Multivariate analysis of lipid distributions in Recent salt marsh sediments

Chemical Geology, 85 (1990) 393-402 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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Multivariate analysis of lipid distributions in Recent salt marsh sediments Ramadan Abu El-Ella a and John Carpenter b aDepartment of Science and Mathematics, Faculty of Petroleum and Mining Engineering, Suez Canal University, Suez (EgYPO bCollege of Science and Mathematics, University of South Carolina, Columbia, SC 29208 (U.S.A..) (Accepted for publication April 6, 1990)

ABSTRACT Abu El-Ella, R. and Carpenter, J., 1990. Multivariate analysis of lipid distributions in Recent Salt marsh sediments. Chem. Geol., 85: 393-402. The alkane and fatty acid fractions in the top 60-cm section of contemporary salt marsh sediment samples were isolated and analyzed by gas chromatography to obtain information on sources contributing organic matter and information on early diagenetic alteration of the organic sediments. The samples were obtained from the salt marsh sediment in an area adjacent to Goat Island near Georgetown, South Carolina, U.S.A. The multivariate analysis of the alkane and fatty acid fractions indicates that the salt marsh sediment lipid distributions can be explained as varying contributions of a limited number of end-members. Most of these end-members patterns are similar to patterns produced by rooted marsh macrophytes, algae and bacteria. In the surface sediments, the rooted macrophyte and bacterial lipids generally dominate. At depths, sample proportions reflect contributions from root material and organic material from surface sources derived through bioturbation. At depth below the zones of rooting and bioturbation, the degraded end-member increases in proportion, indicating the probable modification of organic material by the bacterial community.

1. Introduction

When organisms die, their organic matter undergoes a variety of reactions, some microbial, such as the formation of methane by anaerobes, and some physical and chemical changes, such as dehydration and oxidation. The combined attack of weathering and microbes converts much of the organic matter either to gases that escape into the atmosphere or soluble products that are carried off by groundwater. In the salt marsh, the major source of organic matter has been attributed to Spartina, algae, aerobic and anaerobic bacteria, as well as benthonic organisms and phytoplankton (Howart and Teal, 1979; Marinucci, 1982; Kornder, 1986 ). In Recent sediments a certain 0009-2541/90/$03.50

even carbon-number predominance limited to C16 and C~8 n-alkanes has been observed (Simoneit, 1975 ). When large proportions of terrestrial material accumulated in a restricted limnic environment, n-fatty acids with a range from C24 to C32 and with an even carbon-number predominance are found (Tissot and Welte, 1984). The lipid category includes a broad range of compounds such as: fats, fatty acids, esters, sterols, hydrocarbons (alkanes and alkenes) and pigments. The original lipid composition in the sediment undergoes rapid changes in the early stages of diagenesis. Previous studies have suggested that the fatty acid fraction is altered more rapidly after deposition than are the hydrocarbons (Farrington and Quinn, 1973; Johnson and Calder, 1973 ). In general, diagenesis breaks down large bio-

© 1990 Elsevier Science Publishers B.V.

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polymers to simpler structures which may finally be transformed into geopolymers (Wapies, 1981 ). The modern salt marsh and associated environments are areas of high organic productivity (Haines, 1976; Marinucci, 1982) where large amounts of organic material are available to be incorporated into the sediment. The amount and source of organic material in the salt marsh is dependent on the particular subenvironment and/or vegetation zone. The material that is incorporated is a function of the diagenesis of the original organic matter. The actual amount of diagenetically altered material that is incorporated is a function of the oxidation-reduction potential of that marsh environment. The sterol composition of a contemporary lacustrine sediment has been previously reported (Gaskell and Eglinton, 1976 ) and limited data on the distributions of hydrocarbons and carboxylic acids (for the surface sediment alone) have been included in a comparison of several contemporary sediments (Brooks et al., 1976, 1977). The distributions of hydrocar-

R. ABU EL-ELLA AND J. CARPENTER

bon and fatty acids in a lacustrine sediment have been studied by Cardoso et al. ( 1983 ). These preliminary studies are extended in this paper to ascertain the extent to which the organic constituents in the top 60 cm of marsh sediments could provide information on the sources of organic matter and on the processes of early diagenesis. As an expansion of the earlier study of Kornder and Carpenter (1984), which involved the unmixing data of the surface salt marsh sediments, this study examines both surface and subsurface samples to investigate the source of lipids at depth and the early diagenetic changes in the lipid fraction of marsh sediments.

2. Method of study The core samples were taken from the top 60 cm, at four locations throughout the salt marsh sediments in an area adjacent to Goat Island near Georgetown. South Carolina, U.S.A. (Fig. 1 ). The cores were taken using 2 in. by 20 ft. (5 cm by 6.1 m) aluminum irrigation tubes;

Fig. 1. L o c a t i o n m a p o f the s t u d i e d cores (C-'g:l . . . . . . core No, 1, etc. ).

395

LIPID DISTRIBUTIONS IN RECENT SALT MARSH

TABLE I

Normalized sample compositions of the studied marsh sediments: transformed 6-component Extended Q-model alkane and fatty acid solution and the sand % of the studied samples Depth

End-members (%)

Sand

(cm)

(%) 1

2

3

4

5

6

14 29 6 6 11

7 2 22 23 30

16 16 26 20 18

42 36 36 36 30

6 4 1 4 5

15 13 9 11 6

86 89 91 90 92

10 8 9 1

12 17 10 20

20 19 19 14

13 15 15 16

7 6 5 25

38 35 42 24

94 95 94 96

16 4 22 18 11 17 8 0

10 19 19 48 47 38 52 71

23 32 14 0 16 8 8 11

21 33 25 14 20 17 15 5

9 0 2 11 0 8 9 5

21 12 18 9 6 12 8 8

60 72 75 92 93 94 96 97

9 2 4 11 11 0

11 21 56 26 24 56

19 10 6 8 13 14

37 39 25 37 33 10

8 10 9 15 8 10

16 18 0 3

65 78 92 93 95 96

Core I: 0-5 5-10 10-15 15-20 20-25 Core 2: 0- 5 10-15 20-25 30-35 Core 3: 0-5 5-10 15-20 20-25 25-30 35-40 45-50 55-60 Core 4: 0-5 5-10 15-20 25-30 35-40 45-50

11 10

the method of drilling was described in detail by Lanesky et al. (1979). The sand amounts were determined by placing a 25-g (wet) aliquot of each sample in a preweighed 50-ml beaker. To each beaker were added ~ 25 ml of a Calgon ® dispersant solution (2 g 1-1 ) and the sample placed in an ultrasonic bath until disaggregation. The sample was wet-sieved through No. 10 (2 m m ) and

No. 230 (0.063 m m ) sieves. The fraction > 2 mm consisted mainly of very small amount of organic debris, which was discarded. The sand fraction was captured on the No. 230 sieve (Table I ). The method of lipid extraction was a simultaneous saponification-extraction procedure modified after that of Boehm and Quinn (1978). The lipid extracts of the sediments were separated into three fractions: saturate hydrocarbons (alkanes), aromatics and fatty acids. This investigation deals with the alkane and fatty acid fractions due to the very limited number and concentration of aromatic compounds. A Hewlett-Packard ® 2790 capillary gas chromatograph with a split/splitless all-glass injection system and a flame ionization detector (FID) was employed for alkane analysis. All injections were performed with the splitless injection technique proposed by Grob and Grob (1978). Analysis of the fatty acid was performed on a Hewlett-Packard ® 5840 with FID and an all-glass injection system. The gas chromatograph (GC) had been modified to accept capillary columns following the methods ofSeverson et al. (1980). Compounds of the alkane fraction were identified by comparison of their retention times to those of qualitative standards. For the identification of the fatty acid, qualitative standards and gas chromatography-mass spectrometry (GC-MS) were used. A Finnigan model 4021C GC-MS instrument, with a 25-m fused silica capillary column similar to that in Hewlett-Packard ® 5790, was used to collect all mass spectra. In order to relate the gas chromatographic patterns to their sources, the chromatographic data in this study were examined by a technique previously used by Kornder and Carpenter (1984). The unmixing algorithm procedure requires that the data be a constant sum. This requirement was met by normalizing the chromatographic data of each sample. Because of the

396

R. ABU EL-ELLA AND J. C A R P E N T E R

great differences in the concentrations of alkanes vs. fatty acids (micrograms vs. milligrams) each fraction was normalized to a con-

stant sum separately. The first data set consisted of normal alkanes C15 to C33 plus pristane and phytane. The

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Fig. 2. Plots of the end-membersof the alkanes and fatty acids of the salt marsh sediments in the study area. The order of variables along the X-axis is: normal alkanes ( 15-33 ), pristene (Pr), phytane (Ph), normal fatty acids ( 13-28 ), unsaturated fatty acids ( 13-20, denoted by U) and branched fatty acids ( 14-18, denoted by B). The unsaturated and branched fatty acids represent the sum of all acids of that given carbon number. second contained the normal saturated fatty acids C13 to C2s, unsaturated fatty acids C l 3 to C2o, and branched fatty acids. Data sets could then be submitted to the algorithm for analysis of the alkanes and fatty acids combined. The normalized data sets were submitted to the Extended CABFAC procedure (Miesch, 1976) and then to both the Extended and Fuzzy Q-model procedures (Full et al., 1982) to determine the three parameters associated with any mixture: ( 1 ) the number of components comprising the mixture; (2) the composition of each component; and (3) the relative proportions of each component in each sample. The extended Q-model was used to determine the number of components or end-members comprising the studied mixtures. The data set using fatty acids-alkanes mixtures consisted of 23 samples each with 50 variables (Fig. 2; Table I).

3. Results and discussion Two general categories of hypothesis for the preservation of organic matter have been proposed. The first suggests a selective preservation of lipid-type components; while in the second, biopolymers decompose to monomers which condense into complex macromolecular humic-type substances (Hatcher et al., 1983). Many past studies attempting to investigate the sources of organic matter for Recent sediments approached the problem from two basically different analytical methods. One approach involved the use of (~~3C-values either by themselves (Haines, 1977; Hackney and Haines, 1980) or in conjunction with GC information of the lipid fractions (Johnson and Calder, 1973). A second approach in the investigation of the organic matter of sediments was the identification of specific compounds

398

(biomarkers) a n d / o r the visual inspection of complex spectra of the lipid fraction as determined by GC (Farrington and Quinn, 1971; Jeffries, 1972; Barrick et al., 1980; Requejo and Quinn, 1983; Shaw and Johns, 1985). A thorough discussion of carbon isotope analysis of Spartina marsh sediments is detailed by Ember (1985). Examination of both surface and subsurface samples representing the salt marsh environment using GC has been included in this paper to investigate the source of lipid fractions of the studied sediments by investigating the alkane and fatty acid fractions. From the results of the Extended CABFAC procedure, a six-component (end-member) solution was chosen. Cumulative variation accounted for by this solution was 93.2%. Coefficients of determination for the alkanes were >85% for l0 of 21 variables and >75% for 13 of the 21 variables. For the fatty acids, coefficients of determination were > 85% for 6 of 29 variables and > 75% for 15 of 29. Communalities for the samples were >~90% for all but one sample (core No. 3, 20-25 cm depth). Both Extended and Fuzzy Q-model provided similar output, the results that follow are derived from Extended Q-model. Table I contains the end-member compositions in the studied samples. Fig. 2 shows plots of the six end-members. Two n-C29-alkane-dominated end-members are present (Nos. 1 and 4) (Fig. 2 a + b and c + d). These two end-members are attributed to the rooted macrophytes and the major fatty acid constituents of these patterns are saturated n-Ct 6 and unsaturated n-C~6 and n-C~8 acids. A n-C~7-alkane-dominated pattern is present (end-member 3 ) (Fig. 2c and d ). Comparison of the alkanes and fatty acids of this pattern to a composite derived from blue-green algae, indicates that both contain the same major constituents (Parker et al., 1967). The remaining end-members (Nos. 2, 5 and 6) (Fig. 2 a + b and c + f ) have very prominent normal alkane contributions around n-C25. However,

R. ABU EL-ELLA AND J. CARPENTER

each is quite different from the others with respect to fatty acid distributions. End-member 6 can probably be attributed to bacterial sources because it is n-C25-alkanedominated (Han and Calvin, 1969) and the major portion of the branched fatty acids are CI5 and C~7 (Perry et al., 1979). End-member 5 contains some amount of the alkane n-C~5 and n-Cl7 which might be more characteristic of algae. However,/7-C17 can also occur in bacteria (Han and Calvin, 1969). Therefore, based only on the alkane region, end-member 5 may be representative of a bacterial pattern with a minor amount of an algal pattern. From examining the fatty acid region of end-member 5, it appears the fatty acids do not help in the identification of whether this end-member is derived from bacterial or both bacterial and algal sources. The remaining end-member (No. 2) is unique. The alkane fraction of this end-member is dominated by n-C3~ and has a high odd/ even ratio in this high-molecular-weight region, which characterize the rooted macrophyte type source. The lack of branched and unsaturated fatty acids in this end-member could be explained by early bacterial alteration. Table I presents the normalized sample composition data. After initial examination of the results, the data were further reduced for easier interpretation by combining the sample compositions of end-members representing similar groups (Table II ). End-members I and 4, which are characteristic of rooted macrophyte, were combined. End-member 5, although not conclusively derived from bacteria only, was combined with end-member 6, the most probable bacterial end-member. To facilitate the observation of trends in the end-members, the proportion data for each sample were used to construct plots of the endmembers vs. depth for each core. The endmembers 1 and 4, 2, 3 and 5 and 6 represent the macrophyte, degraded, algal and bacterial groups, respectively (Fig. 3 ). Only core 1 is dominated throughout by the

399

LIPID DISTRIBUTIONS IN RECENT SALT MARSH

TABLE II Normalized sample compositions of the studied marsh sediments: transformed 6-component Extended Q-model alkane and fatty acid solution (with similar end-members combined) Depth (cm)

End-members (%) 1 and 4

2

3

5 and 6

56 65 42 42 41

7 2 22 23 30

16 16 26 20 18

21 17 10 15 11

23 23 24 17

12 17 10 20

20 19 19 14

45 41 47 49

37 37 47 32 31 34 23 5

10 19 19 48 47 38 52 71

23 32 14 0 16 8 8 11

30 12 20 20 6 20 17 13

46 41 29 48 44 10

11 21 56 26 24 56

19 10 6 8 13 14

24 28 9 18 19 20

Core 1:

0- 5 5-10 10-15 15-20 20-25 ( b r e 2:

0- 5 10-15 20-25 30-35 Core 3:

0- 5 5-10 15-20 20-25 25-30 35-40 45-50 55-60 Core 4:

0- 5 5-10 15-20 25-30 35-40 45-50

rooted macrophyte end-member group, while cores 2-4 are generally dominated at the surface by either rooted macrophyte or bacterial end-member type sources. Core 3 is similar to core 2 in that both are dominated by bacterial end-member at the surface. From just below the surface to a depth of ~ 23 cm, the rooted macrophyte-type group dominates, while the remainder of the core is dominated by the degraded or bacterially al-

tered end-member 2. In core 4, the rooted macrophyte end-member group dominates at the surface and throughout much of the core. Similar to cores I and 3, the degraded end-member 2 increases in concentration with depth. Summarizing the end-members vs. depth plots of the cores. Core 2 shows almost constant end-member proportions with depth. In cores 1, 3 and 4, the increase in the degraded end-member appears inversely related to the rooted macrophyte end-member. As the proportion of the degraded end-member increases the proportion of the rooted macrophyte endmember decreases, and the algal and bacterial end-member proportions remain nearly constant. No doubt some of this correlation is the result of the data being constraint to a constant sum.

Considering the grain-size data, with the results obtained in core 2, the dominance of the bacterial end-member is very plausible. The very sandy well-drained sediments are conductive to bacterial oxidation of large amounts of organic material. These bacteria in turn then contribute their own lipids. In cores 1, 3 and 4, the greater percentage of fine-grained sediments has resulted in generally reducing sediment conditions and an enhanced preservation of organic matter. Throughout much of the cores in these locations plant lipids dominate, because of the limits placed on the bacterial community by the reducing conditions. 4. Conclusions

( 1 ) Results derived from Extended Q-model indicate that the studied marsh sediment patterns are composed of four basic lipid types. Three of these: rooted macrophyte, algal and bacterial are derived from living sources. The fourth is attributed to a degraded or bacterially altered pattern which is probably the result of selective removal of the less resistant lipid fraction of the marsh sediment. (2) Observations of the diagenetic alteration of fatty acids and alkanes are complicated

R. ABU EL-ELLAAND J. CARPENTER

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by contributions from root material and the transfer of surface organic debris to the subsurface through bioturbation. Below these zones, early diagenetic changes possibly sug-

gest a slightly enhanced preservation of alkanes over fatty acids. Based on the end-members for the fatty acid fraction, the unsaturated and branched acids appear to be degraded most

LIPID DISTRIBUTIONS IN RECENT SALT MARSH

rapidly, while the high-molecular-weightsaturated acids are the best preserved. Their preservation appears to be most common in the muddy, reducing sediments. (3) The sandy well-drained sediments are ideal for the decomposition of organic debris. This is supported by the dominance of the bacterial and bacterially altered end-members (core 2). The greater percentage of finegrained sediments have resulted in generally reducing sediment conditions and an enhanced preservation of organic matter (cores 1, 3 and 4). Throughout much of the cores in the sediments of the reducing conditions, plant lipids dominate, because of the limits placed on the bacterial community by the reducing conditions. References Barrick, R.C., Hedges, J.I. and Peterson, M.L., 1980. Hydrocarbon geochemistry of the Puget Sound region sedimentary acyclic hydrocarbons. Geochim. Cosmochim. Acta, 44: 41-70. Boehm, P.D. and Quinn, J.G., 1978. Benthic hydrocarbons of Rhode Island Sound. Estuarine Coastal Mar. Sci., 6:471-494. Brooks, P.W., Eglinton, G., Gaskell, S.J., McHugh, D.J. Maxwell, J.R. and Philp, R.P., 1976. Lipids of Recent sediments, Part I. Straight-chain hydrocarbons and carboxylic acids of some temperate lacustrine and subtropical lagoonal/tidal flat sediments. Chem. Geol., 18: 21-38. Broosk, P.W., Eglinton, G., Gaskell, S.J., McHuge, D.J., Maxwell, J.R. and Philp, R.P., 1977. Lipids of Recent sediments, Part II. Branched and cyclic alkanes and alkanoic acids of some temperate lacustrine and subtropical lagoonal/tidal fiat sediments. Chem. Geol., 20: 189-204. Cardoso, J.N., Gaskell, S.J., Quirk, M.M. and Eglinton, G., 1983. Hydrocarbons and fatty acid distributions in Rostherne Lake sediment (England). Chem. Geol., 38: 107-128. Ember, L.M., 1985. Sources of sedimentary organic matter in Spartina-dominated salt marshes. M.Sc. Thesis, University of South Carolina, Columbia, S.C., 75 pp. Farrington, J.W. and Quinn, J.G., 1971. Fatty acid diagenesis in recent sediment from Narragansett Bay, Rhode Island. Nature (London), Phys. Sci., 230: 6769. Farrington, J.W. and Quinn, J.G., 1973. Petroleum hydrocarbons in Narragansett Bay - Survey of hydrocar-

401 bons in sediments and clams. Estuarine Coastal Mar. Sci., l: 71-79. Full, W.E., Ehrlich, R. and Bezdek, J.C., 1982. Fuzzy Qmodel: a new approach for linear unmixing. J. Math. Geol., 14: 259-270. Gaskell, S.J. and Eglinton, G., 1976. Sterols of a contemporary lacustrine sediment. Geochim. Cosmochim. Acta, 1221-1228. Grob, K. and Grob, Jr., K., 1978. Splitless injection and the solvent effect J. High Resolution Chromatogr., 1: 57-64. Hackney, C.T. and Haines, E.B., 1980. Stable carbon isotope composition of fauna and organic matter collected in a Mississippi estuary. Estuarine Coastal Mar. Sci., 10: 703-708. Haines, E.B., 1976. Stable carbon isotope ratios in the biota, solid, and tidal water of a Georgia salt marsh. Estuarine Coastal Mar. Sci., 4:609-616. Haines, E.B., 1977. The origins of detritus in Georgia salt marsh estuaries. Oikos, 29: 254-260. Han, J. and Calvin, M., 1969. Hydrocarbon distribution of algae and bacteria, and microbiological activity in sediments. Proc. Natl. Acad. Sci. U.S.A., 64: 436-443. Hatcher, P.G., Spiker, E.C., Szerenyi, N.M. and Maciel, G.E., 1983. Selective preservation and origin of petroleum-forming aquatic kerogen. Nature (London), 305: 498-501. Howarth, R.W. and Teal, J.M., 1979. Sulfate reduction in a New England salt marsh. Limnol. Oceanogr., 24: 9991013. Jeffries, H.P., 1972. Fatty-acid ecology of a tidal marsh. Limnol. Oceanogr., 17: 433-440. Johnson, R.W. and Calder, J.A., 1973. Early diagenesis of fatty acids and hydrocarbons in a salt marsh environment. Geochim. Cosmochim. Acta, 37:1943-1955. Kornder, S.C., 1986. Examination of the organic matter of Recent salt marsh sediments utilizing multivariate analysis of selected lipids. Ph.D. Thesis, University of South Carolina, Columbia, S.C., 244 pp. Kornder, S.C. and Carpenter, J.R., 1984. Application of a linear unmixing algorithm to the normal alkane patterns from recent salt marsh sediments. Org. Geochem., 7: 61-71. Lanesky, D.E., Logan, B.W., Brown, R.G. and Hine, A.C., 1979. A new approach to portable vibracoring under water and on land. J. Sediment Petrol., 49:191-207. Marinucci, A.C., 1982. Tropic importance ofSpartina alterniflora production and decomposition to the marsh ecosystem. Biol. Conserv., 22: 35-58. Miesch, A.T., 1976. Q-mode factor analysis of compositional data. Comput. Geosci., 1: 147-159. Parker, P.L., Van Baalen, C. and Maurer, L., 1967. Fatty acids in eleven species of blue-green algae, geochemical significance. Science, 155: 707-708. Perry, G.J., Volkman, J.K. and Johns, R.B., 1979. Fatty acids of bacterial origin in contemporary marine sedi-

402 ments. Geochim. Cosmochim. Acta, 43:1715-1725. Requejo, A.G. and Quinn, J.G., 1983. Geochemistry of C25 and C3o biogenic alkanes in sediments of the Narragansett Bay estuary. Geochim. Cosmochim. Acta, 47: 1075-1090. Severson, R.F., Arrendale, R.F. and Chortyk, O.T., 1980. Simple conversion of two standard gas chromatographs to all-glass capillary systems. High Resolution Chromatogr.-Capillary Chromatogr., 3: 3-7. Shaw, P.M. and Johns, R.B., 1985. Organic geochemical studies of a recent Inner Great Barrier Reef sediment,

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I. Assessment of input sources. Org. Geochem., 8: 147156. Simoneit, B.R.T., 1975. Sources of organic matter in oceanic sediments. Ph.D. Thesis, University of Bristol, Bristol, 300 pp. Tissot, B.P. and Welte, D.H., 1984. Petroleum Formation and Occurrence. Springer, Berlin, 699 pp. Waples, D., 1981. Organic Geochemistry for Exploration Geologists. Burgess, Minneapolis, Minn., 245 pp.