The Identification of Milankovitch Cycles in theGrayLevel Series of FeMn Crust from the Central Pacific Ocean and Its Growth Rate Evolution

Key words: 
• FeMn crust

Abstract: A FeMn crust sample (CB14) from a seamount in the central Pacific Ocean was studied in detail. According to the microtexture of the growth profile of the crust, its outer layer (18.7mm in thickness) can be divided into 5 sublayers, no hiatus observed between the adjacent sublayers. Using the technique of digital image processing, the graylevel variation series of the reflectivity of the growth pattern were obtained. The power spectral analysis revealed that most of the prominent cycles identified from graylevel series are corresponding to Milankovitch cycles (eccentricity, obliquity and precession). Through matching and tuning to the Milankovitch cycles, we obtained that the growth rates of sublayer 1 through sublayer 5 are 2.15, 2.70, 2.43, 2.75 and 2.67 mm/Ma, respectively. Their corresponding ages are 1.3, 2.5, 5.9, 6.7 and 7.5 Ma, respectively. Our results show that since late Micocene, the growth rates of FeMn crust changed alternatively, since recent 1.3 Ma, the growth rate of the FeMn crust slowed down significantly. It is considered that the graylevel series of the growth profile of FeMn crusts can be used as a paleoenvironmental indicator, and the application of orbital pacing method on graylevel series is an effective approach to determine the high resolution growth rates of FeMn crust and hence provide important information on paleoenvironmental change.