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In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O2 (∼1 μM), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ∼1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter. [ABSTRACT FROM AUTHOR]

Abstract: In 2004, the US Geological Survey (USGS) and the Geological Survey of Canada (GSC) initiated a pilot study that involved collection of more than 1500 soil samples from 221 sites along two continental transects across Canada and the United States. The pilot study was designed to test and refine protocols for a soil geochemical survey of North America. The two transects crossed a wide array of soil parent materials, soil ages, climatic conditions, landforms, land covers and land uses. Sample sites were selected randomly at approximately 40-km intervals from a population defined as all soils of the continent. At each site, soils representing 0 to 5cm depth, and the O, A, and C horizons, if present, were collected and analyzed for their near-total content of over 40 major and trace elements. Soils from 0–5cm depth were also collected for analysis of organic compounds. Results from the transects confirm that soil samples collected at a 40-km spacing reveal coherent, continental- to subcontinental-scale geochemical and mineralogical patterns that can be correlated to aspects of underlying soil parent material, soil age and climate influence. The geochemical data also demonstrate that at the continental-scale the dominance of any of these major factors that control soil geochemistry can change across the landscape. Along both transects, soil mineralogy and geochemistry change abruptly with changes in soil parent materials. However, the chemical influence of a soil’s parent material can be obscured by changing climatic conditions. For the transects, increasing precipitation from west to east and increasing temperature from north to south affect both soil mineralogy and geochemistry because of climate effects on soil weathering and leaching, and plant productivity. Regional anomalous metal concentrations can be linked to natural variations in soil parent materials, such as high Ni and Cr in soils developed on ultramafic rocks in California or high P in soils formed on weathered Ordovician limestones in central Kentucky. On local scales, anomalous metal concentrations recognized in soil profiles, such as high P in soils from animal confinement sites, are consistent with local anthropogenic disturbances. At a larger scale, the distribution of Hg across the west to east transect demonstrates that it can be difficult to distinguish between natural or anthropogenic contributions and that many factors can contribute to an element’s spatial distribution. Only three samples in a subset of seventy-three 0–5cm depth soil samples from the north to south transect had organochlorine pesticides values above the method detection limit, apparently related to historic usage of the pesticides DDT and dieldrin. [Copyright &y& Elsevier]

A geochronological study of the Chocolay Group at the base of the Paleoproterozoic Marquette Range Supergroup in Michigan, Lake Superior Region, is attempted for the first time. Age data from detrital zircon grains and hydrothermal xenotime from the basal glaciogenic formation, the Enchantment Lake Formation, and the stratigraphically higher Sturgeon Quartzite and its equivalent, the Sunday Quartzite, provide maximum and minimum age constraints for the Chocolay Group. The youngest detrital zircon population in the Enchantment Lake Formation is 2317 ± 6 Ma; in the Sturgeon Quartzite, it is 2306 ± 9 Ma, and in the Sunday Quartzite, it is 2647 ± 5 Ma. The oldest hydrothermal xenotime age in the Enchantment Lake Formation is 2133 ± 11 Ma; in the Sturgeon Quartzite, it is 2115 ± 5 Ma, and in the Sunday Quartzite, it is 2207 ± 5 Ma. The radiometric age data in this study implies the depositional age of the Chocolay Group is constrained to ∼2.3-2.2 Ga, which proves its correlation with part of the Huronian Supergroup in the Lake Huron Region, Ontario, and reveals the unconformity that separates the Chocolay Group from the overlying Menominee Group is up to 325 million years in duration. The source(s) of the ∼ 2.3 Ga detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma because no known rock units of this age are known in the Michigan area. It is speculated that once widespread volcano-sedimentary cover sequences in Michigan were removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages probably reflect basinal hydrothermal fluid flow associated with the period of extension, involving rifting and major dyke formation, that affected the North American provinces between 2.2 and 2.1 Ga. [ABSTRACT FROM AUTHOR]

Although the Great Basin of North America has produced some of the most robust and ancient fiber artifact assemblages in the world, many were recovered with poor chronological controls. Consequently, this class of artifacts has seldom been effectively incorporated into general discussions of early chronological and cultural patterns. In recent years, the Great Basin Textile Dating Project has accumulated direct AMS dates on textiles (bags, sandals, mats, cordage, and basketry) from dry caves in the Great Basin, particularly in the northern and western areas. We focus here on the terminal Pleistocene/early Holocene, to identify chronological patterns in this class of artifacts and to evaluate Adovasio's characterization of the region's earliest basketry as simple and undecorated. New AMS dates now suggest that the region's earliest people had sophisticated textile traditions that incorporated numerous decorative elaborations. Some distinctive structures, including Fort Rock sandals and weft-faced plaited textiles, have limited early temporal ranges and may serve as diagnostic indicators for terminal Pleistocene/early Holocene times. Other basketry forms and structures that appear by about 9000 cal B.P. persist into the historic period, suggesting a stronger thread of continuity (especially in the north) from this time than is apparent in lithic traditions. [ABSTRACT FROM AUTHOR]