Diagenetic Chert Research Articles

The role of Fe(II)-silicate gel in the generation of Archean and Paleoproterozoic chert

Abstract Chert is abundant in Archean and Paleoproterozoic rocks and is commonly densely packed with authigenic Fe(II)-silicate nanoparticles such as greenalite, indicating a close relationship between iron and silica deposition. We investigate the relationship between Fe(II)-silicate minerals and dissolved silica during precipitation, settling, and diagenesis using anoxic synthesis, sorption, and heating experiments. Excess silica is associated with the solid during precipitation, resulting in high molar Si/Fe ratios (<1.52) that exceed that of stoichiometric greenalite (0.67). At pH 8–8.5, silica sorbs to the surface, reaching sorption densities of 0.68 mmol Si per mmol Fe(II)-silicate. Furthermore, excess Si is released upon heating as the Fe(II)-silicate gel crystallizes. We suggest that Fe(II)-silicate minerals acted as an effective Si shuttle between the water column and the sediments in Archean and Paleoproterozoic marine environments, providing sites for the growth of early diagenetic chert, consistent with observations from the sedimentary record. Our results explain the exceptional preservation of greenalite in early chert and indicate that these minerals could provide a robust archive of marine geochemical data.

Pervasive accumulations of chert in the Equatorial Pacific during the early Eocene climatic optimum

Biosiliceous cherts hosting the transformation of biogenic silica to paracrystalline opal-CT, deposited in the low-latitude realms of Jurassic to Neogene, were recently interpreted as developing during episodes of elevated primary productivity along upwelling areas in equatorial and sub-equatorial paleo-latitudes. Geographically widespread distribution of chert and other rock types of biosiliceous origin, including porcellanite and siliceous claystones throughout the Eocene however indicates these diagenetic deposits are not in any case the dependable markers of latitude-driven upwelling regimes. This study aims to explore the dominance of early Eocene chert occurrences in the Equatorial Pacific (EP) sediments, and their contribution to the Cenozoic global climate change. Our new assessment of the age-abundance dispersal of EP Cenozoic chert sections accommodated by Deep Sea Drilling Project (DSDP)- and Ocean Drilling Program (ODP)-cored deposits realized that the cherts transformed from deposited opal-A sediments most commonly in the early Eocene, with a peak in abundance at ∼50 Ma. This age interval of significant chert occurrences coincided with the period of seawater-temperature maxima of the early Eocene climatic optimum (EECO), during which the world ocean was supposedly marked by highly reduced upwelling intensity and siliceous bioproduction. The spatio-temporal distribution model of the EP Cenozoic cherts with the peak occurrences during the EECO presented in this study bears close similarities to the global distribution patterns of chert during the Cenozoic, which strongly correspond to a global paleoclimatic driver rather than upwelling regimes. Following our survey, cherts occurred less frequently in the post-early Eocene, which is characterized by cooling bottom waters and amplified upwelling and bioproductivity. This model suggests the paleo-seawater rich in dissolved silicon (DSi) would have been imperative for peak incidences of early Eocene chert production, despite extreme surface warmth and greenhouse conditions of the EECO, making DSi available to opal-secreting phytoplankton via deep-water ventilation. These imply that the paleo-climate was more impactful to diagenetic chert production than latitudinally mediated ocean upwelling. The chert distribution events during the early Eocene reduced siliceous bioproductivity appear to have been insufficient to compensate for the significant silica input to the basin mainly from climate-controlled continental weathering. The DSi uptake by precipitation of authigenic clays which accompanies opal-A diagenesis in many depositional systems has been operative, with higher rates in carbonate- than clay-dominated sediments, so as to sustain balance between DSi released to the ocean from terrestrial sources and the silica expelled in sediments during the EECO weakened upwelling.

Vase-shaped microfossils and possible trochamminid foraminifera from the Ediacaran Dhaiqa Formation of Saudi Arabia

We report the first discovery of vase-shaped microfossils (VSMs) and possible multichambered foraminifera from the Ediacaran rocks of Saudi Arabia. The Ediacaran successions of Saudi Arabia are well exposed in the northwestern and central part of the Arabian plate but are relatively understudied due to the lack of economic potential. The exposures can be found in different small sedimentary basins distributed along the flanks of Najd Fault system of the western Arabian Plate. The Dhaiqa sub-basin located near the historic city of Al-Ula contains some of the best exposures of Ediacaran Mattar and Dhaiqa Formations with collective exposure almost 400 m thick. These formations (especially the Dhaiqa Formation) were investigated to describe sedimentary facies, depositional environment, and possible microfossils. This study focuses on the identification of vase-shaped microfossils and possible trochamminid foraminifera of Ediacaran age that are herein recognized for the first time in Saudi Arabia. Outcrop samples were thin-sectioned and analyzed to reveal nine sedimentary facies belonging to four facies associations. The observed microfossils were found in a total of four samples from the cherty/siliceous carbonate and sandy microbialite facies. The microfossils are observed to be associated with diagenetic chert, which is abundant in these facies. The two facies contain a sparse microfossil assemblage with vase-shaped microfossils, which are single-chambered microfossils of uncertain taxonomic affinity, and possible multichambered agglutinated foraminifera that resemble trochamminids.

A review of microbial-environmental interactions recorded in Proterozoic carbonate-hosted chert.

The record of life during the Proterozoic is preserved by several different lithologies, but two in particular are linked both spatially and temporally: chert and carbonate. These lithologies capture a snapshot of dominantly peritidal environments during the Proterozoic. Early diagenetic chert preserves some of the most exceptional Proterozoic biosignatures in the form of microbial body fossils and mat textures. This fossiliferous and kerogenous chert formed in shallow marine environments, where chert nodules, layers, and lenses are often surrounded by and encased within carbonate deposits that themselves often contain kerogen and evidence of former microbial mats. Here, we review the record of biosignatures preserved in peritidal Proterozoic chert and chert-hosting carbonate and discuss this record in the context of experimental and environmental studies that have begun to shed light on the roles that microbes and organic compounds may have played in the formation of these deposits. Insights gained from these studies suggest temporal trends in microbial-environmental interactions and place new constraints on past environmental conditions, such as the concentration of silica in Proterozoic seawater, interactions among organic compounds and cations in seawater, and the influence of microbial physiology and biochemistry on selective preservation by silicification.

BIOSIGNATURE PRESERVATION AIDED BY ORGANIC-CATION INTERACTIONS IN PROTEROZOIC TIDAL ENVIRONMENTS

Abstract The preservation of organic biosignatures during the Proterozoic Eon required specific taphonomic windows that could entomb organic matter to preserve amorphous kerogen and even microbial body fossils before they could be extensively degraded. Some of the best examples of such preservation are found in early diagenetic chert that formed in peritidal environments. This chert contains discrete domains of amorphous kerogen and sometimes kerogenous microbial mat structures and microbial body fossils. Our understanding of how these exquisite microfossils were preserved and the balance between organic degradation and mineral formation has remained incomplete. Here, we present new insights into organic preservation in Proterozoic peritidal environments facilitated through interactions among organic matter, cations, and silica. Organic matter from Proterozoic peritidal environments is not preserved by micro- or cryptocrystalline quartz alone. Rather, preservation includes cation-rich nanoscopic phases containing magnesium, calcium, silica, and aluminum that pre-date chert emplacement and may provide nucleation sites for silica deposition and enable further chert development. Using scanning electron microscopy and elemental mapping with energy dispersive X-ray spectroscopy, we identify cation enrichment in Proterozoic organic matter and cation-rich nanoscopic phases that pre-date chert. We pair these analyses with precipitation experiments to investigate the role of cations in the precipitation of silica from seawater. Our findings suggest that organic preservation in peritidal environments required rapid formation of nanoscopic mineral phases through the interactions of organic matter with seawater. These organic-cation interactions likely laid the initial foundation for the preservation and entombment of biosignatures, paving the way for the development of the fossiliferous chert that now contains these biosignatures and preserves a record of Proterozoic life.

Tracing the evolution of dissolved organic carbon (DOC) pool in the Ediacaran ocean by Germanium/silica (Ge/Si) ratios of diagenetic chert nodules from the Doushantuo Formation, South China

The Ediacaran Period (ca. 635–538 Ma) was a critical interval in Earth’s history, bridging the Proterozoic and the Phanerozoic Earth systems. It witnessed the diversification of eukaryotic phytoplanktons in the early stage, the evolution of macroscopic multicellular lives in the late stage, and frequent oscillations of carbonate carbon isotopes of large magnitudes, presumably reflecting large perturbations of the global carbon cycle. It has been proposed that a large dissolved organic carbon (DOC) pool, hundreds to thousands times larger than the present one, was developed in the Ediacaran deep ocean. The wax and wane of the marine DOC pool might be responsible for the secular variation of carbon isotopes of marine dissolved inorganic carbon (DIC) pool, which was recorded in the carbonate carbon isotope chemostratigraphy. However, neither the extent nor the evolutionary history of marine DOC pool in the Ediacaran ocean has been well constrained. In this study, we use Germanium/silica (Ge/Si) ratios of chert nodules from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, to trace the evolution of marine DOC pool. These chert nodules yield abundant microfossils, i.e. acanthomorphic acritarchs, and were precipitated in the early diagenetic replacement of calcareous silty/muddy sediments. Most chert nodules show positive correlations between Al contents and Ge/Si ratios, which can be resolved by a binary mixing between a silica (quartz) endmember and an Al-rich silicate (clay) endmember. On the one hand, the Ge/Si ratios of silica component ((Ge/Si)SiO2) are comparable to the modern seawater composition, suggesting the normal seawater rather than hydrothermal fluids was the major source of Si. On the other, the Ge/Si ratios of the clay component ((Ge/Si)clay) are one to two orders of magnitudes higher than that of normal marine clays. The Ge-enrichment of the clay component could be attributed to the extensive chelation of Ge by organic ligands and massive formation of organic-Ge-Clay complex at high DOC concentration. This observation supports the presence of a large DOC pool in the Ediacaran ocean. In addition, there is an increase of (Ge/Si)clay from the Member II to Member III of the Doushantuo Formation, suggesting an increase of DOC concentration. The expansion of marine DOC pool is coincident with the reduction of the sizes of the upper acanthomorphic acritarch assemblage in the Member III. We suggest that the increase of marine DOC pool might be attributed to the reduction of sizes of particulate organic matter, which favored DOC generation and accumulation in the Ediacaran seawater.

Fossil evidence provides new insights into the origin of the Mesoproterozoic ministromatolites

Although the biogenicity of the very common millimeter-sized ministromatolites in the Proterozoic has been noted, direct biotic evidence is missing, leading to the discussion of a chemical origin that differs from that of the cyanobacteria-related stromatolites in the Phanerozoic. Distinctive ministromatolites were recently discovered in a microbial dolostone interval of the Mesoproterozoic Wumishan Formation, at Lingyuan, North China. Thin section examination of the ministromatolites reveals the presence of abundant coccoidal microfossils ranging 10–30 μm in diameter. Petrofabric analysis shows that the ministromatolites experienced three periods of diagenetic silicification, and that the microfossils are exclusively preserved within early diagenetic chert. This supports the view that silicification penecontemporaneous with mat growth is the key to unique preservation of microfossils. In contrast, the absence of microfossils in most Precambrian carbonate stromatolites may well be interpreted as the lack of hydrochemical conditions favorable for early silicification. Although chemically-formed fibrous carbonate minerals are the dominant components of these ministromatolites, the presence of abundant microfossils indicates that the role of microorganisms in the formation of ministromatolites has previously been underestimated. Therefore, this research suggests that Mesoproterozoic ministromatolites were not solely chemical products but consist of complex associations of both biotic and chemical fabrics.

Structural and chemical heterogeneity of Proterozoic organic microfossils of the ca. 1 Ga old Angmaat Formation, Baffin Island, Canada.

Organic microfossils in Meso- and Neoproterozoic rocks are of key importance to track the emergence and evolution of eukaryotic life. An increasing number of studies combine Raman spectroscopy with synchrotron-based methods to characterize these microfossils. A recurring observation is that Raman spectra of organic microfossils show negligible variation on a sample scale and that variation between different samples can be explained by differences in thermal maturation or in the biologic origin of organic precursor material. There is a paucity of work, however, that explores the extent to which the petrographic framework and diagenetic processes might influence the chemical structure of organic materials. We present a detailed Raman spectroscopy-based study of a complex organic microfossil assemblage in the ca. 1Ga old Angmaat Formation, Baffin Island, Canada. This formation contains abundant early diagenetic chert that preserves silicified microbial mats with numerous, readily identifiable organic microfossils. Individual chert beds show petrographic differences with discrete episodes of cementation and recrystallization. Raman spectroscopy reveals measurable variation of organic maturity between samples and between neighboring organic microfossils of the same taxonomy and taphonomic state. Scanning transmission X-ray microscopy performed on taphonomically similar coccoidal microfossils from the same thin section shows distinct chemical compositions, with varying ratios of aromatic compounds to ketones and phenols. Such observations imply that geochemical variation of organic matter is not necessarily coupled to thermal alteration or organic precursor material. Variation of the Raman signal across single samples is most likely linked to the diagenetic state of analyzed materials and implies an association between organic preservation and access to diagenetic fluids. Variation in the maturity of individual microfossils may be a natural outcome of local diagenetic processes and potentially exceeds differences derived from precursor organic material. These observations stress the importance of detailed in situ characterization by Raman spectroscopy to identify target specimens for further chemical analysis.

A new model for silicification of cyanobacteria in Proterozoic tidal flats.

Microbial fossils preserved by early diagenetic chert provide a window into the Proterozoic biosphere, but seawater chemistry, microbial processes, and the interactions between microbes and the environment that contributed to this preservation are not well constrained. Here, we use fossilization experiments to explore the processes that preserve marine cyanobacterial biofilms by the precipitation of amorphous silica in a seawater medium that is analogous to Proterozoic seawater. These experiments demonstrate that the exceptional silicification of benthic marine cyanobacteria analogous to the oldest diagnostic cyanobacterial fossils requires interactions among extracellular polymeric substances (EPS), photosynthetically induced pH changes, magnesium cations (Mg2+ ), and >70ppm silica.

Biologically mediated silicification of marine cyanobacteria and implications for the Proterozoic fossil record

Abstract Silicification was a major mode of fossilization in Proterozoic peritidal environments, but marine silica concentrations and the chemical and biological mechanisms that drove microbial silicification and formation of early diagenetic chert in these environments remain poorly constrained. Here, we use taphonomy experiments to demonstrate that photosynthetically active cyanobacteria that are morphologically analogous to the oldest cyanobacterial fossil, Eoentophysalis, mediate the formation of magnesium-rich amorphous silica in seawater that is undersaturated with respect to silica. These results show that microbes in Proterozoic tidal environments may have mediated their own silicification at lower silica concentrations than previously assumed.

Epigenetic siliceous rocks from the southern part of the Kraków-Częstochowa Upland (Southern Poland) and their relation to Upper Jurassic early diagenetic chert concretions

The epigenetic siliceous rocks (ESR) are common in the southern part of the Kraków-Częstochowa Upland where they are hosted in Upper Jurassic microbial-sponge megafacies as: (i) coatings on the surfaces of fractures and faults, (ii) irregular bodies enclosed within both the Upper Jurassic limestones and the overlying Cenomanian abrasion platform sediments, and (iii) concentric or irregular rims of silicified limestones on early diagenetic chert concretions. The distribution of ESR is limited to the topmost part of the preserved Upper Jurassic succession. The results of chemical analyses and fluid inclusion microthermometric data clearly demonstrate the hydrothermal origin of the ESR. The epigenetic silicification was triggered by low temperatures and low saline fluids.The age of the ESR is indirectly suggested by: (i) the early diagenetic cherts forming the crystallization nuclei, (ii) the silicification of pressure dissolution structures in limestones and (iii) the silicified carbonate infillings of fractures in chert concretions formed in the local extensional regime, enhancing the migration of thermal fluids during the main, Cenozoic faulting stage of Upper Jurassic sediments.

δ30Si and δ18O of multiple silica phases in chert: Implications for δ30Siseawater of Darriwilian seawater and sea surface temperatures

A promising quantitative method is the use of fluctuations of seawater silicon isotope compositions (δ30Siseawater) to estimate the silicon cycle and silicon isotopic budgets of both the marine and terrestrial systems in a pre-diatom ocean. However, a thorough global calibration of Phanerozoic δ30Siseawater, which is linked to continental tectonics and silicate weathering, has not been established. The silicon and oxygen isotope compositions of Darriwilian cherts (δ30Sichert and δ18Ochert, respectively) have considerable potential as proxies for δ30Siseawater and sea surface temperatures (SSTs), which can expand our understanding of the influence of continental tectonics and temperature changes on the Great Ordovician Biodiversification Event (GOBE). In this study, primary depositional and early diagenetic cherts were sampled from bedded chert, lump-shaped chert, nodular chert, and banded chert from five Darriwilian outcrops in the Kalpin and Yijianfang areas of the Tarim Basin in China. The preservation conditions of these cherts were assessed based on petrographic criteria such as the absence of calcite veins and fractures under macroscopic observation and the absence of aggregate chalcedony (CHA), microquartz (MQ), and cryptocrystalline quartz (MQC) using optical microscopy and scanning electronic microscopy (SEM). The SSTs and δ30Siseawater of Darriwilian seawater were reconstructed using the δ18Ochert and δ30Sichert of well-preserved cherts. Our results show that the SSTs were approximately 32 ± 5 °C in the early Darriwilian and approximately 30 ± 5 °C in the late Darriwilian. The δ30Siseawater was approximately +3.1‰ in the Kalpin and Yijianfang areas during the Darriwilian Stage. This is the first estimation of δ30Siseawater in the Darriwilian Stage, which provides a parameter for the global calibration of the δ30Siseawater of the pre-diatom ocean in the Phanerozoic Eon and validates the hypothesis that the pre-diatom ocean waters had higher δ30Si values than modern seawater.

Late Eocene onset of the Proto-Antarctic Circumpolar Current

The formation of the Antarctic Circumpolar Current (ACC) is critical for the evolution of the global climate, but the timing of its onset is not well constrained. Here, we present new seismic evidence of widespread Late Eocene to Oligocene marine diagenetic chert in sedimentary drift deposits east of New Zealand indicating prolonged periods of blooms of siliceous microorganisms starting ~36 million years ago (Ma). These major blooms reflect the initiation of the arrival and upwelling of northern-sourced, nutrient-rich deep equatorial Pacific waters at the high latitudes of the South Pacific. We show that this change in circulation was linked to the initiation of a proto-ACC, which occurred ~6 Ma earlier than the currently estimated onset of the ACC at 30 Ma. We propose that the associated increased primary productivity and carbon burial facilitated atmospheric carbon dioxide reduction contributing to the expansion of Antarctic Ice Sheet at the Eocene-Oligocene Transition.

Vase-shaped microfossil biostratigraphy with new data from Tasmania, Svalbard, Greenland, Sweden and the Yukon

The recent revision of the Cryogenian lower boundary from 850 Ma to 720 Ma has nearly doubled the duration of the Tonian Period to 280 Myrs, an interval longer than the Mesozoic and Cenozoic eras combined. Although subdivision of the Tonian will likely emphasize chemostratigraphy, the relatively rich fossil record of this time period may also be useful. In particular, vase-shaped microfossils (VSMs) occur globally and often abundantly in a variety of lithofacies, exhibit a variety of preservational modes, and are stratigraphically constrained by the two major carbon isotopic events of the Tonian Period, the Bitter Springs and Islay anomalies. VSMs have been used as informal, grade-level indicators of late Tonian age (e.g. ca. 800–700 Ma) for several decades but here, based on new data from an assemblage in northwestern Tasmania and restudy of samples from Svalbard, Greenland, Sweden and the Yukon, we report a species-level faunal assemblage indicative of the interval ca. 789 to 729 Ma. Species-level VSM biostratigraphy presents an opportunity for potential biostratigraphic subdivision of the lengthy Tonian Period and is particularly useful in successions where chemostratigraphy and radiometric age constraints are absent.Eight taxa are described from shales and early diagenetic cherts of the Black River Dolomite, Togari Group, Tasmania, including seven previously named species (Bombycion micron, Bonniea dacruchares, Bonniea pytinaia, Cycliocyrillium simplex, Cycliocyrillium torquata, Melanocyrillium hexodiadema and Palaeoarcella athanata), and one newly described genus and species Pakupaku kabin n. gen et n. sp. Study of additional units resulted in new recognition of B. pytinaia, C. simplex and C. torquata in the Draken Formation, Spitsbergen, Svalbard, B. dacruchares and P. athanata in the Ryssö Formation, Nordaustlandet, Svalbard, B. dacruchares, B. pytinaia, C. torquata and P. athanata in Bed 18 of the Eleonore Bay Group, Greenland, and B. dacruchares, B. pytinaia and Bombycion micron in the Visingsö Group, Sweden.

New biostratigraphic and chemostratigraphic data from the Ediacaran Doushantuo Formation in intra-shelf and upper slope facies of the Yangtze platform: Implications for biozonation of acanthomorphic acritarchs in South China

Carbon isotopic and microfossil records of the Doushantuo Formation in South China provide a valuable window onto major perturbations of the Ediacaran carbon cycle and the evolution of morphologically complex acanthomorphic acritarchs. Both records exhibit significant geographic heterogeneity related to environmental, taphonomic, and diagenetic variations. Absolute δ13Ccarb values vary by as much as 10‰ between sections of the Doushantuo Formation. The geographic and stratigraphic distribution of microfossils in the Doushantuo Formation is controlled by the occurrence of early diagenetic cherts and phosphorites that preserve microfossils. Early diagenetic cherts of the Doushantuo Formation have been investigated extensively at intra-shelf sections around the Huangling anticline but have been poorly documented elsewhere in South China. Here we present new petrographic and micropaleontological data from three Doushantuo sections to the south of the Huangling anticline, including the Wangzishi and Huangjiaping sections representing intra-shelf settings, as well as the Siduping section representing an upper slope setting. We also present δ13Ccarb data from Wangzishi. Petrographic observations indicate that some chert nodules at Siduping were formed in situ, whereas others were reworked and probably transported from the outer shelf along with olistostromes. Integrated lithostratigraphic and chemostratigraphic correlations suggest that the fossiliferous intervals at the three studied sections belong to the lower Doushantuo Formation. The new micropaleontological data extend the geographic range of Ediacaran acanthomorphs and suggest that elements of the Tanarium conoideum–Hocosphaeridium scaberfacium–Hocosphaeridium anozos biozone recognized in the upper Doushantuo Formation around the Huangling anticline, specifically Hocosphaeridium anozos and Urasphaera nupta, are also present in the lower Doushantuo Formation at the Siduping section. The data suggest that the first appearance of Hocosphaeridium anozos may be in the δ13Ccarb chemostratigraphic feature EP1 and it is necessary to revise the acritarch biozonation developed in the Yangtze Gorges area. This study highlights the importance of integrating δ13Ccarb chemostratigraphic and acritarch biostratigraphic data from multiple facies to develop Ediacaran acanthomorph biozonation in South China and beyond.

Proterozoic microbial mats and their constraints on environments of silicification.

The occurrence of microfossiliferous, early diagenetic chert in Proterozoic successions is broadly restricted to peritidal marine environments. Such coastal environments are amongst the most environmentally variable of marine environments, experiencing both enhanced evaporation and potential influx of terrestrial freshwaters. To better understand potential conditions under which silicification occurs, we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Spectacular preservation of silicified microbial mats, their associated mineral phases, and the petrographic fabrics of the chert itself require that silicification occurred at the sediment-water interface, penecontemporaneously with mat growth. In some cases, silica is the primary precipitated mineral phase and is not associated with replacement of precursor mineral phases. In other cases, silica deposition includes the mimetic replacement of carbonate, gypsum, and halite mineral phases. These petrographic constraints suggest that silicification potentially occurred under a range of fluid chemistries associated with environmental variability in nearshore peritidal environments. Here we provide the first direct thermodynamic modeling of hypothetical Proterozoic seawater solutions, seawater-derived brines, and mixed seawater-freshwater solutions, and demonstrate that peritidal environments are capable of providing a wide range of fluid chemistries under which early diagenetic silica can both precipitate and replace primary mineralogical phases. Despite the thermodynamic potential for silica deposition under a wide range of fluid compositions, chert is not ubiquitous in Proterozoic nearshore environments, suggesting that the kinetics of silica polymerization exert a primary control over deposition.

Greenalite precipitation linked to the deposition of banded iron formations downslope from a late Archean carbonate platform

Banded iron formations (BIFs) were deposited as deep-water facies distal to the late Archean Campbellrand carbonate platform, Transvaal Supergroup, South Africa. They are traditionally interpreted to have formed from iron oxides/hydroxides and silica that precipitated when upwelling hydrothermal water enriched in Fe2+ and silica mixed with cooler, shallow seawater enriched in dissolved oxygen. The ferric oxides/hydroxides were then converted to hematite or, in the presence of organic matter, reduced and incorporated into siderite. New high-resolution microscopy of BIFs from distal facies of the Campbellrand platform questions that interpretation, revealing the presence of abundant greenalite nanoparticles, interpreted to represent primary precipitates from ferruginous seawater, consistent with recent results from equivalent-aged BIFs of the Hamersley Group, Western Australia. The particles locally define primary sedimentary lamination, and are engulfed in diagenetic chert that preserves shrinkage structures, considered to have formed during dehydration and recrystallization of amorphous silica cement. The replacement of greenalite nanoparticles by coarser grained oxide, silicate and carbonate minerals, with destruction of primary textures and structures, indicates that iron-silicate muds were originally much more widespread. Based on our findings we propose a new model for the deposition of the BIFs, involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor. We suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids. Upon mixing with cooler, more alkaline seawater, the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron-silicate nanoparticles over vast areas of the seafloor. The close association between iron and silica during the deposition of BIFs may be explained by the precipitation of iron-silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles. Furthermore, on the seafloor, iron-silicate nanoparticles presumably acted as nucleation sites for dissolved silica, promoting early diagenetic silica precipitation and cementation of the iron-rich muds. Our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of BIFs.

Palaeobiology of the early Ediacaran Shuurgat Formation, Zavkhan Terrane, south-western Mongolia

Early diagenetic chert nodules and small phosphatic clasts in carbonates from the early Ediacaran Shuurgat Formation on the Zavkhan Terrane of south-western Mongolia preserve diverse microfossil communities. Chert nodules contain abundant fragments of organic material, which include organic-walled microfossils and pieces of microbial mats. These fragments are dominated by several Siphonophycus species forming a variety of microbial textures. Spheroids such as Myxococcoides are of secondary importance but dominate one rock sample. Dispersed phosphatic clasts within the carbonate matrix preserve degraded filaments and spheroids. Petrographic characteristics and the composition of the palaeobiological community are similar to those of early Ediacaran assemblages elsewhere. The presence of Salome hubeienesis, previously reported in the Doushantuo Formation of South China and the Krol Group of the Lesser Himalaya, India, and possible multicellular fossils similar to those of Doushantuo, prompts comparisons with the diversity of other biotas, suggesting similarities in regional early Ediacaran communities. The Shuurgat fossils add to the growing evidence for the nature of Ediacaran ecosystems at this critical juncture in Earth history.

Sheet-like microfossils from hydrothermally influenced basinal cherts of the lower Cambrian (Terreneuvian) Niutitang Formation, Guizhou, South China

Early diagenetic cherts in basinal shales of the lower Cambrian (Terreneuvian) Niutitang Formation, exposed in the Yangtiao section, Majiang, Guizhou Province, South China, contain abundant sheet-like microfossils. The sheets are carbonaceous, rich in complex aromatic molecules, sinuously folded and, commonly, tightly enrolled. Their regular thickness, lack of evidence for cellular structure, occasional ornamentation by finger- or hair-like projections, and infrequent attachment to one another eliminate ripped up microbial mats and blade-like algae as biological sources. More likely, the Niutitang sheets represent surficial coverings of animals. In the absence of information on sheet morphology, it is difficult to constrain systematic interpretation further, but other Ediacaran and Cambrian fossils suggest that a cnidarian affiliation is plausible. The siliceous sequence of the lowest Cambrian Niutitang Formation was deposited under a relatively stable anaerobic environment, episodically influenced by hydrothermal fluids. Thus, it is probable that the sheets were transported into the basin from oxic shelves or sank to the bottom from oxygenated surface waters. In either event, these problematic remains provide new evidence of biological diversity in early Cambrian oceans.

Lithostratigraphic Succession and Depositional Dynamics of the Lower Mississippian, Southern Ozarks, Northern Arkansas and Adjacent Areas

The Lower Mississippian interval comprises a single, third-order, eustatic cycle subdivided lithostratigraphically into the St. Joe Limestone (Hopkins 1893) and overlying Boone Formation (Branner 1891, Simonds 1891) with type areas in northern Arkansas. Coeval, homotaxial limestones occur in adjacent southwestern Missouri and northeastern Oklahoma, but neither Arkansas name is applied. To eliminate this “state line fault,” Missouri formation names for the St. Joe interval are recognized in Arkansas as members (ascending order): Bachelor, Compton, Northview, Pierson. The Boone interval in Missouri is represented by the (ascending order): Reeds Spring, Elsey, Burlington-Keokuk undifferentiated, but utilization of those names in Arkansas is problematic. Chert development and characteristics associated with the Boone Formation in northern Arkansas have not been applied to the equivalent succession in Missouri. Consequently, in northern Arkansas, the Boone Formation is subdivided into informal lower and upper members based on chert development: lower with black to gray, penecontemporaneous chert; upper with white to light gray, later diagenetic chert. In adjacent northeastern Oklahoma, the nomenclature is a mixture of the Arkansas and Missouri names, but chert development is not used lithostratigraphically. The St. Joe Limestone rests unconformably on the Chattanooga Shale (Upper Devonian-Lower Mississippian) or older units marking the initial transgression of the Kaskaskian II Cycle (Vail et al. 1977). Thin-bedded, St. Joe crinozoan packstones represent bioclastic sediment and carbonate mud transported from its origin on the Burlington Platform (now Missouri), and down the adjacent northern Arkansas ramp in a lobate manner. Distal limestones are condensed and replaced by shale beyond the ramp. A brief drop in sea level represented by the terrigenous Northview Member, was followed by continued transgression through Pierson deposition, reaching maximum flooding without a break in the lower Boone (=Reeds Spring) represented by calcisiltites and penecontemporaneous chert. Highstand and regression are recorded in the upper Boone as rapidly deposited crinoidal packstones and grainstones with later diagenetic chert replacement.