Comparison of the climate simulated by the CCM3 coupled to two different land-surface models

We present results from a coupled atmosphere-biosphere model CCM3/IBIS (the Community Climate Model coupled to the Integrated BIosphere Simulator), which is designed to study the dynamic interactions between climate and vegetation and the global carbon cycle. We analyze the climate simulated by CCM3/IBIS with fixed vegetation conditions and we compare it to the climate simulated by the standard CCM3, which includes the LSM (land surface model) land-surface package. Important differences between the two models include simple parametrizations of lakes, wetlands and crops in CCM3/LSM not taken into account in CCM3/IBIS. CCM3/IBIS and CCM3/LSM share common biases (compared to observations) in the temperature field in boreal winter and in the precipitation field annually, making the atmospheric model the most probable cause of those biases. The models differ in the temperature field and surface energy balance in the Sahara annually and in the mid-to high latitudes from spring through fall. CCM3/IBIS simulates global annual air temperatures that are on average 1.7 °C higher than CCM3/LSM and 0.5 °C higher than the observed climatology. Differences in albedo and/or snow parametrization explain most of the Sahara and high-latitude temperature disagreement. Our sensitivity study with CCM3/LSM shows that the presence of lakes and wetlands in CCM3/LSM can account for about half of the difference in temperature in summer over the lake and wetland regions of the mid-latitudes. A second sensitivity study shows that higher surface roughness length in CCM3/IBIS can also explain part of the difference in summer surface temperature in the mid-latitudes. Surface roughness length affects the surface temperature through a feedback mechanism linking surface wind speed, planetary boundary layer height, low level cloudiness and radiation     

The effect of crystal orientation on the wetting behaviour of silicate melts on the surfaces of spinel peridotite minerals

. The effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces of spinel, diopside, enstatite and olivine has been determined experimentally by the sessile melt drop technique. The anisotropy, Ãg_SL{\rm \~A}\gamma _{{\rm SL}} , of solid-liquid interfacial energies (%_SL(max)-%_SL(min)) can be related to the wetting angles, N, by Ãg_SL μ | cosy(max)  - cosy(min) | = Pw_{( [(A)\tilde]gSL )}{\rm \~A}\gamma _{{\rm SL}} \propto \left| {\cos \psi (\max )\; - \cos \psi (\min )} \right| = Pw_{\left( {{\rm \tilde A}\gamma _{{\rm SL}} } \right)} . Normalising to the smallest wetting angle gives values of Pw for diopside=0.0728, olivine=0.0574, orthopyroxene=0.0152, and spinel=0.0075. Crystal anisotropy influences grain-scale morphology of small-degree partial melts, permeability and the melt connectivity threshold, J_C. Results show that, at sufficient melt fractions, diopside should increase permeability in a peridotitic matrix, whereas enstatite should lower it. Despite its low anisotropy, spinel contributes positively to permeability and J_C because of its high surface energies. These results suggest that harzburgitic mineral matrices typical of the subcratonic mantle should impede the movement of low-degree partial melts, whereas melts should flow more easily through spinel lherzolites.     

Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas

A model is developed for the origin of ultrapotassic melts by melting of veined lithosphere; the veins are rich in clinopyroxene and mica, whereas the wall-rocks consist principally of peridotites. The veins originate by solidification of low-degree melts which are themselves the results of earlier, deeper, multistage processes ultimately due to the presence of a transition zone between large-scale channelled and porous flow regimes. The melting event producing the ultrapotassic magma begins in the veins due to the concentration of hydrous phases and incompatible elements, but spreads to include the surrounding wall-rocks by a combination of two mechanisms. The alkaline magma composition is thus a hybrid of vein (V) and wall-rock (W) components.

The melt hybridization mechanisms are: (i) Solid-solution melting: Minerals which from extensive solid-solutions are abundant in the vein assemblages (Cr/Al spinel, F/OH mica, amphibole and apatite). The breakdown of these phases take place over a temperature range between the solidus of the vein assemblage and the elimination of the more refractory end-members. This process bridges the temperature gap between the solid of vein and wall-rock, so that a melt component from the wall-rock is added to that from the vein before elimination of all vein minerals. Phlogopite forms the most effective of these sliding reactions, resulting in its stability at near-liquids temperatures in experiments. (ii) Dissolution of wall-rock minerals: The initial melt fraction in the vein infiltrates the surrounding wall-rock due to the dominance of surface energy minimization on melt flow at the intergranular scale. Following infiltration, dissolution of wall-rock minerals occurs at temperatures lower than their melting temperatures, thus imparting a refractory wall-rock component to the melt composition. Dissolution of olivine and/or orthopyroxene occurs preferentially, since these minerals are furthest from equilibrium with the strongly alkaline, vein-derived melt.

Remobilisation of several generations of veins explains the occurrence within a restricted space and time of rocks bearing chemical characteristics which are generally thought to indicate contrasting tectonic settings (e.g. central Italy). The ultrapotassic rocks are explained as being dominanyly vein-derived (i.e. high V/W ratio): further dilution of the V-component by wall-rock, supplemented by asthenospheric melt in advanced cases, leads to the production of more voluminous basaltic rocks bearing incompatible element signatures reminiscent of those of ultrapotassic rocks.     

Nanoscale Chemical Imaging by Photo‐Induced Force Microscopy: Technical Aspects and Application to the Geosciences

Photo‐induced force microscopy (PiFM) is a new‐frontier technique that combines the advantages of atomic force microscopy with infrared spectroscopy and allows for the simultaneous acquisition of 3D topographic data with molecular chemical information at high spatial (~ 5 nm) and spectral (~ 1 cm^?1) resolution at the nanoscale. This non‐destructive technique is time efficient as it requires only conventional mirror‐polishing and has fast mapping rates on the order of a few minutes that allow the study of dynamic processes via time series. Here, we review the method’s historical development, working principle, data acquisition, and evaluation, and provide a comparison with traditional geochemical methods. We review PiFM studies in the areas of materials science, chemistry and biology. In addition, we provide the first applications for geochemical samples including the visualization of faint growth zonation in zircons, the identification of fluid speciation in high‐pressure experimental samples, and of nanoscale organic phases in biominerals. We demonstrate that PiFM analysis is a time‐ and cost‐efficient technique combining high‐resolution surface imaging with molecular chemical information at the nanoscale and, thus, complements and expands traditional geochemical methods.     

Distinct site preferences for heavy and light REE in amphibole and the prediction of Amph/LDREE

New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r₀), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at ^[8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of ^[8]Ca²⁺. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe²⁺ and Mg), we propose a new model for REE incorporation. LREE order at the ^[8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed ^Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE³⁺ incorporation explains why measured and estimated ^Amph/LD_HREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r₀ are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determined ^Amph/LD_REE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE. Received: 17 March 1999 / Accepted: 11 June 1999     

Phase relations and fractionation sequences in potassic magma series modelled in the system CaMgSi2O6-KAlSiO4-Mg2SiO4-SiO2-F2O−1 at 1 bar to 18 kbar

Liquidus phase relations in the system diopside–kalsilite–forsterite–quartz with 3 wt% F were examined at 1 bar and the locations of important invariant points were determined at 18 kbar. At all pressures within this range a large liquidus field for fluorphlogopite (Phl) exists, and has a large influence on both melting and fractionation processes. One eutectic point was found to the silica-rich side of the plane Lc–Fo–Di at Di₁Ks₃₀Fo₂Qz₆₇, where a melt coexists with San, Qz, Phl and Di at 840 °C and 1 bar. Another eutectic point must exist in the silica-poor part of the system because the phase topology determines that thermal barriers must exist. At this point a feldspathoid, either Lc or Ks, must coexist with Fo, Phl and a Ca-bearing phase such as Di. The exact location and phase assemblage were not determined, but the equilibrium melt must have a composition rich in Di (>29 wt%) and extremely poor in Qz (<8 wt%). The composition of the first eutectic moves towards lower SiO₂ contents with increasing pressure (Di₃Ks₄₀Fo₁Qz₅₆ at 18 kbar), whereas the second does not exist at 18 kbar due to the disappearance of Lc as a stable liquidus phase. Liquids which coexist with mafic minerals such as En, Fo, Phl and Di are important for the genesis of potassium-rich mafic rocks by partial melting in the mantle and for the early stages of fractional crystallisation. The equilibrium melt at the invariant point Fo + En + Phl + Di + L at 1125 °C is very poor in Fo and Di components at atmospheric pressure (Di₅Ks₃₇Fo₅Qz₅₃), whereas at 18 kbar the melt contains large amounts of Fo and Di (Di₁₉Ks₃₁- Fo₂₈Qz₂₁), and has a composition close to that of natural lamproites. Kamafugites do not correspond to melts in this system under any of the studied conditions, and appear to require CO₂ in the source. Fractionation processes from primitive potassic basanite melts are controlled principally by the size (and not the mere presence) of the liquidus phase field for phlogopite: at high pressures where the Phl field is large, olivine is eliminated early from the fractionating assemblage and Cpx + Phl fractionation may lead to relatively silica-rich rock differentiates such as trachytes. At low pressures, extensive olivine and restricted Phl crystallisation prevents silica enrichment in the melt, resulting in phonolitic differentiates. Later crystallisation of alkali feldspar accentuates the trends laid down in the early stages of fractionation. Received: 2 February 1999 / Accepted: 14 October 1999     

Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.     

4DVAR data assimilation in the Intra-Americas Sea with the Regional Ocean Modeling System (ROMS)

We present the background, development, and preparation of a state-of-the-art 4D variational (4DVAR) data assimilation system in the Regional Ocean Modeling System (ROMS) with an application in the Intra-Americas Sea (IAS). This initial application with a coarse model shows the efficacy of the 4DVAR methodology for use within complex ocean environments, and serves as preparation for deploying an operational, real-time assimilation system onboard the Royal Caribbean Cruise Lines ship Explorer of the Seas. Assimilating satellite sea surface height and temperature observations with in situ data from the ship in 14 day cycles over 2 years from January 2005 through March 2007, reduces the observation-model misfit by over 75%. Using measures of the Loop Current dynamics, we show that the assimilated solution is consistent with observed statistics.     

The Regional Ocean Modeling System (ROMS) 4-dimensional variational data assimilation systems: Part III - Observation impact and observation sensitivity in the California Current System

The critical role played by observations during ocean data assimilation was explored when the Regional Ocean Modeling System (ROMS) 4-dimensional variational (4D-Var) data assimilation system was applied sequentially to the California Current circulation. The adjoint of the 4D-Var gain matrix was used to quantify the impact of individual observations and observation platforms on different aspects of the 4D-Var circulation estimates during both analysis and subsequent forecast cycles. In this study we focus on the alongshore and cross-shore transport of the California Current System associated with wind-induced coastal upwelling along the central California coast. The majority of the observations available during any given analysis cycle are from satellite platforms in the form of SST and SSH, and on average these data exert the largest controlling influence on the analysis increments and forecast skill of coastal transport. However, subsurface in situ observations from Argo floats, CTDs, XBTs and tagged marine mammals often have a considerable impact on analyses and forecasts of coastal transport, even though these observations represent a relatively small fraction of the available data at any particular time.During 4D-Var the observations are used to correct for uncertainties in the model control variables, namely the initial conditions, surface forcing, and open boundary conditions. It is found that correcting for uncertainties in both the initial conditions and surface forcing has the largest impact on the analysis increments in alongshore transport, while the cross-shore transport is controlled mainly by the surface forcing. The memory of the circulation associated with the control variable increments was also explored in relation to 7 day forecasts of the coastal circulation. Despite the importance of correcting for surface forcing uncertainties during analysis cycles, the coastal transport during forecast cycles initialized from the analyses has less memory of the surface forcing corrections, and is controlled primarily by the analysis initial conditions.Using the adjoint of the entire 4D-Var system we have also explored the sensitivity of the coastal transport to changes in the observations and the observation array. A single integration of the adjoint of 4D-Var can be used to predict the change that occurs when observations from different platforms are omitted from the 4D-Var analysis. Thus observing system experiments can be performed for each data assimilation cycle at a fraction of the computational cost that would be required to repeat the 4D-Var analyses when observations are withheld. This is the third part of a three part series describing the ROMS 4D-Var systems.