铁锰结壳年代学方法及其应用

Review and application of dating methods of ferromanganese crusts

铁锰结壳记录了丰富的古海洋和古气候信息,要分析这些信息,必须先确定各壳层的生长年龄。目前主要使用的定年方法有:Be同位素定年、U系同位素定年、Os同位素定年、经验公式法、古地磁地层学方法和古生物地层学方法等。每种定年方法均各有利弊,其中10Be是年龄小于10 Ma结壳的最佳定年方法;U系同位素定年精度高,但定年范围小于0.5 Ma;Os同位素定年时间尺度长,且可以判断沉积间断,但需要有区域的海水Os同位素演化曲线;Co经验公式因为简单易行,是结壳估算年龄最为常用的方法,但该法无法判断沉积间断,在使用时需特别谨慎,应确保经验公式适用于所研究的样品;如果结壳厚度大、生长速率较快且不存在沉积间断,古地磁地层学方法可以快速地进行定年;此外,如果结壳中古生物化石较多,可考虑采用古生物地层学方法。在分析铁锰结壳年代学方法利弊基础上,本文利用U系定年法、古地磁地层学法和经验公式计算法分别对南海铁锰结壳进行定年,通过比较,认为目前铁锰结壳较优的定年方法为古地磁地层学方法。

Deep-sea ferromanganese crusts are the preferred recorders of paleoceanographic events. To analyze paleoceanographic events recorded by nodules and crusts, first, the age of each layer must be determined. The main dating methods used so far include the following: 10 Be dating, U-series dating, Os isotope dating, empirical equation, magnetic stratigraphy, and biological stratigraphy. Every dating method has its advantages and disadvantages. 10 Be dating is the best method for determining the age of crusts younger than 10 Ma. U-series dating has the best accuracy, but its dating time scale is only 0.5 Ma. Os isotope dating can span long time scales and could determine sedimentation hiatuses, although, first, the 187Os/188Os seawater curve of the local ocean must be obtained. The empirical equation is simple and easy, and it is the most commonly used method for age estimation. However, it cannot determine sedimentation hiatuses. Special care should be taken to ensure that the empirical formula is suitable for the sample. Magnetic stratigraphy can be used to date thick and fast-growth crusts without a sedimentation hiatus. Biological stratigraphy is an effective method for dating nodules and crusts with more fossils. Based on the analysis of the advantages and disadvantages of the dating methods of ferromanganese crusts, the U-series dating method, paleomagnetostratigraphy method, and empirical formula calculation methods were used to date the ferromanganese crusts of the South China Sea. By comparison, it was determined that the current best dating method for ferromanganese crust is the paleomagnetostratigraphy method.