Vertical distribution of major, minor and trace elements in sediments from mud volcanoes of the Gulf of Cadiz: evidence of Cd, As and Ba fronts in upper layers

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Vertical distribution of major, minor and trace elements in sediments from mud volcanoes of the Gulf of Cadiz: evidence of Cd, As and Ba fronts in upper layers ⁎

Lina Carvalhoa, Rui Monteirob,c, Paula Figueiraa,d, , Cláudia Mieirob, Joana Almeidae, Eduarda Pereiraa,b, Vítor Magalhãesf, Luís Pinheirog, Carlos Valec a

Central Laboratory of Analysis (LCA), University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal CESAM and Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, Av. General Norton de Matos s/n 4450-208 Matosinhos, Portugal d CICECO (Aveiro Institute of Materials) and Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal e Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal f Marine Geology and Georesources Division (DivGM), Portuguese Institute for the Ocean and Atmosphere (IPMA), Rua C ao Aeroporto,1749-077 Lisboa, Portugal g CESAM and Geosciences Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Cadiz mud volcanoes Sediment elemental composition Depth profiles Ba, Cd and As fronts

Mud volcanoes are feature of the coastal margins where anaerobic oxidation of methane triggers geochemical signals. Elemental composition, percentage of fine particles and loss on ignition were determined in sediment layers of eleven gravity cores retrieved from four mud volcanoes (Sagres, Bonjardim, Soloviev and Porto) and three undefined structures located on the deep Portuguese margin of the Gulf of Cadiz. Calcium was positively correlated to Sr and inversely to Al as well as to most of the trace elements. Vertical profiles of Ba, Cd and As concentrations, and their ratios to Al, in Porto and Soloviev showed pronounced enhancements in the top 50-cm depth. Sub-surface enhancements were less pronounced in other mud volcanoes and were absent in sediments from the structures. These profiles were interpreted as diagenetic enrichments related to the anaerobic oxidation of methane originated from upward methane-rich fluxes. The observed barium fronts were most likely caused by the presence of barite which precipitated at the sulphate-methane transition zone. Cd and As enrichments have probably resulted from successive dissolution/precipitation of sulphides in response to vertical shifts of redox boundaries.

1. Introduction Submarine mud volcanoes (MVs) have been studied in various continental margins (e.g., Castellini et al., 2006; Chao et al., 2011; Gardner, 2001; Graue, 2000; Hensen et al., 2004; MacDonald et al., 1994; Omoregie et al., 2009; Vanneste et al., 2012) as they transport hydrocarbons from deep to shallow sediments, reaching in certain cases the water column (Dimitrov, 2002). Methane is the dominant hydrocarbon of the fluids expulsed at these seep sites (Judd and Hovland, 2007). Upward transport of methane along conduits fuels the anaerobic oxidation of methane (AOM) and consumes SO42- of pore waters near the seafloor (Boetius et al., 2000; Borowski et al., 1996; Reeburgh, 1976). Removal of downward diffusing SO42- and upward migrating CH4 occurs across a relatively thin depth interval, depending on the balance between the two opposite fluxes. This horizon is often called

⁎

sulphate-methane transition (SMT), where SO42- and CH4 are consumed by anaerobic methanotrophic archaea and sulphate reducing bacteria (D’Hondt et al., 2002; Vanneste et al., 2013). In principle, the upward flux of CH4 and other hydrocarbons should dictate the rate of SO42consumption and the depth of the SMT (Borowski et al., 1996; D’Hondt et al., 2002; Reeburgh, 1976). Consequently, SMT occurs in upper sediment layers at active mud volcanoes. Upward fluxes of hydrocarbons do also affect the geochemical cycle of Ba in marine sediments (eg., Castellini et al., 2006; Dickens, 2001; Torres et al., 1996; Vanneste et al., 2013, 2012). During the ascendant flux, gas-charged fluids containing elevated concentrations of dissolved barium get into contact with sulphate rich pore waters leading to the precipitation of barite (Aquilina et al., 1997; Torres et al., 1996; Vanneste et al., 2013, 2012). As authigenic barite builds up just above the depth of sulphate depletion, its presence indicates the depth of the

Corresponding author at: Central Laboratory of Analysis (LCA), University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal. E-mail address: paulafi[email protected] (P. Figueira).

https://doi.org/10.1016/j.dsr.2017.12.003 Received 15 December 2016; Received in revised form 24 November 2017; Accepted 18 December 2017 0967-0637/ © 2017 Published by Elsevier Ltd.

Please cite this article as: Carvalho, L., Deep-Sea Research Part I (2017), https://doi.org/10.1016/j.dsr.2017.12.003

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multibeam and side-scan sonar surveys (hereby designated by Structures). Table 1 gives the name, reference and coordinates of the 11 sampling sites, water depths and core lengths.

SMT (Dickens, 2001; Snyder et al., 2007). Barite dissolves at the SMT, producing high interstitial Ba2+ concentrations. In diffusive systems, dissolved Ba2+ can move upwards to re-precipitate barite just above the SMT (Aloisi et al., 2004; Dickens, 2001; Torres et al., 1996; Vanneste et al., 2012). Although the Ba cycle is well documented at seep sites (eg., Dickens, 2001; Riedinger et al., 2006; Torres et al., 1996) studies of authigenic barite layers and their linkage to methane fluxes are limited to a few mud volcano fields (eg., Castellini et al., 2006; Vanneste et al., 2013, 2012). At MVs calcium carbonate tends to be the more abundant and widespread authigenic phase. Depending on the availability of bicarbonates produced during the AOM, carbonate precipitates at cold seep sites vary from microcrystalline phases formed within the sediment column to blocks and chimneys scattered over the seafloor (Aloisi et al., 2002; Díaz-del-Río et al., 2003; Gontharet et al., 2007; Vanneste et al., 2012). In this study, we document the major, minor and trace element composition of bulk sediments from the Sagres, Bonjardim, Soloviev and Porto MVs and three structures located on the Portuguese margin of the Gulf of Cadiz. Sediment chemical profiles of 11 gravity cores were used to search for anomalies of trace elements and to examine their relations with the sulphate-methane transition zone and anaerobic oxidation of methane.

3.2. Analytical methodologies After sampling, cores were opened, sliced in intervals of 15–20 cm thickness, and sediment samples were immediately kept at 4 °C on board. Percentage of fine particles was determined by wet sieving using approximately 5 g of dried sediment through a sieve of 63 µm mesh. Organic matter content and composition of major, minor and trace elements (Al, Fe, Ca, Mg, Ti, P, Mn, Sr, Ba, Cr, V, Zn, Li, Ni, Cu, Co, Pb, As, Be, Cd and Hg) were determined in the sediment fraction smaller than 1 mm to remove clasts that varied greatly in size. Samples were powdered and homogenized with a Retsch mill. Organic matter content was estimated by the loss on ignition (LOI) of 100 g of wet sediment at 500 °C during 4 h (Williams, 1985). The time was tested to obtain a relatively constant weight. Total dissolution of approximately 200 mg of sediments was obtained by a mixture of 9 mL of HNO3 (69%) and 3 mL of HF (40%) in closed Teflon bombs using a CEM MARS 5, model 240/50-microwave digestion system, with continuous temperature and pressure monitoring. Then the residue was re-dissolved with 5 mL HCl (1:1 V/V) and evaporated. At the end, the residue was dissolved in 10 mL of HNO3 (20%; 1:5 V/V) and the solutions heated for 30 min, at 150 °C. After cooling at room temperature, the solutions were transferred into 100 mL volumetric flasks and then filled with ultrapure water to the final volume for analysis (adapted from USEPA, 2007). Aluminum, Ca, Mg, Fe, Co, Cr, Li, Mn, Ni, Sr, V and Zn were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Horiba Jobin Yvon Activa M) under the following conditions: forward power 1000 W, argon flow plasma 12 L/ min, sheat gas 0.8 L/min and the Burgener MiraMist nebulizer; the acquisition method using an algorithm background correction was used for quantification. Determinations of As, Ba, Be, Cd, Cu, Pb and Ti were carried out using a quadrupole inductively coupled plasma mass spectroscopy (ICP-MS, Thermo Elemental, X-Series) equipped with a Peltier cooled impact bead spray chamber and a concentric Meinhard nebulizer. Experimental conditions were: forward power 1400 W; peak jumping mode; 150 sweeps per replicate; dwell time: 10 ms; dead time: 30 ns. Polyatomic and isobaric interferences were minimized by setting the ratios 137Ba++/137Ba and 140Ce16O/140Ce to 0.02, under routine operating conditions. Internal standard was 115In. Quality Control and Quality Assurance procedures included the processing and analysis, at each batch, of two blanks and the certified reference material (CRM) MESS-3. For all the elements analyzed, the coefficients of variation between triplicate analyses were always lower than 5% and a 7-point daily calibration was used for the quantification of major, minor and trace elements. Two procedural blanks were prepared and included in each batch of 10 samples. Sediment samples were analyzed in triplicate and differences among them were always lower than 10%. Procedural blanks always accounted for less than 1% of the element concentrations. The measured values in CRM MESS-3 were consistent with the certified ones, with extraction efficiencies ranging from 93% to 120% (Table 2). Mercury (Hg) analyses were performed by atomic absorption spectrometry (AAS) with thermal decomposition, using an Advanced Mercury Analyzer (AMA) LECO 254 (for further details see Costley et al., 2000). The accuracy and precision of the analytical methodologies were assessed by replicate analysis of the CRM MESS-3. The precision of the method ranged between 0% and 10%. The extraction efficiency was 112 ± 4% (Table 2).

2. Area of study The Gulf of Cadiz is located in a tectonically active area that corresponds to a tectonically diffuse transpressive plate boundary between the southern Iberian continental margin and the northern African Atlantic margin (Buforn et al., 1995; García-Lafuente et al., 2006; Pinheiro et al., 2003; Terrinha et al., 2009). This region is characterized by abundant mud volcanism, revealing the existence of gas-rich sediments at depth (Pinheiro et al., 2003; Somoza et al., 2003). Although the majority of mud volcanoes in the Gulf of Cadiz are considered in a dormant stage (Vanneste et al., 2013), records of fluid migration and escape to the seafloor of the middle and lower slope have been documented in various studies (Berndt et al., 2006; Cofaigh et al., 2006; Díaz-del-Río et al., 2003; Gardner, 2001; Huguen et al., 2004; Kenyon et al., 2003; Maestro et al., 2002; Pinheiro et al., 2003; Van Rensbergen et al., 2005). In particular, the presence of methane-derived authigenic carbonates throughout the mud volcano fields in the Gulf of Cadiz suggests the occurrence of episodes of extensive methane seepage in the past (Díaz-del-Río et al., 2003; Magalhães et al., 2012; Vanneste et al., 2012). A clear-cut sulphate-methane transition zone overlapping with high sulphide concentrations was reported by Niemann et al. (2006), suggesting that methane oxidation is mediated under anaerobic conditions with sulphate as electron acceptor. Anaerobic oxidation of methane and sulphate reduction rates showed maxima 20 and 200 cm below seafloor at different mud volcanoes. In comparison to other methane seeps, AOM activity and diffusive methane fluxes in mud volcano of the Gulf of Cadiz are admittedly low to mid-range (Vanneste et al., 2013). 3. Material and methods 3.1. Samples Sediment samples were obtained from the deep mud volcano field of the Gulf of Cadiz, with water depth ranging from 1554 to 3880 m (Fig. 1). Samples were retrieved in the summer of 2007 during the TTR17 scientific cruise, on board of the R/V Professor Logachev in the scope of the Euromargins MVSeis Project. During this cruise, gravity cores were sampled in the deep Portuguese margin of the Gulf of Cadiz to investigate the seepage activity at four known mud volcanoes: Sagres (669G), Bonjardim (675G and 677G), Soloviev (680G) and Porto (681G, 682G, 684G and 686G), and in three new structures (670G, 672G and 674G) that were identified as high reflectivity sites on the

3.3. Data analysis The Spearman rank correlation coefficient (rS) was determined to evaluate the cross-correlations among elements, fine fraction and LOI of 2

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Fig. 1. Map of the Gulf of Cadiz with the location of the retrieved gravity cores within the mud volcanoes field during the scientific cruise Thought Trough Research 17 (TTR17): Sagres, Bonjardim, Soloviev, Porto, Structures I, II and III.

Table 1 Reference, latitude and longitude of the sampling sites, depth of the water column, and length of the gravity cores sampled in the Gulf of Cadiz. Station number

Site

669G 670G 672G 674G 675G 677G 680G 681G 682G 684G 686G

Sagres MV Structure I Structure II Structure III Bonjardim MV Bonjardim MV Soloviev MV Porto MV Porto MV Porto MV Porto MV

Latitude

36°02.209N 35°31.526N 35°30.389N 35°29.666N 35°27.549N 35°27.503N 35°12.770N 35°33.752N 35°33.752N 35°33.769N 35°33.762N

Longitude

08°05.540W 08°20.831W 08°27.605W 08°24.862W 08°59.817W 08°59.811W 09°06.432W 09°30.398W 09°30.394W 09°30.427W 09°30.412W

Depth (m)

1554 2237 2340 2318 3055 3052 3310 3870 3870 3880 3875

Table 2 Mean values of the certified reference material MESS-3 (n=7) and the values experimentally obtained for major and trace elements (mean ± one standard deviation, dry weight).

Core length (cm) 144 427 195 285 137 272 157 196 187 196 317

the sediment samples. The parameters fine fraction, LOI, major, minor and trace element composition and the sample location were compared by a Principal Components Analysis (PCA), using Canoco (ter Braak, 1995). Before performing the PCA, data was transformed, centered and standardized in order to normalize the variables. PCA was based on the correlation matrix of sediment parameters and aimed to outline the main similarities among the different studied MVs and Structures. Whenever the assumptions for parametric statistics failed, the nonparametric correspondent test (Kruskall - Wallis) was performed, followed by nonparametric pairwise multiple comparison procedure (Dunn's test).

Elements

Units

Certified values

Results obtained

Recovery efficiency (%)

Al Ca Mg Fe Ti P Ba Mn Sr Cr V Zn Li Ni Cu Co Pb As Be Cd Hg

%

8.59 ± 0.23 1.47 ± 0.06 1.6 4.34 ± 0.11 0.44 ± 0.06 0.12 926a 324 ± 12 129 ± 11 105 ± 4 243 ± 10 159 ± 8 73.6 ± 5.2 46.9 ± 2.2 33.9 ± 1.6 14.4 ± 2 21.1 ± 0.7 21.2 ± 1.1 2.30 ± 0.12 0.24 ± 0.01 0.091 ± 0.009

8.7 ± 0.6 1.5 ± 0.1 1.7 ± 0. 2 4.0 ± 0. 3 0.38 ± 0.04 0.12 ± 0.8 1021 ± 64 318 ± 41 129 ± 18 102 ± 7 238 ± 15 170 ± 14 74 ± 6 46 ± 3 36 ± 2 14 ± 1 23 ± 1 22 ± 1 2.3 ± 0.3 0.30 ± 0.07 0.098 ± 0.0032

103 ± 8 103 ± 7 110 ± 11 93 ± 8 94 ± 6 107 ± 7 110 ± 7 103 ± 8 109 ± 12 100 ± 7 102 ± 5 107 ± 7 100 ± 9 94 ± 7 105 ± 5 95 ± 6 106 ± 5 105 ± 4 98 ± 12 120 ± 18 112 ± 4

a

mg kg−1

Information value according to Begum et al. (2007).

supplementary figures (Figs. S1 and S2) show the sampling stations at the Bonjardim and Porto mud volcanoes, core logs and images of retrieved gas hydrates.

4. Results 4.1. Description of gravity cores

4.2. Grain size, aluminum and organic matter content

Table 3 gives a short description of sedimentology characteristics of the sampled cores. Observation of mud breccias, lithified and semi-lithified authigenic carbonate clasts, shell fragments, structure of sediments porous due to degassing of gas hydrates, and strong H2S smell indicate recent seepage at the sampled Bonjardim, Sagres, Soloviev and Porto MVs. In the cores from Porto carbonate structures were identified below 65 cm depth. Three cores collected in the structures (670G, 672G and 674G) consisted of clayish hemipelagic sediments without H2S smell or other characteristics related to the active seepage. Two

Table 4 presents the whole dataset of all parameters measured in the analyzed sediments. Sediments from the structures and Porto were composed of abundant silt and finer fraction (FF), 86–96% and 73–96% respectively, while lower proportions of the fraction < 0.63 µm were found in Sagres (71–82%), Bonjardim (65–76%) and Soloviev (66–77%). Samples from the studied MVs showed LOI varying from 10% to 18%, while lower and a wider range of values were observed in the Structures (5.5–17%) as shown in Fig. 2. Aluminum content varied within a narrow interval in Bonjardim, Porto, Sagres, and Soloviev 3

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Table 3 Observations and gravity core description sampled in mud volcanoes (MV) and three Structures in the Gulf of Cadiz. Site

Observations and core description

Sagres MV (669G) Bonjardim MV (675G)

Mud Breccia: 2–144 cm, grey with lithifield and semilithified clayed clasts; strong smell of H2S smell at the bottom of the layer. Mud Breccia: 0–36 cm, brown to grey, clasts represented by claystones and semilithified clay; Mud Breccia: 36–137 cm, dark grey, porous due to degassing, clasts represented by claystones and semilithified clay, strong smell of H2S. Mud Breccia: 0–30 cm, grey, soft with clasts up to 1 cm of diameter; Mud Breccia: 30–272 cm, dark grey, porous due to degassing of gas hydrates, large amount of clasts up to 147 cm of diameter. Mud Breccia: 0–24 cm, brown, oxidized, rich in foraminifera, size of clasts up to 3–4 mm; Mud Breccia: 24–96 cm, grey, with black and dark spots; Mud Breccia: 96 to −157 cm, dark grey, structureless, clasts up to 2–3 cm, strong smell of H2S. Clay: 0–27 cm, brown to greyish, foraminifera; Clay: 27–88 cm, dark grey with foraminifera, pieces of carbonate crust; Mud breccia:: 88–177 cm, light grey with semilithified clasts of mud stone and carbonate clasts from 65 cm towards the bottom; structure of sediments porous due to degassing of gas hydrates; Mud breccia:: 177–196 cm, dark grey with small amount of semilithified clasts. Clay: 0–22 cm, brown to greyish; Clay: 22–84 cm, dark grey with foraminifera, various pieces of carbonate crust from 68 cm towards the bottom; Mud breccia:: 84–89 cm, light grey with semilithified clasts and carbonate clasts; Mud breccia:: 89–177 cm, dark grey with small amount of semilithified clasts, shell fragments, smell of H2S; Mud breccia:: 177–196 cm, dark grey with small amount of semilithified clasts. Clay: 0–24 cm, brown with foraminifera; Clay: 24–66 cm, grey with foraminifera, large pieces of carbonate crust from 65 cm towards the bottom; Mud breccia:: 88–177 cm, dark grey with semilithified clasts and carbonate clasts; Mud breccia:: 177–196 cm, grey with small amount of semilithified clasts, porous due to degasing of gas hydrates, smell of H2S. Clay: 2–14 cm, brownish grey with foraminifera and small shell fragments; Clay: 14–46 cm, grey with foraminifera, shell fragments; Mud breccia:: 46–75 cm, dark grey with semilithified clasts; Mud breccia:: 75–317 cm, grey with clasts and carbonate clasts, structure of sediments porous due to degassing of gas hydrates, smell of H2S. Clay: 0–20 cm, light brown with foraminifera, bioturbated; Clay: 20–427 cm, gradual changes from light brown to light grey, foraminifera, bioturbated, dark spots. Clay: 0–55 cm, greyish to light grey, bioturbated, black spots of organic matter; Clay: 55–195 cm, light grey, bioturbated, dark patches, clasts of semilithified clay. Clay: 4–285 cm, change of colour from brownish to light grey, bioturbated, foraminifera.

Bonjardim MV (677G) Soloviev MV (680G) Porto MV (681G)

Porto MV (682G)

Porto MV (684G)

Porto MV (686G)

Structure I (670G) Structure II (672G) Structure III (674G)

0.90), both elements showing no accentuated variation with depth and among the MVs (Fig. 3). Magnesium varied within a narrower interval (1.1–2.2%) and no significant variations were found among the sampling sites or depth. Calcium varied inversely with Al (R2 = 0.84) and showed no significant relation with Mg.

(6.6–9.2%) with the exception of two samples from Porto (4.3% and 5.2%). Structures displayed statistically significant lower Al content (3.7–6.6%) than the other cores (H=49.103, p < 0.001). 4.3. Calcium, magnesium and strontium Calcium concentrations in the bulk sediments of the Structures (11–28%) were significantly higher than in all the other cores (H = 54.386, p ≤ 0.001) that varied between 1.4–7.1% in Bonjardim, Porto, Sagres and Soloviev with the exception of two samples containing 13% and 20% of Ca and low Al. Small carbonate concretions in the sediments of the Structures and in the two samples in the MVs may explain the elevated values of Ca. Strontium varied positively with Ca (R2 =

4.4. Iron and manganese Iron varied from 3.0% to 4.6% in the MVs Bonjardim, Porto, Sagres and Soloviev, while lower percentages were found in the Structures (2.0–3.5%). A good correlation (R2 = 0.76) was found between Fe and Al, differences of total Fe among sites and depth being partially explained by the Al variation (Fig. 4). Ratios to Al showed small variation

Table 4 Percentage of fine fraction (FF, %), loss on ignition (LOI, %), and concentration of major elements (%) and trace elements (mg kg −1) in bulk sediments of the gravity cores in Sagres (667 G), Bonjardim (675 G, 677 G), Soloviev (680 G), Porto (681 G, 682 G, 684 G, 686 G) and Structures (670 G, 672 G, 674 G) collected in the Cadiz mud volcano field. Sagres (n=7)

FF LOI Al Ca Mg Fe Ti P Mn Sr Cr V Zn Li Ni Cu Co Pb As Be Cd Hg

%

%

mg kg−1

Bonjardim (n=8)

Soloviev (n=3)

Porto (n=27)

Structures (n=28)

Median

Min-Max

Median

Min-Max

Median

Min-Max

Median

Min-Max

Median

Min-Max

73 14 8.2 5.4 1.6 3.9 0.50 0.11 566 296 137 145 104 74 41 40 15 14 8 2.3 0.2 0.05

71–82 13–16 7.3–9.0 4.2–7.1 1.4–1.6 3.0–4.0 0.36–0.56 0.09–0.12 477–712 222–350 132–141 127–172 94–126 65–79 39–44 29–47 13–16 14–16 5.4–22 1.9–2.5 0.1–0.7 0.05–0.06

73 13 8.2 3.0 1.3 3.5 0.63 0.10 648 299 135 149 106 61 47 48 17 15 5.5 2.4 0.2 0.06

65–76 12–15 7.4–8.6 2.5–3.3 1.2–1.4 3.3–3.9 0.60–0.64 0.10–0.11 613–671 236–348 131–141 127–152 88–111 55–64 45–49 45–50 17–18 14–15 4.9–12 2.3–3.0 0.1–0.3 0.05–0.07

75 13 8.5 4.1 1.8 4.2 0.40 0.12 725 275 137 172 112 62 44 41 16 15 5.0 2.0 0.3 0.05

66–77 13–15 8.4–9.2 3.4–4.6 1.7–1.9 4.2–4.6 0.39–0.42 0.12–0.12 607–859 244–302 134–146 160–182 105–114 61–67 43–46 32–43 14–17 14–16 4.6–15 2.0–2.1 0.2–0.9 0.05–0.06

85 14 7.8 5.8 1.4 3.7 0.52 0.10 592 417 120 141 103 59 43 43 15 15 5.9 2.1 0.3 0.05

73–96 10–18 4.3–9.0 1.4–20 1.2–2.2 2.2–4.2 0.21–0.62 0.06–0.11 211–823 251–1225 68–137 87–188 59–115 34–78 24–153 18–72 8.4–17 8.8–19 4.7–34 1.2–2.6 0.1–1.4 0.04–0.07

93 10 5.6 15 1.3 2.8 0.33 0.06 390 694 77 101 72 48 33 33 11 12 11 1.7 0.3 0.03

86–96 5.5–17 3.7–6.6 11–28 1.1–1.6 2.0–3.5 0.16–0.48 0.04–0.11 146–1226 506–1300 49–104 64–119 51–92 35–60 24–46 26–39 8.6–15 8.2–15 4.6–25 0.9–2.0 0.1–0.5 0.02–0.04

4

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Structures were strongly associated with Ca and Sr, while those of Bonjardim, Sagres and Soloviev were preferentially linked to LOI, Al and the associated elements. Porto samples were projected in a larger area of the PC1-PC2 plan, some samples being close to Ca, others to the trace elements, and a group of them to Ba, Cd and As. Several samples from Bonjardim, Soloviev and Porto were highly associated with Mn. 4.6. Comparison of elemental composition between sediments from mud volcanoes and upper continental crust To test whether chemical composition of sediments from MVs of the Gulf of Cadiz differs from upper continental crust, enrichment factor for each element (EF) was calculated by the ratio between Al-normalized value in sediments and the corresponding Al-normalized value in the upper continental crust reported by Rudnick and Gao (2003). Fig. 8 presents the mean and standard deviation of EFs encompassing the 74 sediment samples from all sampled sites. No significant (p < 0.05) differences were found between EFs calculated for MVs and the Structures. Most of the analyzed elements (Mn, Fe, Ni, Co, Pb, Be, Hg, Ti and Ba) showed EFs below 1.2, while other elements (P, Zn, V, Cr, Cu and Cd) have EFs between 1.2 and 1.6. Coefficient variations of EFs were below 20% indicating element/Al ratios relatively uniform both among the MVs and the Structures, as well as between MVs and Structures. Alnormalized values of these elements in MVs and Structures did not differ substantially from the upper continental crust normalized composition. Furthermore, these ratios were similar to the values observed in ancient sediments deposited offshore the south coast of Portugal 900 BCE (Mil-Homens et al., 2016), which eliminate possible doubts about the anthropogenic effect. Low EFs indicates that concentrations of those elements (Mn, Fe, Ni, Cr, Pb, Be, Hg, Ti, P, Zn, V and Cr) were not substantially affected by seepage gas-rich fluids (Magalhães et al., 2012; Niemann et al., 2006; Vanneste et al., 2012; Wang et al., 2015). A different situation was found for Cd, As and Ba since concentrations do vary more significantly among the samples. Variability was higher in MVs, (from 70% for As to 140% for Cd) than in the Structures, (from 30% for Ba to 110% for Cd).

Fig. 2. Depth variation of LOI (%) in cores of the mud volcanoes Sagres, Bonjardim, Soloviev and Porto (black symbols) and Structures (open symbols).

either in the MVs (mean ± sd = 0.46 ± 0.039) or in the Structures (mean ± sd = 0.52 ± 0.059). Manganese concentrations varied within wider intervals than Fe and showed no correlation with Al. In Bonjardim, Sagres, Soloviev and sediment layers of Porto containing mud breccias, Mn ranged from 357 to 859 mg kg−1, while lower values (211–316 mg kg−1) were found in the upper 60-cm clay sections of the four cores collected from Porto (Fig. 5). Concentrations in the Structures ranged from 146 to 506 mg kg−1 with the exception of the surface layer (0–20 cm) of the Structure 670 G (1226 mg kg−1). 4.5. Relations among trace elements and sediment chemical characteristics To identify possible correlations among major, minor and trace elements in bulk sediments, the Spearman rank order correlation was applied to the whole dataset. Iron, Ti, P, Cr, V, Zn, Li, Ni, Cu, Co, Pb, Be, Hg and LOI correlated positively to Al and to one another, while Mn, Ba, As and Cd were poorly correlated to other elements. The positive correlations between several trace elements and Al and the negative correlations with Ca found for most of the analyzed elements are illustrated by the Cr-Al and Cr-Ca plots (Fig. 6). Higher Cr values were found in Bonjardim, Porto, Sagres and Soloviev, while lower Cr and Al and higher Ca concentrations were measured in the Structures. Principal Component Analysis (PCA) was applied to the values of FF, LOI, Al, Ca, Mg, Fe, Ti, P, Mn, Sr, Ba, Cr, V, Zn, Li, Ni, Cu, Co, Pb, As, Be, Cd and Hg in all sediment samples (Fig. 7). First two axes of this ordination analysis account for 79.0% (51.3% PC1 + 27.7% PC2) of the total variance. The ordination pattern confirmed the contrast between two groups of descriptors projected in opposite sides of PC1: Al, LOI and most of the trace elements versus Ca and Sr. Barium, Cd and As were projected in the positive side of PC2. Points corresponding to the

4.7. Depth variations of barium, cadmium and arsenic ratios to aluminum Because Fe, Ti, P, Cr, V, Zn, Li, Ni, Cu, Co, Pb, Be and Hg concentrations followed the corresponding Al content as pointed by the PCA (Fig. 7), depth variations of Al-normalized values were attenuated, showing no vertical distribution pattern. On the other hand, Ba, Cd and As, once normalized to Al, do still show enrichments at certain depths. Depth profiles of Ba/Al ratios and Ba concentrations (Fig. 9) differed among the sampling sites. The ratios in the Structures varied within the interval 57–90 with a median of 68 × 10−4 and concentrations (mg kg−1) varied within the interval 233–519 with a median of 365. Two outliers at sub-surface layers (50–60 cm) were not considered:112 and Fig. 3. Depth variation of Ca (%) and Sr (mg kg−1) in cores of the mud volcanoes Sagres, Bonjardim, Soloviev and Porto (black symbols) and Structures (open symbols).

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Fig. 4. Depth variation of Fe (%) and Fe/Al ratio in cores of the mud volcanoes Sagres, Bonjardim, Soloviev and Porto (black symbols) and Structures (open symbols).

Fig. 5. Depth variation of Mn concentrations (mg kg−1) in cores of the mud volcanoes Porto, Sagres, Bonjardim, Soloviev and Structures.

349 × 10−4, corresponding to 656 and 1599 mg kg−1 respectively. Values in the upper 50-cm sediments of Bonjardim and Sagres (ratios: 41–46×10−4; concentrations: 336–400 mg kg−1) were slightly above the ratios at deeper sediments (ratios: 23–39 × 10−4; concentrations: 179–312 mg kg−1). Depth distribution defined a clearer pattern in the cores from Porto and Soloviev, presenting increases in the upper 50-cm layer (ratios: 240–656×10−4; concentrations: 2003–4157 mg kg−1), which exceeded more than 10 times the values found in deeper sediments consisting of mud breccia (ratios: 30–40 × 10−4; concentrations: 190–592 mg kg−1). Fig. 10 shows the depth profiles of Cd/Al ratios for the same three groups of cores. In the Structures ratios varied between 0.02 and 0.09 × 10−4, corresponding to 0.13 and 0.35 mg kg−1, respectively, with the exception of the values 0.13 × 10−4 (0.49 mg kg−1) and 0.14 × 10−4 (0.53 mg kg−1) between 150 and 200 cm depth. In contrast, the variations in Porto, Sagres and Soloviev were marked by accentuated

increases at 50 cm depth up to 0.19 × 10−4 (1.4 mg kg−1), while most of the other layers displayed Cd/Al ratios below 0.05 ×10−4 (0.34 mg kg−1). Sub-surface enrichment was not registered in cores from Bonjardim. A similar depth distribution was observed for As/Al ratios (Fig. 11). Whereas ratios in the Structures presented a broad variation (ratios: 0.9–5.6 × 10−4; concentrations: 4.6–21 mg kg−1) without a well-defined pattern, most of the cores from the MVs pointed to sub-surface enrichments at 50 cm, reaching 4.6×10−4 (34 mg kg−1) in contrast with the values registered in the other deeper layers (approximately 1.0 ×10−4 or 6 mg kg−1). 5. Discussion The elemental composition of sediments from mud volcanoes in the Gulf of Cadiz points to sub-surface enrichments of Ba, Cd and As. This pattern was better defined in the four cores collected from the Porto Fig. 6. Relations between Cr (mg kg−1) and Al (%) and between Cr (mg kg−1) and Ca (%) in sediments of the mud volcano field in the Gulf of Cadiz; data of mud volcanoes (Sagres, Bonjardim, Soloviev and Porto) and Structures are marked.

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concretions (Ritger et al., 1987) and hence small precipitates may be included in the sediment fraction analyzed in the present study. This may explain the variation of Ca content in the MVs (1.4–7.1%) and high values (13% and 20%) in two sub-surface samples of Porto 686G and Porto 684G (Fig. 3). High Ca content (11–28%) in hemipelagic sediments of the Structures may indicate the precipitation of calcium carbonates namely from biogenic origin, such as calcareous microfossils observed visually in the cores (Table 3). Calcium content is most certainly related to the presence of calcium carbonate, while Al and other elements are associated with clay content. The different mineralogies both Ca and Al are associated with, explain the negative correlations between trace elements and Ca (Fig. 6). Previous studies showed that methane-derived authigenic carbonates may incorporate different quantities of Mg and Sr (Magalhães et al., 2012; Merinero et al., 2012; Vanneste et al., 2012). Results of the present work indicate also a broad interval of Mg: Ca molar ratios in the mud breccias and clays from the four MVs (0.18–1.53, median 0.47). These ratios are higher than values reported for carbonate (0.04–0.08) sampled in the Darwin MV, Gulf of Cadiz (Vanneste et al., 2012) and comparable to ratios reported for other cold seep areas (Bayon et al., 2007; Eichhubl et al., 2000; Gontharet et al., 2007). Moreover, Mg: Ca ratios in sediments above MVs exceeded largely the proportion found in hemipelagic sediments (Structures: 0.08–0.20, median 0.14). Molar ratios of Sr: Ca obtained in the four MVs (0.22–0.93, median 0.35 × 103) were higher than in the Structures (0.18–0.24, median 0.20 × 103). Preferential incorporation of Mg and Sr in authigenic carbonates from MVs may indicate the presence of different minerals relatively to hemipelagic clays. Future studies on the mineralogy may elucidate the presence and proportion of aragonite, Mg-calcite and calcite in sediments.

Fig. 7. Principal Component Analysis (PCA) of the characteristics (fine fraction-FF, LOI and elemental composition) of sediments from the cores SA-Sagres (669 G), BOBonjardim (675 G and 677 G), SO-Soloviev (680 G), PO-Porto (681 G, 682 G, 684 G, 686 G) and S-Structure (670 G, 672 G and 674 G) sampled in the Gulf of Cadiz.

MV. Enrichment was found in the hemipelagic veneer above the mud breccia and authigenic carbonate structures identified by visual observations of the cores (Table 3, Figs. S1 and S2). At Bonjardim and the Structures no enrichments were observed, which might be due to the low sampling resolution. Despite the lack of pore water data, enhanced Al-normalized Ba values suggest Ba cycling at sub-surface layers nearby the STM, which is in agreement with pore water data measured from Bonjardim and Porto MVs as reported in Hensen et al. (2007) and Scholz et al. (2009). The association of Ba front with the STM was observed in other mud volcanoes of the Gulf of Cadiz (Vanneste et al., 2013, 2012) and of other areas of the world (eg., Castellini et al., 2006; Snyder et al., 2007). Barium front in Porto MVs is in agreement with the position of SMT close to the seafloor (Scholz et al., 2009). Pronounced increases of Cd/Al and As/Al ratios slightly below the Ba front, or within the same sediment interval, is interpreted as diagenetic enhancements most likely due to Cd and As sulphide precipitates. To the best of our knowledge Cd and As fronts nearby the SMT of mud volcanoes were not documented before in the Gulf of Cadiz or in other areas of mud volcanism.

5.2. Deficit of Mn in upper sediment layers By studying filtered and unfiltered seawater samples collected above methane seeps in the Gulf of Guinea, Lemaitre et al. (2014) proposed that seepage of methane-rich fluids at continental margins could be accompanied by substantial export of dissolved Fe and Mn forms to seawater. Vertical profiles of total Mn concentrations in sediments from the Porto MV (Fig. 3) indicate much lower values in the upper layers (211–316, median 287 mg kg−1) than below 50-cm depth (357–823, median 623 mg kg−1) and sediments from the other volcanoes (477–859, median 643 mg kg−1). It may be hypothesizes that low values correspond to a Mn deficit in solid sediments as result of the export of dissolved Mn forms to seawater. Characterization of deep seawater, suspended particles and topmost sediments in a future work is needed to test this hypothesis. Pattern observed in four cores of Porto contrasts with the Mn enrichment observed in the first 20-cm of the Structure 670G (Fig. 3). Most likely, it mirrors the diagenetic precipitation of Mn(IV) when Mn(II) diffused from lower layers meets O2

5.1. Variability of calcium, magnesium and strontium with lithology Authigenic carbonates were identified in various layers of the cores collected from the MVs of the Gulf of Cadiz (Table 1). These observations are in line with previous studies in the region (eg., Vanneste et al., 2012; Wang et al., 2015). As gas-rich methane interacts with sulphate of pore waters and releases bicarbonate, calcium carbonates precipitate as proposed in other studies (Aloisi et al., 2002; Liebetrau et al., 2010; Lykousis et al., 2009; Merinero et al., 2012; Vanneste et al., 2012). Size of the carbonate structures may vary from millimeter to centimeter-size

Fig. 8. Means and standard deviation of enrichment factors (EF) calculated as the ratio between Al-normalized element concentration in each sediment sample and the corresponding Al-normalized value in the upper continental crust (Rudnick and Gao, 2003).

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Fig. 9. Depth variation of Ba/Al ratios in the cores of Structures (a), of Sagres and Bonjardim (b), and of Porto and Soloviev (c).

cores (Table 3). Various works (eg., Castellini et al., 2006; Vanneste et al., 2013) have proposed a mechanism to the formation of authigenic barite: Ba-rich ascending fluids meet sulphate rich pore fluids in the shallow subsurface above the SMT; the depth where sulphate is reduced to hydrogen sulphide is influenced by various processes namely the AOM. High values of Ba (2003–4157 mg kg−1) registered in clay layers above mud breccia in Porto and Soloviev MVs are comparable to punctual enrichments in extracted sediments from MVs of the Gulf of Mexico reported by Castellini et al. (2006). Future studies of mineralogy may confirm the presence of abundant barite at those depths. Barium front in sediments from the Porto MV is located above the porous mud breccia with a smell of H2S. Slight enhancement of Ba (381–400 mg kg−1) in the Sagres and Bonjardim MVs may result from the low-resolution sampling or it mirrors weaker AOM.

(eg., Sundby, 2006). Total Fe profiles in Porto and other MVs were relatively uniform with the depth (Fig. 3), probably because only a small fraction of the total Fe was involved in the redox reactions contrarily to Mn (Sundby, 2006).

5.3. Barium front as proxy of sulphate-methane transition The low sampling resolution does not allow to detect thin layer elemental enrichments such as records of short-term diagenetic processes. Absence of chemical data on selective extractions and pore water composition is also a limitation to document undoubtedly ongoing or past post-mobilization processes. Nevertheless, the clear-cut enrichment of Ba in upper or sub-surface layers points to the existence of diagenetic processes influencing the total concentrations in solid sediments. Increase of Ba in the upper 50-cm layer is in line with the Ba enrichment registered in thin sediment layers of Carlos Ribeiro MV in the Gulf of Cadiz (Vanneste et al., 2013, 2012), in mud volcanoes of the Gulf of Mexico (Castellini et al., 2006), and in the mud volcano field of Black Ridge (Dickens, 2001; Snyder et al., 2007). The Ba front reported in these works was attributed to the cycling of this element just above time averaged depths of SO42- depletion. Barium enrichments observed in the present study (Fig. 9) result from a similar process. Although the location of the SMT cannot be determined by the current study because it may vary with the time according to the methane upward flux and subsequent AOM (Niemann et al., 2006), the depths of the Ba fronts registered in the four cores from Porto MV and in Bonjardim MV were comparable to the SO42- depletion depths reported in Hensen et al. (2007) and Scholz et al. (2009). Similar depths were also observed in Sagres and Soloviev MVs despite the Ba enrichments were less accentuated. Ba fronts were above the authigenic carbonates observed in the

5.4. Evidences of diagenetic fronts of Cd and As Sediments at 20–70 cm depth are 5–10 times enriched in Cd and As in five cores of Porto and Soloviev MVs and 3 times in Sagres MV (Fig. 6) points to diagenetic enrichments at those depths. These results suggest Cd and As fronts at the depth or slightly below the Ba front. To the best of our knowledge those observations have not been reported before for sediments above MVs. Sub-surface enrichment of Cd has been registered in sediments of marine environments and attributed to post-mobilization processes (Gobeil et al., 1997, 1987). As organic matter freshly deposited is oxidized, Cd is released to pore water and diffused downward. On the other hand, hydrogen sulphides are regenerated as metal sulphides react with oxides and diffuse upward. The pronounced peaks of Cd in solid sediments of MVs suggest that explanation may be extended to some MVs, such as Porto and Soloviev in the Gulf of Cadiz. Anaerobic

Fig. 10. Depth variation of Cd/Al ratios in the cores of Structures (a), of Sagres and Bonjardim (b), and of Porto and Soloviev (c).

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Fig. 11. Depth variation of As/Al ratios in the cores of Structures (a), of Sagres and Bonjardim (b), and of Porto and Soloviev (c).

Acknowledgments

oxidation of methane is the major producer of sulphides in these environments (Niemann et al., 2006) that diffuse upwards and meet dissolved Cd fluxed downward. Determination of Cd in pore water samples in future campaigns may confirm whether Cd is released to pore waters at topmost sediments as reported in Gobeil et al., (1997, 1987). Opposite fluxes of sulphide and Cd facilitate its sequestration as Cd sulphide. Occurrence of the Cd front nearby the Ba front, and presumably the SMT, suggests that sulphide produced by the AOM is rapidly consumed to sequester Cd and As. Arsenic values in the upper most sediment layer were higher than in layers below 50-cm depth (Fig. 9), this observation differs from the Cd profiles. Whereas Cd may be released to pore waters and diffused downward, As may interplay with Fe and Mn cycling in the oxic to suboxic zone before it interacts with the S reduced forms (Charlou et al., 2003). Slight enhanced values in upper most layers may represent the association with Fe oxides. When Fe(III) is reduced to Fe(II) at sub-oxic layers, As re-dissolves and As(III) diffuses to deeper pore waters layers, where meets hydrogen sulphides resulted from sulphate reduction, as proposed for coastal sediments (Edenborn et al., 1986). Sulphide production may increase in MVs by the AOM as suggested by the lithology of Porto and Soloviev sediments (Table 1) and the data presented in Scholz et al. (2009). Arsenic front was hence observed, as for Cd within or slightly below the Ba front. Cadmium and As fronts between 20 and 70-cm depth indicate these elements are not buried with the time. The most plausible argument is that the boundary between chemical zones where only certain redox reactions can take place may not be steady (Froelich et al., 1979; Kasten et al., 2004). Changes may occur in time triggered by variations of several factors that influence those reactions. In the case of MVs, the location of the sulphate-methane transition zone may vary as deep sourced methane fluxes change (Niemann et al., 2006), creating zones where elements are alternately oxidized and reduced, and where sulphides are alternately precipitated and dissolved, as proposed for Cd in coastal sediments (Gobeil et al., 1997). The fluctuation of the sulphate-sulphide boundary is therefore the most plausible explanation for the maintenance of Cd and As fronts within or near the SMT with a 50-cm thickness in Porto MV.

This work was supported by the Strategic Funding of CESAM (UID/ AMB/50017/2013), CIIMAR (CORAL project), CICECO (UID/CTM/ 50011/2013), Euromargins MVSeis Project (01-LEC-EMA24F) and through national funds provided by FCT/MEC and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020. Rui Monteiro (SFRH/BD/108535/2015) and Cláudia Mieiro (SFRH/BPD/100740/2014) acknowledge their grants to FCT. The manuscript benefited significantly from comments by Fritz Neuweiler, two anonymous reviewers and the editor Igor M. Belkin. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.dsr.2017.12.003. References Aloisi, G., Bouloubassi, I., Heijs, S., Pancost, R.D., Pierre, C., Sinninghe Damsté, J.S., Gottschal, J.C., Forney, L.J., Rouchy, J.M., 2002. CH4-consuming microorganisms and the formation of carbonate crusts at cold seeps. Earth Planet. Sci. Lett. 203, 195–203. http://dx.doi.org/10.1016/S0012-821X(02)00878-6. Aloisi, G., Wallmann, K., Bollwerk, S.M., Derkachev, A., Bohrmann, G., Suess, E., 2004. The effect of dissolved barium on biogeochemical processes at cold seeps. Geochim. Cosmochim. Acta 68, 1735–1748. http://dx.doi.org/10.1016/j.gca.2003.10.010. Aquilina, L., Dia, A.N., Boulegue, J., Bourgois, J., Fouillac, A.M., 1997. Massive barite deposits in the convergent margin off Peru: implications for fluid circulation within subduction zones. Geochim. Cosmochim. Acta 61, 1233–1245. http://dx.doi.org/10. 1016/S0016-7037(96)00402-4. Bayon, G., Pierre, C., Etoubleau, J., Voisset, M., Cauquil, E., Marsset, T., Sultan, N., Le Drezen, E., Fouquet, Y., 2007. Sr/Ca and Mg/Ca ratios in Niger Delta sediments: implications for authigenic carbonate genesis in cold seep environments. Mar. Geol. 241, 93–109. http://dx.doi.org/10.1016/j.margeo.2007.03.007. Begum, Z., Balaram, V., Ahmad, S.M., Satyanarayanan, M., Rao, T.G., 2007. Determination of trace and rare earth elements in marine sediment reference materials by ICP-MS: comparison of open and closed acid digestion methods. At. Spectrosc. 28, 41–50. Berndt, C., Cattaneo, A., Szuman, M., Trincardi, F., Masson, D., 2006. Sedimentary structures offshore Ortona, Adriatic Sea - Deformation or sediment waves? Mar. Geol. 234, 261–270. http://dx.doi.org/10.1016/j.margeo.2006.09.016. Boetius, A., Ravenschlag, K., Schubert, C.J., Rickert, D., Widdel, F., Gieseke, A., Amann, R., Jorgensen, B.B., Witte, U., Pfannkuche, O., 2000. A marine microbial consortium apparently mediating anaerobic oxidation of methane. Nature 407, 623–626. Borowski, W.S., Paull, C.K., Ussler, W., 1996. Marine pore-water sulfate profiles indicate in situ methane flux from underlying gas hydrate. Geology 24, 655–658. http://dx. doi.org/10.1130/0091-7613(1996)024<0655:MPWSPI>2.3.CO. Buforn, E., de Galdeano, Sanz, Udías, A, C., 1995. Seismotectonics of the IberoMaghrebian region. Tectonophysics 248, 247–261. http://dx.doi.org/10.1016/00401951(94)00276-F. Castellini, D.G., Dickens, G.R., Snyder, G.T., Ruppel, C.D., 2006. Barium cycling in shallow sediment above active mud volcanoes in the Gulf of Mexico. Chem. Geol. 226, 1–30. http://dx.doi.org/10.1016/j.chemgeo.2005.08.008. Chao, H.C., You, C.F., Wang, B.S., Chung, C.H., Huang, K.F., 2011. Boron isotopic composition of mud volcano fluids: implications for fluid migration in shallow subduction zones. Earth Planet. Sci. Lett. 305, 32–44. http://dx.doi.org/10.1016/j.epsl.2011.02. 033.

6. Conclusions Depth profiles of sediment element concentrations from Porto, Soloviev and Sagres MVs in the Gulf of Cadiz point to diagenetic enrichments of Ba, Cd and As in the upper sediments. Location of Ba front in Porto MV is in line with the depths of sulphate depletion registered in previous works. Cadmium and As fronts occur near the Ba fronts and did form as a result of AOM. The plotting of three elements along the axis PC2 of the PCA (Fig. 7) is a strong argument that Cd, As and Ba distributions within the sediment cores are highly influenced by the same process, AOM. 9

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