Clay fraction contents distribution in the Senegal River estuary after the building of the anti-salt Diama dam has shown that upstream sediments are very muddy. Clay fraction content is as high as 60 % of sediments in...Clay fraction contents distribution in the Senegal River estuary after the building of the anti-salt Diama dam has shown that upstream sediments are very muddy. Clay fraction content is as high as 60 % of sediments in the middle of the channel with a pronounced downstream decreasing trend on the banks comparing to the contents in the middle of the channel. Towards the river mouth, the fine fraction tends to disappear leading to a shelly sand sediment type essentially. During low water period, clay fraction contents increase particularly at the vicinity of the dam reservoir. In the estuarine reach, downstream the dam, this increasing trend during the low water stage is particularly marked in the middle of the channel. The clay mineral assemblage shows that kaolinite and smectite are the main components, whereas interlayered illite-smectite and illite are minor constituents. The upstream-downstream profile shows a decrease in kaolinite content although it remains the dominant constituent. Floating clay minerals （smectite and illite-smectite） concentrate rather in the middle of the channel and on the right bank where the water depth is higher. According to seasonal variations, the distribution of these clay minerals often follows the concentration of the entire clay fraction; their contents increase during low water stage period. This hydrodynamics sorting of floating minerals, evident in the up-dam zone, tends to disappear in the eastern reach between Diama dam and the mouth. Clay minerals assemblage shows, through the time, a rather homogeneous composition which does not seem to be altered by the seasonal fluctuations.展开更多
Clay mineral assemblages and crystallinities in sediments from IODP Site 1340 in the Bering Sea were analyzed in order to trace sediment sources and reconstruct the paleoclimatic history of the Bering Sea since Plioce...Clay mineral assemblages and crystallinities in sediments from IODP Site 1340 in the Bering Sea were analyzed in order to trace sediment sources and reconstruct the paleoclimatic history of the Bering Sea since Pliocene （the last -4.3 Ma）. The re- sults show that clay minerals at Site U1340 are dominated by illite, with a moderate amount of smectite and chlorite, and minor kaolinite. Sediment source studies suggest that the clay mineral assemblages and their sources in the studied core are controlled primarily by the climate conditions. During the warm periods, clay minerals originated mainly from the adjacent Aleutian Is- lands, and smectite/（illite＋chlorite） ratios increased. During the cold periods, clay minerals from the Alaskan region distinctly increased, and smectite/（illite＋chlorite） ratios declined. Based on smectite/（illite＋chlorite） ratios and clay mineral crystallinities the evolutionary history of the paleoclimate was revealed in the Bering Sea. In general, the Bering Sea was characterized by warm and wet climate condition from 4.3 to 3.94 Ma, and then cold and dry condition associated with the enhanced volcanism from 3.94 to 3.6 Ma. Thereafter, the climate gradually became cold and wet, and then was dominated by a cold and dry condi- tion since 2.74 Ma, probably induced by the intensification of the Northern Hemisphere Glaciation. The interval from 1.95 to 1.07 Ma was a transitional period of the climate gradually becoming cold and wet. After the middle Pleistocene transition （1.07 to 0.8 Ma）, the Bering Sea was governed mainly by cold and wet climate with several intervals of warm climate at -0.42 Ma （MIS 11）, -0.33 Ma （MIS 9） and ~0.12 Ma （MIS 5）, respectively. During the last 9.21 ka （the Holocene）, the Bering Sea was characterized primarily by relatively warm and wet climatic conditions.展开更多
文摘Clay fraction contents distribution in the Senegal River estuary after the building of the anti-salt Diama dam has shown that upstream sediments are very muddy. Clay fraction content is as high as 60 % of sediments in the middle of the channel with a pronounced downstream decreasing trend on the banks comparing to the contents in the middle of the channel. Towards the river mouth, the fine fraction tends to disappear leading to a shelly sand sediment type essentially. During low water period, clay fraction contents increase particularly at the vicinity of the dam reservoir. In the estuarine reach, downstream the dam, this increasing trend during the low water stage is particularly marked in the middle of the channel. The clay mineral assemblage shows that kaolinite and smectite are the main components, whereas interlayered illite-smectite and illite are minor constituents. The upstream-downstream profile shows a decrease in kaolinite content although it remains the dominant constituent. Floating clay minerals （smectite and illite-smectite） concentrate rather in the middle of the channel and on the right bank where the water depth is higher. According to seasonal variations, the distribution of these clay minerals often follows the concentration of the entire clay fraction; their contents increase during low water stage period. This hydrodynamics sorting of floating minerals, evident in the up-dam zone, tends to disappear in the eastern reach between Diama dam and the mouth. Clay minerals assemblage shows, through the time, a rather homogeneous composition which does not seem to be altered by the seasonal fluctuations.
基金supported by National Natural Science Foundation of China(Grant Nos.41476037,41076026,41276051 and 91228207)the National Key Basic Research Program of China(Grant No.2013CB956102)IODP-China
文摘Clay mineral assemblages and crystallinities in sediments from IODP Site 1340 in the Bering Sea were analyzed in order to trace sediment sources and reconstruct the paleoclimatic history of the Bering Sea since Pliocene （the last -4.3 Ma）. The re- sults show that clay minerals at Site U1340 are dominated by illite, with a moderate amount of smectite and chlorite, and minor kaolinite. Sediment source studies suggest that the clay mineral assemblages and their sources in the studied core are controlled primarily by the climate conditions. During the warm periods, clay minerals originated mainly from the adjacent Aleutian Is- lands, and smectite/（illite＋chlorite） ratios increased. During the cold periods, clay minerals from the Alaskan region distinctly increased, and smectite/（illite＋chlorite） ratios declined. Based on smectite/（illite＋chlorite） ratios and clay mineral crystallinities the evolutionary history of the paleoclimate was revealed in the Bering Sea. In general, the Bering Sea was characterized by warm and wet climate condition from 4.3 to 3.94 Ma, and then cold and dry condition associated with the enhanced volcanism from 3.94 to 3.6 Ma. Thereafter, the climate gradually became cold and wet, and then was dominated by a cold and dry condi- tion since 2.74 Ma, probably induced by the intensification of the Northern Hemisphere Glaciation. The interval from 1.95 to 1.07 Ma was a transitional period of the climate gradually becoming cold and wet. After the middle Pleistocene transition （1.07 to 0.8 Ma）, the Bering Sea was governed mainly by cold and wet climate with several intervals of warm climate at -0.42 Ma （MIS 11）, -0.33 Ma （MIS 9） and ~0.12 Ma （MIS 5）, respectively. During the last 9.21 ka （the Holocene）, the Bering Sea was characterized primarily by relatively warm and wet climatic conditions.
