Ich kann das folgende nur ganz kurz andeuten, es steckt aber viel mehr drin in den zitierten Texten:
Im frühen Tertiär (bis ins Eozän) war das Klima in Nordwestamerika subtropisch feucht-warm. Die Grundidee scheint zu sein, daß die westlichen Highlands wie eine Insel inmitten tropischer Bedinungen den 'temperierten' Taxa wie Koniferen und Cupressaceen ein Refugium boten, von dem aus sie später die Sierra Nevada kolonisierten:
Millar 1996, Tertiary Vegetation, Chap.4:
During the earliest Tertiary, the region of the present Sierra Nevada was mostly low plains to low hills, dominated by old marine sediments. In the north, the region was a low plain with a river 5 km (3 mi) wide crossing near the area of Susanville today. Southward, the region consisted of low, rolling hills, with smaller rivers draining across most of the present Sierran axis (Armentrout et al. 1979). In the region of Mount Whitney, the land rose to its highest altitude, which was still quite low compared to current Sierran elevations. The western edge of the Sierra Nevada formed the Eocene Pacific Ocean coastline for all but the northern portion (figure 5.2) (Axelrod 1968; Minckley et al. 1986). Interior to the Sierra Nevada was a large upland region that stretched throughout the northern Great Basin and intermountain areas of Idaho, western Wyoming, and western Colorado (Axelrod 1968; Ruddiman and Kutzbach 1989; Wolfe 1987). This was the only important upland region of western North America, extending to elevations over 1,225 m (4,000 ft) (Axelr 1968) or, by Gregory's recent interpretation (1994), od to 2,500 m (8,000 ft) in Colorado. This high plateau was dominated by volcanic centers and large lakes, and many of the fossil floras in the region are contained in calderas and depositional basins. This upland was unusual not only for western North America but also for temperate latitudes worldwide. As such it was an important relictual area for temperate montane taxa and a source of taxa to the Sierra Nevada in later epochs (references in Millar 1993). Climates for the Sierra Nevada during the Paleocene and Eocene, as inferred from several sources, were different from current climates and from those in epochs before the Tertiary. Although warm-equable climates had typified the late Mesozoic (McGowran 1990; Parrish 1987; Wolfe and Upchurch 1986), temperate latitudes of the early Tertiary experienced unusually high temperatures (figure 5.3) and rainfall (references in Wolfe 1990). The trends toward increasing humidity started in the early Paleocene and continued into the Eocene, reaching maximums by the early Eocene (Savin 1977; Wolfe 1985). Major fluctuations in temperature (greater in magnitude than those of the Pleistocene) characterized the Eocene (figure 5.3), causing conditions in California to alternate between tropical and subtropical (references in Millar 1993). Truly temperate conditions (seasonally cool and dry) did not exist in California except perhaps in a few limited upland areas, and no true deserts or arid areas are known to have existed at this time in the region of the Sierra Nevada (Axelrod 1979). Except in the uplands of the northern Great Basin and Idaho, humid subtropical conditions existed in a broad zone throughout temperate latitudes in North America (Millar 1993; Wolfe 1978) throughout the early Tertiary.
Im Oligozän begann das Klima weniger warm und feucht zu werden (war aber immer noch weit wärmer und feuchter als heute), und bis ins Miozän hinein (in Zeitperioden in der Größenordnung von ca. 1 Mio Jahren) stärker zu fluktuieren.
Millar 1996, Tertiary Vegetation, Chap.5:
Vegetation dynamics of the Miocene and Pliocene of the Sierra Nevada and adjacent regions are complex. Although this undoubtedly reflects the major environmental transitions of the time, it is probably also an artifact of the better records from younger ages. I discuss in turn the Miocene-Pliocene vegetation history under three more-or-less-chronological themes:
1. Early extinctions and vegetation replacements resulting from the Eocene:Oligocene climatic event,
2. High diversity through the middle to late Miocene correlated with summer rain, and
3. Migrations, species turnovers, increasing provincialization, zonation, and late Miocene extinctions due to decreasing summer rainfall through the Miocene-Pliocene.
Early Extinctions and Replacements
(...) The most dramatic changes are the loss of warm-humid-adapted angiosperms and the appearance of cool-temperate-adapted conifers and angiosperms (Axelrod 1977, 1986; Axelrod and Schorn 1994; Raven and Axelrod 1978; Schorn 1984; Wolfe 1969). Almost all of the key subtropical and tropical taxa (e.g., Diospyros, Ficus, Engelhardtia, Magnolia, Viburnum, Cinnamomum, Persea) that are known from Sierran and other western North American Eocene floras are missing from the fossil deposits of the mid-Tertiary. (...) many taxa (or their nearest relatives) that occurred in California during the Eocene are found now in tropical Mexico and others only in eastern Asia (Axelrod 1977; Raven and Axelrod 1978). (...) In addition to taxa currently native to the Sierra Nevada, these Miocene forests contained temperate-adapted taxa now native to other parts of North America. These included conifers and angiosperms currently native to non-Sierran provinces of California (e.g., Chamaecyparis, Sequoia, Picea, Thuja, coastal species of Quercus), as well as species that now grow in eastern North America (e.g., Carya, Ulmus, Juglans, Liquidambar). Many of these temperate-adapted taxa appear to have been present during the early Tertiary on the volcanic plateau of the Great BasinIdaho uplift, which seems to have served as a refugial island during the warm, humid phases of the early Tertiary (Axelrod 1968, 1986; Axelrod and Raven 1985; Millar 1993; Millar and Kinloch 1991).
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Thema: Feuerökologie und Erdgeschichte (Gelesen 60330 mal)