Apr 15, 1995 - peak of 50 mI (higher values of a.f. ,â¢vere impossible to obtain duc ..... data [e.g. Zhou et mi., 1991; Fang, 1991; Lister et mi., 1991; Gasse et mi.
GEOPHYSICALRESEARCHLETTERS,VOL. 22, NO. 8, PAGES953-956,APRIL 15, 1995
Mineral magnetic record of environment in Late Pleistoceneand Holocenesediments,Lake Manas, Xinjiang, China. A. Jelinowska,P. Tucholka,F. Gasseand J.C. Fontes GdR970-CNRSPa16ohydrologie, Pa16oclimatologie continentales, Universit•ParisSud,Orsay,France
Abstract. A studyof magnetic parameters wascarriedouton a Chronology,!ithologyand climatic changes Late Pleistocene andHolocenesedimentary sequence (5 m) from Thechronology is based on 9 AMS 14Cdates measured on LakeManas(northern Xinjiang,China),thathasalready beenanacalciteand/oraragonite or totalorganic matterlRhodes lysedforbulksediment mineralogy, stables isotopes fromcarbon- authigenic agesof sediments arein sequence and ates and biogenicmaterial.Magneticmineralsconfirmmajor et al.,in press].Holocene and150cm.Ages changes in palaeoenvironmental andpalaeohydrological conditions rangefrom10to 0.33.kyrB.P.between-400 betweenlevelswith similarlithofacies. that were deducedfrom other methods.They also provide canthusbe interpolated areobserved (Fig.2). Thepre-Holocene complementary information on changes in thelakesystem during Fourmajorlithofacies (498-400 cm)weredeposited ingenerally' dry,oxidising theearlyandmiddleHolocene. Themostimportant mineralmag- sediments They include,however,a Late Pleistocene lacustrine neticchange results fromtheauthigenic formation of ferrimagnetic conditions. phase (498-460 cm) dated at-32 kyr B.P. This wet phase is folgreigite, Fe3S 4 duringthelacustrine episodes datedfrom-8.7 kyr aridepisode (460-418cm)thatis attributed B.P. to-5.5 kyr B.P. (380-335cm) andfrom -4.5 kyr B.P. to lowedbyanextremely et al.,in press].A returnto -3.5 kyr B.P.(320-285cm)respectively. Ourdataalsoindicatean to theLastGlacialMaximum[Rhodes conditions isregistered by vaguelylaminated to mottled abruptbreakin sedimentary conditionswhichcoincideswith a lacustrine brown and grey clay (418-403 cm). Assuming a sedimentation rate short-term retmx•towardsdryclimaticconditions around3.5 kyr similarto thatof theoverlying sediments, thisLateGlacialhumid B.P.,whichwasonlysuggested fromotherproxydata. episode wouldbeginaround12 kTr B.P. Severalmillimetricfine Introduction sandbedsobserved around402 and399 cm,preceed theestabIn 1991, Lake Manas (45ø45'N, 86ø00'E, 251 m.a.s.l.) was in-
lishment of a permanent lakeandthedevelopment of a steppe ve-
in thecatchmerit area,whichlasted-4,500years. vestigated aspartof a Sino-French palaeoclimate andpalaeoenvi- getation ronmentalprogramin northernXinjiang (CNRS - Academia Sinica,andthe International AtomicEnergyAgency).This paper presents the resultsof a mineralmagneticstudyfrom a 5 m sedimentcorefrom LakeManas.The coreprovidesa Late Pleistocene andcontinuous Holocene recordthathasbeenalreadyanalysed for mineralogy, chronology, stableisotopes,carbonates, pollen,diatoms,andtotalorganicmatter[Rhodeset al., in press]. Lake Manas is the terminallake of the Manas River (-400 km long)whichflowsfromtheTian Shahmountain rangeandliesunder desertconditions(Fig. 1). The lakedrainsa catchmerit areaof
ThisfirstHolocene lacustrine episode datedfrom10to-5.5 •x
B.P. (400-335cm)is recorded by softlacustrine mud,richin calcite and aragonite. Fluvialdeposition of coarsesandsoccurred
from335to 320cm(-5.5 to-4.5 k3xB.P.).Thisis mostlikely due to a return to arid conditions.A secondHolocenelacustrine
episode, datedfrom-4.5 to-2.5 kyr B.P.(320-240cm)is documented by sediments verysimilarto thosedeposited in the 10-5.5 kyrinterval. A dryspellis,however, suggested byboththemineralogyandpollenrecords around a depthof 280 cm(-3.5 kyr B.P.).Above240cm(-2.5 kyrB.P.),thediatom flora,mineralogy and stables isotopes indicate a stepwise establishment of shallow •11,000 km2.Thebedrock oftheupper Manas River catchment basin with hypersaline conditions in Lake Manas, attributed to a consists of granites,sedimentary formations of DevonianandCarboniferous age,andMesozoiclimestone. At thecityof Manas,the fiver entersthe Zunggaryplain whichis coveredby late Quaternary fluvial, aeolianand lacustrinedeposits.To the north, the Manasbasinis limitedby a seriesof NNE-SSW trendingnormal faultswhichaffectcontinental Neogenedepositsandthe underlying petroliferous Mesozoicformationsof Karamai.Lake Manas was a saltlake (10-15 km x 50-52 km), whichdriedup in the 1960's.Todayit is a largesaltplain,exploitedfor halite. Material
generaltrendtowardaridclimate.
Palaeomagneficsampling and analysis Standardsamplingmethodswereusedto obtaina semi-continuous coverageof each sectionof the core. About 300 oriented
and methods
Threecores,-5 m in length(LM I, LM II, and LM Ill), were taken within -lm
of each other from near the centre of the Manas
saltplain,usinga handaugerto penetrate thesaltcrust,andthena Wrightpistoncorer.Analysespresented herewereperformedon cores LM II and LM III. J. C. Fontes deceased
Copyfight 1995 by the AmericanGeophysicalUnion. t•O F,
Paper number 95GL00708 0094-8534/95/95GL-00708503.00
Figure1.Location mapof LakeManasin northwest China. 953
954
JELINOWSKA
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Figure2. Dovm-core variationin a) magnetic susccptibili• •, b) intensiB' of ARM, c) intensityof NRM aftera.f. demagnetisation at 20 mT, d) SIRM, e) S-ratio.f)highfield susceptibiliB'/Ms, g) Mrs/Ms -upperlegend:Her/He lowerlegend,h) Hc -upperlegend:Her -lowerlegend.i) SIRM/susceptibilit3. •. a), b) & c) for coresLM II andLM
III oncommon depth scale: from d)toi) core LMIII. 14Cages measured inLMII. Simplified lithofacies' 1)salt crust,2) marls,mudsolivegreyto dark,3) fluvialsands.4) claywith saltcrystalsandseveralindurated redlayers.
samples of8cm3perspex cubes were obtained from thetwocores.of magneticproperties.Thesezonesare alsoobserYedin the beparameters (Fig.2 f-h). Ms, Mrs, andZI•' Natural remanentmagnetisation (NRM) aas n•easuredwith a haviourof hysteresis massof the sample.for whichthevhave CTF 3-axis SQUID magnetometer. Anhvstereticremanentmag- valuesdependon thedr>.• In the Lake Manassequence, however,the presnctisations(ARM) were appliedby superimposition of a 50 gT beennormalised. saltcrystalswhichhaveno, or bias field on an alternatingfield that decreases smoothlyfrom a enceof relativelylargediamagnetic peakof 50 mI (highervaluesof a.f. ,•vereimpossible to obtainduc negligibleinfluenceon magneticparameters,might in a random to the limits of the usedapparatus).Low field susceptibility (Z) way influencethe massof the smallsamples.Accordinglymass wasmeasured with a BartingtonInstruments MS-2 susceptibility normalisedvaluesof these3 parametersshowlarge incoherent quotientssuch as Mrs/Ms and bridge.Isothermal remanentmagnetisations (IRM) wereappliedto variations.Mass-independent
samples takenfrom10cmintervals in coreLM Ill uptofieldsof XHF/Ms aretherefore used toinvestigate thevariation inmagnetic Mrs/Ms allowsinvestigation of the sizeof grainsin the 0.5 T. Althoughsaturation x•.asgenerallyobtainedin appliedfields properties. smaller than 0.3 T, a field of 0.5 T was used to introducethe satu-
ferrimagnetic fraction andZ}tF/MSgivesinformation ontherela-
tion betweenparamagneticand ferrimagneticfractions.Three the IPG in Paris. zonesare evidentfor theseparameters: (i) 498-380 cm, (ii) 380An AlternatingGradientMagnetometer (Micromag)wasusedto 285 cm and(iii) 285-164 cm (Fig. 2). The bottomof the sequence (498-380 cm, Zone I) is charactermeasurethe hysteresis properties:saturationmagnetisation (Ms). saturationremanentmagnetisation (Mrs), coerciviB'(He), rema- isedby low valuesof X, ARM. NRM and SIRM, whichare sugof ferrimagnetic grains.Low values nentcoercivit).' (Her) and alsohigh field susceptibili.ty (ZItF). gestiveof low concentrations Samples(10-40 mgof clr•.', bulksediment) weretakenat 10 cmin- of Hc (•10 mT) andthe •'steresisloopformsobservedheresugof magnetite[Thompson andOldfield,1986;Petervalst¾omcoreLM III. Curie balanceanalysesand X-ray dif- gestthepresence 1994].Diagrmns of Mrs/MsversusHcrKtc, fraction(XRD) measurements weremadeonmagneticextractsob- tersandThompson, relativegrainsizevariationfor magnetite tainedwith a methodsimilarto that of Papamarinopoulus et al.. usefulfor estimating thepresence of PSDandMD particles [1982]. Thesemeasurements were undertakenat the CFR du La- [Dayet al., 19771,indicate rationIRM (SIRM). All of thesemeasurements wereundertaken at
boratoireMixte CNRS/CEA, in Gif-sur-Yvette.
(squares in Fig. 3a). ThereforeS-ratio(IRM0.3/SIRM)smaller
NRM, ARM, andZ recordsfrom coresLM II andLM III record than 1 (Fig. 2e) andHcr valuesthat are higherthanexpectedfor the samechanges exceptfor a 16 cm gap in coreLM III dueto MD magnetiteIThompsonand Oldfield, 1986], especiallybe-
slippage ofthesediment within thecoring tube.Afterremoval of tween 430and390cm,indicate thepresence ofoxidised magnetthisgap,therecords correlate perfectly. itcorminor amounts ofminerals withhigher coercivities such as goethite or haematite. HighZi_•,/Msvaluesreflectrelatively high
TheLakeManas mineral magnetic record
quantities ofparamagnetic grains (Fig. 2f). Zone[I (380-285 cm) is characterised by highvaluesof Z, ARM,
Variationsof bulk magneticparametersX, ARM, SIRM and NRM andSIRM with theirdropsat 350 and 325 cm. High ARM NRM versusdepth(Fig. 2 a-d)clearlydefinethreedifferentzones valuesaredueto thehighconcentrations of magneticgrainswhen
JELINOWSKA ET AL.' MINERAL MAGNETIC
RECORD
955 c)
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400
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Figure 3. a)Classification ofmagncuc minerals intcrms ofmagnetisation andcocrcivity ratios. Squares - zoneI, - zone circles - zoneu, triangles m. •, -single-domain grams, MD -ntu•t•uomum ...............grains, -pseudo-singledomain grains andSP-superparamagnetic grains. b)X-raydifti-action pattern obtained formagnetic extractfrom coreLM 1IIat a depth 380-360 cm.Pvrite(P) andgreigite (G) dominate withsome quartz (Q) andprobabb' magnetite (M?).c)Thermomagnetic curve ofthesame magnetic extract heated inargon inmagnetic fieldof2.5T. occurring in phasewithpeaksin susceptibili•' (360-380cm,300 cm),andto thepresence of SDparticles at 338cm.Hysteresis parameters(Mrs/Ms and Hcr/Hc) indicatesmall particlesizes thoughout thiszone(circles in Fig.3a),withgrains lyingnearthe SD/PSDboundary, except fortxvopointsat 30l and327cm.The lattercorresponds to a sandlayerandtheformerindicates animportam contribution fromsupcrparamagnctic (SP)grains. S-ratio indicates thatall of thegrainsaresaturated before0.3 T (Fig.2e), suggesting thepresence of a ferrimagnctic mineral. Hoxvever. valuesof Hc (•-,40•nT) are too highfor SD to PSD magnetite {Thompson andOldfield,19861.X-raydiffraction datafromthe separated material (Fig.3b)indicates thepresence of pyrite(FeS2)
areprimarily ducto authigenic formation of thetbrrimagnctic iron sulphide, greigite(Fe3S4). Theformation of authigenic ironsulphides ispossible in alkaline.
anoxicandsulphate-rich environments withsufficient quantit3.' of dissolved iron.Duringearlydiagenesis. ironsulphides areformed
bythereaction ofH2SwithironionslBemer, 1980;Roberts and Turner,19931.Thesecomponents derivefromthe reduction of dissolved sulphate by bacteriawhichuseoxygen(combined in sulphate) asa source of energ) * anddetritalironbearing minerals present in sediments. Significant sulphur canalsobeliberated as
H2Sduring bacterial degradation oforganic matter.
According to Jones andBowser •19781,greigite is anintermediate mineral in the p3Titization reactions which occur duringsulandgreigite (Fe3S4). Thethermomagnetic behaviour of themagof an initialb ' amorphous precipitate of FeS.The neticextractin anargonatmosphere indicates thepresence of a phurization of ironlnonosulphides to psTitewill not occurin mineral thatundergoes a majordecrease in magnetisation behvccn transformation of sulphur. In the absence of sulphur(e.g.in many 300-400øC. which afienvardsis transformedto a f•rrimagnctic theabsence environments) iron sulphideformationwill be limited ironoxide(Fig.3c).Suchbehaviour issimilartothatobserved for freshwater FeSorgreigite. Jones andBowser suggest that greigite if heated inair[Snowball andThompson. 1990:Snowball. tocrs.'ptocrs.'stalline affectsonlythe sulphate reduction process. 1991:Hoffman.1992:RobertsandTumer, 1993:Reynoldset al., theroleof organisms besides thispossibility of inorganic greigiteformation 1994].Considering theverysmallquantity of magnetic material in Therefore, mediated), it is knownthatseveral species of magnetootherzones,CurieBalanceandXRD experiments werepossible (organic intracellular magnetic inclusions of sinonlyin thislithological unit.Hysteresis parameters of greigite in tacticbacteriassmhesise greigite. Thesemagnetotactic bacteria arecommon in LakeManassediments, suchashighvaluesof Hc andsmallpar- gledomain sulphide-rich waterandsediments [Farinaet al., 19901. ticlessizes(SD), confirmobservations of Snowball119911and brackish, Roberts119941.concerning the behaviourof naturalgreigite. Theseproperties areconsistent withhighvaluesof SIRM/z(Fig. Discussion and conclusions 2i), whichare commonly observed in naturalgreigitesamples .•selRobertsand Turner,1993;Reynolds et al., 1994].Hysteresis Mineral magneticzonationof the Lake Manas sedimenta• quence is generally in agreement with the palaeoenviromnental reloopsarecharacterised bywide,opencentral sections asit is also observed in naturalgreigitesamples [Snowball, 1991;Petersand corddeftnedby otherproxydatalRhodeset al., in pressI. periodis clearlycharacterised by a low conTrompsore 19941. LowZHF/MS suggests relatively lowcontribu- The pre-Holocene
centration of magneticparticlesWhichis dominated by PSD and tionsfromparamagnetic grainsin thiszone. In zoneIIl (285-170cm),a dramatic dropof ARM, SIRM, Z and MD detritalmagnetite,with oxidisedmagneticgrainsbetween430 with the preNRM indicatesa decrease in theconcentration of magneticgrains. and390 cm (•12.5-9.5 k).TB.P.). This is consistent The hysteresis parameters and S-ratiocorrespond to thoseex- dominanceof detritalmineraland organicmaterialin the sedi-
withlow13Cand180of carbonates reflecting shortresipected forPSDmagnetite (triangles in Fig.3a).Thelackof direct ments, areinterpreted asbeevidence (ascitedabove)for greigitesuggests thatgreigiteis ab- dencetimeof thewater.Theseobservations with a desert sentfromthispartof thesequence. Smallincreases of ARM andZ ing indicativeof low lakelevelswhichis consistent dominant).Maximum aridity, at 270,240,and190cmaredueto relatively higherconcentrationstype vegetation(Chenopodiaceae as suggested by several environmental indicators [Rhodeset al., in of ferrimagnetic grains. HighZ}rF/Msvalues in thiszonereflect
press],wasrecorded between460 and418 cm, andis tentatively highercontributions of theparamagnetic clays. of oxidised The LakeManassediments thusshowlargevariationsin the size, attributedto theLastGlacialMaximum.The presence grainsin theLakeManasrecordroughlycoincides with concentration andmineralogy of magneticgrains.Thesechanges magnetic
956
JELINOWSKA
ET AL.: MINERAL
MAGNETIC
RECORD
theupperpartof thisclEv episode andthelowerpartof thefollow- References inglacustrine period. The magneticpropertiesof the sedimentsindicatea long term
Bemer.R.A., Early Dirgenexis:A Theoretical Approach,Princeton ' University, Press,Princeton.1980.
episode from---8.7 to-3.5 ,kyrB.P.(380-285cm)whichwaschar- Day, R.. M. Fuller.andV.A. Schmidt, Hysteresis properties of tiacterised by the authigenic formationof greigite.Thiscouldsugtanomagnetites: grain-size andcompositional dependence. Phys. gesthighbiological activitywhichmayfayourauthigenesis of iron Earth Planet. Inter., 13, 260-267, 1977. sulphides (eitherbiomediated or synthesised by bacteria)in stag- Fang,J.Q.,Lakeevolution during thepast30.000yearsin China,and its implication for environmental change, Quaternary Research, nantwaterswith anoxicbottomconditions.The limitsof this epi3e•,37-60, 1991. sodearesharplydefinedby mostof the magneticparameters analysed.The lowerlimit is shiftedby -1.3 •r abovethemajoreli- Farina,M., D.MS. Esquivel.andH.G.P. Linsde Barros,Magnetic iron-sulphur crystalsfrom magnetotactic micro-organisms. mmic evemdeducedfrom the hydroclimatic record,whichcorreNature. 343• 256-258, 1990.
sponds toarapidtransition fromdrytowetconditions at
Gasse,F., M. Arnold,J.C. Fontes.E. Gibert.A. Huc,B. Li. Y. Li, Q.
B.P.(400 cm).Theabruptness of theonsetof thismagnetic interLiu. F. M{51R:res. E. Van Campo,F. Wang,andQ. Zhan,A 13000 val indicatesa suddenchangein the lake metabolism. The magyearclimaterecordfromwestern TibeLNature, 353, 742-745, 1991. neticparameters of thelowerpartof thelacustrine episode( 10 to 8.7 k3•rB.P.) indicatethat a detritalinflux dominatedthe lake Hoffmann, V., Greigite (Fe3S4): magnetic properties andfirstdomain Phys.EarthPlanet.Interiors,70,288-301, 1992. duringthistime.The concentration of magneticgrainsdecreases observations, duringtheve•' wet,warmimerval(7.5 to-6 ks'rB.P.).Thiscould Jones,B.F., and C.J. Bowset,The mineraloe' andrelatedchemistD' of lake sediments,In: Lakes: ('hemistr); GeoloKv.Physics.A. be explainedby the transformation of greigiteto pyritedueto Lerman,ed.,pp. 179-22Z Springer,New York, 1978. completion of the sulphurisation reactions. Suchchangecouldbe Lister. G.S., K. Kelts, C.K. Zao, J.K. Yu, and K. Niessen, Lake due to a decrease of detrital iron influx into the lake or to increased Qinghai,China:closedbasinlakelevelsandtheox?genisotope availabilityof dissolvedsulphate. record for ostracoda since the latest Pleistocene,Palaeogeog.. The sudden influx of fluvial material recorded around 5.5-4.5 kw
Palaeoclim., Palaeoecol., 84, 141-162, 1991.
S., P.W. Readman.Y. Maniatis,and A. B.P. (335-320 cm), is clearlydocurnented in the magneticrecord Papmnarinopoulus. Simopolus, Magneticchamctcrisation and M6ssbaucr spectrosby a sharpdecrease in ARM, Z andotherparameters. This likely copyof magnetic conccntratcs fromGreeklakesediments, Earth reflectsa short-termreturnto dr5.'climate,as suggested by the Planer ,S•'i. Lett.. 5 7. 173-181.1982. pollenrecord.However,theh)pothesisof a tectonicuplift in the Peters,C. andR. Thompson, Qualitativeandqum•titative magnetic Tian Shanmountainscannotbe ruledom [Rhodeset mi.,in pressI. idcmification of iron oxidesandsulphides, Ann. Geophys.. EGS, Conditionssimilarto thoseof the early Holoceneare recorded Suppl.1 to vol. 12, p 166•L 1994. from•4.5 to ,-•3.5•,r B.P. (320-285). The abruptchangeat ---3.5 Reynolds,R.L., F. Tuttle, C.A. Rice, N.S. Fishman.J.A. kyr B.P. coincides with a short-term dr5,'eventsuggested by a sudKaraszewski• and D.M. Shemtan,Magnetization and geochemistry of greigite-bcaring Cretaceous strata.North SlopeBasin, deninputof detritalorganicmaterialto the lake.The carbonate Alaska,Am. d. Sci., 294, 485-528, 1994. contentand the stableisotoperecordsonly documema general shift towardsincreasing aridi•' from that time [Rhodeset mi.,in Rhodes,T.E., F. Gasse,R. Lin, J.C. Fontes,K. Wei, P. Bertrand,E. Gibert.F. M•li•res,P. Tucholka, andX. Sun,A LatePleistocenepress].The magneticdatathusclearlyindicatea majorenvironHolocenelacustrinerecordfrom Lake Manas (northernXinjiang, mentalchange in thelakeandthecatchment areaat---3.5kyr B.P. western China), Palaeogeogr.,Palaeoclim., Palaeoecol., m The lateHolocene(up to 285 cm:3.5 ksx- recent)is similarto press,1994. that of the pre-Holocene period,exceptfor lack of evidencefor Roberts, A.P.,Sedimentas3., greigitc (Fe3S4): Formation, identificaoxidationof magnetite grains.Thelakebecameshallowandbetter tion and magneticcharacteristics. EOS, 7)'ans.AGU Suppl.to vol. 75, (16), p. 124, 1994. mixed,withoutevidenceof greigitetbrmation.The pollenrecord formationof fernmagsuggests instabilityin hydrologyor climaticconditionsin the Roberts,A.P. and G.M. Turner,Dimgenetic neticiron sulphidemineralsin rapidelydeposited marinesedicatchment areawhichis not reflectedm themagneticrecordof the lake sediments.
The Lake Manasrecordis broadlyconsistent with previouspalaeoclimaticdata from westernChina. Different typesof proxy
ments,SouthIsland,New Zealand,Earth Planet. Sci. Lett., 115, 247-173,
1993.
Snowball,I. and R. ThompsomA stablechemicalremanence in Holocenesediments../. Geophys.Res.,95, 4471-4479, 1990.
data[e.g.Zhouet mi.,1991;Fang,1991;Listeret mi.,1991;Gasse Snowball,I.,F., Magnetichysteresis properties of greigite(Fe3S 4) and a new occurrence in Holocene sediments from Swedish et mi.,1991|, indicatethat conditions warmerandwetterthantoLappland, Phys.Earth Planet.Inter., 68.32-40,1991 dayprevailedin westernChinaduringthe early-middle Holocene R., and F. Oldfield.Em,ironmental magnetism• Allen period.Thesedatadocument the rapidestablishmere of monsoon Thompson, and Unwin, London. 1986. precipitation around10kyr B.P.,anda climaticoptimum(wetand warmconditions) around7-6 k3,rB.P., followedby a generaltrend Zhou, S.Z., Chen, F.H.. Pan, B.T., Cao, J.X.. Li, J.J., and Derbyshire,E., Environmentalchangeduring the Holocenein western towardsariditywhichculminated at-•3.5kyr B.P. TheLakeManas China on a millennialtimescale,The Holocene, 1, 151-156, 1991. recordalsosuggests that the smnmermonsoon penetrated westwardovertheZunggaryBasinduringtheearly-middle Holocene. Acknowledgements.We gratefullyacknowledge E.Tric and D. Williamsonfor helpfuldiscussions and comments. This work would
A. Jelinowska,P. Tucholka,Lab. de Physiquede la Terre et des Plan6tes,UA CNRS 1369, Brit.504, UPS, 91405 ORSAY F. Gasse, J.C. Fontes, Lab. d'Hydrologieet de Gdochimic
URA CNRS 723, Brit.504, UPS,91405 ORSAY. nothavebeenpossible without thehelpof thestaffof theLaboratory Isotopique, of Palaeomagnetism of the IPG in Paris and of the CFR m Gif-surYvette.We aregreatlyindebted to reviewers: A. Roberts,J. King and an anonymousreviewer for extensivecommentsand correctionsof our manuscript.This work was fm•dcd by the CNRS, and the
Universit6Paris-Sud, Orsay,France.
(Received:June 10, 1994: revised:November16, 1994; accepted: December1, 1994)
