16:15 〜 16:30
[SEM13-09] Analysis of magnetic mineral composition of a sediment core taken from the Ontong-Java Plateau and its implications to relative paleointensity estimations
キーワード：古地磁気強度記録、ケイ酸塩包有磁性鉱物、生物源マグネタイト、オントン・ジャワ海台
Marine sediments contain considerable amounts and different types of magnetic mineral particles. Magnetic minerals may be statistically aligned to the direction of the ambient geomagnetic field so that potentially preserve geomagnetic intensity records in the past. However, different types of magnetic minerals should preserve the remanent magnetization in different manners. And the compositional variation of magnetic mineral assemblages in marine sediments may hinder us from extracting reliable geomagnetic paleointensity records.
We studied a sediment core taken from the Ontong-Java plateau and managed to extract the geomagnetic paleointensity variation from it. The magnetic mineral assemblages of the sediment core are principally a two-component mixture of terrigenous and biogenic magnetite with changing amounts of carbonates. So it provides an opportunity to assess influence that compositional variations in marine sediments could bring to relative paleointensity (RPI) estimations and thus to distinguish different contributions of the biogenic and terrigenous components to paleointensity signal recording in marine sediments.
RPI obtained by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) shows downcore decreases, and it has an inverse correlation with the ratio of ARM susceptibility (kARM) to saturation isothermal remanent magnetization (SIRM) (kARM/SIRM). This indicates that the RPI signal became apparently weaker with increasing proportion of biogenic to terrigenous magnetic minerals. Moreover, NRM-ARM demagnetization diagrams show curvature, which indicates that the coercivity distributions of NRM and ARM are different. Therefore, we tried to distinguish the different contributions of the biogenic and terrigenous components to the RPI record by recalculating NRM-ARM demagnetization slopes in relatively higher and lower coercivity intervals. If we assume that the magnetization of the higher coercivity interval is carried by biogenic magnetite while that of the lower coercivity interval is carried by the terrigenous component, RPI recording efficiency of the biogenic component may be lower than that of the terrigenous component. The validity of this assumption was investigated by first-order reversal curve (FORC) measurements, transmission electron microscope (TEM) observations, low-temperature measurements, and extraction of silicate-hosted magnetic inclusion from the sediments, and the results proved that NRM of the higher coercivity interval is carried mainly by biogenic magnetite. But our conclusion contradicts with the previous studies using a similar method; some previous studies suggested higher RPI recording efficiency of the biogenic magnetic component than the terrigenous component. Different concentrations of silicate-hosted magnetic inclusions due to different sedimentary environments might be a possible reason for the contradiction; the contribution of silicate-hosted magnetic inclusions to NRM is minor in our sediments. However, this contradiction remains to be studied further. The goal of our research is to figure out the mechanism how magnetic minerals derived from different sources contribute to remanent magnetizations of sediments. Then, it is expected that influences on RPI estimations brought by compositional variations of magnetic mineral assemblages in marine sediments will be able to be known.
We studied a sediment core taken from the Ontong-Java plateau and managed to extract the geomagnetic paleointensity variation from it. The magnetic mineral assemblages of the sediment core are principally a two-component mixture of terrigenous and biogenic magnetite with changing amounts of carbonates. So it provides an opportunity to assess influence that compositional variations in marine sediments could bring to relative paleointensity (RPI) estimations and thus to distinguish different contributions of the biogenic and terrigenous components to paleointensity signal recording in marine sediments.
RPI obtained by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) shows downcore decreases, and it has an inverse correlation with the ratio of ARM susceptibility (kARM) to saturation isothermal remanent magnetization (SIRM) (kARM/SIRM). This indicates that the RPI signal became apparently weaker with increasing proportion of biogenic to terrigenous magnetic minerals. Moreover, NRM-ARM demagnetization diagrams show curvature, which indicates that the coercivity distributions of NRM and ARM are different. Therefore, we tried to distinguish the different contributions of the biogenic and terrigenous components to the RPI record by recalculating NRM-ARM demagnetization slopes in relatively higher and lower coercivity intervals. If we assume that the magnetization of the higher coercivity interval is carried by biogenic magnetite while that of the lower coercivity interval is carried by the terrigenous component, RPI recording efficiency of the biogenic component may be lower than that of the terrigenous component. The validity of this assumption was investigated by first-order reversal curve (FORC) measurements, transmission electron microscope (TEM) observations, low-temperature measurements, and extraction of silicate-hosted magnetic inclusion from the sediments, and the results proved that NRM of the higher coercivity interval is carried mainly by biogenic magnetite. But our conclusion contradicts with the previous studies using a similar method; some previous studies suggested higher RPI recording efficiency of the biogenic magnetic component than the terrigenous component. Different concentrations of silicate-hosted magnetic inclusions due to different sedimentary environments might be a possible reason for the contradiction; the contribution of silicate-hosted magnetic inclusions to NRM is minor in our sediments. However, this contradiction remains to be studied further. The goal of our research is to figure out the mechanism how magnetic minerals derived from different sources contribute to remanent magnetizations of sediments. Then, it is expected that influences on RPI estimations brought by compositional variations of magnetic mineral assemblages in marine sediments will be able to be known.