Takaharu Sato

Quaternary geomagnetic field intensity: Constant periodicity or variable period?

Paleomagnetic results are presented from a core, KH 90-3-5, in the Melanesia basin and are correlated with the previously obtained results from two cores, KH 73-4-7, KH 73-4-8, in the same basin in a period between 0.04 Ma and 1.1 Ma. The sediments in the three cores are composed mostly of calcareous ooze. It has been shown that the variation in the saturation isothermal remanent magnetization (SIRM) in the two cores is caused neither by deposition-rate variation of magnetic minerals nor by rock-magnetic inhomogeneity but almost solely by variations in CaCO3 dissolution. A close similarity between the variation in the SIRM in the two cores and the variation in the initial susceptibility (κ) in KH 90-3-5 has enabled us to correlate among the cores, and also means negligible rock-magnetic inhomogeneity in the core. A time versus depth correlation has been established from the δ18O record of KH 90-3-5. The relative paleointensity record estimated from the natural remanent magnetization (NRM) normalized by κ in KH 90-3-5 and those records in the other two cores are remarkably similar, implying that the records faithfully reflect variations in relative paleointensity of the geomagnetic field for the past 1.1 Ma. We examined on the periodicity of the paleointensity records with wavelets. It is shown that periods of major changes in the records reveal continuous shifts between about 50 Ka and 140 Ka over time. We offer an alternative viewpoint about the periodicity of the paleointensity in which periods of major changes shifted continuously within the time range instead of constant periodicities.

Vertical distribution of sediment phosphorus in Lake Hachirogata related to the effect of land reclamation on phosphorus accumulation

The focus of this work is the change in sediment properties and chemical characteristics that occur after land reclamation projects. The results indicate a higher sedimentation rate in Lake Hachirogata after reclamation, with the rate increasing with proximity to the agricultural zone. In the west-side water samples, higher levels of dissolved total nitrogen and dissolved total phosphorus (DTP) were found in both surface and bottom waters. The increase in P (39–80%) was generally greater than that for N (12–16%), regarding the nutrient supply from reclaimed farmland in the western part of the lake. In the eastern part of the lake, the pore-water Cl− profile showed a decreasing vertical gradient in the sediment core. This indicates desalination of the lake water after construction of a sluice gate in 1961. In the western sediment-core sample, a uniform Cl− profile indicates the mixing of lake water and pore water after reclamation. Considering the sedimentation of P in the last 100 years, there is a trend of increasing accumulation of P and P-activities after the reclamation project. This appears to be an impact from change in the lake environment as a result of increased agricultural nutrients, desalination, and residence. A large amount of mobile phosphorus (42–72% of TP in the western core sample) trapped in sediment increases the risk of phosphorus release and intensification of algal blooms. High sediment phosphorus and phosphorus mobility should be considered a source of pollution in the coastal environment.