The origin of Island mammoths and the quaternary land bridge history of the Northern Channel Islands,California     

Donald Lee Johnson 《Quaternary Research》1978,10(2):204-225

Pygmy proboscidean remains of Mammuthus exilis occur abundantly in late Quaternary deposits on the Northern Channel Islands, California. On the assumption that ancestral elephants could not have swum to the islands and must therefore have walked out, various land bridges have been hypothesized that link the northern islands to the mainland by a peninsula. Geological evidence for a land bridge, however, is lacking, and new evidence shows that elephants are excellent swimmers and skilled at crossing watergaps. The Santa Barbara Channel was narrowed to only 6 km during glacially lowered sea levels. Modern elephants swim much further, and at speeds ranging from 0.96–2.70 km/hr. Motives for California elephants to cross Pleistocene watergaps are inferred from motives that lead modern elephants in Asia and Africa to cross watergaps. These are the visual and olfactory sensing of islands and of insular food during times of drought or fire-induced food shortage. Diminutive size of M. exilis principally reflects lack of island predators, an adaption to periodic food stress in a finite forage area affected by periodic drought and fire, and an adaptation for keeping population numbers high to maintain genetic variability and to ensure survival despite accidents. A late Quaternary scenario describes the environmental setting of the Santa Barbara Channel and the conditions that led to proboscidean dispersal to the preexistent super-island Santarosae.  相似文献   

Potassium variation across the New Britain volcanic arc   总被引：1，自引：0，他引：1  

R.W. Johnson 《Earth and Planetary Science Letters》1976,31(1):184-191

Late Cainozoic volcanoes of the New Britain island arc overlie an inclined Benioff zone that extends to a depth of at least 580 km. The rocks are tholeiitic basalt, andesite, dacite, and rhyolite. Unlike many other examples of island arcs described in the literature, K₂O contents in rocks with the same SiO₂ content do not increase progressively as depth,h, to the New Britain Benioff zone increases. The most complex relationships between K₂O, SiO₂, andh are shown by volcanoes overlying the deeper part of the Benioff zone. In these, the K₂O contents of rocks containing more than about 60% SiO₂,decrease as depth to the Benioff zone increases. The New Britain volcanic arc provides a striking exception to the generalisation thatK-h relationships are essentially similar in all island arcs.  相似文献   

Bamus volcano,Papua New Guinea: Dormant neighbour of Ulawun,and magnesian-andesite locality     

R. W. Johnson  R. P. Macnab  R. J. Arculus  R. J. Ryburn  R. J. S. Cooke  B. W. Chappell 《International Journal of Earth Sciences》1983,72(1):207-237

Bamus and Ulawun are more than 400 m higher than all other major composite volcanoes in the 1000-km-long Bismarck volcanic arc. The two cones are immediately adjacent to each other and have partly coalesced. This close proximity, their similarity of form, and their positions over the same depths (70–160 km) to the New Britain Benioff zone, could be indications that Bamus and Ulawun have had related, or at least similar, eruptive histories. But there are important differences between the two volcanoes. Bamus is thought to have last erupted some time between 1878 and 1894, whereas Ulawun has erupted at least 17 times since the late nineteenth century. In addition, the rocks of Bamus are distinctly different from those of Ulawun. Mafic rocks are found in the older part of Bamus (including a boninite-like, or magnesian-andesite, lava flow), and the younger rocks of Bamus are low-MgO andesites (the most recent products have the highest SiO₂ contents of all the analysed rock samples). In contrast, Ulawun appears to be a uniform pile of basalts and some andesites that have consistently lower amounts of CaO and Zr compared to Bamus rocks. Ulawun basalts are unlikely to represent the parental magmas from which Bamus andesites were derived by simple crystal fractionation, judging by the results of least-squares mixing calculations and the relatively high Zr contents of the Bamus andesites. The magmatic histories of Bamus and Ulawun appear to be unrelated, but because both volcanoes are the highest of all volcanoes in the Bismarck volcanic arc, both may now be susceptible to large-scale gravitational slumping or cauldron subsidence. North- and east-trending lineaments and escarpments on both volcanoes may represent the tectonically controlled scars of former gravitational collapses. Ulawun, in particular, may be at a critical stage because of its great height and steep slopes, and because eruptions in 1978 took place from a new, east-trending (possibly tectonically controlled) fissure low on the eastern flank of the volcano.

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Zusammenfassung Bamus und Ulawun überragen mit mehr als 400 m alle anderen Hauptvulkane im 1000 km langen Bismarck-Vulkanbogen. Beide Kegel liegen sich unmittelbar gegenüber und sind teilweise miteinander verbunden. Die enge Nachbarschaft, die Ähnlichkeit in der Morphologie und die gleiche Entfernung (70–160 km) von der New Britain Benioff-Zone können Anzeichen dafür sein, daß Bamus und Ulawun in Beziehung miteinanderstehen oder letztendlich die gleiche eruptive Geschichte haben. Aber es gibt wichtige Unterschiede zwischen beiden Vulkanen. Man nimmt an, daß Bamus zum letzten Mal zwischen 1878 und 1894 ausgebrochen ist, im Gegensatz dazu brach Ulawun 17 mal im späten neunzehnten Jahrhundert aus. Darüber hinaus sind die Gesteine von Bamus deutlich unterschieden von denen Ulawuns. Mafische Gesteine findet man im älteren Teil von Bamus (einschließlich eines Boninit-ähnlichen Gesteins oder magnesiumreiche Andesite und Lavaausflüsse). Die jüngeren Gesteine von Bamus sind magnesiumarme Andesite (die meisten der jüngsten Produkte haben die höchsten SiO₂-Gehalte aller analysierten Gesteinsproben). Im Gegensatz dazu scheint Ulawun eine einheitliche Anhäufung von Basalten und einigen Andesiten zu sein, diese haben durchgehend geringere Gehalte an CaO und Zr verglichen mit den Bamus-Gesteinen. Es gilt als unwahrscheinlich, daß Ulawun-Basalte ursprünglich Magma repräsentieren, von dem die Bamus-Andesite durch einfache Kristallfraktionierung stammen, bei der Beurteilung der Ergebnisse von least squares Mischungsberechnungen und den relativ hohen Zr-Gehalten der Bamus-Andesite. Die magmatische Geschichte von Bamus und Ulawun scheint beziehungslos, aber da beide Vulkane die höchsten im Bismarck-Vulkanbogen sind, mögen beide für einen weiträumigen gravitativen Kollaps oder für ein calderaartiges Einsinken empfänglich sein. Nord- und ostverlaufende Lineamente und Steilhänge an beiden Vulkanen können tektonisch kontrollierte Merkmale früherer gravitativer Zusammenbrüche sein. Besonders Ulawun könnte in einem kritischen Stadium sein wegen seiner Höhe und seiner steilen Hänge, und aufgrund jener Ausbrüche, die 1978 von einer neuen ostverlaufenden (möglicherweise tektonisch kontrolliert) Kluft tief an der östlichen Flanke des Vulkans stattfanden.

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Résumé Les monts Bamus et Ulawun s'élèvent à plus de 400 m d'altitude au-dessus de tous les autres volcans importants composites de l'archipel volcanique de Bismarck, qui s'étend sur plus de 1000 km en long. Les deux cônes sont contigus et en partie se confondent. Leur contiguité, leur conformation semblable et leur position à l'aplomb de la même profondeur (70–160 km) de la zone de Benioff de la Nouvelle-Bretagne pourraient indiquer que le Bamus et l'Ulawun ont eu une histoire éruptive apparentée ou au moins semblable. Mais il existe des differences importantes entre les deux volcans. On croit que la dernière éruption du Bamus a eu lieu entre 1878 et 1894, tandis que l'Ulawun a fait éruption au moins 17 fois depuis les dernières années du 19ème siècle. De plus, les roches du Bamus sont de composition nettement differénte de celles de l'Ulawun. On trouve des roches mafiques dans la partie plus ancienne du Bamus (y compris une coulée volcanique qui ressemble à une boninite ou andésite magnésienne) et les roches les plus récents du Bamus sont des andésites à faible teneur en MgO (les produits les plus récents contiennent plus de SiO₂ que tous les échantillons jusqu'ici analysés). Par contre, l'Ulawun parait être un amas uniforme de basaltes avec quelques andésites qui contiennent regulièrement moins de CaO et de Zr en comparaison avec les roches du Bamus. Il est peu probable que les basaltes de l'Ulawun représentent les magma-mères d'ôu proviennent les andésites du Bamus par une simple cristallisation fractionnée, à en juger par les calculs du mélange des «moindres carrés» et la quantité relativement élevée de Zr dans les andésites du Bamus. Il n'y a apparemment aucun lien de parenté entre les histoires du magmatisme du Bamus et de l'Ulawun; mais étant donné qu'ils sont les deux volcans les plus hauts de l'archipel volcanique Bismarck, il est possible qu'ils soient maintenant susceptibles d'effondrements par gravité de grande dimension ou de subsidences en caldera. Les linéaments et les escarpements qui s'étendent vers le nord et l'est des deux volcans représentent peut-être les sutures contrôlées par la tectonique d'anciens effondrements par gravité. Surtout l'Ulawun a peut-être atteint une phase critique à cause de sa hauteur et de ses pentes raides, et parce que l'éruption de 1978 dans le flanc inférieur du volcan est sortie d'une fissure en direction de l'est (peut-être sous une influence tectonique).

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Bamus Ulawun , 400 , , 1000 . . , (70– 160) , , , , , . . , Bamus 1878 1894 , a Ulawun 17 19- . Ulawun'a. — , — Bamus. — ; . Ulawun, , , , , . , , Ulawun , Bamus . , , , , . . , , , , . , , . , . , 1978 , .

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Paleopedology ofRamapithecus-bearing sediments,North India     

Dr. Gary D. Johnson 《International Journal of Earth Sciences》1977,66(1):192-216

The utility of using fossil soils in addition to other sedimentologic evidence in reconstructing past environments is considered with a preliminary analysis of the character of pedogenic alteration of selected floodplain deposits which have yielded the hominoid primatesRamapithecus andDryopithecus in the vicinity of Haritalyangar, District Bilaspur, H. P., India. The alluvial paleosols studied reflect variance of properties which are related to original depositional fabric modified by secondary pedogenic processes. The Nagri aged paleosols of Haritalyangar are ferruginous tropical soils or low-grade oxisols developed on typical floodplain toposequences, all reflecting somewhat varied histories as a function of proximity to ancient active stream courses.

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Zusammenfassung Es wird untersucht, inwieweit es für die Rekonstruktion des früheren Bildungsmilieus sinnvoll ist, in Verbindung mit anderen sedimentologischen Erscheinungen fossile Böden mit heranzuziehen, und zwar nach einer Analyse der Merkmale pedogenetischer Veränderungen in bestimmten Ablagerungen der Alluvialebenen in der Umgebung von Haritalyangar, Bilaspur, H. P., Indien, in denen die hominoiden PrimatenRamapithecus undDryopithecus gefunden worden sind. Die untersuchten alluvialen Paleoböden zeigen, verglichen mit der ursprünglichen Ablagerungsstruktur, Abweichungen in ihren Eigenschaften, die durch sekundäre pedogenetische Prozesse bestimmt sind.Die Paleoböden der Nagri-Stufe von Haritalyangar sind eisenreiche tropische Böden oder schwächere Oxiböden, die in den typischen Toposequenzen der Alluvialebenen entstanden sind und die aufgrund ihrer verschiedenen Entfernung zu alten Flußläufen alle eine etwas unterschiedliche Vergangenheit widerspiegeln.

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Résumé On considère ici la possibilité d'utiliser des sols fossiles en plus d'autres évidences sedimentologiques pour reconstruire les environnements du passé. On présente aussi une analyse préliminaire des caractéristiques de modifications pédogéniques de certains sédiments de plaines alluviales, qui ont livré les primates hominoïdesRamapithecus etDryopithecus, au voisinage de Haritalyangar, district de Bilaspur, H. P., Inde. Les paléosols alluviaux étudiés révèlent diverses propriétés caractéristiques de dé pôts originaux, modifiés par des processus pédogéniques secondaires. Les vieux paléosols de Nagri, aux environs de Haritalyangar, sont des oxisols peu évolués, formés sur des séquence topographique typique de plaines alluviales. Tous reflètent quelque peu des stages divers selon leur proximité d'anciens cours fluviatiles actifs.

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, , , , Haritalyangar'a, Bilaspur'a H.P., , . , , . Haritalyangar , , , , .

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Stable isotope compositions of olivine and dolomite in peridotites formed by deserpentinization,Darrington area,North Cascades,Washington     

Michael Johnson  Michael A Dungan  Joseph A Vance 《Geochimica et cosmochimica acta》1977,41(3):431-435

Oxygen isotope analyses of five olivines from the Darrington peridotite, Washington, yield δO¹⁸ values of +7.3 to +8.9%. which are consistent with derivation of these rocks from a serpentinite precursor. The isotopic data are compatible with mineralogical, textural and chemical evidence that most of the Darrington peridotites have formed by deserpentinization. Olivine from a single, petro-graphically distinct peridotite sample has a δO¹⁸-value of +5.2%. which is within the field of high-temperature olivines. The isotopic and textural evidence indicate that this is a partially recrystallized peridotite tectonite.Oxygen and carbon isotope analyses of dolomites from olivine-carbonate rocks indicate that they could have originated by introduction of atmospheric CO₂ via meteoric waters during the formation of ophidolomites or ophicalcites. Subsequent metamorphism and reequilibration have modified the δO¹⁸-values.  相似文献   

Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome     

R.J. Arculus  R.W. Johnson  B.W. Chappell  C.O. McKee  H. Sakai 《Journal of Volcanology and Geothermal Research》1983,18(1-4)

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Discussion on a paper by D. D. liou     

A. G. Collings  E. R. Johnson  D. D. Liou 《地震工程与结构动力学》1983,11(3):437-438

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Brunhes-Matuyama magnetic reversal dated at 790,000 yr B.P. by marine-astronomical correlations     

R.G. Johnson 《Quaternary Research》1982,17(2):135-147

The presence of the Brunhes-Matuyama magnetic reversal in deep-sea core sediments makes possible an alternative to the usual K/Ar radioistope method of dating the reversal as found in rocks. The alternative method uses correlations of Northern Hemisphere summer insolation with oxygen-isotope ratios from tropical cores. The latitude-dependent insolation variations are calculated from planetary mechanics and thus provide a highly accurate astronomical time scale. The insolation variations strongly influence glacial-ice volume fluctuations that dominate the oxygenisotope ratio changes recorded in core sediments. The summer half-year insolation variations are identified with corresponding isotope-ratio changes in cores from the present through glacial Stage 20. Misleading effects of discontinuities or major nonuniformities of sediment deposition are avoided by an analysis of the uniformity of V28-238 and V28-239, the principal cores studied, and by comparisons with other cores. The top section (Stages 1 to 10) of V28-238 is uniformly deposited, and for this section an isotope-ratio time scale is chosen that agrees with the thorium-uranium date for the high sea stand of the last interglacial extreme. Over this interval, major glacial extremes (strong isotope-ratio minima) coincide consistently with major insolation minima at times of low orbital eccentricity. In addition interstadials are directly associated with precessional insolation peaks, and the envelope of isotope-ratio peaks resembles the envelope of precessionally dominated insolation peaks. The assumption that the glacial extremes depended similarly on insolation minima during Stages 10 to 20 permits minor age shifts of strong isotope-ratio minima in the two cores (relative to ages based on uniform overal deposition) to match the ages of low-eccentricity insolation minima. The age shifts reflect residual nonuniformities of deposition. The validity of this matching procedure is supported by a resulting consistent identification of principal isotope-ratio peaks with high- and low-latitude coincident insolation maxima. The Brunhes-Matuyama reversal is found intinterglacial Stage 19, and is dated on the astronomical time scale at 790,000 ± 5000 yr B.P.  相似文献   

The formation of garnet in olivine-bearing metagabbros from the Adirondacks     

Craig A. Johnson  Eric J. Essene 《Contributions to Mineralogy and Petrology》1982,81(3):240-251

A regional study of olivine-bearing metagabbros in the Adirondacks has permitted testing of the P(pressure)-T(temperature)-X(composition) dependence of garnet-forming reactions as well as providing additional regional metamorphic pressure data. Six phases, olivine, orthopyroxene, clinopyroxene, garnet, plagioclase and spinel, which can be related by the reactions: orthopyroxene+clinopyroxene+spinel +anorthite=garnet, and forsterite+anorthite=garnet occur together both in coronal and in equant textures indicative of equilibrium. Compositions of the respective minerals are typically Fo_25–72, En_44–75, En_30–44Fs_9–23Wo_47–49, Pp_13–42Alm_39–63Gr_16–20, An_29–49 and Sp_16–58. When they occur in the same rock, equant and coronal garnets are homogeneous and compositionally identical suggesting that chemical equilibrium may have been attained despite coronal textures. Extrapolating reactions in the simple CMAS system to granulite temperatures and making thermodynamic corrections for solid solutions gives equilibration pressures (using the thermometry of Bohlen et al. 1980b) ranging from about 6.5 kb in the Lowlands and southern Adirondacks to 7.0–8.0 kb in the Highlands for the assemblage olivine-plagioclase-garnet. These results are consistent with inferred peak metamorphic conditions in the Adirondacks (Valley and Bohlen 1979; Bohlen and Boettcher 1981). Thus the isobaric retrograde path suggested by Whitney and McLelland (1973) and Whitney (1978) for the formation of coronal garnet in olivine metagabbros may not be required. Application of the same equilibria gives >8.7 kb for South Harris, Scotland and 0.9 kb for the Nain Complex. Disagreement of the latter value with orthopyroxeneolivine-quartz barometry (Bohlen and Boettcher 1981) suggests that the use of iron-rich rocks (olivines Fa₅₀) results in errors in calculated pressures.Contribution No. 385 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA  相似文献   

Morphology of the Weddell Sea and southwest Indian Ocean     

Dr. G. Leonard Johnson  Prof. Jean-René Vanney  Anders Elverhøi  John L. LaBrecque 《Ocean Dynamics》1981,34(6):263-272

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