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The great oxidation event (GOE), ~2.4 billion years ago, caused fundamental changes to the chemistry of Earth's surface environments. However, the effect of these changes on the biosphere is unknown, due to a worldwide lack of well‐preserved fossils from this time. Here, we investigate exceptionally preserved, large spherical aggregate (SA) microfossils permineralised in chert from the c. 2.4 Ga Turee Creek Group in Western Australia. Field and petrographic observations, Raman spectroscopic mapping, and in situ carbon isotopic analyses uncover insights into the morphology, habitat, reproduction and metabolism of this unusual form, whose distinctive, SA morphology has no known counterpart in the fossil record. Comparative analysis with microfossils from before the GOE reveals the large SA microfossils represent a step‐up in cellular organisation. Morphological comparison to extant micro‐organisms indicates the SAs have more in common with coenobial algae than coccoidal bacteria, emphasising the complexity of this microfossil form. The remarkable preservation here provides a unique window into the biosphere, revealing an increase in the complexity of life coinciding with the GOE.

 

Molybdenum (Mo) in marine sediments has been used as a paleoproxy to provide evidence for past oceanic euxinic and sulfidic conditions through its association with pyrite. Here, we examine the adsorption of Mo to the pyrite precursors mackinawite and greigite and assess the robustness of this association during iron sulfide phase transformations. Tetrathiomolybdate (MoS42–) adsorption experiments were done using mackinawite and greigite that had been characterized using powder X-ray diffraction and Raman spectroscopy. Adsorption of tetrathiomolybdate to mackinawite and to a primarily greigite mixture was similar. Both showed little change to the mineral phase upon adsorption. Relative to previously published data on pyrite, there was a much greater amount of Mo adsorption and a different mode of adsorption. A mackinawite/greigite mixture was also synthesized through an alternative method that more closely mimicked environmental conditions with a brief in situ aging to form an initial phase of iron sulfide, likely highly disordered mackinawite, and the near-immediate addition of MoS42–. X-ray photoelectron spectroscopy results support the adsorption of tetrathiomolybdate and its concomitant reduction to Mo(IV). The Mo-adsorbed mackinawite/greigite mixture was transformed through heating into a greigite/pyrite mixture while monitoring Mo release to the aqueous phase. Here, the sorption of Mo on the solid phase promoted the transformation of mackinawite into pyrite upon heating without diagenetic loss of Mo to the aqueous phase. These results support the early capture of MoS42– to less-stable forms of iron sulfide with negligible diagenetic loss during subsequent transformation. This work continues to point to Mo(VI) as a plausible oxidant of FeS to FeS2 within natural euxinic settings. 

This work is a systematic experimental and theoretical study of the in‐plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type.k‐space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near‐zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model‐based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light–matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum‐)plasmonics and nano‐photonics.