Loess-soil sequences in southern Anhui Province: Magnetostratigraphy and paleoclimatic significance

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1 Chinese Science Bulletin 2003 Vol. 48 No Loess-soil sequences in southern Anhui Province: Magnetostratigraphy and paleoclimatic significance QIAO Yansong 1, GUO Zhengtang 2,1, HAO Qingzhen 1, WU Wenxiang 1, JIANG Wenying 1, YUAN Baoyin 1, ZHANG Zhongshi 1, WEI Jianjing 1 &ZHAOHua 1 1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing , China; 2. SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi an , China Correspondence to Guo Zhengtang ( ztguo@95777.com) Abstract Two parallel loess-soil sequences from Xuancheng and Fanchang in southern Anhui Province are dated using geomagnetic and luminescence methods. The Brunhes/Matuyama (B/M) reversal boundary is recognized within the lower part of the so-called Vermiculated Red Soil (VRS) in the Xuancheng section while the entire Fanchang sequence is of Brunhes age. This indicates that the most recent VRS in southern China, a stratigraphic marker and an indication of extremely warm-humid conditions, was formed during the middle Pleistocene, chronologically correlative with the S4 and S5 soil units in northern China. Microscopic and sedimentologic investigations reveal that eolian deposition started in this region at about 0.85 MaBP, roughly synchronous with the well-known Mid-Pleistocene climate change of global significance. The strengthening of both summer and winter monsoon circulations and the consequent river hydrological changes at that time would have provided favorable conditions for sustained eolian deposition in the middle-lower reaches of the Yangtze River since 0.85 MaBP. Keywords: loess, vermiculated red soil, eolian deposit, magnetostratigraphy. DOI: /03wd0183 The loess-soil sequences in the middle-lower reaches of the Yantgze River are among the best geological records of Quaternary environment that bear information of climate changes for the subtropical zone of China. Type sections containing eolian deposits in this region mostly consist of three parts. They are the upper loess-soil sequence referred to as Xiashu Loess Formation [1], the so-called Vermiculated Red Soil (VRS), a widely used stratigraphic marker at the middle part, and the underlying fluvial deposits or bedrocks. Sedimentological [2 4] and geochemical [2] studies have confirmed the eolian origin of the Xiashu Formation. Part of the parent material for the VRS may have also been derived from eolian deposits [5 8]. Up to date, chronological investigations on these sequences have mostly been concentrated on the Xiashu loess [2,9,10], mainly due to the restriction of the luminescence dating techniques. Although electron spin resonance (ESR) dating was applied in the Xuancheng section [11] and magnetostratigraphic measurements were carried out on a few sections from Jiujiang and Xiushui in Jiangxi Province [11 14], a spatial correlation of stratigraphy and detailed paleoclimatic approaches would require more magneto- and pedo-stratigraphic constraints on these eolian deposits. Correlation with the loess stratigraphy in northern China also needs further efforts. The Xuancheng section is also a paleolithic site in southern China [8]. Similar to the other sections in the middle-lower reaches of the Yangtze River, it is lithologically composed of three parts: the Xiashu loess at the upper part containing three clearly definable soil layers, the VRS at the middle part and the underlying fluvial sediment. Earlier ESR dating [11] suggested a basal age of about 0.8 Ma for the VRS. This section was also tentatively correlated with the loess stratigraphy in northern China [15]. However, magnetostratigraphy has not yet been carried out that would be particularly helpful to a comprehensive chronostratigraphic framework. In this paper, two eolian sequences, one from Xuancheng (118º51hE, 30º54hN) and another from Fanchang (118º09hE, 30º03hN) in southern Anhui Province, are studied based on detailed magnetostratigraphic and sedimentologic analyses. The main objectives are to determine the age of the onset of eolian deposition in this region and that of the VRS formation. The paleoclimatic implications of these events are also discussed. 1 Lithostratigraphy, sampling and analytical methods The Xuancheng section, 11 m thick, was developed on the second terrace (T2) of the Shuiyang River, a branch of the Yangtze River. It consists of 10 field definable lithological units according to their color, structure and interlayer transitions (Fig. 1(a)). From the top to the bottom, these include a cultivated soil (unit 1), the so-called Xiashu loess (units 2 8), the VRS (unit 9), and the sandy-gravel deposits (unit 10). The Fanchang site is located near Chen Village of Fanchang County. The 6.8-mthick section is situated on the first terrace (T1) of the Huanghu River, another branch of the Yangtze River. This section was lithologically divided into 8 units, and the correlation with Xuancheng is shown in Fig. 1. The upper part of the fluvial sandy deposits in both sections was significantly weathered. 96 and 62 oriented paleomagnetic samples were taken at cm intervals from Xuancheng and Fanchang, respectively. Measurements were carried out in the Paleomagnetism Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences. Stepwise thermal demagnetization up to 675k was performed using a MMTD600 Thermal Demagnetizer on 61 samples from Xuancheng and 43 samples from Fanchang with a 2088 Chinese Science Bulletin Vol. 48 No. 19 October 2003

2 Fig. 1. Lithostratigraphy and magnetostratigraphy of the Xuancheng and Fanchang sections. 1, Cultivated soil; 2, loess; 3, nonvermiculated palosol; 4, vermiculated red soil; 5, sandy or gravel deposit; 6, OSL sample position. temperature step of 20 50k. Measurements were made using a 2G three-axis cryogenic magnetometer. Representative demagnetization diagrams are shown in Fig. 2. Because a stable characteristic remanent magnetization (ChRM) can be obtained above k for these samples the other samples were only demagnetized at 300, 350 and 400k. 482 and 265 bulk samples were respectively collected at 2 cm intervals from Xuancheng and Fanchang for magnetic susceptibility measurements. Susceptibility was measured using a Bartington MS2 unit. Grain-size of 154 samples from Xuancheng was measured on a Malvern Mastersizer-2000 laser particle analyzer, with an analytical precision of 1%. Luminescence dating (OSL) was performed on one sample from 0.4 m depth at Xuancheng to determine the top age of the section. The measurement was conducted in the OSL Laboratory, Institute of Geology, China Seismology Bureau. 12 oriented block samples were taken from Xuancheng for micromorphological analysis. 2 Results and discussion Magnetic susceptibility, inclination, declination and geomagnetic polarity of the two studied sections are shown in Fig. 1. The upper 8.4 m portion in the Xuancheng section is of normal polarity while the lower portion shows a negative polarity (Fig. 1(a)). The polarity reversal at 8.4 m depth within the middle-lower part of the VRS evidently corresponds to the Brunhes/Matuyama (B/M) boundary in the standard polarity timescale [16] as is supported by the OSL age of 18B2.5 ka from the top part of the section. This indicates that the fluvial sediment underlying the VRS was older than 0.78 Ma in age. All samples from Fanchang show a normal polarity (Fig. 1(b)), indicating an age younger than 0.78 Ma for the whole sequence, including the bottom fluvial deposits. The geomagnetic results are consistent with the geomorphological locations of the two sections: the Xuancheng site is on T2 of the Shuiyang River while the Fanchang site is on T1 of the Huanghu River, both are first-order branches of the Yantgze River. Our results hence provide new chronological evidence for the terraces of these branches of the Yantgze River: T1 is of Brunhes age while T2 was formed in the Matuyama epoch. According to the magnetic susceptibility data and the lithological features (Fig. 1), the top part of the Fanchang section was eroded. The VRS at Fanchang corresponds to the upper part of the vermiculated soil at Xuancheng. Since weathering during the formation of the VRS strongly modified the texture of its parent materials, an Chinese Science Bulletin Vol. 48 No. 19 October

3 ongoing question is whether the parent material of the VRS was derived from fluvial or eolian deposits. Micromorphological observations on the VRS thin sections show that the coarse fraction in the top part is dominated by quartz grains 100 µm with angular shapes, characteristic of loess deposits. On the contrary, the lower part of the VRS contains a significant proportion of rounded quartz grains as large as µm, indicating a fluvial origin. The boundary between these two types of parent materials is located at 9.1 m depth. This is consistent with the particle analyses showing a sharp increase in grain-size at this boundary (Fig. 3). The similarity of grain-size distribution between the upper Xuancheng samples and the typical Xifeng loess samples also confirms the eolian origin for the upper 9.1 m portion of the Xuancheng section (Fig. 4(a)). On the contrary, the three-peak grain-size distribution mode for samples lower than 9.1 m (Fig. 4(b)) is characteristic of fluvial sediments. Fig. 2. Orthogonal demagnetisation plots of typical normal and reversed samples from Xuancheng and Fanchang., The horizontal component;, the vertical component. X80, X500, X1000 and X1030 indicate samples from Xuancheng at the depths of 0.8, 5.0, 10.0 and 10.3 m, respectively. F320 and F650 represent samples from Fanchang at 3.2 and 6.5 m depths, respectively. Fig. 3. Lithological and the grain-size variations in the Xuancheng section (Legend is the same as in Fig. 1). Fig. 4. Comparison of grain-size distribution between the Xuancheng samples and typical loess samples from Xifeng., Xuancheng sample; ----, Xifeng sample Chinese Science Bulletin Vol. 48 No. 19 October 2003

4 The exact boundary between the eolian and fluvial deposits in the Xuancheng section is therefore located at 9.1 m depth while the B/M boundary is at 8.4 m depth, indicating that eolian deposition started prior to 0.78 MaBP. The OSL age (18B2.5 ka) at 0.4 m depth and the B/M boundary age (0.78 MaBP) suggest an average eolian accumulation rate of about 1.05 cm/ka. Extrapolation according to this accumulation rate yields a basal age of about 0.85 Ma for the aeolian portion at Xuancheng. Eolian deposit older than 0.85 Ma has never been reported from the middle-lower reaches of the Yantgze River, as is also confirmed by our spatial investigation in field. These indicate that eolian deposition started in southern China at about 0.85 MaBP. Although several paleosols present in the Xiashu loess formation, the VRS at the lower part of the two studied sections represents the youngest vermiculated soil in the studied region. The abundant laminated clay coatings with high birefringence in this soil indicate strong clay illuviation process, characteristic of forest soil under humid conditions. At least two kinds of clay coatings are observed, one reddish and another yellowish, indicating a polygenetic nature of the soil. These features, in comparison with those of the younger soils, define warm-humid extremes of climate in the studied region because clay illuvial features are much less abundant in the younger paleosols, which have not any vermiculated characteristics. The above magneto- and pedo-stratigraphic data provide firm evidence for the age of the VRS that is widely distributed in southern China. The fact that B/M boundary is located within the low-middle part of this soil unit indicates a soil formation age younger than 0.78 Ma. The soil-forming processes have affected the parent materials deposited in both Matuyama and Brunhes epochs. This is strongly confirmed by the normal polarity of the entire Fanchang section that contains the same VRS. Eolian dust deposition requires the existence of a source area without significant vegetation cover and winds strong enough to carry dust particles. Dust sources for the loess deposits in the middle-lower reaches of the Yangtze River are still contentious. One view is that the loess deposits of this region may have similar sources to the loess in northern China [5]. An alternative view emphasizes local sources [17]. Several lines of evidence suggest that local sources would be predominant for these eolian deposits. Firstly, thicker eolian deposits in southern China have mostly been reported near river valleys except those close to the eastern shorelines or to the Tibetan Plateau. The geomorphic distribution of these loess deposits is quite similar to that in Europe and North America where loess deposits are usually associated with large rivers in geography. Secondly, the lack of significant loess cover between the Qinling Mountains and the Yangtze River alsosuggests a different source for loess deposits in southern China. Otherwise, loess coverage would be continuous from the Loess Plateau to southern China. Thirdly, earlier study 1) showed significant differences of geochemical characteristics between the Xiashu loess and the loess in northern China, suggesting again a difference in the sources. Finally, the loess at Xuancheng contains a great proportion of particles larger than 32 µm (Fig. 3(b)). This fraction is unlikely to be transported for a long distance according to wind tunnel experiments [18]. We therefore believe that eolian dust deposits in the studied region were primarily derived from the river valleys of the Yangtze River and its branches. A contribution from the deserts in northern China, even though it played a role, would not have been predominant. The onset of sustained loess deposition in the middle-lower reaches of the Yantgze River at about 0.85 Ma BP clearly indicates an important environmental event. It coincides with the well-known Mid-Pleistocene climate change of global significance. Marine δ 18 O records [19] indicate drastic increase in global ice volume at this boundary associated with a shift of climate periodicity, from a 40-ka to a 100-ka dominant frequency. A further uplift of the Tibetan Plateau may also occur near 0.85 MaBP [20,21], which is thought to have led to initial freezing conditions on the plateau [20,22]. The loess-paleosol sequences in the Loess Plateau region indicate that both summer and winter monsoon were significantly strengthened at that time associated with enhanced climate contrasts between the glacial and interglacial times [23,24]. The onset of eolian deposition in the Yangtze River basin evidently resulted from a response of the regional climate to these major geo-environmental changes of global significance. Since this region is under strong influence of the East Asia monsoon, strengthened summer monsoon during the interglacials since 0.85 MaBP would have resulted in more rainfall and greater water flux of the rivers. Abundant fluvial water is, in turn, favorable to the development of enlarged riverbeds and fluvial deposition. The uplift induced erosion in the upper reaches of the Yangtze River would have also helped fluvial deposition. During glacial times, the fine-grained materials in these fluvial sediments would be able to provide a source for eolian dust, due to the decreased river runoff and the sparse vegetation cover under greater aridity. The strengthened winter winds would be strong enough to deflate dust particles and to deposit them in the nearby regions. Thus, the onset of sustained loess deposition in the Yangtze River basin at about 0.85 MaBP appeared to have resulted from a joint effect of tectonic-climate changes 1) Nie, G. Z., The mineral characteristics and formation environment of Xiashu loess in Nanjing district, Master Degree Dissertation of Institute of Geology (in Chinese), Chinese Academy of Sciences, Chinese Science Bulletin Vol. 48 No. 19 October

5 and the consequent hydrological changes of the regional river system. Detailed pedological features of the VRS will be addressed elsewhere. Nevertheless, this soil clearly represents the strongest one since 0.78 MaBP. A reliable correlation of soils between the studied sections and those in northern China would require more chronological data. However, the strong pedogenic intensity of the VRS and its stratigraphic position suggest a synchronicity with the S4 and S5 soil units in the Loess Plateau region, which also represents warm-humid extremes of the middle Pleistocene in northern China [25]. The S4 and S5 soils in the Loess Plateau contain totally four sub-soil units (S4, S5-1, S5-2 and S5-3) [25]. The polygenetic properties of the VRS at Xuancheng and Fanchang seem to be consistent with the multiple soil-forming stages in S4 and S5 soils although individual soil-forming stages are difficult to be resolved in the VRS due to strong weathering. The studied VRS in southern China and the strong S4 and S5 soils in northern China therefore indicate several intervals with unusually strengthened summer monsoon in the middle Pleistocene. These climatic extremes are also of global significance [25]. Although marine δ 18 O record has not any obvious counterparts of these events, their imprints in marine δ 13 C records [26] are quite clear, suggesting that these events may have dynamically linked with global carbon cycles [25]. 3 Conclusion In this study, detailed manetostratigraphic and sedimentologic investigations were made on two loess-soil sequences in southern Anhui Province. The results indicate the onset of eolian deposition in the middle-lower reaches of the Yangtze River at about 0.85 Ma in response to a Mid-Pleistocene climate change of global significance. Strengthening of the summer and winter monsoon circulations, probably induced by a further uplift of the Tibetan Plateau, would have led to greater hydrological contrasts of the rivers between glacial and interglacial times. The increased runoff and fluvial materials during interglacial times, and the stronger aridity and winds during glacial times would have provided favorable conditions for eolian deposition in this region. Our results also provide new evidence for the chronology of the VRS that is usually regarded as a stratigraphic marker in southern China. The most recent VRS was developed in the Mid-Pleistocene after 0.78 MaBP (B/M boundary), correlative with the S4 and S5 soils in the Loess Plateau region in northern China. This contradicts a traditional view that all the vermiculated soils in southern China may have been formed during the Tertiary. 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