Nano‐Magnetite Aggregates in Red Soil on Low Magnetic Bedrock, Their Changes During Source‐Sink Transfer, and Implications for Paleoclimate Studies
Nano‐Magnetite Aggregates in Red Soil on Low Magnetic Bedrock, Their Changes During Source‐Sink Transfer, and Implications for Paleoclimate Studies
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DOI：
10.1029/2020jb020588
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发表时间：
2020-10
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通讯作者：
Qi Zhang;E. Appel;Shouyun Hu;R. Pennington;Jannik C. Meyer;U. Neumann;M. Burchard;Frederik J. Allstädt;Long-sheng Wang;A. Koutsodendris
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作者：
Qi Zhang;E. Appel;Shouyun Hu;R. Pennington;Jannik C. Meyer;U. Neumann;M. Burchard;Frederik J. Allstädt;Long-sheng Wang;A. Koutsodendris
土壤和湖泊沉积物是重要的古气候档案，通常形成源汇环境。为了更好地了解这种环境下的磁性，我们研究了中国鹤庆盆地草海湖（CL）低磁性基岩和近现代沉积物上的红壤。红土是CL沉积物的唯一重要来源材料，它具有高磁性，磁化率（χ）约为10−5 m3/kg。红壤以成土纳米磁铁矿（~10-15 nm）为主，以~100 nm的聚集体排列，颗粒相互作用导致超顺磁（SP）范围内的宽有效粒度分布，最终形成稳定的单域行为。透射电子显微镜和宽带频率 χ(f) 表明，在红土材料转移到 CL 过程中，聚集体部分分解，纳米颗粒向赤铁矿的转变增加。这将磁畴状态行为转变为更小的有效磁晶粒尺寸，从而导致更低的 χfd% 和 χ 值以及 χ(f) 的特征变化。红壤中团聚体的 SP 稳定单域分布可能与气候有关，饱和剩磁与 χ 之比是反映它的潜在基岩特定古气候代理。 CL 沉积物的磁性由纳米颗粒聚集体和较大尺寸的基岩衍生磁铁矿的组合控制。这些结果挑战了之前鹤庆盆地 168 米长的 Core-HQ（900-30 ka）古气候解释的有效性。聚集体的崩解可能导致具有低 χfd% 的 SP 行为，而单个磁铁矿纳米粒子不会消失，并且基于 χfd% 的 SP 磁铁矿溶解假设可能是错误的。
Soil and lake sediments are important paleoclimate archives often forming a source‐sink setting. To better understand magnetic properties in such settings, we studied red soil on low‐magnetic bedrock and subrecent sediments of Caohai Lake (CL) in Heqing Basin, China. Red soil is the only important source material for the CL sediments, it is highly magnetic with susceptibilities (χ) of ~10−5 m3/kg. The red soil is dominated by pedogenic nano‐magnetite (~10–15 nm) arranged in aggregates of ~100 nm, with particle interaction that causes a wide effective grain size distribution in the superparamagnetic (SP) range tailing into stable single‐domain behavior. Transmission electron microscopy and broadband frequency χ(f) suggest partial disintegration of the aggregates and increased alteration of the nanoparticles to hematite during transfer of red soil material to CL. This shifts the domain state behavior to smaller effective magnetic grain sizes, resulting in lower χfd% and χ values, and a characteristic change of χ(f). The SP‐stable single‐domain distribution of the aggregates in red soil could be climate dependent, and the ratio of saturation remanence to χ is a potential bedrock‐specific paleoclimate proxy reflecting it. Magnetic properties of the CL sediments are controlled by an assemblage of nanoparticle aggregates and larger‐sized bedrock‐derived magnetite. The results challenge the validity of the previous paleoclimate interpretation from the 168‐m‐long Core‐HQ (900–30 ka) in Heqing Basin. Disintegration of aggregates could lead to SP behavior with low χfd% without extinction of individual magnetite nanoparticles, and the χfd%‐based assumption of SP magnetite dissolution may be wrong.