow tuning range from 1800 – 975 cm-1 and dispersive artefact distortion of infrared spectra collected in reflection mode. FPA-FTIR imaging provides relatively reliable quantification and material identification for microplastics in environmental samples greater than 20 µm but might provide an imprecise description of the particle shapes. Optical photothermal infrared (O-PTIR) spectroscopy can detect submicron-sized environmental microplastics (0.5-5 μm) intermingled with a substantial amount of biological matrix; the resulting spectra are searchable in infrared databases without the influence of fluorescence interference, but the process would need to be fully automated

Metabolically diverse primordial microbial communities in Earth’s oldest seafloor-hydrothermal jasper

The oldest putative fossils occur as hematite filaments and tubes in jasper-carbonate banded iron formations from the 4280- to 3750-Ma Nuvvuagittuq Supracrustal Belt, Québec. If biological in origin, these filaments might have affinities with modern descendants; however, if abiotic, they could indicate complex prebiotic forms on early Earth. Here, we report images of centimeter-size, autochthonous hematite filaments that are pectinate-branching, parallel-aligned, undulated, and containing Fe2+-oxides. These microstructures are considered microfossils because of their mineral associations and resemblance to younger microfossils, modern Fe-bacteria from hydrothermal environments, and the experimental products of heated Fe-oxidizing bacteria. Additional clusters of irregular hematite ellipsoids could reflect abiotic processes of silicification, producing similar structures and thus yielding an uncertain origin. Millimeter-sized chalcopyrite grains within the jasper-carbonate rocks have 34S- and 33S-enrichments consistent with microbial S-disproportionation and an O2-poor atmosphere. Collectively, the observations suggest a diverse microbial ecosystem on the primordial Earth that may be common on other planetary bodies, including Mars

Characteristics of the carbon cycle in late Mesoproterozoic: Evidence from carbon isotope composition of paired carbonate and organic matter of the Shennongjia Group in South China

The late Mesoproterozoic Era (1300–1000 Ma) was a critical period that witnessed the origin and early evolution of multicellular eukaryotes. Multiple studies have reported observations that the carbon isotope composition of carbonate (δ13Ccarb) underwent a substantial increase after ca. 1250 Ma. The high δ13Ccarb values suggest that the fraction of organic carbon burial (forg) had increased, which, consequently, might have promoted the oxidation of surface environments and the diversification of multicellular eukaryotes. However, details of the carbon cycle during this critical time interval remain elusive, which is pivotal to testing this hypothesis. In this study, we analyzed the stable carbon isotope composition of paired organic matter (δ13Corg) and carbonate, as well as the thermal characteristics of the organic matter, from the Shennongjia Group in South China, which has been constrained to an age of ∼ 1340–1150 Ma. A gradual increase in δ13Ccarb, from ∼–2‰ to >+4‰, is observed in the interval from the middle Shennongjia Group. Above which, the δ13Ccarb declines to ∼–2‰ in the uppermost. The paired δ13Corg values show an extensive range of compositions, from ∼ –22 to –32‰. Raman spectral data revealed that the organic matter in the Shennongjia Group experienced a wide range of thermal history, which might be ascribed to the involvement of detrital organic matter. We reconstructed the Δ13Ccarb-org and forg values in two different scenarios: one included all the δ13Corg data, while the other screened the δ13Corg data to minimize the possible effects of detrital organic matter. The results of these two reconstructions show similar temporal trends. Intriguingly, the Δ13Ccarb-org values are generally larger than the modern counterpart, which can be attributed to higher pCO2. The calculated forg of the Shennongjia Group is generally lower than the modern value, 20%. However, the forg experienced a substantial increase between ca. 1250 and 1190 Ma, suggesting enhanced oxidation of surface environments, which might have paved the way for the diversification of multicellular eukaryotes

Tracing high-pressure metamorphism in the eastern Himalayan syntaxis using detrital zircon and monazite from modern stream sediments

The timing of high-pressure (HP) metamorphism in the eastern Himalayan syntaxis is important for understanding the India-Asia collisional processes, but it remains elusive. To reveal the metamorphic history of the eastern Himalayan syntaxis, we performed a study of geochronology, trace elements, and mineral inclusions of detrital zircon and monazite from modern stream sediments in the eastern Himalayan syntaxis. Detrital zircon comprise magmatic and metamorphic domains with different zoning. Inherited magmatic zircon domains have high Th/U, low (Dy/Yb)N, and retain ages of 1798−360 Ma. Metamorphic zircon domains with low Th/U, high (Dy/Yb)N, and inclusions of garnet, kyanite, and/or clinopyroxene probably formed under HP conditions. They yield age groups of 49−35 Ma, 33−17 Ma, and 12−7 Ma. The low Th/U and low (Dy/Yb)N metamorphic zircon domains probably formed during retrogression and yield age groups of 27−16 Ma and 10−6 Ma. Detrital monazite yield age distributions similar to those of the low (Dy/Yb)N metamorphic zircon except for the 821−402 Ma inherited cores. The (Dy/Yb)N of 31.6−5.7 Ma monazite decreases with increasing Y content, which indicates that it likely formed under the retrograde stage during garnet breakdown. Based on the oldest metamorphic ages, the initial India-Asia collision occurred no later than 50−44 Ma in the eastern Himalayan syntaxis. The multimodal age patterns of the metamorphic zircon and monazite indicate that the Indian continent underwent multistage HP and retrograde metamorphism in the eastern Himalayan syntaxis. The nearly contemporaneous HP and retrograde metamorphism indicate that the Indian continent continued subducting while the earlier HP metamorphic slices detached and exhumed

Hydrothermal dolostones in the Shiqi quarry of Wudongde Hydropower Station, Yunnan

The Wudongde Hydropower Station in Yunan is the third hydropower station in China with a capacity of over 10 million kilowatts.The artificial aggregate used in its construction is derived from the limestone of the Mesoproterozoic Luoxue Formation of downstream the dam site.However, both limestone and dolostone are found in the aggregate survey and mining yard, meanwhile the lithological boundary between cross cuts the stratum.This paper focuses on the limestone-dolomite transition zone and a dolomite bulge remaining in the aggregate mining yard.Detailed geological survey, petrological work and geochemical analysis have been conducted on the samples of the research area.The results show that: ①Morphological features and spatial relationships of the carbonate minerals suggest that the dolostones are formed by dolomitization of limestone precursors. ②The development of zebra structure, saddle dolomite and associated sulfides and carbonaceous material is consistent with hydrothermal dolomitization. ③Fluid inclusions in the saddle dolomite yield homogenization temperatures between 183 and 215 ℃, which are significantly higher than those of fluid inclusions in the surrounding calcite(102-152 ℃).The dolostones are generally depleted in 18O(δ18O=-13.3‰ to -7.8‰), with the saddle dolomites have the lowest δ18O value(-13.3‰), which is consistent with direct precipitation of dolomite from hydrothermal fluids.The other dolomites, which show lower homogenization temperatures and higher δ18O, are probably related to less influence by the hydrothermal fluids. ④The near-vertical strata and the presence of fractures in the Shiqi area might have provided channels for the migration of hydrothermal fluids, while the overlying dolostones of Sinian Dengying Formation probably have acted as a cap layer, allowing prolonged reactions between the hydrothermal fluids and surrounding limestones.The migration direction of the hydrothermal fluids determines the distribution of the hydrothermal dolostones