Mid-Late Quaternary loess-paleosol sequence in Lantian’s Yushan, China: An environmental magnetism approach and its paleoclimatic significance

Abstract

The application of rock magnetism methods to investigating the variations of magnetic minerals in the sediments is an important approach to the reestablishment of paleoclimate evolution. Thus we performed fine magnetic measurements on the loess-paleosol sequence (from L15 upwards to S5, in which L is short for Loess, S is short for Paleosol, the same hereinafter) of Yushan stratigraphic section, which is on the southeastern margin of Chinese Loess Plateau, in Lantian County of China's Shaanxi Province, and the thickness of which is ca. 40 m. Our study shows that the primary magnetic carriers of loess and paleosol in this section are magnetite, maghemite, hematite and goethite. Thermomagnetic analyses on the samples of representative horizons show that the higher pedogenesis degree of the sediments, the smaller variations of magnetization there will be before and after heating, probably related to the pedogenic alteration of loess sediments. Analyses of several magnetic parameters show a significant discrepancy between the paleoclimatic conditions recorded in the strata from the loess unit L15 upward to the paleosol unit S5 in the study area and those recorded in the relative strata of other sections on the Chinese Loess Plateau, and those recorded in marine sediments, indicating the great impact of regional geological background. Similarly, the rapid and intensive change recorded in the segment from L15 to S9-1, and the significant difference between the paleoclimate evolutions of the two periods before and after the change (from L15 to S9-1, and from L9 to the base of S6) indicate the strong alteration of magnetic carriers in the study area as a result of the alternations of summer and winter monsoons in East Asia.Loess Plateau, loess, paleosol, magnetic mineral, paleoclimate change Citation:Wu Y, Zhu Z Y, Rao Z G, et al. Mid-Late Quaternary loess-paleosol sequence in Lantian's Yushan, China: An environmental magnetism approach and its paleoclimatic significance.

A rock magnetic record of Asian cooling and aridification processes during 1.95-0.40 Ma in the southeastern Chinese Loess Plateau

“…In addition, the remanent magnetic coercivities of loess samples are larger than those of paleosol samples (Figure 3(b),(d)). Combined with previously reported thermomagnetic curves of loess and paleosol samples [19], these all indicate that the main magnetic carriers in Yushan loess-paleosol section include ferrimagnetic magnetite and maghemite and antiferromagnetic hematite and goethite. In Figure 4, the Day diagram displays that 12 samples are distributed in the range of pseudo single domain (PSD), which indicates that the mean grain size is constant.…”

“…There are no similar characteristics in the records from other typical sections. The low values in the latter two segments (L15 and L9-L6) are closely related to the upper and lower silty loess units (L9 and L15), which might be due to changes of the dust origin and transportation pathways caused by pulsed growth events of the Qinghai-Tibet Plateau [17][18][19][40][41][42]. The magnetic properties of segment L9-L6 show that after L9 deposition, the environment of about 0.32 Myr in the deposition area was in an overall stable condition, a relatively cold and arid climate.…”

“…This section includes the loess-paleosol units from L27 to L5. The units L15-L5 have been described before [19]. The sequence below L15 is usually easy to recognize in the field, especially in the southern Chinese Loess Plateau.…”

“…Warm deep-sea oxygen isotope stages are oddly numbered in "LR04". Part of the magnetic records of 0-38.8 m have been discussed in [19]. layers were marked.…”

“…The experimental method is basically the same as described in [19]. The magnetic parameters are calculated as [21,22]:…”