Abstract
We present the findings from analysis and modeling of a stratigraphic series of magnetic susceptibility data measured with a portable magnetic susceptibility 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 m 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.
Highlights
The objectives of this work are to (1) document Permian climatic variability and assess Milankovitch characteristics using magnetic susceptibility (MS) data from the Salagou Formation loessite obtained with a portable MS meter, (2) leverage laboratory rock magnetic measurements to better understand the origin of the magnetic signal, and (3) compare this upper Paleozoic record with analogous successions from the Eurasian Quaternary record
Average spectral misfit analysis supports the interpretation of the presence of astronomical forcing frequencies, which are statistically significant in the MS series for optimal sedimentation rates of 9.4 cm/kyr and 13 cm/kyr: similar to the Permian of northwest China (∼9–10 cm/kyr) and the western United States (∼8 cm/kyr)
Rock magnetic data from the Salagou Formation indicates magnetic enhancement wherein we interpret that magnetic particles likely formed by pedogenesis, possibly with a diagenetic overprint
Summary
Loess-paleosol sequences of the Chinese Loess Plateau (CLP; thickness of ∼200 m) and other regions (e.g., Li et al, 2019) are widely regarded as high-resolution and continuous continental records of global Pleistocene climate change (e.g., Liu, 1985; Heller and Liu, 1986; Kukla, 1987; Kukla et al, 1988; Kukla and An, 1989; Maher and Thompson, 1991, 1992; Evans and Heller, 2001; Yang and Ding, 2004; Muhs, 2007; Maher and Possolo, 2013; Maher, 2016) This new evidence supports the hypothesis explored in this work for presumably analogous Milankovitch cycles recorded in the thick (∼1000 m) record of upper Paleozoic loess in eastern equatorial Pangea
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