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

Autor: Tatsuo Nozaki, Gen Shimoda, Qing Chang, Kosuke T. Goto, Hiroshi Kitazato, Adolpho Herbert Augustin, Takashi Toyofuku, Koji Kameo, Jun-Ichi Kimura, Katsuhiko Suzuki
Rok vydání: 2017
Předmět:
Zdroj: Deep Sea Research Part II: Topical Studies in Oceanography. 146:82-92
ISSN: 0967-0645
Popis: 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 Sao 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 Sao 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 Sao 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.
Databáze: OpenAIRE
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