Toshitsugu YAMAZAKI Professor

Speciality: Paleomagnetism and Rock magnetism, Marine Geology and Geophysics

(1) Paleointensity of the geomagnetic field using marine sediment cores.

Studies on elucidating variations of the ancient geomagnetic field intensity (paleointensity) have greatly advanced since the 1990s using marine sediment cores. The variations during the last about three million years have been clarified so far. This revealed that large paleointensity variations occurred even within a stable polarity period between reversals (Fig. 1). It was also revealed that paleointensity lows correspond to documented geomagnetic excursions, large departure of the geomagnetic poles from the spin axis. The establishment of paleointensity master curves enabled inter-core correlation and age estimation of sediment cores using relative paleointensity variations, called "paleointensity-assisted chronostratigraphy". Furthermore, time-series analyses of paleointensity records suggested existence of the periodicity of Earth's orbital parameters, and the hypothesis on "orbital modulation" of the geomagnetic field was proposed. If this is true, this implies existence of an energy source of the geodynamo outside the core. This is fundamentally important for understanding the geodynamo, and heated debate has continued up to today. A strong opposition to the hypothesis is a possibility that paleoclimatically induced changes in sediment properties as a recorder of the ancient geomagnetic field may contaminate into paleointensity records. Our current studies include quantifying the influence of changes in sediment properties on relative paleointensity records, understanding its physical mechanism, and improvement of the method for relative paleointensity estimations. We are also trying to recover relative paleointensity variations for older ages beyond 3 Ma.

Fig.1 Variations in paleointensity of the geomagnetic field last 800 kyr (Sint-800, Guyodo and
Valet, 1999), and correspondence between paleointensity lows and geomagnetic
excursions (arrow).

(2) Application of rock-magnetic technique to paleoceanography (environmental magnetism)

Paleomagnetism and rock magnetism have wide areas of applications, which include paleoceanography and microbiology as well as geology and geophysics. Environmental magnetism utilizes magnetic minerals as tracers and magnetic properties as proxies for studying paleoenvironment. I am strongly interested in magnetites produced by magnetotactic bacteria. Recently, it becomes apparent that biogenic magnetites, called magnetofossil (Fig. 2), is a major constituent of magnetic mineral assemblages in sediments, more common than previously imagined, and biogenic magnetites are quantitatively important as a carrier of remanent magnetization. Our rock magnetic study (Yamazaki and Ikehara, 2012) revealed that biogenic magnetite and detrital (eolian) maghemite constitute magnetic mineral assemblages in sediments of the Southern Ocean, and the former quantitatively surpasses the latter. We estimated that in the Southern Ocean magnetic mineral concentration was increased by iron fertilization in glacial periods; the production of biogenic magnetites was enhanced associated with increased ocean productivity, which was fueled by increased eolian dust flux.

Fig.2 TEM image of magnetites produced by
magnetotactic bacteria extracted
from sediments in the southern Indian

(3) Moving hotspot and mantle dynamics

A paleomagnetic study of cores from the Emperor seamount chain drilled during the Ocean Drilling Program (ODP) Leg 197 indicated that the Hawaii hotspot moved southward about 15‹ between about 80 and 50 Ma (Tarduno et al., 2003). This means that hotspots may not be fixed to deep mantle as previously thought. A mantle wind model was proposed to explain the Hawaii hotspot motion, which allows mantle plumes to move along with mantle convection. To test the mantle wind model, Integrated Ocean Drilling Program (IODP) Expedition 330 (Dec. 2010 to Feb. 2011) drilled the Louisville seamount trail (Fig. 3), which is another major hotspot track that belongs to the Pacific Plate besides the Hawaii-Emperor chain, and I joined the expedition as one of co-chief scientists. The mantle wind model predicts that latitudinal motion of the Louisville hotspot was small. To determine paleolatitudes of the four seamounts drilled, whose ages range between about 74 to 50 Ma, four paleomagnetists who sailed the expedition are jointly conducting post-cruise measurements and analyses. In addition, I am studying paleointensity of the geomagnetic field in late Cretaceous and early Paleogene using the drilled samples.

Fig.3 Drilling sites of IODP Expedition 330 along
the Louisville seamount trail.

(4) Tectonics of Philippine Sea plate
The Izu-Bonin (Ogasawara)-Mariana (IBM) arc occupying the eastern margin of the Philippine Sea (PHS) plate (Fig. 4) is considered to be initiated about 50 Ma, but it is not yet clear why and how it formed. To understand the initiation processes of oceanic arc, it is necessary to know position, shape, and orientation of PHS plate at that time. Studying the history of PHS plate is also important for better understanding of the tectonics of the southwest Japan and Ryukyu arcs, which has been influenced by the subduction of PHS plate underneath them. It is, however, not easy to elucidate the history of PHS plate, in particular before the beginning of the opening of the Shikoku and Parece-Vela Basins. The reasons of the difficulty include that the most part of the plate is submerged, which hinders paleomagnetic studies based on fully-oriented samples, that there is no hotspot tracks that record plate motion, and that a large part of the plate was already subducted from the Ryukyu trench. To clarify the history of PHS plate, we are conducting bathymetrical and geophysical mappings of the southern part of the West Philippine Basin, where modern data are still scarce, as well as a paleomagnetic study of drill cores from the seafloor of PHS plate.

Fig.4 Bathymetric map of Philippine Sea plate.

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