[M-ZZ44] Marine Manganese Minerals: Depositional environments, exploration, and development

[MZZ44-02] Age estimate using ultrafine scale magnetostratigraphy on ferromanganese crusts and development of methodology

*Hirokuni Oda¹, Naoto Fukuyo¹, Masanobu Yamamoto², Akira Usui⁴, Teruhiko Kashiwabara³, Jun Kawai⁵, Miki Kawabata⁵ (1.Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 2.Hokkaido University, 3.JAMSTEC, 4.Kochi University, 5.Kanazawa Institute of Technology)

Keywords:ferromanganese crust, scanning SQUID microscope, magnetostratigraphy, age model

Marine ferromanganese crusts are invaluable archive of past environments of the ocean recorded as internal microscopic structures and chemical compositions including isotopes. One of the key parameters is the dissolved oxygen concentrations in deep Ocean water (Yamamoto et al., presented at this conference). For the reconstruction of the environments, it is quite important to develop a reliable age model for each ferromanganese crust. ¹⁰Be dating is one of the reliable isotopic methods, which is effective back to ~15 Myr. Another independent method for age dating is ultrafine scale magnetostratigraphy (e.g. Oda et al., 2011; Noguchi et al. 2017). In order to further promote scientific outputs, a ferromanganese crust sample from Takuyo Daigo seamount in the North Pacific Ocean (NT09-02_Tak5_01_A; water depth 2987m) was measured with scanning SQUID microscope (SSM) at Geological Survey of Japan, AIST. ¹⁰Be dating on the sample suggest average growth rate of ~2.2mm/Myr for this samples (Usui et al., 2017). A thin section from the sample was measured with the SSM after AF demagnetizations at 0, 5, 10, 15, and 20 mT. All the results show magnetic stripes corresponding to geomagnetic reversal boundaries parallel to the growth layers. Preliminary interpretation of the magnetic stripes seems to be consistent with the age model suggested by ¹⁰Be dating. We also applied dipole subtraction due to contaminated magnetic particles and upward continuation on the magnetic image using software ProcSQMicro developed on Igor Pro (Oda et al., submitted). Especially, a new feature implemented to calculate X and Y from Z components of magnetic field based on an algorithm by Lima&Weiss (2009) exhibit enhanced magnetic stripes in X magnetic field image. By obtaining three components of magnetic field, we can calculate the intensity of spatial differential vectors (ISDV; Seama et al., 1993) along a profile perpendicular to geologic layered structures, which is suitable for identification of boundaries of magnetized bodies with oblique magnetizations and/or interfaces. In the presentation, we explore the optimum post processing treatments to further enhance geomagnetic reversal boundaries recorded in the ferromanganese crust sample for the development of reliable age model based on magnetostratigraphy.

Acknowledgements
This study was supported by JSPS KAKENHI Grant Nos. 21H04523 and 23K13188.

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