Cambrian Trilobites From the Nounan Dolomite and Lower St. Charles Formation (Upper Marjuman to Lower Sunwaptan; Miaolingian to Furongian Series), Smithfield Canyon, Northern Utah

The trilobite faunas that occur with the Steptoean Positive Isotope Carbon Excursion (SPICE) at Smithfield Canyon, Utah, have been reported, but not illustrated. Given the importance of the SPICE at this section for international correlations, the trilobites from new collections from the upper Nounan Dolomite to lower St. Charles Formation at Smithfield Canyon are reported herein and integrated with the previously reported taxa. Trilobite assemblages indicate that the upper Cedaria to the Ellipsocephaloides biozones (Miaolingian Series, Guzhangian Stage to Furongian Series, Jiangshanian Stage) are present stratigraphically below or above the SPICE. Some of the taxa reported herein may represent new species, but they are not represented by well-enough preserved specimens and are left in open nomenclature. However, Kingstonia smithfieldensis n. sp. and Bromella utahensis n. sp. are named on the basis of common and well-preserved specimens. New carbon isotope data from Smithfield Canyon from an overlapping section of the lower St. Charles Formation, that add to the overall shape of the SPICE curve, are presented. The new δ13C values above the Elvinia Biozone range from –0.36‰ to +1.5‰, confirming that the SPICE concludes within the Elvinia Biozone

LA-ICP-MS Analysis of Quartzite from the Upper Gunnison Basin, Colorado

We report the results of LA-ICP-MS analysis of 402 quartzite samples representing 48 collection loci in the Upper Gunnison Basin (UGB), Colorado and determine the extent to which the sources can be geochemically discriminated from one another using this non-destructive technique. The ability to differentiate among the sources would open the door to provenance studies of the quartzite chipped-stone tools and debitage that constitute 95% or more of most of the 3000-plus prehistoric site assemblages documented in the UGB. Our samples represent prehistorically quarried and non-quarried quartzite sources, including outcrop (primary) and gravel (secondary) deposits. The results reveal spatial and chronological trends in quartzite elemental composition that can be exploited for provenance determinations of quartzite artifacts from UGB sites, albeit using an assemblage-based sourcing strategy that differs from the familiar approach of “matching” obsidian artifacts to their statistically likeliest geological source. We offer a preliminary version of a sourcing protocol for UGB quartzite

A Robust Age Model for the Cryogenian Pocatello Formation of Southeastern Idaho (Northwestern USA) from Tandem in situ and Isotope Dilution U-Pb Dating of Volcanic Tuffs and Epiclastic Detrital Zircons

Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤ 670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation. These 670–675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported “reworked fallout tuff,” which we instead conclude provides only a maximum depositional age of ≤ 673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716–660 Ma) and Marinoan (≤ 650–635 Ma) low-latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated

An Emerging Picture of Neoproterozoic Ocean Chemistry: Insights from the Chuar Group, Grand Canyon, USA

Detailed iron, sulfur and carbon chemistry through the > 742 million year old ChuarGroup reveals a marine basin dominated by anoxic and ferrous iron-rich (ferruginous) bottom waters punctuated, late in the basin's development, by an intrusion of sulfide-rich (euxinic) conditions. The observation that anoxia occurred frequently in even the shallowest of Chuar environments (10s of meters or less) suggests that global atmospheric oxygen levels were significantly lower than today. In contrast, the transition from ferruginous to euxinic subsurface water is interpreted to reflect basinal control—specifically, increased export of organic carbon from surface waters. Low fluxes of organic carbon into subsurface water masses should have been insufficient to deplete oxygen via aerobic respiration, resulting in an oxic oxygen minimum zone (OMZ). Where iron was available, larger organic carbon fluxes should have depleted oxygen and facilitated anaerobic respiration using ferric iron as the oxidant, with iron carbonate as the expected mineralogical signature in basinal shale. Even higher organic fluxes would, in turn, have depleted ferric iron and up-regulated anaerobic respiration by sulfate reduction, reflected in high pyrite abundances. Observations from the ChuarGroup are consistent with these hypotheses, and gain further support from pyrite and sulfate sulfur isotope abundances. In general, Chuar data support the hypothesis that ferruginous subsurface waters returned to the oceans, replacing euxinia, well before the Ediacaran emergence of persistently oxygenated conditions, and even predating the Sturtian glaciation. Moreover, our data suggest that the reprise of ferruginous water masses may relate to widespread rifting during the break-up of Rodinia. This environmental transition, in turn, correlates with both microfossil and biomarker evidence for an expanding eukaryotic presence in the oceans, suggesting a physiologically mediated link among tectonics, environmental chemistry and life in the dynamic Neoproterozoic Earth system.Earth and Planetary Science

Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere

This study was funded by NERC grant NE/F016832/1. P.A.E.P.v.S. is supported by NERC fellowship NE/I020571/2.Neoproterozoic (1,000–542 Myr ago) Earth experienced profound environmental change, including ‘snowball’ glaciations, oxygenation and the appearance of animals. However, an integrated understanding of these events remains elusive, partly because proxies that track subtle oceanic or atmospheric redox trends are lacking. Here we utilize selenium (Se) isotopes as a tracer of Earth redox conditions. We find temporal trends towards lower δ82/76Se values in shales before and after all Neoproterozoic glaciations, which we interpret as incomplete reduction of Se oxyanions. Trends suggest that deep-ocean Se oxyanion concentrations increased because of progressive atmospheric and deep-ocean oxidation. Immediately after the Marinoan glaciation, higher δ82/76Se values superpose the general decline. This may indicate less oxic conditions with lower availability of oxyanions or increased bioproductivity along continental margins that captured heavy seawater δ82/76Se into buried organics. Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution.Publisher PDFPeer reviewe