Uranium and thorium contents are low and total rare earth element (REE) contents are generally high. REE ratios and shale-normaiized patterns demonstrate ...
PALEOCEANOGRAPHY,VOL. 8, NO. 2, PAGES293-311,APRIL 1993
TWO MAJOR CENOZOIC
EPISODES OF
PHOSPHOGENESIS RECORDED LN EQUATORIAL PACIFIC
SEAMOUNT
DEPOSITS
James R. Hein,1 Hsueh-Wen Yeh,2 Susan H. Gunn, 1 William
V. Sliter, 1LauraM. Benninger, 3andChung-Ho Wang, 4
rareearthelement(REE) contentsaregenerallyhigh. REE ratiosand shale-normaiized patternsdemonstrate thatthe
accumulation of dissolved phosphorus in thedeepseaduring relativelystableclimaticconditions whenoceanic circulation wassluggish.Fluctuations in climate,sealevel, andupwelling thataccompanied theclimatetransitions mayhavedriven cyclesof enrichment anddepletionof thedeep-sea phosphorus reservoir.As temperature gradients in theoceansincreased, Antarcticglaciafionexpanded andoceaniccirculation and upwelling intensified.Expansion andintensification of the oxygenminimumzonemayhaveincreased thecapacity for midwaterstorageof phosphorus supplied by dynamic upwellingaroundseamounts; however,thebottomwaters neverbecameanoxicduringthephosphogenic episodes. Fluctuations in the CCD andlysocline,CO2 fluxes,and changes in bottomwatercirculation andtemperatures may
REEs and host CFA were derived from seawater. Strontium
have bathed the seamount carbonatesin more corrosive waters
isotopic compositions compared withinferredCenozoic seawater curvesdefinetwomajorepisodes of Cenozoic phosphafization: LateEocene/early Oligocene (39-34Ma) and lateOligocene/early Miocene(27-21Ma); threeminorevents arealsoindicated.The majorepisodes occurred at timesof climatetransition,thefirstfroma nonglacialto glacialearth andthesecond froma predominantly glacialto warmearth. Thepaleoceanographic conditions thatexisted at thosetimes initiatedandsustained development of phosphorite by
which,coupled withincreased supplies of dissolved phosphorus, promoted replacement processes. Thelate Eocene/early Oligocenephosphogenic episoderecorded in seamount deposits is notmatchedby largephosphofite deposits in thegeologicrecord,whereas thelate Oligocene/early Mioceneepisode andmiddleMioceneevent arematched by largedeposits distributed globally.The seamount phosphorires areexposed at thesurface of the
Abstract.Sea_mount phosphorites havebeenrecognized sincethe 1950s,but thisis the first studyto providean in depthexploration of theoriginandhistoryof thesewidespread
deposits.Representative samples fromequatorial Pacific Cretaceous s•mountswereanalyzedfor chemical, mineralogical, andstableisotopecompositions. The phosphorites occurin a widevarietyof forms,butmost commonly carbonate fluorapatite (CFA) replaced middle Eoceneand oldercarbonatesedimentin a deepwater environment(>1000 m). Elementratiosdistinguish seamount
phosphorites fromcontinental margin,plateau, andinsular phosphorites. Uraniumandthoriumcontents arelowandtotal
seamountsand have beenfor mostof the Neogeneand
Oligocene.Thephosphorires donotshowsignsof etching that would indicate substantial undersaturation of seawater
phosphate withrespect toCFA. Massbalance calculations
1U.S.Geological Survey, MenloPark,CA 94025.
indicate thatabout 5.4-19x 1012gofP205arelocked upin
2Hawaii Institute ofGeophysics, University ofHawaii,
equatorialPacificseamount phosphorires. Thatamountis equivalentto about2-7 yearsof thepresentannualinputfrom
Honolulu.
3Geology Department, University ofCalifornia, Davis.
rivers.
4Institute of EarthSciences, Academia Sinica,Taipei, Republicof China.
INTRODUCTION
Copyright1993
Phosphorites generallyoccurin fourgeographic-tectonic settingsin thepresent-day oceanbasins:(1) The beststudied phosphorites occuron continental shelvesandslopes, primarilyoff thewestcoastof continents.Typicalexamples includePeru-Chilemargin,southernCaliforniacontinental
by theAmericanGeophysical Union Papernumber93PA00320 0883-8305/93/93PA-00320510.00
294
Heinetal.: Equatorial Pacific Episodes ofPhosphogenesis
borderland, andNamibiamargin[e.g.,Burnett,1977;Birch, 1979;Loebneret al., 1987]. Thesedeposits formedbeneath zonesof coastalupwellingfromearlydiagenetic processes veryneartheseawater-sediment interfacein an organic matter-richenvironment[Burnett, 1977; Froelichet al., 1988;
studiedof the marinephosphorites.Thesedepositshavebeen recognizedsincethe 1950sandarethoughtto resultfrom replacementof carbonates by carbonatefiuorapatite(CFA) [HamiltonandRex, 1959],but hydrothermal depositionand submerged insularguanodepositshavealsobeenproposed
GlennandArthur,1988]. (2) Phosphorites occurextensively
[Kharin, 1974; Cullen and Burnett, 1986; Rao and Burnett,
on somesubmarineplateaus,ridges,andbanks,thebest studiedbeingBlakePlateauoff the southeastern UnitedStates andChathamRiseoff New Zealand[e.g.,Manheimet al., 1980;yonRadandROsch,1984;Riggs,1989]. Plateau
1992]. Relativelycompletequantitative chemicalanalyses haveonly recentlybeenprovided[Bezrukovet al., 1969; CullenandBurnett,1986;Burnettet al., 1987],andprecise age(s)of phosphatization of seamount phosphorites havenot beendetermineduntilnow. Phosphorites in thePacificand adjacentseashavethusfar beenidentifiedon seamounts locatedbetweenlatitudesof about35øSand42øN [Karpoffet
phosphorires formedfromcementation andreplacement of carbonates in an organicmatter-richenvironment.Several stagesof reworkingof thedepositsarecommon.(3) Phosphorites formon islands,atolls,andwithinatolllagoons [WhiteandWarin, 1964;Tracey,1980]. Theseinsular depositsreplaceandcementreef carbonates withinthe freshwater lens,or withintheseawater-freshwater mixing zone,andmaymarkperiodsof sealevelchange[Roeand Burnett,1985;Piperet al., 1990]. Marine lakesand meromicticlakesmay alsobe environments of insular phosphatization [Bourrouilh-LeJanet al., 1985;Burnettet al., 1989]. The sourceof phosphorus is primarilyguano,butmay alsoincludeweatheringof volcanicrocksandhumicand sapropelic organicmatter[BurnettandLee, 1980;Rodgers, 1989;Piperet al., 1990]. (4) Phosphorites formonmid-plate seamounts andmaybe themostwidelydistributed but least
al., 1980; Heezen et al., 1973; Slater and Goodwin, 1973].
Water depthsof nonreworked depositsvary from several hundredto severalthousandmeters. Seamountphosphorites are also common in the Atlantic and Indian oceans and
adjacentseas[Marlowe, 1971;Kharin, 1974;Baturin,1978; Jonesand Goddard, 19791.
This is the first studythatsystematically determines agesof phosphatization for a suiteof seamountphosphorites that occurovera largepartof the centralPacific. Agesof phosphatization are determined by strontiumisotope stratigraphyandoxygenisotopestratigraphyis testedasa possibleagedatingtool for phosphorites.Depositionalagesof phosphatized limestones weredetermined by CFA-replaced
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Fig. 1. Generallocationof foursamplingareas,bathymetry of samplingsites,andlocationof 12 dredgehaulsfrom ninevolcanicedificesin theequatorialPacificlistedin Table 1' bathymetrymodifiedfromChaseandMenard [1973].
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Heinetal.: Equatorial Pacific Episodes ofPhosphogenesis
295
foraminiferaassemblages. We providedetailedchemical analyses includingmajoroxides,rareearthelements, uranium,
extractCFA for analysis.Six additionalsampleswith 0-15%
thorium,fluorine, chlorine,sulfur,and carbondioxide. We
isotopiccompositionof sampleswith about44d
12.3'N
6.0'N
87e 84e
4.3'N 8.3'N
2.7'S 1.7'N
4.0'N
3.0'S
(•ruise F2-88-HW, Hawaii
D12
155'58.2'W
2900-2490
CruiseL9-84-CP. Marshall Islands
D6 D18
L6kkw6rkw6r Seamount 08'46.1' W6den-K6pakut Guyot 13'54.0'
169'45.3'E 167'38.8' E
2900 1600
CruiseF 10-89-CP, Marshall Islands
D22
Mij-LepSeamount
08'43.7'
163'12.2' E
1270-1245
-_80 f
aSaitoandOzima [1977].
bSchlanger etal.[1984]. CHeinet al. [ 1990b].
dBased onforaminifera age(seeTable3). eDavis et al.[ 1989].
fBased onageofnearby seamounts [Davis etal.,1989; Heinetal.,1990b; Lincoln etal.,1993].
296
Heinetal.:Equatorial Pacific Episodes ofPhosphogenesis
Heinetal.:Equatorial Pacific Episodes ofPhosphogenesis TABLE 3. Semiquantitative X RayDiffractionMineralogy of Seamount Phosphorites Listedin Table2
Sample Number
X RayMineralogy (%) Cruise L5-83-HW
D5-A3-2
100 CFAa
D18-B3-7A
81 CFA, 19 barite
D18-B3-7B
95CFA,5calcite (100CFA) b
D29-Al-la D29-Al-lb D29-Al-lc D29-Al-ld D29-Al-le D29-Al-lf
100 100 100 100 100 100
D29-A7
84 CFA, 14 phillipsite+plagioclase, 2
CFA CFA CFA CFA CFA CFA
calcite(85 CFA, 15
phillipsite+plagiocase) Cruise F7-$6-HW
CD1-6B CD6-12B
65 goethite,35 CFA 55 CFA, 37 plagioclase, 6 smectite, 2 clinoptilolite
CD14-2D CD19-1A
100 CFA 100 CFA
CD21-7H CD21-7I
97 CFA, 3 calcite(100 CFA) 98 CFA, 2 calcite(100 CFA)
CD21-7J
100 CFA
CD21-7K
70 CFA, 30 barite
CD2!-7L
73 CFA, 27 phillipsite+plagioclase
297 Shackleton,1986;Miller et al., 1987;Wright et al., 1992; Zachoset al., 1993]. Thesecurvescommonlydid not yield a uniqueageand if morethanoneagewaspossible,the age closestto that determinedusingstrontiumisotopeswas recordedin Table2. For agedating,it is assumed that structuralcarbonatebehavesisotopicallyexactlylike calcite and thatCFA formedin deepwater. Major oxidecontentsof phosphorite sampleswere determined by X ray fluorescence spectroscopy (Table4) [Taggattet al., 1987]. Carbondioxidecontents were determined by coulometric titration[Englemanet al., 1985], H2O+ by waterevolvedat 950øCasdetermined coulometrically by Karl-Fischertitration[Jackson et al., 1987],andH20- by sampleweightdifferenceat 110øCfor greaterthan1 hour[Shapiro,1975]. Fluorineandchlorine weredetermined by specificion electrodemethodsandsulfur by combustion andinfraredspectroscopy (Tables4 and5) [Jacksonet al., 1987]. Rare earthelementswere determined by inductivelycoupledplasmamassspectrometry (Table6) [Lichte et al., 1987]. Uranium and thoriumwere determined by delayedneutronactivationanalysis(Table4) [McKown and Millard, 1987].
X ray diffractionmineralogy wascompleted ona Philips diffractometer usingCuKc•radiationanda curved-crystal carbonmonochromator (Table 3). The usualPearsonproduct moment correlation coefficient was used to calculate the
correlationcoefficientmatrix. Foraminiferaassemblages were identifiedfrom thin sectionsof thephosphorite samplesand thetime scaleof Berggrenet al. [1985] wasused. RESULTS
Cruise F2-88-HW
D 12-1A D12-5
99 CFA, 1 quartz >99 CFA, < 1 quartz
D6-3A D6-3B D6-3D D18-3E D18-3F
86 CFA, 14 calcite(100 CFA) 68 CFA, 32 calcite(100 CFA) 66 CFA, 34 calcite(100 CFA) 77 CFA, 23 calcite(100 CFA) 63 CFA, 37 calcite(100 CFA)
Cruise L9-84-CP
Cruise F 10-89-CP
D22-1E-A
>99 CFA,
