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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39266609
72347
10.1038/s41598-024-72347-6
Article
Magnetic enhancement in paleosols with hydroclimatic and vegetation cover variabilities (Holocene vs. late MIS 3) in the central Korean Peninsula
Ahn Hyeon-Seon hs.ahn.86@kigam.re.kr

12
Park Sujeong sujeongp@kigam.re.kr

1
Lim Jaesoo 12
1 https://ror.org/044k0pw44 grid.410882.7 0000 0001 0436 1602 Quaternary Environment Research Center, Climate Change Response Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, 34132 Republic of Korea
2 grid.412786.e 0000 0004 1791 8264 Department of Geological Science, University of Science and Technology (UST), Daejeon, 34113 Republic of Korea
12 9 2024
12 9 2024
2024
14 213231 1 2024
5 9 2024
© The Author(s) 2024
2024
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Magnetic susceptibility enhancement (kE) is useful for reconstructing terrestrial paleohydroclimate variabilities. However, kE and its driving process(es) in the Korean Peninsula remain uninvestigated. Therefore, this study investigated two kEs of similar magnitudes, dated MIS 1 (Holocene) and late MIS 3 (~ 29–36 ka), from a paleosol sequence in the upland of paleo-fluvial terrace in the central Korean Peninsula. We observed increased ferri- and antiferro-magnetic mineral components,including ultrafine particles, and stronger chlorite weathering for the two kEs, suggesting pedogenic component predominance. The Fe-bearing (phyllo)silicate weathering mechanism proposed for the Chinese Loess Plateau sequences can explain the pedogenesis-induced kEs for the studied site. Superparamagnetic-domain (SPD) to pseudo-single-domain sized particles of pedogenic magnetite are likely major contributors to kEs. Moreover, we recognized the younger kE interval as more SPD contribution but less in total ferrimagnetic contribution, and more antiferromagnetic contribution than the older ones. The magnetic differences between the periods can result from vegetation cover impact and surrounding hydroclimate conditions, consistent with the recent suggestion for part of the southeast Chinese sites with relatively more rainfall. Consequently, our study provides a baseline for improving the relationship between mineral magnetic signals and local/regional hydroclimatic and environmental variabilities.

Subject terms

Climate sciences
Environmental sciences
Solid Earth sciences
http://dx.doi.org/10.13039/501100003700 Korea Institute of Geoscience and Mineral Resources GP2022-005 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Deciphering hydroclimatic variabilities on diverse spatiotemporal scales over geologically recent periods, including the Holocene1–4, is important for understanding the background of natural variability underlying human influences on hydroclimatic changes5,6, which considerably reshape the environments surrounding human societies, and making efforts to mitigate future hydroclimatic crises.

Natural materials on the Earth’s surface such as soils, sediments, and rocks can preserve information on these paleo-hydroclimatic variabilities because they are in direct contact with climatic conditions during their formation7. These natural samples contain mixtures of magnetic phases potentially of different origins, mineralogical compositions, concentrations, and grain sizes. Many efforts have been made to characterize and quantify the various magnetic mineral components using different magnetic properties [e.g., magnetic susceptibility (k), anhysteretic remanent magnetization, and isothermal remanent magnetization (IRM)] for various topics in paleo-hydroclimatic variability4,8–13.

In particular, paleosols developed in and on loess or fluvio-alluvial sedimentary sequences have received attention as archives recording paleo-hydroclimatic variabilities, where the observation of k enhancement (kE), unraveling its mechanism, and driving hydroclimatic factors have been important keys12–26. The most commonly investigated and typical case is the Chinese Loess Plateau (CLP) paleosol sequence. For the CLP, the kE is observed in paleosols and is generally known to be mainly attributed to in situ formation of nano-sized [corresponding to approximately superparamagnetic-domain (SPD) and single-domain (SD) sized] strongly magnetic (≈ferri- and/or antiferro-magnetic) mineral particles through pedogenesis and/or weathering of the parent loess material14,15,17,22. The k variability has been used as an important proxy for East Asian summer monsoon (EASM; ≈ monsoonal rainfall strength) variability8,15,17,20,27 and is associated with oceanic oxygen isotope variation (the marine oxygen isotope stages, MISs28)14,19,29–31. However, the magnetic mineral types and sizes contributing to such magnetic enhancement remain unclear21,32–41. For the kE and forming magnetic mineralogy, two contrasting, representative mechanisms have been presented: (1) kE linked with soil redox-related formation of Fe2+/Fe3+ ferrimagnet (i.e., magnetite), associated with rainfall (or moisture)21,25,42, and (2) kE linked with formation from ferrihydrite to a maghemite-like phase that can be transformed to hematite (probably through aluminum substitution) upon aging22,36. Recent studies with electron microscopic analyses suggested that the ferrimagnet of the major kE contributor is authigenic (i.e., pedogenic) magnetite (but limited or no maghemite as grains or oxidized rim of the pedogenic magnetite)25,40. These findings strengthen the hypothesis that soil wetting–drying oscillations, correlating to change in monsoonal rainfall, prevailingly drove the kE in well-drained, generally oxidizing soils21,25. On the other hand, it has been reported that, mainly in southern CLP sites with >  ~ 1000 mm/yr in present mean annual rainfall (MAR), soils were magnetically enhanced but k is likely not correlated with rainfall21,43–45. Besides, Hyodo et al.40 found concentrated weathered phyllosilicates that include authigenic particles of not only magnetite but also hematite as a candidate of the significant kE contributor in paleosols. Given that together with another pre-existing hypothesis, chlorite (silicate) weathering of paleosols can result in kE34,36,39,46, it shows a need for further consideration focusing on (phyllo)phyllosilicates (generally paramagnetic) and their weathering. Regarding the magnetic mineral size contributing to kE, even coarser sizes up to a few microns, besides the well-known nanometer scales, in the occurrence of pedogenic ferrimagnetic phase, have been reported in the literature32,37,40. Although sediments and paleosols/soils in and on fluvial or alluvial (or fluvio-alluvial mixed) sedimentary sequences may potentially have more complicated, diverse mechanisms responsible for kE and additional magnetic responses18–20, some of them have been found to have similar kE mechanisms with such pedogenic magnetic mineral predominance evidenced in CLP paleosol sequences23,26. Thus it is generally recognized that magnetic mineral pedogenesis can play an important role in kE responses over different types of parent materials. As the magnetic responses should be differentiated depending on their environments with different geological, geographical, climatic, and vegetative conditions4,12,21,46,47, investigation of previously un-investigated regions in similar ways to the one above may always be helpful.

The Korean Peninsula, situated on the continental margin of East Asia, has been substantially influenced by the East Asian monsoon over the Quaternary period14,48 but would have been under different conditions (e.g., more rainfall and humid) from the continental and monsoonal Chinese regions. On the Peninsula, many upland landscapes and Late Pleistocene‒Holocene flat-lying paleosol sequences have been exposed, including parent sediments of eolian, alluvial, fluvial, or mixed origins49–51. Bulk k data have often been reported for paleosol sequences52–54. Additionally, many paleosol sequences have archeological significance for understanding modern human evolution and settlement history. Over the past two decades, more than a hundred ancient lithic (particularly Paleolithic) sites have been discovered in South Korea, and rich lithic industries have been unearthed55,56. Their archeological significance is postulated to connect hydroclimatic variabilities for the given area at that time2. However, the magnetic mineral particle-forming mechanisms of paleosol sequences in the Korean Peninsula, with clearer identification of contributors to kE and links to paleo-hydroclimatic variabilities, have not been investigated in detail.

Therefore, this study aimed to investigate the kEs obtained in paleosols of the Korean Peninsula in more detail and deepen our understanding of the links between such magnetic records and local/regional hydroclimatic variabilities during geologically recent periods by comparing the well-published cases of CLP sequences.

Study area

The study site, a paleo-fluvial terrace where a paleosol-bearing sequence forms, is situated in Mangwol-dong, Hanam City, Gyeonggi-do Province, approximately 18 km east of the center of Seoul (the west-central part of the Korean Peninsula) (Fig. 1a and b). The site lies near the middle reaches of the Hangang River (one of the largest rivers in South Korea), which flows westward from the Taebaek Mountains to the western coast and discharges into the Yellow Sea (Fig. 1b). Along the middle reach of the river, Quaternary (more specifically, the latest Late Pleistocene‒Holocene) sediments with geomorphological settings of dissected fluvial terraces and alluvial fans are well-developed52,57,58 (Fig. 1c). These Quaternary sediments are surrounded by older basement rocks including Jurassic granites (the Seoul Granite) and a variety of Precambrian gneisses and schists belonging to the Gyeonggi Gneiss Complex59 (Fig. 1c). The mean annual precipitation (MAP) around Hanam City from 1981 to 2010 (30 years; according to the data of Seoul from the service by Korea Meteorological Administration; Accessed 2/2/2022) is approximately 1450 mm/yr (> 1000 mm/yr in mean annual rainfall, MAR) with most precipitation occurring during the summer and early fall seasons (June to September) due to the influence of “Jangma” (a month-long rainy season in which considerably heavy rains are frequent) and typhoons. The mean annual temperature (MAT) from 1981 to 2010 (Korea Meteorological Administration; Accessed 2/2/2022) is approximately 12.5 °C, with seasonal averages ranging from − 0.5 °C during the winter to over 20 °C during the summer.Fig. 1 Maps of the study area with location of the sampling trench site, and photographs of the trenched deposit. (a) Regional map of East Asia with locations of our study area (Hanam City) and reference areas (cited in this study) where records of East Asian summer monsoon (EASM), aridity proxies, and air temperature were previously reported: 1. Dongge Cave with speleothem (EASM62), 2. Hulu Cave with speleothem (EASM63; aridity76), 3. Sihailongwan maar lake (aridity76), 4. Mangshan loess section (air temperature77), and 5. Jingyuan loess section (aridity76). (b) Satellite map image of the sampling site (HNM) and surroundings. Near HNM site, a river (Hanggang River) and several streams (such as Mangwolcheon and Godeokcheon) are flowing. Red star and circle symbols indicate the locations of HNM and two previously investigated sites (Dukso52 and Godeok49) of paleosol sequences (for comparison), respectively. (c) Geological map of the area surrounding the HNM site. The area is same as that shown in (b). The red star and circle symbols are the same as in (b). (d) Photographs of the deposit (the analyzed subject of this study) exposed by a trench investigation at the HNM site, and the sampled vertical profile with lithological subdivision (Units I to V, from top to bottom; see text for details).

Around the study site, Jeong et al.49 reported brownish silty-clayey sequences of eolian origin on colluvial sediments in the Godeok area, Seoul (Godeok site location: 37°33′47.8″ N and 127°09′27.6″ E; Fig. 1b and c). They found narrow ranges of differences in mineralogy, geochemistry, color, grain size distribution, and K–Ar detrital age in multiple sequences with differing geomorphologic settings and bedrock lithology. They also showed eastward trends of increasing weathering degree and fining in grain size and K–Ar detrital age range identical to that of the CLP sequences. Yu et al.52 studied a loess-paleosol sequence in the Dukso area, Namyangju City (Dukso site location: 37°35′35″ N and 127°12′12″ E; Fig. 1b and c), as a recorder of the East Asian monsoon intensity variations. Their k data showed low values in the loess but higher values in the paleosols, resulting in a correlation between the Dukso sequence and the well-established CLP sequence between MIS 7 and 2.

Along the Hangang River, rich and numerous lithic artifacts have been excavated in clayey and/or silty stratigraphic horizons of fluvial and/or eolian origin49,52,56,60,61. These artifacts have archeological significance, for instance, in updating human settlement history with cultural and industrial evolution in South Korea and hominin dispersion in East Asia.

In the study site, an ~ 2.7 m thick sedimentary sequence (37°34′25.0″ N, 127°10′39.8″ E; 21 m altitude at the top; Fig. 1d), dominated by fine-grained (silty and clayey; Fig. 2a–c) sediments with reddish- and yellowish-brown colors, was exposed in an open-air trench site by a trench investigation for archeological excavation61. This sequence is the subject of this study. The features of the sequence are summarized in the Methods. Currently, the sequence is close to two rivers, the Hangang River and the Mangwolcheon Stream (Fig. 1b), and its top surface is positioned at least ~ 15 m higher than the current water level (~ 5.6 m elevation). Recently, an apartment building complex was constructed on the site. The sedimentary sequence of this study was likely deposited by mixed fluvial (or slopewash) and eolian processes, based on the characteristics of the sediment grain size and geomorphological condition of the site. From the bottom up to 1.7 m depth in the sequence (Unit V in Fig. 2), it occurred probably as a transition to a more distal fluvial (or slopewash) environment, and at above 1.7 m depth, the depositional environment probably was relatively stable, sufficient to cause less variability of the sediment grain composition (Fig. 2b–d). The sequence was previously investigated by Park et al.3. Park et al.3 investigated past climate changes during MIS 3 and MIS 1 periods from the same Hanam sequence using total organic carbon (TOC; Fig. 2e) and TOC carbon isotope (δ13C; Fig. 2f) data of bulk sediments along with grain size data (Fig. 2b–d), sedimentary features (Fig. 2a), and age constraints (Supplementary Table S1; Supplementary Fig. S1) via radiocarbon (14C) and optically stimulated luminescence techniques. By comparing with the available records for other areas in South Korea, they concluded that the reconstructed change in vegetation and aridity for Hanam City could be correlated with the EASM intensity variation62,63 and the D-O oscillation64,65 and that the climate responses in South Korea to global climate change possibly have latitudinal differences at millennial timescales.Fig. 2 An array of data profiles as functions of depth for the studied sequence (HNM) in Hanam City. The presented data are those of lithology with unit subdivision, sediment granulometry (clay-, silt-, and sand-size fractions), total organic carbon (TOC) content, carbon isotope (δ13C) of TOC, and radiocarbon (14C) and optically stimulated luminescence (OSL) age estimates from Park et al.3 and low-frequency (kLF), high-frequency (kHF), and frequency-dependent (kFD and kFD%) magnetic susceptibilities (ks), and age-depth relationship curve from this study. (a) Vertical profile photograph and lithological subdivision (Units I to V) of the studied sequence. (b‒d) Relative fractions (in %) of clay, silt, and sand. (e) TOC content (%). (f) TOC δ13C (‰). (g) kLF (in 10−5 SI; black) and kHF (in 10−5 SI; gray). Red circle and triangle symbols represent the individual values in the horizons where further mineral magnetic properties were measured. Two magenta vertical bars represent the intervals of k enhancement (kE) focused on in this study. (h) kFD% (in %; black) and kFD (in 10–5 SI; grass green). The horizons with red circle and triangle symbols are the same as in (g). (i) The previous 14C and OSL age data points with analytical uncertainties3 and revised age-depth relationship model (for details see Methods: Available previous data and chronology). Zones of the lithological unit subdivision are separated by color-shaded boxes and are presented across (a) to (i). (j) kLF, kFD, and kFD% variabilities as functions of age (in ka), converted by the revised age-depth relationship (i), with presentation of the marine isotope stages (MIS 1, 2, and 3)25 and Last Glacial Maximum (LGM)66. Red circle symbols represent the horizons where further mineral magnetic properties were measured. The data presentation is limited to the age range of 0.593–40 ka.

In this study, we used the available age data of Park et al.3 to obtain temporal variations in our measured magnetic properties and TOC and δ13C data to discuss the relationships between our magnetic properties and local and regional hydroclimatic changes during the given period. Here, we developed the age-depth relationship (Fig. 2i) for the Hanam sequence (see Methods for details; the available age data points are listed in Supplementary Table S1, and the results of the age-depth relationship modelling are also introduced in Supplementary Table S2 and Supplementary Fig. S1).

Results

Magnetic susceptibilities (ks)

The various bulk properties of k, i.e., low-frequency bulk k (kLF), high-frequency bulk k (kHF), the absolute frequency dependence of k (the difference between kLF and kHF; kFD), and percentage of kFD (with respect to kLF; kFD%; Fig. 2g–h), are shown as functions of depth in Fig. 2, along with the other available data comprising δ13C; TOC; and relative contents of clay, silt, and sand (from Park et al.3) for comparison. The kLF and kHF (Fig. 2g) had ranges of ~ 65.0–179.9 and ~ 57.8–157.6 ×10−5 SI. The depth-varying profile in kLF (and kHF) exhibited three distinct intervals with kE, i.e., 0.09–0.39, 0.53–0.81, and 1.31–1.61 m depths, with an additional second-order kE interval, 0.97–1.21 m depth (Fig. 2g). The three major enhancements in kLF revealed kLF enhancement of 113%–140% of its respective minimum value, but the second-order kE interval suppressed the enhancement by 39% owing to the relatively high minimum value. The kFD% varied between 5.1 and 20.6 and showed a gradual upward increase throughout the whole sequence with centimeter- and decimeter-scale fluctuations (Fig. 2h).

By using our constructed age-depth relationship for the interval of ~ 0.30–2.10 m depth (Fig. 2i), the temporal variabilities of kLF, kFD, and kFD% were obtained, as shown in Fig. 2j. A period with very low sedimentation rate and/or erosion dominance appeared within MIS 2; in other words, over the last glacier maximum (LGM) period (19–26.5 ka66). We identified two distinct intervals revealing kE during MIS 1 and late MIS 3 (Fig. 2j) corresponding to 0.53–0.81 and 1.31–1.61 m depth, respectively. In this study, we called these two intervals, “kE_MIS1” and “kE_MIS3,” respectively. kE_MIS1 and kE_MIS3 were considered free from the potential effects of human activities on magnetic variabilities. The characteristic features of the two kE intervals are listed in Table 1. The two kE intervals revealed similarities in the depth interval (~ 30 cm), kLF variability range (~ 90 × 10−5 SI), covering temporal duration (~ 7 or 9 kyr), and apparent average accumulation rate (~ 3 or 4 cm/kyr; cf. ~ 20 cm/kyr for a Holocene loess-paleosol sequence at the western CLP, according to Maher et al.10). However, kFD% was higher in kE_MIS1 than in kE_MIS3 by an average factor of ~ 1.4.Table 1 Summarized characteristics of two intervals with magnetic susceptibility enhancement (kE_MIS1 and kE_MIS3) that were focused on in this study.

	kE_MIS1	kE_MIS3	
“Depth interval [cm]”	53–81 (28)	131–161 (30)	
“kLF range [10–5 SI]”	155.9–65.0 (90.9)	164.4–76.0 (88.4)	
“Age range [ka; kyrBP]”	1.716–10.459 (8.743)	28.769–35.868 (7.099)	
“Average accumulation rate [cm/kyr]”	 ~ 3.2	 ~ 4.2	

Moreover, we checked the relationships between kLF and TOC and δ13C in the kE_MIS1 and kE_MIS3 intervals and compared their relationships between the two intervals (Fig. 3). In the kE_MIS1 interval, kLF had statistically significant positive correlations with both TOC (correlation coefficient R = 0.832; Fig. 3a) and δ13C (R = 0.825; Fig. 3b), whereas kLF in kE_MIS3 did not.Fig. 3 Comparison of correlations between kLF and TOC and δ13C for kE_MIS1 (grass green; n = 15) and kE_MIS3 (red; n = 16) intervals. (a) Biplot of TOC vs. kLF. (b) Biplot of δ13C vs. kLF. In (a) and (b), the slope indicates the determined linear regression slope and R indicates the determined correlation coefficient.

Further mineral magnetic properties

Representative subsamples from 11 core horizons (see Methods) were subjected to further magnetic property analyses to determine the origin of the magnetic signals (Supplementary Figs. S2–S4; Figs. 4 and 5), especially by discriminating between those of the kE intervals and the others. All further analyses, including magnetic hysteresis parameter measurements (Supplementary Fig. S2; Fig. 4a), IRM acquisition curves and component unmixing (Supplementary Fig. S3; Fig. 4b–f), and the first-order reversal curve (FORC) diagram (Fig. 5) indicated the dominance of low-coercivity ferrimagnetic mineral phases (such as magnetite and maghemite but not hematite and goethite).Fig. 4 Depth-varying profiles of selected magnetic properties obtained from the hysteresis loop, isothermal remanent magnetization (IRM) progressive acquisition, and IRM unmixing analysis. (a) χhf/Mrs ratio (black) and χhf (magenta). (b) IRM1T intensity. (c) C1% (black) and C1 IRM intensity (magenta). (d) C2% (black) and C2 IRM intensity (magenta). (e) C3% (black) and C3 IRM intensity (magenta). (f) C4% (black) and C4 IRM intensity (magenta). Corresponding kLF, kFD, and kFD% values (11 horizons only) are also shown on the left side of the figure for reference. The two light green shaded boxes represent the two intervals with kE (kE_MIS1 and kE_MIS3) focused on in this study. See text for details.

Fig. 5 Day plot67 of magnetic hysteresis ratios and first-order reversal curve (FORC) diagram results83 from selected subsamples. (a) The biplot of Mrs/Ms vs. Bcr/Bc (Day plot) for the 11 subsamples subjected to the measurements of “Further mineral magnetic properties.” The number adjacent to each data point (squares) represents the sampling depth (in cm). Light green and blue squares indicate the data from the kE_MIS1 and kE_MIS3 intervals, respectively. The reference regions for single-domain (SD), pseudo-SD (PSD), and multi-domain (MD) particles and theoretical curves for mixtures of SD and MD particles (three solid lines with dots) and SD and superparamagnetic domain (SPD; particularly, 10 nm in size) particles (two solid lines with cross symbols) are from Dunlop65,66. (b) FORC diagrams and average horizontal profiles ρ(Bc) of coercivity distribution (at Bu = 0 mT) for four typical subsamples. SF denotes the smoothing factor.

The mass-specific k of non-ferrimagnetic (≈weakly magnetic) fraction χhf and its normalized ratio by saturation remanent magnetization (Mrs) χhf/Mrs obtained from the hysteresis loop (Supplementary Fig. S2) are shown as functions of depth (from the top surface) in Fig. 4a. The χhf/Mrs showed a decreasing upward trend within both kE intervals. Notably, the absolute values of χhf/Mrs were totally higher in the kE_MIS1 interval than in kE_MIS3 interval, and the degree of the χhf/Mrs decreasing upward trend was also higher.

IRM imparted at 1 T of the direct current field (final acquisition step) during the IRM progressive acquisition curve (Supplementary Fig. S3) [IRM1T] is shown as a function of depth in Fig. 4b. In addition, in both kE intervals, the magnetic enhancement of IRM1T was observed. However, IRM1T intensities were lower in kE_MIS1 than in kE_MIS3, unlike the case of k. The IRM component unmixing analysis recognized four magnetic mineral components (referred to as C1, C2, C3, and C4 in ascending order of the median coercivity Bh) with different coercivity spectra in all analyzed subsamples (Supplementary Fig. S3). The Bh and its associated dispersion Dp of the unmixed components C1, C2, C3, and C4 were 3.2–6.1 mT of Bh with 2.2–3.6 mT of Dp; 16.1–26.2 mT of Bh with 2.0–2.2 of Dp; 44.5–131.4 mT of Bh with 1.7–2.1 of Dp; and 174–301 mT of Bh with 1.5–2.3 of Dp, respectively. Four IRM unmixed components were observed in both the parent (or less altered) and paleosol units. The relative proportions (in %; referred to as C1%, C2%, C3%, and C4%) and partial absolute IRM intensities (in units of 10−4 Am2/kg; referred to as C1 IRM, C2 IRM, C3 IRM, and C4 IRM) of the respective components are shown as functions of depth in Fig. 4c–f. C2% was the largest (64–71%) of all analyzed subsamples. Within both kE intervals, increasing upward trends in C1%, C2%, and C4% and decreasing upward trends in C3% were observed. Moreover, C3% and associated kFD% for the two kE intervals had a negative correlation (R = -0.94, n = 6) (Supplementary Fig. S4).

Figure 5 shows the resultant diagrams, which provide information on the particle sizes of the magnetic minerals. The biplot of the combination ratios of the hysteresis parameters (Supplementary Fig. S2), Mrs/Ms and Bcr/Bc, called the Day diagram (Day et al.67; additional interpretations inserted by Dunlop68,69) (Fig. 5a), revealed a differentiation in the effective bulk particle size of subsamples between kE_MIS1 and kE_MIS3 intervals. Although the two groups of data were within the pseudo-SD (PSD) region, the kE_MIS3 data were concentrated close to the SD + multi-domain (MD) curves, but the kE_MIS1 data were clustered on the right side. Figure 5b compares the FORC results from the upper- and lower-part subsamples for the respective kE intervals (HNM-60 cm vs. HNM-80 cm for the kE_MIS1 interval and HNM-150 cm vs. HNM-160 cm for the kE_MIS3 interval). All four FORC diagrams exhibited two Bc peaks at ~ 5 mT and near the origin with vertically suppressed divergence in the contour distribution, where the contours are mostly not closed and intersect the y-axis (Bu axis). These features indicate the presence of SPD–SD transitional (boundary) and PSD-sized magnetic particles13,70,71. However, compared to the two lower-part subsamples, the upper-part subsamples showed a more pronounced peak near the origin in the intensity profile with Bc space (along y = 0), possibly indicating a greater SPD contribution70. This is in agreement with the kFD% variability (Fig. 2h). In addition, the kE_MIS3-belonging diagrams, compared to those belonging to kE_MIS1, have a wider, vertically spreading contour pattern at low Bc levels, indicating a greater predominance of PSD particles.

X-ray diffraction (XRD) analysis

All the analyzed samples (corresponding to the horizons of 45, 55, 65, 75, 85, 115, 135, 155, 175, 195, and 215 cm depths) revealed that quartz is the most abundant (45.6–56.1 wt%), followed by muscovite (13.8–18.7 wt%) and K-feldspar (8.9–17.7 wt%) with minor amounts (generally < 10 wt%) of albite, illite, chlorite, and hematite (Supplementary Table S3). The hematite content (wt %) exhibited an increasing trend within the respective kE intervals, consistent with the variability behavior of the highest-coercivity remanence component, C4. Among the identified minerals, only the wt% of illite showed an increasing trend in both kE intervals. The illite/chlorite (I/C) ratio as a function of the depth is shown in Fig. 6. The I/C ratio was low (0.557–1.046) in non-kE horizons with low k but comparatively higher (0.761–1.357) in the two kE intervals. In each of the two kE intervals, the I/C ratio exhibited an increasing upward trend.Fig. 6 Depth variation in X-ray diffraction (XRD)-derived illite/chlorite (I/C) ratio, together with those of kLF and kFD% and lithology subdivision (Units I to V). Magenta vertical bars and shaded boxes indicate the two kE intervals focused on in this study (kE_MIS1 and kE_MIS3).

Discussion

Our results confirm that the relative contributions of the C1, C2, and C4 components correlate to the kEs of the paleosols is enhanced but that of the C3 component declines (Fig. 4c–f). C1 probably represents magnetite with approximately SPD–SD boundary sizes, which is also confirmed by the FORC diagram contour pattern. The sizes of these contributing ferrimagnetic minerals are considered slightly larger than those of the contributors to the higher kFD% phenomena. C1 is probably of pedogenic origin because of the positive correlation between C1% and kFD%. C2 likely represents pedogenic PSD-sized magnetite13,40. The highest coercivity component, C4, is probably dominated by pedogenic hematite40. C3 is interpreted to represent (partially) oxidized magnetite (i.e., maghemitized magnetite) comprising detrital components, possibly of an eolian origin40,72.

Combined with the kFD% indication, the interpretations of the C1, C2, and C4 pedogenic components suggest that the pedogenic components were predominantly responsible for kEs and further SPD- to PSD-sized (ultrafine and fine) particles of pedogenic magnetite were contributors to kEs. This agrees with previous studies of paleosol sequences for other regions13,23,26,33,35,40,73. This agreement coincides with the recent recognition of the predominant role of similar magnetic mineral pedogenesis in kE mechanism, which is the soil redox-related formation of ferrimagnet, over different types of parent materials23,26. Moreover, our results suggest the presence of pedogenic ferrimagnetic minerals (and antiferromagnetic hematite) in both the kE and non-kE intervals. This implies that the entire studied sequence was formed by an all-time competing process involving the deposition of detrital components and (post-depositional) pedogenesis. This implies that pedogenic magnetic minerals are formed during and shortly after deposition.

Within both kE intervals, the general sense of kFD% enhancement, χhf/Mrs decline, and I/C ratio increase with kE (Figs. 2g and h, 4a and b,6) agrees with one of the magnetic enhancement models in CLP, characterized by the chlorite (silicate) weathering during the soil-forming process that released available Fe and subsequently allowed the production of (ultra)fine-grained strongly magnetic minerals34,36,39,46. The chlorite weathering mechanism plays a more dominant role in the southern part of the CLP, with higher rainfall than in the northern part39,46. This is consistent with the present MAR and MAT of 1450 mm/yr and 12.5 °C on and around our study site. Hyodo et al.40 recently found that weathered phyllosilicates, such as muscovite and chlorite, which contain many elongated, submicron to a few micron-sized (mainly prism-shaped) authigenic magnetite and hematite particles with aspect ratios of >  ~ 4, are the predominant contributors to kE in paleosols of the CLP. In other words, although muscovite and chlorite were observed in both the loessic and paleosol units, weathered magnetite/hematite-bearing phyllosilicates were less abundant in the loessic units. Furthermore, the phyllosilicate-hosted magnetite particles were mainly in the SD and PSD states. In our studied sequences, muscovite and chlorite were observed in both paleosols and others (Supplementary Table S3); however, the current results cannot directly identify such weathered phyllosilicates. Despite the lack of direct identification, we postulate that the occurrence of phyllosilicate-hosted magnetite/hematite particles is another possible kE contributor. The validity of this postulation needs to be confirmed in future research. Through these consideration, we reach an inference that the presence of weathering and pedogenesis of Fe-bearing phyllosilicates (e.g., clay minerals) likely play an important role in the kE of paleosols of the studied sequence.

A major magnetic difference between the two kEs is interpreted to be in the size distribution of the pedogenic ferrimagnetic components: a higher concentration of SPD-sized particles in the kE_MIS1 interval and a higher concentration of particles larger than SPD, capable of carrying IRM, in the kE_MIS3 interval (Figs. 2h, 4b, and 5b). Moreover, kE_MIS1 reveals slightly higher relative contribution of hematite, compared to kE_MIS3 (Fig. 4f). Hu et al.41 observed a similar combination of these magnetic features and proposed their relationships with the soil moisture balance, organic matter, and Al content in a humid environment. This provides additional insights into the link between kEs (via pedogenesis) and hydroclimatic and vegetation conditions for the studied sequence, which is discussed in more detail below.

We determine whether the kEs and associated pedogenesis could be connected with hydroclimatic and vegetation cover proxies at the studied site that currently has a much higher MAP than most CLP sites (Fig. 7). The selected mineral magnetic properties for the studied site as functions of age are shown in Fig. 7a–g. The study site XRD-derived I/C ratio as a function of age is shown in Fig. 7h. Proxies of local and regional referential hydroclimate and vegetation cover (organic matter content) as functions of age are shown in Fig. 7i–n, with the zonation of MISs and LGM (Fig. 7o). Before determining the association, we characterized the local and regional hydroclimatic and vegetation conditions at and around the studied area over past periods corresponding to the two kE intervals, i.e., during the Holocene (MIS 1) and late MIS 3. The local (Hanam) temporal variabilities in vegetation cover (TOC, proposed by Mu et al.74 and Lu et al.75) and aridity (δ13C, proposed by Park et al.3) proxies are shown in Fig. 7i and j. The regional (China) temporal variabilities in aridity76 and EASM intensity62,63 proxies and land air temperature77 are shown in Fig. 7k–m. The insolation variability at 37°N latitude inferred by Laskar et al.78 is shown in Fig. 7n as an additional reference for comparison. Notably, the local aridity variability resembled not only the Chinese aridity variability but also the EASM intensity variability during the Holocene and late MIS 3 (Fig. 7j, k, and l). This resemblance is consistent with the context found by Yang et al.79 that the intensity and location variabilities of the East Asian monsoon rain belt across the CLP could be indicated by the C3/C4 vegetation (δ13C) change.Fig. 7 Comparison of various mineral magnetic properties (kLF, kFD%, IRM1T, {C1 + C2 + C3} IRM, {C1 + C2}%, C2%, C4%, and χhf/Mrs), and XRD-retrieved I/C ratio for HNM with local/regional hydroclimate and environment factors. (a) kLF. (b) kFD in % (kFD%), reflecting relative concentration of pedogenic, ultra-fine (SPD) (ferri-)magnetic components. (c) Total IRM intensity imparted at 1 T direct current field (IRM1T), reflecting total concentration of remanence-carrying magnetic minerals, and IRM intensity of the ferrimagnetic fraction ({C1 + C2 + C3} IRM), reflecting concentration of total ferrimagnetic minerals. (d) Relative contribution of pedogenic ferrimagnetic components ({C1 + C2}%). (e) Relative contribution of pedogenic PSD-sized magnetite (C2%). (f) Relative contribution of pedogenic hematite (C4%). (g) Ratio of mass-specific k of the non-ferrimagnetic (≈ weakly magnetic) fraction to saturation remanent magnetization (χhf/Mrs), reflecting relative concentration of the non-ferrimagnetic (≈ paramagnetic) components. (h) I/C ratio, reflecting the chlorite (silicate) weathering intensity. (i) TOC content at the HNM, indicating the organic matter content/vegetation cover. (j) δ13C at the HNM, indicating the aridity. (k) Regional (China) aridity proxy. (l) Regional (China) EASM intensity proxy (speleothem δ18O). (m) Regional (China) air temperature. (n) Average insolation computed for 37°N latitude. (o) Chrono-climatic division by the globally identified marine isotope stages (MISs) with the Last Glacial Maximum (LGM). The two magenta shaded regions indicate the kE intervals focused on in this study. For reference, the corresponding range of Unit IV is indicated by grayish dashed lines across Fig. 7a to j.

Considering the hydroclimatic and vegetation cover conditions, the kE_MIS1 interval could be correlated with high but temporally decreasing EASM (≈ rainfall) intensity, increasing aridity, consistently high air temperature, and increasing vegetation cover, while the kE_MIS3 interval could be correlated with comparatively high EASM (≈ rainfall) intensity, relatively low aridity, relatively low air temperature, and weak vegetation cover. Our kE_MIS1 and kE_MIS3 magnetic enhancement patterns and ages (Fig. 7a) resemble, for example, those for Holocene soil S0 and late MIS 3 weakly developed soil L1-2 observed at CLP sites by Yang et al.79, but differ from the generally recognized observations of higher k values during every interglacial period (such as MIS 1 and MIS 3; Fig. 7a and o) over the CLP regions14,29–31 and even in the Korean Peninsula52. We also confirm that the kEs are not synchronous with EASM (≈ rainfall) intensity variability (Fig. 7a and l). In other words, the presence of ‘low k’ interval during every interglacial or high EASM period would be problematic in the interpretation of the hydroclimate association. This allows us to consider a possible complexity in the relationship between kE and hydroclimatic variabilities, and/or the possibility of another factor that can contribute considerably to kE, in and around the studied region. Indeed, CLP sites with >  ~ 1000 mm/yr in MAR, mainly in the southern part of the CLP, have been reported to have observations with a similar context: soils were magnetically enhanced but k was likely not correlated with rainfall or temperature21,43–45.

Several studies on the CLP and Tibetan paleosol sequences have proposed a soil water balance or moisture proxy, expressed by a combination of rainfall (e.g., MAR) and potential evapotranspiration (PET), to better decipher the potential pedogenic activity and kE response21,41,78. Hu et al.41 proposed that, in kE, a weaker ferrimagnetic contribution is associated with a relatively higher hematite concentration that was favored for transformation from ferrimagnetic minerals under high soil moisture conditions (high MAR and low PET), and a higher concentration of finer SPD pedogenic particles is aided by a humid environment with more organic matter and higher Al content. Similarly, our kE_MIS1 interval, compared to the kE_MIS3 interval, was characterized by more contribution of pedogenic SPD ferrimagnetic particles, lower total ferrimagnetic contribution, and relatively higher hematite contribution (Fig. 7b, c, and f), accompanied by a relatively stronger EASM (more rainfall) and vegetation cover (higher organic matter content) (Fig. 7l and i). We acknowledge that there may be ambiguity regarding whether the humid environments mentioned in Hu et al.41 and during the kE_MIS1 interval are comparable. Even so, a MAR/PET ratio, as the soil moisture proxy, for the studied site at present can be roughly estimated from the compiled MAR/PET map provided by Orgeira et al.21: the present MAR/PET =  ~ 1.2–1.4, indicating high soil moisture. Therefore, the differences in composition and particle size of magnetic minerals during the MIS 1 and MIS 3 periods (although both periods show similar magnitudes of kE) would result from different degrees of pedogenesis of magnetic minerals controlled by both hydroclimatic and vegetation cover conditions, but probably the potential impact of vegetation cover (especially in condition with low absolute content of organic matter) could be more intense.

On the other hand, we recognize that the age and duration of the kE_MIS3 interval include potentially high uncertainty since its corresponding age-depth relationship relies partly on the single OSL age data point with high analytical uncertainty, although we currently regard the mean OSL age as reliable. Accordingly, another differing age-depth relationship under the determination uncertainty and thus connection between magnetic properties and hydroclimate and vegetation cover conditions may be conceivable. For example, an extreme case of age-depth relationship where the entire Unit IV was supposed to form during ~ 26–30 ka (indicating a higher deposition rate). Then, the kE_MIS3 interval is supposed to correspond to roughly 28.5–30 ka, indicating a very short period around the terminal MIS 3. The corresponding kE_MIS3 period can be characterized by a lower relative concentration of SPD pedogenic particles (Fig. 7b) and a higher relative concentration of total ferrimagnetic particles (Fig. 7c), with entirely low (but slightly increasing) vegetation cover (Fig. 7i) and high EASM intensity excursion (Fig. 7l). Comparison between the kE_MIS1 and kE_MIS3 characteristics allows us to imply that both hydroclimate factor such as EASM intensity and vegetation cover can be related to the kE phenomena, and the magnetomineralogy might be influenced discriminatingly by the EASM intensity and vegetation cover. This implication does not significantly differ from the case of the formal age-depth relationship.

In conclusion, this study provides a novel Korean paleosol example including two kEs (with no human impacts) corresponding to MIS 1 (Holocene) and late MIS 3 (the latest Late Pleistocene), both of which are relatively warm periods with different hydroclimatic and vegetation cover conditions. The two kE intervals represented similarity in kE magnitude but complexity in detailed mineral magnetic responses, which are likely associated with the hydroclimatic and vegetation cover variabilities between them. Our study emphasizes that detailed characterization of various other mineral magnetic properties, in addition to the bulk k measures, allows us to decipher the type, concentration, and grain size of magnetic mineral components and can clarify potential mineral magnetic links to local/regional variabilities represented by combinations of hydroclimatic and vegetation cover conditions. Thus, our findings provide a baseline for deepening our understanding of the magnetic mineral pedogenesis and responses linked to the surrounding hydroclimatic and environmental variabilities in the Korean Peninsula (and potentially southeast China). Additionally, our results will serve as valuable basic data for research investigating whether Fe-bearing phyllosilicates are a key player in the kE mechanism.

Methods

Description of sedimentary features and sampling

The studied sequence, according to a previous geological survey (Won et al.59; Fig. 1c), was deemed a diluvial deposit. The sequence was likely underlain by a gravel-rich layer, presumably of fluvial origin, given its identification at similar stratigraphic levels in some of the other neighboring trench sequences (this was not directly observed at the studied trench due to the depth limitation of the trench investigation; the Report on the excavation of Misa-dong Paleolithic sites in Hanam City, 2014). The sequence could be divided into five sedimentary units (Fig. 1d) based on color, grain size, and sedimentary features (Figs. 2a–d): from top to bottom, (I) 0–0.45 m depth interval, characterized by pale yellow to pale yellowish brown, silt and clay-dominated sediments; (II) 0.45–0.90 m depth interval, characterized by yellowish brown, silt and clay-dominated sediments; (III) 0.90–0.95 m depth interval, characterized by pale yellowish-colored, silt and clay-dominated sediments; (IV) 0.95–1.70 m depth interval, characterized by yellow–brown, silt and clay-dominated sediments, with vertical cracks concentrated at the upper part; and (V) 1.70–2.70 m depth interval, characterized by brown or darker brown, silty/clayey sand sediments. The vertical cracks in Unit IV formed reticular networks in the horizontal planes and were filled with pale yellowish fine-grained materials, similar to the overlying unit (Unit III). In Unit V, the sand size fraction peaked in the lowermost 30 cm and then decreased upward. Above 1.70 m depth, the grain size fractions were less variable, and the proportions of grains of ≤ 2 mm (silt + clay) were larger than 80%. Units II and IV were regarded as paleosols based on visual inspection. Unit V seemed to be largely of fluvial origin.

In the field work, sediment samples were collected using nine stainless steel boxes of 30 cm in length, after removing a few centimeters of the exposed surface, throughout the 2.7 m-deep vertical section. In the laboratory, all box samples were subsampled at 2 cm intervals, and the 2 cm-interval subsamples were air-dried, disaggregated, homogenized using hand, and placed in glass vials for k measurement and storage prior to further magnetic property measurements. For magnetic measurements using an alternating gradient field magnetometer (AGM), subsample subsets of approximately a dozen milligrams each was prepared and tightly packed using aluminum foil. All magnetic measurements were conducted at the Paleomagnetism Laboratory of the Korea Institute of Geoscience and Mineral Resources (KIGAM), South Korea.

Available previous data and chronology

Data on the respective contents of clay, silt, sand, TOC, and δ13C for sediments with different stratigraphic horizons from the studied sequence have been reported by Park et al.3 (Figs. 2b–f).

The chronology of the studied sequence was based on twelve radiocarbon (14C) and one optically stimulated luminescence date (Park et al.3; Supplementary Table S1). An age-depth relationship was provided using these median age data by simple linear interpolations3. Here, the age-depth relationship was developed by applying a statistical analysis technique considering age uncertainties and IntCal2080 in the 14C calibration of 14C dating via Undatable software81. Detailed information on our age-depth modeling procedure and a comparison of age-depth models between that of Park et al.3 and this study is given in Supplementary Materials, Supplementary Table S2, and Supplementary Fig. S1. The age-depth relationship indicates sequence formation from the Late Pleistocene to the Holocene, corresponding to the period ranging from MIS 3 to MIS 1, with very low sedimentation rates or occurrence of erosion-dominant environments during the majority of MIS 2.

Magnetic susceptibility measurements

We measured volume-specific k using an MS2 k meter with an MS2B sensor (Bartington Instruments, UK). Each measured k value was corrected by subtracting the constant k of the glass vial and multiplying it by the ratio of the calibration/effective volume (i.e., 10 cm3 for MS2B) to the subsample volume, which was calculated by assuming a cylinder as the subsample-filling shape and thus using measures of the bottom diameter (constant) and filling height (variable between subsamples). The k measurements for individual subsamples were made at 0.47 kHz (low-frequency) and 4.7 kHz (high-frequency). We named the corrected low- and high-frequency k as “kLF” and “kHF,” respectively (in units of 10−5 SI). Each reported k value (Fig. 2g) represented the average of the corrected individual values derived from three measurements. We also calculated two frequency-dependent susceptibility measures: kFD and kFD%. They are expressed as kFD = kLF − kHF (in 10–5 SI) and kFD% = (kLF − kHF)/kLF × 100 (in %), which are used as a measure for the relative contribution of SPD ferrimagnets11. Sets of kLF, kHF, kFD, and kFD% data were obtained from 134 subsamples (corresponding to 134 individual horizons).

Further magnetic property measurements of selected samples

For each of the subset of subsamples from 11 selected horizons (0.50, 0.60, 0.76, 0.80, 0.90, 1.00, 1.34, 1.50, 1.60, 2.10, and 2.30 m depths), magnetic hysteresis loop, backfield demagnetization of saturation isothermal remanent magnetization (IRM), progressive IRM acquisition, and FORCs were sequentially measured using the AGM-setup machine of a MicroMag Model 3900 dual-head AGM/VSM (vibrating sample magnetometer) system (Princeton Measurements Corporation, Princeton, New Jersey; now Lake Shore Cryotronics, Westerville, Ohio). Each hysteresis loop was measured by applying magnetic fields up to 0.5 T, and the measured data were corrected for para‐ and dia-magnetic contributions using a high‐field slope correction. This allowed the determination of saturation magnetization (Ms), saturation remanent magnetization (Mrs), coercive force (Bc), and high-field k [χhf, an estimate of non-ferrimagnetic (approximately paramagnetic) susceptibility]. Each backfield demagnetization of saturation IRM (SIRM) determined the coercivity of remanence (Bcr). Ms, Mrs, Bc, and Bcr are the hysteresis parameters. Each progressive IRM acquisition was performed in applied direct current fields of up to 1 T with 50 incremental field steps. IRM acquisition data were used for unmixing (ferri- and antiferro-) magnetic mineral components in the coercivity space. This unmixing analysis was performed using the MaxUnmix software82, which obtains the best fit with the total IRM acquisition using the fewest possible numbers of components characterized by the SIRM; the peak field Bh at which half of the SIRM is reached; and the dispersion Dp of its corresponding cumulative lognormal distribution. For each FORC measurement, 169–338 FORCs were made with an average time of 0.2 s per data point, and the measured data were processed to produce the FORC diagram using FORCinel83. The FORC diagrams were smoothed using a simple smoothing functionality with a smoothing factor of 6 or 8, which were chosen based on exploration of the optimal smoothing factor through the operation of a FORCinel-embedded function.

XRD measurements

For the 11 selected horizons (45, 55, 65, 75, 85, 115, 135, 155, 175, 195, and 215 cm depths), each bulk sediment sample was finely ground and then subjected to XRD analysis using a Rigaku (Japan) SmartLab X-ray diffractometer at KIGAM to determine the major mineral components and their compositions. These analyzed horizons involved two distinct intervals with kE and those below and above each kE interval. Cu-Kα radiation was used with a tube voltage of 40 kV and a current of 40 mA. The scan range was 3–65° (2θ) with an interval of 0.01°. Mineralogy quantification from the XRD data for each sample was performed using SIROQUANT v3.0. The identified mineral components were quartz, muscovite, K-feldspar, albite, illite, chlorite, and hematite. The magnetite (and maghemite) crystal structure data were not set as input sources for the determination of the crystalline phases and their compositions (wt%). From these XRD-quantified data, we calculated an index for the weathering degree of chlorite (silicate), I/C ratio46, where a high I/C ratio can be correlated to strong chlorite weathering in paleosols, especially of southern Chinese loess-paleosol sections that have high mean annual precipitation (> ~ 600 mm) and temperature (> ~ 10 °C).

Supplementary Information

Supplementary Figure S1.

Supplementary Figure S2.

Supplementary Figure S3.

Supplementary Figure S4.

Supplementary Legends.

Supplementary Table S1.

Supplementary Table S2.

Supplementary Table S3.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-72347-6.

Acknowledgements

This work was supported by the Basic Research Projects of KIGAM, funded by the Ministry of Science and ICT, Republic of Korea (GP2022-005 and GP2022-006). We would like to thank Editage (www.editage.co.kr) for English language editing.

Author contributions

S.P. and J.L. initiated this study. H.-S.A. developed this study. S.P. collected samples. H.-S.A. and S.P. conducted measurements and analyses. H.-S.A. wrote the manuscript. All authors contributed to data interpretation and approved the final manuscript for publication.

Data availability

The datasets generated and/or analysed during the current study are included in this published article (and its Supplementary Information files) or are available from the corresponding authors on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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