Environmental magnetism and paleoclimatic interpretation of the Sanmenxia loess-paleosol sequence in the southeastern extremity of the Chinese Loess Plateau

Abstract

This is a report of multi-parameter mineral magnetic results of the Sanmenxia loess-paleosol couplets in the SE extremity of the Chinese Loess Plateau (CLP) over the interval of L1-L13 spanning the last 1 Ma. These results show that mass-specific magnetic susceptibility (χ), χ ARM and SIRM and their ratios exhibit parallel stratigraphic variations. Since superparamagnetic (SP) particles have significant contribution to χ but no contribution to remanence, e.g., ARM and SIRM, the strong linear correlations between χ and remanence (e.g., ARM and SIRM) suggest that the pedogenesis-produced magnetic grains are predominantly in small single domain range. Consequently, the significant contribution of SP particles to the bulk χ as supposed in previous studies has been obviously overestimated.χ ARM /χ data in Sanmenxia and median grain size records of typical loess sections from the hinterland of the CLP display a strong resemblance, implying that χ ARM /χ may be regarded as a reliable indicator for monitoring the variations of ferrimagnetic grain sizes. The results of S-ratio and HIRM ((SIRM+IRM −0.3T )/2) measurements indicate that there is significant enhancement of both soft magnetite/maghemite and hard hematite/goethite in paleosols associated with pedogenesis.Presently it is widely accepted that the enhancement of magnetic susceptibility for the Chinese loess-paleosol sequences is primarily due to the formation of ultrafine magnetic particles through pedogenesis [1] . However, there still exist controversies on both the actual contribution of pedogenesis-produced superparamagnetic (SP) particles to the total magnetic susceptibility and the process whereby climate change is encoded in magnetic susceptibility signals [ ] 2-5 . Relying on a specific multiple regression technique to fit the experimental isothermal and anhysteretic remanent magnetization data to corresponding data for synthetic analogues, Maher and Thompson [6] demonstrate that the contribution of magnetite in soils is twice as much as in loess, and over 90% of the susceptibility contrast between soil and loess is due to magnetic grains lying near the SP threshold. Based on low-temperature saturation remanence magnetization analysis, Banerjee and Hunt [7] suggest that the ratios of SP/total saturation remanence at 15 K are as high as about 0.75 in soil but only 0.2 in loess. Results of detailed magnetic susceptibility measurements and grain size analysis by Han and Jiang [8] suggest that the very fine-grained particles (<0.3 μm), which are considered to be of pedogenic origin, contribute little to the bulk magnetic susceptibility, no more than 3%. Recently, Liu et al. [9 11] report thorough enquiries into the grain-size distribution, concentration and enhancement mechanisms of magnetic minerals in Chinese loess by a series of fruitful rock magnetic measurements. These detailed mineral magnetic results from the western Chinese Loess Plateau (CLP) suggest that: 1) single-domain (SD) magnetic particles contribute more than half of the magnetic susceptibi...

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

“…Although the seasonal precipitation possibly shows an increasing trend, the East Asian summer monsoon under its direct influence would manifest a strong overall weakening trend [33]. In addition to the comparison and published environmental magnetic results [14,15,17,18,34,35], non-magnetic evidence from loess sections also clearly indicates the long-term variation trend of the Quaternary climate, for example, strontium isotope records from the Luochuan section [36] and the Jingchuan section [37]. Ao et al [38,39] successfully extracted the severer aridification signal since the Pleistocene period from the fluvial-lacustrine facies sediments in the Nihewan Basin, which also indirectly proved the potential of East Asian continental sediments to reconstruct the contemporaneous global paleoclimate.…”

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

“…The magnetic parameter curves of the paleosol unit S5 in these sections generally display a narrow peak. Compared with other magnetic parameters, the peak value of the χ ARM /χ lf ratio for S5 is not significant, and even lower than those of the paleosol units S6, S13 and S14, probably due to the abundant SP magnetic grains generated in the pedogenesis [47]. As is indicated by the magnetic records, the climate of the study area probably became warm and wet since S6 developed, after experiencing a long-term cold-arid climate during the interval of L9 to S6, and a rapid climate change may occur during the interval of L6 to S5.…”

“…Jingbian [14,44] Jiaodao [13] Luochuan [15,45] Weinan [46] Duanjiapo [45] Yushan (This study) Yanyu [16] Caocun [47] Selected range East Asian summer monsoon increased with its strength being decreased, whereas East Asian winter monsoon progressively intensified 2.6 Ma ago. The differences of climatic trend and transitions recorded in the Lantian loess and Jingbian loess may indicate the complexity of the East Asian monsoon climate evolution on the Loess Plateau.…”

“…The generally small variation of HIRM may just have demonstrated the stability of the absolute content of the coarse-grained antiferromagnetic minerals mainly of a detrital origin. By comparison, the significant transition has not been detected in other contemporaneous parallel records, such as the related magnetic records of the Jingbian [14], Jiaodao [13], Luochuan [45] and Sanmenxia [47] loess sections, and even the Weinan loess section [46]. Only the contemporaneous magnetic record of the Duanjiapo loess section [45] is comparable with that of the Yushan section.…”

“…Significant differences still remain between loess and paleosol in the χ ARM /SIRM and χ ARM /χ lf ratios, the parameters of magnetic mineral grain size, but generally their background values are relatively low. Similarly and comparatively, such stable characteristics have not been revealed in other contemporaneous parallel records, such as the related magnetic records of the Jingbian [14], Jiaodao [13], Luochuan [45], Sanmenxia [47], and Duanjiapo loess sections [45]. The χ lf values of the Weinan loess section [46] may equally indicate that the contemporaneous paleosol units did not experience a strong pedogenesis.…”