Carbonate Stable Isotope Stratigraphy of Well Xike-1, Xisha Islands
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摘要: 西科1井由于矿化重结晶作用和白云岩化作用普遍发育, 无法采用传统的氧同位素地层学方法进行地层年代标定.但是该井δ13C变化曲线与南海及全球主要大洋的碳同位素变化曲线完全相同, 可以用来准确标定200 ka以来的地层年龄.该井0~50 m深度对应全球氧同位素1~7期, 5 m处地层时代为14 ka, 为氧同位素1期的底界年龄; 11.70 m处为氧同位素2期的底界, 年龄为29 ka; 13.90 m深度年龄为57 ka; 到35.65 m为氧同位素6期底界, 年龄为191 ka, 同时δ13C值表现出冰期低而间冰期高的特点, 取自25.21 m的珊瑚U-Th定年年龄为131.062±2.320 ka.通过碳同位素定年发现, 石岛缺失近代5 ka以来的沉积物, 在间冰期向冰期转换时因海平面下降造成碳酸盐台地暴露剥蚀.全球气候变化是石岛碳酸盐台地δ13C值发生突变的主要原因.Abstract: The oxygen stable isotopes in carbonate sediment of well Xike-1 cannot be used to calibrate strata ages due to the destructive effect of recrystallization and dolomitization, while the paired δ13C records can be used to calibrate Xike-1 strata age since 200 ka, because of the similar patterns with those of the South China Sea and other oceans. Accordingly, the 0-50 m sediment of the well corresponds to the oxygen isotope stages (MIS) 1-7, with the MIS 1/2 boundary (14 ka) at 5 m, the MIS 2/3 boundary (29 ka) at 11.70 m, the MIS 3/4 boundary (57 ka) at 13.90 m, the MIS 4/5 boundary (71 ka) at 16.8 m, the MIS 5/6 boundary (129 ka) at 23.8 m and the MIS 6/7 boundary at 35.65 m respectively. Moreover, the δ13C records exhibit low values in glacial periods and high values in interglacial periods. The U-Th dating of coral from 25.21 m is 131.062±2.320 ka. It is concluded from the dating that there are no sediments younger than 5 ka existing on the Shidao Island of Xisha, and the transition from the interglacial to glacial periods with downward movement of global sea-level lead to the exposition and erosion of the local carbonate platform, which indicates that the abrupt changes of the δ13C records of the carbonate sediments from Xike-1 are responses to global climate variations.
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Key words:
- Xisha Islands /
- coral reef /
- stable isotope /
- strata /
- palaeoceanography /
- marine geology
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图 6 西科1井与全球碳同位素变化曲线对比
图a~e引自Li and Wang, 2006;红点处珊瑚U-Th年龄:131.062±2.302 ka
Fig. 6. The δ13C records of the well Xike-1 and comparison with those of the South China Sea and global major oceans
表 1 西科1井地层年龄及δ13C极值及平均值
Table 1. Age of the strata, extreme and mean value of δ13C of the well Xike-1
年龄(ka) 孔深(m) 保存厚度(m) δ13C极值(‰) 氧同位素期次 最大值 最小值 平均值 14 5.00 5.00 2.99 -1.46 0.49 1 29 11.70 7.55 2.34 -3.35 -0.37 2 57 13.90 2.20 2.42 -1.24 0.85 3 71 16.80 2.90 2.02 -0.41 0.94 4 129 23.80 7.00 3.86 -3.89 0.15 5 191 35.65 12.85 3.35 -3.47 -1.18 6 注:年龄值引自Lisiecki and Raymo(2005). -
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