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

Abstract

For understanding the fundamentals of the geodynamo such as the relation between paleointensity and polarity length and time‐averaged field (TAF) structure, continuous records of relative paleointensity (RPI) and inclination anomaly (ΔI) are desired; however, available records older than ~3 Ma are still very limited in time and space. We conducted a paleomagnetic study of the Integrated Ocean Drilling Program 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.

Geomagnetic Field Paleointensity Spanning the Past 11 Myr From Marine Magnetic Anomalies in the Southern Hemisphere

“…Therefore, the correlation between LI ET AL. (Yamazaki & Yamamoto, 2018). our low anomalies and some excursions may indicate that the geomagnetic field intensity was low before and after these excursions over a longer scale.…”

“…At present, the available relatively long, continuous paleointensity records for the past 11 Myr include marine magnetic anomaly records that span the past 5 Myr (Li et al, 2018) and marine sedimentary records for the past ∼8 Myr (Yamazaki & Yamamoto, 2018). Li et al (2018) generated a composite record of magnetic anomaly profiles from the South Atlantic, verified by those from Central Indian, East Pacific and Pacific-Antarctic ridges (Pouliquen et al, 2001) and RPI data of VM93 and PISO-1500 (Channell et al, 2009;Valet & Meynadier, 1993), and identified 14 additional age calibration points (Figure 3).…”

“…For an appropriate comparison, relative paleointensities from sediment records need to be adjusted so that they represent equal time intervals as the magnetic anomaly profiles. We built a model to synthesize a magnetic anomaly profile from the source layer magnetized in the ambient geomagnetic filed of VADMs with the polarity consistent with the GPTS2020 Li et al, 2018), synthetic magnetic profile (green line) of absolute paleointensity calibrated from the relative paleointensity record (API; green dots; Yamazaki & Yamamoto, 2018). The black frames show the boundaries of periods whose names are labeled within them.…”

“…Figure 3. Comparison of our stacked magnetic anomaly record from the three survey areas (1, 2 and 3; red lines) with magnetic anomaly record from the South Atlantic Ridge (SAR; blue line;Li et al, 2018), synthetic magnetic profile (green line) of absolute paleointensity calibrated from the relative paleointensity record (API; green dots;Yamazaki & Yamamoto, 2018). The black frames show the boundaries of periods whose names are labeled within them.…”

A High‐Resolution Paleosecular Variation Record for Marine Isotope Stage 6 From Southeastern Black Sea Sediments

“…1994, Yamazaki et al, 2008) but reaching far back in time, e.g. back to 10 Ma (Yamazaki & Yamamoto, 2018), with sedimentation rates of just some 1 to 2 cm ka -1 . On the other hand, sites closer to the shore such as ODP Site 1233 in the SE Pacific, offshore Chile, can reach sedimentation rates as high as ~175 cm ka -1 (Lund et al, 2006), providing an extremely high-resolution paleomagnetic record, but only for the past about 70 ka.…”