Fine-scale compositional variations in a Co-rich Mn crust from the Marcus-Wake Seamount cluster in the western Pacific based on electron microprobe analysis (EMPA) |
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| Authors: | Xiangwen Ren G P Glasby Jihua Liu Xuefa Shi Jingwu Yin |
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| Institution: | (1) Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, State Oceanic Administration, No. 6 XianXiaLing Road, High-Tech District, Qingdao, 266061, P.R. China;(2) Geological Laboratory Center, China University of Geosciences, 29 Xueyuan Road, Beijing, 100083, P.R. China |
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| Abstract: | The internal structure and composition of a 37-mm-thick Co-rich Mn crust from the Lamont Guyot in the Marcus-Wake Seamount
cluster have been studied in detail by means of electron-probe microanalysis (EPMA). 184 point analyses for 16 elements were
carried out at an average spacing of 0.20 mm and each point was dated using the Co-geochronometer method. Two types of variation
in composition were observed in the crust: long-term trends and short-term erratic variations. The long-term trends were identified
using the fifth order polynomial. Mn and Ni were shown to increase in concentration from 23.3 Ma to a maximum at about 20 Ma
and then decline steadily to the Present, whereas Fe, P and Si showed the opposite trend. By contrast, Co displayed a double
humped pattern with maxima at about 19 Ma and 3.5 Ma and minima at 23.3 Ma, about 11 Ma and at Present. The long-term trends
in element concentrations in the crust lead us to suggest that much of the Fe in western Pacific Ocean ferromanganese crusts
is aeolian in origin and derived from the deserts of central Asia. Cooling of the Asian mainland at about 20 Ma led to an
increase in the flux of Fe to the oceans. This was supplemented by an additional input of Fe into the crusts as a consequence
of the dissolution of biogenic CaCO3 tests at 4.5−10.5 Ma. For the short-term erratic variations, three periods of 0.61, 0.96 and 1.65 m.y. were identified by
spectral analysis. The second harmonics of these periods are 1.22, 1.92 and 3.30 m.y., which may correspond to the highest-order
periods for eccentricity of 1.31, 2.04 and 3.47 m.y. This suggests the possibility that the high-frequency oscillations of
the time series data are linked to climatic changes controlled by the highest-order periods of the Milankovitch cycles. |
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| Keywords: | Co-geochronometer Co-rich manganese crust Electron microprobe analysis Lamont Guyot Marcus-Wake Seamount cluster |
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