Chert Deposits Research Articles

The rise of biogenic silicon cycling and microbial silicification promoted Mesoproterozoic chert deposition

The silicon (Si) cycle is intimately interlinked with other biogeochemical cycles (e.g., carbon and nitrogen) and plays an important role in regulating long-term global climate change and biotic evolution in Earth's history. The abundance of cherts in Mesoproterozoic shallow-marine carbonates implies unique environmental conditions for silica precipitation, but the driving mechanism behind the Si cycle in Mesoproterozoic oceans remains unclear. In this paper, we report an integrated study of the cherts from the ∼1.48 Ga Wumishan Formation of North China. The Wumishan cherts predominantly consist of microquartz (∼90%), with some silica-replaced carbonate (∼5%) and minor pyrite (∼1%) grains, indicating that the cherts were largely formed through primary silica precipitation. High germanium/silicon molar ratios (Ge/Si = 0.71–19.1 μmol/mol; mean = 8.83) and positive europium anomalies (Eu/Eu* = 0.41–2.42; mean = 1.41) of the cherts suggest a considerable contribution from hydrothermally derived Si. Diverse microbial components, including organic-rich filaments, EPS (extracellular polymeric substances) relics, mat fragments and picocyanobacteria fossils, are observed in the Wumishan cherts. These components, together with abundant organominerals resulted from the interactions of microbes and dSi in ambient environments, imply that microbial activities played critical roles in silica deposition. The Si liberated from degraded EPS or EPS-Si complex may have locally increased the dSi concentrations and changed chemical conditions in the substrate and pore-waters, promoting silica precipitation. Given that picocyanobacteria and some other prokaryotes can accumulate significant amounts of silica in their cells and EPS, biogenic silica from decomposed microbial biomass may have exerted an important influence on silica precipitation in shallow-marine environments of the Mesoproterozoic ocean.

Siliceous deposition in limestone-marl alternations of the Yangtze Carbonate platform during the Permian Chert Event

During the Permian Chert Event (PCE), limestone-marl alternations (LMAs), contemporaneously developing in the Yangtze region with chert-mudstone alternations, differed from typical LMAs by their enrichment in authigenic siliceous components (quartziferous components and Mg-phyllosilicates). However, the factors driving the transition from distal chert deposits to proximal siliceous-rich carbonate deposits remain unclear, and the genesis of authigenic siliceous components is puzzling. Resolving these issues could provide a deeper understanding of the paleoclimate and paleoenvironment during the PCE. Optical and SEM observations and geochemical analyses revealed that Si4+, derived from seawater, silica-secreting organisms, and terrigenous clasts, precipitated as authigenic quartziferous components under relatively acidic conditions, while it precipitated as Mg-phyllosilicates through reverse weathering by combining with the dissolved high-Mg calcites under relatively alkaline conditions. Through the facies division of LMAs, its variations describe relative and short-term sea-level fluctuations in the Yangtze Carbonate Platform (YCP). By integrating tendency comparisons and time series analyses of sea level fluctuations, continental weathering intensity, authigenic siliceous content, and relative proportions of quartz vs. Mg-phyllosilicates, four proxies exhibit covariation and similar astronomical signals, indicating that the high sea level period of the YCP, corresponding to intensified continental weathering, favored the deposition of authigenic siliceous components and reverse weathering. Because the facies variations encompass short eccentricity, obliquity, and precession cycles, sea level fluctuations are considered to have been influenced by Gondwana glaciation P3. The seawater in the continental shelf cooled and became rich in silica and nutrients under the effect of upwelling. Interglacial periods may trigger high sea levels and intensified continental weathering in the YCP, facilitating the influx of Si-rich eutrophic seawater and stimulating biological silica secretion by radiolarians. However, the direct precipitation of activated silica resulted in weakened reverse weathering. Consequently, the lithological transformation from distal to proximal areas during the PCE was attributed to differences in silica-secreting organism abundance, upwelling intensity, biological pump activity, reverse weathering intensity, OM abundance, pH conditions, and seawater temperature between the continental shelf and carbonate platform.

Opaline silica precipitations in the Permian Fengcheng Formation indicate hot spring environments in north Mahu Sag, NW China

The Permian Fengcheng Formation represents a series of deposits formed in an alkaline saline lake, characterized by high proportions of cherts, located in the Northwestern Junggar Basin in China. While discussions on silica-rich hot springs as sources of silica for brine and subsequent chert deposition have been extensively conducted for Quaternary and Mesozoic-Cenozoic alkaline saline lakes worldwide, investigations into Paleozoic alkaline saline lakes are scarce due to challenges associated with identifying evidences of hot spring environments. In this study, we present detailed SEM-EDS analyses and examine silica isotope compositions as well as oxygen isotope compositions to discuss hydrothermal-associated silica precipitation processes. Our findings suggest that migrating silica-rich hot springs along deep-rooted faults released silica into the brine, leading to chert deposition during cooling periods and induction of thermophilic microorganism degradation. Microorganism-associated siliceous precipitates primarily consist of opal-A spheres adhered by extracellular polymeric substances (EPS), while numerous microorganism spheres have undergone silicification. Opal-A spheres within these structures were partially transformed into well-crystallized quartz through multiple stages of dissolution-precipitation processes during prolonged burial history. Furthermore, our results indicate that deep-rooted fault-related hot springs can occur both in marginal and subaqueous areas within alkaline saline lakes, with chert formations observed in shallow-marginal saline lake settings as well as deeper regions. The findings of this study provide valuable insights into the paleoenvironmental conditions, which can be further explored in investigating the accumulation of organic matter in chert-rich successions formed within an alkaline saline lake.

Helium‐isotope constraints on palaeoceanographic change and sedimentation rates during precession cycles (Cenomanian Scaglia Bianca Formation, central Italy)

AbstractFor much of the pelagic sedimentary record, time control is limited to the resolution of precession cycles (ca 20 kyr): the Milankovitch parameter that forms the most detailed metronome for the Cenozoic and Mesozoic Eras. The influence of precession is often detected in lithological alternations, where the duration represented by individual lithologies is not well constrained. Here the novel technique of extraterrestrial helium abundance (3HeET) is used to investigate the sedimentation dynamics and palaeoceanography within individual precessional cycles. High‐resolution 3HeET timescales were produced for four precession cycles from the rhythmically bedded Scaglia Bianca Formation, a sequence of Upper Cretaceous (Cenomanian) deep‐marine pelagic limestones from central Italy that are well characterized by cyclostratigraphy. Using 3HeET concentrations as a proxy for sedimentation rate allows instantaneous sedimentation rates and organic‐carbon mass accumulation rates to be calculated for each bed within a precession cycle. Eccentricity is known to modulate the amplitude of precession forcing, and precession cycles deposited under eccentricity maxima and minima were selected for comparison. Lithological changes through these chert–(black shale)–limestone cycles are explained using the concept of ‘palaeoenvironmental thresholds’; these timescale calculations indicate that when the amplitude of precessional insolation forcing was greatest (at eccentricity maxima) the palaeoenvironmental system spent longer in the more nutrient‐rich environment under which siliceous and organic‐rich sediments were deposited, reflecting increased time spent above a ‘threshold’ insolation level. Estimates of primary productivity are relatively elevated for organic‐rich beds. An increase in the flux of terrestrial helium (4Heterr) during the deposition of cherts may have been coincident with an increase in terrestrially derived nutrients. The presented results indicate great potential for the use of 3HeET to understand past oceanographic, climatic and sedimentological processes at high temporal resolution.

Formation conditions of Jixian System cherts in the Qishan area, Ordos Basin: Implications for marine redox conditions and paleoecology

Abundant chert bands and nodules are discovered throughout the Mesoproterozoic Jixian System in the Ordos Basin. These cherts faithfully record the Jixian period oceanic conditions and paleoecology. However, the diagenetic mechanism of the cherts remains unclear and controversial. To understand the origin of these cherts, we performed a multitracer study by combining field reconnaissance, petrological analyses, Si isotope analysis, and major and trace element analysis of chert samples from the Qishan section. The results show that Jixian cherts had a high Al/(Al + Fe + Mn) value (~0.48), showed a flat distribution of rare earth elements in seawater, and most of the Fe/Ti values are <20. A weak negative Ce anomaly (~0.97) indicates the contribution of REY to weakly oxygenated seawater. The high isotopic composition of silicon (δ30Si = 0.74 ‰-1.35 ‰) and the average Siex value of 37.52 indicate that there is a source of biological silica, and the relationship between Eu/Eu* and Y/Ho and δ30Si shows that the hydrothermal and volcanic influences were less and correlated with seawater. The chert bands were closely related to biological activities. The chert nodules were influenced by hydrothermal activity. Secondary cherts were related to diagenesis, but the nature of siliceous fluids remains unchanged, still characterized by biogenic activity origins. The sea oxygen concentration in the Jixian period had increased, with periodic variations in the intensity of biological activities affecting the pH of the water body, or biological photosynthesis binding SiO2 colloid in water, affecting chert sediments, leading to frequent interlayering between dolomite and chert bands. Episodic hydrothermal fluids and late-stage diagenetic processes jointly influenced the formation of cherts. This study bears significant significance in enhancing our understanding of the sedimentary environment and the origin of cherts during the Jixian period in the Ordos Basin.

Lithostratigraphy of the Paleogene Deccan Intra-, Intertrappeans of the Saurashtra, Western India and their Prevalence in Large Igneous Provinces

Abstract The periphery of the Deccan Volcanic Province (DVP) of India comprises sedimentary succession deposited during the waning phase of volcanism across the Cretaceous-Paleogene boundary which preserves the continental biota. The Saurashtra Peninsula, a part of the Saurashtra-Kachchh sub-province, exposes thick intertrappean sedimentary successions, systematically described to understand the stratigraphic framework with respect to the lava flows and their geographic distinctness. The thickness of the exposed sections is measured, and contact and continuities are marked, revealing two different, small geographically isolated but adjacent, coeval basins, named the Ninama Basin and Chotila Basin, initially comprising fine grain sediment, followed by thick conspicuous limestone and chert deposits respectively. Formal lithostratigraphic unit names are proposed for both basins: Ninama Basin comprises lithic arenite, fossiliferous limestone, bedded siltstone, mudstone and claystone, and is divided into Sukhbhadar Formation and Ninama Limestone. Chotila Basin comprises calcareous sandstone, bedded siltstone, silty shale and mudstone, claystone and chert divided into Rangpar Formation, Chotila Chert, and Bamanbor Formation. Lithology and palynofossil evidence suggest restricted continental environments with varying salinities during the Paleogene.

Mesozoic magmatism of Natuna Island, Indonesia: Implications for the subduction history of eastern Sundaland

Subduction beneath eastern Sundaland occurred throughout the Mesozoic forming a wide magmatic arc. This resulted in extensive magmatism, ophiolite formation, and development of a large accretionary complex that stretches north from Borneo, through Vietnam, and into China. However, much of this complex is currently submerged beneath the South China Sea, making the study of longitudinal variations in magmatic and subduction history difficult. This study focusses on the Mesozoic geology of Natuna, a remote island in the South China Sea, and uses fieldwork, whole-rock geochemistry, and zircon geochronology to determine the islands relationship to the subduction margin and how its magmatic history compares to examples from Borneo and Vietnam. Natuna records abundant subduction-related magmatism, with switches from extensional to compressional regimes driven by changes in the angle of subduction and rate of slab roll-back. Inherited zircons reveal the presence of Triassic–Early Jurassic crust beneath Natuna and indicate the formation of an Early Jurassic arc that has since eroded away. This is followed by Middle–Late Jurassic spreading in the fore-arc driven by slab-roll back and formation of the Pulau Tiga Ophiolite coeval with the deposition of cherts in the Bunguran Formation. A regional Late Jurassic–Early Cretaceous slab flattening then drove abundant I-type magmatism, however, the products of this are not observed on Natuna until the Late Cretaceous. Initial I/S-type granodiorite and porphyritic granite magmatism is recorded at 85–86 Ma and formed as mantle melts rose through and became partially contaminated by previously-formed arc crust. Following this, I/S-type granites record a period of crustal thickening and increased contamination from partially melted crust at 71–75 Ma. This crustal thickening marks the end of Palaeo-Pacific subduction beneath eastern Sundaland, compression in the fore-arc, ophiolite obduction, and formation of the Natuna Accretionary Complex along the Lupar Line.

Comparative geochemistry of Early Carboniferous marine red beds (MRBs) and their significance for deep time paleoceanographic reconstructions

Marine red beds (MRBs), also known as oceanic red beds (ORBs), are reddish colored sediments deposited within the marine realm that have been stained during deposition and/or early diagenesis because of changes in the ocean water chemistry in response to paleoceanographic and paleoclimatological factors. Although common throughout the Phanerozoic, marine red beds have mostly been investigated in Upper Cretaceous successions as CORBs (Cretaceous Oceanic Red Beds) - their occurrence and paleoceanographic context within the Paleozoic has yet to be studied in detail. Here we describe and interpret the high-resolution litho- and chemofacies of Early Carboniferous (Late Viséan) marine red beds from the northern coast of Menorca (Balearic Islands, Spain) that mark a rapid transition from siliceous radiolarian chert deposition to re-worked carbonate debrites followed by typical European Culm facies. The succession was deposited within a strait-like narrow basin that formed part of the Alboran-Kabylian-Peloritan-Calabrian terrane in the Western Paleotethys Ocean prior to the onset of the Variscan Orogeny. The marine red bed geochemistry is described in terms of three geochemical endmembers Al-MRBs (clayey), Ca-MRBs (calcareous) and Si-MRBs (siliceous) by using XRF-derived Al2O3, CaO, and SiO2 concentrations. This study finds that Viséan marine red beds from Menorca are predominantly Al-MRBs with subordinate Ca-MRBs. Furthermore, we compare the bulk rock geochemistry of the Viséan marine red beds of this study with the geochemical data of Paleozoic, Mesozoic and Cenozoic marine red beds available in the existing literature (Cambrian, Silurian, Jurassic, Cretaceous, Paleogene and recent Pacific Ocean red clays) to highlight the significance of marine red bed occurrences in relation to (paleo-)bioproductivity fluctuations, deep-water oxygenation and oceanic redox transitions. This study finds that Phanerozoic marine red beds are dominantly Al-MRBs and Ca-MRBs, whilst Si-MRBs are less common. The Early Carboniferous marine red beds of the present study were formed in a distal, deeper-marine environment under oxic-suboxic, oligotrophic conditions. A lack of any significant Fe2O3T enrichment points towards a more complex diagenetic process for the origin of the red color imparted in Phanerozoic MRBs.

The Depositional Mechanism of Hydrothermal Chert Nodules in a Lacustrine Environment: A Case Study in the Middle Permian Lucaogou Formation, Junggar Basin, Northwest China

Although chert deposits are limited in geological distribution, their geological and geochemical characteristics can provide important information to reconstruct paleoenvironmental and diagenetic processes. For the Permian period, cherts are utilized to trace global silicon cycles and hydrothermal activities in relation to the Permian Chert Event. In Northwest China, Permian chert nodules have recently been discovered in both the southeastern and northwestern margins of the Junggar Basin. We conducted an analysis of the mineralogy, petrology and geochemistry of chert nodules of the Lucaogou Formation in the southeastern margin of the Junggar Basin to identify silicon sources and determine the precipitation mechanism of chert nodules. As evidenced by petrology, the chert nodules were mainly composed of crypto-microcrystalline silica (94.33% on average), with development of a soft-sediment deformation structure, indicating the synsedimentary deposition of silicon. Proven by trace elements, high Eu/Eu* ratios (average 2.14), low total rare earth element content (average 6.03 ppm), low LaN/YbN ratios (average 0.17) and low Y/Ho ratios (average 25.25) in chert nodules supports the hydrothermal source of silicon. The wide distribution of authigenic metal-bearing minerals and the significant positive Eu anomalies observed suggest that the chert depositions in the Lucaogou Formation intermittently received high-temperature (&gt;250 °C) hydrothermal fluids, likely associated with the initiation of the Bogda Rift in the middle Permian. Following rapid cooling down and differential compaction, siliceous sediments dehydrated and deformed, finally forming chert nodules.

Exploring the secondary mineral products generated by microbial iron respiration in Archean ocean simulations.

Marine chemical sedimentary deposits known as Banded Iron Formations (BIFs) archive Archean ocean chemistry and, potentially, signs of ancient microbial life. BIFs contain a diversity of iron- and silica-rich minerals in disequilibrium, and thus many interpretations of these phases suggest they formed secondarily during early diagenetic processes. One such hypothesis posits that the early diagenetic microbial respiration of primary iron(III) oxides in BIFs resulted in the formation of other iron phases, including the iron-rich silicates, carbonates, and magnetite common in BIF assemblages. Here, we simulated this proposed pathway in laboratory incubations combining a model dissimilatory iron-reducing (DIR) bacterium, Shewanella putrefaciens CN32, and the ferric oxyhydroxide mineral ferrihydrite under conditions mimicking the predicted Archean seawater geochemistry. We assessed the impact of dissolved silica, calcium, and magnesium on the bioreduced precipitates. After harvesting the solid products from these experiments, we analyzed the reduced mineral phases using Raman spectroscopy, electron microscopy, powder x-ray diffraction, and spectrophotometric techniques to identify mineral precipitates and track the bulk distributions of Fe(II) and Fe(III). These techniques detected a diverse range of calcium carbonate morphologies and polymorphism in incubations with calcium, as well as secondary ferric oxide phases like goethite in silica-free experiments. We also identified aggregates of curling, iron- and silica-rich amorphous precipitates in all incubations amended with silica. Although ferric oxides persist even in our electron acceptor-limited incubations, our observations indicate that microbial iron reduction of ferrihydrite is a viable pathway for the formation of early iron silicate phases. This finding allows us to draw parallels between our experimental proto-silicates and the recently characterized iron silicate nanoinclusions in BIF chert deposits, suggesting that early iron silicates could possibly be signatures of iron-reducing metabolisms on early Earth.

A Pennsylvanian saline-alkaline lake in Gondwana mid-latitude: Evidence from the Piauí Formation chert deposits, Parnaíba Basin, Brazil

The Pangaea assemblage resulted in significant tectonic and eustatic changes, but also in a well-known desertification triggered by the increasing continentalization. While the northern hemisphere basins have been thoroughly studied, several recent studies on southern hemisphere basins bring new insight on how these events affected the Gondwana hinterlands. In this study, sedimentological and geochemical analyses of chert deposits from the Pennsylvanian Piauí Formation of the Parnaíba Basin are examined as new evidence for the onset of continentalization and aridification in the northwestern Gondwana mid-latitudes. The chert deposits correspond to three facies (stromatolites, ooidal arenites and breccias) that together suggest deposition in a shallow water environment and biomarkers analysis of stromatolites indicated predominance of organic matter produced by cyanobacteria in an environment characterized by high salinity and elevated pH. Such evidence suggests that the Piauí Formation cherts deposited in a shallow, saline-alkaline lacustrine environment during a period marked by increasing arid climate conditions.

Sedimentary evolution of tectonically-controlled Cenozoic shallow marine glaucony, fluviatile and lacustrine-palustrine deposits at Bahariya region, Egypt

The Cenozoic basin of the tectonically-active Bahariya region (Western Desert, Egypt) contains a unique paleoenvironmental, paleotectonic and paleoclimatic record displaying a large variety of sedimentary facies, stratigraphic discontinuities, and geochemical changes. This study deals with the sedimentary evolution and tectono-stratigraphic modelling of shallow marine Upper Eocene glaucony facies (Upper Hamra Formation) that changes upward into a sequence of Oligocene continental fluviatile sandstone-calcrete (Radwan Formation) and then into Miocene lacustrine-palustrine deposits (Continental Carbonate Unit). The tectonic instability of the Bahariya basin can be deduced from occurrence of sedimentary discontinuities, development of pedogenic features, and unconformable relations among rock units. Both sedimentation pattern and facies distribution seem to have been controlled by fault activity through time. Two types of glaucony facies (pelloidal and massive smectitic glaucony) were deposited in a narrow sub-basin along main faults reaching the basin center. These faults also conditioned the deposition of the Oligocene fluviatile sandstone and Miocene lacustrine-palustrine carbonates. A transition from greenhouse (global warming event of the Middle Eocene Climatic Optimum) to icehouse climatic conditions (Eocene-Oligocene climatic cooling) is probably represented by the deposition of the shallow marine glaucony facies during the Priabonian. Semi-arid to sub-humid conditions were dominant during the formation of the Oligocene calcretes and the Miocene lacustrine-palustrine carbonates. The latter carbonate deposits consist of basal thin-bedded chert deposits displaying hydro-plastic sedimentary structures that are possibly coeval with the Miocene basaltic extrusions along faulted and low-lying areas. We conclude that both paleoclimate and intraplate tectonic activity related to the Red Sea rifting influenced the pattern of sea level, continental drainage, sedimentation rate and clastic supply since the Priabonian up to the middle Miocene in the Bahariya basin.

Origin of Lower Carboniferous cherts in southern Guizhou, South China

Cherts are generally developed in the Lower Carboniferous Dawuba Formation in southern Guizhou. A further understanding of the origin of cherts is helpful to clarify Early Carboniferous oceanic and depositional conditions. However, the origin of the Lower Carboniferous cherts remains controversial. The Dawuba cherts are rich in sponge spicules (monaxons and triaxon hexactines), radiolarians, and foraminifera (Endothyracea) and can be lithologically divided into the Manchang spicule-bearing chert (MSC), Manchang foraminifera-bearing chert (MFC), Getuhe radiolarian-bearing chert (GRC), and Luogang foraminifera-bearing chert (LFC). The analytical results of redox-sensitive trace elements (Mo, U) covariation pattern and microfossils types indicate that the Dawuba cherts formed in a suboxic–euxinic deep-water environment with some contributions of intermittent calcareous allochthonous sediments from the platform and slope. According to microfossils, Fe/Ti values, Post-Archean Australian Shale normalized rare earth element + Y patterns, Eu anomalies versus Y/Ho cross plot, silicon isotopes (δ 30 Si), and oxygen isotopes (δ 18 O), the Dawuba cherts have biogenic, hydrothermal, and hydrogenetic origins, and the MFCs and LFCs are also characterized by replacement cherts. The silica of Dawuba cherts was mainly sourced from siliceous organisms, hydrothermal fluids, and seawater, followed by river water. The silica diagenesis underwent a sequence of opal-A → opal-CT → quartz via dissolution–precipitation reaction or in-situ replacement without intermediary dissolution steps. Siliceous ooze underwent diagenesis to form biogenic cherts. Silica from hydrothermal fluids, seawater, and river water chemically precipitated to form hydrothermal and hydrogenetic cherts. Silica-rich solutions silicified the calcareous allochthonous sediments to form replacement cherts. Flourishing of siliceous organisms, mixing of hydrothermal, seawater-dissolved, and riverine-dissolved silica, and intermittent transport of calcareous allochthonous sediments, played important roles in the deposition and development of cherts in the trough basin during the Early Carboniferous. • Lower Carboniferous chert formed in a suboxic–euxinic deep-water environment. • Cherts were biogenic and chemical origins, some of them had a replacement origin. • Silica sourced from siliceous organisms, hydrothermal fluids, seawater, river water. • Silica diagenesis via dissolution–precipitation reaction or in-situ replacement. • Chert model on organism, dissolved silica, allochthonous sediment was established.

Factors Controlling the Mineralogical and Geochemical Distribution of Phosphatic Deposits, Western Iraqi Desert

The Iraqi phosphorites distributed in western desert belong to Tethys phosphorites. They are found in Al-Rutba- H3, Al-Ga'ara, Akashat, and Al-Ethna and in the western wadi Aakash areas. The statistical factor analysis shows that the main five factors playing great roles in phosphate formation in the congenital shelf are the more active factors that control the mineralogical and geochemical distributions in phosphorites; they are (1) Phosphorus enrichment factor, which led to apatite (francolite) formation. (2) Depositional environment factor, which represents the deposition of phosphates in continental shelf of the southern part of Tethys Sea. (3) Mg withdrawing from sea water, the factor which controlled by clay mineral transportation like palygorskite and some dolomite that contributes to form apatite. (4) The slightly increased saline factor. (5) The cyclic alternative deposition factor, which represents the sequence deposition of phosphates, carbonates and cherts periodically.

Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica‐depleted Paleozoic oceans

The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic-to-biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ30 Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ30 Si and δ18 O of sponge spicules and radiolarians from Ordovician-Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150μM during early Paleozoic time-a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed.

Hydrothermal Origin of Early Permian Chert Nodules in the Central North China Craton Linked to Northern Margin Cratonic Activation

AbstractSedimentary chert phases from the Archean to the present are widely used to trace sedimentary environments and tectonic settings. Recently, chert nodules occurring within carbonates have been the subject of possible hydrothermal or biogenic origin, in lieu of a diagenetic origin. However, chert nodules from a vast cratonic basin represent extremely rich silica accumulations but less noted is how they respond to submarine hydrothermal activity (and/or surface siliceous organism productivity). The links between the cratonic‐type chert depositions and environmental changes regarding cratonic evolution need to be revisited at a large temporal‐spatial scale. The chert nodules are widespread throughout the Lower Permian Taiyuan Formation in the North China Craton (NCC). Several Taiyuan chert‐rich successions across the NCC have been selected to study possible links between chert deposition and cratonic evolution in scenario of partial cratonic activation of the northern NCC margin during the Late Paleozoic. Based on stratigraphic correlation, the chert nodules are ubiquitously, evenly distributed throughout the Taiyuan Formation at a large craton‐basin scale from the northern to southern interior NCC. Petrological results, elemental abundances, together with silicon and oxygen isotopic compositions of chert samples infer significant hydrothermal contributions for the silica accumulations. Therefore, the cratonic‐scale chert depositions of hydrothermal origin infer a giant and remote silica reservoir, linking to the large igneous province and magmatism in the NCC northern margins. The Taiyuan chert nodules could be unique marine sedimentary archives recording the Late Paleozoic NCC partial activation, which also generated continental records of igneous rocks, bauxites and tuffs. The strong tectonics of the northern margin, south‐dipping topography and northward transgression of the Early Permian NCC facilitated the chert deposition on the shallow marine platform in the cratonic interior.

Origin of chert in the Upper Ordovician–Lower Silurian: implications for the sedimentary environment of North Qilian Orogen

ABSTRACTDuring the Upper Ordovician–Lower Silurian, chert was widely distributed in the Zhongbao Formation in the eastern part of the North Qilian Orogen. The origin and the tectonic setting of these chert were largely unknown. In order to analyse the material provenance, sedimentary environment, their formation and the tectonic setting, we present petrology and geochemical research on chert samples collected from Shihuigou Section. The evidence provided by radiolarite occurrences, Aluminium (Al)–iron (Fe)–manganese diagram and the silicon(Si)/Si + Al + Fe + calcium ratios suggesting a non-hydrothermal input and the biogenic origin chert. The geochemical features and the petrographic signatures have shown that the chert was also influenced by a terrigenous origin. It is considered that the deposition of the Late Ordovician chert is mainly affected by tectonic collision and volcanic ash events. During the Late Ordovician–Early Silurian transition, huge amounts of volcanic ash were released by massive volcanic activity that fell into the ocean, triggering the proliferation of radiolarians. Finally, in the Late Ordovician–Lower Silurian the tectonic setting of the North Qilian Orogen was not a typical deep-water basin, nor a typical continental margin, but a multi-island deep-water basin, which is closed to the mainland.

Origin of biogenic-induced cherts from Permian alkaline saline lake deposits in the NW Junggar Basin, NW China: Implications for hydrocarbon exploration

The Permian Fengcheng Formation is composed of alkaline saline lake deposits in NW China and is tectonically part of the Central Asian Orogenic Belt (CAOB). Cherts are common in this formation and contain abundant fossilized microorganisms. These microfossils occur as individual spheres and clots in chert layers (laterally continuous and commonly parallel to the laminae of the host rocks) and nodules (chert grains formed by shrinkage and desiccation of chert layers). Carbonaceous cell walls have been well preserved in chert layers. Based on petrological and geochemical analyses of chert and their host rocks, the mechanisms of cyanobacterial cell silicification and their significance for hydrocarbon exploration have been discussed. A high concentration of dissolved inorganic silica in brine and high pH values are the premise for microorganism blooms and chert deposition. Hot springs may be an important silica source for chert deposition. Hot springs also create extreme conditions for algal blooms in brine with high temperatures, high salinities and toxic and anoxic conditions . Spheroidal cyanobacterial cells were rapidly silicified, and cell structures, e.g., cell walls and inner division patterns (reproduction of cells), were well preserved. Chert with fossilized spheroidal microorganisms can be considered hydrocarbon source rocks.

On the problematic placement of the fossil arthropod Devonopilio hutchinsoni in Opiliones (Arachnida).

Harvestmen (Arachnida: Opiliones) were among the earliest terrestrial arthropods but their unmineralized exoskeletons are scarce and often fragmentary as fossils (Palencia et al. 2019). Consequently, the discovery and interpretations of fossil harvestmen from the early Palaeozoic can have disproportionate effects on the understanding of evolution in Opiliones. Recently, Devonopilio hutchinsoni Tihelka, Tian Cai, 2020, was described as a new fossil harvestman from the well-known Rhynie chert deposits of Scotland, an important source of information on freshwater and terrestrial ecosystems of the early Devonian (Tihelka et al. 2020). This species would be one of the earliest records of harvestmen. The description was based on a single slide showing fragments of arthropod cuticle, which Tihelka et al. interpreted as a harvestmen penis and unspecified associated body parts. As specialists on harvestman morphology and systematics, we were intrigued by these conclusions. However, based on the material presented by Tihelka et al. we find no compelling evidence supporting the proposal that the specimen is a harvestman.

Silicification and Si cycling in a silica-rich ocean during the Ediacaran-Cambrian transition

The upper Ediacaran-lower Cambrian (551–526 Ma) sedimentary succession of the Yangtze Block, South China, hosts abundant chert that formed contemporaneously with the Neoproterozoic Oxidation Event and the Cambrian Explosion. The effect of biological evolution, especially the rise of silica-secreting organisms, on marine Si cycling during the Ediacaran-Cambrian (E-C) transition remains controversial. This paper describes results of a petrological and geochemical investigation aimed at elucidating the origin of the chert and silica biogeochemical cycling during the E-C transition. Our results reveal that the studied chert is dominated by quartz and carbonate. The widespread presence of relict carbonate in inferred peritidal chert deposits suggests that carbonate replacement was an especially common silicification pattern of this facies, whereas the lack of carbonate precursors in subtidal strata argues in favor of primary and biogenic silica precipitation in deposits of this facies. Silicon isotope compositions, relative abundances of Al, Fe, and Mn and rare earth element (REE) + Y concentrations suggest that seawater rather than high-temperature hydrothermal fluid was the primary silica source of the studied cherts. Secular variation of chert Si isotopes reveals that marine silica cycling was closely linked with biological activity. Although silicification was abiological during the Precambrian, it may have been facilitated by microbial activities, especially the widespread formation of cyanobacterial mats. The flourishing of siliceous sponges and radiolarians in the early Cambrian ocean may have served as a global silica sink thus enhancing biological control of the silica cycle after Cambrian time. The studied chert deposits not only demonstrate the silica-rich nature of the global ocean during the E-C transition, but also record the transition of silica cycling from abiological to biological processes and the effect of the evolutionary radiation of silica-secreting organisms during the Cambrian Explosion on silica cycling.