Sources, transport, and partitioning of organic matter at a highly dynamic continental margin

Marine Chemistry 118 (2010) 37–55

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Marine Chemistry j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m

Sources, transport, and partitioning of organic matter at a highly dynamic continental margin Frauke Schmidt ⁎, Kai-Uwe Hinrichs, Marcus Elvert MARUM, Center for Marine Environmental Sciences, Leobener Straße, D-28359 Bremen, Germany

a r t i c l e

i n f o

Article history: Received 7 May 2009 Received in revised form 14 October 2009 Accepted 19 October 2009 Available online 27 October 2009 Keywords: Lipid biomarkers Lignin phenols BIT index Bacteriohopanepolyols Degradation ratios Galicia–Minho shelf

a b s t r a c t Continental shelves play a major role as transition zone during transport of multiply-sourced organic matter into the deep sea. In order to obtain a comprehensive understanding of the origin and fractionation processes of organic matter at the NW Iberian margin, 40 surface sediment samples were analyzed for a structurally diverse range of lipid biomarkers, lignin phenols, grain size distribution, organic carbon content (TOC), its stable carbon isotopic composition (δ13CTOC), and the organic carbon to nitrogen ratio (TOC/TN). The biomarker inventory reflected a heterogeneous mixture of organic matter from various marine and terrestrial sources. Soil- and vascular plant-derived continental organic matter, indicated by lignin phenols and plant-derived triterpenoids, was primarily associated with the silt fraction and transported by river run-off. The spatial distribution patterns of higher plant-derived waxes, long-chain n-alkanes, n-alcohols, and n-fatty acids suggested distinct different transport mechanisms and/or sources. The branched tetraether index, a molecular proxy expressing the relative abundance of branched dialkyl tetraethers vs. crenarchaeol and considered to signal soilderived organic matter, was not as sensitive as the other molecular indicators in detecting continental organic matter. Hydrodynamic sorting processes on the shelf resulted in a separation of different types of terrestrial organic matter; grass and leaf fragments and soil organic matter were preferentially transported offshore and deposited in areas of lower hydrodynamic energy. Algal lipid biomarker distributions indicated a complex community of marine plankton contributing to organic matter. Spatial and seasonal patterns of phytoplankton growth primarily controlled the distribution of algal organic matter components. The interplay of all of these processes controls production, distribution, and deposition of organic matter and results in three distinct provinces at the Galicia–Minho shelf: (I) fresh marine organic matter dominated the inner shelf region; (II) high inputs of terrestrial organic matter and high TOC content characterized the mid-shelf deposited mudbelt; (III) lower concentrations of relatively degraded organic matter with increased proportions of refractory terrestrial components dominated the outer shelf and continental slope. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Approximately 90% of the global organic carbon stored in modern marine sediments is accumulated at continental margins (Hedges and Keil, 1995). Therefore, they are key locations for organic matter (OM) burial and play an essential role in the global carbon cycle. Nevertheless, due to varying OM composition, sedimentological regimes, and geochemical processes OM accumulation, preservation, and remineralization are not well understood. Common to all continental shelves is the diversity of OM sources. Terrestrial OM from plants and soils accumulate together with marine, estuarine or even riverine OM derived from autotrophic and heterotrophic organisms. Accumulation of OM from these various sources in sediments is further influenced by transport and sinking behaviors of ⁎ Corresponding author. Present address: Helmholtz-Centre Potsdam, German Research Centre for Geosciences (GFZ), Telegrafenberg, D-14473 Potsdam, Germany. Tel.: +49 331 2882812; fax: +49 331 2881782. E-mail address: [email protected] (F. Schmidt). 0304-4203/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2009.10.003

carrier particles and differing chemical reactivity (Wang et al., 1998). Sediment remobilization and resuspension enhance the susceptibility of the OM to degradation due to extended oxygen exposure (Cowie et al., 1995; Moodley et al., 2005; Sun et al., 2002). Within the different OM fractions distinct transport behaviors and preservation potentials are observed under similar environmental conditions and consequently affect the ultimately preserved OM content and quality. For example, pre-aged terrestrial OM from plants and soils is more recalcitrant than fresh algal OM (e.g., Prahl et al., 1997). The existing studies on OM burial in continental shelf sediments (e.g., Goñi et al., 2000; Goñi et al., 2005; Kuzyk et al., 2008; Prahl, 1985; Prahl et al., 1994; Ramaswamy et al., 2008; Sampere et al., 2008; Schmidt et al., 2009; Tesi et al., 2007; Volkman et al., 2008; Xu et al., 2006; Yoshinaga et al., 2008) indicate complex mechanisms behind the OM distribution and strong variations in response to different environmental conditions at each setting. Studies with a comprehensive set of geochemical indicators are needed to obtain a deeper understanding of the diverse processes that control OM distribution and preservation in shelf

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sediments. Bulk properties of sedimentary OM such as its stable carbon isotopic composition (δ13CTOC) and total organic carbon to total nitrogen ratio (TOC/TN) were frequently used to evaluate the sources of OM (e.g., Alt-Epping et al., 2007; Hedges and Parker, 1976; Perdue and Koprivnjak, 2007; Peters et al., 1978; Ramaswamy et al., 2008). However, TOC/TN is seriously affected by the preferential remineralization of nitrogen in marine sediments or nitrogen sorption onto clay minerals (Schubert and Calvert, 2001) and δ13CTOC values of a mixture of C3 and C4 plants could mimic marine algae (e.g., Goñi et al., 1998). Furthermore, both indices cannot provide detailed information about specific OM sources. Lipid biomarkers, on the other hand, can provide such information and, to a certain extent, they allow a determination of the degradation state of OM (e.g., Eglinton and Hamilton, 1963; Poynter and Eglinton, 1990; Ten Haven et al., 1992; Wakeham et al., 1997). In order to overcome the obstacles associated with the interpretation of single OM portions in sediments of continental shelves we analyzed a broad range of lipid biomarkers and lignin phenols in surface sediments from the NW Iberian margin. Our aim was to reveal distributional and early diagenetic patterns of OM from various sources. The NW Iberian

margin with the Galicia–Minho shelf is a high-energy shelf system characterized by high primary productivity in summer due to seasonal upwelling and sediment remobilization during severe winter storms. The material is deposited in the mid-shelf area in a local mudbelt which is primarily fed by material from the Douro River and Minho River (see Fig. 1 and Araújo et al., 2002; Jouanneau et al., 2002). This manuscript links the spatial variations of source-specific biomarkers to bulk sediment properties in order to constrain the relationship between OM deposition and sedimentological processes. 2. Study area The study area is located between 41.33°N and 42.49°N latitude and 8.58°W and 9.45°W longitude at the NW Iberian margin (Fig. 1). The shelf is almost 50 km wide and bordered by sediment-free rocky outcrops in the coastal areas and along the shelf edge in the southern parts, the latter acting as sediment traps. Terrestrial sediment is mainly delivered by the Douro and the Minho River to the shelf, although the supply has been suppressed in recent years due to several dam

Fig. 1. Sampling locations at the NW Iberian margin (numbers represent the last two numbers of the station names GeoB 110..) and the local rivers (MR1 and MR2 at Minho River, CR at Cavado River, UR at Ulla River and RdV at Ría de Vigo). Color code refers to TOC content in the analyzed sediment samples. The mid-shelf mudbelt (red dashed line) and the rocky outcrops (dotted areas) are displayed according to Dias et al. (2002a). Arrows indicate the direction of the bottom currents.

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and 2-methyloctadecanoic acid. Lipid biomarkers were extracted three times with a solvent mixture of dichloromethane:methanol (DCM:MeOH, 2:1, v/v) in a microwave extraction system (MARS X, CEM) at 80 °C. The combined extracts were washed with 0.05 M potassium chloride and residual water was removed from the organic phase with sodium sulfate. The remaining solvent was evaporated under a stream of nitrogen. The total lipid extract was separated in a hexane-soluble (maltene) and insoluble (asphaltene) fraction. The maltene fraction was further separated on SPE cartridges (Supelco LC-NH2, 500 mg sorbent) into four fractions (hydrocarbons, esters and ketones, alcohols, and free fatty acids) according to the protocol by Hinrichs et al. (2000). Prior to analyses, alcohols were reacted with bis-(trimethylsilyl)trifluoroacetamide (BSTFA, Merck) in pyridine to form trimethylsilyl(TMS)-derivatives. Fatty acids (FA) were derivatized with 12% boron trifluoride in methanol (Merck) and analyzed as fatty acid methylesters. Both reactions were conducted at 70 °C for 1 h followed by the removal of solvent and reagent under a stream of nitrogen. All four fractions were stored at −20 °C in the dark until further analysis.

constructions in the rivers (Jouanneau et al., 2002; Oliveira et al., 2002; Vitorino et al., 2002). The smaller rivers along the Galicia–Minho shelf (e.g., Cavado River, Ulla River) contribute only minor amounts of sediment (Araújo et al., 2002), whereas the Rías Baixas located in the north, with islands off the coast, retain most of the sediment supplied by the rivers (Rey Salgado, 1993). The oceanographic regime of the NW Iberian margin is controlled by two current systems; the warm subtropical Eastern North Atlantic Central Waters (ENACW) from <40°N is in exchange with cold less saline subpolar ENACW (>45°N) by a poleward slope undercurrent and on the surface by the Portugal Coastal Counter Current (PCCC). The PCCC is controlled by the predominating wind system: winds from north to northeast in summer direct the PCCC to the south and favor upwelling of cold nutrient-rich subpolar ENACW at the N and NW Iberian margin (Frouin et al., 1990). The movement of these water masses southward in a subsurface front results in enhanced primary productivity of multiple algal groups at the Galicia–Minho shelf (Bode et al., 2002). During autumn and winter the PCCC shifts northwards in response to the southerly winds, creating a downwelling front (Frouin et al., 1990) that results in the deposition of sediment on the shelf. During the frequent winter storm events with increased bottom wave velocities, sediment is remobilized and transported northward by the bottom currents (Vitorino et al., 2002) and eventually deposited in the mid-shelf mudbelt (Fig. 1). Hence, the upper 10 to 15 cm of sediment on the inner shelf (above 100 m water depth) represents a mixing layer which shows a decrease in mixing frequency and depth offshore (Jouanneau et al., 2002). Although the PCCC hinders material export from the shelf in general, a transport of fine material occurs during storm events from the mid-shelf area that is subsequently eroded due to the northward slope current at the outer shelf (Vitorino et al., 2002).

An aliquot of the asphaltene fraction from each sample was prepared for bacteriohopanepolyol (BHP) analysis according to the protocol by Talbot et al. (2001). Briefly, the fraction was acetylated with acetic anhydride in pyridine (1:1, v/v; Roth) for 1 h at 50 °C and left at room temperature overnight. Afterwards, the reagents were removed under a stream of nitrogen and stored at −20 °C in the dark until analysis. Prior to analysis, each sample fraction was dissolved in MeOH:isopropanol (3:1, v/v) and spiked with an injection standard (5α-cholestane) for component quantification.

3. Material and methods

3.5. Branched and isoprenoid tetraether index

3.1. Sampling Sampling was performed with a giant box corer in August 2006 during the GALIOMAR expedition (P342) with the German RV Poseidon at the Galicia–Minho shelf (Hanebuth et al., 2007). Surface sediments were sampled from a depth interval of 0–2 cm for lipid biomarker, lignin phenol and bulk analyses. Additional sediments were collected from the river bank of three local rivers (Minho, Cavado and Ulla Rivers) and from the tidal flat of the Ría de Vigo. All samples were stored in pre-combusted brown glass bottles at −20 °C to avoid OM degradation until further preparation in the home laboratory.

For the branched and isoprenoid tetraether index (BIT) measurements, the polar fraction was isolated from the total lipid extract after the protocol by Hopmans et al. (2004). Aliquots of the total lipid extract were separated over an Al2O3 column (Merck, active basic 70–230 mesh) into an apolar and a polar fraction using hexane:DCM (9:1, v/v) and DCM: MeOH (1:1, v/v), respectively. The solvent was removed from the polar fraction, containing the isoprenoidal tetraethers, under a stream of nitrogen and the fraction redissolved in a defined volume of hexane: isopropanol (99:1, v/v), corresponding to an amount of 2 mg/mL. Prior to analysis, the fraction was filtered through a PTFE filter (pore size 0.45 µm, Roth) in order to remove small particles.

3.2. Total organic carbon, total carbon and total nitrogen

3.6. Lignin extraction

Total organic carbon (TOC), total carbon (TC) and total nitrogen (TN) concentrations were analyzed from the freeze-dried homogenized sediment. TOC was measured with a Leco CS 200. Prior to TOC measurements, 50 mg of each sample was treated with 12.5% hydrochloric acid to remove carbonates. For TN and TC analyses, 25 mg of each sample was packed in tin boats and measured on a Vario EL III Elemental Analyzer. Bulk organic carbon stable isotope analyses were carried out using an Heraeus elemental analyzer connected to a Finnigan MAT Delta Plus isotope ratio mass spectrometer. Prior to analysis, the freeze-dried samples were decalcified with 12.5% hydrochloric acid, homogenized and packed into tin boats. Standard deviation was below 0.1‰ as determined from repeated analysis of a reference sample. Values are quoted in the δ13C notation in per mil relative to the Vienna Pee Dee Belemnite (V-PDB) standard.

The protocol used for lignin extraction was adapted from Hedges and Ertel (1982). Briefly, 1–2 g of solvent-extracted sediment was filled into a microwave vessel followed by the addition of 100 mg ammonium-iron(II) sulfate hexahydrate Fe(NH4)2(SO4)2 ⁎ 6 H2O, p.a. Fluka), 1 g DCM-extracted CuO powder, and 8 mL of nitrogen-purged sodium hydroxide (8% p.a. grade, Merck) under oxygen-free conditions. The oxidation was conducted under nitrogen atmosphere in a sealed extraction vessel in MARS X at 150 °C for 3 h. After the reaction was completed, an internal standard mixture consisting of ethylvanillin and t-cinnamic acid was added to the sediment slurry. The sediment was sonicated three times with 15 mL of 16% sodium hydroxide and each liquid phase was separated from the sediment by centrifugation. The combined extracts were acidified with hydrochloric acid to pH 1 and lignin-derived phenols were extracted from the aqueous solution three times with distilled diethylether, which was treated with Fe(NH4)2(SO4)2 ⁎ 6 H2O in aqueous solution a priori. The combined extracts were dried with sodium sulfate and the solvent was removed under a stream of nitrogen. Prior to analysis by gas chromatography (GC) either coupled to mass spectrometry (MS)

3.3. Lipid extraction 30 g of wet sediment were spiked with a mixture of internal standards consisting of 5α-cholestane, nonadecan-2-one, 1-nonadecanol

3.4. Bacteriohopanepolyols

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or a flame ionization detector (FID), the phenols were reacted with BSTFA in pyridine to form TMS derivatives. In order to check for ester-bound phenols deriving from cutin and suberin, two random pre-extracted sediment samples were subjected to base hydrolysis to release ester-bound phenols. 2–3 g of sediment was heated with 25 mL of 1 M methanolic potassium hydroxide solution at 80 °C for 3 h. After cooling, 5 mL of 22% hydrochloric acid was added and the mixture was sonicated three times with 30 mL of DCM/MeOH (2:1, v/v) for 15 min. Each extract was separated from the sediment by centrifugation and 50 mL of MilliQ was added to the combined extract. Phenols were extracted from the aqueous solution according to the lignin extraction protocol reported above. The extracted, saponified sediment was subjected to CuO oxidation and the phenol composition was compared to the corresponding extracted sediment.

crenarcheol by integrating the peak areas of the [M+ H]+ ions. The BIT index was calculated according to Hopmans et al. (2004). All samples were analyzed in duplicate and the maximum deviation of individual values for the BIT index was 0.01 U.

3.7. Gas chromatography coupled to mass spectrometry or flame ionization detection

3.10. Correlation analysis, cluster analysis, and Ocean Data View

All four lipid fractions and the phenol extract were analyzed by GCMS and GC-FID. The GC (Thermo Electron Trace GC) was equipped with a 30-m Rtx-5MS fused silica capillary column (0.25 mm i.d., 0.25 µm film thickness) and helium was used as carrier gas (flow rate 1.0 mL min− 1). The lipid fractions were injected in hexane using an injection temperature of 60 °C, a temperature ramp of 10 °C min− 1 up to 150 °C followed by a temperature ramp of 4 °C min− 1 up to 320 °C (hold time: 30 min). The lignin phenol extract was injected in pyridine:BSTFA (4:1 v/v) in order to keep all carboxylic groups as trimethylsilyl derivatives. The initial temperature of 100 °C was followed by a temperature ramp of 4 °C min− 1 up to 310 C (hold time: 12 min). BHPs were analyzed on a 30-m DB-XLB fused silica capillary column (0.25 mm i.d., 0.1 µm film thickness). Helium was used as carrier gas with a flow rate of 1.0 mL min− 1 and samples were injected in methanol:isopropanol (3:1, v/v) at an initial temperature of 100 °C. A temperature ramp of 10 °C min− 1 up to 300 °C was followed by a temperature ramp of 4 °C min− 1 up to 360 °C (hold time: 24 min). All components were identified via their mass spectra and concentrations were calculated from their peak area in the FID chromatogram relative to the internal standard with the exception of BHPs which were quantified via an injection standard. Blanks for lipid and lignin extraction were prepared in parallel in the same manner as sediments to check for contaminations during sample preparation. No target components or other substances with similar retention times were detected in the blanks. Three random samples were analyzed in duplicate for lignin phenols in order to detect variations during sample preparation and analysis. Relative standard deviation was below 15% which is in the previously reported range for lignin analysis (Hedges and Ertel, 1982; Houel et al., 2006). The relative standard deviation between lignin phenols obtained from pre-extracted sediments with and without basic hydrolysis was below 13% indicating that ester-bound phenols can be neglected as an important source at the Galicia–Minho shelf. 3.8. High performance liquid chromatography Non-isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) were analyzed on an Agilent 1100 series high performance liquid chromatograph (HPLC) coupled to an Agilent 1200 MSD equipped with automatic injector and HP Chemstation software. A sample volume of 20 µL was injected on an Alltech Prevail Cyano column (150 mm × 2.1 mm; 3 µm) with hexane:isopropanol (99:1, v/v) as eluent at a flow rate of 0.2 mL min− 1. After 5 min of isocratic elution, a linear gradient to hexane:isopropanol 98.2:1.8 (v/v) within 45 min was applied followed by a linear gradient within in 2 min to hexane:isopropanol 95:5 (v/v) that was kept for 8 min. The compounds were ionized in positive APCI mode and expressed as relative abundances of non-isoprenoidal GDGTs compared to

3.9. Grain size analysis The grain size distribution was determined using a laser particle analyzer (Coulter LS 200). Prior to analysis, 0.5 g of pre-extracted sediment was mixed with aqueous sodium pyrophosphate solution (3 mg in 50 mL of water) and heated up to 100 °C in order to avoid the formation of aggregates. Analyses were performed in the range of grain sizes between 0.04 and 1000 µm in triplicate. The silt percentage was calculated based on the granulometric interval of 62.5–3.9 µm.

Correlation and cluster analyses were performed using the software PAST (version 1.75; Hammer et al., 2001). Correlation analysis was performed using Spearman's rank correlation coefficient and for 40 samples a correlation was significant if r > 0.418 for a level of confidence α = 0.01 and r > 0.325 for α = 0.05. Distribution maps were constructed in the software Ocean Data View (ODV; Schlitzer, 2002) using the diva gridding algorithm. 4. Results 4.1. Bulk properties of sediments Table 1 lists the bulk properties of the surface sediments from the NW Iberian margin. Generally, the silt content varied between 11 and 83% corresponding to a mean grain size distribution between 187 µm and 21 µm, respectively. Exceptions from this were the continental slope station GeoB 11033 (92% silt, 9.4 µm mean grain size) and the mid-shelf station GeoB 11011 (7.7% silt, 368 µm mean grain size). The small grain sizes at the continental slope indicate an accumulation of very fine grained, clay-rich sediment in this region, whereas the large grain sizes at station GeoB 11011 suggest a preferential accumulation of carbonate debris which is corroborated by a high total inorganic carbon (TIC) content of 7.3%. Apart from this, the highest silt contents (>50%) were observed in the inner to mid-shelf area (Fig. 2a) confirming the previously reported extensions of the Galicia mudbelt (Dias et al., 2002a). A decrease in the silt content was observed offshore at the southern inner shelf and close to rocky outcrops in the northern shelf area, where the sediment is mainly composed of fine to middle sand. The TOC content varied from 0.14 to 2.21% with the lowest values found at the outer shelf (Fig. 1) and was significantly correlated with the silt distribution (r =0.95) as observed in several studies before (Keil et al., 1994; Mayer, 1994). The mudbelt itself had TOC contents between 0.73 and 2.21%, which is contrasting to all other inner to mid-shelf stations that ranged between 0.28 and 0.54%. Slightly lower values were observed in the sediments from the outer shelf region (0.14–0.49%). The TOC/TN ratio was significantly correlated with TOC and silt content (r = 0.82 and 0.72, respectively), whereas δ13C of TOC was inversely correlated (r = −0.84 and −0.74, respectively). 4.2. Distributions of lipid biomarkers and lignin phenols 4.2.1. Biomarker grouping A correlation analysis was performed to reveal internal relationships. Input parameters were the concentrations of all quantified biomarkers (µg/g dry weight (dw)) and bulk sedimentary properties. In order to reduce the data set and to facilitate data interpretation, the most abundant biomarkers were combined into groups according to their sources (Table 2) and correlation coefficients (Table A1, available electronically at the Marine Chemistry website). For example, all Δ5,22-

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Table 1 Location and bulk properties of samples from the NW Iberian margin and adjacent rivers. Station (GeoB)

Location

Latitude (N)

Longitude (W)

Water depth (m)

TOC [%]

TIC [%]

δ13CTOC (‰)

TOC/TN

Silt [%]

Mean grain size [µm]

11001 11002 11003 11004 11005 11006 11007 11008 11009 11010 11011 11012 11013 11014 11015 11016 11017 11018 11019 11020 11021 11022 11023 11024 11025 11027 11028 11029 11030 11031 11032 11033 11037 11038 11039 11040 11041 11042 11043 11044 MR1 MR2 CR UR RdV

Mudbelt Mudbelt Mudbelt Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Mudbelt Mid-shelf Mudbelt Mid-shelf Outer shelf Outer shelf Mid-shelf Mudbelt Mid-shelf Outer shelf Outer shelf Cont. slope Outer shelf Cont. slope Cont. slope Mid-shelf Outer shelf Mudbelt Mudbelt Inner shelf Outer shelf Outer shelf Cont. slope Inner shelf Inner shelf Mudbelt Mid-shelf Inner shelf Mudbelt Inner shelf Mid-shelf River River River River Tidal flat

42°25 42°17 42°17 42°17 42°17 42°17 42°17 42°36 42°43 42°42 42°53 42°71 42°71 42°71 42°77 42°82 42°52 42°52 42°51 42°51 42°49 42°58 42°71 42°70 42°63 41°97 41°97 41°97 41°97 42°08 42°09 42°17 41°72 41°64 41°55 41°64 41°80 41°72 41°55 41°89 42°05 41°93 41°52 42°52 42°35

9°08 8°99 9°04 9°10 9°18 9°33 9°23 9°22 9°27 9°11 9°20 9°27 9°35 9°46 9°47 9°35 9°24 9°27 9°31 9°32 9°43 9°42 9°56 9°76 9°36 9°18 9°09 9°05 8°99 9°16 9°25 9°56 8°98 8°89 9°08 9°07 9°02 9°02 9°00 9°17 8°52 8°78 8°74 8°71 8°71

136 112 129 141 161 235 184 157 166 119 100 119 130 153 158 132 120 124 147 154 484 289 405 1823 131 137 127 115 94 149 166 1873 80 78 99 99 93 95 84 126 – – – – –

0.92 0.91 1.43 0.49 0.20 0.26 0.30 0.22 0.14 1.06 0.16 0.73 0.49 0.36 0.39 0.28 0.81 0.29 0.32 0.27 0.62 0.43 0.37 0.44 0.50 0.27 1.28 1.32 0.54 0.21 0.25 1.10 0.53 0.35 2.21 0.28 0.51 0.75 0.41 0.36 0.87 0.24 2.80 1.42 4.16

0.76 0.49 0.66 0.18 0.26 1.58 1.31 0.11 0.19 1.08 7.30 1.37 0.97 1.33 0.81 0.92 2.23 0.56 0.22 0.31 0.69 0.92 1.20 2.16 0.89 2.34 1.01 0.61 0.28 0.85 1.50 3.18 0.63 0.98 1.98 1.73 0.68 0.78 0.80 4.17 0.00 0.06 0.53 0.13 0.40

− 23.5 − 23.6 − 23.7 − 22.8 − 21.7 − 22.0 − 21.9 − 22.1 − 21.4 − 23.2 − 21.0 − 22.6 − 22.6 − 22.8 − 22.8 − 22.3 − 22.9 − 22.2 − 22.4 − 22.2 − 22.5 − 22.8 − 22.5 − 23.1 − 22.7 − 22.2 − 24.1 − 24.5 − 23.7 − 21.8 − 22.0 − 22.6 − 22.6 − 22.9 − 25.1 − 22.8 − 23.7 − 23.7 − 23.2 − 22.2 − 27.1 − 25.4 − 26.5 − 25.9 − 26.3

9.55 9.61 10.51 8.93 5.91 8.89 8.06 7.77 4.96 8.99 4.53 8.14 8.07 8.17 8.78 7.20 8.29 7.64 8.05 7.04 12.45 8.83 8.33 7.63 8.20 7.25 11.06 12.62 8.86 7.66 7.41 8.41 8.31 7.97 15.55 7.73 11.02 10.08 9.94 8.36 17.27 6.47 9.76 11.33 12.81

69.2 63.7 83.0 48.7 17.7 22.5 22.7 17.6 10.6 69.6 7.7 53.2 46.1 33.7 40.5 22.7 61.5 31.6 32.4 28.7 45.2 35.0 30.1 41.1 44.2 21.1 76.4 69.9 37.4 17.7 25.2 92.0 44.0 27.2 57.9 20.5 31.1 52.8 30.3 30.2 – – – – –

28.8 34.6 21.4 59.4 122.8 77.6 73.7 145.1 187.3 27.6 367.9 41.3 53.5 73.3 55.8 84.0 48.0 83.5 88.6 104.2 44.7 77.6 66.8 47.4 53.5 103.2 26.0 32.9 58.4 90.6 72.0 9.4 55.9 77.9 45.4 120.7 76.3 47.6 70.7 92.6 – – – – –

Mudbelt samples are indicated by a silt content > 50%, whereas the remaining zonation corresponds to water depth.

Fig. 2. a) Silt content (grain size interval of 62.5–3.9 µm), b) δ13CTOC, and c) TOC/TN ratio at the NW Iberian margin. Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, and 100 m) and station numbers correspond to the last two number of GeoB station codes (cf. Table 1).

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Table 2 Biomarker grouping, internal correlation, and main sources of single biomarkers at the NW Iberian margin. Group/biomarker Components

Correlation

Main source (reference)

Odd numbered C25–C35 n-alkanes

r > 0.72

Monocyclic C25-triene, C25-triene 1, C25-triene 2

r > 0.57

Alkenones LCOH Phytol

C37:2/37:3 Alkenone Even-numbered n-alcohols with a chain length of C22–C32

r = 0.98 r > 0.62

Sterols Dinosterol 24-Methylenecholestadienol Alkyl-diols

27Δ5,22, 27Δ5, 28Δ5,22, 28Δ5, 29Δ5,22, 29Δ5

r > 0.80

Epicuticular plant waxes (Eglinton and Hamilton, 1967) Coccolithophoridae, e.g., Emiliana huxleyi (Volkman et al., 1980) Diatoms from the genus Rhizosolenia, Haslea, Navicula, Pleurosigma (Belt et al., 2000, 2001; Massé et al., 2004) Coccolithophoridae, e.g., Emiliana huxleyi (Conte et al., 1994) Epicuticular plant waxes (Eglinton and Hamilton, 1967) Ester-linked side-chain of chlorophyll-a, degradation of chlorophyll (Johns et al., 1980; Sun et al., 1998) Phytoplankton (Review by Volkman, 2003; Volkman et al., 1998) Dinoflagellates (Boon et al., 1979) Abundant in diatoms (Volkman, 1986 and references therein)

LCalk C37:2-Alkene HBI

28Δ5,24(28) 1,15-C30:1-, 1,14-C30:0-, 1,15-C30:0-Alkyl-diols and 12-hydroxy-C28 -fatty acid Taraxerol, α-amyrin, lupeol

r > 0.75

Eustigmatophytes (Volkman et al., 1999)

r > 0.87

Angiosperm resin

Taraxerone, α-amyrone, lupenone

r > 0.88

Oxidation of triterpenols (Medeiros and Simoneit, 2008)

Even numbered n-FA with a chain length of C22–C32

r > 0.70

Sat.SCFA MUFA PUFA

Saturated C14–C16-fatty acid C16:1ω5-, C16:1ω7-, C18:1ω7-, C18:1ω9-Fatty acid Polyunsaturated fatty acids including C20:4FA, C20:5FA

r > 0.83 r > 0.79 r = 0.81

C18:1ω9-FA C18:1ω7-FA BrFA

i- + ai-C15-, i- + ai-C17, 10-Me-C16-Fatty acid

r > 0.88

Plant-derived triterpenols Plant-derived triterpenones Friedelin LCFA

Angiosperms Epicuticular pant waxes, minor source: microalgae, bacteria (Eglinton and Hamilton, 1967) Unspecific marine (Cranwell, 1982) Alteration (Cranwell et al., 1987) Fresh phytoplankton biomass, mainly from diatoms (Canuel and Martens, 1993; Shaw and Johns, 1985) Zooplankton (Sargent, 1976) Bacteria (Cranwell, 1982) Bacteria (sulfate reducers) (Edlund et al., 1985; Kaneda, 1991; Taylor and Parkes, 1983) Bacteria (Rütters et al., 2001) a) Soil (Prahl, 1985; Prahl et al., 1992) b) Marine OM (Elvert et al., 2001; Venkatesan, 1988) Degradation product of highly functionalized biohopanoids (Innes et al., 1997; Winkler et al., 2001) Degradation product of highly functionalized biohopanoids (Innes et al., 1997; Ourisson and Rohmer, 1992) Bacteria (Ourisson and Rohmer, 1982; Rohmer et al., 1980) Soil (Gordon and Goñi, 2003; Prahl et al., 1994) Vascular plants

C16:0-MAGE Diploptene

C16:0-Mono-O-alkyl glycerol ether

C32-HOH

C30-Hopanol, C32-bishomohopanol

C32-ββ-HA

C32-ββ-Hopanoic acid

BHPs 3,5-Bd/V Lignin

32, 34-Anhydrobacterio-hopanetetrol, bacteriahopanetetrol r > 0.81 3,5-Dihydroxybenzoic acid to vanillyl Vanillyl, syringyl, cinnamyl, p-benzoic phenols r > 0.50 r > 0.70 (for S, V, P) Acetovanillone, vanillin, vanillic acid r = 0.87 Woody gymnosperms (Hedges and Mann, 1979) Acetosyringone, syringealdehyde, syringic acid r = 0.57 Woody angiosperms (Hedges and Mann, 1979) p-Coumaric acid, ferulic acid r = 0.52 Non-woody material (Hedges and Mann, 1979)

Vanillyl phenols Syringyl phenols Cinnamyl phenols p-Hydroxyl phenols

p-Benzophenone, p-benzaldehyde, p-benzoic acid

r = 0.80

n. c.

Lignin (Hedges et al., 1988)

n. c. — no correlation. Correlation indices of syringyl and vanillyl phenols refer to the phenolic and the acidic oxidation product.

and Δ5-sterols in the range of C27 to C29 were correlated (r > 0.80) and therefore combined as indicator of fresh marine biomass (sterols). Due to specific source assignments, sterol biomarkers such as dinosterol (dinoflagellates), 24-methylenecholestadienol (diatoms), the C37:2alkene (coccolithophorids) or the triterpenoid friedelin (angiosperms) were considered individually. All bulk parameters indicating terrestrial input (δ13CTOC, TOC/TN, TOC content, and silt content) generally agree well with the spatial distribution of lignins, BIT index, long-chain C25 to C35 n-alkanes with high odd-over-even carbon number predominance (LCalk), even-numbered long-chain n-alcohols with a chain length of 22 to 32 carbon atoms (LCOH), C16:0-mono-O-alkyl glycerol ether (C16:0MAGE) and to a lesser extent the amount of plant-derived triterpenols, p-hydroxyl phenols and even-numbered long-chain n-fatty acids with a chain length of 22 to 32 carbon atoms (LCFA). Sterols, short-chain nalcohols (SCOH including C16:0- and C16:1-alcohol) and the C18-FA are inversely correlated with δ13CTOC. Several specific terrestrial biomarkers (LCalk, LCOH, LCFA, plantderived triterpenoids, and lignin) as well as C16:0-MAGE and the ubi-

quitous SCOH and C18-FA correlated with the TOC content in the sediment. In contrast, the ratio of 3,5-dihydroxybenzoic acid to vanillyl phenols (3,5Bd/V), the acid to aldehyde ratio of vanillyl ((Ad/Al)V) and the ratio of phydroxyl phenols to syringyl and vanillyl phenols (P/(S+V)) increased with decreasing TOC. Marine lipid biomarkers were not related to TOC in the sediment. Similar patterns were observed in the relationship to the grain size distribution, i.e., most terrestrial biomarkers increased with increasing silt content, whereas marine biomarkers did not correlate with the silt content. LCalk, lignin, and the BIT index were correlated with TOC/ TN ratio, while lignin degradation ratios ((Ad/Al)V, P/(S+V)) were inversely correlated with TOC/TN ratios. 4.2.2. Variation of biomarker distributions between samples Biomarker concentrations were generally highest in the mudbelt samples and decreased with increasing distance from the source rivers along the inner and mid-shelf area to the outer shelf and the continental slope (Table 3). Fig. 3 shows the biomarker distribution of a typical mudbelt sample (GeoB 11002). Highest concentrations of up to 190 µg/g

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

dw were observed for lignin phenols in the southern parts of the mudbelt at station GeoB 11039 (Table 3). In general, concentrations of fatty acids and sterols, both absolute and relative to other biomarkers, were highest in the southern regions, with maximum values of 180 µg/g dw and 33 µg/g dw for short chain fatty acids (SCFA) and sterols, respectively. Both compound groups showed the highest concentration at location GeoB 11042, indicating a strong input of algal OM. LCFA and LCOH yielded concentrations of 1.7 to 11 µg/g dw and 3.1 to 12 µg/g dw in the mudbelt. LCalk was in the same concentration range, maximizing at n-C31 (Fig. 2a). Plant-derived triterpenoids such as α-amyrin, lupeol, taraxerol, their corresponding ketone derivatives, and friedelin were present in even lower concentrations than the plant wax components (LCalk, LCOH, and LCFA), whereas bacteriohopanetetrol (BHT) and anhydrobacteriohopanetetrol (anhydro-BHT) varied strongly in a concentration range from 2.1 to 17 µg/g dw. 4.2.3. Terrestrial biomarker input Within the group of plant wax components, LCFA diverged slightly in the southern parts of the inner and mid-shelf from the distribution of LCalk and LCOH, i.e., showing a relatively balanced distribution (Fig. 4, Table 3), whereas LCalk and LCOH were primarily accumulated at stations GeoB 11039 and 11028. Plant wax constituents were increased at the continental slope with contents of up to 15 µg/g dw. At the northern outer shelf, LCOH showed high values comparable to the nearby mid-shelf area. Long-chain n-alkanes with a chain length of 36 to 40 carbon atoms (n-C36–40) but no carbon number predominance (e.g., Fig. 3a) were observed in all samples. Their spatial distribution diverged only slightly from the plant wax pattern and thus these compounds may be derived from the same sources and/or transported by similar mechanisms. p-hydroxyl phenol concentrations ranged between 2.4 and 37 µg/g dw with the highest amounts found at the northern shelf and were significantly correlated with vanillyl and syringyl phenols (r = 0.831), suggesting an origin of the p-hydroxyl phenols from lignin. The sum of all lignin phenols reached the highest concentration in the southernmost sample GeoB 11039 (190 µg/g dw), followed by station GeoB 11025 (150 µg/g dw) at the northern shelf and samples from the southern mudbelt with concentrations between 110 and 140 µg/g dw (GeoB 11042, 11028 and 11029). The individual phenol groups of vanillyl, syringyl, p-hydroxyl and cinnamyl phenols generally confirmed the partitioning of total lignin phenols, except for the vanillyl phenols at station GeoB 11025. The relative contributions of single lignin phenol groups reflect different types of plant tissue (Table 2; Hedges and Mann, 1979). The ratio of syringyl to vanillyl phenols (S/V) and cinnamyl to vanillyl phenols (C/V) reached rather low values of 0.31 to 0.81 and 0.02 to 0.35 in the sediments of the southern to central inner and mid-shelf (Table 4). Both ratios displayed a general trend to higher values at the outer shelf and in the northern part of the inner and mid-shelf area, with a particular enrichment at the central continental slope station GeoB 11033. The ratio of 3,5-Bd/V, a lignin-based indicator for soil input (Gordon and Goñi, 2003; Prahl et al., 1994), yielded low values in the mudbelt and increased at the outer shelf (GeoB 11020, 11022, 11032, and 11007) and the northern shelf region (e.g., GeoB 11025, and 11016; Fig. 5a). By contrast, the BIT index, proposed as relative marker for terrestrial input (Hopmans et al., 2004) and more recently for soil input (Huguet et al., 2007; Walsh et al., 2008; Weijers et al., 2009) showed only small variations at the Galicia–Minho shelf ranging between 0.04 in the northern outcrop and mudbelt area and 0.10 in the central and southern mudbelt (Fig. 5b, Table 4). Plant-derived triterpenoids varied strongly in the inner shelf and mudbelt samples (0.54 to 6.6 µg/g dw) with enhanced concentrations at stations GeoB 11039 and 11037 in the south and at station GeoB 11010 in the central mudbelt. Friedelin was less abundant and showed an accumulation at the mudbelt stations GeoB 11039 and 11003. Generally, these terrestrial biomarkers showed a decrease at the outer shelf and the continental slope.

43

4.2.4. Marine biomarker input Fig. 6 shows the distribution of lipid biomarkers from marine algae. Dinosterol is widely distributed in the sediments of the mudbelt and inner/mid-shelf area with the highest concentration of 15 µg/g dw at the southern inner shelf station GeoB 11037. Similar distributional patterns were observed for other algal markers such as the alkyl-diols, the sterol group, and 24-methylenecholestadienol. Marine biomarkers decreased offshore with the exception of the central outer shelf station GeoB 11032. Highly branched isoprenoids (HBIs) showed the highest concentration of up to 1.4 µg/g dw in the northern parts of the shelf (GeoB 11016), regardless of grain size distribution or TOC content in the sediment (Fig. 6a). Areas of elevated accumulation of alkenones and the C37:2alkene appeared to be randomly spread over the whole shelf and exhibited the highest concentrations of 1.7 µg/g dw at the central mudbelt station GeoB 11003. SCOH were rather accumulated in the central mudbelt (1.2 to 2.4 µg/g dw) and showed only low values in samples close to outcrops. Polyunsaturated fatty acids (PUFA) and monounsaturated short chain fatty acids (MUFA) showed in general elevated values in the inner/mid-shelf area and decreased offshore. Remarkably high concentrations of bacterial biomarkers (up to 33 µg/g dw) were found for the branched fatty acids (BrFA including i-/ai-C15 and -C17-FAs and 10-methyl-C16-FA), C16:0-MAGE, and C18:1ω7-FA at the southern mudbelt station GeoB 11042 and the northern inner shelf station GeoB 11011. In contrast, BHPs as another indicator of bacterial input showed very high concentrations (up to 38 µg/g dw) at other locations such as the inner/mid-shelf stations GeoB 11016 and 11030 and the central outer shelf at station GeoB 11008 (Table 3). Diploptene and other hopanoids are elevated in several mudbelt samples (GeoB11012, 11028, and 11039) and decline at the outer shelf and continental slope. The general distribution of terrestrial OM vs. aquatic OM is shown in the ratio of SCFA/LCFA (Table 4); the highest values of 15 to 20 were observed at the southern shelf at stations GeoB 11041, 11042, and 11043. In the central outer shelf SCFA was elevated, whereas the northern offshore regions were rather dominated by LCFA. 4.3. Inventory of lipid biomarkers and lignin phenols in the local rivers River bank sediments from three local rivers (Minho, Cavado, and Ulla River) and sediment samples from the tidal flat of the Ría de Vigo (RdV) were analyzed as a reference for river- and land-derived OM input (see Fig. 1 for sampling locations). TOC ranged between 0.24% in the Minho River (MR2) and 4.16% in the RdV sample (Table 1). TOC/ TN and δ13CTOC reflected terrestrial values of 9.8 to 17.3 and −27.1 to − 25.4‰, respectively. The most abundant lipid biomarkers in the river bank sediments were saturated SCFA with highest concentrations of 57 and 76 µg/g dw in MR1 and the Cavado River (CR), respectively. Sterols comprised the second most abundant compound group and were dominated by β-sitosterol. Dinosterol was present in all three river samples in concentrations up to 14 µg/g dw, indicating a migration of dinoflagellates into the Galician rivers, probably in conjunction with red tides (e.g., Figueiras and Ríos, 1993). HBIs and several other algal biomarkers such as alkenones and C37:2 alkenes were either observed at low concentrations or below the detection limit. The BIT index was in the range of 0.72 at MR1 and 0.96 at the Ulla River (UR) indicating a strong impact of soil OM (Table 4). Lignin concentrations in the rivers of Galicia varied from 16 µg/g dw at station MR2 to 110 µg/g dw at the CR. An even higher amount of up to 420 µg/g dw was obtained from the tidal flat sediments of the RdV. 3,5-Bd varied strongly between the samples from 1.8 µg/g dw (MR2) to 13 µg/g dw (UR). Consequently, the 3,5-Bd/V ratio was as low as 0.25 and therefore below typical shelf values (Table 4). Degradation products of BHPs were only present in minor amounts, with C32-ββhopanoic acid (C32-ββ-HA) reaching the highest concentrations of 1.7 µg/g dw at station MR2, whereas total BHPs varied strongly

44

Table 3 Concentrations of lipid biomarkers and lignin-derived phenols (in µg/g dw) in all samples from the mudbelt, inner and mid-shelf, outer shelf and continental slope. Station (GeoB)

n-C36-40

C37:2-Alkene

HBIs

Alkenones

SCOH

LCOH

Phytol

Sterols

2.01 3.31 3.47 5.50 3.63 1.71 5.61 4.16 5.09 3.06

0.16 0.32 0.56 1.69 0.76 0.43 0.78 0.38 0.32 0.30

0.17 0.18 0.15 0.59 0.32 0.14 0.27 0.27 0.65 0.52

0.75 0.82 0.77 1.98 1.90 0.72 1.02 0.78 0.69 0.68

0.19 0.15 1.74 0.80 0.71 0.15 0.39 0.22 0.86 0.70

1.15 2.44 1.44 0.74 0.79 0.53 0.41 2.00 0.76 0.86

3.06 5.71 6.81 12.25 7.20 6.25 9.43 8.33 10.45 7.30

0.25 0.70 0.60 1.65 0.71 0.31 0.41 0.61 0.58 0.69

11.64 17.15 9.29 32.25 17.20 9.12 16.25 12.10 23.92 32.52

Inner/mid-shelf 11011 0.90 11013 2.20 11016 3.11 11018 2.54 11025 1.67 11030 2.74 11037 5.08 11038 2.70 11040 1.42 11041 1.79 11043 1.55 11044 2.09

0.06 0.64 0.80 0.48 0.39 0.18 0.76 0.26 0.14 0.13 0.14 0.30

0.10 0.17 0.59 0.28 0.16 0.30 0.40 0.44 0.32 0.13 0.21 0.22

0.52 0.94 1.38 1.25 0.70 0.64 1.09 0.55 0.41 0.57 0.32 0.67

0.20 0.32 0.74 0.39 0.44 0.53 0.44 0.71 0.19 0.21 0.35 0.55

0.27 1.26 0.58 0.31 0.93 0.61 1.60 0.44 0.30 0.37 0.50 0.35

2.73 5.39 5.54 3.67 6.64 7.61 10.80 8.78 4.75 3.60 4.11 4.74

0.24 0.39 1.67 0.84 1.99 0.46 0.83 0.55 0.04 0.36 0.28 0.39

Outer shelf 11004 11005 11006 11007 11008 11009 11014 11015 11019 11020 11022 11027 11031 11032

1.63 1.25 1.79 1.41 1.55 1.27 1.78 1.68 2.26 1.82 1.99 1.81 1.32 1.38

0.19 0.18 0.29 0.26 0.32 0.21 0.29 0.28 0.47 0.44 0.39 0.39 0.20 0.30

0.12 0.19 0.17 0.23 0.20 0.17 0.25 0.21 0.30 0.18 0.18 0.25 0.18 0.38

0.45 0.40 0.56 0.39 0.35 0.29 0.77 0.73 0.79 0.69 0.69 0.80 0.55 0.54

0.24 0.33 0.30 0.50 0.39 0.32 0.28 0.35 0.37 0.26 0.48 0.26 0.51 0.36

0.77 0.68 0.14 0.22 0.21 0.27 0.38 0.19 0.84 0.45 0.33 0.39 0.30 0.48

4.45 2.52 0.27 4.81 4.17 3.41 5.69 2.36 7.11 4.11 7.05 4.61 3.26 5.62

Continental 11021 11023 11024 11033

slope 3.40 0.91 1.61 2.84

1.35 0.09 0.40 1.95

0.44 0.07 0.11 0.16

0.38 0.12 0.07 0.07

0.34 0.18 0.22 0.22

0.67 0.32 0.20 1.05

6.03 1.99 3.17 3.95

Mudbelt 11001 11002 11003 11010 11012 11017 11028 11029 11039 11042

Dinosterol

24-Methylenecholestadienol

Alkyl-diols

Plant-derived triterpenoids

Friedelin

LCFA

SCFA

2.92 2.63 1.55 7.99 2.67 4.58 2.97 2.12 4.08 3.45

1.71 2.32 1.07 2.90 2.14 3.59 1.31 1.80 2.59 4.26

4.26 5.56 5.24 13.12 7.51 6.28 5.36 5.51 10.06 16.92

1.76 1.77 1.07 4.99 1.56 2.84 4.45 2.02 5.50 2.55

0.06 0.10 0.47 0.06 0.04 0.01 0.18 0.12 0.39 0.11

1.73 4.21 8.22 11.33 5.01 3.74 6.60 7.94 8.17 6.28

16.58 27.29 20.35 66.96 25.34 26.68 47.95 33.30 30.02 176.35

9.36 8.55 16.14 9.75 15.13 23.21 54.05 21.94 11.37 12.11 13.30 10.94

0.99 1.29 2.35 1.94 3.29 3.67 14.87 3.50 3.07 1.62 1.46 2.43

1.04 1.01 1.47 0.88 2.36 3.17 9.09 1.86 1.05 1.32 1.07 0.68

4.17 4.23 10.41 3.69 8.76 13.60 18.70 11.46 7.95 3.71 3.42 3.99

0.56 0.78 1.36 1.22 2.25 2.60 6.69 2.89 1.96 1.20 1.03 0.98

0.01 0.08 0.04 0.04 0.10 0.17 0.03 0.07 0.03 0.04 0.07 0.03

5.08 2.36 7.04 4.47 3.72 6.99 7.77 4.93 6.07 5.54 3.40 5.26

85.86 14.19 53.99 31.33 49.82 80.73 99.98 50.04 50.31 90.51 56.04 25.77

0.19 0.23 0.02 0.20 0.21 0.68 0.24 0.21 0.32 0.14 0.16 0.26 0.24 0.35

10.53 7.28 1.09 6.35 6.26 9.48 6.82 5.36 18.96 8.18 12.77 11.21 6.51 25.90

2.37 1.16 0.12 1.45 1.59 1.15 1.47 0.83 3.78 5.39 3.09 2.72 1.06 3.15

1.57 1.01 0.06 1.35 0.59 0.87 0.71 0.71 1.57 3.43 1.88 1.83 1.31 5.90

2.37 1.16 0.12 1.45 1.59 1.15 1.47 0.83 3.78 5.39 3.09 2.72 1.06 3.15

1.24 0.74 0.06 0.94 1.04 0.63 0.71 0.40 2.26 3.25 1.53 1.42 0.46 1.82

0.05 0.06 0.01 0.05 0.02 0.01 0.09 0.06 0.06 0.02 0.03 0.07 0.02 0.01

5.93 5.09 0.59 3.57 2.27 4.20 5.52 1.77 4.50 2.65 1.39 2.69 2.53 2.66

29.54 27.16 3.48 25.42 20.87 23.18 11.57 18.53 17.17 26.16 11.03 29.38 25.41 47.33

1.42 0.06 0.08 0.08

10.73 2.31 4.98 7.31

2.53 0.46 0.70 0.88

0.71 0.30 0.46 0.92

2.53 0.46 0.70 0.88

1.44 0.24 0.27 0.61

0.01 0.02 0.00 0.02

5.82 1.98 3.75 2.89

26.23 6.33 5.53 17.71

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

LCalk

Station (GeoB)

MUFA

PUFA

C18FA

C18:1ω9FA

C18:1ω7FA

BrFA

C16:0MAGE

Diploptene

C32HOH

C32-ββHA

4.84 6.93 5.50 21.28 4.40 7.09 11.63 8.97 6.63 54.77

6.13 11.63 9.12 27.45 14.82 11.74 22.27 14.23 16.59 79.17

0.05 0.32 1.42 2.33 0.94 0.69 0.92 1.11 1.62 0.72

2.17 4.92 1.22 2.83 2.85 1.89 2.44 2.35 2.25 3.90

1.00 2.18 1.27 3.15 2.63 1.66 2.91 1.94 2.71 6.27

2.02 4.17 2.63 9.20 7.46 3.49 7.60 4.90 7.89 19.90

3.33 2.88 3.90 13.71 2.76 5.06 10.46 6.86 3.79 32.87

0.93 1.80 0.80 1.47 0.94 0.37 0.87 1.13 0.97 2.00

0.14 0.15 0.14 0.11 0.24 0.11 0.28 0.23 0.28 0.19

1.23 1.01 0.52 2.72 1.48 1.82 1.48 1.10 1.65 1.29

0.54 0.66 0.44 1.14 1.31 0.61 0.90 0.74 0.92 2.02

Inner/mid-shelf 11011 22.82 11013 3.47 11016 16.22 11018 11.04 11025 13.70 11030 24.85 11037 27.83 11038 13.48 11040 14.46 11041 27.39 11043 16.48 11044 6.74

34.75 5.74 23.96 11.19 23.03 35.00 50.02 22.91 23.98 40.10 25.71 11.29

0.35 0.06 1.59 0.30 0.23 0.44 1.28 0.34 2.55 0.68 0.52 0.86

1.80 2.41 2.32 1.32 1.01 2.15 4.04 1.46 2.56 6.43 2.98 1.90

3.55 1.29 2.99 1.21 2.09 2.60 6.37 2.46 3.01 6.41 3.41 1.55

10.85 1.83 8.32 3.49 5.78 8.06 15.82 7.29 7.09 12.22 7.55 4.07

23.61 2.10 10.33 7.05 10.81 17.02 15.88 10.89 8.29 14.58 9.83 5.25

0.52 0.75 1.03 0.71 0.58 1.29 0.57 2.14 0.56 0.74 0.98 0.56

0.09 0.16 0.22 0.12 0.12 0.18 0.18 0.15 0.11 0.19 0.09 0.15

0.58 0.81 1.03 0.79 1.42 1.22 2.25 1.21 1.13 0.61 0.60 1.08

Outer shelf 11004 11005 11006 11007 11008 11009 11014 11015 11019 11020 11022 11027 11031 11032

8.53 8.16 0.78 5.48 5.35 7.72 3.21 4.16 5.11 6.19 2.58 6.92 6.54 11.51

12.04 9.74 1.42 10.56 9.16 8.66 4.40 8.85 5.72 11.50 4.73 12.59 10.65 24.45

0.65 0.34 0.05 0.27 0.17 0.62 0.21 0.01 0.24 0.43 0.10 0.28 0.19 0.64

1.37 0.48 0.30 1.69 1.28 1.52 1.66 1.77 0.66 1.91 0.95 0.61 1.61 1.58

1.24 0.59 0.21 1.47 1.45 1.43 0.70 1.74 0.49 1.70 0.87 1.47 1.71 3.62

3.28 1.39 0.44 3.68 2.96 2.79 1.34 2.74 1.58 3.35 1.55 3.75 3.82 8.27

6.69 7.93 0.88 5.72 4.55 4.52 2.02 3.23 5.30 5.89 2.48 8.30 5.97 8.55

0.45 0.39 0.05 0.45 0.40 0.48 0.14 0.27 0.44 0.62 0.78 0.68 0.55 0.76

0.11 0.11 0.50 0.15 0.13 0.08 0.19 0.19 0.17 0.14 0.16 0.18 0.13 0.16

Continental slope 11021 8.42 11023 2.01 11024 1.36 11033 4.24

9.35 1.65 2.05 8.71

0.35 0.00 0.00 0.05

1.02 1.50 1.58 2.96

0.91 0.44 0.32 0.99

2.90 0.46 0.71 2.44

6.62 1.04 0.41 1.80

0.42 0.27 0.19 0.58

0.17 0.07 0.12 0.09

Mudbelt 11001 11002 11003 11010 11012 11017 11028 11029 11039 11042

BHP

3,5Bd

Lignin

Vanillyl phenols

Syringyl phenols

Cinnamyl phenols

p-Hydroxyl phenols

2.84 12.08 4.08 17.11 12.56 4.60 4.33 15.04 7.93 2.05

0.27 2.64 11.17 2.06 4.62 1.40 1.47 2.02 3.67 2.28

43.05 74.35 70.26 78.18 62.89 45.86 107.27 106.70 189.26 136.97

23.86 32.89 24.07 29.31 20.79 17.81 51.38 43.69 86.93 60.95

7.38 14.55 19.34 23.81 11.48 11.08 21.62 34.95 56.48 26.98

2.67 2.13 1.62 8.84 1.34 5.27 5.97 11.72 16.48 18.07

9.14 24.78 25.23 16.22 29.28 11.70 28.30 16.34 29.37 30.97

0.50 0.55 1.00 0.31 0.38 0.31 1.42 0.46 0.92 1.12 0.52 0.84

1.00 0.26 35.73 4.83 4.25 38.48 2.41 6.23 0.53 5.53 1.28 2.33

8.47 0.11 2.91 2.40 53.07 0.55 4.17 1.53 1.29 0.55 1.13 2.28

55.30 6.89 18.23 16.26 153.54 48.73 57.91 13.59 20.75 24.07 23.42 20.18

9.69 1.93 4.35 5.28 33.16 22.05 19.09 4.65 5.00 8.20 8.01 7.08

8.97 1.07 2.96 3.20 61.90 9.61 9.58 2.05 5.14 5.03 4.02 2.78

4.58 1.46 1.41 1.65 21.32 2.24 0.32 1.23 0.38 2.88 0.92 1.94

32.06 2.43 9.51 6.13 37.16 14.83 28.92 5.66 10.23 7.96 10.47 8.38

0.85 0.57 0.10 0.89 0.75 0.52 0.94 0.46 1.37 2.28 1.53 1.28 0.62 1.42

1.08 2.26 0.06 0.61 0.49 0.46 0.66 0.49 0.23 0.48 0.33 0.79 0.45 0.59

2.83 0.55 0.86 2.11 108.51 3.17 4.39 1.90 1.22 2.56 4.51 5.52 8.39 2.06

2.81 1.60 0.58 7.97 3.19 0.73 5.66 3.86 2.94 46.00 40.45 1.11 1.16 5.24

41.25 18.27 14.10 30.06 24.28 5.70 37.07 24.10 20.05 93.39 104.13 13.66 8.12 22.54

16.40 7.07 4.35 8.78 7.49 1.17 14.79 6.78 6.24 25.03 28.99 3.44 2.05 4.37

11.37 3.80 3.71 7.80 5.50 1.08 8.89 4.34 4.73 21.05 23.03 2.63 1.47 6.69

5.40 1.52 1.81 2.75 3.44 0.45 0.85 0.58 1.86 6.74 8.96 1.74 0.58 0.92

8.08 5.88 4.23 10.73 7.85 3.00 12.54 12.40 7.22 40.57 43.15 5.85 4.02 10.56

1.13 0.31 0.47 0.58

0.53 0.21 0.00 0.31

20.57 0.33 1.02 0.00

2.38 1.74 0.28 0.30

21.66 24.85 23.21 6.51

5.50 9.73 5.85 1.18

5.52 7.30 4.57 2.06

2.44 1.80 1.92 0.91

8.20 6.02 10.87 2.36

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

sat. SCFA

45

46

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

Fig. 3. Gas chromatograms (FID) of lipid biomarker fractions and CuO oxidation products from the mudbelt station GeoB 11002; a) hydrocarbon fraction, b) ester and ketone fraction, c) alcohol and sterol fraction, d) fatty acid fraction, and e) lignin fraction. For internal standard (IS) see Sections 3.3 and 3.6.

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

47

Fig. 4. Distribution of terrestrial organic matter on the NW Iberian margin indicated by a) lignin phenols, and b) plant wax components (n-C25–35, LCOH and LCFA). Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, and 100 m), station numbers correspond to the last two numbers of GeoB station codes (cf. Table 1).

between the individual sampling sites from 1.0 µg/g dw at station MR1 to 32 µg/g dw in the RdV. 5. Discussion 5.1. Sources of organic matter at the Galicia–Minho shelf 5.1.1. The composition of terrestrial organic matter on the shelf The broad range of values in TOC/TN (4.5 to 15.6) and δ13CTOC (−21.0 to − 25.1‰) in sediments of the Galicia–Minho shelf is consistent with various degrees of OM mixing from continental and marine sources (see also Schmidt et al., 2009). Elevated TOC/TN values between 8.3 and 15.6 and depleted δ13CTOC values in the central mudbelt and the southern inner shelf may indicate substantial contributions of OM from terrestrial sources (Hedges et al., 1986) delivered by the Douro and Minho Rivers. Particularly in the mudbelt, vascular plant material plays an important role as suggested by the high lignin content. S/V values of 0.32 to 1.87 (Table 4) reflect low contributions from gymnosperms and a predominance of angiosperm material (Goñi and Hedges, 1992; see also Fig. 7). This is supported by a range of plant-derived triterpenoids in the sediment that reflect the vegetation of the hinterland. The catchment area of the Minho and Douro Rivers is predominated by angiosperm vegetation with oak trees and garigue in semi-arid central Spain and deciduous and mixed woodland as well as farmland in the humid north. However, due to recent massive dam constructions in the river course, the plant OM in the shelf sediments is probably mainly derived from the coastal region. The dominating angiosperm vegetation is also mirrored in the plant wax components. The maximum in the LCalk distribution at n-C31, closely followed by n-C29, displayed in the average chain length (ACL) of n-alkanes in Table 4, and the maximum of LCOH at n-C26 is indicative for a mixture of angiosperm grasses and trees (Collister et al., 1994a; Rommerskirchen et al., 2006). Due to the rich, tree-dominated vegetation at the river sampling sites, the maximum for LCalk is shifted to n-C29, a typical indication for angiosperm trees. This trend is evident in C/V values below 0.22 (Table 4 and Fig. 7) for woody material (Goñi and Hedges, 1992). In the shelf sediments, plant waxes account for one quarter of the plant wax concentrations detected in the riverine sediments. LCOH were always most abundant, closely followed by

LCFA and LCalk, indicating rather fresh OM at the shelf (Cranwell, 1981). Moreover, the carbon preference index (CPI) for n-alkanes (Table 5) showed relatively high values that are in the range of fresh plant tissue (4.3 to 40.3; Collister et al., 1994b). The soil-derived fraction of OM is identified by the BIT index and the 3,5-Bd/V ratio in our data set. Based on BIT, the contribution of soil OM to the sediment OM off Iberia appeared to be small. High BIT indices between 0.72 and 0.96 testified to soil OM in riverbank sediments but BIT values in shelf sediments were low and ranged from 0.04 to 0.10. These low BIT values are not unusual for shelf sediments as shown in previous studies (Herfort et al., 2006; Hopmans et al., 2004; Kim et al., 2006) and might be related to a low fraction of soil OM in the terrestrial-derived OM (Walsh et al., 2008). However, the low BIT values in this study are in stark contrast to the soil index 3,5-Bd/V (Fig. 5). 3,5-Bd/V appeared to be a more sensitive indicator of soil OM deposition at the NW Iberian margin and yielded values of 0.01 to 1.84 with generally higher values found offshore. These values are comparable to those reported in studies of other shelf sediments (e.g., Goñi et al., 2000; Prahl et al., 1994). The disparity of the two soil indices suggests that they are associated with different soil fractions. 3,5-Bd was detected in high concentrations in wood and tannic acids (Dickens et al., 2007) and was particularly enriched in the mineral horizon of soils (Houel et al., 2006), whereas non-isoprenoidal GDGTs used for calculation of the BIT index are presumed to be produced by anaerobic soil bacteria (Weijers et al., 2006). Generally, the BIT index is less reliably applied in regions with a high primary productivity and consequently high autochthonous crenarchaeol input, which will lower the BIT values and thus mask the allochthonous signal. Variations in primary productivity strongly affect the BIT index and may misleadingly appear to indicate changes in soil input and river run-off. Inspection of the raw data suggests that the variation of the BIT index is largely governed by the spatial variation of crenarchaeol rather than the branched tetraethers. 3,5Bd/V represents a more robust, absolute measure of the soil input and is therefore more suitable for detecting soil OM in the studied system. 5.1.2. Marine contributions to sedimentary organic matter Particularly in the northern shelf areas and offshore, elevated δ13CTOC values and TOC/TN ratios in the range of the Redfield ratio of ~6.7 (Redfield, 1958) suggested a predominance of marine algal OM

48

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

Table 4 Vegetation and soil ratios in all analyzed surface sediments from the NW Iberian margin and source rivers. Station (GeoB)

Location

ACL

SCFA/LCFA

C/V

S/V

3,5-Bd/V

BIT

11001 11002 11003 11010 11012 11017 11028 11029 11039 11042 11011 11013 11016 11018 11025 11030 11037 11038 11040 11041 11043 11044 11004 11005 11006 11007 11008 11009 11014 11015 11019 11020 11022 11027 11031 11032 11021 11023 11024 11033 MR1 MR2 CR UR RdV

Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mid-shelf Mid-shelf Mid-shelf Mid-shelf Mid-shelf Inner shelf Inner shelf Inner shelf Mid-shelf Inner shelf Inner shelf Mid-shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Cont. slope Cont. slope Cont. slope Cont. slope River River River River Tidal flat

29.8 29.7 29.9 30.0 29.8 29.9 29.9 29.8 29.7 29.6 29.6 29.9 30.0 29.9 30.0 29.7 29.8 29.6 29.7 29.3 29.5 29.9 29.8 29.8 30.4 30.2 30.1 29.9 30.2 29.8 30.1 30.0 30.0 29.9 29.9 29.9 30.2 29.9 29.8 30.2 28.8 28.7 29.4 29.2 30.0

8.98 6.24 2.09 4.93 4.93 6.10 6.23 3.70 4.13 19.20 10.63 5.39 6.25 5.54 10.71 9.51 11.27 7.59 7.49 15.81 15.02 4.07 3.77 3.37 5.19 4.36 6.16 4.72 1.82 8.97 2.78 8.12 6.27 6.87 8.17 15.10 3.39 2.70 1.48 5.60 2.97 16.26 3.58 1.94 0.82

0.11 0.06 0.07 0.30 0.06 0.30 0.12 0.27 0.19 0.30 0.47 0.75 0.32 0.31 0.64 0.10 0.02 0.26 0.08 0.35 0.12 0.27 0.33 0.21 0.42 0.31 0.46 0.39 0.06 0.09 0.30 0.27 0.31 0.51 0.28 0.21 0.44 0.19 0.33 0.77 0.15 0.22 0.04 0.01 0.07

0.31 0.44 0.80 0.81 0.55 0.62 0.42 0.80 0.65 0.44 0.93 0.55 0.68 0.61 1.87 0.44 0.50 0.44 1.03 0.61 0.50 0.39 0.69 0.54 0.85 0.89 0.73 0.93 0.60 0.64 0.76 0.84 0.79 0.76 0.71 1.53 1.00 0.75 0.78 1.74 0.32 0.93 0.45 0.49 0.46

0.01 0.08 0.46 0.07 0.22 0.08 0.03 0.05 0.04 0.04 0.87 0.05 0.67 0.45 1.60 0.02 0.22 0.33 0.26 0.07 0.14 0.32 0.17 0.23 0.13 0.91 0.43 0.63 0.38 0.57 0.47 1.84 1.40 0.32 0.57 1.20 0.43 0.18 0.05 0.25 0.19 0.25 0.10 0.23 0.05

0.09 0.09 0.06 0.05 0.08 0.05 0.08 0.10 0.08 0.08 0.04 0.05 0.04 0.06 0.09 0.07 0.06 0.09 0.05 0.08 0.07 0.05 0.07 0.05 0.05 0.04 0.04 0.04 0.07 0.08 0.06 0.06 0.07 0.05 0.06 0.06 0.06 0.09 0.05 0.05 0.72 0.86 0.82 0.96 0.82

ACL — n-alkane average chain length which is the weighted average number of C atoms in the range from C25 to C33 (Poynter and Eglinton, 1990).

(Fig. 2b, c). The high abundance of sterols and SCFA indicated the accumulation of relatively fresh algal OM, probably as consequence of high marine productivity in areas of increased upwelling. Low stanol to stenol ratios (Δ0/Δ5 < 0.6, Table 5) in all areas reflected fresh unaltered material (Nishimura, 1982; Wakeham, 1989). The distributional pattern of sterols in river and marine sediments suggested that sterol contributions from higher land plants to the Galician shelf are only of minor importance. Land plant-derived β-sitosterol dominated by far in the river sediments, whereas the shelf sediments showed the following sterol abundance: cholesterol > β-sitosterol (29Δ5) > brassicasterol (28Δ5) > dinosterol > cholesta-5,22-dienol (27Δ5,22) ≈ 24-methylenecholestadienol (28Δ5,24(28)) ≈ stigmasterol (29Δ5,22). Although β-sitosterol correlated significantly with the higher plant-derived triterpenoids (r > 0.92) and less pronounced with lignin (r > 0.48) the correlation with the other algal sterols (r > 0.77) and algal compounds (e.g., alkyl-diols, 12-OH-C28-FA), suggests diatoms or haptophyceae as main sources (see Volkman, 2003, and references therein). The correlation of sterols and higher plant-derived triterpenoids is noteworthy. This could indicate a causal relationship between the input of riverine terrigenous material and the production and/or preservation of algal material, i.e., due to fertilization with continent derived nutrients and/or better preserva-

tion of labile OM at elevated sediment accumulation rates. However, this relationship is not reflected in the lignin and grain size distribution (Table A1), and therefore the mechanism behind the strong correlation of sterols and plant-derived triterpenoids remains elusive. More specific markers for marine organisms are HBIs, which are produced only by a restricted range of diatom species (Table 2). Other characteristic biomarkers such as dinosterol indicate important contributions from dinoflagellates, whereas alkenones and C37:2-alkene originate from specific haptophyceae algae such as Emiliania huxleyi. These three algal groups were previously reported as main primary producers on the Galicia–Minho shelf (e.g., Bode et al., 2002; Estrada, 1984). A more recent study found evidence that cyanobacteria account for 55 to 65% of the primary production at the NW Iberian shelf (Lorenzo et al., 2005). However, the low concentrations of hopanes and the lack of specific cyanobacterial markers such as mid-chain methylheptadecanes (Kenig et al., 1995; Shiea et al., 1990) in this study suggest that cyanobacteria are not major contributors to the sedimentary OM. The organisms producing the abundant 1,15-C30:1-, 1,14-C30, and 1,15-C30-alkyl-diols and the 12-OH-C28-FA are not fully constrained. C32-alkyl-diols with a predominance of the 1,15-C32:0- and the 1,15C32:1-isomer were detected in eustigmatophyceae species (Volkman et al., 1992), and diatoms of the genus Proboscia abundantly produce saturated and monounsaturated C28-1,14-diols and 12-OH-C28-FA (Rampen et al., 2007). However, the sources for C30-diols, which usually are the most abundant diols in marine sediments remain unclear (see also review by Versteegh et al., 1997). In late Pleistocene sediments underlying highly productive surface waters in the Southeast Atlantic, the abundance of alkyl-diols was strongly correlated with other major algal biomarkers and TOC, suggesting that the producing algae were major contributors to paleoproductivity (Hinrichs et al., 1999). At the Galicia–Minho shelf, the distributional pattern of alkyl-diols and 12-OH-C28-FA resembled that of sterols but differs from HBIs (Tables 3, Fig. 6a and b), suggesting that the HBIs and alkyl-diols are produced by different groups of organisms. Low TOC/TN values in the range of 5.5 to 7.7 as found at the northern outcrop station GeoB 11011 could indicate elevated contributions of bacterial biomass to the sedimentary OM pool (Fukuda et al., 1998). However, bacterial biomarkers were not correlated with TOC/TN and the origin of several of the bacterial biomarkers, i.e., in situ produced biomass vs. input from the water column or from terrestrial sources, were hard to assign. For example, diploptene correlated with marine-derived HBIs and the C37:2 alkene, which could indicate a dominant marine origin (Elvert et al., 2001; Venkatesan, 1988). On the other hand, it was also significantly correlated with LCalk and was present in the river samples, which would point to a terrestrial source, e.g., from soil OM (Prahl, 1985; Prahl and Hayes, 1992). Likewise, the exact sources of BHPs in the shelf sediments could not be unambiguously identified. Amongst others, BHT was identified in several environments, such as soils, marine sediments and cyanobacterial mats (see Talbot et al., 2008 and references therein). It correlated significantly with anhydro-BHT, a possible degradation product (Schaeffer et al., 2008), but showed a distinct distributional pattern from all other biomarkers. Furthermore, both compounds showed no correlation with degradation products of BHPs, i.e., hopanes, hopanols and hopanoic acids (Table 2). 5.2. Transport and distributional patterns of terrestrial organic matter The major fraction of the terrestrial OM is transported to the shelf in winter and spring during periods of maximum rainfall in the Iberian hinterland and occasional river floods (Dias et al., 2002b). The terrestrial matter is primarily deposited close to the source rivers and redistributed during dynamic winter conditions (Vitorino et al., 2002). High shear wave velocities in winter, particularly during storm events, induce sediment remobilization, northward transport with the bottom currents, and probably also export of a sediment fraction across the shelf break (Dias et al., 2002b; Vitorino et al., 2002). These processes influence the lateral distribution of OM (e.g., Figs. 4 and 7). Apparently, plant debris was

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

49

Fig. 5. Distribution of soil-derived OM on the NW Iberian margin indicated by a) 3,5-Bd/V ratio, b) BIT index, and c) (Ad/Al)V ratio. Color code below refers to typically observed values and was adapted from a) Houel et al. (2006), b) Hopmans et al. (2004), and c) Goñi et al. (1993). Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, and 100 m), station numbers correspond to the last two numbers of GeoB station codes (cf. Table 1).

hydrodynamic sorted and deposited in different areas of the shelf. High C/ V ratios at the outer shelf and continental slope indicated an accumulation of grass and leaf material, i.e., soft plant tissues. Woody material with low C/V ratios was deposited closer to the continent and the source rivers in the mudbelt sediments, thus showing a higher total lignin phenol concentration, whereas the soft plant tissue was effectively transported offshore, probably due to its hydrodynamic properties. Similar sorting processes have been reported from other continental margins, e.g., from the Washington margin (Prahl, 1985), the Gulf of Mexico (Bianchi et al., 2002; Goñi et al., 1998), and the Hudson Bay (Kuzyk et al., 2008) and appeared to be an important factor controlling the OM distribution in this environment. Plant-derived triterpenoids exhibited a partitioning similar

to the woody lignin fraction (Table 3), probably due to an enrichment of resins in wood. Due to the high abundance of LCalk, LCOH and LCFA in epicuticular plant waxes (Eglinton and Hamilton, 1967), which are primarily associated with soft tissues, one would expect a spatial distribution of plant wax components comparable to C/V. However, the high concentrations of plant wax components indicated a general abundance of plant waxes in sediments off NW Iberia (Fig. 4b). Due to their position on the plant surface, wax components are more susceptible to weathering than lignins which are part of the cell walls. Therefore, beside remobilization by water leaching, run-off, and riverine transport, plant waxes are also transported via the atmosphere (e.g., Rommerskirchen et al., 2006; Schefuss et al., 2003). The frequent wild fires in Galicia

Fig. 6. Marine organic matter production at the NW Iberian margin indicated by a) HBIs (diatoms), b) dinosterol (dinoflagellates) and c) C37-alkenones (coccolithophorides). Dashed lines refer to water depth (from the left: 2000 m, 1000 m, 200 m, and 100 m), station numbers correspond to the last two numbers of GeoB station codes (cf. Table 1).

50

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55 Table 5 Biomarker degradation ratios at the NW Iberian margin and source rivers.

Fig. 7. C/V ratio vs. S/V ratio for all marine (station numbers correspond to the last two numbers of GeoB station codes; cf. Table 1) and riverine samples (MR1, MR2, UR, CR, and RdV) analyzed in this study. Squares indicate typical ranges for woody and nonwoody tissue of angio- and gymnosperms, respectively. Symbols refer to sampling location: circles — mudbelt, half circles — inner and mid-shelf, squares — outer shelf, rhomb — continental slope, cross — rivers.

and northern Portugal during the hot and dry summer months could promote a high abundance of plant wax components in the aerosols and the offshore-directed winds from north and northeast in summer favor an aeolian input. Soil OM was mainly transported offshore as indicated by elevated 3,5-Bd/V ratios (Fig. 6a). Coinciding with the increase in 3,5-Bd/V is an increase in (Ad/Al)V offshore (Table 5), likewise observed in soils (Houel et al., 2006), river estuaries (Louchouarn et al., 1997) and on shelves (Prahl et al., 1994). (Ad/Al)V indicates aerobic degradation of lignin. As a consequence of propyl side chain oxidation in lignin macromolecules by white-rot fungi (Goñi et al., 1993), the acidic CuO oxidation products increase and result in (Ad/Al)V ratios >0.5. Low (Ad/Al)V values in the sediments from the inner/mid-shelf area suggested a deposition of fresh plant tissue, whereas values higher than 5 in the outer shelf and the northern outcrop region indicated highly degraded, pre-aged material. This fraction probably derived from degraded plant material that was accumulated in the soils, leached, and transported to the continental margin by water and river run-off as part of soil OM. 5.3. Spatial OM distribution in relation to seasonal plankton ecology Particle transport on the Galicia–Minho shelf occurs preferentially in the surface nepheloid layer (SNL) and the bottom nepheloid layer (BNL), the former enriched in large organic particles and the latter in small inorganic material (Oliveira et al., 2002). The association of terrestrial OM with the silt fraction, as discussed above, implies a distribution of terrestrial OM that is controlled mainly by the BNL, whereas the noncorrelation of marine biomarkers with silt suggests a transport and partitioning that is distinct from the terrestrial OM and occurred most likely in the SNL. However, the distribution of marine biomarkers is not only the result of a transport behavior that is distinct from the terrestrial OM; it also reflects the seasonal and spatial succession of plankton assemblages. The seasonal phytoplankton succession starts in spring with the onset of upwelling resulting in a diatom bloom, followed by a predominance of heterotrophic organisms in summer and a dinoflagellate bloom in autumn, the latter induced by stratified water conditions after the wind relaxation (Figueiras and Ríos, 1993). The high concentrations of HBIs in the northern parts of the shelf (>42.25°N, Fig. 6a) suggested an

Station (GeoB) Location

CPI

HPA Δ0/Δ5 Alkene/ (Ad/Al)V P/(S + V) alkenone

11001 11002 11003 11010 11012 11017 11028 11029 11039 11042 11011 11013 11016 11018 11025 11030 11037 11038 11040 11041 11043 11044 11004 11005 11006 11007 11008 11009 11014 11015 11019 11020 11022 11027 11031 11032 11021 11023 11024 11033 MR1 MR2 CR UR RdV

4.21 3.80 5.62 4.17 3.57 4.05 6.00 5.43 6.11 4.26 2.69 3.53 5.08 5.12 6.07 4.15 4.08 4.23 4.30 3.41 4.37 5.82 4.21 4.12 4.40 4.98 6.89 4.98 4.42 3.38 5.37 4.85 3.47 3.28 4.01 3.61 5.40 4.38 3.51 1.80 6.86 5.36 2.82 9.61 5.14

0.54 0.61 0.64 0.65 0.65 0.71 0.61 0.63 0.62 0.66 0.72 0.69 0.62 0.54 0.74 0.70 0.60 0.72 0.73 0.63 0.70 0.66 0.70 0.64 0.13 0.73 0.69 0.68 0.76 0.56 0.72 0.61 0.76 0.66 0.69 0.79 0.59 0.67 0.65 0.64 0.60 0.63 0.43 0.80 0.70

Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mudbelt Mid-shelf Mid-shelf Mid-shelf Mid-shelf Mid-shelf Inner shelf Inner shelf Inner shelf Mid-shelf Inner shelf Inner shelf Mid-shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Outer shelf Cont. slope Cont. slope Cont. slope Cont. slope River River River River Tidal flat

0.20 0.16 0.41 0.43 0.21 0.44 0.33 0.33 0.23 0.10 0.27 0.21 0.34 0.36 0.33 0.25 0.55 0.30 0.47 0.15 0.13 0.40 0.16 0.19 0.38 0.43 0.38 0.31 0.35 0.29 0.45 0.87 0.25 0.45 0.51 0.41 0.34 0.25 0.12 0.22 0.26 0.19 0.27 0.43 0.31

0.88 1.20 0.09 0.73 0.45 0.92 0.70 1.20 0.75 0.75 0.48 0.54 0.80 0.72 0.37 0.56 0.91 0.61 1.74 0.60 0.58 0.40 0.51 0.57 0.57 0.46 0.52 0.54 0.90 0.59 0.79 0.72 0.37 0.97 0.35 1.05 1.29 0.38 0.52 0.73 – – – – –

0.47 0.08 0.31 0.39 0.41 0.40 0.29 0.40 0.34 0.78 11.56 2.11 0.98 1.26 1.81 0.48 0.09 0.43 0.55 0.88 0.14 0.67 0.47 0.92 0.62 5.73 2.92 14.88 0.24 0.44 1.15 10.83 4.86 1.44 0.92 0.76 1.41 0.47 1.56 0.93 0.53 0.66 0.03 0.11 0.74

0.29 0.52 0.58 0.31 0.91 0.41 0.39 0.21 0.20 0.35 1.72 0.81 1.30 0.72 0.39 0.47 1.05 0.85 1.01 0.60 0.87 0.85 0.29 0.54 0.53 0.65 0.60 1.33 0.53 1.11 0.66 0.88 0.83 0.96 1.14 0.95 0.74 0.35 1.04 0.73 0.19 0.42 0.23 0.45 0.41

CPI — carbon preference index for n-alkanes calculated over the range C24 to C34 (Eglinton and Hamilton, 1963); HPA — higher plant alkanes index based on the concentrations of C24,26,28 n-alcohols to C27,29,31 n-alkanes (Poynter and Eglinton, 1990). P/(V+ S) indicates subaquatic degradation (Dittmar and Lara, 2001).

accumulation of diatom-derived OM close to the NW Iberian upwelling cell further up in the North of the study area. Diatoms dwell under eutrophic conditions that are caused by the nutrient-rich upwelling waters which are dominantly located in the northern parts of the Galicia– Minho shelf during the diatom bloom period in spring (e.g., Bode et al., 2002). The preferential occurrence of HBIs right in front of the Rías Baixas could alternatively indicate a biomass export out of the Rías (e.g., Tilstone et al., 2000). The primary productivity in the Rías Baixas exceeds the shelf production by far due to their favorable growth conditions, i.e., high nutrient input from land and particularly from the upwelling cell into a semi-closed protected environment (Alvarez-Salgado et al., 1999; Alvarez-Salgado et al., 1996; Prego, 1993). Nevertheless, such an export of biomass should be visible in the dinosterol distribution, since dinoflagellates have been reported to account for 13 to 30% of the primary productivity in the Ría de Vigo (Lorenzo et al., 2005). The dinosterol concentration maxima are, however, more or less restricted to the inner/ mid-shelf (Fig. 6b), which correspond to the favored habitat of dinoflagellates in autumn. The high dinosterol concentrations at stations GeoB 11010 (northern mudbelt) and GeoB 11020 (outer shelf) could

F. Schmidt et al. / Marine Chemistry 118 (2010) 37–55

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indicate remobilization and redistribution of dinoflagellate-derived OM during the dynamic winter months. The former station particularly showed also increased concentrations of most of marine and terrestrial biomarkers as well as TOC which is most likely caused by the proximity to rocky outcrops that act as sediment traps (Fig. 1). Haptophytes do not successfully compete with other phytoplankton groups such as diatoms or dinoflagellates and therefore are more adapted to oligotrophic waters (Winter et al., 1994). This explains the fairly high alkenone concentrations farther offshore. In winter, the downwelling front separates two contrasting environments (Castro et al., 1997); the coastal areas exhibit mesotrophic conditions due to the nutrient supply from land and provide an environment for diatoms and larger dinoflagellates (Alvarez-Salgado et al., 2003). The surface mixed layer of the PCCC with its oligotrophic conditions advances towards the continent together with the downwelling front, which shifts the habitat of haptophyceae algae and small flagellates to the mid-shelf region. The elevated alkenone concentrations in the mid-shelf area (Fig. 6c) may reflect the winter conditions and the onshore movement of the oligotrophic plankton assemblage during that time. 5.4. OM provinces on the Galicia–Minho shelf Representative biomarkers were subjected to a cluster analysis (Fig. 8) which revealed a partitioning of the NW Iberian margin into three main regions with distinct OM provenances (Fig. 9): the inner shelf, the muddominated, and the offshore region. The inner shelf region (<130 m water depth) is characterized by a high OM input from marine sources as indicated by the highest values of SCFA/LCFA. The primary productivity in this region is fuelled by nutrient supply from the continent and upwelling, resulting in high concentrations of algal biomarkers such as sterols, alkyldiols, and SCFA. The comparably high abundance of PUFA and MUFA signals fresh OM. Lignin concentrations vary strongly in the inner shelf region, consistent with by-passing of terrestrial-derived OM to the mudbelt region and eventually farther offshore. The terrestrial mud-dominated region extends across the mid-shelf area from 41.8° to 42.8°N, comprising most of the mudbelt. Here, concentrations of most terrestrial lipid biomarkers and lignin are particularly high. Compared to the marine-dominated inner shelf region, the absolute concentrations of marine biomarkers decrease only slightly (e.g., dinosterol, alkyl-diols and 12-OH-C28-FA) or are invariable (e.g., alkenones, phytol, sterols, SCOH, and HBIs). High accumulation of terrestrial OM in the mid-shelf region generally dilutes the regular background sedimentation. Terrestrial OM from the Minho and Douro Rivers is redistributed and accumulated with the sedimentary material in this region, yielding the highest silt and TOC contents. The southernmost branch of this region (station GeoB 11039) is very likely directly fed by the Douro River including sediment remobilization from the Douro mud patch located further in the south (Araújo et al., 1994; Drago et al., 1998). The extension of the terrestrial mud-dominated region to the outer shelf in the North (GeoB 11020, 11022 and 11025) is mainly attributed to the accumulation of lignin and soil OM in these sediments. The transport of terrestrial OM not only northward but also offshore could indicate an export pathway for particulate matter in the northern shelf area as suggested by Dias et al. (2002a,b). A second export pathway runs possibly across the outer shelf, in the north of the rocky outcrops bordering the southern shelf break (Fig. 9) as implied by a biomarker pattern at station GeoB 11032 that is similar to the marine-dominated inner shelf region. Sediment remobilization, resuspension, and ultimately deposition are the main controlling factors in the terrestrial mud-dominated midshelf region. Consequently, increased degradation due to prolonged exposure to oxygen is expected. However, the fairly high values of the higher plant alkane index (HPA, Table 5) in the inner/mid-shelf areas and the low abundance of triterpenones in the sediment (less than 10% of the total plant-derived triterpenoids), suggest low oxidative degradation in the surficial sediments. Moreover, low P/(S+ V) values in the terrestrial mud-dominated region also indicate rather fresh lignin

Fig. 8. Cluster analysis of selected parameters and biomarkers for all sediment samples (station numbers correspond to the last two numbers of GeoB station codes, cf. Table 1) from the NW Iberian margin (variables: water depth, TOC, silt, TOC/TN, δ13CTOC, BIT, LCalk, n-C36–40, HBIs, alkenones, SCOH, LCOH, sterols, dinosterol, 24-methylenecholesterol, alkyl-diols, plant-derived triterpenoids, friedelin, LCFA, sat. SCFA, MUFA, PUFA, C18-FA, C18:1ω9-FA, C18:1ω7-FA, BrFA, C16:0-MAGE, diploptene, 3,5-Bd, as well as vanillyl, syringyl, cinnamyl and p-hydroxyl phenols).

relative to the other OM provinces further corroborating the finding of reduced in situ degradation in this region. The offshore region finally comprises the outer shelf and the continental slope and is dominated by low sedimentation, resulting in a low silt and TOC content of the sediment. The oligotrophic conditions in the PCCC result in a low marine productivity, which is reflected in the low concentrations of sterols and fatty acids. Terrestrial biomarker concentrations in this region were lower on an absolute level but elevated relative to marine biomarkers, a trend that is especially pronounced in the starved offshore regions (Stations GeoB 11006, 11023 and 11024; Fig. 9). This observation can be explained by the lower productivity in combination with the higher water depth that enhances remineralization of marine OM, while pre-aged and chemically more resistant terrestrial OM is more likely to be preserved. 6. Conclusions The multi-biomarker approach applied in this study enabled the identification of the major sources in the heterogeneous sedimentary OM pool at the NW Iberian margin. Accordingly, the continental shelf was classified into three distinct OM regions: ◾

The inner shelf region is predominated by marine OM as indicated by the high abundance of marine lipid biomarkers in sediments with intermediate contents of silt and TOC as well as by TOC/TN and δ13CTOC values consistent with a predominant OM input by marine algae.

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Fig. 9. Organic matter provinces at the NW Iberian margin identified by biomarkers and bulk parameters from the cluster analysis (Fig. 8). Blue regions: marine dominated inner shelf; green region: terrestrial mud dominated mid-shelf mudbelt; red regions: outer shelf and continental slope, yellow regions: starved outer shelf and continental slope. Suggested material trajectories on the shelf are indicated by arrows. Pie charts of representative stations show main OM fractions indicated by selected biomarkers. Bacteria include BrFA, C18:2-FA, C18:1ω7-FA, UFA — unsaturated fatty acids, i.e., MUFA and PUFA, MPP — main primary producers, i.e., dinosterol, alkenones and HBIs. The relative size of the pie charts corresponds to the TOC content in the sediment (see also Table 1).

◾

◾

The terrestrial-dominated mid-shelf region is the main depositional area on the shelf and revealed the highest contents of silt and TOC and the lowest δ13CTOC values, corresponding to the Galicia–Minho mudbelt (Dias et al., 2002b and references therein). The sediments comprise high concentrations of marine and terrestrial lipid biomarkers, and strikingly high lignin phenol contributions at a low degradation state. The offshore region, i.e., the outer shelf and the continental slope receives only little sediment and OM input, resulting in characteristically low concentrations of silt, TOC, and lipid biomarkers. Furthermore, lipid biomarkers provide evidence that starved regions are indicated by an advanced degradation state of marine and terrestrial OM and a relative enrichment of terrestrial lipid biomarkers. This predominance of terrestrial components is, however, not reflected in TOC/TN and δ13CTOC.

The distribution of OM is controlled by a complex and seasonally varying interplay of ocean currents, primary productivity, atmospher-

ic conditions, and river run-off. Controls differ between terrestrial and marine OM pools with hydrodynamic sorting governing the partitioning of terrestrial OM. The transport and deposition of soft tissue is favored offshore, whereas woody material is accumulated in the midshelf mudbelt after shorter transport times. Soil OM behaves in a similar manner to the soft tissue and is preferentially accumulated in the northern parts of the study area and on the outer shelf. On the contrary, the distribution of marine OM reflects the seasonal and spatial succession of plankton assemblages and is directly controlled by the oceanographic conditions. Acknowledgements We thank the crew and shipboard scientific party of RV Poseidon cruise P342 GALIOMAR, particularly the chief scientist Till Hanebuth. We thank Brit Kockisch for assistance with sample collection and for TOC measurements. We are grateful to Gesine Mollenhauer for providing access to her HPLC-MS system for BIT analysis and to Jan-

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Berend Stuut for assistance in grain size analysis. We thank Hella Buschoff for TN and TC analysis, Monika Segl for δ13CTOC measurements, Xavier Prieto and Kevin Becker for support in the lab, and Daniel Birgel and Arne Leider for fruitful discussions and comments on data and the manuscript. We also like to thank Thomas Bianchi and two anonymous reviewers for their helpful comments on the manuscript. Funding was provided by the “Deutsche Forschungsgemeinschaft” through DFG-Research Center/Excellence Cluster “The Ocean in the Earth System” and the Bremen International Graduate School for Marine Sciences (GLOMAR). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.marchem.2009.10.003. References Alt-Epping, U., Mil-Homens, M., Hebbeln, D., Abrantes, F., Schneider, R.R., 2007. 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