Middle Of Holocene Research Articles

Probing for depth‐gradient diversification in the riverine‐spawning Dolly Varden: Insights from a recently discovered ecomorph assemblage

AbstractThe interplay of species could affect the pathways of adaptive diversification in lacustrine fishes. For instance, sockeye salmon is highly effective in transporting biomass from the ocean to the lakes providing abundant food supply for resident dwellers. The largest Asian population of sockeye salmon (>1.5 million spawners annually) reproduces in Lake Kurile (Russia). Here we investigate the possibility of the rapid in‐lake diversification of the tributary‐spawning charr (genusSalvelinus, Salmonidae) that colonised the Lake Kurile habitats >100 m depth and gained access to decomposing sockeye carcasses and eggs. The results confirm the trifurcation of the Lake Kurile charr along with the depth gradient into three reproductively isolated morphs: anadromous Dolly Varden utilising littoral and tributary resources, the resident slope charr and the profundal charr. No feeding specialisation among the morphs was found as the lake is rich in sockeye remains irrespective of depth. Thus, the in‐depth specialisation of the charrs did not result in the archetypical tropho‐morphological adaptations described for the majority of the deep‐water species of fishes. Only the size reduction and some paedomorphic peculiarities characterise the charr permanently dwelling in the 100–300 m depth zone. The lack of morphological differences among the morphs could be explained by the short period of evolutionary diversification. The divergence event is estimated to have occurred after the close of the glacial period in the middle of Holocene, which corresponds to the time of the lake formation. A bottleneck and subsequent rapid expansion can be traced in the demographic history of the profundal morph, suggesting the possibility of modern sympatric diversification in fish without a trophic resource partitioning.

Major Element Geochemistry of LongShan Loess Profile in the Central Shandong Mountainous regions, Northern China

In this paper, the major elements of the Longshan loess profile on the northern piedmont zones and intermountain valleys of mountainous regions in central Shandong Province in northern China, have been systematically tested and been compared with the YHC loess in the Loess Plateau to reveal the geochemical characteristics and material sources of LS loess. It is found that the average chemical composition of Shandong LS profile is similar to that of typical loess at YHC profile. The CIA, Na2O/K2O and Al2O3-CaO+Na2O-K2O triangles show that the LS loess is in moderate weathering stage and the chemical weathering degree is higher than that of the YHC profile in the Loess Plateau, but it is still in the Na, Ca removal of stage. Element activity sequence at the LS profile is Ca> Na> Mg> Si> Al> K> Fe, and Element activity capacity at LS profile is higher than that of at YHC profile. The ratio of TiO2/ Al2O3 and K2O /Al2O3 of the material source index clearly distinguishes LS loess and YHC loess, and it is concluded that the LS loess has different source from the loess plateau loess. It is not the result of dust storm direct from the northwest of China; otherwise, the Yellow River sediments and the North China Plain Material nearby may be its main sources when it was colder and drier during the glacial period. CIA and K2O/Na2O, and the migration rate of Fe, Al, K and Na is restored to the depositional environment of research. The winter monsoon in the late glaciation is strong and the chemical weathering is weak. In the early Holocene winter monsoon is weakened and the chemical weathering is enhanced. In the middle of Holocene, the summer monsoon dominated and the chemical weathering is strongest; in late Holocene, the climate deteriorated again, the chemical weathering is weakened.

Histoire de la végétation et du climat de l’Afrique nord-tropicale au Quaternaire recent*

LATE QUATERNARY HISTORY OF VEGETATION AND CLIMATE OF TROPICAL NORTH AFRICAThe critical examination o f available pollen data from the vegetation of the Sahara allows one to conclude that this vegetation has gone through but few qualitative changes during the last twenty thousand years. In particular, one notices an extension in the Sahara of tropical Sahel taxa about the middle of Holocene. Quantitatively, some pollen and geological data converge to Show that the Saharian plains were extremely arid between about 20 000 and 15 000 years BP and that on the mountains the vegetation became very sparse. A new colonization began on the mountains about 15 000 years ago.The pollen study of Holocene sediments from the central part o f the Chad basin was done in the Tjéri station. The results of this study exhibit a major change near 7 000 years BP, characterized in the Sahel zone by a dramatic extension o f arboreal taxa until about 5 000 years BP, probably corresponding to northward extension of the sahel savanna. One important change took place also at the same time in the wet north tropical zone where, between about 7 000 and 4 000 years BP, there occurred an extension of taxa growing presently on the well-leached soils of interfluves.Such a change near 7 000 years BP also appears in the available stratigraphical, sedimentological and pedological data from tropical north Africa. One observes particularly that, between 15 000 and 7 000 years BP, the