Ca-Eskola incorporation in clinopyroxene: limitations and petrological implications for eclogites and related rocks

Clinopyroxene is an essential mineral in eclogitic rocks. It commonly contains minor amounts of the defect-bearing Ca-Eskola (CaEs, Ca_0.5□_0.5AlSi₂O₆) component, with higher concentrations generally considered to indicate a high-pressure origin at least within the coesite stability field. Changes in pressure and temperature conditions can lead to exsolution of this component as a free SiO₂ phase, which may have a number of petrological implications. This makes it important to understand the factors that maximize CaEs incorporation in clinopyroxene. We have undertaken a series of experiments at high pressures and temperatures (4–10 GPa and 1000–1350 °C) to further investigate the systematics of CaEs incorporation in eclogite-like clinopyroxene and the factors responsible for maximizing CaEs contents. Two simple chemical systems were chosen that allow unambiguous interpretation of the results: (1) CMAS + H₂O and (2) two compositions in the NCMAS system. All experimental products contained clinopyroxene and garnet along with either a free SiO₂ phase or a silicate melt. Coexisting garnet is grossular-rich, generally with X _gr ≥ 0.67. Compositional variations are attributable to the presence or absence of melt and changes in modal amounts of garnet at different pressure–temperature conditions. Even small amounts of H₂O lower the solidus temperature and the presence of a melt reduces the SiO₂ activity, which destabilizes the CaEs component in clinopyroxene. The CaEs and the Ca-Tschermaks (CaTs, CaAl₂SiO₆) components in clinopyroxene decrease with increasing jadeite mole fraction, which is also a function of pressure and bulk Al content. Modeling X-ray powder diffraction data yields a molar volume for the CaEs endmember of V _CaEs = 60.87(63) cm³, which reasonably agrees with a literature value that was estimated from natural samples. In the presence of coexisting coesite, the CaEs and CaTs do not vary independently of each other, being controlled by the internal equilibrium 2CaEs = CaTs + 3SiO₂ (coesite). This relation, observed in simple systems (i.e., CMAS ± Na), is also obeyed by clinopyroxene in more complex, natural analog bulk compositions. An assessment of available experimental data reveals a maximum of 15–18 mol% CaEs in eclogitic clinopyroxene at conditions corresponding to 130–180 km depth. CaEs contents are maximized at high temperatures; i.e., at or near the solidus in the presence of coesite. Thus, this study supports the role of CaEs exsolution in contributing to melt generation during upwelling of eclogite bodies in the mantle, albeit with some caveats. Somewhat higher maximum CaEs contents (~20 mol%) are found in Ca and Al-rich bulk compositions, such as grospydite xenoliths. Such bulk compositions also seem to require the coexistence of kyanite. Other Ca and Al-rich rock types, like rodingites, should have the potential of containing CaEs-rich clinopyroxenes, except that they are SiO₂-undersaturated. This emphasizes the further role of bulk composition, in addition to high temperatures, in achieving maximum CaEs contents in high-pressure clinopyroxene.     

Temporal dynamics and persistence of sponge assemblages in a Central Pacific atoll lagoon

Sponges are important components of coral reef fauna, although little is known of their temporal dynamics. Sponges dominate the lagoon system at Palmyra Atoll in the Central Pacific, which may not be its natural state. Here we examined the temporal variability and recruitment rates of these sponge assemblages to determine if they are stable and examined the evidence that a recent transition has occurred from a coral‐ to sponge‐dominated system. We found 24 sponge species in permanent quadrats in the lagoon between 2009 and 2011, and 11 species on our recruitment panels. The sponge assemblage composition and abundance did not vary significantly between years and appear stable. Many sponge recruits were found in both years that the panels were examined although higher rates were recorded in the second year of the study. While it seems very possible that a change to a sponge‐dominated lagoon is associated with declining environmental quality at Palmyra as a result of modifications over 70 years ago, without pre‐modification data on reef assemblage composition this remains speculative. Our observations of short‐term temporal stability in the sponge assemblages at Palmyra highlight the potential for sponge‐dominated reef states to be maintained in degraded reef environments that are seemingly unsuitable for coral survival.     

Numerical experiments on the long-term morphodynamics of the Colorado River Delta

The interaction among tidal currents, sediment transport, and long-term changes of the sea bottom in the Colorado River Delta have been investigated applying a two dimensional nonlinear hydrodynamic-numerical finite differences model. The system was forced by the dominant M ₂ tidal component at the open boundary. We carried out calculations to study the morphodynamics of the actual bathymetry caused by the bedload sediment transport. To investigate the origin of actual morphological features, we performed experiments using a smoothed bathymetry, in which the islands Montague, Gore, and Pelícano were eliminated. Under the imposed tidal hydrodynamics, the results indicate that the bedload transport contributes significantly in the genesis of sandbanks and in the formation and maintaining of the Montague and Gore Islands.     

Adapting science to a warming world

Climate change is complicating the variables that Alaskans consider when planning for the future. Communities, agencies and other entities have begun to grapple with both the information that they need to adapt to a changing climate and how the processes and practices of science should change to make science more useful. We reviewed sixty-three documents that expressed practical research needs related to climate change in Alaska. Documents nearly unanimously expressed that science, as it is currently practiced, is inadequate to meet the challenges of climate change. They call for processes that are more transparent, collaborative, and accessible. They recommend changed practices including maintaining accessible data-sharing archives, building networks for knowledge sharing, and creating place-based long-term partnerships with communities. They advocate integrating local knowledge, but infrequently address the complexities of how this is best accomplished. They also suggest the need for improved training in interdisciplinary research and changes in the incentive structure of research institutions. This review complements the climate-change literature by providing concrete suggestions about how to increase the utility of science from a region that is experiencing some of the most dramatic climatic change on the planet.     

Bifurcation and generalized mixed loading conditions in geomaterials

The concept of bifurcation has been widely discussed, based, for example, on homogeneous tests in soil mechanics. Essentially two‐dimensional (axisymmetric) conditions have been considered. This paper aims at deriving a bifurcation criterion in geomechanics, independently of the problem's dimension. This criterion is related to the vanishing of the determinant of the symmetric part of the material's (or the discrete system's) constitutive (or stiffness) matrix. The derivation is essentially based on the notion of loading parameters (controlling the loading). Basically, a bifurcation occurs when the existence of a unique solution to the quasistatic problem, involving a given set of loading parameters, is lost. Interestingly, the criterion is shown to be independent of the choice of loading parameters. As a possible extension, the context of structure mechanics is considered, and the close connection between both soil and structure analysis is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super Typhoon Rammasun(2014)

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification(RI)of Typhoon Rammasun.We show that the basic difference stems from subtle differences in initializations of(a)500-850-h Pa environmental winds,and(b)midlevel moisture and ventilation.We then describe how these differences impact on the evolving convective organization,storm structure,and the timing of RI.As expected,ascent,diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI.The evolution of vortex cloudiness from this member is similar to the actual imagery,with the development of an inner cloud band wrapping inwards to form the eyewall.We present evidence that this structure,and hence the enhanced diabatic heating,is related to the tilt and associated dynamics of the developing inner-core in shear.For the most accurate ensemble member:(a)inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air,which is transported to the down-shear sector to feed a developing convective asymmetry;(b)with minimal ventilation,undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud;and(c)this process seems related to a critical down-shear tilt of the vortex from midlevels,and the vertical phase-locking of the circulation over up-shear quadrants.For the member that forecasts a much-delayed RI,these processes are inhibited by stronger vertical wind shear,initially resulting in poor vertical coherence of the circulation,lesser moisture and larger ventilation.Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear,moisture and vortex inner-structure.     

Long-term spatial and temporal trends in frost indices in Kansas, USA

Frost indices such as number of frost days (nFDs), number of frost-free days (nFFDs), last spring freeze (LSF), first fall freeze (FFF), and growing-season length (GSL) were calculated using daily minimum air temperature (T_min) from 23 centennial weather stations across Kansas during four time periods (through 1919, 1920–1949, 1950–1979, and 1980–2009). A frost day is defined as a day with T_min?<?0 °C. The long- and short-term trends in frost indices were analyzed at monthly, seasonal, and annual timescales. Probability of occurrence of the indices was analyzed at 5 %, 25 %, 50 %, 75 %, and 95 %. Results indicated a general increase in T_min from 1900 through 2009 causing a decrease in nFDs. LSF and FFF occurred earlier and later than normal in the year, respectively, thereby resulting in an increase in GSL. In general, northwest Kansas recorded the greatest nFD and lowest T_min, whereas southeast Kansas had the lowest nFD and highest T_min; however, the magnitude of the trends in these indices varied with location, time period, and time scales. Based on the long-term records in most stations, LSF occurred earlier by 0.1–1.9 days/decade, FFF occurred later by 0.2–0.9 day/decade, and GSL was longer by 0.1–2.5 day/decade. At the 50 % probability level, Independence in the south-eastern part of Kansas had the earliest LSF (6 April), latest FFF (29 October) and longest GSL (207 days). Oberlin (north-western Kansas) recorded the shortest GSL (156 days) and earliest FFF (7 October) had the latest LSF (2 May) at the 50 % probability level. A positive correlation was observed for combinations of indices (LSF and GSL) and elevation, whereas a negative correlation was found between FFF and elevation.