- Подробности
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Родительская категория: Публикации
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Категория: Статьи
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Опубликовано: 20.01.2010 08:00
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Просмотров: 914
Binney H.A., Willis K.J., Edwards M.E., Bhagwat S.A., Anderson P.M., Andreev A.A., Blaauw M., Damblon F., Haesaerts P., Kienast F.C., Kremenetski K.V., Krivonogov S.K., Lozhkin A.V., MacDonald G.M., Novenko E.Y., Oksanen P., Sapelko T.V., Valiranta M., Vazhenina L.N.
The distribution of late-Quaternary woody taxa in northern Eurasia: evidence from a new macrofossil database
Quaternary Science Reviews. Vol. 28. Issues 23-24, November, 2009. – P. 2445-2464. http://www.elsevier.com/wps/find/journaldescription.cws_home/636/bibliographic
A database of late-Quaternary plant macrofossil records for northern Eurasia (from 23° to 180°E and 46° to 76°N) comprises 281 localities, over 2300 samples and over 13000 individual records. Samples are individually radiocarbon dated or are assigned ages via age models fitted to sequences of radiocarbon dates within a section. Tree species characteristic of modern northern forests (e.g. Picea, Larix, tree-Betula) are recorded at least intermittently from prior to the last glacial maximum (LGM), through the LGM and late glacial, to the Holocene, and some records locate trees close to the limits of the Scandinavian ice sheet, supporting the hypothesis that some taxa persisted in northern refugia during the last glacial cycle. Treeline dynamics of northern trees show differing spatio-temporal patterns across Siberia: deciduous trees were widespread in the late glacial, with individuals occurring across much of their contemporary ranges, while evergreen conifers expanded northwards to their range limits in the early Holocene. These differences are likely due to an interaction of late-glacial/early-Holocene climate and structural and physiological characteristics of tree species.
Key words: Eurasia; plant macrofossils, Late Quaternary, vegetation history, boreal forest, treeline, refugia.
Представлена база данных позднечетвертичных растительных макрофоссилий, зарегистрированных для Северной Евразии (от 23°-180°в. д. и 46-76°с. ш.), включающая 281 точку наблюдений, свыше 2300 образцов и свыше 13000 отдельных записей. Образцы имеют индивидуальные радиоуглеродные и калиброванные датировки. Лесообразующие породы современных северных лесов (например, ель, лиственница, древовидная береза) зарегистрированы, по крайней мере, от периода последнего позднеледникового максимума (LGM), до голоцена включительно. Небольшое количество местонахождений этих древесных пород сохранилось в пределах Скандинавского ледяного щита, что поддерживает гипотезу о сохранении указанных таксонов в северных рефугиумах в течение последнего ледникового цикла. Бореальные виды древесных пород указывают на развитие различных пространственно-временных моделей Сибири: листопадные деревья были широко распространены в последнем ледниковье и встречались во многих своих современных ареалах, в то время как вечнозеленые хвойные растения расширили свои северные границы лишь в период голоцена.
Ключевые слова: Евразия, растительные макрофоссилии, позднечетвертичное время, история растительности, бореальный лес, граница леса, рефугиум.
- Подробности
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Родительская категория: Публикации
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Категория: Статьи
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Опубликовано: 20.01.2010 07:54
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Просмотров: 882
Akinin V.V., Miller E.L., Wooden J.
Petrology and Geochronology of Crustal Xenoliths from the Bering Strait Region: Linking Deep and Shallow Processes in Extending Continental Crust
Geological Society of America, Special publication 456. 2009. P. 39-68. http://specialpapers.gsapubs.org/content/456/39.abstract
Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province are coupled with high-spatial resolution U-Pb geochronology of their zircons with the SHRIMP-RG yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly mafic xenoliths (constituting less than 2% of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated by magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n = 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (~ 138–60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile additions of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached sillimanite facies to granulite grade within a series of extension-related metamorphic culminations or gneiss domes which developed within the Cretaceous magmatic belt; metamorphic gradients and inferred geotherms (~ 30–50°C/km) from both the gneiss domes and xenoliths are too high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures.
Deep crustal seismic-reflection and refraction data reveal a 30–35 km thick crust, a sharp Moho and reflective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene mafic intrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal reflectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska’s accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events.
На основе изучения гранулитов и габброидов, представленных в нижнекоровых ксенолитах из щелочнобазальтовых лав Северо-Востока Азии и Аляски, дана характеристика глубинной части земной коры. U-Pb SHRIMP датирование циркона и геохимия пород ксенолитов показывают, что существенный объем окраинно-континентальной нижней коры генерирован меловыми постаккреционноми тектоническими и магматическими событиями. Изначально утолщенная кора была существенно модифицирована в мелу при магматическом андерплейтинге, инициированным мантийным магматизмом, и испытала термальное событие в маастрихте - палеоцене. Эти преобразования совпадают с главными импульсами магматизма в регионе.
