[S-EM15] 地磁気・古地磁気・岩石磁気

[SEM15-P02] Magnetic mineral assemblages of a marine sediment core subjected to reductive diagenesis and their contributions to paleomagnetic signals

*李 嘉熙¹、山崎 俊嗣¹、佐藤 雅彦²、黒田 潤一郎¹ (1.東京大学 大気海洋研究所、2.東京大学 理学系研究科 地球惑星科学専攻)

キーワード：堆積物古地磁気、還元性続成作用、古地磁気記録、ケイ酸塩包有磁性鉱物、オントン･ジャワ海台

Diagenesis is ubiquitous in marine sediments, causing sedimentary iron-bearing minerals to undergo a series of redox reactions until they reach equilibrium with reactive chemical components in sediments. Paleomagnetic records in sediments subjected to severe diagenesis may be distorted or lost due to iron mineral dissolution. Some magnetic minerals like silicate-hosted magnetic inclusions can survive the diagenetic iron mineral dissolution and are widely found in marine sediments. Thus, they provide the possibility of preserving paleomagnetic records in sediments subjected to reductive diagenesis. To better understand this issue, we studied on a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. Rock magnetic measurements indicate that severe diagenetic iron-mineral dissolution occurred below about 6 m in depth in the studied sediments, where natural remanent magnetization (NRM) intensity is about 10% of that above 6 m. However, information on paleomagnetic declination and paleointensity could still be recovered. Silicate-hosted magnetic inclusions were separated from bulk sediments by chemical procedures. They contribute about 50% or more of saturation isothermal remanent magnetization (SIRM) in the reduced sediments. High coercivity hematite may be another major remanence carrier in the reduced sediments as indicated by rock magnetic and IRM decomposition results. Transmission electron microscope and scanning electron microscope observations provide direct evidence for the presence of magnetic inclusions and hematite in the reduced sediments. NRM acquisition efficiency of magnetic inclusions will also be discussed based on grain size analysis results.