Pleistocene decalcification of Late Pliocene Red Crag shelly sands from Walton-on-the-Naze, England

Uppermost sands of the Red Crag at Walton-on-the-Naze (Essex) and elsewhere in East Anglia have been decalcified to iron-stained quartz sands. In contrast, lower sands are only minimally altered and contain aragonitic and calcitic shells. Aragonitic shells are slightly dissolved (chalkified), but calcitic shells are unaffected. Cementation is limited to an addition of iron oxides, now mainly haematite, which also coat carbonate grains. Abundant iron-oxide fines in the upper decalcified sands were liberated from the coatings of shells; shells that have since dissolved. The diagenetic nature of the contact between decalcified upper and unaffected lower sands is evident where it transects cross-bedding. The contact is knife-sharp, even smoothly truncating large shells, and is usually planar and subhorizontal. Shelly sands immediately beneath the boundary contain similar amounts of aragonitic material, as do sands further below. Locally the decalcification boundary has been contorted by cryoturbation, implying that carbonate dissolution was a Pleistocene event. Decalcification probably occurred when the area was affected by permafrost. Lower sands were cemented by ice and protected from dissolution. Upper sands were above the ice table and subject to chemically aggressive waters during summer thaws. Decalcification is believed to have taken place during an episode of climate amelioration when downward retreat of the ice table accompanied replacement of tundra by boreal forest. Highly acidic and podzolic soils developed, beneath which shell-carbonate dissolved. Sharp based decalcified zones in Lowestoft Till and Devensian deposits in other parts of England can also be attributed to dissolution associated with permafrost.     

Comparison of point and array-computed rotations for the TAIGER explosions of 4 March 2008

Two large explosions were recorded by a dense array of strong-motion accelerometers and rotational seismometers in northeastern Taiwan associated with a Taiwan Integrated Geodynamics Research long-range refraction experiment. The objective of this experiment was to test the response of the experimental eentec rotational seismometers against calculated array rotations. Computed array rotation rates are seen to have little variation across the array, but point rotation rate measurements obtained from individual rotational seismometers show significant deviations with each other and with the array rotation rates in the ranges of 3?C5?Hz and, especially, 3?C50?Hz. A cross-correlation method was used to compare array-computed rotation rates and point rotation rate measurements in the frequency band of 3?C5?Hz with the result that the absolute value of the normalized maximum correlation coefficient for each station set varied from 0.45 to 0.97 with an average of 0.84. Amplitude differences of the point rotation rate measurements are seen to be factors of 0.2 to 1.8 times the array rotations as well. It is not likely that the differences seen in the point and array-computed rotation rates are due to nonlinear or heterogeneous site conditions under each array element since these effects should also be seen in the acceleration data used to determine rotation rate. A rigorous method for accurately calibrating rotation rate instruments is needed to understand their response in the field.     

Testing the ability of surface arrays to monitor microseismic activity

Recently there has been much interest in the use of data from surface arrays in conjunction with migration‐based processing methods for passive seismic monitoring. In this study we use an example of this kind of data recorded whilst 18 perforation shots, with a variety of positions and propellant amounts, were detonated in the subsurface. As the perforation shots provide signals with known source positions and origin times, the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface. In all but one case the signals from the perforation shots are not visible in the raw or preprocessed data. However, clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events, even when the signals are not visible in individual traces. We find that point source locations are within typically 45 m (laterally) of the true shot location, however the depths are less well constrained (～150 m). We test the sensitivity of our imaging method to the signal‐to‐noise ratio in the data using signals embedded in realistic noise. We find that the position of the imaged shot location is quite insensitive to the level of added noise, the primary effect of increased noise being to defocus the source image. Given the migration approach, the array geometry and the nature of coherent noise during the experiment, signals embedded in noise with ratios ≥0.1 can be used to successfully image events. Furthermore, comparison of results from data and synthetic signals embedded in noise shows that, in this case, prestack corrections of traveltimes to account for near‐surface structure will not enhance event detectability. Although, the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods.     

Mobile ions on carbonate surfaces

Surface ions move during the dissolution and growth of minerals. The present study investigates the density and the mobility of surface ions and the structure of the adsorbed water layer with changes in relative humidity (RH). The time evolution of the polarization force, which is induced by an electrically biased tip of an atomic force microscope, shows that the density and the mobility of surface ions increase with rising humidity, a finding which is consistent with increasing surface hydration. A marked change in the observations above 55% RH indicates a transition from a water layer formed by heteroepitaxial two-dimensional growth at low RH to one formed by multilayer three-dimensional growth at high RH. A comparison of the results of several rhombohedral carbonates (viz. CaCO₃, FeCO₃, ZnCO₃, MgCO₃, and MnCO₃) shows that a long relaxation time of the polarization force at high RH is predictive of a rapid dissolution rate. This finding is rationalized by long lifetimes in terrace positions and hence greater opportunities for detachment of the ion to aqueous solution (i.e., dissolution). Our findings on the density and the mobility of surface ions therefore help to better constrain mechanistic models of hydration, ion exchange, and dissolution/growth.     

Atmospheric and glow images from the Shuttle

The use of simple photographic cameras on early Shuttle missions allowed spacecraft glow to be clearly identified, and its potential for the contamination of weak atmospheric emissions to be estimated. Since those early flights the equipment has been extensively modified so that it is now possible to obtain images with a spectral resolution of 0.1 nm. The early Shuttle glow observations are reviewed and the use of spatially scanned filters to obtain spectral results is described. These glow measurements are discussed in terms of some current ideas for vehicle induced glows and it is suggested that the glow intensity may be controlled by the temperature of the glowing surface. An example of an atmospheric image obtained with the interference filter camera is presented and the limitations in the use of such images are discussed.