Use of lipid biomarker patterns as a proxy of environmental variability in the coastal sedimentary record from the Gulf of Cádiz (SW Spain)

Organic Geochemistry 39 (2008) 958–964

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Use of lipid biomarker patterns as a proxy of environmental variability in the coastal sedimentary record from the Gulf of Cádiz (SW Spain) Laura Sánchez-Garcı´a a,*, J.-Ramón de Andrés a, J.-Antonio Martı´n-Rubı´ a, Francisco-J. González-Vila b, Oliva Polvillo b a b

´ os Rosas 23, 28003-Madrid, Spain Instituto Geológico y Minero de España, IGME, Rı ´a, CSIC, Avda. Reina Mercedes 10, 41080-Sevilla, Spain Instituto de Recursos Naturales y Agrobiologı

a r t i c l e

i n f o

Article history: Received 8 September 2007 Received in revised form 28 February 2008 Accepted 20 March 2008 Available online 1 April 2008

a b s t r a c t Surface sediments from the Gulf of Cádiz were analyzed using gas chromatography–mass spectrometry in order to estimate the origin, spatial distribution and degradation extent of the sedimentary organic matter (OM) from the inner continental shelf of the southwest Iberian Peninsula. A wide variety of lipid assemblages (n-alkanes, n-alkan-2-ones, n-aldehydes, n-fatty acids, a,x-alkanedioic acids, diterpene resin acids and isoprenoids) were extracted with organic solvent and exhibited local variations depending on OM sources. Whereas the sediments located under the Guadiana river’s influence showed dominant continental derived, long chain (>C20) distributions, typically marine-derived short chain (
1. Introduction Sedimentary organic matter (OM) contains indicators that can be used in reconstructing marine and continental palaeoenvironments. Even though OM often represents a minor fraction of sediments, it becomes a valuable source of proxies in the whole sedimentary record, since it represents a geochemical record of sedimentary OM composition and its preservation status (Meyers, 1997). Estuarine zones and continental shelfs constitute particularly interesting scenarios for studying the origin, pathways and fate * Corresponding author. Present address: Department of Applied Environmental Scinces (ITM), Stockholm University, Frescativägen 54 a, SE-10691 Stockholm, Sweden. Tel.: +46 (0)8 674 7339; fax: +46 (0)8 674 7638. E-mail address: [email protected] (L. Sánchez-Garcı´a). 0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.03.013

of sedimentary OM (Hedges and Keil, 1999), as a result of the rapid accumulation of fine sediments and subsequent sealing of these materials, which are preserved from rapid bacterial remineralization. In such characteristic systems, autochthonous and allochthonous inputs, as well as environmental information, are often reflected in the compositional features of the sedimentary OM deposited. Since the information provided by it may be skewed due to diagenetic alteration, a combination of different organic geochemical proxies may help significantly in compensating for potential bias and thereby improve palaeoenvironmental reconstruction. Whereas general sources of OM are inferred from bulk properties such as elemental or stable isotope composition, additional details on its origin can be refined by way of analysis of molecular biomarkers (Meyers, 1997).

L. Sánchez-Garcı´a et al. / Organic Geochemistry 39 (2008) 958–964

The analysis of lipid biomarker compounds has been widely employed to study OM sources, transport and fate in coastal environments (e.g., Goñi and Hedges, 1995; González-Vila et al., 2003), proving to be a valuable geochemical tool. Since little is known about the terrigenous influence on the Gulf of Cádiz inner continental shelf, this study provides a background approach to further understanding of the contribution and fate of continental derived OM in this coastal system. The main goals are to: (i) combine the use of bulk OM properties and lipid biomarkers in determining the different sources contributing to the sedimentary OM in this coastal area, (ii) evaluate the resulting continental influence in terms of lipid terrestrial indicators, (iii) supply local data for the description of the depositional environment. 2. Sampling In an effort to choose a representative site of one of the most important Iberian basin detritus outflows, a suite of 15 surface sediments (upper 20 cm) were collected from the inner continental shelf of the Gulf of Cádiz (Fig. 1), with a Shipeck dredge. Sediments in the area, composed mainly of silt–clay clastic material and a minor sand contribution (Fig. 1), are supplied to the continental shelf from two main sources: (i) the Guadiana river, which is the largest regional sediment source and (ii) the intense existing southeastward littoral drift (González et al., 2004). Samples were dried (40 °C), ground and homogenised to <0.25 mm prior to analysis.

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3. Experimental Total carbon (TC), nitrogen (TN) and sulfur (TS) contents were determined on freeze dried sediments using elemental analysis (EA) of combusted aliquots (30 mg) with an EA Eurovector. Total organic carbon (TOC) was measured on decarbonated samples (3 M HCl) and total inorganic carbon (TIC) was calculated from the difference between TC and TOC (Nieuwenhuize et al., 1994). The 13C composition was measured on decarbonated sediments using an EA Eurovector coupled to an isotope ratio mass spectrometer (Finnigan Isoprime). Similarly, a 1180 Carlo Erba interfaced directly with an isotope reatio mass spectrometer (Micromass CF-ISOCHROM, VG Instruments) was employed to estimate the 15N composition. Results are reported relative to the Pee Dee Belemnite limestone standard (d13C) and to atmospheric N2 (air) standard (d15N) in the usual parts per thousand (‰) notation. Standard deviation was better than 0.05‰ and 0.3‰ for d13C and d15N, respectively. Aliquots (200 g) of sediments were Soxhlet-extracted with CH2Cl2/MeOH (3:1, v/v) for 24 h, using activated (2 M HCl) Cu curls to remove elemental sulfur. Extracts were saponified with 0.5 mol L 1 KOH/MeOH for 2 h under reflux. Neutral and acid fractions were isolated by extraction with hexane before and after acidification to pH < 1, respectively. The acid and polar lipid fractions were methylated with trimethylsilyldiazomethane (TMSCHN2) and silylated with N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) before analysis using gas chromatography (GC; Hewlett–Packard 5730A) and gas

Fig. 1. Map showing location of sampling sites in the Gulf of Cádiz inner continental shelf (modified from González et al., 2006). Solid circles represent sediment samples.

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chromatography–mass spectrometry (GC–MS; HewlettPackard GCD). Separation of acid and neutral compounds was achieved using a SE-52 fused silica column (30 m  0.32 mm i.d., film thickness 0.25 lm). The oven temperature was programmed from 40 to 100 °C at 30 °C min 1 and to 300 °C at 6 °C min 1. He was used as carrier gas at 1.5 mL min 1. Mass spectra were obtained at 70 eV ionizing energy. Individual compounds (n-alkanes, n-alkan-2-ones, n-aldehydes, n-fatty acids, a,x-alkanedioic acids, resin acids and isoprenoids) were identified by displaying traces corresponding to selected ions characteristic for the homologous series and by comparison of mass spectra with spectra in libraries (NIST and Wiley libraries). Total lipid content was determined by gravimetry and expressed relative to TOC. 4. Results and discussion 4.1. Bulk analysis Sediment description and geochemical indicators are given in Table 1. The TOC content varies from 0.50 to 1.40 wt% (of total weight), within the typical low range for coastal sediments (Dickens et al., 2004) and close to the values reported for marine sediments from the Iberian margin of the Atlantic Ocean (0.22–2.60 wt%; Middelburg et al., 1999). TIC ranges from 0.37 to 2.91 wt% and shows enriched carbonate content in those sediments located further from the Guadiana mouth (S-7, S-8, S-9, S-13, S-14 and S-15). Although the samples had no large amount of TS (0.09–0.32 wt%), the high C/S values (C/S 6–23) and the observation of occasional authigenic pyritic remains in microcrystal form, with angular sides and without alteration rests, may be related to the presence of anoxic depositional conditions, in which sulfate reduction would have led to sulfur incorporation into the early diagenetic products (Hadas et al., 2001). The TN content ranges from 0.07 to 0.16 wt% (Table 1). The most diagnostic point relating to this element is the C/ N index (atomic TOC/TN ratio), which allows us to distinguish among potential OM origins (e.g., protein rich marine organisms, C/N = 4–10 and cellulose rich vascular land plants: C/N P 20; Meyers, 1994, 1997). Thus, the C/N values found here, mostly <10, with the exception of samples S-11 and S-12 (Table 1), seem to be attributable to a predominantly marine origin for the sedimentary OM. Nevertheless, one should be cautious with such an interpretation since selective degradation of OM components during early diagenesis and enriched proportions of clay minerals, such as those observed here (Fig. 1), have the potential to modify OM C/N values (Meyers, 1997), reducing for example original terrestrial signatures. In fact, the observed d15N values (2.9–6.1‰, Table 1) might be interpreted as a mixed contribution of both marine and terrestrial OM (estuarine plankton +8.6‰, estuarine land plants, +0.4‰; Peterson and Howarth, 1987). The mean value of 4.9‰ (Table 1) is in agreement with those reported by Meyers and Bernasconi (2005) for Mediterranean Sea bottom sediments (4– 5‰) or by Struck et al. (2001) for modern Mediterranean sediments poor in organic carbon (mean 4.4‰).

Unlike C/N and d15N measurements, d13C values are neither significantly influenced by sediment grain size nor particularly sensitive to environmental or depositional factors, making them useful indicators in reconstructing past sources of OM (Meyers, 1997). The values in Table 1 illustrate typical marine signatures (d13C 20 to 22‰; cf. Meyers, 1994) for samples S-7, S-8, S-9, S-13, S-14 and S15, whereas the rest of the sediments have slightly depleted composition ( 24 to 26‰), that indicate transition from marine to a dominantly terrestrial input. Reflecting their proximity to the coast, samples S-1 to S-6, and to a lesser extent S-10 to S-12, represent greater terrestrial contributions. 4.2. Lipid distributions Main source indicators for different lipid families are shown in Table 1. Total lipid concentration (expressed as %TOC) varies between 0.03 and 0.30%. One of the most abundant lipid constituents are the n-alkanes (Fig. 2), which occur in the C14–C35 range (Fig. 3a–c), showing an overall odd/even carbon number predominance (OEP) and a CPIALK > 1 (carbon preference index, Table 1). This index provides an approach to revealing the nature and evolution status of the OM (Allan and Douglas, 1977). The general OEP distribution of n-alkanes suggests here that the sediments have not undergone extensive diagenesis (González-Vila et al., 2003). According to the n-alkane distributions, three different patterns may are apparent, as illustrated in Fig. 3 for samples S-3, S-11 and S-15: (i) unimodal distribution with strong OEP and maximum in the range C27–C31 (Fig. 3a); (ii)
Table 1 Sample locations, descriptions and geochemical characterization (bulk indicators and lipid biomarkers; for abbreviations see text) S1

Depth (m) Dist-coast (km) TOC (wt%) TIC (wt%) TS (wt%) TN (wt%) C/N d13C (‰) d15N (‰)

13 3.61 1.01 0.81 0.16 0.12 9.8 25.9 4.9

Lipids (% TOC) n-Alkane

n-Fatty acids

n-Alkan-2-ones n-Aldehydes a b c d e f

CPIALKa TARALKb C31/C17 CPIFAMEc C16:1/C16:0 C18:1/C18:0 CPIKETd C25/C27 CPIALDe

0.3 1.6 9.4 5.5 10.5 nmf 0.13 1.7 0.7 1.1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

16 3.89 0.77 0.38 0.32 0.11 8.2 25.3 4.9

12 3.06 0.84 0.56 0.20 0.14 7.0 25.5 5.8

16 5.28 0.75 0.37 0.20 0.12 7.3 25.5 6.1

17 5.83 0.82 0.54 0.26 0.12 8.0 25.7 5.2

15 4.17 0.50 0.47 0.18 0.08 7.3 25.1 4.0

20 8.89 0.64 2.21 0.15 0.11 6.8 23.0 5.2

46 19.72 0.89 2.37 0.16 0.12 8.7 22.8 4.6

53 22.22 0.64 2.91 0.09 0.11 6.8 23.0 4.8

42 14.44 0.98 1.50 0.17 0.16 7.1 24.4 5.4

36 11.94 1.40 1.33 0.32 0.07 23.3 24.9 2.9

27 10.00 1.38 0.55 0.21 0.11 14.6 25.5 5.7

22 11.39 0.72 2.61 0.12 0.10 8.4 22.6 4.9

0.2 2.0 2.0 2.0 8.3 0.02 0.20 3.0 0.7 0.7

0.1 4.2 1670.9 525.7 6.2 0.01 0.13 2.9 0.7 0.4

0.1 1.8 0.2 <0.1 6.8 0.01 0.14 1.5 0.4 0.7

0.2 2.6 0.3 0.1 8.0 0.02 0.19 1.8 1.5 0.4

0.1 1.5 3.4 1.4 7.9 0.02 0.29 1.7 0.7 3.6

<0.1 1.9 1.0 0.6 7.3 0.03 0.15 1.8 1.1 0.4

<0.1 1.0 2.0 nmf 4.7 <0.01 <0.01 nmf nmf 1.0

<0.1 1.1 0.3 0.2 7.9 0.02 0.42 2.0 1.1 1.4

0.1 3.6 0.3 0.1 11.3 0.04 0.19 1.6 1.0 0.7

0.1 1.7 0.9 0.7 7.3 0.02 0.29 2.3 1.0 1.4

0.2 2.6 0.6 0.7 6.8 0.01 0.28 0.7 0.5 1.4

0.1 1.9 0.2 0.1 6.6 0.01 0.15 2.2 1.5 0.3

CPIALK = R[(C23 C31)odd + R(C25 C33)odd]/2  [R(C24 C32)even]. TARALK = (C27 + C29 + C31)/(C15 + C17 + C19). CPIFAME = [2*R(C12 C28)even]/[R(C11 C27)odd + R (C13 C29)odd]. CPIKET = [2  R (C19 C31)odd]/[R(C18 C30)even + R(C20 C32)even]. CPIALD = [2  R(C22 C28)even]/[R(C21 C27)odd + R(C23 C29)odd]. Not measured.

S14

S15

66 21.94 1.12 2.29 0.13 nmf nmf 23.1 4.4

42 18.89 0.69 2.62 0.13 0.13 6.2 22.8 4.8

<0.1 1.0 1.8 0.7 7.9 0.02 0.06 nmf nmf 0.9

0.1 2.4 <0.1 <0.1 9.7 0.03 0.19 1.8 1.0 0.2

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Sample

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100% 80% 60% 40% 20% 0% S-1

S-2

S-3

S-4

n-Alkanes

S-5

S-6

S-7

n-Fatty acids

S-8

S-9 S-10 S-11 S-12 S-13 S-14 S-15

Ketones

n-Aldehydes

Isoprenoids

Fig. 2. Relative abundance of biomarker families in the extracts of the 15 marine sediments.

100%

C C2929

a

C29 C 29

d

100%

C 16 C 16

S-3

10 0%

80%

80%

60%

60%

60%

40%

40%

40%

20%

20%

20%

S- 3

g

80%

C C2323

C

C 1818 C 22 C 22

0% 100%

0%

C1818 C

S-11

CC2323

b

80% 60%

C2929 C 40%

0%

C19 C 19

100%

e

S-11

100%

80%

80%

60%

60%

40%

40%

C 14 C

28 CC28

20%

100%

S-15

c

80%

18

0%

C17 C 17

100%

S-15

f

80%

S- 15

100%

i 80%

60%

60%

60%

40%

40%

40%

20%

20%

20%

C

C

C 1414

0%

0%

14 16 Pr Ph 20 22 24 26 28 30 32 34

24

20%

0%

C C1717

C 24 C

C 18 C

14

20%

0%

S- 11

h

C 1818

0%

14

16 18

20 22

24 26

28 30 3 2 3 4

9

11 13

15 i/a

16 17 in s i/a

18 1 9 in s i/a

2 0 2 2 24 2 6 2 8 3 0 3 2

n-alkanes m/z 85

n-aldehydes m/z 82

n-fatty acids

(m/z = 85)

(m/z = 82)

(m/z = 74)

Fig. 3. Relative abundance (vertical) vs. carbon distribution (horizontal) of n-alkanes (a, b, c), n-aldehydes (d, e, f) and n-fatty acids (as methyl esters; g, h, i) in the studied sediments. The abundances of all individual compounds for each sample are normalized to the most abundant homologue. Pr, pristane; Ph, phytane; i/a: iso/anteiso fatty acid; ins: monounsaturated fatty acid.

related to distinct sources such as marsh vegetation (Simoneit et al., 1984), aquatic macrophyta, microalgae (Ficken et al., 2000) or sea grasses (Jaffé et al., 2001). Thus, the bimodal S-9 n-alkane distribution with major and minor maxima at C18 and C24, respectively, suggests a mixed marine nature, combining both plankton and sea grass signatures. Similarly, samples S-10, S-11 and S-12 display

combined low and high molecular weight n-alkane distributions, denoting a mixed marine and terrigenous input. S-11 shows, for example, a triple contribution from plankton, sea grass and higher plants, according to its relative maxima at C18, C23 and C29, respectively (Fig. 3b). Additionally, two isoprenoid components [pristane (Pr) and phytane (Ph)] were found in the m/z 85

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chromatograms, although in much lower concentration than the n-alkanes (Fig. 3). These acyclic isoprenoids, derived mainly from phytol (Ishiwatari et al., 1999), have been widely used to obtain information about sediment deposition environments, due to the utility of their ratios (Pr/Ph, Pr/C17-alkane or Ph/C18-alkane) as palaeoenvironmental source indicators (Didyk et al., 1978). The low content of Pr in most of the samples produced very low Pr/Ph values (0–0.3), possibly related to anoxic conditions in the active sediment layer (Volkman and Maxwell, 1986). The observed ratios of Pr/C17 and Ph/C18, being generally lower than 1, reinforce the hypothesis of significant microbial activity. Together with the n-alkanes, the n-fatty acids constitute the major components in the extracts (Fig. 2). A suite of C9– C32 n-fatty acids was observed, with a typical EOP and a mean CPI of 7.8 (Table 1). In general, the n-fatty acids show maxima at C16, with a significant contribution from the C18, >C22 or C14 homologues (Fig. 3, m/z 74), depending on the dominant terrigenous or marine influence. Whereas shorter chain components (Fig. 3i) are major lipid components of algae, thereby being considered diagnostic of marine production (Cranwell et al., 1987), C16 acids in particular have been reported as ubiquitous in the biosphere, being was found in land plants, algae, bacteria and other microorganisms (Meyers, 1997). Longer chain C22–C30 homologues, present in several samples (S-2, S-3, S-6, S-12; e.g., Fig. 3g), might be related to the waxy coating of land plant-derived sources (Almendros et al., 1996). The ubiquitous presence of branched fatty acid isomers, with the most abundant homologues being iso-/anteisoC15/C17 (Fig. 3), has been traditionally attributed to microbial activity in sediments (Kaneda, 1991). Accordingly, significant EOP distributions of C9–C26 a,x-alkanedioic acids, which have been suggested as microbiological oxidation products of carboxylic acids (Ishiwatari and Hanya, 1975) or hydroxyacids from cutin and suberin (Cranwell, 1978), were identified in most of the samples, with a dominant maximum at C22. On the other hand, the presence of monounsaturated acids (C16:1 and C18:1; Fig. 3), although in small proportions, suggests that little drastic diagenetic alteration has occurred in the sediments (Cranwell et al., 1987), since they are considered good indicators of recent biogenesis due to their reactive and sensitive nature (Cranwell et al., 1987). Consistent with the n-alkane behaviour, the long chain n-alkanan-2-ones with an OEP, reflecting a strong contribution from higher plants were present in samples S-1, S-2, S-3 and S-6, whereas
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For the aldehydes, three different patterns with an OEP (CPIALD < 1, Table 1) could be established within the C14–C34 n-aldehydes (Fig. 3; m/z 82). Analogous to the n-alkanes, these relate to three groups of sediments: (i) dominant heavy series with maximum at C28–C29 (Fig. 3d), representative of a terrestrial input; (ii) a short chain unimodal distribution, with maximum at C17–C19 (Fig. 3f), typical of algal or microbial sources; and (iii) a combined contribution from both low and high molecular weight components (Fig. 3e), indicative of mixed autochthonous and allochthonous inputs. Since n-aldehydes have been proposed as valuable geochemical markers of the oxidation–reduction level in sediments, the significant presence of these reducing compounds (CostaNeto, 1983) in the samples (Fig. 2) may indicate prevailing reducing conditions in this sedimentary environment. The presence, although at trace level, of diterpene resin acids (dehydroabietic acid, pimaric acid, sandaracopimaric acid and some isopimaric acids), particularly in samples S1, S-2, S-3 and S-6, indicate a contribution from land resin detritus, since the major contributors are higher land plants, particularly conifers (Sukh Dev., 1989). This finding reinforces the conclusion of a greater continental influence on the sediments closest to the Guadiana river. 5. Conclusions A study of lipid assemblages, in combination with bulk geochemical indicators allowed us to classify the 15 surface sediments into three different groups. According to their near shore locations, samples S-1, S-2, S-3 and S-6, displaying important land derived resin acids, depleted d13C compositions and long chain n-alkane, n-alkan-2one and n-aldehyde distributions, reflected a greater Guadiana river derived, continental influence. The miscellaneous sedimentary OM composition observed in samples S-4, S-5, S-8, S-10, S-11, S-12 and S-14, with mixed contribution of algae, seagrass, microbial biomass and higher plants, showed a NW to SE gradient for the terrigenous input, possibly responding to the Guadiana’s fluvial plume. Apparently outside this fluvial influence, sediments S-7, S-9, S-13 and S-15 showed dominant 13C enriched marine OM signatures. The ubiquitous presence of specific bacterial biomarkers, such as iso-/anteiso- C15/C17 branched fatty acids, along with dominantly <1 Pr/C17 and Ph/C18 values, showed the presence of microbial activity. The relative abundance of unsaturated C16:1 and C18:1 acids and strongly reducing naldehydes, together with the high C/S and low Pr/Ph valuess, led us to surmise a dominance of anoxic conditions, with little drastic diagenetic alteration, characteristic of rapid sediment accumulation. Acknowledgement This research Project (REN2002-04602-C01-02) was supported by a grant from the Spanish Ministry of Education and Science (MEC) and a PhD scholarship to the first author. The authors are indebted to Dr. Gélinas, from Con-

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