Recent progress in paleomagnetic studies of marine sediments has revealed long-term (10-100 kyr) variations in geomagnetic field intensity (relative paleointensity). The accumulation of relative paleointensity records has enabled the development of a composite geomagnetic-field intensity stack for time intervals spanning the period from the last few tens of thousands of years to the last few millions of years, and has contributed to establishing an age model for marine sediments. This technique is a powerful tool for synchronizing different geological archives, such as marine sediments and ice cores, by comparing the flux of cosmogenic nuclides. This synchronization is essential for understanding the initiation and propagation of changes in the Earth’s climate system. However, there is some controversy concerning the limits of the use of relative paleointensity records in dating marine sediments. For example, uncertainty may be introduced into the synchronization by the lock-in of a paleomagnetic signal at some depth below the sediment-water interface in marine sediments through the acquisition of post-depositional remanent magnetization (PDRM). This article summarizes the current understanding of the PDRM process and provides examples of relative paleointensity-assisted correlation or dating in marine sediments. Also discussed are possible sources of uncertainty and future prospects for the technique.
The upper part of the Yezo Group, which is widely exposed in southern central Hokkaido, consists of the Saku, Kashima, and Hakobuchi formations, in ascending order. Although the Yezo Group has been well studied in central and northwestern Hokkaido, little is known about the stratigraphy of the group in southern central Hokkaido. To clarify the geological age and microfossil faunal assemblage of the Yezo Group in this region, we studied the geology, litho-biostratigraphy, and microfossil-biostratigraphy of the group in the southern Oyubari and Tomiuchi-Azumi areas. Based on the stratigraphic distribution of benthonic and planktonic foraminifera in the Saku and Kashima formations in the studied areas, we establish four new benthonic foraminiferal zones, as follows (in ascending order): the Haplophragmoides obesus Assemblage Zone (upper Turonian), the Silicosigmoilina futabaensis Interval Zone (upper Turonian? to upper Coniacian), the Nuttallinella florealis-Notoplanulina japonica Concurrent Zone (upper Coniacian to Santonian), and the Lenticulina spp. Interval Zone (lower Campanian). The benthonic foraminiferal assemblage indicates the sequence was deposited at middle to upper bathyal depths from the late Turonian to early Campanian.
A 350.2-m-long sediment core, the Shobu core (GS-SB-1), was recovered from the northern edge of the Omiya Upland, Saitama Prefecture, Central Japan, where the inner part of paleo-Tokyo Bay developed intermittently during the interglacial periods of the middle Pleistocene. Of the nine marine deposits in the core (M1-M9, in descending stratigraphic order), we focus on the M4 and M3 deposits, which are assigned to marine isotope stages (MIS) 11 and 9, respectively, with the aim of inferring the paleoenvironment based on an analysis of fossil ostracodes. Seventy-four ostracode species were identified in these deposits. The M4 deposit contains abundant muddy, enclosed bay species such as Bicornucythere sp. and Nipponocythere bicarinata, and sandy subtidal to sublittoral species such as Loxoconcha optima and Loxoconcha tamakazura. The M4 deposit was divided into lower and upper parts based on ostracode faunal composition and lithology. The paleoenvironment of the lower part was basically subtidal or sublittoral open bay during a transgression. The lower part records at least three cycles of paleoenvironmental change from sandy bay to muddy bay. The paleoenvironment of the upper part was enclosed middle to inner bay during a highstand in sea level. Paleoenvironmental variations were found in the upper part, reflecting changes in the inflow of organic material from the inland areas and changes in the proximity of the source area. The horizon of maximum water depth (20-50 m) is tentatively assigned to the boundary between the lower and upper parts. Neomonoceratina delicata and Bicornucythere bisanensis, the former of which was not found in the M4 deposit, are abundant in the M3 deposit. The paleoenvironment of the M3 deposit was the inner to middle part of an enclosed bay via an open sandy bay. In summary, the paleoenvironment of paleo-Tokyo Bay differed between MIS 11 and MIS 9, as did the ostracode biogeography.
We analyzed the fission-track (FT) thermochronology of zircon in two samples of psammitic schist from the chlorite zone of the Sambagawa metamorphic belt, central Shikoku, Japan. The samples were collected from the lower-grade part of the chlorite zone (pumpellyite-actinolite facies). Detrital zircons from a single locality along the Asemi River are completely annealed, yielding FT ages of 92.6±6.2 Ma (1σ, 13 grains), and those from a single locality along the Dozan River are also completely annealed, yielding FT ages of 47.2±3.8 Ma (1σ, 7 grains). Although both samples were collected from within the chlorite zone, the two FT zircon ages are different, possibly because they represent the timing of peak metamorphism in the former sample, and a D3 thermal event (post peak-metamorphism) in the latter sample.
We obtained a Rb-Sr whole-rock isochron age of 17.7±0.6 Ma from the Nissho Toge granite complex, which intruded the upper portion of a ∼20-km-thick crustal section exposed in the northern Hidaka Mountains, central Hokkaido, Japan. The initial 87Sr/86Sr ratio is 0.70422±0.00009. This age is similar to U-Pb zircon ages reported previously for the Pankenushi gabbro and Magarisawa tonalite (18.5±0.3 Ma and 18.7±0.5 Ma, respectively), which are both located in the lower portion of the crustal section examined in the present study. Our result suggests that almost all of the magmatic bodies in the crustal section were emplaced contemporaneously, during the Early Miocene.