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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 (δ 34 S 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.

The light stable isotope chemistry of barite and acanthite in the Pierina Au-Ag deposit, Peru, provides strong support for microbial involvement at the economically critical supergene oxidation stage. Early hypogene barite yields δ34S values of 23.6‰ to 28.5‰ and δ18O values of 5.8‰ to 10.9‰, and was precipitated with the original precious metal-enriched sulfide assemblage. In contrast, late barite, deposited during formation of supergene goethite and hematite, the main ore-hosts, yields δ34S and δ18O values of 1.4‰ to 14.2‰ and −2.8‰ to 4.7‰, correlated mutually and with the δ34S values of coprecipitated acanthite (δ34S = 0.4‰ to 3.9‰). Low δ34S values of barite and acanthite record sulfide mineral oxidation by meteoric waters. Increasing δ34S and δ18O values of barite and δ34S values of acanthite indicate enrichment of the sulfate reservoir in 34S and 18O, reflecting the preferential utilization of light isotopes by microbes during aqueous sulfate reduction. Continued sulfate reduction locally reduced supergene fluids, remobilizing iron and locally destroying the goethite-hematite assemblage. Precious metals were released but reacted with sulfide in the reduced waters to form Au-rich acanthite.

Stellate crystals of ferroan dolomite in neritic siliciclastic and carbonate sedimentary rocks between Sturtian and Marinoan glaciations in the Mackenzie Mountains are interpreted as replaced glendonites. These pseudomorphs after ikaite indicate that shallow seawater at that time was near freezing. Stromatolites verify that paleoenvironments were in the photic zone and physical sedimentary structures such as hummocky cross-bedding confirm that the seafloor was repeatedly disturbed by storms. Glendonites within these low-latitude, continental shelf to coastal sedimentary deposits imply that global ocean water during much of Cryogenian time was likely very cold. Such an ocean would easily have cooled to yield widespread sea ice and, through positive feedback, growth of low-latitude continental glaciers. In this situation gas hydrates could have formed in shallow-water, cold shelf sediment, but would have been particularly sensitive to destabilization as a result of sea-level change. Co-occurrence of pisolites and glendonites in these rocks additionally implies that some ooids and pisoids might have been, unlike Phanerozoic equivalents, characteristic of cold-water sediments.

Although commonly used as proxies for attributes of ancient ocean waters, the δ 18 O values of brachiopods from modern seas are little studied. To evaluate the utility of brachiopods as recorders of regional oceanography, modern shells from the Lacepede Shelf (25 000 km 2 ) of southern Australia were analyzed for δ 18 O, and the results were compared to the values of ambient seawater. Southern Ocean waters cover this area of extensive cool-water carbonate deposition, but there are distinct sectors of seasonal upwelling and lesser fluvial outflow. δ 18 O values of brachiopods across the environmental spectrum from 40 to 300 m water depth are in general isotopic equilibrium with surrounding seawater. Nevertheless, δ 18 O values from individual sample sites vary as much as 0.60‰. The area of cold-water upwelling in particular is clearly delimited by a group of high δ 18 O values. The range of values across this one shelf, on the order of 2.5‰, is similar to the range of values postulated on the basis of similar results for secular changes in many ancient oceans.

The authors have used the single-zircon Pb-evaporation technique to determine the age of granitoids in the western Flin Flon domain of the Trans-Hudson orogen in Canada and to constrain the timing of molasse sedimentation, deformation, metamorphism, and mesothermal gold mineralization. The plutons were intruded between 1,860 and 1,834 Ma, about 30 m.y. after volcanism, and provide further evidence of a major period of intrusive activity throughout the Trans-Hudson orogen at that time. The older plutons (1,860 to 1,848 Ma) are synchronous with P2 deformation and the early stages of peak metamorphism, whereas the younger plutons are synchronous with or postdate the P3 deformation event. Molasse sedimentary units are deformed and metamorphosed; thus, deposition must have begun at about 1,860 Ma. Mesothermal gold mineralization postdates all plutons and may be related to high-grade peak metamorphism in adjacent terranes at about 1,815 Ma. This study also provides further support for the use of the Pb-evaporation technique as a suitable method for dating single zircons and indicates that the {sup 208}Pb/{sup 206}Pb ratio of zircons may be a useful tool in detecting the presence and source of xenocrystic zircons.

Stable isotope compositions have been determined for modern mullusks and fossil mollusks collected from uplifted marine terraces at three localities in southern California. By using a paleoclimatic model that decouples the temperature and ice-volume signals in ocean water, ocean-water temperatures off southern California are estimated to have been −3.8 °C at ∼85 ka, −3.0 °C at ∼107 ka, and −2.2 °C at ∼125 ka relative to present temperature. These results indicate rather cool conditions during the peak of the last interglacial stage at 125 ka and conflict with results from terrace faunal studies that suggest water temperatures were warm or warmer than at present.