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One of the greatest threats facing the planet is the continued increase in excess greenhouse gasses, with CO2 being the primary driver due to its rapid increase in only a century. Excess CO2 is exacerbating known climate tipping points that will have cascading local and global effects including loss of biodiversity, global warming, and climate migration. However, global reduction of CO2 emissions is not enough. Carbon dioxide removal (CDR) will also be needed to avoid the catastrophic effects of global warming. Although the drawdown and storage of CO2 occur naturally via the coupling of the silicate and carbonate cycles, they operate over geological timescales (thousands of years). Here, we suggest that microbes can be used to accelerate this process, perhaps by orders of magnitude, while simultaneously producing potentially valuable by-products. This could provide both a sustainable pathway for global drawdown of CO2 and an environmentally benign biosynthesis of materials. We discuss several different approaches, all of which involve enhancing the rate of silicate weathering. We use the silicate mineral olivine as a case study because of its favorable weathering properties, global abundance, and growing interest in CDR applications. Extensive research is needed to determine both the upper limit of the rate of silicate dissolution and its potential to economically scale to draw down significant amounts (Mt/Gt) of CO2. Other industrial processes have successfully cultivated microbial consortia to provide valuable services at scale (e.g., wastewater treatment, anaerobic digestion, fermentation), and we argue that similar economies of scale could be achieved from this research. 

In the last few decades, the development of nontraditional isotope (e.g., Mo, Tl, U) measurements of redox sensitive metals provided information about the redox evolution of Earth’s oceans and atmosphere. Rhenium (Re) isotopes have the potential to fill a critical gap in the isotope proxy toolkit. Currently, there are proxies for ocean-basin-scale oxygenated and anoxic (0 uM O2 with no H2S) conditions, but there is not yet a proxy that can detect when large parts of the oceans were in a low-O2 but not anoxic condition, termed ‘suboxic’ (10 ≥ O2 > 0 uM). Detecting suboxic conditions is particularly important because some aerobic organisms can live in extremely low-O2 waters (down to ~10 nM O2; Stolper et al. 2010), and so it is of great interest to know when large parts of the ocean crossed from anoxic to suboxic conditions. Rhenium concentrations have been used as a paleoredox proxy to track suboxic and anoxic marine redox conditions locally, but do not easily extend globally. Because of the long residence time of Re in the oceans, the Re isotope proxy can likely track changes in the extent of suboxic conditions globally in the ocean. Previous publications provided methods for digesting and purifying Re for δ187Re analysis from different materials (e.g., seawater, basalt, sedimentary rocks, chondrites; Miller et al., 2015, Liu et al., 2017, Dellinger et al., 2019, Dickson et al., 2020). These publications set the foundation for creating a δ187Re ocean mass balance. However, there is as yet no method that specifically targets the authigenic Re in shales, which has the potential to directly capture δ187Re of contemporaneous seawater. Here, we report a novel method for digesting samples that is done in a single step that excludes the use of HF, utilizing the well-established Carius tube (CT) digestion technique. By not using HF, this method does not dissolve the silicate portion of samples, allowing the targeted removal of authigenic Re. We also introduce a two-step column chemistry approach that can be utilized to purify Re from large samples with very low Re concentrations. We are applying this new method to characterize δ187Re in modern euxinic and suboxic settings including the Black Sea and the Benguela margin. 

Abstract The Early Mississippian (Tournaisian) positive δ13C excursion (mid-Tournaisian carbon isotope excursion [TICE]) was one of the largest in the Phanerozoic, and the organic carbon (OC) burial associated with its development is hypothesized to have enhanced late Paleozoic cooling and glaciation. We tested the hypothesis that expanded ocean anoxia drove widespread OC burial using uranium isotopes (δ238U) of Lower Mississippian marine limestone as a global seawater redox proxy. The δ238U trends record a large Tournaisian negative excursion lasting ∼1 m.y. The lack of covariation between δ238U values and facies changes and proxies for local depositional and diagenetic influences suggests that the δ238U trends represent a global seawater redox signal. The negative δ238U excursion is coincident with the first TICE positive excursion, supporting the hypothesis that an expanded ocean anoxic event controlled OC burial. These results provide the first evidence from a global seawater redox proxy that an ocean anoxic event drove Tournaisian OC burial and controlled Early Mississippian cooling and glaciation. Uranium and carbon modeling results indicate that (1) there was an ∼6× increase in euxinic seafloor area, (2) OC burial was initially driven by expanded euxinia followed by expanded anoxic/suboxic conditions, and (3) OC burial mass was ∼4–17× larger than that sequestered during other major ocean anoxic events.