Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue2-3.Feb2000:
Organic Geochemistry 31 (2000) 159±167
www.elsevier.nl/locate/orggeochem
Organic geochemistry of recent marine sediments
from the Nansha Sea, China
Yi Duan*
State Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou, Gansu Province
730000, People's Republic of China
Received 29 May 1998; accepted 21 October 1999
(Returned to author for revision 18 December 1998)
Abstract
Recent marine sediments from two cores, collected on the continental slope of the Nansha Sea, China, have been
analyzed for organic matter content, n-alkane and isoprenoid hydrocarbons, fatty acids, n-alkanols and sterols. The
organic carbon contents of the two sediment cores average 0.7 and 0.53%, respectively, and are higher than those of
other sedimentary environments in this region. The distributions of various lipid compounds indicate that most of the
sedimentary organic matter in the two cores is derived from marine plankton and bacteria, with land-derived organic
matter present in relatively small amounts. The decrease in relative abundances of shorter chain lipids (n-alkanes, nfatty acids and n-alkanols) with depth is evident, as is the biochemical conversion of stenols to stanols in some samples.
The low pristane/phytane ratios may result primarily from microbial activity, and re¯ect the occurrence of signi®cant
diagenetic alteration of organic matter in the two cores. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Nansha Sea; Organic carbon content; Lipids; Sedimentary environments; Diagenesis
1. Introduction
Studies of the organic geochemistry of recent marine
sediments are valuable because they provide information about paleoenvironments and paleoceanography
(Keswani et al., 1984; Venkatesan and Kaplan, 1987). A
basic understanding of various lipid sources, their lipid
diagenetic evolution, and the accumulation and preservation of organic matter by such studies, is helpful in
interpreting sedimentary records. To date, such studies
have been concerned with sediments of temperate or
polar regions (Degens and Mopper, 1976; Venkatesan,
1988). The Nansha Sea of China is located in a lowlatitude tropical zone. No previous data have been
reported on the organic geochemical contents from
sediments of this region, although some studies have
been conducted on the organic geochemistry of sinking
* Tel.: +86-931-8827983; fax: +86-931-8418667.
particulate material (Duan et al., 1997a; 1998). This
paper reports the detailed organic geochemical contents
of recent marine sediments from the Nansha Sea. It also
discusses the relationship between the organic carbon
content and sedimentary environment, and the sources
and diagenetic changes of sedimentary organic material
in two sediment cores.
2. Materials and methods
2.1. Sampling site
The Nansha Sea is located to the south of the South
China Sea. The region has a monsoon-type tropical
marine climate, and an annual average ambient temperature of 29.2 C. The water mass in this area consists
mainly of modi®ed water from the West Paci®c Ocean,
with salinity averaging slightly more than 33%. Marine
organisms are very abundant in this area, and diatoms
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(99)00135-7
160
Y. Duan / Organic Geochemistry 31 (2000) 159±167
are the major phytoplankton. The area includes a variety of representative topographies, such as the continental shelf, coral island, island shelf, lagoon,
continental slope and deep-sea basin features.
2.2. Sampling
Sediment cores of station 103(11 120 N, 110 240 E)
and station 102 (12 020 N, 111 000 E) were recovered in
the northwestern Nansha Sea in 1990 by the Multidisciplinary Oceanographic Expedition Team of Academia Sinica (Fig. 1), using a hydraulic piston corer with
minimum disturbance. The sediment cores were divided
into many sections for various analyses, and all samples
used here were kept frozen in the laboratory until analysis.
Station 103 was located in the mid continental slope
at 1584 m water depth. The sediments consisted of silty
clay, with an average carbonate content of 25.4%.
Average sedimentation rate for this area is estimated at
2.4 cm/1000 year (Fen and Luo, 1993). Station 102 is
located in the lower slope at 2884 m water depth. The
sediments were also composed of silty clay, with an
average carbonate content of
www.elsevier.nl/locate/orggeochem
Organic geochemistry of recent marine sediments
from the Nansha Sea, China
Yi Duan*
State Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou, Gansu Province
730000, People's Republic of China
Received 29 May 1998; accepted 21 October 1999
(Returned to author for revision 18 December 1998)
Abstract
Recent marine sediments from two cores, collected on the continental slope of the Nansha Sea, China, have been
analyzed for organic matter content, n-alkane and isoprenoid hydrocarbons, fatty acids, n-alkanols and sterols. The
organic carbon contents of the two sediment cores average 0.7 and 0.53%, respectively, and are higher than those of
other sedimentary environments in this region. The distributions of various lipid compounds indicate that most of the
sedimentary organic matter in the two cores is derived from marine plankton and bacteria, with land-derived organic
matter present in relatively small amounts. The decrease in relative abundances of shorter chain lipids (n-alkanes, nfatty acids and n-alkanols) with depth is evident, as is the biochemical conversion of stenols to stanols in some samples.
The low pristane/phytane ratios may result primarily from microbial activity, and re¯ect the occurrence of signi®cant
diagenetic alteration of organic matter in the two cores. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Nansha Sea; Organic carbon content; Lipids; Sedimentary environments; Diagenesis
1. Introduction
Studies of the organic geochemistry of recent marine
sediments are valuable because they provide information about paleoenvironments and paleoceanography
(Keswani et al., 1984; Venkatesan and Kaplan, 1987). A
basic understanding of various lipid sources, their lipid
diagenetic evolution, and the accumulation and preservation of organic matter by such studies, is helpful in
interpreting sedimentary records. To date, such studies
have been concerned with sediments of temperate or
polar regions (Degens and Mopper, 1976; Venkatesan,
1988). The Nansha Sea of China is located in a lowlatitude tropical zone. No previous data have been
reported on the organic geochemical contents from
sediments of this region, although some studies have
been conducted on the organic geochemistry of sinking
* Tel.: +86-931-8827983; fax: +86-931-8418667.
particulate material (Duan et al., 1997a; 1998). This
paper reports the detailed organic geochemical contents
of recent marine sediments from the Nansha Sea. It also
discusses the relationship between the organic carbon
content and sedimentary environment, and the sources
and diagenetic changes of sedimentary organic material
in two sediment cores.
2. Materials and methods
2.1. Sampling site
The Nansha Sea is located to the south of the South
China Sea. The region has a monsoon-type tropical
marine climate, and an annual average ambient temperature of 29.2 C. The water mass in this area consists
mainly of modi®ed water from the West Paci®c Ocean,
with salinity averaging slightly more than 33%. Marine
organisms are very abundant in this area, and diatoms
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(99)00135-7
160
Y. Duan / Organic Geochemistry 31 (2000) 159±167
are the major phytoplankton. The area includes a variety of representative topographies, such as the continental shelf, coral island, island shelf, lagoon,
continental slope and deep-sea basin features.
2.2. Sampling
Sediment cores of station 103(11 120 N, 110 240 E)
and station 102 (12 020 N, 111 000 E) were recovered in
the northwestern Nansha Sea in 1990 by the Multidisciplinary Oceanographic Expedition Team of Academia Sinica (Fig. 1), using a hydraulic piston corer with
minimum disturbance. The sediment cores were divided
into many sections for various analyses, and all samples
used here were kept frozen in the laboratory until analysis.
Station 103 was located in the mid continental slope
at 1584 m water depth. The sediments consisted of silty
clay, with an average carbonate content of 25.4%.
Average sedimentation rate for this area is estimated at
2.4 cm/1000 year (Fen and Luo, 1993). Station 102 is
located in the lower slope at 2884 m water depth. The
sediments were also composed of silty clay, with an
average carbonate content of