Geochemical Transformations of Sedimentary Sulfur - ACS Publications

Chapter 4

Organic Geochemistry of Sulfur-Rich Surface Sediments of Meromictic Lake Cadagno, Swiss Alps

Downloaded by UNIV OF ROCHESTER on January 13, 2018 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0612.ch004

Anke Putschew, Barbara M. Scholz-Böttcher, and Jürgen Rullkötter Institute of Chemistry and Biology of the Marine Environment, Carl von Ossietzky University of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany

Samples of Recent sediment from Lake Cadagno in the Swiss Alps were examined to study organic facies in a restricted setting and sulfur incorporation into organic matter at a very early stage of diagenesis. Lake Cadagno represents an uncommon lacustrine depositional environment due to a permanently anoxic bottom water column and a constant inflow of sulfate-rich groundwater near the bottom of the lake. The sediment is rich in organic carbon and sulfur. The extractable bitumen contains distinct molecular markers reflecting at least part of the floral and microbial community in the lake and allochthonous higher plant contribution. No low-molecular-weight organo-sulfur compounds were detected. Desulfurization of the heterocomponent and asphaltene fractions in the bitumen with nickel boride, however, yielded hydrocarbons with a strong dominance of phytane and minor concentrations of n-alkanes, steranes and squalane. Carbon isotope ratio mass spectrometry of individual compounds distinguishes phytane of obviously microbial origin in the high-molecular-weight organo-sulfur compounds from free phytane in the bitumen derived from algae or higher plants.

The analysis of organo-sulfur compounds (OSC) in fossil organic matter has become a subject of increasing interest in organic geochemistry. One major reason is the enhanced preservation of biological markers bound into macromolecular sedimentary organic matter via sulfur bonds and thus their potential availability, e.g. for paleoenvironmental reconstruction even at advanced stages of organic matter diagenesis and catagenesis or in crude oils (1-2). Analysis of geological samples suggests that the formation of OSC occurs by intra- or intermolecular addition of reduced inorganic sulfur species into functionalised biogenic lipids (3) leading to compounds of different molecular size.

0097-6156/95/0612-0059$12.25/0 © 1995 American Chemical Society Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR

The sulfur found in OSC comprises single sulfur-carbon as well as polysulfide bonds. Hydrogen sulfide, polysulfides, and elemental sulfur have been suggested as the reactive inorganic sulfur species. Accordingly, a number of laboratory experiments have been carried out in which diagenetic sulfur incorporation has been simulated in a variety of low-molecular-weight organic compounds, e.g. phytol (4-5), phytadienes (4), hop-17(21)-ene (6) and a series of aldehydes and ketones (7). Particularly the successful addition of hydrogen polysulfide to isolated double bonds under mild conditions (4) was supporting evidence of the view that sulfur incorporation can occur at a very early stage of diagenesis. In contrast to this, elemental sulfur probably reacts with sedimentary organic constituents only at elevated temperatures, i.e. during catagenesis (8). The first OSC identified in geological samples mostly had carbon skeletons smaller than C and did not provide much information about their origin (e.g. 9-12). In 1984, Valisolalao et al (13) identified OSC (e.g. a C35 hopanoid containing a thiophene ring) which were structurally related to known biochemical precursors. Since that time a great number of novel low-molecular-weight OSC with structures similar to geologically occurring hydrocarbons and their precursor compounds were identified in bitumens extracted from sediments and in crude oils (14-18). Advanced analytical procedures have been developed which allow the study of organically-bound sulfur compounds in macromolecules and provided new information related to the understanding of OSC formation in sediments. GCpyrolysis of sulfur-rich kerogens using sulfur-specific detectors revealed evidence of the presence of sulfur containing moieties (19-20). Desulfurization of soluble macromolecular material using Raney nickel and nickel boride (Ni2B) released lowmolecular-weight compounds and lead to information concerning the structure of these macromolecules (21-22). The hydrocarbons obtained in this way had structures identical to known biological marker hydrocarbons, but in most cases both the compound distributions and the relative concentrations of stereoisomers differed dramatically from those in the free hydrocarbon fractions indicating that progress of diagenesis - as common for other macromolecularly bound moieties (23) - was slower for the sulfur-bound moieties in the macromolecules than for the free hydrocarbons (24-25). Selective cleavage of di- and polysulfide bonds by MeLi/Mel revealed different modes of carbon-sulfur cross-linking and thus provided clear evidence that polysulfidic reduced inorganic sulfur species are involved in the diagenetic reaction of functionalised lipids (26). Previous investigations used sediments older than 10,000 years to examine the formation of OSC. In the present study fossil organic material not older than 100 years was analysed in an attempt at elucidating the earliest processes of sulfur incorporation into fossil organic matter. 15

Lake Cadagno Lake Cadagno is located south of the St.Gotthard massif in the Ticino Alps of Switzerland (Figure 1) at an altitude of 1923 m above sea level. It is situated in a

Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


Figure 1: Geographic location of Lake Cadagno in the Swiss Alps and schematic cross section of the water body of the lake.


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cirque created by the action of glaciers on pre-Triassic gneisses and mica shists of central alpine crystalline rocks. The southern lake shore follows the contact of the Triassic and early Jurassic rocks folded into the so-called Piora syncline. The rocks comprise tectonized dolomite limestone and gypsum deposits which favor mineral dissolution by groundwater in the fractured subsurface (karst hydrology). The lake surface is about 0.27 km^ the maximal depth is 21 m (Figure 1). The upper part of the water column regularly turned over by weather forces, i.e. the mixolimnion, is oxic and fed by slightly mineralized surface water. This supply is controlled by seasonal changes, e.g. melting waters in the spring. The lower part of the water column not affected by wind stress and other turn-over processes, the monimolimnion, is anoxic. Subaquatic springs highly concentrated in sulfate (2-4 mmol/1) feed this zone (27-28). The density difference between the upper and the lower parts of the water body leads to a stable chemocline at a water depth between 8 and 12 m, which inhibits a vertical mixture of the water column across this boundary. Due to the high sulfate concentration, bacterial sulfate reduction is the main biological process in the monimolimnion. The boundary layer (meromixis) is dominantly populated by phototrophic sulfur bacteria, especially Chromatium (sulfide-oxidizing purple bacteria) (27,29). These bacteria are enriched in the pigment okenone which causes the red color in the water column at the chemocline. Chemotrophic sulfide oxidizing bacteria thrive in the upper part of the meromixis where oxygen is present.


?

Materials and Methods A gravity core of Lake Cadagno sediment, 36 cm long, 7 cm wide, and immediately frozen at the sampling site, was provided by A. Losher (ETH Zurich). Based on the work of Zullig (29) it represents the sedimentary history of the lake over the last 80100 years which corresponds to a sedimentation rate of about 4 mm/a, a value common for moderately to highly productive lacustrine systems with a significant supply of detrital material. The sediments were homogeneously black and had a strong odor of hydrogen sulfide and possibly other volatile sulfur compounds; this odor ceased considerably below a depth of 32 cm. The core was separated into sections of 2 cm thickness which werefreeze-driedand ground. Aliquots of the ground samples were used to determine total carbon (TC) and total sulfur (TS) contents with a LECO SC-444 instrument. Before total organic carbon (TOC) determination with the same instrument, the samples were treated with hydrochloric acid to remove carbonates. Carbonate contents were calculated by difference and expressed as percent calcium carbonate. For molecular organic geochemical analysis the samples were ultrasonically extracted with dichloromethane. The solvent was evaporated and the extract suspended in a small volume of «-hexane. After addition of internal standards (squalane, thianthrene, 5ct(H)-androstan-17-one) asphaltenes (used here and througout this paper as an operational term and comprise humic substances etc.) were precipitated with an approximately 100 fold excess of «-hexane. The compounds soluble in /z-hexane were separated on a column filled with silica deactivated with 5% water. The aliphatic hydrocarbon fraction was eluted with whexane, the aromatic hydrocarbonfractionwith w-hexane/dichloromethane (9:1 by



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Sediments ofMeromictic Lake Cadagno

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volume) and the hetero(NSO)-compounds with dichloromethane/methanol (9:1 by volume). For the investigation of extractable bound fatty acids, alcohols and sterols, aliquots of the NSO fractions were saponified with 3 ml 5% KOH in methanol/water (8:2 by volume) for 2 h at 80°C. Aliquots of the asphaltene and NSO fractions were desulfurized with Ni2B following a published precedure (22). The reaction products were separated by column chromatography on silica gel into an apolar and a polar fraction. The apolar fraction was eluted with w-hexane/dichloromethane (9:1 by volume) and analysed before and after hydrogenation; the polar fraction was eluted with dichloromethane/methanol (1:1 by volume) but not further investigated. For the analysis of compounds bound to the insoluble organic matter, extract residues were saponified with 20 ml 5% KOH in methanol/water (8:2 by volume) for 24 h under reflux. Where appropriate, the extract fractions were derivatized with diazomethane and MSTFA («-methyl-«-trimethylsilyl-trifluoroacetamide) before analysis. GC analysis was carried out on a Hewlett Packard 5890 series II instrument equipped with a temperature programmable injector system (Gerstel KAS 3) and a flame ionization detector (FID) or a sulfur-selective chemoluminescence detector (Sievers SCD). The detection level for sulfur was 17 pg S/ul. A DB-5 (J&W) fused silica capillary column (30 m x 0.25 mm i.d., df = 0.25 um) was used with helium as carrier gas. The temperature of the GC oven was programmed from 60°C (1 min isothermal) to 300°C (50 min isothermal) at 3°C/min. The injector temperature was programmed from 60°C (5 s hold time) to 300°C (60 s hold time) at 8°C/s. GC/MS measurements were performed with the same type of GC system under the conditions described above. The gas chromatograph was coupled to a Finnigan SSQ 710 B mass spectrometer operated at 70 eV with a scan range of m/z 50 to 600 and a scan time of 1 scan/s. Carbon isotope ratios of individual hydrocarbons were determinded using a Finnigan MAT 252 isotope-ratio monitoring mass spectrometer coupled with a Varian gas chromatograph. An Ultrix 2 (HewlettPackard) capillary column (50 m x 0.32 i.d., df = 0.17 urn) was used with helium as carrier gas. The temperature program was identical to the one described before. Structural Assignment of a Major Steroid Olefin in the Aliphatic Hydrocarbon

Fractions. The mass spectrum of the most abundant compound, apart from nalkanes, in most of the aliphatic hydrocarbon fractions of the Lake Cadagno sediments is characterized by a molecular ion at m/z 394 and significant fragment ions at m/z 69, m/z 255 and m/z 257. These data suggest that the compound may be a C steratriene with two double bonds in the ring system and one double bond in the side chain. The mass spectrum of 24R-ethylcholesta-3,5,22-triene, a commercially available standard (Chiron), is similar to that of the hydrocarbon in the Lake Cadagno sediments but the standard has a slightly longer gas chromatographic retention time. Also, thefragmention at m/z 69 is subordinate in the mass spectrum of the standard. Thisfragmentis typical of side chain-unsaturated steroids with a double bond postion at C-22 and a 23,24-dimethyl substitution pattern. It is generated by the formation of an isopentyl ion after double hydrogen transfer and cleavage of the C-23/C-24 bond. This principalfragmentationbehavior has already been described for dinosterol, i.e. 4a,22,23-trimethylcholest-22-en-3P-ol (30), and 23,24-dimethylcholesta-5,22-dien-3P-ol (31). According to the molecular ion and thefragmentat m/z 255, the unknown compound does not carry additional methyl substituents in the ring system. Thus, the steratriene in the Lake Cadagno sediments 29


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appears to be 23,24 dimethylcholesta-3,5,22-triene. After hydrogenation of an aliphatic hydrocarbon fraction of a Lake Cadagno sediment and the standard steratriene, the two resulting steranes coelute. This is in accordance with the interpretation of the steratriene characteristics, because gas chromatographic coelution of 23,24-dimethyl- and 24-ethylcholestanols is known (32). Results and Discussion


Bulk parameters and extract fractionation. The anoxic sediment of Lake

Cadagno is rich in organic carbon and sulfur (Figure 2). The highest TOC value (13.5%) was measured at the sediment surface, the lowest value of 2.9% corresponds to a sample at a depth of 20 cm. The sulfur content shows a trend which at the the first glance seems to grossly covary with that of the TOC values. TS reaches a maximum at a depth of 10 cm (3.3%) and a minimum at a depth of 24 cm (1.3%). The relative variation of the total sulfur content is much smaller, however, than that of the organic carbon which leads to lower TOC/S ratios in those sediment layers which are less enriched in organic carbon (Fig ure 2). This means that the rate of microbial sulfate reduction was not limited by the supply of organic matter. While graded turbidite sequences, caused by episodic inflow of sandy material, have been observed in some parts of Lake Cadagno (28\ no sedimentological evidence (e.g. color change) for dilution of organic matter was observed in the core. Thus, diagenetic processes as well as fluctuations in the supply of organic matter, both qualitatively and quantitatively, are more likely to have affected the distribution of organic matter, bulk sulfur and carbonate concentrations than physical processes. The amount of extractable organic matter is around 10%, normalized to TOC, down to a depth of 16 cm. Below that depth the extract yield drops to about 5% (Table I). This indicates that there may be two sections characterised by differences in the bulk composition/type of the organic matter above and below the boundary of about 16 cm depth despite the fairly uniform relative proportions of the gross extract fractions (Table I). All extract yields are high in view of the early stage of diagenesis of the organic matter, but are typical of sediments rich in sulfur (e.g. 33). As expected, the NSO compounds represent the most important extractfraction(Table I). The relative proportion of the polar fractions is even higher than displayed in Table I, because the aliphatic hydrocarbon fractions include elemental sulfur although this has not been determined quantitatively in all samples. The amount of elemental sulfur decreases with increasing depth. The sample from 6-8 cm depth contains 2.7% elemental sulfur (3.1% TS), whereas the sediment at 30-32 cm depth has an elemental sulfur content of only 0.2% (2.8% TS). Aliphatic Hydrocarbon Fraction. The aliphatic hydrocarbon fractions are dominated by «-alkanes with a strong odd over even carbon number predominance and a maximum at n-C or tf-C (Figure 3). This type of distribution is characteristic for land plant-derived organic matter (34). The most likely source is the grass growing around the lake. 29

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20 -22 24 26 28 + 30 32 34 36

20 22 24 26 28 30 32 34 36

16 Depth 18 (cm)

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Depth 18 (cm)

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8 +

8 10

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0

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3 3

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TS (%) 1,5

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Figure 2. Depth profiles of organic carbon (TOC) and total sulfur (TS) contents in the top 36 cm of lake Cadagno sediments and of the TOC/TS ratio.

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8

TOC(%)



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Table I. Extract yields and composition of total extracts of Lake Cadagno sediments Depth Aliphatic Asphaltenes(% Extract yield Aromatic NSO (cm) (mg/g Core) fraction fraction (%) fraction (%) ) (%) 0-2 7.4 34.8 92 55.3 0.6 2-4 7.2 114 35.1 36.1 4.1 4-6 6.5 34.7 153 0.9 64.8 6-8 13.3 109 55.8 0.7 49.9 8-10 9.6 45.4 116 42.9 0.3 10-12 14.6 66 16.1 78.7 1.6 12-14 11.7 35.1 92 46.7 1.7 14-16 16.8 94 27.1 43.6 1.1 16-18 43 37.4 10.4 47.8 0.9 18-20 63 4.9 30.7 20.8 2.0 20-22 36 30.3 11.3 64.1 2.8 22-24 43 10.3 26.3 55.4 2.8 24-26 14.4 52 24.8 46.4 0.2 26-28 10.7 42 15.7 77.7 0.8 28-30 47 18.4 12.8 63.6 2.4 30-32 39.4 56 9.7 69.4 18.9 32-34 8.7 41 50.0 2.7 40.0 34-36 17.4 46 2.2 74.5 0.5


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2 9

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2 7

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3 3 2 3

2 8 2 6

21 2 4 2 2

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JLJSLA I j L - i 4 0

5 0

J JikJ J 6 0

7 0

i_^^__A_A_

f 8 0

9 0

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Retention time (min)

Figure 3. Gas chromatogram of the aliphatic hydrocarbons of the sample from 28-30 cm depth. n-Alkanes are indicated by their carbon numbers. O = steratriene (see Table II), * = internal standards. Hopanoid hydrocarbons of microbial origin (35) are the second-most important group of constituents of the aliphatic hydrocarbon fractions. Hopanes with the thermodynamically least stable, biogenic 17P(H),2ip(H) configuration and a carbon number range of C - C occur together with high relative amounts of hopenes like 22,29,30-/r/wor-hop-17(21)-ene, hop-17(21)-ene, ra?o-hop-13(18)-ene and hop-22(29)-ene (Figure 4 and Table II). This compounds distribution is typical of immature organic matter and common in Recent sediments, yet it demonstrates the rapid conversion of functionalized precursors into hydrocarbons and their concurrent or subsequent transformation into a mixture of isomers, at least for the unsaturated hopenes. Sterenes are present in low concentrations with the exception of one compound which elutes just before the C w-alkane (Figure 3) and is conspicuously abundant. It was tentatively identified as 23,24-dimethylcholesta-3,5,22-triene, based on comparison with the mass spectrum and relative retention time of a commercially available 24R-ethylcholesta-3,5,22-triene standard and published mass spectra of 4a,22,23-trimethylcholest-22-en-3p-ol (30) and 23,24-dimethylcholesta3,5,22-dien-3P-ol (31) as outlined in the experimental section. Another aspect in favor of the structure of 23,24 dimethylcholesta-3,5,22-triene is the occurrence of a potential precursor compound, namely 23,24-dimethylcholesta-5,22-dien-3P-ol in the NSO fraction of the Lake Cagano sediment extracts. This sterol is known to be characteristic of dinoflagellates (36-37), and dinoflagellates are known to occur in Lake Cadagno (K. Hanselmann, private communication 1994). The amount of the 27

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C

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2 5 23 3 3 21

60:00

i J

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2 6

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LL

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75:001


Figure 4. Partial reconstructed ion chromatogram RIC (top) and mass chromatograms (m/z 191, middle; Im/z (255+257), bottom) of the aliphatic hydrocarbon fraction of the sample from 26-28 cm depth. n-Alkanes are indicated by their carbon numbers (RIC). For compounds identified in the mass chromatograms see Table II. * = internal standards. Note: Retention times of cyclic hydrocarbons relative to n-alkanes differ between GC analysis (see peak O in Figure 3) and GC-MS analysis (see peak O here) due to the effect of the mass spectrometer vacuum on the GC column. Vairavamurthy et al.; Geochemical Transformations of Sedimentary Sulfur ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Table II. Compounds identified in the aliphatic hydrocarbon fractions of Lake Cadagno sediments (see Figure 4) Symbol/Compound a 22,29,30-fr/rtor-hop-17(21)-ene b 17p(H)-22,29,30-/n>wr-hopane c triterpene (m/z 410,395,109) d hop-17(21)-ene e 17P(H),21a(H)-30-«or-hopane f 17p(H),21a(H)-hopane g «eo-hop-13(18)-ene h i j k 1 m n o

triterpene (m/z 410, 395,191,189) 17p(H),21 P(H)-30-«or-hopane hop-21-ene triterpene (m/z 410,218,205,191, 175) 17a(H),21 P(H)-/?o/wo-hopane 17P(H),2ip(H)-hopane hop-22(29)-ene /?0/wohop-22(29)-ene

p 17p(H),21p(H)-/wwo-hopane

Symbol/Compound A des-A-oleanadiene B des-A-oleanadiene C cholesta-5,22-diene D 5a(H)-cholest-2-ene E sterene (m/z 370,355,316,257, 215,147,108) F 24-methylcholesta-5,22-diene G unknown (m/z 368,353,247,213, 159,147) H 24-methylcholestatriene I 24-methyl-5a(H)-cholest-2-ene J C 9-steratriene K C 9-steratriene 2

2

L M N O P Q R S

C 9-steratriene unknown (m/z 394/392,380,253) C29-steratriene 23,24-dimethylcholesta-3,5,22triene C 9-steratriene 24-ethyl-5a(H)-cholest-2-ene unknown (m/z 396,381,275,255, 213,160,147 C^-steratriene 2

2



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steratriene, normalized to organic carbon, generally appears to increase with depth although fluctuations were observed in this trend (Table III). The increase is attributed to a progress in diagenesis, i.e. transformation of a functionalized precursor into the triunsaturated hydrocarbon. In the surface sample of the lake (0-2 cm), the highly branched isoprenoid 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane was detected. Its concentration decreases rapidly with depth. 2,6,10-Trimethyl-7-(3-methylbutyl)-dodecane is typically used as a marker for the green alga Enteromorpha prolifera (38), although more recently highly-branched isoprenoids have been predominantly related to diatoms, at least in the marine environment (39-41). Chlorophyceae and Cryptophyceae were described as the main phytoplankton species in Lake Cadagno (29), but no natural product survey for isoprenoids (or sterols) has yet been performed for these lake communities. Aromatic Hydrocarbon Fraction. Aromatic hydrocarbons are the least abundant fraction of all sediment extracts from Lake Cadagno as common for Recent sediments. The only compound found in higher concentrations was squalene which is a component widespread in many organisms. In these fraction the concentrations of OSC such as thiophenes or thiolanes are below the detection level of the sulfurselective detector. NSO Fraction. The GC-amenable portions of the NSO fractions are dominated by free w-fatty acids with an even over odd carbon number predominance, isoHexadecanoic, w-hexadecanoic, «-octadecanoic acid and fatty acids with higher carbon numbers («-C , «-C , «-C ) are the most abundant compounds (Figure 5a). After hydrolysis of the total NSOfraction,which yields the sum of thefreeand bound extractable acids, the fatty acid distribution pattern changes and is characterized by w-hexadecanoic and w-octadecanoic acid together with their monounsaturated analogs. The concentration of extractable bound fatty acids with carbon numbers higher than eighteen is very low (Figure. 5b; extractable bound fatty acids represent the calculated difference between total extractable fatty acids after hydrolysis and free extractable fatty acids). A distribution pattern similar to that of the extractable bound acids was found for the kerogen-bound fatty acids after saponification of the extract residues (Figure 5c). The total amount of fatty acids bound to kerogen and - to a lesser extent - of the bound extractable acids is much higher than the amount of free fatty acids. The bound fatty acids apparently are mainly derived from algal and bacterial biomass (42). Only the smallfractionof the free fatty acids contains a significant relative proportion of long-chain acids of terrestrial higher plant origin. Unsaturated fatty acids, iso- and anteiso-zcids which are typical of microbial biomass (42) are present in afreeas well as in a bound form. The main sterols identified in the NSO fractions are cholest-5-en-3p-ol, 24ethylcholest-5,22-dien-3P-ol, 23,24-dimethylcholest-22-en-3P-ol, 24-ethylcholest-5en-3p-ol, and 23,24-dimethylcholest-5-en-3P-ol (Figure 6 and Table IV). ^-Alcohols were found in the range of C to C with an even over odd carbon number predominance. Sterols and alcohols were mainly found as free compounds indicating that the major part of the bound fatty acids released by hydrolysis are not derived from wax or steryl esters. The abundance of sterols relative to the concentrations of steroid hydrocarbons is typical of the early stage of diagenesis. Only the 24

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Table III. Concentrations (ug/g C rg) of free phytane and 23,24-dimethylcholesta3,5,22-triene in the aliphatic hydrocarbon fractions and of phytane released by desulfurization 0

Aliphatic hydrocarbon fraction Depth (cm)

Free phytane 17.0 30.0 16.0 4.5 12.0 6.0 3.0 6.0

After desulfurization NSO fraction

Asphaltenes

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Geochemical Transformations of Sedimentary Sulfur - ACS Publications

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