Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle

Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt–associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000–550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 ± 1.4, 2s, n = 6) are similar to those of modern MORB (6.8 ± 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, DFMQ, at 1 GPa) of Archean sample suites (DFMQ –1.19 ± 0.33, 2s) is significantly lower than that of post-Archean sample suites, including MORB (DFMQ –0.26 ± 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth’s mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied by the first “whiffs” of oxygen in sediments of that age

Quantum reduced loop gravity effective Hamiltonians from a statistical regularization scheme

We introduce a new regularization scheme for Quantum Cosmology in Loop Quantum Gravity (LQG) using the tools of Quantum Reduced Loop Gravity (QRLG). It is obtained considering density matrices for superposition of graphs based on statistical countings of microstates compatible with macroscopic configurations. We call this procedure statistical regularization scheme. In particular, we show how the $\mu_0$ and $\bar{\mu}$ schemes introduced in Loop Quantum Cosmology (LQC) emerge with specific choices of density matrices. Within this new scheme we compute effective Hamiltonians suitable to describe quantum corrected Friedmann and Bianchi I universes and their leading orders coincide with the corresponding effective LQC Hamiltonians in the $\bar{\mu}$ scheme. We compute the next to the leading orders corrections and numerical investigation of the resulting dynamics shows evidence for the emergent-bouncing universe scenario to be a general property of the isotropic sector of QRLG.Comment: 22 pages, 4 figures. Two small typos fixed. Conclusions unchange

2-vertex Lorentzian Spin Foam Amplitudes for Dipole Transitions

We compute transition amplitudes between two spin networks with dipole graphs, using the Lorentzian EPRL model with up to two (non-simplicial) vertices. We find power-law decreasing amplitudes in the large spin limit, decreasing faster as the complexity of the foam increases. There are no oscillations nor asymptotic Regge actions at the order considered, nonetheless the amplitudes still induce non-trivial correlations. Spin correlations between the two dipoles appear only when one internal face is present in the foam. We compute them within a mini-superspace description, finding positive correlations, decreasing in value with the Immirzi parameter. The paper also provides an explicit guide to computing Lorentzian amplitudes using the factorisation property of SL(2,C) Clebsch-Gordan coefficients in terms of SU(2) ones. We discuss some of the difficulties of non-simplicial foams, and provide a specific criterion to partially limit the proliferation of diagrams. We systematically compare the results with the simplified EPRLs model, much faster to evaluate, to learn evidence on when it provides reliable approximations of the full amplitudes. Finally, we comment on implications of our results for the physics of non-simplicial spin foams and their resummation.Comment: 27 pages + appendix, many figures. v2: one more numerical result, plus minor amendment

Bianchi I effective dynamics in Quantum Reduced Loop Gravity

The effective quantum dynamics of Bianchi I spacetime is addressed within the statistical regularization scheme in Quantum Reduced Loop Gravity. The case of a minimally coupled massless scalar field is studied and compared with the effective $\bar{\mu}-$Loop Quantum Cosmology. The dynamics provided by the two approaches match in the semiclassical limit but differ significantly after the bounces. Analytical and numerical inspections show that energy density, expansion scalar and shear are bounded also in Quantum Reduced Loop Gravity and the classical singularity is resolved for generic initial conditions in all spatial directions.Comment: 19 pages, 23 figures, 1 tabl

Formation of abiotic hydrocarbon from reduction of carbonate in subduction zones: Constraints from petrological observation and experimental simulation

Abstract Subduction is a key process for linking the carbon cycle between the Earth's surface and its interior. Knowing the carbonation and decarbonation processes in the subduction zone is essential for understanding the global deep carbon cycle. In particular, the potential role of hydrocarbon fluids in subduction zones is not well understood and has long been debated. Here we report graphite and light hydrocarbon-bearing inclusions in the carbonated eclogite from the Southwest (S.W.) Tianshan subduction zone, which is estimated to have originated at a depth of at least 80 kilometers. The formation of graphite and light hydrocarbon likely results from the reduction of carbonate under low oxygen fugacity (∼FMQ - 2.5 log units). To better understand the origin of light hydrocarbons, we also investigated the reaction between iron-bearing carbonate and water under conditions relevant to subduction zone environments using large-volume high-pressure apparatus. Our high-pressure experiments provide additional constraints on the formation of abiotic hydrocarbons and graphite/diamond from carbonate-water reduction. In the experimental products, the speciation and concentration of the light hydrocarbons including methane (CH4), ethane (C2H6), and propane (C3H8) were unambiguously determined using gas chromatograph techniques. The formation of these hydrocarbons is accompanied by the formation of graphite and oxidized iron in the form of magnetite (Fe3O4). We observed the identical mineral assemblage (iron-bearing dolomite, magnetite, and graphite) associated with the formation of the hydrocarbons in both naturally carbonated eclogite and the experimental run products, pointing toward the same formation mechanism. The reduction of the carbonates under low oxygen fugacity is, thus, an important mechanism in forming abiotic hydrocarbons and graphite/diamond in the subduction zone settings

Archaeology of Commons: a Multidisciplinary Approach to the Reconstruction of Multiple Uses and Conflicts on European Uplands

The paper presents a multidisciplinary research project (\u201cArchaeology of Commons: cultural Heritage and Material Evidence of a Disappearing Europe\u201d) on the archaeology of common-lands. The main goal of the project is to investigate, by means of historical and archaeological analysis, the intimate social dimension at the base of the common lands management in the context of southern European mountainous regions. Research investigates the dynamic nature of commons starting from the reconstruction of the present organisation of common lands, going back to the complex transformations of common properties in the nineteenth century and analysing the problem of the archaeological visibility of conflicts since the ancien r\ue9gime. The project will examine how archaeological methods could clarify different aspects of the history of the collective access rights to land, by applying in selected mountain areas methodological approaches based on historical ecology, rural and agrarian archaeology

An examination of the effects of using glycerol and wheat dry distillers grains with soluble in sheep diets

AN EXAMINATION OF THE EFFECTS OF USING GLYCEROL AND WHEAT DRY DISTILLERS’ GRAINS WITH SOLUBLES IN SHEEP DIETS The aims of this research were to assess the effects of feeding glycerol and wheat based dry distillers’ grains with solubles (WDDGS) on rumen metabolism, performance, carcass characteristics and fatty acid composition of adipose tissue of lambs. In study 1, increasing levels of glycerol as replacement of barley grain in in vitro batch cultures did not affect culture pH and total methane (CH4) production, but in vitro dry matter disappearance, total VFA production and propionate proportions were linearly increased. In study 2, increasing levels of glycerol in a forage diet increased total VFA, propionate and butyrate production and DM and neutral detergent fibre (NDF) disappearance in semi continuous fermenters. Methane production was linearly increased despite increases in propionate concentrations as a result of the more reduced state of glycerol as compared to carbohydrates which implies there is no net incorporation of electrons into glycerol when it is metabolised to propionate. In study 3, increasing concentrations of glycerol did not alter in vivo diet digestibility or CH4 production. However, dry matter intake (DMI) was reduced at high glycerol concentrations (210 g/kg DM) and average daily gain (ADG) tended to decrease. Fatty acid profiles were improved by reducing concentrations of palmitic, trans-10 octadecenoic and linoleic acids and the n-6/n-3 ratio and increasing stearic and oleic acids. In study 4, increasing concentrations of WDDGS as direct replacement of barley grain resulted in increased eating rates, DMI and ADG. Linolenic acid in backfat tissue was also increased without altering the n-6/n-3 ratio. However, when WDDGS were included in iso nitrogenous diets as replacement of soybean meal and alfalfa, DM, NDF and CP in sacco effective degradability and in vitro CH4 production were increased. Animal DMI, ADG and hot carcass weight were linearly reduced but feed efficiency was unaffected and trans-10 octadecenoic and linoleic acids were increased