Geochemical And Mineralogical Study On The Co-rich Ferromanganese Crust From The Pacific Ocean And The Palaeoocean And Palaeoenvironment Reconstruction

Ferromanganese crusts compress informations of palaeoenvironment and tectonization since cretaceous with high density and are significant to study palaeoceanic, palaeoenvironmental and global changes.Analysis of single-grain foraminifera from carbonate basements of ferromanganese crusts was successively conducted by a combination of low Sr procedural blank and high precision determination of 87Sr/86Sr ratio, which can be significant for the study and application of Sr isotopic composition of foraminifera. Based on the obtained value of 87Sr/86Sr ratio, the analyzed foraminifera can be dated at 0.91 +0.33/-0.39Ma, which is significant for very difficult-dating foraminifera in carbonate basements of ferromanganese crusts. The foraminifera were analysed for their Mg/Ca with LA-ICP-MS, and with the data of which, the SST of a Magellan seamount in Westpacific was produced at 24.9+1.1/-1.0℃The interelemental correlation in Co-rich crusts from the Mid-Pacific seamounts of WM1 and WX and elemental variation in different layers of the crusts were studied and the study reveals that 1) the amount of hydrosyalites and quartzes increases gradually from lower layers to upper layers,which may be related to evolution of Asian monsoons and the uplift of the Himalaya - Tibetan plateau; 2) the oxidation increase from lower layers to higher layers of crusts, which may be caused by the sink of the seamounts;3) the enrichment of P restrained the adsorption of Co for crusts; 4) phosphatization played an important role in the enrichment of Pt and∑REE in ferromanganese crusts.Dating studies on Mid-Pacific ferromanganese crust were conducted by a combination of dating techniques of Co content, Be isotope as well as Sr isotope in phosphate. The age of the bottom of upper ferromanganese part contacting the upper phosphate layer is 14.9Ma and the crust had been grown continually and without sedimental interruption 0～14.9Ma. There is 4Ma sedimental interruption between the upper ferromanganese part and the upper phosphate layer. Great phosphatizations had happened in this area between 19 and 22.7 Ma BP which might cause the sedimental interruption during the phosphatizaions and 4Ma after them. The crust began to grow 75Ma BP. The growth rate of older crust part is much lower than that of the younger crust part, which can be explained that the environment where the order crust partoccurred is more reducing than that where the younger crust part occurred.Eolian dust in Mid-Pacific was extracted by chemical processes.εNd values of upper part of crust WXD27 (0-15mm depth; 0-6.9Ma) are in the range of those of Chinese loess, however,εNd values of lower part (25-37mm depth; 10.8-14.9Ma) are lower than those of upper part. By analyses of EPM( linear scan of Al) and XRD results of the samples as well as and theirεNd values, it can deduced that the source of upper dust and that of Chinese loess are same which means the upper dust is the result of Asia monsoon input, and that the lower part dust is the result not only monsoon input but also volcano input, which means the volcano activities were intensive 8.8Ma BP, 10Ma BP and especially 11.8Ma BP. Eedimental rate of eolian dust increased drastically 8Ma BP, 4-2.5Ma BP and 2.5Ma respectively. The increase 8Ma BP is the response to the drastic uplift of the Himalaya and Tibetan plateau. The high value of dust sedimental rate 4-2.5Ma BP represents enhance of winter monsoon and also the response to the drastic uplift of the Himalaya and Tibetan plateau. The increase of dust fluxes since 2.5Ma BP reflect that dust load in atmosphere increased, which maight induce the globe went into a great glacial epoch.