Sedimentary characteristics of a drilling core from the Zhongsha atoll lagoon: responses to sea level and provenance changes
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摘要:
基于中沙环礁潟湖的50.5 m钻孔岩芯，采用薄片观察、测年、主微量元素和碳氧同位素分析等方法，探讨了钻孔岩芯的层序地层、沉积微相和地球化学特征。结果显示该处沉积过程对晚第四纪海平面变化和物源有明显响应。结合测年结果，将岩芯的生物礁灰岩划分为10个更新世层序，且可与MIS5—MIS23阶段间冰期海水高位时期相对应。元素Sr和U的含量变化随深度加大而呈现阶梯式下降，这是由于冰期海水低位时期碳酸盐岩台地反复暴露使得老地层接受更长时间淡水成岩作用从而导致元素累积流失更多，且数值下降处可以指示层序界面。该钻孔钻遇的珊瑚礁多发育在高位体系域的进积阶段，推测是由于晚第四纪时期海平面变化速度快，且呈现迅速上升而缓慢下降的特征引起的。物源贡献因子分析结果显示，陆源物质来源、文石和高镁方解石来源、低镁方解石来源和鸟类粪便来源等4类物源影响礁灰岩的主量元素组成。本研究还揭示出现今潟湖钻遇的珊瑚礁地层有对晚第四纪10万a短偏心率周期的海平面变化响应较好，体现潟湖和台地边缘珊瑚礁的差异生长是造成现今环礁潟湖水深较大的原因之一，对解释现代环礁由来有一定的启示意义。
Abstract:Based on thin-section observation, dating results, analysis of major and minor elements and carbon and oxygen stable isotopes, the sequence stratigraphy, sedimentary facies and geochemical characteristics of a 50.5-m drill core in the atoll lagoon were examined and its significance for paleoclimate and paleoenvironment was explored. Results show that sedimentary features well responded to the Late Quaternary sea level change and provenance. Using dating results, ten Pleistocene sequences were correlated with interglacial stages during Marine Isotope Stages 5-23. The gradual decrease in Sr and U values is due to the longer duration of exposure and meteoric diagenesis of the older sequence relative to the younger sequence in the carbonate platform, clearly indicating the existence of a sequence boundary. The reef facies were formed mainly during sea level fall, indicating progradational growth of the reef. Rapid sea level rise and slow sea level fall lead to reef facies preferring to grow in the highstand systems tracts of the sequences. Four potential sources were identified from the combined PMF (positive matrix factorization) factor profiles and factor contributions. Our results indicate the drilling reef layers in the modern lagoon can well reflect the sea level change controlled by the eccentricity cycles of 100 ka and that greater reef thickening of aggradational reef in the marginal platform than that of progradational reef in the lagoon, which enriched our understanding of the evolutional processes of the atoll with a deep lagoon.
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表 1 AMS14C测年结果
Table 1. AMS14C dating results
深度/mbsf 编号 样品 14C 年龄/a BP （13C/12C）/‰ 校正年龄（范围）/cal a BP 校正年龄（中值）/cal a BP 0.01 Beta-530262 有孔虫 440±30 −0.3 130～0 65 0.41 Beta-530263 有孔虫 540±30 +0.1 266～66 166 1.26 Beta-530265 有孔虫 570±30 +0.9 285～111 198 1.68 Beta-530266 有孔虫 820±30 +0.4 513～396 454.5 
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表 2 ZS-01钻孔所取珊瑚的230Th/234U 比值与测年结果
Table 2. 230Th/234U dating results for corals from Core ZS-01
深度/m 样品 U/(μg/g) Th /(μg/g) 234U/238U/10-6 234U 230Th/238U 230Th/232Th/10−6 年龄/a BP 误差（2σ） 4.67 珊瑚 3 472.5±5.2 4 671±94 59.104±0.097 75.2±1.8 0.983 8±0.0017 12 060±243 248 447 2 461 10.60 灰岩 1 784.1±2.3 557±12 58.718±0.104 68.2±1.9 1.028 8±0.0018 54 311±1199 313 774 4 927 22.10 珊瑚 1 149.0±1.5 276±8 56.403±0.105 26.1±1.9 1.039 3±0.0020 71 336±2 086 ＞600 000 \ 26.64 灰岩 2 182.8±2.9 19 175±384 56.508±0.098 28±1.8 1.0448±0.0019 1 961±39 ＞600 000 \ 40.95 珊瑚 1 244.1±1.8 49 541±993 57.131±0.13 39.3±2.4 1.1045±0.0021 457±9 ＞600 000 \
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表 3 ZS-01钻孔87Sr/86Sr比值和定年结果
Table 3. 87Sr/86Sr ratios and dating results for Core ZS-01
深度/m 编号 样品 87Sr/86Sr 2σ 最小年龄/a BP 平均年龄/a BP 最大年龄/a BP 30.63 IS-0599 灰岩 0.709 151 0.000 005 617 000 687 000 773 000 47.5 IS-0600 灰岩 0.709 142 0.000 019 826 000 924 000 1 019 000
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表 4 ZS-01钻孔常量、微量元素相关性系数表（n=186, P＜0.01）
Table 4. The correlations of main and minor elements in ZK-01 carbonate profile
Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Sr U Na2O 1.00 MgO 0.79 1.00 Al2O3 0.23 0.18 1.00 SiO2 0.29 0.14 0.50 1.00 P2O5 0.40 0.67 0.13 0.16 1.00 K2O 0.37 0.18 0.57 0.84 0.21 1.00 CaO −0.87 −0.91 −0.30 −0.25 −0.52 −0.32 1.00 TiO2 0.24 0.08 0.85 0.49 0.07 0.72 −0.23 1.00 MnO 0.31 0.47 0.21 0.45 0.70 0.53 −0.43 0.25 1.00 Fe2O3 −0.08 −0.06 0.59 0.66 0.16 0.82 −0.02 0.70 0.49 1.00 Sr 0.74 0.78 0.12 0.01 0.35 0.03 −0.79 0.00 0.07 −0.19 1.00 U 0.43 0.53 0.12 0.05 0.45 0.08 −0.46 0.02 0.12 −0.01 0.76 1.00
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表 5 不同因子数量的PMF分析结果对比
Table 5. A comparison of the performance of the PMF models with varying numbers of factors
物源贡献因子个数 3 4 5 6 因子 1 SiO2，K2O，TiO2，Fe2O3，Al2O3，P2O5 ，MnO SiO2，K2O，TiO2，Fe2O3， Al2O3 SiO2 SiO2 因子 2 Na2O，MgO Na2O，MgO Na2O，MgO Na2O，MgO 因子 3 CaO，LOI CaO，LOI CaO，LOI CaO，LOI 因子 4 P2O5，MnO P2O5，MnO，Al2O3 P2O5，MnO，Al2O3 因子 5 K2O，TiO2，Fe2O3 K2O，TiO2，Fe2O3 因子6 Na2O
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