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173(1998).
39.We acknowledge and thank DeBeers for providing
diamonds;M.Chaussidon for sulfur isotope stan-dards;A.D.Brandon for providing peridotite samples from Kilbourne Hole;A.Paytan for making her data available to us;NSF,NASA,and American Chemical Society for support to J.F.and B.A.W.;and the NASA Astrobiology program for supporting sulfur isotope studies at UCLA and University of Maryland,College Park.The UCLA ion microprobe facility is partially
supported by a grant from the NSFInstrumentation and Facilities program.J.F.acknowledges editing and insights of L.J.Tuit.
Supporting Online Material
www.sciencemag/cgi/content/full/298/5602/2369/DC1
Materials and Methods Tables S1to S3
20September 2002;accepted 11November 2002
Calibration of Sulfate Levels in
the Archean Ocean
Kirsten S.Habicht,1Michael Gade,1Bo Thamdrup,1Peter Berg,2
夫人英语Donald E.Canfield 1*
The size of the marine sulfate rervoir has grown through Earth’s history,reflecting the accumulation of oxygen into the atmosphere.Sulfur isotope fractionation experiments on marine and freshwater sulfate reducers,to-gether with the isotope record,imply that oceanic Archean sulfate con-centrations were Ͻ200M,which is less than one-hundredth of prent marine sulfate levels and one-fifth of what was previously thought.Such low sulfate concentrations were maintained by volcanic outgassing of SO 2gas,and verely suppresd sulfate reduction rates allowed for a carbon cycle dominated by methanogenesis.It is thought that the Archean Earth had low atmospheric oxygen concentrations (1),low oceanic sulfate concentrations (2),and ele-vated atmospheric concentrations of meth-ane,contributing to possible greenhou warming of Earth’s surface (3).The biogeo-chemistries of the elements are linked,in that low atmospheric oxygen levels suppress the oxidative weathering of sulfides and the delivery of sulfate to the oceans,contributing to the low sulfate concentrations (2).Low sulfate levels could have inhibited sulfate reduction,enhancing methane production (2,4).
This reconstruction depends on our ability to extract reliable sulfate concentration infor-mation from the isotope record of sulfide and sulfate through time.The isotope record reveals small fractionations of generally Ͻ10per mil (‰)between sulfates and dimentary sulfides before 2.5to 2.7billion years ago (Ga)(2).The few available pure culture studies suggest that fractionations become suppresd at a sulfate concentration around 1mM (5,6).Current models link reduced fractionations at low sul-fate concentration to a limitation of sulfate ex-change across the cell membrane (6).In this ca,most of the sulfate entering the cell be-comes reduced,and even with substantial inter-nal enzymatic fractionations,minimal net frac-tionation is expresd.Sulfate limitation also reduces sulfate reduction rates,with half-satu-ration constants (k m )values for marine strains of 70and 200M (7,8)and for freshwater strains,5to 30M (7).If similar sulfate con-centrations limit both fractionation and sulfate reduction rate,then sulfate reducers should maintain substantial fractionation at sulfate con-centrations considerably less than 1mM.
In continuous culture,we explored the fractionations at millimolar and submillimo-lar sulfate concentrations by Archaeoglobus fulgidus grown on lactate at its optimal growth for temperature of 80°C.A.fulgidus is an archaeon and was chon to reprent possible early sulfate reducers from hydro-thermal ttings.We also examined natural
populations of sulfate reducers from a coastal marine diment (natural sulfate concentra-tion,20mM)and a freshwater lake diment (natural sulfate concentration,300M).Freshwater sulfate reducers are especially adapted to low sulfate concentrations (9)and could reflect the behavior of possible early low sulfate–adapted organisms,whereas ma-rine sulfate reducers are adapted to high a-water salinities.In the natural population ex-periments,diment was incubated at 17°C in a rapidly recirculating flow-through plug re-actor (10)with lactate (1mM)as the organic substrate (11).
All three different microbial populations produced high fractionations (11)of up to 32‰with 200M or greater sulfate (Fig.1).The average fractionation for sulfate between 200and 1000M was 22.6Ϯ10.3‰,which is similar to the average for pure bacterial cultures (6)(18Ϯ10‰)and natural popula-tions (6)(28Ϯ6‰)of sulfate reducers uti-lizing 20mM or greater sulfate.By contrast,fractionations were consistently less than 6‰(an average of 0.7Ϯ 5.2‰)with sulfate concentrations less than 50M.Thus,sulfate substantially limited fractionation up to a concentration somewhere between 50M and around 200M.This is also the concen-tration range where sulfate limits rates of sulfate reduction (8,9).
The isotopic composition of dimentary sulfides will,in addition to the bacterial frac-tionation,depend on the extent to which sul-fides form in a zone of sulfate depletion (6,
1
Danish Center for Earth System Science and Institute of Biology,University of Southern Denmark,Campus-vej 55,DK-5230,Oden M,Denmark.2Department of Environmental Sciences,Clark Hall,University of Virginia,VA 22903,USA.
*To whom correspondence should be
addresd.
Fig.1.Isotope fraction-ation as a function of sulfate concentration for freshwater (diamonds)and marine (squares)natural populations of sulfate reducers and for the hyperthermophile A.fulgidus (triangles).For the freshwater and ma-rine populations,hori-zontal bars plot the range of sulfate con-centrations within the reactor,with the higher concentration entering the reactor,and the low concentration exiting the reactor.The sym-bols are positioned on
the bars at the average concentration in the reactor.
20DECEMBER 2002VOL 298SCIENCE www.sciencemag人教版高中英语教材
2372 o n A p r i l 18, 2015
w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o m
12),and 34S-enriched pore water sulfides may be redistributed by diffusion (13).The influence of the diment environment on the final isotopic composition of dimentary sulfides depends on sulfate concentration.Therefore,we explored with a diagenetic model how sulfate concentration influences the isotopic composition of dimentary sul-fides (11).Sulfides prerve,on average,lower fractionati
ons than the bacterial frac-tionations (Fig.2A).Yet,even with 200M SO 42–,a substantial population of sulfides formed,with a ␦34S values approaching the bacterial fractionation values (Fig.2A).Fine-scale single-grain analys should reveal the 34S-depleted sulfides and the highly 34
S-enriched sulfides that were also pro-duced.However,bulk samples and fine-scale lar ablation analysis of sulfides formed be-fore 2.7Ga do not reveal large fractionations (␦34S awater sulfate –␦34S pyrite )of 20to 30‰(14,15).An exception is the biologically produced sulfides from the 3.45-billion-year-old North Pole barites of Western Australia,formed in local sulfate-rich evaporitic condi-tions (16).
From our results,low fractionations (␦34S awater sulfate –␦34S pyrite )of less than 10‰in the sulfur isotope record before 2.7Ga were most likely produced by sulfate reducers living in an ocean with less than 200M sulfate (17).This maximum sulfate con-centration is one-fifth that previously thought
(2,6).The low concentrations are consis-tent with low or negligible oxygen concen-trations suppressing the oxidative weathering of sulfide minerals from land (2).A locally important source of sulfate could come from the oxidation of hydrothermal sulfide in ter-restrial hot springs by anoxygeni
c photosyn-thetic bacteria.However,the most important source of sulfate to the oceans was probably volcanic outgassing of SO 2,followed by ei-ther direct disproportionation to sulfate and sulfide or conversion to sulfate through gas-pha reactions in the atmosphere.A substan-tial role for gas-pha sulfur conversions is indicated by the record of minor sulfur iso-topes,33S and 36S,which prerve consider-able mass-independent fractionations in the Archean (18).The are only known to orig-inate from gas-pha reactions of sulfur com-pounds in an oxygen-free atmosphere (19).Currently,SO 2outgass from volcanoes at a rate of about 1ϫ1011to 3ϫ1011mol year Ϫ1(20,21).Of this,probably about one-half is recycled dimentary sulfur (22),and one-half is from the mantle,producing a man-tle flux of sulfate of 0.4ϫ1011to 1.1ϫ1011mol year Ϫ1,considering that 75%of the SO 2is converted to SO 42–after SO 2dispropor-tionation (Eq.1).
4SO 2ϩ4H 2O 3H 2S ϩ3H 2SO 4
(1)
This mantle sulfate flux is between 1/20th and 1/50th of the prent-day natural (non-pollutive)river sulfate flux to the oceans of 2ϫ1012mol year Ϫ1(23)[compare to a similar calculation in (24)].Thus,with Ar-chean volcanic SO 2degassing rates compa-rable to tho of today,lower sulfate i
nput fluxes would explain lower ocean sulfate concentrations.Rates of volcanic SO 2degas-sing could have been higher than they are today.However,more reducing mantle con-ditions could have substantially reduced the mantle flux of SO 2,thereby reducing the sulfate flux to the oceans,even with a higher degassing rate (1).Therefore,extremely low concentrations of sulfate in the Archean were probably maintained by greatly reduced flux-es of sulfate to the oceans.
Low concentrations of awater sulfate would have been unevenly mixed within the global ocean,somewhat analogous to the un-even distribution of oxygen in the modern ocean.The highest concentrations,though still less than 200M,would have been in surface waters and possibly in proximity to volcanic terraines.Much lower,or even negligible,con-centrations would be expected deeper in the ocean and possibly far away from important source regions,where the supply of sulfate from mixing process would be diminished by sulfate removal through sulfate reduction.Evidence from lake diments suggests that sulfate reduction rates at 200M are substantially suppresd compared to rates at 1mM sulfate,with most of the anaerobic mineralization channeled through methano-genesis (25).This databa,however,is lim-ited,and we therefore cho to explore with a diagenetic model the relations between sul-fate concentration and sulfate reduction rate.We modeled the importance of sulfate reduc-tion in a typical coastal diment supporting only sulfate reduction and methanogenesis expod to various concentrations of sulfate in the overlying water.Using the same basic model parameters as in Fig.2A (11),200M sulfate (Fig.2B)suppresd sulfate reduction by 75%compared to 28mM sulfate (26);and 50M sulfate reduced sulfate reduction rates by over 90%.With a reduced carbon flux more typical of outer slope or continental ri diments (1000to 3000m depth),200M sulfate reduces sulfate reduction by 30%compared to 28mM sulfate (Fig.2B);and 50M sulfate decreas sulfate reduction by about 75%.Our results,ther
efore,demon-strate that rates of Archean sulfate reduction were reduced compared to tho of today (Fig.2B).This is especially true becau 200M is a maximum Archean sulfate concen-tration,and deepwater diments were likely deposited with much lower sulfate concentra-tions than tho from surface waters,reduc-ing rates of sulfate reduction even further.From our model results (Fig.2B),30to 70%of the total carbon mineralization goes through methanogenesis at 200M
sulfate
Fig.2.(A )Model results showing how the average isotopic com-position of pyrite in diments is influenced by sulfate concentra-tion.The upper thin line,follow-ing our experimental results,shows the biological fraction-ations impod on the model,whereas the hatched field shows the isotopic composition of pyrites under average coastal diment conditions (11),with fluxes of reactive organic carbon and diment particulates of 200mol cm Ϫ2year Ϫ1and 0.1g cm year Ϫ1,respectively (lower line),and 20mol cm Ϫ2year Ϫ1and 0.01g cm Ϫ2year Ϫ1(upper line).This range of carbon fluxes rea-sonably brackets tho found in modern diments ranging from the coastal ocean to the outer slope (29).The histogram in the int shows the frequency with which pyrites at different isoto-pic compositions are formed with a sulfate concentration of 200M and the higher organic
carbon flux (200mol cm Ϫ2year Ϫ1).(B )The relative importance of sulfate reduction and methanogenesis as a function of sulfate concentration,assuming that only the two process are involved in organic carbon (C org )mineralization (11).The cloly spaced hatches reprent the relative importance of sulfate reduction for the same high (lower line)and low (upper line)carbon fluxes as ud in (A).The looly spaced hatches reprent the relative importance of methano-genesis for high (upper line)and low (lower line)carbon fluxes.At high carbon flux,sulfate reduction is relatively less important,and methanogenesis relatively more important,as compared to the situation at lower carbon flux.
www.sciencemag SCIENCE VOL 29820DECEMBER 20022373
(and even more at lower sulfate concentra-tions).Although some of the methane would have been reoxidized in the diment by an-aerobic methane oxidation coupled to sulfate reduction(27),considerable methane would have escaped(28)and could have substan-tially contributed to the greenhou warming of the early Earth(3,4).
Sediment-supported rates of sulfate reduc-tion are highly nsitive to sulfate concentra-tions from100to1000M(Fig.2B),and the isotope record(2,6)indicates that sulfate concentrations increa
d beyond200M starting around2.4Ga.The concomitant in-crea in sulfate reduction rate,both in di-ments and in the water column as sulfate became more available,would have reduced methanogenesis substantially,as well as the flux of methane to the atmosphere.This,in concert with a possible ri in atmospheric O
2 providing an incread methane sink,may have led to global cooling and the first known glaciation at around2.4Ga(4).
References andNotes
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sulfur isotope values are available,may have experi-enced sulfate concentrations different from tho of the global ocean.Lower sulfate concentrations may have been possible if the basin experienced restricted exchange with the ocean,and higher concentrations could have been possible if local sulfate sources were available.The lack of high fractionations,except where high-sulfate environments can be documented马的英文
(16),implies that throughout the wide range of dep-
ositional conditions sampled[e data in(2)],sulfate concentrations before2.7Ga were low.We must conclude that low sulfate concentrations were a per-sistent feature of marine depositional environments, and we furthermore emphasize that variability in sulfate concentrations within the global ocean was likely.18.J.Farquhar,H.M.Bao,M.Thiemens,Science289,756
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30.We thank C.Bjerrum for discussions,two reviewers for
helpful comments,and L.Salling and P.Søholt for expert
assistance in the lab.The project was funded by the Danish
National Rearch foundation(Grundforskningsfond)and
the Danish Rearch Council(SNF).
Supporting Online Material
www.sciencemag/cgi/content/full/298/5602/2372/DC1
Materials and Methods
Fig.S1
ggw
References
9September2002;accepted13November2002 Interannual Variability in the
bloodyNorth Atlantic Ocean Carbon
Sink
Nicolas Gruber,1*Charles D.Keeling,2Nicholas R.Bates3
The North Atlantic is believed to reprent the largest ocean sink for atmo-
spheric carbon dioxide in the Northern Hemisphere,yet little is known about
its temporal variability.We report an18-year time ries of upper-ocean
inorganic carbon obrvations from the northwestern subtropical North At-
lantic near Bermuda that indicates substantial variability in this sink.We deduce
that the carbon variability at this site is largely driven by variations in winter
mixed-layer depths and by a surface temperature anomalies.Becau the
variations tend to occur in a basinwide coordinated pattern associated with the
North Atlantic Oscillation,it is plausible that the entire North Atlantic Ocean
may vary in concert,resulting in a variability of the strength of the North
Atlantic carbon sink of aboutϮ0.3petagrams of carbon per year(1petagramϭ
1015grams)or nearlyϮ50%.This extrapolation is supported by basin-wide
estimates from atmospheric carbon dioxide inversions.
The ocean’s contribution to the obrved
interannual variability of atmospheric car-
bon dioxide(CO
2
)
is poorly established.
Estimates bad on atmospheric measure-
ments of CO
2
,oxygen,and stable carbon
isotopes indicate that the variability con-
tributed by the oceanic carbon cycle is
more thanϮ1Pg C yearϪ1(1–4).In con-
trast,estimates bad on direct obrva-
tions of the partial pressure of CO
2
(p CO
2
)
in surface waters(5,6)and on modeling
studies(7,8)indicate a contribution of less
thanϮ0.5Pg C yearϪ1,mainly associated
with tropical Pacific ocean variability
caud by El Nin˜o and La Nin˜a(9).How-
少儿英语培训好不好ever,many uncertainties are associated
with the modeling studies,and the equato-
rial Pacific is the only region where inter-
annual variability in oceanic p CO
2
has been
directly obrved and documented.Given
evidence for substantial extratropical vari-
ability in a surface temperature(SST)
(10)and the ocean’s state(11),other oce-
anic regions may contribute substantially to
the atmospheric CO
2
variability as well.
The North Atlantic Ocean is one of the few
regions where enough data are available to
investigate interannual to decadal variabil-
ity in the extratropical ocean carbon cycle.
Obrvationally bad estimates(12),as
well as forward and inver modeling re-
sults(13),indicate that this region consti-
tutes the largest ocean sink for atmospheric
CO
2
in the Northern Hemisphere,on aver-
age taking up about0.7Ϯ0.1Pg C yearϪ1.八年级英语试题
Obrvations have shown that most of the
interannual to decadal climatic variability in
the North Atlantic basin occurs in broadly
coherent patterns linked to a natural mode of
atmospheric pressure variation known as the
North Atlantic Oscillation(NAO)(14).The 1Institute of Geophysics and Planetary Physics and
Department of Atmospheric Sciences,University of
California,Los Angeles,CA90095,USA.2Scripps In-
stitution of Oceanography,University of California,
San Diego,La Jolla,CA92093,USA.3Bermuda Biolog-
ical Station for Rearch,Inc.,Ferry Reach GE01,
Bermuda.
*To whom correspondence should be addresd.E-
mail:ngruber@igpp.ucla.edu
20DECEMBER2002VOL298SCIENCE www.sciencemag 2374
DOI: 10.1126/science.1078265
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298 Science et al.Kirsten S. Habicht Calibration of Sulfate Levels in the Archean Ocean
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