17:15 〜 18:30
[SEM13-P06] Influence of biogenic magnetite on paleointensity estimations inferred from FORC-PCA of sediments from the western equatorial Pacific
キーワード：FORC図、相対古地磁気強度、生物源マグネタイト
Relative paleointensity (RPI) is estimated from sediments as normalized intensity: natural remanent magnetization (NRM) intensity divided by artificial magnetization intensity such as anhysteretic remanent magnetizaton (ARM). This method requires that magnetic particle composition is uniform and hence normalized intensity depends only on their abundance and geomagnetic intensity. Recent studies have shown wide occurrence of biogenic magnetite in marine sediments, which are originated from magnetotactic bacteria with magnetite chains in their cells. Difference of NRM and ARM acquisition efficiencies between biogenic and terrigenous magnetite needs to be understood for RPI estimations.
In this study, a relation between normalized intensity and magnetic mineral compositions was investigated using three sediment cores taken from the western equatorial Pacific, in which the average normalized intensities are largely different. In the western equatorial Pacific, high solar radiation makes sea surface temperature the highest in the world. The variations of precipitation in the western equatorial Pacific are related with the Inter Tropical Convergence Zone, whose southward migration causes increasing precipitation and fluvial discharge in New Guinea. Undercurrents deliver fluvial particles from New Guinea to the studied region. The ratio of biogenic to detrital magnetic particles is expected to vary with the distance from New Guinea, which helps to estimate the relation between normalized intensity and magnetic particle compositions.
First-order reversal curves (FORCs) are measured by sweeping the inside of a magnetic hysteresis loop, and magnetization (M) is given by function of a reversal field (Br) and a given field (B). FORC distribution ρ(B, Br) is defined as a mixed second derivative of M by B and Br, and plotted in the (Bc, Bu) axes to which (B, Br) is rotated −45°, called FORC diagram. Bc and Bu represent coercivity and magnetostatic interactions, respectively. In geological samples, biogenic and terrigenous magnetites are characterized by a narrow “central ridge” along the Bc axis and by a broad distribution with a significant vertical spread (background component), respectively. Bending and collapse of biogenic magnetite chains also contribute to the background component. This study conducted principal component analysis (PCA) of FORC diagrams and decomposed into the biogenic magnetite and the terrigenous input. It is revealed that the ratio of biogenic and terrigenous magnetite influences the normalized intensity. Larger contribution of biogenic magnetite causes lower relative paleointensity. Even when biogenic magnetite dominates, difference in chain bending could influence the normalized intensity. In conclusion, for estimating relative paleointensity, it is necessary to consider the effect of the variations of biogenic and terrigenous contributions and the difference in the preservation of biogenic-magnetite chain structure.
In this study, a relation between normalized intensity and magnetic mineral compositions was investigated using three sediment cores taken from the western equatorial Pacific, in which the average normalized intensities are largely different. In the western equatorial Pacific, high solar radiation makes sea surface temperature the highest in the world. The variations of precipitation in the western equatorial Pacific are related with the Inter Tropical Convergence Zone, whose southward migration causes increasing precipitation and fluvial discharge in New Guinea. Undercurrents deliver fluvial particles from New Guinea to the studied region. The ratio of biogenic to detrital magnetic particles is expected to vary with the distance from New Guinea, which helps to estimate the relation between normalized intensity and magnetic particle compositions.
First-order reversal curves (FORCs) are measured by sweeping the inside of a magnetic hysteresis loop, and magnetization (M) is given by function of a reversal field (Br) and a given field (B). FORC distribution ρ(B, Br) is defined as a mixed second derivative of M by B and Br, and plotted in the (Bc, Bu) axes to which (B, Br) is rotated −45°, called FORC diagram. Bc and Bu represent coercivity and magnetostatic interactions, respectively. In geological samples, biogenic and terrigenous magnetites are characterized by a narrow “central ridge” along the Bc axis and by a broad distribution with a significant vertical spread (background component), respectively. Bending and collapse of biogenic magnetite chains also contribute to the background component. This study conducted principal component analysis (PCA) of FORC diagrams and decomposed into the biogenic magnetite and the terrigenous input. It is revealed that the ratio of biogenic and terrigenous magnetite influences the normalized intensity. Larger contribution of biogenic magnetite causes lower relative paleointensity. Even when biogenic magnetite dominates, difference in chain bending could influence the normalized intensity. In conclusion, for estimating relative paleointensity, it is necessary to consider the effect of the variations of biogenic and terrigenous contributions and the difference in the preservation of biogenic-magnetite chain structure.