Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments
Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments
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DOI：
10.1029/2022jb024387
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发表时间：
2022-05
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通讯作者：
Jiaxi Li;T. Yamazaki;Y. Usui;T. Sagawa;Y. Kubota;J. Kuroda
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作者：
Jiaxi Li;T. Yamazaki;Y. Usui;T. Sagawa;Y. Kubota;J. Kuroda
海洋沉积物可以保存连续的古地磁强度记录。不同的磁性矿物可能获得不同的剩磁，因此沉积物中磁性矿物组合的成分变化可能会阻碍可靠的相对古强度（RPI）记录的提取。为了更好地了解这个问题，我们对赤道西太平洋翁通爪哇高原的沉积物岩心进行了古和岩石磁研究。通过用非滞留磁化强度 (ARM) 对自然剩磁化强度 (NRM) 进行归一化估计的 RPI 会降低下芯，并与 ARM 磁化率与饱和等温剩磁化强度的比率成反比。这种关系表明，随着生物磁铁矿比例的增加，RPI 信号减弱。获得的NRM-ARM退磁图具有下凹曲率。这些观察结果表明，如果我们假设高矫顽力窗口和低矫顽力窗口的磁化强度分别主要由生物成分和陆源成分携带，则生物成分的 RPI 记录效率低于陆源成分的 RPI 记录效率。这一假设得到了一阶反转曲线测量、透射电子显微镜观察、低温测量以及从沉积物中提取硅酸盐磁性包裹体的支持。先前的研究表明生物磁铁矿的RPI记录效率高于陆源成分的RPI记录效率，这与我们的结果不一致。不同沉积环境中硅酸盐磁性包裹体的不同浓度可能解释了这一矛盾。我们得出的结论是，生物磁铁矿对 RPI 记录的贡献率低于所研究沉积物中未受保护的陆源磁性矿物。改变生物磁铁矿比例会扭曲 ARM 标准化 RPI 估计。
Marine sediments can preserve continuous paleomagnetic intensity records. Different magnetic minerals may acquire remanent magnetizations differently, so that compositional variations of magnetic mineral assemblages in sediments may hinder extraction of reliable relative paleointensity (RPI) records. To better understand this issue, we conducted a paleo‐ and rock magnetic study of a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. RPI estimated by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) decreases downcore with an inverse correlation with the ratio of ARM susceptibility to saturation isothermal remanent magnetization. This relationship indicates that the RPI signal weakens as the proportion of biogenic magnetite increases. The NRM–ARM demagnetization diagrams obtained have concave‐down curvature. These observations indicate that the RPI recording efficiency of the biogenic component is lower than that of the terrigenous component if we assume that the magnetizations of the high‐ and low‐coercivity windows are carried dominantly by biogenic and terrigenous components, respectively. This assumption is supported by first‐order reversal curve measurements, transmission electron microscope observations, low‐temperature measurements, and extraction of silicate‐hosted magnetic inclusions from the sediments. Previous studies have suggested that the RPI recording efficiency of biogenic magnetite is higher than that of the terrigenous component, which disagrees with our results. Different concentrations of silicate‐hosted magnetic inclusions in different sedimentary environments might explain this contradiction. We conclude that biogenic magnetite contributes to RPI records with lower efficiency than unprotected terrigenous magnetic minerals in the studied sediments. Changing biogenic magnetite proportions distort ARM‐normalized RPI estimations.