Signatures Of Deposits Research Articles

Geophysical signatures on gold and base metal mineralisation in the Ibaga area, northeast of Iramba, central Tanzania

This study provides an in-depth geophysical interpretation of the gold and base metal mineralisation of the Ibaga area related to hydrothermal alteration using high-resolution aero-geophysical and induced polarisation ground survey datasets. Despite its potential for gold and base metal and the available artisanal mines, the Ibaga area has remained poorly understood on geophysical signatures of ore deposits and other subsurface geological structures controlling the mineralisation. The magnetic results on the major structures show high magnetic signatures, supported by gravity, radiometric signatures and petrophysical analysis. The potential mineralised structures are interpreted as dyke for gold, and contacts for copper and the depth to the basement varies from 0.113 to 1.556 km. The gravity anomaly shows high values on the gold mineralisation similar to high-density values of dolerite dykes and gabbro and low to medium radiometric concentrations associated with mineralised zones. The magnetic signatures on the copper deposits coincide with the relative positive gravity anomaly of high-density intrusive bodies and relatively moderate conductive bodies with elevated potassium (K) alteration. The copper deposit also has intermediate K and low to intermediate K/Th and K/U ratios caused by K alteration within the geological structures. The good spatial correlation of the hydrothermally altered zones, igneous intrusions and geological structures implies that the geological features associated with gold and copper mineralisation lie within the regions of high-density intrusive bodies and geological structures (faults, fractures and dykes). These features are also associated with relatively moderate conductive bodies, low to intermediate radiometric concentrations and low to moderate magnetic signatures. Thus, the study reveals that the delineated geophysical signatures on gold and base metal mineralisation are associated with quartz–sericite–muscovite ± chlorite hydrothermally altered areas, deformed structures, and intrusive bodies, which act as pathways for the migration and accumulation of hydrothermally altered fluids.

Strontium isotopes and rare earth elements in terrestrial hot-spring deposits: Characterization and geothermal implications

Identifying geothermal reservoir rock types is fundamental in geothermal exploration, but the absence of active surface geothermal manifestations (especially hot springs) in blind geothermal systems makes this identification difficult. Nevertheless, blind geothermal systems may develop early-formed hot-spring deposits. As (bio-)chemical precipitate, these deposits may retain geochemical signatures of spring waters and thus provide insights into reservoir rock types. To assess their geothermal implications, the 87Sr/86Sr and rare earth elements + yttrium (REE + Y) characteristics of hot-spring deposits in two geothermal systems were investigated and compared with those of their reservoir rocks. Results showed relatively uniform 87Sr/86Sr values in hot-spring deposits within each system, but occasional 87Sr/86Sr contamination induced by exogenous detritus input was also observed. The amount of detritus input relates to the potential for interaction with surrounding soils/rocks and is thus environmentally controlled. Hot-spring deposits with high Sr concentrations showed greater 87Sr/86Sr contamination resistance than those with low Sr concentrations, revealing the influence of Sr concentration in hot-spring deposits on detrital 87Sr/86Sr contamination. The detritus input also influenced the REE + Y signatures of the hot-spring deposits, underscoring the necessity of contamination assessment before geochemical interpretation. Excluding samples with significant 87Sr/86Sr and/or REE + Y contamination, the remaining samples closely mirrored their respective reservoir rocks in terms of 87Sr/86Sr, with partial overlap in REE + Y signatures. This suggests that the 87Sr/86Sr and REE + Y characteristics of hot-spring deposits provide valuable insights into reservoir rock types. However, variations in 87Sr/86Sr or REE + Y compositions between some hot-spring deposits and corresponding reservoir rocks indicate additional influencing factors beyond reservoir rock types. Therefore, a comprehensive understanding of reservoir rock types requires integrated geochemical characterization, probably including 87Sr/86Sr, REE + Y, and other parameters. These findings underscore the potential of geochemical characterization of hot-spring deposits for identifying geothermal reservoir rock types, and this geochemical approach can complement geological and geophysical data to improve exploration efficiency, especially in blind geothermal systems.

Fossilized autogenic responses of grain‐size transition to sediment supply and water discharge: Alluvial fan experiments

ABSTRACTAutogenic feedbacks can produce large‐scale, organized stratigraphic patterns in alluvial fans, but autogenic depositional signatures of specific upstream boundary conditions remain challenging to interpret. Here, a combination of theory, experiment and field application is used to explore how autogenic lithofacies changes can be interpreted as stratigraphic indicators of upstream boundary conditions. Six experiments were conducted to test the effects of sediment supply and water discharge rates on autogenic advance and retreat of the lithofacies boundary (grain‐size transition) in an alluvial fan with two dominant grain sizes. Migration of the grain‐size transition caused a short‐term zigzag pattern in the grain‐size transition position in the dip‐directional deposit section. For each experiment, time‐lapse images and laser topographic scans of the fan surface and stratigraphic cross‐sections of the final deposits were used to quantify characteristic timescales of autogenic processes. Timescales for fan‐margin migration, surface wet‐fraction change and grain‐size transition migration generally shorten as sediment supply rate increases and water discharge rate decreases. Increasing the sediment supply rate shortens the duration of the fluvial sediment storage and release cycle, producing higher frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge tends to widen channels and lengthens the duration of the fluvial sediment storage and release cycle, constructing lower frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge also enables more sediment to transport further downstream during release events, leading to higher magnitude zigzags in the grain‐size transition trajectory. These relationships between upstream boundary conditions and the grain‐size transition trajectory demonstrate how autogenic stratigraphic signals could be used as a tool to infer relative changes in boundary conditions.

Mount Etna as a terrestrial laboratory to investigate recent volcanic activity on Venus by future missions: A comparison with Idunn Mons, Venus

The recently selected missions to Venus have opened a new era for the exploration of this planet. These missions will provide information about the chemistry of the atmosphere, the geomorphology, local-to-regional surface composition, and the rheology of the interior. One key scientific question to be addressed by these future missions is whether Venus remains volcanically active, and if so, how its volcanism is currently evolving. Hence, it is fundamental to analyze appropriate terrestrial analog sites for the study of possibly active volcanism on Venus. To this regard, we propose Mount Etna - one of the most active and monitored volcanoes on Earth - as a suitable terrestrial laboratory for remote and in-situ investigations to be performed by future missions to Venus. Being characterized by both effusive and explosive volcanic products, Mount Etna offers the opportunity to analyze multiple eruptive styles, both monitoring active volcanism and identifying the possible occurrence of pyroclastic activity on Venus. We directly compare Mount Etna with Idunn Mons, one of the most promising potentially active volcanoes of Venus. Despite the two structures show a different topography, they also show some interesting points of comparison, and in particular: a) comparable morpho-structural setting, since both volcanoes interact with a rift zone, and b) morphologically similar volcanic fields around both Mount Etna and Idunn Mons. Given its ease of access, we also propose Mount Etna as an analog site for laboratory spectroscopic studies to identify the signatures of unaltered volcanic deposits on Venus.

GWOKM: A novel hybrid optimization algorithm for geochemical anomaly detection based on Grey wolf optimizer and K-means clustering

Identifying the geochemical signatures of valuable mineral deposits using regional geochemical data from stream sediments is a challenging task due to the intricate characteristics of geological formations. Our team is currently investigating the potential of unsupervised clustering analysis (CA) and hybridization with the grey wolf optimizer (GWO) in developing multi-element geochemical models using stream sediment data. To cluster the geochemical data and uncover any unusual patterns, we opted to use the K-means (KM) algorithm due to its straightforward implementation, fast computation speed, and capacity to handle the large datasets. Despite its benefits, the KM method also has notable limitations, such as the random selection of cluster centroids. This can result in higher systematic uncertainty in unsupervised geochemical modeling and longer computation times. To mitigate this concern, we have introduced a new hybrid approach, grey wolf optimizer with K-means so-called the GWOKM algorithm to enhance the delineation of multi-elemental patterns in stream sediment geochemical data. This method incorporates the grey wolf optimization algorithm with KM to optimize the identification of both anomalies and backgrounds using factor analysis and sample catchment basin modeling techniques. This approach was utilized to detect anomalous multi-elemental geochemical patterns indicative of porphyry and skarn copper deposits in the Baft area, Kerman belt, Iran. Upon comparison of the geochemical models derived from KM and GWOKM clustering methods, the latter outperformed the former, as evidenced by its higher prediction rate. The outcomes affirm the efficacy of unsupervised KM clustering analysis (CA) as a means of breaking down geochemical anomaly-background populations. Moreover, the integration of clustering methods with optimization algorithms has proven to be successful for enhancing the credibility of mineralized areas, which could be advantageous in future exploration phases. Based on the results, the GWOKM approach generates more reliable and efficient geochemical anomaly targets.

Multi-wavelength observations and modeling of a microflare: constraining non-thermal particle acceleration

In this work we analyze a small B-class flare that occurred on 29 April 2021 and was observed simultaneously by the Interface Region Imaging Spectrograph (IRIS) and the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray instrument. The IRIS observations of the ribbon of the flare show peculiar spectral characteristics that are typical signatures of energy deposition by non-thermal electrons in the lower atmosphere. The presence of the non-thermal particles is also confirmed directly by fitting the NuSTAR spectral observations. We show that, by combining IRIS and NuSTAR multi-wavelength observations from the corona to the lower atmosphere with hydrodynamic simulations using the RADYN code, we can provide strict constraints on electron-beam heated flare models. This work presents the first NuSTAR, IRIS and RADYN joint analysis of a non-thermal microflare, and presents a self-consistent picture of the flare-accelerated electrons in the corona and the chromospheric response to those electrons.

Cycling of block copolymer composites with lithium-conducting ceramic nanoparticles.

Solid polymer and perovskite-type ceramic electrolytes have both shown promise in advancing solid-state lithium metal batteries. Despite their favorable interfacial stability against lithium metal, polymer electrolytes face issues due to their low ionic conductivity and poor mechanical strength. Highly conductive and mechanically robust ceramics, on the other hand, cannot physically remain in contact with redox-active particles that expand and contract during charge-discharge cycles unless excessive pressures are used. To overcome the disadvantages of each material, polymer-ceramic composites can be formed; however, depletion interactions will always lead to aggregation of the ceramic particles if a homopolymer above its melting temperature is used. In this study, we incorporate Li0.33La0.56TiO3 (LLTO) nanoparticles into a block copolymer, polystyrene-b-poly (ethylene oxide) (SEO), to develop a polymer-composite electrolyte (SEO-LLTO). TEMs of the same nanoparticles in polyethylene oxide (PEO) show highly aggregated particles whereas a significant fraction of the nanoparticles are dispersed within the PEO-rich lamellae of the SEO-LLTO electrolyte. We use synchrotron hard x-ray microtomography to study the cell failure and interfacial stability of SEO-LLTO in cycled lithium-lithium symmetric cells. Three-dimensional tomograms reveal the formation of large globular lithium structures in the vicinity of the LLTO aggregates. Encasing the SEO-LLTO between layers of SEO to form a "sandwich" electrolyte, we prevent direct contact of LLTO with lithium metal, which allows for the passage of seven-fold higher current densities without signatures of lithium deposition around LLTO. We posit that eliminating particle clustering and direct contact of LLTO and lithium metal through dry processing techniques is crucial to enabling composite electrolytes.

A Permian intermediate-sulfidation epithermal Pb–Zn–Ag deposit in the northern margin of North China Craton

Recent Pb–Zn–Ag exploration in the northern margin of North China Craton has made significant new discoveries. Here we report one Pb–Zn–Ag deposit in the Shaolanghe orefield with typical signatures of intermediate-sulfidation epithermal deposits. Zhangjiagou Pb–Zn–Ag mineralization occurs within the Early Permian quartz diorite and Late Permian andesite. Three mineralization stages were identified: white quartz–rhodochrosite–sulfide Pb–Zn–mineralization stage (I), smoky quartz–sulfide Pb–Zn–Ag–mineralization stage (II), and calcite–quartz non–mineralization stage (III). Four sphalerite samples yield the Rb–Sr age of 265.3 Ma, indicating that Pb–Zn–Ag mineralization was associated with the Late Permian andesite. The liquid-rich fluid inclusions of Stage I show low–medium temperatures of 190–270 °C and low salinities (1–3 wt%), and those of Stage III are slightly lower (190–250 °C, 0.5–2 wt%), suggesting a low-salinity, low-temperature H2O–NaCl fluid system for the Zhangjiagou deposit. The H–O–C isotopic signatures indicate a mixed fluid of magmatic water with meteoric water. S and Pb isotopes suggest magmatic source for Zhangjiagou Pb–Zn–Ag mineralization, most likely shallow hydrous intermediate magma. Collectively, these geological, geochronological, and geochemical features indicate that Zhangjiagou is an intermediate-sulfidation epithermal Pb–Zn–Ag deposit.

Editorial: Source or sink? Erosional and depositional signatures of tectonic activity in deep-sea sedimentary systems

Editorial: Source or sink? Erosional and depositional signatures of tectonic activity in deep-sea sedimentary systems

Implications of Late Palaeozoic postglacial marine transgressive‐regressive (T‐R) cycles recorded in the Talchir Formation, Son Valley Basin, peninsular India: A sequence stratigraphic paradigm

The signature of Late palaeozoic glaciation, a pan‐Gondwanaland palaeoclimatic event, is recorded in the Talchir Formation (Late Carboniferous to Early Permian), the basal‐most litho‐unit within the Gondwana Supergroup (Late Carboniferous–Early Cretaceous) in peninsular India. About 200 m thick sedimentary succession of the Talchir Formation, exposed in the Hasdeo River section in the Son Valley Basin, Central India, manifests signatures of deposition by melting of glaciers near to the ice‐grounding line. Evidence of glacial influences is prevalent in the lower‐middle part of the succession and represented by glacial tillites with faceted/striated gravels and bullet‐shaped clasts, dropstones and ice‐rafted debris (IRD), associated with various sediment gravity flow deposits. In the middle‐upper part of the succession, records of post‐glacial marine incursions are preserved as thick sandstone and sandstone‐mudstone heteroliths of a shallow marine shore‐shelf depositional regime. These are characterized by abundant tidal and wave signatures, with marine bivalve fossils and trace fossils of marine invertebrates. The upward decrease in size and abundance of the IRDs indicates a gradual decrease in glacial influences, with landward incursion of the marine depositional systems over the proglacial systems. Multiple stacked progradational and retrogradational stratal stacking patterns formed by alternating glacigenic and marginal marine dominated processes signify multiple transgressive‐regressive (T‐R) cycles, manifesting repeated advancements and retreats of the glacier, associated with sea level rise and marine incursions into the land. The top part of the succession, characterized by fine‐grained siltstone/mudstone without any IRD, suggests sedimentation at a relatively deeper part of the basin, indicating the strong influence of the eustatic rise caused by significant melting of the retreating ice sheet. Stacked T‐R cycles bounded by distinct facies architecture define multiple system tracts confined within three major deglaciation sequences within the studied Talchir Formation, suggesting multiple phases of advance‐retreats of the ice sheet and significant marine onlaps within the continental Gondwanaland under the Late Palaeozoic icehouse‐greenhouse climatic change.

Channel incision into a submarine landslide on a Carboniferous basin margin, San Juan, Argentina: Evidence for the role of knickpoints

AbstractEmplacement of submarine landslides, or mass‐transport deposits, can radically reshape the physiography of continental margins, and strongly influence subsequent sedimentary processes and dispersal patterns. Typically, progressive healing of the complicated relief generated by the submarine landslide occurs prior to progradation of sedimentary systems. However, subsurface and seabed examples show that submarine channels can incise directly into submarine landslides. Here, the evolution of a unique exhumed example of two adjacent, and partially contemporaneous, submarine channel‐fills is documented. The channels show deep incision (>75 m), and steep lateral margins (up to 70°), cut into a >200 m thick submarine landslide. The stepped basal erosion surface, and multiple terrace surfaces, are mantled by clasts (gravels to cobbles) reflecting periods of bedload‐derived sedimentation, punctuated by phases of downcutting and sediment bypass. The formation of multiple terrace surfaces in a low aspect ratio confinement is consistent with the episodic migration of knickpoints during entrenchment on the dip slope of the underlying submarine landslide. Overlying sandstone‐rich channel‐fills mark a change to aggradation. Laterally stacked channel bodies coincide with steps in the original large‐scale erosion surface, recording widening of the conduit; this is followed by tabular, highly aggradational fill. The upper fill, above a younger erosional surface, shows an abrupt change to partially confined tabular sandstones with normally graded caps, interpreted as lobe fringe deposits, which formed due to down‐dip confinement, followed by prograding lobe deposits. Overlying this, an up‐dip avulsion induced lobe switching and back‐stepping, and subsequent failure of a sandstone body up‐dip led to emplacement of a sandstone‐rich submarine landslide within the conduit. Collectively, this outcrop represents episodic knickpoint‐generated incision, and later infill, of a slope adjusting to equilibrium. The depositional signature of knickpoints is very different from existing models, but is probably reflective of other highly erosional settings undergoing large‐scale slope adjustment.

Exploration targeting of copper deposits using staged factor analysis, geochemical mineralization prospectivity index, and fractal model (Western Anti-Atlas, Morocco)

The present work aims to identify the geochemical signatures of copper deposits in the Igherm area (Anti-Atlas, Morocco) using stream sediment data. We applied factor analysis on the data normalized by log transformation. This approach identified three geochemical data associations (F1, F2, and F3) which were then used to calculate the values of the geochemical prospectivity index (GPMI). The results indicated that the large GMPI Cu values correlate strongly with the surface indices. A concentration-surface (C-A) fractal classification was applied to separate the geochemical anomalies from the regional background. To recognize and geo-position the significant geochemical signatures of the deposit, we applied the prediction zone (P-A) plot. A geographic information system (GIS) was used to define the spatial distribution of the anomalies at the sub-watershed level. The results represent the first approach to evaluating potential mineral resources at the scale of the Anti-Atlas: 90% of the anomalous sub-catchments mapped are located along the Precambrian-Cambrian transition (Base Series), 67% are traversed by major tectonic faults, and 40% contain copper showings and/or deposits. The remaining 60% are devoid of any visible surface copper occurrences. They constitute promising areas for future exploration of Cu deposits hidden under the Cambrian cover.

Сравнительный анализ геохимических характеристик источников кайнозойских осадочных отложений по латерали Южного Байкала

Geochemical signatures of Oligocene and Miocene sedimentary deposits from the Khoygot Paleovalley of Vitim Plateau changed successively over time that reflected change in sources of terrigenic material in the context of the migratory nature of the development of the river network. Geochemical signatures of Eocene-Miocene sedimentary deposits from the eastern (Mishikha-Klyuevka) and western (Osinovka) paleovalleys of the Tankhoy tectonic step of South Baikal were uniform that indicates long-term intake of sedimentary material from a common source. Composition of sedimentary material from paleovalleys of the Tankhoy step was controlled by a limited catchment. After the early Pliocene structural reorganization, geochemical signatures of sedimentary deposits from the eastern part of the Tankhoy tectonic step became similar to those of the Pliocene-Quaternary alluvium frpm the Proto-Manzurka valley of the opposite (north-western) coast of Lake Baikal. It is assumed that the common source of Pliocene-Quaternary sedimentary material was located in Jurassic (Pra-Manzurka) and Upper Jurassic-Lower Cretaceous (eastern part of the Tankhoy tectonic step) sedimentary rocks, disintegrated and eroded on the uplifts of the Primorsky and Khamar-Daban ranges.

The premature decay of rifting processes in the northern South China Sea (38–33.9 Ma): insight from 3D seismic and its interpretation of the Lufeng Sag of the Pearl River Mouth Basin

A marked transition in depositional signatures within the second rift phase (RP2, 38–33.9 Ma) in the Lufeng Sag, a proximal domain on the northern continental margin of the South China Sea (SCS) has been previously documented. This study integrates seismic data, fault paleo-throw, strain and depth-to-detachment to demonstrate the structural variability in time and space during the early to late stage of RP2 in the Lufeng Sag and investigates the rifting transition and its relationship with rifting processes on the northern SCS margin. The results indicate that extensional deformation transferred from south to north within the Lufeng Sag during RP2 and the rifting in the South Sub-sag was weaker than in the North Sub-sag during both the early and late stages. This structural variability may be related to the basement uplift in the southeast caused by the remote effect of plate movement and magmatic underplating under the Dongsha Uplift. A change of basin geometry from wedge-shaped to nearly tabular, strongly reduced paleo-throws and strains, and sharply increased depth-to-detachment from the early to late stage indicates the premature decay of rifting before the breakup of the SCS. This transition may be influenced by the opening of the Northeast Sub-basin of the SCS at ca 37 Ma, where extensional strain sharply concentrated, and led to the waned rifting of the Lufeng Sag. The study thus provides new insights into rifting processes along the northern SCS margin that can be applied to similar basins elsewhere. KEY POINTS Basin geometry, fault paleo-throw, strain and depth-to-detachment analysis demonstrate a weakened rifting in the Lufeng Sag (38–33.9 Ma). Structural variability between the South and North sub-sags may be caused by the remote effect of plate movement and magmatic underplating during the Dongsha Uplift. Structural transitions in the Lufeng Sag indicate a premature decay of rifting before the breakup of the SCS, which was caused by thermal subsidence and regional plate subduction.

Marine depositional signatures of the Aptian Oceanic Anoxic Events in the Eastern Tethys, Lower Indus Basin, Pakistan

The depositional signatures of the Cretaceous OAE1a and OAE1b intervals in the Parh Formation, Lower Indus Basin, Pakistan, Eastern Tethys, have been studied to better constrain the Aptian–Albian ocean–climate system. Based on the microfacies attributes, an outer ramp to deep basin is suggested for the deposition of Parh Formation. The quasi-periodic occurrences of radiolarian-rich microfacies and the cyclicity between the outer ramp and deep basin settings throughout the section suggest eutrophication in this part of the Eastern Tethys influenced by Milankovitch cycles. The clastic influx during OAE1a and OAE1b intervals indicate the role of continental runoff in the eutrophication of the basin. The overall negative covariance between radiolarian-rich microfacies and clastic input suggests that upwelling was a major driver of nutrient enrichment rather than continental runoff. The organic enrichment along with pyrite framboids and lack of burrows suggest that dysoxic–anoxic bottom water conditions were common throughout the Aptian and not just in the OAE1a and OAE1b intervals. The poor ventilation of the sea floor was a result of restricted circulation and may have triggered sustained anoxia during Aptian in this part of the Eastern Tethys. Bottom-water currents recharged coarser clastics to the deeper part of the basin during OAE1a and OAE1b intervals. However, the effects of deep ventilated oxic water during the events were reduced by persistent ocean anoxia. Furthermore, the dominance of carbonate sediments throughout the Aptian–lower Albian succession suggests relatively alkaline oceans in this part of Eastern Tethys. A pronounced sea-level rise, as suggested elsewhere was not observed during OAE1a and OAE1b intervals in this part of the Tethys. KEY POINTS The Aptian–Albian hemipelagic sequence of the Parh Formation, Lower Indus Basin, Pakistan, Eastern Tethys has been deposited in an outer ramp to deep basin environment. The quasi-periodic occurrences of radiolarian-rich microfacies and cyclic alternation of depositional environments suggest eutrophication of the basin influenced by Milankovitch cycles. Organic enrichment, pyrite framboids and lack of burrows suggest dysoxic–anoxic bottom-water conditions throughout the Aptian to early Albian.

Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) – Signatures of stratospheric fallout in the Southern Hemisphere

The inventories of 137Cs (503 ± 34 Bq.m−2), 241Am (11.3 ± 1.2 Bq.m−2), 241Pu (33.7 ± 3.4 Bq.m−2), 238Pu (6.82 ± 0.87 Bq.m−2) and 239+240Pu (113.0 ± 5.9 Bq.m−2), sum of 239Pu (100 ± 11 Bq.m−2) and 240Pu (14.5 ± 1.7 Bq.m−2), in the Gambier archipelago (23°S) of the French Polynesia, are well higher the global fallout at this latitude, in unequal proportions for the different radionuclides. (240Pu/239Pu)AR (AR: atomic ratio) of 0.0394 ± 0.0062, and (241Pu/239Pu)AR of (2.03 ± 0.39)10−4, confirm that the overwhelmingly dominant source of these radionuclides comes from local fallout during the 1970s of the French atmospheric tests of Moruroa and Fangataufa located nearly 400 km from Gambier. The signatures of the local fallout were deduced from the excess of its inventory in 137Cs and from the mixing lines established from the signatures of the global fallout, some of the test sites and the isotopic ratios measured in Gambier. Signatures obtained are 2.0 ± 0.4 for 137Cs/239+240Pu, 0.045 ± 0.008 for 238Pu/239+240Pu, 0.031 ± 0.009 for 241Am/239+240Pu, 0.092 ± 0.027 for 241Pu/239+240Pu, 0.0163 ± 0.0049 for (240Pu/239Pu)AR, (0.78 ± 0.23)10−4 for (241Pu/239Pu)AR. The concordance of the mixing lines of the [(240Pu/239Pu)AR, (241Pu/239Pu)AR] and the linear regression of these ratios measured in the stratosphere (40°S) during the 1970s, indicates that the signatures of the close-in deposition are also those of the stratospheric injections of the French tests. The signatures of stratospheric fallout in the Southern Hemisphere were evaluated by considering that the fission energy of these injections represents 11% and that of the Northern Hemisphere represents 89% and that the isotopic ratios of stratospheric injections remained the same over the period 1970–1974. The activity ratios deducted are 21.9 ± 0.1 in 137Cs/239+240Pu, 0.11 ± 0.05 in 238Pu/239+240Pu, 1.03 ± 0.12 in 241Pu/239+240Pu and 0.35 ± 0.04 in 241Am/239+240Pu. The associated atom ratios are 0.157 ± 0.011 for (240Pu/239Pu)AR and (8.33 ± 0.48)10−4 for (241Pu/239Pu)AR. These signatures appear to be consistent with the results of the inventories at Hiva Oa, located more than 1,000 km north of both French test sites, and with those found in the Australian continent, in regions not impacted by UK-test debris. The proportions of close-in tropospheric fallout from the French tests are about 90% in Gambier. They represent a proportion in the inventories of 40% for the 137Cs, 60% for 241Am and in the range between 80 and 90% for Pu isotopes.

Assessment of the Amaruq gold deposit signature in glacial sediments using multivariate geochemical data analysis and indicator minerals

The varying nature and thickness of glacial sediments affect the characteristics of mineralized dispersal trains in the Amaruq gold deposit area in Nunavut, northern Canada. To help evaluate the deposit signature, glacial sediments were collected from frost boils along transects parallel to the main ice-flow direction. A complementary detailed geochemical drift exploration dataset and a new Quaternary map were also used to define and interpret the geometry of the mineralized debris dispersal trains. Principal Component Analysis identifies a gold mineralization signature associating As, Ag, Au, Cu, Pb, Sb, W, S, Fe, and Zn and its mafic-BIF (Banded Iron Formation) host rock. This signature, similar to other lode gold geochemical signatures, follows a general NNW dispersal trend, in agreement with the predominant regional and local ice-flow indicators. In a zone characterized by streamlined landforms composed of thick distally -derived till, the dispersal train shows a higher distance for the first indicator to appear at surface down-ice of the known mineralization (~1 km lag) when compared to other mineralized dispersal trains (~300 m lag). Results from this area also show a lower intensity of deposit signature compared to mineralized dispersal trains located in thin locally-derived till characterized by small transverse moraine ridges. Elevated counts of scheelite and pristine-shaped gold grains are found down-ice of mineralization and their distribution is in general agreement with the dispersal train patterns defined by the till-matrix geochemistry. Our results strengthen the use of PCA and indicator minerals combined with an understanding of ice flow dynamics to map deposit signatures at surface and efficiently guide mineral exploration during glacial sediment surveys at local scales.

The supercritical question for pyroclastic dune bedforms: An overview

AbstractSedimentary structures deposited from dilute pyroclastic currents are largely dominated by the products of metre‐scale dune bedforms containing backset lamination. Since the first seminal works in the 1970s, the vast majority of the literature called these structures antidunes and chutes‐and‐pools. This consensus led to a loose terminology, either as descriptive terms for structures containing backset beds or as an interpretation of hydrodynamic conditions related to Froude‐supercritical flows. Here, the main characteristics of pyroclastic dune bedforms are summarized and classified under four categories. The literature is examined and a discussion is provided regarding a possible interpretation in terms of supercritical‐flow bedforms, including antidunes, chutes‐and‐pools and cyclic steps. The interpretation of pyroclastic dune bedforms as related to supercritical conditions is a possibility among others, yet open questions remain, and under the present state of knowledge alternative interpretations are equally valid. There is consensus that an interpretation of pyroclastic dune bedforms as related to supercritical flows implies deposition from a basal underflow supporting an internal free‐surface formed at a density interface within the overall current structure. Antidunes are likely to be limited to low‐angle, incipient bedforms. Chutes‐and‐pools may occur in the form of series of steep truncations formed by an erosive supercritical ‘chute’, with a depositional signature occurring exclusively in the subcritical ‘pool’ of a Froude jump. Rare examples of cyclic steps may be found as large‐scale undulations with superimposed metre‐scale bedforms, or for short‐scale, fully‐aggrading structures in periodic trains. Additionally, some pyroclastic bedforms may be formed by granular flows passing granular Froude jumps or as frozen granular stationary waves. The dominance of chute‐and‐pool structures over bedforms characterized by relatively more stable morphodynamics (antidunes and cyclic steps) could be the likely result of pulsating flow conditions and abrupt high‐rate deposition, impeding stable flow conditions. In a majority of cases, the growth of pyroclastic bedforms appears triggered by the influence of an inherited bed topography and by bed‐flow feedback effects. Apart from a possible interpretation as supercritical bedforms, other specific dynamics of dilute pyroclastic currents may well explain the characteristics of pyroclastic dune bedform structures. In particular, the inherent turbulence fluctuations, internal organization and highly depositional dynamics may be key to the formation of pyroclastic dune bedforms. In this respect, rather than solely focusing on the Froude dimensionless number of the formative current, theoretical considerations accounting for the Reynolds, Richardson and Rouse dimensionless parameters may be also implemented for the understanding of pyroclastic bedforms.

Bacterial influence on the formation of hematite: implications for Martian dormant life

AbstractPrevious exploration missions have revealed Mars as a potential candidate for the existence of extraterrestrial life. If life could have existed beneath the Martian subsurface, biosignatures would have been preserved in iron-rich minerals. Prior investigations of terrestrial biosignatures and metabolic processes of geological analogues would be beneficial for identifying past metabolic processes on Mars, particularly morphological and chemical signatures indicative of past life, where biological components could potentially be denatured following continued exposure to extreme conditions. The objective of the research was to find potential implications for Martian subsurface life by characterizing morphological, mineralogical and microbial signatures of hematite deposits, both hematite rock and related soil samples, collected from Highland Complex of Sri Lanka. Rock samples examined through scanning electron microscopy-energy dispersive X-ray (SEM-EDX) spectroscopy. Analysis showed globular and spherical growth layers nucleated by bacteria. EDX results showed a higher iron to oxygen ratio in nuclei colonies compared to growth layers, which indicated a compositional variation due to microbial interaction. X-ray diffraction analysis of the hematite samples revealed variations in chemical composition along the vertical soil profile, with the top surface soil layer being particularly enriched with Fe2O3, suggesting internal dissolution of hematite through weathering. Furthermore, inductively coupled plasma-mass spectrometry analyses carried out on both rock and soil samples showed a possible indication of microbially induced mineral-weathering, particularly release of trapped trace metals in the parent rock. Microbial diversity analysis using 16S rRNA gene sequencing revealed that the rock sample was dominated byActinobacteriaandProteobacteria, specifically, members of iron-metabolizing bacterial genera, includingMycobacterium,Arthrobacter,Amycolatopsis,NocardiaandPedomicrobium. These results suggest that morphological and biogeochemical clues derived from studying the role of bacterial activity in hematite weathering and precipitation processes can be implemented as potential comparative tools to interpret similar processes that could have occurred on early Mars.

The depositional signature of high‐aggradation chute‐and‐pool bedforms: The build‐and‐fill structure

AbstractChutes‐and‐pools are hybrid bedforms that occur in upper‐flow‐regime conditions, populating the stability field between antidunes and cyclic steps. Chutes‐and‐pools consist of a superimposition of a train of antidunes on a longer‐wavelength cyclic‐step instability. The presence of a hydraulic jump is more persistent in chutes‐and‐pools than the bores occasionally seen on breaking‐wave antidunes, yet not permanent as over cyclic‐step bedforms. The nature of chute‐and‐pool structures preserved in the depositional record is controlled largely by aggradation rate. This paper documents chute‐and‐pool structures generated beneath highly aggradational sediment density‐flows at the subaqueous toe‐of‐slope of a Pleistocene carbonate wedge. Quarry exposures enable the morphodynamic reconstruction of the sediment‐bed configuration that produced the identified structures. Wave‐breaking on growing antidunes occurred without the destruction of the antidune, which deviates from observations in low‐aggradation subaerial open‐channel flows. The antidune build‐up was followed by hydraulic jump controlled deposition in the antidune trough. Two‐dimensional depth‐resolved numerical simulations have been used to validate this process interpretation. The term aggradational chutes‐and‐pools is proposed for these bedforms, associated with the formation of build‐and‐fill structures consisting of interstratified convex lenses (in‐phase wave regime) and concave lenses (hydraulic jump regime). Such structures contrast with the scour‐and‐fill structures produced by low‐aggradation chutes‐and‐pools frequently described from subaerial open‐channel‐flow deposits. This study demonstrates a potential difference in flow behaviour between open‐channel flows and density flows in the supercritical regime.