Heterogeneities of hydrocarbon compositions in mudstones ... - terrapub

Geochemical Journal, Vol. 42, pp. 151 to 162, 2008

Heterogeneities of hydrocarbon compositions in mudstones of a turbiditic sequence of the Miocene Kawabata Formation in Yubari, central Hokkaido, Japan KAZUKI O KANO and K EN SAWADA* Department of Natural History Sciences, Faculty of Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan (Received March 30, 2007; Accepted September 26, 2007) Biomarker analyses of aliphatic and aromatic hydrocarbons were conducted on the upper and lower parts of individual mudstones in a turbiditic sequence of the Miocene Kawabata Formation, Yubari, central Hokkaido, Japan. The relative abundances of terrigenous biomarkers such as long-chain n-alkanes, C 29 steranes and oleanenes as well as biomarkers produced in coastal areas such as dinosteranes tend to be higher in the lower parts of mudstones than those of the upper ones in the turbiditic sequence. Also, pristane/phytane ratios, which are proxies for depositional environment, significantly vary between the upper and lower parts of mudstones in the turbiditic sequence. These results suggest that these variations reflect the types and phases of transport and redeposition by turbidity systems rather than postdepositional diagenesis. Such heterogeneous distributions in most of biomarker compositions within the mudstone layers may lead to a severe bias for reconstructing paleoenvironment and paleoecology. Nevertheless, the higher plant parameters (HPPs) such as the retene/cadalene ratios are consistently homogenous within the mudstones. Moreover, paleovegetation reconstructed from HPPs are concordant with a previous paleobotanical study. Thus, it seems that these biomarker records in the turbidite sediments are reliable and strongly applicable for reconstructing terrestrial paleovegetation. Keywords: hydrocarbon biomarker, turbiditic sequence, geochemical heterogeneity, terrigenous material transport, paleoenvironment

have been reported (e.g., Hedges and Keil, 1995; Thomsen et al., 2001; Laurier et al., 2003; Treignier et al., 2006), there have been only a few geochemical studies of older turbiditic sedimentary sequences for reconstruction of long time-scale paleoenvironments and material-cycling systems. Hoefs et al. (2002) reported the organic geochemical investigations of Miocene to Pleistocene turbiditic sequences collected by Ocean Drilling Program (ODP) Leg 157 on the Madeira Abyssal Plain (MAP) off northern Africa, which showed variations in biomarker compositions that were attributed to redox conditions (oxygen exposure time) of the depositional environment. Also, organic geochemical and petrographic studies in Miocene turbiditic sequences of central Japan indicated that compositions of kerogens and biomarkers were quite different between layers of turbidite mudstone (Et) and pelagic mudstone (Ep) in the Bouma Sequences, which are typical of turbiditic sequences, of outcrops (Watanabe and Akiyama, 1998; Fujita et al., 1998). These studies of the turbiditic sequences suggested that organic geochemical characteristics were strongly controlled by types and phases of transport, redeposition and postdepositional diagenesis of turbidites. From this fact, it is pointed out that bulk analysis of such sediment may lead to severe bias for the interpretation of organic matter sources and for paleoenvironmental implication. It

INTRODUCTION Lipid biomarkers have been extensively used for reconstructing paleoenvironment and paleoceanographic conditions and for assessing material-cycling systems representing transport of terrigenous materials in landocean interface from coastal to pelagic sediments. Turbidity currents are the principal agents for transport and redeposition of organic matter from coastal to deep-sea provinces, and they produce sedimentary layers, in which lithologic and geochemical features are heterogeneous due to non-steady state deposition (Buckley and Cranstone, 1988; Meyers et al., 1996). The turbidite sediment provides several types of information about both marine and terrestrial environments because it contains autochthonous matter derived from marine organisms and terrigenous matter transported from land areas. However, we should pay attention to the geochemical heterogeneity in turbidite sediments for applying to paleoenvironmental and material-cycling reconstructions. Although many geochemical studies of turbidity currents in the water column and surface sediments of the present-day ocean *Corresponding author (e-mail: [email protected]) Copyright © 2008 by The Geochemical Society of Japan.

151

hence, is necessary to carry out high-resolution biomarker analysis of a sedimentary layer, especially a fine-grained sediment layer in a turbiditic sequence such as a mudstone in which organic matter was well preserved. Turbiditic sequences of the Miocene Kawabata Formation in Yubari area of central Hokkaido, Japan, provide suitable sediments for geochemical studies of biomarkers, because they are clearly formed hemipelagic turbiditic sequences and potentially contain both terrigenous and marine biomarkers. The Kawabata Formation consists of channel-filled turbidite sediments in the Ishikari sedimentary basin, and the depositional environment of this formation is interpreted to be the slope and/or the deep-sea basin (Kawakami et al., 1999). Therefore, this setting is expected to provide the biomarker data associated with the transport from coastal to deep-sea areas as well as the redeposition of organic matter that had been initially deposited as shallow-marine sediments rather than the direct delivery of terrigenous matter from land area. In the present study, high-resolution analyses of hydrocarbon biomarkers were performed in individual mudstone layers of a turbiditic sequence at an outcrop in the Miocene Kawabata Formation of Yubari area. Our goals were to examine the variability of biomarker concentrations and compositions within sediment layer-level, and the effect of transport, redeposition and postdepositional diagenesis in the turbidity system. From these results, we discuss the applicability and reliability of sedimentary biomarker records from turbidite sediments for reconstructing the variations in marine and terrestrial paleoenvironments, and moreover, paleoecology such as a terrestrial paleovegetation. GEOLOGICAL S ETTING The Yubari area (central Hokkaido, Japan; Fig. 1) is located in Ishikari sedimentary basin in the Ishikari– Teshio Belt, in which N-S trending through basins developed (Hoyanagi, 1989). The Ishikari sedimentary basin was filled with thick coarse sediments, which is the middle Miocene Kawabata Formation. The Kawabata Formation consists of turbidites and related coarse clastics, and is interpreted as a channel-filled turbidite facies deposited in slope to deep-sea basin (Kawakami et al., 1999). This formation is subdivided into the Amagiri sandstone and mudstone member and the Higashiyama sandstone and conglomerate member (Fig. 2). There are five key beds of acidic tuffs (K1–K5 beds) in this formation, and the age of the K5 tuff bed is estimated to be 13.2 ± 0.9 Ma by fission-track dating (Kawakami et al., 2002). Biostratigraphy by diatom microfossils suggests that the layer beneath the K4 tuff bed corresponds to Crucidenticula nicobarica to Denticulopsis praedimorpha

152 K. Okano and K. Sawada

(a)

142°E

(b) 144°E

Somokumaisawa-gawa route

44°N

Study outcrop Kuriyama-cho Town

43°N

Doto Expressway

Study area Route274 100 km

Pacific Ocean

Yubari City

Yubari River Yuni-cho Town 100 m

Fig. 1. (A) Location of study area. (B) Sampling location in Somokumaisawa-gawa route, Yubari area, central Hokkaido.

Zones (13.1–11.5 Ma) (Kawakami et al., 2002; Fig. 2). According to paleogeographic reconstruction (Hoyanagi, 1989; Kawakami et al., 1999), the Yubari area was located in slope to basin plain off the paleo-Island of “Hidaka”, corresponding to the Esashi–Hidaka Belt, during Middle to Late Miocene. Furthermore, it was presumed that “paleo-Japan Sea” opened northward like a large bay from 13 to 3.5 Ma (Iijima and Tada, 1990; Koizumi, 1992). Thus, the Yubari area was located on the northeastern edge of the paleo-Japan Sea during the Middle Miocene to Early Pliocene. MATERIALS AND METHODS Sample collection and preparation Sedimentary rock samples were collected from an outcrop (SO11; width about 50 m) in the Soumokumaisawa-gawa of Yubari City and Kuriyama-cho Town in central Hokkaido, Japan (Fig. 1). All samples were mudstones within the Bouma Sequence in the Higashiyama sandstone and conglomerate member of the Kawabata Formation. These mudstone layers are common types, which do not separate Ep and Et layers in the sequences. The outcrop of SO11 includes a K4 tuff bed, and sampling of mudstones was performed from a layer beneath K4 tuff bed to about 45 m below that bed. All samples were collected from 20 cm below surface of outcrop in order to minimize the effect of weathering. We collected the upper and lower parts of individual mudstone layers (Fig. 3). The separate sampling of upper and lower parts of mudstones carried out about 2–5 cm above and below the attached sandstone layer. Before organic geochemical analyses, whole rock samples were cleaned, and any weathered surface was removed by a penknife. The samples were then crushed to a fine powder in an agate mortar.

Hydrocarbon compositions in mudstones of the Miocene turbiditic sequence 153

M.Miocene

Legend

15

10

L.Miocene

Age (Ma)

?

Kawakami et al. (2002)

Pebbly mudstone

not exposed

Lithology

100 m

K4

K5

SO11-5(U)

SO11-13(U, L)

SO11-6(U)

SO11-7(L)

SO11-15(U, L)

SO11-8(U, L)

SO11-9(U)

tuff

sand-mud alternation

mudstone

sandstone

conglomerate

Legend

20

15

10

5

0

Depth (m)

45

40

35

30

25

Fig. 2. Stratigraphy of the Miocene Kawabata Formation in this study area (Kawakami et al., 2002) and lithologic section. Numbers in column indicate sampling points. U: upper part of mudstone layer (mudstone-U), L: lower part of mudstone layer (mudstone-L).

Key tuff bed

Mudstone

Takinoue Formation Mud-rich alternation, sandstone, conglomerate

?

K5(13.2 ± 0.9 Ma) K4 K3 K2 K1 Amagiri Sandstone and Mudstone Member

Higashiyama Sandstone and Conglomerate Member

Oiwake Formation/ Iwamizawa Formation

Formation

Thick-bedded sandstone and conglomerate, sand-rich alternation

Denticulopsis lauta

Denticulopsis hyalina

Crucidenticula nicobarica

Denticulopsis praedimorpha

Thalassionema yabei

Denticulopsis dimorpha

Denticulopsis katayamae

Thalassionema schraderi

Diatom zone

Kawabata Formation

SO11-1(U, L)

SO11-2(U)

SO11-16(U, L)

SO11-3(U)

SO11-14(U, L)

SO11-4(U)

SO11-11(U, L)

SO11-12(U, L)

v/v), respectively. These fractions were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS).

1m

upper part sand

mud

lower part sand

20 cm

Fig. 3. Photographs of a turbiditic sequence in an outcrop of SO11 of the Miocene Kawabata Formation, Somokumaisawagawa area. Sampling points in the mudstone-L and mudstoneU are also shown.

Total organic carbon content A portion of each sediment sample was acidified with 1 M HCl and allowed to stand for half a day to remove carbonates. Carbonate-free samples were then vacuum dried and analyzed for total organic carbon (TOC) content by a J-Science Micro Corder JM10 at the Center for Instrumental Analysis, Hokkaido University. Lipid extraction and separation Lipids were extracted from the sediment samples with dichloromethane (DCM) and MeOH as described by Sawada et al. (1996). d 62-Triacontane were added prior to extraction as an internal standard for quantifying nalkanes and isoprenoid alkanes such as pristane and phytane. The extract was dried in a rotary evaporator and then re-dissolved in hexane. The lipid-containing hexane extract was passed through a silica gel column (95% activated), and the aliphatic and aromatic hydrocarbon fractions were eluted with hexane and hexane/toluene (3/1


Gas chromatography/mass spectrometry (GC/MS) Identification of the lipid fraction was carried out by GC/MS using a Hewlett Packard 6890 attached to a capillary GC (30 m × 0.25 mm i.d. DB-5HT column, J&W Scientific) directly coupled to a Hewlett Packard XL MSD quadrupole mass spectrometer (electron voltage, 70 eV; emission current, 350 µA; mass range, m/z 50–550 in 2.91 s). The GC temperature was programmed as follows: 50°C for 4 min, 50–300°C at 4°C/min and 300°C for 20 min. The lipids were quantified with a Hewlett Packard 6890 capillary GC equipped with a flame-ionization detector (FID), the capillary column and temperature program used being the same as those used for GC/MS. Identification of the compounds was made from mass chromatographic responses (mass fragmentation pattern and molecular ion etc.) and relative retention times in comparison with library data (NIST98) and the literature. Quantification of the compounds was made from the peak areas determined by the FID responses, and/or the responses of individual base peaks (e.g., m/z 57 for n-alkanes) determined from the authentic standards. RESULTS AND DISCUSSION Total organic carbon (TOC) content TOC content (%) of mudstone samples in the turbiditic sequence of the SO11 outcrop range from 0.30 to 1.72 (Table 1 and Fig. 4). The remarkable highest TOC content (1.72%) was observed in the lower part of mudstone (mudstone-L) sampled at 4.7 m depth (SO11-8L). TOC values tended to be higher and were clearly different between the samples of mudstone-L and the upper part of mudstone (mudstone-U) in upper layers (at about 0–15 m depths) of a SO11 turbiditic sequence. In middle to lower layers of the sequence, TOC values were almost similar and hardly different between mudstone-L and mudstone-U samples. In addition, TOC is quite lower (0.3%) in a mudstone-L of a top layer, which had been close to a K4 tuff layer and might have been diluted by the minerals originated from the tuff. n-Alkanes, pristane and phytane Figures 5a and 5b show total ion chromatogram and mass fragmentograms of m/z 57 (n-alkane and acyclic isoprenoid hydrocarbons) of SO11 samples, respectively. Concentrations of n-alkanes and ratios of n-alkanes, pristane, and phytane are shown in Table 1 and Fig. 5. Concentrations of n-alkanes range from 0.62 to 5.88 µg/ g dry sediment, although a quite lower value (0.09) exists in a mudstone-U sample at a top layer, and might be


24.15 24.30 29.40 29.90 30.30 32.40 32.60 34.80 37.80 38.00 40.10 43.70 43.90

0.00 4.50 4.70 8.55 8.70 11.20 14.70 17.85 18.00 20.80

(m)

Depth

upper part of mudstone layer upper part of mudstone layer lower part of mudstone layer upper part of mudstone layer lower part of mudstone layer lower part of mudstone layer upper part of mudstone layer upper part of mudstone layer lower part of mudstone layer upper part of mudstone layer upper part of mudstone layer lower part of mudstone layer upper part of mudstone layer lower part of mudstone layer upper part of mudstone layer upper part of mudstone layer lower part of mudstone layer upper part of mudstone layer upper part of mudstone layer lower part of mudstone layer upper part of mudstone layer upper part of mudstone layer lower part of mudstone layer

Lithology

0.30 0.73 1.72 0.78 1.03 1.23 0.69 0.73 0.42 0.59 0.67 0.61 0.54 0.57 0.66 0.64 0.70 0.58 0.71 0.74 0.58 0.70 1.01

TOC%

0.03 0.28 0.18 0.36 0.39 0.11 0.24 0.58 0.54 0.22 0.57 0.51 0.88 0.10 0.26 0.56 0.76 0.39 0.13 0.08 0.34 0.53 0.11

(µg/mgC)

(µ g/g-rock) 0.09 2.05 3.02 2.79 4.00 1.38 1.63 4.22 2.27 1.32 3.79 3.10 4.73 5.88 1.69 3.61 5.33 2.25 0.93 0.62 1.97 3.74 1.09

Alkane/TOC

Alkane conc.

1.42 2.67 2.81 3.09 3.11 2.72 2.68 2.76 2.76 2.75 2.91 2.95 2.95 2.78 2.51 2.63 2.72 2.77 2.70 3.19 3.50 2.49 2.21

CPI

 1.62 1.84 1.45 1.61 3.29 1.98 1.35 2.29  1.60 1.78 1.58 1.76 1.69 1.65 1.36 1.51 1.59 2.08 1.78 1.32 2.97

Pr/Ph

1.57 1.04 1.78 1.32 0.96 1.71 0.98 1.39 0.94 2.32 1.18 1.10 1.24 1.18 0.54 1.02 0.92 1.22 1.08 1.16 1.03 0.85 1.83

H/L

0.38 0.32 0.29 0.31 0.29 0.32 0.33 0.34 0.32 0.32 0.33 0.32 0.33 0.33 0.33 0.33 0.34 0.35 0.35 0.32 0.35 0.35 0.35

22S/22S + 22R of C31 hopane 0.42 0.57 0.24 0.38 0.38 0.24 0.40 0.35 0.31 0.51 0.39 0.39 0.32 0.39 0.51 0.42 0.41 0.46 0.43 0.36 0.46 0.54 0.36

Sterane 0.18 0.24 0.47 0.26 0.28 0.28 0.24 0.27 0.25 0.22 0.21 0.25 0.28 0.25 0.22 0.25 0.24 0.15 0.21 0.19 0.14 0.17 0.31

Dino/Hop 0.27 0.21 0.57 0.19 0.21 0.59 0.10 0.19 0.40 0.19 0.23 0.23 0.27 0.21 0.14 0.22 0.23 0.18 0.18 0.24 0.18 0.23 0.18

Dino/Ste

C 27/C27 + C29

1.96 0.99 4.17 1.46 1.39 3.78 1.86 1.44 2.13 1.38 0.91 0.88 1.48 1.28 1.23 0.81 0.93 0.71 0.85 1.79 1.19 0.63 3.81

Ole/Hop

HPP 1.00 0.85 0.89 0.53 0.60 0.89 0.88 0.74 0.82 0.99 0.99 0.98 0.69 0.66 0.87 0.72 0.76 0.58 0.70 0.72 0.99 0.97 0.59

alkane conc., alkane concentration; CPI, 1/2{(C 25 + C 27 + C29 + C31)/(C24 + C26 + C 28 + C 30) + (C 25 + C27 + C29 + C31)/(C26 + C 28 + C 30 + C 32)}; Pr/Ph, Pristane/Phytane; H/L, (C 14–C 24)/ (C 25–C 33); Dino/Hop, Dinosterane/C30 Hopane; Dino/Ste, Dinosterane/C 27 Sterane; Ole/Hop, Oleanene/C30 Hopane; HPP, Retene/(Cadalene + Retene).

SO11-9U SO11-8U SO11-8L SO11-15U SO11-15L SO11-7L SO11-6U SO11-13U SO11-13L SO11-5U SO11-12U SO11-12L SO11-11U SO11-11L SO11-4U SO11-14U SO11-14L SO11-3U SO11-16U SO11-16L SO11-2U SO11-1U SO11-1L

Sample No.

Table 1. Data of TOC and aliphatic and aromatic hydrocarbons in mudstones of a turbiditic sequence in an outcrop of SO11 of the Miocene Kawabata Formation, in Somokumaisawa-gawa route, Yubari area, central Hokkaido

Alkane Conc.

TOC

Depth (m) 0

CPI

alkane( g)/TOC(mgC)

22S/22S+22R of C31 hopane

Pr/Ph

H/L

5 10 15 20 25 30 35 40 45 0

0.4

0.8

1.2

1.6

0

1

2

3

4

5

6 0

0.4

0.8

1.2 1

1.5

2

2.5

3

3.5 0

0.5

1.5

2.5 0

1

2

3

0.22 0.26

0.3

0.34

0.38

( g/g-rock)

Fig. 4. Relative depth profiles of concentrations of total organic carbon (TOC), total n-alkanes (Alkane conc.), carbon preferential index (CPI), long chain n-alkans/short chain n-alkanes ratios (H/L), pristane/phytane ratios (Pr/Ph) and ratios of 22S to 22R isomer of C 31 hopanes (22S/[22S + 22R]) in Miocene sediments of Somokumaisawa-gawa route. Samples of mudstone-L and mudstone-U are closed and open circles, respectively.

due to dilution by the minerals originated from the tuff as mentioned above. The maximal peaks were observed in mudstone-L samples at about 30 to 35 m depths, while they do not coincide with those observed in the TOC profile. The concentrations of mudstone-L were slightly higher than those of mudstone-U. The ratios of n-alkanes to TOC are significantly vary (0.03–1.03), and the profile of these ratios is resemble to that of n-alkane concentrations rather than TOC. These ratios are higher from 35 to 25 m depths. However, there are no differences in nalkane/TOC ratios between the mudstone-L and the mudstone-U. The distributions of n-alkanes are dominated by long-chain homologues with odd carbon-numbers such as C 27, C29 and C31 (Fig. 4b). Long-chain n-alkanes are known to be derived from higher plant wax (Eglinton et al., 1962). Also, post-depositional diagenesis forms short to medium-chain n-alkanes from long-chain n-alkanes and biological molecules as fatty acids and amino acids. Carbon preferential indices (CPIs) are almost constant and ranged from 2.5 to 3.5 (Fig. 4), which clearly indicate predominance of odd carbon-number n-alkanes. There were no differences in CPIs between mudstone-L and mudstone-U. The ratios of long-chain to short-chain nalkanes (H/L) were about 0.5 to 2.3, and the ratios of mudstone-L samples were slightly higher than those of mudstone-U samples, especially at upper layers of SO11 turbiditic sequence (Fig. 4). The ratios of pristane to phytane (Pr/Ph) of mudstoneU samples were almost similar about 1.32–1.98 (Fig. 4), while those of mudstone-L samples varied from 1.36 to 3.29. The Pr/Ph ratios of mudstone-L were higher than those of mudstone-U. The Pr/Ph ratio has been proposed as an indicator of redox condition in source sediments (Didyk et al., 1978). In general, high Pr/Ph ratios (>1.0) 156 K. Okano and K. Sawada

indicate oxic conditions of depositional environment, while the low values (3.0) observed for the mudstone-L in an upper layer of the sequence suggests terrigenous organic matter deposited under oxic conditions (Powell, 1988). Hopane isomer ratios (maturity) Figure 5c shows mass fragmentograms of m/z 191 (hopanes). In this study, the ratio of hopane isomers is used for estimating the maturity of the organic matter in the sediments. 17β(H), 21β(H) hopanes, which are derived from bacterial membrane lipids, are predominant with one diastereomer 22R in recent and immature sediment. On the other hand, in ancient sediment and oils, the more stable 17α(H), 21 β(H)-hopane series usually predominates with the mixture of the diastereomers 22R and 22S resulting from transformation of (22R)-17β(H), 21β(H)-hopane during diagenesis. The ratios of 22S to 22R isomers of C31 hopane (22S/(22S + 22R)) are consistently about 0.29–0.35 throughout the SO11 turbiditic sequence, although the ratio is higher (0.38) in a mudstone-U at the top of the sequence because that layer

(a)

(b)

Pr

TIC (SO11-12L:f1)

Pr

m/z 57 (SO11-2U:f1)

n-alkane

Ph

Ph

C29 C17 C19

C21

C27

C23

C25 C31

C15

Relative Abundance

25

35

30

40

45

55

50

65

60

C30

(c)

25

(d)

m/z 191 (SO11-2U:f1)

30

45

50

55

m/z 231 (SO11-12L:f1)

23S,24S

C31 (22S)

(e)

40

23S,24R

C31

53

35

(22R) C32 (22R) (22S)

57

55

23R,24R

C32

59

61

61

18

m/z 218 (SO11-2U:f1)

62

63

64

Retene

(f)

12

23R,24S

m/z 183(–), m/z 219(–) (SO11-3U:f2)

13(18)

Cadalene

52

53

54

55

56

57

26

30

34

38

42

Retention Time Fig. 5. (a) Total ion chromatogram (TIC) and mass fragmentograms of (b) m/z 57 (n-alkanes), (c) m/z 191 (hopanes), (d) 231 (dinosteranes) and (e) m/z 218 (oleanens) of aliphatic hydrocarbon fraction, and m/z 183 and 219 of aromatic hydrocarbon fraction in mudstones of a SO11 turbiditic sequence. Pr: pristane, Ph: phytane. ∆12, ∆13(18) and ∆18 are olean-12-ene, olean13(18)-ene and olean-18-ene, respectively.

had been presumably affected by heating during deposition of K4 tuff layer (Fig. 4). This result indicates that the organic matter is thermally immature in the sequence. This conclusion is confirmed because the 22S/(22S + 22R) ratios of C31 hopanes are almost the same between samples of mudstone-L and mudstone-U. Regular steranes and dinosteranes C27 to C29 steranes were commonly identified by mass fragmentograms of m/z 217 throughout the SO11 turbiditic sequence (Fig. 6). The distribution of steranes is clearly different between mudstone-L and mudstone-U. The C27 and C 28 steroids are mainly derived from aquatic phytoplankton and zooplankton in the marine environment (Volkman, 1986). The C29 steroids have been gen-

erally thought to be characteristic of higher plants (Huang and Meinschein, 1979). As shown in Fig. 6, the relative abundances of C29 steranes are higher relative to C27 and C28 steranes in mudstone-L than those in mudstone-U samples. On the basis of C27, C28 and C29-sterane proportions, we could distinguish between sterane distributions corresponding to the mudstone-L and mudstone-U samples of the turbiditic sequence (Fig. 7). According to ecological zonation proposed by Fleck et al. (2002) in the ternary diagram as Fig. 7, the mudstone-L and mudstoneU samples are plotted in zones of terrestrial area and estuarine or bay, respectively. The C30 4-methyl steranes were identified by the mass fragmentogram of m/z 231 (Fig. 4d). It was found that there were four dinosteranes (4,23,24-


C29 [5 (H), 20R]

C28 1

m/z 217

0.2

C27 [5 (H), 20R] C28 [5 (H), 20R] C28 [5 (H), 20R]

0.6

lacustrine

0.6

C27 [5 (H), 20R]

0.8

C29 [5 (H), 20R]

0.4

0.4

C29 [5 (H), 20R]

(b) SO11-8L


0

Oleanenes, cadalene and retene Oleanenes with a double bond in cyclic structures (olean-12-ene, olean-13(18)-ene, and olean-18-ene) were identified by mass fragmentograms of m/z 218 in a SO11 turbiditic sequence (Fig. 5e). Oleanene is thought to be derived from pentacyclic triterpenoidal alcohols such as β-amyrin, which is in certain angiosperms (ten Haven and Rullkötter, 1988; Moldowan et al., 1994). The ratios of oleanenes to C30 hopane in mudstone-L samples increase in upper layers of the turbiditic sequence (Fig. 8). The ratios of mudstone-L are higher than those of mudstoneU, and therefore, the mudstone-L samples might be strongly affected by terrigenous organic matter. In addi-

higher plant

0.2

trimethylcholestanes), which are known as specific biomarkers for dinoflagellates (Summons et al., 1987). These compounds can be explained as four isomers of (23R, 24R), (23S, 24S), (23S, 24R) and (23R, 24S) in C 30 4 α-methyl steranes. Dinosteranes/C30 hopane and dinosterane/C27 steranes ratios are higher in mudstone-L samples in upper layers of the turbiditic sequence (Fig. 8). This result suggests that the marine algal source organisms might comprise abundant dinoflagellates in these layers. The dinosteranes/C 30 hopane ratios are almost similar between mudstone-L and mudstone-U, while the dinosterane/C 27 steranes ratios could be distinguished between these mudstones, particularly in upper layers of the sequence. (Fig. 8).

terrestrial

0.4

Fig. 6. Mass fragmentograms of m/z 217 of mudstone-U (SO118U) and mudstone-L (SO11-8L) samples.

estuarine or bay

0.6

Retention Time

open marine

0

C27

0.8

C27 [5 (H), 20R]

C27 [5 (H), 20R]

C28 [5 (H), 20R]

1

C28 [5 (H), 20R]

C29 [5 (H), 20R]

plankton

1

0.2

m/z 217

0.8

Relative Abundance

0

(a) SO11-8U

C29

Fig. 7. Ternary diagrams of steranes in the mudstones of the turbiditic sequence in Somokumaisawa-gawa area. Samples of mudstone-L and mudstone-U are closed and open circles, respectively. Lines in the diagram represent the boundaries of depositional environments and organic matter sources reported by Fleck et al. (2002).

tion, the pattern of oleanenes/C30 hopane has a strong resemblance to the depth profile of dinosteranes/steranes ratios. Cadalene and retene were identified in PAH fraction by the mass fragmentogram of m/z 183 and m/z 219, respectively (Fig. 5f). In general, cadalene is an aromatic sesquiterpene that has multiple source organisms such as terrestrial higher plant and algae, while retene is an aromatic diterpene formed from abietic compounds that originate from coniferous gymnosperms (Simoneit, 1985). Van Aarssen et al. (2000) has proposed the higher plant parameter (HPP), which is expressed as the ratio of retene to the sum of retene and cadalene as measured by mass responses of m/z 219 and m/z 183, and used as proxies for paleovegetation and terrestrial paleoclimate. HPPs range from 0.53 to 1.0 in SO11 turbiditic sequence as shown in Fig. 8. Interestingly, it was found that the parameters were almost similar between samples of mudstone-L and mudstone-U, in spite of the difference in dinosterane and oleanene parameters. Variations of biomarker compositions within mudstones of turbiditic sequence In this study, we found that most of biomarker compositions, except CPI, C31 hopane 22S/(22S + 22R) and HPP, significantly varied between samples of mudstoneL and mudstone-U in the turbiditic sequence of the Miocene Kawabata Formation. Previous studies on turbidite sediments (Meyers et al., 1996; Watanabe and Akiyama, 1998; Fujita et al., 1998; Hoefs et al., 2002)

Dino/Hop

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Fig. 8. Relative depth profiles of dinosteranes/C30 hopane ratios (Dino/Hop), dinosteranes/C27 steranes ratios (Dino/Ste), oleanenes/C30 hopan ratios (Ole/Hop) and ratios of retene to cadalene (retene/[retene + cadalene] : HPP) in the mudstones of the turbiditic sequence in Somokumaisawa-gawa area. Samples of mudstone-L and mudstone-U are closed and open circles, respectively.

have suggested that variations and heterogeneities of geochemical aspects in the turbidite sediments could be attributed to transport, redeposition and postdepositional diagenesis. In particular, Meyers et al. (1996) and Hoefs et al. (2002) have emphasized that the geochemical characters of sedimentary organic matter mainly depended on degradation controlled by redox conditions in sediments and sediment-water interface of turbidites in early diagenesis after deposition. Hoefs et al. (2002) observed variability in the ratios of marine to terrestrial biomarkers between oxidized and unoxidized parts of turbidites, and the relative abundances of terrestrial plant biomarkers tended to increase in the oxic part. The terrestrial plant biomarkers are generally more resistant against oxic degradation than marine algal biomarkers, resulting in their being better preserved in sediment (Leenheer and Meyers, 1983). Therefore, terrigenous matter must be more abundantly contained in the mudstone-U corresponding to the oxic part, which was commonly formed in upper part of sedimentary column. Nevertheless, the indicators for terrestrial plant input such as H/L and oleanenes/hopane ratios in mudstone-L are higher than those of mudstone-U in the SO11 turbiditic sequence (Fig. 4). Also, the relative abundances of C29 steranes are higher in mudstone-L samples in the ternary diagram of C27–C28–C29 steranes (Fig. 7). These results suggest that the higher abundances of terrestrial biomarkers in mudstone-L were caused by transport and redeposition rather than postdepositional oxic degradation. In addition, depth profiles of terrestrial and coastal biomarkers (oleanenes and dinosteranes) are

similar to that of TOC content (Fig. 4). These results suggest that the amounts of sedimentary organic matter in the studied area were controlled by delivery of terrestrial organic matter by turbidity currents. We presume that the mudstone-L and mudstone-U layers corresponded to Et (-like) and Ep (-like) layers in the Bouma Sequence, respectively, although their lithologic features are not distinct. The mudstone-L could be rapidly deposited by turbidity flow together with the underlying coarser sediments such as sandstone in the turbiditic sequence, and directly transported terrigenous and coastal organic matter. On the other hand, the mudstone-U might be quietly and continuously deposited in offshore after the abrupt turbidity deposition of mudstone-L, and mainly composed of autochthonous matter of marine primary producers as algae. The results of dinosterane parameters (dinosteranes/ C30 hopane and dinosteranes/C27 steranes ratios) show that the higher values are observed in mudstone-L, which is similar to the trends of terrestrial-plant input parameters. The dinoflagellate is a typical shallow-marine species and their cysts and senescent cells are deposited in coastal and neritic areas, whereas the depositional province of the Kawabata Formation was thought to be slope to deepsea basin from geological investigation (Kawakami et al., 1999). Thus, these dinosteranes might be transported from coastal and neritic provinces by turbidity currents and redeposited in deep-sea basin. The differences of dinosterane parameters within the mudstones of the turbiditic sequence can be explained as contribution to organic matter that was supplied from coastal and neritic


oxic

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Fig. 9. Relationships between sterane ratios (C27/[C27 + C 29]) and pristane/phytane ratios (Pr/Ph) in the mudstones of the turbiditic sequence in Somokumaisawa-gawa area. Samples of mudstone-L and mudstone-U are closed and open circles, respectively. Lines in the diagram represent boundaries of depositional environment and source of organic matter reported by Sawada (2006).

areas by turbidity currents. In addition, the relative abundances of oleanenes and dinosteranes tend to increase and to be more clearly different between mudstone-L and mudstone-U at the upper layers in the sequence, while these are almost similar in both types of mudstones at middle layers (about 20–35 m depths). This observation implies that the types of turbidites might be change from the middle to the upper layers of the sequence, resulting in such differences in these geochemical characters. At the upper layers of the sequence, the turbidite sediments compose of sandstone layers containing conglomerates and relatively thin mudstone layers (Fig. 2). This lithologic feature suggests that the turbidity currents having deposited these layers might be stronger. Hence, it was presumed that the organic matter of terrestrial and coastal areas was more efficiently transported by strong turbidity flows, leading to formation of the mudstone-L that abundantly contained these matter. A diagram of C27 and C29-sterane proportions (C27/ (C 27 + C29)) vs. Pr/Ph ratios (Fig. 9) indicates that the characteristics of organic sources (terrestrial and coastal or pelagic) and redox conditions of depositional environments differ between mudstone-L and mudstone-U samples. The mudstone-L samples have relatively high Pr/Ph and low C27/(C27 + C 29) sterane ratios, which indicates that the depositional environment was oxic to suboxic in shallow-marine to terrestrial areas. Conversely, the mudstone-U samples have low Pr/Ph and high C27/(C27 + 160 K. Okano and K. Sawada

C29) sterane ratios. Therefore, these mudstones were deposited in hemipelagic to pelagic environment under anoxic conditions. The sources and depositional environments in mudstone-L and mudstone-U are concordant with those of Et and Ep reported by Watanabe and Akiyama (1998) and Fujita et al. (1998), respectively. These results supporte that the differences between mudstone-L and mudstone-U can be mainly attributed to those of types and phases for transport and redeposition by turbidity currents. Reliability of biomarker compositions as paleoenvironmental and paleoecological indicators in turbidite We point out that the heterogeneous distributions in most of the biomarker compositions within the mudstone layers of a turbiditic sequence can lead to a severe bias for reconstructing paleoenvironment and paleoecology, particularly, when the amounts of terrigenous material inputs and the marine/terrestrial ratios are evaluated from biomarker compositions. Thus, we suggest that it is necessary to compare the biomarker records in only mudstone-L or mudstone-U in the turbiditic sequences, not in bulk mudstones, when temporal variations of paleoenvironment and paleoecology are reconstructed. In this study, it is important that the HPPs are consistently similar between mudstone-L and mudstone-U. This fact suggests that the “contents” in terrigenous organic matter did not vary at a sedimentary layer-level in the turbiditic sequence, although there were differences in the amounts and relative abundances of terrigenous matter within the mudstones. Therefore, the paleovegetation (and/or terrestrial paleoclimate) records in the biomarker compositions might not be affected by the types and phases of turbidity transport and redeposition. The HPPs in the mudstones have high values of >0.5 (i.e., the abundances of retene are higher than those of cadalene) throughout a SO11 turbiditic sequence. During the deposition of the Kawabata Formation (Middle Miocene) the cadalene could be derived from not only gymnosperms but also angiosperms, and the source organisms of retene were likely to be coniferous gymnosperms. From organic petrographic investigations of SO11 turbidite sediments (K. Sawada, unpubl. data), it was found that the macerals derived from pollens of Pinus species were abundantly present. Therefore, these retenes might mainly originate from Pinus species of conifers in this study area. Thus, the HPP could be an indicator of the contribution of conifer, particularly Pinus in terrestrial higher plants in the Kawabata Formation. The high HPPs values implied that the Pinus species of conifer were presumably dominant in paleoflora in paleo-Hokkaido areas. These results are concordant with paleobotanical reports that subalpine conifers including Pinus were dominant in northern Ja-

pan during the late Middle Miocene (Tanai, 1967). From these results, it is likely that these biomarker compositions in mudstones of the turbiditic sequences are reliable and strongly applicable for reconstructing terrestrial paleovegetation. CONCLUSIONS Hydrocarbon biomarker compositions were examined in the upper and lower parts of individual mudstones in a turbiditic sequence of the Miocene Kawabata Formation in order to investigate the variability of the biomarker records at the sedimentary layer-level and their use in reconstructing paleoenvironment and paleoecology in land-ocean interface and terrestrial areas. We concluded; 1. Most of biomarker compositions such as n-alkanes, pristane, phytane, steranes, dinosteranes, and oleanenes significantly vary between the upper and lower parts of mudstones in the turbiditic sequence. We postulate that these variations result from the types and phases of transport and redeposition by turbidity currents rather than postdepositional diagenesis. 2. It is pointed out that the heterogeneous distributions in most of biomarker compositions, particularly the relative abundances of biomarker compounds produced in terrestrial and coastal areas, within the mudstone layers of the turbiditic sequence may lead to a severe bias for reconstructing paleoenvironment and paleoecology. 3. The higher plant parameters (HPPs) are consistently similar between the upper and lower parts of mudstones. Moreover, paleovegetation reconstructed from HPP are concordant with a previous paleobotanical study. Thus, these biomarker records in the turbidite sediments are reliable and strongly applicable for reconstructing terrestrial paleovegetation. Further examinations will continue to be needed for other several types of turbidite sediments, and it will be possible to strengthen the utility of biomarker records in paleoenvironmental and paleoecological studies of terrestrial and shallow-marine environments from a wider range. Acknowledgments—We thank Prof. N. Suzuki of Hokkaido University for use GC/MS and for his discussion to conduct this work, and Dr. G. Kawakami of Hokkaido University (present: Geological Survey of Hokkaido) for his discussion for geological work. We are also grateful to Mr. T. Kotone for help us sampling and description of route map. I am also grateful to Prof. P. A. Meyers of University of Michigan and an anonymous reviewer for insightful comments which significantly improved the manuscript. This study was supported in part by Grants-in-Aid No. 16740291 (to K. S.) and No. 18684028 (to K. S.) for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. I gratefully acknowledge the 21st Century COE grant by

the Japanese Ministry of Education, Culture, Sports, Science and Technology for the “Neo-Science of Natural History” Program (leader: Prof. H. Okada).

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