We describe in detail the Miocene to Quaternary paleoceanography of the western equatorial Pacific Ocean based on calcareous nannofossils of Hole 805B. The relative abundance of Discoaster which lived in the lower pho...We describe in detail the Miocene to Quaternary paleoceanography of the western equatorial Pacific Ocean based on calcareous nannofossils of Hole 805B. The relative abundance of Discoaster which lived in the lower photic zone under the stable sea with nutricline and thermocline, decreased step by step at NN5/NN6 and NN10/NN11 boundaries. Al- though the size of Reticulofenestra which is strongly influenced by nutrient, increased five times throughout the section, it drastically decreased in NN4-5 zone, NN10, NN12, and in NN15/NN16 boundary. On the basis of the relationship between Discoaster abundance and Reticulofenestra size change, collapse of the stability of the sea surface in the west- ern equatorial Pacific Ocean progressed step by step throughout the Miocene to Quaternary.展开更多
Based on the oxygen and carbon stable isotopic records of benthic foraminifera in nine deep-sea cores of the South China Sea (SCS), the bathymetric profiles of δ 18O and δ 13C since the last glacial maximum (LGM) ar...Based on the oxygen and carbon stable isotopic records of benthic foraminifera in nine deep-sea cores of the South China Sea (SCS), the bathymetric profiles of δ 18O and δ 13C since the last glacial maximum (LGM) are preliminarily established. The bathymetric gradients of deep-water δ 18O and δ 13C in the SCS are obviously greater during the LGM than during the Holocene, showing the existence of the deep thermocline and nutricline at water depth of about 2 000 m. Particularly, the differences in δ 18O and δ 13C between the LGM and Holocene, from which the ice-volume effect and the global mean shift have been subtracted respectively, are positive values at water depths of 1 000-2 500 m in the SCS. This indicates the existence of deep-water mass with relatively cool temperature or higher salinity, better ventilation and more δ 13C within the water depth range of the SCS during the LGM, which is distinctly different from that at present. These changes further confirm the existence of the glacial "North Pacific Deep Water" which, however, is possibly confined to the water depth range of 1 000-2 500 m.展开更多
A great number of calcareous nannofossils have been found in the deep-sea sediments of 2.32 Ma at ODP Site 1143 located in the Nansha area, the southern South China Sea. The number of coccoliths varies from about 0.5 ...A great number of calcareous nannofossils have been found in the deep-sea sediments of 2.32 Ma at ODP Site 1143 located in the Nansha area, the southern South China Sea. The number of coccoliths varies from about 0.5 × 106 up to almost 53 × 106 coccoliths/g sediment, with an average of 16 × 106 coccoliths/g sediment. The accumulation rate of total coccoliths varies from 1 × 106 to 278 × 106coccoliths/cm2 ka. The nannofossil assemblages are usually dominated by a Iower-photic species-Florisphaera profunda, of which the average percentage is about 70% in all samples. The absolute abundance and the accumulation rate of nannofossils as well as the per centage of F. profunda display significant oscillations on two different time scales. One is the fluc tuation coincident with the glacial-interglacial cycle, and the other is the long-term changes on a time scale longer than 100 ka. Six evolutionary stages of calcareous nannofossils could be divided for the last 2.32 Ma, from which we can reconstruct the changes in the depth of nutricline of the Nansha area. In this paper, the possible mechanism resulting in these variations is also discussed.展开更多
The relative abundance of Florisphaera profunda , a calcareous nannofossil species in lower euphotic zone, affords an opportunity to study variations in upper ocean structure such as thickness of mixed layer. As revea...The relative abundance of Florisphaera profunda , a calcareous nannofossil species in lower euphotic zone, affords an opportunity to study variations in upper ocean structure such as thickness of mixed layer. As revealed by the nannofossil analysis in sediments since 20 ka B.P. from three Okinawa Trough cores, the percentage of F. profunda increased significantly from glacial to Holocene, with a declining event ca 4 ka B.P. in its southern part. Its downcore trend is similar to that of Pulleniatina obliquiloculata, a tropical planktonic foraminiferal species which lives in subsurface water below thermocline in the modern ocean. Their similar variations imply an eastward shift of the Kuroshio Current, a reduction in thickness of the mixed layer, a shallowing of thermocline and a possible increase of turbidity in the upper ocean during the glaciation. The eastward shift of the Kuroshio might be repeated again about 4 ka B.P.展开更多
文摘We describe in detail the Miocene to Quaternary paleoceanography of the western equatorial Pacific Ocean based on calcareous nannofossils of Hole 805B. The relative abundance of Discoaster which lived in the lower photic zone under the stable sea with nutricline and thermocline, decreased step by step at NN5/NN6 and NN10/NN11 boundaries. Al- though the size of Reticulofenestra which is strongly influenced by nutrient, increased five times throughout the section, it drastically decreased in NN4-5 zone, NN10, NN12, and in NN15/NN16 boundary. On the basis of the relationship between Discoaster abundance and Reticulofenestra size change, collapse of the stability of the sea surface in the west- ern equatorial Pacific Ocean progressed step by step throughout the Miocene to Quaternary.
文摘Based on the oxygen and carbon stable isotopic records of benthic foraminifera in nine deep-sea cores of the South China Sea (SCS), the bathymetric profiles of δ 18O and δ 13C since the last glacial maximum (LGM) are preliminarily established. The bathymetric gradients of deep-water δ 18O and δ 13C in the SCS are obviously greater during the LGM than during the Holocene, showing the existence of the deep thermocline and nutricline at water depth of about 2 000 m. Particularly, the differences in δ 18O and δ 13C between the LGM and Holocene, from which the ice-volume effect and the global mean shift have been subtracted respectively, are positive values at water depths of 1 000-2 500 m in the SCS. This indicates the existence of deep-water mass with relatively cool temperature or higher salinity, better ventilation and more δ 13C within the water depth range of the SCS during the LGM, which is distinctly different from that at present. These changes further confirm the existence of the glacial "North Pacific Deep Water" which, however, is possibly confined to the water depth range of 1 000-2 500 m.
基金the National Natural Science Foundation of China (Grant No. 49999560) and NKBRSF (Grant Nos. G1999043409 and G2000078500).
文摘A great number of calcareous nannofossils have been found in the deep-sea sediments of 2.32 Ma at ODP Site 1143 located in the Nansha area, the southern South China Sea. The number of coccoliths varies from about 0.5 × 106 up to almost 53 × 106 coccoliths/g sediment, with an average of 16 × 106 coccoliths/g sediment. The accumulation rate of total coccoliths varies from 1 × 106 to 278 × 106coccoliths/cm2 ka. The nannofossil assemblages are usually dominated by a Iower-photic species-Florisphaera profunda, of which the average percentage is about 70% in all samples. The absolute abundance and the accumulation rate of nannofossils as well as the per centage of F. profunda display significant oscillations on two different time scales. One is the fluc tuation coincident with the glacial-interglacial cycle, and the other is the long-term changes on a time scale longer than 100 ka. Six evolutionary stages of calcareous nannofossils could be divided for the last 2.32 Ma, from which we can reconstruct the changes in the depth of nutricline of the Nansha area. In this paper, the possible mechanism resulting in these variations is also discussed.
文摘The relative abundance of Florisphaera profunda , a calcareous nannofossil species in lower euphotic zone, affords an opportunity to study variations in upper ocean structure such as thickness of mixed layer. As revealed by the nannofossil analysis in sediments since 20 ka B.P. from three Okinawa Trough cores, the percentage of F. profunda increased significantly from glacial to Holocene, with a declining event ca 4 ka B.P. in its southern part. Its downcore trend is similar to that of Pulleniatina obliquiloculata, a tropical planktonic foraminiferal species which lives in subsurface water below thermocline in the modern ocean. Their similar variations imply an eastward shift of the Kuroshio Current, a reduction in thickness of the mixed layer, a shallowing of thermocline and a possible increase of turbidity in the upper ocean during the glaciation. The eastward shift of the Kuroshio might be repeated again about 4 ka B.P.
