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731.
The Geelbek Dunes located north of Cape Town, South Africa, are an active, northward migrating dune field. Interdunal deflation hollows temporarily expose former land surfaces that are associated with archaeological sites. These open‐air sites shed light on large‐scale patterns of Middle and Later Stone Age settlement and augment the information gained from well‐stratified, but spatially limited caves, rock shelters, and coastal shell middens. Based on paleopedological and sedimentological parameters, three former land surfaces were identified and associated with different assemblages. A chronostratigraphy of the various land surfaces was established by applying optically stimulated luminescence (OSL) dating. The youngest former land surface is represented by a dune generation (AD2) which stabilized at a maximum of 5 ka. An older dune generation (AD1) shows a more heterogeneous age pattern where deposition started at ca. 27 ka with a maximum estimated age of stabilization at ca. 10 ka. Both of these dune generations overlie a weathered calcrete complex of Middle to Late Pleistocene age. While the third dune generation (AD0) was observed between underlying calcrete layers, samples taken from this unit could not be dated. © 2008 Wiley Periodicals, Inc. 相似文献
732.
Kyu-Myong Kim William K.-M. Lau Yogesh C. Sud Gregory K. Walker 《Climate Dynamics》2010,35(1):115-126
Effects of aerosol radiative forcing on the diurnal and seasonal cycles of precipitation over West Africa and eastern Atlantic
Ocean are investigated for the boreal summer season: June–July–August. An eight year (2000–2007) average of GCM simulated
rainfall data is compared with the corresponding TRMM rainfall data. The comparison shows that the amplitude of the diurnal
cycles of rainfall over land and ocean are reasonably well simulated. Over land, the phase of the simulated diurnal cycle
of precipitation peaks several hours earlier than that of the TRMM data. Corresponding differences over the ocean(s) are relatively
smaller. Some of the key features of the aerosol induced model simulated field anomalies are: (a) aerosol direct radiative
forcing which increases the atmospheric stability and reduces the daytime moist convection and convective precipitation; (b)
the aerosol induced changes in the diurnal cycle of precipitation are out of phase with those of the TRMM data over land,
but are in-phase over the ocean; (c) aerosols reduce the amplitude of the diurnal cycle of precipitation over land and enhance
it over ocean. However, the phase of the diurnal cycle is not affected much by the aerosol radiative forcing both over land
and ocean. During the boreal summer, aerosol radiative forcing and induced circulation and precipitation cool the Sahel and
the southern part of Sahara desert more than the adjacent areas to the north and south, thereby shifting the peak meridional
temperature gradient northward. Consequently, an anomalous easterly jet is found north of its climatological location. This
anomalous jet is associated with increased cyclonic circulation to the south of its axis, resulting in an anomalous monsoon
rain belt in the Sahel. 相似文献
733.
The chemical-temporal evolution of lithospheric mantle underlying the North China Craton 总被引:7,自引:0,他引:7
Previous studies of samples of subcontinental lithospheric mantle (SCLM) that underlay the North China Craton (NCC) during the Paleozoic have documented the presence of thick Archean SCLM at this time. In contrast, samples of SCLM underlying the NCC during the Cenozoic are characterized by evidence for melt depletion during the Proterozoic, and relatively recent juvenile additions to the lithosphere. These observations, coupled with geophysical evidence for relatively thin lithosphere at present, have led to the conclusion that the SCLM underlying the NCC was thinned and modified subsequent to the late Paleozoic. In order to extend the view into both the Paleozoic and modern SCLM underlying the NCC, we examine mantle xenoliths and xenocrystic chromites extracted from three Paleozoic kimberlites (Tieling, Fuxian and Mengyin), and mantle xenoliths extracted from one Cenozoic basaltic center (Kuandian). Geochemical data suggest that most of the Kuandian xenoliths are residues of small degrees of partial melting from chemically primitive mantle. Sr-Nd-Hf isotopic analyses indicate that the samples were removed from long-term depleted SCLM that had later been variably enriched in incompatible elements. Osmium isotopic compositions of the two most refractory xenoliths are depleted relative to the modern convecting upper mantle and have model melt depletion ages that indicate melt depletion during Paleoproterozoic. Other relatively depleted xenoliths have Os isotopic compositions consistent with the modern convecting upper mantle. This observation is generally consistent with earlier data for xenoliths from other Cenozoic volcanic systems in the NCC and surrounding cratons. Thus, the present SCLM underlying the NCC has a complex age structure, but does not appear to retain materials with Archean melt depletion ages. Results for what are presumed to be early Paleozoic xenoliths from Teiling are generally highly depleted in melt components, e.g. have low Al2O3, but have also been metasomatically altered. Enrichment in light rare earth elements, low εNd values (∼−10), and relatively high 87Sr/86Sr (0.707-0.710) are consistent with a past episode of metasomatism. Despite the metasomatic event, 187Os/188Os ratios are low and consistent with a late Archean melt depletion event. Thus, like results for xenoliths from other early Paleozoic volcanic centers within the NCC, these rocks sample dominantly Archean SCLM. The mechanism for lithospheric thinning is still uncertain. The complex age structure currently underlying the NCC requires either variable melt depletion over the entire history of this SCLM, or the present lithospheric material was partly or wholly extruded under the NCC from elsewhere by the plate collisions (collision with the Yangtze Craton and/or NNW subduction of the Pacific plate) that may have caused the thinning to take place. 相似文献
734.
Emily M. Chiappe Richard D. Ash Arto Luttinen Sari Lukkari Jukka Kuva Connor D. Hilton Richard J. Walker 《Meteoritics & planetary science》2023,58(12):1747-1759
The meteorite Lieksa was found in 2017 in Löpönvaara, Finland, and later donated to the Finnish Museum of Natural History. Here, we report siderophile element concentrations, genetic isotopic data, and a metal–silicate segregation age for the meteorite. The ~280 g Lieksa is ~80% metal and ~20% silicate and oxide inclusions by volume, with the inclusions consisting primarily of Fe-rich olivine. Due to Lieksa's silicate content, coupled with a texture characterized by metal enclosing the silicates, it has been classified as a pallasite. Lieksa's olivine and bulk chemical characteristics are distinct from those of the known pallasite and iron meteorite groups, consistent with its classification as ungrouped. The meteorite exhibits a flat, chondrite-normalized highly siderophile element pattern, consistent with an origin as an early crystallization product from a metallic melt with chondritic relative abundances. Molybdenum, Ru, and 183W isotopic data indicate that Lieksa formed in the non-carbonaceous (NC) domain of the solar nebula. Radiogenic 182W abundances for Lieksa yield a model metal–silicate segregation age of 1.5 ± 0.8 Myr after calcium-aluminum-rich inclusion formation, which is within the range established for other NC-type pallasite and iron meteorite parent bodies. 相似文献