The redox state of the mid-Proterozoic oceans, lakes, and atmospheres is still debated, but it is vital for understanding the emergence and rise of macroscopic organisms and eukaryotes. The Appekunny Formation, Belt Supergroup, Montana, contains some of these early macrofossils dated between 1.47 Ga and 1.40 Ga and provides a well-preserved record of paleoenvironmental conditions. We analyzed the iron chemistry and mineralogy in samples from Glacier National Park, Montana, by pairing bulk rock magnetic techniques with textural techniques, including light microscopy, scanning electron microscopy, and synchrotron-based X-ray absorption spectroscopy. Field observations of the Appekunny Formation combined with mineralogical information allowed revised correlations of stratigraphic members across the park. However, late diagenetic and/or metasomatic fluids affected primary iron phases, as evidenced by prevalent postdepositional phases including base-metal sulfides. On the west side of the park, pyrrhotite and chlorite rims formed during burial metamorphism in at least two recrystallization events. These complex postdepositional transformations could affect bulk proxies for paleoredox. By pairing bulk and textural techniques, we show primary records of redox chemistry were preserved in early diagenetic and often recrystallized framboidal pyrite, submicron magnetite grains interpreted to be detrital in origin, and red-bed laminae interpreted to record primary detrital oxides. Based on these observations, we hypothesize that the shallow waters of the mid-Proterozoic Belt Basin were similar to those in modern marine and lacustrine waters: fully oxygenated, with detrital reactive iron fluxes that mineralized pyrite during organic diagenesis in suboxic, anoxic, and sulfidic conditions in sedimentary pore waters.

Pyrite framboid diameters were examined in 31 samples taken from 2 Marcellus Shale cores recovered from Greene County, Pennsylvania, and Upshur County, West Virginia (USA). Analysis of framboid diameters in those samples from the more proximally located Upshur County core suggests that anoxic to anoxic-euxinic conditions persisted during accumulation of the transgressive-regressive cycle (MSS1) that comprises the Union Springs Member of the Marcellus Shale, with intermittent episodes of dysoxia. An increased abundance of large framboids documented from the overlying transgressive-regressive cycle (MSS2), which comprises the bulk of the Oatka Creek Member of the Marcellus Shale, indicates improved bottom-water conditions. Redox conditions recorded by framboid diameters of the MSS1 cycle of the Greene County core are generally similar to those of the Upshur County core; however, conditions in that region of the basin from which the Greene County core was recovered appear to have remained dominantly anoxic to anoxic-euxinic. Furthermore, the presence of small syngenetic framboids and large diagenetic framboids in the same thin section samples suggests that redox conditions fluctuated on a temporal scale beyond that observed at the scale of a centimeter-scale thin section. Framboid diameter trends established for both cores enhance our understanding of how much redox conditions varied both spatially and stratigraphically during accumulation of the Marcellus Shale.

High-Mg Kroenke-type basalts containing 9–11 wt% MgO from the Ontong Java Plateau (OJP) include spinels. These spinels have nearly identical Cr/(Cr + Al) ratios (0.45–0.54), and are generally hosted by olivine phenocrysts with relatively primitive compositions, with Fo contents [100 × Mg/(Mg + Fe 2+ ) in atomic ratios] as high as 88. This implies that the primary OJP magmas were in equilibrium with refractory peridotites (i.e., harzburgites) and were homogenized by large-scale melting and magmatic evolutionary processes. Oxygen geobarometry indicates that the OJP primary magmas record uniform oxygen fugacity ( f O 2 ) conditions and are slightly more oxidized (0.6 log units) than normal mid-ocean ridge basalts, but record less-oxidized f O 2 conditions than ocean island basalts. These data are consistent with previous studies that suggest the OJP primary magmas were generated by large-scale and extensive melting of a mantle source region under relatively oxidized conditions.

This review summarizes and assesses a series of papers presented at the Geological Society of America Annual Meeting in 2011 on the changing composition of Earth's early atmosphere. This is a developing field, with differing views, due largely to the facts of an incomplete rock record and negligible preservation of the gaseous components of the atmosphere. Nevertheless, there are constraints, available through geological proxies in the form of chemical sedimentary rocks that reflect the composition of the hydrosphere, and because the two are directly linked, the atmosphere. A review of the geological constraints on atmospheric conditions is presented for early Earth, from its formation at 4.56 Ga up to 1.8 Ga, followed by a developing model that links changing atmosphere/hydrosphere conditions and biosphere evolution to changes in planetary tectonics, including the evolving supercontinent cycle.

Abstract Organo-clay fabric and physico-chemistry of marine mud play important roles in early sediment diagenesis including the development of mass physical properties, consolidation behavior, and sequestration of organic matter (OM) in sediments over geologic time. Transmission electron microscopy (TEM) images of nano- and microfabric reveal that organic matter is sequestered following enzymatic digestion despite the pervasive openness of pore-fluid pathways observed in 3D rotated images. The locations of sequestered organic matter correspond to those predicted by modeling of the potential energy of interaction. Initial flume experiments on high porosity clay-mineral-rich mud deposited under dynamic flow and static (vertical settlement) conditions demonstrate differences in clay fabric and the distribution of organic matter (we define the term organo-clay fabric as the contiguous association and arrangement of organic matter and clay domains). These differences in organo-clay fabric impact the preservation-degradation mechanisms and dynamics during depositional and burial processes. Organo-clay fabric and physico-chemical modeling of potential energy fields coupled with direct observations of organo-clay fabric, three-dimensional (3-D) clay fabric reconstructions, sediment static and dynamic properties, and controlled flume experiments are providing new insight into the developmental history of sedimentary sequences, nano- to macroscale environmental processes, and diagenesis from unconsolidated mud to shale.

Characteristics of host rocks, secondary minerals, and fluids would affect the transport of radionuclides from a previously proposed repository at Yucca Mountain, Nevada. Minerals in the Yucca Mountain tuffs that are important for retarding radionuclides include clinoptilolite and mordenite (zeolites), clay minerals, and iron and manganese oxides and hydroxides. Water compositions along flow paths beneath Yucca Mountain are controlled by dissolution reactions, silica and calcite precipitation, and ion-exchange reactions. Radionuclide concentrations along flow paths from a repository could be limited by (1) low waste-form dissolution rates, (2) low radionuclide solubility, and (3) radionuclide sorption onto geological media. The chief sources of radioactivity in spent nuclear fuel are americium, plutonium, and neptunium. Therefore, studies have concentrated on their geochemical mobility. Uranium-233, uranium-234, iodine-129, technetium-99, and other radionuclides also have been included in some experiments. Solubilities were determined experimentally in representative Yucca Mountain waters. Sorption coefficients were determined using water, rock, and pure mineral samples from Yucca Mountain. Batch experiments were performed at several pH levels and oxidizing conditions. Dynamic transport-column experiments, diffusion experiments, and solid-rock beaker experiments also were conducted. The batch tests gave slightly lower retardation factors than those derived from column-breakthrough experiments. This finding indicates that using batch-sorption coefficients to predict radionuclide transport will yield conservative results in a performance assessment. Understanding of unsaturated-zone transport is based on laboratory and field-scale experiments. Fractures provide advective transport pathways. Sorption and matrix diffusion may contribute to retardation of radionuclides. Conversely, sorption onto mobile colloids may enhance radionuclide transport.

ABSTRACT New data and review of classic sections from the Middle and Upper Ordovician North American Midcontinent in the Upper Mississippi Valley provide a refined picture of the age, stable isotope geochemistry, faunal composition, and—ultimately—origin of this epeiric ramp succession. Sequence stratigraphic analysis reveals a series of unconformity-bounded, genetically related facies packages. Shallowing and deepening trends are sometimes difficult to resolve due to a paucity of hydrodynamic indicators, yet unconformity surfaces are well marked by hardgrounds and confirmed by negative C-isotope spikes. Recent conodont biostratigraphy, new U-Pb radioisotopic ages for K-bentonites, and correlation of C-isotope profiles to global trends suggest that the succession spans the Darriwilian to Hirnantian epochs. Focus on Platteville to lower Galena Group strata (Sandbian to early Katian) provides a temporally high-resolution look at the onset and evolution of a long-term (>2 m.y.) positive carbon-isotope excursion, short-term perturbations in that record, and relationship to the preservation and diversity of the enclosed fauna and strata. Major changes in authigenic mineral suites and organic carbon content throughout the Upper Ordovician Upper Mississippi Valley suggest at least three major redox cycles. The combined evidence for globally recognized, positive carbon-isotope excursions coincident with these redox cycles, as well as high-frequency, sea-level fluctuations and successive faunal turnover events, suggests far-field responses to multiple global oceanic anoxic events.