Quaternary geomagnetic field intensity: Constant periodicity or variable period?

Abstract

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.