Abnormally abundant alkenone-derived C37 and C38n-alkanes in Miocene Onnagawa siliceous mudstones, northeast Japan

Organic Geochemistry 34 (2003) 1247–1258 www.elsevier.com/locate/orggeochem

Abnormally abundant alkenone-derived C37 and C38 n-alkanes in Miocene Onnagawa siliceous mudstones, northeast Japan Yoshikazu Sampeia,*, Toshinori Inabab,1, Noriyuki Suzukib a

Department of Geoscience, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan b Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan Received 6 January 2003; accepted 12 May 2003 (returned to author for revision 10 April 2003)

Abstract Abnormally abundant n-C37 and n-C38 alkanes, together with a significant amount of the n-C39 alkane, were found in the Miocene Onnagawa siliceous mudstones. The relative abundance of these alkanes resembles that of C37, C38 and C39 alkenones from Emiliania huxleyi and Gephyrocapsa oceanica. The stable carbon isotope ratio of the n-C37 alkane is 23.5%, supporting their planktonic origin. The n-C37-C39 alkanes are likely to be derived from alkenones produced by the family Gephyrocapsaceae during the Middle to Late Miocene period. Total organic carbon concentration of the samples rich in the alkenone-derived n-alkanes is in the range of 1.5– 2.3%, and the samples are characterized by a low pristane/phytane ratio and a high homohopane index, suggesting anoxic depositional conditions. These sediments are relatively immature (Ro=0.33–0.45%), suggesting that the generation of the long-chain n-alkanes from alkenones took place during early diagenesis. The mudstones rich in n-C37-C39 alkanes are generally poor in CaO ( < 1.9%) and rich in SiO2 (77.9–85.3%). There is no relationship between CaO concentration and abundance of the n-C37-C39 alkanes. Since the Onnagawa siliceous mudstones were formed in a deep-sea environment, the low concentration of CaO is most probably due to the removal of calcareous skeletons by dissolution during deposition below the carbonate compensation depth. These alkenonederived n-alkanes are useful biomarkers for detecting contributions of calcareous nannoplankton to deep-sea sediments. # 2003 Elsevier Ltd. All rights reserved.

1. Introduction Long-chain alkenones and alkyl alkenoates are useful paleothermometers for reconstructing geological records of sea surface temperature and palaeoceanographic indicators for the assessment of the input of coccolithophorids closely related to some species of the family * Corresponding author. Tel.: +81-852-32-6453; fax: +81852-32-6469. E-mail address: [email protected] (Y. Sampei). 1 Current address: The Egyptian Petroleum Development Co., Ltd., No.11-10 2-Chome, Minami-Azabu, Minato-ku, Tokyo 106-0047, Japan.

Gephyrocapsaceae (Farrimond et al., 1986; Marlowe et al., 1990; Rostek et al., 1997; Villanueva et al., 1998; Sicre et al., 2000). Although alkenones have been detected in sediments as early as the Cretaceous (Farrimond et al., 1986), it is difficult to apply them as a palaeoceanographic indicator in sediments that are thermally mature, because the alkenones disappear with increasing thermal maturity in sedimentary rocks (Marlowe et al., 1990). Thus, a rather low content of alkenones has been found in Tertiary/Cretaceous mudstones (Farrimond et al., 1986; Brassell, 1993; Yamamoto et al., 1996). Alkenones/alkenoates can be converted to n-alkanes during diagenesis (e.g. Koopmans et al., 1997; Grice et al., 1998). There are few reports of n-C37 and n-C38

0146-6380/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0146-6380(03)00115-3

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alkanes dominating n-C31-C36 alkanes in ancient sediments and crude oils; such distributions have been explained as reduction products of alkenones derived from prymnesiophyte algae (Marlowe et al., 1990). Some analyses of Middle Miocene calcareous claystones (McEvoy et al., 1981), Late Miocene evaporitic marls (Schaeffer et al., 1995) and Early Eocene lacustrine mudstones (Grice et al., 1998) have shown a high proportion of n-C37 and n-C38 alkanes, but their origins and significance as a palaeoceanographic indicator were not fully discussed. In the present study, we show that long-chain n-C37, n-C38 and n-C39 alkanes in Middle to Late Miocene siliceous mudstones are derived from alkenones. The abundant n-C37 and n-C38 alkanes with a significant n-C39 alkane content can be useful indicators for showing a significant production of some species of the family Gephyrocapsaceae when the sediments were deposited. Those species leave a clear imprint in the Miocene siliceous sediments from the Akita basin, northeast Japan.

2. Geological settings The Middle to Late Miocene Onnagawa Formation (ca. 6–12 Ma) is widely distributed in the Akita Basin (Taguchi, 1975; Tada and Iijima, 1983; Aoyagi and Iijima, 1987; Tada, 1994). It is mainly composed of siliceous mudstone and shale with felsic volcaniclastics and marls, often containing siliceous fossils, diatoms and radiolaria (Tada and Iijima, 1983). Marine finegrained diatomaceous rock comprises the major lithology of the Lower Pliocene and Miocene sections of northern Japan (Tada and Iijima, 1983). Siliceous mudstone/shale from the Onnagawa Formation is rich in organic matter (Taguchi, 1975; Aoyagi and Iijima, 1987), and can be correlated with the middle part of the Monterey Formation in California, USA (Aoyagi and Iijima, 1987) These Middle to Late Miocene Formations located in the north circum-Pacific region are characterized by abundant biogenic silica, and have common organic geochemical features such as occurrences of Type II-S kerogen, abundant organic sulfur compounds, and presumed diatom-related 24-norcholestane (Suzuki et al., 1993, 1995).

spectrometry (GC-MS), gas chromatography-combustion-isotope ratio-mass spectrometry (GC-C-IR-MS) and X-ray fluorescence spectrometry were used to analyze the samples. Siliceous mudstone samples were dried at room temperature and pulverized to approximately 200-mesh size. About 10 g of each sample were submitted to bitumen extraction using ultrasonic extraction (15 min, twice) with a dichloromethane/methanol (9:1; v:v) mixed solvent. The extracted bitumen (about 50 mg) was fractionated with silica gel (Wakogel Q-23) column chromatography into three fractions: saturated hydrocarbons (n-hexane fraction), aromatic hydrocarbons (n-hexane/benzene (3:7; v:v) mixed solvent fraction), and NSO-compounds (benzene/methanol (1:1; v:v) mixed solvent fraction). These solvents were removed by evaporation at room temperature. The saturated hydrocarbon fraction was further separated by urea adduction into two fractions: n-alkanes and branched and cyclic alkanes, as follows. The saturated hydrocarbon fraction was dissolved with a small volume of dichloromethane, and added to urea-saturated methanol. After dichloromethane and methanol were completely evaporated at room temperature, the precipitated urea crystals were washed with dichloromethane to recover branched and cyclic alkanes. Then water was added to dissolve the urea crystals, and n-alkanes were recovered by extracting the water with additional dichloromethane. 3.1. GC-MS and GC-C-IR-MS A urea adducted fraction containing n-alkanes was analyzed by GC-MS and GC-C-IR-MS. The Hewlett Packard HP6890 GC was equipped with a fused silica capillary column DB-5 (60 m, 0.25 mm i.d., J&W). The GC was programmed isothermally at 40  C for 2 min, from 40 to 120  C at 20  C/min, from 120 to 300  C at 4  C/min, and held at 300  C isothermally for 25 min. The GC was coupled to a Finnigan MAT 252 isotoperatio mass spectrometer by a Finnigan MAT Interface GC-Combustion III. The d13C value of each component was obtained by the GC-C-IR-MS technique. A CO2 reference gas of known 13C/12C was introduced during the analyses (beginning and end of the run). All d13C values are reported in per mil calibrated against the international PDB standard.

3. Materials and experimental

3.2. TOC, Rock-Eval and vitrinite reflectance

A suite of 19 siliceous mudstone samples was selected from two wells in the north of the Yabase oil field of the Akita basin: 13 and 6 samples from the Tofuiwa-1 and Shin Yabase-1 wells, respectively (Fig. 1). All samples are cuttings from the Onnagawa Formation. Total organic carbon content (TOC), Rock-Eval pyrolysis, vitrinite reflectance, gas chromatography-mass

TOC (%) was measured after HCl treatment using a Yanaco MT-3 CHN-CORDER by combustion. Mudstone samples were analysed using the GEOMECANIQUE Rock Eval V device. It was programmed isothermally at 300  C for 3 min, and from 300 to 650  C at 25  C/ min. Hydrocarbons that were released at 300  C corresponded to those already present in mudstone (S1).

Y. Sampei et al. / Organic Geochemistry 34 (2003) 1247–1258

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Fig. 1. Location map of the Tofuiwa-1 and the Shin Yabase-1 wells with oil fields in the Akita sedimentary basin (based on the map from Inaba et al., 2001).

Hydrocarbons were generated through kerogen pyrolysis (S2) from 300 to 650  C. Mudstone samples were also treated with hydrochloric acid (HCl) and hydrofluoric acid (HF) to remove silicate minerals. A CaBr2 solution (prepared at 1.6 g/cm3) further isolated a comparatively light organic substance that possibly includes plant materials. The floated substance was incorporated in a plastic base and polished carefully vitrinite maceral was identified and the reflectance was measured on the polished sample with a Leitz polarized microscope. 3.3. X-ray fluorescence spectrometry (XRF) Major elements were analyzed using a Seiko Instruments SEA2001 energy-dispersive X-ray spectrometer and pressed powder tablets. The rhodium tube was adjusted at 15 kV and approximately 20 mA.

4. Results and discussion 4.1. Origin of the n-C37, n-C38, and n-C39 alkanes Figs. 2a (A1) and 2b (A2) show GC-MS chromatograms of saturated hydrocarbon fractions from siliceous mudstones at 1350, 1650 and 1700 m depth in the Shin Yabase-1 well and 1660 and 1780 m depth in the Tofuiwa-1 well. All show abundant n-C37 and n-C38 alkanes with a significant amount of the n-C39 alkane. The other layers of the Onnagawa Formation contain low relative amounts of n-C37–C39 alkanes, for example in Figs. 2a (B1) and 2b (B2). The layers with conspicuous amounts of n-C37 and n-C38 alkanes (identified based on the ratio of n-C37 to n-C35 alkane being more than 1) are shown with an asterisk in Table 1. Although higher plant wax-derived long-chain n-alkanes with carbon numbers more than 35 generally decrease in abundance with

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Fig. 2. (a) Typical mass chromatograms (m/z 57) of saturated hydrocarbons in the Middle to Late Miocene Onnagawa siliceous mudstones from the Tofuiwa-1 and Shin Yabase-1 wells. (A1) n-alkane distributions with an abnormally high abundance of n-C37, n-C38 and n-C39 alkanes. (B1) n-alkane distributions not showing enhanced abundance of n-C37, n-C38 and n-C39 alkanes. (b) Total ion chromatograms of the same samples in Fig. 2a. [sterane] A: 13b(H),17a(H)-diacholestane 20S (C27), B: 13b(H),17a(H)-diacholestane 20R (C27), C: 5a(H),14a(H),17a(H)-24-norcholestane 20S (C26), D: 5a(H),14a(H),17a(H)-24-norcholestane 20R (C26), E: 5a(H),14a(H),17a(H)-cholestane 20S (C27), F: 5a(H),14a(H),17a(H)-cholestane 20R (C27), G: 5a(H),14b(H),17b(H)-24-methylcholestane 20R (C28), H: 5a(H),14a(H),17a(H)-24-methylcholestane 20R (C28), I: 5a(H),14b(H),17b(H)-24-ethylcholestane 20R (C29), J: 5a(H),14a(H),17a(H)-24-ethylcholestane 20R (C29).

Y. Sampei et al. / Organic Geochemistry 34 (2003) 1247–1258

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Fig. 2. (continued)

increasing carbon number (e.g. Chouparova and Philp, 1998; Hsieh et al., 2000), n-C37–C39 alkanes dominated by n-C37 alkane in the layers show different distributions. The same layers are also characterized by a minor terrestrial input based on the following indicators: ratios of sterane C27/(C27+C29) higher than 0.5, and low contents of Al2O3 and Fe2O3 ranging from 6.2 to 10.2%

and from 2.1 to 4.3%, respectively (Table 1). Accordingly, the supply of land-derived long-chain n-alkanes with carbon numbers more than 37 into sediments in the layers should be very low or negligible. The d13C value of the long-chain n-C37 alkane from the Onnagawa siliceous mudstones was measured to be 23.5% (1700 m depth of Shin Yabase-1 well; Fig. 4).

1252 Table 1 Data for Rock-Eval pyrolysis (hydrogen index: HI and Tmax), vitrinite reflectane (Ro) measurement, gas chromatography/mass spectrometry and X-ray fluorescence spectrometry of mudstones from the Onnagawa Formation in Akita basin, northeast Japan Well

1560 1580 1600 1650 1660 1700 1720 1730 1780 1800 1870 1890 1900 1350 1400 1550 1650 1700 1900

1.6 2.1a 2.5 1.2 1.8a 1.4 1.0a 1.8 1.5a 1.7 2.2 1.7 1.7 2.0 2.4 1.7 1.6 2.3 1.0

153 169 237 138 244 122 96 137 254 222 445 351 133 370 386 463 373 495 68

403 402a 402 401 412a 399 400a 406 406a 403 399 416 403 418 422 419 420 419 429

– 0.32a – 0.29 – – – – 0.33a – – – – 0.41 0.35 0.29 0.45 0.41 0.42

Gas chromatography-mass spectrometry

X-ray fluorescence spectrometry

Sterane Sterane Hopaned/ C35-HHIf Pr/Phg S/(S+R)b C27/C27+29c Steranee

C37/C35 C37/C38 C38/C39 SiO2 n-alkane n-alkane n-alkane (%)

Al2O3 (%)

Fe2O3 (%)

CaO (%)

0.07 0.08a 0.10 0.17 0.16a 0.16 0.20a 0.12 0.13a 0.13 0.21 0.25 0.31 0.05 0.06 0.19 0.08 0.09 0.16

– 0.8 – – 1.5 – 0.7 – 2.3 – – 0.5 – 6.7 – 0.9 3.3 3.4 0.4

12.9 10.2a 11.0 14.0 6.2a 11.2 11.3a 11.1 8.4a 10.2 6.4 6.0 15.8 10.2 9.5 5.8 6.6 7.3 9.9

4.4 4.9a 4.6 4.9 2.1a 4.3 4.5a 4.5 2.1a 3.8 2.9 2.6 7.2 4.3 3.6 1.7 3.1 2.4 4.1

1.0 0.9 1.1 1.6 0.4 0.8 0.9 0.9 0.5 1.6 2.4 12.9 10.5 1.0 0.8 0.7 1.9 0.6 2.7

0.56 0.55 0.62 0.56 0.52 0.52 0.55 0.58 0.52 0.62 0.63 0.55 0.58 0.56 0.57 0.56 0.62 0.57 0.57

0.29 0.32 0.22 0.71 0.43 0.69 0.75 0.49 0.34 0.30 0.68 0.99 0.97 0.09 0.16 0.49 0.17 0.12 0.81

0.17 0.09 0.20 0.10 0.17 0.09 0.10 0.07 0.17 0.16 0.10 0.15 0.07 0.21 0.12 0.13 0.22 0.26 0.08

– 0.30 0.40 0.08 0.21 0.50 – 0.32 0.10 0.42 0.45 0.09 0.40 0.33 0.70 0.16 – – –

*

*

*

* *

– 1.9 – – 1.4 – 1.5 – 1.2 – – 1.8 – 1.2 – 1.4 1.2 1.2 1.3

– 4.9 – – 6.5 – 3.9 – 9.0 – – 3.6 – 8.4 – 4.2 11.2 9.6 3.2

76.7 77.3a 78.3 76.0 85.3a 78.3 74.3a 76.5 82.4a 80.8 84.4 55.4 58.0 77.9 77.5 89.7 83.5 83.3 74.0

*: n-C37/n-C35 alkane >1. –: below the detection limit. a Inaba et al. (2001). b S/(S+R)=5a,14a,17a(H)-24-ethylcholestane 20S/(5a,14a,17a(H)-24-ethylcholestane 20S+5a,14a,17a(H)-24-ethylcholestane 20R). c C27=5a,14a,17a(H)-cholestane 20R. C29=5a,14a,17a(H)-24-ethylcholestane 20R. d Hopane=17a,21b(H)-30-norhopane+17a,21b(H)-30-hopane+17a,21b(H)-29-homohopane 22S+22R+17a,21b(H)-29-bishomohopane 22S+22R+17a,21b(H)-29-trishomohopane 22S+22R. e Sterane=cholestane+methylcholestane+ethylcholestane 5a,14a,17a(H) 20R+20S and 5a,14b,17b(H) 20S+20R. f C35HHI=17a,21b(H)-29-pentakishomohopane 22S+22R(C35)/[17a,21b(H)-29-homohopane 22S+22R(C31) to 17a,21b(H)-29-pentakishhomohopane 22S+22R(C35)]. g Pr/Ph=pristane/phytane ratio.

Y. Sampei et al. / Organic Geochemistry 34 (2003) 1247–1258

Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Tofuiwa-1 Shin Yabase-1 Shin Yabase-1 Shin Yabase-1 Shin Yabase-1 Shin Yabase-1 Shin Yabase-1

Depth TOC HI Tmax Ro (m) (%) (mg/gC) ( C) (%)

Y. Sampei et al. / Organic Geochemistry 34 (2003) 1247–1258

This value is rather heavier than those of n-C20–C27 alkanes in the same sample which show a decreasing trend from 24.8% in n-C20 alkane to 28.5% in n-C27 alkane with increasing carbon number (Fig. 4). In general, the d13C values of n-C29–C35 alkanes from higher land plants are much lighter as reported by Naraoka and Ishiwatari (1999; 28.1% to 34.6%) than those values. Thus, the n-C37 alkane with the heavy value of d13C from the Onnagawa siliceous mudstones is suggested to be derived from planktonic organic matter. It is noteworthy that the value of d13C of the n-C37 alkane is within the range of the values for diunsaturated C37 alkenones of Gephyrocapsaceae in sediments from 22.5 to 25.8% (calculated from the data of Benthien et al., 2002; avg. 24.1  0.9%, n=29). Consequently, longchain alkenones are likely sources for the n-C37–C39 alkanes in the Onnagawa siliceous mudstones. The synthesis of C37–C39 alkenones is extremely restricted taxonomically and has been documented only within the Haptophyta (Conte et al., 1994). The alkenones are known to comprise di-, tri- and tetra-unsaturated ketones (Volkman et al., 1980; Marlowe et al., 1984b). Relative properties vary between species, but all have abundant C37 and C38 alkenones with a small but significant amount of C39 alkenone. The average relative abundances of C37, C38 and C39 alkenones from Emiliania huxleyi, Gephyrocapsa oceanica, Chrysotila lamellosa and Isochrysis galbana in seawater environments are shown in Fig. 3 (Marlowe et al., 1984b;

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Fig. 4. d13C values of n-C20 to n-C27 alkanes and n-C37 alkane from the Onnagawa siliceous mudstones at 1700 m depth of Shin Yabase-1 well.

Volkman et al., 1995; Schulz et al., 2000), and compare with that of the n-C37–C39 alkanes obtained in the present study. The relative abundance of the n-C37–C39 alkanes rather resembles that of C37-C39 alkenones from E. huxleyi and G. oceanica. E. huxleyi has an approximately similar ratio of C37 to C38 alkenones to those of G. oceanica (Volkman et al, 1995; Sawada et al., 1996; Conte et al., 1998; Table 2). The ratios of C37 to C38 alkenones for E. huxleyi and G. oceanica are about 1, though the other species Chrysotila lamellosa and Isochrysis galbana have extraordinarily high ratios of C37 to C38 alkenones of more than 8 (calculated from Marlowe et al., 1984b; Fig. 3). High ratios

Fig. 3. Relative abundances with range of (a) n-C37, n-C38 and n-C39 alkanes from the Onnagawa siliceous mudstones and C37, C38, and C39 alkenones from (b) Emiliania huxleyi and Gephyrocapsa oceanica and (c) Chrysotila lamellosa and Isochrysis galbana (and Isochrysis sp.). The mean relative abundances of C37, C38, C39 are 11.0, 8.9, 1.0 for the n-alkanes (present study; n=5), 9.1, 8.0, 1.0 for Emiliania huxleyi-type sediments (data from Schulz et al., 2000 excluding low salinity samples <8 PSU; n=8), 6.9, 9.7, 1.0 for cultured Gephyrocapsa oceanica (data from Volkman et al., 1995; n=9), 38.2, 4.3, 1.0 for cultured Chrysotila lamellosa (data from Marlowe et al., 1984b; n=2) and 18.3, 2.0, 1.0 for cultured Isochrysis galbana and Isochrysis sp. (data from Marlowe et al., 1984b; n=3), respectively.

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Table 2 Ratios of C37 to C38 alkenones and n-C37 to n-C38 alkanes Species

C37/C38 alkenones

Standard deviation

Range

Number of samples

Sampling site

References

Emiliania huxleyi Emiliania huxleyi Emiliania huxleyi Emiliania huxleyi Emiliania huxleyi Emiliania huxleyi (average)

1.1 1.3 1.2 1.5 1.5 2.3 (1.5)

0.2 0.1 0.1 0.2 0.2 –

0.8–1.4 1.1–1.5 0.9–1.4 1.3–1.9 1.2–1.7 –

8 82 39 18 5 1

Baltic Sea Arabian Sea Indian Ocean cultures cultures cultures

Schulz et al. (2000) Rostek et al. (1997) Sonzogni et al. (1997) calculated from Sawada et al. (1996) Prahl et al. (1988) calculated from Marlowe et al. (1984)

Gephyrocapsa oceanica Gephyrocapsa oceanica Gephyrocapsa oceanica (average)

1.4 1.1 0.7 (1.1)

0.2 0.2 0.1

1.2–1.7 0.8–1.3 0.6–0.8

8 11 9

cultures cultures

Conte et al. (1998) calculated from Sawada et al. (1996) Volkman et al. (1995)

Chrysotila lamellosa Isochrysis galbana (or sp.)

8.9 10.9 n-C37/n-C38 1.2

– 5.0

8.5–9.3 5.4–15.1

2 3

cultures cultures

calculated from Marlowe et al. (1984) calculated from Marlowe et al. (1984)

0.1

1.2–1.4

5

Onnagawa F.

this study

n-alkane

more than 5 for C37 to C38 alkenones have also been reported for low salinity areas where other species are present (Cranwell, 1985, 1988; Schulz et al., 2000). The ratio of n-C37 to n-C38 alkane is 1.2 on average (Table 2). On the other hand, the ratios of C38 to C39 alkenones are also significantly different between the above two groups, i.e. E. huxleyi and G. oceanica in the group and Chrysotila lamellosa and Isochrysis galbana in the other. The ratios of C38 to C39 alkenones are 8.0 and 9.7 on average from the first group (Fig. 3), but the ratios from the second group are 4.5 and 2.0 on average (calculated from Marlowe et al., 1984b; Fig. 3). The ratio of n-C38 to n-C39 alkane is 8.9 on average (Fig. 3). The ratios of C37 to C38 alkenones for G. oceanica are sometimes slightly lower than those for E. huxleyi (Prahl et al., 1988; Marlowe et al., 1990; Rosell-Mele´ et al., 1994; Volkman et al., 1995; Sawada et al., 1996; Sonzogni et al., 1997), but recent arguments raise doubt about the use of this ratio for distinguishing the two species; E. huxleyi is thought to have branched from G. oceanica during the late Pleistocene (McIntyre, 1970), and some G. oceanica strains have similar ratios of C37 to C38 alkenones to those for E. huxleyi (Volkman et al, 1995; Sawada et al., 1996; Conte et al., 1998). The ratios of C37 to C38 alkenones from E. huxleyidominant sediments and cultured E. huxleyi cover the range 1.1–2.3 as shown in Table 2 (Prahl et al., 1988; Sonzogni et al., 1997; Rostek et al., 1997; Schulz et al., 2000). The ratios of C37 to C38 alkenones found for G. oceanica also vary in some reports: e.g. 0.7  0.1 (0.6–0.8) (Volkman et al., 1995), 1.1  0.2 (0.8–1.3) (Sawada et al., 1996) and 1.4 0.2 (1.2–1.7) (Conte et al., 1998). These variations in the ratios could be due to

nutrient, physiologic status of cells and growth temperature (Conte et al., 1998). The total ion chromatograms in Fig. 2b show abundant steranes as well as n-C37 and n-C38 alkanes; cholestanes and methylcholestanes are abundant. The chromatograms with steranes and n-C37 and n-C38 alkanes are similar to those reported by Koopmans et al. (1997) in their heating experiments. Long-chain n-alkenones from the order Isochrysidales in the class Prymnesiophyceae are accompanied with abundant sterols, e.g. high concentrations of 24-methylcholesta5,22E-dien-3b-ol (Marlowe et al., 1984b). The d13C of methylcholestanes from Yabase oil is close to that of cholestane from 24.4 to 26.3% (Inaba and Suzuki, 2003). These data also support the idea that the longchain n-alkanes have originated from a member of the family Gephyrocapsaceae. Long-chain n-alkenes, i.e. C37:3 heptatriaconta8E,15E,22E-triene and C38:3 octatriaconta-9E,16E,23Etriene, co-occur in some strains of E. huxleyi and G. oceanica (Volkman et al., 1980; Marlowe et al., 1984b; Marlowe et al., 1990). However, a greater diversity of chain lengths with carbon number less than C36 was found in the alkene distribution (Volkman et al., 1980; Marlowe et al., 1984b). Furthermore, longchain n-alkenes are rarely abundant in sediments except for those deposited under cold water (Sikes et al., 1997; Volkman et al., 1998), and long-chain dienes are rapidly degraded in surface sediments (Teece et al., 1998). Accordingly, we suggest that long-chain n-alkenes did not contribute significantly to the abundant n-C37 and n-C38 alkanes in the Onnagawa siliceous mudstones.

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E. huxleyi is considered to have emerged about 265 ky B.P. (Thierstein et al., 1977). Therefore, in the Miocene Onnagawa Formation the origin of the abundant n-C37 and n-C38 with significant n-C39 alkanes should be from an ancestor of E. huxleyi and/or G. oceanica in the family Gephyrocapsaceae. Alkenones have been detected in ancient sediments dating back to Cretaceous (Farrimond et al., 1986), and some alkenones are considered to be characteristic components related to species of the family Gephyrocapsaceae dating back to Eocene (Marlowe et al., 1990). Late Miocene sediments are the oldest ones with characteristic microfossils (Brassell et al., 1980; Marlowe et al, 1984a). 4.2. Diagenetic processes Vitrinite reflectance (Ro) values of the Miocene Onnagawa siliceous mudstones which contain significant n-C37–C39 alkanes range from 0.33 to 0.45% (Table 1). The Rock Eval Tmax and ethylcholestane 20S/ (20S+20R) ratios of the same samples range from 406 to 420  C and from 0.05 to 0.16, respectively (Table 1), implying maturity close to the oil window. Evidence for a palaeo-environment with a stagnant sea-bottom and weak activity of aerobic bacteria is suggested by moderate Rock Eval HI values, i.e. 244 to 495 mg/gC, low hopane/sterane ratios less than 0.5, low pristane/ phytane (Pr/Ph) ratio less than 0.7, and high C35 homohopane indices (Peters and Moldowan, 1991) up to 0.26 (Table 1). The TOC in the same samples is slightly higher than that of the other samples without n-C37–C39 alkanes (1.5–2.3%, avg. 1.8%, n. 5 cf. 0.97–2.5%, avg. 1.7%, n. 14, respectively; Table 1). The conversion of long-chain alkenones to n-alkanes with the same carbon number can be explained according to the view of Koopmans et al. (1997) based on laboratory heating experiments. They showed the n-C37 and n-C38 skeletons were linked to kerogen by sulfur or oxygen bonding at the double bond positions of alkenones; long-chain n-alkanes with the same carbon number were then released with increasing thermal maturity. The sulfur linking of long-chain alkenones to kerogen occurs readily in sediments with abundant H2S at a very early stage of diagenesis. According to Suzuki et al. (1995), the Onnagawa Formation is characterized by Type II-S kerogen, which is formed under stagnant seabottom condition rich in H2S. We suggest that the alkenones in the Onnagawa Formation would have become linked to the kerogen by C-S bonding, with partial reduction, and then the alkenone-derived n-alkanes were probably released under comparatively low maturity because C-S bonds are quite weak (Orr, 1986; Koopmans et al., 1997; Grice et al., 1998). Most of the alkenones would have been transformed to n-alkanes with the same carbon number (Fig. 2b). On the other hand, alkyl alkenoates appear not to be the

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major contributors to the conspicuous long-chain n-alkanes in the Onnagawa Formation, because alkyl alkenoates would produce hydrocarbons with a carbon number one or two fewer than that of original methyl or ethyl alkenoate. 4.3. Paleoceanographic significance of the n-C37, n-C38 and n-C39 alkanes The Onnagawa Formation mainly consists of diatomaceous siliceous mudstones. Diatoms dominate the phytoplankton during upwelling or cold current conditions when nutrients are abundant (Lange et al., 1998; Abrantes and Moita, 1999). In contrast, in warm oligotrophic seawater or during non-upwelling conditions coccolithophorids can become predominant (Abrantes and Moita, 1999; Jordan and Winter, 2000). The longchain n-C37–C39 alkanes in the Onnagawa Formation suggest that a bloom of coccolithophorids occurred instead of diatoms probably due to a depletion in nutrients caused by an invasion of warm currents and/or non-upwelling, considering recent observations that a decrease in primary productivity is associated with the occurrence of coccolithophorid blooms (Balch et al., 1991; Head et al., 1998). A contribution of terrestrial nutrients was also reduced, particularly in the deposition-period of the layers containing abundant n-C37–C39 alkanes, likely according to the small input of terrestrial material as shown in Table 1. The n-C37-C39 alkanes are abundant in the upper and middle parts of the Onnagawa Formation. During the Middle to Late Miocene the fossil cephalopod record (Sakumoto et al., 1996) and geological studies (Tada, 1994) suggest that warm currents sometimes invaded the surface of the Miocene Sea of Japan through the Tsushima channel, southwest of Japan or through a paleo-channel situated in central Japan. We infer that highly stratified seawater within a shallow mixed layer with low or depleted inorganic nutrients occasionally occurred in the Akita basin, in agreement with Garcı´a-Soto et al. (1995), Balch et al. (1991) and Head et al. (1998). This scenario is consistent with a previous study of major/minor elements by Watanabe et al. (1995), which showed a stagnant ocean bottom due to restricted circulation of seawater. The low CaO concentrations (the Tofuiwa-1 well: 0.4–0.5%, the Shin Yabase-1 well: 0.6–1.9%; Table 1) of the Onnagawa siliceous mudstones rich in n-C37–C39 alkanes indicate little CaCO3. The Onnagawa Formation in this area was deposited in a deep sea with water depths to about 2000 m (Iijima and Tada, 1990; Yamamoto et al., 1994), where no CaCO3 would remain under the CCD (carbonate compensation depth). Therefore, the conspicuous n-C37–C39 alkanes are of great importance for palaeoceanographic reconstructions using deep-sea sediments. To date, reconstructions of

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palaeoceanoic environments are often performed using calcareous microfossils, but this application is useful only in shallow marginal seas, oceanic rises and nearocean plateaux where the sediments were deposited above the CCD. Consequently, the specific distribution of n-C37, n-C38 and n-C39 alkanes in sediments lacking CaCO3 will be a useful proxy indicating dissolution of coccoliths deposited under the CCD, and can be widely applicable to comparatively old sediments deposited in a deep-sea environment.

5. Conclusions Abnormally abundant n-C37, n-C38 and n-C39 alkanes were found in the Middle to Late Miocene Onnagawa siliceous mudstones. The n-C37 alkane has a comparatively heavy d13C value ( 23.5%), consistent with a planktonic origin. The distribution of the n-C37–C39 alkanes resembles that of C37-C39 alkenones from E. huxleyi and G. oceanica. We conclude that the n-C37– C39 alkanes in the Onnagawa siliceous mudstones were derived from alkenones of haptophyte algae, which could be the ancestors of species related to present-day E. huxleyi and G. oceanica. The long-chain n-alkanes derived from alkenones in the siliceous mudstones suggest that a warm current depleted in nutrients occasionally invaded the surface Sea of Japan in the Middle to Late Miocene. The absence of calcareous coccolith microfossils in the siliceous mudstones indicates deposition in an environment below the CCD. A conspicuous abundance of n-C37–C39 alkanes in a sediment without coccolith fossils can be a useful proxy for evaluating production of coccolithophorids and the depositional environment in a deep ocean.

Acknowledgements We are grateful to the Teikoku Oil Co. Ltd. for offering the cuttings samples and for permission to publish this paper. We would like to express our thanks to Dr J. K. Volkman, Dr E. Calvo, Dr K. Grice and an anonymous referee for comments which significantly improved the manuscript. We thank Messrs. Gotaro Amenomori and Yoshiya Kamei, Hokkaido University for supporting GC-MS and GC-C-IR-MS analyses. Associate Editor—J.K. Volkman

References Abrantes, F., Moita, M.T., 1999. Water column and recent sediment data on diatoms and coccolithophorids, off Portu-

gal, confirm sediment record of upwelling events. Oceanologica Acta 22, 67–84. Aoyagi, K., Iijima, A., 1987. Petroleum occurrence, generation, accumulation in the Miocene siliceous deposits of Japan. In: Hein, J.R. (Ed.), Siliceous Sedimentary Rock-Hosted Ores and Petroleum. Van Nostrand, Reinhold, pp. 117–137. Balch, W.M., Holligan, P.M., Ackleson, S.G. and Voss, K.J., 1991. Biological and optical properties of mesoscale coccolithophore blooms in the Gulf of Maine. Limnology and Oceanography, 36, 629-643. Benthien, A., Andersen, N., Schulte, S., Mu¨ller, P.J., Schneider, R.R., Wefer, G., 2002. Carbon isotopic composition of the C37:2 alkenone in core top sediments of the South Atlantic Ocean: Effects of CO2 and nutrient concentrations. Global Biogeochemical Cycles 16, 10.1029/2001GB001433. Brassell, S.C., Comet, P.A., Eglinton, G., Isaacson, P.J., McEvoy, J., Maxwell, J.R., Thomson, I.D., Tibbetts, P.J.C., Volkman, J.K., 1980. Preliminary lipid analyses of Sections 440A-7-6, 440B-3-5, 440B-8-4, 440B-68-2, and 436-11-4: Legs 56 and 57, Deep Sea Drilling Project. Initial Reports of the Deep Sea Drilling Project 56 57 (2), 1367–1390. Brassell, S.C., 1993. Applications of biomarkers for delineating marine paleoclimatic fluctuations during the Pleistocene. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 699–738. Chouparova, E., Philp, R.P., 1998. Geochemical monitoring of waxes and asphaltenes in oils produced during the transition from primary to secondary water flood recovery. Organic Geochemistry 29, 449–461. Conte, M.H., Volkman, J.K., Eglinton, G., 1994. Lipid biomarkers of the Haptophyta. In: Green, J.C., Leadbeater, B.S.C. (Eds.), The Haptophyte Algae. Clarendon Press, pp. 351–377. Conte, M.H., Thompson, A., Lesley, D., Harris, R., 1998. Genetic and physiological influences on the alkenone/ alkenoate versus growth temperature relationship in Emiliania huxleyi and Gephyrocapsa oceanica. Geochimica et Cosmochimica Acta 62, 51–68. Cranwell, P.A., 1985. Long-chain unsaturated ketones in recent lacustrine sediments. Geochimica et Cosmochimica Acta 49, 1545–1551. Cranwell, P.A., 1988. Lipid geochemistry of Late Pleistocene lacustrine sediments from Burland, Cheshire, U. K. Chemical Geology 68, 181–197. Farrimond, P., Eglinton, G., Brassell, S.C., 1986. Alkenones in Cretaceous black shales, Blake-Bahama Basin, western North Atlantic. Organic Geochemistry 10, 897–903. Garcı´a-Soto, C., Ferna´ndez, E., Pingree, R.D., Harbour, D.S., 1995. Evolution and structure of a shelf coccolithophore bloom in the Western English Channel. Journal of Plankton Research 17, 2011–2036. Grice, K., Schouten, S., Peters, K.E., Sinninghe Damste´, J.S., 1998. Molecular isotopic characterisation of hydrocarbon biomarkers in Palaeocene-Eocene evaporitic, lacustrine source rocks from the Jianghan Basin, China. Organic Geochemistry 29, 1745–1764. Head, R.N., Crawford, D.W., Egge, J.K., Harris, R.P., Kristiansen, S., Lesley, D.J., Maran˜o´n, E., Pond, D., Purdie, D.A., 1998. The hydrography and biology of a bloom of the coccolithophorid Emiliania huxleyi in the northern North Sea. Journal of Sea Research 39, 255–266.

Y. Sampei et al. / Organic Geochemistry 34 (2003) 1247–1258 Hsieh, M., Philp, R.P., del Rio, J.C., 2000. Characterization of high molecular weight biomarkers in crude oils. Organic Geochemistry 31, 1581–1588. Iijima, A., Tada, R., 1990. Evolution of Tertiary sedimentary basins of Japan in reference to opening of the Japan Sea. Journal of the Faculty of Science, University of Tokyo Section II 22, 121–171. Inaba, T., Suzuki, N., Hirai, A., Sekiguchi, K., Watanabe, T., 2001. Source rock lithology prediction based on oil diacholestane abundance in the siliceous-clastic Akita sedimentary basin, Japan. Organic Geochemistry 32, 877–890. Inaba, T., Suzuki, N., 2003. Gel permeation chromatography for fractionation and isotope ratio analysis of steranes and triterpanes in oils. Organic Geochemistry 34, 635–641. Jordan, R.W., Winter, A., 2000. Assemblages of coccolithophorids and other living microplankton off the coast of Puerto Rico during January–May 1995. Marine Micropaleontology 39, 113–130. Koopmans, M.P., Shaeffer-Reiss, C., de Leeuw, J.W., Lewan, M.D., Maxwell, J.R., Schaeffer, P., Sinninghe Damste´, J.S., 1997. Sulfur and oxygen sequestration of n-C37 and n-C38 unsaturated ketones in an immature kerogen and the release of their carbon skeletons during early stages of thermal maturation. Geochimica et Cosmochimica Acta 61, 2397– 2408. Lange, C.B., Romero, O.E., Wefer, G., Gabric, A.J., 1998. Offshore influence of coastal upwelling off Mauritania, NW Africa, as recorded by diatoms in sediment traps at 2195 m water depth. Deep-Sea Research 45, 985–1013. Marlowe, I.T., Brassell, S.C., Eglinton, G., Green, J.C., 1984a. Long chain unsaturated ketones and esters in living algae and marine sediments. In: Schenck, P.A., de Leeuw, J.W., Lijmbach, G.W.M. (Eds.), Advances in Organic Geochemistry 1983, Organic Geochemistry 6. Pergamon Press, Oxford, pp. 135–141. Marlowe, I.T., Green, J.C., Neal, A.C., Brassell, S.C., Eglinton, G., Course, P.A., 1984. Long chain (n-C37-C39) alkenones in the Prymnesiophyceae- Distribution of alkenones and other lipids and their taxonomic significance. British Phycological Journal 19, 203–216. Marlowe, I.T., Brassell, S.C., Eglinton, G., Green, J.C., 1990. Long-chain alkenones and alkyl alkenoates and the fossil coccolith record of marine sediments. Chemical Geology 88, 349–375. McEvoy, J., Eglinton, G., Maxwell, J.R., 1981. Preliminary lipid analyses of sediments from Sections 467-3-3 and 467-972. Initial Reports of the Deep Sea Drilling Project 63, 763– 774. McIntyre, A., 1970. Gephyrocapsa protohuxleyi sp. n., a possible phyletic link and index fossil for the Pleistocene. DeepSea Research 17, 187–190. Naraoka, H., Ishiwatari, R., 1999. Carbon isotopic compositions of individual long-chain n-fatty acids and n-alkanes in sediments from river to open ocean: Multiple origins for their occurrence. Geochemical Journal 33, 215–235. Orr, W.L., 1986. Kerogen/asphaltene/sulfur relationships in sulfur-rich Monterey oils. In: Leythaeuser, D., Rullko¨tter, J. (Eds.), Advances in Organic Geochemistry 1985, Organic Geochemistry 10. Pergamon Press, Oxford, pp. 499–516. Peters, K.E., Moldowan, J.M., 1991. Effects of source, thermal maturity, and biodegradation on the distribution and iso-

1257

merization of homohopanes in petroleum. Organic Geochemistry 17, 47–61. Prahl, F.G., Muehlhausen, L.A., Zahnle, D., 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochimica et Cosmochimica Acta 52, 2303–2310. Rosell-Mele´, A., Carter, J., Eglinton, G., 1994. Distribution of long-chain alkenones and alkyl alkenoates in marine sediments from the North East Atlantic. Organic Geochemistry 22, 501–509. Rostek, F., Bard, E., Beaufort, L., Sonzogni, C., Ganssen, G., 1997. Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea. Deep-Sea Research 44, 1461–1480. Sakumoto, T., Seto, K., Takayasu, K., 1996. Fossil cephalopods from the Middle Miocene Fujina Formation of southwest Japan and its paleoenvironmental significance. Earth Science (Chikyu Kagaku) 50, 408–413. (in Japanese). Sawada, K., Handa, N., Shiraiwa, Y., Danbara, A., Montani, S., 1996. Long-chain alkenones and alkyl alkenoates in the coastal and pelagic sediments of the northwest North Pacific, with special reference to the reconstruction of Emiliania huxleyi and Gephyrocapsa oceanica ratios. Organic Geochemistry 24, 751–764. Schaeffer, P., Harrison, W.N., Keely, B.J., Maxwell, J.R., 1995. Product distributions from chemical degradation of kerogens from a marl from a Miocene evaporitic sequence (Vena del Gesso, N.Italy). Organic Geochemistry 23, 541–554. Schulz, H.M., Scho¨ner, A., Emeis, K.-C., 2000. Long-chain alkenone patterns in the Baltic Sea- an ocean-freshwater transition. Geochimica et Cosmochimica Acta 64, 469–477. Sicre, M.-A., Ternois, Y., Paterne, M., Boireau, A., Beaufort, L., Martinez, P., Bertrand, P., 2000. Biomarker stratigraphic records over the last 150 kyears of the NW African coast at 25 N. Organic Geochemistry 31, 577–588. Sikes, E.L., Volkman, J.K., Robertson, L.G., Pichon, J.-J., 1997. Alkenones and alkenes in surface water and sediments of the Southern Ocean: Implications for paleotemperature estimation in polar regions. Geochimica et Cosmochimica Acta 61, 1495–1505. Sonzogni, C., Bard, E., Rostek, F., Lafont, R., Rosell-Mele´, A., Eglinton, G., 1997. Core-top calibration of the alkenone index vs sea surface temperature in the Indian Ocean. DeepSea Research 44, 1445–1460. Suzuki, N., Sampei, Y., Koga, O., 1993. Norcholestane in Miocene Onnagawa siliceous sediments, Japan. Geochimica et Cosmochimica Acta 57, 4539–4545. Suzuki, N., Sampei, Y., Matsubayashi, H., 1995. Organic geochemical difference between source rocks from Akita and Niigata oil fields, Neogene Tertiary. Journal of the Japanese Association for Petroleum Technology 60, 62–75. (Abstract in English). Tada, R., 1994. Paleoceanographic evolution of the Japan Sea. Palaeogeography Palaeoclimatology Palaeoecology 108, 487–508. Tada, R., Iijima, A., 1983. Petrology and diagenetic changes of Neogene siliceous rocks in northern Japan. Journal of Sedimentary Petrology 53, 911–930. Taguchi, K., 1975. Geochemical relationships between Japanese Tertiary oils and their source rocks. In: Proceedings of 9th World Petroleum Congress, Volume 2 Geology, Applied Science Publishers, 193–194.

1258

Y. Sampei et al. / Organic Geochemistry 34 (2003) 1247–1258

Teece, M.A., Getliff, J.M., Leftley, J.W., Parkes, R.J., Maxwell, J.R., 1998. Microbial degradation of the marine prymnesiophyte Emiliania huxleyi under oxic and anoxic conditions as a model for early diagenesis: long chain alkadienes, alkenones and alkyl alkenoates. Organic Geochemistry 29, 863–880. Thierstein, H.R., Geitsenauer, K.R., Molfino, B., 1977. Global synchroneity of late Quaternary coccolith datum levels: validation by oxygen isotopes. Geology 5, 400–404. Villanueva, J., Grimalt, J.O., Cortijo, E., Vidal, L., Labeyrie, L., 1998. Assessment of sea surface temperature variations in the central North Atlantic using the alkenone unsaturation index (UK’ 37 ). Geochimica et Cosmochimica Acta 62, 2421–2427. Volkman, J.K., Eglinton, G., Corner, E.D.S., Sargent, J.R., 1980. Novel unsaturated straight chain C37-C39 methyl and ethyl ketones in marine sediments and a coccolithophore Emiliania huxleyi. In: Douglas, A.G., Maxwell, J.R. (Eds.), Advances in Organic Geochemistry 1979, Organic Geochemistry. Pergamon Press, Oxford, pp. 219–227. Volkman, J.K., Barrett, S.M., Blackburn, S.I., Sikes, E.L.,

1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochimica et Cosmochimica Acta 59, 513–520. Volkman, J.K., Barrett, S.M., Blackburn, S.I., Mansour, M.P., Sikes, E.L., Gelin, F., 1998. Microalgal biomarkers: A review of recent research developments. Organic Geochemistry 29, 1163–1179. Watanabe, Y., Yamamoto, M., Watanabe, M., 1995. Geochemistry and paleoceanography of the Onnagawa diatomaceous sediments. Journal of the Japanese Association for Petroleum Technology 60, 15–26. (Abstract in English). Yamamoto, M., Ficken, K., Baas, M., Bosch, H.-J., de Leeuw, J.W., 1996. Molecular palaeontology of the earliest Danian at Geulhemmerberg (The Netherlands). Geologie en Mijnbouw (Kluwer Academic Publishers) 75 (2/3), 255–267. Yamamoto, S., Irino, T., Tada, R., Iijima, A., 1994. Reconstruction of a Miocene Volcanic seamount, the sedimentation depth for siliceous shale of the Onnagawa Formation in the Fujisato area, northern Akita. The Journal of the Geological Society of Japan 100, 557–573. (Abstract in English).