Downward remagnetization of a ∼74-m-thick zone in lake sediments from palaeo-Lake Idaho (NW United States)—Locating the Gauss/Matuyama geomagnetic boundary within a dual-polarity zone
Downward remagnetization of a ∼74-m-thick zone in lake sediments from palaeo-Lake Idaho (NW United States)—Locating the Gauss/Matuyama geomagnetic boundary within a dual-polarity zone
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DOI：
10.1093/gji/ggaa165
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发表时间：
2020-08
影响因子：
2.8
通讯作者：
Frederik J. Allstädt;E. Appel;W. Rösler;A. Prokopenko;U. Neumann;T. Wenzel;J. Pross
中科院分区：
文献类型：
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作者：
Frederik J. Allstädt;E. Appel;W. Rösler;A. Prokopenko;U. Neumann;T. Wenzel;J. Pross
再磁化是古地磁中的一个重要问题。在这里，我们讨论了高斯和松山时之间异常厚（~ 74 m）的双极性过渡带。所研究的演替是从钻芯到古爱达荷湖（美国西北部蛇河平原）的湖相沉积物的演替，湖相沉积物中嵌有玄武岩单元。我们确定了富含钛的碎屑钛磁铁矿和片状出溶体中的磁铁矿是初级剩余物的主要载体，可能来自于蛇河平原喷发的玄武岩。逐步热退磁表明，在过渡区上方和下方分别存在反向和正向极性的单组分剩余磁化。根据岩石磁性结果、显微观察和古爱达荷湖演化过程中已知的事件，过渡带的极性反转成分代表了由磁性矿物蚀变或部分磁铁矿新形成引起的二次化学残余磁化，与原生碎屑磁性矿物的强烈枯竭有关，这种枯竭影响了再磁化事件发生的深度以下的广泛范围。这一再磁化事件很可能与古爱达荷湖水位下降和部分干涸有关。了解再磁化的性质和起源，可以在不寻常的情况下识别极性边界，这种情况下，极性反向的二次磁化在一个序列中向下产生，深度非常大。在已有的年龄信息基础上，观测到的逆转代表高斯/松山边界，为古气候解释演替提供了重要的年龄约束。
Remagnetization is an important issue in palaeomagnetism. Here, we discuss an extraordinarily thick (∼74 m) dual-polarity transition zone between the Gauss and Matuyama Chrons. The studied succession is from a drill core through lacustrine sediments of palaeo-Lake Idaho (Snake River Plain, NW United States of America) that are intercalated with basalt units. We identified detrital Ti-rich titanomagnetite and magnetite in lamellar exsolutions as the main carriers of a primary remanence, likely derived from the basalts that erupted in the Snake River Plain. Stepwise thermal demagnetization revealed a single-component remanent magnetization with reversed and normal polarities above and below the transition zone, respectively. Based on rock-magnetic results, microscopic observations, and previously known events in the evolution of palaeo-Lake Idaho, the reversed-polarity component in the transition zone represents a secondary chemical remanent magnetization caused by magnetic mineral alteration or partial neo-formation of magnetite, in association with strong depletion of the primary detrital magnetic minerals that affected a wide depth range below the level where the remagnetization event occurred. This remagnetization event was most likely related to lake-level lowering and partial desiccation of palaeo-Lake Idaho. Understanding the nature and origin of the remagnetization allows to identify the polarity boundary in the unusual case of a secondary magnetization with reversed polarity produced downward in a sequence to an extraordinary large depth. Based on available age information, the observed reversal represents the Gauss/Matuyama boundary, which provides an important age constraint for palaeoclimatic interpretation of the succession.