Rock magnetic signature of a Miocene playa cycle in Central Asia and environmental implications
Rock magnetic signature of a Miocene playa cycle in Central Asia and environmental implications
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DOI：
10.1007/s00531-019-01761-3
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发表时间：
2019-10
影响因子：
2.3
通讯作者：
V. Verestek;E. Appel;Konstantin Frisch;S. Voigt
中科院分区：
文献类型：
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作者：
V. Verestek;E. Appel;Konstantin Frisch;S. Voigt
中新世中亚干旱化的加剧与该地区湖泊的形成和playa环境的演变时间一致。然而，中新世大陆气候动力学和干旱化强迫仍然没有得到很好的约束。哈萨克斯坦东南部伊犁盆地新近系湖相泥滩沉积提供了一个暴露良好的古气候档案。在此，我们对一个封闭盆地中中新世湖陆旋回进行了详细的岩石磁学研究。利用高分辨率岩石磁性参数、岩性研究和地球化学等手段，重建了该盆地的沉积环境和沉积条件。盐湖旋回的岩石磁性矿物学受赤铁矿和两个细粒磁铁矿相控制。在干燥的泥滩条件下，随着地下水位的降低和干旱程度的增加，磁性浓度会增加。控制观测到的磁浓度变化的潜在过程是沉积期间和沉积后成岩过程的复杂相互作用。数据支持两个磁铁矿相的自生成因，一个形成于沉积固结之前，另一个形成于沉积固结之后。早期成岩作用下微生物作用形成细粒磁铁矿，沉积后形成次生细粒磁铁矿。磁浓度相关参数、ARM/SIRM和s-比值等岩石磁学结果反映了伊犁盆地地下水可利用性和干旱性变化的敏感记录。我们认为，它们不仅可以作为中新世中期伊犁盆地的古环境详细重建的有效代表，也可以作为类似洪泛平原/湖的古环境重建的有效代表。
The increased aridification of Central Asia during the Miocene coincides in time with lake formations and the evolution of playa environments in the region. However, Miocene continental climate dynamics and the forcing of aridification are still not well constrained. Neogene lacustrine mudflat deposits in the Ili Basin in southeast Kazakhstan provide a well-exposed paleoclimate archive. Here, we present a detailed rock magnetic study of a middle Miocene playa cycle deposited in a closed basin. We use high-resolution rock magnetic parameters, lithological studies and geochemistry to reconstruct the playa environment and the depositional conditions. The rock magnetic mineralogy of the playa cycle is controlled by hematite and two fine-grained magnetite phases. Increased magnetic concentrations occur during dry mudflat conditions, with a lower groundwater table and increased aridity. The underlying processes controlling the observed variation in magnetic concentrations are a complex interplay of diagenetic processes during and after deposition. The data support an authigenic origin of both magnetite phases, one formed before and the other after sediment consolidation. Early diagenetic formation of fine-grained magnetite by microbial activity is followed by post-depositional formation of a secondary fine-grained magnetite phase. The rock magnetic results such as magnetic concentration-dependent parameters, ARM/SIRM and s-ratio indicate a sensitive record of (ground) water availability and aridity changes in the Ili Basin. We suggest that they can serve as an effective proxy for detailed paleo-environment reconstruction of playa evolution, not only in the middle Miocene Ili Basin but also in comparable floodplain/playa lake settings.