Burial depth and calcium carbonate content of sediment: impact on the sediment dry bulk density for the South China Sea
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摘要:
在古海洋学研究中，质量堆积速率是反映地质历史时期某种物质沉积通量变化趋势的重要指标。计算质量堆积速率的一个必要参数是沉积物的干密度（DBD）。但是，很多钻孔沉积物的干密度数据往往因采样间隔稀疏而分辨率太低，甚至没有实测数据，为后续研究带来一定限制。前人研究中曾经发现深海沉积物的DBD可以用沉积物中的CaCO3含量进行估算，但这一方法在南海是否可行尚缺乏研究。笔者根据前人经验，对南海ODP184，IODP 349、367、368共4个航次17个站位的DBD、CaCO3含量与样品埋藏深度数据进行了分析，发现沉积物埋藏深度是影响DBD的重要因素，DBD和CaCO3含量仅在南海南部的ODP1143站位表现出良好的关系。而在南海北部及中部可能由于受到不同程度深层流活动的影响，导致沉积环境极不稳定，非碳酸钙组分来源复杂多变，故DBD与CaCO3%的相关性差。在南海南部，以沉积物埋藏深度和CaCO3%为变量，对ODP1143站位用二元多项式拟合出一经验公式，该公式适用于南海南部海域与1143站位有相似沉积环境的沉积物样品干密度的估算。
Abstract:The dry bulk density （DBD） of marine sediment is an important parameter to calculate the mass accumulation rate and estimate the sediment flux in geological history. Due to sparse sampling interval, data of DBD in many borehole sediments are often poor in resolution or even unavailable, which restricts subsequent studies. Previous studies have found that the DBD of deep-sea sediments can be estimated via the content of CaCO3 in the sediments. However, whether this method is feasible for the South China Sea (SCS) sediments remains unknown. In this study, we collected and analyzed data of DBD, CaCO3 content (CaCO3%), and burial depth of downcore sediment samples from 17 sites in four ODP/IODP expeditions of Legs 184, 349, 367, and 368 in the SCS. Results showed that the burial depth is an important factor affecting DBD in the southern SCS where currents are less dynamic as represented by the ODP1143 site, showing significant DBD-CaCO3% relationship; while in the northern and central SCS, relationship between DBD and CaCO3% is poor due probably to dynamic currents and multiple non-CaCO3 inputs, which leads to the unstable sedimentary environments. Using the burial depth and CaCO3% of the ODP1143 site as variables, an empirical binary polynomial fitting equation for DBD was established, which may be applicable for the DBD estimation for the southern SCS where the sedimentary environment is similar to that of the ODP 1143 site.
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Key words:
- South China Sea /
- dry bulk density /
- sediment burial depth /
- CaCO3% content /
- binary polynomial fitting
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表 1 本文站位信息
Table 1. Specification of the study sites
航次 航次时间 站位 经度/（°E） 纬度/（°N） 水深/m 数据点/个 184 1999年2—4月 1143 113.285 9.362 2 771 180 1144 117.419 20.053 2 035 143 1145 117.631 19.584 3 175 60 1146 116.273 19.457 2 091 181 1147 116.555 18.835 3 245 24 1148 116.566 18.836 3 297 196 349 2014年1—3月 U1431 117.000 15.376 4 237 43 U1432 116.391 18.352 3 829 16 U1433 115.047 12.919 4 379 147 U1434 114.923 13.192 4 009 6 U1435 116.610 18.556 3 252 27 367 2017年2—4年 U1499 115.860 18.409 3 760 79 U1500 116.220 18.305 3 801 53 368 2017年4—6月 U1501 115.766 18.885 2 852 94 U1502 116.231 18.465 3 763 32 U1504 116.242 18.849 2 816 16 U1505 115.859 18.918 2 916 79 
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表 2 南海南部不同一元拟合效果比较
Table 2. Comparison of different unary fitting methods for the southern South China Sea
自变量 阶数 R2 RMSE/（g/cm3）
CaCO3%1 0.62 0.10 2 0.62 0.10 3 0.65 0.09 沉积物埋藏深度 1 0.674 0.09 2 0.70 0.08 3 0.74 0.08
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表 3 南海南部不同阶数二元拟合效果比较
Table 3. Comparison of different binary fitting methods for the southern South China Sea
阶数（埋藏深度） 阶数（CaCO3%） R2 RMSE/（g/cm3） 1 1 0.70 0.08 2 1 0.71 0.08 3 1 0.79 0.07 1 2 0.71 0.08 2 2 0.71 0.08 3 2 0.80 0.07 1 3 0.79 0.07 2 3 0.80 0.07 3 3 0.80 0.07 注：加粗一行数据为最终采用的阶数。
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