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The Nonesuch Formation microbiota provide a window into ca. 1075 Ma life within the interior of ancient North America. The Nonesuch water body formed following the cessation of widespread volcanism within the Midcontinent Rift as the basin continued to subside. In northern Michigan and Wisconsin, USA, the Copper Harbor Conglomerate records terrestrial alluvial fan and fluvial plain environments that transitioned into subaqueous lacustrine deposition of the Nonesuch Formation. These units thin toward a paleotopographic high associated with the Brownstone Falls angular unconformity. Due to these “Brownstone Highlands,” we were able to explore the paleoenvironment laterally at different depths in contemporaneous deposits. Rock magnetic data constrain that when the lake was shallow, it was oxygenated as evidenced by an oxidized mineral assemblage. Oxygen levels were lower at greater depth—in the deepest portions of the water body, anoxic conditions are recorded. An intermediate facies in depth and redox between these endmembers preserves detrital magnetite and hematite, which can be present in high abundance due to the proximal volcanic highlands. This magnetic facies enabled the development of a paleomagnetic pole based on both detrital magnetite and hematite that constrains the paleolatitude of the lake to 7.1 ± 2.8°N. Sediments of the intermediate facies preserve exquisite organic-walled microfossils, with microfossils being less diverse to absent in the anoxic facies where amorphous organic matter is more likely to be preserved. The assemblage of cyanobacteria and eukaryotes (both photoautotrophs and heterotrophs) lived within the oxygenated waters of this tropical Mesoproterozoic water body. 

Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history. 

The early‐middle Neoproterozoic is thought to have witnessed significant perturbations to marine P cycling, in turn facilitating the rise of eukaryote‐dominated primary production. However, with few robust constraints on aqueous P concentrations, current understanding of Neoproterozoic P cycling is generally model‐dependent. To provide new geochemical constraints, we combined microanalytical data sets with solid‐state Nuclear Magnetic Resonance, synchrotron‐based X‐ray Absorption Near Edge Structure spectroscopy, and micro‐X‐ray Fluorescence imaging to characterize the speciation and distribution of P in Tonian shallow‐water carbonate rocks. These data reflect shallow water phosphate concentrations 10–100× higher than modern systems, supporting the hypothesis that tectonically‐driven influxes in P periodically initiated kinetically‐controlled CaCO₃deposition, in turn destabilizing marine carbonate chemistry, climate, and nutrient inventories. Alongside these observations, a new compilation and statistical analysis of mudstone geochemistry data indicates that, in parallel, C_organd P burial increased across later Tonian continental margins until becoming decoupled at the close of the Tonian, implicating widespread N‐limitation triggered by increasing atmospheric O₂.

 

Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H₂has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H₂in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H₂fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H₂production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO₂(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H₂production is directly proportional to both the SiO₂(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO₂(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H₂than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H₂production requires a reevaluation of the processes that supplied H₂to prebiotic and early microbial systems, as well as those that balanced ocean–atmosphere redox through time.