The deep subsurface is an enormous repository of microbial life. However, the metabolic capabilities of these microorganisms and the degree to which they are dependent on surface processes are largely unknown. Due to the logistical difficulty of sampling and inherent heterogeneity, the microbial populations of the terrestrial subsurface are poorly characterized. In an effort to better understand the biogeochemistry of deep terrestrial habitats, we evaluate the energetic yield of chemolithotrophic metabolisms and microbial diversity in the Sanford Underground Research Facility (SURF) in the former Homestake Gold Mine, SD, USA. Geochemical data, energetic modeling, and DNA sequencing were combined with principle component analysis to describe this deep (down to 8100 ft below surface), terrestrial environment. SURF provides access into an iron-rich Paleoproterozoic metasedimentary deposit that contains deeply circulating groundwater. Geochemical analyses of subsurface fluids reveal enormous geochemical diversity ranging widely in salinity, oxidation state (ORP 330 to −328 mV), and concentrations of redox sensitive species (e.g., Fe2+ from near 0 to 6.2 mg/L and Σ S2- from 7 to 2778μg/L). As a direct result of this compositional buffet, Gibbs energy calculations reveal an abundance of energy for microorganisms from the oxidation of sulfur, iron, nitrogen, methane, and manganese. Pyrotag DNA sequencing reveals diverse communities of chemolithoautotrophs, thermophiles, aerobic and anaerobic heterotrophs, and numerous uncultivated clades. Extrapolated across the mine footprint, these data suggest a complex spatial mosaic of subsurface primary productivity that is in good agreement with predicted energy yields. Notably, we report Gibbs energy normalized both per mole of reaction and per kg fluid (energy density) and find the later to be more consistent with observed physiologies and environmental conditions. Further application of this approach will significantly expand our understanding of the deep terrestrial biosphere.

23The Proterozoic Eon hosted the emergence and initial recorded diversification of 24 eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than 25 today's, although how much lower is debated. Here, we use concentrations of iodate (the oxidized 26 iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive 27 record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both 28 local oxygen availability and the relative proximity to anoxic waters. To assess the validity of 43

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Identifying temporal trends in deep subsurface geomicrobiology is challenging as it requires both in-depth knowledge of in situ geochemistry and innovative sampling techniques. Subsurface microbial dynamics can only be understood in the context of accompanying geochemistry, and thus, it is imperative to first characterize available microbial habitats and their temporal evolution. Also, samples must be acquired in a clean and consistent manner to avoid artifacts stemming from surface microbes, atmospheric contamination, or external temporal variability. To facilitate these ends, we established the Deep Mine Microbial Observatory (DeMMO) in the Sanford Underground Research Facility (SURF), Lead, SD, USA to sample naturally draining fracture fluids at six spatially distributed sites from the shallowest (800 ft) to the deepest accessible (4,850 ft) depths. Here we report on the installation and subsequent two-year aqueous geochemical monitoring campaign of the DeMMO network. DeMMO fluids have distinct geochemical compositions showing differences with respect to depth, proximity to mine workings, and host rock geology. Most measurements were remarkably stable through the two-year sampling window, illustrating temporal stability of the water sources to each site, including over induced perturbations such as drilling. Interestingly, there was a lack of seasonality even at shallowest sites, indicating limited direct communication with modern meteoric waters. Patterns of fluid geochemistry are distinct between sites, and largely predictable based upon our understanding of the lithology and inorganic geochemistry of the host rocks. Thermodynamic calculations suggest that both inorganic and organic redox reactions can yield energy to, respectively, lithotrophic and heterotrophic microorganisms in this system, although the yields vary considerably by site. We conclude that each DeMMO site represents a unique window into the deep subsurface of SURF, accessing distinct fluid pockets, aqueous geochemistry, and dissolved gas geochemistry-providing stable conditions that facilitate long-term habitation of subsurface fractures and water pockets by distinct microbial communities.

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Projections of future rates of mass loss from the Greenland Ice Sheet are highly uncertain because its sensitivity to warming is unclear. Geologic reconstructions of Quaternary interglacials can illustrate how the ice sheet responded during past warm periods, providing insights into ice sheet behavior and important tests for data-model comparisons. However, paleoclimate records from Greenland are limited: Early Holocene peak warmth has been quantified at only a few sites, and terrestrial sedimentary records of prior interglacials are exceptionally rare due to glacial erosion during the last glacial period. Here, we discuss findings from a lacustrine archive that records both the Holocene and the Last Interglacial (LIG) from Greenland, allowing for direct comparison between two interglacials. Sedimentary chironomid assemblages indicate peak July temperatures 4.0 to 7.0 °C warmer than modern during the Early Holocene maximum in summer insolation. and chironomids in LIG sediments indicate July temperatures at least 5.5 to 8.5 °C warmer than modern. These estimates indicate pronounced warming in northwest Greenland during both interglacials. This helps explain dramatic ice sheet thinning at Camp Century in northwest Greenland during the Early Holocene and, for the LIG, aligns with controversial estimates of Eemian warming from ice core data retrieved in northern Greenland. Converging geologic evidence for strong LIG warming is challenging to reconcile with inferred Greenland Ice Sheet extent during the LIG, and the two appear incompatible in many models of ice sheet evolution. An increase in LIG snowfall could help resolve this problem, pointing to the need for hydroclimate reconstructions from the region.