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Progress in Oceanography 148 (2016) 26–43

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Progress in Oceanography

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

http://dx.doi.org/10.1016/j.pocean.2016.09.0020079-6611/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Departamento de Biología Marina, UniversidadCatólica del Norte, Larrondo 1281, Coquimbo, Chile. Tel.: +56 51 2209802.

E-mail address: [email protected] (P. Muñoz).

Práxedes Muñoz a,b,⇑, Lissette J. Cárdenas c, Dieter Garbe-Schönberg d, Javier Sellanes a, Laurent Dezileau e,Ives Melville a, Stephanie D. Mendes f

aDepartamento de Biología Marina, Universidad Católica del Norte, Larrondo 1281, Coquimbo, ChilebCentro de Estudios Avanzados en Zonas Áridas (CEAZA), Coquimbo, Chilec Laboratorio de Oceanografía, Universidad Católica del Norte, Programa de Magister en Ciencias del Mar, Facultad de Ciencias del Mar, Larrondo 1281, Coquimbo, Chiled Institut für Geowissenschaften, Christian-Albrechts Universität zu Kiel, D-24118 Kiel, Germanye Laboratoire Géosciences Montpellier (GM), Bâtiment 22, 1er étage, Université de Montpellier 2, Place Eugène Bataillon, 34095 Montpellier cedex 05, FrancefDepartment of Earth Science, Webb Hall, UC Santa Barbara, Santa Barbara, CA 93106, USA

a r t i c l e i n f o

Article history:Received 12 November 2013Received in revised form 2 August 2016Accepted 5 September 2016Available online 15 September 2016

a b s t r a c t

This study presents the geochemical composition of superficial sediment under oxic and suboxic bottomwater conditions along the Chilean continental margin (SE Pacific), where evidence for benthicchemosynthetic activity associated with diffuse seeping of chemically reduced fluids has been reported.The exploration was carried out at: (1) the Chilean Triple Junction (CTJ), at a water depth of �2900 m,with the additional indication of hydrothermal activity near a methane-rich cold-seep area (46�S)(German et al., 2010); and (2) the El Quisco methane seep site (EQSS), at �340 m water depth (33�S)(Melo et al., 2007; Krylova et al., 2014). While the deeper CTJ is located within an oxic environment (dis-solved oxygen in the bottom waters: 164 lM), the shallower EQSS lies within a suboxic environment(dissolved oxygen in bottom water: 23 lM), located within the lower limit of the SE Pacific oxygen min-imum zone (OMZ).Pore water from short cores was analyzed for dissolved major, minor, and trace elements (Cl, Na, Mg, K,

Ca, Sr, Si, B, P, Ba, Pb, Mn, Fe, Cd, U, and Mo), d13DIC, sulfide, sulfate, and methane. The solid sedimentfraction was likewise analyzed for total organic carbon (TOC), metals, and redox potential. Elevated sed-iment temperatures were found in superficial sediments (5–13 �C) at the CTJ site, which could be due towarm fluids associated with the proximity of the ridge, where hydrothermal vents may occur. Reducedfluids were also present here, indicated by higher Mn fluxes toward the water column even in oxidizedsediments (RPD > 8 cm), which contrasted with the lower fluxes in reduced sediments of the EQSS site(RPD � 2 cm). 13C-depleted DIC, anomalously low pore water Cl (�15 ppb), and low concentrations ofother major elements may be the result of dilution by fluid seeping and precipitation of major elements,producing authigenic enrichment (Ca, Mg, Sr). The fluid could also: (a) be diluted by pure water producedduring methane hydrate dissociation, as observed in other cold-seep areas; and (b) correspond to claymineral dehydration, as reported in plate subduction systems. The reducing conditions established atthe CTJ conduct the Cd enrichment at a similar magnitude of that seen at the shallower suboxic site(EQSS). Evidence of chimney or vent fauna was not observed. At the EQSS, higher TOC and total sulfidecontents were consistent with enhanced deposition of organic matter and reducing conditions developedin the OMZ, favoring the authigenic enrichment of Cd, U and Pb. The geochemical evidence, based only onmethane concentrations and d13DIC, is insufficient to establish the presence of methane seeps, as previ-ously reported.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

The tectonically active Chilean continental margin is character-ized by the presence of the Atacama trench, resulting from the sub-duction of the Nazca plate beneath the South American Continental

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P. Muñoz et al. / Progress in Oceanography 148 (2016) 26–43 27

plate (SACP). While an erosive, empty trench is present from Northto Central Chile (�20–33�S), an accretionary and sediment-filledtrench develops southwards (>33�S), with sediment input intothe trench controlled by supply from the continent. Subductionof the Chile Rise below the SACP, at �46�S, forms the only placeon Earth where an actively spreading ridge is currently being sub-ducted beneath a continental margin, the Chilean Triple Junction(CTJ); adjacent to this site there is substantial evidence for theoccurrence of hydrothermal activity (Baker and German, 2004).An active subduction front with a well-developed accretionaryprism, and fluid migration from deep within the subductionzone which includes hydrocarbon gas, has been described here(Behrmann et al., 1992; Brown and Bangs, 1995; Lagabrielleet al., 2004). Additionally, massive sulfide deposits altered by sub-sequent diagenesis have been reported in this area (Lindsley-Griffin et al., 1995; Karsten et al., 1996). Results of stratigraphicstudies using seismic techniques between 35� and 46�Shave revealed abundant bottom-simulating reflectors (BSRs), indi-cating the widespread occurrence of shallow deposits of methanegas hydrates (Brown and Bangs, 1995; Brown et al., 1996;Grevemeyer et al., 2003). It has also been suggested that the pres-ence of the accretionary prism and thermal heating of this marginthrough the subduction of the oceanic crust could give rise to flu-ids, generating hydrothermal vents, methane seeps, or even hybridsystems along the margin closest to the CTJ (Brown et al., 1996;German et al., 2010). The hydrate layer close to the CTJ has beenestimated to be 100 m thick, covering an area of at least1000 � 20 km, as indicated by means of BSR analyses (Brownet al., 1996; Díaz-Naveas, 1999; Morales, 2003; Coffin et al., 2006).

Further to the North, the main active methane seep area so faridentified along the Chilean margin is located off Concepción(CMSA, �36�S) (Sellanes et al., 2004, 2008; Sellanes and Krylova,2005). The area is on a slope between water depths of �600–850 m, and it has been suggested as one of the largest knowncold-seep areas on active continental margins, including bothactive 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 Austro-gena nerudai, empty tubeworms of siboglinid polychaetes) andauthigenic 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 margincharacterized by oxygen concentrations below 0.5 mL L�1

(22 lM). This hypoxic water mass and resulting anoxic sedimentare generated by the enhanced primary productivity in upwellingnutrient-rich surface waters, and subsequent remineralization oforganic matter at midwater depths coupled with sluggish circula-tion. The OMZ covers an area of �12 million km2 and narrows sea-wards, from a thickness of about 300 m at 70�W to disappear at103�W (Helly and Levin, 2004; Schneider et al., 2006), influencingthe benthic assemblages and species distribution at this waterdepth in Northern and Central Chile (Veit-Köhler et al., 2008;Quiroga et al., 2010). The biogeochemical processes in thesereduced sediments (Thamdrup and Canfiel, 1996; Muñoz et al.,2004) also favor the formation of authigenic minerals, such asphosphorite, and trace metal enrichment in pore waters (Muñozet al., 2012).

Biogeochemical processes, occurring in both methane seepareas and hydrothermal systems, generate hydrogen sulfide,methane, and other hydrocarbon-rich fluids. At methane seeps,one of the most conspicuous processes indicative of chemosyn-thetic activity is the authigenic formation of barium and carbonateminerals. This process is closely related to the anaerobic oxidationof methane (AOM) (Greinert et al., 2002a, 2002b, and referencestherein) forming irregular crusts, reefs, and different forms of

chimneys, or stromatolites, that are associated with living organ-isms (Boetius et al., 2000; Greinert et al., 2002a; Díaz-del-Ríoet al., 2003). At these sites, dissolved methane and sulfide concen-trations are variable, depending on both the microbial activity andfluxes of sulfate and methane during the AOM. Other dissolved ele-ments, such as Ba, B, and Cl anomalies, can be taken as indicators ofactive seeping fluids rising from deeply rooted sources in sub-ducted sediments (Colwell et al., 2004). In contrast, metalliferoussediments associated with seafloor hydrothermal systems areenriched with authigenic minerals due to the high concentrationsof sulfide and trace metals in the fluids. Dissolved metals risinginside hydrothermal plumes react with oxygenated bottomwaters,leading to the precipitation of Mn and Fe oxyhydroxides, as well asother metals (German and Von Damm, 2006). This study describesthe geochemical characteristics of superficial sediments takenfrom two different settings where chemosynthetic activity hasbeen found along the Chilean margin (Thurber et al., 2010;Krylova et al., 2014). The first, found in the southernmost segmentof the East Chile Rise, at the Chilean Triple Junction (CTJ, �46�S), isa potential sedimented hydrothermal system where discrete sitesof venting have been reported in oxygenated bottoms, at 2900 mwater depth (German et al., 2010, 2011). The second is the ElQuisco Seep Site (EQSS, �33�S), at a shallower depth of �350 m,immersed within the lower boundary of the SE Pacific OMZ. Wedetermine the element concentration distribution in the porewater and in the solid fraction, which is mainly a result ofdiagenetic processes in surface sediments. The diagenesis involvesthe consumption, release, and diffusion of the elements, creatingconcentration gradients and authigenic metal enrichmentsthat indicate the main geochemical processes in the sediments.We estimate the diffusive flux and element enrichment factor inorder to compare the primary geochemical reactions involvedwithin the sediments at both sites; this contributes to knowledgeof the functioning of deep-sea environments in the SE Pacific, aswell as their contribution to the global balance of elements inthe ocean.

2. Methods

2.1. Sampling

Short sediment cores were obtained off the Chilean margin dur-ing a cruise on board R/V Melville (Scripps Institution of Oceanog-raphy) in February–March 2010. The areas investigated during thiscruise were the Chilean Triple Junction (CTJ, �46�S) and El QuiscoSeep Site (EQSS, �33�S) (Fig. 1). Water column conditions, conduc-tivity, temperature, and dissolved oxygen concentrations were allregistered using a CTD-rosette with a SeaBird 911+ systemequipped with dual C-T sensors, a SeaBird dissolved oxygen sensor,and a 25 cm transmissometer.

Sediment samples for pore water and sediment chemical anal-yses were obtained using a TV-guided multicorer (MUC) with thecapacity to simultaneously obtain 8 cores of about 50 cm lengthin soft sediments. The cores obtained at the CTJ were close to theaxis and did not show evidence of seeps, but methanogenic bacte-ria were found (Thurber, comm. Pers.) at the position of MUC04.We were unable to obtain enough pore water for metal analysesfrom this core; instead, we obtained pore water samples fromMUC06, further out but still near the axis. At the EQSS, sedimentsshowed some dispersed clam beds of Austrogena nerudai; here, allcores (MUCs-15b, 19, 20, 21 and 22) were obtained from sedimentswith similar characteristics (Fig. 2, Table 1).

Subsamples of sediment were taken from the original coresusing 3.6 cm i.d. and 20 cm long polycarbonate liners immediatelyafter collection. These short cores were sectioned at 1 cm intervals

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).

28 P. Muñoz et al. / Progress in Oceanography 148 (2016) 26–43

and squeezed under a N2 atmosphere; pore water was extractedfrom the sediment cores using a nylon Reeburgh-type squeezer(Reeburgh, 1967). Solid sediment fractions were kept frozen forchemical analysis of chloroplastic pigment equivalent (CPE), totalorganic carbon (TOC), grain size, and element analysis.

Sediment redox potential (Eh) was measured onboard with aportable Mettler Toledo Eh-meter by inserting a platinum elec-trode directly into the sediment of one core at intervals of 1 cm.

For the determination of dissolved total sulfide (H2S + HS� +S2�) and sulfate content in pore waters the cores were sectioned

at 2 cm intervals. The subsampling for methane analysis was per-formed by inserting cut-off 3-mL plastic syringes, with pre-drilled lateral holes, along the axis of each core. Sediment wasimmediately placed into serum vials partially filled with 2 M NaOHin order to terminate any bacterial activity. The vials were cappedimmediately with a red rubber septum and N2 was added using asyringe connected to a N2 gas cylinder. The samples were shakenand then left to stand, allowing the dissolved hydrocarbon gasesfrom the sediment to equilibrate in the vial’s headspace for a min-imum of 24 h before analysis.

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 ofechiurans 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 ofmaldanid polychaetes.


For trace metal analyses, concentrated ultrapure HNO3 (2 lL)was added to the pore water (>0.5 mL) placed in Supelco� pre-cleaned vials. Samples for dissolved inorganic carbon analysis(d13DIC) were placed in pre-evacuated glass vials (Exetainers LabcoUK). All samples were kept in darkness at 4 �C until further analy-sis; lower temperatures were not desirable as salt precipitationscan occur during freezing.

2.2. Chemical analyses

2.2.1. Pore water analysesQuantitative determinations of dissolved total sulfide (H2S

+ HS� + S2�) and sulfate were performed spectrophotometricallyfor small samples with detection range concentrations of 1–20 mM for sulfide, and 6–20 mM for sulfate (Cord-Ruwisch,1985). A UV–Vis spectrophotometer set at 400 nm and 480 nmwas used to quantify the concentration of sulfate and sulfide,respectively. For sulfate measurements, a calibration curve wasmade using five standards of 0–5-fold dilutions of IAPSO(28.9 mM SO4

2�) standard seawater. For each sample, 200 lL ofsample was added to 4 mL of water and then treated with excessBaCl2; this is a semi quantitative method allowing the establish-ment of the samples’ concentration range (Gieskes et al., 1991).For sulfide measurements, a calibration curve was made usingsix calibrated standards ranging from 0 to 20 mM of sulfide. Foreach sample, 100 lL of sample was added to 4 mL of 2 mmol CuSO4

reagent and immediately placed in the spectrophotometer. Theanalytical errors were 8% and 5% for sulfide and sulfate,respectively.

The analyses of stable isotopes in dissolved inorganic carbon(d13DIC) were performed at Washington State University’s stableisotope facility. A GV Instruments Multiflow Preparation Systemwas used to prepare the samples; they were then placed in sealedvials and acidified with phosphoric acid. The gas liberated in theheadspace was then sampled and analyzed for carbon isotopesusing a GV Isoprime Isotope Ratio Mass Spectrometer (IRMS). Abicarbonate standard calibrated against NBS reference materials

was used as an internal standard in sample runs. The sample erroris about 0.5%.

The methane analyses were performed according to Kinnamanet al. (2007). The gas in the vial’s headspace was quantified usinga Shimadzu GC-14A Gas Chromatography-Flame Ionization Detec-tion (GC-FID). Samples were run on the GC-FID under isothermal60 �C conditions using N2 as the carrier gas through a 120 � 1/800

packed column (n-octane on Res-Sil C). The precision of these anal-yses is ±4%.

Dissolved elements in pore water were analyzed at the ICPMSlaboratory of the Geosciences Institute, Christian Albrechts Univer-sity of Kiel (Germany). Trace metals (Ba, Pb, B, Cd, U and Mo) weredetermined using an AGILENT 7500cs ICP-MS instrument with aprecision that varied between 0.1% and 6%, as determined from 2replicas out of 7 samples. The accuracy was monitored with IAEAW4, NIST1643e, IAPSO seawater, and NASS-4 certified referencematerials (CRM), yielding deviations from certified values between0.4% and 8.4%. Sample preparation and analytical procedures aredescribed in Garbe-Schönberg (1993). Other elements (Sr, Si, Mg,K, Mn, Fe, P, Ca, Na and Cl) were analyzed using a SPECTRO CirosSOP ICP-OES instrument with a precision between 0.3% and 0.7%,as estimated from four replicas; Mn, Fe, and P showed slightlylower precision (0.6–9.2%) due to low concentrations in some sam-ples. Accuracy was monitored with the same suite of CRMs.

2.2.2. Sediment analysesSediment surface CPE (Chlorophyll-a + phaeopigments) content

was analyzed fluorometrically (Plante-Cuny, 1973). Samples werekept frozen at �20 �C and thawed before extraction. Portions of0.2–0.3 g were mixed with 0.1 g of MgCO3 and placed in glass vials.Then, sediment was weighed (precision ±0.1 mg) and 5–10 mL of90% acetone was added. Vials were vigorously shaken within a vor-tex (30 s), then stored overnight at 4 �C in the dark, and later cen-trifuged (10 min at 800g). The supernatant was utilized forspectrophotometric determinations.

Total organic carbon (TOC) and stable isotope (d15N and d13C)analyses in sediment samples were performed at Washington State

Table1

Environm

entalc

ondition

sat

thetw

ostations

stud

ied:

theCh

ilean

Triple

Junc

tion

(CTJ)an

dtheEl

QuiscoSe

epSite

(EQSS

).Major

elem

entco

ncen

trations

(ave

rage

values)didno

tva

rygrea

tlyalon

gtheco

resex

cept

attheCT

J(seeFig.

7).

Sulfide

( RH2S)

andsu

lfateinve

ntoriesweremea

suredin

thefirst17

cmex

cept

inMUC0

4,whe

rethey

wereon

lycalculated

downto

7cm

dueto

thesh

orterco

reav

ailable.

TotalOrgan

icCa

rbon

(RTO

C),pigm

ent(exp

ressed

asch

loroplasticpigm

enteq

uiva

lent

(CPE

))inve

ntories,an

dstab

leisotop

eswerede

term

ined

inthefirst5cm

.Con

centration

rang

es(m

inim

uman

dmax

imum

values)of

metha

ne,C

PE,a

ndC/N

ratios

alon

gtheco

rearesh

own.

Disso

lved

oxyg

enin

thebo

ttom

water

(BDO)was

mea

suredat

each

site,a

ndthesedimen

tgrainsize

determ

ined

alon

gtheco

re(�

25cm

).

Site

Core

Dep

th(m

)La

t.S

Long.

WCl(l

M)

Na(l

M)

RH

2S(m

mol

m�2)

SO4(m

mol

m�2)

CH

4min–m

ax(nM)

BDO

(lM)

RPD

b(cm)

CTJ

MUC01

2628

46�14.21

375

�46.37

723

a95

6a16

4MUC04

2927

46�16.99

075

�48.15

356

447

8>1

0MUC06

3096

46�12.62

875

�49.77

942

234

88

MUC07

2934

46�16.86

475

�48.09

1

EQSS

MUC15

b34

933

�23.11

171

�52.81

022

.2–3

8.6

MUC19

348

33�23.39

471

�52.76

056

447

810

229

8518

.8–3

6.4

234

MUC20

340

33�23.40

771

�52.76

256

447

823

.7–2

7.8

MUC21

342

33�23.43

771

�52.81

053

643

5MUC22

340

33�23.49

871

�52.79

353

643

549

2694

26.3–4

9.8

4

Site

Core

Dep

th(m

)La

t.S

Long.

WCPE

(min–max

)(l

gg�

1)

RCPE

(lgcm

�2)

RTO

C(g

Cm

�2)

C/N

(min-m

ax)(mea

n)

d13C(m

in-m

ax)

d15N

(min-m

ax)

Grain

size

Continua

tion

CTJ

MUC01

2628

46�14.21

375

�46.37

7MUC04

2927

46�16.99

075

�48.15

33–

2752

137

3.9–

11.6

(7.8)

�17.30

to�1

9.82

12.24–

7.68

Finesandto

finesilt

MUC06

3096

46�12.62

875

�49.77

94–

725

195

8.3–

11.1

(9.3)

�19.99

to�2

0.06

7.34

–4.57

Coa

rseto

finesilt

EQSS

MUC19

348

33�23.39

471

�52.76

018

–28

122

568

10.3–1

4.6(11.8)

�20.64

to�2

1.38

5.9–

6.67

Coa

rseto

med

ium

silt

MUC20

340

33�23.40

771

�52.76

2MUC21

342

33�23.43

771

�52.81

012

–15

7238

09.6–

11.2

(10.4)

�19.45

to�2

0.36

7.29

–8.20

Coa

rseto

med

ium

silt

MUC22

340

33�23.49

871

�52.79

3

aIfmea

nva

lues

forsu

lfide

andsu

lfatebe

twee

n7an

d17

cmareco

nside

red,

theinve

ntories

shou

ldincrea

seto

53an

d22

48mmol

m�2,respe

ctively.

bRed

oxpo

tential

discon

tinuitylaye

r(RPD

,con

side

redat

�100

Mv).


University. The samples were dried at 80 �C for 24 h, milled to afine powder, and acidified to remove carbonates. Dry material(0.5–2 mg) was placed into tin capsules and combusted in anEurovector elemental analyzer (Milan, Italy). The resulting N2

and CO2 gases were separated by gas chromatography and admit-ted into the inlet of a Micromass (Manchester, UK) Isoprime iso-tope ratio mass spectrometer (IRMS), to determine 15N/14N and13C/12C ratios. Typical precision of analyses is ±0.5‰ for d15N,and ±0.2‰ for d13C, using atmospheric nitrogen as a standard inthe first case and Peedee belemnite (PDB) in the second. Egg albu-min was used as a daily reference material.

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

2.3. Data analysis

2.3.1. Estimation of dissolved trace metal fluxesThe diffusive fluxes (J) were estimated according to Fick’s first

law (Berner, 1980), dependent on the pore water concentrationgradients (DC/Dz) under steady state condition. The movementof the ions occurs in the volume occupied by the pore wateraccording to the diffusive coefficient of each element:

J ¼ �/DSedDC=Dz ð1Þ

/ is the porosity and the Dsed the diffusion coefficient. The porositywas determined from the water content at each section of the core.Dsed was estimated for seawater (Dsw) from the diffusion coefficientof the solutes in free solution, taking into consideration the temper-ature of the bottom water at the shelf sites and corrected for tortu-osity h2 (Li, 1974; Boudreau, 1997). For calculation details seeSchulz (2006):

Dsed ¼ Dsw=h2 and h2 ¼ 1� lnð/2Þ ð2Þ

The concentration gradients determine the flux direction, i.e.increasing concentrations in depth will result in negative gradientsin the sediment and positive fluxes toward the bottom waters. Onthe other hand, decreasing concentrations will result in negativefluxes i.e. toward the sediments. We calculated the concentrationgradients using only some sections of the core, where the concen-tration gradients were maximum, close to the surface, as these gra-dients control the fluxes to the bottom water. We used theresulting flux calculations for comparison purposes which helpedus to differentiate the main diagenetic processes at each site.

2.3.2. Estimation of elements authigenic enrichment (EF)The element concentrations in the solid sediment fraction were

normalized to Al concentrations (Calvert and Pedersern, 1993;Tribovillard et al., 2006; Calvert and Pedersen, 2007) which is an

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


indicator of aluminosilicate portion within the sediments, unreac-tive during organic matter diagenesis.

The EF (enrichment fraction) of elements was determined inorder to compare the element authigenic enrichment at each areaconsidering the crustal abundance.

EF : ðMe=AlÞsample=ðMe=AlÞdetrital ð3Þwhere (Me/Al)sample is the bulk sample element concentration nor-malized to the Al content, and the denomination ‘‘detrital” indicatesa lithogenic background. Detrital concentrations ([Me]detrital and[Al]detrital) were established by taking into consideration the ele-ment abundance at closer localities from where was possible tohave available information; this is more accurate than using meanEarth crust values (Van der Weijden, 2002). Due to there being noinformation for element concentrations from continental inputsfor the CTJ, we used the element concentrations in deep clays forthis area (Chester, 1990), and we used the element concentrationsin suspended river particles from the Bío Bío river (37�S) for calcu-lations at the EQSS site; this is the closest location where this infor-mation was available for central Chile (Table 2). If EF is more than 1,this means that the element is enriched with respect to detrital val-ues, and if less than 1, it is depleted, constituting a good tool forinterpreting authigenic precipitation (Morford and Emerson,1999; Tribovillard et al., 2006).

3. Results

3.1. Bottom water environmental conditions and underlying sedimentcharacteristics

At the CTJ site the bottom waters presented oxygenated condi-tions (>150 lM), lower temperatures (1.83 �C), and slightly highersalinities (34.68 psu). In contrast, the EQSS site (8.45 �C and34.53 psu), located on the upper slope within the OMZ, exhibitedlower oxygen concentrations (23 lM, Table 1).

Overlying water and surface sediment temperatures were mea-sured immediately after retrieval at the CTJ site using a thermocou-ple. While the temperature of the supernatant water did notgreatly differ in comparison with the bottom temperature mea-sured with the CTD, the sediment temperature showed importantdifferences. In MUC06 the temperature ranged between 4 and5 �C, and in MUC04 the sediment temperature reached 13 �C; inboth cores the sediment temperature was at least 4 �C higher thanthat of the bottom water (1.83 �C).

The redox measurements of surface sediment showed positivevalues between surface and 6 cm depth at the CTJ (Fig. 3). Below

Table 2Bulk element concentrations normalized to Al content (Me/Al ratios � 104) used for eleme

Element Deep sea claysa (lg g�1) Me/Al � 104 Patagonian rivers

Al 95,000 61,675Mg 18,000 1895 –K 28,000 2947 –Ca 10,000 1053 5988Sr 250 26 191P 1400 147 –Pb 200 21 –Ba 1500 158 456Cd 0.23 0.02 –Mn 6000 632 684Fe 60,000 6316 31,879U 2.00 0.21 –Mo 8.00 0.84 –

a Chester (1990) for K at central Chile (33�S) we used the mean concentration in mudb S. Bertrand, unpublished data. Mean concentrations from superficial sediments in Pc J. Muratli, unpublished data; details will be published elsewhere.d Böning et al. (2009).

that Eh quickly reached negative values of around �100 mV and�200 mV, respectively, except in MUC04 which showed positivevalues at all depths. At the EQSS, the redox potential discontinuitylayer (RPD) was observed at 4 cm, reaching �200 mV at 10 cm(Fig. 3); at this site, sediment smelled strongly of sulfide.

The pigment content (CPE) integrated in the first 6 cm of surfacesediment showed higher values at the EQSS (12–28 lg g�1), andconsequently higher inventories (RCPE: 72–122 lg cm�2) andhigher carbon content (RTQC: 380–568 g C m�2), with isotopic val-ues (13C and 15N) indicating their main organic source to be plank-ton (�19–21‰). The deeper CTJ site showed comparably lowerpigment content and TOC inventories, with slightly different iso-topic values than those from the EQSS (Table 1).

3.2. Pore water chemistry

Methane concentrations measured at the EQSS were between18 and 50 nM, with maximum values at the bottom of the cores(Fig. 4); no measurements were done at the CTJ site, as hard bot-

nt authigenic enrichment calculations (EF).

b (lg g�1) Me/Al � 104 Bio Bio riverc (lg g�1) Me/Al � 104

74,961– 22,736 3033– 24,000a 3463a

971 38,525 513931 419 56– 44,205 5897– – 1.56d

74 301 40– 0.46 0.06111 1421 1905169 91,077 12,150– 1.13 0.15– 1.19 0.16

sediments and their correspondent Al content of 69,300 lg g�1.atagonian rivers (�54�Lat. S–70�Long. W); details will be published elsewhere.

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


toms prevented the collection of sediment cores suited for thisanalysis (Fig. 2).

Sulfide concentrations were relatively low at both sites,between 0.3 and 1.2 mM (Fig. 5, Table 1). Sulfate concentrationswere lower at the CTJ site than at the EQSS, showing valuesbetween 15 and 18 mM, and between 21 and 31 mM, respectively(Fig. 5). Comparing the sulfate and sulfide inventories in only thefirst 7 cm, due to the cores showing different core lengths, highersulfate and sulfide inventories occurred at the EQSS site (Table 1).

Stable isotope analyses from pore water DIC samples may sug-gest different sources of carbon, given that more negative d13DICvalues were measured at the CTJ site (�20‰ to �10‰) than atthe EQSS (�10‰ to 3‰) (Fig. 6).

3.3. Dissolved trace metal distribution

The elements distribution analyzed in the pore water of sedi-ment cores from the CTJ and EQSS sites are shown in Figs. 7 and8. The elements were organized according their concentrationand reactivity: first, the major elements including B (higher con-centrations and conservative behavior at water column); followedby minor and nutrient elements (Sr, Si, P; lower concentrations),and trace elements (very low concentrations) related mainly withthe particle transport (organic and inorganic; Pb, Ba, Cd), andoxidation-reduction reactions (Mn, Fe, U and Mo). The mean valuesof Cl and Na are also shown in Table 1.

Both sites presented similar concentrations of Cl and Na exceptin core MUC06, from the CTJ area, which showed lower values par-ticularly for Cl concentrations. Molar Mg/Ca and Sr/Ca ratios indi-cate similar values for both stations (�5 and �0.0083,

respectively) (Fig. 9). The lowest Sr and Mg concentrations,together with the lowest Ca concentrations, were observed at theCTJ, maintaining metal ratios similar to those estimated for theEQSS site. The correlations between major elements were verygood at both sites but higher values were observed at the CTJ(Appendix A), showing a conservative behavior; others, includingnutrient type elements and trace metals, did not show good corre-lations at either site.

The major element concentration profiles analyzed from coreMUC06, taken from the CTJ site, revealed lower metal concentra-tions in the core than those expected for major elements in bot-tom waters. These concentrations dropped between 7 and 15 cmcore depth, reaching half the concentrations of those in surfacecore sections. An example of this can be seen in the case of Cl,where concentrations of 500 mM fall to �200 mM at thesedepths within the core (Fig. 7). Similarly depleted valuesbetween 7 and 15 cm core depths were observed for B, Sr andSi profiles. Si and B showed good correlation as both are closelyrelated with clay composition (r = 0.85, p < 0.01; Appendix A). Atthe CTJ, ten times higher concentrations of Mn were observedthan those at the EQSS, with concentration values increasingwith depth; the highest values occurred below 3 cm core depth,at the same time Fe concentrations decreased to values lowerthan those at the EQSS site. In the surface, Fe at the CTJ sitewas slightly higher than expected for an oxic zone, due to ahigher source of oxides. Other trace elements also showeddecreasing concentrations with increasing depth at the CTJ, prob-ably due to precipitation in the presence of sulfides (CdS), orreduction (U); these concentrations were also lower than thoseat the EQSS site (Figs. 7 and 8).

Fig. 5. Total sulfide (RH2S) 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).


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

Fig. 6. d13DIC distribution in pore waters from sediment cores from (a) the Ch

3.4. Diffusive fluxes

The diffusive fluxes were estimated taking into considerationthe maximum concentration gradients, from the sediment surfaceto deeper sections within the sediment core, adjusting for the bestlineal or exponential curve; alternatively, the difference betweentwo points was considered in those cases where the curve fixingresulted in low r2 estimations. Sediment parameters used for fluxcalculations are shown in Table 3. The estimated fluxes representthe main movement of ions close to the sediment water interface.The major elements (Mg, K and Ca) indicate fluxes toward the sed-iment (negative values) on both sites, but the magnitude washigher at the CTJ (Fig. 10a and b); similarly, Sr showed negativevalues at both sites, also higher at the CTJ. Other elements suchas B and Si showed lower fluxes toward the water column (positivevalues) at both sites, though higher at the EQSS site. Likewise, pos-itive fluxes of P were estimated to be distinctly higher at the CTJ.Those elements (Mn, Fe) sensitive to redox changes precipitatedunder oxygenated conditions showed very high fluxes, positivefor Mn and negative for Fe at the CTJ. In contrast, the EQSS siteshowed positive values for both elements but very low fluxes forMn. Other elements (U, Mo) showed very low negative fluxes atthe CTJ, and higher positive fluxes at the EQSS. Lead, Ba, and Cdshowed similarly negative fluxes at both sites.

3.5. Me/Al ratios and EF estimations

The Me/Al did not show big changes in depth at 21 cm corelength at either site (Appendix B), therefore we use an averagevalue of the EFs calculated at intervals of 1 cm in the core depthfor each element (Fig. 11); this resulted in higher EF values formajor elements at the CTJ, especially for Ca, indicating an enrich-ment that was not observed for these elements at EQSS. Trace ele-ments that precipitate under low oxygen conditions or whenassociated with particle deposition were enriched at the EQSS(Pb, Ba, Cd, U and Mo); Cd and U were also enriched at the CTJbut less than at the EQSS. In contrast, Mn and Fe were not enrichedat either site.

It is important to consider that deep clays have higher Al and Fecontent than the sediments observed at the CTJ, which means theEFs could be underestimated. If the CTJ sediments receive impor-

ile Triple Junction (42�S, CTJ) and (b) the El Quisco Seep Site (33�S, EQSS).

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 andconcentrations used for DC/Dz calculations for flux estimations.


tant continental input, it should be possible to make the compar-ison with Me/Al ratios from Patagonian river sediments; this isquite far from our sampling site however (54�S; Table 2). Usingthe Patagonian sediments, only a slight enrichment of Mn, Fe and

Ba (1.07, 1.11, 1.12, respectively) and similar enrichment of Caand Sr was estimated; therefore, as no other elements were avail-able for Patagonian rivers to calculate the EFs, they were calculatedusing Me/Al ratios from deep sea clays.

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 andconcentrations used for DC/Dz calculations for fluxes estimations.


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).

Table 3Parameters detail for diffusive flux calculations. Concentration gradients were estimated from the best fit model for concentration profiles. The variances of the fitted curves areshown. Diffusive coefficients in pore water (Dsed) were estimated from the diffusive coefficients in seawater (Dsw, Boudreau, 1997) corrected for tortuosity and considering thebottom water temperature at each station. Mean porosity (/): 0.75 and 0.59 for the CTJ and the EQSS, respectively. Tortuosity (h): 1.59 and 2.04 for the CTJ and the EQSS,respectively.

Mg K Ca B Si Sr P Pb Ba Cd Mn Fe U Mo

CTJ (MUC06)DC/Dx (lmol/cm4) 2884 528 767 �158 �227 6 �21 0.01 0.14 0.32 �8 26 0.001 0.02Range of depth for DC/DX (cm) 3–7 3–7 3–7 1–7 0–8 3–7 1–3 1–2 1–3 1–20 2–5 1–3 1–7 3–7Dsed (cm2 y�1) 79 220 87 127 104 87 94 103 95 75 73 78 108 108r2 0.72 0.71 0.75 0.72 No 0.75 No No 0.90 0.90 0.91 No 0.94 0.70

EQSS (MUC19)DC/Dx (lmol/cm4) 2080 420 399 �348 �166 3 �10 0.02 0.03 �0.23 �0.45 �17 �1.06 �0.63Range of depth for DC/DX (cm) 4–7 4–7 4–7 0–2 5–7 4–7 1–4 0–2 0–10 0–2 2–5 0–4 3–6 0–17Dsed (cm2 y�1) 72 201 79 118 89 79 106 95 82 71 67 70 84 81r2 No No 0.87 No No 0.89 No No 0.81 No 0.84 0.99 0.95 0.87

No r2 estimation for gradients indicate low adjusted numbers. In those cases DC/Dx 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).

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 indicateincreasing concentrations at greater depth, resulting in positive fluxes to the water column. Negative values of fluxes indicate the opposite.


4. Discussion

4.1. Sediment characterization and environmental conditions

As expected, at the deeper CTJ site bottom waters had higheroxygen content and surface sediments had lower organic content,

a result of particle fluxes from the water column, consistent withlower RCPE and lower RTOC (Fig. 12); the lowest mean C/N ratio(7.8 and 9.3) indicates the presence of a less refractory materialthan at the EQSS site (11.8 and 10.4; Table 1). The EQSS sedimentsare receiving the particle material from the adjacent shelf, highlyrecycled until their deposition in deeper zones. The sediment con-

Fig. 11. Element authigenic enrichment at surface sediments (0–20 cm), meanvalues ± SD. The enrichment fraction (EF) was calculated comparing Me/Al ratios inthe sample, and lithogenic background using the element content in deep sea claysfor the CTJ site and in suspended particles from Bío Bío river for the EQSS site(Table 2).

Fig. 12. Relationship between inventories of RCPE (lg cm�2) and RTOC (g C m�2)content in surface sediments from the Chile Triple Junction (42�S, CTJ) and the ElQuisco Seep Site (33�S, EQSS). Information of surface sediment from the ConcepciónMethane Seep Area (CMSA, 36�S) was included in the analysis.


ditions at the CTJ were oxidized. Along MUC04 positive redox val-ues were observed, favored by a slightly higher grain size whichallowed for more efficient organic matter degradation; this differedfrom MUC06, near the axis, where more reduced conditions wereobserved below 8 cm core depth (Fig. 3, Table 1). Here, there wasno evidence of seeps or bacteria mats in the vicinity of MUC06,contrasting with the location of MUC04 where methanotrophicbacteria were present in both surface sediments and bottomwaters (Thurber, comm. Pers.). Additionally, anomalies in surfacetemperatures of sediments at the CTJ were observed, with a max-imummeasured sediment temperature of 13 �C at MUC04 and 5 �Cat MUC06, while the bottom water temperature was 1.83 �C atboth coring locations. These constitute a clear indicator of the pres-ence of hydrothermal fluids percolating through the sediment atthis site. Despite this, no chimney structures, crusts, or stroma-tolitic aggregates were observed here; however, hard bottomswere present. This is somewhat to be expected, considering theproximity with a continental margin characterized by fjords andrivers potentially generating a high input of terrigenous material;it is most probable that this site corresponds to a sedimented area

with lower-temperature (<20 �C) diffuse flows, according to pre-liminary observations in this zone during the cruise (Germanet al., 2010, 2011; Thurber et al., 2012).

In contrast with the CTJ area, which is characterized by thepresence of oxygenated waters, the sediments at the EQSS areaffected by the OMZ and therefore show low oxygen content inbottom waters (23 lM, Table 1). The existence of methane seepageand the presence of a cold-seep biological community at the EQSSsite has been evidenced by the presence of living clams of a newchemosynthetic vesicomyid genus, Austrogena nerudai, in a coreretrieved closer to our coring station (�360 m); tubes of siboglinidpolychaetes, probably of the genus Lamellibrachia, as well as authi-genic 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 sulfideshowever pore water analysis did not yield significantly higher sul-fide levels; additionally, lower methane content was measured(Table 1, Figs. 4 and 5). The magnitude of the fluxes here is proba-bly low, but it sustains chemosynthetic clams which inhabit a widerange of reduced environments, developing strategies to adapt todifferent compound concentrations necessary for their metabolism(Roeselers and Newton, 2012).

4.2. Sulfate-sulfide, methane, 13dDIC, 13dTOC

In general, the distribution of sulfate and methane in the sedi-ments is closely related to anaerobic oxidation processes of organicor inorganic compounds. Sulfate is a conservative element in sea-water and can penetrate several meters into the sediment; it isreduced to sulfide by sulfate reducing bacteria, through the oxida-tion of organic matter, diminishing its concentration until it istotally consumed and, consequently, sulfate reduction ceases(Fossing et al., 2000 and references therein). Pore water sulfateconcentrations at the EQSS were in the order of values reportedfor sediments under suboxic and oxic conditions off the Chile –Perú margin (20–30 mM, �100–1000 m water depth; Coffinet al., 2006; Raiswell and Canfield, 1996), and the sulfide concen-trations were lower than expected but within the order of magni-tude reported for anoxic sediments of the Chilean continentalmargin (from 0.3 mM to 1.4 mM at Concepción Bay; Ferdelmanet al., 1997; Thamdrup and Canfiel, 1996). The sulfides should beprecipitating as metal sulfides, reducing the soluble phase(Böning et al., 2005). At the CTJ site sulfide (<0.5 mM) and sulfate(<20 mM) values were lower than at the EQSS; the sulfide is prob-ably rapidly oxidized in surface sediments in contact with oxy-genated conditions of the bottoms (Jørgensen and Revsbech,1983; Fenchel and Bernard, 1995), or else actively oxidized bychemosynthetic activity (Govenar, 2012; O’Brien et al., 2015). Forcomparison, a similar situation is found in the Comau Fjord inSouthern Chile (�42�S), where a shallow vent system of reducedfluids sustains a distinct microbial community; low sulfide concen-trations were found close to the vents due to the rapid oxidation ofsulfide, even in places with steady sulfide fluxes (116–587 lM;Ugalde et al., 2013; Muñoz et al., 2014). Sediments at the CTJ wereless reduced (positive redox potential in the first 5 cm), with higherporosity and grain size than at the EQSS site, favoring sulfide oxi-dation. The sulfate concentrations were lower than expected forsalinity in bottom waters (�27.6 mM), in agreement with the dis-tribution of other major elements, as explained later.

The massive presence of methane deposits along the Chileanmargin, which has been confirmed by seismic surveys (Brownand Bangs, 1995; Brown et al., 1996; Díaz-Naveas, 1999;Morales, 2003; Coffin et al., 2006), seems to be mostly associatedwith the biogenic processes of methanogenesis during the oxida-tion of organic matter. Here, gas hydrates are formed even with


low TOC content (<0.5%), and the methane clearly results fromimportant in-situ sources of carbon (Froelich et al., 1995; Brownet al., 1996). Although the presence of methane seeping has beenproposed at the EQSS (Krylova et al., 2014), we observed lowmethane concentrations (20–60 nM; Fig. 3). At other sites, sedi-mentary methanogenesis can easily produce values of methanein the range of 0.5–6 mM when measured at greater depths withinthe sediments (>100 cmbsf: 0.5–6 mM; Chilean margin 34�–44�S;Treude et al., 2005; Gulf of Mexico; Ussler and Paull, 2008); how-ever, aerobic and anaerobic methane oxidation processes reducethe gas content within sediments closer to the surface (Knitteland Boetius, 2009). This could partially explain the low concentra-tions observed at the EQSS, particularly in our short sediment cores(25 cm) where the oxidants are suitable for AOM (i.e. sulfate,nitrate, among others) reducing the large input of the methaneto the oceans (Caldwell et al., 2008; Reeburgh, 2007; and refer-ences therein).

The presence of gas hydrates and upward fluid migration hasbeen reported in the vicinity of our coring stations at the CTJ(Behrmann et al., 1992), and methanotrophic bacteria is alsorecently being described (Thurber, pers. Comm). Unfortunately,we could not obtain methane measurements within the sedimentsand other parameters, as DIC were performed. The relationshipbetween carbon sources and sulfate consumption determines thecarbon isotopic composition of DIC, and indicates the nature of thisoxidative process (sulfate reduction); very negative d13C-DIC val-ues result from the anaerobic methane oxidation, when the d13Cof methane is 20–75‰ more negative than organic matter(Kastner et al., 2008). This process produces 13C-depleted bicarbon-ate ([HCO3]�), which reacts with seawater Ca to precipitatemethane-derived carbonates, resulting in very negative d13C-DICvalues (<�30‰ to �60‰; Aharon et al., 1992; Teichert et al.,2005). Our measurements were less negative (��20‰ to �10‰)than can be explained by sulfate reduction (<�12‰ to �26‰)and the remineralization of less enriched 13C organic matter in sur-face sediments.

The DIC composition of the pore waters also depends on car-bonate dissolution and re-oxidation of sulfide; based on the sulfatereduction intensity and Ca concentrations in pore waters, two end-members have been described by Walter et al. (2007). The mostnegative values of d13DIC (end-member 1) result from low carbon-ate dissolution producing moderately depleted Ca in pore waters ina closed system of sulfate reduction; this is similar to what weobserved at the CTJ site (MUC 06), where the Ca content(<10 mM; Fig. 7) was lower than expected from the Ca/Cl mea-sured in the adjacent bottom waters (10.19 mM). End-member 2is characterized by very positive values of d13DIC, associated withbioturbated sediments and sulfide oxidation producing Ca excessin pore waters; the d13DIC values at the EQSS site should be com-patible with this scenario, as the pore water Ca concentrationsmeasured (10.4 ± 0.4 mM Ca) were slightly higher than in the bot-tom water, when normalized to salinity (10.14 mM; Fig. 8). Biotur-bation could be an important process here, increasing the isotopicvalues of DIC (more positive). Our samples fall between these twoend-members, indicating the contribution of both organic matterre-mineralization and the balance between carbonate precipitationand dissolution in the pore water. Furthermore, no evidence ofAOM was obtained from the DIC isotopic values, as the measure-ments were carried out in surface sediments.

4.3. Major and minor elements

The lowest concentrations of major elements including sulfate,when compared with those expected for bottom waters werefound at the CTJ site (Fig. 7). All of them maintained a similar pro-portion with Cl along the core, which could be attributed to dilu-

tion by deeper fluid seepage; in the accretionary prism, fluids arenormally less concentrated than seawater due to the depth ofclay-mineral dehydration during tectonic compression (Haeseet al., 2006; Hensen et al., 2007). Additionally, gas hydrate dissolu-tion processes could be occurring at this site due to the decompo-sition reactions involved in the formation of gas hydrates,introducing fresh water which lowers the Cl concentrations in porefluids (Paull et al., 1996 in Borowski, 2004; Gieskes and Mahn,2007). On the other hand, these lower concentrations in porewaters could respond to mineral precipitation, for example, cal-cium carbonate, considering that the lowest d13C-DIC were foundat similar depths (�7–15 cm); however, given their proportionaldecrease, the Sr/Ca and Mg/Ca did not differ significantly at theEQSS site (Fig. 9). This supports the dilution hypothesis, becausethe mineral precipitation establishes different equilibrium concen-trations for each cation, modifying the conservative behavior.Alternatively, low Cl, Mg and sulfate concentrations could beattributed to the upwelling of hydrothermal fluid observed in sur-face sediments (Aquilina et al., 2013).

Boron is removed from seawater via uptake into the igneousoceanic crust, and adsorbed onto detrital sediments (You et al.,1995, and references therein). It is considered a conservative ele-ment and may be enriched in cold seeps and hydrothermal fluidsvia dehydration and desorption (Haese et al., 2006, and referencestherein), reaching values between 3.5 and 15 mM in these environ-ments (Wei et al., 2005); these are much higher than our reportedconcentrations (1–4 mM, Figs. 7 and 8). The lattice bound B des-orption produces higher B concentrations and simultaneouslylower Cl concentrations at very high temperatures and very deepbelow the sea floor (>150 �C; 2.5 km; You et al., 1995); this wasnot comparable with our short cores. Our data indicates lower Cland B concentrations along the core at the CTJ site (Fig. 7), with asimilar distribution pattern to that of major elements, supportingthe idea of pore water dilution as suggested before. It is importantto consider that a substantial proportion of B in expelled fluids islikely reabsorbed or trapped by surrounding sediments as the tem-perature decreases, due to the B adsorption-desorption mecha-nisms being highly dependent on environmental conditions inpore waters. It is therefore possible that adsorption or co-precipitation with carbonates could be favored by the low temper-atures and higher pH at the surface (Vengosh et al., 1991; Youet al., 1995), which should explain the good correlation with Si(r = 0.85; Appendix A). The mean concentrations of B at the EQSSsite do not differ significantly from those at the CTJ, also showingincreasing concentration at depth but without correlation withother elements. The organic content accumulated in sedimentsfrom the CTJ site was at least 30% of that accumulated at the EQSS,therefore the organic carbon rain and the diagenetic reactionsshould be more relevant for some elements at the second site,and highly influenced by terrigenous material diluting the sedi-mented marine organic matter. Phosphorus and Si distributionare highly dependent on organic transport to the bottoms as bothelements form part of phytoplankton skeletal material and soft tis-sue, playing a role in metabolic processes. Similarly, Sr could haveimportant input from biogenic particle deposition to the bottomsin pelagic sediments (Bernstein et al., 1998; Jacquet et al.,2007b), promoting the element enrichment within the sediments(Fig. 11). Adsorption onto oxyhydroxides and co-precipitationoccur in many different environments, including hydrothermalsystems, constituting an important element removal mechanismfrom pore water and column water (Calvert and Pedersen, 2007).This process has been reported relevant for authigenic P, formedfrom their organic phases and oxide associated forms (Wheatet al., 1996; Delaney, 1998). The P profile at the EQSS site showlow, yet significant, correlation with Fe (r = �0.5, p < 0.01; Appen-dix A). This could suggest a relation with the Fe cycle, adsorption,


and subsequent liberation, during reduction of Fe-oxides in anaer-obic organic matter remineralization, producing higher positivefluxes toward column water (Fig. 10). Although P concentrationsin 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 corre-lation with Fe was observed at the CTJ site, however the P fluxeswere three times higher than the fluxes estimated at the EQSSand out of the sediments (Fig. 10), maintaining low P concentrationin the solid fraction with EF < 1. Here the organic matter reminer-alization should be relevant in the nutrient element distribution.

Manganese oxidation maintain low pore water concentrationson the surface at the CTJ site, but very high concentrations werefound (10–30 lM) below the oxidized layer (�5 cm) comparedwith the concentrations observed at the shallower anoxic site(EQSS; 1–2 lM). Consequently, substantially higher benthic efflu-ence of Mn was estimated at the CTJ (Fig. 10) even in oxygenatedconditions, when oxide precipitation occurs. This could beexplained by leaching of basalt during upwelling pore waters,resulting in net sources of Mn and others, such as Fe and Si. Thisprocess could occur on the sea floor in the presence of low-temperature spring waters (2–75 �C; Juniper and Tebo, 1995;Wheat and Mottl, 2000; Wheat and McManus, 2005), as has beenfound here (5–13 �C). Contrarily, precipitation reactions canremove elements from the pore water, diminishing concentrationsof Mn, Fe, and Si related with oxides, silicates and sulfides forma-tion, and in some cases closely related with bacteria activity (Fortinet al., 1998). These processes could explain the diminishing porewater concentrations of Fe below its maximum at the surface(80 lM), which occur in the presence of oxidative conditions; Feis less soluble than Mn and form insoluble phases (monosulfides,pyrite, carbonates). The sequestering of Fe from pore water in oxi-dized layers (Figs. 3 and 7) resulted in very negative fluxes (towardthe sediments) that also imply their accumulation within the sed-iments. However, Fe did not show authigenic enrichment (Fig. 11).At the EQSS site, the reduced conditions in the bottoms providefewer sources of Mn oxides and consequently lower Mn fluxes.This has been observed in reduced sediments off Peru, where theMn and Fe fluxes were a result of the supply of oxides (Böninget al., 2004; Scholz et al., 2011). In the case of Fe, the continentalinput of Fe is probably relevant along the Chilean margin, espe-cially in shallower zones; therefore, these sites did not showenrichment above detrital content (Fig. 11).

4.4. Trace elements

Trace elements generally did not show good correlationsbetween each other. The most relevant were observed for Ba andCd at the CTJ site (MUC06; r = 0.77, p < 0.01; Appendix A). Theaccumulation of these elements within the sediments occurs in avariety of environments with reduced fluid seepage, includinghydrothermal vents and cold seeps, but also where there are highorganic fluxes from biological productivity toward the bottom(Boström et al., 1973; Aloisi et al., 2004; Stevens et al., 2015). Theirbehavior in pore water environments is opposed; while Ba is sol-uble in reduced environments, Cd forms insoluble sulfide com-pounds (You et al., 1996; Castellini et al., 2006; Calvert andPedersen, 2007 and references there in), and no positive correla-tion should be expected for them. Both Ba and Cd presented sim-ilar fluxes toward the sediments at the CTJ and EQSS (Ba: �10 and�2 nmol cm�2 y�1; Cd: �18 and �9 nmol cm�2 y�1; respectively)(Fig. 10), suggesting that these elements are being efficientlyremoved from the pore water. Under reduced conditions, Ba min-erals are unstable and Ba is lost to the bottom water, normally inthe 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 watersat the CTJ could suggest Ba precipitation, however no enrichmentwas observed in sediments (EF < 1). The bulk Ba/Al ratios of thesesediments are higher than at the EQSS (0.008 and 0.004, respec-tively, Appendix B), and not enough to enrich the sediments whencompared to the content of deep clays. At the EQSS site, Ba dis-played a negative correlation with Si (r = �0.63, Appendix A)and, although this is not strong, it is significant (<0.01); this meansthat while Ba is remobilized from the sediments under reducingconditions, Si concentration increases in pore waters. These ele-ments normally correlate well; Si is the best predictor of dissolvedBa in the water column, with the exception of surface waters dueto uptake by plankton, while the precipitation of barite and opalwill control the dissolved Ba content in the bottoms (Jacquetet al., 2004, 2007a, 2007b). Therefore, Ba and Si behavior at theEQSS site imply that the organic flux to the bottom should beimportant for the distribution of these elements within the sedi-ments. The Ba EF value, slightly higher than 1 at the EQSS, cannotbe considered enriched (1.14 ± 0.15 ppm) but, given the close loca-tion to the continental margin, the detrital input could be moreimportant than the organic fluxes to the bottoms and be dilutingthe authigenic Ba concentration. The case of Cd, it was enrichedsubstantially at both sites (EFs > 2), indicating the presence ofreduced fluids being expelled from the sediments and low-sulfide pore water concentrations resulting from the mineralprecipitation.

In general, Mo and U exhibit similar tendencies but have nostrong correlations between them at either site, as their precipita-tion (or reduction) occur under different redox conditions; U isreduced close to the depth at which sulfate reduction occurs andMo needs free sulfides for precipitation (Calvert and Pedersen,2007). These elements are considered conservative in the watercolumn, and enter the sediments by diffusion. They showed dimin-ishing concentrations with depth at the CTJ site and did not showsignificant gradient concentrations with core depth, resulting invery low fluxes toward the sediments (Fig. 10) which indicates thatthese elements are being reduced and precipitated. Above theredox potential boundary layer, marked by the Mn (and Cd) porewater distribution (�5 cm, Figs. 3 and 7), these elements shouldbe oxidized, producing a slight increase of the U and Mo concentra-tions at surface. The decreasing pore water concentrations indepths resulted in very low negative fluxes (�0.1 and�1 nmol cm�2 y�1, respectively), facilitated by higher porosity atthis site, which allows oxygen penetration toward the sediments.Therefore, U fluxes occurred toward the sediments(�0.1 nmol cm�2 y�1) in the order of magnitude observed foranoxic sediments influenced by oxygenated conditions in bottomwaters (permanently or periodically) (�0.1 to �0.6 nmol cm�2 y�1,Morford et al., 2009; Scholz et al., 2011; Muñoz et al., 2012).

In contrast, at the shallow suboxic EQSS site, U and Moshowed higher concentration gradients in pore waters and higheroverall concentrations compared with the deeper CTJ site (Figs. 7and 8), resulting in positive fluxes from the sediment toward bot-tom waters (Fig. 10). The U and Mo presented increasing concen-trations toward deeper sections of the sediment core, and peaksobserved around 4, 10 and 16 cm were concordant with the max-imums observed for the Fe profile. Even though low correlationwas found between these elements, the U and Mo profiles couldbe based on their adsorption/desorption onto oxyhydroxides (Feand Mn). A similar situation was observed at the suboxic siteoff Concepción (Muñoz et al., 2012), and it is also suggested forPeru sediments (Scholz et al., 2011). The shelf area here is highlyinfluenced by seasonal river discharges, constituting a relevantsource of oxides to surface sediments. Moreover, high primaryproductivity produces high organic matter rain, which also servesas U carrier to the bottoms and may have been released during


organic matter diagenesis. Their final accumulation shoulddepend on the oxygen concentration in bottom water, and theoxygen penetration depth within the sediments (Zheng et al.,2002; McManus et al., 2005, 2006; Morford et al., 2009). Weassume that oxygen content in the bottoms of the EQSS site hasa seasonal pattern, most probably similar to that in southernareas such as at the Concepción shelf (36�S), highly influencedby the OMZ. In such systems, the U accumulated in sedimentsat reduced periods may constitute a source during oxygenatedperiods (Morford et al., 2009; Muñoz et al., 2012). In our casethe cruise was carried out in March, when reduced conditionsprevail, and low U pore water concentrations were measured insurface sediment. Uranium enrichment was observed at the EQSSsite, but no higher differences in Me/Al ratios were observedbetween the CTJ and EQSS sites (Appendix B); therefore, the esti-mated detrital U (measured in deep sea clays and suspended par-ticles from Bio Bio river) was determinant in establishing theenrichment (or no-enrichment) at both sites. For Mo, slightlyhigher bulk concentrations and a higher EF value were observedat the EQSS site, but no different from 1 (1.09 ± 0.21), thereforeit has not been considered as enriched at this site (Fig. 11; Appen-dix B). The low oxygen and low redox conditions in the bottomsclearly promote the U enrichment, but are not enough for Moenrichment which needs very anoxic and sulfidic conditions tobe precipitated.

A very low Pb content was recorded at both sites, since thiselement is highly insoluble (Figs. 7 and 8); it is remobilized fromsolids but it is also easily removed by new mineral formation(You et al., 1996), generally showing low concentrations in thewater. At the shallow EQSS site, the Pb content in pore waterwas higher in surface sediments (0–5 cm, Fig. 8) and diminishedquickly at depths in the sediment core, reaching values similar tothose at the CTJ site. This surface maximum occurs due to Pbsurface adsorption onto particles, or co-precipitation with Fe oxi-des in the water column which is then deposited in surface sed-iments and delivered to pore waters. Under reducing conditionsthe oxides are dissolved; however, where adequate sulfide condi-tions are encountered, they are precipitated again as PbS withinthe sediments. A clear enrichment of Pb was estimated at theEQSS, diminishing the pore water sulfide concentrations, and asimilar trend was described for Cd; both are associated withmetal sulfide precipitation, and both enriched in thesesediments.

Uranium, Pb, Mo, and Cd concentrations would be maintainedwithin the sediments in equilibriumwith precipitated sulfide com-pounds (Morford et al., 2009; Aparicio-González et al., 2012), andMo and U distribution could be closely related with the Mn-Feoxide/reduction cycle.

5. Conclusions

Discrete vents sites have been explored in the southern part ofthe Chile Rise (German et al., 2010, 2011; Thurber et al., 2012) and,based on oceanographic observations and recent biological evi-dence, these suggest the occurrence of a sedimented area of diffusehydrothermal fluids at the CTJ site; this is to be expected, given theproximity of a continental margin providing an important flux ofterrigenous material. This study contributes through geochemicalinsights by helping to describe the nature of the reduced environ-ment at the CTJ. Relatively higher temperatures were found in sur-face sediments, and substantially higher Mn concentrations andfluxes toward bottom waters suggest the ascent of reduced fluidsat this site. Additionally, lower pore water concentrations of majorelements (Cl, Mg, Ca, K and sulfate) were observed within the core

compared with those observed in bottom waters, which could berelated to diluted fluid seeping and mineral precipitation. HighCa enrichment was estimated and also, to a lesser extent, U andCd enrichment; both require reduced conditions, but the presenceof sulfide is only necessary in the case of Cd. The reduced condi-tions could not be explained by organic matter diagenesis, sinceoxygenated bottoms are present and low organic rain rate wasestimated from the TOC inventories. Although this does not consti-tute strong evidence for the origin of the reduced fluids observedhere, the presence of low-temperature hydrothermal fluids shouldnot be discarded. The widespread occurrence of shallow deposits ofmethane gas hydrates has also been indicated by geophysical stud-ies in the margin adjacent to the CTJ (Brown et al., 1996;Grevemeyer et al., 2003); however, no evidence of AOM could befound in surface sediments. The same stands for DIC pore watervalues, which were not sufficiently depleted in 13C, alreadyexplained by the calcium carbonate precipitation/dissolutionprocess.

In the EQSS shallow zone, the reduced conditions in the bot-toms and sediments enable trace element enrichment. The verylow Mn fluxes are the consequence of the low oxygen conditionsin bottom waters resulting in low supply of oxides, therefore therewas no enrichment of Mn. The reduced conditions of the sedi-ments promote the enrichment of elements such as Cd, Pb, U,and in some degree Mo. Similarly, Pb and Cd fluxes toward thesediment suggest that their presence in the pore water is con-trolled by the adsorption/desorption processes and precipitationwith sulfides, which also diminish de sulfides concentration inpore waters. At this site, the organic matter remineralization andthe presence of oxygen deficient bottom waters due to the OMZare responsible for the element distribution in pore water andsediments.

The presence of methane seeps at the EQSS cannot be identifiedfrom our geochemical evidence, due to low methane concentra-tions and enriched d13DIC in pore waters; however, the occurrenceof distinctive chemosynthetic fauna, typical of active seep sites,prompts for further integrated exploration of this area in order toclarify the sources of the reduced compounds supporting theseassemblages.

Acknowledgments

We would like to thank the captain and crew of the R/V MEL-VILLE, Scripps Institution of Oceanography, during the Inspire2010 cruise. We also give thanks to Dr. Jesse Muratli and Dr. Sebas-tien Bertrand, for sharing unpublished data from river samples.This work was partially funded by the FONDECYT projects No.1100166 and 1120469 (to J.S.), and the COPAS center from theUniversity of Concepción. Additional funding to P.M. during thewriting phase of this manuscript was provided by FONDECYT pro-ject 1140851 for additional sediment sample analyzes. Other fund-ing, for logistics and ship time, was provided by the NOAA OceanExploration Program (via SCRIPPS Institution of Oceanography,contract NOAA NA17RJ1231) and the Census of Marine Life pro-gram (through its field project COMARGE).

Appendix A

Spearman rank correlation coefficients in MUC06 (CTJ) andMUC19 (EQSS). Values >0.70 (printed in bold) are significant(<0.01 level).

Cl Mg Ca Na K Sr B Si Mn Fe P Mo Cd Ba Pb U

MUC06Cl 1.00 0.99 0.97 0.99 0.99 0.99 0.51 0.64 0.31 �0.01 0.43 0.53 0.06 0.50 0.17 0.51Mg 1.00 0.99 1.00 0.99 0.99 0.51 0.61 0.32 �0.08 0.44 0.55 0.06 0.49 0.14 0.51Ca 1.00 0.99 0.97 0.99 0.53 0.57 0.31 �0.13 0.43 0.59 0.14 0.53 0.15 0.54Na 1.00 0.99 0.99 0.51 0.61 0.32 �0.08 0.44 0.55 0.06 0.49 0.14 0.51K 1.00 0.99 0.50 0.62 0.34 �0.06 0.45 0.52 0.03 0.46 0.11 0.48Sr 1.00 0.52 0.61 0.33 �0.07 0.45 0.54 0.06 0.49 0.13 0.50B 1.00 0.85 0.47 0.01 �0.11 0.41 0.26 0.61 0.08 0.15Si 1.00 0.54 0.35 0.00 0.18 0.05 0.54 0.11 0.12Mn 1.00 �0.08 �0.02 0.06 �0.09 0.26 �0.40 �0.23Fe 1.00 �0.04 �0.42 0.01 0.16 0.29 0.00P 1.00 0.19 �0.05 0.01 0.24 0.27Mo 1.00 0.57 0.59 0.23 0.57Cd 1.00 0.77 0.34 0.56Ba 1.00 0.34 0.51Pb 1.00 0.41U 1.00

MUC19Cl 1.00 0.85 0.70 0.82 0.85 0.77 �0.11 �0.02 0.20 0.13 0.42 0.27 0.35 0.20 0.07 0.02Mg 1.00 0.95 0.98 0.96 0.97 �0.27 �0.20 0.43 0.29 0.36 0.39 0.31 0.18 0.29 �0.02Ca 1.00 0.95 0.89 0.97 �0.24 �0.20 0.53 0.32 0.30 0.40 0.30 0.20 0.38 �0.01Na 1.00 0.94 0.97 �0.19 �0.09 0.39 0.36 0.37 0.40 0.28 0.12 0.31 �0.04K 1.00 0.94 �0.22 �0.21 0.35 0.15 0.34 0.46 0.16 0.18 0.16 0.12Sr 1.00 �0.19 �0.13 0.50 0.31 0.31 0.42 0.22 0.15 0.21 0.08B 1.00 0.67 �0.25 �0.20 �0.26 0.28 �0.24 �0.42 �0.28 0.55Si 1.00 �0.13 0.32 0.20 0.15 �0.31 �0.63 �0.36 0.35Mn 1.00 0.56 0.09 �0.19 �0.13 �0.03 0.07 0.13Fe 1.00 0.51 �0.15 �0.08 �0.45 0.08 �0.16P 1.00 0.30 0.09 �0.24 �0.02 �0.03Mo 1.00 �0.05 �0.21 �0.24 0.61Cd 1.00 0.48 0.64 �0.61Ba 1.00 0.54 �0.34Pb 1.00 �0.61U 1.00


Appendix B. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.pocean.2016.09.002.

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