Composition and Distribution Characteristics and Geochemical Significance of n-Alkanes in Core Sediments in the Northern Part of the South Yellow Sea

1Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221116, China 2School of Resources and Geoscience, China University of Mining and Technology, Xuzhou 221116, China 3Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China 4Guangzhou Marine Geological Survey, Guangzhou 510760, China


Introduction
The organic carbon deposition rate of continental margin sea is 8-30 times higher than that of the ocean, and over 80% deposited organic carbon is buried in the continental shelf and continental slope area [1,2].Therefore, the margin sea plays an important role in global carbon cycle [2].Organic matter source of the margin sea is an important scientific subject in the research on carbon cycle [3,4].The South Yellow Sea is typical semiclosed continental margin sea, into which a large amount of terrigenous matter was transported along streams since Holocene [5,6].Influenced by runoff of Yellow River and Yangtze River, warm current and alongshore current of the Yellow Sea, and tidal action, South Yellow Sea became an important carbon sink with diverse organic matter and changeable sedimentary environment [7,8].Much research has been conducted for modern sediments of South Yellow Sea [9][10][11][12][13][14].
Predecessors conduct a large amount of research on source of organic matter and sedimentary environment of South Yellow Sea with stable carbon isotopes [15], content of elements and minerals [16][17][18][19][20], and biomarkers [5,6,[21][22][23] and achieved abundant outcomes.Provenance study results show that organic matter of western and central part of the South Yellow Sea comes from modern Yellow River and ancient Yellow River; sediments in the eastern part are mainly influenced by the matter in Korean Peninsula; nearshore area in the northern part is mainly influenced by the matter in modern Yellow River [15][16][17][18]24]. Research on organic matter source of sediments in the Yellow Sea shows that nalkanes of South Yellow Sea are dominated by terrigenous inputs [5, 6, 21-23, 25, 26], and the terrigenous n-alkanes are mainly derived from higher plants waxiness [5].Atmospheric deposition inputs account for a small percentage of n-alkanes, but the river input is a main contributor of n-alkanes [6,27].
The distribution of n-alkanes is characterized by three types: predominance of short-chain n-alkanes, predominance of long-chain n-alkanes, and double-peak groups [5,6,22,28].Previous research on n-alkanes of organic matter in modern sediments of South Yellow Sea focuses on surface sediment samples [5,6,22,23,28].Not much research on sediment core samples has been conducted, and the length of sediment cores in the research is short (only 35 cm) [22].There is a lack of research on n-alkanes of long sediment core samples.This paper studies composition and distribution of n-alkanes of a 250 cm long sediment core in the northern part of South Yellow Sea and analyzes the geochemical significance.

Sample Collection.
A 250 cm long sediment core sample was collected in the northern part of South Yellow Sea in August, 2013.Water depth of the sampling station is 40 m, and the sampling locations are shown in Figure 1.
The sample spacing was 10 cm, and 25 sediment core samples wrapped with aluminium foil and plastic bags were brought to the laboratory at the ground temperature and put into the freezer for preservation at −20 ∘ C until organic analysis.Foraminiferal sediments samples were selected from 3 sediments samples at different depths and delivered to Beta Analyses for foraminiferal AMS 14 C dating, and the obtained AMS 14 C dating data were converted into chronological age via the software Calib 5.5.2 (Table 1) [29].Marine04 curve was used [30], the age of regional carbon reservoir was set to be 0 by referring to the method of Kong et al. [31], and the deposition rate at various depths was calculated according to depth and chronological age.It is shown in the results that this 250 cm long sediment core is representative of 3952-yearold sediments.Deposition rate at various depths is between 0.048 and 0.108 cm/a, which is in line with 0.026-0.67cm/a [32,33] or 0.013-0.119cm/a [34,35], the deposition rate range  of South Yellow Sea obtained with 210 Pb isotope method or obtained with foraminiferal dating method by predecessors.

Sample Analysis.
After freezing and drying of the sediment samples, Soxhlet extraction was performed for 20 g crushed sample for 72 h with dichloromethane.The extracting solution was concentrated and was isolated and purified with silica gel/aluminium oxide (1 : 1) chromatographic column and then eluted with mixed liquor of n-hexane and dichloromethane (V : V = 1 : 1).Saturated hydrocarbon in the sample was finally obtained.Saturated hydrocarbon sample was under full scan by Agilent GC-MSD (6890/5975) for analytical determination.The chromatographic column was a DB-5MS capillary column (column length: 30 m, inner diameter: 0.25 mm, coating thickness: 0.25 m; J & W Scientific).The carrier gas was high-purity helium (He), column flow was 1.0 mL/min, and sample size was 1 L with splitless sampling.The temperature of injection port and detector was 280 ∘ C and 300 ∘ C, respectively.Here is the heating procedure: maintain initial temperature 60 ∘ C for 1 min, increase temperature to 180 ∘ C with heating rate 8 ∘ C/min and remain so for 1 min, and increase temperature to 300 ∘ C with heating rate 3 ∘ C/min and remain so for 2 min.The ion source temperature was 250 ∘ C, and ionization energy was 70 eV.The relative standard deviation of the experiment was <±10%.The identification of compound was based on retention time and characteristic ion (/ 85) of the standard sample and comparison with mass spectrometry scheme of reference material and mass spectral library (NIST2005).

Composition and Distribution Characteristics of n-
Alkanes.The n-alkanes with chain length C 12 -C 40 were tested.The C 12 alkane content of 13 samples was too low to be detected, and C 13 alkane content of 2 samples was too low to be detected.The C 37 -C 40 alkane content of some samples was not detected due to low content (Table 2).The gas chromatogram of typical n-alkanes of core sediments is shown in Figure 2. The distribution of n-alkanes is characterized by three types: double-peak groups (Figure 3 The relative amount distribution of n-alkanes with different carbon number is shown in Figure 3. The front peak group of n-alkanes, between C 12 and C 21 , has main peak carbon of C 16 , C 17 , or C 18 and higher content of C 17 -C 20 .The back peak group, between C 22 and C 40 , has main peak carbon of C 29 or C 31 and the highest content of C 31 , followed by C 29 and then C 27 .Among isoprenoid alkanes, the peak height of pristane (Pr) is generally lower than that of phytane (Ph).

∑C 27+29+31 /∑C 15+17+19 Value Distribution Characteristics and Their
Significance.∑C 27+29+31 /∑C 15+17+19 , the ratio of the sum of terrigenous-dominated n-alkanes content to the sum of marine-dominated n-alkanes content, is also a common parameter to evaluate relative contribution of terrigenous n-alkanes and marine n-alkanes [39].Therefore, ∑C 27+29+31 /∑C 15+17+19 can indicate organic matter source of sediments.In Table 2   It is of great significance to further distinguish terrigenous organic matter, that is, ligneous plants dominated or herbaceous plants dominated.The main peak of ligneous plantderived n-alkanes is mostly C 29 , while that of herbaceous plant-derived n-alkanes is mostly C 31 [40,41].The main peak of back peak group of studied samples is mostly C 31 .However, C 29 content is similar to C 31 content with C 31 /C 29 between 0.87 and 1.12, averaging 1.04 (Table 2, Figure 4).Therefore, herbaceous plants and ligneous plants account for similar percentages in the sediment core samples, which is in line with previous research results [5,23,42].
The average carbon chain length (ACL) of n-alkanes reflects vegetation variation.n-Alkanes have smaller ACL when the formation is dominated by ligneous plants and have larger ACL when the formation is dominated by herbaceous plants [43].It is shown in Table 2 that ACL of sediment core samples is distributed between 29.61 and 30.09, averaging 29.92, around 30.This reflects that herbaceous plants and ligneous plants account for similar percentages in the sediment core samples, which is in line with the above results indicated by C 31 /C 29 .

CPI Value Distribution Characteristics and Their Significance.
The CPI value of organic matter is a common index determining the odd-even predominance (OEP) of carbon number of n-alkanes [44].The short-chain n-alkanes group and long-chain n-alkanes group of double-peak n-alkanes have different odd-even predominance, which is defined by CPI1 and CPI2.The corresponding computational formulas and CPI of samples are shown in Table 2 and Figure 4.It is shown in Table 2 that, except for number ZY22 sample (CPI1: 1.09), CPI1 values of short-chain alkanes group of 24 samples are between 0.57 and 0.97 and are all below 1.0 and average 0.80, reflecting the apparent even predominance; CPI2 values of long-chain alkanes group are between 0.7 and 3.95, and the CPI2 values of numbers ZY1, ZY2, and ZY3 samples are 0.7, 0.77, and 1.06, respectively, reflecting the apparent even predominance; CPI2 values of other 22 samples are all above 1.20, and average 2.77, reflecting the apparent odd predominance.
The carbon number of n-alkanes contaminated by oil does not show the apparent odd-even predominance [44,45].However, all the sediments samples in this research show the apparent odd-even predominance.Thus, it is estimated that these samples were not contaminated by oil, and the parameters of n-alkanes of samples could reflect the characteristics of primitive sedimentary environment and source.
The short-chain n-alkanes are mainly from marine floating algae and bacteria [36], which are dominated by C l5 , C 17 , and C 19 [39], and have odd predominance [46].However, the short-chain n-alkanes of 24 samples show even predominance, which shows specialty apparently.
It is found in the previous research that medium-chain n-alkanes in sea areas like Bohai Sea and Yangtze Estuary show even predominance [47], and medium-chain n-alkanes of a few samples in southern part of South Yellow Sea also show even predominance [5], which possibly resulted from microbe, for example, bacteria [47,48] or fresh water and marine macrophyte [40,41].
All the samples in the research contain squalene compound (Figure 5(a)), and the squalene is from the microbe.Thus, it is proved that all the samples suffered from the degradation by microbe.Meanwhile, the unseparation complex mixture (UCM) is widely developed in the n-alkanes of front peak group (Figure 5(b)), and UCM indicates that the organic matter suffered from the degradation by microbe.However, UCM is only shown in the short-chain n-alkanes, but not in long-chain n-alkanes, which could be resulted from two reasons.The first reason is mixture of two periods of organic matter.The organic matter deposited in the early period suffered from the degradation by microbe, and then new terrigenous organic matter migrated here fast, showing the present characteristics after mixing.The second reason is selective degradation by microbe.The microbe degraded short-chain n-alkanes in a selective way.However, determination of genesis needs further study.
The long-chain n-alkanes show apparent odd predominance [37,49], with C 27 , C 29 and C 31 most abundant [37,39], which shows that long-chain n-alkanes of sediment core samples are mainly derived from surface waxiness of continental higher plants.
Additionally, CPI is a parameter indicating the maturity of organic matter.Generally, CPI >1.2 indicates immaturity, but CPI <1.2 does not necessarily indicate maturity.The CPI of sediment cores is between 0.76 and 4.03 (only 3 samples   6) also suggests that studied sediment samples are from marine sedimentary environment with strong reducibility and are at immature stage, which corresponds to characteristics of modern marine sediments.

Conclusions
(1) The distribution of n-alkanes of 25 sediment samples in this research is characterized by three types: n d o n g P e n i n s u la S o u t h e r n Y e l l o w S e a Sampling location C h a n g j i a n g R i v e r O l d Y e l l o w R i v e r ( b e f o r e 1 8 5 5 )

Figure 2 :
Figure 2: Gas chromatogram of typical n-alkanes and isoprenoid alkanes of sediment core samples.

3. 2 . 1 .
Distribution of n-Alkanes and Their Significance.The distribution of n-alkanes of 25 sediment samples in this research is characterized by three types: double-peak groups (Figure 3(a)), predominance of long-chain n-alkanes (Figure 3(b)), and predominance of short-chain n-alkanes (Figure 3(c)).The short-chain n-alkanes (C 12 -C 21 ) reflect marine inputs and long-chain n-alkanes (C 22 -C 40 ) reflect terrigenous inputs.The double-peak suggests that both terrigenous organic matter and marine organic matter contribute to the n-alkanes of sediment core samples in this research (Figure 2).In this research, samples with double-peak groups are influenced by both terrigenous inputs and marine inputs (Figure 3(a)); samples with predominance of long-chain nalkanes indicate higher terrigenous inputs (Figure 3(b)); samples with predominance of short-chain n-alkanes indicate lower terrigenous inputs (Figure 3(c)).

Figure 3 :
Figure 3: The three typical distribution patterns of n-alkane contents.((a) Terrestrial and marine input; (b) stronger terrestrial input; (c) weaker terrestrial input).

∑Figure 4 :Figure 5 :
Figure 4: Vertical distribution characteristics of geochemical parameters of n-alkanes of sediment core samples.

Table 1 :
AMS14C dating data and deposition rate of foraminiferal sediments samples.
3.2.2.∑C 25−35 /∑C 15−21 Value Distribution Characteristics and Their Significance.The sum of C 15 -C 21 content of shortchain n-alkanes, ∑C 15−21 , represents marine alkane content [36].The sum of C 25 -C 35 content of long-chain n-alkanes, ∑C 25−35 , represents terrigenous alkane content [37].The value of ∑C 25−35 /∑C 15−21 can be used to eliminate the influence of particle size and deposition rate and indicate organic matter source in the sediments more accurately [38].The higher ratio indicates stronger influence of terrigenous inputs, while the lower ratio indicates weaker influence of terrigenous inputs.The value distribution of ∑C 15−21 , ∑C 25−35 , and ∑C 25−35 /∑C 15−21 of samples in this research is shown in Table 2 and Figures 4 and 5.It is shown in Table 2 that ∑C 15−21 ranges between 15.90% and 62.61%, averaging 32.58%, with the maximum value 3.9 times of the minimum value; ∑C 25−35 ranges between 31.33% and 72.85%, averaging 55.20%, with the maximum value 2.3 times of the minimum value; ∑C 25−35 /∑C 15−21 ranges between 0.50 and 4.58, averaging 1.92.The above parameters suggest higher terrigenous organic matter input of studied sediment core samples.