Thermal intensification of microbial Fe(II)/Fe(III) redox cycling in a pristine shallow sand aquifer on the Canadian Shield

This investigation evaluates spatial relationships between summer (July) groundwater temperatures and Fe(II)/Fe(III) biogeochemical cycling over a five year period in a shallow pristine sand aquifer at Meilleurs Bay near Deep River, Ontario, Canada. A warm subsurface thermal island of 12.5-16.1 °C, compared to background conditions of 10-11 °C, was manifest in contour maps of average groundwater temperature over the study period. The warm zone coincided with an area of convergent groundwater flow, implicating horizontal heat transfer by advective convection as the reason for elevated temperatures. Additionally, high concentrations of dissolved Fe(II) and Fe(III) overlapped the warm thermal island, indicative of increased rates of bacterial Fe(II)-oxidation and Fe(III)-reduction. A depletion in the modal abundance of Fe(II)-bearing minerals, notably amphibole and biotite, inside the area of the warm thermal island was also observed, suggesting enhanced mineral dissolution owing to chemoautotrophic Fe(II)-oxidation coupled to the reduction and fixation of dissolved inorganic carbon as biomass. Throughout the aquifer, redox conditions were poised in terms of Eh and pH close to equilibrium with respect to the Fe(II)/Fe(OH)₃ couple, feasibly enabling simultaneous bacterial Fe(II)-oxidation and Fe(III)-reduction with an adequate supply of electron acceptors and donors, respectively. The significance of higher groundwater temperature as a determinant of elevated dissolved Fe(II) and Fe(III) concentrations induced by thermal intensification of microbial biogeochemical activities yielded Pearson product-moment correlations in which temperature alone, as a single independent variable, explains almost 30 to nearly 60 percent of the variation in the measured dissolved Fe(II) and Fe(III) concentrations in the groundwater. These results emphasize the important influence of thermal conditions on biogeochemical processes in aquifers coupled to the development of steep gradients in groundwater quality over short distances in shallow unconfined groundwater systems.

SPRITE MRI with prepared magnetization and centric k-space sampling

A technique for imaging materials with short transverse relaxation times and prepared longitudinal magnetization is proposed. The technique is single-point ramped imaging with T1-enhancement (SPRITE) MRI with centric k-space sampling. The effects of transient state behavior on image resolution and signal/noise are estimated. Centric sampling in the basic SPRITE sequence gives increased signal-to-noise and permits a quantitative determination of the MR parameters associated with longitudinal spin preparation. Spin-lock and inversion recovery preparation experiments are presented. Copyright 1999 Academic Press.

Relaxation time mapping of short T*2 nuclei with single-point imaging (SPI) methods

New techniques for quantitative mapping of T1, T2, and T*2 are proposed, based on the single-point imaging (SPI) method, for materials with short nuclear magnetic resonance relaxation times which cannot be imaged with traditional methods. Relaxation times extracted from two-dimensional images of uniform doped agarose phantoms (T*2 approximately 60-210 microseconds) as well as hardened mortar (T*2 approximately 220 microseconds) and polymers (T*2 approximately 20-100 microseconds), using these techniques, agreed with bulk measurements. The method was then applied to a partially dried cylindrical concrete sample (T*2 approximately 115 microseconds).

Evidence for functional sodium and calcium ion channels in the membrane of cultured cardiomyocytes of the adult rat

Characteristics are reported for electrical activity of adult rat cardiomyocytes in long-term primary culture. Cells in vitro for 12 to 28 days have mean membrane potential of -53 mV, are electrically excitable, and some are spontaneously contractile. The action potential of these cells has a slow rate of depolarization and is abolished by methoxyverapamil (D-600) but not by tetrodotoxin (TTX). When cells are hyperpolarized by passage of an inward current, spontaneous action potentials cease and action potentials evoked by depolarizing pulses are then TTX sensitive. Fetal bovine serum is a constituent of the culture medium. Its temporary removal causes spontaneous contractility to cease but the cells remain electrically excitable.

Subjective uncertainty, variability of experience, and the infant's response to discrepancies

4-month-old infants were assessed in a habituation/discrepancy paradigm using random shapes to test several propositions deduced from the discrepancy hypothesis. First, an inverted-U distribution of attention was found for discrepancies produced by random shapes which differ in the density of contours or spatial frequencies rather than in terms of specific overt pattern. Second, the distribution of attention was more similar for discrepancies defined by increases and by decreases in contour density for random shapes than for checkerboard stimuli. This result supported the role of the hypothesized intervening variable of subjective uncertainty, because while simple checkerboards are subsets of more complex checkerboards, simple random shapes are not unmodified portions of more complex shapes. Third, greater variability of experience with stimuli about a given level of standard stimulation reduced attention to simple discrepancies but increased attention to larger magnitudes of more complex discrepancies.

The interfering effect of distracting stimuli on the infant's memory

Infants 3 1/2 months of age were assessed for the possible role of the dissimilarity of the distracting stimulus to the originally learned standard in a modified familiarization-distraction-test paradigm. When data for all subjects were analyzed, there was no evidence that the distraction had any interfering effect at all on the memory for the original standard. However, specialized analyses suggested that interference varied with the likelihood that subjects actually encoded the distracting stimuli might govern the probability or extent that an infant encodes the distracting stimulus rather than affecting the interference process directly.