The Coorong Lagoon is a unique hydrological and depositional system at the terminus of the Murray–Darling Basin, the largest river system in Australia. It exhibits large salinity, nutrient, and organic matter gradients, providing a modern analogue to study and validate the use of δ 15 N and δ 13 C as tracers of past and contemporary geochemical cycles in estuarine environments. To this end, water and surface sediment samples were analyzed for particulate organic nitrogen (PON) and carbon (POC) concentrations, and the respective δ 15 N and δ 13 C signatures of particulate nitrogen and carbon. PON and POC exhibited positive relationships to chlorophyll-a, indicating the dominance of phytoplankton production upon suspended organic matter. There was also a general trend of increasing δ 15 N of PON (δ 15 N PON ) values and decreasing δ 13 C of particulate carbon (δ 13 C PC ) values with increasing salinity and eutrophication in the restricted South Lagoon. In a multiple linear regression for δ 15 N PON , the best two predictors in combination are PON and C:N molar ratio, highlighting the importance of productivity and the type or source of organic matter. For δ 13 C PC , the best two predictors are total dissolved phosphorus and latitude, suggesting influences from productivity and proximity to the ocean. Sediment δ 15 N values across the Coorong Lagoon overlap with the δ 15 N PON in the water column, suggesting that PON derived from algal material represents the main source of nitrogen to lagoon sediments. We hypothesize that limited N loss via denitrification leads to PON being recycled almost exclusively to ammonium, due to low rates of nitrification and dominance of dissimilatory nitrate reduction to ammonium (DNRA). We propose that preferential volatilization of 14 N in ammonia increases the δ 15 N of ammonium assimilated by phytoplankton, thereby increasing the δ 15 N within suspended organic matter and surface sediment in the South Lagoon. By contrast, the gradient exhibited in δ 13 C PC data was countered by a relatively constant sedimentary organic carbon δ 13 C. Data from the Coorong, therefore, suggest that δ 15 N values in sediments can be used to infer palaeoproductivity in this hypereutrophic and hypersaline depositional environment, however, the measured δ 13 C PC may be influenced by δ 13 C DIC or preferential loss of 13 C during sedimentation that alter the sedimentary δ 13 C record of organic carbon.
Sedimentary rocks contain a unique record of the evolution of the Earth system. Deciphering this record requires a robust understanding of the identity, origin, composition, and post-depositional history of individual constituents. Petrographic analysis informed by Scanning Electron Microscope - Energy Dispersive Spectroscopy (SEM-EDS) mineral mapping can reveal the mineral identity, morphology and petrological context of each imaged grain, making it a valuable tool in the Earth Scientist’s analytical arsenal. Recent technological developments, including quantitative deconvolution of mixed-phase spectra (producing “mixels”), now allow rapid quantitative SEM-EDS-based analysis of a broad range of sedimentary rocks, including the previously troublesome fine-grained lithologies that comprise most of the sedimentary record. Here, we test the reliability and preferred mineral mapping work flow of a modern Field-Emission scanning electron microscope equipped with the Thermofisher Scientific Maps Mineralogy mineral mapping system, focusing on mud/siltstones and calcareous shales. We demonstrate that SEM-EDS mineral mapping that implements 1) a strict error minimization spectral matching approach and 2) spectral deconvolution to produce ‘mixels’ for mixed-phase X-ray volumes can robustly identify individual grains and produce quantitative mineralogical data sets comparable to conventional X-ray diffraction (XRD) analysis ( R 2 > 0.95). The correlation between SEM-EDS and XRD-derived mineralogy is influenced by mineral abundance, processing modes and mapped area characteristics. Minerals with higher abundance (>10 wt%) show better correlation, likely the result of increased uncertainty for XRD quantification of low-abundance phases. Automated spectral deconvolution to produce ‘mixels’ greatly reduces the proportion of unclassified pixels, especially in the fine-grained fraction, ultimately improving mineral identification and quantification. Mapping of larger areas benefits bulk mineralogy analysis, while customized area size and shape allows high-resolution in situ mineralogical analysis. Finally, we review SEM-EDS-based mineral mapping applications in the Earth Sciences, via case studies illustrating 1) approaches for the quantitative differentiation of various mineral components including detrital (allogenic), syndepositional (authigenic) and burial diagenetic phases, 2) the origin and significance of lamination, 3) the effectiveness and appropriateness of sequential leaching in geochemical studies, and 4) the utility of mineral maps to identify target grains within specific petrological contexts for in situ geochemical or geochronological analysis.
Abstract Retrograde clay mineral reactions (reverse weathering), including glauconite formation, are first-order controls on element sequestration in marine sediments. Here, we report substantial element sequestration by glauconite formation in shallow marine settings from the Triassic to the Holocene, averaging 3 ± 2 mmol·cm − ²·kyr −1 for K, Mg and Al, 16 ± 9 mmol·cm − ²·kyr −1 for Si and 6 ± 3 mmol·cm − ²·kyr −1 for Fe, which is ~2 orders of magnitude higher than estimates for deep-sea settings. Upscaling of glauconite abundances in shallow-water (0–200 m) environments predicts a present-day global uptake of ~≤ 0.1 Tmol·yr −1 of K, Mg and Al, and ~0.1–0.4 Tmol·yr −1 of Fe and Si, which is ~half of the estimated Mesozoic elemental flux. Clay mineral authigenesis had a large impact on the global marine element cycles throughout Earth’s history, in particular during ‘greenhouse’ periods with sea level highstand, and is key for better understanding past and present geochemical cycling in marine sediments.
The greater McArthur Basin of northern Australia is a vast frontier exploration province for basin-hosted resources, both hydrocarbons (oil and natural gas) and metals (critical metals [e.g. rare earth elements, Co], Cu, Pb, Zn and Au). This basin system covers much of northern Australia and may have included much of North China that lay off northern Australia when the basin formed—ca. 1820–1325 Ma. Hydrocarbon and metal deposits in the basin are largely controlled by host sediment composition and ‘redox traps’ related to ancient water chemistry, which, in-turn, are modulated by biological activity, tectonism and relative sea level change. None of these controls are fully understood or constrained throughout the basin.
<p>The greater McArthur Basin is a regionally extensive Palaeo-to-Mesoproterozic, intra-cratonic, super basin system overlying the North Australian craton. Deposition initiated after the Pine Creek Orogeny whereby the basin extends from Western Australia to northwestern Queensland. Lithostratigraphic units are divided into five coherent packages of similar age, stratigraphic position and facies association. Successions of the basin system are dominated by an assemblage of sedimentary siliciclastics, evaporitic carbonates and organic-rich mudstones with minor intersections of volcanic rocks and records nearly a billion years of Earth&#8217;s history from ca. 1.82 Ga to the Tonian. This period has generally been considered a time of stability within the Earth system and is therefore unfortunately titled &#8216;the boring billion&#8217;. However, compilation of new and existing water chemistry proxies shown in this study reflected the contrary. Notably, shales and carbonates from the greater McArthur Basin chronicled a critical time in Earth&#8217;s history; where the oxygenation of the ocean and atmosphere began and multi-cellular eukaryotes started to thrive within the ecosystem, demonstrating that this interval in the geological record is anything but boring.</p><p>This study applied a multi-proxy approach based on observations of isotopic tracers and elemental variations from an extensive archive of carbonate-rich units throughout the greater McArthur Basin to reconstruct its palaeoenvironment, determine the tectonic setting and establish regional or global correlations. Elucidating the evolution of the basin is essential for understanding the controls of its petroleum and mineral resources as well as how Earth system processes developed during the Proterozoic. Radiogenic and stable isotopes are used to infer palaeo-depositional constraints such as biological productivity, weathering fluxes and provenance sources. Redox-sensitive elemental concentrations can also be used to reflect the changes in water-column chemistry between oxic, anoxic and euxinic conditions.</p><p>Consequently, results from this study illustrate the relationship between the precipitation of metal compounds, production of organic matter and preservation of both systems with large-scale biogeochemical processes. Furthermore, this study also highlights the spatial and temporal variations of water chemistry within the basin. Enrichment in Mo concentrations in the Wollogorang Formation within the Tawallah Group indicated spells of widespread euxinia. Base metal concentrations within the unit showed lithogeochemical, halo-like distribution that is strongly correlated with changes in water column redox conditions. A shift to more radiogenic <sup>87</sup>Sr/<sup>86</sup>Sr values up to &#8764;0.722 in the Fraynes Formation of the Limbunya Group reflected an increase in relative contribution of strontium from old continental crust in contrast to hydrothermal input which is consistent with a transient basin restriction from the open ocean. Rare earth and yttrium (REY) plots of the Dook Creek Formation inferred parts of the basin may have been lacustrine at ca. 1.5 Ga. Further up stratigraphy, the Middle Velkerri showed a shift towards more positive &#949;<sub>Nd(t)</sub> values, representing a change to more juvenile source regions. These mafic provenances are richer in essential nutrients for biological activity such as phosphorus. More juvenile &#949;<sub>Nd(t) </sub>data within the Velkerri Formation coincide with an increase in P concentrations and total organic carbon content (>8 wt. %).</p>
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