Turbidity currents are powerful flows of sediment that pose a hazard to critical seafloor infrastructure and transport globally important amounts of sediment to the deep sea. Due to challenges of ...direct monitoring, we typically rely on their deposits to reconstruct past turbidity currents. Understanding these flows is complicated because successive flows can rework or erase previous deposits. Hence, depositional environments dominated by turbidity currents, such as submarine channels, only partially record their deposits. But precisely how incomplete these deposits are, is unclear. Here we use the most extensive repeat bathymetric mapping yet of any turbidity current system, to reveal the stratigraphic evolution of three submarine channels. We re-analyze 93 daily repeat surveys performed over four months at the Squamish submarine delta, British Columbia in 2011, during which time >100 turbidity currents were monitored. Turbidity currents deposit and rework sediments into upstream-migrating bedforms, ensuring low rates of preservation (median 11%), even on the terminal lobes. Large delta-lip collapses (up to 150,000 m3) are relatively well preserved, however, due to their rapidly emplaced volumes, which shield underlying channel deposits from erosion over the surveyed timescale. The biggest gaps in the depositional record relate to infrequent powerful flows that cause significant erosion, particularly at the channel-lobe transition zone where no deposits during our monitoring period are preserved. Our analysis of repeat surveys demonstrates how incomplete the stratigraphy of submarine channels can be, even over just 4 months, and provides a new approach to better understand how the stratigraphic record is built and preserved in a wider range of marine settings.
•Repeat seafloor surveys reveal how stratigraphy is built in submarine channels.•Flow monitoring, seafloor surveys & coring are integrated.•Active submarine channels preserve 11% of deposits (median).•Infrequent but powerful flows can erase depositional record.•New method can be applied in a wider range of submarine systems.
Motivated by the challenge of capturing complex hierarchical chemical detail in natural material from a wide range of applications, the Maia detector array and integrated realtime processor have been ...developed to acquire X-ray fluorescence images using X-ray Fluorescence Microscopy (XFM). Maia has been deployed initially at the XFM beamline at the Australian Synchrotron and more recently, demonstrating improvements in energy resolution, at the P06 beamline at Petra III in Germany. Maia captures fine detail in element images beyond 100 M pixels. It combines a large solid-angle annular energy-dispersive 384 detector array, stage encoder and flux counter inputs and dedicated FPGA-based real-time event processor with embedded spectral deconvolution. This enables high definition imaging and enhanced trace element sensitivity to capture complex trace element textures and place them in a detailed spatial context. Maia hardware and software methods provide per pixel correction for dwell, beam flux variation, dead-time and pileup, as well as off-line parallel processing for enhanced throughput. Methods have been developed for real-time display of deconvoluted SXRF element images, depth mapping of rare particles and the acquisition of 3D datasets for fluorescence tomography and XANES imaging using a spectral deconvolution method that tracks beam energy variation.
The discovery that Au accumulates in calcrete (pedogenic carbonate or caliche) was made in 1987 by CSIRO. Calcrete is a general term describing accumulation of alkaline earth metals in soils of arid ...and semi-arid terrains around the world. The principal constituent of calcrete is calcite while Au is a noble metal. Calcrete has been a significant tool in a number of Au deposit discoveries, so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery. Numerous laboratory experiments, case histories and exploration models have been published (most from Australia) yet we do not fully understand the mechanisms involved. It is timely, therefore, twenty-five years on since the first publication of this phenomenon, to review this highly unusual but economically important association.
Critical to any review on Au in calcrete is to first consider calcretes themselves. The nature of a particular calcrete, where it has formed and mode of formation is relevant to how, where and why Au accumulates within it. This review commences with a background, nomenclature, history, classification and some examples of calcrete types found near Au deposits. How calcretes form, their origins and the role of biota is considered. Their locations in the regolith and landscape, as well as exploration models for Au in calcrete are discussed. A section on the chemistry of Au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us. Following on is a summary of practical aspects of identifying, collecting and analysing samples for exploration purposes. Selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings. Concluding sections include a summary and implications of this accumulated knowledge to discovering Au deposits.
The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may ...lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river‐fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within Bute Inlet, a fjord in western Canada. We show that 62% ± 10% of the OC supplied by the two river sources is buried across the fjord surficial (30–200 cm) sediment. The sandy subenvironments (channel and lobe) contain 63% ± 14% of the annual terrestrial OC burial in the fjord. In contrast, the muddy subenvironments (overbank and distal basin) contain the remaining 37% ± 14%. OC in the channel, lobe, and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least 3 times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long‐term (>100 years) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient at storing OC supplied by rivers in their near‐surface deposits.
Plain Language Summary
Plants on land use CO2 from the atmosphere to produce organic carbon, which promotes their growth. Rivers transport organic carbon to the sea, where it is either eaten by fauna or buried in the seafloor, thus decreasing atmospheric CO2 levels on Earth over thousands to millions of years. Fjords are recognized as global organic carbon sinks; trapping 18 million tons of organic carbon in their seafloor sediments each year. However, the complex morphology of fjord seafloors was not considered in the calculation of this organic carbon flux. In this study, we determine the distribution and abundance of terrestrial organic carbon across a fjord (Bute Inlet, Canada), which contains a submarine channel network terminating onto a large accumulation of sand (called a lobe). We show that 62% of the organic carbon supplied by the two rivers connected to the fjord is buried across the fjord; the majority of this carbon being held in the lobe. In total, Bute Inlet buries at least 3 times more organic carbon per unit of surface area than other fjords previously studied. Submarine channels in fjords thus appear to promote the storage of land‐derived organic carbon in the seafloor, potentially impacting CO2 levels and food resources for marine fauna.
Key Points
Bute Inlet, a river‐fed fjord incised by turbidity currents, has a contemporary terrestrial organic carbon burial efficiency of 62% ± 10%
Sandy surficial deposits are responsible for 63% ± 14% of the total terrestrial organic carbon burial budget in Bute Inlet, but only cover 17% of the seafloor area
Global estimates based only on the muddy parts of fjords may significantly underestimate organic carbon burial rates by a factor >3
Submarine channels are the primary conduits for terrestrial sediment, organic carbon, and pollutant transport to the deep sea. Submarine channels are far more difficult to monitor than rivers, and ...thus less well understood. Here we present 9 years of time-lapse mapping of an active submarine channel along its full length in Bute Inlet, Canada. Past studies suggested that meander-bend migration, levee-deposition, or migration of (supercritical-flow) bedforms controls the evolution of submarine channels. We show for the first time how rapid (100-450 m/year) upstream migration of 5-to-30 m high knickpoints can control submarine channel evolution. Knickpoint migration-related changes include deep (>25 m) erosion, and lateral migration of the channel. Knickpoints in rivers are created by external factors, such as tectonics, or base-level change. However, the knickpoints in Bute Inlet appear internally generated. Similar knickpoints are found in several submarine channels worldwide, and are thus globally important for how channels operate.
Elevated Au concentrations above bedrock mineralization are commonly closely associated with calcrete in arid residual and semi-residual soils in Australia. The origin of Australian calcrete has been ...argued for many years but there have been very few published studies. Calcrete from Au deposits and prospects, and elsewhere in the Gawler Craton (South Australia) were studied for their geochemical composition, and Sr and C isotope ratios. By comparing carbonate
87Sr/
86Sr ratios with underlying weathered rock and bedrock, it is demonstrated that many samples have overwhelmingly atmospheric Sr (>
94%) and, by inference, Ca (>
98%) rather than a local bedrock or soil mineral origin. The isotopic composition of calcretes at the Challenger Gold Deposit lie on a mixing trend of decreasingly marine Sr from the Nullarbor Plain to Tarcoola, 300 km to the east. The
δ
13C
PDB values (−
7.5‰ to +
0.1‰) suggest that the carbonate C in calcrete has a mixed origin derived from C3 and C4 plants.
A model is presented to explain how the Au–Ca association may form in calcrete at Challenger based on these new Sr and C isotope data and in which the role of plants and atmospherically deposited Sr and Ca combine with pre-existing Au in the soil. The implication for mineral exploration is that, since most of the Ca is derived from marine sources, it is not a pre-requisite that autochthonous soil minerals (derived from underlying weathered bedrock) supply Ca for calcrete to form and for the Au–Ca association to occur. Thus, the independence of the Ca with respect to underlying lithology reaffirms the usefulness of calcrete as a sampling medium for Au exploration.
Objectives: To characterise management of suspected acute coronary syndrome (ACS) in Australia and New Zealand, and to assess the application of recommended therapies according to published ...guidelines.
Design, setting and patients: All patients hospitalised with suspected or confirmed ACS between 14 and 27 May 2012 were enrolled from participating sites in Australia and New Zealand, which were identified through public records and health networks. Descriptive and logistic regression analysis was performed.
Main outcome measures: Rates of guideline‐recommended investigations and therapies, and inhospital clinical events (death, new or recurrent myocardial infarction MI, stroke, cardiac arrest and worsening congestive heart failure).
Results: Of 478 sites that gained ethics approval to participate, 286 sites provided data on 4398 patients with suspected or confirmed ACS. Patients’ mean age was 67 years (SD, 15 years), 40% were women, and the median Global Registry of Acute Coronary Events (GRACE) risk score was 119 (interquartile range, 96–144). Most patients (66%) presented to principal referral hospitals. MI was diagnosed in 1436 patients (33%), unstable angina or likely ischaemic chest pain in 929 (21%), unlikely ischaemic chest pain in 1196 (27%), and 837 patients (19%) had other diagnoses not due to ACS. Of the patients with MI, 1019 (71%) were treated with angiography, 610 (43%) with percutaneous coronary intervention and 116 (8%) with coronary artery bypass grafting. Invasive management was less likely with increasing patient risk (GRACE score < 100, 90.1% v 101–150, 81.3% v 151–200, 49.4% v > 200, 36.1%; P < 0.001). The inhospital mortality rate was 4.5% and recurrent MI rate was 5.1%. After adjusting for patient risk and other variables, significant variations in care and outcomes by hospital classification and jurisdiction were evident.
Conclusion: This first comprehensive combined Australia and New Zealand audit of ACS care identified variations in the application of the ACS evidence base and varying rates of inhospital clinical events. A focus on integrated clinical service delivery may provide greater translation of evidence to practice and improve ACS outcomes in Australia and New Zealand.
Highly anomalous Au concentrations in calcrete were discovered in 1987 at the Bounty Gold Deposit, Western Australia. A strong correlation was noted between the Ca, Mg, Sr and Au in soil profiles ...which have not only attracted the interest of mineral explorers but also chemists, soil scientists, metallurgists and climatologists. Gold has been considered an inert element and so its strong association with the alkaline earth group of relatively mobile elements is both remarkable and intriguing. Despite widespread interest, there have been few published papers on the Au–calcrete phenomenon. Here, we present work conducted on calcareous soil samples from above the Bounty mineralisation in Western Australia, prior to mining.
Using SXRF (synchrotron X-ray fluorescence) and XANES (micro-X-ray absorption near-edge structure), we have shown for the first time the distribution of Au in calcrete and that it occurs in both particulate and ionic form. Much of the ionic Au associated with Br is found in a root tubule. The observations are consistent with an evapotranspiration model for the formation of Au in the calcrete; Au has been mobilised then precipitated as vadose water has been removed from the soil by trees and shrubs. While the association between Au and Ca is very strong in bulk sample analyses down the soil profile, other detailed analyses on sub-samples using wet chemical, LA-ICP-MS (laser ablation inductively coupled mass spectrometry) and SXRF techniques show that it is not apparent at the sub-millimetre scale. This suggests that the Au and Ca are behaving similarly but independently and they do not (at the μm scale) co-precipitate with carbonate minerals.
These results corroborate other studies that suggest biotic influences can affect the mobilisation and distribution of Au in surficial materials. Water-extractable Au in calcrete has been reported previously and the ionic Au described in this study likely represents that soluble component. The presence of easily solubilised Au in soils has been widely discussed and exploited for mineral exploration.
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