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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.