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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 Motivated by the need to interpret the results from a combined use ofin vivobrain Magnetic Resonance Elastography (MRE) and Diffusion Tensor Imaging (DTI), we developed a computational framework to study the sensitivity of single-frequency MRE and DTI metrics to white matter microstructure and cell-level mechanical and diffusional properties. White matter was modeled as a triphasic unidirectional composite, consisting of parallel cylindrical inclusions (axons) surrounded by sheaths (myelin), and embedded in a matrix (glial cells plus extracellular matrix). Only 2D mechanics and diffusion in the transverse plane (perpendicular to the axon direction) was considered, and homogenized (effective) properties were derived for a periodic domain containing a single axon. The numerical solutions of the MRE problem were performed with ABAQUS and by employing a sophisticated boundary-conforming grid generation scheme. Based on the linear viscoelastic response to harmonic shear excitation and steady-state diffusion in the transverse plane, a systematic sensitivity analysis of MRE metrics (effective transverse shear storage and loss moduli) and DTI metric (effective radial diffusivity) was performed for a wide range of microstructural and intrinsic (phase-based) physical properties. The microstructural properties considered were fiber volume fraction, and the myelin sheath/axon diameter ratio. The MRE and DTI metrics are very sensitive to the fiber volume fraction, and the intrinsic viscoelastic moduli of the glial phase. The MRE metrics are nonlinear functions of the fiber volume fraction, but the effective diffusion coefficient varies linearly with it. Finally, the transverse metrics of both MRE and DTI are insensitive to the axon diameter in steady state. Our results are consistent with the limited anisotropic MRE and co-registered DTI measurements, mainly in thecorpus callosum, available in the literature. We conclude that isotropic MRE and DTI constitutive models are good approximations for myelinated white matter in the transverse plane. The unidirectional composite model presented here is used for the first time to model harmonic shear stress under MRE-relevant frequency on the cell level. This model can be extended to 3D in order to inform the solution of the inverse problem in MRE, establish the biological basis of MRE metrics, and integrate MRE/DTI with other modalities towards increasing the specificity of neuroimaging.