Studies On Growth Rates,Elemental Distribution And Phosphate Oxygen Isotopes Of Fe-Mn Deposits From The Western Pacific

The ferromanganese crusts and polymetallic nodules were dredged from the Western Pacific seamount in this study.For the short-time scale,the average growth rate of each sample was estimated by the U-series chronology,the discrepancy between growth rates estimated from Pa and Th isotope ratios was discussed and the relationship between major and trace elements for each sample was analyzed.For the long-time scale,the variation of the phosphate oxygen isotopes was discussed.The main conclusions were as follows:(1)For ferromanganese crusts CAD10,CBD12-2 and CAD25-1 and polymetallic nodules 40V-C1-BC1601 and 40VI-C1-BC 1612a,the average growth rates of 2.21± 0.14mm/Ma,2.25±0.17 mm/Ma,2.01±0.07mm/Ma,1.16±0.07 mm/Ma and 1.31 ±0.07 mm/Ma are estimated from U-series chronology respectively.(2)The discrepancy between Pa-and Th-derived growth rates is more readily explained by the diffusion of Pa and the effective diffusion coefficient of 8×10-9cm2/a for 231Paex in ferromanganese crusts is estimated by one-dimensional diffusion model,therefore the effect of this process on the ancient signal of 231Paex cannot be neglected.At the same time,the spatial distribution of 230Thex/231Paex)A.R in the surface of each sample has a great relationship with the upper marine biological productivity.(3)It is considered that the ferromanganese crusts in this study are all hydrogenous by using the(Co + Cu + Ni)× 10]-Fe-Mn diagram.The content of Mn is 16%to 19%and Fe is 15%to 17%,so Mn and Fe are the most important ore-forming elements in the ferromanganese crusts.By correlation analysis,Co,Ni,Mg,Pb,As,Se are positively correlated with Mn and Cu,Al,Ba,Ag,Sn,Sb are positively correlated with Fe,meanwhile Mn is negatively correlated with Fe.Besides,the spatial distribution for most of the metal elements in the ferromanganese crusts are mainly controlled by Mn and Fe.In addition,Fe in the ferromanganese crusts gradually increases with the water depth,while Mn decreases with the water depth.(4)The low values of ?18Op are mostly contributed by the hydrolysis of organic P and the contribution of debris P is the least in the non-phosphatized ferromanganese crusts.However,in the phosphated ferromanganese crusts,the low values of ?18Op are mainly contributed by the debris P and the contribution of the hydrolysis of organic P is too low to be ignored.The high values of ?18Op in ferromanganese crusts are mainly from authigenic phosphates and the deviation of ?18Op from the equilibrium can record the utilization efficiency of the phosphate for the surrounding seawater.(5)It is found that the potential variation pattern of ?18Op in ferromanganese crusts is related to the long-term climate cycle on the earth through comparison with the growth rate,the chronology and the trend of ?18Op.Therefore,the study of ?18Op in ferromanganese crusts is of great significance for paleoceanography and paleoclimate.