Studies On Growth Rates Of Ferromanganese Crusts From Pacific Seamounts

Ferromanganese crusts grow very slowly on the substrate rock at rates of several mmper Ma and can therefore provide a long-term history of marine environments and globalchanges. In this dissertation, fourteen ferromanganese crusts from Pacific Seamounts havecomprehensively studied to expound the geochemistry of elements and isotopes and theinformation of their genesis using various analytical tools and geochronological methods.The principal results are as follows:(1) The multielemental analyses of CAD15 and MHD59 crusts have been carried outby the Proton Induced X-ray Emission(PIXE) method. The profiles and possibleinterelemental correlation of some elements e.g., Fe, Mn, Co, Ni, Cu, have been discussed.(2) Specific activities of U, Th isotopes and ²³¹Pa were measured in 14 crusts fromthe Pacific Seamounts. The depth profiles of these radioactive isotopes have been used toestimate growth rates which are 3.14 mm/Ma(²³⁰Th_ex method), 3.32 mm/Ma(²³⁰Th_ex/²³²Thmethod), 6.30 mm/Ma(²³¹Pa_ex method) and 1.71 mm/Ma(²³⁴U_ex method) respectively.(3) AMS using HI-13 accelerator of ¹⁰Be/⁹Be ratio and ¹⁰Be depth profiles in theCAD15 and MHD59 crusts from the Pacific seamounts, have showed an exponentialdecrease and yielded growth rates of 2.19 and 2.55 mm/Ma for CAD15 and MHD59respectively.(4) The growth rates and ages of CAD15 and MHD59 crusts have also beencalculated according to the Co chronology method proposed by Puteanus and Halbach andLyle' empirical formula based on Fe and Mn contents. Furthermore, based on the analysisof cobalt contents of CAD15 and MHD59 and their growth rates determination by the ¹⁰Bemethod, a new empirical formula has been derived.(5) The growth rates of the western Pacific have been observed higher than thecentral Pacific for 4 U-series datings. The geographic variation of the growth rates of theferromanganese crusts from the Pacific Seamounts were showed that the growth rates in the south and the west are higher than in the north and in the east respectively. A negative linearcorrelation of the growth rates of the crusts with the water depths has been observed.(6) This study has adopted seven dating methods, including 4 U-series methods, ¹⁰Bemethod, Co chronology, empirical formula based on Fe and Mn contents, to investigatethe growth rates of cobalt-rich crusts from the Pacific Seamounts. ²³⁴U_ex method has beenconsidered as an unbelievable dating method due to a relatively large diffusion coefficienturanium isotope in crusts. Low activity and special chemistry of ²³¹Pa in crusts can bringdifficult analyses and therefore limit the ²³¹Pa_ex method. Due to a constant supply of ²³⁰Thin water column and its strong particle-reactive, ²³⁰Th_ex and ²³⁰Th_ex/²³²Th dating methodscan obtain relatively precise growth rates which are, however, restricted to the last about530 kyr. The application of cosmogenic ¹⁰Be, which appears to give reliable results whennormalized to stable ⁹Be, extends chronologies back to～10 Ma.These dating methods have each advantage and limit so that the selective andcombined application is very essential to obtain a better dating.