| 1. |
- Madanian, H., et al.
(författare)
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The Dynamics of a High Mach Number Quasi-perpendicular Shock : MMS Observations
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 908:1
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Tidskriftsartikel (refereegranskat)abstract
- Shock parameters at Earth's bow shock in rare instances can approach the Mach numbers predicted at supernova remnants. We present our analysis of a high Alfven Mach number (M-A = 27) shock utilizing multipoint measurements from the Magnetospheric Multiscale spacecraft during a crossing of Earth's quasi-perpendicular bow shock. We find that the shock dynamics are mostly driven by reflected ions, perturbations that they generate, and nonlinear amplification of the perturbations. Our analyses show that reflected ions create modest magnetic enhancements upstream of the shock, which evolve in a nonlinear manner as they traverse the shock foot. They can transform into proto-shocks that propagate at small angles to the magnetic field and toward the bow shock. The nonstationary bow shock shows signatures of both reformation and surface ripples. Our observations indicate that although shock reformation occurs, the main shock layer never disappears. These observations are at high plasma beta, a parameter regime that has not been well explored by numerical models.
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| 2. |
- Nisson, D.M., et al.
(författare)
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Hydrogeochemical and isotopic signatures elucidate deep subsurface hypersaline brine formation through radiolysis driven water-rock interaction
- 2023
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier. - 0016-7037 .- 1872-9533. ; 340, s. 65-84
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Tidskriftsartikel (refereegranskat)abstract
- Geochemical and isotopic fluid signatures from a 2.9–3.2 km deep, 45–55 °C temperature, hypersaline brine from Moab Khotsong gold and uranium mine in the Witwatersrand Basin of South Africa were combined with radiolytic and water–rock isotopic exchange models to delineate brine evolution over geologic time, and to explore brine conditions for habitability. The Moab Khotsong brines were hypersaline (Ca-Na-Cl) with 215–246 g/L TDS, and Cl− concentrations up to 4 mol/L suggesting their position as a hypersaline end-member significantly more saline than any previously sampled Witwatersrand Basin fluids. The brines revealed low DIC (∼0.266–∼1.07 mmol/L) with high (∼8.49–∼23.6 mmol/L) DOC pools, and several reduced gaseous species (up to 46 % by volume H2) despite microoxic conditions (Eh = 135–161 mV). Alpha particle radiolysis of water to H2, H2O2, and O2 along with anhydrous-silicate-to-clay alteration reactions predicted 4 mol/L Cl− brine concentration and deuterium enrichment in the fracture waters over a period > 1.00 Ga, consistent with previously reported 40Ar noble gas-derived residence times of 1.20 Ga for this system. In addition, radiolytic production of 7–26 nmol/(L × yr) H2, 3–11 nmol/(L × yr) O2, and 1–8 nmol/(L × yr) H2O2 was predicted for 1–100 g/g 238U dosage scenarios, supporting radiolysis as a significant source of H2 and oxidant species to deep brines over time that are available to a low biomass system (102–103 cells/mL). The host rock lithology was predominately Archaean quartzite, with minerals exposed on fracture surfaces that included calcite, pyrite, and chlorite. Signatures of 18Ocalcite, 13Ccalcite, Δ33Spyrite, 34Spyrite and 87Sr/86Sr obtained from secondary ion mass spectrometry (SIMS) microanalyses suggest several discrete fluid events as the basin cooled from peak greenschist conditions to equilibrium with present-day brine temperatures. The brine physiochemistry, geochemistry, and cellular abundances were significantly different from those of a younger, shallower, low salinity dolomitic fluid in the same mine, and both were different from the mine service water. These results indicate the discovery of one of few long-isolated systems that supports subsurface brine formation via extended water–rock interaction, and an example of a subsurface brine system where abiotic geochemistry may support a low biomass microbial community.
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