Sedimentary dynamics and climatic implications of Cretaceous loess-like red beds in the Lanzhou basin, Northwest China

Journal of Asian Earth Sciences 180 (2019) 103865

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes

Full length article

Sedimentary dynamics and climatic implications of Cretaceous loess-like red beds in the Lanzhou basin, Northwest China

T

⁎

Jiasheng Chena, , Xiuming Liua,b, Xiaojing Liua a Key Laboratory for Subtropical Mountain Ecology (Ministry of Science and Technology and Fujian Province Funded), Institute of Geographical Sciences, Fujian Normal University, Fuzhou 350007, Fujian, China b Department of Environment and Geography. Macquarie University, Sydney, NS 2109, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Cretaceous red beds Fine-grained sediment Grain size Environmental magnetism Sedimentary dynamic

The Chinese loess has been widely studied in order to reconstruct Cenozoic environmental change. Cretaceous loess-like red beds with alternating red and gray layers in the Lanzhou basin, Northwest China are well preserved and can be used for understanding the Mesozoic environmental and climatic changes. However, the red beds and gray clays are less studied and their significance is not clear. Hence, Cretaceous red beds of the basin are studied. The gray layers show horizontal lamination and are deposited in a lacustrine environment, whereas the red layers show gradational change in texture and color, and are analogous to paleosols. The red and gray clays show similar immobile trace elemental compositions and identical grain size distributions, suggesting that the sediments of the two deposits are derived from the same source and carried by identical transporting forces. The dominant 2–10 μm fine grain size component indicates that the deposits are sorted through long-distance transportation. In this context, the magnetic properties of the red beds are mainly determined by post-depositional environmental development. Hematite is the dominant magnetic mineral and some of the gray layers contains small quantities of magnetite. Therefore, the red and gray layers develop on land and under water, respectively. The magnetic parameter IRM-100mT eliminates the effects of ferromagnetic minerals and is sensitive to hard magnetic minerals. Thus, it can be used as a proxy for paleoclimatic change. Its high values in the red layers correspond to a drier paleoclimatic environment.

1. Introduction Fine-grained sediment in Northwest China is a reliable archive of inland environmental changes in Asia. The area between the Northeast corner of the Tibetan Plateau and the western edge of the Chinese Loess Plateau has the most well-preserved fine grain sediment. Records of thickest Quaternary loess (Chen et al., 2014), longest continuous aeolian deposits over the last 22 Ma (Guo et al., 2002), earliest Asian winter monsoonal winds at 34 Ma (Licht et al., 2014), and persistent drying in northwestern China over the last 52 Ma (Fang et al., 2015) were all established with the deposits in this area. This region is important for loess research and has provided excellent paleoclimatic and geological records of the Cenozoic environmental change. In the Cretaceous period, the area was a part of the Qingyang Lake and the ancient Datong River watershed, with a widely distributed red beds (Chen, 1987). Red beds are usually formed by the erosion and redeposition of red lateritic soil in moist tropical climate or deposition under desert conditions, and are of some paleoclimatic significance

⁎

(Turner, 1980; Walker, 1967). Literature on climatic implication of the Cretaceous red beds in China is scarce. Hendrix et al. (1992) conducted detailed stratigraphic, sedimentologic, paleocurrent, and subsidence analyses on the red beds in Tianshan, Northwest China, and briefly discussed its implications on monsoonal circulation and paleo-rain. The climatic implications of the well-preserved red beds on the Northeast corner of the Tibetan Plateau are yet to be studied. The red beds in this area are characterized by alternating loess-like red layers and gray layers and are widely distributed in the Lanzhou basin with an area of 800 km2 (Zhang et al., 2014). It has the potential to extend research on inland loess and paleoclimate from Cenozoic Era into the Mesozoic Cretaceous period. The alternating loess-like red and gray layers of the area have been identified as lacustrine deposits (Chen, 1987). This gray color is a result of gleization and forms in underwater anaerobic conditions (Ponnamperuma, 1972; Retallack, 2008). The red color, however, indicates high hematite content (Torrent et al., 1983), and such red deposits usually develop in tropical soils (Retallack, 2008) rather than in

Corresponding author. E-mail address: [email protected] (J. Chen).

https://doi.org/10.1016/j.jseaes.2019.05.010 Received 18 October 2017; Received in revised form 7 May 2019; Accepted 9 May 2019 Available online 10 May 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

1998). The red beds have weathered into a broad valley and hoodoos with a height of 20–50 m and the ancient planation surface has disappeared (Fig. 2(a)). The clays contain various colors like red, yellow, gray, and less common gray-black, which reflect different developmental environments. Samples were collected at two sites, LZ1 and LZ2 (Fig. 2(b) and (c)), for comparing and including clays in all colors. Fig. 2(b) shows the six layers in section LZ1 and the sampling positions. Two to four samples were collected in each layer with an interval of 1–3 m, and a total of 18 samples were collected from the top to the bottom. The four layers in LZ2 are shown in Fig. 2(c), and 10 samples were collected and numbered 19–28. The sequential numbering from LZ1 to LZ2 is for the convenience of analysis and for comparing the clays properties and does not reflect their continuity in the strata. The red clays show gradational changes in texture and color without graded bed (Fig. 2(d)), which is a characteristic of paleosols (Retallack, 2008). Gypsum layers and veins are found in the red layers, but the gypsum veins crossed each other and are not parallel to the bedding. Therefore, these veins could have formed through the dissolution and precipitation of soluble components after burial of the red beds. The yellow layer is transitioning from red to gray layer in LZ2 (Fig. 2(c)). Gray, and the rare gray-black layers are characterized by horizontal lamination (Fig. 2(e) and (f)); sample 14 is from a gray-black layer. Color reflectance, room-temperature environmental magnetism, thermomagnetic curves, and grain size distribution were measured. Color reflectance was determined using a Color Flex® EZ spectrophotometer (HunterLab, USA). The instrument was calibrated using standard white and black boards, and 5 g samples were evenly spread at the bottom of the dish and flattened. The instrument randomly selected three flat surface areas and measured lightness L*, green–red a*, and blue–yellow b* values. The average values of each of these three measurements were taken as the final results. High-and low-frequency magnetic susceptibilities (χlf and χhf) were measured using a Bartington MS2B susceptibility meter for frequencies of 470 and 4700 Hz. Frequency-dependent magnetic susceptibility was

underwater environments. The sedimentary dynamics and developmental conditions of these red and gray deposits are not yet fully understood. Before the 1980s, Quaternary Chinese loess was interpreted as aqueous deposits. However, evidence such as soil identification, elemental composition, and grain size distribution data revealed that both the red paleosols and brown loess were mainly transported to short distances by the East Asian monsoon from the northwestern desert areas (Ding et al., 2002; Ding et al., 2001; Liu and Ding, 1998; Sun, 2006; Sun et al., 2004). Iron is the fourth most abundant element in the Earth's crust and is very sensitive to changes in environmental redox conditions. The gray and red colors are typically related to variations of ferrous and ferric iron content (Evans and Heller, 2003; Tauxe et al., 2013). In the study of Chinese loess deposits, environmental magnetism and color reflectance are considered as indicators of environment changes (Ji et al., 2001; Liu et al., 2005, 1995; Yang and Ding, 2003). The conventional methods used in Chinese loess studies are helpful for interpreting the sedimentation processes and developmental conditions of these analogous sediments. Analyses of color reflectance, magnetism, grain size, and trace elemental compositions, that have been widely used in the study of Cenozoic Chinese loess, are applied on these Cretaceous alternating red and gray deposits in the Lanzhou basin to explore their similarities and differences in sedimentary dynamics and developmental environments.

2. Materials and experiments Lower Cretaceous red beds with alternating red and gray clays are widely distributed in the Lanzhou basin (Fig. 1) between the Northeast corner of the Tibetan Plateau and the western edge of the Chinese Loess Plateau (Halim et al., 1998). This region lies on the boundary between arid and semi-arid areas. Samples were collected from the west of Kushui Town (36°18.525′N, 103°23.263′E, altitude 1683 m) in Lanzhou City. The Cretaceous paleolatitude of this site was ∼28° N (Halim et al.,

Fig. 1. Geological map of the Lanzhou Cretaceous red beds and the section location. The top left corner is the map of China, and red square marks the study area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 2

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 2. The Lanzhou red beds and photographs of typical deposits; (a) the Google Earth's image of the sampling location. (b) red and gray layers at site LZ1 and the sample number; (c) red and gray layers at site LZ2 and sample number; (d) red layer; (e) gray layer; (f) gray-black layer.

calculated as χfd% = 100% × (χlf − χhf)/lf. Anhysteretic remanent magnetization (ARM) was measured using a Molspin Minispin magnetometer after pre-treatment in a DTECH alternating demagnetization instrument with a peak 100 mT alternating demagnetizing field and a 50 uT direct current field. Isothermal remanent magnetization (IRM) was determined using a Magnetic Measurements Pulse Magnetizer; where the maximum magnetic field strength was 1 T. After measuring the saturation isothermal remanent magnetization (SIRM) under a 1 T field, each sample was placed under opposite-direction magnetic fields of 100 mT and 300 mT (IRM-100 mT, IRM-300mT). Remanence coercivity (Hcr) is the back field that decreases SIRM to zero. The thermal magnetic susceptibility curves χ–T were determined using an Agico KLY 4 Kappabridge and thermal magnetization curves and hysteresis loops were measured using a variable field translation balance instrument. Grain size distributions were measured using a Mastersizer 2000 by

Malvern Instruments after removing organic matter, carbonate, and clay minerals (Sun et al., 2004). These experiments were conducted at the School of Geographical Science, Fujian Normal University. Trace elements were measured using a VP320 X-ray fluorescence spectrometer at Lanzhou University after samples were prepared and ground to pass through a size 200 mesh.

3. Results 3.1. Color reflectance Fig. 3(a)–(c) shows the variations of L*, a*, and b*. In the CIELAB color space, a* ranges between red (60) and green (−60). The redness of soil depends on the content of hematite (Torrent et al., 1983). The parameter b* ranges between yellow (60) and blue (−60) and is 3

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 3. Variations of color reflectance and magnetic parameters; (a), (b), and (c) are color reflectance parameters a*, b*, L*, respectively. (d) Magnetic susceptibility χ, (e) ARM, (f) SIRM, (g) IRM-100mT, (h) IRM-300mT, (i) Hcr and (j) χ–T580−680. The dashed line sperate the samples from LZ1 to LZ2. Table 1 Correlation coefficients between the parameters. Parameters *

a b* L* χ ARM SIRM IRM-100mT IRM-300mT Hcr χ-T580-680

a*

b*

L*

χ

ARM

SIRM

IRM-100mT

IRM-300mT

Hcr

χ-T580-680

1.00 0.84 0.49 0.00 0.24 0.84 0.90 0.06 0.26 0.70

1.00 0.77 0.03 0.21 0.76 0.88 0.21 0.38 0.77

1.00 0.15 0.10 0.44 0.55 0.19 0.29 0.57

1.00 0.05 0.00 0.00 0.14 0.02 0.05

1.00 0.48 0.31 0.11 0.00 0.24

1.00 0.94 0.01 0.18 0.74

1.00 0.09 0.31 0.82

1.00 0.54 0.14

1.00 0.25

1.00

High correlation coefficients (> 0.6) are marked in bold.

associated with the content of goethite (Ji et al., 2001). a* values of the red samples 1–3, 6–11, 16–18, 19–21, and 27–28 in figure (2) are greater than 8. Among these, samples 1–3, 7–11, and 18–21 show very high a* values of approximately 18 (Fig. 3(a)). Gray samples 4, 12–15, and 25–26 show low a* values between 2 and 0. a* values of yellow samples 22–24 vary from 2 to 4.3. The value of b* varies between 8 and 24 (Fig. 3(b)) and is positively correlated with a*; the correlation

coefficient R2 is 0.84 (Table 1). L* represents lightness, and ranges from white (0) to black (100). Soil organic matter and carbonate contents are the dominant factors controlling this parameter (Yang and Ding, 2003; Yang et al., 2001). The lightness L* values of the samples vary between 45 and 75 (Fig. 3(c)). The value of L* is negatively correlated with a* and b*, with R2 values of 0.49 and 0.77, respectively (Table 1). The red layers have 4

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 4. The χ–T curves of typical red and gray samples; (a)−(c) are gray samples 4, 14 and 26; (d)−(f) are red samples 7, 20 and 28.

and are not shown in the figure. Hematite has various coercivities from tens of mT to 1 T (Tauxe et al., 2013), and its signal is within 100–600 mT in the unmixing of backfield IRM curves of typical Chinese loess (Nie et al., 2014). Since a backfield of 300 mT removes part of the remanence of hematite, IRM-300mT is not correlated with IRM-100mT and a*, which include all contributions from hematite. The parameter Hcr represents the remanence coercivity of the sample, and refers to the situation when the reverse magnetic field force for the saturated remanence decreases to zero (Evans and Heller, 2003). High values indicate high contents of hard, antiferromagnetic minerals such as hematite and goethite. The Hcr values of magnetite and maghemite are lower than 100 mT, which only appears in three gray samples (4, 14, and 25); values for all others samples are higher than 200 mT (Fig. 3(i)). These results suggest that antiferromagnetic minerals are the main magnetic carriers. The χ–T curve represents the variation of susceptibility with increasing temperature to 700 °C followed by cooling to room temperature. The Curie or Néel temperature of the χ–T curve reveals the type of magnetic minerals; the χ–T curve of ferrous magnetite shows paramagnetic properties after heating to the Curie temperature of 580 °C, when the susceptibility value rapidly drops to nearly zero. The χ value of hematite drops to zero when it is heated to its Néel temperature of 680 °C. Thus, the integral area of the χ–T curve between 580 °C and 680 °C (χ–T580−680) is calculated to represent the relative content of hematite (Fig. 3(j)). The value of χ–T580−680 is positively correlated with SIRM, a*, and IRM-100mT. The remanence carrier of the Lanzhou red beds is therefore inferred to be hematite. Samples 4, 14, and 26 are gray and show low a* values, whereas samples 7, 20, and 28 are red and show high a* values. These samples are selected for both χ–T and M–T analysis (Fig. 4). The Curie temperature in the χ–T curves of the gray samples 4 and 26 are not detected as these samples contain little magnetic mineral content. The magnetic susceptibility of the gray sample 14 sharply reduces to approximately zero after heating to 580 °C, which indicates that it contained magnetite. The magnetic susceptibilities of all the red samples (7, 20, and 28) decrease rapidly beyond 680 °C as hematite is the dominant magnetic mineral of the red layers. Fig. 5 presents the results of the M–T test for the samples in Fig. 4. The M–T curves of the gray samples 4 and 14 reveal a Curie temperature at 580 °C, which indicates that the dominant magnetic mineral of both samples is magnetite. The Curie temperature of sample 26 is ∼610 °C. The Curie temperature of maghemite is 590–675 °C (Evans

higher content of hematite and the gray layers contain more carbonate.

3.2. Environmental magnetism parameters Magnetic susceptibility is sensitive to the contents of ferrous magnetic minerals, such as magnetite and maghemite. The low-frequency magnetic susceptibility values (Fig. 3(d)) ranges between 0.4 and 1.4 × 107 m3kg−1. The magnetic susceptibility values of typical loess samples from the Xifeng Quaternary loess layer L1 are 3–5 × 107 m3kg−1 and 15–18 × 107 m3kg−1 for the paleosol layer S1 (Liu et al., 1995). The parameter χfd% is sensitive to superparamagnetic particles and is 0–4% for the Lanzhou red beds, whereas it is as high as 10% for the Quaternary paleosols of the Xifeng section (Liu et al., 1990). For Quaternary Chinese loess, χfd% is highly correlated positively with χ (Liu et al., 2007). However, for the Cretaceous red beds, it has no correlation with χ or other parameters (not shown). The ferrous magnetic mineral and superparamagnetic mineral contents of the Lanzhou clays are much lower than those of the typical Quaternary Chinese loess. ARM is sensitive to single-domain ferrous magnetic mineral contents (Tauxe et al., 2013). In the Lanzhou section, except a high value of 4 × 10 m2kg−1 for sample 16, ARM of other samples is 0–3 × 10−5 m2 kg−1 (Fig. 3(e)). The remanence after 1 T magnetic field treatment is typically deemed as the SIRM in conventional laboratory conditions (Evans and Heller, 2003) and contains the contributions of all magnetic minerals: magnetite, maghemite, hematite, and goethite. SIRM (Fig. 3(f)) is not significantly correlated with χ or ARM (Table 1), which indicates low contents of ferrous magnetic minerals. SIRM is positively correlated with a* (R2 = 0.84) and b* (R2 = 0.76) (Fig. 3(f) and Table 1), which suggests that goethite or hematite are the main remanence carriers. The typical coercivity of magnetite is within tens of mT (Tauxe et al., 2013). In typical Quaternary Chinese loess, the ferrous magnetic minerals are anti-aligned with the direction of SIRM after the backward 100 mT field treatment (Nie et al., 2014). The SIRM is partly eliminated by the reverse 100 mT field and the remaining IRM-100mT is contributed by antiferromagnetic minerals. Fig. 3(g) shows that the IRM-100mT correlates positively with SIRM, a* and b*, which are mainly determined by antiferromagnetic minerals. With a backward 300 mT field, all signals of the ferrous magnetic minerals are eliminated from the SIRM, and IRM-300mT is contributed by antiferromagnetic minerals. Fig. 3(h) shows that this parameter does not correlate with SIRM and a*. HIRM and S-ratios, which are related to IRM-300mT, show similar results 5

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 5. The M–T curves of typical red and gray samples; (a)−(c) are gray samples 4, 14 and 26; (d)−(f) are red samples 7, 20 and 28.

distributions of the gray samples 4, 14, and 26 and the red samples 7, 20, and 28 are shown in Fig. 7. All samples show three peaks, at 0–2 μm, 2–10 μm, and 10–100 μm, numbered as Peaks 1, 2, and 3, respectively. The average positions of the three peaks of the gray samples are at 0.57 μm, 3.36 μm, and 50.71 μm, respectively (Table 2). The positions of these three peaks for the red samples are at 0.62 μm, 2.58 μm, and 21.67 μm, respectively. The average peak heights of the gray sample are 1.15%, 4.77%, and 0.33%, whereas those of the red samples are 1.99%, 6.38%, and 0.45%. For both the red and gray samples, the heights of Peak 2 (4–7%) > Peak 1 (1–2%) > Peak 3 (0.5%). The red and gray samples have similar grain size distribution which are dominated by 2–10 μm fine component.

and Heller, 2003). Therefore, sample 26 may contain maghemite. Hematite is the magnetic carrier of the red samples 7, 20, and 28 as their magnetizations decrease sharply after heating to 680 °C. It is consistent with the result of χ–T curves. Hysteresis behavior refers to irreversible changes of the intensity of magnetization after a sample experiences increased and decreased magnetic fields. This parameter is controlled by the magnetic mineral species and the magnetic particle size. The hysteresis loops of the gray sample 4, red sample 7, and gray-black sample 14 from the horizontal beds are shown in Fig. 6. All the curves are corrected for the paramagnetic contribution. The hysteresis curves of samples 4 and 14 are closed under the 300 mT and 100 mT fields. The magnetizations saturated under the low field indicates that ferrous magnetic minerals are the main minerals of these samples. The hysteresis curve of sample 7 close under a strong field of 1 T, which is higher than that of samples 4 and 14. The waist of the hysteresis loop of sample 7 is thin; such “goosenecked” hysteresis loops are associated with mixtures of high contents of high-coercivity hematite and low contents of low-coercivity magnetite (Tauxe et al., 1996). The red samples are dominated by hematite, whereas the gray samples contain small amounts of magnetite.

3.4. Trace elements Samples with a* values greater than 8 are categorized as red clays. The mean trace elemental concentrations of these samples have been compared with those of the gray clays in Fig. 8(a). The trace elemental abundances of the two clays are very similar and the compositions of the red and gray clays are well mixed during their respective transportation processes. The mean elemental concentrations of all red beds samples are similar to those of the upper continental crust (Taylor et al., 1983) (Fig. 8(b)). The transportation processes of the red beds therefore provide an average sample of the upper continental crust. Such characteristics are typical in eolian dust deposits (Ding et al., 2001; Guo et al., 2002). Trace elements such as Y, Zr, Hf, and Nb are incorporated into clastic sedimentary rock and are resistant to alteration by transportation and weathering. Because of their low mobility, they reflect the signature of the parent material, and can be evaluated to determine provenance (Bhatia and Crook, 1986; Holland, 1978; Mclennan et al., 1983). The elemental ratio plots of Zr/Hf vs. Y/Nb and Hf/Nb vs. Zr/Nb can be applied to differentiate provenance based on the compositions of different parent rocks (Hao et al., 2010). The Zr/Hf ratios of the red and gray samples vary within 20–50, the Y/Nb ratio varies between 0.8 and 1.6 (Fig. 8(c)), the Hf/Nb ratio varies between 0.2 and 0.5, and the Zr/ Nb ratio varies between 9 and 12 (Fig. 8(d)). Since the elemental ratios of the red and gray samples have similar ranges, the two clays are derived from the same source. The elemental ratios of the Xifeng loess in northern China and the Xiashu loess in southern China (Hao et al., 2010) are shown in Fig. 8(c) and (d). The Zr/Hf and the Y/Nb ratios range within 30–40 and 0.8–1.6,

3.3. Grain size distributions The loess of the Chinese Loess Plateau is of eolian origin, and its grain size curve obeys Weibull distribution (Sun, 2006; Sun et al., 2004). Weibull peak fit is applied on the Lanzhou clays. The grain size

Fig. 6. Hysteresis loops of red sample 7, gray sample 4 and gray-black sample 14. 6

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 7. Grain size distributions and components; (a)−(c) are gray samples 4, 14 and 26; (d)−(f) are red samples 7, 20 and 28.

loess grain size distributions, Peak 3 represents the dynamics of shortdistance East Asian winter monsoon, Peak 2 represents long-distance dynamics, such as the westerly winds, and Peak 1 is associated with soil development (Sun, 2006; Sun et al., 2004). The Lanzhou dust storm has an alluvial saltation component, Peak 4, it probably reflects short-distance transport from the nearby Yellow River bed. The 0–2 μm superfine component is related to post-depositional soil development while the 2–10 μm and 10–100 μm components depend on transportation processes. Those three components are indicators of sedimentary dynamics. The positions of the peaks of the red and gray clays shown in Fig. 7 are similar to those of typical Chinese loess. It suggests that the Lanzhou clays are also carried by short-distance wind, long-distance wind, and affected by soil development. Peak 2 is the highest in both red and gray clays, and is 10 times higher than Peak 3 (Table 2). This suggests that the red and gray clays have similar sedimentary dynamics, and are mainly carried by long-distance transportation. The East Asian winter monsoon is the short-distance carrier of the Xifeng loess, and the adjacent floodplains are the source of the eolian deposits for the Xiashu loess (Hao et al., 2010). The elemental ratios in Fig. 8(c) and (d) show that their source regions are relatively narrow and limited. The Lanzhou clays have a broader source region, which coincides with the result that they are carried by long-distance dynamics. The red and gray clays have similar sedimentary dynamics, but develop in terrestrial and aquatic environments, respectively. A grain size distribution similar to that of the Lanzhou clays with a dominating 2–10 μm component, has been reported in samples from the middle of a closed modern lake (Sun et al., 2001), as well as for dust fall during non-dust-storm periods (Xiao et al., 2007). The middle of a lake is typically a reducing environment, such an environment cannot account for the formation of red clays of Lanzhou, which are analogous to soil. The strong present-day Asian monsoons originated at 34 Ma. The Tibetan Plateau uplift, began at 55 Ma (Molnar and Tapponnier, 1975), blocked moisture from the Indian Ocean and provided vast amounts of material for short-distant dust transport. It promoted the aridification of inland Asia, and strengthened the East Asian monsoon (An et al., 2001). The uplift of the Tibetan Plateau and strengthening of the East Asian

Table 2 Peak positions and heights of the grain size components of the red beds and typical eolian loess samples. Sample

Peak 1 position (μm)

Peak 2 position (μm)

Peak 3 position (μm)

Peak 1 height (%)

Peak 2 height (%)

Peak 3 height (%)

No.4 (red) No.14 (red) No.26 (red) Average No.7 (grey) No.20 (grey) No.28 (grey) Average Dust storm Loess L1 Paleosol S1 Average

0.52 0.55 0.61 0.56 0.66 0.63 0.58 0.62 0.69 0.65 0.88 0.74

3.01 3.45 3.63 3.36 2.99 2.46 2.3 2.58 3.89 5.09 5.74 4.91

48.67 48.31 52.07 49.68 33.7 16.53 14.77 21.67 31.5 33.2 30.12 31.61

1.27 1.02 1.17 1.15 1.88 2.08 2.02 1.99 0.37 0.45 0.83 0.55

4.31 4.6 5.21 4.71 6.45 6.67 6.02 6.38 0.53 1.69 2.54 1.59

0.37 0.24 0.39 0.33 0.34 0.42 0.6 0.45 5.52 5.77 4.39 5.23

respectively, whereas the Hf/Nb and Zr/Nb ratios range within 0.2–0.3 and 9–11, respectively. The elemental ratios of the loess lie within the range of those of the Lanzhou red beds. The Lanzhou red beds therefore have more diverse parent materials and broader source regions than the Chinese loess. 4. Discussion 4.1. Sedimentary dynamics Fig. 9 presents the grain size distributions of a modern dust storm sample from Lanzhou City and of typical Quaternary loess and paleosol samples from the Xifeng section on the Chinese Loess Plateau. All the samples are of eolian origin and their grain size distribution curves show three peaks at 0–2 μm, 2–10 μm, and 10–100 μm. The average positions of the three peaks are at 0.74 μm, 4.91 μm, and 31.61 μm and their heights are 0.55%, 1.59%, and 5.23%, respectively. Therefore, Peak 3 > Peak 2 > Peak 1, based on height. The grain size distribution of the Lanzhou dust storm has an additional Peak 4, with its height as 2.57% at 100–100 μm. According to previous studies on Chinese 7

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 8. Elemental analysis. (a) Comparison diagram of trace elemental compositions between red and gray clays of Lanzhou; (b) Comparison of trace elemental compositions between the Lanzhou red beds and upper continental crust. Plots of immobile trace elemental ratios (c) Zr/Hf vs. Y/Nb and (d) Hf/Nb vs. Zr/Nb, and comparisons with Quaternary loess in northern and southern China.

dominant magnetic mineral. The parallel horizontal lamination of the gray-black sample is similar to that of mud shale, and indicates a reducing environment. Such conditions typically develop in lacustrine environments with stagnant water. The gray color is the result of gleization, and occurs in anaerobic underwater conditions. Microorganisms in such environment usually produce minerals that contain reduced iron (Fe2+), such as FeS and magnetite (Ponnamperuma, 1972; Retallack, 2008), and result in the presence of small amounts of magnetite in the gray samples. The grayblack and gray layers are inferred to have been submerged under water for a long period of time, and formed in an anoxic lacustrine environment. The χ–T curves of the Quaternary loess and paleosols from the Chinese Loess Plateau are shown in Fig. 10(a) and (b). The Curie temperature of 580 °C indicates that magnetite is the dominant magnetic mineral. Fig. 10(c) shows the χ–T curve of a Tertiary red clays sample. The significant decrease in magnetic susceptibility at 580 °C and 680 °C suggests that the Tertiary red clays contains both magnetite and hematite. The Lanzhou red samples only show a hematite signal. Since the degree of oxidation of hematite is greater than that of

winter monsoon had not yet occurred in the Cretaceous Period, and northwestern China lacked a major source of dust for short-distance transport. The climate of the Lanzhou area was subtropical, warm, and humid at that time, and the meridional temperature gradient was low (Littler et al., 2011). Dust storms are presumed to have been infrequent. Therefore, the grain size distributions of the Lanzhou clays show a low height of Peak 3. 4.2. Sedimentary environment and climate proxy The variations of the χ–T curves and hysteresis loops suggest that the gray samples contain magnetite. The magnetic susceptibility and saturation magnetization of magnetite are 800 and 230 times higher than those of hematite and goethite (O'reilly, 2012), respectively. However, the SIRM of the gray samples is 100 times lower than that of the red samples. The magnetite content of the gray layers is quite low. The color reflectance a*, magnetic parameters IRM-100mT and χ–T580−680 are all related to the content of hematite (Torrent et al., 1980); the red sample shows a 680 °C Néel temperature and “goosenecked” hysteresis loop. All the facts support that hematite is the

Fig. 9. Grain size distributions and the components of (a) a Lanzhou dust storm, (b) Quaternary loess and (c) Quaternary paleosol. 8

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

Fig. 10. The χ–T curves of (a) Quaternary paleosol (b) Quaternary loess and (c) Tertiary red clays from the Chinese Loess Plateau.

suggest relatively dry conditions.

magnetite, the red clays of the Lanzhou section experienced stronger oxidizing conditions as compared to the Chinese loess and Tertiary red clays. The Lanzhou red layers show gradational changes in texture and color, which is a diagnostic characteristic of soil development (Retallack, 2008). Red soil is found mainly in tropical regions, but the red color can also be interpreted as burial dehydration of goethite to hematite (Retallack, 2008). The Tibetan Plateau had not yet uplifted in the Cretaceous period (Molnar and Tapponnier, 1975), and the NeoTethys Ocean with its northern edge at ∼25°N had not yet shrunk (Scotese, 1991). The paleolatitude of Lanzhou was ∼28°N at this time and the area had a zonal climate in a subtropical zone. Climate in the early Cretaceous was characterized by a warm, stable climate with a lower meridional temperature gradient than today (Littler et al., 2011). Presently, red and yellow soils develop at 28°N. Climate of the Lanzhou basin in Cretaceous was therefore suitable for the development of red soil. The Lanzhou red clays contain massive amounts of hematite and develop in oxidative conditions, and is terrestrial paleosol deposits. The gray clays, in contrast, develop in an underwater environment. Despite these differences in sedimentary environment, the red and gray clays have similar source areas and sedimentary dynamics, and both clays are transported by long-distance winds. Similar ratios of weathering resistant trace elements such as Y, Zr, Hf, and Nb of the red and gray samples indicate that the two types of clays are derived from the same source. Their uniformity in grain size distribution with a dominating 2–10 μm fine component suggest that the two types of clays are carried by long-distance wind and are thoroughly mixed before deposit. The red and gray clays are derived from the same broad source and carried by the same sedimentary dynamics prior to deposition. They could have been quite similar to each other in chemical composition before deposit, resembling Chinese loess and paleosol (Deng et al., 2005; Liu and Ding, 1998). The post-depositional pedogenesis is thus the main factor of magnetic differentiation of clays. Therefore, the magnetic parameters of these deposits can be used as indicators of post-depositional climate change. The climate during the Quaternary interglacial period produces high contents of pedogenic magnetite and hematite. Paleosols is stained by pedogenic hematite, its red color indicates a humid climate (Liu et al., 2007). For the Cretaceous red beds, the gray clays forms in underwater environments, and experiences more humid conditions than the red clays. The red color in the Cretaceous red beds therefore represents relatively dry conditions. The magnetic parameters χ, χfd%, and ARM are sensitive to pedogenic ferrous magnetic particles, and are proxies of pedogenic strength and climate change (Liu et al., 2007). The primary magnetic mineral of the Lanzhou clays is hematite, which is weak in both susceptibility and magnetization, and is difficult to detect with the magnetic parameters χ, χfd%, and ARM. These magnetic parameters therefore have limitations in climatic reconstruction for the Cretaceous Lanzhou clays. However, parameters such as IRM-100mT, which eliminate the effect of ferrous magnetic minerals and preserve the signal of hematite, have the potential to be applied as a climatic proxy. Its high values in red layers

4.3. Alternating red/gray color layers and the climate change A major feature of the Lanzhou red beds is the presence of alternating red and gray layers. Red layers forms in warm and arid hinterland climates. Gray and gray-black layers correspond to cooler, more humid hinterland conditions. The alternating red and gray layers could reflect climatic cycles. The Earth's orbital cycles has been reported in some Cretaceous climate records (Herbert and Fischer, 1986). Model simulation suggests that the northern margin of the Tethys Ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations, supporting a link between orbital periodicities and variations of intense precipitation associated with Cretaceous bedding patterns (Barron et al., 1985). It is likely that the red and gray cycles develops in the Milankovitch band. Milankovitch cycles were widely reported in pelagic mid-Cretaceous limestones (Fiet et al., 2001; Herbert, 1992; Herbert and Fischer, 1986; Mayer and Appel, 1999). For most regions the periodicity reflected a combination of precessional and eccentricity climatic forcing (Flögel, 2001; Herbert, 1992; Herbert and Fischer, 1986). The response to obliquity, though generally small (Park and Oglesby, 1991). Episodes of deep-sea “black shale” deposition occurred in minimal eccentricity with minimal seasonal contrast (Herbert, 1992; Herbert and Fischer, 1986). The dominant cycle of the Lanzhou red beds and the connection between black layer and Milankovitch cycle remains to be investigated. 5. Conclusion (1) The color reflectance a*, SIRM, IRM-100mT, and χ–T580−680 are all sensitive to hematite. These parameters are high in the Lanzhou red layers and low in the gray layers. The red layers have high contents of hematite. It is also supported by χ–T and M–T thermomagnetic curves of the red clays. Magnetic parameters, such as χ, χfd%, and ARM are sensitive to ferrous magnetic minerals and have been widely applied in the paleoclimatic reconstruction of Chinese loess, but cannot extend to the Cretaceous red beds in the Lanzhou basin. IRM-100mT is applicable in such scenarios, and its high values in red layers represents drier conditions. (2) The red layers develop in an oxidizing environment and are analogous to soil, while the gray-black and gray layers form in an underwater reducing environment. However, the red and gray clays have similar sedimentary dynamics as evidenced by their similar grain size distribution. The ratios of the immobile elements Zr, Hf, Y, and Nb are indicators of change in source regions, the red and gray clays have similar distribution of these ratios, indicating that they are derived from the same source. The positions of the three peaks the Lanzhou clays grain size are consistent with those of the eolian Chinese loess, but the height of the 10–100 μm grain size component is significantly lower than that of the eolian loess. This can be attributed to the fact that the short-distance 9

Journal of Asian Earth Sciences 180 (2019) 103865

J. Chen, et al.

modern-like East Asian winter monsoon had not yet evolved in the Late Cretaceous. Both the red and gray clays show high peaks of the 2–10 μm grain size component, which indicates that they are carried by a longdistance transporting force. It is further supported by the wide variety of source regions indicated by the elemental data. Both the red and gray clays are well mixed in the source areas and during long-distance transportation. It is likely that the Lanzhou clay is eolian in origin.

deformation in the Tian Shan: evidence from Mesozoic strata of the north Tarim, south Junggar, and Turpan basins, northwest China. Geol. Soc. Am. Bull. 104, 53–79. Herbert, T.D., 1992. Paleomagnetic calibration of Milankovitch cyclicity in Lower Cretaceous sediments. Earth Planet. Sci. Lett. 112, 15–28. Herbert, T.D., Fischer, A.G., 1986. Milankovitch climatic origin of mid-Cretaceous black shale rhythms in central Italy. Nature 321, 739. Holland, H.D., 1978. The Chemistry of the Atmosphere and Oceans. Wiley. Ji, J., Balsam, W., Chen, J., 2001. Mineralogic and climatic interpretations of the Luochuan loess section (China) based on diffuse reflectance spectrophotometry. Quatern. Res. 56, 23–30. Licht, A., van Cappelle, M., Abels, H.A., Ladant, J.B., Trabucho-Alexandre, J., FranceLanord, C., Donnadieu, Y., Vandenberghe, J., Rigaudier, T., Lecuyer, C., Terry Jr, D., Adriaens, R., Boura, A., Guo, Z., Soe, A.N., Quade, J., Dupont-Nivet, G., Jaeger, J.J., 2014. Asian monsoons in a late Eocene greenhouse world. Nature 513, 501–506. Littler, K., Robinson, S.A., Bown, P.R., Nederbragt, A.J., Pancost, R.D., 2011. High seasurface temperatures during the Early Cretaceous Epoch. Nat. Geosci. 4, 169–172. Liu, Q., Deng, C., Torrent, J., Zhu, R., 2007. Review of recent developments in mineral magnetism of the Chinese loess. Quatern. Sci. Rev. 26, 368–385. Liu, Q., Torrent, J., Maher, B.A., Yu, Y., Deng, C., Zhu, R., Zhao, X., 2005. Quantifying grain size distribution of pedogenic magnetic particles in Chinese loess and its significance for pedogenesis. J. Geophys. Res. 110, B11102. Liu, T., Ding, Z., 1998. Chinese loess and the paleomonsoon. Annu. Rev. Earth Planet. Sci. 26, 111–145. Liu, X., Liu, T., Heller, F., Xu, T., 1990. Frequency-dependent susceptibility of loess and Quaternary paleoclimate. Quatern. Sci. (in Chinese) 1, 42–49. Liu, X., Rolph, T., Bloemendal, J., Shaw, J., Liu, T., 1995. Quantitative estimates of palaeoprecipitation at Xifeng, in the Loess Plateau of China. Palaeogeogr., Palaeoclimatol., Palaeoecol. 113, 243–248. Mayer, H., Appel, E., 1999. Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change in the Early Cretaceous Biancone Formation of the Southern Alps, Italy. Cretaceous Res. 20, 189–214. Mclennan, S.M., Taylor, S.R., Kröner, A., 1983. Geochemical evolution of Archean shales from South Africa I. The Swaziland and Pongola Supergroups. Precambrian Res. 22, 93–124. Molnar, P., Tapponnier, P., 1975. Cenozoic tectonics of Asia: effects of a continental collision. Science 189, 419–426. Nie, J., Zhang, R., Necula, C., Heslop, D., Liu, Q., Gong, L., Banerjee, S., 2014. Late Miocene–early Pleistocene paleoclimate history of the Chinese Loess Plateau revealed by remanence unmixing. Geophys. Res. Lett. 41, 2163–2168. O'reilly, W., 2012. Rock and Mineral Magnetism. Springer Science & Business Media. Park, J., Oglesby, R.J., 1991. Milankovitch rhythms in the Cretaceous: a GCM modelling study. Global Planet. Change 4, 329–355. Ponnamperuma, F.N., 1972. The chemistry of submerged soils. Adv. Agron. 24, 29–96. Retallack, G.J., 2008. Soils of the Past: An Introduction to Paleopedology. John Wiley & Sons. Scotese, C.R., 1991. Jurassic and cretaceous plate tectonic reconstructions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 87, 493–501. Sun, D., 2006. Supper-fine grain size components in Chinese loess and their palaeoclimatic implication. Quatern. Sci. 26, 928–936. Sun, D., Bloemendal, J., Rea, D.K., An, Z., Vandenberghe, J., Lu, H., Su, R., Liu, T., 2004. Bimodal grain-size distribution of Chinese loess, and its palaeoclimatic implications. CATENA 55, 325–340. Sun, Q., Zhou, J., Xiao, J., 2001. Grain-size characteristics of Lake Daihai sediments and its paleaoenvironment significance. Mar. Geol. Quatern. Geol. 21, 93–96. Tauxe, L., Banerjee, S.K., Butler, R.F., van der Voo, R., 2013. Essentials of paleomagnetism: second web edition. Tauxe, L., Mullender, T.A.T., Pick, T., 1996. Potbellies, wasp-waists, and superparamagnetism in magnetic hysteresis. J. Geophys. Res. 101, 571–584. Taylor, S.R., Mclennan, S.M., Mcculloch, M.T., 1983. Geochemistry of loess, continental crustal composition and crustal model ages. Geochim. Cosmochim. Acta 47, 1897–1905. Torrent, J., Schwertmann, U., Fechter, H., Alferez, F., 1983. Quantitative relationships between soil color and hematite content. Soil Science 136, 354–358. Torrent, J., Schwertmann, U., Schulze, D., 1980. Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma 23, 191–208. Turner, P., 1980. Continental Red Beds. Elsevier. Walker, T.R., 1967. Formation of red beds in modern and ancient deserts. Geol. Soc. Am. Bull. 78, 353–368. Xiao, S., Chen, F., Qiang, M., Zhang, J., Zhou, A., Sun, D., 2007. Distribution pattern of grain size in surface sediments from Sugan Lake and its potential in recording aeolian dust in Arid China. Acta Geogr. Sin. 62, 1153–1164. Yang, S., Ding, Z., 2003. Color reflectance of Chinese loess and its implications for climate gradient changes during the last two glacial-interglacial cycles. Geophys. Res. Lett. 30, 2058. Yang, S., Fang, X., Li, J., An, Z., Chen, S., Hitoshi, F., 2001. Transformation functions of soil color and climate. Sci. China Ser. D: Earth Sci. 44, 218–226. Zhang, Y., Sun, D., Li, Z., Wang, F., Wang, X., Li, B., Guo, F., Wu, S., 2014. Cenozoic record of aeolian sediment accumulation and aridification from Lanzhou, China, driven by Tibetan Plateau uplift and global climate. Global Planet. Change 120, 1–15.

Acknowledgments We thank the National Science Foundation of China for financial support (NSFC grants 41602190 for J.C. and 412101041 for X.L., respectively) of J.C. and X.L, Special Project of Fujian Public Welfare Research Institutes (2019R1002-5) and Project of Innovation Team of Fujian Normal University (IRTL1705). The data presented here can be found in the supporting information. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2019.05.010. References An, Z.S., Kutzbach, J.E., Prell, W.L., Porter, S.C., 2001. Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times. Nature 411, 62–66. Barron, E.J., Arthur, M.A., Kauffman, E.G., 1985. Cretaceous rhythmic bedding sequences: a plausible link between orbital variations and climate. Earth Planet. Sci. Lett. 72, 327–340. Bhatia, M.R., Crook, K.A.W., 1986. Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins. Contrib. Mineral. Petrol. 92, 181–193. Chen, J., Liu, X., Kravchinsky, V.A., 2014. Response of the high-resolution Chinese loess grain size record to the 50°N integrated winter insolation during the last 500,000 years. Geophys. Res. Lett. 41, 6244–6251. Chen, P.J., 1987. Cretaceous paleogeography in China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 59, 49–56. Deng, C., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q., Shaw, J., Zhu, R., 2005. Mineral magnetic variation of the Jiaodao Chinese loess/paleosol sequence and its bearing on long-term climatic variability. J. Geophys. Res.: Solid Earth 110, 1–17. Ding, Z., Derbyshire, E., Yang, S., Yu, Z., Xiong, S., Liu, T., 2002. Stacked 2.6-Ma grain size record from the Chinese loess based on five sections and correlation with the deep-sea δ18O record. Paleoceanography 17 5-1-5-21. Ding, Z.L., Sun, J.M., Yang, S.L., Liu, T.S., 2001. Geochemistry of the Pliocene red clay formation in the Chinese Loess Plateau and implications for its origin, source provenance and paleoclimate change. Geochim. Cosmochim. Acta 65, 901–913. Evans, M.E., Heller, F., 2003. Environmental Magnetism: Principles and Applications of Enviromagnetics. Academic Press. Fang, X., Zan, J., Appel, E., Lu, Y., Song, C., Dai, S., Tuo, S., 2015. An Eocene-Miocene continuous rock magnetic record from the sediments in the Xining Basin, NW China: indication for Cenozoic persistent drying driven by global cooling and Tibetan Plateau uplift. Geophys. J. Int. 201, 78–89. Fiet, N., Beaudoin, B., Parize, O., 2001. Lithostratigraphic analysis of Milankovitch cyclicity in pelagic Albian deposits of central Italy: implications for the duration of the stage and substages. Cretaceous Res. 22, 265–275. Flögel, S., 2001. On the influence of precessional Milankovitch cycles on the Late Cretaceous climate system: Comparison of GCM-results, geochemical, and sedimentary proxies for the Western Interior Seaway of North America. Christian-AlbrechtsUniversität, p. 192. Guo, Z.T., Ruddiman, W.F., Hao, Q.Z., Wu, H.B., Qiao, Y.S., Zhu, R.X., Peng, S.Z., Wei, J.J., Yuan, B.Y., Liu, T.S., 2002. Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China. Nature 416, 159–163. Halim, N., Cogné, J.P., Chen, Y., Atasiei, R., Besse, J., Courtillot, V., Gilder, S., Marcoux, J., Zhao, R.L., 1998. New Cretaceous and Early Tertiary paleomagnetic results from Xining-Lanzhou Basin, Kunlun and Qiangtang blocks, China: implications on the geodynamic evolution of Asia. J. Geophys. Res. 103, 21025–21045. Hao, Q., Guo, Z., Qiao, Y., Xu, B., Oldfield, F., 2010. Geochemical evidence for the provenance of middle Pleistocene loess deposits in southern China. Quatern. Sci. Rev. 29, 3317–3326. Hendrix, M.S., Graham, S.A., Carroll, A.R., Sobel, E.R., Mcknight, C.L., Schulein, B.J., Wang, Z., 1992. Sedimentary record and climatic implications of recurrent

10