Deep Water Ventilation and Stratification in the Neogene South China Sea
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摘要: 综合南海ODP1148站、1146站和1143站沉积物物性、底栖有孔虫、同位素等资料, 探讨早中新世以来南海深层水的演化特征.结果表明, 在21~17Ma、15~10Ma和1~5Ma3个时间段分别对应3个富含红褐色粘土的岩性单元, 其红色参数(a*) 增高指示南海深层水中溶解氧含量的增加.对比发现, 前两阶段的深层水增氧与南极底层水和北大西洋组合水增强有关, 说明10Ma前南海与外地的底层水基本是相互连通的.10Ma以后, 南海深层水溶解氧降低, 同时分别处于下深层水的1148站和上深层水的1146站之间的CaCO3含量变化加大, 喜氧底栖有孔虫减少, 底栖δ13C在10Ma大幅度减轻, 说明南海当时的深层水受大洋深层水的控制减弱.推测主要是南海海盆自16~15Ma停止扩张以后, 南海逐渐关闭引起本地深层水开始形成的缘故.从6Ma左右开始出现大量的太平洋底层水和深层水的底栖有孔虫标志种, 1148站和1146站在5~3Ma期间的CaCO3含量之差达到40%, 标志南海深层水最大分异期.除了全球气候变冷、北半球结冰引起太平洋深层水扩张的影响之外, 南海海盆由于更强烈向东俯冲而进一步下沉也可能是原因之一.3Ma以来南海深层水演化进入现代模式, 两站之间的CaCO3含量之差稳定在10%左右, 厌氧底栖种丰度增加.太平洋底层水和深层水的标志种相继在1.2Ma和0.9Ma大量减少, 底栖δ13C也同时大幅度变轻到新近纪的最低值, 表明太平洋底层水的影响基本消失, 太平洋深层水的影响也大大减弱.因此, 标准现代模式的南海深层水, 推测主要由于“中更新世气候转型”时期巴士海峡下面的海槛抬升到接近目前~2600m的深度时, 才开始形成.Abstract: Combined data of physical property, benthic foraminifera and stable isotopes from ODP Sites 1148, 1146 and 1143 are used to discuss deep water evolution in the South China Sea (SCS) since the Early Miocene. The results indicate that 3 lithostratigraphic units respectively corresponding to 21-17 Ma, 15-10 Ma and 10-5 Ma with positive red parameter (a*) marking the red brown sediment color represent 3 periods of deep water ventilation. The first 2 periods show a closer link to contemporary production of the Antarctic Bottom Water (ABW) and Northern Component Water (NCW), indicating a free connection of deep waters between the SCS and the open ocean before 10 Ma. After 10 Ma, red parameter dropped but stayed higher than the modern value (a*=0), the CaCO3 percentage difference between Site 1148 from a lower deepwater setting and Site 1146 from an upper deepwater setting enlarged significantly, and benthic species which prefer oxygen-rich bottom conditions dramatically decreased. Coupled with a major negative excursion of benthic δ13C at 10 Ma, these parameters may denote a weakening in the control of the SCS deep water by the open ocean. Probably they mark the birth of a local deep water due to shallow waterways or rise of sill depths during the course of sea basin closing after the end of SCS seafloor spreading at 16-15 Ma. Several Pacific Bottom Water (PBW) and Pacific Deep Water (PDW) marker species rapidly increased since 6 Ma, and from 5 Ma to 3 Ma the local deepwater became strongly stratified, as indicated by up to 40% CaCO3 difference between Sites 1148 and 1146. Apart from a strengthening PDW due to global cooling and ice cap buildup on northern high latitudes, a deepening sea basin due to stronger subduction eastward may also have triggered the influx of more corrosive waters from the deep western Pacific. Since 3 Ma, the evolution of the SCS deep water entered a modern phase, as characterized by relative stable 10% CaCO3 difference between the two sites and increase in infaunal benthic species which prefer a low oxygenated environment. The subsequent reduction of PBW and PDW marker species at about 1.2 Ma and 0.9 Ma and another significant negative excursion of benthic δ13C to a Neogene minimum at 0.9 Ma together convey a clear message that the PBW largely disappeared and the PDW considerably weakened in the mid-Pleistocene SCS. Therefore, the true modern mode SCS deep water started to form only during the "mid-Pleistocene climatic transition" probably due to the rise of sill depths under the Bashi Strait.
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Key words:
- South China Sea /
- Miocene /
- Pliocene /
- Pleistocene /
- deep water evolution /
- ventilation /
- carbonate accumulation /
- oxygen and carbon isotopes
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图 1 南海ODP 1143、1146、1148站位图
Fig. 1. Location map of ODP Sites 1143, 1146 and 1148 in the South China Sea
图 2 南海和邻近海区水流示意图(a) (据Chen et al., 2006修改)和南海与开放型菲律宾海不同深度的实测溶解氧剖面(b) (Li and Qu, 2006)
Fig. 2. Schematic profile showing the major flow pattern in the South China Sea and neighboring sea basins (a) and measured profile of dissolved oxygen levels at different Philippine and SCS sites (b)
图 3 ODP1148站(a)和1146站(b) 新近纪沉积物物性和测井曲线(据Wang et al., 2000修改)
Fig. 3. Physical property and logging curves of Neogene sediments at ODP Sites 1148 (a) and 1146 (b)
图 4 1148站代表性底栖有孔虫的相对丰度(据Zhao et al., 2007)
Fig. 4. Relative abundance of selected benthic foraminifera from Site 1148
图 5 1148站底栖(B) 氧、碳同位素, 浮游(P) 氧、碳同位素, 底栖和浮游同位素差值与全球组合记录相对比
据赵泉鸿等(2001a, 2001b)和翦知湣等(2001); 全球组合记录据Zachos et al. (2001); 箭头指示1148站底栖碳同位素的主要负位移事件; 直线棒表示Δδ13C (P-B) 的主要变化趋势; MMCT代表“中中新世气候转型”
Fig. 5. Comparison between the global composite benthic isotopic records and those of benthic and planktonic records and their differences from Site 1148
图 6 新近纪南海深层水演化主要阶段和全球深层水增强阶段相对比
ODP1148、1146、1143站有关资料见图 3; 浮游有孔虫碎壳率据陈晓良等(2002); 北方组合水(NCW)和南极底层水(AABW) 增强主要据Ramsay et al. (1994)和Hodell and Venz-Curtis (2006); 1148站溶解事件D1~D5据Li et al. (2006)
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