Atomic displacements in the normal-incommensurate phase transition in Co-åkermanite (Ca2CoSi2O7)

New in-situ high-temperature X-ray powder diffraction data on the normal-incommensurate phase transition in Co-åkermanite (Ca₂CoSi₂O₇) are presented. Evidence for the phase transition is found in the abrupt change in the thermal expansivity of the c lattice parameter at 220° C. In addition, the c lattice parameter exhibits premonitory effects of the phase transition through the leveling out of the thermal expansivity at temperatures from 87 to 220° C. The c/a ratio for X₂ZSi₂O₇ incommensurate melilites is shown to be constant for a wide variety of compositions. Correlation of the trends in mean atomic positions with c/a ratio and the sigmoidal variation in the c/a ratio for Ca₂CoSi₂O₇ provide insight into the atomic displacements occurring in the melilite structure as a function of temperature. Three temperature regimes are discussed in detail (i.e. temperatures well above T _c, temperatures just above T _c, and temperatures below T _c). The atomic displacements occurring in each temperature regime are correlated with the changes in anisotropy and orientation of the atomic displacement ellipsoids for each site, as well as spectroscopic evidence for the increasing number of Z- and T-site environments.     

Bluff evolution and long-term recession rates,southwestern Lake Michigan

Where eroding cohesive sediments are present, Lake Michigan bluffs range up to 40 m in height, exposing multiple glacial stratigraphic units. Following the model presented here, bluffs form as a wave-cut terrace erodes inland from a point near the original shoreline. The erosion plane is nearly horizontal, in contrast with the eastward dip of the glacial units inherited from underlying bedrock. Therefore, terraces eroding inland produce progressively higher bluffs and expose successively older units at the toe and beneath the lake. This process was repeated several times as lake levels sequentially dropped to their modern stage. The initial modern shoreline, and hence the width of the wave-cut terrace, was determined from four offshore seismic and bottom-sampling profiles. It was picked as an inflection point in the lake bed, occurring offshore of dipping reflectors intersecting the lake bottom. The calculated average recession rate over a 2500-year duration of the modern stage is 1.5 m/yr in contrast to average rates of approximately 0.6 m/yr measured over the last century. Thus rates decrease through time as the terrace widens and wave energy is dampened. By correlating bluff height to recession distance, a third rate of approximately 2.7 m/yr for the first 940 years of recession is calculated from relict Nipissing bluffs. The three rates define a steeply decaying exponential curve in early stages of bluff retreat, flattening into a nearly linear function after 1000 years.     

Microscopic feather fractures in the faulting process

The nature and development of microscopic feather fractures (mff) are investigated in experimentally deformed intact and precut cylinders of room-dry Tennessee and Coconino Sandstone. All specimens are deformed at 25° C, and at a shortening rate of 10⁻⁴ sec⁻¹ ; the intact ones are at confining pressures from 0.5 to 2.5 kbar; and the precut specimens at 1.0 and 1.5 kbar. Mff occur in grains adjacent to induced shear fractures or faults; they are wedge-shaped and die out within one or two grain diameters from the fault; and they make acute angles with the fault such that arrows directed into the apices of these angles on either side of the fault define its sense of shear. Occurrence of mff only after slip on precut surfaces clearly demonstrates that they form as a result of shear displacement. The average angle between the mff and fault is 10° greater than that between the load axis and the fault, and it increases with increasing confining pressure in initially intact specimens. Data suggest that the abundance of mff (mean number per grain) increases with increasing normal stress across the fault and with displacement. The wedgeshaped character of many mff and their consistent orientation at 10° to the load axis are distinguishing characteristics. Mff are shown to be parallel to the local maximum compressive stress and thus are extension microfractures. They are not to be confused with precursive micro fractures developed prior to macroscopic fracture, nor to Riedel shears developed during faulting.     

A note on time integration of unsaturated soil-moisture transport

The non-linear soil-moisture diffusivity model can be approximately linearized by using values of diffusivity assumed constant for small intervals of space and time. By a series expansion of the diffusivity function and integrating the resulting series of differential equations with respect to time, an improved numerical model is developed. Results from application of this new approach to a sharp wetting-front soil infiltration problem indicates that a 67% saving in numerical effort is achieved at comparable estimation accuracy levels when using the traditional finite timestep Crank-Nicolson approach.     

The use of pyrolysis and barium distributions to assess the areal extent of drilling fluids in surficial marine sediments

The influence of drilling fluid dispersion on the chemistry of surficial sediments was determined by an integrated study of organic and inorganic parameters. Pyrolysis characteristics and barium, organic carbon, and carbonate content were determined for sediments associated with six drilling sites in coastal Gulf of Mexico waters. The areal extent and magnitude of influence of dispersed fluids were related to water depth, prevailing currents, density of the dispersed material, history of the drill site and platform-induced biological activity. The potential use of pyrolysis techniques for defining the impact area and the various types of inputs derived from off-shore drilling operations was demonstrated.     

Great Lakes Hydrology Under Transposed Climates

Historical climates, based on 43 years of daily data from areas south and southwest of the Great Lakes, were used to examine the hydrological response of the Great Lakes to warmer climates. The Great Lakes Environmental Research Laboratory used their conceptual models for simulating moisture storages in, and runoff from, the 121 watersheds draining into the Great Lakes, over-lake precipitation into each lake, and the heat storages in, and evaporation from, each lake. This transposition of actual climates incorporates natural changes in variability and timing within the existing climate; this is not true for General Circulation Model-generated corrections applied to existing historical data in many other impact studies. The transposed climates lead to higher and more variable over-land evapotranspiration and lower soil moisture and runoff with earlier runoff peaks since the snow pack is reduced up to 100%. Water temperatures increase and peak earlier. Heat resident in the deep lakes increases throughout the year. Buoyancy-driven water column turnover frequency drops and lake evaporation increases and spreads more throughout the annual cycle. The response of runoff to temperature and precipitation changes is coherent among the lakes and varies quasi-linearly over a wide range of temperature changes, some well beyond the range of current GCM predictions for doubled CO₂ conditions.     

Statistical mechanical modeling of the kinetics of order-disorder in omphacitic pyroxenes

A kinetic statistical model based upon a one-dimensional Ising lattice reflecting the nature of the pyroxene octahedral strip, and with nearest-neighbor and mean-field interactions, is applied to the order-disorder transformation in omphacitic pyroxenes. The equilibration temperature of a naturally ordered omphacite and the annealing data of Carpenter (1981a) on omphacites from the same sample, when applied to the model, provide time- temperature-transformation relations for both long-range and short-range ordering. Results indicate that cooling times on the order of tens of millions of years are necessary for the development of significant degrees of long-range order under optimum conditions, whereas short-range order is developed on much shorter time scales. These results are in agreement with observations of naturally ordered and disordered omphacites with the exception of omphacites ordered in a low-temperature subduction-zone regime; the results support the hypothesis that these omphacites crystallize in a highly ordered state.     

Identification of a collapsing protostar

The globular molecular cloud B335 contains a single, deeply embedded, far-infrared source. Our recent observations of H₂CO and CS lines toward this source provide direct kinematic evidence for collapse. Both the intensity and detailed shape of the line profiles match those expected from inside-out collapse inside a radius of 0.036 pc. The collapse began about 1.5 × 10⁵ years ago, similar to the onset of the outflow. The mass accretion rate is about 10 times the outflow rate, and about 0.4M should have now accumulated in the star and disk. Because B335 rotates only very slowly, any disk would still be very small (about 3 AU). The accretion luminosity should be adequate to power the observed luminosity. Consequently, we believe that B335 is indeed a collapsing protostar.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Response to Thrift's “The future of geography”

Reviews and observations about the status of the discipline of geography, no matter how positive, invariably raise programmatic concerns. These concerns have a long history that arise from geography's struggles to find an identity that embraces its many parts and yet are consistent with the logic by which the academy partitions knowledge. Pedagogy and research historically claimed by geography is currently being reinvented and relabeled under such headings as “integrated environmental science” and “spatial science”, and these developments have the potential to change the breadth of the “geographic imagination”. Several observations about dominant explanatory perspectives and substantive domains of geographic enquiry are also provided.