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21.
Dominique Schwartz Michel Thinon Stéphanie Goepp Christian Schmitt Julien Casner Thierry Rosique Patrice Wuscher Anne Alexandre Étienne Dambrine Charly Martin Bernard Guillet 《Comptes Rendus Geoscience》2005,337(14):1250-1256
The age of the upland grasslands of the Vosges Mountains is still not well known. On the basis of the study of historical archives, it was assumed that the forest clearings, which led to grasslands establishment, were done by the monks who colonized the Vosges valleys between the 7th and the 8th centuries. Our pedo-anthracological study raises questions about this hypothesis, based on the discovery of Juniperus communis charcoal in soils from the 2nd or 1st century BC. This plant specie is characteristic of grasslands developing into fallows. The occurrence of Juniperus communis charcoals indicates that upland grasslands did exist at least 800 years earlier than it was expected before our study, i.e. at least since the late Iron Age. To cite this article: D. Schwartz et al., C. R. Geoscience 337 (2005). 相似文献
22.
Clay‐rich basins have undergone varying degrees of magnetic transformation during burial, affecting their ability to retain accurate records of Earth's dynamic magnetic field. We propose to bracket the magnetite‐out and pyrrhotite‐in temperatures in shales and slates from Taiwan and the Pyrenees by using a combination of low‐temperature magnetic transitions and geothermometers. For Tburial < 340°C, the magnetic assemblage is dominated by magnetite. Gradually with increasing burial temperature, the concentration of magnetite decreases to a few ppmv. We observe the magnetite‐out isograd at Tburial ~350°C. At Tburial >60°C and Tburial >340°C respectively, fine‐grained and coarse‐grained pyrrhotite develop. In the course of burial, a clay‐rich basin gradually loses its capability to retain a record of Earth's magnetic field. It is only during basin uplift, that coarse pyrrhotite might acquire a thermo remanent magnetization. Our results point out therefore highly contrasted magnetic properties and palaeomagnetic records between deeply buried basins and exhumed ones. 相似文献