Effects of glaciations and permafrost upon the development of karst in Canada

Canadian examples suggest that karst landforms may be divided into eight types in terms of their temporal relationships to the record of repeated Quaternary glaciations. Two types are postglacial, two are subglacial, one type occurs where glacial features are adapted to karstic drainage, and three types display sequences of karstic and glacial action. Glacier effects upon karst landforms and their underlying aquifers display the gamut of possibilities. They may destroy, inhibit, preserve, or stimulate karst development. Where continuous permafrost is maintained when covered by glacier ice, postglacial karst is limited to the active layer epikarst. Where permafrost is thawed beneath ice or during deglaciation there are a variety of postglacial karst developments, depending in part upon climate and in part upon local lithologic and relief conditions.     

Chemical and physical studies of type 3 chondrites, VII. Annealing studies of the Dhajala H3.8 chondrite and the thermal history of chondrules and chondrites

Samples of the Dhajala H3.8 chondrite have been annealed for 10 hours at 600, 700, 800, 900 and 1000°C and at 1000°C for 1, 2, 20 and 100 h and their thermoluminescence (TL) properties measured. The TL sensitivity decreased by a factor of 2 after annealing at < 900°, but at higher temperatures fell by an order of magnitude. An abrupt increase in the temperature of the TL peak from 172 ± 9°Cto231 ± 8°C and a steady increase in the width of the peak from 169 ± 7°Cto212 ± 5°C were caused by the annealing treatment. The TL phosphor in Dhajala is thought to be feldspar predominantly in the high-temperature (disordered) form, but the present data indicate that a contribution from the low-temperature form is also present and that this low-temperature component is converted to the high form by the annealing treatment. The low-temperature feldspar is located in a few of the chondrules ( 20% of those separated from the meteorite) which are also noteworthy for having high TL sensitivities. These chondrules must have suffered greater crystallization of their mesostasis than the other chondrules, and equilibrated to lower temperatures. It is argued that, for compositional reasons, their mesostasis constituted less of a barrier to diffusion and therefore equilibration. Presumably the post-metamorphic cooling rate of the meteorite through the stability field of the low form was slow enough to permit some crystallization, and the width and temperature of the TL peaks for petrologic types 3.5–3.9 are somehow related to cooling rate. Based on TL, there is no indication of a correlation between petrologic type and cooling rate for types 3.5–3.9; this is not consistent with a simple, single internally heated meteorite parent body.     

Chemical and physical studies of type 3 chondrites—V: The enstatite chondrites

We report instrumental neutron activation analysis determinations of 19 major, minor and trace elements in three enstatite chondrites. Based on these, and literature data on the bulk and mineral composition of enstatite chondrites, we discuss the history of the type 3 or unequilibrated enstatite chondrites, and their relationship with the other enstatite chondrites. The type 3 enstatite chondrites have E chondrite lithophile element abundances and their siderophile element abundances place them with the EH chondrites, well resolved from the EL chondrites. Moderately volatile chalcophile elements are at the low end of the EH range and Cr appears to be intermediate between EH and EL. We suggest that the type 3 enstatite chondrites are EH chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock-produced sulfurous liquids. The oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas ~30° higher than the others, or with the loss of a plagioclase-rich liquid. The mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration, or, in some instances, with shock effects.     

Magnetic blocking temperature

Formulae are derived for the 95% and 5% magnetic blocking temperatures in a cooling orogen when the spontaneous magnetization is allowed to vary realistically as a function of temperature. The solution is obtained for an assemblage of identical magnetic moments which can set either exactly parallel to or anti-parallel to a weak applied field.     

Late Pleistocene paleoclimates of North America as inferred from stable isotope studies of speleothems

Some speleothems (CaCO₃ cave deposits) can be demonstrated to have been formed in oxygen isotopic equilibrium with their parent seepage waters and thus a record of relative fluctuations in depositional temperature can be obtained from the δ¹⁸O variations in successive growth layers of such deposits. These temperature fluctuations reflect variations in the average annual air temperature at the surface above the cave, and therefore permit inference of past continental climate changes. Equilibrium deposits have been obtained from caves in San Luis Potosi, Bermuda, Kentucky, West Virginia, Iowa, and Alberta, ranging in age from 200,000 years BP to the present, as determined by ${}^{230}\text{Th}{}^{234}\text{>U}$ dating of the speleothems. The δ¹⁸O time curves for the six sites show the following synchronous climatic fluctuations: warm periods from 190 to 165 and from 120 to 100 Ka, at 60 and 10 Ka, and cold periods from 95 to 65 and from 55 to 20 Ka. The periods of thermal maxima correspond in time to the interglacial periods of the marine foraminiferal isotopic and faunal temperature records and to periods of high sea stand as observed from radiometric dating of raised coral reefs. Maxima and minima in insolation appear to be synchronous with this record as well.     

The age of the Mistastin Lake crater,Labrador, Canada

⁴⁰Ar/³⁹Ar age determinations have been carried out on eight samples of melt rocks and one of the maskelynite from Mistastin Lake impact crater, Labrador. The observed⁴⁰Ar^* evolution spectra of the impact melts fall into distinct groups which correlate with petrographic variations. The release patterns of six of the melt rock samples define an age plateau in the range 34–41 m.y.; the other two have complex spectra which indicate incomplete equilibration of inclusions. Four of the samples with well-defined plateaux exhibit a high-temperature sag in their⁴⁰Ar/³⁹Ar ratio similar to that observed in some lunar samples. Maskelynite gives a partially overprinted spectrum which rises monotonically to a final age near 700 m.y., approximately half the age of the country rocks. The data from the melt samples are interpreted as indicating an age of 38 ± 4 m.y. for the Mistastin Lake impact event. Previously, it had been considered that this crater was 202 ± 25 m.y. old.     

Resource allocation in urban services: Spatial response to financial constraints with reference to health visiting in Kent,England

Part of urbanised South East England is used to discuss some of the spatial allocation problems of a reorganised health service region under conditions of financial constraint. In particular, the role and response of health visiting services are scrutinised so as to examine the relationship between stringent financial control, changing spatial organisation patterns and actual primary health-care requirements.