Late Pleistocene sedimentary sequences and paleoclimate changes in Xunhua basin in the upper reach of Yellow River in China

Abstract

The third terrace of the Yellow River was well developed in Xunhua basin in the north-east margin of the Tibetan Plateau. The terrace was formed at ca 75 ka as dated by the optically stimulated luminescence (OSL) method. On the basis of grain size, magnetic susceptibility and palynological data, six episodes of the climatic change were identified in Xunhua basin; they include very warm and humid period during 120–114 ka, cool and dry period during 114–105 ka, warm and humid period during 105–98 ka, gradually cooling period during 98–85 ka, warm and humid period during 85–75 ka, very cold and dry period during 75–63 ka. The six stages of climatic change recorded in Xunhua basin correspond to the marine oxygen isotope stages (MIS) of 5e, 5d, 5c, 5b, 5a and 4, respectively.

Keywords

sedimentary sequences / river terrace / Xunhua basin / Tibetan Plateau / Late Pleistocene / paleoclimatic change

Introduction

Compared with sediments which could reflect global climate changes, such as deep-sea sediments, ice cores and loess deposits (Liu., 1985), fluvial sediments in a stable crustal uplift area could then record regional fluctuations (Fuller et al., 1998; Nádor et al., 2003). This view has been demonstrated by numerous studies (Vandenberghe, 1993; 1995; Bridgland and Maddy, 1995; Maddy et al, 2001; Chang et al., 2005). Penck and Brückner (1909) inferred a link between changing climate and the formation of fluvial terraces. Bull (1991) concluded that river systems in the region, particularly those in the west and east Atlantic margins, have responded to these climate changes through vegetation-related variations in runoff and sediment supply, and alterations in flood regime resulting from change in storm frequency, magnitude, and seasonality.

The Xunhua basin (elevation ~1900 m at the Yellow River), a north-western oriented depression basin, is located at the north-east margin of the Tibetan Plateau in eastern Qinghai (Pan et al., 1996; Zhang et al., 2010) (Fig. 1). Today, the Yellow River is one of the major rivers flowing from western Qinghai into Bohai sea (Zhang et al., 2004). There have been developed five terraces (T₁-T₅) along the Yellow River valley in the Xunhua basin (Yue et al., 1997). Previous studies mainly focused on the Lanzhou area, and the Lanzhou terraces have been used to reconstruct tectonic movements and regional climate changes repeatedly (Li., 1991; Blum and Törnqvist, 2000; Pan et al., 2007; Wang et al., 2010). Nevertheless few studies about the Xunhua terraces have been reported. In this paper, we conducted the analysis of granularity, magnetic susceptibility, palynology, and optically stimulated luminescence (OSL) dating, in order to reveal climate status of fluvial sediments in Xunhua basin during the Late Pleistocene.

Materials and methods

The third terrace section (35°50′6″ N, 102°30′10″ E) is located at the convex bank of the Yellow River (Fig. 1). The section with a thickness of 28.19 m is mainly composed of fluvial sediments which are covered by 7.28 m-thick secondary loess. A total of 173 samples were taken at interval of 0.15-3 m from this section for magnetic susceptibility, grain size and palynology analysis. Three samples for OSL dating were collected from the secondary loess and the well-sorted blanket sands.

The magnetic susceptibility measurements were carried out in the paleomagnetic laboratory of the Institute of Geophysics and Geomatics, China University of Geosciences in Wuhan, using the Bartington MS2 Magnetormeter. All the samples were tested for three times and the average was adopted to get accurate results. The granularity analysis was tested in the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences. Grain-size was analyzed by laser diffraction on a LS230 Laser Analyzer according to the approach described by Konert and Vandenberghe (1997).The organic matter was removed by boiling with 10% H₂O₂ for about 15 min until the excess peroxide has been destroyed. Then carbonates were removed by boiling for 1 min with 10% HCl. Further, 0.3 g Na₄P₂O₇.10H₂O was added in a 100 mL suspension and boiled for 1 min to disaggregate the components (Vandenberghe et al., 2004).

For palynology analysis, samples were treated with HCl (35%) and HF (70%) to remove carbonates and silica. Separation of the palynomorphs from the residue was by ZnCl₂ (density= 2), following the method of Faegri and Iversen (1989). Most samples proved positive for organic matter but contained low concentrations of palynomorphs.

OSL dating of three samples was conducted in the Luminescence Dating Laboratory, Groundwater Mineral Water and Environmental Supervising and Testing Center, The Institute of Hydrogeology and Environmental Geology, Shijiazhuang. The fine quartz grains (4-11 µm) were separated under weak red light, and the purity was tested by the IRSL (Infra Red Stimulated Luminescence) scanning. The equivalent dose was determined using fine-grained quartz (4-11 µm) on the Daybreak1100 Reader with the single-aliquot regenerative-dose (SAR) proposed by Murray and Roberts (1998). Meanwhile, the contents of uranium, thorium and potassium were determined by neutron activation analysis. Taking into account the moisture content and the effect of cosmic rays, the OSL ages were calculated with a program compiled by R. Grün (2001).

The sequence of the third terrace and its age in Xunhua basin

Sedimentary characteristics of the third terrace

According to the lithologic features, this section can be divided into seven units from top to bottom (Fig. 2):

Unit G: A 7.28 m (0-7.28 m) mauve secondary loess without beddings, palynomorph fossils were dominated by Pinus, Picea, Artemisia and Chenopodiaceae.

Unit F: A 9.43 m (7.28-15.35 m) succession of mauve sandy conglomerates, interbedded with celadon sandstones and jacinth siltstones. This unit is characterized by 88 small cycles. The bedding structures include horizontal bedding, parallel bedding, wedge and tabular cross-bedding. The main palynomorph fossils are Artemisia, Chenopodiaceae, Pinus, Abies, Picea, Cupressus, Pterocarya, Quercus, Carpinus, Juglans and Taxodiaceae.

Unit E: A 0.56 m (15.35-16.97 m) layer of jacinth calcareous mudstones, interbedded with celadon thin-bedded siltstones. This unit is characterized by 13 small cycles. The bedding structures include horizontal bedding. This unit yields palynomorph fossils: Artemisia, Quercus, Pinus and Taxodiaceae.

Unit D: A 5.37 m (16.97-22.24 m) succession of reddish brown, jacinth sandstones, interbedded with celadon mudstones. This unit is characterized by 69 small cycles with positive graded beddings, horizontal beddings, mud cracks and imbricated structures. There are some reddish brown conglomerate layers at the bottom of cycles. This unit contains palynomorph fossils as following: Artemisia, Pinus, Quercus, Juglans, Potamogeton, Amaranthus, Betula, Acer, Carpinus, Polygonum, Taxodiaceae, Gramineae and Ranunculaceae.

Unit C: A 2.04 m (22.24-24.51 m) succession of incanus siltstones, muddy siltstones interbedded with gray-red mudstones, occasionally intercalated with sandstone lenses. This unit could be divided into 8 small cycles from the bottom to top, containing climbing ripple bedding and wavy bedding, and horizontal bedding. The palynomorph fossils are including Artemisia, Chrysanthemum, Quercus, Chenopodiaceae, Juglans, Castanea, Betula, Acer, Carpinus, Polygonum, Taxodiaceae and Gramineae.

Unit B: A 0.5 m (24.51-25.06 m) layer of incanus fine sandstones covered by incanus coarse quartz sandstones. Depositional beddings such as parallel bedding, wedge cross-bedding and wavy bedding, are well developed. Few palynomorph fossils were found in unites A and B.

Unit A: A 3.01 m (25.06-28.19 m) layer of grayish white gravel-pebble layer with good sorting and rounded.

OSL age dating of the third terrace of the Yellow River

The results of OSL dating are shown in Table 1. The age of the overlying re-depositional loess is 62.8±3.6 ka, and the age of the bottom fluvial marginal bank facies sediment is 113.6±8.4 ka. According to the previous studies of the initial deposition age of the loess in third terraces, we suggest that the basal age of the overlying thick loess layer is ca 75 ka (Zhao and Liu, 2003; Pan et al., 2007; Wang et al., 2008). In other words, the third terrace formed finally at ca 75 ka. This is in accordance with the formation ages of third terrace of the Yellow River at Fanjiaping (Wang et al., 2008) and Xunhua . Several previous studies have suggested that the third terrace in the eastern Lanzhou basin was formed during the Gonghe movement (0.15 Ma) before 0.13 Ma (Li, 1991; Pan et al., 2007). This conclusion cannot be validated in our research.

Results

The formation of the third terrace

The formation of the terrace can be divided into three stages as follows (Fig. 3).

The unit A was deposited during the first stage of terrace formed. This riverbed sediment is characterized by well-rounded pebbles. The fine-grained materials, such as siltstones and clays, were taken away owing to sorting and transport. One riverbed sand sample in the unit was obtained an OSL ages of 113.6±8.4 ka, which correlated to marine oxygen isotope stages (MIS) 5e stage during the last interglacial. Ice core records at Guliya suggested that the temperature in MIS 5e was warm and humid with 5°C higher than the present (Yao et al., 1997). Paleocurrent analysis was performed on wedge to planar cross-bedding and imbricated conglomerate clasts (Fig. 2). The analysis indicated that the paleocurrent direction of the Yellow River pointed to NEE at that time. The warm and humid climate could result in bending and migrating of the riverway (Kasse et al., 1995).

During the second stage, the units B-F was deposited, indicating the river has began to lateral erosion and accretion. The river eroded in the concave bank and deposited in the convex bank during its swing in valley. As a result, the meandering channel could be getting wider and wider (Yang, 1985; Sheng and Gong, 1986). Paleocurrent analysis was performed on wedge to planar cross-bedding and imbricated conglomerate clasts (Fig. 2). The paleocurrent direction of the fluvial marginal bank deposits in these units pointed to NNE during the stage. Vandenberghe (1993, 1995) suggested that river was more likely to be down-cutting erosion during the climate transition from warm to cold. The Yellow River down-cutted quickly and formed the third terrace during this stage, when climate was transited from the last interglacial to last glacial (Antoine et al., 2000).

The unit G is the top layer of the third terrace, indicating the last stage of terrace formed. The ealian loess began to deposited and preserved above the third terrace of the Yellow River during this stage (Pan et al., 2007).

Environmental change in Xunhua basin

According to the comprehensive analysis of granularity, magnetic susceptibility and palynology of the third terrace, the environmental change during Late Pleistocene in Xunhua basin can be divided into six stages (Fig. 4).

Stage I (28.185-22.635 m; 120-114 ka): The magnetic susceptibility is the highest, so does the median grain size (Fig. 4). The sediments mainly consist of gravel and sand. The percentage of sand (>63 µm) ranges from 20% to 60%, while concentrations of clay and fine silt fraction are low. The results indicated a strong hydrodynamic condition in this stage. The high proportions of broad-leaved tree taxa (Quercus, Juglans and Betula) with less proportions of xerophytic taxa supported the conclusions that. The climate is humid and warm, which is corresponded to MIS 5e.

Stage II (22.635-18.615 m; 114-105 ka): The magnetic susceptibility is much lower than stageI. The percentage of sand is about 20%, while the fine grain of silt and clay are over 60% in percentage. The content of coniferous taxa is slightly more than stage I, and the content of xerophytic taxa is the highest. The results show a dry and cold climate with small fluctuations, and corresponds to MIS 5d.

Stage III (18.615-16.345 m; 105-98 ka): The magnetic susceptibility is relatively low with a narrow fluctuation range, while the grain size is getting coarser and fluctuates severely. We interpret this as evidence for a relatively strong hydrodynamic environment. In addition, high proportions of broad-leaved trees (Quercus, Juglans and Carpinus) and xerophytes(Artemisia and Chenopodiaceae) imply a humid and warm climate character, which could be correspond to MIS 5c.

Stage IV (16.345-12.625 m; 98-85 ka): The values of magnetic susceptibility during this stage are relatively high and decrease upward. The mean grain size is about 30 µm with a narrow fluctuation range. Compared with the previous stage III the average fraction of clay increased about 15%. In this stage, concentrations of broad-leaved tree taxa such as Quercus and Pterocarya gradually decrease while coniferous taxa such as Picea and Abies increase, along with high proportions of Artemisia (Fig. 5). The climate character of this stage is a relatively warm- cool and weak hydrodynamic environment, which could be corresponded to the environment of MIS 5b.

Stage V (12.625-7.280 m; 85-75 ka) (Fig. 4): This stage is characterized by relatively high value both in magnetic susceptibility and the median grain size. The average fraction of sand is high (45%) with a minor contribution from silt and clay. The median grain size ranges from 20 to 40 µm. . The tabular and wedged cross-bedding can be commonly observed in the gravel-bearing sands at the bottom, indicating a higher hydrodynamic condition. The dominant pollens include Artemisia, Chenopodiaceae, Pinus, Quercus and Betula. Xerophytic taxa pollens decrease to 20% and conifers pollens are few counted. The results show a relatively warm and humid climate, and could be corresponded to MIS 5a.

Stage VI (7.280-0 m; 75-63 ka) (Fig. 4): This stage consists of loess, and characterized by low magnetic susceptibility and fine grains. The mean grain size is 16 µm, and the susceptibility ranges from 2 × 10^-4 to 3 × 10^-4 SI. Only a small amount of pollens, such as Picea and Pinus were found, indicating a relatively cold and dry climate. The basal age of the loess layer is 75 ka, corresponding to MIS 4.

Discussion and conclusions

Generally, magnetic minerals in river sediments have three main sources: eroded bedrock, surface soil erosion, and dust (Thompson, Oldfield, 1986). The study of the Pleistocene river sediments in Hungary shows that magnetic susceptibility values of river sediments contributed mainly by the magnetite content which originated from bedrocks (Nádor et al., 2003). Sand grains in fluvial deposits were brought mainly by river, which means that the magnetic susceptibility in fluvial sediments directly indicate the dynamics of river hydrology, and also indirectly imply the climate conditions (Li et al., 2008). Result of this study shows that the magnetic susceptibility of the third terrace of the Yellow River has a positive correlation with the sand (0-4ф), inverse correlation with the siltstone (4-8ф), and weak correlation to the clay (4-8ф). Therefore, we could draw the conclusion that the magnetic minerals in the sediments of the Xunhua terrace main came from the sand particles (Wang et al., 1996).

The compositions of grain size in clastic sediments record the integrated information of topography, mediums of transportation, hydrodynamic conditions, sources and depositional environment (Shen et al., 2006). At the third terrace in Xunhua basin, the sands and gravels correspond to the strong hydrodynamic condition with warm and humid climate; silt and clay formed in cold and dry climate correspond to weak hydrodynamic condition. These results have been proved by palynology analysis

From above materials and discussion, we can make conclusions as follows.

1) The sedimentary sequence of the Xunhua terrace can be divided into three parts from the bottom to the top: riverbed sands and gravels, point-bar and natural levee sand deposits, secondary loess. The deposition of the third terrace of the Yellow River began to form at about 120 ka, and downcutted quickly during the transition from the last interglacial to the last glacial (about 75 ka), and then the third terrace formed finally.

2) In view of the magnetic susceptibility analysis, particle size analysis, combining the pollen analysis and OSL age dating results, the Late Pleistocene environmental change in the Xunhua basin can be divided into six stages: I: 120-114 ka, strong hydrodynamic condition, with warm and humid climate conditions; II: 114-105 ka, weak hydrodynamic condition with cool and dry climate; III: 105-98 ka, highly hydrodynamic condition and relatively warm and humid climate; Ⅳ: 98-85 ka, weak hydrodynamic condition and cool climate; Ⅴ: 85-75 ka, highly hydrodynamic condition with warm and humid characters; Ⅵ: 75-63 ka, cold and dry climate. The six stages respectively correspond to the climate change patterns recorded by high-resolution ice cores and marine oxygen isotopes.

3) The magnetic susceptibility of the river sediments shows a positive correlation to the sand-sized particles (0-4ф), which suggested that the magnetic minerals mainly come from bedrocks. Gravel deposits could correspond to the warm-humid conditions, silt and clay deposit correspond to cold and dry climate.

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