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)

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 0.20mm 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 23.3Ma to a maximum at about 20Ma 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 19Ma and 3.5Ma and minima at 23.3Ma, about 11Ma 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 20Ma 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 4.5−10.5Ma. For the short-term erratic variations, three periods of 0.61, 0.96 and 1.65m.y. were identified by spectral analysis. The second harmonics of these periods are 1.22, 1.92 and 3.30m.y., which may correspond to the highest-order periods for eccentricity of 1.31, 2.04 and 3.47m.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.