A Geomagnetic Paleointensity Record of 0.6 to 3.2 Ma From Sediments in the Western Equatorial Pacific and Remanent Magnetization Lock‐In Depth

Abstract

Information of past geomagnetic intensity variations is important for better understanding of the geodynamo, and efforts to recover continuous paleointensity records continue, in particular for ages older than 2 Ma. In this study, a new relative paleointensity (RPI) record of 0.6 to 3.2 Ma was obtained from a sediment core in the western equatorial Pacific, which has good age control by the oxygen isotope stratigraphy from 2.0 to 3.2 Ma. The RPI record could well be correlated to existing RPI templates despite the low sedimentation rate of ~5 m/Ma and will be useful for future construction of a stacked paleointensity curve beyond 2 Ma. Magnetic minerals of the core consist mainly of biogenic and terrigenous oxidized magnetite. Some rock magnetic contamination to long‐term RPI changes is recognized; RPI inversely correlates with the ratio of anhysteretic remanent magnetization to saturation remanent magnetization, which may be caused by differences in remanence acquisition efficiency between the biogenic and terrigenous magnetic components. The lock‐in depth of the studied core is estimated to be approximately 0 from a line of evidence including no apparent offset between RPI and cosmogenic beryllium nuclide flux changes across some polarity boundaries. Lock‐in depths vary locally from core to core or even within a single core and have no obvious relation with sedimentation rates and carbonate contents. Lock‐in depths may be controlled by factors that are currently difficult to be evaluated, such as variations in sediment flocculation sizes and biogeochemical remanent magnetization dependent to chemical conditions of sediments.

Relative Paleointensity and Inclination Anomaly Over the Last 8 Myr Obtained From the Integrated Ocean Drilling Program Site U1335 Sediments in the Eastern Equatorial Pacific

“…The dominant magnetofossil morphology in the samples is equant octahedra (~80% of the approximately 1,000 particles counted for each sample), and magnetofossils with other morphologies such as teardrops and parallelepipeds are minor. The predominance of octahedral magnetofossils has been reported from pelagic sediments in oxic environments (Sakuramoto et al, 2017;Yamazaki & Shimono, 2013). Lowtemperature magnetic property measurements support that the studied sediments were deposited under oxic conditions ( Figure S2 in the supporting information); the obscured Verwey transition suggests that magnetites have been at least partially oxidized to maghemites.…”

“…Grain‐size changes in the terrigenous components in pelagic environments such as the study area are considered to be small if they occur and thus would not significantly alter the k ARM /SIRM ratios. The k ARM /SIRM ratios of the studied sediments are high among those reported from deep‐sea sediments in the Pacific and Indian oceans (Sakuramoto et al, ; Yamazaki, ; Yamazaki & Ikehara, ) and near those for intact magnetosome chains produced by cultured magnetotactic bacteria (Moskowitz et al, ). The weaker impulse field of 0.8 T for imparting IRM in this study than the 2.5 T used in previous studies might have caused an overestimation of the k ARM /IRM ratio, but the high S ratios ( S −0.3T ) near 1 (Figure S3 in the supporting information) indicate that this effect should be negligibly small.…”

“…Grain-size changes in the terrigenous components in pelagic environments such as the study area are considered to be small if they occur and thus would not significantly alter the k ARM /SIRM ratios. The k ARM /SIRM ratios of the studied sediments are high among those reported from deep-sea sediments in the Pacific and Indian oceans (Sakuramoto et al, 2017;Yamazaki, 2012;Yamazaki & Ikehara, 2012) and near those for intact b and d). The FORC diagrams of all samples show a sharp ridge along the coercivity axis that is indicative of noninteracting single-domain grains, which are considered to represent intact bacterial magnetite chains.…”

“…Comparison of the relative paleointensity (RPI) of Site U1335 from 0 to 3.2 Ma with virtual axial dipole moment (VADM) variations of the PADM2M model (Ziegler et al, ) and other RPI records: the equatorial Pacific stack EPAPIS‐3Ma of Yamazaki and Oda (), the Integrated Ocean Drilling Program Sites U1308 (black, left axis; Channell et al, ) and U1314 (blue, right axis; Ohno et al, ) in the North Atlantic, and core MR1402‐PC1 in the western equatorial Pacific (Sakuramoto et al, ).…”

“…For the last 2 Myr, numerous RPI records have been obtained thus far, and global RPI stacks (Channell et al, ; Valet et al, ) and the axial dipole field model PADM2M that integrate RPI records with absolute paleointensities from volcanic rocks have been proposed (Ziegler et al, ). For the period from 2 to 3 Ma, an increasing number of RPI records have been reported mainly from the North Atlantic and equatorial Pacific (Channell et al, ; Ohno et al, ; Sakuramoto et al, ; Yamazaki & Oda, ), although a global RPI stack has not yet been established. However, available RPI records older than ~3 Ma are still very limited in time and space: two records from the equatorial Pacific spanning from 14.5 to 18.5 Ma (Ohneiser et al, ) and from 17.5 to 26.5 Ma (Channell & Lanci, ), and Eocene to Oligocene records from the equatorial Pacific (Yamamoto et al, ; Yamazaki et al, ) and the South Atlantic (Tauxe & Hartl, ).…”