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
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
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DOI：
10.1029/2018jb016209
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发表时间：
2018-09
期刊：
影响因子：
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通讯作者：
T. Yamazaki;Y. Yamamoto
中科院分区：
文献类型：
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作者：
T. Yamazaki;Y. Yamamoto
为了理解地球动力学的基本原理，如古强度与极性长度和时间平均场（TAF）结构之间的关系，需要相对古强度（RPI）和倾角异常（ΔI）的连续记录；然而，可获得的~ 3ma以上的记录在时间和空间上仍然非常有限。我们对赤道东太平洋U1335综合海洋钻探计划站点沉积物进行了古地磁研究，获得了~8 Ma以来的连续RPI和ΔI记录。缓慢沉积，~8.4 m/Myr或更低，限制了记录的分辨率，但确实允许确定长期变化。岩磁测量表明，生物成因的磁铁矿在磁性矿物组合中占主导地位，在~4 Ma之前，生物成因与陆源磁性矿物的比例增加。~4 ~ 8 Ma的平均古强度比0 ~ ~4 Ma的平均古强度低约30%。在~4 Ma时，RPI的表观降低约为~50%，但估计其中的~20%是人为的，是由生物成因磁铁矿比例增加引起的。在最后~8 Myr，古强度与极性长度没有关系。在过去的~5 Myr内，在反极性时（4.43°±1.47°），ΔI的量级略大于正极性时（- 0.69°±2.98°），这与该时间跨度的TAF模型一致。在~6 Ma之前，正常时间内ΔI的符号可能会变为正，而在反向时间内ΔI的符号可能会略大于~5 Ma之后的符号。
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.