138 resultados para Symphytum
Resumo:
The Nachtigall clay pit near Holzminden, northern Germany, is located in a subrosional basin filled with 43 m of interglacial, interstadial and stadial deposits adjacent to the Weser River. The succession separates the Older Middle Terrace from the Younger Middle Terrace of the Weser River. Nachtigall core KB1 (1998) mainly contains silt and clay with intercalated peat layers. The layers of fen peat and intercalated humic silt are between 36 and 22.5 m depth. According to palynological studies, the peat layers and some humic silts were deposited during interglacial and interstadial periods marked by forest vegetation, termed Nachtigall 1 and Nachtigall 2. They are subdivided by a stadial, termed Albaxen. The peat of Nachtigall 1 is interrupted twice by silt and clay strata (Allochthonous Unit I, II) which are reworked sediments of older glacial periods, possibly of late Elsterian or early Holsteinian age. The palynological sequences of Nachtigall and Göttingen/Ottostrasse show the same pattern. Moreover, the contemporaneous pollen profiles of Nachtigall and Göttingen/Ottostrasse can be compared with the Velay pollen sequence (France). The Nachtigall core section 36-26.02 m corresponds to Bouchet 2 - Bonnefond - Bouchet 3 in Velay. The profiles of Velay and Nachtigall are independently correlated to the MIS-timescale and correspond to MIS 7c, 7b, and 7a. TIMS 230Th/U-dating shows ages ranging from 227 + 9/-8 to 201 + 15/-13 ka, which are in good agreement with the inferred MIS 7 age.
Resumo:
A 200 m long marine pollen record from ODP Site 658 (21°N, 19°W) reveals cyclic fluctuations in vegetation and continental climate in northwestern Africa from 3.7 to 1.7 Ma. These cycles parallel oxygen isotope stages. Prior to 3.5 Ma, the distribution of tropical forests and mangrove swamps reached Cape Blanc, 5°N of the present distribution. Between 3.5 and 2.6 Ma, forests occurred at this latitude during irregular intervals and nearly disappeared afterwards. Likewise, a Saharan paleoriver flowed continuously until isotope Stage 134 (3.35 Ma). When river discharge ceased, wind transport of pollen grains prevailed over fluvial transport. Pollen indicators of trade winds gradually increased between 3.3 and 2.5 Ma. A strong aridification of the climate of northwestern Africa occurred during isotope Stage 130 (3.26 Ma). Afterwards, humid conditions reestablised followed by another aridification around 2.7 Ma. Repetitive latitudinal shifts of vegetation zones ranging from wooded savanna to desert flora dominated for the first time between between 2.6 and 2.4 Ma as a response to the glacial stages 104, 100 and 98. Although climatic conditions, recorded in the Pliocene, were not as dry as those of the middle and Late Pleistocene, latitudinal vegetation shifts near the end of the Pliocene resembled those of the interglacial-glacial cycles of the Brunhes chron.