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ABSTRACT Quaternary to Recent sedimentary dolomite in groundwater calcrete and saltmarsh sediments of Lake Way and Lake Maitland, Western Australia, provide new information about the formation of low‐temperature dolomite. Dolomite can form through numerous pathways depending on the depositional and diagenetic environment. Many pathways involve microbial processes and/or the presence of a nucleation substrate which help overcome kinetic barriers preventing precipitation in the laboratory. Petrographic, mineralogical, hydrogeochemical and stable isotopic data in this study reveal the importance of Mg‐clays as nucleation sites for dolomite precipitation in a range of aquifer environments. There is also a close association between authigenic Mg‐clays, dolomite and the potassium–uranyl–vanadate ore mineral carnotite in channel and playa uranium deposits. It is interpreted that evaporation‐driven precipitation of dolomite establishes a positive feedback loop promoting the dissociation of aqueous uranyl–carbonate complexes and concomitant increase in carnotite saturation. Critical to this model is the presence of authigenic Mg‐clays because they facilitate dolomite precipitation and promote carnotite nucleation by concentrating potassium ions on clay surfaces via adsorption. This Mg‐clay–dolomite–carnotite relationship is widespread throughout Western Australian channel and playa uranium deposits and has been observed in similar deposits in Namibia and Botswana. In addition to this economic implication, Mg‐clay mediated nucleation of dolomite potentially has global relevance as a precipitation mechanism for low temperature dolomite in sedimentary deposits where detrital and/or authigenic Mg‐clays are present. Maturation of sedimentary dolomite from disordered high‐calcian dolomite to ordered low‐calcian dolomite occurs very early in the meteoric realm making it resistant to alteration during burial diagenesis. Diagenetic resistance may be further increased by early meteoric silicification related to the degradation of associated Mg‐clays. These findings indicate that Mg‐clay associated sedimentary dolomite has potential to retain a primary isotopic signature indicative of its origin, making it a useful recorder of palaeoenvironmental conditions. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Animikie Basin in the Lake Superior region of North America is the benchmark for understanding one of the most significant changes in seawater chemistry in Earth history--the transition to a sulfidic ocean (ca. 1.84 Ga). This transition marks a dramatic increase in ocean sulfide produced by bacterial reduction of sulfate formed through the oxic chemical weathering of pyrite during the Great Oxidation Event. Such a change is thought to have caused the cessation of widespread iron formation deposition and marks the beginning of a long lull in the evolution of eukaryotes. The feasibility of using the Animikie Basin to understand this transition is re-evaluated by comparing new sulfur isotope data ([delta][sup.34][S.sub.pyrite]) from the Michigamme Formation in northern Michigan, United States, to published results from the correlative Rove Formation in Ontario, Canada. These data suggest that microbial sulfate reduction was restricted to organic-rich environments away from the influence of deltas, which delivered low-sulfate river water to the coast. Sulfur isotope data and the patchiness of sulfate reduction within the Animikie Basin suggest strong lateral and vertical gradients in seawater sulfate concentrations. When viewed in the context of the most recent tectonic and sedimentologic models, these observations are best interpreted as reflecting an evolving basin with restricted water circulation, not open circulation with the global ocean as previously postulated. Thus, the Animikie Basin does not contain an accurate record of open-ocean structure and composition during the onset of sulfidic conditions. What is clear is that marginal seas must be carefully evaluated before they are used to model the global ocean, especially given the rarity of preserved Precambrian deep-sea sediments. doi: 10.1130/G30747.1

An ejecta layer produced by the Sudbury impact event ca. 1850 Ma occurs within the Baraga Group of northern Michigan and provides an excellent record of impact-related depositional processes. This newly discovered, ~2-4-m-thick horizon accumulated in a peritidal environment during a minor sea-level lowstand that punctuated a period of marine transgression. Common ejecta clasts include shock-metamorphosed quartz grains, splash-form melt spherules and tektites, accretionary lapilli, and glassy shards, suggesting sedimentation near the terminus of the continuous ejecta blanket. Sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact-generated tsunami wave train. Observed platinum group element anomalies (Ir, Rh, and Ru) within the Sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite. Documenting and interpreting the detailed characteristics of the Sudbury ejecta horizon in Michigan have yielded a fingerprint to identify this chronostratigraphic marker in other Paleoproterozoic basins. For the first time a foundation exists to assess the consequences of the Sudbury impact on Precambrian ocean chemistry and early life. Keywords: bolide, impact, meteorite, ejecta, sedimentology, platinum group elements, Sudbury, Michigan.

Irrespective of geologic age, Phanerozoic coated phosphate grains deposited beneath productive surface waters in organic-rich paleoenvironments are of only two types. Unconformity-bounded grains contain internal discordances and erosional surfaces, attributable to multiple episodes of phosphogenesis and sedimentary reworking during periods of stratigraphic condensation. Redox-aggraded grains consist of concordant concentric phosphate laminae that are intimately interlayered with circumgranular layers containing pyrite, chamosite, or barite, recording in situ diagenetic mineralization driven by changes in pore-water redox potential. Such changes can be attributed to variations in biological oxygen demand within suboxic pore-water environments resulting from fluctuations in sedimentation rate of organic carbon. Redox-aggraded grains are thus sensitive indicators of variations in organic carbon export and record changes in primary productivity and/or ecological dynamics of the surface ocean. This concept of coated-grain formation necessitates a long residence time just below the sediment-water interface. If sedimentation rate is too high, grains are rapidly buried and so removed from the zone of active phosphate precipitation. Coated phosphate grains can therefore be considered the granular equivalents of condensed beds. These concepts are equally applicable to the interpretation of other types of coated grains and concretions that contain Eh-sensitive minerals, such as iron-bearing ooids and polymineralic concretions. Keywords: phosphorite, coated grains, redox changes, stratigraphic condensation.

The Sokoman Formation is a ca. 100-m-thick succession of interbedded iron formation and fine-grained siliciclastics deposited at 1.88 Ga. Accumulation occurred on a dynamic paleoshelf where oxygen stratification, coastal upwelling of hydrothermally derived Fe and Si, microbial processes, tide and storm currents, diagenesis, and low-grade prehnite–pumpellyite metamorphism controlled lithofacies character and produced complex associations of multigenerational chert, hematite, magnetite, greenalite, stilpnomelane and Fe carbonate. Hematite-rich facies were deposited along suboxic segments of the coastline where photosynthetic oxygen oases impinged on the seafloor. Hematitic, cross-stratified grainstones were formed by winnowing and reworking of freshly precipitated Fe-(oxyhydr)oxide and opal-A by waves and currents into subaqueous dunes. Magnetite-rich facies contain varying proportions of greenalite and stilpnomelane and record deposition in anoxic middle shelf environments beneath an oxygen chemocline. Minor negative Ce anomalies in hematitic facies, but prominent positive Ce and Eu anomalies and high LREE/HREE ratios in magnetite-rich facies imply the existence of a weakly oxygenated surface ocean above anoxic bottom waters. The Fe isotopic composition of 31 whole rock (−0.46 ⩽ δ 56 Fe ⩽ 0.47‰) and 21 magnetite samples (−0.29 ⩽ δ 56 Fe ⩽ 0.22‰) from suboxic and anoxic lithofacies was controlled primarily by the physical oceanography of the paleoshelf. Despite low-grade metamorphism recorded by the δ 18 O values of paragenetically related quartz and magnetite, the Sokoman Formation preserves a robust primary Fe isotopic signal. Coastal upwelling is interpreted to have affected the isotopic equilibria between Fe 2+ aq and Fe-(oxyhydr)oxide in open marine versus coastal environments, which controlled the Fe isotopic composition of lithofacies. Unlike previous work that focuses on microbial and abiotic fractionation processes with little regard for paleoenvironment, our work demonstrates that depositional setting is paramount in governing the Fe isotopic composition of iron formations irrespective of what Fe-bearing minerals precipitated. [ABSTRACT FROM AUTHOR]

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fl uvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fl uvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution. [ABSTRACT FROM AUTHOR]