Early diagenesis of bacteriohopanoids in Recent sediments of Lake Pollen, Norway

PII:

Org. Geochem. Vol. 29, No. 5±7, pp. 1285±1295, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0146-6380(98)00108-9 0146-6380/98 $ - see front matter

Early diagenesis of bacteriohopanoids in Recent sediments of Lake Pollen, Norway HELEN E. INNES, ANDREW N. BISHOP, PAUL A. FOX, IAN M. HEAD and PAUL FARRIMOND* Fossil Fuels and Environmental Geochemistry (Postgraduate Institute): NRG, Drummond Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K. AbstractÐThe distribution of free bio- and geohopanoids were studied in a short (46 cm) core from Recent sediments of Lake Pollen, Norway. Bacteriohopanepolyols are abundant components of the sediments (totalling up to 321 mg/g dry sediment; 7485 mg/g TOC), being dominated by C-32,33,34,35 tetrafunctionalised hopanoids. Signi®cantly, bacteriohopanetetrol only accounts for between 7 to 62% of these (24% on average), indicating that composite bacteriohopanoids are more abundant sedimentary constituents, and thus more important precursors of geohopanoids than previously recognised. Downcore pro®les of bio- and geohopanoid abundance show maxima at ca. 25 to 30 cm, with a marked decrease in abundance above this depth which coincides with a well-documented change in environment from fjord to isolated lake. A second maximum in biohopanoid abundance occurs at ca. 5 to 12 cm depth. Speci®c biohopanoids were a€ected to di€erent extents by the change in environment, suggesting that they have, at least partly, di€erent sources. Our data thus indicate that biohopanoids, and their geohopanoid products, may be sensitive to changes in depositional environment and that they have potential to act as markers for environmental conditions. # 1998 Published by Elsevier Science Ltd. All rights reserved Key wordsÐhopanoids, recent sediments, Lake Pollen, alkenones, composite hopanoids, environmental change, bacteriohopanepolyols

INTRODUCTION

The hopanoids, pentacyclic triterpanes used extensively as biomarkers in ancient sediments and oils, have potential to provide valuable palaeoenvironmental information due to their ubiquity in Recent and ancient sediments. Their biological precursors, the biohopanoids, are synthesised as membrane lipids by bacteria of diverse taxonomic groups (e.g. Rohmer et al., 1984), and comprise variations on C30 (diplopterol (I) and diploptene (II)) or C35 structures, the latter consisting of C-32,33,34,35 tetra- (III), C-31,32,33,34,35 penta- (IV) and C30,31,32,33,34,35 hexafunctionalised (V) polyols, aminopolyols and composite hopanoids (e.g. Rohmer, 1988, 1993). Little is known about the distribution of biohopanoids in Recent sediments however, with only few previous studies (Rohmer et al., 1980; Boon et al., 1983; Ries-Kautt and Albrecht, 1989; Buchholz et al., 1993; Innes et al., 1997). Conversely, the distribution of geohopanoids (compounds formed through diagenesis of biohopanoids including hopanoic acids, hopanols, most hopenes and hopanoidal aldehydes and ketones) in Recent sediments has been relatively well studied (e.g. Dastillung et al., 1980a,b; Rohmer et al., 1980; *To whom correspondence should be addressed. E-mail: [email protected]

Quirk et al., 1984; Venkatesan, 1988; Buchholz et al., 1993; Dehmer, 1993; Innes et al., 1997). Nonetheless, our understanding of their early diagenetic formation from polyfunctionalised biohopanoids remains unclear. Our research addresses these gaps in understanding by studying both the composition and diagenetic fate of hopanoids in a short (46 cm) core of anoxic sediment from Lake Pollen, Norway. We describe the distribution and abundance of both bio- and geohopanoids downcore and compare these with our previous ®ndings in another lake (Priest Pot, U.K., Innes et al., 1997). The present study extends our understanding of the in¯uence of depositional environment upon hopanoid composition, as the Lake Pollen core contains sediments deposited during a well-constrained change in environment from fjord to isolated lake (evidenced by the distribution of alkenones from marine prymnesiophyte algae). EXPERIMENTAL

Sampling A core of anoxic sediment from Lake Pollen (20 km south of Oslo, Norway) was collected in June 1992 with a simple gravity corer (from 18 m water depth, 46 cm long, 4.4 cm diameter, Ficken, 1994). It was immediately sealed and frozen, before

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being transported to the laboratory. The core was stored at ÿ208C before being extruded whilst still frozen using a steel rod (June 1996), and cut into 1 cm sections with a mechanical hacksaw. Samples were freeze-dried and ground to a ®ne powder before analysis. Total organic carbon (TOC) and total sulphur (TS) values TOC values were obtained by the method of KroÈm and Berner (1983). One aliquot of sediment (approximately 100 mg) was analysed directly using a Leco carbon±sulphur analyser to determine the total carbon and sulphur content of the samples. A second aliquot was ashed before analysis, to combust all the organic carbon, in a furnace at 4508C for 24 h. The TOC value was obtained by taking the di€erence between the total carbon and the carbon remaining after ashing. Reproducibility of the procedure was determined by running samples from two di€erent depths in the core in triplicate at random points in the sample set. For the TOC and TS determination procedures 18 samples from the core were studied. Twelve of these were selected at random for hopanoid analysis. Extraction and derivatisation of hopanoids Twelve freeze-dried sediment samples (0.4±1.2 g) were selected from intervals downcore and were extracted using a Gerhardt Soxtherm apparatus, with chloroform/methanol (150 ml; 2:1 v/v). Elemental sulphur was removed by the addition of activated copper turnings to the cellulose thimbles prior to extraction. Extracts were evaporated to near dryness, transferred to preweighed vials and blown dry under nitrogen. An internal standard of 1-eicosene and squalane was added for quanti®cation. Our experimental scheme uses split derivatisation steps to target speci®c hopanoid groups and thus avoid problems associated with the handling of amphiphilic, highly functionalised hopanpolyols. The more highly functionalised penta- and hexafunctionalised polyols and aminopolyols, and tetrafunctionalised composite hopanoids, are not amenable to direct detection by gas chromatography-mass spectrometry (GC±MS). Analysis of these could only be performed after treatment with periodic acid and sodium borohydride. After redissolving in warmed chloroform/methanol (2:1 v/v), the total extract was split into aliquots (10±20 mg) for derivatisation by methods targeted at speci®c hopanoid groups. Individual derivatisation procedures are detailed in Innes et al. (1997). Procedure 1. Methylation of hopanoic acids to their methyl esters using boron tri¯uoride/methanol. Procedure 2. Acetylation of hopanols using acetic anhydride/pyridine.

Procedure 3. Cleavage of the hopanoid sidechains using periodic acid and sodium borohydride. This method yields bb hopanol (VIc), bb homohopanol (VIb) and bb bishomohopanol (VIa) products from C-30,31,32,33,34,35 hexa-, C-31,32,33,34,35 penta- and C-32,33,34,35 tetrafunctionalised precursors respectively. These products are then acetylated prior to analysis by GC±MS. This allows the bulk of the biohopanoids to be quanti®ed, although it does not allow us to distinguish between individual hopanoids with similar side-chain functionalities. Furthermore, the reaction will also cleave any hopanoids bound to solvent-soluble macromolecular organic matter in the sediments which retain two adjacent free hydroxyl groups between the point(s) of attachment and the hopanoid nucleus. There is currently no means of distinguishing these from free hopanoids, although their contribution is likely to be minor. Gas chromatography±mass spectrometry (GC±MS) analysis of hopanoids GC±MS was carried out on a Fisons GC 8000 ®tted with a 15 m DB5-HT fused silica capillary column (0.25 mm i.d., 0.1 mm ®lm thickness) interfaced to a Fisons TRIO 1000 mass spectrometer (ionisation energy 70 eV). On-column injection was used with helium as carrier gas (30 kPa). The oven was programmed from 50 to 2758C at 108C/min, and from 275 to 3508C at 58C/min, followed by 8 min at 3508C. Elevated ion source (2708C) and transfer line (3508C) temperatures were used to optimise the response to the functionalised hopanoids. The front end of the column was trimmed (ca. 15 cm) every three samples to improve overall reproducibility, although the original column was used throughout the sample set. Hopanoids were identi®ed from full scan (m/z 50±950) runs of selected samples, by comparison with authentic standards and published spectra and by relative retention times. They were quanti®ed using restricted scan mode (m/z 180 to 210; 1 cycle per second) from peak areas in the m/z 191 chromatogram against the m/z 183 peak area response of the squalane standard. The results were corrected for di€erences in compound responses using a standard containing known concentrations of hopanoids (diploptene, diplopterol, bb bishomohopanol and bacteriohopanetetrol) and squalane. Response factors used for hopanoids identi®ed in the samples were as follows; bacteriohopanetetrol from bacteriohopanetetrol standard, all other hopanols and hopanoic acids from the bb bishomohopanol standard, and hop-17(21)-ene from the diploptene standard. The combined hopanoid standard was run throughout the set of analyses after the column had been trimmed to monitor the performance of the mass spectrometer. Analytical errors (%, one standard deviation) were determined by analysing one

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Fig. 1. Gas chromatogram showing alkenones identi®ed in a Lake Pollen sediment sample from the bottom of the core (45 cm). The peak labelling refers to the length of the carbon chain and the degree of unsaturation.

sample from each derivatisation method in triplicate at random points in the sample set (see appropriate Figure legend for values). Analysis of alkenones Alkenones were analysed using a Carlo Erba Mega Series 5300 HR GC ®tted with a 15 m DB5HT fused silica capillary column (0.25 mm i.d.; 0.1 mm ®lm thickness). Hydrogen was used as carrier gas (30 kPa) with on-column injection. Samples were dissolved in dichloromethane (DCM) prior to injection. The oven temperature was programmed from 508C (2 minutes) to 3108C at 38C/min, and held isothermally for 42 minutes. The front end of the column was trimmed (ca. 15 cm) every three samples, with subsequent blank DCM injections, to improve peak shape and overall reproducibility. Alkenones were identi®ed by comparison of relative retention times with those from a previous study on Lake Pollen (Ficken, 1994). They were quanti®ed from methylated aliquots of the total extracts using peak areas from GC against the peak area of a squalane standard. Analytical reproducibility was determined by analysing one sample in triplicate at random points in the sample set (see legend to Fig. 2 for values).

RESULTS AND DISCUSSION

Lake Pollen: Change in environment and alkenone distribution Pollen is a small lake situated 20 km south of Oslo. The lake is strati®ed from May to September, with an anoxic water column extending from the bottom sediments (18 m maximum depth) to the chemocline at 7 m. In November, when the water column is overturned, mixing occurs to a depth of 10 m, but below this the water column and bottom sediments remain permanently anoxic (Ficken, 1994). Lake Pollen was once part of the Oslofjord system connected to the sea, but became isolated in the late 18th century due to isostatic uplift follow-

ing the removal of glacial ice sheets (Ficken, 1994). This change in environment from fjord to isolated lake is recorded in the organic geochemical record, particularly in the alkenones (Ficken, 1994). These long chain unsaturated ketones are believed to derive mainly from marine prymnesiophyte algae (e.g. Brassell et al., 1980; Volkman et al., 1980), although low abundances have been found in lacustrine environments (Cranwell, 1985, 1988; Volkman et al., 1988). A gas chromatogram showing the alkenones identi®ed in Lake Pollen sediments is shown in Fig. 1. The distribution of alkenones is typical of a cool marine environment, since features often associated with lacustrine settings (i.e. higher relative abundance of C39 and C40 alkenones and the C37:4 methyl ketone; Cranwell, 1985; Brassell, 1993; Li et al., 1996; Thiel et al., 1997), are not seen here. A depth pro®le of total alkenone abundance (Fig. 2) shows the disappearance of alkenones between 25 and 30 cm depth, indicating the boundary between marine (below this depth) and lacustrine (above) conditions. Both the positioning of the boundary and the absolute amounts of alkenones agree with the previous organic geochemical work on other cores in this environment (Ficken, 1994). Bulk geochemistry Total Organic Carbon (TOC) values for the Lake Pollen sediments are in the range of 1.9% (20.04%) to 8.3% (20.17%), and re¯ect high productivity in the water column and good conditions of preservation (i.e. permanently anoxic water column below 10 m). Total Sulphur (TS) values are in the range of 1.1% (20.01%) to 2.6% (20.03%). Downcore plots of TOC and TS (Fig. 2) show more variation in samples deposited under fjord conditions (below 25 to 30 cm), suggesting that redox conditions may have stabilised after isolation of the lake. Hopanoid abundances are quoted both in terms of mg/g dry sediment and mg/g TOC. The former method allows the deposition and diagenetic fate of individual hopanoids to be followed downcore, rather than their distribution relative to total carbon in the sediments. For comparisons between

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Fig. 2. Downcore total organic carbon (TOC) and total sulphur (TS) pro®les for the Lake Pollen core. A plot of the downcore abundance of C37 and C38 alkenones is also shown (212% analytical error), with the change in environment from fjord to isolated lake evidenced by their disappearance at ca. 25 to 30 cm depth.

di€erent environments however, concentrations in mg/g TOC are more appropriate, since the abundance of organic matter can vary considerably. Hopanoid Analysis Chromatograms showing the GC-amenable hopanoids in Lake Pollen samples derivatised by di€erent procedures are shown in Fig. 3. The abundance and downcore distribution of these bio- and geohopanoids are discussed in the following sections. At present it is dicult to draw conclusions regarding the source speci®city of hopanoids, since a clear relationship between the phylogeny of bacteria and the nature of the hopanoids that they synthesise has not been determined. According to the present understanding of the distribution of hopanoids within bacterial taxa, it is currently accepted that strictly anaerobic bacteria do not synthesise biohopanoids (Ourisson et al., 1987; Rohmer, 1988). Some anoxygenic phototrophic organisms have been shown to be hopanoid-producers (Rohmer et al., 1984; Neunlist et al., 1985; Barrow and Chuck, 1990), however we are working under the assumption that there is no input of hopanoids from bacteria within the sediments. Downcore pro®les shown here are therefore assumed to re¯ect diagenesis of hopanoids sourced from water column or allochthonous bacteria. Biohopanoids - diplopterol and diploptene. The C30 biohopanoids diplopterol (I) and diploptene (II) have previously been widely reported in sediments (e.g. Rohmer et al., 1980; Robinson et al., 1986; Cranwell et al., 1987; Venkatesan, 1988; Cranwell and Koul, 1989; Cranwell, 1990; Venkatesan et al., 1990; Spooner et al., 1994). They could not be posi-

tively identi®ed against authentic standards in Lake Pollen sediments suggesting that, if present, they were below the detection limit of the analytical methods used or, in the case of diploptene, may be rapidly isomerising to other products (e.g. hop17(21)-ene (VIII); see later). Biohopanoids - bacteriohopanetetrol. Bacteriohopanetetrol (IIIa) was the only extended biohopanoid measured directly in Lake Pollen sediments, at levels of 5±66 mg/g dry sediment (or 137±1462 mg/g TOC; see Table 1). The tetrol has previously been identi®ed in soils (Ries-Kautt and Albrecht, 1989) and cyanobacterial mats (Boon et al., 1983), but previous to our work, has been quanti®ed in only one other Recent sediment study, at levels of 0.8± 2.0 mg/g dry sediment (ca. 5±50 mg/g TOC; Rohmer et al., 1980). We previously reported bacteriohopanetetrol in concentrations of 20±90 mg/g dry sediment (or 110±400 mg/g TOC) in Priest Pot, a small highly productive lake in the English Lake District (Innes et al., 1997; see Table 1). The high concentrations reported in the present study and in Priest Pot, relative to previous work, are thought to re¯ect the high levels of bacterial productivity and conditions favourable to the preservation of organic matter in these environments. However, they may also re¯ect our optimised experimental scheme which has been developed to minimise losses of bacteriohopanetetrol during sample processing (by avoiding aqueous phases and rinsing glassware with warm solvents) and increase GC±MS response (see earlier). Biohopanoids - polyols, aminopolyols and composite hopanoids. Composite hopanoids (those with a biochemical functionality at C-35) and more highly

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Fig. 3. m/z 191 chromatogram showing hopanoids identi®ed in aliquots of a surface sediment sample derivatised by the three di€erent procedures. These are representative of those seen throughout the core. Retention times di€er due to trimming of the column between sample sets. BHT is bacteriohopanetetrol. Peak C demonstrates GC±MS characteristics typical of a hopanoid but is, as yet, unidenti®ed.

functionalised bacteriohopanepolyols and aminopolyols (with ®ve or six functional groups), which are known to be synthesised by certain bacteria (e.g. Rohmer, 1988, 1993), could not be analysed directly by GC±MS. Aminobacteriohopanetriol (IIIb) is GC-amenable (eluting just before the tetrol) but was not detected here. The more highly functionalised biohopanoids could only be analysed after treatment with periodic acid and sodium borohydride had yielded (by 1,2-diol cleavage) terminal alcohol products which are amenable to GC±MS. This treatment gave bb bishomohopanol (VIa), bb homohopanol (VIb) and bb hopanol (VIc) products from C-32,33,34,35 tetra- (III), C-31,32,33,34,35 penta- (IV) and C-30,31,32,33,34,35 hexafunctionalised (V) precursor biohopanoids respectively. Data from this treatment showed that bacteriohopanepolyols are abundant in Lake Pollen sediments (totalling up to 321 mg/g dry sediment; 7485 mg/g TOC). Prior to our work, the only other environment where the periodic acid/sodium borohydride cleavage method had been applied to Recent sediments was Lake Constance, Germany (Buchholz et al., 1993), where bb bishomohopanol was the only reported product, at concentrations of up to two orders of magnitude lower than those reported here

(up to 1.5 mg/g dry sediment or ca. 20 mg/g TOC). Absolute amounts of biohopanoids in Lake Pollen sediments are similar to those which we previously reported in Priest Pot (Innes et al., 1997; see Table 1) and again, re¯ect the high productivity in these environments combined with conditions favourable to the preservation of organic matter. In Lake Pollen the biohopanoids are dominated by C-32,33,34,35 tetrafunctionalised species (see Table 1). Signi®cantly though, bacteriohopanetetrol only accounts for between 7 to 62% of the total tetrafunctionalised hopanoids (24% on average downcore). Since the recovery of bacteriohopanetetrol is good (ca. 45±65%), and the di€erence in abundance between the tetrol and the total tetrafunctionalised hopanoids is considerable, the data suggest that composite hopanoids are likely to be abundant sedimentary constituents. This compares with data from Priest Pot where bacteriohopanetetrol was found to account for only 2±9% of the total tetrafunctionalised hopanoids (Innes et al., 1997). Taken together, these studies suggest that composite hopanoids are likely to be more important precursors to sedimentary geohopanoids than previously recognised. Furthermore, the abundance of C31 and C30 hopanols produced after side-chain cleavage also

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Table 1. Abundances of bacteriohopanetetrol (BHT) and major biohopanoid groups detected after periodic acid/sodium borohydride treatment for the Lake Pollen and Priest Pot cores

mg/g dry sediment Lake Pollen Priest Pot Lake Constancea mg/g TOC Lake Pollen Priest Pot Lake Constancea

BHT

Total tetrafunctionalised biohopanoids

Total pentafunctionalised biohopanoids

Total hexafunctionalised biohopanoids

5±66 20±90 n.m.

22±238 620±1900 0.1±1.5

4±73 150±500 n.m.

3±103 290±750 n.m.

137±1462 10±400 n.m.

1104±5282 2880±8250 2.0±21.1

227±1394 1000±2130 n.m.

144±1983 1820±3270 n.m.

a

Data from Buchholz et al. (1993); n.m. refers to biohopanoid groups which were not measured in that study.

suggests that the more highly functionalised hopanoids (penta- and hexafunctionalised respectively) are also abundant components of the Lake Pollen sediments, and may thus also be important precursor compounds. Clearly, bacteriohopanetetrol may not the most important precursor to sedimentary geohopanoids in these environments. Geohopanoids. Geohopanoids are also abundant in Lake Pollen sediments (totalling up to 307 mg/g dry sediment or 5271 mg/g TOC) and are dominated by bb bishomohopanoic acid (VII; 19±74% of total geohopanoids) and hop-17(21)-ene (VIII; 18±70% of total geohopanoids) with small amounts of bb bishomohopanol (VIa; 2±16% total geohopanoids). Hopanoidal aldehydes and ketones were not detected, although their concentrations may have been below the detection limit (ca. <3 mg/g dry sediment; 40 mg/g TOC) as trace levels have previously been found in Recent sediments (Dastillung et al., 1980a; Robinson et al., 1986; Cranwell et al., 1987). Similarly, hopanoids methylated at C-2 or C3 were also not detected in the m/z 205 chromatogram, despite having being found previously in Recent sediments (Dastillung et al., 1980b; Spooner et al., 1994). bb Bishomohopanoic acid was detected in high abundance (10±228 mg/g dry sediment or 194± 3912 mg/g TOC), along with smaller amounts of bb bishomohopanol (1±25 mg/g dry sediment or 35± 546 mg/g TOC). These values, whilst up to two orders of magnitude higher than found in some environments studied (Rohmer et al., 1980; Buchholz et al., 1993), are consistent with data from Priest Pot (Innes et al., 1997) and some other Recent sedimentary environments including microbial mats (bb bishomohopanol up to 82 mg/g dry sediment; Dobson et al., 1988) and peats (bb bishomohopanol up to 352 mg/g dry sediment; Quirk et al., 1984). The high concentrations of geohopanoids in Lake Pollen and Priest Pot are suggested to re¯ect the high bacterial activity in these environments, combined with favourable conditions for preservation of organic matter in the anoxic sediments. High abundances of hop-17(21)-ene observed here and in Priest Pot (Innes et al., 1997), combined with the apparent absence of the biohopanoid diploptene in both these environments, suggests

that the latter may be isomerising to the more stable geohopanoid. Laboratory experiments have shown that diploptene adsorbed onto clays can readily isomerise to various hopene products (including hop-17(21)-ene) under mild conditions (Ageta et al., 1987; Moldowan et al., 1991). Of the hopanoic acids and hopanols, only the C32 species were detected. This predominance of C32 components has previously been observed in Recent sediments (e.g. Dastillung et al., 1980b; Ries-Kautt and Albrecht, 1989; Buchholz et al., 1993; Innes et al., 1997), and supports the suggestion made by Zundel and Rohmer (1985) that early diagenetic reactions may mimic those occurring during periodic acid/sodium borohydride treatment (i.e. 1,2diol cleavage). This suggestion was given further support by the identi®cation of a C-30,32 hopandiol from a cyanobacterial mat (Dobson et al., 1988). This diol was suggested to derive from the diagenesis of a known bacteriohopanepentol with a C30,32,33,34,35 substituted side-chain, with the product directly analogous to that expected from periodic acid/sodium borohydride treatment. Similar reactions would explain the high concentrations of C32 hopanoids in Pollen sediments, since the biohopanoid precursors are predominantly C-32,33,34,35 tetrafunctionalised species (40±89% of total biohopanoids). However, although penta- and hexafunctionalised precursors are also abundant in the sediments, we could not detect the expected C31 and C30 geohopanoid products. These have previously been detected in Recent sediments (e.g. Rohmer et al., 1980; Robinson et al., 1986; Ries-Kautt and Albrecht, 1989; Innes et al., 1997), but may be in abundances below detection limits in Lake Pollen. Downcore hopanoid abundance pro®les. The geohopanoids were present in substantial quantities in the uppermost sediment sample (hop-17(21)-ene 90 mg/g dry sediment or 1192 mg/g TOC, bb bishomohopanoic acid 22 mg/g dry sediment or 287 mg/g TOC, and bb bishomohopanol 3 mg/g dry sediment or 41 mg/g TOC) suggesting that reactions leading to their formation from precursor biohopanoids are commencing at a very early diagenetic stage, either in the uppermost sediment samples or in the water column of the lake.

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Fig. 4. Downcore pro®les of bacteriohopanetetrol (*; 215%) and biohopanoids detected after treatment with periodic acid and sodium borohydride ((q) C-32,33,34,35 tetrafunctionalised hopanoids (213%); (w) C-31,32,33,34,35 pentafunctionalised hopanoids (216%); (W) C-30,31,32,33,34,35 hexafunctionalised hopanoids (214%)). Data for the tetrafunctionalised hopanoids are corrected for naturally occurring bb bishomohopanol. The change in environment from fjord to isolated lake is evidenced by the disappearance of alkenones at ca. 25 to 30 cm depth (Fig. 2).

Downcore plots of bio- and geohopanoid abundance (Figs 4 and 5) show similar pro®les. A maximum in both plots occurs towards the bottom of the core. In the geohopanoids, this is particularly marked by bb bishomohopanoic acid. A marked decrease in abundance above 25 to 30 cm coincides with the change in environment from marine to lacustrine conditions and thus may relate to a change in bacterial populations. A second maximum in hopanoid abundance occurs between 5 to 12 cm. In addition to ¯uctuations in the absolute abundance of hopanoids downcore, compositional variation of biohopanoids is also seen. The composition of the total tetrafunctionalised hopanoids, for example, varies considerably, with bacteriohopanetetrol accounting for between 7 to 62% downcore. Such variability was previously observed in Priest Pot sediments (Innes et al., 1997) where we suggested that it showed that di€erent biohopanoids had, at least partly, di€erent sources. Data from Lake Pollen allows this idea to be taken further. Here, bacteriohopanetetrol is in greater abundance with respect to the total tetrafunctionalised hopanoids in sediments deposited under fjord conditions compared to lacustrine conditions (Table 2). In a purely marine environment (Oman Margin; Innes et al., unpublished) the tetrol can account for up to 100% of the total tetrafunctionalised hopanoids. Taken together, these data strongly suggest that signi®cant source and/or environmental information

may be preserved in sedimentary biohopanoid compositions. Furthermore, in Lake Pollen, the C30,31,32,33,34,35 hexafunctionalised hopanoids comprise a considerably smaller proportion of the total bacteriohopanoids in samples deposited under fjord conditions compared to those deposited under lake conditions (see Table 2). This again suggests partly di€erent sources for the di€erent biohopanoids, with the bacteria producing hexafunctionalised hopanoids being less abundant in the fjord setting than the lake. At present though, it is dicult to ascribe speci®c bacterial sources to speci®c biohopanoids since the relationship between the phylogeny of bacteria and their ability to synthesise hopanoids is not clearly understood. This is complicated further by the fact that only a limited number of taxa have been shown to produce penta- and hexafunctionalised hopanoids (Table 3). The hexafunctionalised hopanoids have been identi®ed in several bacterial strains after periodic acid/sodium borohydride side-chain cleavage (Rohmer et al., 1984). In most of these bacteria however, they are only minor components of the total bacteriohopanoids produced. The only group of bacteria currently known to synthesise signi®cant amounts of hexafunctionalised hopanoids are type I methanotrophs. Based on the current understanding of the bacterial distribution of di€erent biohopanoids therefore, hexafunctionalised hopanoids may be interpreted as potentially re¯ecting a contribution

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Fig. 5. Downcore abundance pro®les of the major geohopanoids ((Q) bb bishomohopanoic acid (210%); (W) hop-17(21)-ene (212%); (q) bb bishomohopanol (213%)) in Lake Pollen sediments. A plot of the abundance of total bio- ((*) all hopanols from periodic acid/sodium borohydride treatment (225%)) and total geohopanoids ((w) three major geohopanoid species (220%)) is also shown. The change in environment from fjord to isolated lake is evidenced by the disappearance of alkenones at ca. 25 to 30 cm depth (Fig. 2).

from type I methanotrophic bacteria to the sediments. Notably in Pollen, the relative abundance of hexafunctionalised hopanoids increased in sediments deposited after the change in environment to isolated lake had occurred. This is as expected if these hopanoids do derive from type I methanotrophic bacteria since, although the oceans are known to be supersaturated with respect to atmospheric methane (Conrad and Seiler, 1988), the low turnover rates of methane in ocean surface waters suggest that methanotrophs are not very active in these environments (e.g. Kiene, 1991; Murrell and Holmes,

1995). In lakes however, methanotrophs are known to be widespread, and this may explain the relatively higher amounts of hexafunctionalised hopanoids present in the Pollen lacustrine samples. Since the biohopanoids appear to respond to environmental change, and bacteria are known to synthesise a wide range of side-chain functionalities (e.g. Rohmer, 1988, 1993), it seems that hopanoids have the potential to store valuable information about depositional environments. Whether or not remnants of this information will survive diagenetic reactions in the sediment (and incorporation and subsequent cleavage from kerogen) and be available

Table 2. Compositional variation in biohopanoid abundance downcore. BHT refers to bacteriohopanetetrol, ``composite tetras'' refers to composite C-32,33,34,35 tetrafunctionalised hopanoids, ``pentas'' refers to C-31,32,33,34,35 pentafunctionalised hopanoids and ``hexas'' refers to C-30,31,32,33,34,35 hexafunctionalised hopanoids. Data refers to individual biohopanoid groups as a percentage of the total Sample depth (cm)

%BHT

% Composite tetras

%pentas

%hexas

Lake

1.2 3.5 5.8 9.2 12.7 16.1 20.7 25.3

8 19 14 4 10 4 9 22

46 21 42 56 59 44 46 56

18 22 23 14 11 8 15 10

28 38 21 26 20 44 30 12

Fjord

29.9 34.5 39.1 44.9

13 29 46 14

76 55 28 65

8 10 15 15

3 6 10 6

11 26

46 56

15 12

27 6

Average in lake Average in fjord

Early diagenesis of bacteriohopanoids

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Table 3. Possible precursor biohopanoids, with currently known bacterial sources, of the C30, C31 and C32 hopanols produced after periodic acid/sodium borohydride treatment. These are undoubtedly not the exclusive bacterial sources for the various biohopanoids, since no clear phylogenetic relationship is currently known and relatively few bacteria have been analysed for hopanoids Cleveage products

Possible biohopanoid precursors

bb hopanol (VIc)

Aminobacteriohopanepentol (V)

bb homohopanol (VIb)

bb bishomohopanol (VIa)

Possible bacterial source

Methylotrophic bacteria (Rohmer et al., 1984; Zundel and Rohmer, 1985) Bacteriohopanepentol (IVa) Cyanobacteria (Rohmer and Ourisson, 1976; Zhao et al., 1996) Aminobacteriohopanetetrol (IVb) Methylotrophic bacteria (Neunlist and Rohmer, 1985; Zundel and Rohmer, 1985) Bacteriohopanepentol ether (IVc) Nitrogen-®xing bacteria (Vilcheze et al., 1994) Diverse bacterial groups including Bacteriohopanetetrol (IIIa), Aminobacteriohopanetriol (IIIb), Composite methylotrophs, cyanobacteria and purple nonsulphur bacteria (e.g. Rohmer et al., Hopanoids (IIIc) 1984; Rohmer, 1988, 1993)

for interpretation in ancient sediments and oils, is however, currently unclear.

CONCLUSIONS

Concentrations and distributions of bio- and geohopanoids have been determined in a short (46 cm) core from Recent sediments of Lake Pollen, Norway. The response of the hopanoid record to a well-constrained change in environment has also been investigated. (1) Both bio- and geohopanoids are present in high concentrations in Lake Pollen sediments (up to 321 mg/g dry sediment or 7485 mg/g TOC and 307 mg/g dry sediment or 5271 mg/g TOC respectively). (2) Downcore pro®les of bio- and geohopanoids show similar pro®les with maxima around the base of the core, and a marked decrease in abundance above 25 to 30 cm. This correlates with the depth in the core where a well-documented change in environment from fjord to isolated lake is evidenced by the disappearance of alkenones. A second maximum in biohopanoid abundance occurs at ca. 5 to 12 cm depth. These observations suggest that both biohopanoids, and their geohopanoid products may be sensitive to changes in depositional environment. (3) The composition of the biohopanoids shows a marked change in association with the shift in environmental conditions from fjord to isolated lake. Speci®cally, bacteriohopaneterol increased in abundance with respect to the tetrafunctionalised composite hopanoids in sediments deposited under fjord conditions. In these sediments the hexafunctionalised biohopanoids are also notably less abundant in the fjord sediments. This suggests that they may derive partly from a speci®c bacterial source, possibly type I methanotrophic bacteria, which became relatively more abundant when the environment changed to a strati®ed lake. The compositional changes downcore suggest that di€erent biohopanoids have, at least partly, di€erent bacterial sources, and that they thus have potential to act as markers for environmental conditions.

(4) The biohopanoids in Pollen sediments are dominated by C-32,33,34,35 tetrafunctionalised species. As for the sediments of another lake (Priest Pot, U.K., Innes et al., 1997), bacteriohopaneterol only accounts for a small proportion of these, indicating that composite tetrafunctionalised hopanoids are more important precursors to sedimentary geohopanoids than previously thought. (5) The geohopanoids comprised bb bishomohopanoic acid, hop-17(21)-ene and smaller amounts of bb bishomohopanol. No other homologues of the hopanoic acids and hopanols, or ab hopanoids, were detected. AcknowledgementsÐThis work was funded by Saga Petroleum (Norway), The Royal Society, The Natural Environmental Research Council and the University of Newcastle upon Tyne. We thank Dr Kath Ficken (University of Bristol, U.K.) and Professor Mo. Abdullah and colleagues (University of Oslo) for collection of Pollen samples. We also thank Michel Rohmer (Universite Louis Pasteur, Strasbourg) for standards of diplopterol, diploptene, C32 hopanol and aminobacteriohopanetriol, as well as advice regarding methodology, and Bob Moreau (United States Department of Agriculture, Philadelphia) for the bacteriohopanetetrol standard. Finally, we are grateful to Paul Donohoe at Newcastle for help with GC± MS. REFERENCES

Ageta, H., Shiojima, K. and Arai, Y. (1987) Acid-induced rearrangement of triterpenoid hydrocarbons belonging to the hopane and migrated hopane series. Chemical and Pharmaceutical Bulletin 35, 2705±2716. Barrow, K. D. and Chuck, J. (1990) Determination of hopanoid levels in bacteria using high-performance liquid chromatography. Analytical Biochemistry 184, 395±399. Boon, J. J., Hines, H., Burlingame, A. L., Klok, J., Rijpstra, W. I. C., de Leeuw, J. W., Edmunds, K. E. and Eglinton, G. (1983) Organic geochemical studies of Solar Lake laminated cyanobacterial mats. In Advances in Organic Geochemistry 1981, ed. M. Bjorùy et al., pp. 207±227. John Wiley, London. Brassell, S. C., Comet, P. A., Eglinton, G., Isaacson, P. J., McEvoy, J., Maxwell, J. R., Thomson, I. D., Tibbetts, P. J. C. and Volkman, J. K. (1980) Preliminary lipid analysis of sections 440A-7-6, 440B-3-5, 440B-8-4, 440B68-2 and 436-11-4: legs 56 and 57, Deep Sea Drilling Project. Initial Reports of the Deep Sea Drilling Project 56/57, 1367±1390.

1294

H. E. Innes et al.

Brassell, S. C. (1993) Applications of biomarkers for delineating marine paleoclimate ¯uctuations during the Pleistocene. In Organic Geochemistry: Principles and Applications, ed. M. H. Engel and S. A. Macko, pp. 699±738. Plenum, New York. Buchholz, B., Laczko, E., Pfennig, N., Rohmer, M. and Neunlist, S. (1993) Hopanoids of a recent sediment from Lake Constance as eutrophication markers. FEMS Microbiology Ecology 102, 217±223. Conrad, R. and Seiler, W. (1988) In¯uence of the surface microlayer on the ¯ux of non conservative trace gases (CO, H2, CH4, N2O) across the ocean-atmospheric interface. Journal of Atmospheric Chemistry 6, 83±94. 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. Cranwell, P. A. (1990) Paleolimnological studies using sequential lipid extraction from recent lacustrine sediment: recognition of source organisms from biomarkers. Hydrobiologia 214, 293±303. Cranwell, P. A., Eglinton, G. and Robinson, N. (1987) Lipids of aquatic organisms as potential contributors to lacustrine sediments. II. Organic Geochemistry 11, 513± 527. Cranwell, P. A. and Koul, V. K. (1989) Sedimentary record of polycyclic aromatic and aliphatic hydrocarbons in the Windermere catchment. Water Research 23, 275±283. Dastillung, M., Albrecht, P. and Ourisson, G. (1980a) Aliphatic and polycyclic ketones in sediments: C27±C35 ketones and aldehydes of the hopane series. Journal of Chemical Research (S), 166±167. Dastillung, M., Albrecht, P. and Ourisson, G. (1980b) Aliphatic and polycyclic alcohols in sediments: Hydroxylated derivatives of hopane and of 3-methylhopane. Journal of Chemical Research (S), 168±169. Dehmer, J. (1993) Petrology and organic geochemistry of peat samples from a raised bog in Kalimantan (Borneo). Organic Geochemistry 20, 349±362. Dobson, G., Ward, D. M., Robinson, N. and Eglinton, G. (1988) Biogeochemistry of hot spring environments: extractable lipids of a cyanobacterial mat. Chemical Geology 68, 155±179. Ficken, K. J. (1994) Lipid and sulphur geochemistry of Recent sediments from oxic and anoxic environments. Ph.D. thesis, University of Newcastle, 222 pp. Innes, H. E., Bishop, A. N., Head, I. M. and Farrimond, P. (1997) Preservation and diagenesis of hopanoids in Recent sediments of Priest Pot, England. Organic Geochemistry 26, 565±576. Kiene, R. P. (1991) Production and consumption of methane in aquatic systems. In Microbial Production and Consumption of Greenhouse Gasses: Methane, Nitrogen Oxides and Halomethanes, ed. J. E. Rogers and W. B. Whitman, pp 15±33. American Society for Microbiology, Washington, DC. KroÈm, M. D. and Berner, R. A. (1983) A rapid method for the determination of organic and carbonate carbon in geological samples. Journal of Sedimentary Petrology 53, 660±663. Li, J., Philp, R. P., Pu, F. and Allen, J. (1996) Long-chain alkenones in Qinghai Lake sediments. Geochimica et Cosmochimica Acta 60, 235±241. Moldowan, J. M., Fago, F. J., Carlson, R. M. K., Young, D. C., Van Duyne, G., Clardy, J., Schoell, M., Pillinger, C. T. and Watt, D. S. (1991) Rearranged hopanes in sediments and petroleum. Geochimica et Cosmochimica Acta 55, 3333±3353.

Murrell, J. C. and Holmes, A. J. (1995) Molecular ecology of marine methanotrophs. In Molecular Ecology of Aquatic Microbes, ed. I. Joint, pp 365±390, NATO ASI Series. Springer. Neunlist, S. and Rohmer, M. (1985) Novel hopanoids from the methylotrophic bacteria Methylococcus capsulatus and Methylomonas methanica (22S)-35-aminobacteriohopane-30,31,32,33,34-pentol and (22S)-35-amino3b-methylbacteriohopane-30,31,32,33,34-pentol. Biochemical Journal 231, 635±639. Neunlist, S., Holst, O. and Rohmer, M. (1985) Prokaryotic triterpenoids. The hopanoids of the purple nonsulphur bacterium Rhodomicrobium vannielii: an aminotriol and its aminoacyl derivatives, N-tryptophanyl and N-ornithinyl aminotriol. European Journal of Biochemistry 147, 561±568. Ourisson, G., Rohmer, M. and Poralla, K. (1987) Prokaryotic hopanoids and other polyterpenoid sterol surrogates. Annual Review of Microbiology 41, 301±333. Quirk, M. M., Wardroper, A. M. K., Wheatley, R. E. and Maxwell, J. R. (1984) Extended hopanoids in peat environments. Chemical Geology 42, 25±43. Ries-Kautt, M. and Albrecht, P. (1989) Hopane-derived triterpenoids in soils. Chemical Geology 76, 143±151. Robinson, N., Cranwell, P. A., Eglinton, G., Brassell, S. C., Sharp, C. L., Gophen, M. and Pollingher, U. (1986) Lipid geochemistry of Lake Kinneret. Organic Geochemistry 10, 733±742. Rohmer, M. (1988) The hopanoids, prokaryotic triterpenoids and sterol surrogates. In Surface structures of microorganisms and their interactions with the mammalian host, ed. E. Schrinner, M. H. Richmond, G. Siefert and M. Schwarz, Vol. 18, pp. 227±242. Verlag Chemie Weinheim. Rohmer, M. (1993) The biosynthesis of triterpenoids of the hopane series in the eubacterium: a mine of new enzyme reactions. Pure and Applied Chemistry 65, 1293± 1298. Rohmer, M. and Ourisson, G. (1976) DeÂriveÂs du bacteÂriohopane: variations structurales et reÂpartition. Tetrahedron Letters 40, 3637±3640. Rohmer, M., Bouvier-Nave, P. and Ourisson, G. (1984) Distribution of hopanoid triterpenes in prokaryotes. Journal of General Microbiology 130, 1137±1150. Rohmer, M., Dastillung, M. and Ourisson, G. (1980) Hopanoids from C30 to C35 in Recent muds. Naturwissenschaften 67, 456±458. Spooner, N., Rieley, G., Collister, J. W., Lander, M., Cranwell, P. A. and Maxwell, J. R. (1994) Stable carbon isotopic correlation of individual biolipids in aquatic organisms and a lake bottom sediment. Organic Geochemistry 21, 823±827. Thiel, V., Jenisch, A., Landmann, G., Reimer, A. and Michaelis, W. (1997) Unusual distributions of longchain alkenones and tetrahymanol from the highly alkaline Lake Van, Turkey. Geochimica et Cosmochimica Acta 61, 2053±2064. Venkatesan, M. I. (1988) Diploptene in Antarctic sediments. Geochimica et Cosmochimica Acta 52, 217±222. Venkatesan, M. I., Ruth, E. and Kaplan, I. R. (1990) Triterpenols from sediments of Santa Monica Basin, Southern California Bight, U.S.A.. Organic Geochemistry 16, 1015±1024. Vilcheze, C., Llopiz, P., Neunlist, S., Poralla, K. and Rohmer, M. (1994) Prokaryotic triterpenoids: new hopanoids from the nitrogen-®xing bacteria Azotobacter vinelandii, Beijerinckia indica and Beijerinckia mobilis. Microbiology 140, 2749±2753. Volkman, J. K., Eglinton, G., Corner, E. D. S. and Forsberg, T. E. V. (1980) Long-chain alkenes and alkenones in the coccolithophoroid Emiliania huxleyi. Phytochemistry 19, 2619±2622.

Early diagenesis of bacteriohopanoids Volkman, J. K., Burton, H. R., Everitt, D. A. and Allen, D. I. (1988) Pigment and lipid compositions of algal and bacterial communities in Ace Lake, Vestfold Hills, Antartica. Hydrobiologica 165, 41±57. Zhao, N., Berova, N., Nakanishi, K., Rohmer, M., Mougenot, P. and JuÈrgens, U. J. (1996) Structures of two bacteriohopanoids with acyclic pentol side-chains

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from the cyanobacterium Nostoc PCC 6720. Tetrahedron 52, 2777±2788. Zundel, M. and Rohmer, M. (1985) Hopanoids of the methylotrophic bacteria Methylococcus capsulatus and Methylomonas sp. as possible precursors of C29 and C30 hopanoid chemical fossils. FEMS Microbiology Letters 28, 61±64.

APPENDIX