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The Genesis and Time Distribution of Two Distinctive Proterozoic Stromatolite Microstructures
Abstract
Stromatolites are commonly viewed as sedimentary proxies for microbial communities. In consequence, secular variation in stromatolite form has been attributed to evolutionary change in mat organisms and/or their interactions with global environments. This interpretation requires that one be able to identify features of stromatolite macrostructure or microstructure that are under direct biological influence and use them to document the sedimentological or petrological consequences of microbial evolution. Well-preserved stromatolites in Proterozoic carbonates of the Siberian Platform contain true distinctive types of microstructure that enable us to address such issues. Preserved microstructures in Baicalia lacera and Tungussia confusa from lower Neoproterozoic (lower Upper Riphean) platform carbonates of several widely separated regions clearly reflect the biology of underlying mat communities; mm-scale laminae of densely interwoven calcified. filaments alternate with filament-poor microspar. Comparable carbonate microstructures are known from a number of other Neoproterozoic stromatolites, but are as yet unreported from older successions. Filamentous cyanobacteria formed mats throughout the Proterozoic Eon; thus, the temporal distribution of this microstructure appears principally to reflect secular and environmental variations in carbonate cementation and diagenesis. Omachtenia omachtensis is the characteristic stromatolite of Mesoproterozoic (Lower and, occasionally, Middle Riphean) successions of the Siberian. Platform and elsewhere. Its distinctive microstructure consists of mechanically deposited event laminae separated by thin organic films that served as nucleation sites for micritic and, less commonly, fibro-radiate carbonate precipitates. Microbial mats may have stabilized sediments between events, but there is little evidence that mats played an active role in the trapping and binding or precipitation of laminae. Thus, physico-chemical factors must also be responsible for the distribution. of these stromatolites in time and space. Analysis of these two microstructures suggests that, in general, secular trends in stromatolite microstructure may encrypt important information about environmental change through the Proterozoic Eon. Microbial evolution may also play a role in determining the stratigraphic distributions of particular Proterozoic stromatolites, but this remains to be demonstrated. Evolution. may be most important in driving the progressive environmental restriction of stromatolite-forming microbial communities through time. Organismic and Evolutionary Biology
... Precambrian dolomite can be exquisitely fabric-retentive (also called "mimetic dolomite"), commonly preserving early marine fibrous cements, radial oolitic fabrics, spherulites precipitated in stromatolite laminae, filamentous microfossils, and even fine mudstones with gypsum swallowtails (Plate 6, B, D, E, F); Phanerozoic dolomite, by contrast, often exhibits coarse fabrics characterized by coarse, interlocking, rhombohedral crystals, evidence of dissolution and reprecipitation. (Tucker, 1977(Tucker, , 1982(Tucker, , 1984Zempolich et al., 1988;Burdett et al., 1990;Knoll and Swett, 1990;Sibley, 1991;Zempolich and Baker, 1993;Knoll and Semikhatov, 1998;Wright, 2000;Corsetti et al., 2006). Our results replicate these textural observations to a limited degree (Fig. S1) although we note that high-resolution, petrographic studies of Precambrian and Phanerozoic dolomite are better suited to describe dolomitic fabric than Microstrat, which focuses on larger-scale facies. ...
... We hypothesize that the replacement of Ω-increasing (anaerobic) with Ω-decreasing (aerobic or otherwise oxygen-dependent) metabolisms leaves a signature in the carbonate textural record. Consistent with earlier reports (Grotzinger and Read, 1983;Hofmann and Jackson, 1987;Kah and Grotzinger, 1992;Sumner and Grotzinger, 1996;Kah and Knoll, 1996;Semikhatov and Raaben, 1996;Knoll and Semikhatov, 1998;Grotzinger and Knoll, 1999;Grotzinger and James, 2000;Higgins et al., 2009;Bergmann et al., 2013), Microstrat shows that seafloor macro-and micro-precipitates are abundant in Archean and Paleoproterozoic carbonate deposits across the observed depth gradient, but are uncommon thereafter. We suggest that the abundance of seafloor precipitate textures is consistent with the presence of precipitation-promoting anaerobic metabolisms dominating an anoxic sediment-water interface during the Archean and Paleoproterozoic intervals. ...
... The success of stromatolite stratigraphy in the broad correlation of Proterozoic sedimentary successions and the evident facies relationships of stromatolite forms within individual basins indicate that the morphologies of these boundstones vary spatially and temporally in Precambrian carbonate successions (Cloud and Semikhatov, 1969;Bertrand-Sarfati and Trompette, 1976;Walter, 1976;Bertrand-Sarfati and Moussine-Pouchkine, 1988;Knoll and Semikhatov, 1998;Kah et al., 2006Kah et al., , 2009Murphy and Sumner, 2008). Our database corroborates these broad changes in stromatolite morphologies (Fig. 4); we hypothesize that these changes are explained by changes in the style and loci of carbonate precipitation. ...
Carbonate minerals have precipitated from seawater for at least the last 3.8 billion years, but changes in the physical and chemical properties of carbonate rocks demonstrate that the nature, loci, and causes of this precipitation have varied markedly through time. Biomineralization is perhaps the most obvious time-bounded driver. However, other changes in the sedimentological character of Archean and Proterozoic carbonates offer an under-considered record of fluctuations in Earth’s chemical, biological, and physical systems before and during the rise of animal life. Geobiologists and geochemists have successfully used large geochemical datasets to track changes in Earth’s surface chemistry through time, but inconsistencies between proxies make the recognition of clear spatial-temporal patterns challenging. In order to place new constraints on the environmental history of Archean and Proterozoic oceans, especially with regard to redox state, we present for the first time a high-resolution database of global Precambrian and Cambrian carbonate sedimentation.
Our initial datasbase, named Microstrat, comprises carbonate-bearing successions dating from c. 3.4 billion to ~500 million years old; these include more than 130 carbonate-bearing formations, digitized at the highest stratigraphic resolution possible and classified by environment of deposition. Lithofacies details are recorded at the finest scale available, including a range of microbial and other depositional fabrics, mineralogy, environment of deposition, age and location. We interrogate the Archean, Proterozoic, and Cambrian carbonate records, testing both long-standing and novel hypotheses about changes in Earth’s carbonate system through time. Changes in carbonate sedimentology illuminate global changes in bioturbation and microbial community structure; the depth-dependent timing of marine oxygenation and the transition from anaerobic to aerobic respiration at and within the seafloor; and temporal changes in patterns of dolomitization. Shifts in both the loci of carbonate precipitation and in markers of animal activity are consistent with time- and depth-dependent trends in ocean oxygenation. We also note changes through time in the morphologies of stromatolites and thrombolites, which we suggest reflect secular shifts in carbonate precipitation, clastic sediment transport, and eukaryotic evolution. Observations from the database and petrographic studies confirm that the nature and extent of Proterozoic dolomite contrasts with that of dolomite in the Phanerozoic, consistent with dolomite being an early or even, in some cases, primary precipitate prior to the Cambrian Period. Because this database preserves environmental context and the stratigraphic relationships of carbonate features rather than tracking the occurrence of single features through time, it permits new and detailed analyses of the physical sedimentology of Precambrian carbonates and correlations in time and space among different features.
We also consider how to build upon this initial dataset. Capturing and analyzing sedimentological data is difficult. Analysis becomes more challenging when data have been collected by multiple researchers working across Earth’s entire history and surface. Field observations have a great deal of power to describe and explain changes in Earth’s surface environments, yet they are often reported in idiosyncratic ways that make it difficult to compare them, obscuring their true explanatory power. Microstrat is an effort to pool idiosyncratic sedimentological data in hopes of illuminating Archaean, Proterozoic, and Cambrian carbonate depositional environments, yet it is unavoidably limited. We describe how other researchers can help to build Microstrat into a community-curated database by reporting and sharing data.
... Although a biogenic origin for stromatolites has been recognized for over 100 years (Kalkowsky, 1908) and has not been challenged in rocks <2 Ga, an unequivocally biogenic origin for many Archean and some early Proterozoic stromatolites remains somewhat contentious (Grotzinger & Rothman, 1996;McLoughlin et al., 2008), in part due to higher degrees of recrystallization and metamorphism in the older rocks (e.g. Knoll & Semikhatov, 1998). ...
... The effects of diagenesis may also have an impact upon the frequency of banding and may obscure the original biological imprint on the sample (e.g. Knoll & Semikhatov, 1998). The striking patterns highlighted in this study suggests that hyperspectral imaging may provide information that can be used in combination with other lines of evidence to link patterns of layering in stromatolites to ancient environmental conditions. ...
... Diagenetic or metamorphic changes can obscure, change and, in some cases, obliterate biologically imprinted signatures within stromatolites, thus complicating their interpretation (e.g. Knoll & Semikhatov, 1998). For example, evidence from hyperspectral analysis of Sample 3, suggests that diagenetic recrystallization of the dolomite could obscure a subtle biological imprint from this setting. ...
Hyperspectral imaging (400–2496 nm) was used to quantitatively map surface textures and compositional variations in stromatolites to determine whether complexity of textures could be used as evidence to support biogenicity in the absence of preserved biomarkers. Four samples of 2.72–2.4 Ga stromatolites from a variety of settings, encompassing marine and lacustrine environments, were selected for hyperspectral imaging. Images of the sawn surfaces of samples were processed to identify reflectance and mineral absorption features and quantify their intensity (as an index of mineral abundance) using automated feature extraction. Amounts of ferrous iron were quantified using a ratio of reflectance at 1650 and 1299 nm. Visible near infrared imagery (400–970 nm) did not reveal additional textural patterns to those obtained from visual inspection. Shortwave infrared imagery (1000–2496 nm), however, revealed complex laminar and convoluted patterns, including a distinctive texture of sharp peaks and broad, low troughs in one sample, similar to living tufted microbial mats. Spectral analysis revealed another sample to be composed of dolomite. Two other samples were dominated by calcite or chlorite � illite. Large variations in amounts of ferrous iron were found, but ferric iron was exclusively located in the oxidation crust. Hyperspectral imaging revealed large differences between parts of a sample of biogenic and non-biogenic origin. The former was characterized by calcite with varying amounts of ferrous iron, distributed in lenticular, convoluted patterns; the latter by Mg-Fe chlorite with large amounts of aluminium silicate, distributed as fine laminar layers. All minerals identified by hyperspectral imaging were confirmed by thin section petrography and XRD analyses. Spatial statistics generated from quantitative minerals maps
showed different patterns between these different parts of the sample. Thus, hyperspectral imaging was shown to be a powerful tool for detecting structures in stromatolites that could be used, together with other lines of evidence, to support biogenicity.
... Many ancient stromatolites and stromatolitic facies, therefore, are sufficiently well recrystallized to preclude reconstruction of lamination processes. Fortunately, the record also includes numerous stromatolites that have suffered only minimal recrystallization so that primary textures are well preserved (Figure 3a, 3b, 3c), in some cases, with preservation of cyanobacterial filaments and molds (Figure 3d, 3e, 3f; see also Knoll & Semikhatov 1998, Semikhatov et al 1979, Walter et al 1988. In certain cases, fabrics are so well preserved that they imply neomorphic inversion of aragonite and high-Mg calcite directly to dolomite without a low-Mg calcite intermediary (Grotzinger & Read 1983). ...
... Strong support for this interpretation was provided by Hofmann and Jackson (1987), who discovered primary crystal textures that had been silicified before neomorphic inversion or recrystallization to more stable mineral phases. Comparable textures have subsequently been observed in a number of other Precambrian stromatolites (Bartley et al 1999, Kah & Knoll 1996, Knoll & Semikhatov 1998, Sami & James 1996, mostly of Paleo-and Mesoproterozoic age (Figure 4c, 4d ). Another mineral texture consistent with in situ mineral precipitation, herringbone calcite, commonly encrusts thin films of organic matter interpreted as former microbial mats (Sumner 1997, Sumner & Grotzinger 1996a). ...
... Horodyski 1975, Walter et al 1988. Over the past decade it has become increasingly clear that in situ mineral precipitation is an important accretion mechanism in ancient stromatolites (Figures 2-6; see also Bartley et al 1999, Grotzinger 1986a, Grotzinger & Read 1983, Hofmann & Jackson 1987, Kah & Knoll 1996, Knoll & Semikhatov 1998. In some remarkably wellpreserved stromatolites of late Archean age, it can be observed that the only components in the stromatolite structures were microbial mats, early marine cement, and later porosity-occluding burial cement-sedimentary particles are completely absent (Sumner 1997). ...
Stromatolites are attached, lithified sedimentary growth structures, accretionary
away from a point or limited surface of initiation. Though the accretion process is
commonly regarded to result from the sediment trapping or precipitation-inducing
activities of microbial mats, little evidence of this process is preserved in most Precambrian
stromatolites. The successful study and interpretation of stromatolites
requires a process-based approach, oriented toward deconvolving the replacement
textures of ancient stromatolites. The effects of diagenetic recrystallization
first must be accounted for, followed by analysis of lamination textures and deduction
of possible accretion mechanisms. Accretion hypotheses can be tested
using numerical simulations based on modern stromatolite growth processes. Application
of this approach has shown that stromatolites were originally formed
largely through in situ precipitation of laminae during Archean and older Proterozoic
times, but that younger Proterozoic stromatolites grew largely through the
accretion of carbonate sediments, most likely through the physical process of microbial
trapping and binding. This trend most likely reflects long-term evolution
of the earth’s environment rather than microbial communities.
... However, there is also evidence that encrusting cements are capable of forming stromatolites and associated morphologies without a clear biogenic component (e.g. Grotzinger and Knoll, 1999;Grotzinger and Rothman, 1996;Knoll and Semikhatov, 1998). Grotzinger and Rothman (1996) used the Kadar-Parisi-Zhang (KPZ) equation to demonstrate that some stromatolite forms (such as domal morphologies) can be created abiogenically. ...
... This is inconsistent with accretion in which crystal nucleation and growth is controlled by microbial mats that exhibit spatial heterogeneity (e.g. Bartley et al., 2000;Grotzinger and Knoll, 1999;Knoll and Semikhatov, 1998;. ...
... Raaben, 2006) or microdigitate stromatolites (e.g. Grotzinger, 1989;Knoll and Semikhatov, 1998;Petryshyn and Corsetti, 20ll;Riding, 2008). The laminations of these stromatolites alternate between radial-fibrous and non-radial fibrous laminae (Fig. 9) so that they appear similar to hybrid sparry fine-grained crusts described by Riding (2008). ...
Stromatolites can form from either biogenic or abiogenic accretionary processes. Mesoproterozoic (1.1 Ga) carbonate stromatolites from the nonmarine Copper Harbor Conglomerate in the upper peninsula of Michigan have been described as biogenic although they lack internal microfossil evidence and exhibit a problematic radial-fibrous calcite that may indicate direct abiogenic precipitation. If shown to be biogenic, they will represent one of the earliest examples of terrestrial microbial activity. Therefore, it is important to determine the biogenicity of these stromatolites and refine a methodology for testing stromatolite biogenicity in the absence of microfossil evidence. Stromatolite samples were collected from four localities within the formation that represent abandoned fluvial channels within an alluvial fan system. The stromatolites are often found in thin (< 0.5 m) oolitic grainstone beds that drape clastic cobbles. They are also found in mudstone lenses that display soft-sediment deformational structures. These stromatolites contain mudstone laminations and are found draping shale rip-up clasts. One method of determining biogenicity of the stromatolites will be to measure growth angles of cobble-draping stromatolites to determine if a preferential growth direction towards sunlight occurred. A new biosignature method using magnetic susceptibility will also be used to test the biogenicity of these stromatolites. The magnetic susceptibility is based on the distribution of detrital material across single laminae. The theory behind this methodology is that biogenic stromatolites are better at trapping sediment (including magnetic minerals) than abiogenic stromatolites because of the adhesive nature of microbial mats. Petryshyn et al. (2011) found support for this in both laboratory experiments and the measured susceptibility in ancient stromatolites. If the susceptibility is similar regardless of growth angle then the hypothesis that the Copper Harbor stromatolites are biogenic will be supported. If the susceptibility varies with different growth angles then the hypothesis that the stromatolites are biogenic will not be supported. The results of this study will have important implications for interpreting the early history of life on Earth, particularly in terrestrial settings.
... However, there is also evidence that encrusting cements are capable of forming stromatolites and associated morphologies without a clear biogenic component (e.g. Grotzinger and Knoll, 1999;Grotzinger and Rothman, 1996;Knoll and Semikhatov, 1998). Grotzinger and Rothman (1996) used the Kadar-Parisi-Zhang (KPZ) equation to demonstrate that some stromatolite forms (such as domal morphologies) can be created abiogenically. ...
... This is inconsistent with accretion in which crystal nucleation and growth is controlled by microbial mats that exhibit spatial heterogeneity (e.g. Bartley et al., 2000;Grotzinger and Knoll, 1999;Knoll and Semikhatov, 1998;. ...
... Raaben, 2006) or microdigitate stromatolites (e.g. Grotzinger, 1989;Knoll and Semikhatov, 1998;Petryshyn and Corsetti, 20ll;Riding, 2008). The laminations of these stromatolites alternate between radial-fibrous and non-radial fibrous laminae (Fig. 9) so that they appear similar to Fig. 9. Thin section photomosaic of Dan's Point-type stromatolite with alternating radial-fibrous and non radial-fibrous laminae. ...
... This lithofacies is regarded as a type of non-stromatolitic microbial carbonate, and warrants further research. Interestingly, if the clotted fabric of this lithofacies is a postdepositional thrombolite (c.f., Riding, 2000) or incipient oncolite (c.f., Zhao, 1992), then it may well be one of the oldest recorded thrombolites as shown in Figure 3a (Kennard and James, 1986;Aitken and Narbonne, 1989;Knoll and Semikhatov, 1998;Grotzinger and James, 2000;Riding, 2000). In addition, the microdigital stromatolites making up the Mopanyu stromatolite sub-assemblage (Liang et al., 1984;Zhu et al., 1993Zhu et al., , 1994 would be one of the youngest stromatolites that resulted from the precipitation of carbonate directly on sea floor, since this form of stromatolite is chiefly developed in the Archaean and Palaeoproterozoic (Knoll and Semikhatov, 1998;Grotzinger and James, 2000). ...
... Interestingly, if the clotted fabric of this lithofacies is a postdepositional thrombolite (c.f., Riding, 2000) or incipient oncolite (c.f., Zhao, 1992), then it may well be one of the oldest recorded thrombolites as shown in Figure 3a (Kennard and James, 1986;Aitken and Narbonne, 1989;Knoll and Semikhatov, 1998;Grotzinger and James, 2000;Riding, 2000). In addition, the microdigital stromatolites making up the Mopanyu stromatolite sub-assemblage (Liang et al., 1984;Zhu et al., 1993Zhu et al., , 1994 would be one of the youngest stromatolites that resulted from the precipitation of carbonate directly on sea floor, since this form of stromatolite is chiefly developed in the Archaean and Palaeoproterozoic (Knoll and Semikhatov, 1998;Grotzinger and James, 2000). ...
Carbonate strata of the Mesoproterozoic Wumishan Formation in the Jixian area near Tianjin are ~3300 m thick and were deposited over some 100 million years (from ~1310±20 Ma to ~1207±10 Ma). Metre-scale cycles (parasequences) dominate the succession. They are generally of the peritidal carbonate type, and mostly show an approximately symmetrical lithofacies succession with thick stromatolite biostromes and small thromboliteoncolite bioherms constituting the central part and tidal-flat dolomites forming the upper and lower parts. Lagoonal-supratidal dolomitic shales with palaeosol caps make up the topmost layers. The boundaries of the Wumishan cycles are typically exposure surfaces, and there is abundant evidence for fresh-water diagenesis. Widespread 1:4 stacking patterns indicate that the individual Wumishan cycles are sixth-order parasequences, with 4 parasequences constituting one fifth-order parasequence set. Locally, 5–8 beds or couplets, can be discerned in some of the cycles. The regular vertical stacking pattern of beds within the sixth-order parasequences, forming the fifth-order parasequence sets, are interpreted as the result of environmental fluctuations controlled by Milankovitch rhythms, namely the superimposition of precession, and short and long-eccentricity. The widespread 1:4 stacking pattern in the cyclic succession, as well as the local 1:5–8 stacking patterns of the beds within the cycles, suggest that the Milankovitch rhythms had similar ratios in the Mesoproterozoic as in the Phanerozoic. Based on the cycle stacking patterns, 26 third-order sequences can be distinguished and these group into 6 second-order, transgressive-regressive megasequences (or sequence sets), all reflecting a composite, hierarchical succession of relative sea-level changes.
... Prior to the radiation of skeleton-forming eukaryotes, CaCO 3 and SiO 2 both accumulated preferentially along the margins of the oceans-in tidal flats and coastal lagoons, where evaporation drove precipitation (Maliva et al. 1989;Knoll and Swett 1990). Bacteria undoubtedly facilitated this deposition-the microscopic textures preserved in silicified stromatolites include both encrusted filaments and radially oriented crystal fans nucleated within surface sediments (e.g., Knoll and Semikhatov 1998;Bartley et al. 2000)-but the overall distribution of Proterozoic carbonates and silica reflects primary physical controls on precipitation. In this regard, it is worth noting that surface textures of Proterozoic sandstone beds record the widespread distribution of microbial mats in environments where no stromatolites accreted (e.g., Hagadorn and Bottjer 1997;Noffke et al. 2002); stromatolites formed where carbonate was deposited, not the reverse. ...
... More likely, however, the fossils record unmineralized thalli whose relatively decay resistant walls were cast by encrusting carbonates soon after burial. Cyanobacterial sheaths preserved in similar fashion occur widely if sporadically in Neoproterozoic rocks around the world (e.g., Knoll and Semikhatov 1998). Ca. 600 million year old rocks of the Doushantuo Formation, China, contain extraordinary fossils of multicellular red and green algae preserved in anatomical detail, and these include florideophyte reds interpreted as stem group members of the (now) skeleton-forming Corallinales. ...
... Stromatolites of the Kerpyl' Group are represented by local endemics and transit Middle-Upper Riphean form genera and species, whereas basal beds of the Lakhanda Group show a higher diversity of transit taxa and enclose Inzeria tjomusi Kryl., Jurusania cylindrica Kryl., and Baicalia lacera Semikh., the form species typical of the lower Upper Riphean (Krylov, 1975;Semikhatov and Serebryakov, 1983;Semikhatov, 1995). Microstructure of the last form that is widespread in basal Upper Riphean horizons exemplifies the particular stage in evolution of cyanobacterial ecosystems and global environments of carbonate accumulation (Knoll and Semikhatov, 1998). ...
... The Member I 6 is of a roughly known age. Stromatolites from the underlying Member I 5 (Bertrand-Sarfati, 1972;Knoll and Semikhatov, 1998) are characteristic of the lower Upper Riphean (~1030-850 Ma), and the Rb-Sr isochron dates for illite fractions ~2 μm separated from both members are 890 ± 37 and 874 ± 23 Ma, respectively (Clauer, 1981). However, these dates are applicable only for the approximate evaluation of the age, because the fraction ~2 μm usually contains, as is established now (Gorokhov et al., 2001, and references therein), the non-cogenetic illite generations. ...
The results obtained contribute much to the reconstruction of secular Sr isotope variations in seawater of the late Middle-early Late Riphean and elucidate factors that determined low values of the Sr-87/Sr-86 ratios in the World Ocean at the formation time of Rodinia (Grenville orogenic cycle). The new curve that depicts the Sr isotope variations in seawater 1050-1000 Ma ago is much more detailed than others characterizing the Precambrian. It is plotted after investigation of limestones collected from the Kerpyl' and Lakhanda groups of the East Siberian Uchur-Maya region and of correlative limestones from the Turukhansk region of Central Siberia. In order to obtain the confident analytical data determining the curve configuration, the selection of limestone samples was controlled by petrographic and strict geochemical criteria (Mn/Sr less than or equal to0.20, Fe/Sr less than or equal to5.0, Mg/Ca less than or equal to0.024) that ensure the least alteration degree of rocks. In addition, all the selected samples were preliminary treated in 1N solution of ammonium acetate for the partial removal of epigenetic carbonate phases. We took into consideration the lowest Sr-87/Sr-86, ratios obtained for dolomites only as evaluating the maximum limit of this parameter in the sedimentation medium. After generalization of all Sr-isotopic data available for the Middle-early Late Riphean, we established that the Sr-87/Sr-86 ratios in seawater did not exceed 0.70490 about 1300 Ma ago and began to rise afterward to become as high as 0.70518-0.70549 approximately 1200 Ma ago. At the end of the Middle Riphean (1050-1030 Ma), the ratios reached a relative peak of values corresponding to 0.70563-0.70592, after which they decreased. During the earliest Late Riphean (1030-1000 Ma), they varied between 0.70519 and 0.70569, generally decreasing with time, and became equal to 0.70523-0.70527 1000 Ma ago. Within the time span of 900-800 Ma, the Sr-87/Sr-86 ratios were likely not greater than 0.70525-0.70585. The following factors and events appear to be responsible for the persistently low Sr-87/Sr-86 ratios in seawater of the Grenville and post-Grenville time: a high proportion of the pre-Grenvillide mantle rocks in the crust of Grenvillides; peculiar features of metamorphism, exhumation, and unroofing of different lithotectonic units in the Grenville orogens; an active influx of fresh juvenile material into oceans of the Grenville time; the Late Grenville sea-level rise; and, possibly, the partial locking up of the continental runoff in inner continental depressions at the beginning of the Late Riphean.
... Recent work has demonstrated the importance of the 'precipitating model' in forming stromatolite laminae. Although cited as having largely occurred in the Neoarchean and Paleoproterozoic, precipitate stromatolites are found throughout the rock record (Hofmann and Jackson, 1987;Knoll and Semikhatov, 1998). Arguments exist as to whether the precipitation is driven by abiotic or biotic processes (Grotzinger and Knoll, 1999). ...
... Stromatolite taxonomists interested in interbasinal correlation primarily use microstructure (Cloud and Semikhatov, 1969;Raaben, 1969;Knoll and Semikhatov, 1998). Therefore, the ethological changes recorded show some temporal constraint. ...
Stromatolites are trace fossils that record the interaction between microbial communities and sediments. Macroscopically, the shape of the stromatolite may represent stronger environmental controls. Study of the scalar attributes can yield insight into microbialethology, and foster biostratigraphic and facies analysis over the past 3500 million years. Stromatolites have largely been treated as structures analogous to colonial organisms or eukaryotic algae, even though only a fraction of fossil occurrences contains an organic component and none are made by a singular colonial animal orindividual plant. Stromatolites and other microbialites are primarily composed of mechanical sediments and cements produced through the activities of a microscopic consortium. The internal fabric and macroscale attributes of stromatolites have been used for biostratigraphic correlation and facies analysis, even though the taxonomic affinities of the microbial community are usually unknown. Indeed, it is quite probable that a particular microbial ecosystem may be responsible for more than one type of microbialite, and several different microbial consortia may construct a unique microbialite.
... Data on the 87 Sr/ 86 Sr ratio of seawater at the onset of the late Riphean were obtained during investigation of limestone samples from the Neryuen Formation, the basal one in the Lakhanda Group of the Uchur-Maya region in Siberia. Organic walled microfossils (Veis, 1988;German, 1990;Sergeev et al., 2010) and stromatolites (Semikhatov and Serebryakov, 1983;Knoll and Semikhatov, 1998) described from this for mation are typical of the lower upper Riphean depos its, and the Pb-Pb age of the formation limestones corresponds to 1025 ± 40 Ma . The Lakhanda Group base representing the middle-upper Riphean boundary is defined at the level of 1030 Ma Dopol neniya…, 2000). ...
... However, such frac tions usually contain noncogenetic illite generations different in age and cannot be regarded as valid geochronometers. Stromatolite taxa identified in Member I 5 underlying Member I 6 are typical of the lower part of the upper Riphean (Ber trand Sarfati, 1972;Knoll and Semikhatov, 1998), and the Atar Group used to be correlated with middle hori zons of the upper Riphean. According to results of the Re-Os systematics recently published by Rooney et al. (2010), organic rich sediments of the Tourist For mation in Central Mauritania, which are correlative with Member I 5 , turned out to be surprisingly older, corresponding in age to 1109 ± 12 and 1105 ± 37 Ma. ...
Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the 87Sr/86Sr variations in the Riphean and Vendian oceans. During the time span of 1600–1250 Ma, the 87Sr/86Sr variations were in a narrow range corresponding to 0.70456–0.70494, but approaching the date of about 1030 Ma, the 87Sr/86Sr ratio rose to 0.70601–0.70611 and then quickly declined to 0.70519–0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521–0.70535 to values of 0.70874–0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561–0.70575 at approximately 760 Ma and to 0.70533–0.70538 at 670–660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812–0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806–0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.
... The shapes of modern marine stromatolites range from stratiform to dm-to m-sized clubs, ridges, cauliflower heads or small domes (Logan et al., 1964;Gebelein, 1969;Playford, 1980;Burne and Moore, 1987;Spadafora et al., 2010). Stromatolites with much more diverse shapes and sizes were present during the Precambrian, exhibiting predictable forms on individual carbonate platforms (e.g., Hoffman, 1974;Horodyski, 1975;Grey and Thorne, 1985;Altermann, 2008) and likely tracking the chemical, biological and physical changes through time (e.g., Cloud and Semikhatov, 1969;Semikhatov et al., 1979;Walter and Heys, 1985;Awramik and Riding, 1988;Kah and Knoll, 1996;Semikhatov and Raaben, 1996;Knoll and Semikhatov, 1998;Grotzinger and Knoll, 1999;Raaben, 2006;Tice et al., 2011;Bosak et al., 2013). When biological, chemical and physical regimes that contribute to stromatolite textures or shapes can be reconstructed, stromatolites can be used to recognize and interpret broader environmental and evolutionary trends. ...
... Current models and interpretations of stromatolite growth and form rely on studies of sub-mm scale laminae and textures (for a small sample of these, see : Hofmann, 1975;Monty, 1976;Burne and Moore, 1987;Knoll and Semikhatov, 1998;Visscher et al., 2000;Sprachta et al., 2001;Vasconcelos et al., 2006;Bosak et al., 2009;Bontognali et al., 2010;Bosak et al., 2010;Petryshyn and Corsetti, 2011;Mata et al., 2012), sub-mm or mm-scale angles of fossil biofilms and stromatolite laminae (Tice et al., 2011;Petroff et al., in review), mm-wide and tall tufts (Buick, 1992;Sim et al., 2012) or clumps in photosynthetic mats , and mm-scale laminae packed with trapped and bound grains (Black, 1933;Reid et al., 2003). A model that explores competition for nutrients in diffusion-limited mats can account for the somewhat larger, cm-scale spacing of small conical stromatolites . ...
Stromatolite shapes, sizes, and spacings are products of microbial processes and interactions with topography, sedimentation, and flow. Laboratory experiments and studies of modern microbial mats and sediments can help reconstruct processes that shaped some typical stromatolite forms and some atypical microbially influenced sediments from Neoproterozoic cap carbonates. Studies of modern, cohesive microbial mats indicate that microbialaminite facies in the lower Rasthof Formation (Cryogenian) formed in the presence of very low flow and were not deformed by strong waves or currents. Giant wave ripples, corrugated stromatolites, and tube-hosting stromatolites in basal Ediacaran cap carbonates record interactions between microbes, flow, and evolving bedforms. Preferential cementation in and close to the giant ripple crests is attributed to interactions between flow and local topography. These interactions pumped alkaline porewaters into ripple crests and helped nucleate elongated stromatolites. The similar textures of giant wave ripples and elongated, corrugated, and tube-hosting stromatolites suggest growth in the presence of organic-rich, rounded particles and microbial mats, and in flow regimes that permitted mat growth. These hypotheses can be tested by experiments and models that investigate lithification and the macroscopic morphology of microbial mats as a function of the flow regime, preexisting topography, redox-stratification in sediments, and delivery of organic-rich particles. The widespread microbially influenced textures in Cryogenian microbialaminites and basal Ediacaran cap dolostones record a strong reliance of carbonate deposition on the presence of organic nuclei, supporting carbonate accumulation rates comparable to those in modern reefs. Therefore, the unusual macroscopic morphologies of microbially influenced facies in Neoproterozoic cap carbonates may not reflect oceans that were greatly oversaturated with respect to carbonate minerals.
... Turner et al. (1993) Défarge et al. (1996) Knoll and Semikhatov (1998) Riding and Sharma (1998) Duane and Al-Zamel (1999) Seong-Joo and Golubic (1999) Bartley et al. (2000) Seong-Joo et al. (2000) Turner et al. (2000) Riding (2000) Raaben (2006) Riding (2008) *Note the overlapping of meanings and how it constantly repeats. When interpreted as isolated terms, they are comprehensive (although with some exceptions); however, the overview of the literature is a mixed, confusing glossary. ...
Abstract Stromatolites are laminated biosedimentary structures of great importance for paleobiological, paleoecological, and paleoenvironmental analyses, mainly in Precambrian rocks. Their value is related to the glimpse of past life recorded in their lamination, fabric, and, eventually, due to the preservation of organic matter, including microfossils, and because their deposition is directly influenced by environmental conditions. Although stromatolites are widely described in microscopic scale, there is a lack of standardization of their nomenclature, precluding better paleoenvironmental and paleobiological interpretations. In this study, we propose a guide for the microscopic analysis of fossil stromatolites and, possibly, thrombolites, and provide a review of specialized literature and the bibliometric context of main terms. The goal is to contribute to the improvement of their application through systematization of microscopic data, in the face of novel paleoecological and paleobiological approaches and for astrobiological prospection for microbialites in therock record of Mars.
... Fine-grained textures occur intimately associated with sparry carbonate in many of Archean and Proterozoic stromatolites (Riding, 2008(Riding, , 2011Riding and Virgone, 2020). They are referred to as ribboned and striated (Hoffman, 1969), streaky (Walter, 1972), filmy (Knoll and Semikhatov, 1998), spar-micrite couplets (Sami and James, 1996) or hybrid crusts (Riding, 2008(Riding, , 2011Riding and Virgone, 2020). Hybrid crusts are known from the Archean and may have been the dominant fabrics in Paleo-and Mesoproterozoic stromatolites (Riding, 2011). ...
Fibrous aragonite crusts occur in two consecutive Pleistocene successions in the Danakil Depression (Afar, Ethiopia). Lateral transitions between pristine and altered fibrous aragonite crusts document changes in texture associated with diagenesis. Crusts formed as essentially abiotic seafloor precipitates at the transition from marine to evaporitic conditions. Diagenesis started with the dissolution of aragonite fans at the interface between single fans in non-laminated crusts and along lamination planes in isopachous, irregular, or crudely laminated crusts. Incomplete dissolution resulted in the development of secondary porosity within a matrix of undissolved aragonite fibers. Subsequently, the porosity was filled with calcite that systematically encased remaining aragonite crystals. This was followed by the dissolution of remnant aragonite fibers, producing a network of elongated inter- and intracrystalline pores that were eventually filled with low-Mg calcite. The stepwise substitution of fibrous aragonite by low-Mg calcite resulted in sparry, sparry-cloudy, sparry-micritic (including clotted micrite), and peloidal textures, which obscure the fibrous nature of the original deposits. Stable C- and O-isotope compositions suggest that early diagenesis was driven by meteoric and evaporative fluids. These observations unequivocally demonstrate destructive diagenesis, resulting in secondary textures, which mimic micritic and grumous (peloidal and clotted) textures associated with sparry microfabrics. This suggests that these textures, classically interpreted as primary microbial precipitates and used as evidence of biogenicity in ancient microbialites, might be diagenetic products in some cases, even though at some stage, microbial processes and/or degradation of organic matter could have been involved in the diagenetic process.
... Stromatolites from the Horse Thief Springs Formation have previously been identified as Baicalia and Conophyton (Cloud & Semikhatov, 1969;Howell, 1971;Maud, 1979Maud, , 1983. All stromatolites from the Horse Thief Springs Formation form wide domes several centimetres to decimetres in width, resembling Mesoproterozoic to Neoproterozoic Baicalia Lacera described from Late Riphean shallow water platform carbonates (Knoll & Semikhatov, 1998). Variability in stromatolite morphology indicates changing environmental conditions related to water depth, wave energy, flow energy and turbulence, and rates of sediment supply (Hofmann, 1976;Horodyski, 1977;Grotzinger, 1989;Andres & Reid, 2006). ...
The sedimentary record of the Pahrump Group in Death Valley comprises well‐exposed successions of mixed carbonate and siliciclastic deposits. Despite the abundance of studies focussing on the depositional dynamics of mixed carbonate – siliciclastic deposition in the Phanerozoic, the record of similar Proterozoic examples is comparatively sparse. Using high‐resolution stratigraphy and microfacies analyses, this study investigates the Tonian Horse Thief Springs Formation within the Pahrump Group of Death Valley, California, in order to propose first‐order constraints on the interplay between carbonate and siliciclastic deposition in the early Neoproterozoic. The mixed successions are unlike many previously studied examples interpreted as being caused by glacio‐eustatic sea‐level changes, thus arguing for a more relevant tectonic influence on siliciclastic deposition. This interpretation is supported by detailed microfacies analyses of siliciclastic‐rich dolostones, which show abundant soft‐sediment deformation features suggesting sudden pulses of sandstone deposition onto a shallow marine carbonate shelf. The Tonian carbonate factory recovered quickly after being smothered by siliciclastics, particularly due to abundant stromatolite growth. A new relationship between siliciclastic input, carbonate fabric diversity, and carbonate preservation is established based on microfacies analyses, putting forward that siliciclastic deposition had a significant impact on the formation and preservation of later‐stage diagenetic dolomite cements, as well as on stromatolite morphology and carbonate fabric diversity within the microbialites. This study shows how repeated siliciclastic incursions had a significant impact on the Proterozoic carbonate factory of the Horse Thief Springs Formation, as well as on diagenetic modification and preservation of shallow marine carbonates, and establishes previously unexplored relationships between carbonate and siliciclastic strata in the Proterozoic.
... aragonite crystal fans) that characterize the Neoarchean to Paleoproterozoic (>1.6 Ga) carbonate successions worldwide (Grotzinger and Knoll, 1999). As abiotic processes can contribute to form these circularly concentric and radial patterns, an abiotic carbonate precipitation scenario is plausible for the accretion of MDSs (Grotzinger and Reed, 1983;Hofmann and Jackson, 1987;Knoll et al., 1993;Sergeev et al., 1995;Grotzinger and Knoll, 1999;Riding, 2008), although heterotrophic bacterial metabolism may have facilitated carbonate precipitation and nucleation during MDS formation (Knoll and Semikhatov, 1998). However, the ultrastructures of such radial-fibrous textures in carbonate precipitates have been rarely uncovered, although their microstructures are often observed under optical microscopes. ...
Stromatolites have been widely reported from the Archean and Paleoproterozoic successions worldwide, and they could represent one of oldest life forms on Earth. Of these, a group of small stromatolites occur as microdigitate low-relief columns, and are also conspicuous in the field. However, biogenicity of these microdigitate stromatolites (MDSs) has long been disputed due to to the abundance of radial-fibrous texture and a lack of convincing microfossils. New examples of MDS are documented from the Mesoproterozoic Wumishan Formation of the Jixian area, North China. Vertically oriented fibrous fabrics are conspicuous and penetrate laminae as well as microscopic spheroids, which point to an abiotic genesis for this specific fabric. Stromatolite laminae contain abundant spheroids, typically 15–30 μm in diameter, with single or double outlines and they occur as solitary coccoid-like microfossils or in small aggregated colonies. Spheroids show strong fluorescence under both green and purple exciting lights, consistent with their composition of organic matter. Spheroids are abundant in the Wumishan stromatolites and they are categorized into two types. The first kind comprises micrite nuclei surrounded by sparitic sheaths, without nano-particle coatings. A smooth to grainy spheroidal surface defines the first kind of spheroids that also has a distinct rounded opening, which is often broken probably due to diagenesis and silicification. The second kind of spheroids is usually covered with nano-particles and lacks circular opening on surface. These spheroids possess large nuclei of single sparitic calcite coated with thin sparry sheaths. Overall, the Wumishan spheroids resemble coccoidal microorganisms reported from other Archean-Paleoproterozoic strata worldwide, but they are also better preserved. The rounded opening on spheroid surface is interpreted as division point of unicells during reproduction of the life cycle of bacteria akin to Myxococcoides grandis. Clump-like micro-particle aggregates in nuclei could represent daughter cells released from the parental envelope, similar to the reproduction process and life cycle suggested for similar spheroidal microfossils from other similar Precambrian occurrences. The Wumishan spheroids therefore may represent fossilized prokaryotes that could have contributed to construct the MDS. Moreover, filamentous microfossils are occasionally present in the coloumns of stromatolites, and they resemble filamentous cyanobacteria, but may not be major constructors of MDS due to their rarity in the buildups. Three types of nano-particles are also conspicuous: (1) putative organic relics, such as fragmented filaments and mucuslike biofilms (purported EPS), (2) organominerals, including nanoglobules, polyhedrons, and their aggregates, and (3) dumbbell-shaped nano-particle aggregates. All of these nano-particles are interpreted to be likely biogenic in origin, and many of them were found from the radial-fibrous fabrics of carbonate precipitates in the MDS, implying that some heterotrophic bacteria may have afficliated the precipitation of radial fibers in deep-time radial-fibrous carbonate precipitates. Therefore, abundant and diverse biosignatures (spheroids, tubular filaments, and nano-particles) are identified in the Wumishan MDSs, and we conclude that diverse filamentous and coccoidal micro-organismscontributed to the formation of the Wumishan stromatolites.
... However, the increase in calcimicrobial reefs from the Cambrian must reflect global changes. Reports of calcified filaments in Mesoproterozoic (Kah and Riding, 2007) and Neoproterozoic (Knoll et al., 1993;Knoll and Semikhatov, 1998) stromatolites are rare. Kilometre-scale microbial reefs in the lower Neoproterozoic Little Dal Group, Northern Territories, Canada, include poorly preserved Girvanella and Renalcis-like calcified microbes (Turner et al., 1993(Turner et al., , 2000. ...
A dramatic shift in microbial reefs occurred around the Ediacaran–Cambrian boundary. Here we describe changes in the composition, construction, and texture of microbial reefs in the Zavkhan Terrane of Gobi-Altai Province, western Mongolia, during the late Ediacaran and early Cambrian. Stromatolites consisting of peloids, micritic clots, and homogeneous lime mud without calcified microbes (calcimicrobes) are characteristic of the upper Ediacaran (units 9 and 16A of the Zuun-Arts Formation). In contrast, abundant thrombolites with stromatolites occur in the lowest Terreneuvian (units 17A and 17 of the Bayan Gol Formation). These Cambrian microbial reefs are made up of micritic clots and homogeneous lime mud in close association with calcimicrobes including Korilophyton, Renalcis, and Tarthinia. The thrombolites and calcimicrobial reefs studied herein occur directly stratigraphically above strata that record a strong negative shift in δ¹³C values and are dominated by small shelly fossils; these are the earliest known calcimicrobial reef representatives of the Phanerozoic. These microbial reefs changed almost simultaneously with drastic fluctuations in environmental conditions (e.g., seawater chemistry, Ca concentration, carbonate saturation, and oxygen level). These changes would have been influenced by the evolution of calcimicrobes and skeletal metazoans across the Ediacaran–Cambrian boundary. The present work provides crucial geobiological information on substantial shifts at the Ediacaran–Cambrian boundary and how calcimicrobes and related textures appeared in tandem with the innovation of biomineralisation.
... Microbially induced carbonate precipitation can influence the shapes and sizes of carbonate grains 93,94 , and even form carbonate grains and stromatolites within siliciclastic sediments [95][96][97] . These structures and grains can preserve organic inclusions that trace lamination, as defined by the different sizes of carbonate crystals or their purity 13,83,84,98 . Biological influence on sedimentation and sedimentary textures can also be inferred from individual grains or thin layers of quartz, clay and other minerals enclosed within the precipitated carbonate that suggest a former presence of sticky organic surfaces (biofilms). ...
The recognition of past habitable environments on Mars has increased the urgency to understand biosignature preservation in and characterize analogues of these environments on Earth. In this Review, we examine the detection and interpretation of potential biosignatures preserved in deposits rich in carbonates, silica and clay. Many of the earliest chemical, textural and morphological evidence of life on Earth are found in carbonates and carbonate-hosted phases. Early diagenetic chert within carbonate deposits can exceptionally preserve microbial body fossils, and clay minerals that form in ultramafic terrains can protect organic matter. On Mars, similar deposits older than 3.5 billion years could contain biosignatures or remnants of prebiotic processes that have long been erased from Earth. Terrestrial analogues for the deposition of magnesium carbonate minerals in Jezero crater, Mars, present patterns that can guide the collection of samples with the highest astrobiological potential by the Perseverance rover. Continued characterization of terrestrial analogue sites and rigorous examination of the processes that impact the preservation of isotopic signals, organic compounds, and microbial textures and fossils will advance the interpretation of Martian deposits.
... Stromatolites are laminated organosedimentary structures produced by the interaction of microbial communities, detrital sediment and autochthonous carbonate precipitation (see Grey & Awramik, 2020 for a detailed discussion). Analysis of stromatolite diversity typically focuses on morphology, or macroscopic structure, of individual stromatolitic forms (Bertrand-Sarfati & Moussine-Pouchkine, 1985;Bertrand-Sarfati & Awramik, 1992;Grey & Awramik, 2020), often with only secondary focus on mesoscopic elements, such as laminae structure (Kah et al. , 2009Bartley et al. 2015), or the microscopic elements that comprise laminae (Bertrand-Sarfati, 1976;Turner et al. 1993;Knoll & Semikhatov, 1998;Bartley et al. 2000;Riding, 2008). Here, we examine both synoptic relief and inheritance (Hofmann, 1969;Grey & Awramik, 2020) as comprising macroscopic morphology (i.e. ...
Time-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantine et al. (2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.
... Stromatolites occur from the shoreface to the offshore zones of the mixed carbonate-siliciclastic Caboclo ramp (Fig. 13). The wide distribution and abundance of stromatolites seems to be a feature of the Precambrian (Hofmann, 1973;Knoll and Semikhatov, 1998;Grotzinger and Knoll, 1999;Grotzinger and James, 2000;Riding, 2000;Allwood et al., 2006). The occurrence of stromatolitic deposits of the Caboclo Formation is similar to the Early Proterozoic stromatolitic carbonate platform of Campbellrand subgroup (Beukes, 1987), the Lower Proterozoic Pethei Group carbonate platform (Hoffman, 1974) and the 2.72 Ga stromatolite biofacies of Tumbiana Formation (Coffey et al., 2013). ...
Records of shallow-marine ramps with the mixing of carbonate and siliciclastic sediments are common throughout the geological time. All these records have pure carbonate and pure siliciclastic deposits as end members, occurring contemporaneously in distinct depositional regions along the ramp, and transitional hybrid facies between them. The two end member can mix in different scales and can alternate in time due to climatic changes and regressions and transgressions. This work presents a detailed reconstruction of a Mesoproterozoic storm-dominated mixed carbonate-siliciclastic ramp composed of hybrid sediments and whithout the presence of pure siliciclastic or carbonate deposits, a rare example in the geological record. Based on a high resolution logged section (in 1:20 scale) and qualitative thin sections, eleven lithofacies were identified and grouped into three lithofacies associations (offshore, offshore transition and shoreface), which are stacked vertically forming a transgressive–regressive cycle. This faciological distribution indicates a low relief ramp with wide microbial colonization from shallow to relatively deep waters (below storm-wave base level). In offshore low-energy distal areas, microbial mats spread laterally over large distances with little or no interference from currents, while in the offshore transition the morphology of the bioherms is shaped by currents induced by waves. In turn, the high wave energy in the shoreface inhibits the formation of stromatolites, restricting their occurrence to thin layers of microbial carpets or intraclastic lags. The mixing occurs in compositional scale and is relatively homogeneous along the whole logged interval, independent of the shifts in lithofacies or lithofacies associations. This compositional homogeneity is linked to the wide distribution and regularity in the input of siliciclastic sediments during the sedimentary accumulation. Strong currents induced by storms allow the transport and mixing of siliciclastic sediments with carbonate grains generated in the basin during fair-weather periods.
... Micritic to microclotted fabrics of microbial carbonates represent calcification of the extracellular polymeric substances of sulphatereducing bacteria (Riding, 2011). Very thin microbial layers are similar to the "filmy structure" of Precambrian stromatolites (Bertrand-Sarfati, 1976;Knoll and Semikhatov, 1998) from the primary growth of microbial organisms (Harwood and Sumner, 2012). ...
Lower Ordovician stromatolite-like columns and thrombolite-like mounds, composed of fossilised keratose sponges (keratolites) and microbial carbonates, are reported from the Tremadocian Mungok Formation, Yeongwol, Korea. The stromatolite-like columns, which are up to 10 cm wide and high, consist of an inner core with low-angled (10–45°) layers that are covered by high-angled (>45°) layers. The inner core is made up of millimetric layers of alternating keratolite and microbial carbonate, and microbial carbonate dominantly comprises the outer cover. The entire columns are surrounded by bioclastic packstone to grainstone. The thrombolite-like mounds are domes a maximum of 100 cm high and 40–60 cm wide embedded within lime mud and shale. These mounds consist of keratolite–microbial carbonate clots and minor lithistid sponge–microbial carbonate clots. The stromatolite-like columns were formed in a high-energy subtidal setting, in which laminoid keratolite and microbial carbonate formed the tight laminar frame columns. Continued growth of the column narrowed the intercolumnar space, resulting in higher-energy hydrodynamic conditions that limited the growth of sponges but promoted growth of microbial organisms. In contrast, thrombolite-like mounds developed in a low-energy environment below fair-weather wave base, where irregular to bulbose keratose sponges with minor lithistid sponge–microstromatolite associations formed cluster reefs. There appear to have been ecological and/or environmental factors that affected the distribution of these sponges; keratose and lithistid sponges rarely occur together in the Mungok reefs, whereas lithistids are pervasive within coeval intermediate-energy microbial reefs elsewhere. These results demonstrate the importance of hydrodynamic controls on overall reef morphology and configurations during the Early Ordovician, and suggest that keratosan–microbial consortia may have been an integral component of the Great Ordovician Biodiversification Event, together with the lithistid sponge–microbial consortium.
... Accretion was by a combination of microbial mat-stabilized silty lime mud plus microbially induced precipitated micrite (Horodyski (1976(Horodyski ( , 1977. The precipitated laminae are more finely crystalline than the micrite in lime mudstone beds (e.g., Knoll and Semikhatov, 1998;Le Ber et al., 2015), and their irregular and discontinuous nature suggests the microbial mat surfaces possessed some topographic relief. Domical stromatolites are present, but those described by White (1984) are folds due to synsedimentary deformation. ...
Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America include the Haig Brook, Tombstone Mountain, Waterton and Altyn formations which crop out in the eastern Rocky Mountains of northwestern Montana, southwestern Alberta and southeastern British Columbia. They record the development on the present-day northeastern side of the Mesoproterozoic Belt Basin of a carbonate platform early in the basin history while it was still relatively deep. The Waterton–Altyn succession documents a shallowing-upward, westward-prograding, broadly ramp-style platform composed of mixed carbonate–siliciclastic sediments. Five main facies types are recognized: Laminite, Ribbon, Grainstone, Oolite and Stromatolite. The first two were formed of lime muds deposited on the ramp and outer platform under low-energy conditions. The Grainstone facies consists of sand-sized peloids, aggregates and intraclasts plus admixed quartz and feldspar sand, microspar grains, radial ooids and silicified oolite and anhydrite. The Oolite facies is dominated by ooids with a concentric cortex. These are allochthonous coarse-grained particles interpreted to have been transported westward to a marginal belt and outer platform mainly by tsunami off-surge from the platform interior, coastal sand shoals and tidal-flat sabkhas, outcrops of which are not preserved. Absence of hummocky or swaley cross-stratification suggests that the platform was not affected by strong storms. Instead, flat-pebble conglomerates on the ramp are ascribed to episodic, tsunami-induced wave action which caused localized rupture and imbrication of flat pebbles. Scouring by tsunami off-surge produced intraclasts in the outer platform. There, this sediment lay undisturbed, but in the shallower marginal belt it was reworked by strong tidal currents which generated variably directed cross-lamination from dune and ripple migration and, locally, large sand bars with northwest-dipping clinoforms. Deformation features caused by synsedimentary earthquakes are common, with the various seismite types reflecting the facies-specific rheology of the sediment. Seismites in the lower Altyn Formation appear not to be linked to individual tsunami-lain dolograinstone beds in the outer platform, suggesting that these two sets of features were not generated by the same faults. The carbonate factory shut down when the platform was suffocated by siliciclastic mud sourced from the west, and tidal activity diminished as the whole basin shallowed. The interpretation of the carbonate platform presented herein is radically different from previous views that considered the Waterton and Altyn formations to be predominantly of shallow-subtidal and tidal-flat origin. These rocks are relevant to the appreciation of other carbonate platforms, especially in that tsunamis may be an under-appreciated agent of erosion and sediment transport offshore.
... Lacustrine and spring microbialites have also been described as early as the Archean, though smaller primary spatial extents limit the number of known locations (Buck, 1980;Buick, 1992;Djokic & others, 2017;Wilmeth & others, 2019). Many researchers have developed biostratigraphic schemes for correlating Precambrian deposits (e.g., clouD & semikhatov, 1969;knoll & semikhatov, 1998;semikhatov & raaBen, 2000) though not without controversy, as discussed later in this chapter. ...
Microbially induced, lithified structures known as microbialites are both geologically and biologically significant, forming extensive sedimentary, geochemical, and microbiological records in modern and ancient environments. Depositional settings range from deep ocean hydrocarbon seeps, hydrothermal vents, and whale falls, to cool water carbonate banks—abundant occurrences within peritidal zones and,
finally, to non-marine environments such as lakes, rivers, and springs. More than three billion years of Earth’s biosphere is primarily
recorded within microbialites, including the oldest macrofossils on the planet. Even with diminished diversity and abundance during the Phanerozoic, periodic microbialite resurgences after mass extinctions are used as indicators for relative environmental recovery. Microbialites are also targeted by astrobiology studies for their ability to form in harsh environments and their capacity to preserve specific biosignatures. Yet, despite the broad scientific significance of microbialites, many authors note a lack of consistent terminology, while others address the challenges of differentiating microbialites from numerous abiogenic sedimentary deposits.
This chapter aims to provide the reader with a basic working guide for field and laboratory descriptions of microbialites, and to synthesize the various terminologies present in the literature. As a guide, this contribution is meant to complement the various review articles that focus more specifically on the fossil record of microbialites.
... This type of deposition has occurred since the Proterozoic and forms reefs and carbonate sediments, including stromatolites, thrombolites, dendrites and oncolites. Many case studies are available in the literature (Golubic 1973(Golubic , 1976Zhang and Hofmann 1982;Hofmann 1987;Zhuravlev and Wood 1996;Feldmann Edited Knoll and Semikhatov 1998;Myrow 2002;Riding 1991Riding , 2000Riding and Liang 2005;Rowland and Shapiro 2002;Kruse and Zhuravlev 2008;Hicks and Rowland 2009;Adachi et al. 2011). The Lower Ordovician marine carbonate deposits in the Yangtze Plate developed continuously, and the associated microbialites and reefs are widely distributed (Zhu et al. 1990(Zhu et al. , 1995(Zhu et al. , 2006Mei and Lu 1991;Li and Wu 2001;Yang 2018;Xiao et al. 1993Xiao et al. , 1995Xiao et al. , 2004Xiao et al. , 2011Adachi et al. 2011;Wang et al. 2012). ...
This study is the first systematic assessment of the Lower Ordovician microbial carbonates in Songzi, Hubei Province, China. This paper divides the microbial carbonates into two types according to growth patterns, namely nongranular and granular. The nongranular types include stromatolites, thrombolites, dendrolites, leiolites and laminites; the granular types are mainly oncolites and may include a small amount of microbiogenic oolite. According to their geometric features, the stromatolites can be divided into four types: stratiform, wavy, columnar and domal. Additionally, dipyramidal columnar stromatolites are identified for the first time and represent a new type of columnar stromatolite. The thrombolites are divided into three types: speckled, reticulated and banded. The grazing gastropod Ecculiomphalus and traces of bioturbation are observed in the speckled and reticulated thrombolites. This paper considers these two kinds of thrombolites to represent bioturbated thrombolites. These findings not only fill gaps in the field of domestic Ordovician bioturbated thrombolites but also provide new information for the study of thrombolites. Based on the analysis of the sedimentary characteristics of microbialites, the depositional environments of the various types of microbialites are described, and the distribution patterns of their depositional environments are summarized. The relationship between the development of microbialites and the evolution and radiation of metazoans during the Early to Middle Ordovician is discussed. Consistent with the correspondence between the stepwise and rapid radiation of metazoans and the abrupt reduction in the number of microbialites between the late Early Ordovician and the early Middle Ordovician, fossils of benthonic grazing gastropods (Ecculiomphalus) were found in the stromatolites and thrombolite of the study area. It is believed that the gradual reduction in microbialites was related to the rapid increase in the abundance of metazoans. Grazers not only grazed on the microorganisms that formed stromatolites, resulting in a continuous reduction in the number of stromatolites, but also disrupted the growth state of the stromatolites, resulting in the formation of unique bioturbated thrombolites in the study area. Hydrocarbon potential analysis shows that the microbialites in the Nanjinguan Formation represent better source rocks than those in the other formations.
... The aforementioned examples show that purely agglutinated microbialites are quite rare during the Phanerozoic (Table 2), but their scarcity is much more remarkable throughout their 3000 Myr-long Precambrian history. Throughout this long period, trapping and binding was an infrequent accretion process (Awramik and Riding, 1988;Sami and James, 1996;Knoll and Semikhatov, 1998;Altermann, 2008;Planavsky and Grey, 2008;Bosak et al., 2013), with few mentions of microbialites that only partially or locally include grains within their microfabrics (Walter, 1972;Horodyski, 1976;Fairchild, 1991 Suarez-Gonzalez, et al. Earth-Science Reviews 194 (2019) 182-215 In summary, the literature review presented here shows that fossil agglutinated microbialites have been continuously rare through Earth history, in comparison with the abundant and diverse spectrum of fossil microbialites. ...
Trapping and binding of allochthonous grains by benthic microbial communities has been considered a fundamental process of microbialite accretion since its discovery in popular shallow-marine modern examples (Bahamas and Shark Bay). However, agglutinated textures are rare in fossil microbialites and, thus, the role of trapping and binding has been debated in the last four decades. Recently, renewed attention on this subject has produced new findings of fossil agglutinated microbialites (those mainly formed by ‘trapping and binding’ and analogous to modern examples), but they are still few and geologically recent (mainly post-Paleozoic) when compared to the 3.5 Gyr long record of microbialites. In order to better understand this discrepancy between modern and fossil examples, an extensive literature review is presented here, providing the first thorough database of agglutinated microbialites, which shows that all of them are formed in shallow-marine environments and most under tidal influence. In addition, a Lower Cretaceous example is described, including very diverse microbialites, each of them formed in a particular paleoenvironment. Some of these microbialites developed in grainy settings, but only those formed in marginal-marine tide-influenced environments accreted mainly by trapping and binding the surrounding grains, being analogous of modern agglutinated microbialites, and matching the environmental pattern observed in the literature database. The combination of the literature review with the case study allows to discuss the factors that control and enhance ‘trapping and binding’: a) occurrence of grains in the microbialite environment; b) frequent currents that mobilize the grains and supply them onto the microbialite surface; c) high concentration and diversity of electrolytes in the water to increase the adhesiveness of the extracellular polymeric substances (EPS) of the microbialite surface; and d) a CaCO 3 saturation state not high enough to promote early and strong carbonate precipitation within EPS, which would eventually decrease its availability to adhere grains. Therefore, this review shows that the keys to solve the ‘trapping and binding’ debate may be environmental, because the conjuction of these hydrodynamic and hydrochemical parameters is preferentially achieved in shallow-marine settings and especially in those influenced by tides, at least since Mesozoic times. This explains the limited environmental and stratigraphic distribution of microbialites mainly formed by ‘trapping and binding’, and opens new ways to look, geologically and microbiologically, at this process, so often cited and yet so rare.
... In some Precambrian coniform stromatolites, sparry layers changing laterally in thickness alternate with finer-grained thinner laminae. The spar layers may represent sea-floor precipitates whereas the darker thinner laminae have been interpreted as lithified microbial mats (Knoll & Semikhatov, 1998;Riding, 2008). The present-day composition of these Precambrian hybrid crusts does not match that of the Afar examples, but recrystallization of randomly oriented aragonite fans and alteration of Mgsilicates would lead to similar fabrics. ...
Pleistocene fibrous aragonite fabrics, including crusts and spherules, occur in the Danakil Depression (Afar, Ethiopia) following the deposition of two distinctive Middle and Late Pleistocene coralgal reef units and pre‐dating the precipitation of evaporites. Crusts on top of the oldest reef units (Marine Isotope Stage 7) cover and fill cavities within a red algal framework. The younger aragonite crusts directly cover coralgal bioherms (Marine Isotope Stage 5) and associated deposits. Their stratigraphic position between marine and evaporitic deposits, and their association to euryhaline molluscs, suggest that the crusts and spherules formed in restricted semi‐enclosed conditions. The availability of hard substrate controls crust formation with crusts more often found on steep palaeo‐slopes, from sea‐level up to at least 80 m depth, while spherules mainly occur associated to mobile substrate. Crusts reach up to 30 cm in thickness and can be microdigitate, columnar (branching and non‐branching) or non‐columnar, with laminated and non‐laminated fabrics. Two different lamination types are found within the crystalline fabrics: (i) isopachous lamination; and (ii) irregular lamination. These two types of lamination can be distinguished by the organization of the aragonite fibres, as well as the lateral continuity of the laminae. Scanning electron microscopy with energy dispersive X‐ray spectroscopy analyses on well‐preserved samples revealed the presence of Mg‐silicate laminae intercalated with fibrous aragonite, as well as Mg‐silicate aggregates closely associated to the fibrous aragonite crusts and spherules. The variety of observed fabrics results from a continuum of abiotic and microbial processes and, thus, reflects the tight interaction between microbially mediated and abiotic mineralization mechanisms. These are the youngest known isopachous laminated, digitate and columnar branching fibrous crusts associated with a transition from marine to evaporitic conditions. Understanding the context of formation of these deposits in Afar can help to better interpret the depositional environment of the widespread Precambrian sea‐floor precipitates. This article is protected by copyright. All rights reserved.
... In marine environments, carbonate encrustations generally precipitate soon after cell death, in association with bacterial decay (Chaftez and Buczynski, 1992;Bosak and Newman, 2003). Carbonate encrusted sheaths are uncommon in rocks older than about 1000 Ma, not because cyanobacteria were absent but because seafloor precipitation of carbonate crystals obliterated potential fossils (Knoll and Semikhatov, 1998). Although carbonate encrustation fossilizes mat-building filaments, it rarely preserves morphological details that might permit fine-scale taxonomic resolution. ...
... At the same time, some other species of these stromatolite genera are present in younger strata (Middle Riphean, Lower Riphean, and, even, Vendian) of some regions, representing elements of stratigraphically significant assemblages (Semikhatov et al., 1991 and references therein). In this regard, Baicalia lacera, which characterizes lower members of the Upper Riphean sections on continents, is the most remarkable taxon (Knoll and Semikhatov, 1998). The sequences with upper Yusmastakh stromatolites are limited by dates ranging from 1420-1400 tõ 560 Ma. ...
The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic‒geochronological study of globular phyllosilicates (GPS) of the glauconite‒illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb‒Sr and K‒Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb‒Sr, 1485 ± 13 Ma; K‒Ar, 1459 ± 20 Ma) and Yusmastakh (Rb‒Sr, 1401 ± 10 Ma; K‒Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.
... Yet, filamentous forms of cyanobacteria must have evolved soon after (possibly during or just after the GOE) since they were prominent components of microbial mats during most of the Proterozoic Eon (2.5-0.54 Gyr; Knoll and Semikhatov 1998). Comparative genome analyses of present-day cyanobacteria tend to confirm hypotheses about their terrestrial origin. ...
Although numerous marine microorganisms can exploit solar energy for photosynthesis or photoheterotrophy, cyanobacteria and microalgae are the only ones able to perform oxygenic photosynthesis and to produce organic carbon, an essential brick of life that sustains the whole marine trophic web. Here we review recent advances in the investigation of marine oxygenic microorganisms, with a special focus on cyanobacteria. We discuss novel insights into the ecology, evolution and diversity of Synechococcus and Prochlorococcus, the two most abundant and certainly the best known oxyphototrophs at all scales of organization from the gene to the global ocean. A particular emphasis is also made on diazotrophic cyanobacteria, which constitute an important source of bioavailable nitrogen to oceanic surface waters, possibly the most important external nitrogen source, before atmospheric and riverine inputs. Diazotrophic cyanobacteria are polyphyletic and display a remarkably large range of physiologies and morphologies. These include both multicellular cyanobacteria, such as the colonial Trichodesmium or the heterocyst-forming Calothrix, Richelia and Nodularia, and unicellular cyanobacteria belonging to three major groups: the symbiotic Candidatus Atelocyanobacterium thalassa (UCYN-A), the free-living Crocosphaera sp. (UCYN-B) and the UCYN-C cluster that notably encompasses Cyanothece. Whereas some of these species can form immense blooms (Nodularia, Trichodesmium), others can also have a major ecological impact even though they represent only a minor fraction of the bacterioplankton (UCYN-C). After about one billion years of evolution, which led them to colonize any single marine niche reached by solar light, cyanobacteria appear as truly fascinating organisms that constitute a major component of the marine microbial communities and are the matter of an ebullient research area. The considerable amount of omics information recently becoming available on both isolates and natural populations of marine oxyphototrophs provide a solid basis for investigating their molecular ecology, their contribution to biogeochemical cycles, as well as their possible utilization in biotechnology, data mining, or biomimetics.
... Calcified algal-bacterial communities and stromatolites mainly occur in hypersaline environments with high carbonates concentration where differentiation between biogenic and abiogenic types of carbonate sedimentation is difficult (Merz-Preiß, 2000). Calcareous microfossils were scarce during cyanobacteria dominance in Precambrian (Komar, 1979;Knoll et al., 1993;Knoll & Semikhatov, 1998). However, calcareous cyanobacteria and eukaryotic algae became abundant near Precambrian-Cambrian boundary as well as in Palaeozoic and Mesozoic (Riding, 1991). ...
A monographic account is presented on the fossil Proterozoic cyanobacteria. It chronicles the 60 years of history of investigations on the Precambrian microfossils. The researches on Precambrian microfossils have revealed a new, earlier unknown, world of oldest microorganisms and divulged the steps in life’s evolution on the earth. Documented records show that cyanobacteria occupied all available ecological niches of the Precambrian biosphere and filamentous and coccoidal cyanobacteria were the dominant microbial community. Extinct fossilized cyanobacteria in diagenetic cherts of the Precambrian are comparable in morphology and behavior with extant forms. These oxygenic phototrophic microorganisms were masters for at least first 3.0-3.5 billion years of the Earth history and almost did not change for billion years. The unprecedented evolutionary conservatism of the cyanobacteria is established so much so that modern systematics of cyanobacteria can be applied on Proterozoic forms at least, up to the family level. More than half a century of research on Precambrian microfossils demands refinement in taxonomy and allows differentiation between products of taphonomy and primarily biological features of fossilized cyanobacteria as well as those features formed as a result of postmortem degradation and subsequent diagenetic alternations. The paper embodies all cyanobacterial taxa broadly accepted by most of the researchers and provides complete revision of all Precambrian fossil cyanobacterial remains. It presents a comprehensive information on the taxonomy of cyanobacterial and related microorganisms along with emendations with due considerations of possible processes of post-mortem alterations. Detailed analysis of fossil cyanobacteria populations has revealed 50 genera and 92 species as truly acceptable forms. Of this, more than 10 genera and 18 species are recognized as problematic cyanobacterial taxa that could be alternatively interpreted as Protista. The present review contains diagnosis and descriptions of genera as well as type and some other very important species. The information on other species (size, type specimen, distribution) is given in the table format along with the described genera. All valid taxa described from the Proterozoic microbiotas are incorporated in this work. Problematic remains of Archaean (?) cyanobacteria are not included because of their uncertain and disputable biogenic origin. The relevant data of molecular biology and other methods applied in systematics of modern cyanobacteria are discussed in the paper. Besides, main taxonomic part and relevant discussion on the morphology of microfossils the palaeobiology, palaeoecology and geological history of cyanobacteria are also provided. The present paper contains following taxa: Family- CHROOCOCCACEAE: Brachypleganon, Coniunctiophycus, Corymbococcus, Eoaphanocapsa, Eogloeocapsa, Eosynechococcus, Gloeodiniopsis, Gloeotheceopsis, Gyalosphaera, Sphaerophycus, Tetraphycus; Family- ENTOPHYSALIDACEAE: Coccostratus, Eoentophysalis; Family- DERMOCARPACEAE: Polybessurus; Family- HYELLACEAE: Eohyella; Family- PLEUROCAPSACEAE: Palaeopleurocapsa, Scissilisphaera; Family- XENOCOCCACEAE: Synodophycus; Family- OSCILLATORIACEAE: Calyptothrix, Cephalophytarion, Cyanonema, Eomicrocoleus, Eoschizothrix, Filiconstrictosus, Heliconema, Obruchevella, Oscillatoriopsis, Palaeolyngbya, Partitiofilum, Siphonophycus, Uluksanella; Family- NOSTOCACEAE: Eosphaeronostoc, Veteronostocale; Family- SCYTONEMATACEAE: Circumvaginalis, Ramivaginalis; Order- NOSTOCALES OR STIGONEMATALES: Archaeoellipsoides, Orculiphycus, INSERTAE SEDIS: Animikiea, Chlorogloeaopsis, Chuaria, Clonophycus, Glenobotrydion, Gunflintia, Huroniospora, Leiosphaeridia, Leptoteichos, Myxococcoides, Phanerosphaerops, Polysphaeroides, Polytrichoides.
... These age determinations are approximate, however, because the studied fractions contain non-cogenetic illite generations (Gorokhov et al., 1994. In this work, the age of 870 Ma consistent with data on stromatolites from a lower part of the Atar Group (Bertrand-Sarfati, 1972;Knoll and Semikhatov, 1998) is conventionally adopted for Member I 6 . Using this value in calculations, we obtain the 87 Sr/ 86 Sr initial ratio of 0.70558 to characterize limestones of Member I 6 . ...
... Whereas stromatolite microfabrics are generally believed to refl ect a combination of microbial community growth, decomposition and lithifi cation (Golubic, 1976;Bertrand-Sarfati, 1983;Turner et al., 1993;Knoll & Sergeev, 1995;Kah & Knoll, 1996;Knoll & Semikhatov, 1998;Grotzinger & Knoll, 1999;Lee & Golubic, 2000;Reid et al., 2000), stromatolite morphology appears to be affected more strongly by physical depositional factors, particularly water depth, wave energy and sediment infl ux (Cloud & Semikhatov, 1969;Semikhatov et al., 1979;Semikhatov & Raaben, 1994Andres & Reid, 2006). As a result, stromatolitic laminae, which record both microscale and macroscale growth processes, are arguably the most fundamental aspect of stromatolite morphology (Walter, 1992). ...
... These age determinations are approximate, however, because the studied fractions contain non-cogenetic illite generations (Gorokhov et al., 1994. In this work, the age of 870 Ma consistent with data on stromatolites from a lower part of the Atar Group (Bertrand-Sarfati, 1972;Knoll and Semikhatov, 1998) is conventionally adopted for Member I 6 . Using this value in calculations, we obtain the 87 Sr/ 86 Sr initial ratio of 0.70558 to characterize limestones of Member I 6 . ...
The presented Rb-Sr systematics of carbonates from the Karatau Group, the Upper Riphean stratotype of southern Urals, elucidates important details of secular variations of 87Sr/86Sr ratios in the Late Riphean seawater, which have been formerly unknown. Samples selected for analysis satisfy all the strict geochemical criteria characterizing the least altered carbonate rocks (Mn/Sr ≤ 0.2, Fe/Sr ≤ 5.0, Mg/Ca ≤ 0.024 in limestones and Mn/Sr ≤ 1.2, Fe/Sr ≤ 3.0, Mg/Ca ≥ 0.608 in dolostones), and they all have been preliminary treated in IN solution of ammonium acetate for a partial removal of epigenetic carbonate phases. A verified curve of secular variations of 87Sr/86Sr ratio in the Late Riphean ocean is plotted based on new data and Sr isotope parameters formerly known for carbonates of the Upper Riphean key sections. As is established, that ratio was nearly constant, ranging from 0.70519 to 0.70566, within the time span of 1030-810 Ma and next rose up to 0.70611 about 775 Ma ago. Afterward, between 765 and 740 Ma, it decreased down to 0.70561-0.70575 and then, within the time span of 740-690 Ma, it ranged from 0.70646 to 0.70686 with a short-term drop down to 0.70620 about 720 Ma ago. At the end of the Late Riphean (660-640 Ma), the ratio lowered to 0.70538-0.70580 to become growing up to 0.70840-0.70860 during the Vendian and initial Cambrian. The established variations of the 87Sr/86Sr ratio in the Late Riphean ocean have been controlled by a combination of geodynamic factors, magmatic events, sea-level oscillations, and compositional changes in provenances, which were subjected to erosion at that time. An additional influence of climatic fluctuations over supercontinent and its fragments is also admissible.
Microbialites are microbial sedimentary structures that constitute some of the oldest traces of life on Earth. By their deposition in a wide range of sedimentary environments and their presence throughout most of geological time, the sedimentological and geochemical signatures they preserve represent important paleoenvironmental archives for understanding Earth’s biological and geochemical co-evolution. Here we present a large microbialite collection containing more than 1370 curated specimens, covering all continents except Antarctica and spanning more than 3.5 Ga of Earth history, that is accessible to the international scientific community for examination and sampling at the Muséum National d’Histoire Naturelle (MNHN) in Paris, France. After cataloguing and evaluating the samples for their lithology, biogenicity, and inferred depositional environments, we characterized the collection for selected geochemical parameters, notably carbonate stable carbon and oxygen isotope ratios, as well as major, trace, and rare earth element compositions. Finally, we explore the different geochemical proxies analyzed with regards to their utility for reconstructing evolving Earth surface environments and/or microbial metabolisms via comparison of geochemical data from the MNHN Microbialite Collection to a compilation of similar proxy data for carbonates worldwide. We demonstrate that certain temporal trends previously recognized in carbonates worldwide (e.g., with respect to variations in C and O stable isotope compositions and redox sensitive trace element enrichments) are well reflected in this collection. Our findings highlight the utility of the MNHN Microbialite Collection and microbialites more generally for reconstructing the conditions associated with habitable environments in deep time and for tracing the response of microbial communities to the geochemical evolution of Earth’s surface.
This chapter reviews and summarizes the early ideas on the existence of Mesoproterozoic components in the Arabian-Nubian Shield (ANS). It sheds more light on their significance in crustal evolution of the ANS. Nowadays, there is a general agreement among geologists that the Late Mesoproterozoic to Cambrian period bears the most remarkable geological record in Earth’s history. It began with the assembly of Rodinia supercontinent (1300–900 Ma) and ended by the amalgamation of Gondwana supercontinent (around 550 Ma). It is widely accepted that the ANS were formed by juvenile magmatic arc accretion and subsequent shield-wide post-tectonic magmatism. However, the recorded Mesoproterozoic crust, with subordinate Palaeoproterozoic and rare Archaean components, proves that ANS is less juvenile than previously thought, and may provide important clues for the reconstruction of the crustal evolution of the ANS. Recent studies by several workers have shown that inherited zircon grains from igneous rocks within the ANS may provide evidence for Mesoproterozoic crustal growth. The Mesoproterozoic components of the ANS were interpreted as pervasively reworked pre-existing crust during the Pan-African orogeny, which was responsible for the final juxtaposition of the collage of terranes now seen, e.g. reworked pre-Neoproterozoic crustal fragments at Sa’al Mesoproterozoic rocks, Wadi Rutig volcano-sedimentary succession and Wadi Solaf metapsammitic and granodioritic biotite gneisses in Sinai (Egypt); Khida subterrane in eastern Saudi Arabia; the arc–gneiss collages of the Precambrian basement of Yemen and the eastern Ethiopian–northwestern Somalian crustal block. Sediments are good natural samplers of the existing continental crust at the time of erosion and sedimentation. Several workers have presented detrital zircon isotopic dataset from the Palaeozoic sedimentary sequences of the ANS (e.g. Meinhold et al. 2020). They interpreted the Ediacaran to middle Tonian zircon grains as being derived from igneous rocks of the ANS, whereas Palaeoproterozoic and Archaean grains were interpreted as a primary derivation from Palaeoproterozoic and Archaean basement found in some parts of the ANS or crustal components of the eastern Saharan Metacraton. This is consistent with palaeocurrent indicators, which suggest sediment transport from the hinterland of Gondwana. In the light of the above, future efforts to understand ANS crustal evolution will require more zircon geochronology and related studies to answer two main questions: (1) how isotopically juvenile Neoproterozoic magmas were contaminated with pre-Neoproterozoic zircon? and (2) was zircon assimilated from unexposed older crust that underlies the contaminated shield or derived from Neoproterozoic sediments sourced from a neighbouring pre-Neoproterozoic craton?
Abstract
A couplet comprising a Boxonia-bearing stromatolite unit and phosphatic layers occurs widely at the base of the Zuun-Arts Formation in the Zavkhan Terrane of Gobi-Altay Province, western Mongolia. The stromatolite unit is late Ediacaran in age and forms bioherms of several kilometres in lateral extent. The stromatolites consist of two parts: the lower columnar Boxonia stromatolites (ca. 7 m thick) change abruptly into the upper domed stromatolites (ca. 4.5 m thick) in the Bayan Gol Gorge. The columnar stromatolites are made up of columnar structures (2–6 cm in diameter), formed by the accumulation of upward-convex laminae, and partly protrude laterally to form bridges between neighbouring columns. In contrast, the domed stromatolites (30–60 cm in width) are composed of accumulations of low convex laminae. Both types of stromatolite are characterized by alternating darker and lighter laminae, which consist of peloids, two types of micritic clot, homogeneous lime mud, and spar-filled fenestral fabrics. These stromatolites are inferred to have been deposited in a subtidal setting below the wave base, with no evidence of sediment displacement by strong currents. The stratigraphic transition from columnar to domed growth forms reflects deepening of the basin. The change in column diameter observed within the columnar stromatolites might reflect fluctuations in microbial activity. The appearance of inter-columnar bridges in the columnar stromatolites might indicate more active colonization of microbial mats, which expanded into the inter-columnar areas. Neither type of stromatolite includes microbial remains, such as filamentous or coccoidal cells. However, the various micritic components of the stromatolites (clots, peloids, and homogeneous lime mud) originated from in situ precipitation through microbial activity, and reflected differences in the timing and intensity of microbial calcification and degradation. These stromatolites, including the variety of micritic components, may provide clues to the variations in microbial metabolic activity and degradation processes that were associated with stromatolite formation.
Stromatolites are abundant in shallow marine sediments deposited before the evolution of animals, but in the modern ocean they are restricted to locations where the activity of animals is limited. Overall decline in the abundance of stromatolites has, therefore, been attributed to the evolution of substrate-modifying metazoans, with Phanerozoic stromatolite resurgences attributed to the aftermaths of mass extinctions. Here we use a comprehensive stratigraphic database, the published literature, and a machine reading system to show that the rock record–normalized occurrence of stromatolites in marine environments in North America exhibits three phases: an initial Paleoproterozoic (ca. 2500 Ma) increase, a sustained interval of dominance during the Proterozoic (2500–800 Ma), and a late Neoproterozoic (700–541 Ma) decline to lower mean prevalence during the Phanerozoic (541–0 Ma). Stromatolites continued to exhibit large changes in prevalence after the evolution of metazoans, and they transiently achieved Proterozoic-like prevalence during the Paleozoic. The aftermaths of major mass extinctions are not well correlated with stromatolite resurgence. Instead, stromatolite occurrence is well predicted by the prevalence of dolomite, a shift in carbonate mineralogy that is sensitive to changes in water-column and pore-water chemistry occurring during continent-scale marine transgressive-regressive cycles.
Structures and textures of the peloidal wackestones, as well as size, shape, and composition of peloidal grains, from the Mesoproterozoic (Middle Riphean) Sukhaya Tunguska carbonate platform in the Turukhansk Uplift (Siberia) are considered. It is shown that these grains formed in the course of diagenesis were closely associated with the microsparitic replacement and the formation of molar tooth (MT) structures. Diagenetic transformations of rocks were related to the activity of anaerobic microbial communities inside the buried carbonate silt layers. The microbial activity during diagenesis was governed by the carbonate sediment composition and conservation mechanism of the high-molecular organic matter of primary producers therein, since this organic matter was the nutritious substrate for the primary anaerobe communities.
Thrombolites are a common component of carbonate buildups throughout the Phanerozoic that are usually described as microbialites with an internally clotted texture. A wide range of thrombolite textures have been observed and attributed to diverse processes, leading to difficulty interpreting thrombolites as a group. Interpreting thrombolitic textures in terms of ancient microbial and metazoan ecosystems requires understanding diverse processes, specifically those due to microbial growth and metazoan activity. Many of these processes are reflected in thrombolites in the Cambrian Carrara, Bonanza King, Highland Peak and Nopah formations, Great Basin, California, USA; they comprise eight thrombolite classes based on variable arrangements and combinations of depositional and diagenetic components. Four thrombolite classes (hemispherical microdigitate, bushy, coalescent columnar and massive fenestrated) contain distinct mesoscale microbial growth structures that can be distinguished from surrounding detrital sediments and diagenetic features. By contrast, mottled thrombolites have mesostructures that dominantly reflect post-depositional processes, including bioturbation. Mottled thrombolites are not bioturbated stromatolites, but rather formed from disruption of an originally clotted growth structure. Three thrombolite classes (arborescent digitate, amoeboid and massive) contain more cryptic textures. All eight of the thrombolite classes in this study formed in similar Cambrian depositional environments (marine passive margin). Overall, this suite of thrombolites demonstrates that thrombolites are diverse, in both internal fabrics and origin, and that clotted and patchy microbialite fabrics form from a range of processes. The diversity of textures and their origins demonstrate that thrombolites should not be used to interpret a particular ecological, evolutionary or environmental shift without first identifying the microbial growth structure and distinguishing it from other depositional, post-depositional and diagenetic components. Furthermore, thrombolites, by definition, are fundamentally different from stromatolites and dendrolites in which the laminae and dendroids reflect a primary growth structure, because clotted textures in thrombolites do not always reflect a primary microbial growth structure. This article is protected by copyright. All rights reserved.
A multidisciplinary approach was used to study the Lower Riphean deposits in the Siberian hypostratotype of the Riphean in the Uchur region of the southeastern Siberian platform and to clarify their relationships with the Lower Proterozoic (Upper Karelian) rocks. As is shown, the regional section of the Lower Riphean begins with the Uyan Group divisible into the Birindya, Konkula, and Adargai formations, the latter distinguished for the first time. An important unconformity is established below basal beds of the Uyan Group overlying the deeply eroded surface of weathered acid volcanics of the Elgete Formation of the Upper Karelian Ulkan Group, which are intruded by granites having the U-Pb isotopic ages of 1676-1721 Ma. The Uchur Group including the Gonam, Omakhta, and Ennin fort-nations terminates the Lower Riphean in the Uchur-Maya region and overlies deposits of the Uyan Group with a stratigraphic hiatus and azimuthal discordance. The basal conglomerate of the Gonam Formation includes pebbles of stromatolitic dolomites derived from the underlying Adargai Formation. This geological boundary separates the Ulkan and Uyan rock complexes of contrast lithological composition. The boundary is geologically distinct and close in isotopic age to the lower boundary of the Riphean that is accepted to be 1650 50 Ma old in the Precambrian stratigraphic scale of Russia.
Early Neoproterozoic reefs of the Little Dal Group, Northwest Territories,are built by stromatolites and thrombolites containing calcified filamentous cyanobacteria and interstitial cement. Micritic and microcrystalline carbonate grew in or on extracellular cyanobacterial sheaths, preserving filaments when mineralization was early relative to sheath degradation, or grumeaux when mineralization was later. Filamentous microstructure is volumetrically predominant in the reefs; less common are micritic and grumelous microstructures already known from late Proterozoic stromatolites and Phanerozoic thrombolites. Textural intergradation of filamentous-calcimicrobial microstructure with these non-filamentous microstructures reflects microstructural variation developed through differential preservation at the scale of individual filaments and laminae. Textural gradients from filaments to grumeaux, and from calcimicrobial to stromatolitic and thrombolitic microstructure types, imply that a wide variety of microbialite microstructure types can be derived from a single progenitor community. This suggests that taphonomic variables may be as important in the development of microbialite microstructure as the biology of the microbial mat community. It also challenges recent suggestions that the Neoproterozoic increase in thromboids was related to the rise of multicellular organisms. These conclusions have broad implications for the interpretation of fossil microbialites, many of which might have been more closely related in, origin than hitherto suspected.
Stromatolites from the Mesoproterozoic Changcheng System dolomite in the Jiashan area of Xingcheng are reported for the first time. The stromatolite bed had previously been attributed to the top of the Changzhougou Formation. From the bottom up, the stromatolites occur in the section as Stratifera sp., Cryptozoon sp., Eucapsiphora sp. . Both this stromatolite assemblage and those from the Tuanshanzi Formation in adjacent area of Huludao belong to the type of stromatolite zones in the Tuanshanzi Formation in Yanshan and Taihang Ranges. The presence of the stromatolites in the section provides important biostratigraphic evidences to conclude that the beds belong to the Tuanshanzi Formation. According to the lithostratigraphic characteristics, and the relationship between the vertical distribution rule of stromatolites' morphology and the paleoenvironment, the sedimentary environment history of the Tuanshanzi Formation in Xingcheng area should be fromregression (shallows to intertidal zone and supratidal zone) to transgression (intertidal zone and supratidal zone to intertidal zone and subtidal zone).
The authors have dealt with a sequence of rhythmic deposition consisting alternatively of algal mats, algal balls, algal filaments, algal pellets and micrites in the strata of the Mesoproterozoic Yanshan aulacogen in Pingquan County, Hebei Province. Based on correlation and analysis on features and structural types of a series of specimens (also thin section), the present authors suggest that the rhythmic deposition represent a sub-abysmal algal sedimental system of storm flow turbidite below storm wave base. The algal sedimentary succession consists of those formed at shallow-sea and at bathyal part. Their different features could serve as a reliable basis for recognizing their sedimentary environments. The present paper presents a new view on the existing depth limit of algal mats.
Precambrian stromatolites are unique objects in Earth history. The predominance of microbes in ecosystems that they document, their specific global environments, and the scale of their evolution have no counterparts in the Phanerozoic. Among several, basically different stromatolite classifications known in the literature, the current version of the traditional system is most extensively employed in the study of Precambrian buildups. It is artificial in nature but follows conventional rules of paleontological classification and requires definition of a hierarchy of taxa: forms (form-species), groups (form-genera), and types. At present, there is a common understanding of stromatolite characteristics, providing the most efficient basis for definition and identification of the traditional system taxa. The types are based on the most general features of the buildup’s morphology. Groups are based on particular combinations of morphological characteristics defined by the mode of accretion and shape of stromatolite laminae (plus some general features of the microstructure in several cases). Forms are predominantly or exclusively based on microstructure. The stratigraphic potential of Precambrian stromatolites, revealed by empirical time-and-space distribution data of the distinctive assemblages, is evident. Stromatolites are not suitable for the subdivision of the Proterozoic, but provide paleontological characterization of units which have been defined by other methods and significantly contribute to their correlation especially within the limits of particular stromatolite provinces. Interprovincial stromatolite-based correlations are of lower reliability and time-resolution due to strong variations in the taxonomic composition of coeval stromatolite assemblages across provincial boundaries. Precambrian stromatolites demonstrate distinctive directional secular changes in taxonomic composition and diversity which were defined by the evolution of both global environmental and biological factors relevant to the construction and habitat restrictions of these biolites.
The carbonate Malgina Formation, a constituent of the Riphean hypostratotype in Siberia, corresponds, according to paleontological evidence, to the terminal Middle Riphean horizons, and present-day geochronological data indicate that it is younger than 1300 ± 5 Ma and older than 1025 ± 40 Ma. Eight limestone samples (Mg/Ca < 0.066) selected for the Pb-Pb age determination contain less than 5.8% of silicate components and are insignificantly altered, as it is evident from geochemical criteria (Mn/Sr ≤ 0.2, Fe/Sr ≤ 5, Rb/Sr ≤ 0.001) and minimum difference between 87Sr/86Sr ratios (≤0.0007) measured in the primary and secondary carbonate phases. Secondary carbonates were removed using the step leaching of samples in HBr of variable concentration. The Pb-Pb age of 1043 ± 14 Ma (MSWD = 7.8) that is calculated for 18 data points characterizing various carbonate phases appears to be best approximating the time of early diagenesis in the Malgina limestones. With due account for stratigraphic position of studied samples, this and other Pb-Pb dates obtained for the Middle-Upper Riphean carbonate formations of the Urals and Siberia are concordant with ages measured by other methods and form altogether a regular series of five successive values. The Pb-Pb isochron dates of stratigraphic significance can be obtained only after a strict procedure of sample selection followed by the removal of secondary carbonate phases with the help of step-leaching method.
The evolution of opinions about the isotope age of the general stratigraphic subdivisions of the Upper Proterozoic in Russia (Lower, Middle, Upper Riphean and Vendian) during the second half of the 20th century and the beginning of the 21st century is considered, and the current data on the age of these subdivisions are analyzed in detail. It is shown that the isotope age of the lower boundaries of the Lower, Middle, and Upper Riphean and Vendian should be estimated at 1750 ± 50, 1400, 1030, and 640 ± 5 Ma, respectively. The role of the historical–geological criteria in the substantiation of these boundaries is studied.
Silicided microfossils from the Middle Riphean Svetlyi Formation of the Riphean hypostratotype in the Uchur-Maya region of southeastern Siberia are described for the first time. Cherts are discovered to enclose only simple filamentous and coccoidal microorganisms: hollow sheaths of four hormogonian cyanobacteria species of the Siphonophycus genus (S. robustum, S. typicum, S. kestron, and S. solidum), filaments of the oscillatorian cyanobacteria Palaeolyngbya catenata, and spheroidal microfossils Myxococcoides sp. of the unclear taxonomic affinity. The Svetlyi microbiota differs from most assemblages of Middle-Late Riphean (Mesoproterozoic) age by the lack of ellipsoidal representatives of the Archaeoellipsoides genus. The low taxonomic diversity of silicified microfossils from the Svetlyi Formation and their organic-walled analogues from the underlying Lower Riphean Omakhta Formation suggest that the successive microfossil assemblages in the section of the Uchur-Maya Region cannot represent a standard for the entire Riphean succession. Description of microfossils from cherts of the Svetlyi Formation is presented.
δ13C and δ18O values of calcite and dolomite have been determined for 229 sedimentary carbonate rock samples from Paleoproterozoic supracrustal belts of the Fennoscandian Shield. In addition, δ13C values of organic carbon have been analysed for 28 black shale samples collected from formations directly connected to selected sedimentary carbonate units. It is suggested that the positive carbon isotope shift observed in sedimentary carbonates of the Fennoscandian Shield was related to an increased relative rate of burial of organic matter on a global basis. The shift was possibly enhanced by local burial of isotopically unusually light organic matter. As high burial rates of organic carbon are probably followed by a large flux of free oxygen, the positive carbon isotope shift may be fundamentally connected with the significant rise in atmospheric oxygen levels at about 2.0 Ga. -from Author
New data for the isotopic composition of carbon in carbonate sediments
deposited between 2.6 and 1.6 Ga indicate that the value of
δ13C in these sediments underwent a very large positive
excursion between 2.22 and 2.06 Ga. A reassessment of the earlier
δ13C data for carbonate sediments shows that this
excursion was probably worldwide, and that it was preceded and followed
by several hundred million years during which the δ13C
of carbonate sediments differed little from that of modern carbonates.
The large δ13C excursion between 2.22 and 2.06 Ga was
probably related to an abnormally high rate of organic carbon
deposition, which generated an abnormally high rate of O2
production. We estimate that the total excess O2 produced
during the excursion was between 12 and 22 times the present atmospheric
O2 inventory. The δ13C data therefore
suggest that the O2 content of the atmosphere increased very
significantly between 2.22 and 2.06 Ga. This inference is supported
strongly by several other lines of evidence.
Basinal facies in the Neoproterozoic Little Dal Group contain large
reefs up to 300 m in height and 8 km in diameter. In contrast to the
stromatolitic composition typical of earlier Proterozoic buildups,
Little Dal reefs consist mainly of three different calcareous
microfossils. A tubule-thread microfossil, analogous to the calcimicrobe
Girvanella in Paleozoic reefs, formed
laminar-reticulate structures, like those in Ordovician bioherms. A
clotted to saccate microfossil, comparable morphologically to Paleozoic
Renalcis , constructed botryoidal to encrusting
masses. An enigmatic component of multicellular appearance that has
attributes of an early calcified metaphyte exhibits the encrusting
sheetlike habit characteristic of middle to late Paleozoic algae. These
reefs represent an intermediate stage between Proterozoic stromatolitic
buildups and Paleozoic metazoan-calcimicrobial reefs and biogenic
mounds.
Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (∼0.711). Consequently, the non-radiogenic value at ∼900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.
A brief review of the geochemistry of U, Th, and Pb in oceanic water and marine carbonates is given, and the U-Th-Pb systematics in limestones and dolomites of the Sukhaya Tunguska Formation in the Turukhansk Uplift is studied. The analysis of phases with different solubilities in ammonium acetate and hydrochloric acid and the plotting of available results along the {sup 207}Pb/{sup 204}Pb-{sup 206}Pb/{sup 204}Pb coordinates yield an age of 1035{plus_minus}60 Ma. This value matches the stratigraphic position of the Sukhaya Tunguska Formation rocks at the Middle-Late Riphean boundary and may be interpreted as the age of an early diagenesis of sediments. Evidence for the temporal compatibility of the processes of sedimentation, dolomitization, and early diagenesis is provided. Isotope-geochemical data, which indicate a recent uranium loss by carbonate rocks, are in good agreement with results of the study of Cenozoic evolution in the Turukhansk Uplift. The average {sup 238}U/{sup 204}Pb value (8.22) calculated for the source of the Sukhaya Tunguska carbonate deposits suggests that Pb from mantle sources or a relatively young crust was abundant in the near-contact fluid phase that governed the composition of carbonate rocks 1035{plus_minus}60 Ma ago.
Stromatolites are described from the Upper Eleonore Bay Group in Canning Land, central East Greenland. Comparison with Spitzbergen forms and microstructural analysis suggest a Vendian age. The following new taxa are described: Poludia boreica, P. tyrrellina, Eleonora ramosa and Inzeria groenlandica.
The approximately 3400-million-year history of the Archaean and Proterozoic Eons is rich in the fossil remains of organosedimentary structures called stromatolites, built primarily by cyanobacteria. Stromatolites first appear 3500 Ma ago and argue well for the presence and therefore great antiquity of cyanobacteria. The presence of cyanobacteria in such ancient rocks indicates that the evolution of the major prokaryotic phyla had occurred by Early Archaean time.
Although rare in the Archaean and first 300 million years of the Proterozoic, stromatolites undergo diversification and increase in abundance in the late Early Proterozoic due, in large part, to the oxygenation of the atmosphere-hydrosphere system, permitting cyanobacteria to disperse, colonize, and thrive in shallow continental shelf-like environments produced during earlier and contemporaneous periods ofcratonization. A second diversification occurred in the Early to Middle Riphean (approximately 1500 to 1200 Ma ago), and might in some way be due to the appearance of eukaryotes. A sharp drop in stromatolite diversity occurs during the Vendian (675 to 570 Ma ago) and is probably due to the activity of metazoans. Two diversity plateaus occur, one separates the two diversifications and the other occurs after the last diversification and before the Vendian decline.
Stromatolites are the products of the complex interactions of microbial, sedimentary, and environmental factors. While stromatolites are not well understood from a biogeological per-spective, they do provide valuable evidence for ancient life, they are useful for biostratigraphy and palaeoecology, and their distributional and diversity patterns provide insight into the first 3 billion years of the history of life on this planet.
This chapter describes the approaches to the classification of stromatolites. The necessity of classifying and naming stromatolites is imposed by the possibility of using them in practical work; the same constructions can and must have the same names. Stromatolites are classified formally and named in the Linnean fashion, in conformity with all the rules of nomenclatural codes. Most convenient is the International Code of Botanical Nomenclature in its paleobotanical part. The morphological variety of stromatolites within one bioherm has definite limits. It consists of regular, persistent associations of definite constructions. These associations (bioherm series) are stable and can be recognized for the majority of the Riphean key forms of stromatolites in various regions of the U.S.S.R. Finally, the use of Linnean nomenclature is meaningful only when the strictest rules are observed to maintain its stability. This is a problem of utmost importance in the present state of stromatolite studies.
Precambrian stromatolites are widely distributed both in Peninsular and Extra-Peninsular India (Valdiya, 1969; Schnitzer, 1971). Although these bio-sedimentary structures have been used for stratigraphic correlation of various Precambrian sedimentary basins in India, their petrographic and chemical features have been hardly touched.
Much is now known about the compositions of modern stromatolite-building communities (see review by Golubic 1976). Such stromatolites, although from widely separated areas, are built by a limited number of micro-organisms. These often possess special adaptations which allow them to survive under the rather special conditions in which they live. Many forms inhabiting intertidal settings for example produce gel envelopes which enables them to resist desiccation, while other forms exhibit motility which helps prevent burial by sediment (Golubic and Campbell 1979).
In the Atar Group of Mauritania (about 900 Ma), a shift from positive (+0.3 to +2.8 permil) to variably negative (-0.2 to -3.3 permil) carbon isotope values of carbonate carbon occurs between Formation I-5 with its Conophyton-Jacutophyton biostromes and Formation I-6 in which the stromatolites display tussocky microstructure. No biologically-related 13C fractionations can be recognized by comparison of stromatolitic micrites with marine cements so in principle stromatolites seem suitable for studies of secular change in δ13C. Diagenesis of the initially metastable marine Proterozoic precipitates poses the main problem for such studies. A suggested strategy for isotope stratigraphy is to seek out deliberately the diagenetic patterns of alteration and to correct for them. -from Authors
The alternation of two major climatic types, the glacial and nonglacial, in the Phanerozoic history of the Earth is well-documented at present. These climates affected the state of the biosphere to such an extent that cool (glacial) and warm (nonglacial) biospheres can be distinguished. The main features of the cool biosphere can be determined from its state at present and during the Pleistocene. The cool biosphere is characterized by features such as the following: permanent glacial or ice polar caps, occasional glacial covers or permafrost occurrences in the temperate labtudes, an oceanic psychrosphere, low temperature and intense circulation in the atmosphere and hydrosphere, low oceanic level, high rates of erosion and sedimentation, a low concentration of atmospheric carbon doxide, contrasting climate and landscape zonation, clearly pronounced biogeographic and ecological differentiation, rapid fluctuations of the above features, frequent biotic crises, and so on. Evidently, the warm biosphere exhibited different characteristics, perhaps even opposite to those of the cold biosphere. However, our understanding of the properties of the warm biosphere is far from being clear and complete, even though this type of biosphere has sharply prevailed in geological history. Defining these properties is a principal goal of historical geological and ecological studies . When solved, this problem will be of prognostic and general methodic importance for the earth sciences, allowing the applicability of the actualistic method to be specified.
This chapter discusses the effects of the physical, chemical and biological evolution of the earth. This chapter emphasizes on the application of recent data, the most important of these qualifying conditions being: (1) the importance of bacteria in the Early Precambrian stromatolites; (2) the importance of both environmental and evolutionary factors in explaining the change in stromatolite structures; (3) the effects of the absence of metaphytes and metazoans on the distribution and diversity of stromatolites in the early and middle Precambrian; (4) the effects of the development of metaphytes and metazoans on the Phanerozoic distribution and diversity of stromatolites; and (5) the importance of the development of calcareous red algae in modifying the role of blue–green algae in Mesozoic and Cenozoic reefs. It is possible to apply the data on recent stromatolites to the solution of many of the remaining problems in the study of stromatolites.
The focus of this work was to resolve the age of the Ulkan volcanoplutonic complex by applying U-Pb technique utilizing. These rock composites were uncovered within the Ulkan Graben at the southeast end of the Aldan shield on the South Aldan deep fault. Volcanics from Ulkan graben was subdivided into four groups of rock formations: the Toporikan, Ulkachan, Elgetey and Birindinsk. Based on the analysis of data, the volcanics of Elgetey formation and granitoids were of the same age confirming that they are comagmatic. In addition, corresponding age of alkalic granite from North Uchur pluton and granosyenite from the South Uchur pluton implied the igneous rocks of both plutons originated from a single intrusive complex. Moreover, the Ulkan igneous rocks were dated between 1727 ± 11 to 1703 ± 18 m.y. which indicated an age between Upper and Lower Proterozoic on the Siberian craton.
The dynamics of the global diversity of Proterozoic stromatolites is analyzed at specific and generic levels on the basis of separate records from each of six superregions (the data on three of them are discussed herein), with a subsequent generalizing examination of the superregional taxa samples. The analyzed eleven time intervals of comparable duration include four of the Early Proterozoic and seven of the Late Proterozoic. The samples of the identified stromatolitic taxa from Africa, Australia, and North America are of comparatively low volume. Their diversity (and abundance) peaks are distinguished at 2.0-1.8 Ga ago (Australia, North America), in the Early Burzyanian (Africa, Australia), and in the Early (Africa) or Late Karatavian (Australia, North America). The histogram of the global stromatolite diversity is principally based on the taxa distribution in the largest samples from China and Northern Eurasia. It is of a trimodal character (the peaks at 2.3-2.0 Ga in the Early Burzyanian and Early Karatavian), and demonstrates three periods of low taxonomic diversity at the very beginning (2.5-2.3 Ga) and end (2.0-1.65 Ga) of the Early Proterozoic and during the Late Burzyanian. A sharp diversity decrease is notable in the Late Karatavian (0.85-0.65 Ga), and this trend smoothly extends into the Vendian and, probably, Cambrian time. The secular changes in the Proterozoic stromatolite diversity are depictable against the background of discrete global tectonic events, variations of Sr-isotope composition in the sea water, significant climatic fluctuations, directed changes in pCO2 and sea water saturation with respect to Ca and Mg carbonates. In addition, during the Late Riphean (Karatavian) and Vendian changes in stromatolite diversity were correlative with secular variations in the volume ratios between the buried carbonate and organic carbon, as well as with some important evolutionary changes in the biota. The dynamics of the global diversity (and abundance) of Proterozoic stromatolites reflects a directed evolution of Proterozoic ecosystems. Copyright © 1996 by MAEe cyrillic signK Hayκa/Interperiodica Publishing.
This chapter discusses environmental diversity of middle Precambrian storm atolites. Stromatolites occur in a wide variety of facies —carbonate and noncarbonate, shallow and deep-water, marine and non-marine. Mere presence of stromatolites is not in itself sufficient to interpret environment of deposition, but specific types of stromatolites may be environmentally diagnostic. Certain types of stromatolites may be diagnostic of specific facies. Extensive stromatolite biostromes occurring in the upper parts of stacked regressive cycles are typical of the interior of a carbonate shelf. Large stromatolite bioherms separated by tidal channels are typical of the outer margin of a carbonate shelf. The distinctive columnar stromatolite Conophyton may be an exclusively subaqueous form, and poorly laminated Conophyton-like columns may grow in foreshelf basins of water tens or even hundreds of meters deep. The elongation of stromatolites in plan is parallel to paleocurrent direction, and commonly normal to the depositional strike. Stromatolite growth by direct precipitation, rather than sediment trapping, occurs in non-marine and deep-water marine basins, where the particulate sediment is terrigenous.
This chapter presents an attempt to classify late Precambrian stromatolite microstructures. The stromatolite primary lamination reflects the growth pattern of the algal coenose and the habit of the carbonate precipitated or trapped within the filament framework, among other things. The fossil stromatolite microstructure includes furthermore the imprint of subsequent diagenesis. A carefully detailed description of the actual fabric of the microstructure mixing up original and diagenetic features leads to an excessive number of microstructural patterns, and a very artificial classification. The need of modern models is great because of the excessive attention paid by the geologists to the sedimentary and physical processes. Well-preserved microstructures were proposed for the classification of stromatolites that were well adapted: (1) simple microstructures where the laminations or the dominant fabrics follow the rhythmical growth pattern of the coenose; and (2) complex microstructures where the historical succession of laminations presents microstructural changes due to seasonal differentiation of the algal coenose.
Publisher Summary The Belt Supergroup, a thick Middle Proterozoic sedimentary sequence consisting of argillaceous, arenaceous, and calcareous rocks, extends from western Montana and northern Idaho into adjacent parts of Alberta, British Columbia, and Washington State. The Altyn Limestone is of restricted distribution, occurring only in a narrow outcrop band that immediately overlies the Lewis Overthrust on the east side of Glacier National Park and an adjacent part of Canada. The interval exposed in Glacier National Park attains a thickness of several hundred meters and is composed predominantly of silty and sandy dolostone. Stromatolites growing under more closely spaced conditions and forming relatively more horizontal biohermal growth surfaces developed into less divergently branched forms. Lateral expansion of these stromatolites is inhibited by adjacent columns and the growth of relatively more erect branches is favored by the more nearly horizontal nature of the bioherm growth surface. The inclined elongate forms apparently developed under relatively strong current conditions, their elongate nature probably being a streamlining effect caused by unidirectional currents.
Precambrian carbonates generally have not been examined from the perspective of platform evolution. The major facies and stratigraphic relations of several carbonate sequences ranging from the early Archean through late Proterozoic are examined and discussed in terms of platform construction and the dominant factors that influenced patterns of sedimentation and the production of carbonate. Eustasy apparently influenced the stratigraphic architecture of many platforms, causing rim backstepping, incipient shelf drowning, and the development of third-order sequences of "Grand Cycles'. It also was the likely cause of parasequences or "small-scale cycles' that characterize peritidal environments in late Archean through late Proterozoic sequences. -from Author
Microbialites in the 2521±3 Ma Gamohaan and Frisco formations, South Africa, consist of three components: very thin, filmy laminae that are interpreted as remnants of microbial mats; irregular surfaces that acted as supports over which filmy laminae draped, which also are interpreted as microbial in origin; and cement-filled voids that formed contemporaneously with microbialite growth. The structure of the microbialites varies with the proportions of filmy laminae and supports, disruption of had filmy laminae, and abundance of voids. Seven end-member morphologies have been defined: planar laminae, contorted laminae, tented microbialites, cuspate microbialites, irregular columnar microbialites, plumose structures, and herringbone calcite beds. Similar structures have not been reported in younger rocks, but they may have partial analogs in Yellowstone hot springs, ice covered lakes in Antarctica, and Proterozoic conoform stromatolites. Herringbone calcite, a fibrous marine cement, precipitated contemporaneously with microbial growth. It preferentially precipitated on the supports over the laminated mat as demonstrated by: the concentration of herringbone calcite near supports; growth banding in herringbone calcite, which indicates that calcite nucleated on and grew away from supports but not laminated mat; and the abutment of herringbone calcite coatings against laminated mat attached to supports. These observations suggest that the laminated mat inhibited nucleation of calcite crystals. The differences in the morphology and relationships to herringbone calcite in the supports and laminated mat imply that they were composed of distinct microbial communities that interacted with their environment differently. Also, the complexity of the microbialites demonstrates that a remarkably diverse set of microbe-substrate interactions evolved by late Archean time.
Columnar buildups found in a tidal channel off Lee Stocking Island, Exuma Cays, Bahamas, have been interpreted as modern giant stromatolites growing in a subtidal normal marine environment. However, these organically-formed columns reveal three discernible internal structures: (1) prokaryotic stromatolites comprised of alternating layers of coarse-grained ooids and peloids, and fine micrite that formed exclusively by microbial activity; (2) eukaryotic stromatolites comprised of microbially-induced micritic layers alternating with detrital layers accumulated, bound, and cemented by eukaryotic algae; and (3) thrombolites displaying irregular, clotted fabrics and formed by microbes, algae, and metazoans. Phanerozoic thrombolites, in contrast, have been interpreted as unlaminated stromatolites constructed by cyanobacteria. Eukaryotic organisms overgrow all of the columns at present. Thus, the contemporaneous formation of prokaryotic stromatolites, eukaryotic stromatolites, and thrombolites under identical conditions within the present environment appears unlikely. We suggest that the prokaryotic stromatolites represent forms that began to develop in an intertidal setting with the Holocene flooding of the Great Bahama Bank. The thrombolites, however, began to form under the present, normal-marine subtidal conditions. The eukaryotic stromatolites represent intermediate forms between prokaryotic stromatolites and thrombolites. There is evidence for a gradual change from stromatolite to thrombolite reefs associated with rising sea-level. With the deepening, there would have been a decrease in salinity, an increase in energy, and possibly an increase in nutrient supply; all factors that favor thrombolite growth. We propose that the co-existing stromatolites and thrombolites found off Lee Stocking Island did not grow contemporaneously, but reflect a response to changing environmental controls with changing sea-level.
Early Organic Evolution is the proceedings of the ninth Alfred Wegener Conference, the final meeting of IGCP Project 157 held in Germany in 1988. Over the past 15 years, Project 157 has promoted the blending of organic geochemistry, economic geology, and evolutionary biology. This IGCP publication covers a diverse set of topics and truly reflects the interdisciplinary nature of the field of early organic evolution. In the second and largest section, seventeen papers on organic matter in ancient sediments discuss the chemical analysis of early sediments, gas, and oil. The reader is treated to a review of carbon isotope chemistry and a [delta][sup 13]C walk through the past 3.8 billion years, and even deeper yet into the mantle. Following this is a series of papers carefully describing elemental, isotopic, and organic geochemical (particularly biomarker) data from ancient sediments found throughout the earth. This section ends very strongly with the paper by Fowler on the influence of a single alga on Ordovician oils and rocks from Canada. He first gives a detailed account of the considerable chemical and microscopic evidence showing that minimally reworked Gloeocapsomorpha prisca is the main contributor of organic matter to the oil and rock and then goes on to discuss the nature of the organism. In general, this book reviews information presented in other places, but still serves as a good resource for those interested in the evolution of the Earth.
Spheroidal and filamentous organic-walled microfossils have been detected in ca. 1.7 Ga old cherts of the Hornby Bay Group, Northwest Territories, Canada. The majority of the spheroidal forms range from 1 to 4 mu m in diameter, are referable to the genus Sphaerophycus, and probably represent the preserved sheaths of chroococcacean cyanophytes. A single, robustwalled, 27 mu m diameter, spheroidal microfossil of undetermined affinity is also present. The filamentous forms are tubular, unbranched, and range from 1 to 8 mu m in diameter. They appear to represent the preserved sheaths of nostocalean cyanophytes or filamentous bacteria. The filaments illustrate the relationship between matrix mineralogy and the fidelity of preservation of organic-walled microfossils. Where they occur in silica the filaments are preserved as three-dimensional tubular microstructures, which are readily recognized as microfossils. In contrast, where they extend from silica into adjacent dolomite they are highly compressed and not readily discernable as microfossils. This compression appears to have been caused by aggrading neomorphism and pressure dissolution of the carbonate minerals, and it illustrates the contribution of diagenesis, in addition to decomposition of organic material, in causing the paucity of microfossils in Precambrian carbonate rocks. -Authors
The mode of formation and environmental setting of stromatolites from the lower Missoula Group (ca. 1.1·10⁹ years old) in Glacier National Park, Montana, have been determined. The stromatolite-bearing interval in the lower Missoula Group was deposited in a shallow, intermittently exposed setting of very low relief, the stromatolites forming during periods of submergence. In situ carbonate precipitation was the dominant process involved in the formation of encrusting stromatolitic laminae. This precipitate was deposited within, and probably beneath, algal mats, most likely as a result of the photosynthetic removal of carbon dioxide by the mat-building microscopic algae. Calcite also was precipitated in several types of open-space structures occurring within these stromatolites. Other laminae were produced by the organic stabilization of detrital particles; by the solely physical accumulation of terrigenous material; and probably, by bacterially induced precipitation of iron sulfide which was later oxidized to form hematite layers.
Stromatolites are now used more and more frequently in attempts to solve Precambrian stratigraphic problems. Abundant new data that have become available during the last two years support the view that the methods are valid and useful, provided that rigorous taxonomic procedures are followed. Contrary to earlier indications that Early and Late Proterozoic stromatolites are identical, none of the 53 forms (‘form-species’) now known from the Early Proterozoic are also reported from younger rocks. Nonetheless, it has become increasingly apparent that correlations based only on group (‘form-genus’)-level identifications can be misleading. The problems of establishing identity at the form-level indicate the need to substantially improve descriptive methods, and in particular to make more use of numerical techniques.Stromatolite assemblage zones distinguished in Middle to Late Proterozoic sequences are 100–350 Ma in duration, with boundaries at 1650 ± 50, 1400 ± 50, 1050 ± 50, 650–680 ± 20 and 570 ± 20 Ma.
In an effort to narrow down the first-and second-order variations in isotopic composition of the Early Paleoproterozoic (2.25 {plus minus} 0.25 Ga) seawater, the authors report comprehensive study to mineralogy, chemistry, and isotopic composition of the Malmani Dolomite (Transvaal Supergroup, South Africa), Duck Creek Dolomite (Wyloo Group, Australia), and Bruce Limestone' Member of the Espanola Formation (Huronian Supergroup, Canada). Based on trace element data, their diagenetic rank increases in the order Duck Creek < Bruce < Malmani. The interpolation of alteration trends to best' values yields an estimate of 0.70550 for â¸â·Sr/â¸â¶Sr, comparable to previously published value of 0.70496. For δ¹³C, the measured range of 0 {plus minus} 1.5 {per thousand} PDB is similar to that observed for Phanerozoic marine carbonates. In contrast, the best' δ¹â¸O value for dolostones is - 5{per thousand} PDB, depleted in ¹â¸O relative to Phanerozoic counterparts, but comparable to the estimates obtained by similar approach for Archean facies. The isotope geochemistry and mineralogy of Bruce Limestone' Member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment, possibly related to the recession of the Bruce glaciers. If such interpretation proves unacceptable from a sedimentological standpoint, the origin of the unusual isotopic signatures must be sought, despite the above trace element interpretation, in as yet unspecified post-depositional alteration phenomena. In contrast to Phanerozoic carbonates, the Early Paleoproterozoic - as well as the previously described Archean - counterparts are enriched in Mn and Fe.
Statistical evaluation of 3056 δ13C measurements in carbonate rocks and fossils shows that they record a 2‰ 13C depletion from the late Proterozoic to the early Paleozoic, a 2.5‰ enrichment to the Permian, and a 1.5‰ depletion to the Cenozoic. These variations, not controlled primarily by facies or alteration phenomena, correlate negatively with the δ34S sulfate secular trend, as confirmed by collation of 1083 δ34S measurements. The correlation suggests that the biologically mediated redox fluxes of the C and S cycles have been approximately balanced through this long span of geological time, generally levelling available oxygen. Such a redox system is consistent with the controlling mechanism proposed by Garrels and Perry (1974). Consequently, the sedimentary reservoirs of Corganic as well as Sbacteriological'have varied through geological time.
The Late Proterozoic glacial record is reviewed on a global basis as a setting and introduction for that part of the symposium concerned with West African tillites. No unequivocal glacial deposits have yet been recorded in the interval 1.0–2.0 Ga. Late Proterozoic glacial epochs have been grouped into three main periods namely: Lower Congo (c. 0.9 Ga), Sturtian (c. 0.8 Ga) and Varangian (c. 0.65 Ga), and a possible final Late Sinian Epoch near the Precambrian-Cambrian boundary. The Ediacaran Epoch, with its metazoan fauna follows Varangian and in part at least precedes Late Sinian. The Sturtian and Varangian glacial periods each comprise at least two main glacial epochs, the tillites of which are widespread. Chronometric ages are currently disputed and the values given are for identification in the literature rather than to provide chronometric definition.Of all the glacial epochs, only those of the Varangian glacial period appear to be represented in West Africa. It has been argued that Varangian tillites were of global extent, many forming in tropical latitudes, though some authors prefer a hypothesis of polar wandering to limit glaciation to polar latitudes.
As part of a continuing biostratigraphic study, stromatolites are described and named from the following units: Amelia Dolomite, Tooganinie Formation and Emmerugga Dolomite (McArthur Group), and the Balbirini Dolomite (Nathan Group), McArthur Basin; Lochness Formation (Haslingden Group), Mount Isa Province; Yardida Tillite, Georgina Basin. The latter is Late Proterozoic and all others are Middle Proterozoic. To the previous record of Conophyton garganicum Korolyuk from the McArthur Basin we add Kussiella kussiensis Krylov, Omachtenia omachtensis Nuzhnov, Acaciella emmerugga f. nov. and the components of the Balbirina prima bioherm series, Bukalara tortuosa gr. et f. nov. and Australoconus abnera gr. et f. nov. We introduce a formal taxonomic procedure for bioherm series. ?Segosia isa f. nov. is described from the Mount Isa Province and Yardida delicata gr. et f. nov. from the Georgina Basin. The new groups and forms at present are unknown from elsewhere and so have no biostratigraphic significance at this stage, but the assemblage of stromatolites from the McArthur Basin allows correlation with the Lower Riphean of the USSR, which is consistent with the available geochronologic constraints.
Traces of glaciations are unevenly distributed in the stratigraphic column and exhibit rhythmicity of several ranks. This rhythmicity makes it possible to establish the corresponding hierarchy of climatic events (in descending order): glacio- and thermo-eras, glacial and interglacial periods and epochs, as well as smaller events which appear, to be compared to the most important climatic fluctuation during the Pleistocene. The direct factors causing initiation and termination of glaciations were climatic fluctuations. The palaeolatitudinal position of the continents controlled a degree of asymmetry and, perhaps, a certain diachronism of glaciations in different hemispheres. During the glacial periods in high and middle latitudes of the Earth the glacial and cold—temperate climates either were predominant, or alternated. The thermo-eras and at least some interglacial periods were characterized by warm—temperate and warm climates at high and middle latitudes. Glaciations have been numerous and important geological events; they affected all outer spheres of the Earth from asthenosphere to atmosphere. They apparently stimulated the biological evolution too. The study of ancient glaciations is important from both the scientific and economical points of view.
Stromatolites are laminated, lithified, sedimentary growth structures that accrete away from a point or limited surface of attachment. They are commonly, but not necessarily, of microbial origin and calcareous composition. Although familiar to geologists, they remain enigmatic as to origin and uncertain as to their full potential for historical geology. The authors summarize here the results of collective inquiry and discussion concerning central problems of stromatolite morphogenesis. Focus is on relations between microstructure, laminar details, and gross morphology of ancient, probably biogenic, stromatolites and on the microbial composition and laminar characteristics of analogous modern microbial mats and sedimentary structures produced by them.
Siliciclastic rocks of the Ruyang Group, southern Shanxi, and the broadly equivalent Gaoshanhe Group, Shaanxi, contain exceptionally well-preserved, large (Ø ≈ 150 μm) acanthomorphic acritarchs recently interpreted as late Neoproterozoic (Sinian, c. 800-544 Ma) in age. This biostratigraphic interpretation is based on the presence of large acanthomorphs in Sinian successions of South China and elsewhere and the perceived absence of comparable forms in older rocks; it casts doubt on the long-accepted interpretation of Ruyang and correlative rocks as Mesoproterozoic in age (1600-1000 Ma). In contrast, thick marine dolomites in overlying units contain abundant radial fibrous fabrics and a narrow range of δ13C values (c. 0 ± 1‰ vs. PDB), features which characterize unambiguously Mesoproterozoic carbonates elsewhere on the North China Platform and on other continents. Age estimates based on petrofabrics and chemostratigraphy are corroborated by a UPb zircon age of 999 Ma (no recorded error) for granites which intrude overlying carbonates. Thus, in combination, the available data constrain the Ruyang siliciclastics and overlying carbonates to be older than about one billion years, making Shuiyousphaeridium Yan and other large process-bearing acritarchs from these units among the oldest known distinctly ornamented eukaryotic microfossils.