Paleomagnetic study of ferromanganese crusts recovered from the northwest Pacific — Testing the applicability of the magnetostratigraphic method to estimate growth rate

Scanning SQUID microscopy of a ferromanganese crust from the northwestern Pacific: Submillimeter scale magnetostratigraphy as a new tool for age determination and mapping of environmental magnetic parameters

“…The boundaries between major polarity chrons (i.e., Brunhes normal, Matuyama reversed, Gauss normal, and Gilbert reversed chrons) were primarily used for the correlations (thick dotted lines in Figure ). We were able to assign age control points with higher resolutions than those obtained using sliced samples or the 10 Be/ 9 Be method [ Noguchi et al ., ]. For Chron C2 (1.778 – 2.581 Ma), although the longer “normal” interval of ~16 – 18 mm was correlated to Chron C2n (1.778 – 1.945 Ma) while the shorter “reversed” interval of ~15 – 16 mm was correlated to Chron C2r (1.945 – 2.581 Ma), this may be caused either by contamination of the normal magnetic field originating from strongly magnetized grains with positive magnetic anomalies, positive shift of the baseline, or slower growth rate during the reversed Chron C2r.…”

“…We processed the raw image from each magnetic scan by applying linear drift correction and median filtering (3 × 3 points) (see Figure S3 for details). Rock magnetic properties were measured using samples taken from three sliced samples (depth intervals of 0–1.4, 2.7–3.3, and 27.9–28.3 mm) collected from the Takuyo‐Daigo Seamount, which were used in the previous study by Noguchi et al []. Samples were placed in a 3.0 T magnetic field at 300 K to put saturation isothermal remanent magnetization (IRM), cooled down to 5 K, and warmed up to 300 K while measuring magnetizations with Magnetic Property Measurement System (MPMS, Quantum Design) at the Center for Advanced Marine Core Research, Kochi University, Japan.…”

“…However, the thickness of the slices (2 mm) appeared to be too large to resolve multiple paleomagnetic polarity records in individual slices and as a result gave an inaccurate magnetostratigraphy. Noguchi et al [] conducted paleomagnetic measurements using a series of thinner slices, with thicknesses of 0.5–1.0 mm, for crust samples from five locations in the northwest Pacific (Ryukyu Trench, Ryusei Seamount, Hanzawa Seamount, Takuyo‐Daigo Seamount, and Nosappu Fracture Zone). The magnetostratigraphic method indicated constant growth rates of 1.49, 2.54, 3.56, and 3.67 mm/Ma for four samples, three of which are consistent with those estimated using 10 Be/ 9 Be dating.…”

“…Scanning SQUID microscope (SSM) has been successfully applied in various paleomagnetic studies [e.g., Weiss et al ., , ; Fukuzawa et al ., ]. Here we demonstrate SSM on a ferromanganese crust sample taken from the Takuyo‐Daigo Seamount that was used by Noguchi et al [] for magnetostratigraphic correlation. We also demonstrate the applicability of 2‐D mapping of rock magnetic parameters for environmental magnetic studies with the use of a simple inversion of isothermal remanent magnetization (IRM).…”

“…Geochronology based on cosmogenic nuclides, such as 10 Be/ 9 Be, is increasingly popular [e.g., Nielsen et al, 2011;Nishi et al, 2017;Usui et al, 2017] for age models younger than~15 Ma. In recent years, magnetostratigraphy, which is a completely different geophysical method based on paleomagnetism, has emerged as an alternative method to estimate the age models of ferromanganese crusts [e.g., Joshima and Usui, 1998;Oda et al, 2011;Noguchi et al, 2017].…”