Paleointensity of the geomagnetic field during the last 2.5 million years

自从布朗日(SINT-800)建立了叠加式地磁古地磁强度记录以来，对相对古地磁强度的研究一直致力于获得最近约100KYR的高分辨率记录，如NAPIS-75KYR，并将记录延伸到比布朗日更老的年龄。这项研究旨在阐明3 Ma以来相对古强度的变化。对赤道西太平洋西卡罗林盆地和南太平洋马尼希基高原的6个沉积岩芯进行了古地磁研究。测量包括磁化率、自然剩余磁化强度(NRM)、无滞后剩余磁化强度(ARM)和等温剩余磁化强度(IRM)。对U形通道采集的样品进行去卷积，以提高时间分辨率。根据ARM归一化的NRM强度估计了地球磁场的相对古强度。沉积岩芯的年龄控制主要是通过将磁浓度变化(磁化率和ARM)与南中国海ODP1143站的氧同位素曲线相关联来进行的。采用部分颜色反射率进行相关分析。对于一个岩心，测量了氧同位素比值，并将其用于年龄估计。所有岩心的相对古强度变化在不确定的年龄范围内(几万年)吻合得很好。通过对这些记录进行年龄的小幅调整，我们建立了过去3年的古强度曲线。这种变化的特点是在整个记录过程中强度低的准周期出现，其中许多似乎与以前报道的地磁漂移或极性边界相对应。这些特征类似于在布伦斯时间期间在SINT-800记录中观察到的特征。未观察到“不对称锯齿状”现象。

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.