MAGNETOSTRATIGRAPHY OF HYDROGENETIC MANGANESE CRUSTS FROM NORTHWESTERN PACIFIC SEAMOUNTS

Fine-scale dating is crucial to understanding the growth and environme nts of formation of marine manganese deposits. The paleomagnetic metho d of dating of manganese nodules and crusts has been attempted but wit h no success so far. We measured remanent magnetization (RM) on up to 75-mm-thick hydrogenetic crusts at intervals of 2.5 mm after mineralog ical and chemical examination, careful separation, and alternative fie ld demagnetization. Stable sharp RM and well-correlated polarity-chang e patterns between four samples made it possible to identify the Plioc ene-Quaternary magnetic chrons in them. These data show that the three crusts have grown continuously at rates of 14-17 mm/m.y. since the Ea rly Pliocene. This is three or four limes faster than those for Centra l Pacific seamount hydrogenetic crusts. Estimates of growth rate based on determination of Co-flux are within this range. Magnetostratigraph y therefore appears to be a powerful method for dating hydrogenetic ma nganese crusts when supported by conventional dating methods. These re sults encourage finer-scale investigations which promise more detailed paleoceanographic reconstruction. (C) 1998 Elsevier Science B.V. All rights reserved.