Compositional similarities of non-solvent extractable fatty acids from recent marine sediments deposited in differing environments

0016-7037/87/53.w + .oo

Compositional similarities of non-solvent extractable fatty acids from recent marine sediments deposited in differing environments MITSUGU NISHIMURA**~ and EARL W. BAKER Organic Geochemistry Group, College of Science, Florida Atlantic University, Boca Raton, FL 3343 I, U.S.A. (Received May 22, 1986; accepted in revisedform Febmary 24, 1987) Abstract-Five recent sediment samples from a variety of North American cominental shelves were analyzed for fatty acids (FAs) in the ~lvent~xt~c~ble (SOLEX) lipids as well as four types of non-solvent extractable (NONEX) lipids. The NONEX lipids were operationally defined by the succession of extraction procedure required to recover them. The complete procedure included (i) very mild acid treatment, (ii) I-IF digestion and (iii) saponification of the sediment residue following exhaustive solvent extraction. The distribution pattern and various compositional parameters of SOLEX FAs in the five sediments were divided into three different groups, indicating the difference of biological sources and also diagenetic factors and processes among the three groups of samples. Nevertheless, the compositions of the corresponding NONEX FAs after acid treatment were surprisingly very similar. This was also true for the remaining NONEX FA groups in the five sediment samples. The findings implied that most of the NONEX FAs reported here are derived directly from living organisms. It is also concluded that a large part of NONEX FAs are much more resistant to biodegradation than we have thought, so that they can form the large percentage of total lipids with increasing depth of water and sediments. I~RODU~~N

1977; MATSUDA and KOYAMA, 1977; LEE et al., 1977, 1979, 1980; CRANWELL,1978, 1980; ALBAIGES ef al., 1984). The possible process may include entrapment or loose linkage within sedimentary elements such as carbonate, clays and humic materials, relatively tight adsorption on or formation of chemical binding with them (e.g., MEYERS and QUINN, 1971a,b, 1973; MORRIS and CALVERT, 1975; HEDGES, 1977; SCHNITZER, 1978; LAHANN and CAMPBELL, 1980; ISHIWATARI ef al., 1980b; HARVEY et al., 1983). Another possible origin for NONEX lipids is structured constituents originally present in living organisms (e.g., GASKELLand EGLINTON, 1976; BRCXKSet al., 1976; CRAWELL, 1978, 1979; ISH~ATARI et al., 198Oa; LEE et al., 1980; FEVRIERet al., 1983). To gain further clues as to the relative importance of the two possibilities, the present investigators Carried out the first comparative survey of lipid compositions in various types of NONEX lipid fractions extracted exhaustively from five recent marine sediments from a wide variety of North American continental shelves. To this end, we applied three extraction procedures sequentially to obtain four different groups of NONEX lipids from one sediment sample. Special attention was paid to whether there are qualitative and/or quantitative relationships between SQLEX and NONEX components. This first report deals with the distribution of fatty acids fFAs) in SOLEX and NONEX lipid groups obtained from the five recent marine sediments. NIS~IMU~,

IN ORDERTO OBTAINmore Collective ~nfO~atiOn on the biogeochemistry of lipid components in recentlydeposited sediments, additional extraction procedures were applied to the sediment residue after Conventional solvent extraction. Alkaline and acidic hydrolysis procedures over relatively lengthy periods at solvent-refluxing temperature have been used by many investigators (e.g., FARRINGTONand QUINN, 197 I; GASKELL and EGLINTON, 1976; BROOKSet al., 1976; FARRINGTON et al., 1977; NISHIMURA, 1977; MATSUDA and KOYAMA, 1977; LEE ez al., 19’77, 1979, 1980; CRANWELL, 1978,1980,ISHIWATARIet al., 1980a; MEYERS et al., 1980, FEVRIER et a/., 1983; ALBAIGESef al., 1984). The application of these hydrolytic procedures to various recent aquatic sediments commonly yielded significant quantities of various lipid components, and this fact indicates that non-solvent extractable (NONEX) lipids are important as one of the major lipid sinks in recent aquatic sediments. However, still lacking is the geochemical background (viz., origin, existing form and diagenetic behavior) essential to understanding the significance of the NONEX geolipid constituents in relation to the corresponding solvent extractable (SOLEX) ones, which serve as tracer for the study of marine processes and molecular fossiIs in the interpretation of paleoenvironments. There has been controversy as to whether NONEX lipids originate diagenetically from SOLEX lipids by interaction with sedimentary elements (e.g., FARRINGTONand QUINN, 1973; FARRINGTONet al., 1977;

EXPERIMENTAL Sample

* Present address: Geochemistry Unit of Chemistry, Department of Liberal Science, Aichi Gakuin University, Iwasaki, Nisshincho, Aichi-gun, Aichi 470-01, Japan. DAuthor to whom correspondence should be addressed.

Based on geographical and geochemical data, five recent sediment samples were selected from the continental shelves surrounding the North American continent in such a way as to represent different depositional environments. i365

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M. Nishlmura and E. W. Baker

FIG. I. Locations (* marks) of samphng stations OII the North America continental shelves. For detuleci locations, see Table I.

Sampling locations for the five manne timents are shown in Fig. I, and the detailed description of sampling locations. sedimentological features and some geochemical data are listed in Table I.

Sediment samples from the Bering Sea near Ummak island were collected with gravity corer. The sample from San Nicolas basin off California was collected by piston corer as was the sediment sample from the area of the Mississippi River outlet

Marine sediment fatty acids in the Gulf of Mexico. Sediment samples from Davis Strait and Smith Sound located near Greenland were collected with a grab sampler. All of these sediment samples were frozen at the time of collection and kept frozen until analyzed. Extractionprocedures A flow sheet for the procedures used to obtain SOLEX and NONEX lipid groups is given as Fig. 2. a) Solvent-extractablelipids fSOL.EX]. After frozen sediment samples were thawed at room temperature, most of the water in the sediments was removed on a glass-f&ted funnel with suction. The partially wet sediments, all between I50 g and 300 gin dry weight, were sequentially extracted with 4UO600 mi each of acetone, two different mixtures of acetone and benzene (8:2 and 64 v/v) and then with benzene alone. Each extraction was carried out for 2 hours by shaking in an oneliter bottle with 2 cm diameter ceramic balls. The extract obtained was separated from the sediment residue on a glassfritted funnel by suction. The sediment residue was further extracted with 400 ml benzene-ethanol (7:3 v/v) using a Soxhlet apparatus until little lipid material was obtained in the extraction in a 24hour period. In a typical case, it took from 5- 10 days to remove the SOLEX lipids. During the exhaustive extraction, the solvent was changed every one or two days. All extracts were combined as SOLEX fraction. b) Non-solventextr~table iipid obtains by acidic solvent extraction~~O~~-i~. T%esolventextracted sediments (IOO200 g dry weight) were put with a mixture of benzene and ethanol (7:3 v/v), and 20-150 ml of 6 N HCl was added to the solvent until it reached pH 2-3. The sediments were then extracted by shaking as described previously. Further extractions with the same solvent mixture acidified to pH 2-3 were carried out with a l-2 hr extraction period until no significant GC peak was shown from the extracts. The extracts were then filtered with a glass-fritted funnel from the sediment residue and immediately neutralized with 2 N NaOH. Subsequently, most of the solvent was evaporated at 40-4YC under reduced pressure, and the aqueous solution was extracted three times with 150 mi diethyl ether. The sediment residue was washed with 300 ml methanol to remove most acidity and subsequently extracted with 4OQmI benzeneethanol (7:3 v/v) using a Soxhlet apparatus for 4-6 days until ne~i~ble quantities of lipid materials were extracted in a 24-hour period. During this time tbe solvent was changed everyday. The former diethyl ether extracts and the Soxhlet extracts were combined and then called herein NONEX-I. c) Non-solventextractablelipidsreleasedajier IIF digestion (NON&Y-II).After the extraction of NONEX-I fraction, the sediment residues were treated with an increasing concentration (O-30%) of HF in a Teflon vessel surrounded by ice water,

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while stirring with a Teflon stirring bar. Concentrated HF was added to the aqueous sediment solution until the addition produced no further evidence of reaction. The HF-sediment mixture was then stirred for 20 hours, washed with distilled water and, following vigorous shaking and settling, the supematant was discarded. This process was repeated until the supematant reached neutml. Most of the water in the sediment residue was removed by centrifugation, and the residue was dried in vacuum at room temperature. The pulverized residue was extracted with 400 ml benzene-ethanol (7:3 v/v) in a Soxhlet apparatus for lO- IS days at which negligible quantities of lipid materials were recovered in a 24hour period. The solvent was changed every day for the ftrst 5 days and every 2-3 days for the next 5- 10 days. The combined extracts were named NONEX-II. d) Non-solventextructablelipids released by refrwcin alkaline solution (NONEX-III). Subsequently, the sediments remaining after extraction of NONEX-II were refluxed in 0.5 N KOH-methanol-benzene (300 ml of 0.5 N KOH in 95% methanol f 180 ml benzene/30 g dry sample) with stirring by stirring rod under N2 flow for 12 hours. The solvent was replaced every 2 hours for the first 6 hours. The extract was separated by suction from the sediment residue on a glassf&ted funnel and immediately neutralized by 6 N HCl. After most solvent was evaporated from the extracts at 40-45”C under reduced pressure, the aqueous solution was extracted three times with 200 ml diethyl ether. The combined diethyl ether extracts are called herein NONEX-III. ej Another non-solventextractable tipidfraction releospd bv ret&x in alkalinesolutionINONEX-WI. The same extraction procedure as that for NONEX-III wad applied to the sediment residue, from which NONEX-I was extracted, to obtain NONEX lipids. The NONEX lipid fraction obtained was named herein NONEX-IV. Isolation,identificationand quantificationof FAs Details of these techniques are presented elsewhere (NISHand BAKER, 1986). In brief, each lipid fraction was separated into a neutral fraction and an acid fraction using a KOH-impregnated SiOz column (MCCATHY and DUTHIE, 1962). The total acid fractions were derivatised to their methyl esters by treatment with 14% 3Fs in CHPH. The crude FA methyl ester was chromat~ph~ on an alumina column. They were then analyzed by fused silica capillary gas chromatography (CC) and combined gas chromatography-mass spectrometry (CC-MS). Each complete control experiment employing the four extraction procedures for SOLEX and NONEX FAs was run by using NIH mixtures D and F (Supelco, Inc., Bellafonte, PA, USA). All the procedures gave a recovery of over 93% IMSIJRA

-i-l

Reflex in 0.5N KRi-UEUI-Benzene.

(taE+Ilf) LIPIDS

FtG. 2. Flow chart of the extraction procedures for one extractable and four non-solvent extractable lipid fmctions.

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and 84% for saturated (C,, and Cj4) and mono-unsaturated (C,, and Cjs) FAs, respectively. A blank experiment of the analytical procedures from the extraction through CC-MS showed negligible contaminants. Additionally, there does not appear to be significant transformation of n-alkanes and n-aikanois into FAs during the procedures, because of no siicant loss of them added during control experiments and of very different carbon number distributions of their NONEX or SOLEX components from NONEX FAs in the five sediments examined here (NISHIMURA and BAKER, in preparation)

Detailsof these techmques are presented elsewhere (NISHfMURA and BAKER, 1986). RESULTS

Concentrations

qfF.&

The total concentrations of FAs obtained from the five lipid groups are presented in Table i. Table 3 gives the relative abundance of NONEX FAs to the concentrations of SOLEX FAs. The amounts of saponifiable FAs in SOLEX lipids from the five sediment samples were, in ait cases, very fow relative to those of free FAs. From this, SOLEX FAs refer herein to free FAs. Total fatty acid concentrations of each NONEX group in the five sediments were consistently found to be present in significant concentrations. Some of them were almost equal to or greater than the corresponding SOLEX FAS. The concentrations of each group of the NONEX FAs show major variations between samples and did not correlate with the corresponding SOLEX FAs. This is also true for the totai concentrations of FAs in ati the NONEX groups (Tables 2 and 3).

In both the Bering Sea (BS) and San Nicolas (SN) sediment samples, the FAs were characterized by a unimodal distribution with a maximum at CZh (Fig. 3). On the contrary, both the distributions of SOLEX FAs in the Davis Strait (DS) and Smith Sound (SS) sediments were unimodal with a maximum at Cl6 and were remarkably dominated by C,, and C‘lx monounsaturated FAs. In the Gulf of Mexico (GM) sediments the SOLEX FA com~s~tion was intermediate

between the above two carbon number dist~buiion~ (Fig. 3). Thus, the distribution was bimodal with max, ima at Cl6 and C28. Furthermore, the FAs were characterized by the striking predominance of branched CIs and CIi FAs (Fig. 3). Various compositional parameters of FAs (see fable 4) were also divided into three groups corresponding to such distribution patterns of the five sediment samples. hj ,-Vi?NEA’ F.&S

Despite the very different compositions of- SOLEX FAs in the three groups of the five sediment samples. unexpectedly, the compositions of the corresponding NONEX-I FAs in the five sediment samples were quite similar to each other. Figure 4 gives the distributions of NONEX FAs in the three sediment samples ()_I:.” BS, GM and DS) in which the corresponding SOLEX FAs are strikingly different from each other, as shown in Fig. 3. The same distribution pattern was also found for the remaining samples. This group was characterized by the saturated FAs ranging from Ct4 to CrZ with a predominant of Ct6 and C1r components and minor or trace unsaturated and branched FAs. Su~~s~n~~~ this was also true for the remaining three NONEX F4 groups in the five sediment samples (Fig. 4’1. Similarities were also found in various compositional parameters for each NONEX FA group from the five samples (Table 4). The L/H ratios (see Table 4 for abbreviation) for NONEX-I, NONEX-II, NONEX-III and NONEX-IV FAs fell in a very narrow range from 1.6 to 2.3. 1.3 to 2.7. 3.3 to 4.4, and 3.5 tc~ “.!,. IZspectively (Table 4). The same trend was also observed for the Branch/LMW, E/O and LJnsatJSat. ratms fT3ble 4). DISCUSSIOh

The significant abundance of’ FAs in rhz *arious NONEX fractions recovered by different extraction

procedures indicates that FAs in marine sediments exist in various chemical forms which are not solvent-cxtractable. Before investigating the possible origin of NCINEX FAs found in the five marine surface sediments, ‘we discuss the origin of SOLEX FAs. which provides tnformation on the biological source and the diagenetic. factors and processes influencing the corn~s~tl~~ns of

Marine sediment fatty acids

co-existing organic constituents. The discussion is made by dividing the five samples into three groups (viz., BS & SN, GM and DS & SS), based on the distribution patterns of the FAs (see Fig. 3).

SOLEX 100

LLJ BS

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Environmental factors controlling SOLEX FAs compositions a) Bering Sea (BS) and San Nicolas (SN) sediments

0

lLl_L

“.

SN

LB 26

I

y. LLI2

I

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FIG. 3. Relative abundance of saturated n-fatty acids (solid line), branched fatty acids (iso- and anteiso-; dashed line) and monounsaturated n-fatty acids (dotted line) in SOLEX fractions from the five sediment samples.

(i) Organic sources. In the southeastern shelf of the Bering Sea including our sampling location, the water column rate productivity amounted to 1400 g C/m’/ yr (MOTODA and MINODA, 1974). This rate is extremely high, and thus it can be expected that the majority of FAs and other organic components in the surface sediments are of plankton origin. This consideration is further substantiated by the estimate of the balance of sedimentary material in the Bering Sea provided by LISITZIN(1969); this estimate suggested that more than 90% of the flux of sedimentary material into the sea was plankton-derived material. In fact, according to the microscopic investigation of the sediments, substantial amounts of sediment particles were composed of the aggregates of various kinds of diatom tests with a small portion of unknown fine colorless particles. This microscopic observation is in accord with the fact that plankton in the Bering Sea are mostly diatomaceous phytoplankton (LISITZIN, 1969; MoTODA and MINODA, 1974). Although detailed data on the flux of sedimentary material into the San Nicolas sampling area are not available, microscopic examination of this sediment sample showed a result similar to that of BS sample in terms of sedimentary constituents. Thus, various kinds of plankton detritus made up a considerable percentage of the sediment particles, reflecting relatively high productivity ( 1SO-250 mg C/m’/day) in the area (KOBLENTZ-MISHKEet al., 1970). A large part of the detritus disappeared by adding 3 N HCl to the sediments. This indicates that the plankton consists mainly of zooplankton with carbonate frustules like foraminifera. The relatively high CaC03 content in the BS sample (Table 1) could well be attributed to such zooplankton detritus. In order to obtain more reliable clues as to marine or terrigenous origin of the organic matter in the sediments, the isotopic ratios of carbon (6°C values) for total organic carbon (TOC) and kerogenous and humic substances were analyzed. The d13Cvalues of TOC from the BS and SN samples were -22.2% and -22.3%, respectively, while those of kerogenous materials were -20.5L and -22.2%, respectively (Table 1). This is also the case for the b13C values of humic acid (Table 1). Based on many previous reports on 613C values of organic matter in various geochemical samples (e.g., NISSENBAUM,1974; GOH et al., 1976, 1977; STUERMERef al., 1978; PETERS et al.. 1978), the above data on 6°C value also suggest that large amounts of organic matter in the BS and SN

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M. Nishimura and E. W. Baker

sediment were derived from a marine source. The 6°C of kerogenous and humic substances in a sediment core (O-35 cm) obtained from the San Nicolas basin very close to our sampling location was measured by another group (VENKATESAN et al., 1980). All the values throughout the core also indicated a marine source for the two organic geopolymers which were composed of major parts of sedimentary organic matter. Nevertheless, the compositions of FAs in the two sediment samples were predominantly of higher molecular weight (HMW: Cz4-C& FAs characteristic of terrigenous input (Fig. 3). The ratio of lower molecular weight (LMW: C1,&rB) to HMW FAs (L/H ratio) was 0.2 (Table 4) and extremely low in comparison to those found in various marine living organisms. The disagreement between the geochemical data on possible organic sources (Table 1) and the composition of SOLEX FAs in the BS and SN samples (Fig. 3) leads us to consider that most of the LMW FAs were preferentially removed relative to the HMW FAs during the early sedimentation processes at the two locations, as suggested by many investigators (e.g.. MATSUDA, 1978: KOYAMAet al., 1979; SASSEN, 1979: VOLKMANet al., 1983; DE BAAR et al., 1983; WAKEHAMet al., 1984). This means that the HMW FAs in both the BS and SN sediments might have derived principally from plankton (perhaps, diatom) and/or tenigenous sources. as discussed in the following section. (ii) Diagenetic factors and procexwr. Recently, many sediment trap experiments have been carried out to understand the vertical flux of organic detritus in the oceans (e.g., MCCAVE, 1975; HONJO and RoMAN, 1978; KNAUER et al.. 1979: PRAHL and CARPENTER, 1979; HONJO, 1980; WAKEHAMef al., 1980). The results suggest that the dominant mechanism of vertical transport is by rapid settling of large (260~) particles such as fecal pellets, marine snow, zooplankton carcasses and exuviae, phytoplankton remains and the like (e.g., MCCAVE, 1975; HONJO, 1980). The FA compositions in such particulate matter collected in traps deployed in various oceanic regimes and depths, in all cases reported, were predominantly marine LMW FAs (C14-C20) with minor to trace amounts of HMW FAs (G4-Cr8) (WAKEHAM et al., 1980, 1983, 1984; TANOUE and HANDA, 1980; DE BAAR et al.. 1983). This means that the composition of FAs at the earliest stage of their sedimentary diagenesis is of mainly a marine source (C,,-C,,). In the light of sedimentary

compositions and b13Cvalues. as discussed before, this can be also true for our sampling locations of BS and SN. Thus, the difference of FA compositions between particulate matter collected during various trap studies reported and our sediment samples from the two lo cations represents a diagenetic transformatron after deposition. The comparison between the two indicates the disappearance of most LMW FAs and the striking predominance of HMW (Cz4, Ctb and Cza) FAs ut both the sediment samples. This resuh should have heen caused by the preferential removal of marine SOLEX FAs (namely, LMW FAs) in the BS and SN sediments. MATas suggested by many investigators (.q SUDA, 1978; KOYAMA et ul., 1979; SASSER. 1979: VOLKMANet al., 1983). Recent sediment trap studies demonstrated that such a diagenetic process operates to a considerable extent even on particulate matter rapidly sinking to the ocean floor (DE BAAK P[
Marine sediment fatty acids

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M. Nishimura and

MAN er al., 1980). The relative abundance of such HMW FAs to total FAs amounts to 2.5% and the ratio of the HMW FAs to C,,:, is 0.13. Hence. the concentration appears to be significant enough to take into consideration in investigating the mechanism for the preferential removal. Another reason for the preferential removal of LMW FAs may come from the difference of tissue structure between the debris of plankton and higher plants. Unlike the bulk material of plankton, that of higher plants is composed mostly of polymerized and cross-linked structure of cellulose, lignin and other compounds. This fact suggests that the debris of higher plants is likely to be much more resistant to mechanical, chemical and biochemical degradation than that of plankton. Thus, the relatively high susceptibility of LMW FAs in plankton to biological utilization could lead to a decrease in the ratio of LMW/HMW FAs. In general, the major agents resulting in such a qualitative and quantitative modification of organic constituents after arrival at the sea bottom are micro- and macro-benthic organisms. The Bering Sea is well known as one of the finest fisheries for food benthos such as the tanner, king crab, shrimp and mollusks (e.g., ALTON, 1974). This suggests that the activities of micro-benthic organisms (mainly, bacteria and fungi), coupled with those of such macro-benthic organisms, should also have been highly vigorous. Accordingly, such depositional environments favorable to the vigorous activities of benthic organisms in the Bering Sea as well as the long period of time after burial, as implied by the sediment depth, probably brought about the great modification of organic compositions. Many kinds of bathypelagic coelentrates were found near the bottom of the San Nicolas basin (EMERY. 1960), and active bioturbation in the sediments was suggested by the very advanced age of surface sediments (EMERY and BRAY, 1962). The same conclusion was also reached by VENKATE~ANet al. ( 1980). based on the analytical results of some organic compounds in the San Nicolas surface sediment core. Judging from the radiocarbon ages measured in this region (EMERY and BRAY, 1962), it is likely that the sedimentation rate was very slow. Thus, the sedimentary organic materials should have been exposed to catabolism by various benthic organisms activities, though relatively low. for a long period of time. in conclusion, the striking predominance of HMW FAs in the BS and SN sediments might have originated mainly from the preferential removal of LMW FAs by the activities of micro- and macro-benthic fauna as well as by the long period of time after deposition.

b) Gulfofhlexico (GM) sedrmentr (i) Organic sources. In the areas west of the Mississippi River mouth near the Mississippi trough, including the sampling location of the present sediment sample, there is an upwelling of the Caribbean Current through the Yucatan Strait and consequently a very

E. W. Baker

large standing crop of plankton diatoms (BWUANW ct al., 1968). Surprisingly few, if any, diatom t’rustuies and/or debris of other decomposing plankton could be seen microscopically. This could be due tlr the dolution of such biogenic materials caused by the large charge of tenigenous sedimentary materials consisting mostly of silt and clay carried by the Mississippi River. From this, one might expect the great contribution of tenigenous organic matter. However, the 6°C values for the GM kerogen, humic acid and TOC imply a large input from a marine source to the sediments II! terms of organic contribution. This suggestion does not necessarily agree unh the compositional feature of FAs in the sediments (Fig. 3). The FAs were characterized by a bimodal distribution with maxima at C,b and Czs and the predominance of branched C15 and C,, FAs (Fig. 3). One might think that the large quantity of HMW FAs in this sample resulted from the combination of a large original input of tenigenous FAs and the preferential removal ofmatine LMW FAs. From the geographical situation and the overwhelming abundance of silt and clay in the sampling location (Table 1). however, the major source of the HMW FAs is probably from higher land plants. Large amounts of LMW FAs were also found. Judging from the remarkable predominance of branched c‘! i and Cl7 FAs, typical of some bacteria (LEO and PARKER, 1966; COOPERand BLUMER, 1968: PERRY (JIa/ I979), bacteria together with plankton could also be a major contributor to the LMW FAs in the GM se&ments. Thus, it is thought that the dominant source of the LMW FAs is bacteria and plankton. while that of the HMW FAs is higher land plants. A very srmilar result was also obtained for the biological sourer of nalkanes in the GM sediments (NISHIMURAand BAKEF.. i986). (ii) Diagenetic fhctors and processes. The FA composition (Fig. 3) is very different from that expected by the 6’%Z values for the kerogen, humic acid and TOC (Table 1). The 6°C values (-20.4%-_‘i.X%) imply that the composition of FAs which actually reached the sediment surface from the overlying water column should have been of a predominantly marine planktonic source (Cr.+&). Accordingly, it is thought that a considerable transformation of the marine FAs took place under a certain condition. The condition should have been relatively anoxic as compared to those of the remaining sediments, because the ratio o! pristane to phytane (Pr/Ph) in the GM sediments (Table I) is considerably low (DIDYK et al., 197Xj. This might be owing to the high rate of sedimentatron of clay minerals carried by the Mississippi River In the light of the large quantities of iso- and anteiso-C, Tand -Cl7 FAs, the distribution of LMW FAs in the GM sediments appears to be very much the result of bacterial working of the original algal FAs after deposition. As aforementioned, a major portion of HMW (C:?-Cq2) FAs in the sediments could be continentally denved mainly through the Mississippi River. The high relative concentration (Fig. 3) could as well result tiorr!

Marine sediment fatty acids the preferential loss of marine LMW FAs, as discussed earlier. c) Davis Strait (OS) and Smith Sound (SS) sediments (i) Organic sources. The sampling area of the DS sediments is located at the confluence of the Labrador Current and the cold down-current from the Arctic Ocean through Smith Sound. A relative high productivity in the area (KOBLENTZ-MISHKEet al., 1970) may be due to the sea-current confluence. All the values for 6t3C of the TOC, kerogen and humic acid fractions fall in the range from -20% to -22% (Table l), implying large amounts of organic matter in the sediments from a marine source. The same was true for the SS sediments. Under microscopic investigation, the DS sediment particles contained various kinds of diatom frustules, zooplankton debris and fine clay minerals. The former was somewhat more abundant than the latter two. A similar composition of sediment particles was found in the SS sediments. This suggests a large contribution of organic matter from diatom to both the sediment samples. Both the compositions of FAs in the DS and SS sediments were remarkably dominated by CL6and C18 mono-unsaturated FAs with the corresponding saturated ones (Fig. 3). This composition is basically similar to that in marine sediments with a major lipid input from diatom (VOLKMANet al., 1980). This agrees with the implication from sedimentary compositions mentioned above. Therefore, one might expect that large amounts of FAs in the DS and SS sediments originated from diatoms. However, based on the discussion in the next section on diagenesis, it is concluded that original algal FAs were almost entirely replaced by bacterial FAs through the reworking processes of certain bacteria. This conclusion is consistent with that drawn for the source of n-alkanes in the same sediment samples (NISHIMURAand BAKER, 1986). The lack of FAs ranging from CZ4to C3Z, typical of higher land plants, in the sediment samples could be due to the scarce population of land plants in the arctic areas surrounding the sampling locations. (ii) Diagenetic factors and processes. The FA compositions in the DS and SS sediments (Fig. 3) seem at a glance to be similar to those of marine algae. Together with this, the extremely high abundance of C16 and Cta mono-unsaturated FAs might be due to very little influence of diagenetic modification on algal FAs during the vertical transport to the ocean floor and after deposition. The possibility, however, is not consistent with the absence of labile poly-unsaturated FAs such as Czas and CZo:s, typical of many marine algae including diatoms, in the samples (Fig. 3), as discussed by some authors (e.g., VOLKMANet al., 1980; SMITH et al., 1983). Moreover, it is hard to consider that the remarkable predominance of mono-unsaturated FAs originated from plankton has been still preserved in the O-40 cm sediment depths with little diagenetic degradation, based on geochemical data on the fate of

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FAs during early sedimentation (e.g., PARKERand LEO, 1965; BOON et al., 1975). Particularly in the diatomaceous ooze from the Walvis Bay shelf, the relative abundance of C16:, FAs to Cr6:0 FAs has greatly decreased as compared to those of various diatOmS (BOON et al., 1975). In addition, the FA composition in the DS and SS samples is considerably different from those observed as generally common to sediment traps deployed in various oceanic regimes, as mentioned earlier. Hence, it is not likely that the DS and SS FAs have undergone little d&genetic alteration. More likely; the process responsible for producing the FA compositions with the striking predominance of Ct6:, and Cls:, is an almost entire replacement of algal FAs by bacterial reworking in the two sediments. This possibility is supported by the high degree of similarity in comparison with the sedimentary FA distribution with those reported for some marine bacteria (OLIVER and COLWELL, 1973; PERRY et al., 1979; VOLKMAN et al., 1980). Such reworking processes should have operated in oxic condition at least relative to that in the GM sediments, based on the comparison of the Pr/Ph values between the two samples. From the above discussion on organic sources and diagenetic factors and processes, the following conclusions are drawn. 1) Major biological sources of SOLEX FAs in the five sediment samples are probably quite different among at least the three sample groups (i.e., BS & SN, GM and DS & SS) as summarized in Table 5. 2) Diagenetic factors and processes determining the FA compositions might have been quite different among the three sample groups of sediments (i.e., BS & SN, GM and DS & SS) as summarized in Table 5. Such a difference based on the earlier discussion almost entirely corresponds to that divided by the ranges of various compositional parameters of FAs (Table 4) as well as Pr/Ph ratios (Table 1) in the five samples. This strongly supports the above conclusion. Compositional features

ofNONEX

FAs

Figure 4 gives the distributions of NONEX FAs in the three sediment samples (i.e., BS, GM and DS) in which the corresponding SOLEX FAs are strikingly different from each other, as shown in Fig. 3. As discussed above, biological sources and diagenetic factors and processes determining the FA compositions might have been quite different among at least the three groups of sediments. Nevertheless, the compositions of the corresponding NONEX-I FAs were surprisingly similar among the five samples (Fig. 4). This group is characterized by the saturated FAs ranging from Cl4 to CsZ with a predominance of Cl6 and Cl8 components and minor or trace unsaturated and branched FAs. Unexpectedly, the remaining three NONEX FA groups in the five samples were also strikingly similar (Fig. 4). Similarities were also found in various compositional

M. Nishimura and E. W. Baker

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ranging from CZ4to C3,, observed m the NONEX FAs from the BS, SN and GM sediment samples. despite the impressive predominance and great abundance ot the HMW FAs found in the SOLEX fractions from these samples? The same question can be raised in the cases of the branched FAs in the GM sediments and the unsaturated FAs in the DS and SS sediments. The most probable answer is that little formation of the NONEX FAs takes place at least in the sediments, as suggested previously through an incubation experiment (NISHIMURA, 1977). But it seems probable that the Possible origin of NONES FAs formation of the NONEX FAs found in the five seda) Diageneric transformation qfSOLE_Y FAs rnto iments is the result of processes briefly operative in the NONEX F.4s overlying water column. One of the possible processes in the waters is the transformation of dissolved FAs There has been controversy as to whether NONEX into NONEX FAs by chemical and/or physical interFAs originate diageneticaily from SOLEX FAs by interaction with sedimentary elements (e.g., FARRING- action with an organic and/or inorganic matrix menTON and QUINN, 197 1: NISHIMURA, 1977; MATSUDA tioned earlier. A number of investigators (e.g.. ~EFFREl~. 1970; BLUMER, 1970; STAUWER and MAC‘~~‘T>‘R~, and KOYAMA, 1977; LEE et al., 1977; FARRINGTONet 1970; TREGUER et al., 1972: PAMUS and AC’RMAN, al., 1977; VAN VLEET and QUINN, 1979; ALBAIGESer 1977; GOUTX and SALIOT, 1980) have reported that al., 1984). The possible process may include entrapconsiderable amounts of dissolved FAs ranging from ment or loose linkage within sedimentary elements Cl2 to CZ2with a maximum at Cl6 were found m eusuch as carbonate, clays and humic materials, relatively photic zones and the underlying water of various matight adsorption on or formation of chemical binding with them (MEYERSand QUINN, 197 la,b, 1973: MOR- rine environments. RIS and CALVERT, 1975: HEDGES. 1977: SCHNITZER. Another possibility for the formation of NONEX 1978; ISHIWATARIet al.. 1980b; LAHANNand CAMP- FAs in sea waters is the interaction of FAs in organic BELL, 1980). If this is the case for our NONEX FAs, particulates including plankton and also dissolved FAs during ingestion by zooplankton. This means that the one would expect a correlation of the total concentrations of any NONEX FAs with carbonate. clay mininteraction may be produced in the intestinal tract h> erals or humic materials. But no such correlation was the particulate and dissolved organic matter, and the found between carbonate contents and the NONEXNONEX FAs formed may be released as fecal pellets I FA concentration in the five sediment samples (Tables into sea waters and settle rapidly to the bottom. The same may also hold for macro-benthic orgamsms as 1 and 2). This is also true for clay minerals (compare the result of the ecological food system. the ratio of the sediment weight before and after HF However, if the NONEX FAs originated mainly digestion in Table 1 with NONEX-II FA concentrations in Table 2) and humic + kerogenous materials from such diagenetic processes in sea waters, then it is hard to explain how various parameters (especially. (see Tables I and 2). These results, however. do not diagenetic parameters) for each group of NONEX FAs necessarily rule out the possibility of diagenetic transwere so similar among the five samples (Table 3). deformation of SOLEX into NONEX FAs through inspite the very different depositional environments m teraction of FAs with such sedimentary elements, because specific kinds of carbonates. clay minerals and at least the three groups of the sampling locations (1.~’ BS & SN, GM and DS and SS). Additionally. the IIkerogenous and humic materials may play a major role in the existence of NONEX FAs. alkanes found in the same NONEX fractions revealed, Moreover, if such a transformation does take place. in most cases, a very similar distribution pattern with a remarkable even-to-odd predominance as those rewhy is there no remarkable predominance of the FAs for each NONEX FA group from the five samples (Table 4). Thus, irrespective of the difference of biological sources and also diagenetic factors in the five locations, a great similarity is noted in the distribution patterns as well as these compositional parameters in each group of NONEX FAs from the five sediments. Such great similarities provide a new, and promising clue to the origin, chemical form and diagenetic behavior of NONEX lipids. parameters

Marine sediment fatty acids

1375

ported previously by NISHIMURA and BAKER (1986) (NISHIMURAand BAKER, in preparation). Such n-alkanes are extremely rare in sea waters. Thus, it is not likely that the NONEX FAs originated mainly from certain diagenetic processes in water environments.

among the five samples (Table 4). This finding strongly indicates that a large part of the NONEX FAs is more resistant to biogeochemical degradation than we have thought. This resistance does not vary in terms of the possible chemical forms of NONEX FAs in organisms discussed above. b) Structured constituents originally present in living One of the present authors previously investigated organisms the occurrence of NONEX lipids in some organisms such as diatoms and higher land plants (NISHIMURA, A more likely origin for the NONEX lipids is struc1977). All the samples studied yielded a very small tured constituents originally present in living organamount of NONEX lipids (three orders of magnitude isms. Some authors reported that addition of acid or less than the amount of the corresponding SOLEX alkaline to algal samples prior to or during the usual lipids). This indicates that NONEX lipids in some spesolvent extraction increases significantly the yields of various lipid components including FAs (e.g., GELL- cies of organisms, the organic contributors to sediments, are extremely low in amount compared with ERMAN and SCHLENK,1965,1972; ALLENet al., 1970; GONZALES and PARKS, 1977; DUBINSKYand AARON- the corresponding SOLEX lipids. The more rapid SON, 1979; WANNIGAMAet al., 1981). This result is (preferential) degradation of SOLEX lipids, however, can leave behind a large percentage of NONEX lipids probably due to the hydrolytic release of a lipid portion during sedimentation processes. linked via ester or amide bonds to non-solvent extractIn the BS, DS and SS samples, one of the possible able complexes with polar components such as carmajor organic sources is consistently diatom, as disbohydrates, amino acids, phosphoglycerols and the like, forming cell membranes and walls (e.g., KATE& 1964; cussed earlier. Based on the compositional parameters GELLERMAN and SCHLENK, 1965; KOLATTUKUDY, (Table 2), the extent of diagenetic alteration experienced by the FAs in the BS sample appears to be con1980). The liberation of lipid molecules entrapped siderable relative to those in the DS and SS samples. within polymerized and cross-linked structures of such complexes following partial hydrolysis may also be re- One can then expect that a higher relative concentrasponsible. If such is the case for the NONEX FAs ex- tion of NONEX to SOLEX FAs in the BS sample is much higher than those in the DS and SS samples. amined here, a possible interpretation for the great This was in fact true, except for NONEX-I FAs, as similarities in compositional parameters for the NONEX FAs can be based on the following analogy. It is seen in Table 3. The same was virtually true of the remaining two samples (Table 3). This therefore implies known that the composition of FAs (mainly, Cla and CIB) forming phospholipid in the cell membrane of that even though NONEX lipids occur at an extremely low concentration in living organisms, the lipids make various animals is very similar, irrespective of the anup a large percentage of the total lipid in a sample with imal species (CHAPMAN, 1975). The same is found in increasing depth of water and sediments. In addition, cuticular plant wax polymers such as cutin and suberin (KOLATTUKUDY, 1980). By analogy, this could well if such a preferential degradation of SOLEX FAs op erates as suggested by several authors (e.g., NISHIMURA, be applied to FA portions bound to and/or entrapped 1977; MATSUDA and KOYAMA, 1977; FARRINGTON within non-solvent extractable complexes common to et al., 1977; VAN VLEET andQUINN, 1979; CRANWELL, major organic contributors (presumably, marine or198 1), then the distributions of FAs likely to be found ganisms) to the five sediments, and such FA portions in ancient geological samples will probably reflect the may be major sources of various NONEX FAs. The NONEX forms. The similarities in NONEX FAs will great difference existing between the NONEX-I, -II and -111(or -IV) FAs is probably in the kinds (or extent) of be similar even in different depositional areas as well chemical bonds and/or entrapment relating to the for- as for differing organic matter source types. From the same standpoint, NONEX FAs may also be an immation of each NONEX FA group. portant part of major lipid sinks responsible for the This will be confirmed by investigating the NONEX hydrocarbon-producing potential of sediments during lipid fractions obtained directly from living organisms. catagenesis. The investigation, however, may not be easy, because all the lipid fractions in various organisms are not always in a detectable concentration, as mentioned later. CONCLUSION Diagenetic

behavior of NONEX

FAs

As discussed earlier, the extent of diagenetic alteration experienced by SOLEX FAs during early sedimentation processes is remarkably different among at least the three groups of the five sediment samples. Nevertheless, the compositional parameters for the corresponding NONEX FAs are surprisingly similar

Despite the considerable difference in biological sources, diagenetic factors and processes at least in the three groups of the five locations, the distributional patterns as well as compositional parameters of NONEX FAs proved to be very similar in all five sediment samples. This great similarity led the authors to conclude the following as to the origin and diagenetic behavior of NONEX FAs.

M. Nishimura and E. W. Baker

1376

(1) It is unlikely that NONEX FAs were produced through a diagenetic transformation of SOLEX FAs by interacting with certain sedimentary elements. A more possible origin for NONEX FAs is structured constituents originally present in living organisms. Such NONEX FA portions in living organisms appear to be in the form bound to and/or entrapped within nonsolvent extractable complexes common to major organic contributors. (2) NONEX FAs are much more resistant to biogeochemical degradation than we have thought. As a result, NONEX FAs probably make up a large percentage of the total FAs in a sample with increasing depth of water and sediments. Acknowledgements-We thank P. Jones, K. Kvenvolden and E. E. Bray for providing sediment samples, and J. W. Farrington, J. K. Volkman and S. G. Wakeham for valuable comments on the original manuscript. We also acknowledge E. McKnight for generous technical assistance. This work was supported by the Petroleum Research Fund administered by the American Chemical Society and the Sunmark Exploration Company. Editorial handling: J. W. de Leeuw

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