中太平洋富钴结壳WXD27的年代学研究

CHRONOLOGICAL STUDY OF MID-PACIFIC FERROMANGANESE CRUST WXD27

通过针对系统取自中太平洋富钴结壳WXD27的顸部至其底部（1mm～2mm为间隔取样）样品进行的Co含量（中子活化方法）测试，拟合出不同深度壳层的富钴结壳生长速率曲线，以此推导富钴结壳Co含量的定年积分公式，并利用10Be定年方法，对Co含量定年的准确性进行检验。对超微量Sr同位素测试得到的富钴结壳磷酸盐年龄进行分析认为：（1）上部富钴结壳与上层白色磷酸盐化层接触部位的年龄为14．9Ma；（2）自14．9Ma以来，结壳的生长是连续的，沉积间断发生的可能性非常小，该样品可以用来进行14．9Ma年以来古海洋、全球变化的重建工作；（3）上层白色磷酸盐化层与其上部的富钴结壳之间存在4Ma的沉积间断；（4）19Ma～22．7Ma左右在该区发生过大规模的磷酸盐化作用，磷酸盐化作用可能造成磷酸盐化时期（19Ma～22．7Ma）和之后4Ma（19Ma～14．9Ma）富钴结壳的沉积间断；（5）结壳的初始生长年龄在75Ma左右；（6）老结壳的生长速度要远低于新结壳的生长速度。

Ferromanganese crusts play a significant role in palaeo-oceanic and palaeoclimatic studies. However,to date the crusts,which is essential for those studies,is very difficult by only a single dating method. Neutrino Activation analysis was used to measure the content of Co in different depth of a ferromanganese curst from Mid-Pacific. An eighth-order polynomial curve of the ferromanganese curst growth rate vs. its depth was deduced through using the analysis results of Co content. An integral formula of the age of the ferromanganese crust vs. its depth was also produced based on the polynomial curve. The accuracy of Co-dating results was examined through 10Be dating method. The research leads to following conclusions： A） The age of the ferromanganese contacting with the upper phosphate layer is 14. 9 Ma B） The crust had been grown continually without sedimentation interruption since 14. 9 Ma,suggesting that it can be used to rebuild the environment of palaeo-oeean and palaeo-clirnate from 14. 9 Ma to 0 Ma; C） The sedimentation interruption between the ferromanganese and the upper phosphate layer lasted around 4 Ma D） Between 19 Ma and 22.7 Ma, great phosphatization event happened in this area, which might cause the sedimentation interruption from 22.7 Ma to 14.9 Ma; E） The ferromanganese crust began to grow from 75 Ma. F） The growth rate of older crust is much slower than that of the younger crust.