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ItemCryogenian iron formations: glaciation and oxygenationLechte, Maxwell ( 2018)The Cryogenian Period (720–635 Ma) experienced extreme glaciations broadly coincident with a transformation of the Earth’s surface oxidation state, supercontinent breakup, and the evolution of complex animal multicellularity. However, the cause-and-effect relationships of these events are unresolved. The Cryogenian ice ages, known as ‘Snowball Earth' events, would have placed important constraints on the biosphere, and it remains unclear what role global refrigeration played in setting the stage for eukaryotic diversification and the origin of animals. The Cryogenian also experienced the deposition of iron-rich marine chemical sediments (iron formations), representing the first episode of global iron formation deposition in over one billion years. This shift in iron cycling highlights complexities in seawater chemistry and oxidation state during this time, and these iron formations offer valuable insights into Cryogenian palaeoenvironments. Iron formations from Cryogenian glacial successions in Namibia, USA and Australia were studied in order to investigate Cryogenian iron formation genesis and elucidate the relationships between glaciation, ocean chemistry, oxygenation and biotic evolution. In-depth sedimentology, stratigraphy and petrography reveals that these iron formations are intimately associated with Sturtian glacial sediments and are interpreted have been deposited in a range of glaciomarine environments. Geochemical analysis of these chemical sediments permits the reconstruction of Cryogenian ocean chemistry and the synglacial palaeoredox landscape. Multiple geochemical proxies, including rare earth element and iron isotope systematics, indicate widespread marine anoxia with increasing seawater oxidation with proximity to the ice shelf grounding line. A genetic model is proposed whereby the mixing of oxygenated glacial fluids with ferruginous seawater led to the deposition of iron formations in glacial successions during the Cryogenian. Atmospheric oxygen trapped in glacial ice was likely an important oxidant source, delivered to Cryogenian glaciomarine environments via subglacial meltwater outwash. This meltwater supply may have been crucial in establishing oxygenated marine habitats for eukaryotes, including early animals, during Snowball Earth. Multi-million-year oxidation of the oceans via this mechanism may have also set the stage for a Neoproterozoic marine oxygenation event.
ItemIsotopic disequilibrium in granitic systems: the origins of heterogeneity in granites and implications for partial melting in the crust and petrogenetic modelsIles, Kieran Anthony ( 2017)Unravelling the processes involved in granite magmatism is essential to our understanding of the continental crust, its formation and evolution. Radiogenic isotope systems are commonly employed to this end, but the behaviour of these systems may not be as simple as is often assumed. Understanding the causes of isotopic complexity in granite suites is the aim of this research. By modelling the disequilibrium partial melting of isotopically heterogeneous protoliths the different compositions of the melt, source and restite for a range of hypothetical scenarios have been calculated. Results demonstrate that the melt produced may have Sr, Nd and Hf isotope compositions distinct from both the protolith and restite. A corollary is that restite-bearing magmas may exhibit different isotope compositions than their melts, a feature which should be preserved as a difference between the Hf isotope compositions of bulk-rock samples and their magmatic zircon populations. The same modelling also suggests that a single source rock can produce melts with diverse isotope compositions. The predictions of this modelling have been tested by analysing S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, including iconic examples of restite-bearing rocks. Comparisons of Hf isotope compositions between bulk-rocks and their magmatic zircons reveal discrepancies (ΔεHfbulk-zircon) ranging from -0.6 to +2.5 ε units for I-type granites. This intra-sample Hf isotopic heterogeneity is interpreted to represent disequilibrium between the melt and restite assemblage. The ΔεHfbulk-zircon values are consistent with calculated ΔεHfmagma-melt values (from -4.2 to +7.4) based on the disequilibrium amphibole dehydration melting of 0.5-1.0 Ga meta-igneous protoliths. S-type granites also record differences between their bulk-rock and magmatic zircon Hf isotope compositions; however, the disparity is more subtle. Both positive and small negative ΔεHfbulk-zircon values are observed, consistent with modelling the partial melting of isotopically heterogeneous meta-sedimentary protoliths. In addition to low-temperature granites, case studies of two high-temperature I-type granitoid suites (Boggy Plain and Wallundry) have also been conducted. Both display a weak coupling between geochemical parameters that have been interpreted previously to indicate the involvement of assimilation and fractional crystallisation (AFC) processes. Positive ΔεHfbulk-zircon values obtained in the Boggy Plain Suite support the existing petrogenetic model in which basaltic melt becomes variously contaminated by material derived from the continental crust. The positive value is explained by retention of earlier-crystallised, more radiogenic phases in isotopically evolved, more felsic samples. In contrast, the Wallundry Suite is characterised by negative ΔεHfbulk-zircon values caused by the presence of unmelted components of its contaminant. A complex interplay of contamination, crystallisation, melt segregation and interaction between magma batches is required to account for the Wallundry Suite isotope data. The results of this study indicate that disequilibrium partial melting can produce within-suite isotopic variability without recourse to assimilation or mixing processes (1) in mafic to felsic samples caused by the progressive separation of melt from its isotopically distinct restite assemblage; and (2) via the extraction of multiple batches of isotopically distinct melts produced from a single source as anatexis proceeds. Furthermore, the isotope variation resulting from restite unmixing may be distinguished from magma mixing by decoupling of the Rb-Sr, Sm-Nd and Lu-Hf isotope systems. Importantly, the isotopic discrepancy between bulk-rock granite samples and their magmatic zircon populations suggests that the most mafic bulk-rock granite samples of a given suite, not magmatic zircon, provide the most accurate estimate of source rock Hf isotope compositions. This raises concerns regarding the ubiquitous use of zircon Hf isotope data to constrain crustal growth models.
ItemAerosol contributions to speleothem geochemistryDredge, Jonathan ( 2014)There is developing interest in cave aerosols due to the increasing awareness of their impacts on the cave environment and speleothems. This study presents the first multidisciplinary investigation into cave aerosols and their potential contribution to speleothem geochemistry. Aerosols are shown to be sourced from a variety of external emission processes, and transported into cave networks. Both natural (marine sea-spray, terrestrial dust) and anthropogenic (e.g. vehicle emissions) aerosol emissions are detected throughout caves. Internal cave aerosol production by human disruption has also been shown to be of importance in caves open to the public. Aerosols produced from floor sediment suspension and release from clothing causes short term high amplitude aerosol suspension events. Cave aerosol transport, distribution and deposition are highly variable depending on cave situation. Cave morphology, ventilation, and environmental conditions will influence how aerosols are distributed through cave networks. Aerosol deposition monitoring in Obir Cave, Austria has shown the significance of cave chamber size in aerosol transport, with large open chambers presenting higher levels of deposition. Modern monitoring of suspended aerosol concentrations, CO2 and temperature in Gough’s Cave, Cheddar Gorge have presented a strong relationship with cave ventilation processes. Temporal variations of aerosol levels have demonstrated the ability of aerosol monitoring to record seasonal ventilation shifts, beyond anthropogenic influences. Aerosol minima (based on 24 hours) provide a representation of natural aerosol baseline conditions without diurnal anthropogenic influences. Aerosols have shown a quicker recovery to natural background levels when compared to CO2 and T, making aerosols a sensitive and effective monitoring tool. When used in combination with more established monitoring methods, suspended aerosol monitoring is a beneficial addition to cave environmental studies. Theoretical modelling and calculations based on modern aerosol monitoring have established that aerosol contributions are highly variable. In some instances, modern aerosol supply is sufficient to account for speleothem geochemistry concentrations entirely. Aerosol contributions are of greatest significance under slow growth or hiatus scenarios and high aerosol deposition scenarios. Geochemical and stratigraphical analysis of a flowstone core from Gibraltar has highlighted the importance of hiatus events for future aerosol studies. Hiatus events provide a unique opportunity to investigate the type and amount of aerosol deposition and accumulation. Marine aerosol contributions have been quantified in the Gibraltar flowstone core and account for 18.5% of speleothem Sr. Sr isotopic analysis has confirmed the significance of marine aerosol contributions. Flowstone analysis has also demonstrated the ability of speleothems to record shifts in the supply of highly radiogenic terrestrial dust. Bio-aerosol deposits and bacterial colonisation have been identified as a potential source of trace element bioaccumulation and flowstone coloration in Yarrangobilly Caves, Australia. Bio-aerosols have shown to be deposited throughout cave networks. Inorganic aerosol deposition may provide a nutrient supply to cave surfaces allowing for, and sustaining microbial colonisation.
ItemStratigraphy and sedimentology of Cryogenian carbonates, Flinders Ranges, South AustraliaFromhold, Thomas Alexander ( 2011)The Adelaide Geosyncline of South Australia contains a Neoproterozoic-aged sedimentary succession consisting of a complex accumulation of sedimentary formations and units recording a diverse and unique depositional record. A detailed stratigraphic and sedimentological investigation of the interglacial period within the Cryogenian-aged Umberatana Group of the Northern and Central Flinders Ranges reveals a complex array of sedimentary successions lying between the Sturtian and Marinoan glacial deposits. In the Northern Flinders Ranges a major unconformity separates the Sturtian and Marinoan-aged sedimentary successions in the area. This forms a sub-aerial erosion surface with terrestrial and marginal marine sediments directly above the Angepena and Balcanoona formations in their respective localities. This exposure surface is here correlated with the previously documented submarine unconformity between the Yankaninna Formation and the underlying deep marine Tapley Hill Formation. This erosional event provides a chronostratigraphic marker horizon that coincides approximately with the previously defined Sturtian-Marinoan time series boundary in the Northern Flinders Ranges. These stratigraphic relationships also constrain lateral facies relationships between the Oodnaminta Reef Complex (Balcanoona Formation) and the Angepena Formation. Similarly, the shallow water Weetootla Dolomite is correlated with the deeper water carbonates of the Yankaninna Formation. In the Northern Flinders Ranges the Angepena Formation occurs as a marginal marine red-bed succession consisting of supratidal mudstones which are interbedded with subtidal and intertidal carbonates. The Angepena Formation is interpreted as a coastal mudflat succession that formed as a shoreward, laterally equivalent facies of the extensive carbonate platforms (reefs) of the Balcanoona Formation. Sedimentological and geochemical investigation of the Angepena Formation reveal that the unit contains a diverse accumulation of shallow marine carbonates including ooidal sands, tepee buckled algal mats, intraformational breccia (palaeo-caliche) and fenestral-bearing microbial deposits. The stratigraphic and sedimentological relationship within the interglacial successions of the Umberatana Group of the Northern Flinders Ranges are found to extend well over a hundred kilometres southwards into regions of the Central Flinders Ranges. The post-glacial Sturtian-aged Tapley Hill Formation records a near-identical depositional record to the Tapley Hill Formation of the Northern Flinders Ranges. In the Central regions, the Tapley Hill Formation is overlain by deep-marine carbonates and calcareous shales of the Wockerawirra Dolomite and Sunderland Formations respectively. The base of the Wockerawirra Dolomite is defined by an erosional surface, which is directly correlated to the unconformity found overlying the Tapley Hill Formation in the Northern Flinders Ranges (Sturtian-Marinoan series boundary). This stratigraphic relationship indicates the Wockerawirra Dolomite and Sunderland Formations of the Central Flinders Ranges are direct correlatives of the Yankaninna Formation of the Northern Flinders Ranges. The regionally widespread carbonate platform complexes of the Balcanoona Formation in the Northern Flinders Ranges preserve a unique history of the depositional record within the middle Umberatana Group of the Adelaide Geosyncline. Cessation of reef development coincides with a major regression event situated immediately below the Sturtian-Marinoan boundary. The regional consistency of the stratigraphic features found at the Sturtian-Marinoan boundary (i.e. unconformities) suggests that regional scale mechanisms, such as glacio-eustasy, were probably active during this otherwise ‘interglacial’ succession of the Cryogenian-aged Umberatana Group.