海洋沉积物中自生黄铁矿研究的体视镜挑选与铬还原处理方法对比——来自南海北部陆坡Site 4B站位的研究

黄铁矿是大陆边缘海相沉积物中常见的自生矿物类型之一,对沉积环境变化、早期成岩作用和天然气水合物均有重要的指示意义.体视镜挑选和铬还原处理两种方法是目前研究海相沉积物中自生黄铁矿的最常用方法,但少有对这二种方法进行对比研究的报道.本文采用上述二种方法对取自南海北部陆坡神狐海域Site 4B站位的浅表层沉积物中的黄铁矿进行了测试与分析,结果表明二种方法获得的黄铁矿相对含量及其硫同位素值的主要变化特征均有良好的同步性,且二种方法结果的差值基本保持在一定的误差范围内,说明任何一种方法都可以有效地反映沉积物中黄铁矿相对含量及其硫同位素值的主要变化规律.然而,上述二种方法从操作性和经济实用性上存在较明显的差别,铬还原处理方法从理论上更为科学和准确,但是体视镜挑选方法具有明显的易操作、经济性和实用性.本文研究后建议在实际应用过程中不妨将两种方法结合,即初期样品处理采用体视镜挑选方法,获得整个沉积物中黄铁矿产出变化规律,后期对变化明显区段采用铬还原处理方法进行更精确研究.

Method Comparison of Authigenic Pyrite Analysis between the Handpicking under Binocular Microscope and the CRS in Shallow Cored Marine Sediments: An example from the Site 4B,northern continental slope of the South China Sea

As one of the most common authigenic minerals in continental margin sediments, pyrite has been proved as an important indicator of sedimentary environment, early diagenesis and gas hydrates. So far, the identification and handpicking under binocular microscope and the analysis of Chromium reduction sulfur (CRS) are the two most commonly used methods for the analysis of authigenic pyrite in marine sediments, however there is yet rare study made for comparisons of those two methods. In this paper, both methods are employed to the shallow sediments of Site 4B, northern continental slope of the South China Sea for comparison in authigenic pyrite analysis. The results show that 1) the pyrite contents and their sulfur isotopic values obtained by both methods exhibit great synchronicity in the major variations; 2) the discrepancies between two methods are limited to a certain small range with little changes throughout the sampled interval. Therefore, both methods can inform us of the major changes of pyrite contents and their sulfur isotopic values in the studied sediments. However, both methods have their advantages and disadvantages. While theoretically, CRS seems more scientific and accurate, observation and recognition under the binocular microscope are much easier operated and more economical and practical. We suggest that it might be a better choice to combine both methods in the practice of analyzing authigenic pyrite in marine sediments, which means that the binocular microscope for the overall variation of pyrite concentration and CRS for more precise and versatile analysis within the critical intervals with major changes.