# Geochemical characterization of two distinctive systems with evidence of chemosynthetic activity, explored at the SE Pacific margin off Chile (46 degrees S and 33 degrees S)

Praxedes Munoz, Lissette J. Cardenas, Dieter Garbe-Schoenberg, Javier Sellanes, Laurent Dezileau, Ives Melville, Stephanie D. Mendes

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Praxedes Munoz, Lissette J. Cardenas, Dieter Garbe-Schoenberg, Javier Sellanes, Laurent Dezileau, et al.. Geochemical characterization of two distinctive systems with evidence of chemosynthetic activity, explored at the SE Pacific margin off Chile (46 degrees S and 33 degrees S). Progress in Oceanography, 2016, 148, pp.26-43. 10.1016/j-pocean.2016.09.002. hal-01412700

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Geochemical characterization of two distinctive systems with evidence of
chemosynthetic activity, explored at the SE Pacific margin off Chile (46°S
and 33°S)

Práxedes Muñoz¹,², Lissette J. Cardenas³, Dieter Garbe-Schönberg⁴, Javier Sellanes¹,

Laurent Dezileau⁵, Ives Melville¹, Stephanie D. Mendes⁶.

7

¹Departamento de Biología Marina, Universidad Católica del Norte, Larrondo 1281,

Coquimbo, Chile.

²Centro de Estudios Avanzados en Zonas Áridas (CEAZA), Coquimbo, Chile.

³Laboratorio de Oceanografía, Universidad Católica del Norte, Programa de Magister en
Ciencias del Mar, Facultad de Ciencias del Mar, Larrondo 1281, Coquimbo, Chile

⁴Institut für Geowissenschaften, Christian-Albrechts Universität zu Kiel, D-24118 Kiel,
Germany

⁵Laboratoire Géosciences Montpellier (GM), Bâtiment 22, 1er étage, Université de
Montpellier 2, Place Eugène Bataillon, 34095 Montpellier cedex 05, France

⁶Department of Earth Science, Webb Hall, UC Santa Barbara, Santa Barbara, CA 93106,
USA

19

20

21

22
1 Abstract

2

3 This study presents the geochemical composition of superficial sediment under oxic and

4 suboxic bottom water conditions along the Chilean continental margin (SE Pacific), where

5 evidence for benthic chemosynthetic activity associated with diffuse seeping of chemically

6 reduced fluids has been reported. The exploration was carried out at: (1) the Chilean Triple

7 Junction (CTJ), at a water depth of ~2900 m, with the additional indication of hydrothermal

8 activity near a methane-rich cold-seep area (46°S) (German et al., 2010; Thurber et al.,

9 2010); and (2) the El Quisco methane seep site (EQSS), at ~ 340 m water depth (33°S)

10 (Melo et al., 2007; Krilova et al., 2014). While the deeper CTJ is located within an oxic

11 environment (dissolved oxygen in the bottom waters: 164 µM), the shallower EQSS lies

12 within a suboxic environment (dissolved oxygen in bottom water: 23 µM), located within

13 the lower limit of the SE Pacific oxygen minimum zone (OMZ).

14 Pore water from short cores was analyzed for dissolved major, minor, and trace elements

15 (Cl, Na, Mg, K, Ca, Sr, Si, B, P, Ba, Pb, Mn, Fe, Cd, U, and Mo), δ¹³DIC, sulfide, sulfate,

16 and methane. The solid sediment fraction was likewise analyzed for total organic carbon

17 (TOC), metals, and redox potential. Elevated sediment temperatures were found in

18 superficial sediments (5–13°C) at the CTJ site, which could be due to warm fluids

19 associated with the proximity of the ridge, where hydrothermal vents may occur. Reduced

20 fluids were also present here, indicated by higher Mn fluxes towards the water column even

21 in oxidized sediments (RPD > 8 cm), which contrasted with the lower fluxes in reduced

22 sediments of the EQSS site (RPD ~ 2 cm). ¹³C-depleted DIC, anomalously low pore water

23 Cl (~15 ppb), and low concentrations of other major elements may be the result of dilution
1 by fluid seeping and precipitation of major elements, producing authigenic enrichment (Ca,

2 Mg, Sr). The fluid could also: (a) be diluted by pure water produced during methane

3 hydrate dissociation, as observed in other cold-seep areas; and (b) correspond to clay

4 mineral dehydration, as reported in plate subduction systems. The reducing conditions

5 established at the CTJ conduct the Cd enrichment at a similar magnitude of that seen at the

6 shallower suboxic site (EQSS). Evidence of chimney or vent fauna was not observed. At

7 the EQSS, higher TOC and total sulfide contents were consistent with enhanced deposition

8 of organic matter and reducing conditions developed in the OMZ, favoring the authigenic

9 enrichment of Cd, U and Pb. The geochemical evidence, based only on methane

10 concentrations and δ¹³DIC, is insufficient to establish the presence of methane seeps, as

11 previously reported.

12

13 Keywords: marine sediment geochemistry; cold seeps; reducing systems; deep sediments,

14 trace elements, Chilean Triple Junction, Chilean continental margin

15

16 *Corresponding author*: praxedes@ucn.cl, Tel. 56-51-209802, Fax. 56-51-209812

17
1

1. Introduction

2

The tectonically active Chilean continental margin is characterized by the presence of the

3

Atacama trench, resulting from the subduction of the Nazca plate beneath the South

4

American Continental plate (SACP). While an erosive, empty trench is present from North
to Central Chile (~20 to 33°S), an accretionary and sediment-filled trench develops

5

southwards (>33°S), with sediment input into the trench controlled by supply from the

6

continent. Subduction of the Chile Rise below the SACP, at ~46°S, forms the only place on

7

Earth where an actively spreading ridge is currently being subducted beneath a continental
margin, the Chilean Triple Junction (CTJ); adjacent to this site there is substantial evidence

8

for the occurrence of hydrothermal activity (Baker and German, 2004). An active

10

subduction front with a well-developed accretionary prism, and fluid migration from deep
within the subduction zone which includes hydrocarbon gas, has been described here

11

(Behrmann et al., 1992; Brown and Bangs, 1995; Lagabrielle et al., 2004). Additionally,

13

massive sulfide deposits altered by subsequent diagenesis have been reported in this area

14

(Lindsley-Griffin et al., 1995; Karsten et al., 1996). Results of stratigraphic studies using

15

seismic techniques between 35° and 46°S have revealed abundant bottom-simulating

16

reflectors (BSRs), indicating the widespread occurrence of shallow deposits of methane gas

17

hydrates (Brown and Bangs, 1995; Brown et al., 1996; Grevemeyer et al., 2003). It has

18

also been suggested that the presence of the accretionary prism and thermal heating of this
margin through the subduction of the oceanic crust could give rise to fluids, generating

19

hydrothermal vents, methane seeps, or even hybrid systems along the margin closest to the

20

CTJ (Brown et al., 1996; German et al., 2010). The hydrate layer close to the CTJ has been
estimated to be 100 m thick, covering an area of at least 1000 x 20 km, as indicated by

22

23
means of BSR analyses (Brown et al., 1996; Díaz-Naveas, 1999; Morales, 2003; Coffin et al., 2006).

Further to the North, the main active methane seep area so far identified along the Chilean margin is located off Concepción (CMSA, ~36°S) (Sellanes et al., 2004; Sellanes and Krylova, 2005; Sellanes et al., 2008). The area is on a slope between water depths of ~600 to 850 m, and it has been suggested as one of the largest known cold-seep areas on active continental margins, including both active and inactive sites (Klaucke et al., 2012). More recently, another probable cold-seep site was reported off El Quisco (~33°S); biological evidence (i.e. chemosymbiotic clams of *Austrogena nerudai*, empty tubeworms of siboglinid polychaetes) and authigenic carbonates, both indicative of a methane seep habitat, have been reported in this area (Melo et al., 2007; Krylova et al., 2014). The site is located within the oxygen minimum zone (OMZ), a significant midwater feature along the Chilean margin characterized by oxygen concentrations below 0.5 mL L⁻¹ (22 µM). This hypoxic water mass and resulting anoxic sediment are generated by the enhanced primary productivity in upwelling nutrient-rich surface waters, and subsequent remineralization of organic matter at midwater depths coupled with sluggish circulation. The OMZ covers an area of ~12 million km² and narrows seawards, from a thickness of about 300 m at 70°W to disappear at 103°W (Helly and Levin, 2004; Schneider et al., 2006), influencing the benthic assemblages and species distribution at this water depth in Northern and Central Chile (Veit-Köhler et al., 2008; Quiroga et al., 2010). The biogeochemical processes in these reduced sediments (Thandrup and Canfield, 1996; Muñoz et al., 2004) also favor the formation of authigenic minerals, such as phosphorite, and trace metal enrichment in pore waters (Muñoz et al., 2012).
Biogeochemical processes, occurring in both methane seep areas and hydrothermal
systems, generate hydrogen sulfide, methane, and other hydrocarbon-rich fluids. At
methane seeps, one of the most conspicuous processes indicative of chemosynthetic activity
is the authigenic formation of barium and carbonate minerals. This process is closely
related to the anaerobic oxidation of methane (AOM) (Greinert et al., 2002a, b, and
references therein) forming irregular crusts, reefs, and different forms of chimneys, or
stromatolites, that are associated with living organisms (Boetius et al., 2000; Greinert et al.,
2002a; Díaz-del-Río et al., 2003). At these sites, dissolved methane and sulfide
concentrations are variable, depending on both the microbial activity and fluxes of sulfate
and methane during the AOM. Other dissolved elements, such as Ba, B, and Cl anomalies,
can be taken as indicators of active seeping fluids rising from deeply rooted sources in
subducted sediments (Colwell et al., 2004). In contrast, metalliferous sediments associated
with seafloor hydrothermal systems are enriched with authigenic minerals due to the high
concentrations of sulfide and trace metals in the fluids. Dissolved metals rising inside
hydrothermal plumes react with oxygenated bottom waters, leading to the precipitation of
Mn and Fe oxyhydroxides, as well as other metals (German and Von Damm, 2006). This
study describes the geochemical characteristics of superficial sediments taken from two
different settings where chemosynthetic activity has been found along the Chilean margin
(Thurber et al., 2010; Krilova et al. 2014). The first, found in the southernmost segment of
the East Chile Rise, at the Chilean Triple Junction (CTJ, ~46°S), is a potential sedimented
hydrothermal system where discrete sites of venting have been reported in oxygenated
bottoms, at 2900 m water depth (German et al., 2010, 2011). The second is the El Quisco
Seep Site (EQSS, ~33°S), at a shallower depth of ~350 m, immersed within the lower
boundary of the SE Pacific OMZ. We determine the element concentration distribution in
1 the pore water and in the solid fraction, which is mainly a result of diagenetic processes in
2 surface sediments. The diagenesis involves the consumption, release, and diffusion of the
3 elements, creating concentration gradients and authigenic metal enrichments that indicate
4 the main geochemical processes in the sediments. We estimate the diffusive flux and
5 element enrichment factor in order to compare the primary geochemical reactions involved
6 within the sediments at both sites; this contributes to knowledge of the functioning of deep-
7 sea environments in the SE Pacific, as well as their contribution to the global balance of
8 elements in the ocean.

9

10
## 1. 2. Methods

## 2. 2.1. Sampling

Short sediment cores were obtained off the Chilean margin during a cruise on board R/V

Melville (Scripps Institution of Oceanography) in February-March 2010. The areas
investigated during this cruise were the Chilean Triple Junction (CTJ, ~46°S) and El

Quisco Seep Site (EQSS, ~33°S) (Fig. 1). Water column conditions, conductivity,
temperature, and dissolved oxygen concentrations were all registered using a CTD-rosette
with a SeaBird 911+ system equipped with dual C-T sensors, a SeaBird dissolved oxygen
sensor, and a 25 cm transmissometer.

Sediment samples for pore water and sediment chemical analyses were obtained using a TV-guided multicorer (MUC) with the capacity to simultaneously obtain 8 cores of about 50 cm length in soft sediments. The cores obtained at the CTJ were close to the axis and did not show evidence of seeps, but methanogenic bacteria were found (Thurber et al., 1987) at the position of MUC04. We were unable to obtain enough pore water for metal analyses from this core; instead, we obtained pore water samples from MUC06, further out but still near the axis. At the EQSS, sediments showed some dispersed clam beds of *Austrogena nerudai*; here, all cores (MUCs-15b, 19, 20, 21 and 22) were obtained from sediments with similar characteristics (Fig 2, Table 1).

Subsamples of sediment were taken from the original cores using 3.6 cm i.d. and 20 cm long polycarbonate liners immediately after collection. These short cores were sectioned at 1 cm intervals and squeezed under a N₂ atmosphere; pore water was extracted from the sediment cores using a nylon Reeburgh-type squeezer (Reeburgh, 1967). Solid sediment
1 fractions were kept frozen for chemical analysis of chloroplastic pigment equivalent (CPE),
2 total organic carbon (TOC), grain size, and element analysis.

3 Sediment redox potential (Eh) was measured onboard with a portable Mettler Toledo
4 Eh-meter by inserting a platinum electrode directly into the sediment of one core at
5 intervals of 1 cm.

6 For the determination of dissolved total sulfide ($H_2S$ + $HS^-$ + $S^{2-}$) and sulfate content in pore
7 waters the cores were sectioned at 2 cm intervals. The subsampling for methane analysis
8 was performed by inserting cut-off 3-mL plastic syringes, with pre-drilled lateral holes,
9 along the axis of each core. Sediment was immediately placed into serum vials partially
10 filled with 2M NaOH in order to terminate any bacterial activity. The vials were capped
11 immediately with a red rubber septum and $N_2$ was added using a syringe connected to a $N_2$
12 gas cylinder. The samples were shaken and then left to stand, allowing the dissolved
13 hydrocarbon gases from the sediment to equilibrate in the vial's headspace for a minimum
14 of 24 hours before analysis.

15 For trace metal analyses, concentrated ultrapure $HNO_3$ (2 µL) was added to the pore water
16 (>0.5 mL) placed in Supelco® pre-cleaned vials. Samples for dissolved inorganic carbon
17 analysis ($\delta^{13}$DIC) were placed in pre-evacuated glass vials (Exetainers Labco UK). All
18 samples were kept in darkness at 4°C until further analysis; lower temperatures were not
19 desirable as salt precipitations can occur during freezing.

20
## 1 2.2. Chemical Analyses

### 2 2.2.1. Pore water analyses

3 Quantitative determinations of dissolved total sulfide ($H_2S$ + $HS^-$ + $S^{2-}$) and sulfate were performed spectrophotometrically for small samples with detection range concentrations of 4 1 mM to 20 mM for sulfide, and 6 mM to 20mM for sulfate (Cord-Ruwisch, 1985). A UV-5 Vis spectrophotometer set at 400nm and 480nm was used to quantify the concentration of 6 sulfate and sulfide, respectively. For sulfate measurements, a calibration curve was made 7 using five standards of 0 to 5-fold dilutions of IAPSO (28.9 mM $SO_4^{2-}$) standard seawater. 8

9 For each sample, 200 µL of sample was added to 4 mL of water and then treated with 10 excess $BaCl_2$; this is a semi quantitative method allowing the establishment of the samples' 11 concentration range (Gieskes et al., 1991). For sulfide measurements, a calibration curve 12 was made using six calibrated standards ranging from 0–20 mM of sulfide. For each 13 sample, 100 µL of sample was added to 4mL of 2mmol $CuSO_4$ reagent and immediately 14 placed in the spectrophotometer. The analytical errors were 8% and 5% for sulfide and 15 sulfate, respectively.

16 The analyses of stable isotopes in dissolved inorganic carbon ($\delta^{13}$DIC) were performed at 17 Washington State University's stable isotope facility. A GV Instruments Multiflow 18 Preparation System was used to prepare the samples; they were then placed in sealed vials 19 and acidified with phosphoric acid. The gas liberated in the headspace was then sampled 20 and analyzed for carbon isotopes using a GV Isoprime Isotope Ratio Mass Spectrometer 21 (IRMS). A bicarbonate standard calibrated against NBS reference materials was used as an 22 internal standard in sample runs. The sample error is about 0.5%.
1 The methane analyses were performed according to Kinnaman et al. (2007). The gas in the
2 vial's headspace was quantified using a Shimadzu GC-14A Gas Chromatography-Flame
3 Ionization Detection (GC-FID). Samples were run on the GC-FID under isothermal 60 °C
4 conditions using N₂ as the carrier gas through a 12' x 1/8" packed column (n-octane on
5 Res-Sil C). The precision of these analyses is ±4%.

6 Dissolved elements in pore water were analyzed at the ICPMS laboratory of the
7 Geosciences Institute, Christian Albrechts University of Kiel (Germany). Trace metals (Ba,
8 Pb, B, Cd, U and Mo) were determined using an AGILENT 7500cs ICP-MS instrument
9 with a precision that varied between 0.1% and 6%, as determined from 2 replicas out of 7
10 samples. The accuracy was monitored with IAEA W4, NIST1643e, IAPSO seawater, and
11 NASS-4 certified reference materials (CRM), yielding deviations from certified values
12 between 0.4 and 8.4%. Sample preparation and analytical procedures are described in
13 Garbe-Schönberg (1993). Other elements (Sr, Si, Mg, K, Mn, Fe, P, Ca, Na and Cl) were
14 analyzed using a SPECTRO Ciros SOP ICP-OES instrument with a precision between 0.3
15 and 0.7%, as estimated from four replicas; Mn, Fe, and P showed slightly lower precision
16 (0.6–9.2%) due to low concentrations in some samples. Accuracy was monitored with the
17 same suite of CRMs.

18
19 2.2.2. Sediment analyses

20 Sediment surface CPE (Chlorophyll-a + phaeopigments) content was analyzed
21 fluorometrically (Plante-Cuny, 1973). Samples were kept frozen at -20°C and thawed
22 before extraction. Portions of 0.2 – 0.3 g were mixed with 0.1 g of MgCO3 and placed in
23 glass vials. Then, sediment was weighed (precision ± 0.1 mg) and 5 – 10 ml of 90%
acetone was added. Vials were vigorously shaken within a vortex (30 sec.), then stored
overnight at 4°C in the dark, and later centrifuged (10 min at 800 x g). The supernatant was
utilized for spectrophotometric determinations.

Total organic carbon (TOC) and stable isotope (δ¹⁵N and δ¹³C) analyses in sediment
samples were performed at Washington State University. The samples were dried at 80°C
for 24 h, milled to a fine powder, and acidified to remove carbonates. Dry material (0.5 – 2
mg) was placed into tin capsules and combusted in an Eurovector elemental analyzer
(Milan, Italy). The resulting N₂ and CO₂ gases were separated by gas chromatography and
admitted into the inlet of a Micromass (Manchester, UK) Isoprime isotope ratio mass
spectrometer (IRMS), to determine ¹⁵N/¹⁴N and ¹³C/¹²C ratios. Typical precision of
analyses is ±0.5 ‰ for δ¹⁵N, and ±0.2 ‰ for δ¹³C, using atmospheric nitrogen as a standard
in the first case and Peedee belemnite (PDB) in the second. Egg albumin was used as a
daily reference material.

Trace elements in the solid sediment fraction were analyzed at the Laboratoire Géosciences
of the Université de Montpellier 2 (France) using ultra-pure techniques. ~50 mg of
sediment were placed into PTFE beakers, previously cleaned using different acid baths, and
then digested with supra pure acids (e.g., HNO₃, HF, HClO₄, HCl) in consecutive steps.
The samples were placed on a hot plate at ~160 °C and left closed overnight, then
evaporated on a hot plate at ~90°C to dryness; we used MAG-1 as reference material. Once
the sediments were dissolved, they were diluted in HNO₃ (2%) to a dilution factor of about
4000 in order to measure the samples in an Agilent 7700x quadrupole inductively coupled
plasma mass spectrometer (ICP-MS). For internal standardization, ultra-pure solution
enriched in In (1 ppb) and Bi (1ppb) were used, to avoid mass-dependent sensitivity
variations of the sample matrix and instrumental origin. Polyatomic interferences were
1 controlled by running the machine at an oxide production level < 1%; typical analytical
2 precisions attained by this technique were generally between 1–3% relative standard
3 deviation.

4

5 2.3. Data analysis

6 2.3.1. Estimation of dissolved trace metal fluxes

7 The diffusive fluxes (J) were estimated according to Fick's first law (Berner, 1980),

8 dependent on the pore water concentration gradients (ΔC/Δz) under steady state condition.

9 The movement of the ions occurs in the volume occupied by the pore water according to
10 the diffusive coefficient of each element:

$$
11 \quad J = - \oint_D D_{\text{sed}} \Delta C / \Delta z \qquad (\text{eq. 1})
$$

12 $\phi$ is the porosity and the $D_{\text{sed}}$ the diffusion coefficient. The porosity was determined from
13 the water content at each section of the core. $D_{\text{sed}}$ was estimated for seawater ($D_{\text{sw}}$) from the
14 diffusion coefficient of the solutes in free solution, taking into consideration the
15 temperature of the bottom water at the shelf sites and corrected for tortuosity $\theta^2$ (Li, 1974;
16 Boudreau, 1997). For calculation details see Schulz (2006):

$$
17 \quad D_{\text{sed}} = D_{\text{sw}}/\theta^2 \quad \text{and} \quad \theta^2 = 1 - \ln(\phi^2) \quad \text{(eq. 2)}
$$

18 The concentration gradients determine the flux direction, i.e. increasing concentrations in
19 depth will result in negative gradients in the sediment and positive fluxes towards the
20 bottom waters. On the other hand, decreasing concentrations will result in negative fluxes
21 i.e. towards the sediments. We calculated the concentration gradients using only some
22 sections of the core, where the concentration gradients were maximum, close to the surface,
23 as these gradients control the fluxes to the bottom water. We used the resulting flux
1 calculations for comparison purposes which helped us to differentiate the main diagenetic
2 processes at each site.

3

4 2.3.2. Estimation of elements authigenic enrichment (EF)

5 The element concentrations in the solid sediment fraction were normalized to Al
6 concentrations (Calvert and Pedersen, 1993; Tribovillard et al., 2006; Calvert and Pedersen,
7 2007) which is an indicator of aluminosilicate portion within the sediments, unreactive
8 during organic matter diagenesis.

9 The EF (enrichment fraction) of elements was determined in order to compare the element
10 authigenic enrichment at each area considering the crustal abundance.

11

$$
\text{EF: } (\text{Me/Al})_{\text{sample}} / (\text{Me/Al})_{\text{detrital}} \quad (\text{Eq. 3})
$$

12 Where (Me/Al)sample is the bulk sample element concentration normalized to the Al content,
13 and the denomination “detrital” indicates a lithogenic background. Detrital concentrations
14 ([Me]detrital and [Al]detrital) were established by taking into consideration the element
15 abundance at closer localities from where was possible to have available information; this
16 is more accurate than using mean Earth crust values (Van der Weijden, 2002). Due to there
17 being no information for element concentrations from continental inputs for the CTJ, we
18 used the element concentrations in deep clays for this area (Chester, 1990), and we used the
19 element concentrations in suspended river particles from the Bío Bío river (37°S) for
20 calculations at the EQSS site; this is the closest location where this information was
21 available for central Chile (Table 2). If EF is more than 1, this means that the element is
22 enriched with respect to detrital values, and if less than 1, it is depleted, constituting a good
1 tool for interpreting authigenic precipitation (Morford and Emerson, 1999; Tribovillard et

2 al., 2006).

3

4
## 1 3. Results

## 2 3.1. Bottom water environmental conditions and underlying sediment characteristics

3. At the CTJ site the bottom waters presented oxygenated conditions (> 150 µM), lower temperatures (1.83°C), and slightly higher salinities (34.68 psu). In contrast, the EQSS site (8.45°C and 34.53 psu), located on the upper slope within the OMZ, exhibited lower oxygen concentrations (23 µM, Table 1).

7. Overlying water and surface sediment temperatures were measured immediately after retrieval at the CTJ site using a thermocouple. While the temperature of the supernatant water did not greatly differ in comparison with the bottom temperature measured with the CTD, the sediment temperature showed important differences. In MUC06 the temperature ranged between 4 to 5 °C, and in MUC04 the sediment temperature reached 13°C; in both cores the sediment temperature was at least 4°C higher than that of the bottom water (1.83°C).

14. The redox measurements of surface sediment showed positive values between surface and 6 cm depth at the CTJ (Fig. 3). Below that Eh quickly reached negative values of around -100 mV and -200 mV, respectively, except in MUC04 which showed positive values at all depths. At the EQSS, the redox potential discontinuity layer (RPD) was observed at 4 cm, reaching -200 mV at 10 cm (Fig. 3); at this site, sediment smelled strongly of sulfide.

19. The pigment content (CPE) integrated in the first 6 cm of surface sediment showed higher values at the EQSS (12 to 28 µg g⁻¹), and consequently higher inventories (ΣCPE: 72–122 µg cm⁻²) and higher carbon content (ΣTOC: 380–568 gC m⁻²), with isotopic values (¹³C and ¹⁵N) indicating their main organic source to be plankton (~19–21‰). The deeper CTJ site
1 showed comparably lower pigment content and TOC inventories, with slightly different
2 isotopic values than those from the EQSS (Table 1).

3.2. Pore water chemistry

4 Methane concentrations measured at the EQSS were between 18 and 50 nM, with
5 maximum values at the bottom of the cores (Fig. 4); no measurements were done at the CTJ
6 site, as hard bottoms prevented the collection of sediment cores suited for this analysis (Fig.
7 2).

8 Sulfide concentrations were relatively low at both sites, between 0.3 and 1.2 mM (Fig. 5,
9 Table 1). Sulphate concentrations were lower at the CTJ site than at the EQSS, showing
10 values between 15 and 18 mM, and between 21 and 31 mM, respectively (Fig. 5).
11 Comparing the sulfate and sulfide inventories in only the first 7 cm, due to the cores
12 showing different core lengths, higher sulfate and sulfide inventories occurred at the EQSS
13 site (Table 1).

14 Stable isotope analyses from pore water DIC samples may suggest different sources of
15 carbon, given that more negative δ¹³DIC values were measured at the CTJ site (-20‰ – -
16 10‰) than at the EQSS (-10‰ – 3‰) (Fig. 6).

3.3. Dissolved trace metal distribution

19 The elements distribution analyzed in the pore water of sediment cores from the CTJ and
20 EQSS sites are shown in Figs. 7 and 8. The elements were organized according their
21 concentration and reactivity: first, the major elements including B (higher concentrations
22 and conservative behavior at water column); followed by minor and nutrient elements (Sr,
1

Si, P; lower concentrations), and trace elements (very low concentrations) related mainly

2

with the particle transport (organic and inorganic; Pb, Ba, Cd), and oxidation-reduction

3

reactions (Mn, Fe, U and Mo). The mean values of Cl and Na are also shown in Table 1.

4

Both sites presented similar concentrations of Cl and Na except in core MUC06, from the

5

CTJ area, which showed lower values particularly for Cl concentrations. Molar Mg/Ca and

6

Sr/Ca ratios indicate similar values for both stations (~5 and ~0.0083, respectively) (Fig. 9).

7

The lowest Sr and Mg concentrations, together with the lowest Ca concentrations, were

8

observed at the CTJ, maintaining metal ratios similar to those estimated for the EQSS site.

9

The correlations between major elements were very good at both sites but higher values

10

were observed at the CTJ (Appendix A), showing a conservative behavior; others,

11

including nutrient type elements and trace metals, did not show good correlations at either

12

site.

13

The major element concentration profiles analyzed from core MUC06, taken from the CTJ

14

site, revealed lower metal concentrations in the core than those expected for major elements

15

in bottom waters. These concentrations dropped between 7 and 15 cm core depth, reaching

16

half the concentrations of those in surface core sections. An example of this can be seen in

17

the case of Cl, where concentrations of 500 mM fall to ~200 mM at these depths within the

18

core (Fig 7). Similarly depleted values between 7 and 15 cm core depths were observed for

19

B, Sr and Si profiles. Si and B showed good correlation as both are closely related with clay

20

composition (r=0.85, p<0.01; Appendix A). At the CTJ, ten times higher concentrations of

21

Mn were observed than those at the EQSS, with concentration values increasing with depth;

22

the highest values occurred below 3 cm core depth, at the same time Fe concentrations

23

decreased to values lower than those at the EQSS site. In the surface, Fe at the CTJ site was

24

slightly higher than expected for an oxic zone, due to a higher source of oxides. Other trace
1 elements also showed decreasing concentrations with increasing depth at the CTJ, probably
2 due to precipitation in the presence of sulfides (CdS), or reduction (U); these concentrations
3 were also lower than those at the EQSS site (Figs. 7 and 8).

4 The major elements measured in core MUC19 from the EQSS site showed slightly higher
5 concentrations than at the CTJ site, with a relatively constant concentration to depth
6 correlation (Mg, K and Ca) (Fig. 8). Similarly, Sr, Si and P showed no relevant changes
7 with depth; Sr and Si concentrations were slightly higher than those at the CTJ, and P
8 showed lower values in surface sediments (0 – 5 cm). Trace elements showed increasing
9 concentrations with depth, except for Ba, Pb and Cd (Figs. 7 and 8).

10 3.4. Diffusive fluxes

11 The diffusive fluxes were estimated taking into consideration the maximum concentration
12 gradients, from the sediment surface to deeper sections within the sediment core, adjusting
13 for the best linear or exponential curve; alternatively, the difference between two points was
14 considered in those cases where the curve fixing resulted in low r² estimations. Sediment
15 parameters used for flux calculations are shown in Table 3. The estimated fluxes represent
16 the main movement of ions close to the sediment water interface. The major elements (Mg,
17 K and Ca) indicate fluxes towards the sediment (negative values) on both sites, but the
18 magnitude was higher at the CTJ (Fig. 10a, b); similarly, Sr showed negative values at both
19 sites, also higher at the CTJ. Other elements such as B and Si showed lower fluxes toward
20 the water column (positive values) at both sites, though higher at the EQSS site. Likewise,
21 positive fluxes of P were estimated to be distinctly higher at the CTJ. Those elements (Mn,
22 Fe) sensitive to redox changes precipitated under oxygenated conditions showed very high
23 fluxes, positive for Mn and negative for Fe at the CTJ. In contrast, the EQSS site showed
1 positive values for both elements but very low fluxes for Mn. Other elements (U, Mo)

2 showed very low negative fluxes at the CTJ, and higher positive fluxes at the EQSS. Lead,

3 Ba, and Cd showed similarly negative fluxes at both sites.

4

5 3.5. Me/Al ratios and EF estimations

6 The Me/Al did not show big changes in depth at 21 cm core length at either site (Appendix

7 B), therefore we use an average value of the EFs calculated at intervals of 1 cm in the core

8 depth for each element (Fig. 11); this resulted in higher EF values for major elements at the

9 CTJ, especially for Ca, indicating an enrichment that was not observed for these elements at

10 EQSS. Trace elements that precipitate under low oxygen conditions or when associated

11 with particle deposition were enriched at the EQSS (Pb, Ba, Cd, U and Mo); Cd and U

12 were also enriched at the CTJ but less than at the EQSS. In contrast, Mn and Fe were not

13 enriched at either site.

14 It is important to consider that deep clays have higher Al and Fe content than the sediments

15 observed at the CTJ, which means the EFs could be underestimated. If the CTJ sediments

16 receive important continental input, it should be possible to make the comparison with

17 Me/Al ratios from Patagonian river sediments; this is quite far from our sampling site

18 however (54°S; Table 2). Using the Patagonian sediments, only a slight enrichment of Mn,

19 Fe and Ba (1.07, 1.11, 1.12, respectively) and similar enrichment of Ca and Sr was

20 estimated; therefore, as no other elements were available for Patagonian rivers to calculate

21 the EFs, they were calculated using Me/Al ratios from deep sea clays.

22

23
1 4. Discussion

2

3 4.1.- Sediment characterization and environmental conditions

4 As expected, at the deeper CTJ site bottom waters had higher oxygen content and surface
sediments had lower organic content, a result of particle fluxes from the water column,
5
6 consistent with lower ΣCPE and lower ΣTOC (Fig. 12); the lowest mean C/N ratio (7.8 and
9.3) indicates the presence of a less refractory material than at the EQSS site (11.8 and
7
8 10.4; Table 1). The EQSS sediments are receiving the particle material from the adjacent
shelf, highly recycled until their deposition in deeper zones. The sediment conditions at the
9
10 CTJ were oxidized. Along MUC04 positive redox values were observed, favored by a
slightly higher grain size which allowed for more efficient organic matter degradation; this
11
12 differed from MUC06, near the axis, where more reduced conditions were observed below
13 8 cm core depth (Fig. 3, Table 1). Here, there was no evidence of seeps or bacteria mats in
14 the vicinity of MUC06, contrasting with the location of MUC04 where methanotrophic
bacteria were present in both surface sediments and bottom waters (Thurber, comm. Pers.).
15
16 Additionally, anomalies in surface temperatures of sediments at the CTJ were observed,
with a maximum measured sediment temperature of 13°C at MUC04 and 5°C at MUC06,
17
18 while the bottom water temperature was 1.83 °C at both coring locations. These constitute a
19 clear indicator of the presence of hydrothermal fluids percolating through the sediment at
this site. Despite this, no chimney structures, crusts, or stromatolitic aggregates were
20
21 observed here; however, hard bottoms were present. This is somewhat to be expected,
considering the proximity with a continental margin characterized by fjords and rivers
22
potentially generating a high input of terrigenous material; it is most probable that this site
23
corresponds to a sedimented area with lower-temperature (<20 °C) diffuse flows, according to preliminary observations in this zone during the cruise (German et al., 2010; German et al., 2011; Thurber et al., 2012).

In contrast with the CTJ area, which is characterized by the presence of oxygenated waters, the sediments at the EQSS are affected by the OMZ and therefore show low oxygen content in bottom waters (23 µM, Table 1). The existence of methane seepage and the presence of a cold-seep biological community at the EQSS site has been evidenced by the presence of living clams of a new chemosynthetic vesicomyid genus, *Austrogena nerudai*, in a core retrieved closer to our coring station (~360 m); tubes of siboglinid polychaetes, probably of the genus *Lamellibrachia*, as well as authigenic carbonates have also been described (Krylova et al., 2014). This is similar to the fauna found in a southern area off Concepción, where a methane seep area was described (~36°S; Sellanes et al., 2004, 2008). The reduced sediments at the EQSS smell of sulfides however pore water analysis did not yield significantly higher sulfide levels; additionally, lower methane content was measured (Table 1, Fig. 4, 5). The magnitude of the fluxes here is probably low, but it sustains chemosynthetic clams which inhabit a wide range of reduced environments, developing strategies to adapt to different compound concentrations necessary for their metabolism (Roeselers and Newton, 2012).

## 4.2. Sulphate-sulfide, methane, ¹³δDIC, ¹³δTOC

In general, the distribution of sulfate and methane in the sediments is closely related to anaerobic oxidation processes of organic or inorganic compounds. Sulfate is a conservative element in seawater and can penetrate several meters into the sediment; it is reduced to
1 sulfide by sulfate reducing bacteria, through the oxidation of organic matter, diminishing its
2 concentration until it is totally consumed and, consequently, sulfate reduction ceases
3 (Fossing et al., 2000 and references therein). Pore water sulfate concentrations at the EQSS
4 were in the order of values reported for sediments under suboxic and oxic conditions off the
5 Chile - Perú margin (20-30 mM, ~100-1000 m water depth; Coffin et al., 2006; Raiswell
6 and Canfield, 1996), and the sulfide concentrations were lower than expected but within the
7 order of magnitude reported for anoxic sediments of the Chilean continental margin (from
8 0.3 mM to 1.4 mM at Concepción Bay; Ferdelman et al., 1997; Thamdrup and Canfield,
9 1996). The sulfides should be precipitating as metal sulfides, reducing the soluble phase
10 (Böning et al., 2005). At the CTJ site sulfide (< 0.5 mM) and sulfate (< 20mM) values were
11 lower than at the EQSS; the sulfide is probably rapidly oxidized in surface sediments in
12 contact with oxygenated conditions of the bottoms (Jørgensen and Revsbech, 1983;
13 Fenchel and Bernard, 1995), or else actively oxidized by chemosynthetic activity (Govenar,
14 2012; O'Brien et al., 2015). For comparison, a similar situation is found in the Comau
15 Fjord in Southern Chile (~42°S), where a shallow vent system of reduced fluids sustains a
16 distinct microbial community; low sulfide concentrations were found close to the vents due
to the rapid oxidation of sulfide, even in places with steady sulfide fluxes (116-587 µM;
17 Ugalde et al., 2013; Muñoz et al. 2014). Sediments at the CTJ were less reduced (positive
18 redox potential in the first 5 cm), with higher porosity and grain size than at the EQSS site,
19 favoring sulfide oxidation. The sulfate concentrations were lower than expected for salinity
20 in bottom waters (~27.6 mM), in agreement with the distribution of other major elements,
21 as explained later.
22
1 The massive presence of methane deposits along the Chilean margin, which has been
2 confirmed by seismic surveys (Brown and Bangs, 1995; Brown et al., 1996; Díaz-Naveas,
3 1999; Morales, 2003; Coffin et al., 2006), seems to be mostly associated with the biogenic
4 processes of methanogenesis during the oxidation of organic matter. Here, gas hydrates are
5 formed even with low TOC content (<0.5%), and the methane clearly results from
6 important *in-situ* sources of carbon (Froelich et al., 1995; Brown et al., 1996). Although the
7 presence of methane seeping has been proposed at the EQSS (Krilova et al., 2014), we
8 observed low methane concentrations (20 – 60 nM; Fig.3). At other sites, sedimentary
9 methanogenesis can easily produce values of methane in the range of 0.5 – 6 mM when
10 measured at greater depths within the sediments (> 100 cmbsf: 0.5 – 6 mM; Chilean margin
11 34°–44°S; Treude et al. 2005; Gulf of Mexico; Ussler and Paull, 2008); however, aerobic
12 and anaerobic methane oxidation processes reduce the gas content within sediments closer
13 to the surface (Knittel and Boetius, 2009). This could partially explain the low
14 concentrations observed at the EQSS, particularly in our short sediment cores (25 cm)
15 where the oxidants are suitable for AOM (i.e. sulfate, nitrate, among others) reducing the
16 large input of the methane to the oceans (Caldwell et al., 2008; Reeburgh, 2007; and
17 references therein).
18 The presence of gas hydrates and upward fluid migration has been reported in the vicinity
19 of our coring stations at the CTJ (Behrmann et al., 1992), and methanotrophic bacteria is
20 also recently being described (Thurber pers. Comm). Unfortunately, we could not obtain
21 methane measurements within the sediments and other parameters, as DIC were performed.
22 The relationship between carbon sources and sulfate consumption determines the carbon
23 isotopic composition of DIC, and indicates the nature of this oxidative process (sulfate
reduction); very negative δ¹³C-DIC values result from the anaerobic methane oxidation, when the δ¹³C of methane is 20–75‰ more negative than organic matter (Kastner et al., 2008). This process produces ¹³C-depleted bicarbonate ([HCO₃⁻]), which reacts with seawater Ca to precipitate methane-derived carbonates, resulting in very negative δ¹³C-DIC values (< -30 – -60‰; Aharon et al., 1992, Teichert et al., 2005). Our measurements were less negative (~ -20 – -10 ‰) than can be explained by sulfate reduction (< -12 – -26‰) and the remineralization of less enriched ¹³C organic matter in surface sediments.

The DIC composition of the pore waters also depends on carbonate dissolution and re-oxidation of sulfide; based on the sulfate reduction intensity and Ca concentrations in pore waters, two end-members have been described by Walter et al. (2007). The most negative values of δ¹³DIC (end-member 1) result from low carbonate dissolution producing moderately depleted Ca in pore waters in a closed system of sulfate reduction; this is similar to what we observed at the CTJ site (MUC 06), where the Ca content (< 10 mM; Figs. 7) was lower than expected from the Ca/Cl measured in the adjacent bottom waters (10.19 mM). End-member 2 is characterized by very positive values of δ¹³DIC, associated with bioturbated sediments and sulfide oxidation producing Ca excess in pore waters; the δ¹³DIC values at the EQSS site should be compatible with this scenario, as the pore water Ca concentrations measured (10.4±0.4 mM Ca) were slightly higher than in the bottom water, when normalized to salinity (10.14 mM; Fig. 8). Bioturbation could be an important process here, increasing the isotopic values of DIC (more positive). Our samples fall between these two end-members, indicating the contribution of both organic matter re-mineralization and the balance between carbonate precipitation and dissolution in the pore
water. Furthermore, no evidence of AOM was obtained from the DIC isotopic values, as
the measurements were carried out in surface sediments.

3

4.3. Major and minor elements

The lowest concentrations of major elements including sulfate, when compared with those
expected for bottom waters were found at the CTJ site (Fig.7). All of them maintained a
similar proportion with Cl along the core, which could be attributed to dilution by deeper
fluid seepage; in the accretionary prism, fluids are normally less concentrated than seawater
due to the depth of clay-mineral dehydration during tectonic compression (Haese et al.,
2006; Hensen et al., 2007). Additionally, gas hydrate dissolution processes could be
occurring at this site due to the decomposition reactions involved in the formation of gas
hydrates, introducing fresh water which lowers the Cl concentrations in pore fluids (Paull et
al., 1996 in Borowski, 2004; Gieskes and Mann, 2007). On the other hand, these lower
concentrations in pore waters could respond to mineral precipitation, for example, calcium
carbonate, considering that the lowest δ¹³C-DIC were found at similar depths (~7–15 cm);
however, given their proportional decrease, the Sr/Ca and Mg/Ca did not differ
significantly at the EQSS site (Fig. 9). This supports the dilution hypothesis, because the
mineral precipitation establishes different equilibrium concentrations for each cation,
modifying the conservative behavior. Alternatively, low Cl, Mg and sulfate concentrations
could be attributed to the upwelling of hydrothermal fluid observed in surface sediments
(Aquilina et al., 2013).

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

Boron is removed from seawater via uptake into the igneous oceanic crust, and adsorbed
onto detrital sediments (You et al., 1995, and references therein). It is considered a
conservative element and may be enriched in cold seeps and hydrothermal fluids via
1 dehydration and desorption (Haese et al., 2006, and references therein), reaching values
2 between 3.5 and 15 mM in these environments (Wei et al., 2005); these are much higher
3 than our reported concentrations (1–4 mM, Figs. 7 and 8). The lattice bound B desorption
4 produces higher B concentrations and simultaneously lower Cl concentrations at very high
5 temperatures and very deep below the sea floor (>150°C; 2.5 km; You et al., 1995); this
6 was not comparable with our short cores. Our data indicates lower Cl and B concentrations
7 along the core at the CTJ site (Fig. 7), with a similar distribution pattern to that of major
8 elements, supporting the idea of pore water dilution as suggested before. It is important to
9 consider that a substantial proportion of B in expelled fluids is likely reabsorbed or trapped
10 by surrounding sediments as the temperature decreases, due to the B adsorption-desorption
11 mechanisms being highly dependent on environmental conditions in pore waters. It is
12 therefore possible that adsorption or co-precipitation with carbonates could be favored by
13 the low temperatures and higher pH at the surface (Vengosh et al, 1991; You et al., 1995),
14 which should explain the good correlation with Si (r=0.85; Appendix A). The mean
15 concentrations of B at the EQSS site do not differ significantly from those at the CTJ, also
16 showing increasing concentration at depth but without correlation with other elements. The
17 organic content accumulated in sediments from the CTJ site was at least 30% of that
18 accumulated at the EQSS, therefore the organic carbon rain and the diagenetic reactions
19 should be more relevant for some elements at the second site, and highly influenced by
20 terrigenous material diluting the sedimented marine organic matter. Phosphorus and Si
21 distribution are highly dependent on organic transport to the bottoms as both elements form
22 part of phytoplankton skeletal material and soft tissue, playing a role in metabolic
23 processes. Similarly, Sr could have important input from biogenic particle deposition to the
bottoms in pelagic sediments (Bernstein et al., 1998; Jacquet et al., 2007b), promoting the element enrichment within the sediments (Fig. 11). Adsorption onto oxyhydroxides and co-precipitation occur in many different environments, including hydrothermal systems, constituting an important element removal mechanism from pore water and column water (Calvert and Pedersen, 2007). This process has been reported relevant for authigenic P, formed from their organic phases and oxide associated forms (Wheat et al., 1996, Delaney, 1998). The P profile at the EQSS site show low, yet significant, correlation with Fe (r=~0.5, p<0.01; Appendix A). This could suggest a relation with the Fe cycle, adsorption, and subsequent liberation, during reduction of Fe-oxides in anaerobic organic matter remineralization, producing higher positive fluxes toward column water (Fig. 10). Although P concentrations in the EQSS sediments (~1400 ppm) were higher than at the CTJ (~800 ppm; Appendix B), they did not show P enrichment (Fig. 11), probably due to a higher detrital input. No relevant correlation with Fe was observed at the CTJ site, however the P fluxes were three times higher than the fluxes estimated at the EQSS and out of the sediments (Fig. 10), maintaining low P concentration in the solid fraction with EF<1. Here the organic matter remineralization should be relevant in the nutrient element distribution.

Manganese oxidation maintain low pore water concentrations on the surface at the CTJ site, but very high concentrations were found (10 – 30 µM) below the oxidized layer (~5 cm) compared with the concentrations observed at the shallower anoxic site (EQSS; 1 – 2 µM). Consequently, substantially higher benthic effluence of Mn was estimated at the CTJ (Fig. 10) even in oxygenated conditions, when oxide precipitation occurs. This could be explained by leaching of basalt during upwelling pore waters, resulting in net sources of
1 Mn and others, such as Fe and Si. This process could occur on the sea floor in the presence
2 of low-temperature spring waters (2-75°C; Juniper and Tebo 1995, Wheat and Mottl, 2000,
3 Wheat and McMannus, 2005), as has been found here (5 - 13°C). Contrarily, precipitation
4 reactions can remove elements from the pore water, diminishing concentrations of Mn, Fe,
5 and Si related with oxides, silicates and sulfides formation, and in some cases closely
6 related with bacteria activity (Fortin et al., 1998). These processes could explain the
7 diminishing pore water concentrations of Fe below its maximum at the surface (80 µM),
8 which occur in the presence of oxidative conditions; Fe is less soluble than Mn and form
9 insoluble phases (monosulfides, pyrite, carbonates). The sequestering of Fe from pore water
10 in oxidized layers (Fig. 3, 7) resulted in very negative fluxes (towards the sediments) that
11 also imply their accumulation within the sediments. However, Fe did not show authigenic
12 enrichment (Fig. 11). At the EQSS site, the reduced conditions in the bottoms provide
13 fewer sources of Mn oxides and consequently lower Mn fluxes. This has been observed in
14 reduced sediments off Peru, where the Mn and Fe fluxes were a result of the supply of
15 oxides (Böning et al., 2004; Scholz et al., 2011). In the case of Fe, the continental input of
16 Fe is probably relevant along the Chilean margin, especially in shallower zones; therefore,
17 these sites did not show enrichment above detrital content (Fig. 11).

18

19 4.4. Trace elements

20 Trace elements generally did not show good correlations between each other. The most
21 relevant were observed for Ba and Cd at the CTJ site (MUC06; r=0.77, p<0.01; Appendix
22 A). The accumulation of these elements within the sediments occurs in a variety of
23 environments with reduced fluid seepage, including hydrothermal vents and cold seeps, but
also where there are high organic fluxes from biological productivity towards the bottom
(Böstrom et al., 1973; Aloisi et al., 2004; Stevens et al., 2015). Their behavior in pore water
environments is opposed; while Ba is soluble in reduced environments, Cd forms insoluble
sulfide compounds (You et al., 1996; Castellini et al., 2006; Calvert and Pedersen, 2007
and references there in), and no positive correlation should be expected for them. Both Ba
and Cd presented similar fluxes towards the sediments at the CTJ and EQSS (Ba: -10 and -
2 nmol cm⁻²y⁻¹; Cd: -18 and -9 nmol cm⁻²y⁻¹; respectively) (Fig. 10), suggesting that these
elements are being efficiently removed from the pore water. Under reduced conditions, Ba
minerals are unstable and Ba is lost to the bottom water, normally in the sulfate /methane
transition or sulfate depleted horizons, re-precipitating under the presence of sulfate at
surface (Torres et al., 1996, 2002, Aloisi et al., 2004). Depleted sulfate pore waters at the
CTJ could suggest Ba precipitation, however no enrichment was observed in sediments
(EF<1). The bulk Ba/Al ratios of these sediments are higher than at the EQSS (0.008 and
0.004, respectively, Appendix B), and not enough to enrich the sediments when compared
to the content of deep clays. At the EQSS site, Ba displayed a negative correlation with Si
(r= -0.63, Appendix A) and, although this is not strong, it is significant (<0.01); this means
that while Ba is remobilized from the sediments under reducing conditions, Si
concentration increases in pore waters. These elements normally correlate well; Si is the
best predictor of dissolved Ba in the water column, with the exception of surface waters due
to uptake by plankton, while the precipitation of barite and opal will control the dissolved
Ba content in the bottoms (Jacquet et al., 2004; 2007). Therefore, Ba and Si behavior at the
EQSS site imply that the organic flux to the bottom should be important for the distribution
of these elements within the sediments. The Ba EF value, slightly higher than 1 at the
EQSS, cannot be considered enriched (1.14 ± 0.15ppm) but, given the close location to the
1 continental margin, the detrital input could be more important than the organic fluxes to the
2 bottoms and be diluting the authigenic Ba concentration. The case of Cd, it was enriched
3 substantially at both sites (EFs > 2), indicating the presence of reduced fluids being
4 expelled from the sediments and low-sulfide pore water concentrations resulting from the
5 mineral precipitation.

6 In general, Mo and U exhibit similar tendencies but have no strong correlations between
7 them at either site, as their precipitation (or reduction) occur under different redox
8 conditions; U is reduced close to the depth at which sulfate reduction occurs and Mo needs
9 free sulfides for precipitation (Calvert and Pedersen, 2007). These elements are considered
10 conservative in the water column, and enter the sediments by diffusion. They showed
11 diminishing concentrations with depth at the CTJ site and did not show significant gradient
12 concentrations with core depth, resulting in very low fluxes towards the sediments (Fig. 10)
13 which indicates that these elements are being reduced and precipitated. Above the redox
14 potential boundary layer, marked by the Mn (and Cd) pore water distribution (~5 cm, Fig.
15 3,7), these elements should be oxidized, producing a slight increase of the U and Mo
16 concentrations at surface. The decreasing pore water concentrations in depths resulted in
17 very low negative fluxes (-0.1 and -1 nmol cm⁻² y⁻¹, respectively), facilitated by higher
18 porosity at this site, which allows oxygen penetration towards the sediments. Therefore, U
19 fluxes occurred toward the sediments (-0.1 nmol cm⁻² y⁻¹) in the order of magnitude
20 observed for anoxic sediments influenced by oxygenated conditions in bottom waters
21 (permanently or periodically) (-0.1 – -0.6 nmol cm⁻² y⁻¹, Morford et al., 2009; Scholz et al.,
22 2011, Muñoz et al., 2012).
In contrast, at the shallow suboxic EQSS site, U and Mo showed higher concentration gradients in pore waters and higher overall concentrations compared with the deeper CTJ site (Figs. 7 and 8), resulting in positive fluxes from the sediment toward bottom waters (Fig. 10). The U and Mo presented increasing concentrations toward deeper sections of the sediment core, and peaks observed around 4, 10 and 16 cm were concordant with the maximums observed for the Fe profile. Even though low correlation was found between these elements, the U and Mo profiles could be based on their adsorption/desorption onto oxyhydroxides (Fe and Mn). A similar situation was observed at the suboxic site off Concepción (Muñoz et al., 2012), and it is also suggested for Peru sediments (Scholz et al., 2011). The shelf area here is highly influenced by seasonal river discharges, constituting a relevant source of oxides to surface sediments. Moreover, high primary productivity produces high organic matter rain, which also serves as U carrier to the bottoms and may have been released during organic matter diagenesis. Their final accumulation should depend on the oxygen concentration in bottom water, and the oxygen penetration depth within the sediments (Zheng et al., 2002; McManus et al., 2005, 2006; Morford et al., 2009). We assume that oxygen content in the bottoms of the EQSS site has a seasonal pattern, most probably similar to that in southern areas such as at the Concepción shelf (36°S), highly influenced by the OMZ. In such systems, the U accumulated in sediments at reduced periods may constitute a source during oxygenated periods (Morford et al., 2009; Muñoz et al., 2012). In our case the cruise was carried out in March, when reduced conditions prevail, and low U pore water concentrations were measured in surface sediment. Uranium enrichment was observed at the EQSS site, but no higher differences in Me/Al ratios were observed between the CTJ and EQSS sites (Appendix B); therefore, the estimated detrital U (measured in deep sea clays and suspended particles from Bio Bio
river) was determinant in establishing the enrichment (or no-enrichment) at both sites. For

Mo, slightly higher bulk concentrations and a higher EF value were observed at the EQSS

site, but no different from 1 (1.09 ± 0.21), therefore it has not been considered as enriched

at this site (Fig. 11; Appendix B). The low oxygen and low redox conditions in the bottoms

clearly promote the U enrichment, but are not enough for Mo enrichment which needs very

anoxic and sulfidic conditions to be precipitated.

A very low Pb content was recorded at both sites, since this element is highly insoluble

(Fig. 7, 8); it is remobilized from solids but it is also easily removed by new mineral

formation (You et al., 1996), generally showing low concentrations in the water. At the

shallow EQSS site, the Pb content in pore water was higher in surface sediments (0 – 5 cm,

Fig. 8) and diminished quickly at depths in the sediment core, reaching values similar to

those at the CTJ site. This surface maximum occurs due to Pb surface adsorption onto

particles, or co-precipitation with Fe oxides in the water column which is then deposited in

surface sediments and delivered to pore waters. Under reducing conditions the oxides are

dissolved; however, where adequate sulfide conditions are encountered, they are

precipitated again as PbS within the sediments. A clear enrichment of Pb was estimated at

the EQSS, diminishing the pore water sulfide concentrations, and a similar trend was

described for Cd; both are associated with metal sulfide precipitation, and both enriched in

these sediments.

Uranium, Pb, Mo, and Cd concentrations would be maintained within the sediments in

equilibrium with precipitated sulfide compounds (Morford et al., 2009, Aparicio-Gonzalez

et al., 2012), and Mo and U distribution could be closely related with the Mn-Fe

oxide/reduction cycle.
## 5. Conclusions

Discrete vents sites have been explored in the southern part of the Chile Rise (German et al., 2010; German et al., 2011; Thurber et al., 2012) and, based on oceanographic observations and recent biological evidence, these suggest the occurrence of a sedimented area of diffuse hydrothermal fluids at the CTJ site; this is to be expected, given the proximity of a continental margin providing an important flux of terrigenous material. This study contributes through geochemical insights by helping to describe the nature of the reduced environment at the CTJ. Relatively higher temperatures were found in surface sediments, and substantially higher Mn concentrations and fluxes toward bottom waters suggest the ascent of reduced fluids at this site. Additionally, lower pore water concentrations of major elements (Cl, Mg, Ca, K and sulfate) were observed within the core compared with those observed in bottom waters, which could be related to diluted fluid seeping and mineral precipitation. High Ca enrichment was estimated and also, to a lesser extent, U and Cd enrichment; both require reduced conditions, but the presence of sulfide is only necessary in the case of Cd. The reduced conditions could not be explained by organic matter diagenesis, since oxygenated bottoms are present and low organic rain rate was estimated from the TOC inventories. Although this does not constitute strong evidence for the origin of the reduced fluids observed here, the presence of low-temperature hydrothermal fluids should not be discarded. The widespread occurrence of shallow deposits of methane gas hydrates has also been indicated by geophysical studies in the margin adjacent to the CTJ (Brown et al., 1996; Grevemeyer et al., 2003); however, no evidence of AOM could be found in surface sediments. The same stands for DIC pore
1 water values, which were not sufficiently depleted in ¹³C, already explained by the calcium
2 carbonate precipitation/dissolution process.

3 In the EQSS shallow zone, the reduced conditions in the bottoms and sediments enable
4 trace element enrichment. The very low Mn fluxes are the consequence of the low oxygen
5 conditions in bottom waters resulting in low supply of oxides, therefore there was no
6 enrichment of Mn. The reduced conditions of the sediments promote the enrichment of
7 elements such as Cd, Pb, U, and in some degree Mo. Similarly, Pb and Cd fluxes toward
8 the sediment suggest that their presence in the pore water is controlled by the
9 adsorption/desorption processes and precipitation with sulfides, which also diminish de
10 sulfides concentration in pore waters. At this site, the organic matter remineralization and
11 the presence of oxygen deficient bottom waters due to the OMZ are responsible for the
12 element distribution in pore water and sediments.

13 The presence of methane seeps at the EQSS cannot be identified from our geochemical
14 evidence, due to low methane concentrations and enriched δ¹³DIC in pore waters; however,
15 the occurrence of distinctive chemosynthetic fauna, typical of active seep sites, prompts for
16 further integrated exploration of this area in order to clarify the sources of the reduced
17 compounds supporting these assemblages.

18
# 6. Acknowledgments

We would like to thank the captain and crew of the R/V MELVILLE, Scripps Institution of

Oceanography, during the Inspire 2010 cruise. We also give thanks to Dr. Jesse Muratli and

Dr Sebastien Bertrand, for sharing unpublished data from river samples. This work was

partially funded by the FONDECYT project No. 1100166 (to J.S.), and the COPAS center

from the University of Concepción. Additional funding to P.M. during the writing phase of

this manuscript was provided by project VRIDT2010 UCN and the FONDECYT project

1120469. Other funding, for logistics and ship time, was provided by the NOAA Ocean

Exploration Program (via SCRIPPS Institution of Oceanography, contract NOAA

NA17RJ1231) and the Census of Marine Life program (through its field project

COMARGE).

12

13
1 7. References:

2

3 Aharon, P., Graber, E.R., Roberts, H.H., 1992. Dissolved Carbon and 13C Anomalies in

4 the Water Column Caused by Hydrocarbon Seeps on the Northwestern Gulf of

5 Mexico Slope. Geo-Marine Letters 12, 33 – 40.

6 Aloisi, G., Wallmann, K., Bollwerk, S., Derkachev, A., 2004. The effect of dissolved

7 barium on biogeochemical processes at cold seeps. Geochimica et Cosmochimica

8 Acta 68, 1735 – 1748.

9 Aparicio-González, A., Duarte, C.M., Tovar-Sánchez, A., 2012. Trace metals in deep ocean

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1. Figure captions

2.

3. Fig. 1. Study area showing the position of the sampling stations. Chilean Triple Junction (42°S, CTJ) and El Quisco Seep Site (33°S, EQSS).

5. Fig. 2. TV guided multicore images from: the CTJ site at ~2900 m water depth, (A, B) MUC01 and MUC06 showing deep sea fauna (arrows): echinoderms and holes of echiurans worms; and the EQSS site at ~340 m water depth, (C, D) MUC19 showing soft sediments, a clam bed of *Austrogena nerudai* (chemosynthetic fauna), and tubes of maldanid polychaetes.

10. Fig. 3. Redox potential (mV) measurements in sediment cores at (a) the Chilean Triple Junction (42°S, CTJ) and (b) the El Quisco Seep Site (33°S, EQSS).

12. Fig. 4. Methane concentrations (nM) measured in sediment cores from the El Quisco Seep Site (33°S, EQSS).

14. Fig. 5. Total sulfide ($\Sigma H_2S$) and sulfate concentrations in sediment cores from (a) the Chile Triple Junction (42°S, CTJ) and (b) the El Quisco Seep Site (33°S, EQSS).

16. Fig. 6. $\delta^{13}$DIC distribution in pore waters from sediment cores from (a) the Chile Triple Junction (42°S, CTJ) and (b) the El Quisco Seep Site (33°S, EQSS).

18. Fig. 7. Element distribution in pore waters from sediment cores, from the Chile Triple Junction (42°S, CTJ). The lines are the fitted curves over the ranges of depths and concentrations used for $\Delta C/\Delta z$ calculations for flux estimations.

21. Fig. 8. Element distribution in pore waters from sediment cores, from the El Quisco Seep Site (33°S, EQSS). The lines are the fitted curves over the ranges of depths and concentrations used for $\Delta C/\Delta z$ calculations for fluxes estimations.

24. Fig. 9. Mg/Ca and Sr/Ca ratios in pore waters from sediment cores, from (a) the Chile Triple Junction (42°S, CTJ) and (b) the El Quisco Seep Site (33°S, EQSS).

26. Fig. 10. Diffusive fluxes of major (a), minor and trace elements (b) at each site (nmol cm$^{-2}$ y$^{-1}$). Negative values of the concentration gradients in pore water indicate increasing concentrations at greater depth, resulting in positive fluxes to the water column. Negative values of fluxes indicate the opposite.

30. Fig. 11. Element authigenic enrichment at surface sediments (0 – 20 cm), mean values ± SD. The enrichment fraction (EF) was calculated comparing Me/Al ratios in the
1

2

3

4

5

6

7

8

9

sample, and lithogenic background using the element content in deep sea clays for the CTJ site and in suspended particles from Bío Bío river for the EQSS site (Table 2).

Fig. 12. Relationship between inventories of ΣCPE (µg cm⁻²) and ΣTOC (gC m⁻²) content in surface sediments from the Chile Triple Junction (42°S, CTJ) and the El Quisco Seep Site (33°S, EQSS). Information of surface sediment from the Concepción Methane Seep Area (CMSA, 36°S) was included in the analysis.
1 Table Captions

2

3 Table 1. Environmental conditions at the two stations studied: the Chilean Triple Junction (CTJ) and the El Quisco Seep Site (EQSS). Major element concentrations (average values) did not vary greatly along the cores except at the CTJ (see Fig. 7). Sulfide ($\Sigma H_2S$) and sulfate inventories were measured in the first 17 cm except in MUC04, where they were only calculated down to 7 cm due to the shorter core available. Total Organic Carbon ($\Sigma TOC$), pigment (expressed as chloroplastic pigment equivalent (CPE)) inventories, and stable isotopes were determined in the first 5 cm. Concentration ranges (minimum and maximum values) of methane, CPE, and C/N ratios along the core are shown. Dissolved oxygen in the bottom water (BDO) was measured at each site, and the sediment grain size determined along the core (~25 cm).

13

14 Table 2. Bulk element concentrations normalized to Al content (Me/Al ratios $\times 10^4$) used for element authigenic enrichment calculations (EF)

15

16

17 Table 3. Parameters detail for diffusive flux calculations. Concentration gradients were estimated from the best fit model for concentration profiles. The variances of the fitted curves are shown. Diffusive coefficients in pore water ($D_{sed}$) were estimated from the diffusive coefficients in seawater ($D_{sw}$, Boudreau, 1999) corrected for tortuosity and considering the bottom water temperature at each station. Mean porosity ($\phi$): 0.75 and 0.59 for the CTJ and the EQSS, respectively. Tortuosity ($\theta$): 1.59 and 2.04 for the CTJ and the EQSS, respectively.

18

19

20

21

22

23

24
1 Appendix A. Spearman rank correlation coefficients in MUC06 (CTJ) and MUC19

2 (EQSS). Values >0.70 (printed in bold) are significant (<0.01 level).

3

4 Appendix B. Element concentrations in sediments (solid fraction) and elements normalized

5 to Al concentrations (Me/Al) at the CTJ and EQSS sites.

6

7

8

9

10

11
<figure><img src="image_2.png" /><figcaption>80°W 78°W 76°W 74°W 72°W 70°W 68°W</figcaption></figure>

Long W (°) Long W (°) Long W (°) Long W (°) Long W (°) Long W (°)

C11 023.7 80.149 75.015 12.71 1.21 1.21 1.21 1.21 1.21 1.21

E22 181.3 23.487 71.925 72.4 4.41
<figure><img src="image_3.png" /></figure>

A

<figure><img src="image_4.png" /></figure>

B

Sea Cam 02:48:51 2010/03/14

Sea Cam 02:49:35 2010/03/14

<figure><img src="image_5.png" /></figure>

C

<figure><img src="image_6.png" /></figure>

D
<figure><img src="image_7.png" /><figcaption>Fig. 3</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 4</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 5</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 6</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 7 (continued)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 7</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 8 (continued)</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 8</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 9</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 10</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. 11</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Fig. 12</figcaption></figure>
<table><caption>Table 1.</caption><thead><tr><th>Site</th><th>Core</th><th>Depth (m)</th><th>Lat. S</th><th>Long. W</th><th>Cl (µM)</th><th>Na (µM)</th><th>ΣH₂S (mmol m⁻²)</th><th>SO₄ (mmol m⁻²)</th><th>CH₄ min-max (nM)</th><th>BDO (µM)</th><th>RPD<sup>b</sup> (cm)</th></tr></thead><tbody><tr><td rowspan="4">CTJ</td><td>MUC 01</td><td>2628</td><td>46°14.213</td><td>75°46.377</td><td></td><td></td><td>23<sup>a</sup></td><td>956<sup>a</sup></td><td></td><td>164</td><td></td></tr><tr><td>MUC 04</td><td>2927</td><td>46°16.990</td><td>75°48.153</td><td>564</td><td>478</td><td></td><td></td><td></td><td></td><td>&gt; 10</td></tr><tr><td>MUC 06</td><td>3096</td><td>46°12.628</td><td>75°49.779</td><td>422</td><td>348</td><td></td><td></td><td></td><td></td><td>8</td></tr><tr><td>MUC 07</td><td>2934</td><td>46°16.864</td><td>75°48.091</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="4">EQSS</td><td>MUC15b</td><td>349</td><td>33°23.111</td><td>71°52.810</td><td></td><td></td><td></td><td></td><td>22.2 – 38.6</td><td></td><td></td></tr><tr><td>MUC 19</td><td>348</td><td>33°23.394</td><td>71°52.760</td><td>564</td><td>478</td><td>102</td><td>2985</td><td>18.8 – 36.4</td><td>23</td><td>4</td></tr><tr><td>MUC 20</td><td>340</td><td>33°23.407</td><td>71°52.762</td><td>564</td><td>478</td><td></td><td></td><td>23.7 – 27.8</td><td></td><td></td></tr><tr><td>MUC 21</td><td>342</td><td>33°23.437</td><td>71°52.810</td><td>536</td><td>435</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>MUC 22</td><td>340</td><td>33°23.498</td><td>71°52.793</td><td>536</td><td>435</td><td>49</td><td>2694</td><td>26.3 – 49.8</td><td></td><td>4</td></tr></tbody></table><p><em>Continuation</em></p><table><thead><tr><th>Site</th><th>Core</th><th>Depth (m)</th><th>Lat. S</th><th>Long. W</th><th>CPE (min-max) (µg g⁻¹)</th><th>ΣCPE (µg cm⁻²)</th><th>ΣTOC (gC m⁻²)</th><th>C/N (min-max)(mean)</th><th>δ<sup>13</sup>C (min-max)</th><th>δ<sup>15</sup>N (min-max)</th><th>Grain size</th></tr></thead><tbody><tr><td rowspan="4">CTJ</td><td>MUC 01</td><td>2628</td><td>46°14.213</td><td>75°46.377</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MUC 04</td><td>2927</td><td>46°16.990</td><td>75°48.153</td><td>3 – 27</td><td>52</td><td>137</td><td>3.9 – 11.6 (7.8)</td><td>-17.30 – -19.82</td><td>12.24 – 7.68</td><td>fine sand to fine silt</td></tr><tr><td>MUC 06</td><td>3096</td><td>46°12.628</td><td>75°49.779</td><td>4 – 7</td><td>25</td><td>195</td><td>8.3 – 11.1 (9.3)</td><td>-19.99 – -20.06</td><td>7.34 – 4.57</td><td>coarse to fine silt</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="4">EQSS</td><td>MUC 19</td><td>348</td><td>33°23.394</td><td>71°52.760</td><td>18 – 28</td><td>122</td><td>568</td><td>10.3 – 14.6 (11.8)</td><td>-20.64 – -21.38</td><td>5.9 – 6.67</td><td>coarse to medium silt</td></tr><tr><td>MUC 20</td><td>340</td><td>33°23.407</td><td>71°52.762</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MUC 21</td><td>342</td><td>33°23.437</td><td>71°52.810</td><td>12 – 15</td><td>72</td><td>380</td><td>9.6 – 11.2 (10.4)</td><td>-19.45 – -20.36</td><td>7.29 – 8.20</td><td>coarse to medium silt</td></tr><tr><td>MUC 22</td><td>340</td><td>33°23.498</td><td>71°52.793</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<footnote><sup>a</sup>If mean values for sulfide and sulfate between 7 and 17 cm are considered, the inventories should increase to 53 and 2248 mmol m⁻², respectively.</footnote>

<footnote><sup>b</sup>Redox Potential disconitnuity layer (RPD, considered at -100 Mv)</footnote>
<table><caption>Table 2.</caption><thead><tr><th rowspan="2">Element</th><th colspan="2">Deep Sea clays<sup>a</sup></th><th colspan="2">Patagonian Rivers<sup>b</sup></th><th colspan="2">BioBio River<sup>c</sup></th></tr><tr><th>(µg g<sup>-1</sup>)</th><th>Me/Al x 10<sup>4</sup></th><th>(µg g<sup>-1</sup>)</th><th>Me/Al x 10<sup>4</sup></th><th>(µg g<sup>-1</sup>)</th><th>Me/Al x 10<sup>4</sup></th></tr></thead><tbody><tr><td>Al</td><td>95000</td><td></td><td>61675</td><td></td><td>74961</td><td></td></tr><tr><td>Mg</td><td>18000</td><td>1895</td><td>--</td><td>--</td><td>22736</td><td>3033</td></tr><tr><td>K</td><td>28000</td><td>2947</td><td>--</td><td>--</td><td>24000<sup>a</sup></td><td>3463<sup>a</sup></td></tr><tr><td>Ca</td><td>10000</td><td>1053</td><td>5988</td><td>971</td><td>38525</td><td>5139</td></tr><tr><td>Sr</td><td>250</td><td>26</td><td>191</td><td>31</td><td>419</td><td>56</td></tr><tr><td>P</td><td>1400</td><td>147</td><td>--</td><td>--</td><td>44205</td><td>5897</td></tr><tr><td>Pb</td><td>200</td><td>21</td><td>--</td><td>--</td><td>--</td><td>1.56<sup>d</sup></td></tr><tr><td>Ba</td><td>1500</td><td>158</td><td>456</td><td>74</td><td>301</td><td>40</td></tr><tr><td>Cd</td><td>0.23</td><td>0.02</td><td>--</td><td>--</td><td>0.46</td><td>0.06</td></tr><tr><td>Mn</td><td>6000</td><td>632</td><td>684</td><td>111</td><td>1421</td><td>190</td></tr><tr><td>Fe</td><td>60000</td><td>6316</td><td>31879</td><td>5169</td><td>91077</td><td>12150</td></tr><tr><td>U</td><td>2.00</td><td>0.21</td><td>--</td><td>--</td><td>1.13</td><td>0.15</td></tr><tr><td>Mo</td><td>8.00</td><td>0.84</td><td>--</td><td>--</td><td>1.19</td><td>0.16</td></tr></tbody></table>

<footnote><sup>a</sup>Chester 1990. for K at central Chile (33°S) we used the mean concentration in mud sediments and their correspondent Al content of 69300 µg g<sup>-1</sup>.</footnote>

<footnote><sup>b</sup>S. Bertrand, unpublished data. Mean concentrations from superficial sediments in Patagonian rivers (~54°Lat.S – 70°LongW); details will be published elsewhere.</footnote>

<footnote><sup>c</sup>J. Muratli, unpublished data; details will be published elsewhere.</footnote>

<footnote><sup>d</sup>Bönning et al., 2009.</footnote>
<table><caption>Table 3.</caption><thead><tr><th></th><th>Mg</th><th>K</th><th>Ca</th><th>B</th><th>Si</th><th>Sr</th><th>P</th><th>Pb</th><th>Ba</th><th>Cd</th><th>Mn</th><th>Fe</th><th>U</th><th>Mo</th></tr></thead><tbody><tr><td colspan="16"><strong>CTJ (MUC06)</strong></td></tr><tr><td>ΔC/Δx (μmol/cm<sup>4</sup>)</td><td>2884</td><td>528</td><td>767</td><td>-158</td><td>-227</td><td>6</td><td>-21</td><td>0.01</td><td>0.14</td><td>0.32</td><td>-8</td><td>26</td><td>0.001</td><td>0.02</td></tr><tr><td>range of depth for ΔC/ΔX (cm)</td><td>3-7</td><td>3-7</td><td>3-7</td><td>1-7</td><td>0-8</td><td>3-7</td><td>1-3</td><td>1-2</td><td>1-3</td><td>1-20</td><td>2-5</td><td>1-3</td><td>1-7</td><td>3-7</td></tr><tr><td>D<sub>sed</sub> (cm<sup>2</sup>y<sup>-1</sup>)</td><td>79</td><td>220</td><td>87</td><td>127</td><td>104</td><td>87</td><td>94</td><td>103</td><td>95</td><td>75</td><td>73</td><td>78</td><td>108</td><td>108</td></tr><tr><td>r<sup>2</sup></td><td>0.72</td><td>0.71</td><td>0.75</td><td>0.72</td><td>no</td><td>0.75</td><td>no</td><td>no</td><td>0.90</td><td>0.90</td><td>0.91</td><td>no</td><td>0.94</td><td>0.70</td></tr><tr><td colspan="16"><strong>EQSS (MUC19)</strong></td></tr><tr><td>ΔC/Δx (μmol/cm<sup>4</sup>)</td><td>2080</td><td>420</td><td>399</td><td>-348</td><td>-166</td><td>3</td><td>-10</td><td>0.02</td><td>0.03</td><td>-0.23</td><td>-0.45</td><td>-17</td><td>-1.06</td><td>-0.63</td></tr><tr><td>range of depth for ΔC/ΔX (cm)</td><td>4-7</td><td>4-7</td><td>4-7</td><td>0-2</td><td>5-7</td><td>4-7</td><td>1-4</td><td>0-2</td><td>0-10</td><td>0-2</td><td>2-5</td><td>0-4</td><td>3-6</td><td>0-17</td></tr><tr><td>D<sub>sed</sub> (cm<sup>2</sup>y<sup>-1</sup>)</td><td>72</td><td>201</td><td>79</td><td>118</td><td>89</td><td>79</td><td>106</td><td>95</td><td>82</td><td>71</td><td>67</td><td>70</td><td>84</td><td>81</td></tr><tr><td>r<sup>2</sup></td><td>no</td><td>no</td><td>0.87</td><td>no</td><td>no</td><td>0.89</td><td>no</td><td>no</td><td>0.81</td><td>no</td><td>0.84</td><td>0.99</td><td>0.95</td><td>0.87</td></tr></tbody></table>

no r² estimation for gradients indicate low adjusted numbers. In those cases ΔC/Δx was calculated using the difference between two points of concentration.

The curves predicted from the concentration gradients considered for calculation of fluxes are shown in the graphs (Figs. 7 and 8)
## Chilean Triple Junction

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

Evidence of methanogenic bacteria and authigenic mineral formation (cold seeps, seeping fluid)

<figure><img src="image_21.png" /></figure>

Presence of chemosynthetic fauna, reduced conditions. Low methane concentrations and higher trace element enrichments

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

short sediment cores: 25 cm

Pore water and sediments
ΣH₂S, TOC
Redox potential

Element enrichment fraction

<figure><img src="image_24.png" /></figure>

## Central Chile margin

<figure><img src="image_25.png" /></figure>