江苏中部海岸晚第四纪沉积物的粒度与磁化率特征及其古环境意义

Characteristics of grain size and magnetic susceptibility of the Late Quaternary sediments from core 07SR01 in the middle Jiangsu coast and their paleoenvironmental significances

    摘要
  • 摘要: 对位于江苏中部海岸的07SR01钻孔沉积物进行了粒度和磁化率测量，分析了粒度与磁化率的变化特征及其两者间的相关关系，并结合北半球晚更新世气候与相对海面变化以及该孔年代框架和沉积相的研究认识，进一步揭示了研究区的古环境变化。研究表明，MIS 5晚期至MIS 3时期，江苏中部海岸先后经历了3个阶段的环境演化：① MIS 5晚期为较高海面、较强水动力的潮汐河口（边滩、河床）阶段（36.10～26.65 m），该阶段沉积物主要受古长江物源影响，粒度粗，分选性波动大（0.55～2.35），粒度频率分布曲线呈极正偏的窄峰（主峰位于3 Φ附近）且“拖细尾”，磁化率较高且波动小(5.81～42.16)×10−8 m3·kg−1，与砂组分（＜4 Φ）呈强正相关；② MIS 4-3时期为冷干转为暖湿、海面先下降后上升和较弱水动力的淡水与滨岸湖沼阶段（26.65～15.77 m），该阶段沉积物细，分选性稳定（1.51～3.03），粒度频率分布曲线呈正偏的宽峰（主峰位于4.75 Φ附近），磁化率低且稳定(6.46～20.04)×10−8 m3·kg−1，主要与粗粉砂组分（4～5 Φ）呈弱正相关；③ MIS 3时期为较高海面、较强水动力的潮汐河口（分流河道）阶段（15.77～0 m），该阶段沉积物受MIS 3苏北古黄河的影响增强，粒度较粗，分选性波动大（0.94～2.82），粒度频率分布曲线呈极正偏的窄峰（主峰位于3.75 Φ附近）且“拖细尾”，磁化率较高且波动大(10.21～57.25)×10−8 m3·kg−1，与砂和粗粉砂组分（＜5 Φ）呈弱正相关。揭示海岸沉积物粒度和磁化率组合指标的古环境指示意义，将为进一步深入研究这一指示意义的形成机理提供必要的基础。

     

    Abstract: The grain size and magnetic susceptibility of sediments from core 07SR01 in the middle Jiangsu coast were measured, as well as their variation and correlation were analyzed. Combined with the chronological framework and sedimentary facies data from the core, plus the research results of the Northern Hemisphere climate and relative sea level changes since the Late Pleistocene, the paleoenvironmental evolution of the study area was further established. It is found that the study area has experienced three periods of environmental changes from late MIS 5 to MIS 3: (1) The period of marginal bank and riverbed developed in the tidal estuary during late MIS 5 (36.10～26.65 m) which was under relatively high sea level and strong hydrodynamic conditions, the sediments deposited during this period were mainly affected by the provenance of old Yangtze River and characterized by coarse grain size, dramatical variations in sorting coefficients (0.55～2.35), extremely positive deviation with a high-narrow peak near 3 Φ and a thin tail in the frequency curve, and high magnetic susceptibilities with small fluctuations in the range of (5.81～42.16)×10−8 m3·kg−1 and strongly and positively correlated with sand (＜4 Φ); (2) The period dominated by fluviolacustrine and littoral environments with weak hydrodynamics during MIS 4-3, in which the climate changed from cold and dry to warm and humid as the sea level rose after a drop (26.65～15.77 m), the sediments deposited at this period are characterized by fine grain size, stable sorting coefficients (1.51～3.03), positive deviation with a low-broad peak near 4.75 Φ in the frequency curve, low values and small variations of magnetic susceptibilities (6.46～20.04)×10−8 m3·kg−1, which are weakly and positively correlated with coarse silt (4～5 Φ); (3) The period of distributary channel in the tidal estuary with relatively high sea level and strong hydrodynamics during MIS 3 (15.77～0 m), the sediments deposited at this period were strongly influenced by the old Yellow River of North Jiangsu and characterized by relatively coarse grain size and large variations in sorting coefficients (0.94～2.82), extremely positive deviation with a high-narrow peak near 3.75 Φ and a thin tail in the frequency curve, and relatively high magnetic susceptibilities with great fluctuations in the range of (10.21～57.25)×10−8 m3·kg−1 and weakly and positively correlated with sand and coarse silt (＜5 Φ). To reveal the paleoenvironmental significances for combination of grain size and magnetic susceptibility of coastal sediments, will provide a necessary basis for further study on their genetic mechanisms.

     

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