Rock Magnetic Cyclostratigraphy of Permian Loess in Eastern Equatorial Pangea (Salagou Formation, South-Central France)
Rock Magnetic Cyclostratigraphy of Permian Loess in Eastern Equatorial Pangea (Salagou Formation, South-Central France)
复制标题
DOI：
10.3389/feart.2020.00241
复制
发表时间：
2020-06
期刊：
影响因子：
--
通讯作者：
Lily S. Pfeifer;L. Hinnov;C. Zeeden;C. Rolf;Christian Laag;G. Soreghan
中科院分区：
文献类型：
--
作者：
Lily S. Pfeifer;L. Hinnov;C. Zeeden;C. Rolf;Christian Laag;G. Soreghan
本文介绍了利用便携式质谱仪对法国中南部二叠纪萨拉古组黄土岩的一系列磁化率（MS）数据进行分析和建模的结果。结果显示，整个萨拉沟组具有明显的米兰科维奇尺度的古气候变化，记录了赤道东泛大陆深时黄土的天文强迫气候变化。最佳沉积速率估计在9.4 cm/kyr（下萨拉沟组）和13 cm/kyr（中上萨拉沟组）之间。存在一个持续的10米厚的周期，可能代表了中晚期（约285-275 Ma）的轨道偏心率尺度（~ 100 kyr）变化。厚度为~ 3.3 ~ 3.5和~ 1.8 m的次高频周期似乎代表了倾角和岁差尺度的变率。如果磁增强的驱动因素是成土作用，那么在剖面上一致的~ 10 m厚旋回可能代表了萨拉沟组黄土-古土壤联的厚度。实验室岩石磁性数据显示，与类似的欧亚第四纪黄土矿床相比，磁性增强一般较低。这与萨拉沟组中赤铁矿占主导地位（信号明显弱于磁铁矿或磁赤铁矿）有关，这可能是由于形成条件不同（例如，同沉积过程，更干旱和/或氧化气候条件），而不是现在的欧亚大陆和/或沉积后磁铁矿和磁赤铁矿的氧化作用。
We present the findings from analysis and modeling of a stratigraphic series of magnetic susceptibility (MS) data measured with a portable MS meter from the Permian Salagou Formation loessite (south-central France). The results reveal discernible Milankovitch-scale paleoclimatic variability throughout the Salagou Formation, recording astronomically forced climate change in deep-time loessite of eastern equatorial Pangea. Optimal sedimentation rates are estimated to have ranged between 9.4 cm/kyr (lower Salagou Formation) and 13 cm/kyr (mid-upper Salagou Formation). A persistent 10-m-thick cyclicity is present that likely represents orbital eccentricity-scale (∼100 kyr) variability through the middle to late Cisuralian (ca. 285—275 Ma). Subordinate, higher frequency cycles with thicknesses of ∼3.3–3.5 and ∼1.8 m appear to represent obliquity and precession-scale variability. If the driver of magnetic enhancement is pedogenic, then the ∼10 m thick cyclicity that is consistent over ∼1000 m of section may represent the thickness of loessite–paleosol couplets in the Salagou Formation. Laboratory rock magnetic data show generally low magnetic enhancement compared to analogous Eurasian Quaternary loess deposits. This is related to the predominance of hematite (substantially weaker signal than magnetite or maghemite) in the Salagou Formation which may be explained by different conditions of formation (e.g., syn depositional processes, more arid, and/or oxidizing climate conditions) than in present Eurasia and/or post depositional oxidation of magnetite and maghemite.