Magnetostratigraphy of the ferromanganese crust in the northwest equatorial Pacific

Abstract

In the northwest equatorial Pacific, around 20 °N, a lot of seamount and guyot covered by ferromanganese (Fe-Mn) crust are distributed. In particular, Fe-Mn crusts in this region are mainly observed in water depths of <3,000 m and show high Co contents. Because of the wide surface area (e.g., surface area of MnO2 ~325 m2/g) and slow growth rate (several mm/My), significant quantity of valuable metals are efficiently absorbed from seawater and Fe-Mn crust is regarded as the archive recording a longest history of deep-sea environmental changes since the Late Cretaceous or Paleogene. To decipher the crust growth related deep-sea environmental changes, a high resolution age dating method is essential, which can trace continuous growth rate. Recently, Oda et al. (2011) proposed an ultrafine-scale magnetostratigraphy of Fe-Mn crust with <0.1 mm resolution and the accuracy of <40 ky, using a scanning superconducting quantum interference device magnetometer (SSM). In the present study, we collected Fe-Mn crusts in the Magellan Seamounts (~20 °N) and selected a well-laminated crust with a thickness of ~10 cm. Upper layers of the crust, above the phosphate precipitated layer, was prepared and attempted to trace growth rate using magnetostratigraphy by SSM measurements. Natural remanent magnetization perpendicular to the growth lamella of the Fe-Mn crust shows more than 30 ancient geomagnetic field reversal events. Comparisonh Co contents. Because of the wide surface area (e.g., surface area of MnO2 ~325 m2/g) and slow growth rate (several mm/My), significant quantity of valuable metals are efficiently absorbed from seawater and Fe-Mn crust is regarded as the archive recording a longest history of deep-sea environmental changes since the Late Cretaceous or Paleogene. To decipher the crust growth related deep-sea environmental changes, a high resolution age dating method is essential, which can trace continuous growth rate. Recently, Oda et al. (2011) proposed an ultrafine-scale magnetostratigraphy of Fe-Mn crust with <0.1 mm resolution and the accuracy of <40 ky, using a scanning superconducting quantum interference device magnetometer (SSM). In the present study, we collected Fe-Mn crusts in the Magellan Seamounts (~20 °N) and selected a well-laminated crust with a thickness of ~10 cm. Upper layers of the crust, above the phosphate precipitated layer, was prepared and attempted to trace growth rate using magnetostratigraphy by SSM measurements. Natural remanent magnetization perpendicular to the growth lamella of the Fe-Mn crust shows more than 30 ancient geomagnetic field reversal events. Comparison