CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHIC DATING OF A FERROMANGANESE CRUST FROM SCHUMANN SEAMOUNT

Calcareous nannofossil biostratigraphy was employed to determine the a ge and growth rates of the different layers within ferromanganese crus ts from Schumann Seamount, 200 miles north of the island of Kauai, Haw aii. This work is part of a study to reconstruct the paleoceanographic history of crusts using various geochemical indicators. The methods a re based on the identification of coccolith imprints, not actual cocco lith remains. Age constraints on the time span of a particular horizon are defined either (1) by the presence of a species which was extant for only a limited geological time interval or (2) by the presence of two organisms whose time ranges overlapped. Age resolution is optimize d by short time ranges or narrow overlaps. In the crust studied there was considerable inter-horizon variability in the preservation of cocc olith imprints, in imprint abundance and in species diversity. Some of the crust layers examined were narrowly constrained temporally (1-4 M yr), while others were less so (> 10 Myr). Our results indicate that t he minimum age for the interval at 27 mm depth is at least Eocene. Thi s is the maximum age many believe any crust can be. The position of th is interval, however. is only about one half of the way through the cr ust. While the data support previous estimates of average crust growth rates (0. 5 mm/Myr). the actual apparent growth rate appears to have varied by an order of magnitude (0.1 to 2 mm/Myr).