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JÓN EIRÍKSSON PETER H. KRISTENSEN HOLGER LYKKE-ANDERSEN KATHRYN BROOKS REW MURRAY KAREN LUISE KNUDSEN CHRISTOPHER GLAISTER 《Boreas: An International Journal of Quaternary Research》2006,35(2):320-331
A unique sequence of Late Saalian, Eemian and Early Weichselian strata is exposed in a coastal outcrop at Mommark in the western Baltic. The sedimentary facies and faunas reflect palaeoenvironmental changes from an initial freshwater lake followed by marine transgression and interglacial deposition in a palaeo-Baltic sea. The upper part of the Eemian marine record indicates regression followed by lacustrine sedimentation and deposition of Early Weichselian aeolian sediments, which are truncated by an erosional unconformity overlain by a till bed. The lower and middle parts of the sequence have previously been correlated with the European glacial-interglacial stratigraphy on the basis of pollen analysis, while the upper part has been dated for the present study using optically stimulated luminescence (OSL) of samples from the aeolian and glacial deposits. A similar complete glacial-interglacial-glacial succession has not previously been recorded from this area. The Mommark sequence of conformable strata has been subjected to lateral compression, evidenced by folding and low-angle reverse faults. Seismic records from the adjacent waters in the western Baltic reveal a system of buried Quaternary valleys in the area. It is suggested that the interglacial deposition took place in a basin within one of these valleys and that a slab constituting the Mommark sequence, originating from the margin of a valley, has been glaciotectonically displaced northwestwards to the present location. 相似文献
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STEFAN ENGELS KARIN F. HELMENS MINNA VÄLIRANTA STEPHEN J. BROOKS H. JOHN B. BIRKS 《Boreas: An International Journal of Quaternary Research》2010,39(4):689-704
Engels, S., Helmens, K. F., Väliranta, M., Brooks, S. J. & Birks, H. J. B. 2010: Early Weichselian (MIS 5d and 5c) temperatures and environmental changes in northern Fennoscandia as recorded by chironomids and macroremains at Sokli, northeast Finland. Boreas, Vol. 39, pp. 689–704. 10.1111/j.1502‐3885.2010.00163.x. ISSN 0300‐9483. A 25‐m‐long sediment record spanning the time from the Eemian to the Holocene was recovered from Sokli, northeast Finland. This study focuses on a 6‐m‐long sediment interval that is dated to the Early Weichselian period (MIS 5d and 5c) and consists of lacustrine and fluvial deposits. Using chironomid remains, botanical and zoological macroremains as well as sediment lithology, we were able to reconstruct past changes in the environment, including climate. The results indicate that the site was situated on a flood‐plain during the latter stages of MIS 5d (Herning Stadial) and that summer temperatures might have been ～6 °C lower than at present. Although this value should be treated with caution, as numerical analysis shows that it has a very poor fit‐to‐temperature, this low reconstructed value concurs with several other reconstructions that are available from western Europe. During MIS 5c (Brørup interstadial), the depositional environment changed into a lake system, initially with stratification of the water and subsequently with complete mixing and a strong influence of streams. Both chironomid‐based and macroremain‐based temperature inferences indicate past July air temperatures that were significantly higher than at present. This result is in contrast to other (low‐resolution) reconstructions from northern Fennoscandia that indicate past temperatures 6–7 °C lower than present using fossil coleopteran assemblages. However, several central European sites indicate that there was a phase during the Brørup interstadial that was characterized by high summer temperatures, and a comparison between the high‐resolution reconstructions from western Europe and the results presented in this study suggests that the north–south July air temperature gradient between the mid‐ and high‐latitudes was much weaker during the Brørup interstadial than it is at present. High solar insolation values (particularly the obliquity) during the Brørup interstadial might explain the low summer temperature gradient over the European continent. A return to fluvial conditions occurred in the upper parts of the sediment sequence, and, after a brief interval of gyttja deposition under cooling conditions, the site became glaciated during MIS 5b. 相似文献
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Intraplutonic Quench Zones in the Kap Edvard Holm Layered Gabbro Complex, East Greenland 总被引：1，自引：2，他引：1
The Kap Edvard Holm Layered Gabbro Complex is a large layeredgabbro intrusion (>300 km2) situated on the opposite sideof the Kangerdlugssuaq fjord from the Skaergaard Intrusion.It was emplaced in a continental margin ophiolite setting duringearly Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thicknessof >5 km, have a typical four-phase tholeiitic cumulus mineralogy:plagioclase, clinopyroxene, olivine, and Fe–Ti oxides.The cryptic variation is restricted (plagioclase An81–51,olivine Fo85–66, clinopyroxene Wo43–41 En46–37Fs20–11) and there are several reversals in mineral chemistry.Crystallization took place in a low-pressure, continuously fractionatingmagma chamber system which was periodically replenished andtapped. Fine-grained (0•2–0•4 mm) equigranular, thin(0•5–3 m), laterally continuous basaltic zones occurwithin an {small tilde}1000 m thick layered sequence in theTaco Point area. Twelve such zones define the bases of individualmacrorhythmic units with an average thickness of {small tilde}80m. The fine-grained basaltic zones grade upwards, over a fewmetres, into medium-grained (>1 mm) poikilitic, olivine gabbrowith smallscale modal layering. Each fine-grained basaltic zoneis interpreted as an intraplutonic quench zone in which magmachilled against the underlying layered gabbros during influxalong the chamber floor. Supercooling by {small tilde}50C isbelieved to have caused nucleation of plagioclase, olivine,and clinopyroxene in the quench zone. The nucleation rate isbelieved to have been enhanced as the result of in situ crystallizationin a continuously flowing magma. The transition to the overlyingpoikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typicalfor an open magma chamber system: olivine (Fo77–68 5)and plagioclase cores (An80–72) show a zig-zag crypticvariation pattern with no overall systematic trend. Olivinehas the most primitive compositions in the quench zones andmore evolved compositions in the olivine gabbro; plagioclasecores show the opposite trend. Although plagioclase cores arebelieved to retain their original compositions, olivines re-equilibratedby reaction with trapped liquid. Some plagioclase cores containrelatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones varyover a range from 10 to 18% MgO although the mg-number remainsconstant at {small tilde}0•78. The average composition(47•7% SiO2, 13•3%MgO, 1•57% Na2O+K2O) is takenas a best estimate of the parental magma composition, and isequivalent to a high-magnesian olivine tholeiite. The compositionalvariation of the quench zones is believed to reflect burstsof nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separatetranquil influxes which flowed along the interface between alargely consolidated cumulus pile and the residual magma. Theresident magma was elevated with little or no mixing. At certainlevels in the layered sequence the magma drained back into thefeeder system; such a mechanism is referred to as a surge-typemagma chamber system. 相似文献
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Evolution of the Kap Edvard Holm Complex: a Mafic Intrusion at a Rifted Continental Margin 总被引：1，自引：0，他引：1
The Kap Edvard Holm Layered Series forms part of the East GreenlandTertiary Province, and was emplaced at shallow crustal level(at depths corresponding to a pressure of 1–2 kbar) duringcontinental break-up. It consists of two suites: a gabbro suitecomprising olivine and oxide gabbros, leucocratic olivine gabbrosand anorthosites, and a suite of wehrlites that formed fromthe intrusion of the gabbros during their solidification bya hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxenereveal chemical contrasts between the parental melt of the wehrlitesuite and that of the gabbro suite. Thin sills (1–2 mthick) of the wehrlite suite, however, have clinopyroxene compositionssimilar to the gabbro suite, and were formed by interactionwith interstitial melts from the host layered gabbros. All evolvedmembers of the gabbro suite have elevated Nd, Zr and Sr concentrationsand Nd/Yb ratios, relative to the melt parental to the gabbrosuite. These characteristics are attributed to establishmentof a magma chamber at depths corresponding to a pressure of10 kbar, where melts evolved before injection into the low-pressuremagma chamber. Anorthosites of the gabbro suite are believedto have crystallized from such injections. The melts becamesupersaturated in plagioclase by the pressure release that followedtransportation to the low-pressure magma chamber after initialfractionation at 10 kbar. The most evolved gabbros formed bysubsequent fractionation within the low-pressure magma chamber.Our results indicate that high-pressure fractionation may beimportant in generating some of the lithological variationsin layered intrusions. KEY WORDS: fractionation; ion microprobe; layered intrusions; rift processes; trace elements
*Corresponding author. 相似文献
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HANSON R. BROOKS; SORENSEN SORENA S.; BARTON MARK D.; FISKE RICHARD S. 《Journal of Petrology》1993,34(1):23-62
A record of > 100 million years of fluid flow, alteration,and metamorphism in the evolving Sierra Nevada magmatic areis preserved in metavolcanic rocks of the Ritter Range pendantand surrounding granitoids. The metavolcanic rocks consist of:(1) a lower section of mostly marine volcaniclastic rocks, lavas,and intercalated carbonate rocks that is Triassic to Jurassicin age, and (2) an upper section comprising a subaerial caldera-fillcomplex of mid-Cretaceous age. Late Cretaceous high-temperaturecontact metamorphism (2 kbar, >450–500C) occurredafter renewed normal faulting along the caldera-bounding faultsystem juxtaposed the two sections. The style and degree of alteration and 18O values differ amongthe rocks of the upper and lower sections and the granitoids.Rocks of the lower section show pervasive lithologically controlledalkali alteration, local Mn and Mg enrichment, and oxidation.Some ash flow tuffs now contain up to 10% K2O by weight. Therocks of the upper section show lesser extents of alkali alteration.Granitoids that cut both sections are generally unaltered. Mostmetavolcanic rocks of the lower section have high 18O values(+ 11 to + 16%; whole rock and quartz phenocrysts); however,lower-section rocks within the caldera-bounding fault systemhave low 18O values of + 4 to +7. The metavolcanic rocks ofthe upper section also have low 18O values of + 2 to + 7. Granitoidshave 18O values of + 7 to + 10, typical of unaltered Sierrangranitoids. The lower section contains discontinuous veins ofhigh-temperature (450–500C) calc-silicate minerals. Theseveins are typically <5 m long, do not cross intrusive contacts,and postdate the pervasive alkali alteration. Late veins aretypically > 10 m long, formed at temperatures of less than450–500C, and cross intrusive contacts. Veins have similar18O values to those of the local host rocks. The nature of the alteration and the high oxygen isotopic valuesof the rocks of the lower section indicate that these rocksinteracted extensively with seawater at temperatures <300C,probably in superposed marine hydrothermal systems associatedwith coeval volcanic centers. Metavolcanic rocks of the uppersection evidently interacted with meteorie waters, probablyin a hydrothermal system associated with the Cretaceous caldera;rocks of the lower section that were adjacent to the calderawere also affected by this alteration. The preservation of thesignatures of these earlier events, the nature of the earlyveins, and results from numerical models of hydrothermal flowthat include fluid production indicate that during progradecontact metamorphism, the rocks of the pendant primarily interactedwith locally derived fluids. Fluid flow was predominantly upwardand away from intrusive contacts and down-temperature. Permeabilitiesare estimated to have been between 0•1 and 1µD, whichis that necessary for maintenance of lithostatic fluid pressures.In hydrothermal models with such permeabilities, large-scalecirculation of meteoric fluids develops after prograde metamorphismceases. The nature of the late veins in the Ritter Range pendantsuggests that such a flow pattern evolved only after the pendantand granitoids had cooled below 450–500C. The long-termhistory of alteration documented in the Ritter Range pendantis probably typical of wall rocks in most batholiths
*Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721 相似文献
6.
Scholarly literature on newly incorporated municipalities (nims ) often focuses on why nims form. Instead of asking why nims formed, however, we ask why nims stopped forming. We first establish a temporal context for nims from 1950 to 2010, revealing an 86.2 percent decline in nim formation. The decline, triggered by stricter laws, smaller annexations, declining suburbanization, and boundary ossification, has profound implications for metropolitan fragmentation and public choice. We then establish a state‐level spatial context, revealing distinct high‐nim , low‐nim , and flux states due to boundary ossification, growth, and state/regional policy stimuli such as consolidation efforts, grants, and growth management provisions. 相似文献
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This paper presents the first chironomid-based climatic reconstruction for the UK Lateglacial, obtained from a lake basin in southeast Scotland, Whitrig Bog. Comparison of chironomid, lithological, geochemical, and pollen data reveals a marked vegetation lag behind the warming following deglaciation; warm-water chironomid taxa (e.g. chironomus) had replaced cold-water chironomid taxa (e.g. Paracladius) prior to the arrival of shrubs and trees and when local soils were still poorly developed. In addition to clear evidence for the Younger Dryas Stadial, chironomid data also reveal two short-term cold episodes that punctuate the Lateglacial Interstadial, both of which are also reflected in pollen and lithological data as reversals from open birch scrub/woodland to open herbaceous tundra coinciding with inwash of minerogenic matter into the basin. © 1997 John Wiley & Sons, Ltd. 相似文献
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KATHRYN B. BICKNELL RICHARD J. BALL ROSS CULLEN HUGH R. BIGSBY 《New Zealand geographer》1998,54(2):4-11
Sustainability has become a primary goal for much of the legislation which governs resource management in New Zealand. A major difficulty associated with sustainable development objectives, however, is the absence of reliable indicators to measure progress towards the goal of sustainability. The ‘ecological footprint’ provides an estimate of the amount of ecologically productive land required on a continuous basis to sustain current levels of resource consumption and waste assimilation for a given population. By comparing the ecological footprint of a community with the amount of land available, we can more clearly determine whether our current consumption patterns are likely to be sustainable. This paper explores the use of ecological footprint analysis within a New Zealand context. Modifications to the existing procedure for calculating an ecological footprint are proposed, and estimates based on the modified procedures are presented for New Zealand. 相似文献