Comparison and Calibration of Laser Grain Size Analysis with Pipettesieve Method: A solution for the underestimation of the clay fraction of surface sediments from the Eastern South China Sea
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摘要: 沉降法与激光法粒度分析的主要差异是黏土粒径，这个差异将直接影响到黏土、粉砂含量和沉积物类型。本文对激光法黏土粒径和含量及沉积物类型进行系统校正。南海东部水深>2 000 m海域沉降法得出的砂、粉砂、黏土含量分别为3.66%、42.43%、53.91%，激光法砂、粉砂、黏土含量分别为9.26%、61.11%、29.64%，粉砂、黏土含量相差达约20%。沉降法得出沉积物类型主要是粉砂质黏土(69.81%)，其次是黏土质粉砂(19.81%)，其他类型只占11.38%。激光法主要是黏土质粉砂(89.62%)，其次是砂质粉砂(10.38%)，没有其他类型。两种方法得出的相同沉积物类型占21.69%，不相同占78.31%，主要差别是黏土含量。如果不校正激光法结果，粒级含量和沉积物类型没有可比性。激光法黏土粒径校正为<0.01 mm(粉砂粒径校正为0.063~0.01 mm)，黏土、粉砂的平均含量为54.16%、36.17%，接近沉降法的黏土(52.07%)和粉砂(42.28%)含量；校正后激光法与沉降法的相同沉积物类型占91.51%，不相同占8.49%，大部分黏土质粉砂已校正为粉砂质黏土；这些说明对黏土粒级的粒径和含量以及沉积物类型的三种校正已基本达到目的。相反，激光法按回归方程校正后的黏土、粉砂平均含量仍相差约20%，相同沉积物类型只占21.69%，不相同占78.31%，表明按回归方程对粒级含量和沉积物类型的校正没有实际意义。研究表明把激光法黏土粒径校正为<0.01 mm(粉砂粒径0.063~0.01 mm)，就能使黏土、粉砂含量和沉积物类型与沉降法基本相符合，达到校正基本目的。Abstract: Classically, the grain size of sediment samples is determined by the sieve method for the coarse fractions and by the pipette method, based on the Stokes' sedimentation rates, for the fine fractions. Results from the pipette method are compared with results from laser diffraction size analysis using a set of randomly selected 106 surface sediments which are collected from the Eastern South China Sea. The aim of this research is to calibrate systematically the grain size and content of clay fraction and sediment type achieved from laser diffractometry.
〓〓The contents of sand, silt, and clay in sediments of eastern South China Sea with depth more than 2 000 m were 3.7%, 42.4% and 53.9%, respectively, obtained from standard sedimentation method, and 9.3%, 61.1%, 29.6% respectively from laser diffractometry. The deviation of contents of silt and clay fraction obtained from the two methods is about 20%. The sediment types obtained from the eastern South China Sea obtained from sedimentation method are essentially silty clay of 69.8%, and secondly is clay 19.8%. There is very small amount of other types, only 11.4%. The main sediment type based on laser diffractometry is clayey silt of 89.6%, and secondly is sandy silt of about 10.4%, there is no any other types obtained. The agreement of sediment type between the two methods is only 21.7%, which means there are significant discrepancies for the contents of clay fraction achieved from two different methods.
〓〓Therefore, it is not possible to compare the contents of different grain size and sediment type obtained from the two methods without calibration. The grain size of clay fraction from laser method was calibrated to <0.01 mm, and 0.063~0.01 mm for silt correspondingly. The contents of clay and silt fraction are 54.2% and 36.2%, respectively, after calibration, which is very close to the result of 52.1% for the clay fraction and 42.3% for the silt fraction from the sedimentation method. The agreement between sedimentation method and calibrated laser method is 91.5%. Most of clayey silt is calibrated to silty clay. The correlation coefficients are 0.3203 for the silt fraction and 0.3347 for the clay fraction after calibration. The relationship appears to be positively strong, indicating that the calibration for grain size and content of clay fraction is successful.
〓〓However, there is no distinct improvement for the content of clay and silt fraction calibrated according to regressive equation. The contents are 29.1% and 62.2%, respectively. The discrepancy between the content of clay and silt fraction from two methods is 20%, but there is lower amplitude of variation. The agreement of sediment type with regressive method is only 21.7%, which means the discrepancy arrives at 78.3%, indicating that calibration with regressive equation for content of clay and silt fraction and sediment type. The research suggested that after the grain size were calibrated to <0.01 mm for clay fraction and 0.063~0.01 mm for silt fraction, the contents of clay and silt fraction and sediment type agreed well with those from sedimentation method. There is almost no improvement using regressive calibration. -
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