Transport and sediment accumulation of210Pb and137Cs in two Southeast Alaskan fjords     

Susan F. Sugai 《Estuaries and Coasts》1990,13(4):380-392

Sediment profiles of²¹⁰Pb and¹³⁷Cs in cores collected at increasing distances from the heads of Smeaton Bay and Boca de Quadra fjords indicate that watersheds influence the inventories of radioisotopes present and that the steep topographies of the fjords enhance sediment redistribution. Episodic deposition of terrestrially derived sediment was responsible for roughly 50% of the¹³⁷Cs and 45% of the²¹⁰Pb inventories in shallower (less than 180 m) locations in Wilson and Bakewell arms of Smeaton Bay.²¹⁰Pb sedimentation rates at shallower sites when corrected for episodic deposition were less than sedimentation rates obtained in the deep basins of the fjords where sediment focusing and increased primary productivity in the overlying water column occur. Higher fluxes of dissolved Mn from surficial sediments and subsequent reoxidation in the overlying water may have enhanced scavenging of²¹⁰Pb in basin locations, resulting in higher inventories. Episodic events have occurred frequently in Smeaton Bay and Boca de Quadra suggesting that steady-state conditions with respect to sedimenting particles can be achieved only when averaged over long time periods approaching the time over which¹³⁷Cs and²¹⁰Pb are useful.  相似文献   

Barnacle-dominated limestone with giant cross-beds in a non-tropical, tide-swept, Pliocene forearc seaway, Hawke's Bay, New Zealand     

Peter J. J. Kamp  Fraka J. Harmsen  Campbell S. Nelson  Susan F. Boyle 《Sedimentary Geology》1988,60(1-4):173-195

Pliocene, non-tropical, widespread and locally thick (up to 100 m) limestones occur in Hawke's Bay, eastern North Island, where they are intimately associated with very thick ( > 5 km), terrigenous-dominated, Neogene sequences that formed in a tectonically active convergent margin setting. The non-tropical character of the limestones is shown unequivocally by (1) the complete dominance of skeletal calcarenites and calcirudites, (2) the occurrence of oyster banks as the only in situ organic structures, (3) the dominance of barnacles, epifaunal molluscs, bryozoans, echinoderms, foraminifers, brachiopods and calcareous red algae as skeletal components, and (4) the preponderance of calcite over aragonite in the mineralogy of the skeletal grains and cements. The abundance of barnacle fragments in the limestones, and the related exclusive occurrence of only one major organic association, a barnacle-(epifaunal) bivalve-bryozoan assemblage, is striking and unusual given the extent of the limestones. Pecten and oyster valves acted as substrates for barnacle attachment, and their growth was promoted by strong tidal paleocurrents that swept the depositional setting: a long (450 km), narrow (30–50 km) forearc basin seaway, which formed between an actively deforming subduction complex to the east and an uplifting structural ridge to the west. Synsedimentary deformation promoted limestone formation on the margins of the seaway by creating current-swept, clastic-free submarine ridges that acted as the sites of carbonate production. Tidal flows dispersed the carbonate constituents and organised them into a wide spectrum of tide-influenced, cross-bedded and horizontal structures. Most spectacular are occurrences of giant tabular cross-beds, with sets 10–40 m thick and foreset dips of 7–36°, some interpreted as the deposits of major sand bars on carbonate deltas marginal to the mouths of saddles traversing the rising antiforms, and others analogous to modern linear sand ridges. The small- to large-scale planar and trough cross-beds, and the horizontal and lenticular beds that are invariably associated with the giant cross-beds and dominate most sections, represent mainly the deposits of sand waves and sand sheets at inner- to mid-shelf depths in the seaway.  相似文献   

U-Pb systematics of garnet: dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes,Manitoba, Canada   总被引：2，自引：0，他引：2  

K. Mezger  G. N. Hanson  S. R. Bohlen 《Contributions to Mineralogy and Petrology》1989,101(2):136-148

This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured ²⁰⁶Pb/²⁰⁴Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce ²⁰⁷Pb/²⁰⁶Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.  相似文献   

Energetics of radiation damage in natural zircon (ZrSiO4)     

Susan Ellsworth  Alexandra Navrotsky  Rodney C. Ewing 《Physics and Chemistry of Minerals》1994,21(3):140-149

Transposed temperature drop calorimetry at 1000 °C was performed on natural zircons (ZrSiO₄) from Sri Lanka that were partially to completely metamict due to α-decay event damage (0.06 to 11.7×10¹⁵ α-decay events/mg). The enthalpy of annealing at room temperature (ΔH_anneal) varies sigmoidally as a function of radiation dose. ΔH_anneal reaches a saturation plateau at radiation doses greater than 5×10¹⁵ α-decay events/mg. The annealing of several samples to a crystalline structure with broadened diffraction peaks does not significantly affect the enthalpy of annealing. The large magnitude of the enthalpy of annealing plateau, ?59±3 kJ/mol, suggests that the damage to the structure is pervasive on the scale of Angstroms, consistent with the loss of mid-range order characteristic of a glass. The energetics are consistent with, but do not require, chemical heterogeneity caused by micro-domains of amorphous SiO₂-rich and ZrO₂-rich regions in the metamict state.  相似文献   

Complete Pp-Waveform Modelling For Determining Crust and Upper Mantle Structure     

Susan Y. Schwartz  Thorne Lay 《Geophysical Journal International》1993,112(2):210-224

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The privatisation of secondary schooling and the desegregation of central Johannesburg     

Susan Parnell  Heather Webber 《GeoJournal》1990,22(3):267-273

African education controlled by the state's Department of Education and Training has collapsed. Insufficient spending, low standards and persistent student boycotts have made township schools undesirable places of learning. African students with any financial means now seek secondary education at multi-racial private schools in affluent white suburbs or at street-academies in the centre of Johannesburg. The establishment of schooling services for African pupils in the traditionally white central business district contributes to the wider process of desegregation of South Africa's major urban area, Johannesburg.  相似文献   

Editorial     

Susan H. Treagus  Editor 《Journal of Structural Geology》1990,12(8)

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National Research Council, Managing Coastal Erosion, National Academy Press, Washington, D.C., USA (1990), p. 200 Hardcover, xv, USS24.50. ISBN 0-30904143-0.     

H. Hanson 《Engineering Geology》1991,31(3-4):371-373

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The working nation white paper: whither industrial relations reform?     

Susan McGrath‐Champ 《The Australian geographer》1994,25(2):132-141

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Telecommunications and the Large City-Small City Divide: Evidence from Indiana Cities*     

Patrick Alles  Adrian Esparza  Susan Lucas 《The Professional geographer》1994,46(3):307-316

Urban systems are increasingly dominated by large cities that house a disproportionate share of telecommunications infrastructure. This paper investigates whether telecommunications is defining a technological divide between large and smaller cities. The paper provides a conceptual basis for large-city dominance and presents the results of surveys that investigate the use of telecommunications in medium and small Indiana cities. The survey data indicate that telecommunications is used by the set of Indiana cities, but is less prevalent among the set of smallest cities. Chi-square tests, however, suggest that city size does not adequately explain these variations in use.  相似文献