Deep-slab fluids fuel extremophilic Archaea on a Mariana forearc

14 Pages·2003·1.07 MB·English

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Research Letter
GG
GGeeoocchheemmiissttrryy Volume 4, Number11
GGeeoopphhyyssiiccss 14November 2003
9009,doi:10.1029/2003GC000588
GGeeoossyysstteemmss
ISSN: 1525-2027
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
Deep-slab fluids fuel extremophilic Archaea on a Mariana
forearc serpentinite mud volcano: Ocean Drilling Program
Leg 195
Michael J. Mottl
Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822, USA ([email protected])
Stephen C. Komor
104 Berkshire Road, Ithaca, New York 14850, USA
Patricia Fryer
Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii 96822, USA
Craig L. Moyer
Biology Department, Western Washington University, Bellingham, Washington 98225, USA
[1] As the Pacific plate subducts beneath the Mariana forearc it releases water that hydrates the overlying
mantle wedge, converting it to serpentinite that protrudes to form mud volcanoes at the seafloor. Excess
H O ascends through these mud volcanoes and exits as cold springs at their summits. The composition of
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this deep-slab derived water has been determined by drilling on two of these seamounts. It has a pH of
12.5 and, relative to seawater, is enriched in sulfate, alkalinity, Na/Cl, K, Rb, B, light hydrocarbons,
ammonia,18O,anddeuterium,anddepletedinchloride,Mg,Ca,Sr,Li,Si,phosphate,and87Sr.Withinthe
upper 20 m below seafloor at South Chamorro Seamount a microbial community operating at pH 12.5,
made up overwhelmingly of Archaea, is oxidizing methane from the ascending fluid to carbonate ion and
organic carbon, while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia.
Components: 5892words, 9figures, 1table.
Keywords: Subductionfactory;porewater; serpentinite;forearc; mudvolcano;Ocean DrillingProgram.
Index Terms: 1030 Geochemistry: Geochemical cycles (0330); 1050 Geochemistry: Marine geochemistry (4835, 4850);
1615GlobalChange: Biogeochemical processes(4805).
Received 10June2003;Revised 20 August2003;Accepted15 September2003;Published14 November2003.
Mottl, M. J., S. C. Komor, P. Fryer, and C. L. Moyer, Deep-slab fluids fuel extremophilic Archaea on a Mariana forearc
serpentinite mud volcano: Ocean Drilling Program Leg 195, Geochem. Geophys. Geosyst., 4(11), 9009, doi:10.1029/
2003GC000588, 2003.
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Theme: Trench to Subarc: Diagenetic and Metamorphic Mass Flux in Subduction Zones
Guest Editors: Gary Bebout and Tim Elliot
Copyright 2003bythe American GeophysicalUnion 1of 14
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1. Introduction lighthydrocarbons,sulfate,bisulfide,Na/Cl,K,Rb,
and B (Table 1). Near-surface gradients in chloride
[2] The Mariana subduction complex is formed indicatethatthiswaterwasupwellingat1–10cm/yr
between the northwestward subducting Pacific [Mottl, 1992]. Also drilled during Leg 125 was
plate and the overriding Philippine plate. Volatiles Torishima Forearc Seamount, an inactive serpen-
released from the downgoing Pacific plate hydrate tinite mud volcano in the Izu-Bonin forearc near
the overlying mantle wedge of the Philippine plate 31(cid:1)N. Pore water recovered from serpentinite on
and convert depleted harzburgite to low-density this seamount reflects reaction of cool (4–11(cid:1)C)
serpentinite. The resulting serpentinite mud, con- seawaterwithharzburgiteandcontrastsgreatlywith
taining variably serpentinized harzburgite clasts, that from the active Conical Seamount (Table 1).
ascends buoyantly along fractures and extrudes at The distinctive composition of water upwelling at
the seafloor, where it forms large (50 km diameter, Conical Seamount implies that it originates by
2kmhigh)mudvolcanoesalongtheouterMariana dehydration of the subducting Pacific plate,
forearc, in a band that extends from 30 to 120 km (cid:1)29 km below the seafloor based on earthquake
behind the trench axis [Fryer et al., 1985, 1995, depths [Hussong and Fryer, 1981; Seno and
2000].Thesemudvolcanoesarebuiltfromflowsof Maruyama,1984].TheupwellingH Oisinexcess
2
poorly consolidated sedimentary serpentinite fed of that which, during its ascent, hydrates the over-
through a central conduit. Cold ((cid:1)2(cid:1)C) spring lying mantle wedge, and represents one of the
waters fresher than seawater have been sampled earliestreturnsofsubductedvolatilestotheoceans.
on several of these mud volcanoes, by manned Unlike deeply sourced water sampled from other
submersibles [Fryer et al., 1990; Fryer and Mottl, subductionzones,thiswaterhasnotinteractedwith
1997], deep drilling [Mottl, 1992], and piston and an accretionary sedimentary prism, as the Mariana
gravity coring [Fryer et al., 1999]. We report here subduction zone is non-accretionary; the origin of
themostcomprehensivesamplingofsuchwatersto this water is thus easier to infer because it has
date,onOceanDrillingProgram(ODP)Leg195in reacted mainly with a relatively simple matrix of
2001, and the discovery that they support a com- harzburgite during its ascent.
munityofextremophilicmicrobes,mainlyArchaea,
within serpentinite mud 0–20 m below seafloor
2. Results
(mbsf). This community operates at pH 12.5.
[ ] ODP Site 1200 was drilled on the summit of
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[ ] Thefirstserpentinitemudvolcanodrilledinthe South Chamorro Seamount, 85 km arcward of the
3
Mariana forearc, on ODP Leg 125, was Conical trench near 14(cid:1)N (Figure 1 and Table 1) and
Seamount,90kmarcward(west)ofthetrenchnear (cid:1)27 km above the top of the subducting Pacific
19.5(cid:1)N (Figure 1 and Table 1) [Fryeret al., 1992]. plate. Dives in 1996 discovered and sampled three
In 1987 divers in the submersible Alvin discovered cold springs with carbonate crusts and chimneys
chimneysupto3.5mtallatthesummit,composed and an abundant biota of mussels, small tube-
of aragonite, calcite, and amorphous Mg-silicate; worms, whelks, and galatheid crabs [Fryer and
whenonechimneywasdisturbeditbegantoemita Mottl, 1997]. Pore water from Site 1200 was
slow flow of cold ((cid:1)1.5(cid:1)C) water with pH 9.3 and recoveredbysqueezingserpentinite mudfromfour
elevateddissolvedcarbonate,methane,sulfate,and holes that form a transect from one of the springs.
reduced sulfur relative to the surrounding seawater Holes 1200A and 1200E are located within a few
[Fryer et al., 1990]. Bacterial mats and small meters of the spring and Holes 1200F and 1200D
limpets and gastropods were collected from the are 20 m north and 80 m northwest of the spring,
chimneys. Although none of these chimneys could respectively (Figure 2). Temperature gradients
be located during Leg 125, drilling at the summit were low such that none of the pore water
recoveredunusualporewaterthathadlessthanhalf exceeded 3.0(cid:1)C at the sampling depth, compared
the chloride and bromide of seawater, pH of 12.6, with a bottom water temperature of 1.67(cid:1)C
and was highly enriched in dissolved carbonate, [Shipboard Scientific Party, 2002].
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GeochemistryGG mmoottttll eett aall..:: ddeeeepp--ssllaabb fflluuiiddss 1100..11002299//22000033GGCC000000558888
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Figure 1. Location of Conical and South Chamorro Seamounts, two active serpentinite mud volcanoes in the
Mariana forearc that have been drilled.
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Table 1. Composition of Pore Water From Three Serpentinite Seamounts Versus Seawater
Seamount Conicala SouthChamorroa TorishimaForearcb Seawaterc
ODP Site 780 1200 783, 784
Latitude 19(cid:1)32.50N 13(cid:1)47.00N 30(cid:1)57.86,54.490N
Longitude 146(cid:1)39.20E 146(cid:1)0.20E 141(cid:1)47.27,44.270E
Deepest sample (mbsf) 130 71 400
Chloride (mmol/kg) 260±25 510±5 550±5 542
Sulfate (mmol/kg) 46±1 28± 1 9 ±3 28.0
Alkalinity (meq/kg) 52±13 62± 8 1.3 ±0.4 2.3
Carbonate Alkalinityd 35±15 45± 7e 1.1 ±0.4 1.9
pH at25(cid:1)C 12.5 ±0.1 12.5± 0.1 9.6 ±0.4 8.1
Na (mmol/kg) 390±10 610± 10 460±20 466
Na/Cl (molar) 1.5±0.1 1.2± 0.02 0.84 ±0.02 0.860
K (mmol/kg) 15±1 19± 1 5 ±1 10.1
Mg (mmol/kg) 0.003±0.002 <0.01 1 ±1 52.4
Ca (mmol/kg) 1 ±0.5 0.3 ±0.1 55±5 10.2
ChargeBalance (calc.) 3.0 ±25 1.6± 10 7.7± 20 0.9
CH (mmol/kg) 2000±1000 2000±1000e 2 ±1 0.0004
4
Li(mmol/kg) 1.6±0.5 0.4 ±0.1 16±5 26
Rb(mmol/kg) 7.8±0.6 11 ±2 1± 0.2 1.37
Sr(mmol/kg) 20±10 10± 2 200±50 90
Ba (mmol/kg) 0.1± 0.05 0.4? 1.6 ±0.7 0.14
B(mmol/kg) 3900±100 3200± 200 180±80 410
Si (mmol/kg) 60±30 70±20 10±8 190
F(mmol/kg) not determined 47± 3 not determined 67
Mn (mmol/kg) <0.01 0.01 0.4 ±0.2 0
Fe(mmol/kg) 2 ±1 2± 0.5 1± 0.9 0
Phosphate (mmol/kg) 0.4±0.3 0.2 not determined 2.8
NH (mmol/kg) 265±5 220± 10 140±20 0
3
Reduced S(mmol/kg) <250 <250 0 0
C H (mmol/kg) 7 2 0 0
2 6
C /C (molar) 290±15 780± 10
1 2
87Sr/86Sr <0.7062 0.70535 ±0.0001f <0.7070 0.7091
d34SO (%CDT) 13.7 ±0.3 not determined 31.2±0.4 20.5
4
d18O(%SMOW) 4.0±0.5 2.5 ±0.5 (cid:3)0.4 ±0.2 0.0
dD(%SMOW) 3 ±2 12± 2 (cid:3)2±2 0.0
aAsymptoticcompositionofdeepupwellingfluidfromactivemudvolcano.ConicaldataarefromMottl[1992],MottlandAlt[1992],Haggerty
andChaudhuri[1992],andBenton[1997].
bProductofharzburgite-seawaterreactionwithininactivemudvolcanointheIzu-Boninforearc[Mottl,1992].
cEstimatedcompositionoflocalbottomwater.
dmeq/kg,calc’dfromPHREEQC[ParkhurstandAppelo,1999].
ed13CH =(cid:3)11±5%VPDBindeepfluid;d13C-DIC=(cid:3)16%VPDBat15mbsf,stillwithinmicrobialzone.
4
fBickfordandSiegel(personalcommunication,2002).
[ ] Depth profiles of pore water composition (Figure 4) approach asymptotic values within 3–
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(Figures 3 and 4) show the typical convex-upward 4 mbsf in Hole 1200E at the spring. Gradients
shape and steep near-surface gradient indicative of become progressively less steep at Holes 1200F
upwelling at several cm/yr of a fluid composition- and 1200D to the north, implying that upwelling
ally different from seawater. Chloride profiles also slows away from the spring, from about 3 cm/yr at
show irregularities down to 30 mbsf, especially Hole 1200E to 0.2 cm/yr at Hole 1200D, based on
in Hole 1200D (Figure 3). These probably result the K profile. However, the profiles approach the
from leaching of chloride from iowaite, a Mg-Fe same asymptotic value with depth, thereby defin-
hydroxy-chloride hydrate mineral thought to form ingasingledeepcompositionatallthreelocations,
at shallow depth by interaction of seawater with reported in Table 1. Profiles for these chemical
brucite [Heling and Schwarcz, 1992; Fryer and speciesreflectmainlyverticalupwellingandchem-
Mottl, 1992]. Profiles of Na, Na/Cl, K, Rb, B, ical diffusion because these species are relatively
d18O (Figure 3), 87Sr/86Sr (not shown), and pH unreactiveoverthesampleddepth.Aloneamongthe
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Figure 2. Location of ODP Site 1200 drillholes on the summit of South Chamorro Seamount. Holes 1200A and
1200E are within a few meters of an active cold spring. Minutes shown on the axes of this map are for 13(cid:1)N and
146(cid:1)E.
Figure 3. Composition of pore water from ODP Site 1200 on the summit of South Chamorro Seamount versus
depth.Convex-upwardprofilesresultfromupwellingofdeepfluidatseepagevelocitiesrangingfrom3cm/yratHole
1200Enearthespringto0.2cm/yratHole1200D80mNNWofthespring,asestimatedfromtheKprofilesat2(cid:1)C,
porosity 0.51, formation factor 3.7, D for K 0.0334, and D 0.0177 m2/yr.
w sed
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Figure 4. Composition of pore water from ODP Site 1200, South Chamorro summit, compared with microbial
(archaeal and bacterial) biomass, versus depth. Sulfate reduction by Archaea at 1 to 3 and 13 mbsf produces
alkalinity, dissolved reduced sulfur (virtually all bisulfide at this high pH), and ammonia.
chemical species measured, K, Rb, and d18O plot are shown by Mg, Ca, Sr, Mn, and Li, all of which
linearly against one another for all three holes decrease sharply with depth and are thus highly
(Figure 5) and thus define a mixing line between depleted in the deep fluid (Figure 6). Plots of these
theend-memberascendingfluidandbottomseawa- elements against the conservative element K (not
ter. As alkali elements of intermediate atomic shown)indicatethatallfivearetakenupbyserpen-
weight,KandRbcanbeexpectedtoreactsimilarly tinite over the sampled depth, and all are more
to one another, but not to oxygen isotopes. The rapidly reactive than the elements that are mobi-
mutual linear relationships thus imply that these lized, in the order (Si>)Ca = Sr > Mg > Li. Mn is
three species are the least reactive. These are fol- furthercomplicatedbyasharpmaximumwithinone
lowedbyBandNa,whicharelinearwithKonlyfor meter of the seafloor that results from mobilization
Holes1200Eand1200F(Figure5).AtHole1200D underreducing conditions.
farthest from the spring, where slower upwelling
would allow the effects of reaction to become [ ] Depthprofilesofotherchemicalspecies,includ-
6
obvious, some points plot above the mixing line, ingalkalinity,ammonia,sulfate,bisulfide(Figure4),
implyingthatBandNaareleachedfromthesolids Fe, Si, phosphate, and F (Figure 7), again imply a
overpartofthesampleddepth.Exceptforchloride, singledeepfluidcomposition,butthesespeciesare
whichisdepletedby6%relativetobottomseawater, greatlyaffectedbyadifferentsetofreactionswithin
all of these species are highly enriched in the deep the upper 20 mbsf. The first five species are those
ascending fluid. Similar but mirror-image profiles typicallyaffectedbymicrobialoxidationoforganic
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matter in marine sediment and the accompanying maximaataboutthesamedepthsasthetwosulfate
reduction of seawater sulfate and precipitation of minima, as does alkalinity in Hole 1200F, whereas
Fe-sulfide minerals. Sulfate approaches zero Fe shows minima at these depths reflecting precip-
between2and20mbsfinHole1200Eatthespring, itation of Fe-sulfide. Sulfur content of the serpen-
butshowstwominimainHoles1200Fand1200D, tinite mud is <0.01 wt.% except for a near-surface
at1–3and13mbsf, indicatingthatsulfateisbeing enrichment by Fe-sulfide precipitation (Figure 8).
reduced at these two depths, supplied from the
overlyingseawaterattheshallowerdepthandfrom 3. Discussion
the deep upwelling water at the greater depth.
Bisulfide (virtually the only form of dissolved 3.1. Origin of the Ascending Fluid
reduced sulfur at pH 12.5) and ammonia show
[ ] The deep upwelling fluid within the two active
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mud volcanoes is generally similar, except for
much lower chloride and Na and higher sulfate at
Conical Seamount (Table 1). Relative to seawater,
both fluids have higher to much higher sulfate,
alkalinity, pH, Na/Cl, K, Rb, B, light hydrocar-
bons, ammonia, d18O, and dD; and lower to much
lower chloride, Mg, Ca, Sr, Li, Si, phosphate, and
Sr isotopic ratio. Ba, Mn, Fe, and bisulfide are low
in both deep fluids. By contrast, pore water from
TorishimaForearcSeamount,producedbyreaction
of cool seawater with harzburgite, shows the
opposite direction of change for chloride, sulfate,
alkalinity,K, Rb, Ca, Sr,B, d18O, anddD. Na/Cl is
unchanged; pH, methane, Si, and ammonia are not
nearly as high; and Ba, Li, and Mn are much
higher. The deeply sourced water upwelling within
the two active mud volcanoes clearly cannot have
originated by simple reaction of seawater with
peridotite. Considering that depleted harzburgite
contains essentially no alkali elements, the source
ofthefreshwaterisalmostcertainlydehydrationof
sediment and altered basalt at the top of the
Figure 5. (opposite) K versus Rb, d18O, B, and Na in
pore waters from ODP Site 1200 on the summit of
SouthChamorroSeamount.CorrelationsarelinearforK
versus Rb and d18O for all the holes, implying that K,
Rb, and d18O are relatively non-reactive over the
sampled depth interval. By contrast, B and Na deviate
fromlinearity,butonlyatHole1200Dfarthestfromthe
spring where the upwelling speed is slowest. This
deviationpresumablyresultsfromleachingofBandNa
from the serpentinite mud in Hole 1200D, over depth
intervals from 3 to 12 mbsf for B and from 3 mbsf to
greaterthanthemaximumdepthsampledof29mbsffor
Na. These reactions presumably take place in Holes
1200E and 1200F as well, but faster upwelling at these
holes overwhelms the effects of these reactions on the
composition of the pore waters.
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Figure 6. Composition of pore water from ODP Site 1200 on the summit of South Chamorro Seamount versus
depth. These elements are all present at low concentrations in the deep upwelling water relative to seawater, except
for Mn, which displays a large maximum just below the seafloor caused by the onset of reducing conditions. The
slight recovery of Ca and Sr below 20 mbsf in Hole 1200E results from the decrease in alkalinity over this depth
range and implies that the solutions are saturated with CaCO .
3
subducting Pacific plate. The high carbonate alka- eters characteristic of the Mariana arc, steady state
linity, and its absence in pore waters from mud thermal models suggest temperatures of 150–
volcanoes closer to the trench, imply that beneath 250(cid:1)C at 0.8 GPa and 27 km depth [Peacock,
South Chamorro and Conical Seamounts dehydra- 1996; Kincaid and Sacks, 1997]. This range is
tion is accompanied by decarbonation or carbonate consistent with mineral assemblages of blueschist
dissolution [Fryer et al., 1999]. facies metabasites recovered from Conical Sea-
mount (150–250(cid:1)C and 0.5–0.65 GPa) [Maekawa
[ ] These are probably the most pristine slab- et al., 1993] and from South Chamorro Seamount
8
derived fluids recovered to date from a subduction (<350(cid:1)C for P < 0.8 GPa) [Fryer et al., 2000], as
zone, given the simplicity of their harzburgitic wellaswithaserpentineassemblagedominatedby
matrix. Because the depth of origin and the phys- lizardite at South Chamorro Seamount summit
ical conditions there are poorly known, specific [Shipboard Scientific Party, 2002] and by chryso-
sources of fluid components and the processes that tile at Conical Seamount summit [Heling and
generate them are obscure. For subduction param- Schwarcz, 1992; Fryer and Mottl, 1992]. It is also
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Figure 7. Composition of pore water from ODP Site 1200 on the summit of South Chamorro Seamount versus
depth. These species show multiple maxima and minima, indicating that they are highly reactive over the sampled
depth interval.
consistent with high K and Rb in solution, asthese dehydration reactions are accompanied by break-
elements typically are leached fromoceanic crustal down of heavier hydrocarbons to methane in sub-
rocks only above (cid:1)150(cid:1)C; at lower temperatures ducted sediment at 60–150(cid:1)C [Peacock, 1990]
they are taken up into alteration minerals [Seyfried and decarbonation of sedimentary and vein carbo-
andBischoff,1979;Magenheimetal.,1995].Asfor nates at >400(cid:1) to >540(cid:1)C at 0.8 GPa [Kerrick and
the source of the solutions, compactive dewatering Connolly, 2001a, 2001b].
cantakeplaceatanytemperature, butthechemical
dehydrationthatproducesfreshenedfluidsrequires [ ] Sedimentandalteredoceaniccrustatthetopof
9
elevated temperatures, over the range 1) 30–80(cid:1)C the subducting slab beneath Conical and South
for expulsion of interlayer water and conversion of Chamorro Seamounts are therefore hot enough to
opal-A to opal-CT, 2) 50–150(cid:1)C for expulsion of supply H O and light hydrocarbons to the ascend-
2
interlayer water and conversion of smectite to ing fluids but not CO , which must be supplied by
2
illite, 3) >250(cid:1)C for dehydration of sedimentary dissolution of carbonate minerals at the source
minerals,mainlyclaysandzeolites,and4)>450(cid:1)C rather than by decarbonation. An alternative to
for dehydration of hydrous alteration phases in thermogenicproductionoflighthydrocarbonsfrom
the basaltic oceanic crust [Peacock, 1990]. These organic matter is reduction of dissolved carbonate
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Figure 8. Partial composition of serpentinite mud cored at ODP Site 1200 on the summit of South Chamorro
Seamount and at Site 780 on the summit of Conical Seamount versus depth. Water content is typical of that for
serpentinite; smaller concentrations near the seafloor result from dilution of serpentinite by CaCO (typically
3
aragonite needles) that precipitates at a reaction front between the upwelling water, rich in carbonate alkalinity and
poorinCa,anddownward-diffusingbottomseawaterthatisrelativelyrichinCaandpoorinalkalinity.Hole1200E
containspyritizedforaminiferaat12and16mbsf,whereCaCO isenriched,indicatingthatserpentinitemudatthese
3
depths was exposed at the seafloor for extended periods prior to burial by more serpentinite mud; some of this
enrichment also results from CaCO precipitation at a similar reaction front when mud at these depths was at the
3
seafloor. The sulfur enrichment near the seafloor at Site 1200 results from Fe-sulfide minerals that precipitate as a
result of microbial (archaeal) sulfate reduction (Figure 4). These sulfides typically blacken the core. The zone of
sulfurenrichmentextendsto25mbsfinHole1200Eatthespring,toatleast15mbsfinHole1200F20maway,and
to only 5 mbsf in Hole 1200D 80 m from the spring. It is essentially absent at Site 780 on the summit of Conical
Seamount, which also lacks a macrofauna even though it has a much higher content of organic carbon in the
serpentinitemudthanispresentatSite1200.Theenrichmentoforganiccarbonat0–3mbsfinHole1200Dfarthest
from the spring correlates with a high concentration of foraminifera [Shipboard Scientific Party, 2002]. Except for
this enrichment, organic carbon is extremely low in the serpentinite mud at Site 1200.
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