Calcareous nannofossil biostratigraphic dating of a ferromanganese crust from Schumann Seamount

Calcareous nannofossil biostratigraphy was employed to determine the age and growth rates of the different layers within ferromanganese crusts from Schumann Seamount, 200 miles north of the island of Kauai, Hawaii. This work is part of a study to reconstruct the paleoceanographic history of crusts using various geochemical indicators. The methods are based on the identification of coccolith imprints, not actual coccolith 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 optimized by short time ranges or narrow overlaps. In the crust studied there was considerable inter-horizon variability in the preservation of coccolith imprints, in imprint abundance and in species diversity. Some of the crust layers examined were narrowly constrained temporally (1–4 Myr), while others were less so (> 10 Myr). Our results indicate that the minimum age for the interval at 27 mm depth is at least Eocene. This is the maximum age many believe any crust can be. The position of this interval, however, is only about one half of the way through the crust. 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).