Chen et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00416-9(2021) 8:23

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessA potential suite of climate markers oflong-chain n-alkanes and alkenones
preserved in the top sediments from the
Pacific sector of the Southern Ocean
Xin Chen1,2*, Xiaodong Liu1, Da-Cheng Lin3,4,5, Jianjun Wang2, Liqi Chen2, Pai-Sen Yu6, Linmiao Wang7,
Zhifang Xiong7 and Min-Te Chen3,4,5,8*Abstract
Investigating organic compounds in marine sediments can potentially unlock a wealth of new information in theseclimate archives. Here, we present pilot study results of organic geochemical features of long-chain n-alkanes andalkenones and individual carbon isotope ratios of long-chain n-alkanes from a newly collected, approximately 8 mlong, located in the far reaches of the Pacific sector of the Southern Ocean. We analyzed a suite of organic
compounds in the core. The results show abundant long-chain n-alkanes (C29–C35) with predominant odd-overeven carbon preference, suggesting an origin of terrestrial higher plant waxes via long-range transport of dust,
possibly from Australia and New Zealand. The δ13C values of the C31 n-alkane range from −29.4 to −24.8‰, inwhich the higher δ13C values suggest more contributions from C4 plant waxes. In the analysis, we found that the
mid-chain n-alkanes (C23–C25) have a small odd-over-even carbon preference, indicating that they were derived
from marine non-diatom pelagic phytoplankton and microalgae and terrestrial sources. Furthermore, the C26 andC28 with lower δ13C values (~−34‰) indicate an origin from marine chemoautotrophic bacteria. We found that theabundances of tetra-unsaturated alkenones (C37:4) in this Southern Ocean sediment core ranges from 11 to 37%,
perhaps a marker of low sea surface temperature (SST). The results of this study strongly indicate that the δ13C
values of long-chain n-alkanes and Uk37 index are potentially useful to reconstruct the detailed history of C3/C4
plants and SST change in the higher latitudes of the Southern Ocean.
Keywords: Southern Ocean, Pacific Ocean, n-alkane, Carbon isotopic, SSTUk37

1 Introduction
The Southern Ocean plays an important role in global
climate and the carbon cycle related to westerly winds
and the Antarctic Circumpolar Current (ACC, Fischer
et al. 2010; Marshall and Speer 2012). Mid-latitude westerly winds are essential in transporting mineral dust
* Correspondence: chenxust@mail.ustc.edu.cn; mtchen@mail.ntou.edu.tw
1Anhui Province Key Laboratory of Polar Environment and Global Change,
School of Earth and Space Sciences, University of Science and Technology ofChina, Hefei 230026, Anhui, China
3
Institute of Earth Sciences, National Taiwan Ocean University, Keelung
20224, Taiwan
Full list of author information is available at the end of the article

from the continent of Australia and New Zealand to theSouth Pacific sector of the Southern Ocean (Lamy et al.
2014). The westerly winds and ACC’s location and intensity directly control the exchange of heat, salt, nutrients, and freshwater between low and high latitudes(Pahnke and Zahn 2005; Toggweiler and Russell 2008;
Shevenell et al. 2011; Toyos et al. 2020). Thus, environmental fluctuations in the Southern Ocean play a vitalrole in global climate change.
Well-preserved organic matter in marine sediments is
a direct indicator of environmental conditions at thetime of sedimentation and thus is important for paleo-© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate ifchanges were made. The images or other third party material in this article are included in the article's Creative Commonslicence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commonslicence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

environmental studies (Meyers and Ishiwatari 1993).
Among these, lipid organic biomarkers have been widelyused to reconstruct past environmental and climaticconditions in oceans and lakes (Eglinton and Eglinton
2008; Holtvoeth et al. 2019). Long-chain n-alkanes (C25–
C35) are important components of the epicuticular wax
in higher terrestrial plants, and these n-alkanes are
eroded from leaf surfaces and soil by winds and thentransported to the Southern Ocean (Bendle et al. 2007;
Martínez-Garcia et al. 2009, 2011; Lamy et al. 2014;
Jaeschke et al. 2017). Short- and mid-chain lengths are
the major components of n-alkanes in the surface ocean
sediments around Antarctica. These compounds aremainly derived from phytoplankton and bacteria basedon carbon preference index (CPI) and specificcompound carbon isotopic values (Harada et al. 1995;
Bubba et al. 2004). Relatively high carbon isotopic valuesof C31 n-alkane in the surface sediments from the Australian sector of the Southern Ocean suggest significant
contributions of C4 higher vascular plant waxes or conifer resin (Ohkouchi et al. 2000). Altered or recycled material mixed with modern marine input is also an
important source for long-chain n-alkanes with low CPI
values in ocean sediments in the Ross Sea region (Kvenvolden et al. 1987; Venkatesan 1988; Duncan et al.
2019). Although the high latitudes of the Southern
Ocean are usually considered to be little influenced byriver and continent soils, based on the above results, thesources of n-alkanes in the ocean sediments are thought
to be complicated; thus, their eco-environmental implications are still being explored.
Subtropical to polar sea surface temperature (SST)
gradient has been related to the position and intensity of
the westerly winds and ACC in the Southern Ocean(Lamy et al. 2010; Kohfeld et al. 2013). Therefore, quantitative SST records from the past are essential for evaluating the importance of the Southern Ocean for the
global climate. However, the most widely used organic
geochemical SST index, alkenone paleothermometry, hasonly rarely been employed in high latitudes of the Southern Ocean. The U k37 = ([C37:2 − C37:4]/[C37:2 + C37:3 +C37:4]) index has been proposed to quantify the degreeof alkenone unsaturation (Brassell et al. 1986), which is afunction of SST. Because C37:4 is often absent in open
ocean sediments when SSTs are higher than 12 °C (Prahl0
and Wakeham 1987), the index was simplified to U k37 =0([C37:2]/[C37:2 + C37:3]). In recent decades, the U k37 index
has been widely used in middle and low latitude marine
environments. However, our knowledge on the application of alkenone paleothermometry in the high latitudesof the Southern Ocean is still largely insufficient. A few
examples exist, such as that C37:4 methyl alkenone was
not detected in the 10–12 °C waters. Even in the 1.5 °C

Page 2 of 13

waters, the abundance was still very minor (Sikes andVolkman 1993), while it was detected in most surface
sediment samples at 3.5 °C in spring cruise samples in
the Southern Ocean (Sikes et al. 1997). The relative
abundance of C37:4 alkenone in the surface sediments
showed no significant relationship with modern SST,0suggesting that U k37 index may be more proper than U k37
when used in sea surface temperature estimations, evenin cold conditions (Sikes et al. 1997; Jaeschke et al.
2017). However, Ho et al. (2012) found that U k37 records
display better agreement with planktic foraminifera δ18O
and other SST records at the same sites, suggesting that
U k37 is more suitable for SST reconstructions in the subantarctic Pacific. Data on alkenone paleothermometry is
still largely lacking, and these various results of U k37 and0
U k37 -SSTs indicate that more investigations are stillneeded in the high latitudes of the Southern Ocean.
Considering the importance of the position andstrength of westerly winds and the Antarctic Circumpolar Current, reconstructing surface ocean hydroclimaticchanges using organic biomarkers (e.g., long-chain n-alkanes and alkenones) is necessary to better understandthe role of the Southern Ocean in the context of global
climate change. Before carrying out such work, it is crucial to determine the source of organic matter and to estimate whether U k37 index could be useful or not in theSouthern Ocean. Here, we analyze the organic geochemical features of long-chain n-alkanes, alkenones, and thecompound-specific carbon isotope of long-chain n-alkanes (C23–C31) in the ocean sediments from one corein the South Pacific sector of the Southern Ocean (R23,66° 13′ 47.16″ S, 168° 11′ 8.34″ E). Our main objectives
are (a) to evaluate the source of long-chain n-alkanes
based on their chain length distributions and individualcarbon isotopes, (b) to report the distributional featuresof di-, tri-, and tetra-unsaturated alkenone, and (c) to assess the applicability of the alkenone indices in the highlatitudes of the Southern Ocean for reconstructing past
climate changes.

2 Materials and methods
2.1 Materials

The gravity core R23 was drilled at 168° 11′ 8.34″ E, 66°
13′ 47.16″ S at a water depth of 2967 m during the“31th Chinese National Antarctic Research Expedition(CHINARE)” cruise in 2014–2015 (Fig. 1). The sedimentcore is 819 cm long, with a top 10 cm soupy layer characterized by high water content. The core was subsampled at an interval of 2 cm. The color of the sediments
varies among olive, brown, and gray throughout the profile. Based on the wet and dry sieving experiments, thecore mainly consists of homogenous clay with minor
proportion of sand (63–2000 μm), some ice-rafted debris

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

Page 3 of 13

Fig. 1 Map of the Southern Ocean and the continent of Antarctica. The red pentagram denotes the core R23 in our study. Red triangles indicate
sites of sea surface temperature or sea subsurface temperature records reported in previous studies. The SST record in the PS75/034-2 sedimentcore was used for the Uk37 index (Ho et al. 2012), whereas other sediment cores (ODP 1098, JPC 10, and MD03-2601) were used for TEX86(Shevenell et al. 2011; Kim et al. 2012; Etourneau et al. 2013). The thick blue line indicates the Antarctic Circumpolar Current (ACC). The solid dots
denote ice core locations in the Antarctic, including Dome C and Vostok(IRD; >2 mm), and some foraminifera randomly found,but sponge spicules are present throughout the corewith relatively high abundance. No obvious bioturbations were observed in this core. In this pilot study, weonly focus on the source identification of n-alkanes withdifferent chain lengths and then evaluate the possibilityof C37 alkenones used as a suitable proxy for calculating
the past sea surface temperature in this region. To study
the potential of sedimentary organic geochemical features for paleoclimate reconstruction, we choose 12
samples at every ~ 80 cm interval for n-alkane and alkenone analysis in this pilot study. All samples were stored
under −20°C in the lab before analysis.
2.2 Lipid biomarker extraction

The lipid analysis procedure followed the methods ofYamamoto et al. (2000). Briefly, all sediment samples
were freeze-dried and then homogenized and powdered.
Samples (2–3 g) were weighed and extracted two times
with an Accelerated Solvent Extractor (DIONEX ASE350) using dichloromethane-methanol (6:4 v/v) and thenconcentrated. The total lipid extract was separated into
four fractions using column chromatography (SiO2 with

5% distilled water; internal diameter, 5.5 mm; length, 45mm) based on the degree of polarity: F1 (hydrocarbons,
4 ml hexane); F2 (aromatic hydrocarbons, 4 ml hexanetoluene (3:1 v/v)); F3 (ketones, 4 ml toluene); F4 (polar
compounds, 4 ml toluene-methanol (3:1 v/v)). N-C24D50
and n-C36H74 were added as internal standards for theF1 and F3 fraction, respectively. Compounds were quantified using an internal standard n-C24D50 and n-C36H74
for n-alkanes and alkenones, respectively.
2.3 n-alkane and alkenone analysis

Quantification of compounds was performed on a Hewlett Packard 6890 GC-FID system with a Chrompack
DB-1MS column (length, 60 m; i.d., 0.25 mm; thickness,
0.25 μm). The oven temperature was programmed from70 to 290 °C at 20 °C/min, 290 to 310 °C at 0.5 °C/min,and then held at 310 °C for more than 30 min. Helium
was used as the carrier gas, with a flow rate of 30 cm/s.
Selected samples were performed using GC-MS forcompound identification. The GC column and oventemperature program were the same as GC-FID. The
mass spectrometer was run in full scan mode (m/z 50–
650). Electron ionization (EI) spectra were obtained atChen et al. Progress in Earth and Planetary Science(2021) 8:23

70 eV. Alkenones were identified using an external
standard by GC retention times by analogy with a synthetic standard (provided by M. Yamamoto, Hokkaido
University, Japan) and characteristic mass fragments. Nalkanes were identified by comparing mass spectra andretention times with those of the standards and published data.
The carbon preference index (CPI; Bray and Evans
1961) of C26–C34 homologs and the average chain length(ACL) of odd C27–C35 homologs (Duan and He 2011)used in this study were as follows:

CPI ¼


C27 þ C29 þ C31 þ C33 C27 þ C29 þ C31 þ C33=2þ
C26 þ C28 þ C30 þ C32 C28 þ C30 þ C32 þ C34

ð1Þ
ACL ¼

27  C27 þ 29  C29 þ 31  C31 þ 33  C33 þ 35  C35
C27 þ C29 þ C31 þ C33 þ C35

ð2Þ
The [C26–C35] are concentrations of odd and even nalkane. The U k37 = ([C37:2–C37:4]/[C37:2 + C37:3 + C37:4])
index has been proposed to quantify the degree of alkenone unsaturation (Brassell et al. 1986), which is a function of SST. Because C37:4 is often absent in open ocean
sediments when SSTs are higher than 12 °C (Prahl and0
Wakeham 1987), the index was simplified to U k37 =([C37:2]/[C37:2 + C37:3]). We converted the index values
to SST using the widely used Emiliania huxleyi culturebased calibration proposed by Prahl et al. (1988), U k37 =00.04 T – 0.104 (r2 = 0.98) and U k37 = 0.034 T + 0.039 (r20= 0.99), and the simplified U k37 calibration was based on
global core top compilations (Conte et al. 2006).
2.4 Carbon isotope analysis

The carbon isotope ratio of n-alkanes was performed
using a gas chromatograph with a DB-5MS column (60m × 320 μm × 250 μm) interfaced to a Thermo Scientific MAT-253 isotope-ratio mass spectrometer via a
combustion interface (960 °C) consisting of an alumina
reactor containing nickel and platinum wires. Helium
was used as the carrier gas with a flow rate of 1.2 ml/
min using splitless injecting. The oven temperature wasprogrammed from 80 to 100 °C at 10 °C/min, 100 to 220°C at 4 °C/min, 220 to 280 °C at 2 °C/min, and then heldat 280 °C for 15 min. All samples are injected one time
for carbon isotope analysis. The analytical error was calculated based on the reproduced analytical results of an
external standard, injected once after every sixth sample
injection, and had an analytical error of 0.7‰ (1σ). The
pre-calibrated isotopic composition of CO2 was used asa standard. All δ13C values were expressed versus VPDB.
Based on the isotopic values of n-alkanes, we canquantify the percentage source of long-chain n-alkanes

Page 4 of 13

from C3/C4 plants using a binary isotope mass balance
model (Thomas et al. 2014):

δ13 CS ¼ f  δ13 CC3 þ ð1 − f Þ  δ13 CC4

ð3Þ

where δ13Cs is the long-chain n-alkanes from sediments, δ13CC3 and δ13CC4 are the carbon isotopic valuesof long-chain n-alkanes from C3 and C4 terrestrial
higher vascular plants, respectively, and f is the proportion of long-chain n-alkanes from C3 plants. We set the
carbon isotopic values of long-chain n-alkanes for C3
and C4 plants to be −36‰ and −22‰, respectively (Chikaraishi and Naraoka 2007; Vogts et al. 2009). C31 n-alkane abundance is relatively higher than C29 and C33 nalkanes; thus, we use the carbon isotopic values of C31
n-alkane to calculate the percentage source of longchain n-alkanes from C3/C4 plants.

3 Results
3.1 Concentration and distribution of long-chain nalkanes

In the 12 pilot samples from the core R23, we found asignificant change in the concentrations of total longchain n-alkanes (C23–C35) in the sediment profile, ranging from 295–787 ng/g sediment dry weight (Table 1,
Table S1, Fig. 2). The distribution pattern of long-chain
n-alkanes (C23–C35) in each sediment sample was similar, with bimodal distributions peaking at C23–C25 andC27 or C31 (Fig. 3). However, there was no apparent predominant odd-over-even carbon preference, and CPI27–
33 varied from 1.1 to 2.5, with an average of 1.7 (Fig. 2).
The distribution patterns of long-chain n-alkanes weredivided into two types. One is mid-chain n-alkanes(C23–C27), with no predominant odd-over-even carbon
preference (CPI ~1), and the other is long-chain n-alkanes (C29–C35) with dominant odd-over-even carbon
preference. The ACL values of long-chain n-alkanes(C27–C35) were in the range of 29.3–30.7. The ACL
values are strongly correlated with CPI (Fig. 4).
3.2 Concentration and distribution of alkenones

C37:4, C37:3, and C37:2 alkenones were all detected in the
12 pilot samples, with total concentrations ranging from
12.6 to 104.2 ng/g sediment dry weight (Table 2). The
distribution pattern of three unsaturated alkenones revealed significant differences among the subsamples, andthe relative abundance of C37:4, C37:3, and C37:2 varied
from 11 to 37%, 27 to 87%, and 3 to 48%, respectively.
The tri-unsaturated alkenone (C37:3) was the most abundant in the sediments. Interestingly, a high abundance oftetra-unsaturated alkenone was found in the sediment
samples. The SST estimates we inferred from the U k37

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

Page 5 of 13

Table 1 Concentrations, δ13C values, and typical indices based on n-alkanes in the subsamples with different sediment depth ofcore R23. The relative contribution of long chain n-alkanes from C3 and C4 plants are calculated by carbon isotopes of the C31 nalkane
δ13C23
(‰)

Depth
(cm)

δ13C24(‰)

δ13C25(‰)

δ13C26(‰)

δ13C27(‰)

δ13C28(‰)

δ13C29(‰)

δ13C31(‰)

n-alkanesa(ng/g)

C4
(%)

C3
(%)

ACLb

CPIc

16

−29.6

−28.8

−29.9

−33.2

−28.5

−32.2

−27.4

−28.3

445

55

45

30.1

1.6

88

−29.5

−31.8

−27.3

−30.0

−26.3

−33.1

−25.1

−25.5

328

75

25

30.3

2.0

166

−25.4

−26.4

−26.7

−32.5

−28.3

−38.0

−27.1

−27.8

295

59

41

30.1

2.1

243

−27.1

−26.8

−26.8

−30.4

−26.7

−33.3

−29.8

−28.3

362

55

45

30.3

2.5

323

−29.8

−31.2

−30.6

−35.0

−28.3

−32.1

−29.4

−27.9

324

58

42

29.3

1.3

398

−31.5

−35.9

−30.0

−36.7

−28.6

−34.6

−25.0

−24.8

787

80

20

29.6

1.3

482

−26.8

−28.4

−29.3

−30.3

−26.8

−30.1

−27.4

−26.6

411

67

33

30.7

1.9

550

−27.1

−30.7

−29.3

−35.1

−27.8

−31.2

−27.8

−25.4

420

76

24

30.1

1.6

626

−30.4

−32.2

−31.7

−34.6

−29.6

−30.9

−26.4

−25.8

344

73

27

29.6

1.6

698

−28.6

−30.6

−29.0

−32.7

−27.8

−31.8

−27.0

−25.8

490

73

27

29.3

1.1

762

−28.8

−29.3

−30.0

−35.3

−26.7

−32.1

−30.4

−29.4

455

47

53

29.8

1.6

818

−28.8

−28.7

−32.3

−37.4

−30.1

−35.8

−27.7

−27.2

503

63

37

30.1

1.7

a

Total concentrations of C23-C35 n-alkanesb
ACL27-35 =∑(i × Xi)/∑Xi, where X is abundance and i ranges from C27 to C35 odd n-alkanesc
CPI27-33 = 0.5 × ∑(C27–C33)/(C26–C32) + 0.5 × ∑(C27–C33)/(C28–C34)
0

and U k37 indexes are between −1.7 to 8.4 °C and −0.4 to
17.9 °C, respectively.
3.3 The carbon isotope values of individual n-alkanes

Our n-alkane-specified carbon isotope analysis of the 12
pilot samples shows a significant change. Therefore,
based on the chain length of the n-alkanes, we divided
n-alkanes into two endmembers (Table 1). One is midchain n-alkanes, which had δ13C values ranging from

−31.5 to −25.4‰ and −32.3 to −26.7‰, with an averageof −28.6‰ and −29.4‰ for C23 and C25, respectively.
The other is long-chain n-alkanes (C27, C29, and C31),
with δ13C values from −30.1 to −26.3‰ (C27, averaging−28.0‰), −30.4 to −25.0‰ (C29, averaging −27.5‰), and−29.4 to −24.8‰ (C31, averaging −26.9‰). The average
δ13C values of long-chain n-alkanes (C27, C29, and C31)
were higher than mid-chain n-alkanes (C23 and C25). C26
and C28 n-alkanes had the lowest δ13C values averaging~ −34‰. The percentage source of long-chain n-alkanes

Fig. 2 The concentrations (C23–C35), ACL (C27–35–C35), and CPI (C27–C33) values of n-alkanes at different depths of the core R23

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

Fig. 3 The relative abundance of long chain n-alkanes (C23–C35) at different depths of the core R23

Fig. 4 Relationship between ACL27–35 and CPI27–33 of long chain n-alkanes in the core R23

Page 6 of 13

(2021) 8:23

Chen et al. Progress in Earth and Planetary Science

Table 2 The relative abundance and concentrations of C37:4,
0
C37:3, and C37:2 alkenones and based on Uk37 - and Uk37 -SST in thesubsamples with different sediment depth of core R23
0

Depth
(cm)

C37:4
(%)

C37:3
(%)

C37:2
(%)

Alkenones(ng/g)

Uk37 -SSTa
(°C)

16

12

85

4

90.7

0.6

−0.1

88

11

87

3

104.2

0.7

−0.4

166

30

40

30

13.2

2.8

11.5

243

21

51

28

15.3

4.4

9.2

323

35

48

17

14.7

−1.7

6.6

398

16

64

19

50.1

3.5

5.6

482

15

80

6

65.1

0.5

0.7

550

37

38

24

12.6

−0.6

10.3

626

30

44

26

14.0

1.8

9.9

698

23

59

18

19.1

1.2

5.7

762

25

27

48

30.2

8.4

17.9

818

30

36

34

19.0

3.6

13.2

Uk37 -SSTb
(°C)

a k0

U37 = C37:2/(C37:2+ C37:3)
U37 = (C37:2-C37:4)/(C37:2+ C37:3+ C37:4)

b kfrom C3/C4 plants using C31 δ13C values varied from 47
to 80% for C4 plants (Table 1).

4 Discussion
4.1 Source of mid-chain n-alkanes

Our pilot study indicates that the mid-chain n-alkanes(C23–C25) are abundant with no predominant odd-overeven carbon preference in the sediment profile (CPI ~1;

Page 7 of 13

Fig. 3), and the contamination of petroleum may cause
this during coring and sampling. However, our sampling
procedures by the crew of the R/V Xuelong in the 31st
CHINARE have been devised to prevent any possible
contamination by petroleum, and no signs of petroleum
contamination have been observed while treating sediment samples in the laboratory. All labware was baked
at 450 °C in a furnace before using to prevent contamination during the analysis of the samples. Blank experiments were also analyzed, and negligible contamination
was found. Furthermore, the average δ13C values of n-alkanes with different chain lengths are different (Table 1,
Fig. 5). For example, the average δ13C values of midchain n-alkanes (C23–C25) were similar to the n-alkanes
from marine phytoplankton (Ashley et al. 2020). Therefore, it is very unlikely that these n-alkanes were due topetroleum contamination during coring and sampling.
The abundant mid-chain n-alkanes (C23–C25) with nopredominant odd-over-even carbon preference were natural characteristics in the sediment samples of core R23.
Several studies have shown that ocean phytoplankton
can produce mid-chain n-alkanes and n-alkanoic acids(e.g., Volkman et al. 1998). N-alkanoic acids are biosynthesized in the acetogenic pathway, and then, they
are converted to n-alkanes by enzymatic decarboxylation; thus, they have similar distributions (Diefendorf
and Freimuth 2017). Mid-chain n-alkanoic acids (C22–
C24) can be produced by marine plants, such as marine
microalgae, diatoms, and seaweed (Naraoka and Ishiwatari 2000). Therefore, phytoplankton may be a significant

Fig. 5 The average δ13C values of n-alkanes. The error bars represent 1 standard deviation of 12 samples in the core R23 (not analytical errors)

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

source for these mid-chain n-alkanes with no predominant odd-over-even carbon preference (CPI ~ 1). A previous study has reported that the average δ13C values ofn-alkanoic acids produced by marine phytoplankton
were about −28‰ (Ashley et al. 2020). In our pilot analysis of the 12 samples, the average δ13C values of n-alkanes with different chain lengths vary greatly. The δ13C
values of mid-chain n-alkanes (C23–C25; ~ −29‰) are in
the range of n-alkanes from marine organisms, and soil
samples (~−28‰) in the McMurdo Dry Valleys (Hayes
et al. 1990; Ishiwatari et al. 1994; Matsumoto et al.
2010), but lower than lake sediments (~−15‰) withshallow water depth from East Antarctica (Chen et al.
2019). Thus, the terrestrial organic matter from ice-freeareas of Antarctica transported by ice-rafted debris(IRD) and aeolian may also contribute to mid-chain nalkanes (Chewings et al. 2014). Still, the source fromshallow lake sediments at higher latitudes was considered negligible. The δ13C values of C26 and C28 are lower
than other long-chain n-alkanes (Fig. 5), suggesting they
may have other sources. Moreover, the δ13C values of
C26 and C28 in our study samples are also obviously depleted relative to marine organisms and soil samples
from Antarctica. The C26 and C28 may likely originate
from chemoautotrophic bacteria because they have relatively low δ13C values and have no odd-over-even predominance (Hayes et al. 1990; Collister et al. 1994).
Thus, from the above discussion, we believe that midchain n-alkanes (C23 to C25) have mixing sources, including marine (non-diatom pelagic phytoplankton andmarine microalgae) and terrestrial organic matter, but
C26 and C28 n-alkanes might be originated mainly fromchemoautotrophic bacteria.
4.2 Sources of long-chain n-alkanes

There are three major sources for long-chain n-alkanes(C27–C35) in the South Pacific sector of the Southern
Ocean sediments, including long-range transport of dust
from lower latitudes, ocean plankton, and sediments
eroded from Antarctica. Previous studies have shownthat short- and mid-chain n-alkanes are predominant inPleistocene age ocean sediments, modern water columnsuspended particulate matter in the Ross Sea and Antarctic margin, while long-chain n-alkanes have a minor
contribution (Harada et al. 1995; Hayakawa et al. 1996;
Cincinelli et al. 2008). Moreover, the δ13C values of n-alkanes ranged from −28.5 to −26.2‰, suggesting that
their major source was possibly derived from marine organisms (Harada et al. 1995). In the Ross Sea, abundant
long-chain n-alkanes with low CPI values in ocean sediments have suggested that the organic matter was
mainly originated from altered or recycled material
mixed with modern marine input (Kvenvolden et al.
1987; Venkatesan 1988; Duncan et al. 2019). Long-range

Page 8 of 13

transport of terrestrial organic matter and higher plantleaf waxes is also an important source for long-chain nalkanes in the Pacific sector of the Southern Ocean(Bendle et al. 2007; Martínez-Garcia et al. 2009, 2011;
Lamy et al. 2014; Jaeschke et al. 2017). For example,
Bendle et al. (2007) studied organic geochemical characteristics in Southern Ocean aerosol samples, and theirresults showed that the abundant long-chain n-alkanes
with relatively low δ13C values (−37 to −30.8‰) represented a regional background of well-mixed higher vascular plants through long-range transportation.
The core R23 is near the Antarctic continent, and so
the organic matter from Antarctica may be a potential
source of long-chain n-alkanes at our site. However,there are no vascular plants in the Antarctic, except forlimited terrestrial vegetation (moss and lichen) in relatively low latitudes of the Antarctic Peninsula. Dust contribution from terrestrial material through aeoliantransportation is negligible due to the lack of exposed,
mature soils in the McMurdo Dry Valleys and Victoria
Land (Nylen et al. 2004; Lewis et al. 2008; Lewis andAshworth 2016), as well as the long-distance of the coresite from the coast. Moreover, Matsumoto et al. (2010)
have reported that the chain length of n-alkanes ranging
from C15 to C37 was found in McMurdo Dry Valley soil,
with the majority as C23, C25, and C27 n-alkanes, butwith extremely low abundance of C29 and C31 n-alkanes.
Recently, Chen et al. (2019) reported that abundant
long-chain n-alkanes with highly enriched carbon isotopic ratios (~−25 to −12‰) in shallow lake sedimentsfrom East Antarctic (no vascular plants are present in
the surrounding landmass) were predominantly derived
from heterotrophic microbes. However, the average δ13C
values of long-chain n-alkanes (C27, C29, and C31) varying from ~ −28 to −27‰ in the R23 sediments are lower
than these in the lacustrine sediments from East Antarctica. Therefore, the sources of long-chain n-alkanes(C27–C35) from ice-free soils and shallow lacustrine sediments in East Antarctica via dust transport and ocean
phytoplankton is negligible.
The ACL of long-chain n-alkanes refers to the average
number of carbon atoms/molecule and can indicate theirsource (Poynter and Eglinton 1990). The ACL values oflong-chain n-alkanes (C27–C35) ranged from 29.3 to 30.7
in the sediment samples, similar to Southern Ocean
ACL values with a range of 29.1–30.6 in the surface sediments, both indicating the significant contribution ofhigher plants (Jaeschke et al. 2017). A significant linear
relationship was observed between ACL and CPI (n =12, r2 = 0.54, p < 0.01; Fig. 4). Generally, relatively high
CPI values (CPI > 3) indicate long-chain n-alkanes fromhigher vascular plants, while low CPI values (CPI ~1)
may imply post-depositional degradation and mature organic matter inputs (Eglinton and Eglinton 2008;

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

Duncan et al. 2019). According to leaf litter degradation
experiments, the odd-over-even predominance of n-alkanes was observed to decline. The long-chain n-alkane
ratios (e.g., C31/C29) were tended to ~ 1 (Zech et al.
2011). Based on this result, it is reasonable to infer thatn-alkanes in the dust might have experienced a certain
degree of degradation during long-range transportationand post-deposition, which could result in low CPI
values. Furthermore, relatively low CPI values of 1.1 to
2.5 present in the R23 sediment core may also be considered to microbial degradation under very low sedimentation rates < 2 cm/ka (Tiedemann 2012; Jaeschke
et al. 2017; Duncan et al. 2019). Previous studies haveshown that the average sedimentation rates were as low
as 1.18 cm/ka in Prydz Bay (Wu et al. 2015) and 1.00
cm/ka in ODP 1167 (Theissen et al. 2003). Low CPIs
and low sedimentation rates in the DSDP 274 sedimentcore from the northwest Ross Sea suggest that longchain n-alkanes have been extensively degraded by bacterial activity in the seabed surface layers (Duncan et al.
2019). Under the condition of degradation, the δ13C
values of long-chain n-alkanes have no obvious difference (Huang et al. 1997; Li et al. 2017); thus, it could be
useful to trace the sources of organic matter and reconstruct the paleoecological changes. The high abundanceof long-chain even n-alkanes (C26 and C28) with lower
δ13C values in the R23 sediment core indicates microbial(chemoautotrophic) activity in this region. Altered or
recycled organic matter from Antarctica that has beeneroded by glaciers and transported by IRD is importantin the study region (Chewings et al. 2014; Duncan et al.
2019). Therefore, we suggest that the long-chain n-alkanes (C29–C35) primarily originated from terrestrial
higher plant waxes via long-range transport of dust from
Australia and New Zealand and altered or recycled organic matter from Antarctica may be another secondary
source.
Our results are consistent with previous studies in theSouthern Ocean. Long-chain n-alkanes were reported tooriginate mainly from long-range transport of dust from
Australia and New Zealand by prevailing westerlies(Martínez-Garcia et al. 2011; Lamy et al. 2014). For example, relatively enriched carbon isotopic ratios of C31
n-alkane in the surface sediments from the Australian
sector of the Southern Ocean suggest significant contributions of C4 higher vascular plant waxes (Ohkouchi
et al. 2000). More recently, Jaeschke et al. (2017) havereported that the CPI values of long-chain n-alkanes
ranged from 1.1 to 10 in the Pacific sector of the Southern Ocean, indicating the contribution of higher plantleaf waxes. Because the location of surface sediments inour study site is far from the potential source regions(New Zealand and Australia), it is reasonable to believethat the long-chain n-alkanes in the R23 sediment core

Page 9 of 13

are primarily derived from terrestrial higher plant leaf
wax through long-range aeolian transportation.
4.3 Estimation of C3/C4 plant fraction

As discussed above, the long-chain n-alkanes (C27, C29,
and C31) in R23 sediments are primarily derived fromhigher plant leaf waxes by long-range transport of dust.
Interestingly, the δ13C values of long-chain n-alkanes
were 5–10‰ higher than those in C3 plants. This difference indicates that considerable amounts of n-alkanes
are derived from C4 plant waxes, which have significantly higher carbon isotopic values. The relative contributions of long-chain n-alkanes (C27, C29, and C31) fromC3 and C4 plants are significantly different in the sediment samples. For the carbon isotopic values of C31 nalkanes, 80% originated from C4 plants in the 398 cm
section; however, only 47% originated from C4 plants inthe 762 cm section (Table 1). Ohkouchi et al. (2000) reported that the relative contributions of C31 n-alkanes
from C3 and C4 plants are about 60% and 40% in thesurface sediments from the Australian sector of theSouthern Ocean respectively (Ohkouchi et al. 2000). Previous studies have demonstrated that the primary drivers
for the distributions of C3/C4 plants are climatic and atmospheric pCO2 etc. (Huang et al. 2001; Edwards et al.
2010). Compared with C3 plants, C4 grasses usually favor
relatively lower pCO2 and arid conditions due to theirgreater water use efficiency and carbon-concentrating
mechanism (Edwards et al. 2010). Therefore, the different contributions of C3/C4 plants may be related to climate change (e.g., temperature and precipitation) in thesource regions (Huang et al. 2001). Based on the above
discussion, it is reasonable to infer that the source of thelong-chain n-alkanes was mainly derived from longrange transport of dust from New Zealand and Australia(Neff and Bertler 2015). Therefore, these results indicatethat the δ13C values of long-chain n-alkanes could be
used to reconstruct the past changes of C3/C4 plants inthe source area and then investigate climatic variations.
0

4.4 Assessing Uk37 and Uk37 -derived SST records

C37:4 alkenone was found in the R23 sediments withrelative abundance ranging from 11 to 37%. This is similar to a previous study in the higher latitude of the Pacific sector of the Southern Ocean (Sikes et al. 1997) but
significantly higher than the sedimentary abundancefrom the lower latitudes of the Southern Ocean(Jaeschke et al. 2017). Previous studies have shown that
C37:4 is often absent in open ocean sediments where
SSTs are higher than 12 °C (Prahl and Wakeham 1987).
The modern annual SST in our study site is about 0 °C;
thus, the high abundance of C37:4 alkenone may be related to the extremely low temperature. Numerous studies have demonstrated that a high abundance of C37:4 inChen et al. Progress in Earth and Planetary Science(2021) 8:23

surface sediments is related to low-temperature andlow-salinity surface water masses in the Arctic (Sicre
et al. 2002; Bendle et al. 2005; Harada et al. 2006, 2012).
Analysis of 106 surface water and sediment samples
from the Atlantic, Pacific, and the Southern Ocean indicated that the relative abundance of C37:4 methyl alkenone had no apparent relationship with SST andsalinity. Still, it might respond to some other environmental factors, including growth rate, light, or nutrients(Sikes and Sicre 2002). For example, %C37:4 showed anegative linear correlation with sea surface salinity (SSS),
nutrients, and late summer SST in the suspended particles and sediment profiles from the Bering Sea (Haradaet al. 2012). However, SST and SSS showed a strong
negative linear relationship in the north Atlantic and theBering Sea because of sea ice melting during the summer season, suggesting that the strong relationship of%C37:4 and salinity may be the artifact of the good correlation of salinity and temperature (Sikes and Sicre
2002). Moreover, up until now, most samples were fromthe Atlantic and Pacific Oceans, and there were few
studies on the distributional characteristics of alkenones
in the high latitudes of the Southern Ocean.
To determine whether SST affects the relative abundance of C37:4 methyl alkenone, we calculated the sea
0surface temperature based on the U k37 and U k37 indexes
using the formula reported by Prahl et al. (1988) andPage 10 of 13

Conte et al. (2006), respectively (Table 2). Our results0
show that SST data between U k37 - and U k37 -SST were, as
we expected, obviously different (Fig. 6). When the relative abundance of tetra-unsaturated alkenone was0higher, we found that U k37 -SST was warmer than U k37-SST in 166, 243, 323, 550, 626, 762, and 818 cm sedi0

ment sections, and the difference between U k37 and U k37-SST is in the range of 4.8–10.9 °C. However, the SST
difference calculated by these two indexes is relatively
small in the sediment sections of the lower abundanceof C37:4 alkenone. Based on the average summer SST
from the World Ocean Atlas (WOA09) data set (Locarnini et al. 2010), the modern sea surface temperature in
our study site was about 0–1 °C. For the historicalperiod, the highest subsurface temperature is about 4–5°C during the Holocene at similar latitudes, including
the eastern Antarctic continental margin (Kim et al.
2012) and western Antarctic Peninsula (Etourneau et al.
2013). According to modern observation and SST reconstruction during the late Pleistocene, we suggest that thehighest SST in our study site should be lower than 5 °C
0in the warmer periods, which was much lower than U k37-SST. Therefore, all these results indicating higher%C37:4 are most likely controlled by extremely low SST

0

Fig. 6 The relative abundance of C37:4, C37:3, and C37:2 alkenones and the calculated sea surface temperature based on Uk37 and Uk37 index at
different depths of the core R23. The light gray and light blue bands represent modern average summer SST based on the World Ocean Atlas(WOA09) data set (Locarnini et al. 2010) and highest SST during the Holocene at the same latitude of the Southern Ocean from previous studies(Kim et al. 2012; Etourneau et al. 2013), respectively

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

in the R23 sediments, and the U k37 index is more feasible0
than U k37 in the relatively higher latitudes of the Southern Ocean.
0
Ho et al. (2012) also found that the U k37 -SST records
were significantly warmer in glacial periods and that the
U k37 index is a more suitable SST proxy in the subAntarctic and higher latitude Pacific (Ho et al. 2012;
Haddam et al. 2018). Other studies have shown a significant relationship between the relative abundance of C37:4
and temperature (Prahl et al. 1988). Moreover, several
other studies indicate %C37:4 is closely related to cold
water mass expansion (Bard et al. 2000; Martínez-Garcia
et al. 2010). Although the influencing factors on the relative abundance of C37:4 alkenone are complex, a statistically significant relationship between U k37 index and SST
has been found in the surface sediments from high latitudes of the North Atlantic Ocean (Rosell-Melé et al.
1995). This result further validates the general applicability of the U k37 as a reliable climatic proxy for SST reconstructions in the relatively cold climate regions(Rosell-Melé et al. 1994, 1995). The latitude was relatively high at our study site, and the modern annual
summer SST was lower than 1 °C. The marine algae
may synthesize more C37:4 alkenones to adapt to the extremely cold conditions. Notably, there are few U k37 -SST
records in the Southern Ocean at latitudes higher than
60°S. Therefore, all these results indicate that the usageof the U k37 index is feasible for reconstructing past SST
in the Southern Ocean, but more studies on surface
water and sediment samples in high latitudes are required to confirm the relationship between C37:4 alkenones and sea surface temperature.

5 Conclusions
We have presented pilot results of the relative distribution and individual δ13C values of long-chain n-alkanes
and the organic geochemical characterization of alkenones in 12 samples selected from a sediment core collected from the Pacific sector of the Southern Ocean.
Our results suggest that the abundant long-chain n-alkanes (C27–C35) with a significant odd-over-even carbon
preference might have originated from terrestrial higher
plant waxes, possibly via long-range transport of dust
from Australia and New Zealand. The mid-chain n-alkanes (C23–C25) preserved in the sediments have low
odd-over-even carbon preference, perhaps indicating
mixing of marine (non-diatom pelagic phytoplankton
and marine microalgae) and terrestrial sources. The C26
and C28 n-alkanes with relatively low δ13C values indicate an origin from marine chemoautotrophic bacteria.
The δ13C values of long-chain n-alkanes (C27–C31) range
between −30.8 and −24.8‰ in the sediments,

Page 11 of 13

approximately 5–10‰ higher than in terrestrial C3
higher plants. Furthermore, the relative abundance oftetra-unsaturated alkenone in the sediments varies from11 to 37%, higher than those previously reported in thelower latitudes of the South Pacific Ocean. We conclude
that tetra-unsaturated alkenones are sensitive markers of
low SSTs, suggesting the feasibility of using U k37 in further SST reconstructions in the Pacific sector of theSouthern Ocean.

6 Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40645-021-00416-9.
Additional file 1. Supplementary material.

Acknowledgements
We are grateful to the crew of the R/V Xuelong for their assistance withsample collection in the 31st CHINARE, and thanks to Chinese Projects for
Investigations and Assessments of the Arctic and Antarctic (CHINARE20122020 for 01-04, 02-01, and 03-04). We also acknowledge Prof. S. Emslie forediting and Prof. Tiegang Li for providing samples. We are grateful to Dr.
Yusuke Okazaki and two anonymous reviewers whose comments significantly improved the quality of the manuscript.
Authors’ contributions
MC, XL, and XC proposed the topic, conceived and designed the study, andthey wrote the draft of this paper. XC and DL conducted the experiments.
All the co-authors contributed to the discussion and edited and commented
on the paper. All authors read and approved the final manuscript.
Funding
This study was supported by grant numbers 41776188, 41576183, 41476172,
and 41772366 from the National Natural Science Foundation of China, andthe Fundamental Research Funds for the Central Universities. This work wasalso partly supported by the Joint Projects of MOST (Ministry of Science andTechnology) to MTC and the Chinese Projects for Investigations andAssessments of the Arctic and Antarctic (CHINARE2012-2020 for 01-04, 02-01,
and 03-04) to LQC.
Availability of data and materials
The datasets in the current study are available from the corresponding
author on reasonable request.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Anhui Province Key Laboratory of Polar Environment and Global Change,
School of Earth and Space Sciences, University of Science and Technology ofChina, Hefei 230026, Anhui, China. 2Key Laboratory of Global Change andMarine-Atmospheric Chemistry (GCMAC) of Ministry of Natural Resources(MNR), Third Institute of Oceanography (TIO), MNR, Xiamen 361005, China.
3
Institute of Earth Sciences, National Taiwan Ocean University, Keelung
20224, Taiwan. 4Center of Excellence for the Oceans, National Taiwan Ocean
University, Keelung 20224, Taiwan. 5Center of Excellence for Ocean
Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan.
6
Taiwan Ocean Research Institute, National Applied Research Laboratories,
Kaohsiung 80143, Taiwan. 7Key Laboratory of Marine Geology and
Metallogeny, First Institute of Oceanography, Ministry of Natural Resources,
Qingdao 266061, China. 8Pilot National Laboratory for Marine Science andTechnology, Qingdao 266061, China.

Chen et al. Progress in Earth and Planetary Science(2021) 8:23

Received: 30 July 2020 Accepted: 5 March 2021

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