18. PHOSPHATE CONTENT OF SEDIMENTS FROM DEEP-SEA SITES 259 TO 263,EASTERN INDIAN OCEAN1

Peter J. Cook, Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia

ABSTRACTPhosphate determinations carried out on material from Sites 259

to 263, in the Eastern Indian Ocean, revealed the presence of severalphosphate maxima, particularly in the Mesozoic. A thin high-gradephosphorite was intercepted at Site 259. In general, the Cenozoic cal-careous oozes are more phosphatic than the Mesozoic non-calcareous sediments. Abundance of biogenic (generally calcareous)material has a significant effect on the amount of phosphate present.Variation in the rate of sedimentation is also believed to be impor-tant in determining the phosphate content of sediments. Some phos-phate peaks may be associated with important time breaks.

INTRODUCTIONPhosphorus is one of the most important nutrients

found in the oceans. Its abundance in the water columnis reflected both in the phosphatic and biotic content ofthe bottom sediments. Despite its importance, little sys-tematic work has previously been undertaken on thephosphorus content of sediments obtained as part of theDeep Sea Drilling Project. In order to help remedy thisdeficiency, a sample was taken for phosphate analysisfrom every core-section obtained from the five holesdrilled on Leg 27 (Figure 1).

A total of 680 samples was dried, crushed, and colori-metrically analyzed for phosphate at the AustralianMineral Development Laboratories, using the molyb-denum blue method .

RESULTSThe phosphate values from the five sites are shown in

Figures 2 and 3. The P2O5 content is generally in therange of 700 to 1500 ppm, but is less than 200 ppm in afew samples. The maximum value was 22.4% P2O5 atSite 259, at the top of sedimentary Unit IV. Mean phos-phorus contents for Mesozoic and Cenozoic sedimentsare summarized in Table 1. If the single high value of22.4% is excluded, the P2O5 content of the Mesozoicsediments is similar at each site and averages 1000 ppm.The P2O5 content of the Cenozoic sediments showsgreater variation between holes, and the mean P2O5value of 1530 ppm is somewhat higher than the averageMesozoic P2O5 content.

Repeat analyses on 10 samples indicated a precisionof ±3.8%. Accuracy is claimed by the laboratory to beabout ±5%, and the results reported here comparedwith values obtained by El Wakeel and Riley (1961) ondeep-sea sediments and support this level of accuracy. Itshould perhaps be pointed out that the sampling tech-nique involved taking a small plug of material, with adiameter of approximately 2.5 cm, every 1.5 meters.

100

CUV I ERABYSSAL

4. PLAIN

WALLABY f~4

^-~y PLATEAUS

- 30°

1 Published with the permission of the Director, Bureau of MineralResources, Geology and Geophysics, Canberra, Australia.

• Leg 27 d r i l l sites~—2—-BathYMETRic contours (thousand m)

Figure 1. Locality map.

Consequently, despite the fact that a large number ofsamples were obtained, only a small percentage of thetotal sequence was in fact sampled. It is thereforeprobable that some phosphate peaks were missed.

The sample containing 22.4% P2O5 was examined indetail. X-ray diffraction analysis revealed that it is com-posed primarily of carbonate fluorapatite. In thinsection it consists, for the most part, of a structurelessgroundmass of cryptocrystalline collophane. Darkcollophane pellets showing sharp margins, and "blebs"with diffuse margins, are scattered throughout (Figure4). It is uncertain whether the darker coloration of thepellets is due to staining by organic material or by ironoxides. Some poorly defined apatite hexagons arepresent in vugs. Small, high-relief cubic crystals are alsopresent in vugs; they could not be positively identified in

455

ON

Figure 2. Vertical variation in phosphorite content of the Mesozoic-Cenozoic sediments encountered at Sites 259, 260, 261, and 263.

PHOSPHATE CONTENT OF SEDIMENTS, SITES 259 TO 263

DEPTH

METERS

2 0 0

262UNIT AGE

Ooze —

2

PLEISTOCENE

PpO•= CONTENT (ppm)460Q

For reftrenct St* figure 2

Figure 3. Vertical variation in phosphate con-tent of the Plio-Pleistocene sediments en-countered at Site 262.

thin section but may be fluorite. Phosphatized frag-ments of coccoliths, radiolarian spines, and ?fora-miniferal fragments are scattered sparsely throughoutthe collophane groundmass. A small patch of ?glau-conite is present; there are also scattered silt-size quartzgrains, some showing marginal replacement by apatite.

TABLE 1P-Oc Content of Leg 27 and Other Pelagic Sediments

Site

259260261262263

Mean Value

Mean ^>fi>c content

of pelagic sediments

Phosphate Content(ppm P2O5)

Mesozoic

928a

9851127

9601001

Calcareous

Argillaceous

Siliceous

Oceanic average

Cenozoic

13953047903

1338974

1530

1550

1400

2700

1600

Excluding the highly phosphatic sample containingc If this sample is included mean Mesozoic

value for Site 259 is 2410 ppm P 2 °bAfter El Wakeel and Riley (1961).

Figure 4. Photomicrographs of the phosphorite from Site262. (Sample 17-4, 29-31 cm), (a) Rounded phosphatepellet with well-defined margins. Ordinary light. Scale re-presents 0.10 mm. (b) Poorly defined phosphate "bleb"with a diffuse margin. Ordinary light. Scale represents0.15 mm. (c) Structureless mass ofcollophane with someincluded microfossils. Scale represents 4.0 mm.

457

P. J. COOK

With the exception of fish fragments, no otherexample of apatite or collophane was noted in thinsections or smear mounts or detected by X-raydiffraction.

DISCUSSIONThe phosphate content of sediments is influenced

considerably by the phosphate content of the adjacentwaters, whether oceanic or pore water. Other importantcontrolling factors include the rate of sedimentation, thebiota, and the lithology of the sediments. In somecircumstances the phosphate content of a sedimentcomes primarily from allochthonous apatite and collo-phane reworked from some older unit, such as Adams etal. (1961) have described from the Cenozoic of Mary-land. However, it is unlikely that allochthonous sourcesof phosphate make any important contribution toabyssal zone sediments, under most circumstances.Much discussion on the geochemistry of phosphorus insediments, and particularly phosphorites, has revolvedaround the question of whether the phosphate is a pri-mary precipitate or a secondary replacement deposit(see Kazakov, 1937; Dietz et al., 1942; Emigh, 1958).Most of these arguments have little relevance to the Leg27 sediments which are not phosphorites, with thenotable exception of the sediment on the Unit Ill/UnitIV boundary at Site 259. This phosphorite, which is de-scribed above, is lithologically unlike the well-knownpelletal phosphorites of, for instance, the PhosphoriaFormation of the western United States. It is somewhatsimilar to the collophane mudstones or microsphoritesof Riggs and Freas (1965) and also has some features incommon with phoscretes (Cook, 1972). All these fine-grained phosphorites are believed to have formed undervery shallow water or in some circumstances, subaerialconditions, but it is unlikely that the fine-grained phos-phorite at Site 259 formed under such conditions. Inthin section there is evidence that the phosphorite is, inpart, a replacement deposit. The nature of the sedimentbefore phosphatization is uncertain. Included fossilfragments (Figure 4) suggest it was in part calcareous;however, both siliceous and argillaceous sediments alsoundergo phosphatization under marine conditions. Thedepth of seawater at the time of deposition is unknown,but it was probably considerably shallower than thepresent-day water depth. Phosphatization of sedimentscan take place in water with a low P2O5 concentration ifthe rate of deposition is sufficiently slow. Conse-quently, it may not be necessary to invoke high phos-phate values in the water, such as those encounteredduring upwelling, to account for the Site 259 phos-phorite. It seems probable that this phosphorite formedby early diagenetic phosphatization of calcareous andargillaceous sediments, at a time when a slow rate ofdeposition prevailed. Although there is only one phos-phorite known from the Leg 27 drill holes, there arenevertheless marked vertical variations in the phos-phate content in the drill holes.

The background phosphate level is higher in theCenozoic calcareous oozes than in the Mesozoic sili-ceous and argillaceous clays. It is likely that thisCenozoic/Mesozoic difference results primarily fromthe change in lithology between the sediments of these

two eras. Foraminifera and other organisms, which con-tribute a significant percentage of material to the oozes,incorporate some phosphate in their tests as a result ofmetabolic processes. In addition, calcareous material isreadily phosphatized at the sediment-water interfaceeven in aqueous solutions with a comparatively lowphosphate content (Ames, 1959). As a result of these pri-mary and early diagenetic processes, the Cenozoic cal-careous sediments are enriched in phosphate relative tothe predominantly noncalcareous Mesozoic sediments.

There is also some evidence of late diageneticphosphatization within the Cenozoic column, partic-ularly at Site 262. This and other diagenetic features ofSite 262 are discussed by the writer in a subsequentchapter in this volume. It appears that the downwardincrease in phosphate content at Site 262 (Figure 3) maybe a function of late-stage diagenetic phosphatizationprocesses, possibly associated with dolomitization. Thewriter has previously found that phosphatization may insome circumstances be related to dolomitization in theshallow marine environment. It appears that this samecorrelation may hold in deep-sea sediments of the TimorTrough, with the dolomite content as determined by X-ray diffraction showing a good positive correlation withthe P2O5 content. There is also strong evidence linkinghigh P2O5 values in interstitial waters with a high alka-linity, though the reason for this interrelationship is notknown.

Figure 2 shows that there are a number of phosphatepeaks within the Mesozoic (Jurassic-Cretaceous) se-quence. At all four sites where Mesozoic rocks werepenetrated, the sediments are uniform clays and clay-stones. Phosphate maxima do not appear to be relatedto an influx of calcareous beds. Also the magnitude ofthe peaks is considerably in excess of that which wouldresult solely from the influx of calcareous ooze. Somepeaks may represent a fortuitous event such as the incor-poration of fragments of whale bones or shark teeth intothe abyssal zone sediments. Late diagenesis is unlikely toproduce peaks of the type evident in Figure 2, butwould, instead, give a regular vertical trend of increas-ing (e.g., Figure 3) or, in some circumstances, de-creasing phosphate content. Sheldon (1964) has pointedout that paleolatitude is an important limiting param-eter for the formation of phosphorites.

Latitudinal changes in response to sea-floor spread-ing could conceivably produce a change in the phos-phate content of abyssal zone sediments. Figure 5 showsthe change in phosphate content in deep Indian Oceanwaters but the changes are small. Similarly, changes inbathymetry could conceivably alter the phosphate con-tent of sediments. It is apparent from Figure 6 that thereis a phosphate maximum in the water column at about1000 meters, but below this depth the phosphate rapidlystabilizes at about 2.7 µg atom P/1. Again the mag-nitude of the phosphate changes likely to be induced bybathymetric variation appears to be insufficient toaccount for the observed phosphate maxima in the sedi-ment column.

Oceanic upwelling is believed to be an importantmechanism for the localization and concentration ofphosphate (Kazakov, 1937; Baturin et al., 1972). Upwel-ling has the effect of bringing cold nutrient-rich water tothe surface, particularly on the west coast of continents,

458

120° 100°

H Phosphorus abundance at 2000m depth in mgm atoms / m 5

Equatorial Scale

0 1000 2000 3000 4000 5000 8000 7000 8000 km1 1 1 I I I I I I

Figure 5. Phosphorus content of Indian Ocean waters at a depth of 2000 meters (after Redfield, 1958).

- 30 *

- 48° zHWHOTI

WO

W

Ito

P. J. COOK

P content («.g αtoms/l )2

1000 -

2000

3000 -

4000 -

5000 -

Figure 6. Vertical distribution of phosphorus in the IndianOcean (after Bruneau et al, 1953). Station location4°47'S, 88°18'E.

e.g., off Peru. In some areas, it is a comparativelyshallow process with, for instance, upwelling only affect-ing the top 200 meters of the ocean off Southern Cali-fornia. It is evident from Figure 6 that upwelling fromdeep ocean water (particularly of that below 500 m)would result in the influx of phosphate-rich waters intothe euphotic zone. Intermixing of water below 500meters will have no appreciable effect because of theessentially uniform phosphate profile below that depth.Even upwelling into the shallow euphotic zone canproduce higher phosphate levels in abyssal sedimentsbecause of an increased influx of dead biogenic mate-rial. However, at least part of the Mesozoic section isbelieved to be of comparatively shallow-water origin,where localized, fairly near-shore upwelling may havehad some influence on the phosphate content of sedi-ments, particularly a high-grade phosphorite such asthat encountered at Site 259.

It is likely that some upwelling occurs off the westAustralian coast because of its position on the west sideof a continent. However, Rochford (1967) points outthat the upwelling is relatively minor and does not occureast of about long 110°E. It is possible that some up-welling has occurred at various times since the devel-opment of the Indian Ocean in the Late Jurassic-EarlyCretaceous. This is supported by the presence of Cre-taceous phosphorites in the Toolangi Calcilutite in theonshore section of the Carnarvon Basin.

It is unnecessary to invoke an upwelling origin for allphosphatic concentrations, for such concentrations alsocommonly occur on unconformity and disconformitysurfaces in shallow marine sediments. In the absence ofsubaerial weathering, this hiatus-phosphate associationresults primarily from the lack of sediments whichwould otherwise dilute the phosphatic sediments for-ming at, or just below, the sediment-water interface. AtSite 259, a prominent phosphate peak is located on theAlbian-Paleocene hiatus, a time break of about 35 m.y.An additional phosphate peak is located on the markedlithologic change between Units III and IV, which mayalso represent a hiatus. It is uncertain whether a hiatuscan be invoked for other phosphate peaks as the chrono-stratigraphic resolution is generally insufficient to allowcorrelations between phosphate peaks and discon-formities. However, the lack of a reasonable alternativemakes a hiatus-phosphate association the most attrac-tive hypothesis at this stage.

CONCLUSIONS1. Considerable variation in phosphate content

occurs in the sediments encountered in Leg 27 drilling; athin Cretaceous phosphorite is present at Site 259.

2. The Cenozoic calcareous oozes are more phos-phatic than the Mesozoic argillaceous and siliceousclays. This difference is believed to result primarily fromthe abundance of biogenic material in the Cenozoicsequence.

3. Some diagenetic phosphatization of calcareoussediments, possibly associated with dolomitization, mayhave taken place in the Cenozoic sediments at Site 262.

4. Several prominent phosphate maxima are presentin the Mesozoic sequence. Although some couldperhaps be related to oceanic upwelling, there is no evi-dence for this. There is evidence that at least some of thepeaks may be related to major time breaks within theMesozoic. Most of the phosphate peaks may ultimatelyprove to be associated with disconformity surfaces.

REFERENCESAdams, J. K., Groot, J. J., and Hiller, N. W., 1961. Phos-

phatic pebbles from the Brightsea Formation of Mary-land: J. Sediment. Petrol, v. 31, p. 546.

Ames, L. L., Jr., 1959. The genesis of carbonate apatites:Econ. Geol., v. 54, p. 29-41.

Baturin, G. N., Merkulova, K. I., and Chalov, P. I., 1972.Radiometric evidence for recent formation of phosphaticnodules in marine shelf sediments: Marine Geol., v. 13, p.37.

Bruneau, L., Jerlov, N. G., and Koczy, F. F., 1953. Physicaland chemical methods, Appendix Table 1: Swedish Deep-Sea Exped. Rept., v. 3, p. 99.

Cook, P. J., 1972. Petrology and geochemistry of the phos-phate deposits of northwest Queensland: Econ. Geol., v.67, p. 1193.

Dietz, R. S., Emery, K. O., and Shepard, F. P., 1942. Phos-phorite deposits on the sea floor off southern California:Geol. Soc. Am. Bull., v. 53, p. 815.

El Wakeel, S. K. and Riley, J. P., 1961. Chemical and minera-logical studies of deep-sea sediments: Geochim.Cosmochim. Acta, v. 25, p. 110.

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PHOSPHATE CONTENT OF SEDIMENTS, SITES 259 TO 263

Emigh, G. D., 1958. The petrography, mineralogy and origin Riggs, S. R. and Freas, D. H., 1965. Stratigraphy and sedi-of phosphate pellets in the Phosphoria Formation: Idaho mentation of phosphate in the central Florida phosphateBur. Mines Pamphlet 114. district: Am. Inst. Min. Eng. Ann. Mtg., Section H84.

v \ A \/ i n n TU u u u c • Λ .L Rochford, D. J., 1967. The phosphate levels of the majorKazakov, A. V., 1937. The phosphorite facies and the genesis c ' e ., T J• J; A . , * ×* -

e , , •' , . •V r- i r^ n ^ T J surface currents of the Indian Ocean: Australian J. Mar.of phosphorites: Internatl. Geol. Cong., 17th, Leningrad, p. c , . D 1O ,gc e v Freshwater Res., v. 18, p. 1.

Sheldon, R. P., 1964. Palaeolatitudinal and palaeogeographicRedfield, A. C , 1958. The biological control of chemical distribution of phosphorite: U.S. Geol. Surv. Prof. Paper

factors in the environment: Am. J. Sci., v. 56, p. 205. 501-C, p. 106.

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