Clays, clay minerals

Abstract

n REPORTS nm-1 (Park Scientific)]. Images were obtained in the constant-force mode with filters off, an integral gain of 3.0, a proportional gain of 7.0, and a look-ahead gain of 0.0. The "d" scan head was used, which has a maximum scan range of 12 pLm by 12 pLm by 4.4 pLm. The scan rate of the tip during image acquisition ranged from 25 to 60 Hz, and the applied tip-sample force was maintained at Ft,p ? 10 nN in solution. The AFM experiments were performed in aqueous solutions with a fluid cell (Digital Instruments) consisting of a quartz body with ports for fluid entry and exit. Large single crystals of HT, about 10 to 20 pLm in diameter, were allowed to adsorb from a water suspension onto a freshly cleaved mica substrate. The samples were then removed from the suspension and dried at 1 20?C for 1 hour. The mica substrate was then attached to a magnetic stainless steel AFM sample disk. We positioned the AFM tip above single HT crystals, using an optical microscope , before imaging. 13. The hexagonal symmetry and the lattice constant a' 6.2 A are also evident from Fourier analysis of the AFM data. 14.. 26. The PKa values listed for MBSA in this report were determined by the titration of 1.6 mmol of MBSA in aqueous solution with NaOH. 27. The absorption of MBSA1-and MBSA2-on HT has been verified by several independent analyses. X-ray photoelectron spectroscopy reveals the presence of MBSA in amounts approximating those expected for monolayer coverages. Infrared spectra indicate the presence of MBSA1-or MBSA2-on HT, which is infrared-transparent between 1000 and 1300 cm-1. Adsorption was also confirmed frorm the depletion of MBSA1-or MBSA2-from solutions to which HT had been added. 28. We optimized the MBSA structure depicted in Fig. 4 using MOPAC and the CAChE molecular modeling program. The optimized structure compares favorably with that determined from single-crystal x-ray diffraction of guanidinium 5-benzoyl-4-hydroxy-2-methoxybenzenesulfonate (V. A. Russell and M. D. Ward, in preparation). The infrared spectral features of MBSA anions on HT in the region between 1000 and 1300 cm-1 are nearly identical to those observed for the guanidinium salt of MBSA1-in which the sulfonate group is hydrogen-bonded to guani-dinium protons and resides in a site of threefold C3V symmetry. This arrangement is identical to that observed for a series of guanidinium sulfonates [V. A. Russell, M. C. Etter, M. D. Ward, J. Am. Chem. Soc. 116, 1941 (1994)]. 29. The... ABAB... pattern is also evident from Fou-rier analysis of the AFM data, which reveals two reciprocal space components along the reciprocal lattice vector a* at 0.06 A-1 and 0.12 A-1, consistent with the supercell depicted in Fig. 5C. 30. The coverages and orientation surmised from the AFM data, for both MBSA1-and MBSA2-adsor-bates, are in agreement with previously reported chemical analyses and x-ray diffraction data of fully intercalated materials (24). Intercalation of MBSA1-and MBSA2-into the nitrate form of HT results in the expansion of the HT layers and the interatomic spac-ings of 21 and 13 A, respectively. These values are consistent with the vertical orientation of the MBSA molecule, which has a height of about 11 A. The larger interatomic spacing for MBSA1-can be attributed to bilayer packing in which interdigitation of MBSA1-ions on opposing HT layers is prohibited by the dense packing of the organic molecules. In contrast , the lower coverage of the MBSA2-ions permits interdigitation of opposing adsorbate layers, leading to a smaller interatomic spacing. Investigation of tree growth in Isle Royale National Park in Michigan revealed the influence of herbivores and carnivores on plants in an intimately linked food chain. Plant growth rates were regulated by cycles in animal density and responded to annual changes in primary productivity only when released from herbivory by wolf predation. Isle Royale's dendrochronology complements a rich literature on food chain control in aquatic systems, which often supports a trophic cascade model. This study provides evidence of top-down control in a forested ecosystem. Terrestrial food chains of length three-plants, herbivores, and carnivores are found throughout the temperate zone of the Northern Hemisphere, yet their establishment , pervasiveness, and stability are enigmatic subjects of debate among community ecologists (1). According to one hypothesis, depletion of green plants by herbivores occurs only in exceptional circumstances because carnivores usually control herbivores (2). However, systems in which increases in the density of a species at one trophic level accompany increases at higher, dependent trophic levels support a counterargument (3). The top-down (trophic cascade) model predicts that changes in density at one tro-phic level are caused by opposite changes in the next higher trophic level and that such inverse correlations cascade down a food chain. Accordingly, effects such as changes in primary productivity (the energy flow to plants) become noticeable only when higher , masking trophic levels are removed. After removal of carnivores from a three-level system, the control of density relationships is passed down the chain, to herbivores. The bottom-up model predicts that positive correlations occur between density changes at all trophic levels and especially between adjacent trophic levels, that changes in primary productivity affect higher trophic levels , and that extinction of the top trophic level does not change density patterns in lower levels. We investigated food chain control in a large mammal system through observation of three trophic levels a living system that is unlike two-level models of ungulate dynamics , which simulate the herbivore-plant interaction but are insensitive to parameter changes in a carnivore equation (4), and is also unlike models for ungulates that ignore the effects of vegetation change (5). Tree-ring analyses were used to characterize the interaction between an herbivore population and its winter forage in a system with an apparent cycle between predator and prey: the wolves (Canis lupus) and moose (Alces alces) in Isle Royale National Park, Michi-gan, the largest island (544 kiM2) in Lake Superior (6, 7). Ring width in balsam fir (Abies balsamea), a tree that makes up 59% of winter moose diet (8), provided an index of the herbivore food base, even though it is not optimal forage for moose. We assumed that the annual wood accrual for fir was proportional to its foliar biomass, which is an approximate measure of standing forage crop. Balsam fir covers a large area of Isle Roy-ale (Fig. 1), although its relative abundance in the overstory has declined since the arrival of moose early in the 1900s, from 46% in 1848, to 13% in 1978, to-5% today (9). The decline is attributed to the effect of moose herbivory; forests on small nearby islands that are less accessible to moose still have a large fir component (10).