ALBERT

Description: To investigate the growth rates and absolute time stratigraphy of marine hydrogenetic ferromanganese encrustations, we performed 10Be profiling and ‘Co chronometry’ of crustal layers, as well as 87Sr86Sr and δ18O analysis of phosphatised limestone (francolite) within a ∼ 9.5 cm thick ferromanganese crust from Schumann Seamount in the Hawaiian Archipelago. Together with microfossil stratigraphy, our results indicate that some seamount crusts greatly exceed the commonly accepted Miocene maximum age, in this case probably approaching the Cretaceous age of the seamount. In addition to the unconformity at the crust-substrate boundary, at least eight major disconformities are indicated in the Schumann Seamount crust which probably represent depositional hiatuses or episodes of crust erosion. Three of the six upper disconformities can be placed at the Plio-Pleistocene, Middle Miocene and Paleocene-Eocene based on 10Be, microfossil and Co chronometer evidence. 87Sr86Sr and δ18O values of purified francolite from an inclusion-rich layer between the depths of 44 and 49 mm suggest apparent ages that approach those of Eocene-Late Paleocene microfossils reported in overlying layers, whereas francolite vein infillings in the lower part of the crust and in the basaltic substrate yield values that, if interpreted as ages of phosphatization, suggest a minimum Oligocene age. Paleotracking suggests the phosphogenesis observed here and on other Central Pacific seamounts could not have resulted from upwelling enhanced productivity associated with equatorial divergence if the Oligocene and Middle Miocene isotopic ages reported here and elsewhere are correct; however, a maximum Late Paleocene age for the phosphogenesis, consistent with the stratigraphy, would place these seamounts within 10°N of the equator. Paleotracking also suggests northeast tradewind transport of aluminosilicates in the Cenozoic, in agreement with other evidence for the antiquity of this ferromanganese crust.