29Si and 1H NMR spectroscopy of high-pressure hydrous magnesium silicates

We present NMR spectroscopic data, obtained by ¹H MAS, ¹H static spin-echo, and ²⁹Si{¹H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg₂SiO₄, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow ²⁹Si NMR peaks and display the most de-shielded Si^VI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The ¹H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The ¹H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static ¹H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved ¹H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P2₁ mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg₂SiO₄, coexisting with phases B and superhydrous B, appears to give ²⁹Si{¹H} CP-MAS signal, indicating that it contains significant H concentration. The ²⁹Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.     

Diatom-based interpretation of sediment banding in an urbanized lake

Sediment stratigraphy and diatom succession were studied in an 80.5-cm core taken from the deepest part of Third Sister Lake, a small kettle hole in a recently urbanized landscape of southeastern Michigan. Alternating light clay and dark organic bands documented sporadic inputs of clay from outside the basin during rain events, rather than annual laminations. Urban construction activity also disrupted the inflow stream bed and facilitated transport of clay into the lake to generate non-rhythmic banding in the lake's deep hole. Diatom analysis revealed dramatic changes in predominant taxa with sediment depth verifying the non-annual nature of the sediment bands. Observation of halophilic diatom taxa also documented effects of human activity such as road salting on this small, urban lake.     

Evidence for rapid avian-mediated foraminiferal colonization of Lake Winnipegosis, Manitoba, during the Holocene Hypsithermal

A Holocene ecological succession was documented using palynological, foraminiferal, and molluscan faunas sampled from an excavated trench on the margin of Bell River Bay, Lake Winnipegosis, Manitoba. The palynological data record the known gradually isostatically-induced shift from aquatic to terrestrial conditions at the site, and clearly delineates the Holocene Hypsithermal maximal warm interval (commencing here about 5500 years BP). Concurrent with this warming the site became occupied by the extinct salt tolerant gastropod Marstonia gelida and the marine foraminifer Cribroelphidium gunteri by at least 5430 years BP. Water fowl-assisted colonization of non-marine habitats by foraminifera has previously been suggested as a dispersal mechanism for other non-marine foraminiferal occurrences. However, as this relatively warm-water foraminifer (presently found as far north as Cape Cod, MA on the Atlantic USA coast, and Vancouver, BC on the Canadian Pacific coast but also found in Canadian Maritime provinces during the Hypsithermal) did not inhabit the area either prior to or following the Hypsithermal warm interval, this occurrence indicates the efficiency with which foraminifera can utilize non-selective avian transport to colonize new non-marine and marine habitats. It may be that only a few years were required for colonization of the site to occur (2000–3000 km distant from native populations); this suggests that avian transport is a much more important foraminiferal dispersal mechanism than previously realized. The appearance of foraminifera at this site may also constrain models designed to determine the time required for hydraulically injected glacial freshwater to be flushed from normally brine producing aquifers in the region.     

Slope basins, headless canyons, and submarine palaeoseismology of the Cascadia accretionary complex

A combination of geomorphological, seismic reflection and geotechnical data constrains this study of sediment erosion and deposition at the toe of the Cascadia accretionary prism. We conducted a series of ALVIN dives in a region south of Astoria Canyon to examine the interrelationship of fluid flow and slope failure in a series of headless submarine canyons. Elevated head gradients at the inflection point of canyons have been inferred to assist in localized failures that feed sediment into a closed slope basin. Measured head gradients are an order of magnitude too low to cause seepage-induced slope failure alone; we therefore propose transient slope failure mechanisms. Intercanyon slopes are uniformly unscarred and smooth, although consolidation tests indicate that up to several metres of material may have been removed. A sheet-like failure would remove sediment uniformly, preserving the observed smooth intercanyon slope. Earthquake-induced liquefaction is a likely trigger for this type of sheet failure as the slope is too steep and short for sediment flow to organize itself into channels. Bathymetric and seismic reflection data suggest sediment in a trench slope basin between the second and third ridges from the prism's deformation is derived locally. A comparison of the amounts of material removed from the slopes and that in the basin shows that the amount of material removed from the slopes may slightly exceed the amount of material in the basin, implying that a small amount of sediment has escaped the basin, perhaps when the second ridge was too low to form a sufficient dam, or through a gap in the second ridge to the south. Regardless, almost 80% of the material shed off the slopes around the basin is deposited locally, whereas the remaining 20% is redeposited on the incoming section and will be re-accreted.
     

Analysis of fracture network connectivity using percolation theory

Connectivity aspects of fracture networks are analyzed in terms of percolation theory. These aspects are of fundamental importance in characterization, exploitation, and management of fractured formations. General connectivity and power law relationships are determined that characterize the density of fractures and average number of intersections per fracture necessary to ensure network connectivity, the likelihood of a fractured formation being hydraulically connected, and the probability that any specific fracture is connected to the conducting portion of the network. Monte Carlo experiments with a two-dimensional fracture network model confirm the percolation theory predictions. These relationships may prove useful in formulating theoretically tractable approximations of fracture nerworks that capture the essential system properties.     

Constraints on the structure and dynamics of the β-cristobalite polymorphs of SiO2 and AlPO4 from 31P, 27Al and 29Si NMR spectroscopy to 770 K

Nuclear magnetic resonance spectroscopic data are presented for the cristobalite polymorphs of AlPO₄ and SiO₂ from RT to 770 K, through their respective α-β transitions. The nuclear magnetic resonance (NMR) data include chemical shifts for ³¹P, ²⁷Al, and ²⁹Si, ²⁷Al quadrupole coupling parameters, and ³¹P and ²⁷Al spin-lattice relaxation rates. Also presented are electron diffraction patterns of β-cristobalite AlPO₄ that show diffuse scattering similar to that reported previously for SiO₂. For the α-phases of both AlPO₄ and SiO₂, the chemical shifts decrease approximately linearly with increasing temperature from RT to T_c and discontinuously by -2 to -3 ppm from α to β. This result is consistent with a small, continuous increase in the mean T-O-T angle (〈θ〉) of the α-phases with increasing T and an increase of 〈θ〉 by about 4° across the α-β transition for both cristobalite and its AlPO₄ analogue. Based on the ²⁹Si chemical shifts, the mean Si-O-Si angle for β-cristobalite is 152.7±1° near T_c. For AlPO₄-cristobalite, the ²⁷Al nuclear quadrupole coupling constant (C_Q) decreases approximately linearly from 1.2 MHz at RT to 0.94 MHz near T_c (493±10 K). At the α-β transition the ²⁷Al C_Q approaches zero, in agreement with the cubic average structure observed by diffraction. The satellite transitions retain a small frequency distribution above the α-β transition from electric field gradients attributed to defects. The short-range cubic symmetry of the Al-site and non-linear Al-O-P angle support a dynamically disordered model of the β-cristobalite structure. Complete averaging of the ²⁷Al quadrupole coupling in the β-phase indicates that the lifetime of any short-range ordered domains must be shorter than about 1 μs.     

Heat capacity studies of phase transitions in langbeinites II. K2Mg2(SO4)3

Heat capacity measurements have been made on a synthetic sample of langbeinite, K₂Mg₂(SO₄)₃, from 13 to 342 K in an adiabatic calorimeter. Three phase transitions, at 51.0, 54.9 and 63.8 K, have been observed in this material. Our study is the first to report the existence of such phase transitions in K₂Mg₂(S0₄)₃ and disputes predictions that none would take place below 77 K. Two models which have been proposed to explain the transition in potassium langbeinites are discussed in light of these results.