[S-EM18] Geomagnetism, paleomagnetism and rock magnetism

[SEM18-06] Paleomagnetic study of the IODP Site U1335 sediments in the eastern equatorial Pacific - relative paleointensity and inclination anomaly over the last 8 Myr

*Yuhji Yamamoto¹, Toshitsugu Yamazaki² (1.Center for Advanced Marine Core Research, Kochi University, 2.Atmosphere and Ocean Research Institute, University of Tokyo)

Obtaining continuous records of relative paleointensity (RPI) and inclination anomaly (ΔI) is inevitable to understand the fundamentals of the geodynamo, but available records older than ~3 Ma are still very limited in time and space. We performed a paleomagnetic study of the Integrated Ocean Drilling Program (IODP) Site U1335 sediments in the eastern equatorial Pacific to obtain continuous RPI and ΔI records since ~8 Ma. Slow deposition, ~8.4 m/Myr or less, limits the resolution of the records but did allow for determination of long-term variations. Rock-magnetic measurements showed that biogenic magnetite dominates the magnetic mineral assemblages, and the proportion of biogenic to terrigenous magnetic minerals increases prior to ~4 Ma. The average paleointensity between ~4 and 8 Ma is approximately 30% lower than that from 0 to ~4 Ma. The apparent reduction of RPI at ~4 Ma reaches approximately ~50%, but ~20% of this is estimated to be artificial, induced by the increase in the proportion of biogenic magnetite. No relation between paleointensity and polarity length is recognized for the last ~8 Myr. The magnitude of ΔI is slightly larger during reversed polarity chrons (4.43° ± 1.47°) than normal polarity chrons (0.69° ± 2.98°) over the last ~5 Myr, which agrees with the available TAF models of this time span. Prior to ~6 Ma, the sign of ΔI during the normal chrons might have switched to positive, and ΔI during reversed chrons might have been slightly larger than that after ~5 Ma.