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Abstract Demagnetizing effects and internal stress are difficult to distinguish in natural magnetite samples, but quantitative stress estimates can provide valuable information about microstructure formation, surface oxidation, impacts, tectonic stresses, or interface properties in exsolution structures. Quantifying demagnetizing effects informs about magnetite particle shape, magnetostatic interaction, or anisotropic texture. Here, we establish an improved measurement workflow to separate demagnetizing effects from internal stress for natural magnetite. The method is based on temperature‐dependent hysteresis measurements, and for natural samples require accurate estimates of Curie temperature and temperature‐dependent saturation magnetization to ensure that near‐end‐member magnetite is the dominant magnetic mineral, and to calibrate the temperature‐dependent scaled reversible work (SRW). SRW is the fundamental quantity to determine stress and demagnetizing factor. The improved SRW method is applied to three natural samples with different stress histories where it proves that large magnetite crystals in the metamorphosed Modum complex (Norway) have low internal stress (<100 MPa), while in highly exsolved magnetite‐ilmenite intergrowths from Taberg (Sweden) and Bushveld (South Africa) the magnetite component is highly stressed (>230 MPa). This confirms experimentally that interface strain in complex microstructures due to spinodal decomposition and partial oxidation creates large average internal stress in the magnetite minerals. Because sister specimens have similar internal stress but noticeably (>20%) different demagnetizing factors, textural, and shape anisotropy contribute substantially to SRW in these samples.
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Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal-bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020): AUTHIGENIC MAGNETITE OFFSHORE SHIMOKITA
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Abstract Methane seeps are highly productive deep‐sea ecosystems reliant on chemosynthetic primary production. They are increasingly affected by direct human activities that threaten key ecosystem services. Methane seepage often generates precipitation of authigenic carbonate rocks, which host diverse microbes, and a dynamic invertebrate community. By providing hard substrate, even after seepage ceases, these rocks may promote a long‐lasting ecological interaction between seep and background communities. We analyzed community composition, density, and trophic structure of invertebrates on authigenic carbonates at Mound 12, a seep on the Pacific margin of Costa Rica, using one mensurative and two manipulative experiments. We asked whether carbonate macrofaunal communities are able to survive, adapt, and recover from changes in environmental factors (i.e., seepage activity, chemosynthetic production, and food availability), and we hypothesized a key role for seepage activity in defining these communities and responses. Communities onin situcarbonates under different seepage activities showed declining density with increasing distance from the seep and a shift in composition from gastropod dominance in areas of active seepage to more annelids and peracarid crustaceans that are less dependent on chemosynthetic production under lesser seepage. Response to changing environmental context was evident from altered community composition following (1) a natural decline in seepage over successive years, (2) transplanting of carbonates to different seepage conditions for 17 months, and (3) defaunated carbonate deployments under different seepage regimes over 7.4 yr. Seep faunas on transplants to lesser seepage emerge and recover faster than transition fauna (characterized by native seep and background faunas, respectively) and are able to persist by adapting their diets or by retaining their symbiotic bacteria. The macrofaunal community colonizing defaunated carbonates deployed for 7.4 yr developed communities with a similar successional stage asin siturocks, although trophic structure was not fully recovered. Thus, macrofaunal successional dynamics are affected by habitat complexity and the availability of microbial chemosynthetic productivity. This multi‐experiment study highlights the interaction between biotic and abiotic factors at methane seeps at different time scales along a spatial gradient connecting seep and surrounding deep‐sea communities and offers insight on the resilience of deep‐sea macrofaunal communities.more » « less
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We present a method for thephotochemical conversion of the inverse spinel iron oxides in which the mixed-valent magnetite phase (Fe 3 O 4 ) is accessed from the maghemite phase (γ-Fe 2 O 3 ) via a stable, colloidal nanocrystal-to-nanocrystal transformation. Anaerobic UV-irradiation of colloidal γ-Fe 2 O 3 nanocrystals in the presence of ethanol as a sacrificial reductant yields reduction of some Fe 3+ to Fe 2+ , resulting in a topotactic reduction of γ-Fe 2 O 3 to Fe 3 O 4 . This reduction is evidenced by the emergence of charge-transfer absorption and increased d -spacing in UV-irradiated nanocrystals. Redox titrations reveal that ∼43% of Fe in < d > = 4.8 nm nanocrystals can be reduced with this method and comparison of optical data indicates similar reduction levels in < d > = 7.3 and 9.0 nm nanocrystals. Addition of excess acetaldehyde during photoreduction shows that the extent of reduction is likely pinned by the hydrogenation of acetaldehyde back to ethanol and can be increased with the use of an alkylborohydride sacrificial reductant. Photochemical reduction is accompanied by increased magnetization and emergence of magnetic features characteristic of Fe 3 O 4 . Overall, this work provides a reversible, post-synthetic strategy to obtain Fe 3 O 4 nanocrystals with well-controlled Fe 2+ compositions.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/2017GC006943
