V

ELSEVIER Marine Geology 139 (1997) 201-217

MARINE GEOLÖGY

lHrERN4TIC"L JOURN4L OF MARINE GEOLOGY. GEDCHEMISTRY ØND GEOPHYSICS

Phosphorus pathways in atolls: interstitial nutrient pool, cyanobacterial accumulation and Carbonate-Fluoro-Apatite

(CFA) precipitation F ougerie C. Jehl b, J. Trichet /.

a Oceanographie, ORSTOM, BP 529 Papeete, Tahiti, French Polyiiesia Laboratoire HEA, ORSTOM, 911 Averiire Agropolis, BP 5045, 34032 Monipellìer cedex I, Frairce

Laboratoire de Géologie de la Matière Organique, Uiziversitè d'Orléans, U R A CNRS 724, B. P. 6759, 45067 Orlému cedex 2, France

Received 13 March 1995; received in revised form 15 January 1996; accepted 19 January 1996

Abstract

Significant dissolved phosphate concentrations were measured in reef interstitial waters and in small brackish ponds (free water and underlying interstitial water) occupying the emerged rim of the Tuamotu atolls, French Polynesia. These ponds are the sites of accumulation of organic matter of pure microbial origin, in the form of thick stromatolitic cyanobacterial mats called kupara by Tuamotu natives. The profiles of dissolved phosphate interstitial waters seem to show a positive trend with depth. Within kopara, the phosphorus pathway after biological uptake is studied by measuring the C and P concentrations in raw material and in humic substances (humin, fulvic and humic acids) in order to examine the liberation of P from organic matter. For comparison, the same analysis is led on the residual organic matter of phosphate samples from Mataiva deposit (Tuamotu). The results show a relative enrichment in phosphorus with age of kopara, and a similar distribution of humic substances in kopara and phosphate samples. Other work on hydrocarbons from kopara and phosphate samples demonstrates the strong relationship of these materials. If such evolution of organic matter can be observed in kopara ponds, it should be possible to generalize to closed atoll-lagoons which function as mega kopara ponds and where phosphorus-rich layers are present (e.g., the Niau atoll). The different steps leading to atoll phosphogenesis, viewed as phosphorus sink in cyanobacterial mats to final massive CFA accumulation, could then be rationally explained. The interstitial nutrient pool can be a necessary and sufficient factor to provide phosphorus for the kopara growth and accumulation. O Elsevier Science B.V. All rights reserved

Keyvords: Interstitial nutrients; Cyanobacterial mats; Apatite precipitation

1. Introduction have long ago subsided under the sea. Today, the reef rims of these atolls encircle their central lagoons and maintain a delicate outcropping against the oceanic swell, thanks to the high gross productivity of their algo-coral communities, which fix 1-5 g C/m2/day and 10-20 g Caco,/ m2/day (Hatcher7 1990). Conversely7 the primary productivity of the Tropical South Pacific Ocean

Open-ocean atolls such as those forming the Tuamotu Archipelago (French Polynesia) are the last witnesses of ancient volcanic islands which

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l? Roiigerie et al. 1 hforim Geology 139 (1997) 201-217

is very low (0.1 g C/m2/day) due to the scarcity of dissolved nutrients in the euphotic layer (0-150 m) (Rougerie and Wauthy, 1993). This oligotrophy, expressed by values of inorganic phosphate and nitrate around 0.1 pM, results from a stratification of the oceanic column, a state typical of the tropical ocean with a maximum in the South Pacific gyre (Rougerie and Rancher, 1994). At depths below 500 m, there is a rich nutrient reser- voir in the Antarctic Intermediate Water (AIW); this AIW is permanently separated from the oligo- trophic surface layer by a thick thermocline. In specific locations where AIW moves upward and reaches the euphotic layer, it triggers an outstand- ing production and a greening of surface waters; these zones of upwelling are restricted to the west coast of South America and inside the equatorial belt, mainly in central and east Pacific. It is well established that upwelling conditions are unfavor- able for coral reef development, due to the turbid- ity and temperature of water, rapid growth of macro-algae, increase of bio-erosion, etc. (Hallock and Schlager, 1986). At the opposite, endo-upwell- ing is a thermo-convective process within the car- bonate piles of barrier reefs and atolls (Rougerie and Wauthy, 1986, Rougerie and Wauthy, 1993; Rougerie et al., 1992) that allows the nutrient-rich interstitial water intruding from AIW to ascend and reach the coral reef rim (Fig. 1). Autotrophic coral-algal communities could then be fed by a low but permanent flow of new nutrients. But direct evidence of the quantitative role of this interstitial reservoir of nutrients is still lacking, as the measurement of the flow of new nutrients able to reach the different compartments at the top of the atoll structure (algo-coral community, pinna- cles, lagoon, brackish ponds, groundwater, etc).

The 76 atolls of the Tuamotu archipelago sys- tematically show several emerged parts of their reef platform (from IO-100% of their perimeter) slightly above (1-5 m) sea level. These low islets or motu are composed of carbonate sediments (sand and rubble) and covered with vegetation, notably coconut trees. In these motu, rain water can be stored as groundwater in the Ghyben-Herzberg lens. The volume and salinity of this freshwater reservoir depends on several parameters, including evapotranspiration, sea

level, intensity of the rainfall and mixing by tides. The freshwater lens rests on and mixes with the saline interstitial water that saturates the porous and permeable carbonate reef structure (Fig. 1 ). On these motu, the low zones and depressions let the phreatic water outcrop and form permanent brackish ponds. These ponds are systematically colonized by thick cyanobacterial mats called kopara by Tuamotu natives (Figs. 2 and 3). A survey of 50 atolls indicates the presence of kopara ponds in 47 of them, with areal extents from ten to several hundred m2. Some enclosed lagbons, as in the Niau atoll (57 km2), are permanently brack- ish and colonized by kopara mats which can reach several meters in thickness (Landret, 1977). A typical kopara pond has an extent of about 100 m2, and the red cyanobacterial mats are 10-80 cm thick. Similar mats exist in brackish lakes in Palau in Micronesia (Burnett et al., 1989) and in the lagoon of the Aldabra atoll in the Indian Ocean (Potts and Whitton, 1980).

Cyanobacterial mats and phosphate rock both occur in the lagoon of the Mataiva atoll, in the western part of Tuamotu archipelago (Delesalle et al., 1985). Large kopara ponds are located on the brackish lagoonal shore, in close vicinity with a phosphate deposit, now covered by some meters of carbonate sediments. The phosphate deposit represent 15-25 million tons with an average of 38% P205 (Rossfelder, 1990), and is expected to be exploited as in other atolls like Makatea (Tuamotu) and Nauru. The occurrence of kopara mats and dissolved phosphorus enrichment (up to apatite saturation) has been noted in the Niau atoll (Fikri, 1991), Palau lakes (Orem et al., 1991; Burnett et al., 1993) and Aldabra lagoon, where a phosphate deposit has been exploited (Stoddart and Scoffin, 1983). On the Tuvalu archipelago (Central Pacific Ocean), coralligeneous sand and rubble are aggregated by phosphate cement in motu depressions, which were long ago covered by groundwater (Rodgers, 1992). Phosphatization has occurred in the vadose zone during at least the last 4000 yr. Phosphorus is organically bound and driven in phreatic water by humic complexes (Rodgers, 1994a,b), and microbial (cyanobacterial and bacterial) mucilages are still present in phos- phate cement, indicative of a possible kopara

, - KOP

Fig. and (mol lagol

pres stuc

mic. Prol mici mer

invc in i

vegt act soh to d inso Tua

ph o

sou1

5-11

LAGOON

F. Roiigevie et al. I Marine Geology 139 (1997) 201-217

MOTU

Rain

203

OCEAN

Oligotrophic ocean

-50 m

IV \

-100 m

\ \

THERMAL CONVECTION

Lagoonal sediment Porous limestone

Fig. I . Interstitial waters circulation by thermal convection (endo-opwelling) in Tuamotu atolls. Deep oceanic nutrients (P, N, Si) and in situ recycling maintain a rich nutrient reservoir (Rougerie and Wauthy, 1993). Brackish ponds occur on the emerged rim (motu), with thick (0.1-1 ni) cyanobacterial mats (kopara). An extreme expension of kopara can be observed in brackish enclosed lagoons as in Niau atoll, where kopara occupies 45 kmz with a thickness of several meters.

presence before phosphate rock formation. In a study of the organic matter trapped in insular phosphate rocks, Fikri (1991) has shown the microbial origin of this organic material and has proposed that the origin of the phosphorus is in microbial accumulations forming in lagoonal sedi- ments, with the interstitial water as a main nutrient source. Other phosphorus inputs are traditionally invoked to try to balance the phosphorus budget in atolls, i.e., bird dropping (guano), decay of vegetation, volcanic fallout. All these factors could act in periods of marine regression but cannot solve qualitative and quantitative problems related to diagenesis and sequestration of phosphorus (as insoluble apatite) at the time-space scale of the Tuamotu atolls (from 5-50 lo6 yr and from L innn km2) (Cullen and Burnett, 1986). Their

influence on the ponds studied, as well as in the Tuamotu atolls in general, seems negligible (Jehl and Rougerie, 1995).

A logical implication of our endo-upwelling model is that the nutrients brought up by the internal convective flow can enter the different atoll compartments (i.e., reef, lagoon, brackish ponds) as functions of their specific hydrogeologic characteristics (Andrie et al., 1992). These nutri- ents can then accumulate inside the compartments protected from oceanic swells, as confined ponds and closed lagoons, with the possibility of a direct precipitation of apatite (Rougerie and Wauthy, 1989). This hypothesis of a deep oceanic phos- phorus source to explain the phosphate deposit in some uplift atolls such as Nauru or Makatea, has been supported by Bernat et al. (1991), who

204 R Rougerie er cil. 1 Morirre Geo1og.v 139 (1997) 201-217

Fig. 2. Kopara pond, with desiccation polygons: the high productivity is liinited by \vater level, but importiiiit quantities 01' organic matter are produced and preserved: several tens of centimeters laying out o11 several hundred square meters.

showed that trace elenient and rare earth element patterns indicate oceanic and basaltic origins. A recent evaluation of the nutrient budget in atoll (Rougerie, 1995) shows that half the nutrient content of the interstitial water is brought up by internal advection flow (new nutrients) and that the other half is regenerated in situ, with oxygen consumption.

This study is part of a global research program which aim is to link the diifèrent aspects of phos- pliogenesis in atolls. The work presented here tries to establish the plausibility of the phosphorus pathways, from the interstitial reservoir to cyano- bacterial accumulation and apatite precipitation.

2. Methods

Cyanobacterial inats (kopara) have been col- lected at the Tikehau atoll (Fig. 4), from brackish

ponds relatively well protected from oceanic or lagoonal disturbances. Phosphorite samples have been collected in the neighbouring atoll of Mataiva, where an exploration pit was dug ia 1980. MTV-PH was prepared by iiiixing and homogeneization of several dense phosphorite samples, and MTV-PP consisted of pellets with 0.4-1 111111 dianieter. These two samples were crushed to a fine powder.

Several 2 ni long well points have been piit inside kopara ponds (Fig. 5) in order to p~inip two types of water samples: free brackish water (surface water) (a) and interstitial water (under the kopara layer, within the sediinent-limestoiie bedrock) (c). Sainpling was done during 199 1 to 1993, generally every 3 or 4 months, in order to eliminate the seasonal artefacts. Kopara samplings were siniply done by using cylinder collectors, which gave cylindrical cores (Fig. 3) of 10-30 ciii length and 5- 15 cni diameter. The slices cut from kopara

ì

Fig. "PP1 orga

corc wal ana wer the kop OR

nic

01'

ve of in Id ¡te t 11 're

F. Rougerie et cil. 1 MmYiie Geology 139 (1997) -701-217

. ..

205

Fig. 3. Kopxa core showing stroiiiatolitics properties: superposition o f CaCO, laiiiinations (white) and red organic-rich layers in the uppermost part. Under tlie first iwtotrophic centimeter highly reducing conditions prevail, that allows a good preservation of organic matter.

cores were centrifugated 20 min to extract pore water (b) which was then filtered at 0.45 pM before analysis. After centrifugation, the kopara slices were dried and crushed to fine powder, as were the phosphorite samples. All samples (waters and kopara) were refrigerated and stitdied in the ORSTOM laboratory at the Tikehau atoll.

Dissolved phosphate was measured from the three kinds of water samples (a, b, c) by spectro- photometry after reaction with ammonium molyb- date. Total dissolved phosphorus (organic and inorganic) was measured after complete oxidation of organic matter by UV radiation. Dissolved organic phosphate (DOP) was obtained by differ-

206 F. Rougerie et al. 1 Marine Geolog)) 139 (1997) 201-21 7

IlES p k a r u a

!OD

,Rurutu

' Rimatara

Tropique du Capricorne - - - - - - - - - . .

50 ud

Anuanuraro Vanavana. &teia

1[

15 SUI

20'

25'

Fig. 4. French Polynesia islands (five archipelagos) and location of studied ponds on Tikehau atoll (Tuamotu archipelago).

ence between total dissolved phosphate and dis- solved inorganic phosphate (DIP). In kopara interstitial waters (b), DOP was not measured because the centrifugation of kopara samples induces artefacts by solubilizing organic matter.

The humic substances are composed by humin, acid-soluble fraction and humic compounds (fulvic

and humic acids). The phosphorite samples were subjected to acid treatment (2 N HCl) till total decarbonation (end of effervescence). The acid solution was kept and named "acid-soluble frac- tion". From the resulting powder and kopara samples, humic substances were extracted and separated according to the following protocol: 20 g

Fig. pon

F. Rougerie et al. 1 Marine Geology 139 (1997) 201-217 207

ere )tal cid

.ira ind

ac-

o g Fig. 5. Water sampling at different levels in kopara ponds; three different types of water are sampled: brackish free water (a), kopara pore water (b), interstitial water ( e ) .

208 E Rougerie et al. / Marine Geology 139 (1997) 201-217

of each sample were treated with 50ml 0.05 N NaOH and centrifuged for 30 min at 3500rpm. The solution was filtered at 0.45 pm and the opera- tion was repeated until they were uncolored. These alkaline extracts were acidified to p H 2 by 2 N HC1 and left for four days in cold storage to let the humic acids precipitate. Then, fulvic and humic acids were separated by centrifuging at 5000 rpm and washed twice with 2 N HCl. Humic acids were redissolved in 4 N NaOH.

Organic carbon measurement in such samples was carried out by combustion at 1100°C in a Carmhograph after elimination of inorganic carbon by decarbonation (2 N HCl). Phosphorus was measured with ICP-AES at the Laboratoire de Géologie de la Matière Organique of Orleans University. In extracted organic substances, total P values measured correspond to P more or less weakly associated with the organic fraction (by covalent linkage, complexations). In this case, total P can be considered equivalent to organic P, and has been used for C/P ratio calculations for the organic matter.

3. ,Results

3. I . Pliospliate contents in brackislz p e e water (a ) , iii kopara pore water ( b ) aizd in interstitial water ( c )

The phosphate contents in these three compart- ments are given in Fig. 6. Analysis were performed on waters a, b, c taken from several ponds (Fig. 4) of variable depths and kopara thicknesses. Nutrient values other than P and physico-chemical parameters (salinity, dissolved oxygen, pH, alkalin- ity) are given elsewhere (Jehl and Rougerie, 1995):

In most ponds, dissolved inorganic phosphate concentrations in (a) are between 0.3 and 2.5 pM; these values are very significatively above lagoonal and oceanic ones (0.2 pM). The dissolved organic phosphate (DOP) has higher values (4-9 pM) than the inorganic forms. Water stratification has not been detected, even in the GM Ura pond (Fig. 4), where the depth of the free brackish water oscillates between 0.5 and 1 m. The vertical profiles show that in kopara pore waters (b), the first

centimeter has the lowest values of inorganic P. Deeper, a positive gradient appears, with values rising in the red layer of the kopara mat (0.9-1.5 pM in GM Ura and Aramuramu ponds; to 10.5 pM in Pahitomo pond).

In the interstitial water (c), phosphate contents are generally in the 1-8 pM range, with a positive trend with depth (3 out of 4 profiles).

3.2. Carbon mid pkospkorus distribution in Iiunzic coiiipouizds

Values for organic carbon in the raw material and in each humic fraction are given in Table 1. In the raw kopara, organic carbon contents decrease with depth (i.e., age) as inorganic carbon content increases (calcite and magnesian calcite are the main minerals). In the phosphorite samples from the Mataiva atoll, the organic carbon content is very low. Pellets are slightly richer in organic carbon (0.86% t- 0.02) than dense boulders (0.63% &O.OZ) (data are the average of three meas- urements). In the kopard, the main organic frac- tion is insoluble humin while an acid-soluble fraction dominates in phosphorite samples. In both materials studied, fulvic acids are dominant among humic compounds.

P205 concentrations (Table 2) show an average value of O. 11 % in the cyanobacterial mats. In the phosphorite samples, the phosphorus content appear to be related to their external aspect: MTV-PH is typical of dense boulders, very rich in phosphorus (44% P,05) while MTV-PP (carbon- ate-fluor-apatite and CaCO,) has only 16% P20,.

Values of the atomic ratio C/P (Table 3) give information on the variability in the phosphorus content within the humic fractions, with the organic carbon content used as an indicator of organic matter content. These C/P ratios decrease with depth in raw kopara mat, and in humin and fulvic acids. This trend reveals a relative phos- phorus enrichment with time. Conversely, humic acids (HA) lose P with time (increasing kopara thickness). In a general way the values of C/P ratios are lower in fulvic acids (FA) than in humic acids (HA) and the difference increases with the age of the organic matter.

v e 5 D -

- E,

8 . 5

- E

5 - B

Fig abo seci

4.

4. I

i

ricl the inc ren ko1 the act thi: go' ces sul 195

c P. lues mat nds;

ents tive

C

,rial e 1. :nts )on :ite des ent nic ers as- ¿iC-

ble j th 'ng

'ge ihe :nt ct: in

In- )5.

ive

li e of se lid

i ic ra /P iic he

'US

)S-

1 I I

I I

1 1

1 i

1

I

, !

I , I

I

ì

F. Rougerie et al. 1 Marine Geology 139 (1997) 201-217

Pahitomo 1

O 5 10 15

GM Ura 1

O 5 10

209

GM Ura 2

O 2

Kereto Aramuramu

O 5 10 O 2 4 6 O or--- -

-5

D -80 -100 -120 -25

Concentrations (IIM) Concentrations @M)

Fig. 6. Concentrations (pM) in dissolved inorganic phosphate (DIP), dissolved organic phosphate (DOP) in free brackish water (a: above the first dotted line), kopara pore water (b: between the two dotted lines) and carbonate rock interstitial water (c: under the second dotted line) in four kopara ponds of the Tikehau atoll (GM Ura and Pahitomo ponds ace represented by two sets of data).

4. Discussion

4. I. Pliosphoswjlows ìFi the kopam ponds

As already noted, the brackish free water (a) is richer in inorganic phosphate than waters from the lagoon or ocean (0-11 50 m deep). This situation indicates. a specific phosphorus trapping and remineralization after bioconcentration. Inside the kopara pore water (b), the phosphate depletion in the first centimeter is the result of the autotrophic activity in this layer by living cyanobacteria. Under this green layer, the bacterial activity is rapidly governed by heterotrophy and degradation pro- cesses. The free oxygen is totally consumed and sulfate is reduced, with production of H2S (Jehl, 1995). Because of the gelatinous property of the

organic lattice, reducing conditions can persist and result in a good preservation of the organic mate- rial. However, with aging of the kopara, some organic phosphorus can become unbound from organic matter and released in inorganic form, as indicated by higher phosphate values (5-10.5 pM) in the kopara red layer. In the interstitial waters (c), as for the other chemical elements (NH:,

Na', Cl-, SO;-, Ca2+, Mg", K') of this system (Jehl, 1995), a positive gradient of inorganic and organic phosphate seems to appear with depth. Data from bore wells drilled in the reef rim of this atoll reveal that the influence of the phreatic-brackish system can reach a depth of ten meters during the rainy season (Rougerie and Wauthy, 1993). The observed positive gradient of nutrient with depth can imply a permanent flow

210 F. Rougerie et al. 1 Marine Geology 139 (1997) 201-217

Table 1 Weight percentages of organic carbon in raw material and humic substances extracted from two kopara mats (Pahitomo and Matiti, Tikehau atoll) and two phosphate samples from Mataiva atoll

~ ~~

Samples Depth Raw Humin Acid-soluble Fulvic Humic (cm) fraction acids acids

Pahitomo

PII-RI 3.0 16.9 15.6 0.7 0.6

6

I PII-v 0.5 23.2 18.5 2.9 1.9

PII-R2 7.5 10.9 10.5 0.3 o. 1 PII-K1 12.0 9.3 9.1 o. 1 0.2 PII-r 19.5 6.2 5.8 0.3 o. 1 PII-= 27.0 4.5 4.3 0.2 o. 1

MII-V 0.5 15.9 11.4 3.7 0.8

MII-K 30.0 5.8 5.6 O. 1 O. 1

MTV-PH 0.63 0.04 0.52 0.06 0.01 MTV-PP 0.86 0.12 0.65 0.07 0.02

Matiti

MII-R 8.5 9.2 8.3 0.7 0.3 MII-r 19.0 7.2 6.1 0.3 0.3

Mataiva

MTV-PH grind mixture of several consolided phosphate samples, MTV-PP: pellets; acid-soluble fraction has not been extracted from kopara mats.

Table 2 Weight percentages of PzO, in raw material and humic substances extracted from two kopara mats (Pahitomo and Matiti, Tikehau atoll) and two phosphate samples from Mataiva atoll

Samples Depth Raw Humin Acid-soluble Fulvic Humic (cm) fraction acids acids

Pahitomo PII-v 0.5 0.14 0.10 24.0 19.8

E-3 E-3 PII-RI 3.0 0.11 0.10 5.0 E-3 4.2 E-3 PII-R2 7.5 0.12 o. 10 13.1 1.6 E-3

E-3 PII-K1 12.0 0.10 0.10 2.5 E-3 0.3 E-3 PII-r 19.5 0.09 0.07 22.9 0.5 E-3

PII-K2 27.0 0.09 0.07 E-3 18.8 1.1 E-3 E-3

Matiti MII-V 0.5 0.13 0.10 26.1 5.9 E-3

E-3 MII-R 8.5 0.15 0.14 8.2 E-3 1.7 E-3 MII-r 19.0 0.10 0.09 6.7 E-3 2.2 E-3 MII-K 30.0 0.08 0.07 12.6 0.1 E-3

E-3 Mataiva MTV-PH 43.92 3.71 39.41 0.77 0.02 MTV-PP 16.41 4.59 11.21 0.61 1.1 E-3

MTV-PH: grind mixture of several consolided phosphate samples, MTV-PP: pellets; acid-soluble fraction has not been extracted from kopara mats. In phosphate samples, P,O, values for acid-soluble fractions are calculed by difference between P,05 in raw samples and sum of P,O, from fulvic acids, humic acids and humin.

Tab1

sub: Mal Mat

San-

C ß

-

Pali PII- PII- PII- PII- PII. PII- Ma MI1 MI1 MI1 MI1 Ma M-l M-l

M-l san- of I

P v. to I

-

of w Il

gr; tic UP ele shg CY rai ar; Pa en0 ab

Pr f u Pa

tht

mi

Wa

W¿

Ph

CO

.\titi,

imic ds

F. Rougerie et al. 1 Marine Geology 139 (1997) 201-21 7 21 1

Table 3 C/P weight ratio in organic matter of raw material and humic substances extracted from two kopara mats (Pahitomo and Matiti, Tikehau atoll) and two phosphate samples from Mataiva atoll

i

t

I !

.ted -

lau

- iic

-

,3-3 E-3

5-3 E-3

z-3

:-3

:-3 i-3 i-3

5-3

ed -

.1W

Samples Depth Raw Humin Fulvic Humic (cm) acids acids

Pahitomo PII-v PII-RI PII-R2 PII-KI PII-r PII-K2 Matiti MII-V MII-R MII-r

Mataiva MII-K

MTV-PH MTV-PP

0.5 3.0 7.5

12.0 19.5 27.0

0.5 8.5

19.0 30.0

773.3 881.0 623.7 704.2 709.1 636.4 419.2 477.3 104.9 422.7 428.4 181.8 310.0 391.9 60.0 236.8 417.5 48.8

567.9 532.7 649.1 278.8 271.2 393.3 327.3 336.7 178.6 362.5 491.2 62.9

0.07 0.06 0.36 0.24 0.12 0.53

437.8 666.7 714.3 800.0 833.3

1000.0

888.9 483.9 714.3

5000.0

2.29 84.03

~ -~

MTV-PH: grind mixture of several consolided phosphate samples, MTV-PP: pellets. In phosphorite samples, C/P ratio of raw material and humin must not be taken into account, as P values are flawed by P from apatite. The same problem seems to occur in fulvic acids, but probably a t a lower level.

of nutrients towards the top of the atoll structure, where they are diluted by mixing with meteoric water and consumed by biological uptake. This gradient can be due either to a chemical stratifica- tion (with the richest layers at the bottom) or an upward input of elements. A study of rare-earth elements in kopara (Jehl and Barsczus, 1996) showed that concentration patterns recorded by cyanobacteria are very likely originating from AIW rather than from subsurface oceanic water. This argument confirms the reality of the qualitative participation of endo-upwelling in the gradient encountered in the groundwater of atolls. The abundance of nutrients in interstitial and free waters of ponds can be sufficient to sustain the productivity of the cyanobacterial mats. However, further work is needed to test and quantify this pathway, particularly in terms of flow rate of new phosphorus and gross and net productivities of the mats. The brackish ponds where kopara thrives constitute windows opened on the interstitial medium, so the cyanobacterial mats are in perma-

nent hydraulic contact with the atoll's internal nutrient pool. Autotrophic metabolism of the kopara depends on the uptake of inorganic phos- phate (and nitrate or ammonia), a form that we have detected in great amounts in the interstitial compartment of atolls. The thickness of cyano- bacterial mats implies evidence for fixation of interstitial phosphate.

4.2. Distribution of organic carbon

In the phosphorite samples, concentrations of organic carbon are low, in the same range as values obtained by Fikri (1991) on Mataiva phos- phorite samples (0.5-1.1%). Analyses of several insular phosphorite samples from South Pacific islands (Aharon and Veeh, 1984; Fikri, 1991) gave similar low organic carbon contents. They show a further decrease with depth of sampling, within each deposit. In the case of recent phosphorites such as those labelled "guano crusts" (but without clear evidence of guano origin), organic carbon contents can be more important: from 9% in Ebon Island (Aharon and Veeh, 1984) to 17% in the Fangataufa atoll (Fikri, 1991). Dating of phospho- rite samples from the Clipperton almost-atoll gave an age of 2600yr (Roe and Burnett, 1985) and organic carbon values from 0.5-6.4% (Fikri, 1991). The overall trend revealed by these sets of data is that young phosphatic crusts and ores contain more organic carbon and lower phos- phorus percentages than huge old deposits ( 104-107 yr). This implies a process of phosphorus enrichment with time, while organic matter undergoes degradation.

A discrepancy exists with the FA/HA ratios; our data show ratios >1, while Fikri (1991) found values -= 1 in half his insular phosphorite samples, with HA values between 3 and 18% of the TOC ( 1.6-2% in ours). Epicontinental phosphorites are also Lharacterized by a large range of FA/HA ratios values: values > 1 from Oulad Abdoun ores (Morocco) in conjunction with high organic carbon content (Benalioulhaj, 1989). In Israeli phosphorites, organic matter is dominated by HA, then FA and last kerogen, but acid-soluble com- pounds were not extracted (Amit and Bein, 1982).

. .

212 F Rolrgerie et al. /Marine Geology 139 (1997) 201-21 7

In Gafsa Basin (Tunisia), HA represents 4040% of the TOC (Belayouni and Trichet, 1983).

Such distribution of humic substances in our phosphorite samples is characteristic of immaturity of the trapped organic material, with a slight effect of diagenetic processes. Fikri (1991) considered three possibilities to explain this pattern:

Entrapment during apatite precipitation of a part of the immature organic matter which constituted the phosphorus source. Enrichment of phosphate rock in acid-soluble compounds by inheritance from phosphatized biogenic carbonate grains (Benalioulhaj, 1989). Formation of secondary acid-soluble com- pounds, resulting from microbial or oxidative degradation of the organic matter trapped in phosphate rock, during dissolution and repre- cipitation processes ( Benalioulhaj, 1989; Nathan, 1990; Trichet et al., 1990). Such secondary acid-soluble compounds would be more stable than primary ones, as they are the principal “rescued” compounds of the succes- sive dissolution-reprecipitation phases which affect insular phosphate rocks. Fig. 7a shows that the distribution of HA, FA

and humin are relatively similar in both phospho- rite and kopara samples. This analogy is also valid when comparing distribution of humic substances in the phosphorite from Mataiva and in the kopara from the Hao atoll (Defarge, 1983) (Fig. 7b). In these samples the acid-soluble fraction of the organic matter has been obtained by acid treatment (2 N HCI) (and so decarbonation) before extrac- tion of humic compounds (NaOH treatment). Acid-soluble compounds turn out to be the domi- nant organic fraction, followed by fulvic acids and humin, and finally humic acids. The fact that these distributions are very similar between two organic matters originating from two different materials (cyanobacterial mat and phosphate rock) and from two different atolls (Tikehau and Mataiva) strongly support the hypothesis of kopara-like organic matter as a phosphorite precursor.

4.3. C/P ratios

C/P ratios for humin and acid-soluble fraction are not given because they are distorted by solubili-

sation of phosphorus and/or carbon from apatite. In both phosphorite samples, phosphorus is more concentrated in FA than in HA. That difference reaches a maximum in the pellets where the C/P ratio values are 0.53 in FA and 84 in HA. This excessively low ratio for FA from pellets is proba- bly due to inorganic phosphorus dissolved from CFA during basic treatment. Pellets were very carbonated compared to dense phosphorite samples, so their apatite (CFA) must have a greater solubility (Nathan et al., 1990) than that of a dense sample. HA from pellets contain very little phosphorus, while those from dense phosphorites are a hundred times more enriched. A similar fact has already been observed in phosphorite samples from Tunisia (Trichet et al., 1990) where the HA from the samples richest in phosphorus have the lowest values of C/P ratios. As our dense phospho- rite samples have been subjected to many phases of apatite dissolution-reprecipitation, their organic carbon content has been reduced. If the amount of organic phosphorus linked to organic material decreases more slowly than the organic carbon (as seen in kopara evolution: decrease of C/P values with depth), the phosphorus inherited on retained humic and fulvic acids after apatite dissolution and reprecipitation would allow very low values of C/P ratios in altered samples, which seems to be the case. These results are convergent with observations by Nissenbaum ( 1979), i.e., C/P ratios greater in HA than in FA.

Our results from kopara samples (Table 3) indi- cate that the aging and decay of the organic matter is accompanied by a greater loss of carbon than of phosphorus (C/P ratios in raw material decrease with kopara depth).

Similar analyses performed on marine sediments from off Peru (Reimers, 1982) show that C/P ratios in raw organic material fluctuate between 100 and 300 in the zone of apatite precipitation, where phosphorus concentrations are the same as in kopara (0.03-0.12% of total weight). In the same region of active submarine phosphogenesis, Sandstrom (1990) observed that C/P ratios of raw sediments decrease from 36-1 9 with increasing depth (0-27 cm).

However, in anoxic marine sediments, Burdige (1991) found a C/P ratio increase with depth

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F. Rougerie et al. 1 Maririe Geology 139 (1997) 201-217

a: kopara from PAHITOMO (P) and MATITI (MI, phosphorite from MATAIVA atoll (MTV)

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b: Kopara from HA0 atoll (HI (Defarge, 1983) and phosphorite from MATAIVA atoll (MTV)

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Fig. 7. Carbon content in humic substances (% T.O.C.) in two phosphorite samples from Mataiva atoll (MTV-PH and MTV-PP), compared with values in kopara samples from Tikehau atoll (ponds Pahitomo and Matiti) (a) (data from Table 1) and Hao atoll (pond H: Defarge, 1983) (b), which kopara has been subjected to another extraction procedure.

during organic matter mineralisation. In fact, the values and evolution of C/P ratios are linked to sedimentation rate (Ingall and Van Cappellen,

1990): low rates (complete oxidation of metaboli- sable organic matter, but presence of refractory detrital compounds enriched in phosphorus) as

214 F. Rougerie et al. Murine Geology 139 (1997) 201-21 7

well as high rates (preservation of organic matter with anoxia) induce low C/P ratios. Intermediate sedimentation rate allows preferential regeneration of phosphorus during the incomplete decomposi- tion of metabolizable organic matter. According to these results, C/P values for raw kopara class are similar to those of anoxic sediments with a high/intermediate productivity, and those for an average accumulation rate.

In other marine organic materials, Nissenbaum (1979) found lower C/P values in FA than in HA. The figures given by this author are in the same range as those obtained in our kopara samples (C/P values from 65-670 in FA, and from 216-910 in HA). His C/P values for HA also increase with depth of marine sediment (from 400-961 in 35 m), but slower than in kopara.

Another conclusion is that FA and HA have a fair capacity for phosphorus fixation, with C/P

- ratio significantly lower than in humin. But, as humin is quantitatively the dominant form in kopara (80-97% of the TOC), this organic fraction bears the major part of the total amount of phosphorus.

During the degradation of organic matter, in a first stage, phosphorus remains complexed on fulvic and humic acids, which minimizes the loss to pore-water. If HA are formed from FA, values of the C/P ratio indicate that the transformation is accompanied by a loss of phosphorus. If, as proposed by Nathan (1990) and Vanderbroucke et al. (1985), the oxidation of humin leads to secondary HA, phosphorus is also released during this transformation, as indicated by the strong increase of C/P values from humin to 'HA. Whatever the nature of HA (primary or second- ary), their formation favours a loss of phosphorus.

5. Conclusions

Our results show that the thick cyanobacterial mats which colonize the brackish ponds of the Tuamotu atolls can be sustained in nutrients by the underground water of the atoll carbonate sediment. These nutrients can be produced by in situ remineralization of decaying organic matter and by transport from the rich interstitial reservoir

contained in the atoll structure (Rougerie and Wauthy, 1993). As stated by Rodgers (1994b), the fresh-brackish water lens has a central role in redistributing phosphorus in an atoll ecosystem (Fig. 8). The positive vertical gradient observed in this groundwater can be due either to a chemical stratification or to an upward input of elements. In the case of closed atolls, with a brackish lagoon colonized by kopara, the ground water lens under the emerged part plays a secondary role compared with the interstitial nutrient reservoir which is in direct contact with kopara at the bottom of lagoonal sediments.

After its fixation in the organic matter of kopara during autotrophic growth of cyanobacteria, phos- phorus is first indiscriminately associated with various humic fractions, and later with fulvic acids. The highly reducing conditions prevailing below the first centimeter of the mat allow long-term preservation of the organic material. Degradation is mostly due to bacterial degradation by anaerobic bacterial activity (sulphate-reducing and methane- producing bacteria). At the interface with the rock foundation, the presence and circulation of sub- oxic interstitial water may however induce a slight oxidation and degradation of the deepest and oldest layers of kopara (Jehl, 1995). The phos- phorus remineralized and liberated by this oxida- tion can be efficiently entrapped by humic and fulvic fractions, which possess a high affinity for phosphorus. This diagenetic process induces C/P ratios to decrease in the raw material and humic compounds, except for humic acids. In the con- fined systems of the kopara ponds, the trans- formations of FA to primary HA and humin to secondary HA lead to a loss of phosphorus into the interstitial water which becomes gradually enriched in phosphorus. When saturation is sur- passed, apatite can precipitate with its classical cohort of substituted elements of deep oceanic origin (Bernat et al., 1991).

In the phosphorite samples that we studied, the C/P ratios in FA are lower than those of HA, with this trend rising in response to the degree of alteration undergone by phosphate rock. Such evolution in the C/P ratios with rock alteration seems to be also linked to the incorporation of the phosphorus released by mineralisation of the most

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unstable organic matter into residual organic matter previously phosphorus enriched.

Comparison of organic matter from different atolls and different materials shows that the distri- bution of humic substances in phosphate rock is similar to that observed in kopara. This similarity is strengthened by an identical signature in rare earth elements (Jehl and Barsczus, 1996), in hydro- carbon composition and distribution (Fikri, 1991; Jehl, 1995) and confirms and reinforces the hypothesis that the cyanobacterial accumulations (kopara) are the precursors of atoll phosphorites (Fikri, 1991; Rougerie and Jehl, 1993). Where the cyanobacterial mats are large, as at the Niau atoll (Tuamotu) in which the thickness of kopara reaches several meters, maturation and final oxida- tion of millions of tons of kopara can lead to a phosphate deposit greater than IO6 tons, as in the

neighbouring atolls of Makatea and Mataiva. As established by Trichet ( 1967) and Trichet and Defarge ( 1995), the stromatolitic properties of kopara are highly favorable to the nucleation of carbonate minerals as carbonate-fluor-apatite (CFA), which is the most widely occurring mineral in phosphorites. This property also confirms that phosphatized oncolitic pellets described by Fikri (1991) have cyanobacterial precursors, i.e.,.kopara mats. Growth of primary apatite is then sustained by the phosphorus pool present in the interstitial water. Microscopic observations and analysis have shown the importance of the dissolution-reprecipi- tation of phosphate rock, with a phosphorus enrichment of the secondary apatites (Fikri, 1991; Jehl, 1995). This implicates a continuity of phos- phorus enrichment in the phosphate deposit through time, which can be due to many stages of

T -A-

216 F. Rougerie et al. Marine Geology 139 (1997) 201-21 7

kopara growth and degradation (linked to eustatic changes) and/or a permanent supply of phos- phorus from the interstitial reservoir.

We propose a model of phosphogenesis in atolls that takes into account, in a coherent way, the data and conclusions of different authors. This model outlines a specific phosphorus pathway, depending on the presence of a vast interstitial water pool, where phosphate (inorganic dissolved P) originates both from vertical advection (endo- upwelling) and in situ remineralization of atoll detritic biologic material (recycling). This nutrient pool can allow rapid and efficient biological fixa- tion of P (as N, C, ...) by cyanobacterial mats (kopara) whose thickness can reach meters in confined lakes or brackish closed lagoons. The phosphorus pathway continues by anoxic seque- stration of the organic matter produced, oxidative- release of phosphorus, interstitial water enrich- ment, and final precipitation of CFA (Fig. 8). As shown by our data, diagenetic processes within the mats tend to enrich the old deep layers of kopara in phosphorus and the first phosphatic minerals formed. These deposits can be viewed as by-products of the atoll functioning, because they form and accumulate in confined lagoons. Generalization of this model to other phosphatized carbonate structures, as phosphorites located on the Florida peninsula, where there is also an internal thermo-convective circulation (Kohout, 1965), seems to be a logical suggestion.

Acknowledgements

We are indebted to Jean-Louis Cremoux and Joel Orempuller for their support in the atolls and analytical activity in the ORSTOM laboratories of Tikehau and Tahiti. We thank Corinne Ollier for the drawing of figures and Alfred Fagerstrom for his encouragement and english editing. This work is part of a cooperative program between ORSTOM (TOA Department) and the University of Orleans (Laboratory of Organic Matter Geology). This program has lead to a thesis at French University of Pacific with financial assis- tance of French Ministery of Research. PROE- CPS (New Caledonia and Samoa) are also

aknowledged for their financial support in the study of phreatic atoll system. This work is a contribution to the International Geological Correlation Program, Project 325 (Phosphorites). (G.M.F.)

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