Geochemical diversity of hydrothermal systems: Thermodynamic constraints

on biology

Wolfgang BachUniversity of Bremen, Germany

InterRidge Theoretical Institute: Biogeochemical Interaction at Deep-Sea VentsWoods Hole Sept 10-14, 2007

Lithosphere-Biosphere Interactions in Hydrothermal Vents

Driven by Driven by geochemicalgeochemical

energy energy ––powered by

geofuels

Highly diverse: chemicallyand biologically

Dynamically linkedDynamically linkedwith plate tectonicswith plate tectonics

Plate Tectonic - Igneous Genesis

1. Mid-ocean Ridges2. Intracontinental

Rifts3. Island Arcs4. Active Continental

5. Back5. Back--arc Basinsarc Basins6. Ocean Island 6. Ocean Island

BasaltsBasalts7. Miscellaneous Intra7. Miscellaneous Intra--

Continental ActivityContinental ActivityMargins

Winter (2003)

3,5) Subduction zonesRecycling ofvolatiles (and solutes) in Re-gassing the mantle(C,S,H) Wet melting High f O2

1) Mid-ocean ridgesMelting of depleted and degassed upper mantle

Magmas have low gas contentsDry melting Low f O2

6) Ocean island volcanoesMelting of deeper, less degassed mantle

Magmas have moderate gas contents(C,H) Low f O2

Winter (2003)

Plate Tectonic - Igneous Genesis

Geofuel diversity in submarine geotectonic settings

Serpentinite-hostedBasalt-hosted

Arc-related

MOR, Ocean IslandsMOR, Ocean IslandsBasaltBasalt--hostedhosted-- pH 3 to 5pH 3 to 5-- Relatively oxidizing Relatively oxidizing conditionsconditions-- Variable HVariable H22--COCO22 volatile volatile

flux flux -- Global relevanceGlobal relevance

MOR, SubductionMOR, SubductionFelsicFelsic rockrock--hostedhosted-- pH <1pH <1--33-- Very oxidizing conditions Very oxidizing conditions -- Variable HVariable H22OO--COCO22--SOSO22--

HClHCl volatile flux volatile flux -- Variable water (variable Variable water (variable

boiling temperatures)boiling temperatures)

Diversity of systems: physical/chemical differences

MOR, SubductionMOR, SubductionPeridotitePeridotite--hostedhosted-- pH 3 to 12pH 3 to 12-- Reducing conditionsReducing conditions-- Negligible magma volatile Negligible magma volatile

flux?flux?-- AbioticAbiotic organics? organics? -- Early Earth analogsEarly Earth analogs

Serpentinization and hydrogen

a. F420 autofluorescence of a biofilmb. Biofilm cells encased in EPSc. F420 autofluorescence of methane-metabolizing Archaead. FISH photomicrograph confirming that the F420-fluorescent cells are Archaea

Brazelton et al., 2007

Boetius, 2005N&V

??

0 2 4 6 8 10 12 14 16

Temperature (ºC) / Concentration (mmol/kg)

T/100

Cl/100

Si/2

H2S*2

CO2

CH4*5

H2

Mafic/Ultramafic Vent Fluid Chemistry Comparison

RainbowLogatchevMARKTAG

Peridotite-Hosted Hydrothermal Systems

Data: Charlou et al. (2002)

Peridotite-Hosted Hydrothermal Systems

2FeO + H2O Fe2O3 + H2

4H2 + CO2 CH4 + 2H2O

??

Recharge zone

Upflow zone

Reaction zone

Peridotite (± altered)Gabbro intrusion

Serpentinization: Geochemical ModelingGeochemical reaction path model (EQ3NR/6)

-7

-6

-5

-4

-3

-2

-1

0

100 200 300 400

aSiO

2,aq

(M)

Ol+

Tlc+

WSe

rp

Ol+

W

Serp

+Brc

BrcSerp

SerpTlc

OlSerp

TlcQtz

MgO-SiO2-H2O phase relations at 500 bar

Temperature (°C)

Serpentinization of the oceanic lithosphere

OlTlc

2

34

1Internally buffered

Externally controlled

1

Phase relations

Serpentinization: Geochemical Modeling

0.01

0.1

1

125 225 325 425

Temperature (°C)

Mol

es M

iner

als

OlivineMagnetiteSerpentineBruciteCpxTremoliteTalcAndradite

Water/Rock ratio4 2.5 2.0 1.25 1.0

12

34

-6

-5

-4

-3

-2

-1

0

125 175 225 275 325 375 425

Temperature

Log

Mol

es S

olut

es/M

iner

als

SiO2H2Magnetite

Serpentinization: Geochemical Modeling

12

3

4

Fluid compositions

Hydrogen yields in serpentinization are high, because phase assemblages are low silica activity

1.5Fe2SiO4 + H2OFe3O4 +

1.5SiO2(aq) + H2(aq)

Arc volcano hydrothermal system

Gary Massoth et al.

Embley et al. (2004)

Mariana and other arc systemssulfuric and carbonic acid springs

Manus Basin, Papua New Guinea

New Britain

New Ireland

North Su

Active flows of native sulfur on seafloor

Hundreds of acid-sulfate springs, fluid pH as low as 0.9

0

200

400

600

800

1000

0 10 20 30 40 50 60Mg (mmol/kg)

SO4 (

mm

ol/k

g)

NS1NS2D1D2

“Acid Springs” Manus Basin

magmatic fluid

SW

DESMOS

North Su

SO42-

Magmatic SO2 Disproportionation

4SO2 + 4H2O = H2S + 3H2SO4

3SO2 + 2H2O = S0(s) + 2H2SO4

Slide from Jeff Seewald

oooo022 )()( SaqHaqSHr GGGG Δ−Δ−Δ=Δ

H2(aq) + S0 → H2S(aq)

rrr QRTGG ln+Δ=Δ o

)()(

)()(

)(

)(

aqHaqH

aqSHaqSH

SaqH

aqSHr

22

22

02

2

mm

aaa

Qγγ

==

Geochemistry – Biology relations: Gibbs Free Energy

( ) dPVTdCTdTC298TSGGP

1

T

298P

T

298P298fP,T ∫∫∫ +−+−−Δ=Δ oooooo ln

Slide from Jan Amend, redrawn from McCollom & Shock (1997)

Energetics from mixing hydrothermal fluids with seawater

Carbon fixation rate = f(ΔG, T, …)Growth,Maintenance

Unstable phase/species

Me(II) Me(III)KineticKineticBarrierBarrier

CO2,aq

Me2+ + ¼ O2,aq + H+ = Me3+ + ½ H2O (Step of overall reaction that can be mined for energy)

Stable phase/species

Enzymaticcatalysis

ASSIMILATION

The basic principle

ΔG = ΔGo + RT ln QQ = (aMe3+ a½

H2O)/(aMe2+ aH+ a¼O2,aq)

DISSIMILATION

Thermodynamics Tracing the energy across the interface between geology and biology

• What is life in terms of thermodynamics?- Sustaining disequilibria … and- increasing entropy … by- using energy!

• What is the source of the energy?– Light (photosynthesis)– Chemical bonds in organics (heterotrophy)– Electron transfers (chemosynthesis)

Thermodynamics in Biology

ΔG > 0ΔS < 0

ΔG < 0ΔS > 0

ΔGtotal < 0ΔStotal > 0

Zygote Dog

Dog food Dog shit

ΔG = ΔH -TΔS

ΔG > 0ΔS < 0

ΔG < 0ΔS > 0

ΔGtotal < 0ΔStotal > 0

e-donor (food), e-acceptor (e.g., O2)

ΔG = ΔH -TΔS

C,N,H,O,S DNA, RNA,Proteins, Lipids

ASSIMILATION

Reactionproducts &Heat

DISSIMILATION

Only a fraction of the energy can be used to make biomass

Energetic costs of building biomass (from McCollom & Amend, 2005)

In theory (McCollom & Amend, 2005)1.4 kJ / g cellular biomass (anaerobes)18.4 kJ / g cellular biomass (aerobes)Anabolic advantage of living anaerobically

In reality (Heijnen & van Dijken, 1992)30-40 kJ / g cellular biomass (anaerobes using H2 as e-donor)80-170 kJ / g cellular biomass (others)

Only about 10% efficiency90% get lost in form of heat and waste productsThis is increasing entropy in the surrounding

Bioenergetics

Chemolithoautotrophic biomass production estimates

• Axial hydrothermal vents: ~1013 g C/ yr (McCollom and Shock, 1997)

• Hydrothermal plumes: ~1012 g C/ yr (McCollom 2000)

• Ridge flanks/weathering: ~1012 g C/ yr (Bach and Edwards, 2003)

• Photosynthetic: ~1017 g C/ yr (Whitman et al., 1997)

• Sulfate reduction in marine sediments: ~2 x 1012

g C/ yr (Bach and Edwards, 2003 based on sulfate flux rates from D’Hondt et al., 2002)

What reactions are exergonic?

McCollom & Shock (1997)

What reactions support how much biomass?

Max. biomass production (mg / kg vent fluid)0 20 40 60 80 100

Hydrogen ox.

Methane ox.

Fe(II) ox.

Sulfide ox.

Sulfur ox.

Acid-sulfatespringsBasalt-hosted

Peridotite-hosted

??

Hydrogen can be generated abiotically by:(1) Reactions between C-O-H(-S) species in melts and vapors(2) Decomposition of methane at T>600°C(3) Radiolysis of water by radioaktive decay of U, Th, and 40-K(4) Catalysis of silicates under stress in the presence of water(5) Hydrolysis by ferrous minerals in mafic and ultramafic rocks

Away from magmatic centers, only pathways 3-5 are viable. (3) Radiolysis can support on the order of 1,000-10,000 cells per

cm3 in marine sediments (Spivack et al., in press)(4) Happens, but is likely not a continuous source of hydrogen(5) Investigated here

Sources of dihydrogen

Hydrogen demand and supply

Growth

MaintenanceSurvival

Hoehler et al. (2004) Example for methanogens

• Source: Water-rockInteraction @ T=2-120°C

• Source: Water-rock interaction @>350-400°C, followed by mixing with cold seawater

1

10

100

1000

0 20 40 60 80 100 120

Temperature

Wat

er-to

-rock

ratio

0

20

40

60

80

100

120

0 20 40 60 80 100 120Temperature (°C)

Hydrogen demand and supply

Growth

MaintenanceSurvival

Hoehler et al. (2004) Example for methanogens

Seawater-basaltinteraction

Seawater-peridotiteinteraction

Hydrogen demand and supply

Sulfur isotope systematicsbasalt-serpentinite comparison

0%

10%

20%

30%

40%

50%

60%

-60 to -50

-50 to -40

-40 to -30

-30 to -20

-20 to -10

-10 to -5-5 to 0

0 to 55 to 10

10 to 20

20 to 30

δ34S

Freq

uenc

y

Freshbasalt

Sea water

Serpentinite(n=30)

Basalts(n=133)

Isotopic record of sulfate reduction in basalt and serpentinite

Data: Jeff Alt

Growth

MaintenanceSurvival

All redox-rxnsallowed

All redox rxnssuppressed

Hydrogen demand and supply

Logatchev fluidmixed with sea water

H2-O2 is allowed to equilibrate

Seawater – vent fluid mixing

Growth

MaintenanceSurvival

1 mM H2(aq)

12 mM H2(aq)[Logatchev HF]

5 mM H2(aq)

Hydrogen demand and supply

Seawater – vent fluid mixing

Growth

MaintenanceSurvival

0.3 mM O2(aq)

0.1 mM O2(aq)

0.2 mM O2(aq)

Hydrogen demand and supply

Seawater – vent fluid mixing

Summary

• Vent fluid compositions differ fundamentally in different plate tectonic settings.

• Phase equilibria in the deepest and hottest part of hydrothermalsystems set the fluid chemistry that is further modified by mixing with cold seawater, creating disequilibria

• Another way of creating habitable conditions for microbial life is in water-rock reaction systems running at temperatures too low for the system to reach equilibrium

• Life is ineffcient in harvesting the geochemical energy, but relations between the amount of energy and the production of biomass can be expected

• The primary fuel for microbial biomass production is: H2S in basalt-hosted, H2 and CH4 in serpentinization, and S and Fe in felsic rock hosted systems

• In mixing environments, the transition between anoxic and oxic conditions depends on the compositions of the mixing fluids and the kinetics of the H2-consuming redox reactions

Acknowledgments

For discussions and materials:• Tom McCollom, Dionysis Foustoukos, Jan Amend, Art Spivack

(Thermodynamics and bioenergetics)• Katrina Edwards (Deep biosphere)• Jeff Seewald, Paul Craddock, Eoghan Reeves (Manus Basin fluid

chemistry)• Frieder Klein, Michael Hentscher (Geochemical modeling)• Ron Frost (Petrology)• Nicole Dubilier (Geofueled vent biology)

For funding:• NSF and DFG

• InterRidge folks for invitation

-7

-6

-5

-4

-3

-2

-1

0

100 200 300 400

Temperature (°C)

aSiO

2,aq

(M)

Ol+

Tlc+

WSe

rp

Ol+

W

Serp

+Brc

BrcSerp

SerpTlc

OlSerp

TlcQtz

MgO-SiO2-H2O phase relations at 500 bar

Silica-metasomatism in the oceanic lithosphere

Ol+BrcSerp

OlTlc

Serp+EnTlc

Experiments (Allen&Seyfried. 2003)

Nature (Logatchev, Rainbow)