ALBERT

Description: By constructing an empirical model of the spectral and latitudinal distribution of ion cyclotron waves on the basis of Cassini datasets, we investigate the resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn. Calculations based on quasi-linear bounce-averaged diffusion coefficients show that at Saturn ion cyclotron waves can efficiently pitch angle scatter 〉∼1 MeV to tens of MeV electrons into the loss cone thereby inducing precipitation loss, while the mixed and momentum scattering effects are typically negligible. The resultant electron loss timescales range from a few to tens of minutes, which in fact decrease significantly with increasing L-shell at L = 4–6. We also find that the kinetic effects introduced by pick-up ring particles cause distinct changes in pitch angle scattering efficiency for lower energy electrons (〈3 MeV at L = 5). Our results demonstrate that ion cyclotron waves play a significant role in the dynamics of Saturn's radiation belt electrons.

Description: On September 5, 2022, an Ms6.8 earthquake hit the Luding County in Sichuan Province, China, and caused 93 deaths and 24 people missing. The spatial distribution of the relocated Luding earthquake sequence reveals an NNW-trending aftershock zone with long axis about 55 km along Moxi segment, the southern segment of Xianshuihe fault zone (XSH). The aftershock zone is wider in the middle and narrower in the southern and northern sections. The Ms6.8/Mw6.44 mainshock with centroid depth at 3.0 km and the two Ms≥5.0 aftershocks occurred in the middle section. The strike, dip and rake angles of the nodal planes I and II of the focal mechanism for the mainshock are 75°/90°/155° and 165°/65°/0°, respectively. The areal strains As exhibit a pure strike-slip faulting style both for the mainshock and the aftershocks in the southern and northern parts, while the aftershocks in the middle section are of pure or oblique extensional. Based on the spatial distribution of aftershocks, the focal mechanism solutions of the entire sequence and the structural trending in the aftershock zone, we conclude that the seismogenic fault of the Ms6.8 Luding earthquake is the NNW-trending Moxi fault on the southern segment of XSH. The seven Ms≥3.1 aftershocks in the middle section of the aftershock zone and the four Ms≥3.4 events in the 2016 Luding earthquake swarm with pure or oblique extensional behaviors imply the possible existence of a series of SSE (NNW)-striking normal faults, some of which might contribute to the occurrence of aftershocks there.

Description: Solute isotopes, such as 〈sup〉87〈/sup〉Sr〈sup〉/86〈/sup〉Sr, δ〈sup〉11〈/sup〉B, δ〈sup〉7〈/sup〉Li and δ〈sup〉44/40〈/sup〉Ca, have been widely used to trace the sources of dissolved solutes in water, because of the large isotopic variation of the potential sources. Sorption–desorption and ion exchange commonly occur in a water–rock system that contains clay. However, the contribution or the impact of sorption–desorption and ion exchange on isotopic composition of water have not been well identified. Shale is enriched in clay and comprehensive sorption–desorption and ion exchange occurs within a short time. Accordingly, the flowback water following hydraulic fracturing of shale gas reservoirs is a unique material for studying the mechanism affecting the specific isotopic composition (〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr, δ〈sup〉11〈/sup〉B, δ〈sup〉7〈/sup〉Li and δ〈sup〉44/40〈/sup〉Ca). This study also determined all potential endmembers (concentration and isotopic composition/ratio for water-soluble fraction, exchangeable fraction, and carbonate minerals). The results showed that desorption of B and Li from shale rock, isotopic re-equilibrium between the dissolved Sr and exchangeable Sr, and isotopic fractionation during cation exchange (〈sup〉40〈/sup〉Ca was favored over 〈sup〉44〈/sup〉Ca) mainly controlled the specific isotopic composition of flowback water beyond mixing between fracturing fluids and local formation water. However, the increases in 87Sr/86Sr ratios were previously interpreted as the dissolution of silicates with a high 87Sr/86Sr ratio. While clay minerals are commonly distributed in most water–rock systems and the findings of this study are important and useful for tracing water–rock interactions and sources of specific solutes in water systems that could be impacted by asorption-desorption and ion exchange.

Description: Magnetic flux ropes are bundles of twisted magnetic field that harbor free magnetic energy and can be progenitors of coronal mass ejections (CMEs). However, identifying flux ropes on the Sun can be challenging. In this work, we show Hinode EUV Imaging Spectrometer observations of an active region that exhibits a sigmoidal configuration (indicative of a flux rope). We analyze the coronal plasma composition as the sigmoid (flux rope) forms before its eruption. Plasma with photospheric composition was observed in coronal loops close to the main polarity inversion line during episodes of significant flux cancellation, suggestive of the injection of photospheric plasma into these loops driven by photospheric flux cancellation. Concurrently, the increasingly sheared core field contained plasma with coronal composition. As flux cancellation decreased and a sigmoid/flux rope formed, the plasma evolved to an intermediate composition in between photospheric and typical active region coronal compositions. Finally, the flux rope contained predominantly photospheric plasma just before the CME. Hence, plasma composition observations strongly support models of flux rope formation by photospheric flux cancellation forcing magnetic reconnection first at the photospheric level then at the coronal level. We compare and contrast these findings with a flux rope that formed via reconnection directly in the corona, which exhibited coronal plasma composition and was determined to be located at a higher altitude. These results show that plasma composition provides key information on the specific configuration of a flux rope, which in turn affects its stability and the processes involved in CME onset.

Description: The correlation between mantle flow and seismic anisotropy emphasizes its key role in understanding tectonic processes. While global geodynamic modelling of asthenospheric mantle flow results in a good correlation, the large spatial variation of seismic anisotropy beneath continents indicates a considerable complex contribution of the shallow lithosphere, which is not yet well understood. A detailed imaging of anisotropy with depth is therefore an important challenge for seismic investigation.Here, we present our current advances in improving the depth resolution based on receiver function and SKS-splitting techniques applied at the continental margins of the European Alps and Central Appalachians. Classical shear-wave splitting techniques are mostly used to infer a single anisotropic layer. This becomes misleading for continental margins with complex deformation. An azimuthal variation of splitting parameters is an indication for vertical layering of anisotropy and can be analyzed systematically providing insight into these complexities. However, this approach allows no direct constraint on the depth distribution. Recent developments involve the calculation of sensitivity kernels for Splitting Intensity observations, which allows us to consider the laterally broadened sensitivity for the anisotropic structure with depth. A requirement of this tomographic technique is a dense station spacing, which is satisfied by a growing number of seismic deployments.The increased lateral stress in deformational regimes at shallow levels results in a possible contamination of shear-wave splitting by crustal anisotropy. We suggest a stepwise approach in which receiver functions are examined for their harmonic variation with backazimuth to determine and correct for significant anisotropic crustal effects.

Description: Magnetotelluric (MT) signal is a natural electromagnetic (EM) response that can penetrate up to hundreds of kilometers deep in the earth's interior. When deployed at the seafloor, ocean bottom MT receivers provide us quality EM data that can decipher both regional and large-scale conductivity structures underneath the seafloor. Knowledge of such structures is critical in the studies of Earth's interior and marine geohazards. A key step in interpreting these deep sea EM data is accurate modelling of the data over various plausible marine conductivity models, where the existence of highly conductive sea water poses as a main challenge in improving the modelling accuracy. The accuracy degeneration issue caused by the seawater becomes more severe for three-dimensional (3D) studies in which fine discretizations of seawater layer are often required. To ensure modelling accuracy, we have developed high-order finite element (FE) modelling techniques in conjunction with unstructured triangular (2D) and tetrahedral (3D) meshes. Using the high-order FE methods, excessively large meshes are avoided since the seawater layer does not need to be finely discretized. Our modelling code has been tested and can provide accurate modelling results of marine MT data for a large range of frequencies. The modelling accuracy is demonstrated using both 2D and 3D examples. Using the newly developed code, we aim to study deep conductivity structures over various seafloors of interest.

Description: The Gulf of St. Lawrence is a semi-enclosed system that is subject to large freshwater runoff and is strongly influenced by oceanic water originating from the Arctic and Atlantic Oceans. It is also characterised by the presence of deep channels whose deep waters (〉200 m) are poorly ventilated. The variable topography and drivers lead to large temporal and spatial variability of biogeochemical conditions. Different regions, but particularly deep ones, show important deoxygenation, changes in carbonate system metrics, and warming in recent decades. These trends are also projected to continue in the future with an expansion of the areas presenting critical ocean acidification (OA) conditions for the health of some commercial species. In this study we describe the current state and variability of the carbonate system and how it impacts different traits of the American lobster (Homarus americanus). In order to determine which carbonate system parameters are detrimental and at which level to different traits and stages of this species, we first conducted an extensive literature review and synthesis study. A threshold analysis using broken-line regression models was then performed on the data. We used projections of OA conditions obtained from a regional downscaling system to determine the vulnerability of the American lobster to ongoing OA in the Gulf of St. Lawrence. This project is part of the Ocean Acidification Research for Sustainability program of the Ocean Decade.

Description: We discuss two types of ice shelf firn aquifers present in coastal Antarctica: melt-derived and brine-infiltration. Mapping of melt-derived firn aquifers and regions of melt-saturated firn using both passive and active L-band microwave data has identified perennial melt-derived aquifers in the southwestern Antarctic Peninsula, and several seasonal melt-saturation regions there and in widely separated sites around the ice sheet fringe. Extensive melt aquifers are identified on the Wilkins Ice Shelf, the northern George VI Ice Shelf, and the Müller Ice shelf, and are suspected on the former Jones and Wordie ice shelves. Field work has confirmed the aquifer presence in at least two sites on both the Wilkins and Müller shelves. We also present an improved assessment of the extent of brine aquifers based on their characteristics in modern airborne ice-penetrating radar profiles. Brine aquifers are relatively widespread and are found in shelf areas with porous firn at the waterline, as was previously recognized by Cook et al.(2018). Our study also assesses the relative risk of hydrofracture for ice shelves bearing melt and brine aquifers under various scenarios of firn density, aquifer depth, and brine density. We find that ponded surface melt poses the greatest threat, but that the increased density of brine may also be an issue if the brine layer is relatively shallow in the shelf. We also discuss the likely future expansion of melt aquifers on ice shelves under warming conditions, and consider the process of transitioning from a brine aquifer to a melt aquifer.

Description: Platinum-group minerals (PGM) are important host phases for platinum-group elements (PGE) in magmatic sulfide deposits. Recent studies have demonstrated that some PGM, such as sperrylite, can crystallize early from sulfide liquids. It is important to determine whether and how these PGM particles affect the enrichment and differentiation of PGE in Cu-Ni-PGE deposits. The magmatic sulfide deposits in the Yangliuping area (mainly consisting of the Yangliuping deposit and Zhengziyanwo deposit) are the largest Cu-Ni-PGE deposits in the Emeishan large igneous province, SW China. Primitive-mantle normalized PGE patterns and fractional crystallization modelling show that massive ores in the Yangliuping deposit are enriched in Pd and Pt. In contrast, massive ores, breccia ores and Cu-rich ores in the Zhengziyanwo deposit are depleted in Pt. Scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), focused ion beam (FIB) and transmission electron microscopy (TEM) were used to constrain the occurrence of PGE in the massive ores from the two deposits. Mass balance calculation shows that 〈7% of Ir, Rh and Pt occur in base metal sulfides (BMS, mainly pyrrhotite, pentlandite and chalcopyrite) as solid solution in both the Yangliuping and Zhengziyanwo massive ores. 17% and 91% Pd are present in solid solution within BMS in the Yangliuping and Zhengziyanwo massive ores, respectively. In the Yangliuping massive ores, PtAs2 and Pd-PGM particles were commonly found within BMS crystals, and Ir-Rh-(Pt) sulfarsenides enclosed in cobaltite-gersdorffite solid solution (CGSS) are enriched in Pt. In contrast, in the Zhengziyanwo massive ores, no PtAs2 particles and fewer Pd-PGM particles were found, and Ir-Rh-(Pt)AsS particles enclosed in CGSS are poor in Pt. The distribution, mineralogy and microstructures of Ir-Rh-(Pt)AsS, PtAs2 and Pd-PGM particles in both the Yangliuping and Zhengziyanwo deposits suggest that they mainly crystallized from sulfide liquids or formed from Pd-Pt-semimetal-rich droplets. We suggest that the incorporation of Pd-PGM and PtAs2 particles into MSS cumulates led to the enrichment of Pd and Pt in the Yangliuping massive ores. In the Zhengziyanwo deposit, early fractional crystallization of PtAs2 reduced the Pt content in sulfide liquids and resulted in the formation of Pt-depleted sulfide ores and low-Pt Ir-Rh-(Pt)AsS particles, but no Pt-rich sulfide ores have been found to date. As a result, the role of PGM particles should be considered when using PGE to trace the formation process of Cu-Ni-PGE deposits. This study also provides additional evidence that the behavior of Pt in sulfide liquids is influenced by thermodynamic conditions and As, and suggests a potential for Pt-rich sulfide ores exploration in the Zhengziyanwo deposit.

Description: Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter content, bedding planes, natural fractures, porosity and stress regime among others), external factors like wellbore orientation and stimulation design play a role. In this study, we present a series of true triaxial hydraulic fracturing experiments conducted on Lushan shale to investigate the interplay of internal factors (bedding, natural fractures and in situ stress) and external factors (wellbore orientation) on the growth process of fracture networks in cubic specimens of 200 mm in length. We observe relatively low breakdown pressure and fracture propagation pressure as the wellbore orientation and/or the maximum in situ stress is subparallel to the shale bedding plane. The wellbore orientation has a more prominent effect on the breakdown pressure, but its effect is tapered with increasing angle of bedding inclination. The shale breakdown is followed by an abrupt response in sample displacement, which reflects the stimulated fracture volume. Based on fluid tracer analysis, the morphology of hydraulic fractures (HF) is divided into four categories. Among the categories, activation of bedding planes (bedding failure, BF) and natural fractures (NF) significantly increase bifurcation and fractured areas. Under the same stress regime, a horizontal wellbore is more favorable to enhance the complexity of hydraulic fracture networks. This is attributed to the relatively large surface area in contact with the bedding plane for the horizontal borehole compared to the case with a vertical wellbore. These findings provide important references for hydraulic fracturing design in shale reservoirs.