Lisa M. Pratt

Oxidation of the Ediacaran Ocean

Oxygenation of the Earth’s surface is increasingly thought to have occurred in two steps. The first step, which occurred ∼2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period (∼800–542u2009Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates, and may have led to oxygenation of the deep ocean. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (∼635 to ∼548u2009Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period, our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon and sulphur cycles that occurred during this significant period of Earth’s history.

Challenges for Coring Deep Permafrost on Earth and Mars

A scientific drilling expedition to the High Lake region of Nunavut, Canada, was recently completed with the goals of collecting samples and delineating gradients in salinity, gas composition, pH, pe, and microbial abundance in a 400 m thick permafrost zone and accessing the underlying pristine subpermafrost brine. With a triple-barrel wireline tool and the use of stringent quality assurance and quality control (QA/QC) protocols, 200 m of frozen, Archean, mafic volcanic rock was collected from the lower boundary that separates the permafrost layer and subpermafrost saline water. Hot water was used to remove cuttings and prevent the drill rods from freezing in place. No cryopegs were detected during penetration through the permafrost. Coring stopped at the 535 m depth, and the drill water was bailed from the hole while saline water replaced it. Within 24 hours, the borehole iced closed at 125 m depth due to vapor condensation from atmospheric moisture and, initially, warm water leaking through the casing, which blocked further access. Preliminary data suggest that the recovered cores contain viable anaerobic microorganisms that are not contaminants even though isotopic analyses of the saline borehole water suggests that it is a residue of the drilling brine used to remove the ice from the upper, older portion of the borehole. Any proposed coring mission to Mars that seeks to access subpermafrost brine will not only require borehole stability but also a means by which to generate substantial heating along the borehole string to prevent closure of the borehole from condensation of water vapor generated by drilling.

Introduction to geochemistry of metalliferous black shales

1. Metalliferous black shales Metalliferous black shales exhibit a wide range of physical and chemical characteristics. They can be enriched in many different metals, and the metals can be in a variety of geochemical states. Depositional and geological histories of these shales are diverse. Some metalliferous black shales contain relatively high concentrations of organic matter whereas others do not. Because of elevated carbonate contents, some are calcareous shales or marlstones. Metal enrichments can result from redox reactions mediated by bacteria during early diagenesis and also from those associated with hydrothermal processes after deep burial. However, unifying characteristics of metalliferous black shales are sufficiently well established to justify combining a diverse number of fine-grained sedimentary rock types under a common term that reflects enhanced metal content, dark color and an inferred association with organic matter (e.g., Huyck, 1990). Moreover, the term metalliferous black shales is sufficiently accepted that Working Group 254 of the International Geological Correlation Program (IGCP) has that title and focus. This special issue of Chemical Geology brings together reports describing characteris