Recent sedimentary hopanoids in the northwestern Paci®c
alongside the Japanese Islands Ð their concentrations and
carbon isotopic compositions
Hiroshi Naraoka *, Keita Yamada, Ryoshi Ishiwatari
Department of Chemistry, Tokyo Metropolitan University, 1-1, Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan
Received 4 May 1999; accepted 5 July 2000 (returned to author for revision 15 December 1999)
Abstract
Two hopanoids, 17b(H),21b(H)-hop-22(29)-ene (diploptene) and 17b(H),21b(H)-bishomohopanoic acid (bbC32-HA), are the most abundant among the pentacyclic triterpenoids found in Recent sediments of the Paci®c Ocean alongside the Japanese Islands. The concentration of diploptene normalized to organic carbon content is higher in sediments where C37polyunsaturated alkene (a biomarker of Haptophytes) is in high concentration, suggesting that the diploptene may be associated with the accumulation of marine organic matter. In open marine settings,d13C values of bbC32-HA range from ÿ23.8 to ÿ19.4% (relative to PDB), being enriched in13C relative to diploptene (
ÿ31.6 to
ÿ26.3%) by 5±9%. The isotopic dierence indicates the presence of at least partially dierent sources for the two hopanoids. While diploptene is derived from cyanobacteria and chemotrophic bacteria in the water column or sedi-ment, bishomohopanoic acid may be produced mainly by heterotrophs in the sediment using marine organic matter. In contrast, thed13C values of the two hopanoids from river and bay sediments are similar (
ÿ31 toÿ29%), indicating a common source derived from soil components (terrestrial plants or bacteria in soils).#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Hopanoids; Recent sediments; Carbon isotopic compositions; Bacterial activity; Western North Paci®c
1. Introduction
Hopanoids are an important class of biomolecules consisting of a pentacyclic triterpenoid skeletal struc-ture, including hydrocarbons as well as functionalized homologues such as alcohols and carboxylic acids. They are found in prokaryotes such as cyanobacteria and methanotrophs (e.g. Rohmer et al., 1984) as well as in terrestrial vascular plants (Ageta et al., 1964). For pro-karyotes the diversity is sometimes used to clarify the taxonomy of bacteria (Rohmer et al., 1984; Ourisson et al., 1987). Moreover, these hopanoids are found widely in Recent as well as ancient sediments (e.g. Van Dors-selaer et al., 1974; Rohmer et al., 1980; Innes et al., 1997). Molecular geochemical study of hopanoids is
important to understand bacterial activity in water column and sediments, depositional environment and biogeo-chemical cycles of the Earth's surface environment.
The origin of diploptene (17b(H),21b(H)-hop-22(29)-ene), one of the most abundant hydrocarbons bearing a hopanoid structure is, however, still controversial for Recent marine sediments. Prahl (1985) suggested a terrestrial origin for sedimentary diploptene from the southern Washington continental shelf and slope. On the other hand, Venkatesan (1988) reported the presence of diploptene in signi®cant amounts in sediment cores from the Antarctic region, and suggested an origin from marine sources since input of terrestrial organic matter in the Antarctic region is expected to be unimportant. These studies suggest that diploptene in sediments may be both autochthonous and terrestrial in origin.
Hopanoic acids ranging in carbon number from 28 to 33 are also ubiquitous components in sediments. Of
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* Corresponding author. Fax: +81-426-77-2525.
these, 17b(H),21b(H)-bishomohopanoic acid (bbC32-HA) was found to be the most abundant in Recent sediments and soils (Rohmer et al., 1980; Ries-Kautt and Albrecht, 1989) as well as ancient sediments (Van Dorsselaer et al., 1974). The origin of hopanoic acids is not well speci®ed yet, but one of the precursors is sug-gested to be bacteriohopanetetrol and related compounds which are common in prokaryotes (Rohmer et al., 1984; Ourisson et al., 1987). Early diagenetic processes may cause oxidation to carboxylic acids (Rohmer et al., 1980, 1992).
Carbon isotopic compositions of individual hopanoids may have the potential to clarify their sources and biolo-gical activity in sediments. Prahl et al. (1992) examined the carbon isotopic composition of sedimentary diploptene from southern Washington, and assigned a terrestrial source for diploptene in the marine sediments. A marine origin for diploptene has, however, been proposed on the basis of its isotopic composition in water column and sediment samples from the Cariaco Trench and Black Sea (Freeman et al., 1994). In addition, the activity of methanotrophic bacteria can be recognized in sediments from the isotopic compositions of diploptene depleted in 13C (Prahl et al., 1992; Freeman et al., 1994; Spooner et
al., 1994). Yamada et al. (1997) also found isotopically lighter diploptene (ÿ56%) in a core sample from the Japan sea, and inferred the high activity of methano-trophs during the last glacial period.
On the other hand, no work related to the isotopic composition of hopanoic acids in Recent sediments has been reported yet, although a study reported the d13C values in Miocene sediments (Huang et al., 1996). In this study, we report stable carbon isotopic compositions of sedimentary diploptene and bishomohopanoic acid in the Paci®c Ocean along the Japanese Islands to infer sources of the hopanoids.
2. Samples and experimental methods
The samples used in this study consisted of 10 sediments collected from the northwestern Paci®c along the Japanese Islands, including 7 from open marine settings, and 3 from riverine, bay and coastal environments (Fig. 1). Sample descriptions are given in more detail elsewhere (Naraoka and Ishiwatari, 2000). The extraction procedure, gas chromatography (GC) and carbon isotope analyses of hopanoids is the same as used forn-fatty acids andn -alkanes described in Naraoka and Ishiwatari (1999). Brie¯y, sediments were saponi®ed with 0.5 M KOH/ methanol solution. The neutral fraction was subjected to silica gel column chromatography to obtain a hydro-carbon fraction, followed by isolation of unsaturated hydrocarbons using AgNO3/silica gel (10% w/w) col-umn chromatography. The acid fraction was esteri®ed with 14% BF3/MeOH to convert the free acids to the
methyl ester derivatives which were subsequently frac-tionated by silica gel and AgNO3-impregnated (10% w/ w) silica gel column chromatography to obtain satu-rated acid methyl ester derivatives (Naraoka et al., 1994). Molecular identi®cation was carried out using a GC/MS (Varian 3400 GC/Finnigan MAT INCOS 50 mass spectrometer or HP6890 GC/MSD5972A). Compound-speci®c carbon isotopic compositions of hopanoids were analyzed using gas chromatography/combustion/ iso-tope ratio monitoring mass spectrometry (GC/C/IRMS, Varian 3400 or HP5890 GC/Finnigan delta S with an ISODAT data processing system). Isotopic composition was determined by the deviation from the coinjected n -alkanes as isotopic internal standard. Using mass balance calculation, the d13C value of bishomohopanoic acid was corrected for the eects of an additional carbon atom from the CH3OH (d13C=
ÿ37.4%) used for deri-vatization (Naraoka et al., 1994). The d13C value is de®ned as (Rsample/Rstandardÿ1)1000 (%), whereRstandard is the de®ned carbon isotope ratio of a standard (Peedee Belemnite). Carbon isotopic compositions shown in Table 1 are averaged values for duplicate or triplicate injections. Standard deviations were generally less than 0.7%.
3. Results
Diploptene and 17b(H),21b(H)-bishomohopanoic acid (bbC32-HA) are the most abundant hopanoids found in the hydrocarbon and acid fractions, respectively. Other hopanoids such as 17a(H),21b(H)-bishomohopanoic acid were also detected. Their abundances, however, are less than10% ofbbC32-HA. Such a molecular distribution is similar to that of hopanoids reported from several Recent sediments (Boon et al,. 1978; Rohmer et al., 1980). While bbC32-HA ranges from 5 to 290 (mg/g org.C) in concentration, diploptene is less abundant than bbC32-HA by up to two orders of magnitude (Table 1). Five open marine sediments with >0.7 wt.% TOC have relatively high concentrations of both hopa-noids relative to other sediments (open circles in Fig. 2). Carbon isotopic compositions of both hopanoids are shown in Table 1 with those of a C37:3alkene which is a biomarker of marine phytoplankton (Haptophytes; Conte et al., 1994). In open marine settings the d13C value of diploptene varies fromÿ31.6 toÿ26.3%(ave.
ÿ28.41.8%), being more depleted in13C by 5 to 9% than bbC32-HA, which varies from ÿ23.8 to ÿ19.4% (ave.ÿ21.91.6%) (Table 1).
On the other hand, in riverine and bay environments (OR3 and OB2), the isotopic composition ofbbC32-HA (ÿ31.2 toÿ30.2%) is similar or only slightly depleted in 13C relative to diploptene (
Fig. 1. Sample location of the sediments used in this study.
Table 1
Bulk chemical characteristics as well as concentrations and carbon isotopic compositions of two hopanoids and a C37:3alkene in
sediments from the Paci®c Ocean along the Japanese Islandsa
Sample Org. C Org. C/N d13C
bulk Bishomohopanoic acid Diploptene C37.3Alkene
(wt.%) (by weight) (%) (mg/g org. C) (%) (S.D.) (mg/g org. C) (%) (S.D.) (mg/g org. C) (%) (S.D.)
Open marine
SR6S 2.62 9.2 ÿ20.8 132 ÿ22.4 (0.5) 12.0 ÿ31.6 (0.4) 5.2 ÿ23.3 (0.4)
SR68 1.85 9.1 ÿ20.6 135 ÿ22.7 (0.5) 8.0 ÿ27.6 (0.1) 4.0 ÿ22.9 (0.3)
LM8 1.57 8.0 ÿ20.7 133 ÿ19.4 (0.8) 14.6 ÿ28.4 (0.3) 9.1 ÿ23.0 (0.4)
LMSP 0.94 6.3 ÿ20.9 219 ÿ22.2 (0.3) 13.0 ÿ28.0 (0.1) 0.34 n.d.
LM6 0.77 8.0 ÿ21.0 113 ÿ20.6 (1.1) 9.4 ÿ26.3 (0.5) 1.6 ÿ22.4 (0.8)
SR72 0.67 7.1 ÿ20.4 5 ÿ23.8 (0.4) 0.1 n.d.c 0.04 n.d.
LM4 0.34 4.4 ÿ20.3 121 ÿ23.3 (0.9) 1.3 n.d. ±b ±
Riverine, bay and coastal
OR3 3.19 10.8 ÿ26.4 26 ÿ31.2 (0.7) 2.9 ÿ29.1 (0.3) ± ±
OB2 3.10 11.7 ÿ25.5 34 ÿ30.2 (0.1) 5.1 ÿ29.4 (0.6) ± ±
SOB2 0.88 7.2 ÿ22.2 292 ÿ24.7 (0.2) 6.5 ÿ24.9 (0.5) 1.8 n.d.
a Averaged carbon isotopic compositions and standard deviations (S.D.) are calculated by duplicate or triplicate injections.
b ± Under detection limit.
the hopanoids of the riverine and bay sediments by 4± 6%(Table 1).
4. Discussion
4.1. Molecular concentrations
The concentration of diploptene in open marine sedi-ments ranges from 0.1 to 14.6 (mg/g org. C), which is comparable to Washington coastal sediments (6.2±17 mg/g org. C; Prahl et al., 1992), but somewhat lower than in Antarctic sediments (4.4±59 mg/g org. C; Ven-katesan, 1988). The riverine-coastal sediments contain only 2.9±6.5 mg/g org. C diploptene, whereas the Columbia River sediments have higher amounts (2116 mg/g org. C; Prahl et al., 1992). If the diploptene is trans-ported from a terrestrial environment, the concentration relative to TOC is expected to be lower seaward. Actually, concentrations of terrestrial biomarkers such as lignin-derived phenols and cutin-lignin-derived acids decrease seaward (Naraoka and Ishiwatari, 1999). While concentrations of diploptene in riverine and bay sediments range from 2.9 to 5.1 (mg/g org. C), those in marine sediments (SR65, SR68, LM8, LM5P and LM6) are very high (up to 14.6 mg/g org. C). The molecular distribution may suggest that the input of diploptene from a terrestrial environment is relatively small in this study.
There is a general positive correlation in the con-centrations of diploptene and bbC32-HA with a few exceptions (SOB2 and LM4) (Fig. 2).bbC32-HA is more abundant than diploptene by an order of magnitude (Table 1). In bacteria, bacteriohopanepolyols are more abundant than C30 hopanoids (diploptene and diplop-terol) by up to 2 orders of magnitude (Rohmer et al., 1984), because bacteriohopanetetrol and related com-pounds are important as membrane stabilizers (Rohmer et al., 1984; 1992). Since no organisms have been found to contain hopanoic acids, sedimentary bbC32-HA is likely to be derived from bacteriohopanepolyols as a result of oxidation of side-chain polyols (Boon et al., 1978; Rohmer et al., 1980). Probably the relative abun-dance of bbC32-HA becomes higher with increasing early diagenesis and/or oxidative degradation (Rohmer et al., 1980; Innes et al., 1997). In SOB2 and LM4 sedi-ments, the high abundance ofbbC32-HA compared to diploptene may suggest that these sediments have undergone more diagenesis or degradation.
The two hopanoids are more abundant relative to TOC in open marine sediments along the Japanese Islands (LM5P, LM6, LM8, SR65 and SR68) than in riverine and bay sediments. In pelagic sediments such as LM4 and SR72, however, their abundance is less than that of riverine and bay sediments. In particular, the concentration of diploptene is correlated with that of the C37:3 alkene in sediments underlying nutrient-rich
sea-surface water such as o-Sanriku regions (SR65, SR68 and LM8 in Fig. 3), suggesting that the produc-tion of this hopanoid is associated with the accumula-tion of marine organic matter in sediments. The sea surface of the o-Sanriku region is well known to be rich in nutrients such as P and N (Fujiwara and Tsu-bota, 1993) and has good ®sheries, suggesting a high marine productivity. The relatively high amount of the two hopanoids might be associated with the high marine productivity, probably being the two hopanoids not derived from the terrestrial environment.
4.2. Isotopic signature with relevance to sources
In open marine sediments, diploptene is more deple-ted in 13C than bbC
32-HA by 5±9% (Fig. 4). The iso-topic dierence indicates at least partly dierent biological sources for the two hopanoids. Ishiwatari et al. (1997) reportedd13C values of diploptene in sinking particles of 4000 m and9000 m in depth from the Japan Trench (close to LM6) using sediment trap sam-ples. The uniform isotopic composition (ÿ23.20.3%) is richer in13C than the sedimentary diploptene of this study (ÿ31.6 to ÿ26.3%). Yamada et al. (1997) also found a similard13C value of diploptene (
ÿ23.9%) in a sea surface sediment of the Japan sea, and inferred its source to cyanobacteria. Such a source may be consistent with
the d13C value of C37:3 alkene (
ÿ23.3 to ÿ22.4% in Table 1), a marine algal biomarker. On the other hand, diploptene of this study is more depleted in13C by 3±8% than the putative lipid compounds of sea surface auto-trophs. Freeman et al. (1994) reported d13C values of
ÿ39.8 andÿ37.4%for diploptene in the anoxic water column and surface sediment of the Black Sea and Cariaco Trench respectively, and suggested that the diploptene was synthesized in deeper marine environment by chemo-autotrophs. Also in the case of this study,13C-depleted diploptene could be incorporated into the sediments. The isotopic distribution may be due to a mixing between sea surface photoautotrophs and deeper che-motrophs such as nitrifying or methanotrophic bacteria. Because the isotopic composition of bbC32-HA is slightly enriched in13C relative to C
37:3alkene by 0.2± 3.6%,bbC32-HA could not have been produced in the sea surface environment by primary marine autotrophs. While the concentration of diploptene has a weak posi-tive correlation with that of C37:3alkene, that ofbbC32 -HA does not (Fig. 3). In addition bbC32-HA is much more abundant by an order of magnitude than diploptene and C37:3alkene. Such molecular abundance suggests that bbC32-HA may be produced by hetero-trophic bacteria in water column and/or sediment. The slight enrichment in13C ofbbC32-HA (up to
ÿ19.4%) relative to a marine biomarker is likely to be consistent
Fig. 3. Concentration relationships of diploptene and 17b(H),21b(H)-bishomohopanoic acid compared to C37:3alkene (a marine
with the isotopic shift by heterotrophs utilizing marine organic matter (DeNiro and Epstein, 1978; Hayes, 1993).
On the other hand, in riverine and bay environments, the isotopic composition of bbC32-HA is similar or slightly depleted in13C relative to diploptene. Here the similar isotopic signature may indicate a common source for the two hopanoids. Thed13C values of river-ine and bay hopanoids (ÿ31 to ÿ29%) are similar to those of diploptene in forest soils (ÿ33.8 to ÿ31.0%) and coastal sediments (ÿ31.8 toÿ30.4%) from Colum-bia River-Washington Coast area (Prahl et al., 1992). The isotopic similarity indicates that both hopanoids of this study may be derived from soil components. In the present study, however, it has not been clari®ed whether the soil hopanoids have an origin of terrestrial plants or bacteria in soil, becaused13C values of hopanoids from terrestrial plants have not been reported. The isotopic range ofÿ31 toÿ29%could be consistent with a puta-tive isotopic composition of lipid molecules from C3 plants (Prahl et al., 1992). The coastal hopanoids of this study are more enriched in 13C (
ÿ24.8%) by about 5% than the riverine and bay hopanoids. The heavy isotopic composition indicates that the hopanoids are
not derived from terrestrial soils, but from heterotrophic bacterial activity or autotrophs in the marine environ-ment as described above.
5. Summary
1. Two hopanoids, hop-22(29)-ene (diploptene) and 17b(H), 21b(H)-bishomohopanoic acid, are observed to be the most abundant pentacyclic tri-terpenoids found in sediments of the Paci®c Ocean along the Japanese Islands.
2. The concentrations of both hopanoids are higher in sediments underlying nutrient-rich areas of the sea surface, indicating that the hopanoids of the o-Sanriku region are associated with the accu-mulation of marine organic matter.
3. In open marine settings, the d13C values of diploptene and bishomohopanoic acid range from
ÿ31.6 to ÿ26.3%, and from ÿ23.8 to ÿ19.4%, respectively. The isotopic dierence of up to 9% may re¯ect at least partly dierent sources for the two hopanoids.
4. On the other hand, thed13C values of both hopanoids in riverine and bay sediments are similar (ÿ31 to
ÿ29%), probably indicating a common source derived from soil components. The coastal hopanoids of this study are more enriched in13C (
ÿ24.8%), being derived mainly from the marine environment.
Acknowledgements
The crew of R/V Hakuho-Maru and scientists on board are acknowledged for their cooperation in sampling LM sediments. SR sediments were kindly donated by Dr. S. Montani. The authors thank Drs. P. Farrimond, H. P. Nytoft and D. Brincat for improving the manu-script. This work was supported by the Grant-in-Aid for Scienti®c Research from the Japanese Ministry of Edu-cation, Science and Culture (HN and RI), and Nissan Science Foundation grant (HN).
Associate EditorÐP. Farrimond
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