Radiocarbon dating of East african lake levels.

摘要: The fluctuations of the key East African lakes discussed are summarized in Fig. 4 which also includes available evidence from Lake Rukwa ( 42 ) and Chad 43 Exceot for Victoria, all these now lack surface outlets situated much drier climates than major Western Rift Valley, remain filled to their overflow levels. apparent differendes among partly due nature or intensity research both, although there must have been important local differences histories Yet consistencies far more striking, most notably coincidence early Holocene high stands. Between 10,000 8,000 years ago, it seems that many parts tropical Africa were greatly enlarged. Where previous span time is well resolved, appears transgressions leading this stand began about 12,000 evidende three basins (Victoria, Nakuru, Chad) indicates a pause minor recession just at before ago. Wherever information period preceding can consistentlybe shown small-er . Several (Rudolf, show traces earlier phases lake expansion. not yet dated but occurred then 20,000 record subsequent maximum 10,00 ago complex. Three an apparently concordant, positive oscillation some point between 6000 4000 uncertain how widely episode represented. Although closed overflowing least once during late Quaternary, evident periods expansion short-lived compared with contraction levels near those today. This pattern may be accord fragmentary lower middle Pleistocene formations, such as Olduvai( 44 )and Paninj 45 ), within relatively short-term expansions documented, any marked long-term departure balance evaporation precipitation similar present one Further, brief moist pulsations, duration perhaps 2000 5000 years, suggested by other sequences (based primarily on geomorphological palynological evidence) Saharan area, Angola, South 46 ). In default radiometric dating, complex successions intervals provide few stratigraphic markers broad applicability. This, together fact vegetation, weathering processes, montane glaciers, size, salinity, so forth likely reflect diverse aspects Climatic change differently, underscores strictures Cooke 2 Flint 3 against use pluvials intrlvasas basis subdividing Quaternary Africa. Positive correlations high-latitude glacial advances maxima demonstrated areas mid-latitude North America Eurasia 47 patterns often regarded probable well. However, above shows notable considered here. If glaciation connected all, complex-delayed, multiplefactor, inverse-relationship sought 48 renders introduction new climato-stratigraphic terms hypothermal interstadial 49 questionable value whereas so-called pluvial higher latitudes probably reduced 50 our computations Nakuru Naivasha, oscillations Rudolf Victoria recent decades, suggest where associated modest significant increase precipitation.