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The chemical and mineralogical compositions of ferromanganese crusts collected from surfaces of northwestern Pacific guyots during Ocean Drilling Program (ODP) Leg 144 show that they are mainly hydrogenous deposits; that is, they have been formed by the slow accumulation of trace metal enriched oxides directly from the seawater column. The formation of crusts occurred over a period of 30 (and possibly even 60-90) m.y., when guyots were carried "piggy-back" by the oceanic plates across the equatorial zone of the central Pacific. The guyots simultaneously experienced vertical displacement caused by isostatic adjustment that exposed them to the influence of water masses having different chemical compositions. Crust precipitation may have occurred when current velocity over the tops of these guyots oscillated between 1 and 3 8 cm/s and the surface of the guyots subsided through the water column from ~400 m to ~1050 m water depth (i.e., when the guyot surfaces were located within the low-oxygen zone, or near its lower boundary). Two mineralogical assemblages dominated by (1) asbolane-buserite at the northern guyots and (2) Fe-vernadite at the southern guyots record differences in bioproductivity during crust precipitation. The presence of both mineral assemblages in crusts at the Wodejebato Guyot demonstrates the complexity of processes and the importance of the local environment on crust deposition. Increased concentrations of Co, Ni, Mn, and P were noted in crusts deposited in the equatorial high-productivity belt. Both positive and negative Ce anomalies were observed in samples of crusts collected at the same depth level, but at different locations on the same guyot. The behavior of Ce is covariant with Mn and antipathic with P. The trace element and rare earth element (REE) analyses presented by this study suggest that the chemical composition of crusts is more strongly influenced by the combined effects of bioproductivity, relative position of the oxygen-minimum zone, and the current velocity, rather than by the water depth where the crusts precipitate. Our interpretation of the composition of Fe-Mn crust and nodules, as paleoceanographic indicators, suggests that the Late Cretaceous-early Tertiary (90-60 Ma) was a period of lower biogenic productivity over the equatorial Pacific than during the Tertiary, extrinsic to low intensity of upwelling and equable climate. Several periods of increased bottom current velocity and bioproductivity during the late Paleocene-early Eocene (60-50 Ma) indicate an increase in upwelling as the higher latitudes began to cool, thus developing a steeper thermal gradient in the southern ocean. Increased bioproductivity and an increased depositional rate of pelagic carbonate at the equatorial belt during the late Oligocene-early Miocene smothered Fe-Mn crust precipitation at the southern guyots.

Composition of Ferromanganese Crusts and Nodules at Northwestern Pacific Guyots and Geologic and Paleoceanographic Considerations