Paleointensity of the geomagnetic field during the last 2.5 million years

自从Brunhes Chron（Sint-800）期间叠加地磁古强度记录建立以来，相对古强度的研究一直致力于获得最近约10年的高分辨率记录。 100 kyr，例如 NAPIS-75，并将记录扩展到早于 Brunhes Chron 的年龄。本研究旨在澄清 3 Ma 以来的相对古强度变化。对取自西赤道太平洋西卡罗琳盆地和南太平洋马尼希基高原的六个沉积物岩心进行了古地磁研究。这些核心覆盖了最后 1.2 至 3 m。 y。测量包括磁化率、自然剩磁 (NRM)、非滞留剩磁 (ARM) 和等温剩磁 (IRM)。对 u 通道采集的样本应用反卷积以提高时间分辨率。地磁场的相对古强度是根据 ARM 归一化的 NRM 强度估算的。沉积物岩心的年龄控制主要通过将磁浓度变化（磁化率和ARM）与南海1143号ODP站点的氧同位素曲线相关联来进行。部分颜色反射率用于相关性。对于一个岩心，测量了氧同位素比并用于年龄估计。所有岩心的相对古强度变化在年龄的不确定性（几万年）内非常一致。通过叠加这些记录并稍微调整年龄，我们建立了最后 3 m 的古强度曲线。 y。这些变化的特点是整个记录中准周期性出现强度低点，其中许多似乎与先前报道的地磁偏移或极性边界相对应。这些特征与 Brunhes Chron 期间 Sint-800 记录中观察到的特征相似。没有观察到“不对称锯齿图案”。

Since the establishment of a stacked geomagnetic paleointensity record during the Brunhes Chron (Sint-800), researches on relative paleointensity have been directed to obtaining high-resolution records during the last ca. 100 kyr such as the NAPIS-75,and to extending records towards ages older than the Brunhes Chron. This study was aimed to clarify relative paleointensity variations since 3 Ma. A paleomagnetic study was conducted on six sediment cores obtained from the West Caroline Basin in the western equatorial Pacific and the Manihiki Plateau in the South Pacific. These cores cover the last 1.2 to 3 m. y. The measurements include magnetic susceptibility, natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM). Deconvolution was applied for samples taken by u-channels to increase time resolution. Relative paleointensity of the geomagnetic field was estimated from NRM intensity normalized by ARM. Age control of the sediment cores was performed mainly by correlating magnetic concentration changes (magnetic susceptibility and ARM) to the oxygen isotope curve of ODP Site 1143 in the South China Sea. Partly color reflectance was used for the correlation. For one core, oxygen isotope ratios were measured and used for the age estimation. Relative paleointensity variations of all cores agree well within uncertainty of age (a few tens of thousand years). By stacking these records with minor adjustments of ages, we have established a paleointensity curve during the last 3 m. y. The variations are characterized by quasi-periodic occurrence of intensity lows throughout the record, and many of them seem to correspond to previously reported geomagnetic excursions or polarity boundaries. These features are similar to those observed in the Sint-800 record during the Brunhes Chron. The "asymmetric sawtooth pattern" was not observed.