Facies analysis of the Middle and 3

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Climate and Tectonic Influence on Alluvial Dynamics in the Weihe Basin, Central China Rits, D.S.

2017

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Rits, D. S. (2017). Climate and Tectonic Influence on Alluvial Dynamics in the Weihe Basin, Central China.

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Abstract

A detailed study of the uppermost 221 m of drill core LYH-1 - located in the northern part of the Weihe Basin - reveals considerable environmental variety over the past 1 Myr. Studied parameters include lithofacies, sedimentological properties and micropaleontological content of selected core-samples. Present-day environments in the research area have been used as analogues for past conditions. The paleo-environments documented in the sedimentary record are interpreted as shallow lakes, fluvial settings, soils and eolian environments. The temporal variation of the environments indicates a rapid alternation between climate-driven wet and dry phases, suggesting that these fluctuations are related to changes in monsoon intensity. However, on a larger temporal scale, the sedimentation and accumulation of water is also affected by local tectonics. End-member modelling shows that the siliciclastic grain-size distributions can be ascribed to the mixing of six distinct end-members. Most of the sediments in core LYH-1 have similar characteristics to the loess deposits from the Central Loess Plateau. However, the much higher sedimentation rates and sedimentary structures suggest that these sediments consist for a large part of fluvially reworked loess.

Based on: Rits, D.S., Prins, M.A., Troelstra, S.R., Van Balen, R.T., Zheng, Y., Beets, C.J., Wang, B., Li, X.Q., Zhou, J., Zheng, H.B. (2016) “Facies analysis of the Middle and Late Quaternary sediment infill of the northern Weihe Basin, Central China” Journal of Quaternary Science 31(2) 152-165.

Facies analysis of the Middle and 3

Late Pleistocene sediment infill

of the northern Weihe Basin

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Facies analysis of the Middle and Late Pleistocene sediment infill of the northern Weihe Basin

A broad subdivision of the core into three main intervals can be made, tentatively dated by magnetostratigraphy. From 162 to 221 m (690-1000 ka), brackish wetland conditions with a regular influence from fluvial and/or distal alluvial fan systems prevailed. The interval 90- 162 m (330-690 ka) shows more terrestrial settings with occasional hypersaline lakes. From 0 to 90 m (0-330 ka), the environment was a shallow wetland, albeit with a relatively low diversity of aquatic and terrestrial microfossils compared to the lower interval.

3.1 Introduction

Decades of research on loess deposits in the Central Loess Plateau (CLP) in China have resulted in an extensive sedimentary record of the history of the Asian monsoon (e.g. Liu, 1985; Ding et al., 1998; An et al., 2001; Porter, 2001; Roe, 2009; Sun et al., 2010; Nie et al., 2015). The sediments on the CLP are mostly composed of windblown dust with a dominant source in the Northern Tibetan Plateau, Gobi Altay Mountains, proximal deserts and the Yellow River floodplain (Nie et al., 2015; Licht et al., 2016 and references cited therein). The Quaternary sediments are characterized by alternating series of light yellowish loess layers and dark reddish finer grained paleosols, representing glacial-interglacial cyclicity (An et al., 1991; Porter, 2001). This alternating pattern is the result of the variable interplay between the East Asian Winter Monsoon (EAWM) and the East Asian Summer Monsoon (EASM) in the region (Liu and Ding, 1998). During interglacials the summer monsoon is more intense, with higher temperature and precipitation, which results in the formation of well-developed soils. Dryer and colder conditions during the glacial periods lead to more intense spring and autumn dust storms delivering large quantities of eolian dust and resulting in loess formation on the CLP (Roe, 2009).

The CLP is located on the Ordos Block (Fig. 3.1a), and is surrounded by a sequence of tectonic depressions, which have played a significant role in the development of the Yellow River and its tributaries. The Weihe Basin, directly south of the CLP, is one of these depressions (Fig. 3.1). It is located at a transition zone separating arid north-western China from humid southeastern China (Sun and Wang, 2005). The basin contains a thick sedimentary infill, deposited under similar climatic conditions as on the CLP; therefore, additional information may be obtained from the sedimentary record regarding local paleoclimate. Studies in other basins around the Ordos Block have shown that important paleoclimatic data can be derived from the fluvio-lacustrine sediments that fill tectonic depressions. For example, several paleolakes were recognized in the sedimentary sequence of the Yinchuan-Hetao Graben (Chen et al., 2008b; Yang et al., 2008), to the north-west of the Ordos Block (Fig. 3.1). From these lacustrine sequences, an Early to Middle Holocene climate record was derived that was interpreted in terms of East Asian monsoon history (Zhao et al., 2012). Because as the Weihe Basin is an active rift basin, the sedimentary infill could also contain valuable information about past rifting events, which played a crucial role in the development of the Yellow River and its tributaries. Therefore, core LYH was drilled near Luyang Lake in a wetland complex along the northern flank of the Weihe Basin.

The objectives of this chapter are to describe and interpret the sedimentary history of

the uppermost part of core LYH. We combine several parameters, such as the presence /

absence of sedimentary structures, color reflectance, grain size, bulk carbonate content

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Climate and tectonic influence on alluvial dynamics in the Weihe Basin, Central China

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and micropaleontological content to distinguish different lithofacies. Modern sedimentary environments in the Weihe Basin are used as analogues for the interpretation of the lithofacies in terms of paleo-environmental and paleoclimatic conditions.

3.2 Setting

The drill-site of the core is located in a fault-bounded depression in the north-eastern part of the Weihe Basin (Fig. 3.1a) (Rao et al., 2014), relatively close to the erodible loess deposits of the CLP (Fig. 3.1b,c), and between the Jing River to the west and the Luo River to the east. The Sichuan River is closer to the coring location, but currently has no water. The drill-site is a wetland complex with small ponds that are scattered over an area of 20 km by 10 km, characterized by extreme evaporation and poor drainage, resulting in water salinity values that match marine levels and alkaline soils. The Weihe Basin is a tectonic depression bordered by the Qinling Mountains in the south and the CLP and Ordos Block in the north (Fig. 3.1). The origin and evolution of the basin are closely related to the uplift of the Tibetan Plateau and the formation of the Qinling Mountains (Peltzer and Tapponnier, 1988). The development of the basin is mostly controlled by its southern and northern bounding faults, but several faults run through the basin as well (Zhang et al., 1995). According to Rao et al.

(2014), the Weihe Basin can be classified as a half-graben rift basin, filled with over 7000 m of Cenozoic sediment.

Several major and minor rivers transport sediment to the Weihe Basin. The Yellow River, originating from the northeastern Tibetan Plateau, flows through the eastern part. The Wei River runs west to east in the southern part of the basin and joins the Yellow River near the Sanmen Gorge in the east. The Wei River also originates from the north-eastern Tibetan Plateau and it is the largest tributary of the Yellow River. The Jing and Luo Rivers drain the CLP and are the largest tributaries of the Wei River.

Climate in the Weihe Basin is characterized by hot, humid summers and cold, dry winters.

The area is semi-arid (573 µm a -1) with highest precipitation in June. It is influenced by both the EASM and the EAWM (Sun and Wang, 2005). Regional precipitation decreases gradually from the south-east to the north-west (Du and Shi, 2012). The average temperature is highest in July and August. Dust storms frequently occur in the region during spring (Ding et al., 2001), bringing large amounts of windblown material to the basin.

Figure 3.1. (a) Schematic tectonic overview depicting the active major faults as distributed over the central and eastern part of China. Large strike-slip faults run from east to west separating the North and South China Blocks. The Ordos Block is surrounded by rift basins, which are dominated by normal faulting. The Yinchuan–Hetao and Weihe Basins are indicated with a red square and circle, respectively. The Luyang Lake coring site is located in the Weihe Basin. Modified after Zhang et al. (1995). (b) A digital elevation model of the Weihe Basin and its surroundings. The red dots indicate the locations where surface samples were taken. LYH-1 stands for Luyang Lake core 1 and is also the place where 17 surface samples from small ponds were taken (LYH-DSP), W1/2 for Wei River 1/2, J for Jing River, B for Ba River, L for Luo River, YR for Yellow River, O1/2 for Oxbow 1/2, Y for Yuncheng Salt Lake and D for sand dunes. (c) An approximately north–south cross-section through the Weihe Basin, showing the location of the LYH-1 coring site in relation to the Central Loess Plateau, the Wei River and the Qinling Mountains.

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3.3 Materials and methods

3.3.1 Core LYH

The drilling of core LYH started on 16 March 2012 and lasted until 21 October 2012. The core was located at 34 ̊480 43.4900 N, 109 ̊310 53.9500 E at an altitude of 371 m a.s.l. and reached a depth of 1097.18 m. The PVC core diameter used for the drilling was 80 µm. The total recovery rate was over 95%. The core was divided into three parts (LYH-1, 2, 3), but for this thesis we focus on the top 221 m of core LYH-1, corresponding to a considerable part of the Quaternary as it covers the most recent 1 Myr (Fig. 3.2).

3.3.2 Core scanning, photography and sampling

Core sections of 1.5 m were split into a working and an archive half by means of a Geotek Core Splitter. The archive half was used for non-destructive spectral reflectance analyses, carried out at the ‘Key Laboratory of Surficial Geochemistry’ of the Nanjing University using an Avaatech

XRF Core Scanner. The working half of the core was used for digital photography, description and subsampling. Careful preparation of the core surface is imperative and was accomplished by scraping the sediment surface with a sharp blade. Following the cleaning, the core was described in detail and digital photographs were made. Subsequently, subsamples were taken for additional analyses such as paleomagnetism, grain size, loss-on-ignition, X-ray diffraction, pollen and micropaleontological analyses.

3.3.3 Paleomagnetism

Paleomagnetic measurements were used to establish a preliminary age model (Fig. 3.2). As can be seen from the figure, the uppermost 221 m corresponds approximately to 1 Ma. Three magnetic reversals were observed. The Blake event, corresponding to approximately 120 ka (Laj and Channell, 2007) was found at 43 m. The depth interval 0-43 m is thus characterized by a sedimentation rate of 35.8 cm ka

^-1

. The Brunhes-Matuyama boundary and the onset of the Jaramillo event (corresponding to 780 and 990 ka) (Cande and Kent, 1995) were found at 180.0 and 220 m, respectively. Linear sedimentation rates over the corresponding intervals are 20.8 and 19.0 cm ka

^-1

.

3.3.4 Sampling of modern environments in the Weihe Basin

In total 31 sediment samples from a variety of modern sedimentary environments in the study

area were collected (Fig. 3.1b). Lacustrine samples were taken from the Yuncheng Salt Lake

in the north-east of the Weihe Basin and from shallow circular ponds, close to core LYH, over

an area of <1 km

²

. Two abandoned meanders of the Wei River were sampled to represent

freshwater conditions. A small core of 40 cm was drilled and sampled at 10-cm resolution

to determine the sediment characteristics of the local fluvial deposits. Salinity levels were

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measured at the site by a portable salinity meter. In addition, sediment samples were taken from levees of the Yellow River, Wei River, Luo River, Jing River and Ba River (Fig. 3.1).

Four sediment samples were taken from sand dunes in the center of the Weihe Basin. Finally, data from a loess section at Duanjiapo from Prins et al. (2007) and modern dust data from Sun et al. (2003) are used to interpret our results.

3.3.5 Laboratory analyses

Micropaleontology

Our micropaleontological data are primarily used to discriminate between aquatic and terrestrial environments. Aquatic biota comprises molluscs, ostracods, brine shrimps and algae; these organisms also provide a measure of paleo-salinity. Terrestrial indicators are wood fragments and land snails. The state of preservation of the microfossils is an indication of transport or level of oxygenation.

All pond and oxbow samples collected from the modern environments (n = 16) were analysed for their micropaleontological content. The samples were washed over a >63-µm sieve, dried and studied under a binocular microscope. Mineral components and biota were recorded and semi-quantified. In total, 232 samples from the upper 221 m of core LYH-1 and 16 samples from modern environments were selected for micropaleontological analyses. The samples were left standing overnight in a diluted (1 liter) Na

₄

P

₂

O

₂

•10H

₂

O solution to disperse the sediment. Subsequently, they were washed and dried and analysed as described above.

Figure 3.2 - Age model of core LYH-1 (upper 250 m) based on palaeomagnetic reversals, derived from inclination data. The black and white intervals indicate normal and reversed magnetic polarity, respectively. The Blake Event (~120 ka) is at 43 m, the Bruhnes/Matuyama boundary (~780 ka) is at 180 m and the excursion to the Jaramillo Event (~990 ka) is at 220 m. Based upon the age control points (dashed lines), sedimentation rates are calculated and indicated in the figure.

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Grain size analysis

Grain size analyses of samples from the upper 221 m of core LYH-1 (n = 2127; at 10-cm resolution) and from the extant sedimentary environments (n = 31) were carried out in the sediment laboratory of the VU University Amsterdam. Sample preparation was in accordance with the method described by Konert and Vandenberghe (1997). First, ~2 g of sediment was oxidized with 5-10 mL of 30% hydrogen peroxide (H

₂

O

₂

) to remove organic components.

Subsequently, 5-10 mL of 10% hydrochloric acid (HCl) in a 100-mL suspension was added and heated to boiling point to remove carbonates. After careful rinsing of the goblet glass, the dissolved cations were removed with deionized water and left standing overnight. To prevent aggregation of grains, 300mg of tetrasodium pyrophosphate decahydrate (Na

₄

P

₂

O

₂

•10H

₂

O) was added in a 100-mL suspension and subsequently heated to boiling point. Grain size analysis (range 0.1-2000 µm) was done with a Sympatec HELOS/KR laser diffraction particle sizer, equipped with an advanced wet disperser (QIXEL).

The complete grain-size distribution dataset was decomposed into a series of end-members using an inversion algorithm for end-member modelling of compositional data (EMMA;

Weltje, 1997). EΜMA has been designed to provide the simplest possible decomposition of the measured grain-size distribution dataset. It does not require any case-specific assumptions, i.e. the number of end-members and their distributions do not have to be specified beforehand.

For a detailed description of the technical aspects of EΜMA, we refer to Weltje and Prins (2003) and Prins et al. (2007).

Thermo gravimetric analysis

Thermo gravimetric analysis using a Leco TGA701 analyser was performed in the sediment laboratory of the VU University Amsterdam. Samples were prepared by powdering in a mortar, following an initial overnight drying in an oven at 50˚C. In a controlled environment, weight loss is determined as a function of temperature. In general, a clear separation of the loss of carbon dioxide derived from organic matter and carbonates can be achieved by executing a first ignition step from 105-550˚C during which the organic matter is oxidized under a pure oxygen atmosphere. The second heating step, performed under a pure carbon dioxide atmosphere, from 550 to 1000˚C, causes the different types of carbonate to dissociate.

In this way the carbonate and organic matter contents of 2093 samples from core LYH-1 and 31 samples taken from the present-day sedimentary environments of the Weihe Basin were measured.

3.4 Results

3.4.1 Modern environments

The present-day Weihe Basin hosts a variety of lacustrine, fluvial and eolian environmental

settings (Fig. 3.3). In this section, these environments are described and their deposits

characterized to provide modern analogues for past environmental conditions recorded in

core LYH-1.

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Figure 3.3. This assemblage of photos shows six modern sedimentary (sub-) environments across the Weihe Basin.

(a) Yuncheng Salt Lake (Y in Fig. 3.1b), depicting hypersaline conditions. (b) Brackish pond near core LYH-1 (LYH- DSP). (c) Another pond near core LYH-1, which is covered by pondweed, indicating relative fresh conditions. (d) Dried up oxbow lake (O1 in Fig. 3.1b) containing many mud cracks, representative of a shallow freshwater lake. (e) The Luo River (L in Fig. 3.1b), one of the eight fluvial sample locations. Samples were taken from levees or point bars of a river. (f) Sand dunes with yellow and white sediments (D in Fig. 3.1b). A total of four dunes were sampled.

Sample ID Long Lat Facies n Salinity(‰) Carb. (wt%) D50 (µm)

Y 34°59’25.1 111°01’00.1 Lacustrine (saline) 4 36-61 (49) 9-16 (12) 9-15 (11) LYH-DSP 34°48’46.3 109°31’22.0 Lacustrine (brackish) 10 5-41 (19) 13-43 (22) 4-17 (11)

O1 34°34’23.1 109°37’59.6 Fluvial 1 0.4 16 6.0

O1 (10 cm) 34°34’23.1 109°37’59.6 Fluvial 1 - 15 6.9

O1 (20 cm) 34°34’23.1 109°37’59.6 Fluvial 1 - 16 6.9

O1 (30 cm) 34°34’23.1 109°37’59.6 Fluvial 1 - 16 45.5

O2 34°36’51.5 108°59’53.7 Fluvial 1 0.4 16 7.0

YR 34°36’56.1 108°59’53.7 Fluvial 1 0.6 9 58.5

(J)inghe 34°27’06.7 108°59’53.7 Fluvial 1 0.7 11 41.0

(L)uohe 34°46’31.3 109°52’49.7 Fluvial 1 0.6 10 50.4

(B)ahe 34°11’17.7 109°29’12.4 Fluvial 1 0.1 1 323.7

(W)eihe1 34°26’06.1 109°00’34.9 Fluvial 1 0.5 8 111.5

(W)eihe2 34°22’01.6 108°48’08.1 Fluvial 1 0.4 7 165.7

(W)eihe@O1 34°34’26.5 109°38’37.6 Fluvial 1 0.4 17 38.8

(W)eihe@O2 34°36’18.2 109°45’42.2 Fluvial 1 0.4 14 31.5

Dunes 34°37’56.8 109°47’30.3 Eolian 4 - 2-7 (3) 180-418 (264)

Table 3.1 - Facies types with basic sediment properties of sampled locations within the Weihe Basin

* Letters in parentheses are the abbreviations of rivers, shown in Figure 3.1b. Values in parentheses are mean values of multiple samples. D50, median grain size.

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Shallow lacustrine environment

Two lacustrine sites were sampled.

The lakes in Yuncheng (Fig. 3.3a) are characterized by strongly elevated salinities, ranging from brackish to hypersaline conditions over relative short lateral distances (Table 3.1). The white color of the sediment between the water pools is characteristic for the site. The carbonate content ranges from 9 to 16 wt% (Table 3.1). Figure 3.4a shows the grain size distributions. The modal values range between 9 and 25 µm with an (average) median of 11 µm (Table 3.1). Large amounts of interstitial gypsum crystals are present in two of the four samples. Table 3.2 shows the microfauna and flora of the samples.

Most contain eggs of the crustaceans

Artemia salina (the brine shrimp) and Ephippia sp. Several samples contain

ostracods. Vegetation in the area is scarce with minor grasses and reeds at the fringe zones of the investigated ponds.

Near the drilling site, small ponds in swamp-like conditions were sampled (Fig. 3.3b, c). Grasses and reeds cover this area. The nutrient levels of most ponds are high as shown by the excessive growth of algae, and in some cases the pondweed Potamogeton sp. Salinity is much lower than at Yuncheng and can be regarded as brackish (Table 3.1). The sediments contain between 13 and 43 wt% carbonate (Table 3.1). Notable is the bimodal character of the grain size

Figure 3.4. Grain size distributions from recent sedimentary environments within the Weihe Basin.

(a) Yuncheng Salt Lake (Y). (b) Drilling site ponds (DSP). (c) Oxbow lakes from the Wei River (O). (d) Additional fluvial facies: levee samples of the Yellow River (YR), Wei River (W), Luo River (L), Jing River (J) and the Ba River (B). (e) Eolian facies: sand dunes (D) in the Weihe Basin, Late Quaternary loess deposits at Duanjiapo (Prins et al., 2007) and Xi’an (Sun et al., 2002) and modern dust from Liquan (Sun et al., 2003).

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distribution with modes at 5 and 25 µm (Fig. 3.4b, c). Here no salt flakes or Artemia eggs were found in the samples, but a larger floral and faunal diversity is present (Table 3.2).

Field code Gp. Art. Ep. Ostr.

Ins. Mol. F.r. Rhi. Cha. Pot. Car.

Ca. Il.

Y1 +++ + +

Y2 ++ + +

Y3 + + +?

Y4 + +

LYH-DSP-

CANAL + + + + +

LYH-DSP1 + + + +? + + +

LYH-DSP2 + +

LYH-DSP3 + + + +

LYH-DSP4 + + (+) +

LYH-DSP5 + + + + + +

LYH-DSP6 + + +

LYH-DSP7 + (+) +

LYH-DSP8 + + +

LYH-DSP9 + + + + + + + +

Oxbow-1 + ^†

Oxbow-2

Table 3.2 - Fossil remains from modern depositional environments in the Weihe Basin

*A plus sign in parentheses means that the sample contained a low amount of fossils under consideration; additional plus signs indicate a significantly larger amount within the sample. Gp., gypsum (interstitial); Art., Artemia; Ep., Ephippia;

Ostr., ostracods; Ca., Candona; Il., Ilyocypris; Ins., insects; Mol., molluscs; F.r., fish remains (bone fragments); Rhi, rhizo- liths; Cha, oogonia of Chara; Pot, Potamogeton; Car., carbonate fragments.

†Lynaea sp.LYH-DSP4, 5 contains black chara; LYH-DSP7 contains white chara; LYH-DSP8 contains gaspropod frag- ments; LHY-DSP9 contains black and white chara;

Fluvial environment

Two oxbow lakes of the Wei River were sampled (Fig. 3.1b). Oxbow-1 is characterized by abundant mud cracks on the dry sides of the lake (Fig. 3.3d). Both oxbow lakes are freshwater bodies with a salinity of 0.4‰. The carbonate content of the sediment in both oxbow lakes is 16wt%. A poor grain size sorting with a mode of ~10 µm and a median of 6-7 µm characterizes the surface samples of both oxbows (Fig. 3.4d). Deeper samples from a small core are characterized by a coarser grain size with a mode slightly over 50 µm, a median of 46 µm and a much better sorting compared to the surface samples. Sediment samples from both sites lack microfauna, but they do contain some plant detritus. The sample from the mud-cracked surface of oxbow-1 contained abundant Lymnaea shells, a freshwater gastropod (Table 3.2).

Samples from the levees (Fig. 3.3e) of the Yellow, Wei, Jing, Luo and Ba Rivers show large

differences in grain size distributions. Samples from the Ba River and the upper reaches of

the Wei River are much coarser than those from lower reaches of the Wei River and the Luo

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and Jing River (mode of 45-80 µm) (Fig. 3.4d). Salinity is low (0.5‰) and carbonate content has an average value of ~10 wt%, varying between 1 and 17 wt% (Table 3.1).

Eolian environment

Four samples were taken from dunes in the center of the Weihe Basin. Remarkably, one dune has a white color while its position is directly adjacent to one of the other three, yellow- colored dunes (Fig. 3.3f). The color appears to be linked to carbonate content, as the three yellow-colored sand dunes have a carbonate content of around 2 wt%, while the white- colored dune has a carbonate content of over 7 wt%. The dunes are mainly composed of angular quartz and feldspar grains with low to medium sphericity. Charcoal fragments are scattered through this sediment. The yellow sand dunes have a median grain size ranging from 180 to 232 µm and modal size ranging from 210 to 250 µm. The white sands are much coarser with a median (mode) of 418 µm (500 µm) (Fig. 3.4e). Grain size decreases to the north, which may indicate a source in the south. This is probably the floodplain of the Wei River, given the close proximity and the angular shape of the grains.

Figure 3.4e also shows the grain size distributions of three loess-paleosol samples collected near Duanjiapo village (Prins et al., 2007) (Fig. 3.1b). The three samples cover a time span ranging from the last glacial to the present (L1-S0). Figure 3.4e shows poorly sorted silt, a negative skew (metric scale) and modal values of 20-25 µm. Modern dust data from Sun et al. (2002; 2003) are also included in Figure 3.4e. This has similar characteristics to the Duanjiapo loess samples.

3.4.2 Analyses of core LYH-1

Lithofacies

Five main lithofacies can be distinguished within the studied core section based on color (macroscopic and spectral reflectance), sedimentary structures, textures and grain size, carbonate content and micropaleontology (Fig. 3.5). Figure 3.6a shows a composite core photograph and directly adjacent the stratigraphic distribution of the different lithofacies (Fig. 3.6b), in relation to micropaleontological (Fig. 3.6c) content, spectral reflectance (Fig.

3.6d), bulk carbonate content (Fig. 3.6e), median grain size (Fig. 3.6f) and the grain size end-member contributions (Fig. 3.6g; discussed below). Table 3.3 lists the general facies characteristics. The age constraint based on paleomagnetic analysis is shown in Fig. 3.6h.

Facies 1 is characterized by homogenous yellowish silt that lacks sedimentary structures (Fig. 3.5a). In the top of the core, a very gradual color change (a little more reddish) is associated with a slightly finer grain size. Only fragments of molluscs are found in Facies 1 (Table 3.3). Bulk carbonate ranges from 5.7 to 23.9 wt% and median grain size varies from 10 to 30 µm. Facies 1 is present in the uppermost 7 m, between 20 and 25 m, at 34 m and at intervals of the middle section of the core (from 90 to 120 m) (Fig. 3.6b).

Facies 2 consists of coarser, silty to sandy layers. It often displays distinct sedimentary

structures, mostly caused by loading (Fig. 3.5b, c) and thick-bedded structures of about 2 cm

between layers with slightly varying colors and grain sizes (Fig. 3.5d). The coarser deposits

are often accompanied by fine clay deposits and are rich in greenish micas (Fig. 3.5c). No

biota is present in Facies 2. Carbonate content ranges from 7.4 to 16.1 wt% (Table 3.3).

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Median grain size ranges from 2.8 to 145.0 µm. Facies 2 can be found in intervals in the lowermost (145-221 m) and upper (10-60 m) sections of the core, but is absent in the middle interval (Fig. 3.6b).

Facies 3 is distinguished from Facies 2 by a finer grain size and thinner-bedded structures of ~1 cm (Fig. 3.5e). Lamination (up to 0.5 cm in thickness) is also common in this facies (Fig. 3.5f). The color ranges from yellowish to reddish brown (Fig. 3.5e) to pale greyish (Fig.

3.5f). This facies is rich in micro-fauna, with abundant ostracods, molluscs and fish remains (bone fragments). In the lowermost section of the core, from 195 to 221 m, oogonia of the freshwater algae Chara sp. are common, as well as micro-laminated gypsum, pyrite and wood fragments. The carbonate content ranges between 3.5 and 23.1 wt%, and the median grain sizes are within a much narrower range between 3.1 and 17.5 µm (Table 3.3). Facies 3 is present throughout the core, except for the top 30 m where Facies 1 and 2 are dominant (Fig.

3.6b). On the other hand, Facies 3 dominates the lowermost part of the core (~195-221 m).

Facies 4 is generally composed of greenish grey clays. The deposits sometimes show orange mottling structures (Fig. 3.5g). A fractured texture is occasionally present (Fig. 3.5h).

Slight banding may be visible in the greenish grey clays (not displayed), but more often the sediments are homogeneous. The facies is characterized by an abundance of ostracods and molluscs. In the central part of the studied core interval, this facies also contains numerous faecal pellets of the brine shrimp Artemia salina. Interstitial gypsum crystals (platy, rosettes, shards and laminated) are abundantly present. The carbonate content is significantly higher than in other lithofacies units and varies between 11.6 and 57.1 wt%. The median grain size is similar to Facies 3 and ranges from 4.2 to 17.2 µm. Facies 4 is present from 190 to 195, 180-185, 157-165 (not continuous), 120-134, 92-112 (not continuous), 57-60 and 42-47 m

Figure 3.5 - Sedimentary facies recognized in the upper ~225 m of core LYH-1. (a) Facies 1: homogenous yellowish brown silt, resembling eolian loess. Facies 2 contains characteristic sedimentary structures in relative coarse sediments, with typical examples. (b) Loading structures of fine sand in a clayey matrix. (c) Loading structure of coarse silt layer in a thick fine clay matrix. (d) Thick bedded structures of approximately 2-cm thickness between alternating yellowish and greenish silt. Facies 3 is characterized by finer sediments that often display a banding structure or laminae. (e) Centimeter-scale thin-bedded structures between fine and relatively coarser silt. (f) Laminated (mm scale) to thin-bedded layers in very fine clayey sediments. Facies 4 is mainly characterized by reducing colors, and is rich in carbonates. (g) Dominantly greenish grey mud with contrasting yellowish orange mottles/spots. (h) Massive greenish grey mud. (i) Organic-matter-rich layer. Between the black bands, dark greenish grey mud is deposited. Facies 5 is composed of irregular structures in dominantly reddish brown clay to silty sediments. (j) Pale yellowish carbonate concretions. (k) Strongly fractured reddish clay. (l) Vertically grown gypsum crystals, sometimes accompanied by greenish to pale blue mottling structures.

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Figure 3.6. Facies model and several sediment characteristics from core LYH-1 are plotted against depth. (a) Digital photo of the core. (b) Facies model (see description of the facies in the main text). (c) A simplified model of biotic content derived from micropalaeontological analyses. (d) Lightness index. (e) Bulk carbonate content. (f) Median grain size. (g) Proportional contribution of six grain size end-members (see Figure 3.7 for end- member compositions). (h) Palaeomagnetic data (see Figure 3.2 for explanation).

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(Fig. 3.6b). At the bottom of the studied core-section the organic matter content is slightly elevated (Fig.

3.5i). Facies 4 often alternates with Facies 3, implying a genetic relationship.

Facies 5 consists of yellowish to reddish clay and silty clay. The most characteristic aspect of this facies is the deeply fractured texture (Fig. 3.5j). Interstitial gypsum crystals penetrate the deposits in many places (Fig.

3.5k). Mottling structures of greenish grey clays in a more reddish background are present as well. Other irregular patterns are caused by carbonate concretions (Fig. 3.5k). Apart from these concretions, the carbonate content is generally low, ranging from 2.0 to 16.6 wt%.

The median grain size is 2.3-21.6 µm (Table 3.3).

Facies 5 is found in many intervals of the core (Fig.

3.6b), ranging from thin to relative thick deposits.

Biota are rare in this facies.

3.4.3 End-member modeling results

The grain-size distributions are decomposed in to a series of end-members using the inversion algorithm EMMA. The r

²

statistics show that the LYH-1 grain size data are adequately described as mixtures of six end-members (Fig. 3.7a, b), because the six-end- member model explains on average 95% of the total variance in the dataset. Figure 3.7c shows the six end- member distributions with EM1 being the coarsest and EM6 the finest. The end-members have modes of 150 (EM1), 75 (EM2), 50 (EM3), 30 (EM4), 15 (EM5) and 5 µm (EM6).

Figure 3.6g shows the relative contribution of all end- members in relation to their stratigraphic distribution.

The distribution of EM1 and EM2 corresponds dominantly with Facies 2 intervals. EM1 contributes the least of all end-members over the 221-m interval (~1%) and is exclusively present in Facies 2 (Table 3.3). It shows a series of spikes (coarse events) up to 50% between 145 and 221 m and between 24 and 65 m; EM1 is nearly absent between 65 and 145 m and in the top 24 m. EM2 shows a similar pattern, although its contribution is much higher (0-4%). EM2 is nearly absent in the central interval from 65 to 145 m, but in contrast to EM1, EM2 has a significant contribution in the top 24 m.

FaciesGp.Art.Ost.Mol.F.r.Cha.ML (%)CO3 (wt%)MGS (µm)PEM1PEM2PEM3PEM4PEM5PEM6Rdep (m) 1+45.0-74.9 (58.5)5.7–23.9 (13.0)9.0–49.6 (17.2)0.000.000.170.490.300.04~94 2+45.5–61.4 (54.4)7.4–16.1 (10.8)2.8–145.0 (21.6)0.010.070.210.290.200.22~148 3(+)++++++‡34.7–68.7 (56.1)3.5–23.1 (14.5)3.1–17.5 (9.3)0.000.010.120.260.270.34~208 4++++†+++++45.2–99.0 (73.2)11.6–57.1 (25.1)4.2–17.2 (10.9)0.000.010.090.390.320.19~130 5++34.2–65.9 (53.2)2.0–16.6 (8.4)2.3–21.6 (8.1)0.000.000.040.250.260.45~72 Table 3.3 - General facies characteristics of sediments from core LYH-1* * A plus sign in parentheses means that the sample contained a low amount of fossils under consideration, a plus sign indicates presence, a double plus sign indicates abundantly present. †Only present in the middle section; ‡Only present in the interval from 221 to 195 m; Gp., gypsum (interstitial); Art., Artemia (faecal pellets); Ostr., ostracods; Mol., molluscs; F.r., fish remains (bone fragments); Cha., Chara (oogonia); M, micas; L, lightness; CO3, bulk carbonate; MGS, median grain size. PEM, end-member proportion. RDep, representative depth. Numbers in parentheses indicate average values.

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The average contribution of EM3, EM4 and EM5 is 15, 33 and 25%, respectively.

EM3 fluctuates between 0 and 30%, but it frequently reaches values up to 50-60%. EM3 is found in sediments with loading and bedding structures (Facies 2), but also in layers where these structures are less clear (Facies 1). EM4 is present throughout the core.

EM6 is dominantly present in both the compacted dense clays and reddish brown (often fractured) clays of Facies 3 and 5 (Table 3.3). EM6 mainly fluctuates between 0 and 30% (average: ~15%) with frequent maxima of over 50%. At 70 and 55 m, a relative thick (several meters) and more continuous interval is dominated by EM6.

3.5 Discussion

3.5.1 Genetic interpretation

Facies 1

Based on its color, grain size and structureless character, Facies 1 strongly resembles the loess deposits from the CLP (Pye, 1995; Porter 2001). Facies 1 is dominated by EM3, EM4 and EM5 (Table 3.3). Figure 3.8a shows a comparison of these end-members to the modern dust data of Sun et al. (2003).

EM3 resembles the composition of dust from spring storms at Huanxian, being relatively coarse. EM4 resembles the grain size distribution of dust related to lower energy storms, such as those occurring at Liquan during spring. The grain size of EM4 is comparable to the Late Quaternary loess at Duanjiapo, implying that similar dust input occurred in the southern Weihe Basin during this time interval. EM5 is very

Figure 3.7 - Results from the end-member modelling on grain size data for the uppermost 221 m of core LYH-1.

(a) The mean coefficient of determination (r²) across the full size range as a function of the number of end-members. A six-end-member model explains on average 95% of the observed variance in the grain- size data set. (b) Coefficient of determination (r²) statistics for each size class for end-member models with two to six end-members (EMs). (c) Modelled end-members according to the six-end-member model with end-members representing medium sand (EM1, modal size 150mm), very fine sand (EM2, modal size 75 mm), coarse silt (EM3, modal size 50 mm), medium silt (EM4, modal size 25 mm), fine silt (EM5, modal size 15 mm) and clay (EM6, modal size 4–6 mm).

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similar to sediments sampled at 50-m altitude at Huanxian (Sun et al., 2003), regarded as high atmospheric dust transported by the westerly atmospheric circulation. Prins et al. (2007) studied Late Quaternary dust deposition on the CLP, also using grain size end-member modelling. The close resemblance of EM3, EM4 and EM5 to end-members in their model (Fig. 3.8b) implies similar sediment sources and processes.

They relate the variability of the two coarsest end-members to variability in winter monsoon strength. Their finest end-member is attributed to long-term suspension processes and constant background sedimentation related to a high atmospheric circulation, such as the westerlies.

We conclude that Facies 1 probably represents eolian deposition in a relatively dry environment. In the top 11 m of the core, sediments of Facies 1 are underlain by a reddish soil with finer grained sediments and much lower carbonate content. This is very similar to the loess-paleosol sequences from the CLP, where pedogenesis during warmer and wetter periods causes calcite leaching.

Facies 2

The bedding and loading structures in Facies 2 are clear signs of fluvial deposition. The presence of micas is another indicator for a fluvial or alluvial origin, as these platy minerals are rare in eolian sediments from the CLP (Pye, 1995) and must thus have been transported by flowing water. Table 3.3 shows that Facies 2 is the only facies containing EM1 and a considerable contribution of EM2. EM1 and EM2 cannot be of eolian origin, as grain size distributions measured on the loess sequence of the CLP did not show such

Figure 3.8 - Comparison of the modelled end-members with sediments of known origin. (a) EM1 and EM2 compared to grain size distributions of sediment obtained from the Ba, Yellow, Luo and Wei rivers currently draining the Weihe Basin. (b) EM3, 4 and 5 compared to grain size distributions of modern dust deposited during different seasons near Huanxian (at different altitudes) and during spring at Liquan (data from Sun et al., 2003). (c) EM3, 4 and 5 compared to end-member results from a study of a series of Late Quaternary loess–paleosol sequences from the CLP (data from Prins et al., 2007). (d) EM6 compared to a series of grain- size distributions from a pond near the drilling site.

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coarse sediments (Sun et al., 2002; Prins et al., 2007; Yang and Ding, 2008). The presence of EM1 and EM2 indicates fluvial transport, bringing coarser sediments to the core site. EM1 and EM2 show strong similarities to the levee samples from the Yellow and Luo Rivers, respectively (Fig. 3.8c). However, given the position of core LYH and the current course of the Yellow River, we can exclude that this river was responsible for depositing the sediment, leaving only the Luo River as a possible source. The overall small contribution of EM1 and EM2 (8% combined) indicates that energetic fluvial transport took place only sporadically over the past 1 Ma.

EM3, EM4, EM5 and EM6 dominate in Facies 2 (Table 3.3). In the discussion of Facies 1, the strong similarity of loess deposits with the three coarsest end-members was highlighted.

However, the fact that these end-members are also present in sediments with clear bedding suggests that eolian material on the CLP was eroded and fluvially redeposited at the coring location. In addition, the cumulative thickness of EM3, EM4 and EM5 is much larger than the thickness of contemporaneous loess deposits on the CLP. For example, the 1-Ma boundary in the Lantian loess-paleosol section in the southern Weihe Basin is located at ~60-m-depth (Zheng et al., 1992), whereas the cumulative thickness of EM3, EM4 and EM5 in the core is 161 m (calculated by multiplying the relative contribution of the combined end-members by the total thickness).

The lack of faunal remains is significant, indicating that these relatively coarse sediments were deposited in either shallow and low-energy waters or a dry environment. Most of the sandy layers of Facies 2 are yellowish brown, implying deposition in a subaerial (oxidizing) environment. We conclude that Facies 2 represents fluvial deposits in a distal floodplain or alluvial fan setting. An important part of Facies 2 consists of reworked loess deposits.

Facies 3

The lamination and bedding of Facies 3 indicate gentle and periodic sedimentation in a relatively larger water body. However, most of the regular bedded structures are in brown to greyish brown sediments and organic components are minimal, implying aerobic, shallow- water conditions. In contrast to the first two facies, Facies 3 contains an abundance of flora and fauna, mainly ostracods and molluscs. The dominant occurrence of the ostracod taxa

Ilyocypris sp. and Candona sp. can generally be regarded as an indicator of freshwater

environments, although they can survive in slightly brackish conditions, as long as there is an occasional inflow of freshwater (Li et al., 2010). The flora and fauna, representative for Facies 3, are very similar to the shallow and brackish ponds that currently cover the area where the core was taken (Tables 3.2; 3.3).

Facies 3 consists mainly of end-members EM4, EM5 and EM6 (Table 3.3). The absence of

the three coarsest end-members (especially EM1 and EM2) in this facies points towards a

lower energetic environment compared to the fluvial/alluvial genesis of Facies 2, presumably

stagnant water. EM4 and EM5 probably originate from the CLP, again indicating that erosion

of sediments from the CLP played a significant role in the sedimentation in the northern Weihe

Basin. EM6 is a significant element in Facies 3. It is the finest grained end-member resembling

the samples taken from small ponds near the location of core LYH (Fig. 3.8d). Although

these clearly show mixed sedimentation, expressed by a bimodal grain size distribution, a

fine-grained clay fraction is present with an identical mode as EM6. Dietze et al. (2012)

obtained an end-member with a similar distribution as EM6 in sediments from Lake Donggi

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Cona in the northeastern Tibetan Plateau. They interpreted this as suspended sediment from fluvial and alluvial processes. EM6 is also very similar to low-energy suspension load in abandoned channel fills of the Lower Rhine in Europe (Toonen et al., 2015). We conclude that the presence of EM6 in Facies 3 indicates lacustrine suspension deposition after input from fluvial or alluvial flooding events in a lake environment. Facies 3 probably represents small areas of ponding water, similar to the sampled ponds that are present at the coring site (Fig. 3.3b).

Facies 4

Facies 4 consists mainly of greenish grey mud layers, which is reflected in the lightness curve (Fig. 3.6d). The greenish color implies that the depositional environment was (partly) sub- to anoxic. The micropaleontological content of the samples, including ostracods and molluscs, indicates aqueous conditions. The central part of the studied core interval contains faecal pellets of the brine shrimp Artemia salina, pointing to a hypersaline lacustrine setting (Djamali et al., 2010).

An outstanding feature of Facies 4 is that the carbonate content is much higher than the other facies (Table 3.3; Fig. 3.6e). This is explained by enhanced evaporation, causing carbonates to precipitate (Cohen et al., 2007). The carbonate faunal remains cannot solely explain these high carbonate values, as there is no evidence of an increased number of ostracods, molluscs or other calcite-secreting organisms compared to the other facies. High evaporation rates can also explain the presence of salt lake paleo-environments in the middle part of the core.

The current Yuncheng Lake in the eastern part of the Weihe Basin (Fig. 3.3a) with high- salinity shallow lakes and a white carbonate-rich shoreline is a present-day analogue for the depositional environment of Facies 4. The grain size composition of Facies 4 is almost similar to Facies 3. This implies a comparable origin of the siliciclastic fraction. However, the proportion of EM4 is higher in Facies 4, while the proportion of EM6 is significantly lower. We attribute this to eolian activity. After strong evaporation the sediments are sub- aerially exposed and vulnerable to deflation, eventually causing the finest fraction to be (partly) blown away, a common process in playa settings (Holliday et al., 2008).

Although reducing conditions prevail, oxidation spots are common (Fig. 3.5g), indicating the formation of wetland soils (Richardson and Vepraskas, 2001). The sediments resemble gleysolic soils, which are formed by prolonged water saturation in the soil profile and a fluctuating groundwater level (Bedard-Haughn, 2011). The redoximorphic features are created by partly reducing conditions. An Fe-hydroxide mineral has a distinct greenish color when Fe

³⁺

is reduced to Fe

²⁺

(Feder et al., 2005). Ponding must have occurred for sufficiently long periods to develop the anaerobic conditions necessary; hence the greenish grey mud layers must form in periods of enhanced moisture supply. Nevertheless, deep lake conditions must have been absent to create the oxidation spots. We therefore conclude that this facies represents shallow lacustrine deposition with subsequent evaporation and pedogenesis, similar to playa settings (Yechieli and Wood, 2002; Cohen et al., 2007; May et al., 2015, ).

Facies 5

Facies 5 consists of deposits with irregular non-horizontal structures. The reddish fractured

clay resembles soils and were therefore created under subaerial conditions. Rhizoliths (moulds

of plant roots) and greenish grey mottling structures in reddish brown clayey silt indicate past

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vegetation and confirm pedogenic modification of the sediment (Fisher et al., 2007). After plants die, their organic matter is decomposed by bacteria, turning Fe

³⁺

into the more soluble and reduced form of iron, Fe

²⁺

. Gypsum growth would form by evaporative concentration in shallow groundwater (Fisher et al., 2007). Through capillary rise and evaporative pumping, gypsum crystals were formed in vacancies that were left behind after decomposition of roots.

In contrast to the gleysolic soils of Facies 4, the abundance of interstitial gypsum indicates the formation of gypsisols (Florea and Al-Joumaa, 1998). However, because of frequent flooding, these types of soil could not develop completely.

Facies 5 is mainly composed of EM6 and to a lesser extent of EM4 and EM5 (Table 3.3). This would indicate that soil formation in the wetland preferentially takes place in the relative fine sediments derived from lacustrine suspension plumes. Authigenic clay production during pedogenesis is unlikely, because sedimentation rates are too high to be explained by this phenomenon exclusively.

3.5.2 Depositional model

A schematic depositional model of the northern Weihe Basin, based on the present-day morphology and down-core interpretation of the sediments in core LYH-1, is displayed in Figure 3.9. The facies interpretations and morphology indicate that the coring area must be regarded as a distal part of an alluvial fan system, with shallow ephemeral lakes in periods of reduced fan activity and increased wetness due to, respectively, reduced soil erosion in the CLP (caused by the stabilizing effects of vegetation) and increased precipitation in the basin.

The accommodation space for sediment accumulation is created by the tectonic subsidence of the Weihe Basin (Zhang et al., 1995). In addition, as shown by studies of Rao et al. (2014) and Lin et al. (2015), a series of west-east trending faults and folds developed in the northern part of the Weihe Basin, bounding local subdued depressions, such as the coring area (Fig.

3.9). These local fault- and fold-bounded depressions allow for local shallow lake (pond) formation; a deep lake could not exist here, which explains why core LYH-1 did not record

Figure 3.9 - Schematic overview of the depositional mechanisms responsible for the different facies as recognized from core LYH-1. It shows that different facies exist laterally in space. The figure also indicates two faults, which are responsible for ongoing subsidence and thus creation of accommodation space for sedimentation. The main detachment fault, which marks the northern boundary of the Weihe Basin, is the Beishan Piedmont Fault (BPF). Note:

according to Lin et al. (2015) there are two more flexural folds between the CLP and the core site, but for describing the deposition model it was not necessary to include this in the model.

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any deep lake facies. The depressions are local and shallow sediment traps on a gently sloping surface of what is essentially on a large scale an alluvial fan system (Lin et al., 2015).

Sediments from the alluvial fans mostly derive from the CLP, which explains the lack of widespread sandy deposits that are common in other (distal) alluvial fan settings (e.g.

Blair, 2000; Gutiérrez-Elorza et al., 2005; Paik and Kim, 2006; Fisher et al., 2007). Coarser deposits (EM1 and EM2) point to periods of increased fluvial activity, associated with floods, perhaps during intense rainfall episodes. In addition to alluvial fan processes, flooding by nearby rivers might have had an influence on the sedimentation processes. The Luo River flows relatively close to the core location and could have deposited sediments onto a former floodplain extending to the site. However, it is difficult to distinguish between alluvial fan and fluvial floodplain sedimentation processes, as they essentially result in a similar sedimentary record (North and Davidson, 2012).

The size of the wetland area depends on the areal extent of the alluvial fans, which in turn is related to the climatic state in the region (Werner and Zedler, 2002). Expansion of alluvial fans during periods of enhanced transport capacity and sediment availability causes a reduction of the size of the wetland area. This might have happened during glacial times, when the vegetation cover was at a minimum.

Erosion of the CLP and subsequent sediment accumulation is reduced when vegetated soils cover the loess deposits. Soil formation occurs during interglacials when climate is warm and wet (Kemp, 2001; Porter, 2001). At the same time, the EASM becomes stronger bringing increased amounts of moisture to the region. Reducing conditions in shallow ponds can be explained by a decreased sedimentation rate in a more humid climate. Deposition of carbonates and evaporites indicates a lower moisture supply due to seasonal variation, resembling playa lake conditions. It can be assumed that the size of the wetland was relatively large during interglacials as the Blake event (S1; Marine Isotope Stage 5e) and Brunhes-Matuyama (S7;

Marine Isotope Stage 19) boundary are found where Facies 3 and 4 dominate. Nevertheless, sufficient subsidence must have occurred to let the wetland grow in pace with climate.

3.5.3 General environmental evolution during the last 1 Myr

Unit 3 - 162-221 m (~690-1000 ka)

Facies 3 (lacustrine environment) is dominant over this interval, interrupted by two 5-m-thick playa-like deposits (Facies 4). Conditions were almost continuously wet and freshwater conditions prevailed, which is testified by the consistent occurrence of aquatic species and the presence of terrestrial remains such as wood fragments. The presence of pyrite, micro- laminated gypsum and blackened ostracods (possibly due to pyrite coating) are further signs of reducing conditions. Alluvial fan deposition is more frequent, inferred from the regular presence of Facies 2. A more frequent supply of freshwater could maintain a freshwater environment in the semi-arid (steppe) climate of the Weihe Basin.

Unit 2 - 90-162 m (~330-690 ka)

This section contains intervals with eolian sequences (Facies 1) and is largely devoid of

microfauna, except for a central section (~135-105 m) with common faecal pellets of the

brine shrimp Artemia salina in playa deposits (Facies 4). Except for the hypersaline lakes,

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it seems that this interval shows more terrestrial settings and is much drier than the previous interval. This is surprising, because independent climate proxies in the basin point to an enhanced EASM (Sun, 2005; Bloemendal et al., 2008). The drier conditions during this time interval are therefore assigned to local tectonics. A decreased subsidence results in a diminished accumulation space for water and sediments. As sedimentation proceeds, the shallow depression on the alluvial fan fills up and eventually causes sediments to be flushed down to deeper parts of the basin.

Unit 1 - 0-90 m (~0-330 ka)

The upper section contains varying amounts of ostracods and gastropod fragments.

Conditions must have become wetter, although the absence of terrestrial elements such as wood indicates that the catchment became more open and the lake showed less reducing periods compared to the oldest interval. Facies 2 is absent from 90 to 65 m, indicating that alluvial deposition was negligible in the lower part of this section. It is over this particular interval that a massive package of reddish soils could develop. However, microfauna and the presence of EM6 indicate occasional ponding. The top 7 m of the core shows signs of eolian deposition and thus a relative drier climate.

3.6 Conclusions

Based on grain size, color, texture, sedimentary structures, carbonate content and biotic content, five lithofacies were recognized in the sediments of core LYH-1. After comparison to present-day environments within the Weihe Basin and current geomorphology, these facies can be seen as representing eolian deposition, fluvial deposition (distal alluvial fan), shallow ponds, playa lakes and subaerially exposed soils.

Tectonic vertical motions resulted in accommodation space for sediments and the formation of shallow ponds and playas during warm and wet interglacial periods. Alluvial deposits quickly covered these playa settings during colder/dryer climates, when a decreased soil cover in the catchment could not prevent erosion. Eventually, the sediments became subaerially exposed and soils developed, with in some cases eolian deposition.

An end-member analysis of the grain size data indicates that the bulk of the sediments in core LYH-1 resemble the loess deposits of the CLP. However, the high sedimentation rates and sedimentary bedding structures reveal that these sediments were mostly deposited by fluvial processes and are only partly of a primary eolian origin.

Based on the distinguished facies, a general subdivision into three parts can be made.

Relatively fresh, shallow brackish lake conditions in a reducing environment were present from ~690 to 1000 ka (from ~162 to 221 m core depth). Over the subsequent period, between

~330 and 690 ka (~90-162 m), more terrestrial conditions prevailed, which were interrupted

by periods with hypersaline lakes. The youngest interval from ~0 to 330 ka (~0-90 m) shows

that lacustrine conditions prevailed again, but the lower diversity of aquatic and terrestrial

remains indicates that the surroundings of the basin were more open, compared to the oldest

section described here.

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