Preservation and diagenesis of hopanoids in Recent lacustrine sediments of Priest Pot, England

Pergamon Ors. Geochem. Vol. 26, No. 9/10, pp. 565-576, 1997

© 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: S0146-6380(97)00017-X 0146-6380/97 $17.00 + 0.00

Preservation and diagenesis of hopanoids in Recent lacustrine sediments of Priest Pot, England

HELEN E. INNES, ANDREW N. BISHOP, 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.

Abstract--Absolute concentrations of various bio- and geohopanoids have been determined in a 24 cm long core from anoxic Recent sediments of Priest Pot, a small highly productive lake in the English Lake District. Samples were Soxtherm extracted, derivatised by methods targeted at specific hopanoid groups, and quantified by GC-MS. Periodic acid/sodium borohydride treatment was used to cleave the side-chains of highly functionalised hopanoids, producing products more amenable to our detection methods. Biohopanoids are extremely abundant (totalling up to 3000 pg/g dry sediment), particularly those with composite or highly (penta- and hexa-) functionalised side-chains, suggesting that these are important precursors to sedimentary hopanoids in Priest Pot. Downcore biohopanoid abundance profiles show a general diagenetic decline with a marked dip at 5-8 cm, which we interpret as a change in bacterial input. Since this is most marked in the composite hopanoids rather than bacteriohopanetetrol, we suggest that these two hopanoid groups have at least partly different sources, and may thus have potential as environmental markers.

The geohopanoids are also abundant (totalling up to 1700 #g/g dry sediment) being dominated by the fl/~ and :~ bishomohopanoic acids, f ie and ~/~ bishomohopanol and hop-17(21)-ene. Downcore plots show no apparent conversion of free bio- to geohopanoids with depth, which suggests that incorporation into kerogen may be commencing at an early diagenetic stage in the sediments. ~ 1997 Else- vier Science Ltd

Key words - -Recen t sediments, Priest Pot, early diagenesis, biohopanoids, geohopanoids, bacteriohopanetetrol, composite hopanoids, bacterial markers, kerogen incorporation

INTRODUCTION

Hopanoids found in geological samples are the diagenetic products of 'biohopanoids', compounds predominantly biosynthesised as membrane constituents of certain bacteria. As one of the most abundant groups of complex organic molecules in the geosphere (Ourisson et al. , 1984) and having a ubiquitous occurrence and resistant carbon skel- eton, hopanoids (particularly the hydrocarbons; hopanes and hopenes) are used extensively as biomarkers in ancient sediments and oils (Peters and Moldowan, 1993). They have the potential to pro- vide information regarding the contribution of bacterial biomass to sediments, and the palaeoenvironmental conditions prevailing at the time of deposition. At present, however, this information is limited by a poor understanding of the origin, distribution, and diagenesis of hopanoids in Recent sediments. The distribution of biohopanoids in bacteria is, in contrast, better understood. Hopanoids have been isolated from bacteria of widely ranging taxonomic groups (Rohmer e t al. , 1984; Ourisson et al. , 1987; Rohmer, 1988, 1993; Ourisson and Rohmer, 1992), and include simple C3o hopanpolyols such as diplopterol (I; hopan-22- oi) and diploptene (II; hop-22(29)-ene), extended

C35 bacteriohopanepolyols such as bacteriohopanetetrol (Ilia) and aminobacteriohopanetriol (lIIb) and a range of composite hopanoids where the side- chain is linked to a complex polar moiety (Ille). Hopanoids with five (IV) or six (V) functional groups in the side-chain also occur.

Our lack of understanding of the occurrence and diagenetic fate of biohopanoids in Recent sediments is due largely to the fact that the extraction and analysis of functionalised C35 biohopanoids is relatively complex; they are generally not amenable to the routine techniques applied in conventional organic geochemical studies. Consequently, C35 biohopanoids have so far only been identified in a limited number of environments, including Recent muds (Rohmer e t al., 1980), soils (Ries-Kautt and Albrecht, 1989) and cyanobacterial mats (Boon et

al. , 1983). The C30 biohopanoids diploptene and diplopterol are more easily analysed and have been more widely reported (e.g. Rohmer e t al. , 1980; Robinson et al., 1986; Cranwell et al., 1987; Venkatesan, 1988; Cranwell, 1990; Venkatesan et al. , 1990). "Geohopanoids" (the diagenetic products of biohopanoids; i.e. not direct products of biosynthesis), such as hopanoic acids, hopanols, most hopenes and hopanoidal aldehydes and ketones,

565

566 Helen E. innes et al.

have also been widely studied, and have been reported in Recent sediments (Dastillung et al., 1980a,b; Rohmer et aL, 1980; Venkatesan, 1988; Buchholz et al., 1993) and peats (Quirk et al., 1984; Dehmer, 1993). However, the formation of these early diagenetic products from their polyfunctiona- lised precursors remains poorly understood and rep- resents one of the weak links in applying hopanes as biomarkers in ancient sediments and oils. Only by increasing our understanding of these precursor product relationships, can geohopanoid distributions be interpreted with confidence.

We report here, the analysis of hopanoids from Recent sediments of Priest Pot, a small highly productive lake in the English Lake District. This lake is extremely shallow (maximum 4 m depth) and becomes thermally stratified in the summer, resulting in the development of an epi-, meta- and hypo- limnion (Fig. 1). We describe the distributions and abundances of bio- and geohopanoids within a short (24 cm) core from the anoxic bottom sediments of the lake.

Our experimental scheme uses split derivatisation steps to target specific hopanoid groups. Analysis is performed by optimised gas chromatography (GC) and gas chromatography-mass spectrometry (GC MS). For the more highly functionalised (IVa, IVb, V) and composite hopanoids ( I lk) , which are not amenable to direct analysis by these methods, we use treatment with periodic acid/sodium borohydride to produce simple hopanol products (by 1,2- diol cleavage) which are more easily detected. This gives indirect quantification for the bulk of the biohopanoids, although it does not allow us to dis- tinguish between bacteriohopanpolyols and composite hopanoids with similar side-chain functionalities. This method has been applied widely by Rohmer and co-workers to cleave bacterial hopanoids (e.g. Rohmer et al., 1984), but has also recently been applied to sediments of Lake Constance, Germany (Buchholz et al., 1993). In addition to tiff bishomohopanol found in the latter work, we find the/3/~ homohopanol and/3/~ hopanol

products. These alcohols are markers for hopanoids with differing side-chain functionalities; i.e. bishomohopanol from C-32,33,34,35 tetrafunctionalised hopanoids (III), homohopanol from C- 31,32,33,34,35 pentafunctionalised hopanoids (IV) and hopanol from C-30,31,32,33,34,35 hexafunctionalised hopanoids (V). By examining downcore bio- and geohopanoid abundances we demonstrate that hopanoids have significant potential as markers of bacterial input.

EXPERIMENTAL

Sampling

Cores of the upper 24 cm of sediment were taken from Priest Pot (English Lake District; 54°22'N, 3°00'W) in September 1995 with a Jenkin corer. Duplicate cores were taken from the lake bottom ( < l i n g O2/1; 4 m depth) and the lake edge (oxic; 1 m depth), after an oxygen profile had been determined using an oxygen meter (Fig. 1). These were stored in a cool room (4°C; 24 h), then sectioned under nitrogen into slices (1 cm depth; 7 cm diam- eter). The sub samples were frozen immediately at -20°C. Prior to analyses, samples were freeze-dried and crushed to a fine powder. A core from 4 m depth was analysed in the present study.

Total Organic Carbon ( T O C ) values

TOC values were obtained by the method of Kr6m and Berner (1983). One aliquot of sed iment (approximately 100 mg) was analysed directly using a Leco Carbon-Sulphur Analyser to determine the total carbon content of the samples. A second aliquot was ashed before analysis, to combust all the organic carbon, in a furnace at 450°C for 24 h. The TOC value was obtained by taking the difference between the total carbon and the carbon remain ing after ashing. Reproducibility of the procedure (+2%) was determined by running two samples in triplicate at random points in the sample set.

1

r~ 3

/ Epilln~ion

M~!llnmion

Hypolinmion

. . . . i . . . . I . . . . I . . . . I . . . . I . . . . I . . . . I . . . .

0 10 20 30 40 30 60 70 80

Priest Pot Cron Section (m)

Fig. 1. Priest Pot cross section showing the dissolved oxygen profile (taken by mooring buoys near the lake centre; September 1995) and water column structure.

~ g ~ (m~) 0 1 2 3 4 5 6 7 8

OI...,I,, . .I. . , ,h,,,l , , , ,I. , . .I. , , ,I. . .d / 4

Preservation and diagenesis of hopanoids 567

Extraction and derivat&ation of hopanoids

Twelve sediment samples (0.8 g) from intervals

downcore were freeze-dried and extracted, using a

Gerhardt Soxtherm apparatus, with chloroform/

methanol (150ml; 2:1 v/v). Elemental sulphur was removed by the addition of activated copper turn-

ings to the cellulose thimbles prior to extraction. Extracts were evaporated to near dryness, transferred to pre-weighed vials and blown dry under nitrogen. An internal standard of 1-eicosene and squalane was added for quantification. After redis- solving in warmed chloroform/methanol (2:1 v/v), the total extract was split into aliquots (10-20 mg)

REACTANTS PRODUCTS

OH OH

2""°~ ''J'~~ ~" OH Bacteriohopanetetrol (Ilia) OH OH

Z,,,,~/~.. ~ [l~ bishomohopano[ (Via)

V-r1. L~lrl 2

Aminobactenohopanettiol (Ill]a)

C-32,33,34,35 Tetrafunctionalisad Hopanoids (III)

OH OH

Bacteriohopanepentol (IVa) OH OH ~ ' ~

OH OH NH 2

Armnobacteriohopanetetrol (l'Vb)

C-31,32,33,34,35 Pentafunctionalised Hopanoids (VI)

1313 homohopanol (VIb)

OH OH OH

OH OH Nit 2 1313 hopanol (Vie)

Aminobacteriohopanepentol (V)

C-30,31,32,33,34,35 Hexafunctionalised Hopaaoids (V)

Fig. 2. Hopanol products obtained from the periodic acid/sodium borohydride treatment of hopanoids with various side-chain functionalities. The ultimate product results from cleavage of the side-chain at the position shown. In addition to the C-32,33,34,35 tetrafunctionalised hopanoids indicated, any with

composite groups at C35 (i.e. IIIe) will also cleave to give the same C32 product. OG 26/9-10 C

568 Helen E. lnnes et al.

for derivatisation by methods targeted at specific hopanoid groups.

Procedure 1 Methylation to convert hopanoic acids to their methyl esters. An aliquot of the total extract was refluxed for 1 h with boron trifluoride/ methanol (15 ml; 14% BF3) in chloroform/methanol (10 ml). After addition of water (25 ml), the mixture was extracted with chloroform (15ml; x3). Care was taken to allow full separation of the organic phase to avoid losses of functionalised hopanoids at the organic/water interface. The combined extracts were rotary evaporated to near dryness, transferred to pre-weighed vials, blown down under nitrogen and weighed.

Procedure 2 Acetylation ()1 hopanols. An aliquot of the total extract was transferred to a large vial and acetylated overnight at room temperature, after initial warming to 5ffC for l h, with acetic anhydride/pyridine (4 ml; 1:1 v/v). The derivatised extracts were rotary evaporated to near dryness+ transferred to pre-weighed vials and blown down under nitrogen before being weighed. Samples were treated with N,O-bis(trimethylsilyl)trifluoroaceta- mide (BSTFA) prior to analysis to derivatise unreacted functional groups.

Procedure 3 Treatment ~?[ tile total extract with periodic" acid and sodium boroh)'dride. An aliquot of the total extract was treated with periodic acid and sodium borohydride to convert bacteriohopanpolyols and composite hopanoids to more easily ana- lysable terminal alcohols (Fig. 2). The aliquot was stirred for l h at room temperature with periodic acid (HslO++; 300 rag) in tetrahydrofuran/water (3 ml; 8:1 v/v). This oxidised any l+2-diols present to yield aldehyde products. After addition of water (10ml) the mixture was extracted with petroleum ether (15mk x3). The combined extracts were rotary evaporated to near dryness and residual water was removed by further evaporation as an azeotropic mixture with acetone. The resulting extract was stirred for l h at room temperature with sodium borohydride (NaBH4; 100 rag) in etha- nol (3ml) to produce terminal alcohols by reduction. After careful addition of a solution of potassium dihydrogen phosphate (KH:PO4; 15 ml: 100 mM) the mixture was extracted with petroleum ether (15 ml; x3). The resulting extracts were combined, rotary evaporated to near dryness and transferred to large vials for acetylation of the hopanols by procedure 2 above.

Procedure 4 - - Separation of the methylated fraction by thin-layer chromatograph), (TLC) . Separ- ation was performed on four samples to allow screening for hopanoidal aldehydes and ketones and hopanoid hydrocarbons. Samples were methylated (procedure 1) prior to TLC separation on silica gel (20 cm x 20 cm; 0.5 mm thickness; Kieselgel 60G). Dichloromethane as eluent gave four fractions: hydrocarbons and aldehydes (Rr 0.70-1.00),

ketones and methyl esters of hopanoic acids (Rf 0.40 0.70), alcohols (Rf 0.03-0.40) and polar compounds (Rf 0.00 -0.03). Band positions were visual- ised with rhodamine 6G dye. The first three fractions were recovered with dichloromethane, transferred to pre-weighed vials and blown down under nitrogen before being weighed. The fourth fraction was recovered with dichloromethane and methanol (1:1 v/v) and retained. Alcohol fractions (fraction 3) were acetylated by procedure 2 before analysis.

A test of the efficiency of the methods to extract bacteriohopanetetrol was carried out on a sample of freeze-dried sediment. This was spiked with a known amount of bacteriohopanetetrol standard, then extracted and acetylated (procedure 2). Extraction efficiency was determined by comparing the abundance of bacteriohopanetetrol against that obtained from an unspiked sample of the same sediment (ca. 65%).

Gas chromatography ( GC)

GC was carried out on a Carlo Erba Mega Series 5300HR GC fitted with a 15m DB5-HT fused silica capillary column (0.25mm i.d.; 0.1 #m film thickness) with on-column injection and an FID at 360C. Hydrogen was used as carrier gas (30 kPa). The oven temperature was programmed from 50°C to 275~C at 10:'C/min and from 275"C to 350°C at 5 C/rain, with 8 rain isothermal.

Gas chromatography mass spectromet O, (GC MS)

GC MS was carried out on a Fisons GC 8000 fitted with a 15 m DB5-HT fused silica capillary column (0.25 mm i.d.; 0.1 #m film thickness) inter- faced to a Fisons TRIO 1000 mass spectrometer (electron energy 70 eV). On-column injection was used with helium as carrier gas (30 kPa) and an oven temperature profile as above. Elevated ion source (270+C) and transfer line (350~C) tempera- tures 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 identified from full scan (m/-_ 50 950) runs of selected samples, by comparison with authentic standards and published spectra and by relative retention times. They were quantified using restricted scan mode (m/- 180 210; 1 cycle per second) from peak areas in the m/z 191 chroma- togram against the m/z 183 peak area response of the squalane standard. The results were corrected for differences in compound responses using a standard containing known concentrations of hopanoids (diploptene, diplopterol, / ~ bishomohopanol and bacteriohopanetetrol) and squalane. Response factors used for hopanoids identified in the samples


TOC (%) 10 20 30 40

0 . . . . I . . . . I , , ,L . . . .

5 ! .-, 1 0 -

1 5 -

I

20- i

25

Fig. 3. Downcore TOC profile for the Priest Pot core (errors of +2%).

were as follows; bacteriohopanetetrol from bacteriohopanetetrol standard, all other hopanols and hopanoic acids from the tiff 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 running an acetylated and a methylated sample in triplicate at points in the sample set.

RESULTS AND DISCUSSION

Bulk geochemistry

High TOC values (16-25%; Fig. 3) are con- sidered to be the result of Priest Pot being highly productive and having good conditions for preservation (i.e. oxygen poor, shallow water). The TOC gradually declines with depth with no sharp fluctu- ations, suggesting a general diagenetic trend with no significant changes in organic matter deposition. This makes Priest Pot an ideal environment in which to study the diagenetic fate of hopanoids within the sediment column.

Hopanoid analysis

Hopanoids identified from the methylated, acetylated and periodic acid/sodium borohydride treated fractions of the surface sample are indicated in Fig. 4. Variations in the abundances of these bio- and geohopanoids are presented and discussed in the following sections.

METHYLATED FRACTION (Procedure I) I hop- 17(2 l)--ene ._I//cq3 bishomohopanoic 1313 homohopanoic acid

[ acid \ , / | 13~ bishomohopanoic acid

ACEI'YLATED FRACTION (Procedure 2) BHT

l i • a[3 bishomohopano[ ~ " ~ ' " /

.+ c

PERIODIC ACID/SODIUM BOROHYDKIDE TREATEDI(Procadur e 3) ~ homohopanol | ~ bishomohopanol

I ]

Scan Number Fig. 4. M/z 191 chromatograms showing GC-amenable hopanoid distributions for a sample from the surface sediment (0-1 cm) in fractions derivatised by the different methods. These are representative of those seen throughout the core. Retention times are not shown, as they differ due to trimming of the

column between samples.

570 Helen E. Innes et al.

To date, hopanoids have not been detected in strictly anaerobic bacteria (Ourisson e t a l . , 1987). Since the sediments studied here are anoxic (although overlying bottom waters become oxyge- nated during the winter due to overturning of the water column), we are currently working under the assumption that there is no contribution to the hopanoid signal from bacteria within the sediments. Downcore profiles are therefore assumed to reflect diagenesis of hopanoids which have been sourced from bacteria in the overlying water column at the time of sediment deposition.

B i o h o p a n o i d s . The only biohopanoid which could be quantified directly using our analytical pro- cedures, was bacteriohopanetetrol (Ilia; 20-90/tg/g dry sediment), which shows an overall decrease in abundance downcore (Fig. 5). Bacteriohopanetetrol has previously been reported in sediments at levels of 0.8-2.0/~g/g dry sediment (Rohmer e t a l . , 1980). Our higher concentrations, probably reflect the high levels of bacterial productivity and conditions of enhanced preservation in Priest Pot. Diplopterol and diploptene, C30 biohopanoids which are more widely reported in sediments (e.g. Rohmer e t al . ,

1980; Robinson e t a l . , 1986; Cranwell e t al . , 1987; Venkatesan, 1988; Cranwell, 1990; Venkatesan e t

a l . , 1990) could not be positively identified.

Besides bacteriohopanetetrol, no other extended functionalised biohopanoids were detected directly, although we know that aminobacteriohopanetriol is GC amenable from a standard which elutes just before bacteriohopanetetrol. We do not expect to find composite C-32,33,34,35 tetra-, C- 31,32,33,34,35 penta- and C-30,31,32,33,34,35 hexafunctionalised hopanoids using the analytical conditions in the present study. Detection of these could only be achieved by measuring the products generated from the periodic acid/sodium borohydride side-chain cleavage method (Procedure 3; Fig. 2). Hopanols obtained from this treatment are present in high abundances; i.e. /3/3 bishomohopanol (Via) 620-1900/~g/g dry sediment, fl~8 homohopanol (Vlb) 150-490 gg/g dry sediment and fl/~ hopanol (Vie) 290-750/~g/g dry sediment. Based on the various hopanoid side-chain configurations currently known to occur (e.g. Rohmer, 1988, 1993), it is possible to constrain the biohopanoid structures which gave rise to these hopanol products: /~/~ bishomohopanol from the C-32,33,34,35 tetrafunctionalised hopanoids bacteriohopanetetrol (llla), aminobacteriohopanetriol (Illb) or a range of composite hopanoids (i.e. Ille), /3/~ homohopanol from the C-31,32,33,34,35 pentafunctionalised hopanoids bacteriohopanepentol (IVa) or aminobacteriohopanetetrol (IVb), and /~/3 hopanol from the only

,._, 10

20

25

Bacteriohopaneterol (lag/g dry sediment) 20 40 60 80 100

o , , , , I , ~ , , [ . . . . L . . . . I . . . . 0

x._._

< / o_

5-

',0-

:5

Biohopanoids (~tg/g dry sediment)

0 500 1000 1500 2000 . . . . t . . . . I . . . . f . . . .

Fig. 5. Downcore profiles of bacteriohopanetetrol (e; +14% error) and biohopanoids detected indirectly from the periodic acid/sodium borohydride treatment (11 = C-32,33,34,35 tetrafunctionalised hopanoids, • = C-31,32,33,34,35 pentafunctionalised hopanoids, [] = C-30,31,32,33,34,35 hexafunctionalised hopanoids; +6% error). The curve for the C-32,33,34,35 tetrafunctionalised hopanoids was plotted

after correcting the results for naturally occurring/~/~ bishomohopanol.


known C-30,31,32,33,34,35 hexafunctionalised hopanoid, aminobacteriohopanepentol (V).

The only other environment where the periodic acid/sodium borohydride method has been applied to hopanoids extracted from Recent sediments is Lake Constance, Germany (Buchholz et al., 1993). In that study, tiff bishomohopanol was the only reported product, at concentrations of up to only 1.5#g/g dry sediment. Identification of this C32 hopanol, plus the C30 and C31 homologue products in Priest Pot demonstrates the presence of hopanoids with more highly (penta- and hexa-) functionalised side-chains. These compounds have, to date, only been isolated from a small number of bacteria; i.e. bacteriohopanepentol from the cyanobacterium Nos toc m u s c o r u m (Rohmer and Ourisson, 1976; Zhao et al., 1996), and aminobacteriohopanetetrol and aminobacteriohopanepentol from certain methylotrophic bacteria (Neunlist and Rohmer, 1985; Zundel and Rohmer, 1985). These groups of bacteria are likely to be present in our sample environment, indeed there is direct evidence for methylotrophs due to the presence of isotopically light hopanes in Priest Pot sediments (Spooner et al., 1994). Our incomplete knowledge of the bacterial distribution of different biohopanoids makes it impossible, however, to confidently ascribe specific structures to particular sources.

The C-32,33,34,35 tetrafunctionalised hopanoids dominate the biohopanoids, although bacteriohopanetetrol only accounts for a small portion of these (2-9%; compare concentrations in Fig. 5). Since no aminobacteriohopanetriol is present and we are confident that we are getting good recovery of bacteriohopanetetrol (ca. 65%), this indicates that the remainder of the tiff bishomohopanol product must be sourced from composite hopanoids (llle). On this basis, our results suggest that the tetrafunctionalised composite hopanoids are the most abundant sedimentary constituents and hence are, potentially, far more important precursors of geohopanoids than previously thought. Similarly, since the tiff homohopanol and tiff hopanol products are more abundant than bacteriohopanetetrol itself (Fig. 5), the aminobacteriohopanetetrol, bacteriohopanepentol and aminobacteriohopanepentol from which they derive, must also be important precursors for sedimentary hopanoids in Priest Pot.

The downcore concentrations of biohopanoids (i.e. hopanol cleavage products; Fig. 5) show a general decline, consistent with a diagenetic decrease, but with a superimposed dip at 5-8 cm which is particularly marked in the C-32,33,34,35 tetrafunctionalised hopanoids. Since a sedimentation rate of 0.7-1.0cm/yr has been reported for Priest Pot (Cranwell et al., 1987; Cranwell and Koul, 1989), this relates to an event approximately 5-8 years ago. This change in biohopanoid abundance and composition is interpreted to be input related rather

than due to a change in preservational conditions within the sediment, since the TOC (Fig. 3) and geohopanoid profiles (see later) show no compar- able shifts. The hopanoid content of certain bacteria has been shown to vary with growth conditions in terms of absolute amounts (e.g. Poralla et al., 1984; Rohmer et al., 1984; Schmidt et al., 1986) but not, to date, in terms of composition. We suggest, therefore, that the biohopanoid data reflect a change in bacterial populations in the overlying water column, rather than a change in hopanoid composition in the same bacteria, probably in response to a fluctuation in environmental conditions. The shift is most marked in the profiles of the cleavage products (particularly the C- 32,33,34,35 tetrafunctionalised hopanoids) rather than bacteriohopanetetrol. This suggests a change in input from bacteria producing composite hopanoids and, to a lesser extent, the penta- and hexafunctionalised hopanoids, and is evidence of at least partly different sources for the different biohopanoids in Priest Pot sediments. Our data thus indicate that biohopanoids in Recent sediments have potential as markers for different bacterial inputs and/or different environmental conditions, although whether or not this signal will survive subsequent diagenesis to be preserved in the sedimentary record is unclear, and a greater understanding of the source specificity of different biohopanoids is needed.

Geohopanoids . The most abundant geohopanoids (i.e. those compounds not directly biosynthesized but formed from biohopanoids through diagenesis) in the Priest Pot sediments are hopanoic acids, hopanols and a hopene (Fig. 4). Of these, the hopanoic acids are generally the most abundant species (25-48% of total geohopanoids). They are dominated by tiff bishomohopanoic acid (VIIb; 80- 600 #g/g dry sediment) although significant quantities of ~fl bishomohopanoic acid (VIIIa; 60- 300/~g/g dry sediment) are also seen. Small amounts of tiff homohopanoic acid (VlIc; 10-30#g/g dry sediment) and tiff trishomohopanoic acid (VIIa; ca. < 5 #g/g dry sediment) were detected, the latter only in the fraction afforded by TLC (Rr 0.40- 0.70).

tiff Bishomohopanol (Via) is a significant com- ponent (11S-240/~g/g dry sediment; 11-27% of total geohopanoids), although the C30 and C31 homologues could not be detected. :tfl Bishomohopanol (VIIIe) was also present in significant quantities (80-190#g/g dry sediment; 9-17% of total geohopanoids).

Hop-17(21)-ene is also abundant (IX; 150- 530 #g/g dry sediment; 22-44% of total geohopanoids). Other hopanoid hydrocarbons (ctfl norhopane, fir3 hopane, ~tfl homohopane (22S and R), tiff homohopane, ~fl bishomohopane (22S and R), :tfl trishomohopane (22S and R)) could only be seen in


the TLC purified hydrocarbon fraction. The most significant of these are the 22R ~/~ homohopane (VIIlb) and ~/~ norhopane (ca. 50 pg/g dry sediment and 40/~g/g dry sediment, respectively; all others ca. < 15 Itg/g dry sediment). Hopanoidal aldehydes and ketones and methylated hopanoids were not present in sufficient quantities for detection (i.e. ca. < 10 pg/g dry sediment).

The C32 components dominate the hopanoic acids and hopanols, as in previous studies of other Recent environments (e.g. Dastillung et al., 1980b: Ries-Kautt and Albrecht, 1989: Buchholz et al.. 1993), although the concentrations of hopanoids we report are higher than those seen before. [~l~ Bishomohopanol and /~/~ bishomohopanoic acid have been previously found in Recent lacustrine sediments at concentrations of 0.4-1.5pg/g and 0.8 3.0 pg/g dry sediment, respectively (Rohmer et al., 1980; Buchholz et al., 1993). Much higher abundances of fl]3 bishomohopanol have only been reported in microbial mats (e.g. up to 82/~g/g dry sediment; Dobson et al., 1988) and peats (up to 352 pg/g dry sediment; Quirk et al., 1984). The high levels of these geohopanoids in Priest Pot are interpreted as resulting from a combination of high bacterial productivity, and conditions favourable to the preservation of lipids within the sediments.

The predominance of the bishomohopanols and bishomohopanoic acids, which we assume to derive from extended C~5 precursor biohopanoids (hopanoids synthesised by bacteria are almost exclusively either C30 or C~5 structures), suggests that diagenesis may proceed via natural oxidation/reduction steps within the water column and sediments. If so, the underlying process may be analogous to our laboratory cleavage methods (i.e. hopanols formed by oxidative cleavage followed by reduction; hopanoic acids by oxidation). The idea that the periodic acid/sodium borohydride reaction may mimic chemical diagenesis of sedimentary hopanoids has previously been suggested by Zundel and Rohmer (1985). Support came from a study on cyanobacterial mats (Dobson et al., 1988) in which a diol was suggested to derive from the diagenesis of a bacteriohopanepentol with a C-30,32,33,34,35 substituted side-chain (again directly analogous to the expected periodic acid/sodium borohydride reaction product). Certainly, similar chemically related diagenetic reactions would explain the high levels of C~2 hopanoids which we and others report, with cleavage of C-32,33,34,35 tetrafunctionalised biohopanoids appearing to occur preferentially between C- 32 and C-33. However, given the relatively high concentrations of penta- and hexafunctionalised biohopanoids in Priest Pot sediments, it is surpris- ing that we do not see correspondingly large amounts of C30 and C~l hopanols and hopanoic acids.

The origin of the large quantities of ~/~ bishomohopanol, ~/~ bishomohopanoic acid and ~/~ homohopane, in the TLC fraction, is unclear. Bacterial hopanoids reported to date, are exclusively of the 17/3(H) configuration (e.g. Rohmer et al., 1992). Elevated levels of geohopanoids with the 17~(H) configuration have previously been observed in peats (Quirk et al., 1984), where high levels of ~/~ homohopane, 2fl homo- and bishomohopanoic acids and ~[/ bishomohopanol were reported. The presence of ~[:t homohopane (22R) in Recent sediments has been ascribed to Sphagnum sources, and may be associated with the microbially-mediated decay of this moss (Quirk et al., 1984). The 2/~ bishomohopanoic acid, ~/~ bishomohopanol and ~/~ homohopane (22R) which we report in Priest Pot, could also be sourced from Sphagnum introduced by run-off from the catchment area. An alternative hypothesis for the early diagenetic origin of ~// hopanoids involves acid catalysed isomerisation of a 17/3(H) precursor, via a A17(21) intermediate (Van Dorsselaer et al., 1977). We have no evidence to favour one or the other theory.

The hopanes and hopenes we report from the TLC purified fractions, are likely to have diverse origins. The presence of large quantities of hop- 17(21)-ene, combined with the apparent absence of the biohopanoid diploptene, suggests that the latter may be undergoing rapid isomerisation to the more stable geohopanoid in the Priest Pot sediments. Laboratory experiments performed on diploptene adsorbed onto clays have shown that it readily iso- merises to various hopenes (including hop-17(21)- ene) under mild conditions (Ageta et al., 1987; Moldowan et al., 1991). The fi/~ hopanes we report may be the early diagenetic products of biohopanoids in Priest Pot, although a source from Sphagnum, along with :~/~ homohopane (22R), can- not be excluded (see earlier). The ~fl hopanes (excluding the C31 ~/3 22R) are likely to be petroleum derived, as they have characteristically high (60:40) 22S to R ratios (Seifert and Moldowan, 1980). These are likely to have an origin from either airborne pollution or run-off from roads and/or agricultural activity around the lake.

Downcore concentration profiles of the major geohopanoids are shown in Fig. 6. All these geohopanoid species were found in significant quantities in the uppermost sediment sample (0 l cm depth: total hopanoic acids 325 l~g/g dry sediment; hop- 17(21)-ene 280/~g/g dry sediment: [~[~ bishomohopanol 115 ttg/g dry sediment, ~/3 bishomohopanol 80 pg/g dry sediment) demonstrating that their formation from biohopanoid precursors begins at the very earliest stages of diagenesis (i.e. in the water column or topmost sediment).

A plot of the total biohopanoids and total geohopanoids with depth (Fig. 6) shows that biohopanoids dominate over geohopanoids, and that both

Preservation and diagenesis of hopanoids

5-

,--., l O -

..C:l

15-

2 0 -

2 5

Geohopanoids (lttg/g dry sediment) 0 200 400 600 800 I000

. . . . I . . . . I . . . . I . . . . I . . . .

Hopanoids (p,g/g dry sediment) o lO00 2000 3000 4000

0 I . . . . I . , , , I . . . . I . . . .

t 5

10

15

2O

25 ~

Fig. 6. Downcore profiles of the major geohopanoids, with errors of +8% for hop-17(21)-ene (©), ___9% for total bishomohopanols (e; //fl and g//), +23% for total hopanoic acids ( i /~/~ bishomohopanoic acid, ~fl bishomohopanoic acid and/~/~ homohopanoic acid). Downcore profiles for the total geohopanoids (Q; hopanoic acids, bishomohopanols and hop-17(21)-ene; +26%) and the total biohopanoids

( , ; all hopanols from periodic acid/sodium borohydride treatment; -+10%) are also shown.

573

decrease in abundance downcore. Our data indicate that biohopanoid diagenesis is not simply a matter of conversion of free biohopanoid precursors to free geohopanoid products; other processes must be involved. Conversion to non-hopanoid components is one possibility (degradation), although incorporation of both bio- and geohopanoids into macromolecular material is more likely. Indeed, macromolecularly-bound hopanoids have been shown to be abundant in ancient sediments and oils, through a range of chemical degradation and pyrolysis techniques (e.g. Eglinton and Douglas, 1988; Sinninghe Damst6 and de Leeuw, 1990; Standen et al. , 1991; Richnow et al. , 1992; Trifilieff et al. , 1992; Sinninghe Damst6 et al. , 1995). Furthermore, studies of the Messel Oil Shale show that whilst bacteriohopanetetrol could be released by chemical degradation of the kerogen (Mycke et

al. , 1987), it was not present in the soluble low-molecular-weight fraction (Rohmer e t al. , 1980). This demonstrates that incorporation of free functionalised hopanoid molecules into kerogen may be an important route to preserving valuable source information which would otherwise be lost with subsequent diagenesis.

CONCLUSIONS

Absolute concentrations of various bio- and geohopanoids have been quantified for a 24 cm long core from anoxic Recent sediments of Priest Pot, England. From this study we conclude that:

1. Both bio- and geohopanoids are abundant in Priest Pot sediments in comparison to previous work in other environments. This is interpreted to be the result of Priest Pot having very high bacterial productivity in the water column, and good conditions of preservation.

2. Bacteriohopanetetrol was the only extended biohopanoid measured directly by GC-MS. Treatment with periodic acid/sodium borohydride gave ///~ bishomohopanol, /3// homohopanol and/~/~ hopanol products which were used as markers for C-32,33,34,35 tetrafunctionalised hopanoids, C-31,32,33,34,35 pentafunctionalised hopanoids and C-30,31,32,33,34,35 hexafunctionalised hopanoids, respectively.

3. Bacteriohopanetetrol only accounts for a small percentage of the total C-32,33,34,35 tetrafunctionalised biohopanoids, suggesting that those with composite groups attached are more abundant, and of more importance as precursors for sedimentary hopanoids than previously recog-


nised. Similarly, o ther extended functionalised b iohopanoids (bac ter iohopanepento l , aminobac- te r iohopanete t ro l and aminobac te r iohopanepentol) are more a b u n d a n t than bac ter iohopanete- trol in Priest Pot sediments.

4. A p ronounced dip in b iohopano id concent ra t ion between 5 to 8 cm depth is interpreted as record- ing a change in bacterial input. This change in abundance is most marked in the data obta ined from the periodic ac id /sodium borohydr ide side- chain cleavage method, and not in the bacterio- hopanete t ro l profile, suggesting tha t different b iohopanoids in the Priest Pot sediments have at least part ly different sources. This observat ion offers encouragement for the use of hopanoids as markers for specific bacterial inputs in the future.

5. The geohopanoids were domina ted by/~tq and c~tq b i shomohopano ic acids, jq/~ and c~/~ b i shomoho- panol and hop-17(21)-ene. These are already a b u n d a n t in the surface sediment sample (0 1 cm) demons t ra t ing the onset of early diagenetic reactions in the water column or upper sediment on a t ime scale of less than one year.

6. Both bio- and geohopanoids decrease in abundance with depth. Al though convers ion of free bio- to geohopanoids is p robab ly occurring, our data suggest tha t o ther processes are operating. Incorpora t ion of free hopano id molecules into macromolecula r material is likely to be an im- por t an t process.

Aeknowledgements This work was funded by Saga Petroleum (Norway), The Royal Society and the University of Newcastle upon Tyne. We thank Roger Pickup at the Institute of Freshwater Ecology, Windermere, England for help with sampling. We also thank Michel Rohmer (Universit6 Louis Pasteur, Strasbourg, France) for standards of diplopterok diploptene, C32 hopanol and aminobacteriohopanetriol, as well as advice regarding methodology, and Bob Moreau (United States Department of Agriculture, Philadelphia, U.S.A.) for the bacteriohopanetetrol standard. Finally, we are grateful to Martin Jones and Paul Donohoe at Newcastle for help with GC MS, and to Paul Fox for helpful discussions.

REFERENCES

Ageta, H., Shiojima, K. and Arai, Y. (1987) Acid-induced rearrangement of triterpenoid hydrocarbons belonging to the hopane and migrated hopane series. Chemieal and Pharmaceutical Bulletin 35, 2705-2716.

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, eds. M. Bjor~y et al., pp. 207-227. John Wiley, Chichester.

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 and Ecology 102, 217-223.

Cranwell, P. A. (1990) Paleolimnological studies using sequential lipid extraction from recent lacustrine sediment: recognition of source organisms from biomarkers. I4ydrobiologia 214, 293-303.

Cranwell, P. A., Eglinton, G. and Robinson, N. (1987) Lipids of aquatic organisms as potential contributors to lacustrine sediments - - 1I. Organic Geoehemistt 3, 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 q/' 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-methylho- pane. Journal o[" Chemical Research S, 168 169.

Dehmer, J. (1993) Petrology and organic geochemistry of peat samples from a raised bog in Kalimantan (Borneo). OJ~ganie 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.

Eglinton. T. I. and Douglas, A. G. (1988) Quantitative study of biomarker hydrocarbons released from kero- gens during hydrous pyrolysis. Energy and Fuels 2, 81 88.

Kr6m, M. D. and Berner, R. A. (1983) A rapid method for the determination of organic and carbonate carbon in geological samples. Journal 0/" Sedimentao' Petrology 53, 660-663.

Mycke, B., Narjes, E. and Michaelis, W. (1987) Bacteriohopanetetrol from chemical degradation of an oil shale kerogen. Nature 326, 179-181.

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. Geoehimiea et Cosmoehimiea Acta 55, 3333-3353.

Neunlist, S. and Rohmer, M. (1985) Novel hopanoids from the methylotrophic bacteria Methylococeus capsulatus and Methylomonas methanica (22S)-35-aminobac- teriohopane-30,31,32,33,34-pentol and (22S)-35-amino- 3/3-methylbacteriohopane-30,31,32,33,34-pentol. Biochemical Journal 231,635 639.

Ourisson, G. and Rohmer, M. (1992) Hopanoids 2. Biohopanoids: a novel class of bacterial lipids. Accounts o f Chemical Research 25, 403 408.

Ourisson, G., Albrecht, P. and Rohmer, M. (1984) The microbial origin of fossil fuels. Scient([ie American 251, 44 51.

Ourisson, G., Rohmer, M. and Poralla, K. (1987) Prokaryotic hopanoids and other polyterpenoid sterol surrogates. Annual Review of Microbiologr 41, 301-333.

Peters, K. E. and Moldowan, J. M. (1993) The Biomarker Guide. Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, New Jersey, 363 pp.

Poralla, K., HS_rtner, T. and Kannenberg, E. (1984) Effect of temperature and pH on the hopanoid content of Bacillus acidocaldarius. FEMS Microbiological Letters 23, 253 256.

Quirk, M. M., Wardroper, A. M. K., Wheatley, R. E. and Maxwell, J. R. (1984) Extended hopanoids in peat environments. Chenzical GeologLv 42, 25-43.


Richnow, H. H., Jenisch, A. and Michaelis, W. (1992) Structural investigations of sulphur-rich macromolecular oil fractions and a kerogen by sequential chemical degradation. Organic Geochemistry 19, 351-370.

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 Geochemisto' 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, eds. E. Schrinner, M. H. Richmond, G. Eeibert and U. Schwarz, Vol. 18, pp. 227 242. Verlag Chemic, Wienheim.

Rohmer, M. (1993) The biosynthesis of the triterpenoids of the hopane series in the Eubacteria: a mine of new enzyme reactions. Pure and Applied Chemistry 65, 1293-1298.

Rohmer, M. and Ourisson, G. (1976) D6riv6s du bact6rio- hopane: variations structurales et r+partition. Tetrahedron Letters 40, 3637 3640.

Rohmer, M., Bisseret, P. and Neunlist, S. (1992) The hopanoids, prokaryotic triterpenoids and precursors of ubiquitous molecular fossils. In Biological Markers in Sediments and Petroleum, eds. J. M. Moldowan, P. Albrecht and R. P. Philp, pp. 1-17. Prentice Hall, Englewood Cliffs, New Jersey.

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 C3o to C35 in Recent muds. Naturwissenschaften 67, 456-458.

Schmidt, A., Bringer-Meyer, S., Poralla, K. and Sahm, H. (1986) Effect of alcohols and temperature on the hopanoid content of Zymomonas mobilis. Applied Microbiology and Biotechnology 25, 32-36.

Seifert, W. K, and Moldowan, J. M. (1980) The effect of thermal stress on source rock quality as measured by hopane stereochemistry. In Advances in Organic Geochemistry 1979, eds. A. G. Douglas and J. R. Maxwell, pp. 229 237. Pergamon Press, Oxford.

Sinninghe Damst6, J. S. and de Leeuw, J. W. (1990) Analysis, structure and geochemical significance of orga- nically-bound sulphur in the geosphere: State of the art

and future research. Organic' Geochemistry 16, 1077- 1101.

Sinninghe Damst6, J. S., van Duin, A. C. T., Hollander, D., Kohnen, M. E. L. and de Leeuw, J. W. (1995) Early diagenesis of bacteriohopanepolyol derivatives: Formation of fossil homohopanoids. Geochimica et Cosmochimica Acta 59, 5141 5147.

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.

Standen, G., Boucher, R. J., Rafalska-Bloch, J. and Eglinton, G. (1991) Ruthenium tetroxide oxidation of natural organic macromolecules: Messel kerogen. Chemical Geology 91,297 313.

Trifilieff, S., Sieskind, O. and Albrecht, P. (1992) Biological markers in petroleum asphaltenes: Possible mode of incorporation. In Biological Markers in Sediments and Petroleum, eds. J. M. Moldowan, P. Albrecht and R. P. Philp, pp. 350 369. Prentice Hall, Englewood Cliffs, New Jersey.

Van Dorsselaer, A., Albrecht, P. and Ourisson, G. (1977) Identification of novel 17-~t(H) hopanes in shales, coals, lignites, sediments and petroleum. Bulletin de la SociOt~ chimique de France. 1, 165-170.

Venkatesan, M. I. (1988) Diploptene in Antarctic sediments. Geochimica et Cosmochimica Aeta 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.

Zhao, N., Berova, N., Nakanishi, K., Rohmer, M., Mougenot, P. and Jfirgens, U. J. (1996) Structures of two bacteriohopanoids with acyclic pentol side-chains 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 Microbiological Letters 28, 61-64.

576 Helen E. Innes et al.

A P P E N D I X

, i I

A ! : . . ~ ~ =- ~ Hopane 3

\ _ : ~ ' ' / ~ O H I -L _ 2 ~ ' ' / f f "/ II

x x

~ * ' , , 1 ~ 5

x x

x x x

? ' " , , ~ y

l]/a X = OIL Y = OH I]11o X = OH, Y = NH: [lie X = OH, Y = Composite groups (e.g. sugars, amino acids)

IVa X = OH, Y = OH IVb X = OH, Y = NH2

V X =OH, Y = N H :

Via X = (CH:)20H VIb X = CI-I:OH VIc X = OH

Villa X = CH:CO:H VIIIb X = CH3 VIIIc X = (CH2)2OH

IX

VIIa X = (CH2)2CO:H VlIb X = CH2CO2H VIIc X = CO2H

Preservation and diagenesis of hopanoids in Recent lacustrine sediments of Priest Pot, England

Documents

Transcript of Preservation and diagenesis of hopanoids in Recent lacustrine sediments of Priest Pot, England