Late Pleistocene Paleoceanographic Characteristics of the Southwestern South China Sea since 500 ka
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摘要: 对湄公河口外MD01-2392孔浮游有孔虫的定量分析, 并采用FP-12E转换函数、MAT现代类比法及温跃层转换函数的计算, 结合氧同位素分析结果, 揭示了南海南部晚更新世近500ka以来的古海洋学演化特征.发现冰期MIS12、MIS8、MIS2-4冬季表层水温明显高出相邻的间冰期, 特别是间冰期MIS9、MIS5、MIS1表层水温都较低.温跃层在MIS5与MIS1最浅, MIS9其次.主要表现在浮游有孔虫深层高营养种的含量增高, 表明上升流增强.间冰期的低水温很可能主要是由于上升流影响所致, 当然表层盐度由于多雨而降低也可能影响到间冰期的水温估算.冰期时较高的表层水温, 喜暖高盐型次表层种Pulleniatina obliquiloculata的大量繁殖, 说明冰期时南部海区受来自北部强冬季风的制约使上升流活动减弱, 海平面降低后与邻区通道的关闭也造成水体置换明显减弱, 可能有淡水盖层发育, 最终导致上层海水分层增强和冬季表层水温保持相对较高.晚更新世时期的南海南部由于冰期低海平面造成半封闭的海盆环境和季风变化, 是影响其浮游有孔虫对冰期旋回响应与北部和开放大洋不同的根本原因.Abstract: Planktonic foraminifera from IMAGES core MD01-2392 near the mouth of the Mekong River were quantitatively studied. Together with SST and thermocline analyses, the faunal results reveal major evolutionary paleoceanographic characteristics in the southwestern South China Sea over the last 500 ka. A higher winter SST was found for glacial periods, especially MIS12, MIS8 and MIS2-4, while interglacials including MIS9, MIS5 and MIS1 with a relatively low SST were documented. As marked by a high abundance of the nutrient-loving, deep dwelling species, the thermocline reached its shallowest depths during MIS5 and MIS1, followed by MIS9, indicating that the estimated low SST for interglacials was due to stronger upwelling. The relatively low estimated SST, however, may also have been influenced by low salinity due to frequent and stronger rainfall during interglacials which was not considered in the SST calculation. The relatively high winter SST during glacial periods favored the prolific growth of Pulleniatina obliquiloculata, a subsurface species preferring warm and saline environments. This implies that during glacial periods, upwelling in the southern South China Sea was suppressed by a winter monsoon flowing from the north, and water exchange weakened substantially because inter-basin passages were shut down during sea level lowstands and the possible development of a freshwater cap, subsequently leading to stronger stratification in the upper water column and the retention of a relatively warmer SST during glacial winters. The main reason why the response of planktonic foraminifera to Late Pleistocene glacial cycles in the southern South China Sea differs from the northern part and open oceans is likely to be because of the semi-enclosed sea basin and monsoon variability during glacial periods.
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Key words:
- Pleistocene /
- glacial cycle /
- planktonic foraminifera /
- paleo-SST /
- monsoon /
- South China Sea /
- MD01-2392
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图 1 MD01-2392和邻区有关钻孔的站位
虚线为28 ℃温度线; 阴影部分为现代上升流区域, 箭头为夏季海流
Fig. 1. Location map, showing MD01-2392 and other site localities
图 2 MD01-2392孔氧同位素分期与邻近海区结果对比
ODP 1144孔据Bühring et al. (2004); ODP 1143孔据Tian et al. (2002); MD97-2141孔据Oppo et al. (2003)
Fig. 2. δ18O results for MD01-2392, as compared to other records from neighboring sea areas
图 3 主要浮游有孔虫含量、粗组分与碎壳率
Fig. 3. Percentage content, coarse fraction (> 0.063 mm) and test fragmentation of main planktonic foraminifera
图 4 估算的古水温与有孔虫比值及表层暖水种、次表层暖水种、冷水种与氧同位素对比
表层暖水种包括G. sacculifer, G. conglobatus, G. ruber; 次表层暖水种包括G. menardii, G. tumida, G. aequilateralis, G. conglomerata, P. obliquiloculata, 冷水种包括G. inflata, N. pachyderma, G. quinqueloba, G. bulloides
Fig. 4. The estimated paleo-SST, species ratios, relative abundance of warm-water, surface- and subsurface-dwelling species and cool to cold water species, as compared to δ18O results
图 5 温跃层、表层水温温差、上升流种含量与不同水层浮游有孔虫含量
表层低营养种: G. ruber, G. sacculifer, G. glutinata; 次表层至深层高营养种: N. dutertrei, N. pachyderma, G. menardii; 上升流种: N. dutertrei, N. pachyderma, G. bulloides.1~4为温跃层深度变化分段
Fig. 5. Estimated thermocline depth changes, ΔSST and abundance variations of groups confined to different water layers
表 1 MD01-2392的表层水古温度与温跃层深度
Table 1. The estimated SST and thermocline depth for MD01-2392
表 2 南海3钻孔同位素6期以来的平均冬季表层古水温比较
Table 2. Comparison of estimated average winter SST since MIS6 between 3 South China Sea cores
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