IGU Commission on Geographical Education

The International Geographical Union Reports

IGU Commission on Geographical Education     

Diffusion of 40Ar in metamorphic hornblende

Isothermal, hydrothermal experiments were performed on two compositionally contrasting hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 m. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 m. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic ⁴⁰Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 m, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on hornblendes. A comparison of results for 34 m splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 m and 34 m splits of ⁴⁰Ar/³⁹Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of ⁴⁰Ar in hornblende (60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for ⁴⁰Ar in hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for ⁴⁰Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.     

A transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence,North Wales,UK

High-resolution transmission electron microscopy (HRTEM) measurements of the thickness of white mica crystallites were made on three pelite samples that represented a prograde transition from diagenetic mudstone though anchizonal slate to epizonal slate. Crystallite thickness, measured normal to (001), increases as grade increases, whereas the XRD measured 10 Å peak-profile, the Kubler index, decreases. The mode of the TEM-measured size population can be correlated with the effective crystallite size N₍₀₀₁₎ determined by XRD. The results indicate that the Kubler index of white mica crystallinity measures changes in the crystallite size population that result from prograde increases in the size of coherent X-ray scattering domains. These changes conform to the Scherrer relationship between XRD peak broadening and small crystallite size. Lattice strain broadening is relatively unimportant, and is confined to white mica populations in the diagenetic mudstone. Rapid increases in crystallite size occur in the anchizone, coincident with cleavage development. Changes in the distribution of crystallite thickness with advancing grade and cleavage development are characteristic of grain-growth by Ostwald ripening. The Kubler index rapidly loses sensitivity as an indicator of metapelitic grade within the epizone.     

The use of<Superscript>7</Superscript>Be and<Superscript>210</Superscript>Pb<Subscript>xs</Subscript> to differentiate fine suspended sediment sources in South Slough,Oregon

Suspended sediment was collected in the South Slough, National, Estuarine Research Reserve, Oregon, over 8 tidal cycles during and following a single runoff event. The sediment was analyzed for its radionuclide signature to determine the relative contributions of different sources of sediment to the efflux from the estuary. Suspended sediment in the estuary is a mixture of sediment from three potential sources: the river system, Coos Bay, and the estuarine bed. Each source material has a distinctive⁷Be:²¹⁰Pb_xs ratio. The ratios of the source sediments decreased, in magnitude in the following order: riverine >bay>bed. The ratios, of the suspended sediment collected within a subsection of the South Slough estuary reflected the relative mixture of the source areas. The⁷Be:²¹⁰Pb_xs ratios provided a means of not only differentiating, between resuspended bed sediment and freshly delivered sediment from both the river system and Coos Bay, but also calculating the relative amount of resuspended bed sediment in the suspended sediment collected in the estuary. The sampled subsection of the South Slough estuary was a net sink of sediment during a 100-h sampling period associated with the runoff event, but the radionuclide analysis suggests that approximately 39% of the sediment efflux was resuspended bed sediment.     

Spawning sockeye salmon fossils in Pleistocene lake beds of Skokomish Valley, Washington

An assemblage of fossil sockeye salmon was discovered in Pleistocene lake sediments along the South Fork Skokomish River, Olympic Peninsula, Washington. The fossils were abundant near the head of a former glacial lake at 115 m elevation. Large adult salmon are concentrated in a sequence of death assemblages that include individuals with enlarged breeding teeth and worn caudal fins indicating migration, nest digging, and spawning prior to death. The specimens were 4 yr old and 45-70 cm in total length, similar in size to modern sockeye salmon, not landlocked kokanee. The fossils possess most of the characteristics of sockeye salmon, Oncorhynchus nerka, but with several minor traits suggestive of pink salmon, O. gorbuscha. This suggests the degree of divergence of these species at about 1 million yr ago, when geological evidence indicates the salmon were deposited at the head of a proglacial lake impounded by the Salmon Springs advance of the Puget lobe ice sheet. Surficial geology and topography record a complicated history of glacial damming and river diversion that implies incision of the modern gorge of the South Fork Skokomish River after deposition of the fossil-bearing sediments.     

Book Review: Earthquake Microzoning By A. Roca and C. Oliveira (Eds.), Birkhäuser Verlag, 2001, 298 pp., softcover,Euro 52, ISBN 3-7643-6652-4

Studia Geophysica et Geodaetica -     

Nutrient Budgets,Dynamics and Storm Effects in a Eutrophic,Stratified Baltic Lake

Between 1989 and 1998 the small eutrophic stratified Lake Belau was investigated intensively and multidisciplinarily. This article is a short, comprehensive summary and re‐evaluation of the hydrochemistry of the lake, with focus on nitrogen and phosphorus. In several aspects the lake can be regarded as a typical example of the glacial north German lakes. The 1960's and 1970's are characterised by heavy nutrient inputs and fast eutrophication. During the last two decades the external nutrient load, especially the phosphorus load into Lake Belau was significantly reduced. But phosphorus‐rich sediments and large areas with summerly anoxic sediment surface conditions cause intensive release of phosphorus from older deeper sediment layers. Annual budgets reveal that despite an average sediment accumulation of 3 mm a^?1 the lake has lost its function as net phosphorus sink and it is very likely that internal eutrophication by the sediments will keep the lake in its eutrophic state during the next decades. Despite that, monthly budgets of five vertical layers show that the main phosphorus supplier for the phosphorus depleted epilimnion during summer is the creek Alte Schwentine. The annual nitrogen budget indicates groundwater and interflow water as well as atmospheric input as additional important nitrogen sources. 36% (98 μmol m ^?2 h^?1 N) of all nitrogen input is lost to atmosphere mainly due to denitrification. The example of a heavy storm shows that about 10% of the annual nitrogen loss to the atmosphere can take place during a single day and in form of ammonia. The storm further made obvious that these unpredictable events can have strong impact on nutrient cycling and ecology in Lake Belau and the lake can become an unexpected nutrient source for downstream systems.     

Cracking up: faulting on Earth and Mars

Joyce Vetterlein and Gerald P Roberts describe research in progress on the structural evolution of Cerberus Fossae, Mars, with implications for cryosphere cracking in recent Mars history.     

Spherical harmonics in texture analysis

The objective of this contribution is to emphasize the fundamental role of spherical harmonics in constructive approximation on the sphere in general and in texture analysis in particular. The specific purpose is to present some methods of texture analysis and pole-to-orientation probability density inversion in a unifying approach, i.e. to show that the classic harmonic method, the pole density component fit method initially introduced as a distinct alternative, and the spherical wavelet method for high-resolution texture analysis share a common mathematical basis provided by spherical harmonics. Since pole probability density functions and orientation probability density functions are probability density functions defined on the sphere Ω^{3 3} or hypersphere Ω^{4 4}, respectively, they belong at least to the space of measurable and integrable functions ¹(Ω^d), d=3, 4, respectively.

Therefore, first a basic and simplified method to derive real symmetrized spherical harmonics with the mathematical property of providing a representation of rotations or orientations, respectively, is presented. Then, standard orientation or pole probability density functions, respectively, are introduced by summation processes of harmonic series expansions of ¹(Ω^d) functions, thus avoiding resorting to intuition and heuristics. Eventually, it is shown how a rearrangement of the harmonics leads quite canonically to spherical wavelets, which provide a method for high-resolution texture analysis. This unified point of view clarifies how these methods, e.g. standard functions, apply to texture analysis of EBSD orientation measurements.