Geo-neutrinos and Silicate Earth Enrichment of U and Th

The terrestrial distribution of U, Th, and K abundances governs the thermal evolution, traces the differentiation, and reflects the bulk composition of the earth. Comparing the bulk earth composition to chondritic meteorites estimates the net amounts of these radiogenic heat-producing elements available for partitioning to the crust, mantle, and core. Core formation enriches the abundances of refractory lithophile elements, including U and Th, in the silicate earth by ~1.5. Global removal of volatile elements potentially increases this enrichment to ~2.8. The K content of the silicate earth follows from the ratio of K to U. Variable enrichment produces a range of possible heat-producing element abundances in the silicate earth. A model assesses the essentially fixed amounts of U, Th, and K in the approximately closed crust reservoir. Subtracting these sequestered crustal amounts from the variable amounts in the silicate earth results in a range of possible mantle allocations, leaving global dynamics and thermal evolution poorly constrained. Terrestrial antineutrinos from {\beta}-emitting daughter nuclei in the U and Th decay series traverse the earth with negligible attenuation. The rate at which large subsurface instruments observe these geo-neutrinos depends on the distribution of U and Th relative to the detector. Geo-neutrino observations with sensitivity to U and Th in the mantle are able to estimate silicate earth enrichment, leading to a more complete understanding of the origin, accretion, differentiation, and thermal history of the planet.Comment: published version: 21 pages, 3 figures, 5 table

Tectonic evolution of a continental collision zone: A thermomechanical numerical model

We model evolution of a continent-continent collision and draw some parallels with the tectonic evolution of the Himalaya. We use a large-scale visco-plasto-elastic thermomechanical model that has a free upper surface, accounts for erosion and deposition and allows for all modes of lithospheric deformation. For quartz/olivine rheology and 60 mm/yr convergence rate, the continental subduction is stable, and the model predicts three distinct phases. During the phase 1 (120 km or 6% of shortening), deformation is characterized by back thrusting around the suture zone. Some amount of delaminated lower crust accumulates at depth. During phase 2 (120 km–420 km or 6%–22% of shortening), this crustal root is exhumed (medium- to high-grade rocks) along a newly formed major thrust fault. This stage bears similarities with the period of coeval activity of the Main Central thrust and of the South Tibetan Detachment between 20–16 Myr ago. During phase 3 (>420 km or 22% of shortening), the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge. Deformation is localized around frontal thrust faults. This kinematics should produce only low- to medium-grade exhumation. This stage might be compared with the tectonics that has prevailed in the Himalaya over the last 15 Myr allowing for the formation of the Lesser Himalaya. The experiment is conducted at constant convergence rate, which implies increasing compressive force. Considering that this force is constant in nature, this result may be equivalent to a slowing down of the convergence rate as was observed during the India-Asia collision

Numerical simulation of clouds and precipitation depending on different relationships between aerosol and cloud droplet spectral dispersion

The aerosol effects on clouds and precipitation in deep convective cloud systems are investigated using the Weather Research and Forecast (WRF) model with the Morrison two-moment bulk microphysics scheme. Considering positive or negative relationships between the cloud droplet number concentration (Nc) and spectral dispersion (ɛ), a suite of sensitivity experiments are performed using an initial sounding data of the deep convective cloud system on 31 March 2005 in Beijing under either a maritime (‘clean’) or continental (‘polluted’) background. Numerical experiments in this study indicate that the sign of the surface precipitation response induced by aerosols is dependent on the ɛ−Nc relationships, which can influence the autoconversion processes from cloud droplets to rain drops. When the spectral dispersion ɛ is an increasing function of Nc, the domain-average cumulative precipitation increases with aerosol concentrations from maritime to continental background. That may be because the existence of large-sized rain drops can increase precipitation at high aerosol concentration. However, the surface precipitation is reduced with increasing concentrations of aerosol particles when ɛ is a decreasing function of Nc. For the ɛ−Nc negative relationships, smaller spectral dispersion suppresses the autoconversion processes, reduces the rain water content and eventually decreases the surface precipitation under polluted conditions. Although differences in the surface precipitation between polluted and clean backgrounds are small for all the ɛ−Nc relationships, additional simulations show that our findings are robust to small perturbations in the initial thermal conditions. Keywords: aerosol indirect effects, cloud droplet spectral dispersion, autoconversion parameterization, deep convective systems, two-moment bulk microphysics schem

Evidence of micro-continent entrainment during crustal accretion

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Integration of natural data within a numerical model of ablative subduction: A possible interpretation for the Alpine dynamics of the Austroalpine crust

A numerical modelling approach is used to validate the physical and ge- ological reliability of the ablative subduction mechanism during Alpine con- vergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model pre- dictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P-T peak conditions, relative chronology of peak and exhumation events, P-T-t paths, thermal gradients and the tectonic evolu- tion of the continental rocks. These findings suggest that a pre-collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono-metamorphic config- uration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central-Eastern Alps. Based on the natural data available in the literature, a critical discus- sion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include varia- tions in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the oc- currence of continental collision and an oblique convergence.Comment: 11 Figures and 3 Tabe

Inherited crustal deformation along the East Gondwana margin revealed by seismic anisotropy tomography

Acknowledgments We thank Mallory Young for providing phase velocity measurements in mainland Australia and Tasmania. Robert Musgrave is thanked for making available his tilt-filtered magnetic intensity map. In the short term, data may be made available by contacting the authors (S.P. or N.R.). A new database of passive seismic data recorded in Australia is planned as part of a national geophysics data facility for easy access download. Details on the status of this database may be obtained from the authors (S.P., N.R., or A.M.R.). There are no restrictions on access for noncommercial use. Commercial users should seek written permission from the authors (S.P. or N.R.). Ross Cayley publishes with the permission of the Director of the Geological Survey of Victoria.Peer reviewedPublisher PD

Southern Ocean warming: Increase in basal melting and grounded ice loss

We apply a global finite element sea ice/ice shelf/ocean model (FESOM) to the Antarctic marginal seas to analyze projections of ice shelf basal melting in a warmer climate. The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM. Results from their 20th-century simulations are used to evaluate the modeled present-day ocean state. Sea-ice coverage is largely realistic in both simulations. Modeled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM. Projections for future ice shelf basal melting are computed using atmospheric output for IPCC scenarios E1 and A1B. While trends in sea ice coverage, ocean heat content, and ice shelf basal melting are small in simulations forced with ECHAM5 data, a substantial shift towards a warmer regime is found in experiments forced with HadCM3 output. A strong sensitivity of basal melting to increased ocean temperatures is found for the ice shelves in the Amundsen Sea. For the cold-water ice shelves in the Ross and Weddell Seas,decreasing convection on the continental shelf in the HadCM3 scenarios leads to an erosion of the continental slope front and to warm water of open ocean origin entering the continental shelf. As this water reaches deep into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases by a factor of three to six compared to the present value of about 100 Gt/yr. Highest melt rates at the deep FRIS grounding line causes a retreat of > 200km, equivalent to an land ice loss of 110 Gt/yr