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Marine carbonates record geochemical information on the state of Earth's surface environment in the geological past, but recrystallization during burial can make the records difficult to interpret. Here, we report shore to basin gradients in δ44Ca values in matrix limestone and burrow dolomite across a sequence of Late Ordovician (Katian Stage) burrow-mottled carbonates in the Williston Basin, an intracratonic epeiric marine basin in North America. Both gradients are interpreted to have diagenetic origins. The gradient in limestone δ44Ca values developed in a bathymetrically controlled sediment- to fluid-buffered diagenetic system operating between the center and the edges of the basin. Two dolomitizing events formed the gradient in dolomite δ44Ca values. The sediment-buffered end of the limestone gradient preserves the δ44Ca value of the original carbonate mud (∼–1.28 ± 0.10 ‰). The fluid-buffered end of the gradient conserves the δ44Ca value of contemporaneous seawater (–0.47 ± 0.10 ‰), because: (1) pore water exchange drivers operated with greater variety and intensity in shallow water settings, (2) shallow nearshore deposits exchanged more Ca with seawater during recrystallization to diagenetic calcite than deeper offshore deposits, and (3) diagenetic calcite formed with negligible fractionation under equilibrium conditions. The gradient conserves the local value of Δ sed (–0.81 ± 0.20 ‰) despite its diagenetic origin, and the magnitude of Δ sed is consistent with primary calcite precipitation in a 'calcite sea'. Field assessment of the equilibrium Ca isotope fractionation factor between calcite and dolomite of –0.41 ± 0.17 ‰ at ambient temperature is applied to correct basin-centered dolomite for fractionation, yielding –1.62 ± 0.27 ‰, which is very different from the –0.25 ± 0.07 ‰ value of Ca in evaporatively concentrated seawater from which the dolomite originally formed. Applying the same fractionation factor to correct literature data on early diagenetic dolomite in the Great Basin, North America, yields –0.69 ± 0.22 ‰ for the δ44Ca value of seawater, which is ∼0.2 ‰ lower than 'normal marine' waters in stratigraphically equivalent deposits in the Willison Basin, and 0.44 ‰ lower than evaporatively concentrated seawater during a brief period of basin restriction and anhydrite deposition. The nearly 0.5 ‰ variation in the δ44Ca value of Katian seawater falls within the range of scatter in the brachiopod reconstruction of seawater Ca isotopes. Here, we propose that the true ocean record of secular change lies towards the bottom of the range of scatter, and that the higher values in the record reflect local Ca cycling and circulation restriction in epeiric seas. Epeiric seas cultivate the fluid-buffered diagenetic conditions required to promote Ca isotope exchange between reactive carbonate sediments and seawater through carbonate mineral dissolution/recrystallization, crystal coarsening by way of Ostwald ripening, and replacive dolomitization. It is not the apportioning of kinetically fractionated aragonite and calcite in the carbonate deposition flux that drove secular changes in global value of Δ sed between 'calcite' and 'aragonite' seas, but rather the replacement of kinetic fractionations with equilibrium fractionations during fluid-buffered diagenesis with seawater. [ABSTRACT FROM AUTHOR]

The Maastricht Formation is a mostly calcarenitic unit that belongs to the Chalk Group but is unlike the typical North Sea chalk in that it is much coarser and at times contains a significant terrigenous component. The formation was deposited between the late Maastrichtian and the early Danian in a proximal zone of the Chalk Sea immediately north of the Anglo-Brabant and Rhenish massifs. The formation crops out in South Limburg, the Netherlands, in the German state of North Rhine–Westphalia, near Aix-la-Chapelle (Aachen), and in the Belgian provinces of Liège and Limburg, and it is present in the subsurface in the Campine Basin and in the Roer Valley Graben, in Belgium and the Netherlands.

Abstract The Late Devonian extinction ranks as one of the 'big five' Phanerozoic extinctions affecting up to 80% of marine species and is characterized by two major extinction events that are separated by <3 My. The leading hypotheses explaining both extinction events are global cooling and/or widespread marine anoxia. We test the marine anoxia hypothesis by analyzing uranium isotopes (δ 238 U) across a ∼7 My interval of well-dated Upper Devonian marine limestones from the Devil's Gate Formation in Nevada, USA. The measured δ 238 U curve shows no co-variation with local sediment-derived redox trends, water-depth dependent facies changes, redox-sensitive metals, TOC, or diagnostic elemental ratios. From these relationships, we infer that the δ 238 U curve represents global seawater redox conditions. Two negative δ 238 U excursions (indicating more reducing seawater) are observed with durations of ∼3.8 My (late Frasnian) and ∼1.1 My (early Famennian), respectively. Steady-state modeling of the observed −0.2 to −0.3‰ shifts in δ 238 U points to a ∼5–15% increase in the total area of anoxic seafloor during these excursions. The late Frasnian negative excursion is broadly coincident with the first extinction event (late rhenana Zone or lower Kellwasser event), whereas the early Famennian negative excursion (lower-middle triangularis zones) occurs after the most intense Frasnian–Famennian boundary (F–F) extinction event (upper Kellwasser event). Compilations of local sediment redox conditions from Upper Devonian marine deposits with conodont zone-level age control indicates that the extinction events were coincident with widespread anoxic deposits accumulating in subtropical epeiric sea and some open ocean settings, supporting previous interpretations that widespread marine anoxia had an important influence on the Late Devonian extinctions. The temporal relationships between global-ocean redox trends represented by the δ 238 U curve and the newly compiled subtropical marine redox sediment trends indicates that local epeiric seawaters carried a similar U-isotopic composition as the open ocean for the majority of the studied interval except for a brief interval (<500 ky) spanning the F–F boundary. Highlights • Late Devonian δ 238 U trends from marine limestones record two widespread anoxic events. • Late Frasnian global anoxia coincides with lower Kellwasser extinction event. • Epeiric sea anoxia but not global ocean anoxia associated with Upper Kellwasser extinction event. • Late Devonian epeiric seas in relatively good redox communication with open oceans. [ABSTRACT FROM AUTHOR]

Ordovician epeiric sea carbonates in intracratonic basins of Laurentia are enigmatic due to their unique depositional settings and the absence of modern analogs. Integrated ichnologic and sedimentologic analysis of the Upper Ordovician Stony Mountain Formation in the Williston Basin of Canada allows recognition of neritic-marine, nearshore-marine, open-lagoon, restricted-lagoon, peritidal sand-shoal and peritidal-flat subenvironments. The Cruziana ichnofacies occurs in neritic (between fair-weather and storm wave bases) and nearshore (around fair-weather wave base) marine environments. The depauperate Cruziana ichnofacies is present in open- and restricted-lagoon environments, indicating a shift from fully marine to stressed conditions. In the open lagoon, composite ichnofabrics related to omission surfaces illustrate the low rates of background sedimentation interrupted by event deposition and early cementation, illustrating the Glossifungites and Trypanites ichnofacies. The decreased size of discrete burrows in the restricted lagoon is attributed to reduced oxygenation under stagnation rather than hypersalinity. The peritidal complex includes high-energy sand shoals and low-energy tidal flats. Only monospecific colonization took place sporadically in associated subtidal environments within the peritidal complex deposits. [ABSTRACT FROM AUTHOR]

Epeiric seas were common in deep times, but modern analogues are rare. Large-scale reconstruction of ancient examples is usually limited by available data, which also hampers the conceptualization of their variability and key controlling parameters. In this study we summarize and integrate a large stratigraphic dataset of a relatively small, semi-restricted Cretaceous epeiric sea in southwestern Gondwana to identify key sedimentary and biological signals that are used to discuss oceanographic connection with adjacent oceans, areal distribution of sediments entering the marine basin, sediment dispersal pathways within it, and the resulting stratigraphy. The Valanginian–Hauterivian Pilmatué-Lower Centenario study interval represents an exceptional record of well-dated marine and continental sediments deposited in the Neuquén Sea, which occupied a back-arc setting and was semi-connected with the proto-Pacific Ocean across a volcanic arc. The study interval is organized in three transgressive-regressive sequences (Lower, Middle, and Upper) defined by the integration of genetically linked depositional systems, macrobenthic bioevents, and stratal patterns. Detailed paleogeographic reconstructions show a consistent east-west proximal-distal depositional trend and a major fluvial system located in its southeastern apex. This continental-scale delivery system was responsible for the vast volume of sands and muds brought into the sea. The degree of connection between the Neuquén Sea and the adjacent ocean was evaluated by considering onlap stratal relationships, incursion of pandemic fauna, onset and deactivation of tidal offshore fields, and contribution of plankton-derived carbonate mud (a proxy to paleoproductivity). These criteria indicate that the maximum volume of watermasses within the semi-restricted sea was reached not during initial flooding (i.e., base of Lower Sequence), but most probably near the Valanginian/Hauterivian boundary (base of Middle Sequence). Dispersal pathways within the distal marine settings of the Neuquén Sea were reconstructed by integrating geographical thickness variations, composition of muds, and seafloor oxygenation levels. Results allow defining a non-uniform, along-depositional strike scenario, with a locus of siliciclastic deposition westwards of major deltaic systems, and lower accumulation towards the northwestern and southwestern areas. The northwestern region, having maximum carbonate contribution and lower oxygen levels, is inferred to have been more prone to water-column stratification over time. Some of the key attributes reconstructed for the Neuquén Sea (namely significant restriction, major sediment supply in one apex, asymmetric distribution of sediments in the distal settings) are comparable with configurations observed in the present Persian Gulf and the Adriatic Sea and departing from others such as the Baltic Sea and the Gulf of Carpentaria. Comparisons with ancient examples such as the Western Interior Sea and the North American Seaway suggest contrasting paleogeographic models, but at the same time reinforce the value of these key parameters to produce better discrimination between the possible spectrum of epeiric seas. In that sense, building classifications for epeiric seas based on these key parameters, such us size, degree of connection with the open ocean, sediment entry points, and sediment dispersal pathways, appears as the next challenge for more comprehensive reconstructions of these environments worldwide. [ABSTRACT FROM AUTHOR]

Oxygenation of Earth's oceans in the Proterozoic was a protracted process that began prior to the Great Oxidation Event (GOE) and culminated in deep ocean ventilation following Neoproterozoic glacial episodes, with both of these major thresholds accompanied by profound changes in the evolution of Earth's biosphere. Our understanding of the redox structure of Mesoproterozoic oceans, however, is considerably less certain. Limited proxy investigations of oceanic redox in the Mesoproterozoic have focused largely on early Mesoproterozoic shelf to basinal settings, with comparatively little attention paid to epeiric sea environments that dominate the late Mesoproterozoic sedimentary record. Here we report multi-proxy shale redox data from epicratonic and pericratonic environments of the 1.1 Ga Atar and El Mreiti Groups, Taoudeni Basin, Mauritania. These strata were deposited across the West African craton during global sea level highstand, and provide insight into the redox structure of Mesoproterozoic epeiric seas. Iron speciation, pyrite sulfur isotope, and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions, with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments. We suggest that a protracted increase in biospheric oxygenation in the late Mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow, nearshore areas, contributing to the fragmentary pattern of early eukaryotic evolution and diversification. [ABSTRACT FROM AUTHOR]

Abstract: Although the principle of uniformitarianism may be applied to the Precambrian sedimentary record as a whole, certain periods of the Archaean and Palaeoproterozoic witnessed a changing pattern of prime influences controlling the depositional systems. This paper examines the major controls on sedimentation systems and environments during the Archaean and Palaeoproterozoic within the broader perspective of Earth evolution. Earth''s earliest sedimentary system (4.4?-3.7 Ga) was presumably comprised of deep oceanic realms and probably influenced primarily by bolide impacts, major tsunamis, localized traction and global contour current patterns, and bathymetry. As continental crust began to form, the impact-dominated, tsunami type sedimentation gave way to wider varieties of sedimentary environments, known from the oldest sedimentary records. During early continental crustal evolution (c. 3.7–2.7 Ga), sedimentation was essentially of greenstone-type. Volcanic and volcaniclastic rocks were the major components of the greenstone belts, associated with thin carbonates, stromatolitic evaporites, BIF, pelites and quartzites and lesser synorogenic turbidites, conglomerates and sandstones. Volcanism and active tectonism (reflecting dynamic depositional settings during island arc and proto-continental nucleus formation) were the predominant factors influencing sedimentation during this phase of Earth evolution. Transgressions and regressions under the combined influence of tectonics and eustasy are reflected in fining- and coarsening-upwards successions from the proto-cratonic settings; low freeboard enabled the transgression to affect large areas of the proto-cratons. As the earliest, relatively stable craton formed, through a combination of plate tectonic and mantle-thermal processes, continents and supercontinents with the potential for supercontinental cycles started to influence sedimentation strongly. Major controls on Neoarchaean–Palaeoproterozoic sedimentation systems (2.7–1.6 Ga) were provided by a combination of superplume events and plate tectonics. Two global-scale ‘superevents’ at c. 2.7 Ga and c. 2.2–1.8 Ga were accompanied by eustatic rise concomitant with peaks in crustal growth rates, and large epeiric seas developed. The operation of first-order controls leading to development of vast chemical sedimentary platforms in these epeiric seas and concomitant palaeo-atmospheric and palaeo-oceanic evolution combined to provide a second-order control on global sedimentary systems in the Neoarchaean–Palaeoproterozoic period. The supercontinental cycle had become well established by the end of the Palaeoproterozoic, with the existence of large cratons across broad spectrums of palaeolatitude enabling erg development. The entire spectrum of sedimentary systems and environments came into existence by c. 1.8 Ga, prime influences on sedimentation and depositional system possibly remaining essentially uniform thereafter. [Copyright &y& Elsevier]

Abstract Positive shifts in global seawater δ[sup 13] C[sub DIC] are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ[sup 18] O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ[sup 13] C and δ[sup 18] O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and [sup 13] C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate [sup 13] C and [sup 18] O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas. [ABSTRACT FROM AUTHOR]