Spin Gauge Theory of Gravity in Clifford Space: A Realization of Kaluza-Klein Theory in 4-Dimensional Spacetime

A theory in which 4-dimensional spacetime is generalized to a larger space, namely a 16-dimensional Clifford space (C-space) is investigated. Curved Clifford space can provide a realization of Kaluza-Klein theory. A covariant Dirac equation in curved C-space is explored. The generalized Dirac field is assumed to be a polyvector-valued object (a Clifford number) which can be written as a superposition of four independent spinors, each spanning a different left ideal of Clifford algebra. The general transformations of a polyvector can act from the left and/or from the right, and form a large gauge group which may contain the group U(1)xSU(2)xSU(3) of the standard model. The generalized spin connection in C-space has the properties of Yang-Mills gauge fields. It contains the ordinary spin connection related to gravity (with torsion), and extra parts describing additional interactions, including those described by the antisymmetric Kalb-Ramond fields.Comment: 57 pages; References added, section 2 rewritten and expande

Extreme sensitivity in Snowball Earth formation to mountains on PaleoProterozoic supercontinents

During the PaleoProterozoic 2.45 to 2.2 billion years ago, several glaciations may have produced Snowball Earths. These glacial cycles occurred during large environmental change when atmospheric oxygen was increasing, a supercontinent was assembled from numerous landmasses, and collisions between these landmasses formed mountain ranges. Despite uncertainties in the composition of the atmosphere and reconstruction of the landmasses, paleoclimate model simulations can test the sensitivity of the climate to producing a Snowball Earth. Here we present a series of simulations that vary the atmospheric methane concentration and latitudes of west–east-oriented mountain ranges on an idealised supercontinent. For a given methane concentration, the latitudes of mountains control whether a Snowball Earth forms or not. Significantly, mountains in middle latitudes inhibited Snowball Earth formation, and mountains in low latitudes promoted Snowball Earth formation, with the supercontinent with mountains at ±30° being most conducive to forming a Snowball Earth because of reduced albedo at low latitudes. We propose that the extreme sensitivity of a Snowball Earth to reconstructions of the paleogeography and paleoatmospheric composition may explain the observed glaciations, demonstrating the importance of high-quality reconstructions to improved understanding of this early period in Earth’s history

Neighbourhood ethnic density effects on behavioural and cognitive problems among young racial/ethnic minority children in the US and England: a cross-national comparison

Studies on adult racial/ethnic minority populations show that the increased concentration of racial/ethnic minorities in a neighbourhood—a so-called ethnic density effect—is associated with improved health of racial/ethnic minority residents when adjusting for area deprivation. However, this literature has focused mainly on adult populations, individual racial/ethnic groups, and single countries, with no studies focusing on children of different racial/ethnic groups or comparing across nations. This study aims to compare neighbourhood ethnic density effects on young children’s cognitive and behavioural outcomes in the US and in England. We used data from two nationally representative birth cohort studies, the US Early Childhood Longitudinal Study-Birth Cohort and the UK Millennium Cohort Study, to estimate the association between own ethnic density and behavioural and cognitive development at 5 years of age. Findings show substantial heterogeneity in ethnic density effects on child outcomes within and between the two countries, suggesting that ethnic density effects may reflect the wider social and economic context. We argue that researchers should take area deprivation into account when estimating ethnic density effects and when developing policy initiatives targeted at strengthening and improving the health and development of racial and ethnic minority children

The faint young Sun problem revisited with a 3-D climate–carbon model – Part 1

International audienceDuring the Archaean, the Sun's luminosity was 18 to 25 % lower than the present day. One-dimensional radia-tive convective models (RCM) generally infer that high concentrations of greenhouse gases (CO 2 , CH 4) are required to prevent the early Earth's surface temperature from dropping below the freezing point of liquid water and satisfying the faint young Sun paradox (FYSP, an Earth temperature at least as warm as today). Using a one-dimensional (1-D) model, it was proposed in 2010 that the association of a reduced albedo and less reflective clouds may have been responsible for the maintenance of a warm climate during the Ar-chaean without requiring high concentrations of atmospheric CO 2 (pCO 2). More recently, 3-D climate simulations have been performed using atmospheric general circulation models (AGCM) and Earth system models of intermediate complexity (EMIC). These studies were able to solve the FYSP through a large range of carbon dioxide concentrations, from 0.6 bar with an EMIC to several millibars with AGCMs. To better understand this wide range in pCO 2 , we investigated the early Earth climate using an atmospheric GCM coupled to a slab ocean. Our simulations include the ice-albedo feedback and specific Archaean climatic factors such as a faster Earth rotation rate, high atmospheric concentrations of CO 2 and/or CH 4 , a reduced continental surface, a saltier ocean, and different cloudiness. We estimated full glaciation thresholds for the early Archaean and quantified positive radiative forcing required to solve the FYSP. We also demonstrated why RCM and EMIC tend to overestimate greenhouse gas concentrations required to avoid full glaciations or solve the FYSP. Carbon cycle-climate interplays and conditions for sustaining pCO 2 will be discussed in a companion paper

Ubiquitous occurrence of basaltic-derived paleosols in the Late Archean Fortescue Group, Western Australia

The 2.76 Ga old Mount Roe Basalt paleosols (MR#1 and MR#2), recognized near the base of the 2.76-2.69 Ga Fortescue Group in Western Australia, represent some of the oldest definite examples of Archean paleoweathering profiles. The loss of Fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the Late Archean atmosphere, respectively. However, the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post-weathering alteration over primary soil profiles. Here we report new exposures of the MR#1 paleosol as well as a number of new basalt-derived paleosol occurrences distributed in the Mount Roe Basalt and the 2.73 Ga old Kylena Formations of the Fortescue Group. We show that all these paleosols, including MR#1 and MR#2, were strongly affected by post-weathering reductive alteration. Nevertheless, we discovered early lithologic units, locally preserved within the hydrothermally altered paleosols, which feature distinct chemical and mineralogic compositions. These include: (i) C-13-depleted carbonaceous-rich, diaspore-pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and (ii) green hard core material, mostly composed of Fe-sericite associated with authigenic sphene and containing early relics of sulfate-bearing iron-rich smectite or berthierine. We argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage, while berthierine and sphene formed during diagenesis through the circulation of reductive fluids. Because the depletion of iron observed in the Fortescue paleosols is not solely due to pedogenesis, but also to post-weathering alteration, particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer. Recognition of the regional-scale, syn-depositional alteration of the Fortescue subaerial basalts over the north Pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the Pilbara craton during continental break-up and rifting. Such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and/or lacustrine systems, favoring the development of microbial life in shallow waters

Petrology and geochemistry of scandium in New Caledonian Ni-Co laterites

The growing demand for scandium (Sc), essential for several modern industrial applications, drives the mining industry to develop alternative Sc sources. In such context, significant Sc concentrations (~100 ppm) were recently reported in several Ni-Co lateritic oxide ores developed after mafic-ultramafic rocks. This contribution examines the distribution of Sc in Ni-Co laterites from New Caledonia, the sixth largest Ni producer worldwide. Representative lateritic profiles were selected based on the protolith type and include dunite, harzburgite and lherzolite protoliths, wherein the Sc content, determined by the relative proportion of olivine and pyroxene, ranges from 10 ppm in lherzolite. In Ni-Co laterites, dissolution and leaching of primary Mg-rich silicates leads to the residual enrichment of iron as ferric oxides/oxyhydroxides in the upper horizons. Downward remobilization and trapping of Ni and Co lead to their local enrichment to economic concentrations, with maximum grades reached in the saprock/saprolite and in the transition horizons, respectively. In contrast, maximum Sc enrichment occurs in the yellow limonite horizon, where Sc-bearing goethite contains about ten times the Sc content of the parent rock. Consequently, harzburgite- and lherzolite-derived yellow limonites yield maximum Sc concentrations up to 100 ppm, together with moderate Ni and Co concentrations. There, Sc is potentially a valuable by-product that could be successfully co-extracted along with Ni and Co through hydrometallurgical processing. In addition to peridotite-hosted laterites, hornblende-rich amphibolites yield elevated Sc up to 130 ppm. The saprolitization of amphibolites leads to the formation of a goethite-gibbsite-kaolinite mixture with Sc concentrations > 200 ppm. There, goethite is the main Sc carrier with up to 800 ppm Sc. Therefore, despite their relatively limited volumes, amphibolite-derived saprolites may also represent attractive targets for Sc in New Caledonia. It is proposed that three main factors control the distribution and intensity of Sc enrichment in laterites derived from mafic and ultramafic rocks: (i) the initial Sc content of the parent rock, (ii) the development of goethite-dominated, yellow limonite after long-lived tropical weathering, and (iii) the local remobilization of Sc from the uppermost horizons through dissolution/recrystallization of goethite and partial replacement of goethite by hematite, thus leading to downward Sc concentration in the yellow limonite

Early traces of life investigations in drilling Archean hydrothermal and sedimentary rocks of the Pilbara Craton, Western Australia and Barberton Greenstone Belt, South Africa.

International audienceThe Pilbara Craton of Western Australia and the Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, contain some of the oldest and best preserved Archaean rocks and microfossils in the world. Two stratigraphic horizons in the Pilbara Craton were drilled as part of a collaborative effort between France and Australia (the Pilbara Drilling Project) during August 2004, including the 3481 Ma Dresser Formation (Warrawoona Group) and 2724 Ma Tumbiana Formation (Fortescue Group). A new diamond drill hole was cored in August 2008 through part of the ∼3250 Ma Fig Tree Group in the Barberton Greenstone Belt as part of a joint project between France and South Africa. These pristine diamond drill cores present a unique opportunity to constrain the chemistry of the earliest ocean, the composition of the atmosphere, and the settings and types of microbial ecosystems spanning the Archean Eon. These drill core samples can also provide new clues on the earliest metabolic pathways