M. Joshima et A. Usui, MAGNETOSTRATIGRAPHY OF HYDROGENETIC MANGANESE CRUSTS FROM NORTHWESTERN PACIFIC SEAMOUNTS, Marine geology, 146(1-4), 1998, pp. 53-62
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.