________________________________________ O_C_E_A_NO __ LO_G __ ' C_A_A_C_T_A_,_'_9_B_' ,_N_._S_P~~~-------

Phosphoritcs 1'Rleoceanogr.tphy

Phosphorites and paleoceanography T",,~;;:,:::: PhosphorÎtes

Palëodanogrl.phie Trans&rcssÎOn

Climat

ABSTRACT

RÉSUM É

M . A. Arthur ", H . C. Jenkyns b

• US Gcological Survey. Box 25046. MS 940 . Denver Federal Ce nter , Denver. Colorado 80225. USA. , Department of Ocology and M ineralogy, Universit y of Oxford, Parks Road. Oxford QXI 3PR, GUR .

1) Phosphori tc gcnesis corre laies in a gc neTal way with eJevalcd sea leveJ and warm dimale and is therefore influcnced by plate Icctonics and continental drift. 2) l'hosphoritc genes is correlales more specific<lll y wilh transgression - il rcsul! of the interac tion of continental hypsomctry. lectonic perturbation and cuslatic effec ls - whcrcby il small number of shallow s lope and sheU s ites are available for phosphate fixation . Suc h shallow transgressive seas are typiClllly highly fertile. and lend 10 sequester nUlrients. accounting for the common associalion of phosphorite wilh organic-ric h black shales. Poorly·oxygenated shallow seas also provide a shorler trans it for organic ma!ler from prod uctive surfllce walers 10 sed iments. result ing in less oxidation and grenter initial retention of organic phosphorus. 3) Phosphorite gencsis correlates with warm climnte . which may be related to a) increased n ux of phosphorus to the ocean during lime of increased chemical weathering on land : and b) developme nt of widespread oxygen-depleted waters because rates of oceanic circula tion and 0z-solubilit y are reduced. Consis tent with the hypothesis of incrcased nUlrient suppl y 10 the oceans at . o r just before, major Cenozoic phosphorogenic episodes, is the coeval de velopment of widespread biogenic sil ica deposil5. 4) Oceanic anoxic events (OAE's : episodes of world-wide deposit ion and preservation of organic carbon). which ulso corre lute with particularly high seu level s tancWi , genemll y do I lUl

coincide wilh major phosphorogenic episodes. This may he due to the faet thal during OAE's abundant phosphorus is fixed in the deep sea wilh well.preserved organic carbon and is henee unavai luble 10 form major shallow s lope and shelf deposits. Furthcrmore when 'excessive' 3rellS of shalJow shelr exist in parallel wilh numerous potential si tes of phosphate fixation. multiple small deposits te nd to be produced ralher thull li few of economic importance: they th us may e lude not ice or classification. However. phosphatized seamount limestoncs and phosphlltic hardgrounds in shel f carbonate sequences may be formed during ÛAE·s. 5) ln contrast 10 the assertions above. the permian Phosphorill Formation was deposited during a marked overall global low-stand of sea level . However. the phosphalic seque nces we re I,dd down during brief !TlInsgressive episodcs. The anomalous ly high concentration of phosphorus in Ih,,! uni l probably cx iSh because Ihal part of North AmcriCli provided one of the few avai lable sites for the fi xat io n of phosphllte during the Permian. If global waters had withdrawn to the top of the continental slope. or if contine nlal margins slood high. only a parlicularly low-Iying continenllii area would be covered by the sen llnd could act as a s ink for mosl of the ocea n's ava ilable phosph<l le .

OCeiJ/lol. A c ta . 1981. Proceedin~s 26'h International Gculogiçal Congress, Geolo~y of oceans symposium. PMis. July 7-17. 1980.83-96.

Phos phorites et paléocéanographie.

1) Lli phosphaloge nèse correspond dans ses grandes lignes à des périodes de transgression ou de c limat c hllud . Elle est donc influencée par la tectonique des pl<lques et la dérive des cont ine nts.

83

M. A. ARTHUR, H. C. JENKYNS

INTRODUCTION

2) En période de transgression (combinaison du relief continental. des accîdenb tectoniques et des phénomènes eusmtiques). le fond de la mer présente cerhlins s ites, ta nI sur le plateau que sur des pente~ douces. qui permettent 1,1 fixation des phosphates. Les mer~ transgressives peu profondes sont très fenile s et tendent à retenir les sels nutritifs, expliquant ilinsi l'a!>soc iation fréquente de phosphorites avec les vases organiques. De plus dans les mers peu profondes e t appauvries en oxygène di ~!>ous. le transfert de 1<1 matrice organique de s lones productives de la !>urface vers les sédiments. e~t plus rapide. L 'oxydation y est réduite, ayant pour conséquence une rétention initiale plus l:Irande du phosplmte orl:lanique. 3) La phosphatogenèse. qui peut étre mise en corrélation avec un climat chaud. peUl s'expliquer soit par un accroissement de l'apport de phosphore à J'océan consécutif à une altération chimique plu~ intense à terre. ~oi t à une généralisation de la diminution de teneurs en oxygène dans les eaux. liée au ralentissement de la circullition océanique ou à la baisse de solubilité de l'oxygène. La généralisation des dépôts de silice biogénique. qui va de pair avec les grands épi~odes de phosphatogenè!.e au Cénozoïque. est compatible avec cette hypothèse d'un accroissement en ~els nutritifs à ces même~ époques. 4) Les épiSodes anoxiques océaniques (OAE : période où le dépôt et la pré~e rvation du !.:arbone organique cons tituent un phénomène général à l'échelle du globe). période~ qui sont en oulre souve nt corrélables avec un niveau élevé des mers, ne coïncident pas avec les grand~ épisodes phosph:ttés. Cela peut tenir au f,\it que durant les,. OAE '", une quantité importante de phosphore es t retenue dans l'océan profond avec le carbone organique bien conservé: le phosphore n'étant plu~ alors disponible pour constituer des acc umulations importante s tant ~ur la pente que le pla teau . Bien plus. quand la ~urfacc des eaux peu profondes e st considérable. offrant a insi de nombreux ~i te s potentiels de fix a tion de phosphate. la tendance est à la multiplication de petits gisement s plutôt qu'li la formation d'accumulations peu nombreuse~ et d'importance économique . Toutefois , durant les OAE, la phosph,lIisation de calc,lires des sealllounts et le développement des hllrdl:lrounds phosphatés dans les séries carbonatées du plateau continental peut se produire. 5) En contradiction avec ces hypothèse!>. on constate que la " pho~phoria formation .. du Permien s'est déposée à une époque caractérisée par un niveau général des mers particulière­ment bas. bien que les niveaux de forte concentration à l'intérieur de celle unité se soient accumulés durant de brèves périodes de transgression. La concentration anorm,llelllent élevée de phosphore dans celte unité a dû ~e produirc parce que celle partie de l'Amérique du Nord constituait un de!> I1lfes sites de fixation pour le phosphate au Permien. En effet. s i les eaux de l'océan global n'atteignent plus que le sommet de la pente, ou si les marges continentales sont trop élevées, seule une zone contine ntale exceptionnelle ment ba!>se pourra être recouverte par la mer el agir comme un piège pour l'e~~entiel du phosphate océ:tnique disponible.

Oce(j/I()I. A CIll. 1981. Actes 26< Congrè~ International de Géologie . colloque Géologie de~ océans. P-dris. 7-17 juil. l<)KO. lB·%.

The availability of dissolved pho~ph!lte in the oceans in part controls primary producti vity. At present. the concenlra­tions of biologically useful ni troge n :md phosphorus in the oceans relative to other clements ~ ul!:ge st tha t thesc are the limiting nutr ients in phytoplankton production (Broecker. 1974 : Holland . 1978). Froelich el (II. (1981 : cf. Burnetl. 1980) have quantified Ihis pre~ent .day linkage belween the oceanic phosphorus and carbon cycles (Fig. 1). ahhough these flu xes are s tiJl only poorly known . ln considering past variations in glObal oceanic producti vi ty. and to e~ timatc future trend!>, it i!> important tO understand the po~sible effects of various paleoceanographic and climatic fa ctors on the geochemical cycle of phusphorus. Thal significa nl variat ion!> in the phosphorous cycle have occurred in the geologic pas t i~ sugges ted by da ta compiled by Cook and McElhinny (1979). which show larKe differences in the mass of sedimentary phosphate depos its represenling various parIS of I!eologic time (Fig. 2 and 3). However.lhe reported absolute va lue uf tOlal pho!>phoru~ cXlmctcd could bc ;I~ much as a n order of maKnitude in error (or the foll owing reasons: 1) these data arc a record of what is presenl(>(Jand exposedwhich may not adequatel y reflect what wa~ initiall y

Figure 1

c; "" ..... """' .. c; .. ' •• 0'

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.[ ' J." .[!!j .-"D cae:o,

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...... ,01 _.~l (>eUN

... _. , " j<". 'C'"'U - -

Th t in/trac/ion uf /ht curt",n and phospho,.us ryclno (modifitd afler Froelich CI al. . 1981. rf. Bu,"tII. 1980). Estimattd flu.ttl und rUnctnmuions of Pond C /0 und from "arious rtSln'oi fS art ShUN'" (su Ta bll 1). NOlllhal ltss Ihan 10'1 of Iht pUl"iol phosphofll s flll.'1: appartnrly tnrers stdim f"'O I")' phosphorill dtposih" .. ·"rld ... ·idt /lnaer i'iferrta prtst,,'·doy SIt udy,slm e conditiOlls. Thl proportio­nation of plwsphofll s inlO indi"id"o/ rtsen 'uirl> mij:hl hm·t bun q/lill ai/ll'rflll at lime~' in IIIt piISI . TCO~ dfnolfS 10101 dlua/I"td t·u,han.

84

PHOSPHORITES AND PALEOCEANOGRAPHY

~.

fo-' ,,- ..... _ .......... -@"' -;;:' 0 ® ®

........ '-" . Figure 2 L/J/t Mtsowir pa/cou/Jnullraphir p/J rClm~f~r$ , 1It/1. r htmkCl/ Cluumu//Jf;un rlllts, (md ~ntr/JUud marint Slrlllillraphy.

1 1 ___ .c. ---:

-_. ..0" ! -, , Cu/wmn 1 : J/Jr/usk and C,~It. CtO.lS a/lfr \'lm

Hinu, 1976a, b; Cu/umn 2 : J/lrrusk mudifitd Cl/frr Hal/am, 1978; VCl ô/ Cial., 1977: Cr~laCtoUS a/lrr HOllcurk ond KClu//mCln , 1979; VClil el al. , 1977; Cu/umn J : Cl/1fT Ar/hur, 19S1 ; J~nkyns, 1980: Sddonll~r wld Itmlqn,f, 1976; CululIln" : a/ur Sch/III~ ond Arrllur, 1980; Colll/lln 5 : a./ru Cook and Mcëllû" ny, 1979, (A bbrr~ialilms ; Nto. rom _ Ntocllm i,UI : Tllil _ Thit/WIII/HI : Trius . .. Tria.uic).

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deposited : and 2) they represe nt only li tatty of economic deposits and do nol consider the amount of phosphate locked up in sedimentary orga nie malter or as absorbed phosphate or dissemi nated phosphate particles in other noneconomic sedimentary accumulations. Nonetheless,

' Iarge relative <lifferences in phosphate mass between sec· tio ns of the geologic record (if they exis t) should reflect real changes in rates of phosphate ex traction from the oceans, If we accept that the mass of sedimentary phosphate deposits dacs change, then this implies that there have been changes in Ihe concentration of dissolved phosphate in the oceans. The variations could have occurred because of changes in the rate of fi xing of phosphate in sediments nnd/or in the flu x of phosphorous to the oceans from various sources (sec

- , .. ~,= ~

.=:

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Figure 3 Ctnozoic pa/toc~unol:raphic param~tus. 1I~(J(: hemical accumulutiun 'UI~S , and ~ne,aliltd m/lrillt Siruti. Rruphy. Co/umn J : a/rtr 8trggrtn, 1972; Cofu".,, 1 : T. trans· gresslon ; R ; regrtssion, i"dicatt $tCl'/tI'~/ rtltdi,'~ chull. gt$ ; Cl/lU Voit el a l. . 1977; Colum" J : a/ter Arthur . 1981 ; Columll".' a/Ur Wors/~y and Dol' i~s, 1981. u"d Arthur, /981 , compited /ra", "umtrous sllurcts : Co/um" 5 : a/lU Ltintn. 1979, und u/her sOI/ rets ; Cu/mllll 6 : o/Irr Cook and McE/hi"" y, 1979: CO/Ul/In 7: a/lu SI"';n. 1977 : Frakts . 1979 : and Olh~rs, L - Lal~ .' M _ Midd/t ; E _ ëar/y.

Table 1 and Fig . 1). The purpose of this paper is 10 consider possible pas t variations in the geochemical cycle o f phos· phorus, particularly during the Mesozoic·Cenozoic, a nd to suggest that a number o f factors can account fo r the major changes in phosphate sedimentation. The ge neral model developed here is inlended to explain the episodici ty of major phosphorite deposits. but is not intc nded to accaunt for ail large phosphorite nccumulations. A numbcr of recent papers have thoroughly considered the ' phosphate problem ' (e.g. Kolod ny, 1980 ; Ournen , 1980 : Sheldon, 19800, b; Denlor. 1980 ; Cook, McElhinny, 1979; Slansky, 1980). We do not reitera te here any discus­s ion of the fi ne-scale detai ls of forma tion of these sedi· ments. In general, we accept the idea thM phosphatization is

M.A. ARTHUR, H.C. JENKYNS

Table 1

Mujo, P't~", sou'us uml sinks in the geodlt'm/Cuf c:yck of phosphorotu·

Eslimatcd r rc~idcncc lime in oceans: 100,000 yr

Sources 1. ))(,c~wutcr di$sol~cû phosphutc : 2. HlI~ial dissoh'cd phosphatc from rock ... ·cathcring :

J. Vokanic cm:mations : 4. Imentitial watcr renliJl from ~dimen1S: 5. Regeneration ~ia C-org oJlidatÎOn in water wolllmn:

Sinks 1. Uptakc in C·org by phytoplanklOn and zooplankton and burial in

scûimcnh 2. Burial of up<lti lic bonc Iccth und :;cales: J. Buna! in biogenic carbonates: 4. Adsorption on iron-oxidc-rich dccp-liea sediments 5. Incorporat ion in phosphatK: sediment

9:><IOI-gP 1.7 X .olt gPly

0.07 X JOu gP/y Cil. 5 X 1011 gP/y

ca. 0 .73 x 1011 gP/y Cil. 0.04 X 1011 grly ca. 0.54 x 1012 g Ply ca. 0.2 X 1O1l g P/y ca. 0.11 x 1012 gPly

(average cone. 2.2 mM/ kg)

(range of cSlimatcs : 0.4-2.0 x 1011 gP/y) [atmosphcrK: contribution negligible] IHydrothcrmal ridge circulation ncgligibleJ

(> 90 0;1, of downward nUlI)

• I:stimatcd Slcad)'·stalc vallics bascd on data from Froclich rf pl. (1981) ; Baturin . 1978: Morse in UlIrncu and Shl:ldon. 197':01.

mainly a diagenelic process. Upwelling of reJal tvely nutrient-ric h decp waters le'l(.ls to production of organie maller in near-surface waters and the increased downward flux of organic maller results in accumulation of organic carbon-rich sediment ; Ihe release of phosphorus from this organic mltller by oxidation and D.1cterial processes. IInd to sorne extenl the phosphate suppl ied by dissolution of phosphatic ri~h IInd other bone debris. inlO the sediment pore waters causes phospha lization , either by replaceme nt of carbonate. by replacement of fecal pellels, or by precipi­talion of çarbonate fluorapalite within the sediment or nt the sedimentJwater interface. The required conditions for pho~­phalization are therefore a fairly continuous suppl Y of phosphorus to neM-surface waters . the effective burial of orgllnic m:llter in sedimenh. and the regeneration of suffi­cienl phosphorus from organi c mlltter in pore wllters tO aJlow precipitation of phosphate minerais at or near the sedimcnt-water interface. Howe ver. we also ac~nowled.l;e that ~ome ancient phosphorites may he e ither primary predpLt!lle~. such as the oolit lc phosphorites of part of the Phosphoria Formation (Sheldon. 1980 b), or replacemenh of caroonale sedime nts in certain regions. ~uc h as on sellmount s or b ... U1~s within the oxygen-minimum zone arcas Ihllt were perhaps bathed in unusual!y phosphate-rich water~.

THE PHOSPHOR ITE PROBLEM ; A CR IT IQUE OF PRE VIOUS MODELS

Other researchers have recognized that a large increa~e in the abundance of phosphate. a~ apparently represented by the Permilln Phosphoria Formiliion of the western United States. mil;ht repre~ent a significtLnl increase in the oçeanie phosphate concentration (e.g. Sheldon , 1980 a : Senior, 1980). The estimated mll~~ of phosphoru~ (P) in the econo­mie deposils of the Phosphoria Formation (7 x 10" g) rcprescnts as muçh as tha t estimllted to enter the oceans from land-derived fluvial supply in ca. 004 x 10~ yrs (see Table 1) and COnlrlins nearly 10 times riS muçh P as in the modern oce:.n. If Ihe Meade Pea~ r-,'Iember of the Phospho· fia Formation represents deposition over about 4 m.y.

86

(WnrdllLw. pers. comm .. 1980) over ca. 0.2 % of the sea-floor, then more than 5 % of the s teady-sta le fl uvial P flux would have been stored there ilnnuaJly (this amounl equals about la % of the annual riverine flux eSlimaled by Burnell and Oas. 1979). At present, much less tha n about 10 % of lLnnual P input to the oçellns from rivers (based on s teady-state considerations) is s tored annually in sedimen­ta ry phosphale deposits ail over the world (sec Fig. 1 ; Froclich et al., 1981). ClelLrly then , Phosphoria time was anomulous in lerms of the marine phosphorus cycle. Il is emphas ized that Ihcse calcu lation~ do nOlla~e into acçount the amount of P contained in the noneconomic parts of the Phosphoria Formation or other age-equivalent rocks.

Previous models may weil account for some of the variation in the mass of sed imentary phosphate deposited through geologic lime . We are intrigued with the suggestion of Piper and Codbpoti (1975) that the former presence of widespread anoxie oceanic deep-waler masses could have Icd 10 increa­~cd rates of nitrate reduction and Ihe con~equcnt rcmo\al from the entire system of large amounts of combined nitrogen (nitrale) necesSllry for phyto- and lOoplllnkton produclivity : this. il is hypothesized - given thal biological uptake of N and Pis in Il constant rat io (the ' Rcdfield ratio' on aVeT<Lge C:N: P " 106:16:1 - would lead to eleva\ed phosphate concentrations and precipitation of marine apa­tite . Much of the phosphate would hllve to he Încorporated in the hard le~ts of the phytoplan~ton in order to reach the sediment. We find two problems with thi~ hypothesis; 1) Ihere is II relalively poor correlation between inferred 'ocea nic anoxic events' in the Meso7.oic IInd limes of known economic pho~phate dcposits as we discus~ helow (Fig. 2) ; IInd. 2) al present. pho~phoritc deposition IIppears to be closel)' rclatcd to production of organic matter and ib burial in sediments: thus any decfea~e in the flux of orl!anic matter 10 sediments. suc h as thal proposed by Piper and Codispoli (1975) would result in a decrellsed suppl Y of phosphorus as weil. and an increase in the OJlygen conce n­tration in deep-water masses. thereby decreasing the rate of nitrate reduction and reducing organic matter preservation. thus inhihiting phosphorite formation. Transport of pho~· phate to sediment via ~ ~e l clal matter i~ nOI an efficient mechanism s ince much of thi ~ material is not TlLpidly

rege nerated to liberate phosphate. Only if inorganic phos­phorite formation occurred as in the Kazakov (1937) hypo­thesis after buildup of dissolved phosphate in the oceans to sorne criticallevel. would the 'nitra te-reduction' model lead to more widespread fo rmation of phosphatic deposits . Consideration of apatite solubil ity in relation 10 the che mis­try of seawater (pMticularly inhibition by Mgl ' ) suggests thnt inorganic precipilalion is e"ceedingly difficuh , even al supersaturation (Atlas, 1979: in Sheldon, Burnett : discus­sio n in Bentor , 1980 : Burnet!. 1980). Thercfore, we beJieve that although Ihe ·nitrate·reduction · mode! describes a mechanism for increasing the phosphate conlent of the oceans it does nOI e"plain the sedimentary associations of phosphorites in the geologic record .

Sheldon (1980 b), fol lowing a suggestion of Berger and Roth (1975) ltnd Fischer ,lnd Arthur (1977), proposed lin 'episodic circula tion' model in whieh generation of major phosphorite deposits is re la ted to the residence lime of occanic dcep waler. He suggeSIS Ihat deep-waler overturn might he more sluggish during warm dimatic episodes, thus allowing deepwater phosphate contents to inerease grodually. Shel­don's contention is that abrupt climulic-changes, specifi· ca ll y mHjor gl01>.11 cooling, would resull in more rapid deep-water turnover which would s timulate org,lnic produc­li vil y and lead to the ex traction of the large resid ual phosphate buildup inlo phosphori te deposits. This is a plausible mecha nism, but there arc problems with this fairly simple modeJ. It dues not seem to work, for e"ample, at the Eocene/Oligocene transition which appears to be a time of a major readjustment of deep-water c ircula tion (Benson. 1975; Shackleton, Kennet!, 1975: Berggren, Hollister, 1977 : Moore el Cil .. 1978): there are no known major Oligocene phosphorite deposÎls. Nor dues the model e"plain the apparent episodicity of phosphorile formation during the Mesozoic when Ihere were no major climat ic perturba­tions and no s ignifica nt glacial epochs. Another problem with the 'episodic circulation' model is thm at present there arc about 9 x 1016 gl> in the deep-water rescrvoir and sol ubility co nsiderations sugges t Ihal this is neur the point of saturation wit h respect 10 apatite (Sheldon, 1980 b). Il is certainl y not likely that the ocean has mainl.lÎned ~ I deep-w,lIer reservoir of more Ihan 2 or] times that amoun!. which is slili ] times less than that contained in the economic inleryals of the Phosphoria Formation. lt is, furthetmore. unlikely that the ocean could be entirely s tripped of ilS phosphate.

An idea of Ihe ma"imum excursion of the deep-sc,1 dissol­ved phosphate content during the Cenozoic may be provi­ded by examinai ion o f SuC values of total dissolved carbon in the deep sea as recorded by bcnthic fOrllminifers of various ages . These values will only upproximl'llely repre­Sent Ihe deep-sea total dissolved carbon SIlC composition for a number of reasons whieh will nOI bc de,llt with here. A compilation of average beOlhic fomminifer SuC values for the Cenozoic (sec Arthur . 1981) suggeSIS that deep-water tOlal dissolved carbon (TOC) was never much more nega­tive than about 1 %~ which is only about 1 ~ lighter than todays average TOC. Deep-water phosphate concentrations and tillC v\ilues of 10t,11 dissolved carbon in the samc deep-water mass are negatively correlated (e.g. Broecker. 1974). CO~ is added to deeper water masses during organic carbon o"idation. The So C values of organic CMbun is much more negative (average - 23 %,,) tha n thal of t01ll1 dissolved carbon. As organic matter is o/l idi zed then , both phosphate and isotopically light earbon arc added to the water mass. The correspo ndence between phosphate and the SIJC of the

87

PHOSPHORITES AND PALEOCEANOGRAPHY

TOC of average deep waler is such thlll an average PO~l conce ntration of _about 2.2 .... M!kg corresponds to a 51 C val ue of about 0 %o.. Assuming thal the average TOC in the deep sea does not change much through time and that organism~ generalt)' fix phosphorus to carbon in the Red­field ralio . then al %.. change in the deep-water TOC corresponds to uoout a 1 .... M/kg change in the PO': concentration. If the record of 51lC flu ctuations given by an average bcnth ie SuC signal is val id , then we may say that. a t le:..s t during Ihe Cenozoic , the deep-waler phosphale concentral ion did not vary by more than a fa ctor of two,

These arguments muke the 'episod ic circulation' model an unlikely e"planation for ail widespread phosphorite depo­s its. Another problem is Ihul it is not certain that rates of deep-water turnover would necessarily decrease during times of warm, equable climate, At these times masses of dense saline water, deri ved from wide shelves in low la ti tudes or from res lric ted low-Iatitude basins, might sl ide basinward, st imulat ing rapid deep-water turnover.

1t is therefore suggested that most pre vious models have been too simplistic . This paper discusses the factors which we think are Ihe most importanl in consideration of long,term and short-term changes in the marine phosphorus cycle, These are: 1) the expanse of rela t ively sh!lllow seas which is some fun ction of global sea-level changes and continental hypsometry: and 2) glObal d imate which eonlrol s weathering rates and the input of phosphorus 10 the oceans. Both o f these parameters vary in time. bUI do nOI neeessarily vary together. They are also superimposed on the tableau of drihing continents which is an anolher sign ifieant factor (Cook, McElhi nny . 1979). We ha ve anempted to evaluate Ihesc pammeters for the late Meso­zoic,Cenozoie where the best record of changes in cl imale . oceanography, and sedimentation is preserved, and show how this mode! explains other phosphlilic 'anomalies' in the geologie record. The emphasis is on e"plaining the lime relations of large deposits, but olher types or phosphate mineraliZc:ltion such as those of chalk hardgrounds (Ken· nedy , Garrison, 1975 : Jarvis. 1980) are also bricfly conside­red , We fi rs t describe the relationship bctween major 'phosphate events' or what we will term ' phosphori te giants' in the conte"l of Ihe paleoceanographic record . We ne"t describe our modcl ; it is complex. but we believe that the interaction of several parameters best explains the variations in the marine phosphate cycle.

PHOSPHOR ITE 'G!A NT S' AND l'ALEOCEANO­G RAPHY

The dis tribution of ' phosphorite giants' of late Mesozoic­Cenozoic age (Cook, McElhinny, 1979) is shown in Figures 2 and ]. The compilation shows no important phosphorites formed in the Triassie, a mid-lurassic increase in phosphorite formation, a major episode during the L,I le Jurassic-earlies t Cretaeeous, a re lative low in the Eurl y to mid-Cretaceous followed by a major ulte Cretaceous­l'nleocene ' phosphori tc giant' , a dearth of Oligocene phos· phorite, ,1 major Miocene peak in phosphori te accumula­tion, and a significant deeline in for.mation through the Plio-Pleistoceoc. We believe, as do Cook and McElhinny, th,ll these varia tions in amount of eeonomic phosphorite deposits through t ime lire s ignifieanl age trends relatcd to flu ctuations in the conditions that favor phosphorite accu­mulation. Let us cons ider varia tions in a number of paleo­cea nographic parmnetcrs in order to see how they nlight bear on the formation of 'phosphorite giants·.

M.A. ARTHUR, H.C.JENKYNS

Sea le\'cl

The main Lalc Mesozoic~Cenozoic 'phosphorite giants' arc labelled as follows (Fig. 2 and 3) : episode 1 (ccntered on Oxfordian-Callovian at aboul149 m.y. BP); episode 2 (cen­tered on the Cretaceous-Tcrtiary boundary a t approxima­te ly 65 m.y. BP with two diSl inct peaks centered on Santonian-Campanian, about 77 m.y. BP, and Lale-Paleo­cene-Early Eocene at about 54 m. y. BP); and episode 3 (Early to carly Late Miocene, centered on about 14 m.y. BP). The major PMt of each episode lasted perhaps 30 m.y. or so with the exception of episode 3 which was relatively short-lLved at perhaps 20 m.y. durat ion.

Each of the phosphorogenic episodes can be shown to have occurred during a period of relative highstand of sea-Ievel (Fig. 2 and 3; based on Vail et al., 1977; Hancock. Kauffman. 1979 ; and Hallam, 1978). The correlation is good but not perfect. Episode 1 apparently began during a period of r ising sea level during the Callovian and ended probably by Berriasian-Valanginian lime with a lowstand. The Early to middle Cretaceous sea-Ievel Tise (Barremian through Turonian) was apparently not accompanied by major phosphorite deposition, although phosphatic levels occur locally in European chalks (e.g. Kennedy, Garrison, 1975). Episode 2, which began during the Coniacian­Santonian but peaked during the Campanian, corresponded to the highest Cretaceous stands of sea level. A decrease in the amoun! of economic phosphorite of carly Paleocene age appare ntly corresponded to a lowering of sea level a l the end of the Maestrichtian. The relatively high sea levels of the Late Paleoce ne-Middle Eocene were ag:lin in phase with fo rmation of ' phosphorite giants'. The end of episode 2 occurred during the mid to Late Eocene, but this does not appear to have corresponded precisely to a global sea-Ievel drop. A sea-Ievel fall appare ntl y occurred in the latest Eocene but the major drop occurred in mid-Oligocene time. The entire Oligocene is chllraclerized by a lack of signifieant economic phosphorile deposits. Episode 3 occurred in conjunction with rising sea level ; the peak of this epi~ode nearl y coi ncided with the middle Miocene highstand. Falling or rapidly fluctuating sea level characterizes the more phosphoritc-poor P!iocene-Pleistocene. Burnett (1980) has shown for the Peru-Chile margin that even though the relative amounts of phosphorite formed during the Pleisto­cene are low, the periods of formation coincide weU with interglacial sea level highstands. Although therc arc periods of relatively high sea level during the Triassic, there are no known major phosphorites of that age. The exception that cannot be ignored is the Permian 'phosphorite gian!' formed during a period of extremely low sea level (Vail et (II .. 1977). However, the main phosphal ic members of the Phosphoria Formation may have formed during relatively brief trans­gressions within this prcdominantly regressive time (McKel­vey el al .. 1959).

Oceanlc ano ... 1c 1'\'l'nls

The 'phosphorite gian!' episodes do not correspond weil with so-caJJed 'oceanic ano;.;ic events ' (OAE ·s of Schlanger. J enkyns. 1976). These times of better preservation and more rapid accumulation of orga nic matter in deep marine set­tings (Fig. 2 and 3 ; after Arthur. 1981), related to times of apparent widespread deep- and intermediate-water oxygen deficits. are apparently not necessarily conducivc to forma­tion of major phosphate deposits. A possible Toarcian

88

(Early Jurassie) OAE (Jenk yns, 1980) does not correlate wilh a major . phosphorite giant' episode although some phosphorite deposils of that general age are known. Episode 1 does not correspond to a well-developed OAE. The major OAE's of the Early to middle Cretaccous arc also not accompanied by major phosphorile deposition. A less widespread (mainly epicontinental sea and deeper Tethyan­Atlantic-Caribbean) ConÎacian-Sanlonian OAE was not entirely in phase wilh the carly peak in episode 2, and the main part of the OAE precedes the main part of episode 2. The Late Paleocene-middle Eocene portion of episode 2 was accompanied by an increase in accumulation ra tes of organic carbon in marine basins aecord;ng to a survey of DSDP s ites (Fig. 3). The Oligocene was a time of very low accumulat ion rates of organic carbon and no major phos­phorite accumulations. Episode 3 corresponds to wiJcs­pread more or less shallow marine deposits (slopes and shallow basins) ric h in organic carbon which were apparen­tly deposited within wel1-developed oxygen-minimum zones (e. g. see Jenkyns. 1978 ; Anhur, 1979). There was also a slight increase in the global deep-sea accumulation rate of organic carbon al this ti me. Pliocene and Pleistocene time brought gready increased rates of accum ulation of organie carbon in deep-sea setlings appare ntly without the develop­ment of widespread low-oxygen conditions or phosphorites. ln this case the preservation of relatively more organic matter is probably relaled 10 greatly increased sedimenta­tion rates of terrigenous det ritus in the ocean basins because of increased glacial erosion on land . The ' phosphorite gian!" episodes have only a moderate correlation with the average carbon isotope composition (SllC) of coeval pelagie carbo­na tes. which in part must refleCI the relat ionship to carbon cyeling and OA E's (e.g. Berger. 1977 ; Fischer, Arthur, 1977; Scholie. Arthur . 1980 ; Vincent el al., 1980). The ' phosphori te giants ' tend 10 have corresponded 10 more positive SllC values in pelagie carbonates.

The 'phosphorite giaots· are commonly associated with 'black shales ' in a given depositional seuing (e.g. Piper. Codispoti, , 975). However, widespread organic carbon-rich sediments formed during OAE's were not always associa ted in time or space with major phosphorite deposition. 'Phos­phorite giants'. in fact , tend to occur near the beginning or end of an O AE.

Climate

Climate is another variable that seems in some way to bear on Ihe genesis of 'phosphorite gianu·. The major episodes genera!!y correspond to times of warm. equable global climate (as also pointed out by Slansky, 1980). This correla­tion works weil for the Cenozoic, but changes in climate arone cannot explain the variability of phosphori te deposi­tion during the warm. equable Mesozoic when overa ll climatic varia tions were apparenlly minor (e.g. Hallam. 1975). Colder, possibly dry intervals were, however. cha­racterized by a lack of major phosphori te deposits as shown in Figure 3.

Carbonate IIccumuhlliOIl lInd fluctuat iolls ln Ihe CCV

Accumulation of carbonate in the deep sea (Fig. 3 ; after Worsley. Davies, 1981; compilation of DSDP re~u1t~

through Leg 44, biased loward~ the ocean floor below 3 km depth ; Whitman, Davies, 1979) is :mOlher parameter which

does not show n conSistent relalionship to ' phosphorite giants'. al leas t in the Cenozoic where the most adequate data exis\. However. in gener"l. relatively high carbonate accumulation rates occurred in (he deep,sea during non-phosphorogenic periods suc h as the Oligocene and Pleistocene and probllbly during the Early Cretaceous (pre-middle-Aptinn) as weil. The middle Cretaceous was ch:lTacterized by low phosphorite and low deep-sea carbo­nate accumulat ion rates. A dip in the global rate of accumulation of deep-sea carbonate also occurred in the Early Paleocene. corresponding with a possible decrease in phosphorite accumulation during the middle of episode 2. Relativel y low nues of accumulat ion o f carbonate in the deep sea correlate with format ion of 'phosphorite gianu·. with the exception of the Early to Middle Eocene which was chnractcrized by an anomalously high carbonate accu mula­tion ra te.

Fluctulltions of the Curbonate Compenslnion Depth (CCO) through time ure now weil established . In the Cenozoic li

s hallow CCO corresponded 10 formation or 'phosphorite gianls ' whereas li relat ively deep CCD pre vailed at other times . The Cre taceous. however, was characterized by a relatively shallow eco Ihroughout . except possibly during the Late Maestrichti lln lmd during pre-Aptian time.

Association Viith sllieeous sedlmerllS

Biogenic siliceous marine deposits ttppc:ar to have becn more widespread during deposition of ' phosphorite giants' (e.g" Fig . 4). Although the cri tical quantitative data remain unavaiJable. there !lppear to have been interval s of increa­sed nlte and area of accumulation of radiolari!!n. sponge. and diatom-rich stra ta during the Early to Middle Eocene (possibly beginning in the Late Paleocene) and the Early to early Laie Miocene (e .g .. LeClaire . 1974; Fisc her. Anhur. 1977; Leinen. 1979; Ramsay. 1977). These periocJs corres­pond to part of 'phosphori te giant' episodes 2 and 3. The relatively phosphate-poor intcrvals of rhe Early Paleocene. the Oligocene. and Plio-Pleistocene are marked by apparent

Fi~ure " Sfr/Jfl/(r"phir: and palto~(>t>:rophic stl finll of phosphatk facÎU in North A fric/J . A) $ frllf i/traphy of Morocc/Jn Mueslrich fion-Paleogtne dtpos;ls (mo(lified a/ter HouifJ. 1980 ; global sta Ifll·tl a/m V/lii et al. . 1977). NIJ/e Ille 8('1I'ral /lS.fllCiafio" of Ille rie/Iut phosphorile btds k,itlr /n", .fell-Iere/ Jf(mds. probu-bly nl" ... sellli"lt re"'orkill fl 0/ pre,'ioruly-deposifeol pIIOS-plWlic u olimenU. PJwsp/ul/i(' beus occur ",aillly rll rela/i"ely pIHJrl)'-Q,(idiud marly wll/s chllrl/Cleri. ed by /lbundlltll /lSsociaud hifJ/ll'nlc j·ilic/J. Sedimenta/ion r/llts "'Ut /0'" /hroughouf deposilion of Ihe sequttrce. bUI illc,ellSeû greully during deposilion of LUltfiUII cluhon/J/es "'hich art onl)' sUghl/y phosp/ra/ic. 1. foui/ilerous. slighlly f(} 1I01l-p /rosplrulic relati"ely shol_ IOk'-..-mu limes /one 1. ",Im/lrily morl)'. sl/ir:eQlts biOKelli(- Illdes J. p/WSII/wllc bcds : /1) > 28 /'/0 , . b) < 28 Çf 1'10, 4. primllfily marls /o llc. c l/lys/onc . IInd I(If SllndSIUlre. 0) PlIleogeoll,aphy of pari 01 nor/lrerll Iremüplrere ffJr Yprulan /imt (afttr Fi, s/brool!. et al .. 1979) k';/Ir Sle" ..,ali­u d d,'Slribution 01 land and seo and ""slulaled surface cu ,nnfs (after Riech. O'on Rad. 1979) 1. hypo/hl'lical surlacl' currl'II' P/JU'''' 2. DSDP silu ..-ilh 10 ... ·" 10 middl .. Eoctnl' biolle"ic sil/ctQUS facÎU J. pruen/-da)' con/Îne ,,/(II lIutlillt 4. gemlmliu d LO"'tf Eocell .. plrosplru/e dellosi/s 5. foell/ imr (Jf "," jar U'k'U El/cclle p lrospllatt dfposi/ $.

89

PHOSPHOAITES AND PALEOCEANOGAAPHY

decreases in Ihe area and rates of accumulat ion of biogc:nic s iliceous malerial. The Cretaceous. however. shows a relatively poor correlation belween phosphorite and sili­ceous sediments. Dc:ep-sea cherts and Jess altered sil iceous biogenic sediments are common in pre-middle Aplian pela­gic sequences and in Upper Albian rhrough Conacilln­Santon;an deposits. The Campanian-Maestricht ian pelagie sequences contain rehrtive!y less biogenic si liceous mate­rial. Upper Jurussic Tethyan pelagic sequences are chamc­terized by widespread radiolarian chert horizons; howe ver. it is not yel clear whelher such s iliceous rocks exist on Atlantic and Pacific deep-sea floors . although Jurassic rlldiolari tes are pre valent in the Franciscan Complex of California. Obviously there is not a perfeci correla tion betwecn ' phosphorite giants' and siliceous biogenic sedi­ments in the Upper Mesozoic.

Inrerplay of variables

So fa r this d iscussion hus considered mainly the relatio nshi p belwee n deposition of phosphate and the pelagie deep-sea record. The possible correlations ha ve been considered ont: at a time - an arl ificial Wlly of examining the interaction between paleocea nography and sediment,lIion . In aCluali ly there is a general interdependence of many of the variables considered (see Berger. 1977: Fisc her. Arthur. 1977; Arthur. 1979; and references therein). For example. warm dimale generalJy accompanies relnti vely high sea level. . Anoxie events' occur during Ihese limes as weil. Sea level and the nrea of shallow seas also. in part, determines the overalJ patterns of sedimentat ion , large[y via so-calJed shelf-basin fract ionalion (e.g .. Berger . Winterer . 1974 : HlIy. Sou tham . 1977 ; Worsley, OllVies. 1979) . These two fac tors are especialJy pertinent for carbonate accumulat ion as one expc:cts deep-sea accumulat ion rates 10 increase dunng low sea-Ievel a nd decrease during high s tands in which lar!,ocr carbonate plat(orms andlor extensive shallow c halk seas exis!. The presence of abundant carbonate -rich shalJow seas may also he rerJected in lm ele valed CC O in the

,-. ~ . ,

•

,Cl •• .---: , 0

•

M. A. ARTHUR. H.C.JENKYNS

world ocean. However. as <;hown in Figures 2 and 3 :lnd discussed above. there :Ire exceptions (0 many of t he~e generalizations. We di~cuss in the next section the possible reasons for the correlations, bot h positive and negative, and present li somewhat complex but, we hope. real istic model to explain the apparent variations in phosphorite ~edimenta­tion throogh time.

A RATIQNALE FOR ' PHOSI'HOR ITE G IANTS'

T he mech:ln isms fo r variation in phosphori te sedimentation and widespread phosphatizal ion through time can be sepa­mled into firSI- and second-order effee ts (see Table 2). In addition 10 conlinental position ably argued by Cook and MeElhin ny (1979). li first-o rder control o n the occanic phosphate cycle is sea level (Fig. 2: and 3). This is beca use re lative sea level controls suppl y of phosphale to Ihe ocean and secondly beciluse il de termines. 10 a major degree, suitabJe s ites for fo rmation of phosphate-rich dcposits

Phospha tic deposits are interpreted a~ having been formed under a genera l1 y Jow rate of sedimentation (particularly low det rÎ tal influx), a high rate of suppl y of phosphate to surface waters through sorne sort of upweJ1i ng, and a consequent large sopply o f phosphate-beari ng organic malte r to the sediment. Thus. the common model of upwel1i ng zones a lo ng westward-facing coa~ts in the low-Iatitude Trade Wind Belts has evolved (e.g. Sheldon. 1980 a. h ; Cook. McEl hi nny, 1979). The coasts of ancien! east-west seaways in equatorial zones were also common sites of phosphorite sed ime ntation. as revealed by paJeocontinenlal reconstruc­t ions (e.g. Sheldon. 1964 : 1980 (1. b : Cook. McElhinny, 1979). Veeh el 01. (1973). Ilurnell (1977 and 1980), and Piper and Codispot i (1975) amonl: othe!s have suggested a rellllion bclween oxygen·minimum zones impingi ng on lhe continen­tal slope a nd sites of phosphorite fo rmation. This rcla­tion~hip implies thal the low-oxygen conditions are neces­sary for the preservation of large amounls of organic carbon : this carTÎes in organic pho~phoru~ compou nd~

Table 2

which can be liberaled in pore waters by microbial oxidation of the orgaoic malter eventually 10 form interstitial diagene­tic phosphate (usually carbonate fl uorapalite) within the sed ime nt. If too much organic matter is oxidized in tra nsit throogh the water column. the necessary phosphorus is losl to dceper water masses or made avai lable to Ihe biomll~~. Much solid phosphate (fi sh debris and other vertebrale skeletal matter), which is al50 often a major ~ource of phosphate in economic pho~phorite deposits. might lilso bc redissolved and either become available for inoro:anic rixa­tion as carbonate floorapatile or diffuse out of sedimenl~ if pore wa ters a re not near to satura tion with re specl to carbonate f) uompatite . Skeletal phosphate is known to dissolve extensively in most oxidized or dY~llerobic liedi­ments (De Vries. 1979). but is often well prese rved in anoxic environments . soch as in the most intense oxygen deficib wilhin the oxygen-mi nimum zone . Therefore. the distance from productive surface waters to the sed ime nVwater interface and the oXYgen levels in the water col um n Ihrough which the organic matter must faU is crilical. Sholkowitz (1973) has demonstrated rnpid phosphate depletion in org:l­nÎC matter falling through the water column and al the sediment water inle rf:lce where the water is cven only s lightly oxygenated. Milliman (1977) has pointed out the same effect in a brief comparison of the presen t-day Namibiiln coastal area (e .l!:. Walvis Bay) of South Afri ca with thal of the Spanish Sahara (NW Arrica). According 10

his dala the rate of upwelling (fe rtil ity) and producti vi ty is s imilar at both places - c lassiclll upwelling coaSIS and candida tes fo r format ion of phosphorite deposits. However , much more org'lnic carbon is apparenlly buried in the Nambian coastal region because the high productivi ty is centered over a broad. ~ha llow shelf: the shelf off Spanish Sahara is very narrow, and the major opwell ing area is localed over Ihe slope . Thi~ causes the organic maller 10 fa ll

through a deeper column of relat ively oxygenated water . and Ihere is more recycling of C, N. and P within the water column . Relatively less orga nic matter reaches slope sed i­ment, and thlll which dnes fall is depleled in phosphoru~ . T hus o nly in the region around W .. lvis Bay lire recenl

Main tltmen/s QI'ht • J>hosplwrilt Gionl. mQdtl

Fi .... 1 order eff.::.::u

Co"/III(',,,al posi/ioll Ma jor phosphorilc dcpo$ih occur along ooaSlS favorably orienled for signirlCant up",elling and high surface f.::rtili ty (equl'Iorial and "'ebl ... ·.,rd-fHcing COaSl$ .... ilhin trade wind bcIIS).

EXIt'" 01 shâl5tU5 Influences siles for phosphorite deposi lÎon 1"msg,tss.on (a) may bring upper part of Ol minimum zone inlo

shelf. (b) fa"ors nUlr;ent rccycling and C-org burial on socl,·cs and high l'lIle of P dctivcry to sedimcnl$. (c) favon lropping of u:rrigcnou$ sediment neanhorc and 10 .... sedimcntalion l'lIles offsh<Jre .

Ik8Tt'~i,m (a) following or ",ilhin a tmnsgTCssivc .::pisod.:: il allo .... ~ rcworl.:ing and concenlration of phosphalic sediment 10 foon eoonomic dcposiu. (b) increase arca of ... ·"othcring and erosÎOn and allo .... s '::"enlual ,,,turn of sorne fixcd P 10 oceilns. (e) dcnics major marin", ",nvironrnenlal siles for phos· phorite dcpo~ition.

90

Second ordcr cffcclS

RUlts 01 ""unal P qding

(a) RaItS o/octanjc ol·t,mm.- highcr rates encouragc morc rap;d J' cycling 10 surface ""l'ters : changing raIes may. in l'lIn . inOucnce cpisodic phosphorilc dcposilion. (b) Dup-wafu P COIIC(·lIlralio".- elcvalcd concc:ntrulion may incrcase amount availablc for fixation in pho~phu t i e sediments. AClUal varialions may bc limi,.;:d by a factor of 2 ovcr Ih" concenlration of P in QCcuns tOOa y. (e) Clulnges 01 P flux from Ultmal SOIUUS.- incrcased rivcrinc nl.lX may .::nhanee phosphale fixat ion in scdimcnl5. Vo1canÎc Oux is presumed minor.

Global ClitMlL (a) \Vtl>l~r.n& II>/e : .... arm. humid, equabJc global climalc ma)' favor more inlense .... eathering and increascd P supply 10 oceans. (b) Ocean C.rc"lalioll und dup ... attr oxY&e" dtficils; expansion and inten5irlCat;on of ollygen dertcil5 in dec:p-walcr masses .ppears relat.::d 10 warmer, more equuble elimatcs. TheS!: arc gcnerally conditions more favorable \0 ph05phorite gcncsis.

phosphorites being gc neraled (Baturin. Bezruhkov. 1979 : Bireh, 1980). Surface productivity in offs hore Peru is about twice Ihat of Walvis Bay and offshore Spanish Sahara surface produclivity. and presumably the suppl y of phos· phate to surface waters is about twice as high as weil. Off Peru Ihe oxygen-minimum zone is highly developed. abun­dant orsunic matter is preserved on the s lope (especially belween about 400-600 m below sea level). and phosphorite nodules form on the slope in Ihal region (BurneH. 1977). The elevated produclivily offshore Peru is presumably part of the reason for the difference bclween the Spanish Sahara s lope lInd the Peru s lope .

Very low oxygen conditions, such as those fou nd in the mids t of an o.l:ygen-minim um zone. also tend to e liminate burrowing benthic macro·organisms (e.g. Rhoads. Morse . 1971 : Cal ver t. 1964). Laminated carbonaceous muds asso· ciated wit h phosphorites are frequent in ancient deposits . and we suspect Ihat the lack of ' irrigating' activities of <ln infauna lI110ws buildup of phosphate in porc waters over Ihal of biolurbated zones (e.g. Goldhaber I!I 01., 1977 ; Aller. 1980). It is likely Ihat major phosphorile formation is favored in regions of fluc tuating redox conditions.

T he Influence of sea le,-el

Rela tive seu level direetly influences phosphorile deposi­tion , in so far liS il correlales with transgression and regressio n. Given favorable continental hypsometry. high stanu s fa yor 'phosphorite gianls' by flooding ayailable shelves, and a llowinS more upwelling localities to be eenle­red over shelf seas. Thus. the organic maller and skeletal phollphale must fall o nly a short distance through the water column. suffering relatively Jess degradalion th:tn if upwel· ling was centered over the s lope alone. Ouring transgression terrigenouil sediment is lrapped in estuaries and nearshore regions, Ihus preventing extensive dilution of organic­carbon rieh chemient sediments on the outer shelf by detrilal mllterial (Hay, Soulham , 1977). This same phenomenon might also lead 10 localized phosphorus recyding in shallow seas by trapping the nUlrient·bearing runoff from rivers as weil liS from upwelled dissolved phosphale in highly produc. tive inner·shelf regions. Finally. rill ing sell ievei mighl also rtllow the oxygen-minimum zone tO impinge on the outer parts of !<hel ves and exlend inlO epieont inenl:ll seas (Fis·

Figure .5 ElftclS of ua·lntl ("htmgts ail phosphaft dtfXJsilion on gtncrally IIJ .... la/U"J" shtl .·ts (su al.fll G/lrrison CI al .. 1981). A ) TrmlSgrtssion or gtntral sta-ll ,· .. 1 JrIR/rstlllul. NUle UPlllrded Il:cygtn.mi,linrrwr lont mrd gntJIer /lftll of /o ... ·.,:cygetr ... mu III/US impinging 1111 a !'ooded .(he/f. 1. ZIJ/ltS of IIIOSI inun.n! Jiagenrtic pllQsp/roritt furmutillll {transI/limaI Eh (lU condirimu. su luI) 2. Oxygtn·minimlllll zone a) 0: > 0.5 < I.U mfll b) 0 1 < 0.5 "'",

,. .----------•

J. Lumilltl/ed bluck shale and sili("toUJ b/ugenir: !ar:it s 4. Currtnl·rt"·IJrktd phospllorilt dtposits

.--8) Rtgnuicm or stu·lt.·e/lowsland. No/t impinJ,'t'mtnl of o.l:»;tn .lllini".,,,,,, tOlit onl)" on "pp" slopt. milll" ptnCCOII. ItmpOralltOlI.f phosphorilt dtpIJsi/s. bul possiblt mQjllr teononlie dt pmi/l' Iormtd by ... ;nno ... ing QI Qlder phtJJpJru­Iir: ma/trial in shallo ... • .. ·ultr tnVirOllllllll1J C) Re/illion.fhip bit ... ..,,, ... a/tr·/IIiloU IIXygtlllllioll and h"r_ ro .. 'lng !wmils (aflt r CI/II'trt. 1964) in muds rl("h ln IJI'/(unic CM/1U1i mrd hioJtcrric si/kil. Tllflslrold lor lJiologle (jetj" ity j.f III I/bout 0.5 mlll 0 1.

,­-----.

91

PHOSPHORITES AND PALEOCEANQGRAPHV

cher . Arthur. 1977; Jenk ynll. 1980), thereby aiding in initial preservation of organic matter and phosphate in sediments (Fig. 5).

Burncn (1980) has noled the concentration of phosphatic material as fine.grained diagenelic apatile and as phosphalic crUSIS mllinly near the top and bottom boundaries o f the present oxysen-minimum zone. Studies of ancient deposils. such as Ihe Upper Crelaceous and Paleocene phosphorites of North Africa. suggest an association of 'blac k shates' (organie-carbon rich sediment) with phosphorites (e.s. Gar­rison el al .• in press; Slansky, 1980). However, the mOSI organic.carbon rich beds do not coincide wilh those of high phosphale content. In facl. from study of dinoflagellate cYSt preservation (Fauconnier. Slansky. 1979) and of the charae­ter of amorphous m,trine organie matter (Belayouni. Tri­chel, 1980) in the Eocene phosphates of Tunisia. it appears that very s ligh t ly oxidizing environments favor Ihe greales t mobili zalion and redeposition of phosphate. CYIII preserva­tion is poorer and organic matter has a higher oxygen inde ,.; and lowcr hydrogen index in the phosphate beds. whereas interbcdded more organic-carbon rich bcds show Ihe oppo· sile effects. Thus, il is likely Ihat phosphorite formalion is favored in slightly o,.;ygenated environments (or alternllling anoxic-slightly oxie zones) perhaps near the 10p or bottom of an oxygen·minimum zone (Fig. 5) where pH and Eh thresholds obtain (Vee h el al .. 1973 : Manheim et al.. 1975 : Ilurnelt. 1980). These will bc points at which drastic changes in Ihe solubility of carbonale fluorll palite will occur, and where suppl y of phosphate from both upward­diffusing pore waler and from within Ihe waler column will be s ignifican\. Thus, transgressive seas favor initial format ion of phospho­rite deposils. while regressions generally do not: thm is, mueh phosphate may bc fixed in shelf sedimenlS. bUI nOI necessarily concenlrated inlo economic intervals during transgressions. Sea Jevel changes hll ve one (urther effect : 5helf phosphales formed durins hishs tands have a good chance of beinS reworked during drops in sea level (Fig. 4 and 5). whereas slope phosphorites do not . Therefore many 'phosphorite giants', Ihal now appellr ,.s reworked conglo­merates of phosphatic debris muy hllVe bec n initial1y formed during transgressions as organic-rieh . phosphalic muds th:lI were periodicall y winnowed and concenlmted during one or more marine retreats during and fo tlowing the transgres-

"Uv, .. ,-." , . ~ ... ... _.,.,

.--_"C-' ___ _

M. A. ARTHUR , H. C. JENKYN$

s ion. This method of enrichment has been ~ugges ted for Namibian phosphori te deposits which were reworked during Pleistocene sea-Ievel drops during glacials (Baturin. 1971). and for the Florida 'bone'-phosphates (Riggs. 1980) among others (Coole 1967 ; 1976 ; Garrison el al .. in press). Lacally. however . ~ helf bouom waler concent raI ions of pho~phll1e may bu ild up 10 the point that primary precipita­tion of carbonate fluorapalile oceurs. purticularly where other phosphatic nudei are exposed on the sea floor. This process could expia in oolitic textures found in sorne ancient phosphoriles.

T he inlluence or globll l di mate

As we described in a previous section. sea-Ievel variations cannOI explaio the cntire piclure of Mesozoic-Cenozoic ' phosphorite giants·. although it i~ certai nly a first-orde r factor . Global climale seems also 10 have a strong effect on phosphorite sedimentation . Times of more equable. warm climates seem 10 correlate weil with formation of 'phospho­rite gian ts·. as weil as with relative highstands of sea level. We propose that globally Warm climates promote pho~pho­rite sediment!ltion in a t least two ways: 1) by leading to an increase in chemical weathering on land and a con!>equent increase in suppl Y of phosphorus to the ocean. and 2) by infJuencing the format ion. oxygen content. and circulation of water masses thereby leading , in part. to the development of more imense oxygen-minimum zones which are known 10 be llssoc ia ted with si tes of recent phosphorite fo rmation.

The amount and dis tribution of rainfall was probably much different during times when gIOb.1! climate WliS warmer and more equable. Warm. humid climates were undoubledl y more widespread latitudinally during. for example, the LlIIe Cretaceous. the Laie Paleocene- Early Eocene. and the Elirl y 10 Middle Miocene (Savin. 1977: Savi n et al .• 1975 : see also Frakes. 1979). Higher average global temperature is most likely accompanied by higher evaporation rates over the oceans and higher average rainfall on the continents (see Holll\lld. 1978. p. 56-nI). The combination of higher tempe­rature und moisture most Jjkely means increused chemical weatheri ng rates. especially at higher latitudes . depend ing o n vegetation cover. For example. oxysols (approaching laterites) of probable Lower Eocene age have been found at a nomalously high latitudes in North Americ., (Abbott et al .. 1976 : Singer , Nkedi-Kizza. 1980). A similar increase in intensity of welHhering occurred in the Southeastern United State~ during the Early to Middle Miocene (see Frakes, 1979 . fo r review). Wc propose that thi s phenomenon is imporlant for the phosphorus as weil as other geoc hemical cycles. During ~uch times of increased c hemical wealhering (relmive 10 today) , particularly that of widespread limes­tone . a larger suppl y of dissol ved phosphorus . silica. calcium. magnesium . iron . and bicarbonate ions among others is proba bly carr ied to the ocean via rivers . Thus , in terms of suppl Y the oceans were possibly more al kaline and more fertile during cJ imatic optima such as during the Late Paleocene-Elirl y Eocene. This increase in suppl Y would a llowa greater rate of sedimentat ion of phosphates. organic carbon. biogenic and inorganic silica. a nd carbonme in marine settings. Similar arguments were made by LeClaire (1974) tu explain Late Cretaceous and Eocene expa nsions of s il k a producti vi ty. Lei us cons ider the later part of " phosphori te gi;;nt" episode 2. in the Paleocene-Eocene. Figure 4 shows the s tratigraphic ~e tting . general sediment accumulation rates. and phos phatk zones in the "phosphori te giant" provi nce represented by shallow-marine basins of north Africa during

92

the Paleocene. We may compare these relationships with the changes in deep-sea accumulation rates lInd generalized global s trat igraphy of deep-sea sediment s shown in Figure 3. The moSI intense period of phosphorite genesis in the Late Paleocene-Early Eocene coi ncided with the Plileo­cene cJimatic optimum. It also coincidC'd wit h widespre;ld transgressive shelf-sea milfls and muds rich in bilUminous matter as weil as with maximum rates of accumulation of organic carbon in the deep-sea. Biogen ic sitica (now mostl y cherI) is also an important component of Upper Paleocene­Lower (a nd Middle) Eocene shel ' and deep marine deposils (Slansky, 1980: see papcrs in BGRM. Doç. 24. 1980). New formed silica-rich minerais are also extremely common. particularly zeoli tes and montmorilloni te (LeClai re. 1974 : Slansky . 1980). Many of these are magllesium-rich: in addi tion. dolomite is a common const ituent of carbonale sequences . This sugges ts an enhanced suppl y of Mgl

•

whkh at the same time was being extracted into Mg-rich mineraI phases. The associa tion of Mg-rich phases and phosphorile deposits is a common one (8entor, 1980). Rcmoval of Mgl ' inlo other phases . including initial adsor­ptio n on biogenic opal (DonneUy, Merill. 1977). may be necessary in order to generate phosphatic sediments beeause the presence of Mgl~ has a deleterious effeet on phosphate equilibria (Martens. Harris. 1970 : Atlas. Pytko­wÎçz. 1977 : Atlas. 1979 ; cf. Surrleu . 1980). This Mgl '

remova! is <lpp,uentl y an important process . !l nd it is now thought Ihat dolomite can precipitate in anoxie sed iments following removal of sulfa te via bacterial sulfate reduction and formation of metal sulfides (Suess. 1979 ; Kelts , in press: Baker, Kas tner. 1980). This is yel .. nother facet of the anoxia-phosphori te associa tion. The rock assemblages described above differ greml y from those of the Paleoce ne and the La te Eocene-Oligocene . C" rMnate seùimentation peaked in the Middle Eocene (Lutetian) both on shelves and in deep-sea seuings follo­wing the main phase of phosphorite form ation. The anoma­lously high rates of açcumulation of carbonate in bot h settings suggests a gremer than normal supply of bicarbo­nate and calcium tu the oceans. With conSla nt input of Ca~~ and HC01 ~ one would actually expeci a decrease in deep-sea accumulation rates to accompany an increase in the area of carbonate sedimentation in ~helf seas such as occurred in the Early Eocene (e.g. , so-called shelf-basin fractiona tion ; Worsley . Davies. 1981). Wc bel ieve that ail of these relations suggest a link between glObal dimate, ra tes of weathering on land . and suppl y of chem ical element s to the sea. Cooling duri ng the La te Eocene and Oligocene . espec iall y :1I high latitudes, apparently led to more ar id conditions a nd reduced weathering and erosion (Berger. 1979 : Davies et al., 1977). Warmingin the Early 10 Middle Miocene ag'lin led to increased chemical we!lthering and greater flux of phosphorus and silica 10 the oceans. In lIddi tion. relatively IitfJe phosph!lte was deposited as rock phosphate from the Late Eocene Ihrough the Oligocene. a period of about 16 to 18 m.y. During much ofthi s time large areas of the shelves were exposed to weathering and erosion. At least sorne of the phosphate deposÎted during cpisode 2 was expo~ed. possibly redissolved and returned to the sea. This could have led to a graduaI increase in oceanic phosphate conlent untiJ the Miocene episode 3. The combi­nation of the undepleted oceanÎc and the weathering sources during the Miocene climatic optimum and tran sgression made thi ~ an extremely fertile pho~phate-rich period . The "e pisodic-circuJation " modeJ may also partly account for the change in pattern~ of phosphorile sedimenttltion begin­ning in Miocene time. as it appears thm coas tal and

equatorial upwelling were intensified (Dieste r-Hliass, Schrader, 1979: Leinen , 1979), perhaps related to hÎgh-la ti­tude cooling, as argued by Sheldon (1980 a).

An addendum to the weatheri ng model Îs that phosphate tends to form relati vely insoluble complexes with iron. manganese and alumÎnum (e.g., Sturnm , Morgan, 1970). sa that oxysols might tend to hold a subs tanti lil amounl of phosphate. However. when these a re e roded and washed into low oxygen-high s il iea envÎronments on flooded shelves where iron and manganese are reduced and aluminum enlers into s ilicate minerai equilibrium. phosphate would be libera­ted to shelf waters (e.g. Patrick et al .• 1973). Th is is a further mechanism for replenÎshing phosphate to Ihe deple­ted oceans during and after a " phosphorite s iant" episode, and parlicularly for enriching estuarine and inner shel f waters with phosphate.

T here IS a tendcncy for warm global cl imate to coincide with oxygen-deficiencies in oceanic deep-water masses_ For example. the oxygen-minimum zones might expand and intensif y (Sc hlanger, Jenk yns. 1976; Fischer. Arthur. 1977). This oxygen depletion may. in part , be due to the increased fertility and production of organie matter in surface waters. Il is also probably due to the decrease in solubil ity of oxygen with inereas ing temperature (and sali­nit y) of seawater (Kinsman et al .. 1973; Berger. 1979). Thus, the oxygen demand is probably higher and available d issolved oxygen lower during genera ll y warm, eq uable global climates, so areas of mari ne oxygen-deficits beeome more widespread. This deficieney a ids in the preservation of organie matter and phosphate on their way 10. and on, the seaflooL The accumulat ion rates of organic carbon for the LaIe Paleocene through Early Eocene interval show the possible effects of the generally lower dissolved oxygen levels (Fig_ 3). However, the Miocene Episode differs gready because the cold deep-water sphere had developed by then (Savin et tll .. 1975 ~ Shac kl eton, Kennett . 1975). deep circulat ion may have been more rapid. dissolved oxygen levels higher , and therefore deep-sea accumulation rates of organic carbon lower than that during the Eocene (but higher than Oligocene rates). The main effects of the Miocene episo<le were probabJy seen as oxygen-deficiencies in midwater masses (ex panded Oz-minimum zones) which impinged on the outer shelf a nd middle to upper s lope. An example is the circum-Pacific Mioccne conti nental margin deposits rieh in organic carbon such as the Monterey Formation and equivalen ts (e.g. Jenkyns, 1978). T he pre-Oligocene deep circulation was probably very different from that of the Neogene, and was largely domi nated by salinity struc ture in the absence of temperature-induced density dirrerences (Brass et al .. 19!11)_

Although warm climates, flooded sheJves, und marine oxygen-defic iency closely coincide , the periods of most inte nse decp-water oxygen deplet ion. or OAE 's . do not seem to be eonsislen tly accompanied by depos it ion of " phosphorite giants"_ We propose that this is beca use under very widcsprcad anoxie conditions. such as during the Early to mid-Cre taceous. organie malte r is preserved over a much larger area of both the deep and sha llow sea-f[ ooL This effectively diminishes the amount of phos­phate avai l:lble 10 any one a rea. Therefore , ahhough phos­phatie crusts, hardgrounds. nod ules, replacements. and skeletal material might be common to these inter vals (e.g. Kennedy. Garrison, 1975: Jarvis, 1980) . economic deposits a re rare or llbsent. Pe rhllps, also, the excessive area of shallow seas and numerous avai lable sites of fiX'ltion did not a llow concentra tion of phosphate at a few specifie loca-

93

PHOSPHORITES AND PALEOCEANOG RAPHY

t io ns : hence subs ta nt ial deposits could not be produced. ln addi t ion. if conditions were extremely anoxie. preservation of organie malter eould have bee n enhanced in many areas to the point that insufficient phosphate was liberated to pore waters for fo rmation of s igni fi cant phosphate minerais. Thus, we favor times of wax ing and waning of deeper-wlLter oxygen defieits as candidates for " phosphorite giants'· . just as precipitation of ca rbonate fluorapatite is probabl y favo­red in areas of fl uctuating redox potential, such as at Ihe margi ns of the 0 2-mi nimum zone.

CONCLUSIONS

It has been shown that there is no o ne-to-one correlat ion between the major episodes of phosphate deposit ion and any s ingle variable such as cl imate , oeeanic circulat ion patte rns. sea level, ano xie events, or cont inental pos ition , at least for the LaIe Mesozoic-Cenozoic. Instead , the se para­meters together determine the suppl y of phospha le to the ocean and its rate of extraction 10 the sediment reservoir. These fa ctors are shown in Table 2 and Figure 5.

Major phosphori te deposits do not necessarily resul! whe ne­ver continents a re situated favorably_ The paucity of Pleistocene a nd Holocene phosphorlte attesls 10 this. However . il is probably true tha t cont inental position is important in re lation to o ther facto rs, and given thal favorable s ites a re available for phosphorite formation . ot her variables must come into play. Most important of these is the relationship between sea level. cont inental hypsometry and tectonies in eontrolling the area of epiconti­nental seas, in part expounded by Riggs (1980)_ This determines the availabili ty of sites for production, preserva­tion and buriaJ of organic matter incl uding phosphorus. Low sea-Ievel s tands limit the a rea of slope under the oxygen· minimum zone and increase the vertical distunce thlLt o rganie matler produced in surface waters must fall through the water column. Therefore, ail o ther Ihings being equal. more organic matter would be oxidized in the water col umn before burial and the flux of phosphorus to the sediment would be lowered _ High sea level and greater expanse of shelf and epieontinental seas provides li greate r area for org.mic matte r preserva tion and phosphate depositÎon on the outer shelf and s lope. a shorter dista nce and . in sorne regions, a Jess-weil oxygenated water col umn for organie matter in transit to the bollom. and . fu rthermore, a te n· dency to lock-up the nutr ient fl ux to the oceans from land in est uaries and on the shelves and upper slope. Thus trans­gressions favor more widespread deposition of major phos­phorltc depos ils while lowstands a re less likely to produce widespread phosphorite horizons. If. however, a pcriod of relatively high sea level coincides with major "oceanie anox ic events" , sueh as ocçurred in the Early to midd Je Cretaceous. the widespread oxygen deficiencies in oceanie deep water masses would lead to preservation of organic matte r over a much greater a rea of the sea floor . thereby possibly diminishing the amount of organic matter and related phosphate available at any one s ite. At these times wc might expect more numerous small phosphate deposi ts, but much of the phosphorous might remain locked up in buried organic matter. In the same vein, during low sea-Ievel sla nds there may still bc one or more ideal senings for phosphorite formation_ Any o ne of these, over a period of a few million years, might extract all "excess" oceanie phosphate available ; we envis ion the Pcrmian Phosphoria SC,I in Western North America as an example of such an flnomaly.

M. A. ARTHUR, H. C. JENKYNS

Alrhough a rise in the CCD and a consequent decrease in rhe area of carbonate sedimentation on deep-sea fl oors mighr release some phosphorus from the deep-water carbonate s ink (e. g. Bentor, 1980), it is likely that the phosphorus would be transferred either 10 Ihe shaJlow-waler carbonale s ink. since an e levated CCD generally occurs with high sea-level stands. andlor more phosphorus would be absor­beJ on Ihe greater exp'lnse of red clays or metalliferous clays below the CCD (e.g. Berner. 1973: Froelich ell/I .. 1977). Thus, it is difficult to envis ion an entirely carbonate control of phosphate depos ilion.

The aforementioned controls notwithslanding. the amount of phosphate being sequeslered in phosphori te deposits at an y given time is more or less a balance belween input and output. Therefore any coorrol s on input would also act to limit the system. Since Ihe amount of phosphorus supplied to the ocean Ihrough weathering and fluvial transport allows u net storage of phosphorus in marine sediments. any variation in Ihis flux would have the greatest long-term effect on the amount of phosphorite deposited over a given time interva!. Climate (temperature and rainfall/humidity) , in part cOntrols ra ies of wealhering. Thus, during limes of more equable global cl imate the inçreused temperatures and probably increased rainfall would lead 10 greater rates of chemical wearhering, especially al low but eXlending to higher latitudes. The late Paleocene-middle Eocene and E,\Tly to Midd le Miocene global c1 imates. for example. were charaçterizcd by spread of warm. humid cond ilions to higher latitudes and more intense lateritic weatheri ng to higher paleolati tudc. Thcsc wcre probably rimes of greater ehemical flux through ri vers to the ocean. and there is cvidence during Ihese limes for much higher th:\O ilverage rates of acc umulation o f carbonate and s ilica in the deep-sea and on shelves and also for concomitant accumulation of major phosphorire dcposits. T hese were also tmles of relalively high se~ -levcl st'lOd s. Here the s igni fieance of continental posit ion becomes app~rent. If continents arc c1ustered in lower latiludes a greater suppl Y of phosphate through chemical weathering might be expected . Howe ver, rhe existence of large ·· ~upcrcontinents·· , such as Pangaea during the Permo-Triassic might also allow for greilter continental aridit y and internai drainage sys tems. During these times a lower flu vial phosphorous flu x to oceans might oblain. Thi s, in part , might explain the pau,;it y of Triassic phosphorites, ahhough it is d ifficult , ;n this regilrd, to account for the Permian phosphoritcs . More widespread Tethyan shallow-warcr Iimestones may have actcd as the main s ink (or P during the Triass ie.

The lack of exact correlation of "phosphorite giilnt s" with any one or even s e ver~1 factor s is undoubtedly due to Ihe comple x inlerplay of these variables. In addition wc expect some time lags in, for example , the response of phosphorus inpui by weathering during climate change and ilS cycl ing through the ocean and biOla. This may amoun! 10 several mjJIion years. Furthermore. many major phosphorite depo­sits are mechanical concentrates . for example the Pliocene phosphates of Florida. Though much of the phosphilte minerais and other phosphat;c biogenie de bris were proba-

REFERE NCES

Aboon P. L., Minch J. A. , Peterson G. I .. , 1976. Pre· Eocene p:.lt:o~oJ ~o ut h of Tijuan:. . Baja California. Mexico. J. Sedimettr. Pet .• 46. 355-361.

94

bly formed during il stage of relatively high sea le vel in o ne of our "phosphorite giant" epochs _ the /"-"! iocene -concentratÎon of thi s materiil l is of younger age a nd relaled 10 winnowing and sorting during a lowstand of l'ca level. Thus the majorit y of major phol> phorite deposit l> are proba­bly meehanical concentrates reJated lO'relati ve seil Je vel changes that occurred during and afler the main episodc of phosphalization .

We have attempted , in our mode], to account for the major variability in phosphorite deposilion Ihrough time , at teasi in the late Phanerozoic. There arc, of course , other plausible models fo r the genes is of phosph~te deposits . such as those resulling from bird gUilno or from phosph~t i zation of hialUs surfaces (see Sheldon , Burnett, 1980, for outline of possible models). Vet , we are impressed by the greal flu c tuations in volume of "economie" deposits through time. In our model we have not considered possible hydrothermal or volcilnie flux of phosphate to the oceilns (e.g. Riggs, 1980) bec~use this flux is very poorly known and is probably minOT (Baturin, 1978 ; see Table 1). Global changes in rates of sea-floor spreild ing may also be a faclOr in the phosphorus cycle beciluse Ihe area of metalliferous clays probably varies direclly :IS u fu nction of spreading rates . and becll\JSe these clays are a n important s ink for P. However. if there is a relat ion between seil level ilnd spreading rate. as proposed by numerous researchers (e.g. Pitman , 1978; Vogt , 1979) , Ihen we doubt Ihal past varia tions in Ihe area of metallife­rous clilys could have been sign ificant, because ·· phospho­rite gianls" orten occur with trilnsgression limes of propo­sed as fast spreading ilnd increased ilreil of metalliferous clays. The sea-floor spread ing rate and hydrothermal circu­lation through the mid-ocean ridge might be important 10 the phosphorus cycle in anolher way , however. Mg l

' is selecti­vely scavenged during this process , thereby possibly lowe­ring the Mg2• concenlralion in the oceans and the inhibition of carbonate f1uorapatite precipitation . We have not consi­dered possible varial ions in Ihe relUrn flux of phosphale from sediment pore waters as important, since th is is probably rclilted 10 the silme factors thar dctermine ··phos­phorite gianC' episodes. ObvÎously, wc hilve only outlined Ihe general patterns of global phosphilte sedimentilt ion here. Much more detailed sedimenlological, geochemical , and paleoenvironmental s iudy of ancient phos phorites is cer­tilinly necessary.

Acknowledg.-mcnts

An earlier draft of this manuscripl was reviewed by W. H. Berger , W. C. Burnett. R. E. Garrison. E. K. Milughan. J . T . Parrish, R. P. Sheldon. ilnd H . R. Thierstein . Wc appreciate the many helpful comments and c riticisms they provided us and Ihe enthu­s iasm with which they attacked our manuscript. We also thank C. R. Wenkam who carefully Teild and criticized an earlier draft. Sl imulating disc uss ions on the sed imentology of the phosphatic sediments and the geochemis lry of phosphates with W. H. Berger. W. C. Burnett. P. N. Froc­lich. R. E. Garri son. S. R. Riggs. and R. P. Sheldon are grateful ly acknowledged.

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