Early diagenetic processes in deep Labrador Sea sediments: reactive and nonreactive iron and phosphorus

MARC LUCOTTE Centre de recherche en gdochimie isotopique et en gdochronologie (GEOTOP), Universitd du Qudbec d Montrdal,

P. 0. Box 8888, Station A, Montrdal, QC H3C 3P8, Canada

ALFONSO MUCCI Centre de recherche en gdochimie isotopique et en gdochronologie (GEOTOP), Department of Earth and Planetary Sciences,

McGill University, 3450 Universitd Street, Montrdal, QC H3A 2A7, Canada

AND

CLAUDE HILLAIRE-MARCEL AND SOPHIE TRAN Centre de recherche en gdochimie isotopique et en gdochronologie (GEOTOP), Universitd du Qudbec d Montrdal,

P. 0. Box 8888, Station A, Montrdal, QC H3C 3P8, Canada

Received January 7 , 1993 Revision accepted October 30, 1993

A sequential extraction procedure was applied to separate the oxides and lithogenous phases of iron and manganese and the organic and inorganic phosphorus phases in four box cores and one piston core from the slopes and rises of the Labrador Sea. Sedimentation rate, rather than the location in the basin, appears as a master variable of the diagenetic transformations of Fe, Mn, and P. High sedimentation rate, characteristic of two of the box cores, led to the creation of zones near the redox boundary of partial reprecipitation of dissolved Fe, Mn, and P released in the deeper reducing portions of the sediments. In contrast, surficial sediments from box cores with 10 times lower sedimentation rate only have sufficient reductive capacity to remobilize Mn hydroxides while leaving the Fe oxyhydroxides intact. Under these conditions, there is evidence for a redis- tribution of reactive inorganic P leading to the crystallization of carbonate fluorapatite in the top 30 cm of the cores. Gradual transformation of buried orthophosphate to authigenic apatite under suboxic conditions is also observed in the top 400 cm of the high sedimentation rate piston core. As in the box cores, the reaction is complete after several thousand years of burial and occurs in sediments characterized by low CaCO, content. Depending upon the sedimentation rates, the carbonate fluor- apatite crystallization may be superimposed on the changes in detrital sedimentary fluxes accompanying the onset of deglaciation. A proxy indicator of paleo-redox conditions, and thus of biodegradable organic matter accumulation at the sediment -water interface, is given by the ratio of iron oxides to the reactive inorganic P (solid orthophosphate plus authigenic apatite).

Un prockdk d'extraction sequentielle est applique pour identifier les oxydes de fer et de mangankse, ainsi que les phases organiques et inorganiques du phosphore dans quatre carottes B boite et une carotte ?i piston des talus et des glacis de la mer du Labrador. La suite des transformations diagenetiques Bun endroit donne est d6termink par les taux de sedimentation plut6t que par la position du site dans le bassin. Lorsque ces taux sont Blevts, comme dans deux des carottes B boite, ils mknent 2 la crkation, prks de la frontibre r d o x , de zones de reprkcipitation des Fe, Mn et P relarguts plus profondernment en condi- tions rauctrices. Par contre, seuls les oxydes de mangankse sont remobilisBs et partiellement reprecipites dans les deux carottes B boite ayant des taux de saimentation dix fois plus faible. Dans ces environnements, le P inorganique reactif recris- tallise sous la forme de fluorapatite carbonatke dans les premiers 30 cm des carottes. On observe la m6me cristallisation dans la carotte B piston, pourtant caracterisk par de forts taux de sedimentation, mais cette reaction est maintenant repartie sur les premiers 400 cm. En rkgle gtnkrale, la formation d'apatite authigkne n'est compl6tee qu'au bout de plusieurs milliers d'annees, dans des sediments presentant de faibles teneurs en CaCO,. Selon la vitesse de saimentation, la rtaction peut se superposer aux changements de flux saimentaires dktritiques qui accompagnent la mise en place de la periode de deglacia- tion. Le rapport des oxydes de Fe et du P inorganique reactif (phosphate plus apatite authigkne) peut semir d'indicateur de palko-conditions r a o x , donc de l'enfouissement de matiBre organique biodegradable.

Can. J. Earth Sci. 31, 14-27 (1994)

Introduction Complex diagenetic processes decouple the sedimentary burial

rate of reactive elements from their initial fluxes to the sea floor (e.g., Emerson and Hedges 1988; Ingall and Van Cappellen 1990; Lyle et al. 1992). In searching for indicators of paleo- productivity in recent and more ancient sediments, transition metals and nutrients have often been identified as key elements because of their more efficient preservation compared with their biogenic carrier phase (e.g., Moody et al. 1988; Shaw et al. 1990). To be useful indicators, one must be able to decipher the various interdependant diagenetic transformations of these elements from their variable initial sedimentary fluxes. The superimposition of processes occurring on very different time scales complicate the interpretation of the sedimentary

record. For example, zones of rapid metal oxide reduction and concomitant release of adsorbed elements may be juxtaposed with layers of rapid oxic reprecipitation or depth intervals of slower authigenic recrystallization. A proper interpretation of the sedimentary profiles requires a clear understanding of the mobility and preservation of the chosen key elements under various environmental conditions (Rabouille and Gaillard 1991).

Contrary to their lithogenous counterparts, Fe and Mn oxides fractions incorporated in pelagic sediments react to changing redox conditions. Following the depletion of free O2 in surface sediments, NO3 and Mn02 become the next most efficient oxidants, well before Fe203 (Froelich et al. 1979). For this reason, the depth of Mn reduction usually appears above that of Fe reduction.

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LUCOTTE ET AL. 15

Phosphate may be released to the pore waters following microbial degradation of organic matter or reductive solubili- zation of Fe (111) phases to which it is associated (e.g., Krom and Berner 1981; Van Raaphorst et al. 1988; Lucotte and d7Anglejan 1988). ~ h o s ~ h o n k released to the pore waters may diffuse through the sediment column, escape to the overlying water, or associate to Fe oxides precipitating at or accumulat- ing above the oxic-anoxic boundan. Part of the dissolved phosphorus may also reprecipitate *;thin the sediment as an authigenic apatite-like phase (Froelich et al. 1988; Ruttenberg and Berner 1993).

The Labrador 'sea is presently one of the world's most productive pelagic environments, and as such offers unique opportunities to study the geochemical cycling of Mn, Fe, and P following a sustained deposition of organic matter. The deep oceanic circulation of this area has also undergone major changes, such as the reinitiation of the Western Boundary Undercurrent at the Holocene as a result of climate change (Wu and Hillaire-Marcel 1994). As a consequence, the nature and intensity of the sedimentary fluxes in this subarctic marginal sea have drastically shifted during the last glacial -interglacial transition.

The relative importance of both early and late diagenetic processes on the remobilization of Fe, Mn, and P is evaluated in this study by comparing cores taken at nearby locations but influenced to various degrees by bottom currents, and thus characterized by a one order of magnitude difference in sedi- mentation rates. At the same time, ittempts to generalize the geochemical observations reported at one location will be made by comparing sampling sites on both sides of the Labrador Sea. Temporal changes in the nature and intensity of the sedimen- tary fluxes will be assessed by comparing two low sedimenta- tion rate box cores and one high sedimentation rate piston core which penetrated the last deglaciation period and beyond.

By distinguishing the various reactive or nonreactive frac- tions of Fe, Mn, and P in four box cores and one piston core of the Labrador Sea, we intend to (i) identify the major dia- genetic reactions (mineralization, dissolution,-and repiecipita- tion under the same form or as a new authigenic mineral); (ii) identify the environmental sedimentary parameters (organic patter content, bioturbation, sedimentation rate, etc.) that con- trol the diagenetic redistribution of reactive elements on different time scales; (iii) establish a link between the nature and rate of the sedimentarv fluxes and the burial of reactive elements in pelagic sediments; and (iv) propose a first-order reconstruc- tion of paleoproductivity from the sedimentary record.

Methods and analyses I

Sediment sampling and core description Four box cores (stations 01 1,017,020, and 027) and a piston

core (station 013) collected during CSS Hudson cruise 90-013 were used for this study (Fig. 1). The detailed oceanographic setting is given in other contributions in this volume (e.g., Hillaire-Marcel et al. 1994~). Upon recovery of the 0.5 m2 box cores, sediments were sampled using 15 cm diameter PVC push cores. The cores were then extruded under a nitrogen atmos- phere in a glove box. For each 1 cm depth interval sequentially subsampled, 10-20 g of sediment was transferred to poly- ethylene vials. The description of the measurement of the redox potential and collection of pore waters is given in Hillaire-Marcel et al. (1990). The sampled sediments were immediately frozen and later freeze-dried and homogenized in

FIG. 1. Location of sediment box and piston cores collected during CSS Hudson cruise 90-013 in the Labrador Sea.

the laboratory prior to determining their solid phase compo- sition.

All collected cores lie in well-oxygenated bottom waters. The sandy surficial sediments of box core 011, situated at 2800 m depth on the Greenland slope, are swept by the strong Western Boundary Undercurrent (Wu and Hillaire-Marcel 1994; Lucotte et al. 1994). Accelerator mass spectrometry (AMS) radiocarbon stratigraphies revealed that the sediments accumu- lated very slowly during the Holocene, at a rate of - 2 cmlka (Wu and Hillaire-Marcel 1994; Hillaire-Marcel et al. 1994b). Not only does the surface layer appear to be mixed over - 5cm, but the most recent sediments seem to be missing. The sedimentation rate jumps sharply to - 15 cmlka below 22 cm depth ( - 12 000 years).

One order of magnitude faster Holocene sedimentation rate characterizes the nearby clayey sediments (box core 017 and piston core 013) taken slightly deeper on the Greenland rise at -3370 m depth. With an accumulation rate of -32 cmlka, the base of box core 017 was deposited less than 1000 years ago (Wu and Hillaire-Marcel 1994; Hillaire-Marcel et al. 1994b). The sister piston core 013 exhibits similar Holocene sedimen- tation rates of - 30 cmlka. Accordingly, the glacial -interglacial transition lies at around 440 cm depth. With a slower sediment accumulation rate of - 12 cmlka, isotopic stage 2 extends down to - 600 cm. The Holocene CaC03 content of core 013 is made up mostly of coccoliths carried laterally to the site of deposition (de Vernal et al. 1994). The gradual decrease of

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16 CAN. J. EARTH SCI. VOL. 31, 1994

biogenic carbonate with depth (Mucci et al.') is believed to reflect the reduced biogenic productivity at the early stage of the last deglaciation rather than a gradual dissolution.

Box cores 020 and 027 lie within a few tens of nautical miles from each other on the Labrador slope at about the same depth ( - 2900 m). Despite their proximity, core 020 is charac- terized by a one order of magnitude greater sedimentation rate (-22.5 cm/ka) than core 027 (2.9 cmlka) (Wu and Hillaire-Marcel 1994; Hillaire-Marcel et al. 19943). These authors suggest that intense local bottom currents might be responsible for the low accumulation rates of silts in core 027 and may even have led to the erosion of the top few centi- metres of this core. Under these conditions, the base of core 027 represents an early phase of the deglaciation transition. In contrast, the fine clayey sediments at the base of core 020 are at most 1500 years old. Intense bioturbation, marked by numerous worm holes, also characterizes this core.

Laboratory analyses The partitioning of the various solid phases of Fe and P

was achieved using a sequential extraction procedure devised by Lucotte and d'Anglejan (1985), which was calibrated with a number of reference materials and well-defined mineral phases. The precision of the method is better than f 7 % at the 95 % confidence level for each step. The citrate - dithionite - bicarbonate (cdb) method makes it possible to distinguish iron oxyhydroxides (Fecdb) from more refractory forms of iron. The same method was also applied to determine the concentration of manganese oxides (Mncdb). The Fecdb and Mncdb extracts were compared to those obtained with a 1 N/HC1 leach by Mucci et i.' The difference between total Fe or Mn and these weak acid extracts (Few, and Mnwa) is believed to represent lithogenous phases of iron and manganese (Felith and Mnlim). The cdb procedure was also designed to selectively extract reactive inorganic phosphorus (o&hophosphate, Pcdb) asso- ciated with iron oxides. A new extraction step using 1 M Na acetate buffered to pH 4 with acetic acid as reagent followed by a MgC12 wash (Ruttenberg 1992) was added between the orthophosphate and apatite extractions. This allows the distinc- tion of carbonate fluorapatite phosphorus (PC,) from detrital apatite phosphorus (Papa). Following the apatite extraction, organic bound phosphorus (POrg) was liberated after high- temperature combustion and a subsequent acid leach.

Cores 01 1 and 027, which are characterized by low sedimen- tation rates, display large fluctuations in biogenic carbonate content (Mucci et al.'). The CaC03 accounts for between less than 1 and up to 40 weight percent of the total sediment, thus the absolute concentrations of the various nonreactive elements appear artificially low when incorporated in CaC03-rich sedi- ments. Consequently, we present the concentration profiles of the various detrital elements in this study, namely Papa and Felith, normalized on a carbonate-free basis.

Results Redox-sensitive elements in high sedimentation rate box cores

The top part of the redox reaction sequence described by Froelich et al. (1979) for pelagic sediments is observed in box cores 017 and 020, but without reaching sulphate reduction or methanogenesis (Hillaire-Marcel et al. 1990). Given the absence

'A. Mucci, M. Lucotte, C. GariCpy, and C. Hillaire-Marcel. Early diagenetic processes in deep Labrador Sea sediments: a comprehen- sive model. In preparation.

of a concentration gradient in the Cow content, we cannot determine a rate of organic matter degradation despite active electron transport provided by the recycling of sedimented Mn and Fe, and reflected by decreasing Eh.

The oxic-suboxic transition at the bottom of core 017 is followed by the reduction of Fe and Mn oxides. These reac- tions give rise to high Mn and Fe concentrations in the pore waters, respectively, below 17 and 25 cm (Mucci et al.'). Part of the dissolved Fe and Mn diffuses upward and is reprecipi- tated as oxides when entering the O2 penetration zone (Fig. 2). A strong Mncdb peak occurs between 8 and 10 cm in the oxic zone, just above the depth of maximum Mn pore-water con- centration. This enriched Mn layer is characteristic of pelagic sediments underlying oxic bottom waters (Shaw et al. 1990). The Fecdb peak is observed between 20 and 22 cm, coincident with the oxic-anoxic transition and just above the zone of Fe-enriched pore waters. A variable fraction of phosphate is released to the pore waters after dissolution of the Fe oxides in the reducing layer or to a lesser extent by mineralization of organic matter (Krom and Berner 1981; de Lange 1986; Van Raaphorst et al. 1988). The solubilized P diffuses upward and is then partly accumulated at the depth of Fe reprecipita- tion (Fig. 3a).

Although core 020 has a sedimentation rate similar to that of core 017, its redox gradient is steeper than that of core 017. Pore-water measurements show that Mn is solubilized below 12 cm depth, whereas Fe, requiring more reducing conditions, is released below 25 cm (Mucci et al.'). The presence of two deeper paleo-oxidation fronts inferred from the U and Ni dis- tributions in core 020 (GariCpy et al. 1994; Mucci et a1.l) could not be clearly identified by the Fecdb and M Q ~ profiles. When compared to core 017, the whole early diagenetic sequence in core 020 is compressed, and the oxidation and precipitation of both Fecdb and Mncdb occur simultaneously at 10 - 12 cm depth, right at the oxic -anoxic boundary (Fig. 2). A clear concomitant zone of orthophosphate (Pcdb) accumula- tion is again observed (Fig. 3a).

Above the Fe-oxide reprecipitation layer in both cores 017 and 020, the Fecdb/Pcdb atomic rate varies between 20 and 26 (Fig. 3b). This narrow range of values appears as a characteristic of modern oxic pelagic and fast-accumulating sediments, as it exactly corresponds to those found in sediments of the Nares Abyssal Plain (de Lange 1986). Below 15 cm depth in core 020, Fecdb/Pcdb ratios increase significantly ( > 30) (Fig. 3b) and indicate the rupture of the initial phosphate to iron oxides equilibrium association.

Redox-sensitive elements in the high sedimentation rate piston core

The top 30 cm of the selected redox-sensitive elements profiles from piston core 013 is characterized by significant reprecipi- tation of Mn and Fe oxides, along with a clear Pcdb accumula- tion (Figs. 4a-4c). These profiles are similar, although less well defined, to those described for box core 017 taken at the same location. Most of the diagenetic redistribution of reactive elements occurs in the top 30 cm, along with the gra- dients in redox conditions. Below the depth, Fecdb concentra- tions are nearly constant throughout isotopic stage 1, down to 440 cm depth (avg. 97.1 pmollg, sd = 9.6) (Fig. 4a). Below 440 cm, the passage from stage 1 to stage 2 is marked by sig- nificantly higher but still uniform Fecdb concentrations (avg. 112.5 pmollg, sd = 12.3 over the 450-600 cm interval).

In contrast with the constant Fecdb concentrations, Pcdb con- centrations gradually decrease from - 8 to - 2 pmol/g since

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norm. CaCO, 5oo]

700 HU90-013-027

500 600$ r-.

0 5 10 15 20 25 30 35 o s 10 15 20 25 30 35

Depth (crn)

) 150

HU90-013-017 -A- Mncdb

-s- Mn,,,,, norm. CaC03 100

50

- 5 0 5 10 1 5 20 25 30 35 -

0

Depth (cm)

FIG. 2. Sediment profiles of (a) iron oxides (Fe,,), weak acid extractable iron (Few,), amorphous iron - silicium minerals (Fe,,, = Few, - Fe,,,), Fe,,,, normalized on a carbonate-free basis (Fe,,, norm. CaCO,); and (b) manganese oxides (Mn,,,), weak acid extractable manganese (Mn,,), and CaC0,-free normalized lithogenous manganese (Mn,,,, norm. CaCO,) in box cores 017-020 and 01 1-027.

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CAN. J. EARTH SCI. VOL. 31, 1994

0 5 0 1 1 n n 7 1 n 1 1 1 1 ~ n n n I n , 1 u 1 1 u 1 n ! n ' n 1 1 1 n n n I 0 S 10 15 20 25 30 35 0 5 10 15 2o 25 30 35

Depth (cm)

FIG. 3. Sediment profiles of (a) orthophosphate (PC,,), authigenic apatite phosphorus (PC& and organic phosphorus (Po,,); and (b) atomic rates of iron oxides to orthophosphate (Fe,,/Pc,) in box cores 017, 011, 020, and 027.

HU90-013-027

70- 7 0 7

50-

40-

10- 10:

0 . . , . . . . . . . . . , . , , . , , , , , , , , , , . , I 0 , , , . l " " l " " l " " , t I I I I I I u l ] u I I I I 0 : lb 1'5 Q 2 5 30 35 0 5 10 15 20 25 30 35

Depth (cm)

- 60-

HU90-013-020 I 60:

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HU90-013-013

-t- FeClay norm. CaC03

1 " " 1 1 " ' 1 " " 1 " " 1 " " 1 " " 1 o / , , , , l " " l " " l " , , I , ~ ! ! I I I I I ~ 0 100 200 300 400 500 600 0 100 200 300 400 500 600

Depth (cm)

FIG. 4. Sediment profiles of (a) iron oxides (Fe,,), weak acid extractable iron (Few,), amorphous iron - silicium minerals (Fe,,,), Fe,,,, normalized on a carbonate-free basis (Fe,,, norm. CaCO,); (b) manganese oxides (Mq,,) and weak acid extractable manganese (Mn,,); (c) orthophosphate (PC,), authigenic apatite phosphorus (PC,), and organic phosphorus (P,,,); and (d) atomic ratios of iron oxides to ortho- phosphate (Fe,,,/P,,,) and to initial reactive inorganic phosphorus (FeCdb/(Pcdb + AP,,) or R ratio, see text) in piston core 013.

1 the onset of deglaciation (Fig. 4c). This behaviour corresponds 1 to a singularly sharp increase of the Fecdb/Pcdb ratio from 20

to 50 with depth (Fig. 4d). The strong correlation between Pcdb and CaC03 concentrations, from 50 to 560 cm depth,

I combining both isotopic stages 1 and 2 (r = 0.767, n = 45),

i may be indicative of an unusual reactivity of Pcdb under low I carbonate content.

Unlike the constant Fecdb concentrations observed within each isotopic stage, Mncdb below the surface redox transition zone in core 013 seems related to the carbonate content (r = 0.735, n = 45) (Fig. 4b). This suggests that Mn(I1) is rapidly adsorbed on carbonate after Mncdb is subjected to reducing conditions.

Redox-sensitive elements in low sedimentation rate box cores The sedimentary profiles of cores 011 on the Greenland

slope and 027 on the Labrador rise recorded the change from high to low pelagic bioproductivity over the last deglaciation (Hillaire-Marcel et al. 1994~) . The rapid decrease in COrg in both box cores should therefore not be attributed to an intense degradation of the sedimentary organic matter but rather to a drastic shift with time in the organic sedimentary fluxes at these locations.

The broad Mncdb peak observed at around 18 cm depth in core 027 (Fig. 2b) coincides approximately with the depth at which the redox potential becomes slightly negative (Hillaire- Marcel et al. 1990). In a similar manner to the previously described box cores, this peak corresponds to a zone of partial reprecipitation of Mn oxides remobilized from deeper suboxic layers (Shaw et al. 1990). As fully reducing conditions are not established at the bottom of core 027, the increasing Fecdb con- centration below 10 cm depth (Fig. 2a) most probably represents Fedb-rich glacial sediments, rather than diagenetically remobi- lized Fe oxides. Reactive inorganic P (Pcdb) increases along with Fecdb, but without showing a narrow peak as previously reported for cores 017 and 020 near their zone of Fecdb reprecipitation (Fig. 3a).

The redox potential in core 011 is constant and positive (> 100 mV) down to 32 cm depth (Hillaire-Marcel et al. 1990). Suboxic conditions deeper within the sediment are neverthe- less suggested by the well-defined Mqdb diagenetic peak at 25 cm depth (Fig. 2b), as it is usually situated just above the oxic - anoxic boundary (Shaw et al. 1990). Like for core 027, the simultaneous increases in FeCdb and Pcdb concentration below 15 cm and beyond in core 01 1 (Figs. 2a and 3a) should represent glaciogenic sediments of different composition rather

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20 CAN. J . EARTH SCI. VOL. 31, 1994

than a broad zone of authigenic reprecipitation. But in both cores 027 and 01 1, the resulting Fecdb/Pcdb atomic ratio (Fig. 36) is surprisingly much higher than the previously reported average common value for oxic pelagic sediments.

Weak acid extracts In all five cores studied, Few, concentrations are systemati-

cally two to three times higher than Fedb (Fe oxide) concentra- tions (Figs. 2a and 4a). This discrepancy indicates that 50 -70 % of the weak acid leachable iron is in the form of amorphous Fe-Si minerals, carbonates, and (or) iron monosulfides, all of which are not extracted by the citrate-dithionite reagent (Lucotte and d'Anglejan 1985; Ruttenberg 1992). In all cores, the normalized sulfur concentrations are on the order of 20 -60 pmollg . Thus, Fe-monosulfides cannot account for a major fraction of the Few, (A. Mucci, unpublished data). The FQ, minus Fedh fraction also does not appear to be associated with a carbonate phase: it is at the bottom of cores 011 and 027, and below 440 cm depth in core 013, that the differences in concentrations between Few, and Fecdb are the largest, and it is also at these depths that the carbonate contents are the lowest in all five cores. Consequently, we propose that most of the Few, minus Fecdb is in the form of amorphous Fe-Si clay minerals and it will be referred to in the text as FeClay. Based on this conclusion, the Feclay flux to the sea floor is mostly detrital, and the concentrations must be normalized to the CaC03 content, similar to that for Felith and Papa (Figs. 2a and 4a).

In box cores 017 and 020, which are both characterized by high sedimentation rates, the normalized [Feclay] is fairly con- stant at -250 pmollg (Fig. 2a). These concentrations are similar to those observed in the top 440 cm of piston core 013 (isotopic stage 1) (Fig. 4a). Below that depth, the normalized Feclay concentrations increase by - 60% up to - 400 pmollg. In the low sedimentation rate box cores 011 and 027, Feclay surface concentrations are again close to those reported for cores 017 and 020, and for isotopic stage 1 of core 013.

In the five cores investigated in this study, the Mncdb con- centrations are very similar to those of Mn,, (Figs. 2b and 4b). This suggests that most of the weak acid leachable Mn is present as oxides only, and not as carbonates or clays, since these phases are not extracted with the dithionite buffer. Never- theless, the Mn oxides remaining after surface remobilization seem to be preferentially retained or adsorbed onto buried car- bonates, as suggested by the correlation between Mncdb or Mn,, and CaC03 contents in the 50-500 cm depth interval of core 013 (r = 0.751 and 0.740, respectively).

Detrital fractions The Felim and Papa concentrations, normalized to a carbonate-

free sediment, are fairly constant down to the bottom of both high sedimentation rate box cores 017 and 020 (Fig. 5a). In these two cores, the lithogenous iron fraction represents 60-70% of the total iron content. In contrast with Fe, the lithogenous Mn fraction is negligible when compared with the Mn oxide fraction (MnCdb or Mn,,; Fig. 2b). Froelich et al. (1979) already noted that in pelagic sediments almost all Mn was reactive and remobilizable, in contrast with the small por- tion of the total Fe which is present as ferric oxyhydroxides.

In piston core 013, the normalized Felim, Mnlith, and Papa concentrations remain fairly constant down to a depth of - 240 cm (Figs. 6a - 6c). It appears obvious that these detrital fluxes are independent of the bioproduction in the water column,

as biogenic CaC03 decreases by more than 50% over the same depth interval. The Felith, Mnlith, and Papa concentrations in the top 240 cm of core 013 are comparable to those of its sister core 017. The Felim/Papa atomic ratio of - 60 -70 seems to be characteristic of the detrital sedimentary flux on the Greenland rise (cores 013 and 017) for the last few thousand years (Figs. 5b and 6d). In contrast, the detrital sedimentary flux on the Labrador rise has a somewhat smaller characteristic Feli*/Papa ratio of - 50 (core 020, Fig. 5b), mainly because of a smaller Felith content.

Below 240 cm depth, and down to 360 cm, the detrital sedimentary flux in core 013 shifts abruptly. The simultaneous drop in [Feli,] and increase in [Mnlith] and [Papa] correspond to the minimal CaC03 content of isotopic stage 1 (Mucci et al. I). Under these conditions, the Felith/Papa ratio reaches minimal values of - 45. The sediments below the transition to glacial conditions (400 - 600 cm depth) are characterized by the return of a fairly homogeneous detrital fraction composition. Along with the higher content in Feclay previously reported, terrigenous inputs are slightly depleted in Felith and Papa with respect to present-day values. The Feli,/Papa ratio during this glacial period oscillates between 60 and 80.

In the top centimetres of box cores 01 1 and 027, the normal- ized Papa and Feli, concentrations are comparable to those obtained from the most recent sediments of cores 017 and 013 (Fig. 5a). The corresponding Feli,lPapa ratio ( - 35 -40, Fig. 5b) appears to be characteristic of the coarser detrital sedi- ments accumulating today in the Labrador basin. The combined increase in [Felih] (by 30%) and decrease in [PapJ obtained for deglaciation sediments found at the bottom of both box cores 01 1 and 027 precisely replicate concentrations in core 013 for the same period, that is to say, below 400 cm depth. In addition, the Felith/Papa ratios ( - 70 - 80) are similar to those of core 013 below 400 cm.

Authigenic apatite fraction In both high sedimentation rate box cores 017 and 020, Pcfa

concentrations are fairly constant throughout the entire core (Fig. 3a). This average Pcfa concentration of 6 pmollg must represent the sedimentary flux of some preformed minerals. A quasi-instantaneous authigenic precipitation of Pcfa at the sediment - water interface is unlikely because the surface pore waters in these cores are undersaturated with respect to car- bonate fluorapatite (CFA). This is in contrast with the observa- tions of Jahnke et al. (1993) and Froelich et al. (1988) who reported near-surface apatite precipitation in Mexican and Peruvian margin sediments. It should be noted, however, that the acetate buffer used for the selective extraction of authigenic CFA also releases P associated to CaC03 or to clays like smec- tite (Ruttenberg 1992). The proportion of phosphorus which is then detrital (the apatite plus the preformed Pcfa) is on the order of 60 -70%.

The Pcfa concentrations in the top 50 cm of piston core 013 are also of the order of - 6 pmollg, as previously reported for cores 017 and 020 (Fig. 4c). This might again represent the original amount of P brought by a constant carbonate and (or) smectite sedimentary flux to the ocean floor. From 50 to 440 cm, however, the [Pcfa] increases steadily up to - 12 pmollg. This increase could be interpreted as a progressive rise with time of the carbonate - smectite fluxes to the Labrador basin ocean floor. It may also represent a long-term authigenic precipita- tion of Pcfa. If this is the case, it would be the first time that

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LUCmTE ET AL.

20 - - 2w -1 000

- HU90-013-017 - HU90-013-011

800

mk ;::: :: ;600

10- Cb

-400 - 400 3 L

5 - 5- 5 -200 - -200 - 6

0

S 0 ~ l l l l l l l r l l l 1 1 l 1 1 l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I , , I , , , , I , , , , 0 c 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 39 g

20 - - 20- c

-1000 LL - HUQD-013-020 - HU9C-013-027

- 800 15 - t

600

10 - 0 ;:;-* /400

5 - S 00 - 200

0 1 1 1 1 , 1 , , , 1 , , , , , , , , , 1 , , , , 1 , , , , 1 , , , , 0 0 , , , , , " " I " " , " ' , ~ I , , , , , , I , , T , , , 0 0 5 10 IS 20 25 30 3 5 0 5 10 15 20 25 30 35

Depth (cm)

1001 1 oo-,

*:L-, 0

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Depth (cm)

FIG. 5. Sediment profiles of (a) CaC0,-free normalized detrital apatite phosphorus (Papa norm. CaCO,) and lithogenous iron (Fe,,,, norm. CaCO,); and (b) atomic ratios of lithogenous iron to apatite phosphorus (Fe,,,lPaPa) in box cores 017, 011, 020, and 027.

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22 CAN. J. EARTH SCI. VOL. 31, 1994

1000- ( a 1 50 - - FEI,,~ norm. CaC03 + MI-,,,,, norm CaC03

40

' 30

400- 20

200- 10

- g 0 O 1 1 1 , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I , I I I I I I ,

0 O 0 100 200 300 400 500 600 5 - 25 120 - - P- norm. C ~ C O ~ : - F e ~ i i h f p a ~ ( d

b

100- 20

I

15

10 40 -

A 5 20 1 , , , , I

0 100 200 300 400 500 600 0 100 ZOO 300 400 500 600 Depth (crn)

FIG. 6. Sediment profiles of (a) CaC0,-free normalized lithogenous iron (Fe,, norm. CaCO,), (b) CaC0,-normalized lithogenous manganese (Mn,, norm. CaCO,), (c) CaC0,-normalized detrital apatite phosphorus (P,, norm. CaCO,), and (d) atomic ratios of lithogenous iron to apatite phosphorus (FeIith/P,,) in piston core 013.

authigenic precipitation of CFA is reported for environments other than recent shelf sediments (e.g., Jahnke et al. 1983; Ruttenberg and Berner 1993; Lucotte 1993).

In both low sedimentation rate box cores 011 and 027. the Pcf, concentrations increase sharply from -4-5 to - 11 - 15 pmollg with depth (Fig. 3a). As argued for core 013, these increases might be accounted for by a larger sedimentary flux of smectite during deglaciation or by authigenic forma- tion of CFA.

General discussion Detn'tal fluxes to the sediments of the Labrador Sea

The Felim and Papa profiles reflect fluctuations in the input of detrital temgenous particles superimposed onto the diagenetic profiles of reactive Fe and P, and are only affected by smooth- ing resulting from bioturbation in the surface sedimentary layers. In the sedimentary record of the last 1000-2000 years, the average detrital Fe and P input to the sediments in the Labrador Sea is directly related to the local sedimentation rate. On either side of the Labrador basin, fine-grained particles that accumu- lated in high sedimentation rate areas (cores 013, 017, and 020) have a fairly uniform detrital composition, being syste- matically enriched in Felith and depleted in Pap, with respect to the coarser particles of low sedimentation rate areas (cores 01 1 and 027).

During the last glaciation, normalized concentrations of refractory P and Fe are very uniform in the three cores which penetrated that period (cores 01 1 , 013, and 027) on either side of the Labrador Sea. This is probably related to the fact that similar sedimentation rates on the order of 10 cmlka are observed at all three locations during this period (Wu and Hillaire-Marcel 1994). Glacial weathering inputs are clearly distinguished from present-day inputs by their Feclay enrich- ment and depletion in Felim and Papa.

During the interglacial to glacial transition, a sudden and temporary weakening of the Western Boundary Undercurrent has been proposed based on paleontological and chronostrati- graphic evidence (Rochon and de Vernal 1994; Wu and Hillaire- Marcel 1994). This period of very low biogenic productivity is clearly recorded in the detrital fraction of the sediments (from 240 down to 360 cm depth) of core 013. Simultaneous drops in [Felim] and [Feclay] and increases in [Mnlim] and [Pap] probably reflect a major shift from distal to proximal sources in the terrigenous origin of the detrital sedimentary flux. In the low sedimentation rate cores 01 1 and 027, the record of that particular event is totally missing, which suggests that these sites must have experienced periods of erosion since the last deglaciation. Local bioturbation may also have had an impor- tant smoothing effect on the sedimentary profiles of these cores (Wu and Hillaire-Marcel 1994), since the transition between

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L U C ~ E ET AL. 23

deglaciation and more recent sediments is spread out over 10- 15 cm.

No significant Felih increase matching a Fecdb decrease could be found throughout any sampled core. This means that amor- phous Fe - Si mineral precipitation, precluding pyrite forma- tion in pelagic environments, is not observed (de Lange and Rispens 1986; Raiswell et al. 1988). Even if this precipitation did occur under sulfate reducing conditions, it would be masked by the overwhelming presence of lithogenous iron relative to pyrite. The Felith/S atomic ratio in the five cores is always > 15 (Hillaire-Marcel et al. 1994a), and thus authigenic FeS2 represents at most 3% of the lithogenous Fe fraction.

Early redox diagenesis in high sedimentation rate cores Although cores 017 and 020 are characterized by equivalent

sedimentation rates, the subsurface Corg content in core 020 (- 1 %) remains twice that of core 017 (<0.5%, Mucci et al.'). This observation does not fit with the reported correlation between the deposition flux of organic carbon in marine sedi- ments and the sedimentation rate (e.g., Canfield 1989; Ingall and Van Cappellen 1990). Despite higher oxidation rates sub- stantiated by more negative Eh in core 020, the better preser- vation of organic carbon might be partly attributed to the limited diversity of bacterial grazers (Lee 1992). It might also be seen as the result of the observed intense worm activity in core 020, which can enhance the entrainment of surface organic matter in deeper sediment layers (Berner and Westrich 1985; Emerson and Hedges 1988). In that context and according to Ingall and Van Cappellen (1990), the high Corg/Po,, atomic ratios (300-400) found for the sediments of core 020 could reflect the poor degradation of the buried organic matter, whereas the values ( < 200) found for core 017 could represent a nearly complete oxidation of the metabolizable organic matter.

In the three fast sedimentation rate cores (box cores 017 and 020, top of piston core 013), the gradual microbial degrada- tion or organic matter triggered the classic sequence of reduc- tive dissolution and oxidative reprecipitation of metal oxides and proposed by Froelich et al. (1979). The concordance of the Mncdb and Fecdb maxima with the change in redox condi- tions indicates that the reprecipitation zones steadily follow the upward migration with time of the sedimentary oxic - anoxic boundary, with no turbidity current disrupting the Eh profile (Buckley and Cranston 1988).

Not only does the remobilization of redox-sensitive elements in core 020 all appear to take place within the thin 6- 10 cm transition interval, but the surface of this core is bioturbated by intense worm activity. Bioturbation may inhibit the develop- ment of oxide peaks, because it will tend to redistribute the oxides (Aller 1990). This may explain why Fe and Mn oxide peaks do not appear as sharp in core 020 as in core 017. In contrast, for elements like uranium, bioirrigation seems to favour a flux of dissolved species from the overlying waters and accumulation in subsurface zones (GariCpy et al. 1994).

In all three cores, negligible amounts of Porg are mineral- ized. Consequently, most PO4 ions released to the pore waters come from the gradual dissolution of Fe oxides in the reducing layer. Under increasingly reducing conditions, like at the bottom of core 020, the progressive increase in the Fecdb/Pcdb ratio, from its initial oxid equilibrium value of 20-26 to above 30 (Fig. 3b), indicates that the PO4 release is more intense at the beginning of the Fecdb dissolution. This behaviour could be explained by the fact that the finest and least crystallized Fe (111) oxyhydroxides fractions are more readily dissolved and

that they carry more surface-bound phosphate than the more crystalline fractions, which are more resistant to dissolution.

The dissolved orthophosphate diffuses upward in the three high sedimentation rate cores and invariably accumulates at the depth of FeCdb reprecipitation. At this depth corresponding to the oxic transition, the fixation of important amounts of Pcdb could be explained by the presence of abundant benthic bacterial communities (Ingall et al. 1993). The newly reprecipi- tated and poorly crystallized Fe oxides can also serve as active readsorption surfaces for free phosphate ions (Froelich et al. 1988). This process appears to be an effective barrier to diffuse loss of inorganic phosphorus from the sediments, particularly when Fedb reprecipitation is optimized under gradual redox gradient as in core 017.

Long-term evolution of redox conditions in piston core 013 Analysis of the distribution of the reactive Fe, Mn, and P

fractions in piston core 013 provides additional information on the long-term diagenetic remobilization of reactive elements, once steady state conditions are established near the present oxic -anoxic boundary (below 50 cm depth). This informa- tion, however, must be deciphered from the long-term changes in the nature of the sedimentary fluxes.

Since the last glacial episode, pelagic bioproductivity on the Greenland rise has risen drastically (de Vernal et al. 1994), leading to an increased flux of organic particles to the sea floor. Despite the fact that suspended organic aggregates or fecal pellets are active scavengers of Fe oxides and, as such, the main carriers of Fedb to the sediments (e.g., Fowler and Knauer 1986), no decrease in Fedb flux matching that of C,,, is seen in the sediments of core 013 between 50 and 440 cm depth.

In contrast, while the overall bioproduction in the Labrador Sea dropped, the recorded Pdb concentrations decreased from - 8 to -2 pmollg over the 50-440 cm depth interval. (Fig. 4 4 . It is difficult, however, to directly link both observa- tions, since at times of high biological production, the demand for bioavailable phosphorus is greatly enhanced and, thus, a fraction of the Pcdb adsorbed onto sinking particles could be used (Froelich et al. 1982; Manning 1989). Consequently, if the Pcdb flux to the ocean floor was to change in times of higher bioproduction, it should be reduced rather than increased.

On the other hand, the possible changes over time of the Fecdb and Pcdb sedimentary fluxes may also have been smoothed out by early diagenetic redox reactions. Easily redu- cible Fe (111) must have continuously been remobilized from the anoxic sedimentary layers. In addition, and in contrast with Mn (11), remobilized ferrous ions do not reprecipitate as coatings on biogenic carbonates in reducing sediments (Piper and Williamson 1977; Boyle 1983) (Fig. 4b). The constant Fedb concentrations covering several thousand years of sedi- mentation then become an indication of the uniform reducing state probably attained in the sediments of core 013 over that time. In fact, there are few reasons to believe that more reducing conditions than today could have developed in the Labrador basin sediments during the deglaciation. Firstly, the organic matter fluxes towards the sea floor were much smaller at that time, and assuming that the nature of the organic matter reach- ing the sediment-water interface did not vary significantly over that period, it translates into a reduced oxygen demand for biodegradation. Secondly, the COrg/Porg atomic ratios during deglaciation (200 - 300) are smaller than those of more recent conditions ( - 400). According to Ingall and Van Cappellen (1990) and Ingall et al. (1993), these results could suggest that

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24 CAN. J. EARTH SCI. VOL. 31, 1994

oxic conditions allowed a nearly complete degradation of sedi- mentary organic matter during deglaciation, as opposed to an incomplete decomposition of COT, and a preferential regenera- tion of Po,, under more reducing conditions in recent time.

Authigenic precipitation in piston core 013 Considering the lack of solubilization of Fe oxides under the

ambiant redox conditions of the last deglaciation in core 013, it is difficult to explain the sharp Pcdb decrease with depth as a gradual release of phosphate. In this respect, the Fecdb/Pbcdb ratios (up to 50 at depth) are far too high to simply represent the progressive dissolution of better crystallized oxyhydroxide fractions.

Instead of interpreting the decreasing Pcdb concentration with depth as a loss of phosphorus from the sediments of core 013, it might be that Pcdb was reprecipitated in a form not extractable by the dithionite buffer. This hypothesis arises from a consideration of the concomitant increase in Pcfa (APcfa = [PCfJ observed - [PcfJ at the surface) and decrease in Pcdb concentrations in the top 440 cm of the piston core. If one calculates the Fecdb/(Pcdb + APcfa) atomic ratio (R) instead of Fecdb/Pcdb, it falls between 15 and 20 (Fig. 4d). This new ratio is typical of an initial association between reactive inor- ganic phosphorus and iron oxides under oxic or slightly sub- oxic conditions, similar to that found in the surfaces of box cores 017 and 020 (Fig. 3b).

Authigenic precipitation of carbonate fluorapatite (CFA) from seawater has been induced in laboratory experiments (Gulbrandsen et al. 1984). It has also been observed on some occasions in modern continental shelf sediments such as the Peruvian upwelling area (Froelich et al. 1988), the Mexican margin (Jahnke et al. 1983; Van Cappellen and Berner 1988), Long Island Sound and the Mississippi Delta (Ruttenberg and Berner 1993), and the Gulf of St. Lawrence (Lucotte 1993). In all cases, authigenic apatite precipitation seems associated with moderate- or high-productivity environments with elevated organic matter burial and with subsequent intense degradation of organic phosphorus in the sediments. To our knowledge, authigenic apatite precipitation has not been reported as result- ing from longer term diagenesis in pelagic sediments. To be fully convinced of the authigenic precipitation of apatite, direct confirmation by X-ray diffraction would be required. The maximum size of these new particles, however, is not expected to exceed a few tenths of a micrometre to a few micrometres (Van Cappellen and Berner 1991).

The precipitation of CFA requires free Ca2+ and F- ions from the pore waters. On the other hand, elevated carbonate ion concentration in pore waters has an inhibitory influence on apatite precipitation, as their coprecipitation enhances the solu- bility of apatite (Jahnke 1984; Baumer et al. 1986). High Mg2+ concentrations have also been found to slow down the growth rate of authigenic apatite by blocking the growth sites at the crystal surfaces (Gulbrandsen et al. 1984; Van Cappellen and Berner 1989). More importantly, the adsorption of phos- phate onto CaC03 may not allow the build up of high [PO4] in the pore waters (De Kanel and Morse 1978; Burton and Walter 1990). In piston core 013, the amount of presumed authigenic CFA increases from the surface of the core down to -460 cm depth, whereas the CaC03 content decreases gradually over the same depth interval.

Apatite precipitation also appears limited by its slow crystal growth kinetics. In core 01 3, the complete precipitation seems to be achieved only after several thousands of years. This is rem-

iniscent of the slow CFA precipitation proposed for continental margin sediments (Ruttenberg and Berner 1993) or the slow fluorapatite particle size incremental growth rate calculated from laboratory experiments by Van Cappellen and Berner (1991). These observations contrast with the quasi-instantaneous apa- tite precipitation in sediments of coastal upwelling areas where a large number of crystallization sites and high degrees of super- saturation are produced periodically (Froelich et al. 1988).

When apatite formation occurs near the sediment surface, C032- and F- ions may diffuse directly from the overlying waters (Froelich et al. 1988). In piston core 013, the source of carbonate is most probably pore-water alkalinity produced during organic matter decomposition (Mucci et al.'). On the other hand, the diffusion of fluoride through the overyling 450 cm of sediment may partially support the inferred CFA formation, given the very slow precipitation rate. In addition, fluorine may come directly from the recrystallization of illite particles, which may contain up to 800 ppm of F (Matthies and Troll 1990).

Below 450 cm depth, or throughout isotopic stage 2 of core 013, Pcfa concentrations reach a plateau, whereas Pcdb con- centrations increase with depth. Two reasons could be invoked to explain why authigenic apatite precipitation is not as com- plete as during isotopic stage 1: (1) The glacial conditions are characterized by reduced reactive and nonreactive element inputs from ice-covered adjacent lands. In association with the much slower sedimentation rates (Wu and Hillaire-Marcel 1994), reduced organic matter burial probably led to more oxidized sedimentary environments. The weaker remobilization of Fedb below 450 cm depth could be responsible for a stronger retention of Pcdb, and its limited availability to reprecipitate as CFA. (2) During glaciation, the reduced fluxes of lithogenous Papa may have limited the number of growth sites for authigenlc apatite.

Redox reactions and authigenic precipitation in low sedimenta- tion rate box cores 011 and 027

In box cores 01 1 and 027, the development of highly reducing conditions immediately below the sampled depths is improbable, since the deeper sediments are made up of organic-poor sandy sediments from the deglaciation period, and thus with minimal reductive capacity. In this context, MnCdb is the only reactive species that responds to suboxic or mildly reducing conditions. As in high sedimentation rate cores, the zone of Mn-oxide reprecipitation follows the slowly upward migrating oxic- anoxic transition zone. In contrast with cores 017 and 020, this zone of Mn reprecipitation becomes an active scavenger of Cu, released during the oxidation of the organic matter, in the absence of high residual Corg which usually plays a role (Mucci et al.').

The oxic to suboxic environments of cores 011 and 027 allow the preservation with no diagenetic remobilization of all iron oxides deposited throughout the entire sampled depth. The progressive increases in Fecdb concentrations with depth thus reflect an enriched sedimentary flux of that element during cold periods, equivalent to the one observed in piston core 013 below 400 cm depth. Nevertheless, a fraction of the decrease in the recent Feed, content could simply be attributed to dilu- tion by the strong enrichment in biogenic CaC03 content in modern sediments.

At the surface of both cores 01 1 and 027, the present deposi- tion of reactive inorganic phosphorus to the sea floor is related to that of Fecdb in the same proportions as those observed for

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LUCOTTE ET AL. 25

cores 013, 017, and 020. This sets an average Fecdb/Pcdb atomic ratio of 20 -27 for all particles presently settling in the Labrador Sea under oxic conditions. Below the surface of cores 01 1 and 027, this ratio increases rapidly to values larger than 30 (Fig. 36). Unlike what has been previously shown for core 020, however, they cannot be attributed to a weaker association of Pcdb onto Fecdb under increasingly reducing con- ditions (Eh 2 0 throughout cores 01 1 and 027). Taking into account that Fedb has not been diagenetically remobilized, the increase in the Fecdb/Pcdb ratio primarily reflects important deficiencies in Pcdb with respect to the expected oxic equilib- rium ratios. As pointed out earlier for core 013, these Pcdb depletions cannot result from a more intense consumption of Pcdb by the scarce bioproduction in the water column during cold periods.

Coincident with the Pcdb deficit, both cores 011 and 027 exhibit strong increases in Pcfa concentrations with depth. When the absolute Pcfa increase (APcf,) is added to the Pcdb concentrations, the calculated R atomic ratio (Fecdb/(Pcdb + APcfa)) falls between 20 and 30 throughout cores 01 1 and 027. Thus, it seems that the deficiency in Pcdb is almost entirely found as newly precipitated authigenic apatite. As in core 013, the presumed CFA precipitation occurs when the carbonate content is low and even undergoing partial dissolution (Mucci et al.').

In contrast with the top 450 cm of core 013, Pcfa formation is not accounted for by a clear and equivalent Pcdb decrease. This observation is attributable to the fact that the [Pcdb] pro- file must have originally increased, like [Fecdb] still does, dur- ing the transition of Holocene to deglaciation. One should then see the resulting uniform rather than increasing Pcdb profiles with depth of cores 011 and 027 as effective decreases with time of the initial amount of Pcdb delivered to the sediments. In addition, the sharp shifts in Pcdb concentrations between modern and deglaciation sediments may have been considerably smoothed out by the reported bioturbation at each site (Wu and Hillaire-Marcel 1994).

Conclusions Early diagenetic redox transformations

The role of sedimentation rate as a master variable in the early diagenetic processes, underlined by Canfield (1989) and Ingall and Van Cappellen (1990), is particularly applicable to the five Labrador Sea cores studied. A remarkable resemblance exists between the distribution and absolute concentrations of the diagenetic products at locations more than 500 krn apart on opposite sides of the Labrador Sea, and with similar sedimen- tation rates. In contrast, differences in the extent of diagenetic reactions observed between sites located within 100 krn of each other on both sides of the Labrador Sea can be attributed to the one order of magnitude difference in sedimentation rate.

In equivalent high sedimentation rate Greenland rise core 017 and Labrador slope core 020, the remobilization of redox- sensitive elements like Fe and Mn oxides follows the upward- migrating oxidation front, and the sharpness of the redox gradient controls the depth and the intensity of their reprecipi- tation. In core 020, Eh drops rapidly and both zones of Fe and Mn reprecipitation are superimposed and close to the surface. When the redox gradient is more gradual, as in core 017, the depths of MIL,, and Fecdb reprecipitation are distinct from each other and much stronger in intensity. The freshly reprecipi- tated Fecdb oxide layer in core 017 constitutes a most efficient

barrier to the Pcdb diffusing upward following its release after the dissolution of Fe oxides at depth.

In low sedimentation rate box cores 01 1 and 027, misinter- pretation of metallic oxide diagenesis can easily be drawn if one does not take into account the overlapping change in the nature of the sedimentary fluxes with time. The reducing capacity of surface sediments in both cores leads to the reduction of buried Mn oxides, but is insufficient to remobilize Fe oxides. Under these conditions, a clear increase in Fecdb in the vicinity of the Mncdb reprecipitation zone should not be interpreted as the result of the diagenetic remobilization of Fe oxides. It cor- responds to a higher iron oxide content of the sediments deposited during glacial times. Although the sediment accumu- lation has been very slow since the onset of deglaciation, the increase in Fecdb appears fairly gradual because of bioturba- tional mixing.

Slow kinetics of authigenic carbonate fluorapatite precipitation The authigenic carbonate fluorapatite precipitation proposed

in this paper, throughout the interglacial stage of the high sedimentation rate piston core 013 on the Greenland rise, and in the two low sedimentation rate pelagic box cores 01 1 and 027 on both sides of the Labrador Sea, is the first documented observation of the occurrence of such a reaction in deep pelagic sediments.

Unlike the authigenic apatite precipitation reported in shelf sediments (Jahnke et al. 1983; Froelich et al. 1988; Ruttenberg and Berner 1993; Lucotte 1993), the reaction in Labrador Sea sediments is characterized by much slower reaction kinetics spanning over several thousand years. Furthermore, the CFA precipitation in pelagic sediments is not fueled by the minerali- zation of P,,,, as found by Ruttenberg and Berner (1993) in the Long Island Sound or in the Mississippi Delta, but from the gradual reprecipitation of reactive inorganic particulate phosphorus (Pcdb) The Pcdb is buried bound to iron oxides under suboxic conditions. In all three cores where CFA precipi- tation is presumed to occur, Fe oxides are not dissolved under reducing conditions. The Pcdb release must then come from a gradual recrystallization of the fine-grained and poorly crys- tallized Fe(II1) oxides, which frees some of the Pcdb initially bound to them. All together, there is no net loss of P after its burial below the surface redox transition, as the Pcdb decrease corresponds quantitatively to the Pcfa increase.

The CFA precipitation is independent of the depth of Pcdb burial: it extends to the first 400 cm of the fast-sedimenting piston core 013 or is restricted to the top 30 cm of very low sedimentation rate cores 01 1 and 027. The reaction occurs at a nearly constant rate, despite noticeable fluctuations with time of the detrital fluxes of P and Fe. This indicates that Pcfa for- mation depends mainly upon the time of crystallization and the availability of reactive inorganic P. In accordance with labora- tory experiments that emphasize the inhibiting effect of car- bonate ions on apatite precipitation in seawater (Jahnke 1984), CFA precipitation occurs in the sedimentary layers where the CaC03 content is the lowest. These results are reminiscent of the observed maximum P accumulation in carbonate-free pelagic sedimentary layers of the Pacific during the last 10 Ma (Moody et al. 1988).

Long-term diagenetic equilibrium and paleoproductivity The average authigenic apatite accumulation on continental

margins has been suggested to proxy for high initial C,, burial during marine transgressions, and this on several million year

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26 CAN. J. EARTH SCI, VOL, 31, 1994

time scales (Moody et al. 1988; Compton et al. 1993). Although this observation has recently been questioned (Filippelli and Delaney 1992), it is commonly accepted that Pcfa accumulation on continental margins reflects elevated paleoproductivities. In the pelagic sediments of the present study, the CFA precipita- tion appears totally decoupled from modern high surface produc- tivities but instead represents a long-term reequilibration of Pcdb deposited under oxic to suboxic conditions.

The long-term pelagic burial of Pcdb does not either represent a direct tracer of paleonutrient consumption or a straight- forward index of paleoproductivity, because of the diagenetic redistribution reported above. Nevertheless, because of its sensitivity to ambiant redox conditions, the degree of associa- tion of reactive inorganic P (the sum of Pcdb plus the fraction that has recrystallized as PCfJ to iron oxide may serve as a proxy indicator of paleoredox conditions. When the calculated R atomic ratio (Fecdb/Pcdb + APcfa) is low ( < 25), it reflects fairly oxidized paleosedimentary conditions, where reactive inorganic P remained adsorbed onto iron oxides. In contrast, R ratios larger than 35 record a greater demand for electron acceptors below the sediment surface. The R ratio is independ- ent of the absolute initial sedimentary Fecdb or Pcdb fluxes which are variable through time. When deep oceanic waters are well oxygenated, basin-wide sedimentary oxygen consump- tion may be related to biogenic particle mineralization (Jahnke and Jackson 1987; Betts and Holland 1991). Under these con- ditions the R ratio could be a first-order estimate of the amount of C,,, initially deposited on the sediments.

For piston core 013, the R ratio, which remained between 15 and 20 in the top 600 cm, suggests that oxic to suboxic con- ditions prevailed throughout isotopic stage 1 and probably stage 2. Iron oxides kept the equilibrium they originally attained below the surface diagenetic layer, with no detectable pyrite nor Fe carbonate precipitation. Subsurface organic matter bio- degradation at site 013 on the Greenland rise was never intense enough over the past thousands of years to lead to fully reduc- ing conditions, and most biodegradable C,,, that reached the sea floor must have been oxidized at or near the sediment - water interface by aerobic bacteria or benthic organisms. For this reason, in most studies, the accumulated sedimentary C,,, cannot be used as a quantitative record of paleoproductivity (e.g., Emerson and Hedges l 9 h ; Jahnke 1990; Lyle et al. 1992). The oxic to suboxic paleoconditions deduced for piston core 013, however, allow us to link the decreasing C,,, con- tent downcore to a reduced initial delivery of organic matter rather than enhanced biodegradation. For example, the mini- mum C,,, concentrations in the 240-360 cm depth interval (Mucci et a1.l) may be interpreted as a period of minimal organic matter sedimentary flux after mid-deglaciation. This conclusion could not have been drawn from the absolute abun- dance of micropaleontological indicators, such as foraminifers or coccoliths, as dissolution of their calcite tests has occurred. To fully reconstitute paleofluxes of C,,, to the sea floor from the amount of C,, buried, bioturbation processes must also be considered as indicated by the differences in C,,, burial rates between cores 017 and 020 (Berner and Westrich 1985; Emerson and Hedges 1988; Lee 1992).

Acknowledgments

This research was supported by a Collaborative Special Project grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). Financial support for

laboratory and analytical expenses was also provided by NSERC through operating grants to the authors and a team grant from the Fonds pour la Formation de Chercheurs et 1'Aide ?i la Recherche (FCAR) of Quebec. The authors greatly appreciated the critical reviews provided by Drs. Kathleen Ruttenberg, Philippe Van Cappellen, and Lewis Fox of a previous version of this paper.

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