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11.
The late Cenozoic climate of Africa is a critical component for understanding human evolution. African climate is controlled by major tectonic changes, global climate transitions, and local variations in orbital forcing. We introduce the special African Paleoclimate Issue of the Journal of Human Evolution by providing a background for and synthesis of the latest work relating to the environmental context for human evolution. Records presented in this special issue suggest that the regional tectonics, appearance of C(4) plants in East Africa, and late Cenozoic global cooling combined to produce a long-term drying trend in East Africa. Of particular importance is the uplift associated with the East African Rift Valley formation, which altered wind flow patterns from a more zonal to more meridinal direction. Results in this volume suggest a marked difference in the climate history of southern and eastern Africa, though both are clearly influenced by the major global climate thresholds crossed in the last 3 million years. Papers in this volume present lake, speleothem, and marine paleoclimate records showing that the East African long-term drying trend is punctuated by episodes of short, alternating periods of extreme wetness and aridity. These periods of extreme climate variability are characterized by the precession-forced appearance and disappearance of large, deep lakes in the East African Rift Valley and paralleled by low and high wind-driven dust loads reaching the adjacent ocean basins. Dating of these records show that over the last 3 million years such periods only occur at the times of major global climatic transitions, such as the intensification of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker Circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1-0.7 Ma). Authors in this volume suggest this onset occurs as high latitude forcing in both Hemispheres compresses the Intertropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. These periods of extreme climate variability may have provided a catalyst for evolutionary change and driven key speciation and dispersal events amongst mammals and hominins in Africa. In particular, hominin species seem to differentially originate and go extinct during periods of extreme climate variability. Results presented in this volume may represent the basis of a new theory of early human evolution in Africa. 相似文献
12.
Summary At the Devonian/Carboniferous boundary, major climatic and oceanographic changes influenced sedimentation on carbonate platforms
and in peri-platfrom asreas. Three deep-water carbonate successions in Moravia, which were selected to represent different
paleotectonic settings, have been studied with the aim of testing the influence of eustatic, climatic and tectonic controls
on sedimentation and conodont paleoecology and taphonomy.
On the slopes of the wide carbonate platforms of the Moravian Karst Development (Lesní lom and Grygov sections), an exemplary
highstand shedding systems developed in the upper Famennian (expansa Zone), marked by a pronounced thickness of their respective
calciturbidite successions and an abundance of shallow-water skeletal grains.Palamatolepis— andBispathodus-dominated conodont assemblages contain an admixture ofPolygnathus representing a transported, near-shore component. The eustatic sea-level fall in the praesulcata Zone and the lowstand conditions
at the D/C boundary resulted in a decline of carbonate platform production and condensed deposition or nondeposition. In the
Lesní lom section, a condensed sequence of turrbiditic calcarenites and shales (Middle praesulcata—lowermost sulcata Zone)
was followed by lime mud calciturbidites (sulcata and duplicata Zones). In the conodont assemblages, the first event in the
Lower praesulcata Zone was associated with the reduction of ‘mesopelagic’Palmatopic and a bloom of epipelagicPolygnathus communis. The second event in the Middle praesulcata Zone corresponds to the onset of polygnathidprotogranthodid biofacies, indicating
a carbonate slope environment. In the Grygov section, a pronounced thickening and upward-coarsening succession of tubiditic
calcilutites through calcarenites and intraclast breccias, with poor palmatolepid-bispathodid connodont assemblages (expansa
Zone), indicates a progradation of the calciturbidite system associated with sea-level highstand. After a break in sedimentation,
covering the interval from the Lower praseulcata to the base of Lower crenulata Zone, thick-bedded, fine-grained calciturbidites
were deposited in the Lower crenulata Zone, and are associated with poor, mixed assemblages where siphonodellids and polygnathids
predominate. At the isosticha-Upper crenulata/Lower typicus boundary, coasre grained, turbiditic calcarenites and breccias
rich in clastic quartz grains and mixed conodont assemblages with reworked Frasnian and Famennian conodonts indicate a deep
erosion of the source area, presumably due totectonic uplift (relative lowstand).
In the Jesenec section, on the flanks of the volcanic seamount (the Drahany Development), a deep-water Upper Famennian condensed
succession of calciturbidites and presumably winnowed pelagic limestones is marked by conodont assemblages of palmatolepid-bispathodid
biofacies. More proximal calciturbidites with mixed deep-water and shallowwater conodonts prograde at the top of the Upper
Famennian succession (Middle to Upper expansa Zone). A striking hiatus, covering the interval from the Early preaesulcata
to the base of Lower crenulata Zone, resulted from extreme condensation and submarine bottom current erosion due to sea-level
lowstand in the late Famennian and early Tournaisian. The renewed middle Tournaisian calciturbidite sedimentation with strong
evidence of erosion at the source area indicates global eustatic rise and tectonic uplift of the Drahany Development seamounts
(relative lowstand). The earlier occurrence of the uplift in the Jesenec area, relative to the Grygov section, shows the advance
of tectonic processes over time in the Moravian-Silesian basin (orogenic polarity) as a consequence of Variscan orogenic movements. 相似文献
13.
Summary The Sikhor Formation (new) is a predominantly siliciclastic sediment package intercalated between the marly-silty Baghamshah
Formation (below) and the calcareous Esfandiar Limestone and Qal'eh Dokhtar Limestone formations (above). All stratigraphic
evidence points to an Early Callovian age of the formation. The Sikhor Formation is restricted to the southern and central
Shotori Mountains and consists of two members: The Kuh-e-Neygu Member (new) is composed of fluvialdeltaic conglomerates, sandstones,
and siltstones grading into marly silt of the Baghamshah Formation. The overlying Majd Member (new) is characterised by mixed
siliciclastic-carbonate sediments that record the interfingering of carbonate ramp sediments with fluvialdeltaic sands and
silts. Evidence of erosional truncation of the underlying Baghamshah Formation and confinement of the siliciclastic sediment
to a comparatively narrow, NNW-SSE elongated strip suggest that the formation had its origin in the asymmetric uplift of a
westdipping tilted fault block in the southern Shotori Mountains that shed its sediment predominantly in a northern and eastern
direction. After erosional levelling, the former uplifted areas were overgrown by the highly productive Esfandiar Carbonate
Platform. The Sikhor Formation thus is evidence of an extensional tectonic pulse in the early Callovian and underlines that
this area of the Tabas Block was a tectonically highly unstable area during most of the Jurassic. 相似文献