Matching Martian crustal magnetization and magnetic properties of Martian meteorites

Abstract— Magnetic properties of 26 (of 32) unpaired Martian meteorites (SNCs) are synthesized to further constrain the lithology carrying Martian magnetic crustal sources. Magnetic properties of ultramafic cumulates (i.e., Chassigny, Allan Hills [ALH] 84001) and lherzolitic shergottites (ALH 77005, Lewis Cliff [LEW] 88516) are one or two orders of magnitude too weak to account for the crustal magnetizations, assuming magnetization in an Earth‐like field. Nakhlites and some basaltic shergottites, which are the most magnetic SNCs, show the right intensity. Titanomagnetite is the magnetic carrier in the nakhlites (7 meteorites), whereas in most basaltic shergottites (11 meteorites) it is pyrrhotite. Dhofar (Dho) 378, Los Angeles, and NWA 480/1460 and 2046 are anomalous basaltic shergottites, as their magnetism is mainly due to titanomagnetite. Pyrrhotite should be among the candidate minerals for the magnetized Noachian crust.     

An analysis of the vertical structure equation in sigma coordinates

Abstract

An analysis of the vertical structure equation of sigma coordinate primitive equation models is given that brings together and extends the work of several authors. We derive the vertical structure equation, and obtain its solution for a two‐parameter family of vertical structure profiles that includes those of previous studies. For this family, it is shown that in the limiting case of an unbounded atmosphere the spectrum becomes partially continuous, rather than entirely discrete as in the bounded case. A criterion is obtained for the validity of the linearization used to derive the vertical structure equations, and it turns out that this criterion is satisfied by all but one of the previous studies. Asymptotic expansions are derived and used to explain two observations of Wiin‐Nielsen (1971a), viz. why the equivalent depths of the internal modes are relatively insensitive to the precise choice of lower boundary condition, and why one choice in particular leads to the elimination of the external mode; these asymptotic expansions also yield surprisingly accurate numerical values for the equivalent depths. Finally, the projection of atmospheric data onto modes found by direct numerical approximation of the vertical structure equation is shown, particularly for the least grave modes, to be very sensitive to resolution; consequently care must be exercised when interpreting the results of data projection studies that use this approach.     

Ground-based measurements of local cloud cover

Clouds are believed to reflect temporal climate changes through variations in their amounts, characteristics, and occurrence. In addition, they reflect both weather and climate in a region. In this work, a methodology to determine the local cloud cover (LCC) is proposed using sky images obtained from a ground-based instrument. Three years of sky images from an urban, tropical site were obtained and analyzed through that methodology. Monthly average LCC varied from 3 to 96 %, while seasonal average values were 68 % for summer, 54 % for spring, 46 % for fall, and 23 % for winter. LCC results show a clear seasonal dependence and a fair agreement (r ² = 0.72) with satellite data, which typically underestimate the cloud cover in relation to LCC. Our analysis also suggests the possibility of a measurable link between LCC and natural events like the El Niño Southern Oscillation.     

Impact‐related noncoaxial deformation in the Pułtusk H chondrite inferred from petrofabric analysis

Petrofabrics in chondrites have the potential to yield important information on the impact evolution of chondritic parent asteroids, but studies involving chondritic petrofabrics are scarce. We undertook an analysis of the Pu?tusk H chondrite regolith breccia. Measurements of anisotropy of magnetic susceptibility and quantitative tomographic examination of metal grains are presented here and the results are compared with petrographic observations. The major fabric elements are in Pu?tusk shear fractures cutting the light‐colored chondritic clasts as well as brittly and semibrittly deformed, cataclased fragments in dark matrix of regolith breccia. Cataclasis is accompanied by rotation of silicate grains and frictional melting. Fabric of metal grains in chondrite is well defined and coherently oriented over the breccia, both in the clasts and in the cataclastic matrix. Metal grains have prolate shapes and they are arranged into foliation plane and lineation direction, both of which are spatially related and kinematically compatible to shear‐dominated deformational features. We argue that the fabric of Pu?tusk was formed in response to impact‐related noncoaxial shear strain. Deformation promoted brittle cataclastic processes and shearing of silicates, and, simultaneously, allowed for ductile metal to develop foliation and lineation. We suggest that plastic flow is the most probable mechanism for the deformation of metal grains in the shear‐dominated strain field. The process led also to the formation of large metal nodules and bands in the dark matrix of breccia.     

Reactivity at (nano)particle-water interfaces,redox processes,and arsenic transport in the environment

Massive deleterious impacts to human health are resulting from the use of arsenic-bearing groundwaters in South-East Asia deltas and elsewhere in the world for drinking, cooking and/or irrigation. In Bangladesh alone, a fifth of all deaths are linked to arsenicosis. In the natural and engineered subsurface environment, the fate of arsenic is, to a large extent, controlled by redox potential, pH, as well as total iron, sulfur and carbonate content, via sorption and coprecipitation on a variety of natural and engineered (nano)particles. In the present article, we address: (1) new insights in the sorption mechanisms of As on Fe(II) and Fe(III) nanophases recognized to play an important role in the microbial cycling of As and Fe; (2) artifacts often encountered in field and laboratory studies of As speciation due to the extreme redox sensitivity of the Fe-As-O-H phases; and (3) as a conclusion, the implications for water treatment. Indeed the specific reactivity of nanoparticles accounts not only for the As bioavailability within soils and aquifers, but also opens new avenues in water treatment.     

Prey preferences among the community of deep-diving odontocetes from the Bay of Biscay, Northeast Atlantic

Long-finned pilot whales (Globicephala melas), Risso's dolphins (Grampus griseus), melon-headed whales (Peponocephala electra), Cuvier's beaked whales (Ziphius cavirostris), Sowerby's beaked whales (Mesoplodon bidens), northern bottlenose whales (Hyperoodon ampullatus), sperm whales (Physeter macrocephalus), dwarf sperm whales (Kogia sima) and pygmy sperm whales (Kogia breviceps) make up the large community of deep-diving odontocetes occurring off the Bay of Biscay, northeast Atlantic. The ecology of these toothed cetaceans is poorly documented worldwide. The present study described their prey preferences from stomach content analysis and showed resource partitioning within the assemblage. The majority of the species appeared to be mostly teutophageous. Fish was an important food source only for the Sowerby's beaked whale and, to a lesser extent, for the long-finned pilot whale. In terms of foraging habitats inferred from prey composition, either pelagic oceanic or demersal neritic habitats were exploited by toothed whales in the Bay of Biscay, with only the long-finned pilot whale foraging in the two habitats. Finally, with more than 14,000 identified cephalopods from 39 species, the present study highlighted also the poorly known deep-sea cephalopod community off the Bay of Biscay using top predators as biological samplers.     

Planform-asymmetry and backwater effects on river-cutoff kinematics and clustering

River bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long-term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast-migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub-population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel-flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering-river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd.