Environmental Earth Sciences

, Volume 64, Issue 7, pp 1951–1963

Environmental evolution recorded by lipid biomarkers from the Tawan loess–paleosol sequences on the west Chinese Loess Plateau during the late Pleistocene

Authors

    • Institute of Geology and GeophysicsGraduate University, Chinese Academy of Sciences
  • Shuyuan Xiang
    • Faculty of Earth Sciences, State Key Lab of Geological Processes and Mineral ResourcesChina University of Geosciences
  • Kexin Zhang
    • Faculty of Earth Sciences, State Key Lab of Geological Processes and Mineral ResourcesChina University of Geosciences
  • Yulin Lu
    • Development and Research CenterChina Geological Survey
Original Article

DOI: 10.1007/s12665-011-1012-1

Cite this article as:
Zeng, F., Xiang, S., Zhang, K. et al. Environ Earth Sci (2011) 64: 1951. doi:10.1007/s12665-011-1012-1

Abstract

This study provides a reconstruction of the environmental evolution since 128 ka recorded by the lipid biomarkers of the C15–C35n-alkanes, the C13–C33n-alkan-2-ones and the C12–C30n-alkanols isolated from the Tawan loess section, Northwest China. Variations in paleoenvironment are reconstructed from the values of the carbon preference index (CPI), the average chain length (ACL), the L/H (ratio of lower-molecular-weight to higher-molecular-weight homologues), the n-alkane C27/C31 ratios, and the n-alkan-2-one C27/C31 ratio. These parameters indicate the dominance of grasses over the west Chinese Loess Plateau (CLP) during the late Pleistocene. Lower values of the CPI and the ACL values, respectively, indicate stronger microbial reworking of organic matter and changes in plant species, which are both indicative of a warmer-wetter environment. Furthermore, the fluctuations of environment recorded in the Tawan section exhibit ten phases that show obvious cycles between warm periods and cold intervals. This study reveals that changes in the biomarker proxies agree well with changes in the magnetic susceptibility and grain size, and it indicates a huge potential for paleoenvironmental reconstructions by using the n-alkan-2-one and n-alkanol proxies.

Keywords

LoessLipid biomarkerLinxiaLate PleistoceneEarth environment

Introduction

The Chinese loess sediment deposited during the Quaternary is an important archive of global change (Heller and Liu 1984; Liu 1985; Liu and Ding 1998). So far, researchers adopted various proxy indicators to discuss the environmental evolution recorded in loess–paleosol sediments, including grain size (Xiao et al. 1995; Liu and Ding 1998), magnetic susceptibility (An et al. 1991; Liu and Ding 1998), paleontological proxies (Lu et al. 1996; Sun et al. 1997), and inorganic geochemical indices (Gallet et al. 1996; Chen et al. 1999).

More recently, loess sequences and the environment in China have been studied by applications based on molecular biology (Xie et al. 2002, 2003a, 2004; Zhang et al. 2006, 2008; Bai et al. 2009). However, use of multiple biomarker proxies of the biomarkers coupled with other proxies (e.g., magnetic susceptibility and grain size) to reconstruct the paleoenvironment histories in loess deposits is still uncommon. In addition, the same biomarker environmental proxies in Chinese loess deposits, such as carbon preference index (CPI) and average chain length (ACL), sometimes have different significances in different sections (Xie et al. 2004; Zhang et al. 2006; Bai et al. 2009), because of complications presented by organic biological mechanism and the various types of plants in different parts of the vast Chinese Loess Plateau (CLP, 2.7 × 105 km2; Liu 1985). This study aims to reconstruct the paleoenvironment in the east Linxia area during the late Pleistocene based on multi-proxies, including the proxies (CPI, ACL, and others) of lipid biomarkers (n-alkanes, n-alkan-2-ones, and n-alkanols), grain size and magnetic susceptibility (Zeng et al. 2007) from the Tawan section.

Setting and stratigraphy of Tawan loess section

Linxia is an area quite sensitive to environmental variations (Li et al. 1988). It links together the wet region of eastern China, the arid/semi-arid region of Northwest China, and the alpine region of Tibet Plateau (Fig. 1). The climate transitions recorded in the terrigenous and lacustrine sediments of the west Linxia deposited in the Cenozoic have been widely studied (Li et al. 1977; Dettman et al. 2003; Fan et al. 2006; Hong et al. 2007). In contrast, research in the east part of Linxia is relatively limited.
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Fig. 1

Sketch of climate zones in China, showing location of Tawan section. 1 Wet region of eastern China, 2 arid/semi-arid region of Northwest China, 3 Alpine region of Tibet Plateau (simplified after Xi et al. (1984), the spatial map of loess was based on Liu (1985))

The Tawan section (35º38′N, 103º58′E) is situated in the eastern Linxia area, west CLP (Fig. 1). At present, the local annual average temperature and precipitation are around 7°C and 565 mm, respectively. Moreover, 57.4% of the annual rainwater falls in July, August and September (Chinese Natural Resources Database 2009) in association with the East Asian summer monsoon that brings moisture from the Pacific Ocean to inland China.

It is characteristic of loess stratigraphy to contain alternating occurrences of loess (L) and paleosol (S) that are the products of regional and global environmental change during cold and warm climate periods, respectively (Liu 1985). The Tawan loess–paleosol sequences are about 40 m thick and are underlain by the Neogene Maogou Formation (Nm), which is pebbly mudstone (Zhang and Zhu 2006). The loess–paleosol units of the Tawan section are divided into S0, L1 and S1 based on field observation and their magnetic susceptibility records (Liu 1985; Zhou et al. 1990; Zeng et al. 2007). S0 (0–2 m) is a cultivated soil; hence, it was not sampled. L1 (2–33.4 m), a loess complex layer, consists of L1L1 (2–15.8 m), L1S1 (15.8–17.8 m, a weak paleosol), L1L2 (17.8–19.8 m), L1S2 (19.8–21.4 m, a weak paleosol), and L1L3 (21.4–33.4 m). S1 is composed of three paleosols S1S1 (33.4–35 m), S1S2 (36.2–37.8 m), and S1S3 (38.9–40 m), with two loessic interbeds S1L1 (35–36.2 m) and S1L2 (37.8–38.9 m) (Fig. 2).
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Fig. 2

Correlations of strata and magnetic susceptibility (MSUS) records between Tawan and Jiuzhoutai sections. Dates of L1S1 top and 14C shown for the Jiuzhoutai section are from Chen et al. (1993) and Xie et al. (2003a), and those of S0 base (11 ka), L1S1 base (59 ka), S1 top (73 ka), and S1 bottom (128 ka) in the square brackets are derived from the grain-size time series of loess–paleosol sequences on the CLP (Ding et al. 2002) and benthic δ18O record (Pisias et al. 1984; Voelker 2002). The age of the uppermost sample in the Tawan section (depth 2.8 m) was estimated to be 12.6 ka from that of the parallel Jiuzhoutai section

The ages of loess–paleosol sequences are usually obtained via correlations of their magnetic susceptibility curves with δ18O records in deep-sea sediments (Kukla et al. 1988). In addition, correlation of an undated section with another one that has been dated is also a common method. Some age control points of the Tawan section were therefore obtained by correlating with the Jiuzhoutai section (36º03′N, 103º53′E) (Chen et al. 1993; Xie et al. 2003a), about 90 km northwest of the Tawan profile. Consequently, the S1 bottom and S1 top for the Tawan section are correlated to 128 and 73 ka, corresponding to the boundaries of Marine Isotope Stages (MIS) 6/5 and MIS 5/4, respectively. The L1L3 top and L1L1 bottom are correlated to 59 and 27 ka, respectively, corresponding to the MIS 4/3 and MIS 3/2 boundaries. The uppermost sample in L1L1 (MF22-1, at 2.8 m depth) is correlated to 12.6 ka (Fig. 2). The ages for other samples are obtained by linear interpolation depending upon the sedimentation rates.

Analysis of lipid biomarkers

Twenty-five samples for biomarker analysis were selected from the Tawan loess–paleosol sequences, as listed in Table 1. All samples were Soxhlet-extracted with chloroform after grinding to finer than 180 μm. Each sample (about 80 g) was extracted for 72 h, and the extracts were concentrated using a rotary evaporator under reduced pressure and then transferred to a small vial. The total lipid extracts were fractionated by silica gel flash-column chromatography into aliphatic hydrocarbons and non-hydrocarbons eluted with n-hexane and methanol, successively. The hydrocarbons were analyzed by gas chromatography (GC). The non-hydrocarbons containing alcohols and ketones were derivatized with N,O-bis (trimethylsilyl) trifluoroacetamide (BSTFA) for 1.5 h before gas chromatography and mass spectrometry (GC/MS) analysis.
Table 1

Proxies and dominant compounds of n-alkanes for the Tawan loess–paleosol sequences

Sample no.

Depth (m)

Stratum

n-alkanes

Cmax

CPI

ACL

L/H

C27/C31

C23/C31

MF22-1

2.8

L1L1

18, 31

9.36

29.94

0.21

0.14

0.05

MF21-1

5.4

L1L1

17, 31

7.92

29.92

0.40

0.18

0.05

MF20-1

6.2

L1L1

18, 31

9.58

30.04

0.15

0.11

0.05

MF19-2

7.4

L1L1

17, 31

9.72

29.76

0.19

0.12

0.06

MF19-1

8.2

L1L1

17, 31

9.43

30.04

0.31

0.15

0.04

MF18-1

9.4

L1L1

18, 31

10.83

30.09

0.15

0.12

0.03

MF17-1

11

L1L1

17, 31

8.03

30.10

0.32

0.15

0.04

MF16-4

13.4

L1L1

17, 31

11.69

30.53

0.22

0.11

0.02

MF16-1

15.8

L1L1

18, 31

9.48

29.73

0.12

0.10

0.06

MF15-2

16.2

L1S1

17, 31

8.65

30.07

0.30

0.17

0.04

MF15-1

17

L1S1

18, 31

6.98

29.60

0.24

0.18

0.08

MF14-3

18.2

L1L2

17, 31

4.71

28.91

0.53

0.12

0.18

MF14-2

19

L1L2

17, 31

6.72

29.40

1.19

0.20

0.08

MF13-1

20.2

L1S2

18, 31

5.67

29.27

0.33

0.24

0.13

MF12-1

21

L1S2

18, 31

8.10

29.56

0.26

0.17

0.08

MF10-3

24.2

L1L2

17, 31

12.00

30.14

0.21

0.10

0.02

MF9-1

26.6

L1L2

17, 31

10.78

30.42

0.29

0.12

0.02

MF8-2

27.8

L1L2

18, 31

9.10

30.05

0.14

0.12

0.05

MF6-3

31

L1L2

17, 31

14.04

29.97

0.11

0.27

0.03

MF6-1

32.2

L1L2

17, 31

8.40

29.92

0.17

0.27

0.04

MF5-1

33

L1L2

18, 31

6.66

29.40

0.33

0.23

0.11

MF4-1

33.8

S1S1

17, 31

5.31

28.82

0.34

0.43

0.16

MF3-2

37

S1S2

17, 31

7.99

29.61

0.57

0.18

0.07

MF2-1

38.6

S1L2

18, 31

6.29

29.38

0.27

0.20

0.10

MF1-1

39.4

S1S3

17, 31

3.82

28.11

0.50

0.57

0.32

Cmax dominant compounds, CPI (n-alkanes) = ∑C23–35(odd)/∑C22–34(even) (Xie et al. 2003a), ACL = ∑(i × Ci)/∑Ci, i is the carbon number, i ≥ 22, Ci is the compound, L/H = ∑(≤C21)/∑(≥C22) (Xie et al. 2003b)

The hydrocarbons were analyzed using a Shimadzu GC-2010 gas chromatograph fitted with a DB-5 capillary column (30 m length × 0.25 mm inner diameter; 0.25 μm film thickness). The operating conditions were as follows: temperature program increased from 70 to 300°C, rate 3°C/min, and maintained at the final temperature for 30 min. The carrier gas was helium. The relative abundances of n-alkanes with different carbon atoms were obtained via comparisons with internal standards.

GC/MS analysis for non-hydrocarbons was as described by Xie et al. (2003a), the only difference being hold at the final temperature of 300°C for 30 min. Compounds were identified by comparisons of mass spectra with those of reference compound at specific mass-to-charge ratio (m/z), and they were quantified by calculating the peak area of the GC/MS total ion current of the target compound.

Results

Lipid biomarkers detected in the samples from the Tawan section include C15–C35n-alkanes (ALK), C13–C33n-alkan-2-ones (KET) and C12–C30n-alkanols (ACH), containing both lower-molecular-weight (LMW, ≤C21) and higher-molecular-weight (HMW, ≥C22) components (Figs. 3, 4). The biomarker proxies used to reconstruct the environmental history in the Tawan section are listed in Tables 1, 2. The distributions of the lipid biomarkers are addressed as follows.
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Fig. 3

Representative gas and mass chromatograms showing the typical distribution of biomarkers identified from the Tawan section. a, b Gas chromatograms of n-alkanes, c mass chromatograms of n-alkanols, d mass chromatograms of n-alkan-2-ones. Pr pristane, Ph phytane, C18iso C18 isoprenoid ketone, m/z mass-to-charge ratio, the number above the peak is carbon atoms of the homologue

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Fig. 4

A bar graph of the relative abundances of n-alkanes (ALK), n-alkan-2-ones (KET) and n-alkanols (ACH) from the Tawan loess–paleosol sequences. Horizontal axis number of carbon atoms, longitudinal axis relative abundance of the homologue

Table 2

Dominant compounds and parameters of n-alkan-2-ones and n-alkanols in the Tawan loess–paleosol sequences

Sample no.

Depth (m)

n-alkane-2-ones

n-alkanols

Cmax

CPI

ACL

L/H

C27/C31

Cmax

CPI

ACL

L/H

MF22-1

2.8

15, 31

3.50

29.49

0.19

0.34

18, 28

9.67

25.93

0.38

MF21-1

5.4

15, 29

2.65

28.96

0.19

0.54

20, 28

9.60

26.33

0.29

MF20-1

6.2

15, 31

3.28

29.45

0.14

0.38

18, 26

9.90

25.69

0.43

MF19-2

7.4

15, 31

2.86

29.14

0.19

0.53

18, 26

9.44

25.73

0.31

MF19-1

8.2

15, 31

2.83

29.24

0.10

0.42

18, 28

9.77

26.49

0.44

MF18-1

9.4

15, 31

2.78

29.00

0.20

0.57

14, 28

9.09

26.24

0.37

MF17-1

11

15, 31

2.56

29.15

0.12

0.47

18, 28

7.87

26.13

0.20

MF16-4

13.4

15, 29

3.24

29.08

0.18

0.42

18, 28

11.04

26.29

0.42

MF16-1

15.8

15, 31

3.76

29.49

0.14

0.27

18, 26

11.82

26.22

0.24

MF15-2

16.2

15, 31

2.28

28.88

0.12

0.58

18, 28

8.72

26.43

0.33

MF15-1

17

15, 29

2.24

28.46

0.21

0.72

18, 26

7.63

25.05

0.56

MF14-3

18.2

15, 29

4.41

28.95

0.67

0.38

18, 26

11.25

26.47

0.77

MF14-2

19

17, 29

4.04

28.51

0.32

0.68

18, 28

10.48

26.10

0.33

MF13-1

20.2

15, 29

2.39

28.12

0.24

0.85

18, 26

7.04

25.07

0.40

MF12-1

21

15, 29

3.05

28.69

0.20

0.60

18, 26

8.92

25.41

0.45

MF10-3

24.2

15, 31

4.18

29.70

0.05

0.27

18, 28

11.04

26.60

0.25

MF9-1

26.6

15, 31

3.24

29.40

0.11

0.38

18, 28

12.11

27.09

0.15

MF8-2

27.8

15, 31

3.16

29.24

0.19

0.42

18, 26

7.95

25.48

0.58

MF6-3

31

17, 31

3.06

29.02

0.08

0.56

18, 28

8.58

26.52

0.19

MF6-1

32.2

17, 31

3.09

29.12

0.11

0.47

18, 28

9.90

26.61

0.23

MF5-1

33

15, 31

1.99

28.34

0.49

0.68

18, 28

8.67

26.57

0.38

MF4-1

33.8

15, 29

2.69

28.48

0.17

0.86

18, 26

10.27

25.97

0.36

MF3-2

37

17, 29

2.24

27.52

0.17

1.38

18, 26

6.64

25.34

0.70

MF2-1

38.6

19, 31

3.83

29.25

0.21

0.37

18, 28

16.08

27.55

0.19

MF1-1

39.4

15, 29

2.32

28.19

0.52

0.93

18, 26

7.69

25.64

0.92

Cmax dominant compounds, CPI (n-alkan-2-ones) = ∑C23–33(odd)/∑C22–32(even), ACL = ∑(i × Ci)/∑Ci, i is the carbon number, i ≥ 22, Ci is the compound, L/H = ∑(≤C21)/∑(≥C22) (Xie et al. 2003b), CPI (n-alkanols) = ∑C22–30 (even)/∑C21–29 (odd) (Xie et al. 2003a)

n-Alkanes

The n-alkanes from the Tawan loess–paleosol sequences have two maxima, one at C17 or C18, and the second at C31, showing an almost bimodal pattern and strong odd-over-even predominance greater than C22 (Table 1; Fig. 4).

Generally, algae, fungi and bacteria produce mostly the C17n-alkane (Han et al. 1968; Wakeham 1990), whereas higher plants produce large proportions of C27, C29 and C31n-alkanes (Eglinton and Hamilton 1967; Cranwell 1973; Ficken et al. 2000). Measurable amount of C17n-alkane and the largest proportion of C31n-alkane in the sequences indicate a mixed source from microorganisms and higher plants. In addition, n-alkanes strongly resist post-depositional reworking because they have no oxygen-containing functional groups and double bonds (Meyers and Eadie 1993). Consequently, n-alkane distributions and their proxies should be reliable to trace changes in paleoenvironment.

n-Alkan-2-ones

Alkan-2-ones are relatively rarely detected in leaf waxes (Rieley et al. 1991; Szafranek and Synak 2006; Xie et al. 2008), but they are widely found in sediments and soils (Simoneit et al. 1979; Volkman et al. 1981; Xie et al. 2003a; Bai et al. 2009). The n-alkan-2-ones identified from the Tawan section display an odd-over-even predominance and a bimodal pattern; the earlier and later peaks are dominated by C15 (or C17), and C29 (or C31), respectively (Table 2; Fig. 4). Additionally, abundant C18 isoprenoid ketone (C18iso) is detected in all samples.

The origin of n-alkan-2-ones is disputable. Their character of predominant odd-carbon-number components has been interpreted to indicate that they are either the microbial oxidation products of n-alkanes (Allen et al. 1971; Cranwell et al. 1987; Rieley et al. 1991; Xie et al. 2003a), or produced from n-fatty acids by β-oxidation and decarboxylation (Duan and Ma 2001). The distribution of n-alkan-2-ones is similar to that of n-alkanes throughout the Tawan section, implying possibly that these n-alkan-2-ones originate from n-alkanes (Fig. 4). However, they are different in two important aspects: n-alkanes maximize at the C31 component, whereas 36% of the major n-alkan-2-one is the C29 component; yielding the very weak positive correlation between CPI-alk and CPI-ket (R2 = 0.04). Therefore, the origin of n-alkan-2-ones cannot be simply explained by the microbial oxidation of n-alkanes. Although the origin of n-alkan-2-ones is still debatable, n-alkan-2-ones have been successfully applied in paleoenvironmental reconstructions in late Quaternary (Xie et al. 2003a, 2008).

n-Alkanols

The C18 and C28 (or C26) n-alkanols maximize in the LMW and HMW components, respectively, showing a strong even-over-odd predominance (Table 2; Fig. 4). The predominant amounts of C26 or C28 suggest that higher plant waxes were the major source, since the homologues greater than C22 are proposed to be derived from vascular plant waxes (Eglinton and Hamilton 1967; Vioque et al. 1994; Logan et al. 1995), while the low-molecular-weight homologues (<C21) appear to originate from microbial sources (Cranwell 1980; Simoneit and Mazurek 1982), similar to the origin of n-alkanes for the Tawan section.

Discussion

Biomarker proxies and their paleoenvironmental significance

CPI values

The carbon preference index (CPI) is the ratio to evaluate the proportions of the biomarkers with an odd number of carbon atoms relative to those with an even number of carbon atoms (Bray and Evans 1961; Cranwell 1973). Previous studies have observed that the n-alkanes from higher plants almost always show a strong odd-over-even predominance and give high CPI values (Cranwell 1973; Rieley et al. 1991; Ficken et al. 1998; Cui et al. 2008), while those from microorganisms display a weak odd-over-even predominance and have low CPI values (Clark and Blumer 1967; Freeman and Colarusso 2001). Furthermore, the CPI values for n-alkanes (CPI-alk) from higher plants gradually decrease as they are reworked by microbial activity in sedimentary settings (Huang et al. 1996; Freeman and Colarusso 2001; Xie et al. 2003a).

The CPI-alk values range from 3.82 to 14.04 throughout the Tawan section (Table 1). Notably, the lower values correlate to MIS 5 (the last interglacial) and MIS 3 (interstadial), and the higher values correspond to MIS 4 and MIS 2 (the last glacial). Most of lower CPI-alk values are present in the paleosol horizons, whereas the higher values almost always appear in the loess layers, except for the low value of 4.71 (MF14-3) in L1L2 and the high value of 7.99 (MF3-2) in S1S2. The maximum appears in L1L3, and the minimum occurs in S1S3 (Fig. 5). The average CPI-alk value is 6.65 in the palosols (S), clearly lower than that average of 9.15 in the loess layers (L). This difference suggests that stronger microbial activity accelerates the decomposition of the plant organic matter during formation of the paleosols, resulting in the decrease of the CPI-alk values in the loess–paleosol sequence. This result supports the interpretation that microbial activity is ordinarily stronger in a humid/warm environment than in a dry/cold one (Lavee et al. 1998; Bai et al. 2006). A similar pattern is also seen in the Xunyi (Zhang et al. 2006) and Chaona (Bai et al. 2009) sections.
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Fig. 5

Environmental parameters of n-alkanes for the Tawan loess–paleosol sequences (MIS marine isotope stage, dash-dot line no sample determined)

The CPI values for the n-alkan-2-ones and n-alkanols have patterns similar to the CPI-alk values. The values for n-alkan-2-ones (CPI-ket) and n-alkanols (CPI-ach) vary from 1.99 to 4.41 and from 6.64 to 16.08, respectively (Table 2). However, in contrast to the CPI-alk values, the variations of CPI-ket and CPI-ach values more strongly alternate between the loess and paleosol layers. The lower values corresponding to paleosols evidently formed in the warm periods in association with strong microbial reworking, while the higher values correlate to loess layers that accumulated in the cold intervals throughout the Tawan section (Fig. 7).

L/H values

L/H is the ratio of LMW biomarkers to HMW biomarkers, which is a proxy to assess the relative proportion of contributions from microorganisms versus higher plants (Han et al. 1968; Xie et al. 2003b). This proxy has been successfully applied to reconstruct the paleovegetation and paleoclimate recorded in a stalagmite (Xie et al. 2003c) and river sediments (Xie et al. 2003b) in south China.

The L/H values for n-alkanes oscillate from 0.11 to 1.19 throughout the Tawan section (Table 1). Except for the maximum in the L1L2 sample (MF14-2, depth at 19 m), the rest of the values are less than one, indicating a predominant input from higher plants. In addition, most of higher L/H-alk values occur in the paleosol samples, except for the youngest one. There is a weak negative correlation (R2 = 0.25, P = 0.012) between the value of L/H-alk and CPI-alk throughout the Tawan section.

The n-alkan-2-one L/H values (L/H-ket) range from 0.05 to 0.67 (Table 2). Similar to the n-alkane L/H values, the mean value of L/H-ket in samples S1 and L1S1–L1S2 (corresponding to warm periods MIS 5, and MIS 3) is 0.28, which is higher than that of 0.17 in samples L1L3 and L1L1 (corresponding to cold periods MIS 4 and MIS 2). Meanwhile, the highest L/H-ket value appears in the L1L2 (at depth of 18.2 m), which lags behind highest peak of the L/H-alk values in the L1L2 (depth at 19 m) (Fig. 7).

The L/H values for n-alkanols (L/H-ach) in the Tawan section vary from 0.15 to 0.92 (Table 2). Their highest value occurs in sample S1S3 (MF1-1, 39.4 m), which is considered to be correlated to the warmest and wettest period since the late Pleistocene.

Based on these patterns, the higher L/H values might record larger populations of microorganisms in soil ecosystem and stronger microbial activities during the warm periods, consistent with the finding that L/H-alk values are generally higher in warm-wet southeast and southwest China than in cold-dry northwest China (Bai et al. 2006; Wang et al. 2007). The higher values are also in agreement with the finding that more bacteria exist in the paleosols than in the loess layers (Maher and Thompson 1995; Jia et al. 1996). However, the high L/H values for the biomarkers in sample L1L2 might result from some contribution from fungi, and such a possibility needs to be studied further.

ACL values

Average chain length (ACL) is a weight-averaged number of the carbon atoms of lipid biomarkers in a biogeological sample (Poynter et al. 1989; Jeng 2006). Variations of n-alkane ACL values (ACL-alk) related to environmental conditions have been observed in modern higher plants (Cui et al. 2008; Lei et al. 2010), and loess sediments (Zhang et al. 2006, 2008; Bai et al. 2009). Because the ACL-alk values in grasses are higher than those in trees (Lei et al. 2010), the variation of ACL-alk values can to some degree sensitively record changes in plant species.

The ACL-alk values calculated for the HMW components in the Tawan section vary between 28.11 and 30.53 (Table 1). Excursions to higher values in samples L1L2 (depth at 13.4 m) and L1L3 (26.6 m) could be caused by increases in the amounts of grass plants, which produce large proportions of the C31n-alkane (Cui et al. 2008; Lei et al. 2010), whilst lower values recorded in samples S1 and L1S1–L1S2 might result from a relative increase in the warm-adapted woody plants that produce large amounts of the C27 and C29n-alkanes (Cranwell 1973; Lei et al. 2010) (Fig. 5). Additionally, ACL-alk is strongly positively correlated with CPI-alk throughout the Tawan section (R2 = 0.69; Fig. 6a). A similar trend in the ACL-alk values was also observed in the loess–paleosol sequences at Chaona (Bai et al. 2009). However, a different pattern was found in the Xunyi loess–paleosol sequences on the CLP (Zhang et al. 2006). Significantly, the n-alkane C23/C31 ratios (C23/C31-alk) range from 0.02 to 0.32 throughout the Tawan section (Table 2; Fig. 5), showing a highly strong negative correlation with the ACL-alk values (R2 = 0.91; Fig. 6b). Although the n-alkane C23/C31 ratio has been demonstrated to quite sensitively record variations of vegetation and climate in peat deposits (Nott et al. 2000; Xie et al. 2000), the value of this proxy in the loess deposits is still unknown. Accordingly, the source of the n-alkane C23 in the loess deposits and how n-alkane C23/C31 ratios respond to variations of vegetation and environment on the CLP merits further research.
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Fig. 6

Correlations of environmental parameters of n-alkanes (alk), n-alkan-2-ones (ket) and n-alkanols (ach) in all samples from the Tawan section. R2 Pearson’s correlation coefficient of linear regression. P a level of significance

The n-alkan-2-one ACL values (ACL-ket) range from 27.52 to 29.70 (Table 2). Resembling the CPI-ket throughout the section, the ACL-ket values display large variations between loess and paleosol layers, showing lower values in the samples from paleosol horizons (Fig. 7). A decrease in ACL-ket values would be produced by variations of the HMW n-alkan-2-ones, which reflects a change of higher plant species. Furthermore, the trend of ACL-ket values from MIS 5 to MIS 2 is well consistent with the variations of the East Asian monsoon (An et al. 1991).
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Fig. 7

Environmental proxies of n-alkan-2-ones and n-alkanols for the Tawan loess–paleosol sequences in Northwest China

The values of ACL for n-alkanols (ACL-ach) vary from 25.05 to 27.55 (Table 2), showing a positive correlation with CPI-ach (R2 = 0.59; Fig. 6d). Compared to the ACL-ket, their significance in recording the alternate occurrence of loess and paleosol layers is more obvious, but it is less significant in indicating the variations of East Asian monsoon in general trends and patterns (Fig. 7).

C27/C31 ratios in n-alkanes and n-alkan-2-ones

Woody plants typically have n-alkane distributions dominated by the C27 or C29n-alkanes, whereas grassy plants mainly have distributions dominated by the C31n-alkane (Cranwell 1973; Rieley et al. 1991; Maffei 1996; Marseille et al. 1999; Cui et al. 2008; Lei et al. 2010). Moreover, the validity of the n-alkane C27/C31 ratios (C27/C31-alk) as a record the relative change between woody plants and grassy plants has been confirmed in lake sediments (Cranwell 1973; Brincat et al. 2000), red-earth soil (Xie et al. 2003d) and loess–paleosol sequences (Xie et al. 2002; Bai et al. 2009).

The values of C27/C31-alk range from 0.10 to 0.57 (Table 1; Fig. 5). These low values and the dominance of the C31n-alkane in all the distributions (Fig. 4) jointly indicate the important presence of grassy vegetation on the CLP since the last interglacial (Lu et al. 1996; Xie et al. 2002). Furthermore, the n-alkane C27/C31 ratios display variations between loess and paleosol horizons, stronger than the values of CPI-alk and ACL-alk do. Therefore, the C27/C31-alk proxy sensitively records increases in the amount of woody plants in the formation of paleosols during the warm intervals on the CLP.

Additionally, the n-alkan-2-one C27/C31 ratio (C27/C31-ket), which was used to investigate the paleoenvironmental evolution, ranges from 0.27 to 1.38 (Table 2). Notably, it has a very strong inverse correlation with the ACL-ket (R2 = 0.91; Fig. 6c). This finding suggests that this proxy also records information about the variation of plant species induced by environmental change. In comparison with the proxy of C27/C31-alk proxy, the C27/C31-ket is more sensitive in reflecting paleovegetational information.

Environmental evolution reconstructed by the multi-proxies

The environmental evolution recorded in the Tawan loess section was reconstructed by using proxies of the lipid biomarkers, magnetic susceptibility and grain size (Fig. 8). Ten phases of environmental evolution were identified based on the environmental parameters.
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Fig. 8

Environmental evolution (Environ) reconstructed by proxies of the lipid biomarkers, magnetic susceptibility (Zeng et al. 2007) and coarse fraction (>30 μm) for the Tawan section

Phase 1 (MIS 5e, 128–118.8 ka): the paleosol S1S3 developed. The lowest value of ACL-alk, lower ACL-ket value, and the highest n-alkane C27/C31 ratio indicate woody plants probably flourished in this period, possibly the warmest and wettest time during the late Pleistocene. The lipid biomarker proxies correlate well with the magnetic susceptibility and grain size, which have been widely used as proxy indicators of the East Asian summer and winter monsoon, respectively (An et al. 1991; Xiao et al. 1995).

Phase 2 (MIS 5d, 118.8–109.7 ka): the loess S1L2 accumulated. Compared to phase 1, all proxies consistently record a deteriorating change from the warmest-wettest environment to a dry-cold one. The increases of ACL values in n-alkanes, n-alkan-2-ones and n-alkanols probably indicate the lessening presence of woody plants, consistent with the decrease of C27/C31 ratios in n-alkanes and n-alkan-2-ones. Meanwhile, the L/H values of the biomarkers are lower, implying that weak microbial activity occurred in this phase.

Phase 3 (MIS 5c, 109.7–96.3 ka): the S1S2 paleosol horizon formed. Low ACL-ket and ACL-ach values, high values of C27/C31-ket suggest an improving environment. However, the n-alkane proxies are not as sensitive to change as those of n-alkan-2-ones and n-alkanols in this phase.

Phase 4 (MIS 5b, 96.3–86.3 ka): the S1L1 loess layer deposited. Absence of lipid biomarker information, the low magnetic susceptibility and percentage of coarse fraction (>30 μm) suggest a weaker East Asian summer monsoon and winter monsoon during this cool-humid interval.

Phase 5 (MIS 5a, 86.3–73 ka): the paleosol S1S1 formed. The lower values ACL-ket, higher n-alkane C27/C31 ratio and higher values of magnetic susceptibility record a warmer-wetter stage.

Phase 6 (MIS 4, 73–59 ka): the deposition of the L1L3 loess layer. The proxies of C27/C31-alk, ACL-ach and ACL-ket record frequent changes of plant species in the early phase, similar to the variations of magnetic susceptibility and coarse grain size. The environment is dry and cold, perhaps starting with relative more precipitation and higher temperatures.

Phase 7 (MIS 3c, 59–49.9 ka): the L1S2 weak paleosol formed. High ratios of C27/C31-alk and C27/C31-ket, low values of ACL-ket and ACL-alk suggest an increase of woody plants in the ecosystem, indicating the environment is warm-humid.

Phase 8 (MIS 3b, 49.9–38.4 ka): the loess L1L2 accumulated. The decreasing values of C27/C31-alk and C27/C31-ket indicate that the population of the woody plants diminished. Furthermore, the L/H values of n-alkanes and n-alkan-2-ones increase to their highest numbers, probably recording a larger fungal input to the loess sediments. The environment of this phase was probably cool and moist.

Phase 9 (MIS 3a, 38.4–27 ka): the L1S1 weakly developed paleosol formed. Relatively high C27/C31-alk and C27/C31-ket values, and lower ACL-ket, ACL-ach and ACL-alk imply the recovery of woody plants. The environment is warm and humid in this interval.

Phase 10 (MIS 2, 27–12.6 ka): the L1L1 loess accumulated. Lower C27/C31-alk values and higher ACL-ket and low L/H values indicate fewer woody plants and weaker microbial activity under a cold-dry environment in this period. Low-amplitude and high-frequency changes of the environmental proxies are observed during this phase.

Conclusions

The abundant n-alkanes, n-alkan-2-ones and n-alkanols identified in the Tawan loess–paleosol section sensitively record environmental variations that are consistent with the magnetic susceptibility and grain size proxies. This study reveals that the C27/C31-ket ratios reflect quite sensitively climate-induced variations of plant species on the Chinese Loess Plateau. Additionally, the n-alkan-2-ones and n-alkanols appear to be more sensitive than n-alkanes to indicate the environmental evolution for the Tawan section, showing their great potential in future paleoenvironmental reconstructions.

A strong negative correlation between the n-alkane C23/C31 ratios and n-alkane ACL values was observed throughout the Tawan section. Finally, ten environmental phases in the Tawan section are defined on the basis of the multi-proxies and are in accordance with variations of East Asian monsoon during the late Pleistocene (An et al. 1991; Lu et al. 1996; Liu and Ding 1998).

Acknowledgments

Supported by China Geological Survey (No. 1212010610103), National Natural Science Foundation of China (No. 40921062). We sincerely thank Dr. James W. LaMoreaux and the anonymous reviewers for the constructive comments. We thank Xinmin Ma, Lin Chen, Jingfang Lu, Guoqiao Xiao, Jingwei Cui for assistance and discussion. F. Zeng is very grateful to Prof. Shucheng Xie for providing helpful suggestions and GC/MS apparatus. We acknowledge Prof. Philip A. Meyers and Dr. Xianyu Huang for their invaluable work in polishing the manuscript.

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© Springer-Verlag 2011