Contents lists available at ScienceDirect Deep–Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Paleoceanographic conditions on the São Paulo Ridge, SW Atlantic Ocean, for the past 30 million years inferred from Os and Pb isotopes of a hydrogenous ferromanganese crust Kosuke T. Goto a, ⁎ , Tatsuo Nozaki b , Takashi Toyofuku c , Adolpho H. Augustin d , Gen Shimoda a , Qing Chang e , Jun–Ichi Kimura e , Koji Kameo f , Hiroshi Kitazato c , Katsuhiko Suzuki b a Geological Survey of Japan, AIST, Tsukuba, Ibaraki 305-8567, Japan b Research & Development Center for Submarine Resources, JAMSTEC, Yokosuka, Kanagawa 237-0061, Japan c Department of Marine Biodiversity Research, JAMSTEC, Yokosuka, Kanagawa 237-0061, Japan d Institute of Petroleum and Natural Resources, PUCRS, Porto Alegre, Partenon 90619-900, Brazil e Department of Solid Earth Geochemistry, JAMSTEC, Yokosuka, Kanagawa 237-0061, Japan f Department of Earth Sciences, Chiba University, Chiba, Chiba 263-8522, Japan ARTICLE INFO Keywords: Ferromanganese crust Osmium isotope stratigraphy Lead isotope southwestern Atlantic Ocean Paleoceanography ABSTRACT Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the São Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn–Fe–10×(Ni+Co+Cu) and rare-earth element plus yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained 187 Os/ 188 Os data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over ~30 Myr with growth rates of 0.5–3 mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the São Paulo Ridge over the past ~30 Myr. The Pb isotopic compositions show very limited ranges (e.g., 206 Pb/ 204 Pb=18.80–18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the São Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since ~30 Ma. 1. Introduction Hydrogenous ferromanganese crusts (Fe-Mn crusts) are authigenic deep-marine sediments composed mainly of iron (Fe) and manganese (Mn) oxides that are distributed on the slopes of topographic highs such as seamounts, guyots, and plateaus (Bonatti, 1972; Glasby, 2006; Hein et al., 2000, 1987). The growth rate of hydrogenous Fe-Mn crusts is typically 1–10 mm/Myr (Glasby, 2006; Hein et al., 2000), which is much slower than that of other marine sediments. The deposition of the crusts is therefore limited to areas with low rates of detrital and biogenic sedimentation or areas with strong bottom currents that can remove detrital and biogenic sediments (Bonatti, 1972; Glasby, 2006; Hein et al., 2000, 1987). Hydrogenous ferromanganese crusts are also characterized by strong enrichment of transition metals such as Te, Co, and Mo relative to the average upper continental crust (UCC) (Halbach, 1986; Hein et al., 2000, 2003). As most of the enriched elements are incorporated from ambient seawater (e.g., Halbach 1986), thick Fe-Mn crusts have been used as archives for monitoring long-term evolution of seawater chemistry during the Cenozoic (e.g., Frank, et al., 1999, 2002; Nielsen et al., 2009; Reynolds et al., 1999). Paleoceanographic studies using hydrogenous Fe-Mn crusts are largely dependent on the accuracy of the method used to determine the timing and growth rates of the crusts (e.g., Klemm et al., 2005; Nielsen et al., 2011). Hydrogenous Fe-Mn crusts preserve the osmium isotopic http://dx.doi.org/10.1016/j.dsr2.2016.10.010 ⁎ Correspondence to: Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan. E-mail address: k.goto@aist.go.jp (K.T. Goto). Deep–Sea Research II xx (xxxx) xxxx–xxxx 0967-0645/ © 2016 Elsevier Ltd. All rights reserved. Available online xxxx Please cite this article as: Goto, K.T., Deep–Sea Research II (2016), http://dx.doi.org/10.1016/j.dsr2.2016.10.010