Rock‐magnetic artifacts on long‐term relative paleointensity variations in sediments
Rock‐magnetic artifacts on long‐term relative paleointensity variations in sediments
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DOI：
10.1002/ggge.20064
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发表时间：
2013-01
期刊：
影响因子：
3.7
通讯作者：
T. Yamazaki;Y. Yamamoto;G. Acton;E. P. Guidry;C. Richter
中科院分区：
文献类型：
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作者：
T. Yamazaki;Y. Yamamoto;G. Acton;E. P. Guidry;C. Richter
地磁场强度的长期变化，包括可能依赖于极性间隔长度，为理解地球发电机提供了根本上重要的信息。此前，大西洋深海钻探计划站点 522 的渐新世（约 23-34 Ma）相对古强度记录表明，古强度与极性间隔长度之间存在正相关性，这是迄今为止针对该年龄间隔发布的唯一连续古强度数据集。我们对东赤道太平洋三个地点的始新世至渐新世沉积物进行了古地磁研究。我们的目标包括重新审视古强度-极性长度相关性问题。沉积物的磁性符合可靠的相对古强度估计的常用标准。尽管与极性边界和可能的地磁偏移相关的短波长归一化剩磁强度波动在三个地点之间一致，但长期变化不一致。归一化强度与极性长度之间存在明显的正相关关系，但归一化强度与滞回剩磁（ARM）与等温剩磁（IRM）之比存在明显的反相关，其主要受生物和陆源磁性矿物相对丰度的控制。此外，归一化强度与沉降速率相关。这些事实表明归一化强度记录存在岩性污染。对 ARM/IRM 和沉降速率的依赖性在站点 522 也很明显。据推断，沉降速率的变化和生物磁铁矿相对丰度对沉积剩余磁化强度获取效率的影响可能无法通过归一化得到很好的补偿。因此，根据当前可用的标准化强度记录得出在较长极性间隔期间记录到更强地磁场的结论还为时过早。
Long‐term changes of geomagnetic field intensity, including possible dependence on lengths of polarity intervals, provide fundamentally important information for understanding the geodynamo. A positive correlation between paleointensity and polarity interval length was previously suggested from an Oligocene (ca. 23–34 Ma) relative paleointensity record at Deep Sea Drilling Program Site 522 in the Atlantic Ocean, which is the only continuous paleointensity data set published so far for this age interval. We have conducted a paleomagnetic study of Eocene to Oligocene sediments at three sites in the eastern equatorial Pacific Ocean. Our objectives include revisiting the issue of the paleointensity‐polarity length correlation. Magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation. Although short‐wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites, long‐term changes are inconsistent. Apparent positive correlation between normalized intensity and polarity length was observed, but the normalized intensity has an obvious anti‐correlation with the ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals. Furthermore, the normalized intensity correlates with sedimentation rate. These facts indicate a lithological contamination on the normalized intensity records. The dependence on ARM/IRM and sedimentation rate is also evident at Site 522. It is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization. It is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records.