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ABSTRACT Quaternary to Recent sedimentary dolomite in groundwater calcrete and saltmarsh sediments of Lake Way and Lake Maitland, Western Australia, provide new information about the formation of low‐temperature dolomite. Dolomite can form through numerous pathways depending on the depositional and diagenetic environment. Many pathways involve microbial processes and/or the presence of a nucleation substrate which help overcome kinetic barriers preventing precipitation in the laboratory. Petrographic, mineralogical, hydrogeochemical and stable isotopic data in this study reveal the importance of Mg‐clays as nucleation sites for dolomite precipitation in a range of aquifer environments. There is also a close association between authigenic Mg‐clays, dolomite and the potassium–uranyl–vanadate ore mineral carnotite in channel and playa uranium deposits. It is interpreted that evaporation‐driven precipitation of dolomite establishes a positive feedback loop promoting the dissociation of aqueous uranyl–carbonate complexes and concomitant increase in carnotite saturation. Critical to this model is the presence of authigenic Mg‐clays because they facilitate dolomite precipitation and promote carnotite nucleation by concentrating potassium ions on clay surfaces via adsorption. This Mg‐clay–dolomite–carnotite relationship is widespread throughout Western Australian channel and playa uranium deposits and has been observed in similar deposits in Namibia and Botswana. In addition to this economic implication, Mg‐clay mediated nucleation of dolomite potentially has global relevance as a precipitation mechanism for low temperature dolomite in sedimentary deposits where detrital and/or authigenic Mg‐clays are present. Maturation of sedimentary dolomite from disordered high‐calcian dolomite to ordered low‐calcian dolomite occurs very early in the meteoric realm making it resistant to alteration during burial diagenesis. Diagenetic resistance may be further increased by early meteoric silicification related to the degradation of associated Mg‐clays. These findings indicate that Mg‐clay associated sedimentary dolomite has potential to retain a primary isotopic signature indicative of its origin, making it a useful recorder of palaeoenvironmental conditions. [ABSTRACT FROM AUTHOR]

The Green Point Formation of the Cow Head Group in western Newfoundland includes the Global Boundary Stratotype Section and Point (GSSP) for the Cambrian-Ordovician (є--O) boundary on Laurentia. This formation is dominated by slope deposits of alternating lime mud-stones and shale interbeds. Here, we employ isotopic/ elemental data (e.g., Mo, U, pyrite S, and I/Ca ratios) of these sediments to better understand the local depositional environments and the marine redox state at the (є--O) transition. The wide ranges of the shale authigenic δ98Moauth (-0.40 to +3.16%o) and the limestone δ98Mocarb (-0.30 to +2.34%o), along with the relatively low shale [Mo]auth (3.97 ± 10.26 µg/g, 2σ) and limestone [Mo]carb (0.25 ± 0.80 µg/g, 2σ), suggest that both shales and limestones of this interval were deposited under dysoxic to anoxic conditions. This interpretation agrees with the substantial low limestone I/Ca ratios (0.02 to 0.33 μmol/mol), which are well below the Proterozoic Eon background I/Ca values of ~0.5 to 1 μmol/ mol, suggesting shallow marine oxyclines along the regional continental slope. An inverse correlation between the shale δ98Moauth and δ238Uauth values, analogous to those observed from modern semi-restricted basins, indicates that the local basin was probably partially isolated from the open ocean. Consistent with this inference, significant inverse relationships between the shale pyrite δ34Spy signals and the contents of terrigenous elements (Al, Th, and ∑REE) imply that the terrestrial input could have played an important role in regulating the sulfate availability of the local water body. Despite the evidence for basin isolation, the shallow basin water δ238U signals (-0.70 ± 0.24‰, 1σ), inferred from the limestone δ238Ucarb, were probably close to coeval open oceans. With this presumption, the three-sink U-isotope mass balance model predicts that the є--Otransition was a time of expanded marine anoxia with anoxic/euxinic water covering 1.0 to 21.1% of the seafloor. [ABSTRACT FROM AUTHOR]

Understanding how animals respond to large-scale environmental changes is difficult to achieve because monitoring data are rarely available for more than the past few decades, if at all. Here, we demonstrate how a variety of palaeoecological proxies (e.g. isotopes, geochemistry and DNA) from an Andean Condor ( Vultur gryphus ) guano deposit from Argentina can be used to explore breeding site fidelity and the impacts of environmental changes on avian behaviour. We found that condors used the nesting site since at least approximately 2200 years ago, with an approximately 1000-year nesting frequency slowdown from ca 1650 to 650 years before the present (yr BP). We provide evidence that the nesting slowdown coincided with a period of increased volcanic activity in the nearby Southern Volcanic Zone, which resulted in decreased availability of carrion and deterred scavenging birds. After returning to the nest site ca 650 yr BP, condor diet shifted from the carrion of native species and beached marine animals to the carrion of livestock (e.g. sheep and cattle) and exotic herbivores (e.g. red deer and European hare) introduced by European settlers. Currently, Andean Condors have elevated lead concentrations in their guano compared to the past, which is associated with human persecution linked to the shift in diet.