Paleovegetation inferred from the carbon isotope composition of long-chain n-alkanes in lacustrine sediments from the Song-nen Plain, northeast China

Abstract

Abundant n-alkanes were identified by GC/MS analysis in core sediments from Xianghai Lake and the Huola Basin, on the Song-nen Plain, northeast China. The n-alkanes extracted from Xianghai Lake samples showed unimodal and bimodal distribution. The main peaks of unimodal distribution were at n-C29 or n-C31, and the mid- and long-chain n-alkanes had odd-carbon-number predominance, suggesting they were derived mainly from terrestrial higher plants. Bimodal distributions of n-alkanes had maximum values centered at n-C17 and n-C31 in all samples. The short-chain n-alkanes with a maximum at n-C17 showed no odd–even predominance, however there was a strong odd-carbon-number predominance of long-chain n-alkanes, with a maximum at n-C31. These results suggest that organic matter in Xianghai Lake was derived from mixed sources, including bacteria, algae and terrestrial plants. The n-alkanes extracted from Huola Basin sediments were characterized by a unimodal distribution, with the maximum value at n-C31, and the long-chain n-alkanes had an odd-carbon-number predominance, indicating that they were derived mainly from terrestrial higher plants. In addition, the compound-specific carbon isotope composition was determined for C27, C29 and C31n-alkanes in the core sediments, and the relative contributions of C3 and C4 plants were estimated using a binary model. Calculations indicated that C3 plants were the dominant input during the late glacial and Holocene. The relative abundance of C3 and C4 plants changed significantly through time, likely determined by cool versus warm climate conditions.

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1 23
Journal of Paleolimnology
ISSN 0921-2728
Volume 54
Number 4
J Paleolimnol (2015) 54:345-358
DOI 10.1007/s10933-015-9856-0
Paleovegetation inferred from the carbon
isotope composition of long-chain n-
alkanes in lacustrine sediments from the
Song-nen Plain, northeast China
Zhifu Wei, Yongli Wang, Baoxiang Wu,
Zixiang Wang & Gen Wang

1 23
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ORIGINAL PAPER
Paleovegetation inferred from the carbon isotope
composition of long-chain n-alkanes in lacustrine sediments
from the Song-nen Plain, northeast China
Zhifu Wei .Yongli Wang .Baoxiang Wu .
Zixiang Wang .Gen Wang
Received: 9 April 2014 / Accepted: 1 September 2015 / Published online: 5 September 2015
ÓSpringer Science+Business Media Dordrecht 2015
Abstract Abundant n-alkanes were identified by
GC/MS analysis in core sediments from Xianghai
Lake and the Huola Basin, on the Song-nen Plain,
northeast China. The n-alkanes extracted from Xiang-
hai Lake samples showed unimodal and bimodal
distribution. The main peaks of unimodal distribution
were at n-C
29
or n-C
31
, and the mid- and long-chain n-
alkanes had odd-carbon-number predominance, sug-
gesting they were derived mainly from terrestrial
higher plants. Bimodal distributions of n-alkanes had
maximum values centered at n-C
17
and n-C
31
in all
samples. The short-chain n-alkanes with a maximum
at n-C
17
showed no odd–even predominance, however
there was a strong odd-carbon-number predominance
of long-chain n-alkanes, with a maximum at n-C
31
.
These results suggest that organic matter in Xianghai
Lake was derived from mixed sources, including
bacteria, algae and terrestrial plants. The n-alkanes
extracted from Huola Basin sediments were charac-
terized by a unimodal distribution, with the maximum
value at n-C
31
, and the long-chain n-alkanes had an
odd-carbon-number predominance, indicating that
they were derived mainly from terrestrial higher
plants. In addition, the compound-specific carbon
isotope composition was determined for C
27
,C
29
and
C
31
n-alkanes in the core sediments, and the relative
contributions of C
3
and C
4
plants were estimated using
a binary model. Calculations indicated that C
3
plants
were the dominant input during the late glacial and
Holocene. The relative abundance of C
3
and C
4
plants
changed significantly through time, likely determined
by cool versus warm climate conditions.
Keywords n-Alkanes d
13
C of long-chain n-
alkanes Paleovegetation C
3
and C
4
plants 
Northeast China
Introduction
Lacustrine sediments are excellent archives for study-
ing high-resolution paleoclimate changes because of
their precise chronology and the large variety of
proxies contained within them (Smol and Cumming
2000; Fagel et al. 2008). Organic molecules are
increasingly used in paleolimnological investigations
as they provide identifiable environmental informa-
tion from different sources (Cranwell et al. 1987;
Ficken et al. 2000; Huang et al. 1999; Castan
˜eda et al.
Z. Wei (&)Y. Wang (&)B. Wu Z. Wang 
G. Wang
Key Laboratory of Petroleum Resources, Gansu Province/
Key Laboratory of Petroleum Resources Research,
Institute of Geology and Geophysics, Chinese Academy
of Sciences, 730000 Lanzhou, People’s Republic of China
e-mail: williamwei2011@hotmail.com
Y. Wang
e-mail: wyll6800@lzb.ac.cn
Z. Wang G. Wang
University of Chinese Academy of Sciences,
100049 Beijing, China
123
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DOI 10.1007/s10933-015-9856-0
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2007). Based on knowledge of n-alkane distributions
in plants, proxies such as the n-alkane average chain
length (ACL), ratio of non-emergent aquatic macro-
phytes to emergent aquatic macrophytes and terrestrial
plants (Paq), the ratio of trees to grasses (n-C
27
/n-C
31
),
and the carbon preference index (CPI) have been
developed to infer climate-induced changes recorded
in lake sediment and peat sequences (Cranwell et al.
1987; Ficken et al. 2000; Meyers 2003). Besides the n-
alkane distributions, compound-specific d
13
C values
can be used to estimate relative contributions of C
3
and
C
4
plants and infer paleoclimate changes, and to
examine past primary productivity (Huang et al. 1999;
Castan
˜eda et al. 2007).
The potential for n-alkanes and compound-specific
carbon isotope proxies to track specific environmental
information and disentangle processes led to paleo-
climate studies in different regions of China, including
the northern South China Sea (Zhou et al. 2012),
northeastern China (Zhou et al. 2010) and the Qinghai-
Tibet Plateau (Zhu et al. 2008; Aichner et al. 2010;
Mu
¨gler et al. 2010; Duan et al. 2011; Wang and Liu
2012). Changes in the relative abundances of C
3
/C
4
plants throughout geologic history were studied using
the carbon isotopic composition of long-chain n-
alkanes in loess/paleosol sequences (Zhang et al.
2003), lake sediments (Street-Perrott et al. 1997;
Huang et al. 2001; Lane et al. 2011) and marine
deposits (Yamada and Ishiwatari 1999; Huang et al.
2007).
Study area
The Song-nen Plain (43°300–48°410N, 121°300–
127°00E) is one of the main regions for grain
production and animal husbandry in northeast China.
The plain is surrounded by the Da Hinggan, Xiao
Hinggan and Changbai Mountains. The area is com-
posed of alluvial, lacustrine and aeolian deposits.
Tectonically, the plain was a large Mesozoic sediment
basin developed on the base of Paleozoic folds and
part of the Cenozoic Song-Liao Fault Basin (Sun
1990). It has a temperate, semi-arid continental
monsoon climate, with an average annual air temper-
ature of 4.9 ±1.5 %, average annual precipitation of
450 ±50 mm, and average annual evaporation of
1450 ±203 mm (Yang 1996). Its hydrologic envi-
ronment is unique in that there are out-flows formed by
the Nenjiang River and the Songhuajiang River. The
most common soil types in the area include black soil
and chernozem, but there are also meadow soils,
swamp soils, halic soils, sandy soils, and paddy soils.
Owing to agricultural expansion, grasslands are
mainly distributed throughout the west of the Song-
nen Plain and interlace with farmland.
Northeastern China has a mix of C
3
and C
4
plants and
is a zone that is sensitive to climate and vegetation
changes. The area possesses a number of lakes and
sediment cores from these water bodies can be analyzed
to reveal these vegetation and climate changes, inferred
from the distribution and compound-specific carbon
isotopic composition of n-alkanes. We analysed the
distribution and compound-specific d
13
Cofn-alkanes in
sediment cores from Xianghai Lake and the Huola
Basin, on the Song-nen Plain, northeast China. We also
investigated the distribution of paleovegetation and the
relative contribution of C
3
and C
4
plants during the late
glacial and Holocene. These data provide important
information for understanding the vegetation distribu-
tion pattern in the regional environment under a global
warming trend.
Study site
Xianghai Lake is located in the Xianghai Wetland
Nature Reserve (44°550–45°090N, 122°050–122°310E),
a freshwater wetland that covers an area of 360 km
2
in
the downstream reaches of the Huolin River (Fig. 1).
The wetland lies at low altitude (156–192 m asl) and
relatively high latitude. The average annual temper-
ature is *5.1 °C. Water and sediment in marshes are
frozen from late October to early April, but start to
melt in late April. Mean annual rainfall is 408 mm. As
the wetland is located in the semi-arid climate zone
and borders the Keerqin Desert, the main hydrologic
input (about 55 %) to the Xianghai wetlands, except
for rainfall, comes from the Huolin River. Because of
the complex landscape, there are diverse plant and
animal resources. According to preliminary field
investigations, there are [600 higher plant species,
of which 263 are medicinal plants belonging to 256
genera in 76 families.
The Huola Basin is located in the north Da Hinggan
Mountains, and lies in the cold-temperate continental
climate zone. Conditions for cold artesian water exist
in the basin. The average temperature in this area is
-49 °C, with an annual temperature range of[75 °C.
The lowest temperatures are typically -45 to -52 °C,
346 J Paleolimnol (2015) 54:345–358
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and highest values are 30–37 °C. The average yearly
ground temperature is -4.2 °C. The frost-free period
is \100 days, and the freeze period is as long as
8 months. Over 80 % of the annual rainfall occurs in
the months of June to September.
Materials and methods
Sediment coring and radiocarbon dating
Two sediment cores were recovered from Xianghai
Lake and Huola Basin in December 2012 (Fig. 1). The
Xianghai Lake core site was at 45°04027.1200N,
122°19034.3200E and the recovered core was 1420 cm
long. The Huola Basin lacustrine sediments were
collected from the Gulian River Open Pit Coal Mine,
Da Hinggan Mountains (53°00038.8800N,
121°57048.2400E) and the core was 300 cm in length.
Twenty-five samples were taken at varying intervals
from each core for analysis of total organic carbon
(TOC), distribution of n-alkanes and compound-
specific carbon isotope composition of n-alkanes.
Five charcoal samples in each sediment core were
collected for accelerator mass spectrometry (AMS)
14
C dating at the Australian Nuclear Science and
Technology Organisation Laboratory, Australia. All
samples underwent a standard hydrochloric acid wash
to remove carbonates. Radiocarbon ages were cali-
brated using CALIB software (Reimer et al. 2009).
Fig. 1 Location of study area and the cores
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Lipid and carbon isotope analysis
Samples were powdered (80–100 mesh) and extracted
with chloroform in a Soxhlet apparatus for 72 h, and
the solvent was removed by distillation. The extracts
were condensed and weighed. Asphalt fractions were
collected through precipitation separation with petro-
leum ether, and aliphatic, aromatic and resin fractions
were eluted by using silica gel-alumina column
chromatography with n-hexane, benzene and ethanol,
respectively. The organic matter analysis was carried
out in the Key Laboratory of Gas Geochemistry,
Institute of Geology and Geophysics, Chinese Acad-
emy of Sciences.
GC–MS analysis was performed using an HP 5973
MSD (Agilent Technologies, Wilmington, DE, USA)
interfaced to an HP 6890 gas chromatograph that was
fitted with a 30 m 90.25 mm-i.d., fused silica cap-
illary column coated with a film (0.25 lm) of 5 %
phenyl-methyl-DB-5. For routine GC analysis, the
oven was programmed from 80 to 300 °Cat3°C/min
with a final hold time of 20 min. Helium was used as
carrier gas at a linear velocity of 32 cm/s, with the
injector operating at a constant flow of 0.9 mL/min.
The MS was operated with an ionization energy of
70 eV, a source temperature of 230 °C and an electron
multiplier voltage of 1900 V over a range of 35–550
Dalton.
The carbon isotopic values of individual n-alkanes
were determined using a gas chromatography-isotope
ratio mass spectrometry (Thermo Scientific MAT 253)
system. d
13
C values of long-chain n-alkanes were
measured by GC (HP6890) with an HP-5 MS silica
capillary column (60 m 90.32 mm 90.25 lm),
connected to an isotope ratio mass spectrometer (GV
Instruments IsoPrime mass spectrometer). The oven
temperature was programmed to be initially held at
80 °C for 3 min, increased to 300 °C at a rate of
3°C/min and held for another 30 min. Each sample
was analyzed twice, and final averaged results were
expressed as %relative to the VPDB (Vienna Peedee
Belemnite) standard.
Calculation of C
3
and C
4
plant percentages
The relative contribution of C
3
plants can be calcu-
lated with a binary model for C
3
and C
4
plant wax n-
alkanes. Long-chain n-alkanes produced by C
3
and C
4
plants have characteristic d
13
C values: -32 to -39 %
and -18 to -22 %, respectively (Rieley et al. 1991;
Collister et al. 1994; Kuypers et al. 1999; Chikaraishi
and Naraoka 2003). In this study we chose -36 %for
C
3
plant n-alkanes and -21 %for C
4
plant n-alkanes
as end members. These values are well accepted and
used for similar calculations (Zhao et al. 2000). The
percent C
3
plant contribution (x) is calculated from the
following formula:
xð36 &Þþð1xÞð21 &Þ¼d13Cmean
ð1Þ
where d
13
C
mean
is the weighted mean average of d
13
C
of C
27
,C
29
and C
31
n-alkanes, in order to reconstruct
vegetation change:
d13Cmean ¼d13 C27 C27 þd13C29 C29 þd13C31

C31Þ=ðC27 þC29 þC31 Þð2Þ
where C
27
,C
29
and C
31
are the relative abundances of
n-C
27
,n-C
29
and n-C
31
.
Results
Lithology and carbon content
Sediments of the Xianghai Lake core were composed
mainly of interbedded sand and mud (Fig. 2). The
TOC values of samples were relatively low, ranging
from 0.04 to 1.11 %, with an average value of 0.25 %
(Table 1). The Huola Basin core is composed mainly
of lacustrine silt (Fig. 2), and the TOC values of the
profile samples ranged from 0.56 to 3.68 %, with an
average value of 1.41 % (Table 1).
Core chronologies
Five charcoal samples from each sediment core were
dated by radiocarbon analysis (Table 2). The age at
the core top was assumed to be zero in both cases, and
age models were derived by linear interpolation
between AMS
14
C dates on the five charcoal samples
in each sediment core. The age-depth relationship in
the two sediment cores is shown in Fig. 2.
Distribution of n-alkanes
Abundant n-alkanes were detected in the core sedi-
ments from Xianghai Lake and the Huola Basin
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(Table 1). The n-alkanes in the Xianghai Lake sam-
ples represent a suite of components ranging from n-
C
13
to n-C
33
, with either unimodal or bimodal
distribution, and maxima at n-C
29
or n-C
31
,orn-C
17
and n-C
31
, respectively. In contrast, the carbon
number distribution of n-alkanes in Huola Basin
deposits ranges from n-C
14
to n-C
33
, and is character-
ized by a unimodal distribution, with the maximum at
n-C
31
(Fig. 3).
Compound-specific carbon isotope composition
of n-alkanes
Compound-specific d
13
C values of the odd-carbon-
number C
27
to C
31
n-alkanes are listed in Table 3.In
the Xianghai Lake core, d
13
C
27
values are between
-34.0 and -28.5 %(average -30.7 %), d
13
C
29
values are between -35.7 and -29.3 %(average
-32.2 %), and d
13
C
31
values are between -36.0 and
-31.3 %(average -32.7 %). The Huola Basin core
showed more
13
C-depleted values. There, the d
13
C
values of the three primary long-chain n-alkanes
ranged from -35.7 to -32.0 %,-36.7 to -32.4 %,
and -36.9 to -32.9 %, and had average values of
-33.2, -34.3 and -34.3 %, respectively for n-C
27
,n-
C
29
and n-C
31
. The d
13
C values of the two cores show
that the n-alkanes get systematically more
13
C-
depleted with increasing chain length.
Discussion
Origin of the sediment n-alkanes
The n-alkanes are widely present in plants and other
organisms. The source of organic matter can be traced
by distribution characteristics of n-alkanes because
different biological sources of n-alkanes possess differ-
ent distribution characteristics. Previous studies showed
that n-alkanes from lower organisms range from n-C
15
to n-C
20
,oftenwithn-C
17
or n-C
19
as the dominant
compounds, and without obvious odd-over-even pref-
erence (Cranwell et al. 1987). In contrast, n-alkanes
from modern terrestrial higher plants are mainly long-
chain compounds, i.e. n-C
27
,n-C
29
and n-C
31
, and show
an apparent odd-over-even preference, with CPI values
Fig. 2 Age-depth model
based on a linear
interpolation between dates
of charcoal samples
aXianghai Lake core,
bHuola Basin core. The
core top was assigned an age
of zero
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Table 1 The TOC and biomarker parameters of the Xianghai Lake and Huola Basin core sediment samples
Depth (cm) TOC (%) Most abundant
compound
CPI
17–21
a
CPI
23–31
b
OEP
27–31
c
ACL
27–33
d
Rn-C
21
-
/Rn-C
22
?
The Xianghai lake core samples
115 0.24 21 1.11 2.02 2.71 29.1 0.87
213 0.39 29 1.13 2.58 4.12 29.3 0.72
283 1.11 29 1.19 6.37 7.16 29.4 0.26
337 0.87 29 1.55 8.49 10.17 29.5 0.24
425 0.56 31 1.11 10.6 11.68 29.8 0.10
477 0.49 31 1.13 11.84 12.99 29.8 0.07
497 0.42 31 1.10 10.91 12.35 29.7 0.08
515 0.25 31 1.13 5.63 6.93 29.7 0.23
529 0.24 31 1.13 7.53 8.90 29.7 0.17
559 0.22 31 1.13 4.61 5.87 29.7 0.37
609 0.06 17/31 1.13 2.46 3.13 29.4 1.58
639 0.11 17/31 1.13 3.01 3.38 29.5 0.96
731 0.05 17/31 1.13 1.70 2.07 29.6 1.60
781 0.04 17/31 1.13 1.95 2.92 29.6 2.56
833 0.05 17/31 1.13 3.60 5.09 29.8 1.12
851 0.06 17/31 1.13 4.65 5.85 29.8 0.64
875 0.05 17/31 1.13 3.58 4.34 29.9 0.90
905 0.06 17/31 1.13 4.72 5.66 29.9 0.44
925 0.06 17/31 1.13 4.82 5.65 30.1 0.49
1067 0.30 17/31 1.13 6.99 7.64 29.8 0.08
1135 0.14 17/31 1.13 5.75 6.37 29.7 0.21
1207 0.06 17/31 1.13 3.90 5.51 30.0 1.07
1347 0.09 17/31 1.13 6.07 6.55 29.9 0.29
1356 0.28 17/31 1.13 6.67 7.24 29.9 0.15
1385 0.09 17/31 1.13 3.13 3.60 29.5 0.70
Average 0.25 1.15 5.34 6.31 29.7 0.64
The Huola Basin core samples
18 1.88 31 1.50 4.16 4.80 29.8 0.24
39 1.51 31 1.38 4.37 5.44 29.5 0.21
65 3.14 31 1.33 4.16 5.28 29.6 0.27
86 2.09 31 1.35 4.14 5.06 29.6 0.31
105 1.72 31 1.43 4.77 5.59 29.5 0.19
111 1.94 31 1.57 4.50 5.36 29.5 0.18
140 1.21 31 1.31 4.62 5.27 29.8 0.14
156 1.26 31 1.42 4.08 4.71 29.8 0.14
175 1.14 31 1.30 4.34 5.26 29.7 0.16
181 1.38 31 1.26 3.36 4.31 29.6 0.16
186 1.19 31 1.28 4.74 6.28 29.8 0.18
190 1.09 31 1.27 5.09 6.19 29.9 0.18
193 0.57 31 1.24 4.76 5.83 29.8 0.17
198 1.16 31 1.38 4.95 6.06 30.0 0.14
202 1.80 31 1.45 5.29 6.70 29.9 0.14
206 1.11 31 1.29 5.28 6.48 30.1 0.15
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generally[5 (Eglinton and Hamilton 1967; Rieley et al.
1991). The n-alkane distribution exhibits high odd-to-
even predominance in long-chain (C
25
–C
35
)n-alkanes,
which characterizes hydrocarbons from vascular land
plants versus those in petroleum and bacteria (Gearing
et al. 1976; Farrington 1980). One common variable
derived from this predominance is the carbon prefer-
ence index (CPI). The CPI is an indication of n-alkane
source. Hydrocarbons composed of a mixture of
compounds originating from land plant material show
a predominance of odd-numbered carbon chains with
CPI =5–10 (Rieley et al. 1991; Hedges and Prahl
1993), whereas petrogenic inputs have a CPI of about
1.0 (Saliot et al. 1988; Pendoley 1992). CPI values close
to 1 are also thought to indicate greater input from
marine microorganisms and/or recycled organic matter
(Kennicutt et al. 1987). In organic geochemistry, CPI is
used to indicate the degree of diagenesis of straight-
chain geolipids, and is a numerical representation of
how much of the original biological chain length
specificity is preserved in geological lipids (Meyers and
Ishiwatari 1995).
Table 1 continued
Depth (cm) TOC (%) Most abundant
compound
CPI
17–21
a
CPI
23–31
b
OEP
27–31
c
ACL
27–33
d
Rn-C
21
-
/Rn-C
22
?
210 0.67 31 1.37 4.51 5.74 30.0 0.15
220 0.59 31 1.16 4.5 5.77 29.7 0.26
228 0.56 31 1.13 4.69 6.11 29.7 0.27
235 0.80 31 1.13 4.59 5.82 29.6 0.25
243 0.84 31 1.12 4.28 5.06 30.0 0.20
248 1.03 31 1.17 4.35 5.00 29.8 0.25
262 3.68 31 1.32 3.8 4.93 29.3 0.40
289 1.68 31 1.30 4.39 5.36 29.5 0.21
296 1.16 31 1.17 4.21 5.12 29.6 0.17
Average 1.41 1.31 4.48 5.50 29.7 0.25
a
CPI
17–21
=0.5 9[(C17 ?C19 ?C21)/(C16 ?C18 ?C20) ?(C17 ?C19 ?C21)/(C18 ?C20 ?C22)]
b
CPI
23-31
=0.5 9[(C23 ?C25 ?C27 ?C29 ?C31)/
(C22 ?C24 ?C26 ?C28 ?C30) ?(C23 ?C25 ?C27 ?C29 ?C31)/(C22 ?C24 ?C26 ?C28 ?C30 ?C32)]
c
OEP
27–31
=(C27 ?69C29 ?C31)/[4 9(C28 ?C30)]
d
ACL
27–33
=(27 9C27 ?29 9C29 ?31 9C31 ?33 9C33)/(C27 ?C29 ?C31 ?C33)
Table 2 Dates in two cores Lab. code Depth (cm) AMS
14
C age (a BP) Calibrated age (a BP) Material
Xianghai Lake core
Ansto-XH-1 160–162 1800 ±40 1550 ±62 Charcoal
Ansto-XH-2 410–412 3400 ±40 3800 ±73 Charcoal
Ansto-XH-3 890–892 6800 ±45 7600 ±84 Charcoal
Ansto-XH-4 1150–1152 9880 ±40 10,620 ±93 Charcoal
Ansto-XH-5 1394–1396 12,580 ±40 13,410 ±102 Charcoal
Huola Basin core
Ansto-HL-1 18–20 120 ±40 80 ±31 Charcoal
Ansto-HL-2 120–122 3590 ±40 3905 ±82 Charcoal
Ansto-HL-3 160–162 5780 ±45 6568 ±106 Charcoal
Ansto-HL-4 186–188 7050 ±40 7888 ±70 Charcoal
Ansto-HL-5 296–298 19,270 ±40 19,800 ±63 Charcoal
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The n-alkanes extracted from Xianghai Lake sam-
ples show unimodal and bimodal distribution
(Table 1). The main peaks of unimodal distribution
were at n-C
29
or n-C
31
, and the long-chain n-alkanes
had an obvious odd-carbon-number predominance
(CPI
23–31
: 2.58–11.84, average: 7.62; odd–even pre-
dominance (OEP)
27–31
:4.12–12.99, average: 8.91),
indicating that they were mainly derived from terres-
trial higher plants. The characteristic bimodal distri-
bution of n-alkanes had maximum values centered at
n-C
17
and n-C
31
in all samples. The short-chain
alkanes showed no obvious OEP, with a maximum
at n-C
17
(CPI
17-21
: 0.91–1.18, average: 1.07). In
contrast, the long-chain alkanes had a strong odd-
carbon-number predominance of long chain n-alka-
nes, with a maximum at n-C
31
(CPI
23–31
: 1.70–6.99,
average: 4.20). These results suggest that the organic
matter was derived from mixed sources of lower
bacteria and algae, as well as terrestrial higher plants.
The n-alkanes extracted from Huola Basin were
characterized by a unimodal distribution, with the
maximum value at n-C
31
, and the long-chain n-alkanes
had an obvious odd-carbon-number predominance.
Calculated OEP
27–31
values throughout the entire
section ranged from 4.31 to 6.70 and had an average
value of 5.42 (Table 1), indicating that they were
mainly derived from terrestrial higher plants.
Paleovegetation types of the study area
Modern organic geochemistry of molecules shows that
the ratio Rn-C
21
-
/Rn-C
22
?
reflects the proportion of
lower organisms such as bacteria and algae relative to
higher plants (Xie et al. 1999,2003; Xie and Evershed
2001). As shown in Table 1, the ratio Rn-C
21
-
/Rn-
C
22
?
ranged from 0.07 to 2.56 (average 0.64) and 0.14
to 0.40 (average 0.25), respectively, for the Xianghai
Lake core and Huola Basin core sediments, suggesting
that terrestrial higher plants were the main source of
organic matter during the late glacial and Holocene.
From 8.0 to 6.0 cal ka BP, however, the ratio in
Xianghai Lake was[1.0 (Fig. 4), indicating relatively
greater input from bacteria, algae and aquatic plants
under warmer climate and lower lake level. During the
late glacial and late Holocene, the ratio was \1.0,
suggesting that higher plants dominated under colder
climate conditions. The ratio of these n-alkanes in the
Xianghai Lake core sediments was high in the interval
11.5–8.0 cal ka BP (Fig. 4), indicating that higher
plants were replaced as an organic matter source by
bacteria, algae and aquatic plants. During
8.0–5.0 cal ka BP, the ratio declined, indicating that
bacteria, algae and aquatic plants were replaced by
higher plants as an organic matter source, whereas
from 5.0 cal ka BP to present, the ratio increased,
Fig. 3 The distribution of
n-alkanes in the Xianghai
Lake core and Huola Basin
core
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Table 3 The d
13
C values
of three primary long-chain
n-alkanes (n-C
27
,n-C
29
and
n-C
31
) and calculated
relative contribution of C
3
and C
4
plants of the
Xianghai lake core and
Huola Basin core
Depth (m) d
13
C(%)C
3
(%) C
4
(%)
n-C27 n-C29 n-C31 C
mean
The Xianghai lake core samples
115 -30.3 -32.3 -32.5 -31.7 71 29
213 -29.3 -29.3 -31.8 -30.2 61 39
283 -29.0 -31.0 -32.6 -31.1 67 33
337 -28.4 -30.2 -31.8 -30.3 62 38
425 -30.7 -32.4 -33.1 -32.5 76 24
477 -31.5 -32.8 -33.7 -33.0 80 20
497 -30.9 -32.8 -33.4 -32.7 78 22
515 -31.5 -32.1 -33.0 -32.4 76 24
529 -32.4 -32.9 -33.8 -33.2 81 19
559 -32.0 -32.2 -32.1 -32.1 74 26
609 -30.0 -33.3 -32.6 -32.2 74 26
639 -32.5 -33.2 -33.0 -33.0 80 20
731 -28.5 -31.2 -31.5 -30.6 64 36
781 -30.1 -32.7 -32.6 -32.0 73 27
833 -31.6 -33.0 -33.5 -32.9 80 20
851 -30.0 -31.8 -32.0 -31.6 70 30
875 -29.4 -31.9 -30.8 -30.9 66 34
905 -31.1 -31.4 -33.1 -32.1 74 26
925 -29.3 -30.5 -31.3 -30.8 65 35
1067 -31.2 -32.3 -32.7 -32.2 75 25
1135 -34.0 -35.7 -36.0 -35.5 97 3
1207 -30.1 -32.3 -32.6 -32.1 74 26
1347 -31.1 -33.0 -33.0 -32.7 78 22
1356 -32.2 -33.0 -33.2 -33.0 80 20
1385 -29.8 -31.0 -31.8 -31.0 67 33
Average -30.7 -32.2 -32.7 -32.1 70 30
The Huola Basin core samples
18 -32.1 -33.3 -32.9 -32.8 78 22
39 -32.6 -33.9 -34.0 -33.5 83 17
65 -32.2 -32.4 -33.2 -32.7 78 22
87 -33.7 -34.4 -33.3 -33.7 85 15
105 -32.0 -32.8 -33.0 -32.6 78 22
111 -33.8 -35.5 -35.5 -34.9 93 7
139 -32.2 -33.1 -33.4 -33.0 80 20
157 -32.2 -33.7 -33.8 -33.3 82 18
176 -33.2 -34.7 -34.6 -34.2 88 12
180 -33.0 -34.7 -34.6 -34.1 87 13
186 -35.7 -36.7 -35.6 -35.9 99 1
190 -32.6 -33.4 -33.8 -33.4 82 18
193 -33.4 -34.6 -34.7 -34.3 89 11
198 -32.8 -34.1 -34.2 -33.8 85 15
202 -33.0 -34.9 -34.6 -34.3 89 11
206 -32.8 -33.5 -34.1 -33.7 84 16
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suggesting that bacteria, algae and aquatic plants were
again dominant as the source of organic matter. Ratios
in Huola Basin sediments were \1.0 throughout the
record and fluctuated little, suggesting that terrestrial
higher plants were the main source of organic matter
during the late glacial and Holocene.
The n-alkane average chain length (ACL) value is
the concentration-weighted mean chain length of the
C
27
,C
29
,C
31
, and C
33
n-alkanes (Poynter et al. 1989).
In warmer climates, land plants biosynthesize longer-
chain compounds with higher melting points for their
waxy coatings, whereas in cool, temperate regions,
somewhat shorter-chain compounds are produced
(Gagosian and Peltzer 1986). The ACL values of n-
alkanes from plants that grow in warm climates are
consequently larger than those of plants from cooler
regimes (Zhou et al. 2010). A fundamental assumption
for using ACL as a proxy for past vegetation is that leaf
lipids derived from grasslands, on average, have
longer carbon chain lengths than leaf lipids from
forest plants (Cranwell 1973). But a comprehensive
review by Bush and McInerney (2013) summarized
ACL values for alkanes in modern plants from around
the world, and found that ACL was unable to
distinguish graminoids (grasses) from woody plants.
Wang et al. (2015) argued that considerable caution is
necessary in using ACL values as a proxy indicator for
vegetation dynamics, and for interpreting ACL vari-
ation in terms of past changes in environment and
climate.
The ACL values in the Holocene sediments are
overall relatively larger than those of the late glacial
deposits (Fig. 4). The ACL values of the two cores
display an increasing tendency during the late glacial,
whereas during the Holocene, the ACL values show a
decreasing trend. The ACL values of n-alkanes
derived from Xianghai Lake core sediments increased
during the interval 11.5–9.0 cal ka BP (Fig. 4), indi-
cating that woody plants were progressively replaced
by grasses, but that trend reversed from 9.0 to
6.0 cal ka BP, as ACL values declined, indicating
grasses were replaced by woody plants. In the interval
6.0–5.0 cal ka BP, the values again increased, sug-
gesting a replacement of forest by grassland plants, but
from 5.0 cal ka BP to present, the ACL value
decreased, suggesting that grasslands gave way to
the spread of woody plants. In contrast, the ACL
values of Huola Basin only ranged from 29.4 to 29.8
throughout the record, without significant fluctuation
(Fig. 4).
Seki et al. (2012) demonstrated that ACL values
can distinguish trees from shrubs and sedges, in that
shrubs and sedges have higher ACL values ([29) than
trees (*27), as summarized by Kirkels et al. (2013).
The ACL values of Xianghai Lake and Huola Basin
samples are [29.0 and range from 29.1 to 30.1
(mean =29.7) (Table 1; Fig. 4), indicating that veg-
etation types of the study area were mainly shrubs and
sedges during the late glacial and Holocene.
Paleovegetation composition of the study area
The d
13
C records of long-chain alkanes have been
used to estimate the relative abundances of C
3
and C
4
plants at some sites (Huang et al. 2006; Castan
˜eda and
Schouten 2011; Seki et al. 2010; Sun et al. 2013).
Table 3 continued Depth (m) d
13
C(%)C
3
(%) C
4
(%)
n-C27 n-C29 n-C31 C
mean
210 -34.4 -35.4 -33.2 -34.1 87 13
220 -34.0 -35.7 -36.9 -35.7 98 2
228 -33.5 -35.1 -35.2 -34.7 91 9
235 -33.4 -34.0 -34.2 -33.9 86 14
244 -33.0 -34.1 -34.6 -34.1 87 13
248 -33.4 -34.1 -35.0 -34.3 89 11
263 -33.4 -34.3 -34.1 -33.9 86 14
290 -33.3 -34.0 -34.1 -33.8 86 14
297 -33.5 -34.2 -34.7 -34.2 88 12
Average -33.2 -34.3 -34.3 -34.0 86 14
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Long-chain n-alkanes mainly derive from terrestrial
higher plants. Terrestrial higher plants assimilate
atmospheric CO
2
mainly via two photosynthetic
pathways, i.e. the C
3
and C
4
pathways. The C
4
or
Hatch-Slack pathway has evolved as a CO
2
-concen-
trating mechanism in which CO
2
initially combines
with phosphoenol pyruvate to form a 4-carbon acid,
oxaloacetate (Raven et al. 1999). This CO
2
-concen-
trating mechanism gives C
4
plants a competitive
advantage under low pCO
2
conditions (Collatz et al.
1998). It is also generally agreed that C
4
plants have
greater water-use efficiency than C
3
plants (Raven
et al. 1999). Thus, modern C
4
plants are commonly
distributed in hot and dry environments. Warm-season
grasses and sedges use the C
4
pathway. Virtually all
trees, most shrubs, herbs, cool-season grasses and
sedges use the C
3
pathway.
In this study, the percentages of C
3
and C
4
plants
(Table 3) were calculated using Eq. (1). Calculated C
3
plant percentages in the Xianghai Lake core varied
from 61 to 97 %, with an average value of 70 %,
whereas in the Huola Basin core, the percentage of C
3
plants ranged from 78 to 99 %, with an average value
of 90 % (Table 3). These calculations indicate that C
3
plants were a dominant input during the late glacial
and Holocene. The percentages of C
3
plants in the late
glacial are overall greater than those of the Holocene
(Fig. 5), and thus the percentages of C
4
plants in the
late glacial sediments are overall relatively smaller
than percentages in Holocene deposits. During the late
glacial, the percentages of C
3
plants in the two cores
rose, whereas during the Holocene, percentages of C
3
plants in the two cores decreased through time.
Fluctuations in the percentages of C
3
and C
4
plants
in the two cores displayed differences during the
Holocene (Fig. 5). Highest C
3
plant percentages,
however, were recorded in both cores during the
interval 11.5–10.5 cal ka BP, indicating an especially
cold and moist local climate in the Pre-Boreal portions
of the Xianghai Lake and Huola Basin sequences.
From 10.5 to 9.0 cal ka BP, relative abundance of C
3
plants in the Xianghai Lake core decreased dramati-
cally, while C
4
plants increased, indicating the spread
of grasslands at the expense of forest. This transition
occurred in the Huola Basin from 10.5 to 8.0 cal ka
BP. Between 9.0 and 7.0 cal ka BP, C
3
and C
4
plant
Fig. 4 Depth profiles of the variation in the ratio of Rn-C
21
-
/
Rn-C
22
?
and the ACL values of n-alkanes. aXianghai Lake
core, bHuola Basin core
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percentages in the Xianghai Lake core fluctuated on a
sub-millennial timescale, suggesting unstable climate.
Such fluctuations occurred in the Huola Basin record
between about 8.0 and 6.0 cal ka BP. From 7.0 to
5.0 cal ka BP, the C
3
plant percentage in the Xianghai
Lake core increased strongly relative to percentages
for C
4
plants, indicating the expansion of forest at the
expense of grassland. From 5.0 to 2.0 cal ka BP,
however, relative abundance of C
3
plants declined,
indicating expansion of grasses on the landscape. In
contrast, from about 6.0 to 1.0 cal ka BP, C
3
and C
4
plant percentages in the Huola Basin core fluctuated
little, with no discernible trend.
Conclusions
The n-alkanes and d
13
C values of long-chain n-alkanes
were analyzed in lacustrine sediment samples from
cores taken in Xianghai Lake and the Huola Basin, on
the Song-nen Plain, northeast China. The n-alkanes
extracted from the Xianghai Lake core were from a
mixed source composed of bacteria, algae and terres-
trial higher plants, whereas the n-alkanes extracted
from the Huola Basin sediments were derived mainly
from terrestrial higher plants. The carbon isotopic
composition of C
27
,C
29
and C
31
n-alkanes in the core
sediments yielded information about the relative
contribution of C
3
and C
4
plants to the sediment
organic matter. C
3
plants were the dominant input
during the late glacial and Holocene, but the relative
abundances of C
3
and C
4
plants displayed fluctuations
through time, probably a response to alternating warm
and cool climate conditions. The percent of C
3
plants
increased during the late glacial, while the percentage
for C
4
plants decreased. During the Holocene, how-
ever, the percentage of C
4
plants increased, while the
relative abundance of C
3
plants decreased.
Acknowledgments We gratefully acknowledge Prof. Mark
Brenner and two anonymous reviewers for thoughtful and
constructive comments. This research was supported by the
Chinese Academy of Sciences Key Project (Nos.
XDB03020405, XDA05120204), the National Science
Foundation (41172169, 41572350, 41503049), Western Light
General Project, Western Light Joint Scholars Project, and the
Key Laboratory Project of Gansu Province (Grant No.
1309RTSA041).
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