22 research outputs found

    Reconstructing annual inflows to the headwater catchments of the Murray River, Australia, using the Pacific Decadal Oscillation

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    The Pacific Decadal Oscillation (PDO) is a major forcing of inter-decadal to quasi-centennial variability of the hydroclimatology of the Pacific Basin. Its effects are most pronounced in the extra-tropical regions, while it modulates the El Nino Southern Oscillation (ENSO), the largest forcing of global inter-annual climate variability. PalaeoPDO indices are now available for at least the past 500 years. Here we show that the \u3e500 year PDO index of Shen et al. (2006) is highly correlated with inflows to the headwaters of Australia\u27s longest river system, the Murray-Darling. We then use the PDO to reconstruct annual inflows to the Murray River back to A.D. 1474. These show penta-decadal and quasi-centennial cycles of low inflows and a possible 500 year cycle of much greater inflow variability. Superimposed on this is the likely influence of recent anthropogenic global warming. We believe this may explain the exceptionally low inflows of the past decade, the lowest of the previous 529 years

    Palaeo-dust records: a window to understanding past environments

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    Dust entrainment, transport over vast distances and subsequent deposition is a fundamental part of the Earth system. Yet the role and importance of dust has been underappreciated, due largely to challenges associated with recognising dust in the landscape and interpreting its depositional history. Despite these challenges, interest in dust is growing. Technical advances in remote sensing and modelling have improved understanding of dust sources and production, while advances in sedimentology, mineralogy and geochemistry (in particular) have allowed dust to be more easily distinguished within sedimentary deposits. This has facilitated the reconstruction of records of dust emissions through time. A key advance in our understanding of dust has occurred following the development of methods to geochemically provenance (fingerprint) dust to its source region. This ability has provided new information on dust transport pathways, as well as the reach and impact of dust. It has also expanded our understanding of the processes driving dust emissions over decadal to millennial timescales through linking dust deposits directly to source area conditions. Dust provenance studies have shown that dust emission, transport and deposition are highly sensitive to variability in climate. They also imply that dust emissions are not simply a function of the degree of aridity in source areas, but respond to a more complex array of conditions, including sediment availability. As well as recording natural variability, dust records are also shown to sensitively track the impact of human activity. This is reflected by both changing dust emission rates and changing dust chemistry. Specific examples of how dust responds to, and records change, are provided with our work on dust emissions from Australia, the most arid inhabited continent and the largest dust source in the Southern Hemisphere. These case studies show that Australian dust emissions reflect hydro-climate variability, with reorganisation of Australian dust source areas occurring during the mid to late Holocene. Dust emissions are shown to sensitively map the structure of the Last Glacial Maximum in Australia, demonstrating that this period was associated with enhanced, but also variable dust emissions, driven by changing sources area conditions. Finally we show how dust emissions have responded to the arrival of Europeans and the associated onset of broad-scale agriculture across the Australian continent

    Arctic Bay Formation, Borden Basin, Nunavut (Canada): basin evolution, black shale, and dissolved metal systematics in the Mesoproterozoic ocean

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    The Arctic Bay Formation (Nunavut, Canada) represents a late Mesoproterozoic muddy terrigenous ramp and contains >200 m of black shale. The formation was studied in order to decipher the tectonostratigraphic and geochemical evolution of the basin, address the origin of metal enrichment, and determine whether this frontier basin has the potential to host sedimentary-exhalative or polymetallic black shale deposits. Samples were analysed in the laboratory for major and trace elements, total organic carbon (TOC), 4-step loss-on-ignition (LOI), and Pb isotopes. Non-calcareous black shale exhibits neither Ce nor Y anomalies, reflecting euxinia in the lower water column, whereas slightly dolomitic black shale has both Ce and Y anomalies, reflecting the dolomite\u27s probable origin as a precipitate in the upper water column. The stratigraphic distribution of the rare earth elements (REEs) indicates an evolving sediment provenance, and Pb isotopic data indicate that the source of clay in the black shale was dominated by weathered, juvenile, mantle-derived material. Base metals and redox-sensitive metals, expressed as enrichment ratios relative to conservative lithophile elements, are elevated and exhibit coherent covariations in the black shale. Enrichment in the redox-sensitive elements, such as Mo and U, correlates with dolomite content of the shale, rather than with organic C or Fepy. From a deep-time ocean evolution perspective, this important observation suggests that enrichment in these metals cannot necessarily be attributed to metal incorporation at an interface between sediment and euxinic water. Instead, in Arctic Bay Formation black shale, the metals were either scavenged onto dolomite as it precipitated in the water column, or secondarily re-distributed within the sediment according to its dolomite content. The base metals that are concentrated in the black shale (e.g., Zn) were probably sourced from diffuse hydrothermal venting, and although there is no evidence at the studied location for a nearby point source of metals (vent), persistent bottom- water euxinia would have ensured the effective scavenging of any dissolved metals supplied, and so the basin has at the very minimum a hypothetical potential for SEDEX and polymetallic mineralisation. Whole-rock U-Th-Pb isotope analysis of black shale yielded a date of 1092 ? 59 Ma, which is considered to be the Arctic Bay Formation\u27s depositional age

    Trace-element systematics of sediments in the Murray-Darling Basin, Australia: Sediment provenance and palaeoclimate implications of fine scale chemical heterogeneity

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    A high-resolution dataset of trace element concentrations is presented for the Murray-Darling Basin, Australia, Australia's most important river system. The data were obtained by solution quadrupole ICP-MS resulting in concentrations for 44 elements. Of these, 21 were determined with a long-term external precision of better than 1% and a further 13 at a precision better than 2%. Trace element maps for the surface sediments constructed from such high precision data reveal small but coherent variations in the four major sub-catchments of the basin, even in ratios of elements with very similar geochemical behaviour, such as Y/Ho, Nb/Ta and Zr/Hf. The origin of these chemical fingerprints of drainage systems are discussed in terms of the geochemical character of the upper continental crust. The potential of trace element maps for palaeo-environmental and climatic reconstruction is then illustrated. First, a sample of dust collected in a trap located in the far southeastern corner of the study area is used to pinpoint the location of the dust source. Next the fine-scale change in down-stream alluvial sediment chemistry is analysed to estimate the importance of sediment contribution from tributaries with a view to reconstructing river flow dynamics. Finally, the chemistry of dune sediments is compared with surrounding floodplain alluvium to estimate relative age of deposition. These examples demonstrate that in low-elevation river systems, such as the Murray-Darling Basin, extended trace element maps of sediment offer substantially more applications than radiogenic isotope data alone

    Petrographic Microscopy with Ray Tracing and Segmentation from Multi-Angle Polarisation Whole-Slide Images

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    ‘Slide scanners’ are rapid optical microscopes equipped with automated and accurate x-y travel stages with virtual z-motion that cannot be rotated. In biomedical microscopic imaging, they are widely deployed to generate whole-slide images (WSI) of tissue samples in various modes of illumination. The availability of WSI has motivated the development of instrument-agnostic advanced image analysis software, helping drug development, pathology, and many other areas of research. Slide scanners are now being modified to enable polarised petrographic microscopy by simulating stage rotation with the acquisition of multiple rotation angles of the polariser–analyser pair for observing randomly oriented anisotropic materials. Here we report on the calibration strategy of one repurposed slide scanner and describe a pilot image analysis pipeline designed to introduce the wider audience to the complexity of performing computer-assisted feature recognition on mineral groups. The repurposed biological scanner produces transmitted light plane- and cross-polarised (TL-PPL and XPL) and unpolarised reflected light (RL) WSI from polished thin sections or slim epoxy mounts at various magnifications, yielding pixel dimensions from ca. 2.7 × 2.7 to 0.14 × 0.14 µm. A data tree of 14 WSI is regularly obtained, containing two RL and six of each PPL and XPL WSI (at 18° rotation increments). This pyramidal image stack is stitched and built into a local server database simultaneously with acquisition. The pyramids (multi-resolution ‘cubes’) can be viewed with freeware locally deployed for teaching petrography and collaborative research. The main progress reported here concerns image analysis with a pilot open-source software pipeline enabling semantic segmentation on petrographic imagery. For this purpose, all WSI are post-processed and aligned to a ‘fixed’ reflective surface (RL), and the PPL and XPL stacks are then summarised in one image, each with ray tracing that describes visible light reflection, absorption, and O- and E-wave interference phenomena. The maximum red-green-blue values were found to best overcome the limitation of refractive index anisotropy for segmentation based on pixel-neighbouring feature maps. This strongly reduces the variation in dichroism in PPL and interference colour in XPL. The synthetic ray trace WSI is then combined with one RL to estimate modal mineralogy with multi-scale algorithms originally designed for object-based cell segmentation in pathological tissues. This requires generating a small number of polygonal expert annotations that inform a training dataset, enabling on-the-fly machine learning classification into mineral classes. The accuracy of the approach was tested by comparison with modal mineralogy obtained by energy-dispersive spectroscopy scanning electron microscopy (SEM-EDX) for a suite of rocks of simple mineralogy (granulites and peridotite). The strengths and limitations of the pixel-based classification approach are described, and phenomena from sample preparation imperfections to semantic segmentation artefacts around fine-grained minerals and/or of indiscriminate optical properties are discussed. Finally, we provide an outlook on image analysis strategies that will improve the status quo by using the first-pass mineralogy identification from optical WSI to generate a location grid to obtain targeted chemical data (e.g., by SEM-EDX) and by considering the rock texture

    Petrographic Microscopy with Ray Tracing and Segmentation from Multi-Angle Polarisation Whole-Slide Images

    No full text
    ‘Slide scanners’ are rapid optical microscopes equipped with automated and accurate x-y travel stages with virtual z-motion that cannot be rotated. In biomedical microscopic imaging, they are widely deployed to generate whole-slide images (WSI) of tissue samples in various modes of illumination. The availability of WSI has motivated the development of instrument-agnostic advanced image analysis software, helping drug development, pathology, and many other areas of research. Slide scanners are now being modified to enable polarised petrographic microscopy by simulating stage rotation with the acquisition of multiple rotation angles of the polariser–analyser pair for observing randomly oriented anisotropic materials. Here we report on the calibration strategy of one repurposed slide scanner and describe a pilot image analysis pipeline designed to introduce the wider audience to the complexity of performing computer-assisted feature recognition on mineral groups. The repurposed biological scanner produces transmitted light plane- and cross-polarised (TL-PPL and XPL) and unpolarised reflected light (RL) WSI from polished thin sections or slim epoxy mounts at various magnifications, yielding pixel dimensions from ca. 2.7 × 2.7 to 0.14 × 0.14 µm. A data tree of 14 WSI is regularly obtained, containing two RL and six of each PPL and XPL WSI (at 18° rotation increments). This pyramidal image stack is stitched and built into a local server database simultaneously with acquisition. The pyramids (multi-resolution ‘cubes’) can be viewed with freeware locally deployed for teaching petrography and collaborative research. The main progress reported here concerns image analysis with a pilot open-source software pipeline enabling semantic segmentation on petrographic imagery. For this purpose, all WSI are post-processed and aligned to a ‘fixed’ reflective surface (RL), and the PPL and XPL stacks are then summarised in one image, each with ray tracing that describes visible light reflection, absorption, and O- and E-wave interference phenomena. The maximum red-green-blue values were found to best overcome the limitation of refractive index anisotropy for segmentation based on pixel-neighbouring feature maps. This strongly reduces the variation in dichroism in PPL and interference colour in XPL. The synthetic ray trace WSI is then combined with one RL to estimate modal mineralogy with multi-scale algorithms originally designed for object-based cell segmentation in pathological tissues. This requires generating a small number of polygonal expert annotations that inform a training dataset, enabling on-the-fly machine learning classification into mineral classes. The accuracy of the approach was tested by comparison with modal mineralogy obtained by energy-dispersive spectroscopy scanning electron microscopy (SEM-EDX) for a suite of rocks of simple mineralogy (granulites and peridotite). The strengths and limitations of the pixel-based classification approach are described, and phenomena from sample preparation imperfections to semantic segmentation artefacts around fine-grained minerals and/or of indiscriminate optical properties are discussed. Finally, we provide an outlook on image analysis strategies that will improve the status quo by using the first-pass mineralogy identification from optical WSI to generate a location grid to obtain targeted chemical data (e.g., by SEM-EDX) and by considering the rock texture

    Long-range dust transport from eastern Australia: a proxy for Holocene aridity and ENSO induced climate variability

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    We report rates of Australian dust deposition in New Zealand over the last 8000 years using records extracted from an ombrotrophic peat bog. The trace element chemistry of deposited dust is used to identify the Australian source areas, and to calculate Australian dust deposition rates in New Zealand. This is used to infer patterns of aridity and climate variability in eastern Australia during the Holocene. Prior to 4800 cal. BP, dust deposition patterns imply that the Australian climate was relatively wet due to an active/persistent monsoon. Southern Australian source regions supplied the majority of dust transported to New Zealand at this time. After 4800 cal. BP the Australian climate became significantly more arid and variable, attributed to the onset of ENSO-type conditions. The Lake Eyre Basin switched on as a dust source, while overall Australian dust deposition in New Zealand increased 4 times. Rates of Australian dust deposition were found to match patterns of increased ENSO variability, which is attributed to the combined effect of enhanced sediment supply to Australian dust source areas (during wet La Nina events) and preferable conditions for dust transport (during El Nino induced drought). Results also show that Australian dust deposition is a significant sedimentary process in New Zealand and is likely to influence both geochemical cycles and soil development suggesting that it may contribute up to 90% of material in the bog

    Comment on Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170,000 years by P. De Deckker, M. Norman,

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    A recent paper by De Deckker et al. (2010; Lead isotopic evidence for an Australian source of Aeolian dust to Antarctica at times over the last 170,000. years, Palaeogeography, Palaeoclimatology, Palaeoecology, 285(3-4), 205-223) reports radiogenic isotope data of fluvial sediment samples from the Murray Darling Basin (MDB) in southeastern Australia. Some of these display Pb-isotope ratios similar to ancient, pre-industrial pollution dust retrieved from the EPICA Dome C ice core, Antarctica. The authors proposed that this result supports a dust provenance from the Darling sub-basin of the MDB. De Deckker et al. (2010) apparently did not appreciate the extent of anthropogenic Pb contamination of their potential Australian dust sources. In addition, they also neglected to analyze many of the other important Australian dust sources. By contrast, it is demonstrated here that the surface sediments are severely polluted, that other potential dust sources from Australia also need to be considered and that it is not possible to unequivocally distinguish Australian from South American dust using Pb-isotopes alone. C 2010 Elsevier B.V

    Comment on "Lead isotopic evidence for an Australian source of aeolian dust to Antarctica at times over the last 170,000 years" by P. De Deckker, M. Norman, I.D. Goodwin, A. Wain and F.X. Gingele

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    A recent paper by De Deckker et al. (2010; Lead isotopic evidence for an Australian source of Aeolian dust to Antarctica at times over the last 170,000 years, Palaeogeography, Palaeoclimatology, Palaeoecology, 285(3-4), 205-223) reports radiogenic isotope data of fluvial sediment samples from the Murray Darling Basin (MDB) in southeastern Australia. Some of these display Pb-isotope ratios similar to ancient, pre-industrial pollution dust retrieved from the EPICA Dome C ice core, Antarctica. The authors proposed that this result supports a dust provenance from the Darling sub-basin of the MDB. De Deckker et al. (2010) apparently did not appreciate the extent of anthropogenic Pb contamination of their potential Australian dust sources. In addition, they also neglected to analyze many of the other important Australian dust sources. By contrast, it is demonstrated here that the surface sediments are severely polluted, that other potential dust sources from Australia also need to be considered and that it is not possible to unequivocally distinguish Australian from South American dust using Pb-isotopes alone. (C) 2010 Elsevier B.V. All rights reserved

    Long-range dust transport from eastern Australia: A proxy for Holocene aridity and ENSO-type climate variability

    No full text
    We report rates of Australian dust deposition in New Zealand over the last ~ 8000\ua0years using records extracted from an ombrotrophic peat bog. The trace element chemistry of deposited dust is used to identify the Australian source areas, and to calculate Australian dust deposition rates in New Zealand. This is used to infer patterns of aridity and climate variability in eastern Australia during the Holocene. Prior to 4800\ua0cal. BP, dust deposition patterns imply that the Australian climate was relatively wet due to an active/persistent monsoon. Southern Australian source regions supplied the majority of dust transported to New Zealand at this time. After 4800\ua0cal. BP the Australian climate became significantly more arid and variable, attributed to the onset of ENSO-type conditions. The Lake Eyre Basin switched on as a dust source, while overall Australian dust deposition in New Zealand increased > 4 times. Rates of Australian dust deposition were found to match patterns of increased ENSO variability, which is attributed to the combined effect of enhanced sediment supply to Australian dust source areas (during wet La Niña events) and preferable conditions for dust transport (during El Niño induced drought). Results also show that Australian dust deposition is a significant sedimentary process in New Zealand and is likely to influence both geochemical cycles and soil development suggesting that it may contribute up to 90% of material in the bog
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