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Aquatic Geochemistry5: 227–248, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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EH, pH Diagrams for Mn, Fe, Co, Ni, Cu and Asunder Seawater Conditions: Application of TwoNew Types of EH, pH Diagrams to the Study ofSpecific Problems in Marine Geochemistry

GEOFFREY P. GLASBY and HORST D. SCHULZUniversität Bremen, Fachbereich Geowissenschaften, Postfach 330 440, D-28334 Bremen, Germany

(Accepted 8 January 1999)

Abstract. EH, pH diagrams have been calculated using the PHREEQC programme in order to estab-lish the predominance fields of Mn, Fe, Co, Ni, Cu and As in bottom waters from the Angola Basin.Predominance fields are presented separately for both aquatic species and solid mineral phases inorder to simplify interpretation of the data. The diagrams show significant differences from standardEH, pH diagrams for these elements calculated for freshwater at 25◦C and 1 bar which assumean element concentration of 10−6 M. In particular, our diagrams show that Mn2+ and NiCO0

3 arethe predominant aquatic species for Mn and Ni in bottom seawater and FeOOH, Fe2O3, Fe3O4,CoFe2O4, CuFe2O4, CuFeO2, and Ba3(AsO4)2 the predominant solid phases for Fe, Co, Cu andAs, respectively. Mn and Ni are therefore undersaturated and Fe, Co, Cu and As supersaturated inbottom seawater from the Angola Basin. Neither rhodochrosite (MnCO3) nor siderite (FeCO3) canform in this marine environment in equilibrium with seawater. A mixed Mn-Ca carbonate is thereforeformed within the pore waters of reducing sediments. The high Ni/Cu ratios in cobalt-rich manganesecrusts formed adjacent to the oxygen minimum zone may be explained by the change from Cu2+ toCuCl2−3 as the dominant aquatic species of Cu in seawater at an EH of +0.48 V.

Key words: EH, pH diagrams, seawater

1. Introduction

EH, pH diagrams have been in use for more than 30 years (Garrels and Christ, 1965)and have played an important role in understanding marine geochemical processes.Brookins (1988) performed a valuable service in preparing such diagrams for 75elements. Unfortunately for the marine geochemist, these diagrams were calculatedfor natural waters at 25◦C and 1 bar, generally assuming an element concentra-tion of 10−6 M. We have therefore computed these diagrams for a number of keyelements under seawater conditions using element concentrations prevalent in thedeep waters of the Angola Basin as an example. The aim of this paper is to see ifthere are any significant differences between these new EH, pH diagrams and thosepresented by Brookins (1988) and, if so, to assess their geochemical implications.

228 GEOFFREY P. GLASBY AND HORST D. SCHULZ

2. Methods

Calculation of the EH, pH diagrams presented here involved the use of PHREEQE,a computer programme initially developed by Parkhurst et al. (1980, 1990) andlater modified by Kölling (1992). Subsequently, Parkhurst (1995) substantiallyrevized and amended this programme to produce a new version PHREEQC inorder to eliminate many of the deficiencies and limitations in PHREEQE. We haveused this new version which includes revized datasets of thermodynamic constantsin the calculations presented here. Unfortunately, no data were available for Coin PHREEQC. Calculations for Co were therefore made with PHREEQE usingthermodynamic data from the EQ3/EQ6 thermodynamic data set (Wolery, 1992).

One problem with these calculations is that so much information is producedthat it is no longer realistic to present all the data for each element on a singleEH, pH diagram. It was therefore decided to split the data to show the relationsbetween the various aquatic species and those between the various minerals whichare predicted to be supersaturated in the associated solutions. These results arethen presented in individual diagrams side by side which clearly show that theaquatic species and the solid mineral phases with which they coexist are initiallyindependent of each other. This challenges the implicit assumption of EH, pH dia-grams that aquatic species cease to exist at the boundary where solutions becomesupersaturated with respect to an associated mineral phase. Supersaturation doesnot necessarily mean precipitation and solutions can be supersaturated with respectto more than one mineral. In addition, the stability fields of related solid phaseswhich very often overlap are shown because the precipitation of solid phases isoften controlled by kinetic rather than thermodynamic factors. We believe thatthese diagrams therefore represent a more realistic statement of the geochemistryof individual elements than the more traditional EH, pH diagrams.

The concentration data used to calculate these diagrams were based on meas-ured seawater concentrations for bottom water from the Angola Basin and thetemperature was taken to be 2◦C. However, the diagrams were calculated at oneatmosphere pressure and no correction was applied for pressure. These data aresummarized in Table 1. For the solid phases, only oxides, hydroxides, sulphidesand carbonates were considered in these calculations. About 2000 points wereused in the calculation of each of these diagrams with repeat calculations madeat distances in pH of 0.1–0.21pH and in EH of 0.02–0.05 V. Nonetheless, someparts of the diagrams are not very meaningful such as those corresponding to pHvalues<4 and>9. However, the results calculated by PHREEQC represent themost modern and complete set of thermodynamic data available for equilibriumconstants and saturation equilibrium constants for individual species. In our inter-pretation of these diagrams, we have assumed an Eh of +0.4V and a pH of 8 forseawater. These are the values normally given in the literature (Garrels and Christ1965, Fig. 11.2; Brookins 1988, Fig. 1).

EH, PH DIAGRAMS FOR MN, FE, CO, NI, CU AND AS UNDER SEAWATER CONDITIONS 229

Table I. Listing of the various parameters (density, temperat-ure and element concentrations) used in the calculation of theEH, pH diagrams in seawater. Data for deep seawater takenfrom our own analyses in the Angola Basin. Concentrationsof Cu, Ni, As, Zn and Co taken from Bruland (1983).

Density (mg mm−3) 1.023

Temp. (◦C) 2.0

Ca (ppm) 427

Mg (ppm) 1 284

Na (ppm) 10 760

K (ppm) 390

Si (ppm) 7.8 As SiO2Cl (ppm) 19 350

C (ppm) 36.4 As C

S (ppm) 2 600 As SO2−4N (ppm) 2.5 As NO−3P (ppm) 0.2 As PO3−4F (ppm) 1.1

Sr (ppm) 7.1

Fe (ppb) 0.021

Mn (ppb) 0.012

Cu (ppb) 0.38 As Cu

Ni (ppb) 0.0017

As (ppb) 1.7 As As

Ba (ppb) 0.02

Zn (ppb) 0.52

Only in use with PHREEQE:

Co (ppb) 0.4

There were several reasons why two separate EH, pH diagrams are required toshow the distribution of aquatic species and the saturation of minerals. Althoughthe definition of the boundary for aquatic species is quite clear and the specieswith the highest concentration is always shown, aquatic species also occur whereminerals are supersaturated. Where several minerals are involved, the degree ofsupersaturation becomes greater and less space is available to show the distributionof the aquatic species. A single EH, pH diagram therefore becomes too complex. Inaddition, many minerals are supersaturated in parts of the diagram. Supersaturationdoes not necessarily lead to the precipitation of the mineral, only the possibility ofprecipitation. The solid phase diagrams show lines where the degree of saturationfor each mineral is zero. Minerals with the same constituents but different free en-


ergies of formation such as MnO2 have lines which are close together and parallelto one another because they are calculated using the same precipitation/dissolutionreactions but with different equilibrium constants.

The EH, pH diagrams presented here are specific for the deep ocean watersof the Angola Basin. They are not generally valid for all marine systems. Foreach specific situation, a new EH, pH diagram must be calculated. The calcula-tion of these diagrams depends on the thermodynamic dataset used, in this casethe most recent version of PHREEQC (Parkhurst, 1995; available on the inter-net http://wwwbrr.cr.usgs.gov/projects/GWCcoupled/). This dataset is compar-able with the WATEQ4 and MINEQL datasets which are also available in thePHREEQC format on the same web site. Each of these three datasets uses the mostrecent thermodynamic data and may be considered ‘state of the art’. Of course,the calculated diagrams present only equilibrium relationships and do not considerreaction rates or pathways. It is the decision of the author which mineral phases aredisplayed. In general, these are chosen to address a specific scientific question, inthis case the formation of deep-sea manganese nodules. Only a limited area on theEH, pH diagram is relevant to this specific problem. This choice of topic led us toconsider six elements relevant to the study of the deposition of the ferromanganeseoxide phases and the uptake of the various metals in the deep sea (Glasby, 1999).Co was chosen because it has two principal modes of uptake into the ferroman-ganese oxide phase, either in the divalent or trivalent state. Ni and Cu are takenup dominantly in the divalent state but Cu is of particular interest because of itsknown depletion in Co-rich manganese crusts taken below the oxygen minimumzone. As is of interest because it is an anionic species in seawater which is takenup in deep-sea manganese nodules predominantly in the iron oxyhydroxide phase.Self evidently, these diagrams are not valid for sulphide formation in anoxic basinswhere element concentrations are at least an order of magnitude higher than thoseconsidered here.

In particular, two diagrams were prepared for each element because too muchinformation is generated for one diagram. These diagrams may be considered to bepredominance diagrams, that is they show the predominant species present at eachpoint on the diagram. For the solid phases, the diagrams may also be considered tobe saturation diagrams. Each aquatic species is, in principle, present throughout thediagram, albeit in vanishingly small concentrations in some cases. In calculatingthese EH, pH diagrams, we consider only inorganic aqueous species as is standardpractice. It is known that a significant proportion of each element is present asorganic complexes in seawater and, more particularly, in sediment pore waters.However, this situation has only a minor influence on the predominance fields ofthe remaining inorganic aqueous species and affects the saturation boundaries ofthe solid phases only slightly. One significant conclusion to emerge from this studyis the need to calculate specific EH, pH diagrams in order to answer specific sci-entific questions about the conditions under which mineral formation/dissolutiontake place in the marine environment.


3. Results

The calculated EH, pH diagrams for Mn, Fe, Co, Ni, Cu and As are presented inFigures 1–6. They show significant differences from those presented by Brookins(1988).

For Mn, Mn2+ is the principal aquatic species of Mn in seawater as suggestedby Hem (1972), Brookins (1988), Gramm-Osipov et al. (1991) and Gammons andSeward (1996). MnCO03 becomes the predominant aqueous species at pH values>9.1 but MnCl+ is never a predominant species as indicated by Bruland (1983).In the solid phase, various MnO2 phases and MnOOH appear to be stable. Thiscontrasts with the of finding of Brookins (1988) that the main solid phases areMnO2, MnO and Mn3O4. δMnO2, one of the principal minerals occurring in deep-sea manganese nodules, has a standard free energy of−108.9 kcal/mole (Bricker1965). It can be shown that the stability boundary forδMnO2 has the same gradientas the boundaries for nsutite and pyrolusite and lies between these two lines. Non-etheless, these solid Mn oxyhydroxide phases are not thermodynamically stableunder deep sea conditions. Rhodochrosite (MnCO3) does not occur as a stablesolid phase. The formation of MnS requires high activities of dissolved sulphideand Mn coupled with a low activity of Fe (Böttcher and Huckriede, 1997). Thepredominance field of MnS therefore lies outside the predominance diagram ofMn presented here.

For Fe, Fe(OH)03 is the principal aquatic species in seawater. FeCl2+, FeSO+4 ,FeF2+, Fe(OH)+2 and Fe(OH)−4 are the other preferred forms of Fe in the trivalentstate and Fe2+ in the divalent state. The species identified here are the same as thoseindicated by Bruland (1983) to predominate in seawater (Fe(OH)0

3, Fe(OH)+2 andFe2+) with the exception of FeCl+ which is not observed. Fe(HS)0

2 and Fe(HS)−3 arethe stable aqueous species under reducing conditions. Goethite (α-FeOOH), ma-ghemite (γ -Fe2O3) and magnetite (Fe3O4) appear to be the stable solid phases un-der deep-sea conditions. Pyrite is the stable solid phase under reducing conditionsbut siderite (FeCO3) appears to be undersaturated in seawater.

For Co, Co2+ is the principal aquatic species in seawater. CoCO−3 and CoCl+

do not appear to be dominant species as indicated by Bruland (1983). Cosovicet al. (1982) established the dominant form of Co in seawater to be Co2+ withlesser amounts of CoSO0

4 and CoCl+ and minor amounts of CoOH+, Co(OH)02and CoCO0

3 based on polarographic studies but these additional species are notpredominant on this diagram. CoFe2O4 appears to be the stable solid phase underseawater conditions. This differs from the findings of Brookins (1988) that Co3O4

is the stable solid phase at the EH and pH of seawater with CoCO3 stable underslightly less oxidizing conditions and of Hem et al. (1985) that CoOOH is thestable solid phase at the EH and pH of seawater.

For Ni, NiCO03 is the principal aquatic species in seawater and not Ni2+ as

indicated by Bruland (1983) and Brookins (1988). NiCl+ is also not predominant.All solid phases appear to be undersaturated in seawater.

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Figure 1. EH, pH diagrams the aquatic species and solid phases of Mn calculated for deep-seawater conditions in the Angola Basin (Table 1). The solidphases shown in the diagram are nsutite (γ -MnO2), birnessite (Na0.7Ca0.3Mn7O14·2.8H2O but calculated from the data set as MnO2), pyrolusite (β-MnO2),bixbyite (α-Mn2O3), hausmannite (Mn3O4) and manganite (γ -MnOOH).

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S233Figure 2. EH, pH diagrams the aquatic species and solid phases of Fe calculated for deep-seawater conditions in the Angola Basin (Table 1). The solid phases

shown in the diagram are goethite (α-FeOOH), hematite (α-Fe2O3), magnetite (Fe3O4), maghemite (γ -Fe2O3), mackinawite (Fe9S8 but calculated from thedata set as FeS) and pyrite (FeS2).

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ZFigure 3. EH, pH diagrams the aquatic species and solid phases of Co calculated for deep-seawater conditions in the Angola Basin (Table 1). The solidphases shown in the diagram are cattierite (CoS), linnaeite (Co3S4) and Co-spinel (Co3O4).

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S235Figure 4. EH, pH diagrams the aquatic species and solid phases of Ni calculated for deep-seawater conditions in the Angola Basin (Table 1). The solid phase

shown in the diagram is millerite (NiS).

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ZFigure 5. EH, pH diagrams the aquatic species and solid phases of Cu calculated for deep-seawater conditions in the Angola Basin (Table 1). The solidphases shown in the diagram are cupric ferrite (CuFe2O4), cuprous ferrite (CuFeO2), chalcopyrite (CuFeS2), chalcocite (Cu2S), covellite (CuS) and cuprite(Cu2O).

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S237Figure 6. EH, pH diagrams the aquatic species and solid phases of As calculated for deep-seawater conditions in the Angola Basin (Table 1). The solid

phases shown in the diagram are realgar (AsS) and orpiment (As2S3).


For Cu, CuCl2−3 and Cu(OH)2 are the principal aquatic species in seawater butCu2+ becomes the dominant species at an EH above +0.48 V. This differs from thefindings of Bruland (1983) that the principal forms of Cu in seawater are CuCO0

3,CuOH+ and Cu2+. Cupric and cuprous ferrite (CuFe2O4 and CuFeO2) are the over-saturated solid phases under deep-sea conditions. Various sulphides occur underreducing conditions. This contrasts with the diagram of Brookins (1988) whichshows that the stable solid form of Cu at the EH and pH of seawater is CuO andignores the occurrence of CuCl2−

3 as an aquatic species.For As, HAsO2−

4 is the principal aquatic species in seawater. H3AsO03 becomes

stable under more reducing conditions. Ba3(AsO4)2 is the dominant solid phaseunder deep-sea conditions.

In the above diagrams, Mn and Ni are thermodynamically stable in the aqueousphase in under deep sea conditions demonstrating that the solid phases are under-saturated and Fe, Co, Cu and As in the solid phase demonstrating that the solidphases are supersaturated.

4. Discussion

The validity of EH, pH diagrams is, of course, dependent on the thermodynamicdata from which they were calculated. This is a problem in most natural environ-ments where many solid phases are fine grained, non stoichiometric and impure.The use of themodynamic data for crystalline, pure solids is therefore not strictlyaccurate. This is particularly true for the solid phases of Mn and Fe, MnOOH,MnO2 and goethite, reported here (Giovanoli, 1980; Giovanoli and Arrhenius,1988). This problem has been discussed by Glasby (1974), Gramm-Osipov et al.(1984) and Skinner and Fitzpatrick (1992). For Mn, there are a number of defectoxides. The occurrence of cryptocrystalline oxides suggests that these depositshave lower nucleation energies or lower free energies of formation than the morecrystalline oxides (Burns and Burns, 1979), although the differences in free en-ergy are small. Accurate thermodynamic data for these oxides are virtually im-possible to obtain (Burns and Burns, 1979), although they are available forδMnO2

(Bricker, 1965). For Fe, goethite (α-FeOOH) is the thermodynamically most stableFe oxyhydroxide mineral under seawater conditions (Gramm-Osipov et al., 1984;Waychunas, 1991; Schwertmann and Fitzpatrick, 1992) but ferrihydrite (FeOOH)(Murad and Schwertmann, 1988; Puteanus et al., 1991) or akagenéite (β-FeOOH)(Johnston and Glasby, 1978; Murray, 1979; Chen and Yao, 1995) are more likelyto occur in marine minerals because of kinetic factors. However, thermodynamicdata are not available for these minerals. Thermodynamic equilibria are, of course,dependent on temperature. In supercritical, hydrothermal fluids, Mn and Fe aretransported in the form of MnCl−3 and FeCl−3 (Uchida et al., 1995) (cf. Gammonsand Seward, 1996). This would facilitate the migration of Mn and Fe in submarinehydrothermal fluids.


For Mn, the role of MnOOH in formation of Mn oxides has long been known.For example, Bricker (1965) showed that MnOOH was precipitated first on ox-idation of Mn2+ and then immediately oxidized giving the appearance of directprecipitation to the more highly oxidized phase (cf. Grill, 1982; Pokrovskiy andSavenko, 1995; Gramm-Osipov, 1997). The MnOOH present in the marine en-vironment is probably feitknechtite (βMnOOH) (Hem and Lind, 1983; Hem et al.,1989). However, the reaction pathway proposed by Giovanoli (1980) does not showβMnOOH as an intermediate in the formation of marine manganese nodules asdoes that of Bricker (1965). Crerar et al. (1980) pointed out that Mn(III) is unstablewith respect to Mn(II) and Mn(IV) and therefore considered that Mn is present infeitknechtite as Mn(II) and Mn(IV) in equal amounts (cf. Gramm-Osipov et al.,1992). However, new evidence suggests that Mn inβMnOOH is present mainly asMn (III) (Murray et al., 1985; Manceau et al., 1992; Junta and Hochella, 1994).

From the EH, pH diagram, Mn appears to be undersaturated in seawater withrespect to MnOOH (cf. Crerar and Barnes, 1974). The phases precipitated on theseafloor may therefore be more stable than those prepared in the laboratory onaccount of their fine grain size and uptake of trace metals (Glasby, 1974). Onkinetic grounds, it is known that efficient surface catalysis or microbial mediationis required for Mn2+ to be oxidized to MnO2 on a realistic time scale (Diem andStumm, 1984; Tebo et al., 1997). Burns and Burns (1977) have demonstrated therole of FeOOH in initiating the deposition of Mn oxides in the marine environ-ment. Autocatalytic oxidation on Mn oxides then becomes more rapid (Stumm andMorgan, 1970).

For Fe, goethite is the stable phase at the redox conditions of seawater. Thiswould account for the fact that Fe is present in seawater in both the particulateand soluble form (Bruland et al., 1994). Because of their different stability fields,FeOOH is less readily mobilized but more readily precipitated than MnO2 in themarine environment (Crerar and Barnes, 1974; Davison, 1993). This means thatMn is more mobile than Fe in both submarine hydrothermal (Hannington andJonasson, 1992; Lilley et al., 1995) and shallow marine (Glasby et al., 1997) envir-onments.

These diagrams show that neither rhodochrosite (MnCO3) nor siderite (FeCO3)can form in the marine environment in equilibrium with seawater. However, asshown by Berner (1981), rhodochrosite does occur in both anoxic sulphidic andnon-sulphidic environments whereas siderite is restricted to marine and non-marineanoxic non-sulphidic methanic environments. In fact, both these minerals are re-stricted to rapidly-accumulating, fine-grained, organic-rich sediments where CO2

is produced as a result of oxidation of organic matter, in part by Mn and Fe oxyhy-droxides (Robbins and Callender, 1975; Emerson, 1976; Murray et al., 1978; Suess,1979; Balzer, 1982; Manheim, 1982; Röper, 1988; Bruno et al., 1992; Huebneret al., 1992; Davison, 1993; Schulz et al., 1994; Thamdrup et al., 1994; Torreset al., 1995). Such sediments are characterized by high alkalinities and high dis-solved Mn(II) and Fe(II) contents. In some cases, anoxic pore waters taken several


metres below the sediment-water interface can have alkalinities nearly two ordersof magnitude higher than in the overlying seawater (Schulz et al., 1994).

In most cases, a mixed Mn—Ca carbonate rather than pure rhodochrosite isformed (Suess, 1979; Manheim, 1982; Pedersen and Price, 1982; Middelburg et al.,1987; Aller and Rude, 1988; Jakobsen and Postma, 1988; Okita et al., 1988; Okitaand Shanks, 1992; Calvert and Pedersen, 1993; Salonen et al. 1995; Huckriedeand Meischner, 1996, Carman and Rahm, 1997; Friedl et al., 1997; Sternbeck andSohlenius, 1997; Böttcher, 1998). Pure rhodochrosite can not form within marinesediments because the compositions of the pore waters do not lie within a rangein which they would be in equilibrium with pure rhodochrosite (Middelburg et al.,1987). Unfortunately, no thermodynamic data are available for the mixed Ca—MnCO3.

Calvert and Pedersen (1993) concluded that Ca—MnCO3 is precipitated fromthe interstitial waters of sediments rather than the overlying seawater since theaqueous Mn concentrations must be five orders of magnitude higher than those inoxic sea water for Ca—MnCO3 to form. Only there would the Mn concentrationbe high enough to exceed the solubility product of MnCO3 (20–120µmole/l)(Pedersen and Price, 1982). However, this calculation overestimates the concen-tration of Mn required since it is the thermodynamically more stable Ca—MnCO3

rather than the pure MnCO3 which precipitates. The precipitation of Ca—MnCO3

was assumed to occur only when the bottom sea water is oxic because only thencan the surface sediments accumulate Mn oxyhydroxides to supply the Mn tobe enriched in the interstitial waters of subsurface sediments on burial (Calvertand Pedersen,1993). These authors therefore proposed that the presence of Ca—MnCO3 can be used as a reliable index of sedimentation under oxygenated bottomwater conditions. However, this model does not appear to be strictly applicable tothe formation of Ca-rhodochrosite in the anoxic deeps of the Baltic Sea. Jakobsenand Postma (1989) have shown that pore waters in the sediments from these deepsare greatly supersaturated with respect to both rhodochrosite and calcite. The dis-solved Mn contents in the bottom waters of the deeps (> 12µmole/l) would notbe high enough to facilitate deposition of Ca-rhodochrosite (Glasby et al., 1997).Both Huckriede and Meischner (1996) and Sternbeck and Sohlenius (1997) havetherefore proposed that the formation of discrete horizons of Ca-rhodochrositewithin the sediments of the Baltic deeps is related to periods of stagnation of thedeeps. During periodic flushing of the deeps by well-oxygenated North Sea water,oxidation of Mn2+ in the bottom waters takes place to produce particulate MnO2.During the subsequent stagnation, manganese is again reduced and enriched in thenear-bottom waters from which it is then precipitated as a layer of Ca-rhodochrositein the sediments. These data indicate that Ca-rhodochrosite is formed in the BalticSea deeps only when stagnant conditions develop again following the intermediatedeposition of particulate MnO2 at the sediment surface in these deeps. Manganeseis known to be easily leached from sediments in the Baltic Sea under reducing


conditions (Glasby et al., 1997). The ultimate source of the manganese in BalticSea ferromanganese ores is believed to be glacial till (Zhamoida et al., 1996).

Several authors have concluded that the formation of authigenic MnCO3 de-posits is driven by microbial methanogenesis within the sediment column (Hein etal., 1987; Matsumoto, 1987; Curtis, 1995). According to the equations presentedby Curtis (1995), manganese and iron reduction resulting from methanogenesis inthe presence of sulphate ions produces rhodochrosite and pyrite respectively. Onlyin the absence of sulphate ions is siderite (FeCO3) produced (Carman and Rahm,1997). This explains why siderite is not often reported in marine deposits.

Diagenetic processes play a key role in recycling Mn and Fe in marine sedi-ments. Recent papers describing these processes in detail have been presented bySantschi et al. (1990), Schulz et al. (1994), Thamdrup et al. (1994), Dhahar andBurdige (1996), Soetaert et al. (1996), Van Cappellen and Wang (1996), Haese etal. (1997) and Luther et al. (1998).

For Co, there has been considerable speculation on its mode of uptake in deep-sea manganese nodules and crusts as a consequence of its tendency to be associatedwith either Mn or Fe oxyhydroxides (Dillard et al., 1982, 1984; Halbach et al.1982;Crowther et al., 1984; Hem et al., 1985; Giovanoli and Arrhenius, 1988). Accordingto Giovanoli and Arrhenius (1988), Co may subsitute as exchangeable Co2+ in mar-ine manganates and then be oxidized to Co3+ by Mn4+. Co3+ may also substitutefor Fe3+ in Fe oxyhydroxide minerals in the nodules. Crowther et al. (1983), on theother hand, demonstrated that Co is incorporated in synthetic birnessite as Co(II)to pH 7 and as Co(III) above pH 8. Based on synthetic experiments, Hem et al.(1985) proposed that CoOOH is the principal solid phase in deep-sea manganesenodules. The fact that CoFe2O4 and not CoOOH is the stable solid phase of Counder seawater conditions complicates this picture somewhat.

For Ni, the occurrence of NiCO03 as the principal aquatic species in seawaterwould have implications for the sorption of Ni on deep-sea manganese nodulessince it is normally assumed that sorption of Ni2+ takes place on the surface of thenodules (Glasby and Thijssen, 1982).

For Cu, the boundary between Cu2+ and CuCl2−3 as the dominant species inseawater lies slightly above the normal range of redox conditions in seawater (EH

of +0.48 V). At an EH of +0.48 V, the concentration of Cu2+ in seawater would stillbe sufficient for it to be incorporated into manganese nodules by sorption on thesurface of negatively charged MnO2 (Glasby, 1974; Glasby and Thijssen, 1982). Atsomewhat lower EH values, however, the concentration of Cu2+ in seawater woulddecline drastically and the anionic species CuCl2−

3 would be the dominant species.Its sorption on MnO2 would be inhibited by charge considerations. This may wellexplain the high Ni/Cu ratios observed in cobalt-rich manganese crusts (max. 15)formed adjacent to the oxygen minimum zone where less oxidizing conditionsprevail (Halbach et al., 1983, 1989; Dymond et al., 1984; Aplin and Cronan, 1985;Mangini et al., 1987; Meylan et al., 1990). Varentsov et al. (1985) has pointed outthe role of hydrostatic pressure in controling the uptake of metal ions in manganese


nodules from the World Ocean but our study would suggest that other factors aremore important in the case of Cu. Interestingly, other authors have not consideredthe possibility of CuCl2−3 being a predominant species in seawater (Bruland, 1983;Varentsov et al., 1985; Zaitseva and Varentsov, 1990).

For As, it is generally assumed that sorption of an anionic As species, probablyarsenate or HAsO2−4 , on ferric oxyhydroxide takes place in the marine environment(Kunzendorf and Glasby, 1992; Metz and Trefry, 1993; Fuller et al., 1993; Rudnickiand Elderfield, 1993; Lilley et al., 1995; Widerlund and Ingri, 1995). In particular,Pokrovski et al. (1996) have suggested that H3AsO0

3 may be the dominant arsenicspecies in hydrothermal fluids in the pH range 0-8 at temperatures of 20–300◦C(cf. Tossell, 1997). Our data indicate the presence of HAsO2−

4 in seawater whichwould explain the sorption of arsenic on ferric oxyhydroxides (cf. Vink, 1997).Nonetheless, the occurrence of Ba3(AsO4)2 as the stable solid phase at the pHof seawater does suggest that this mineral could occur in submarine hydrothermaldeposits. This may explain the association of Ba and As in submarine hydrothermaliron-rich crusts (e.g., Binns et al., 1993; Stoffers et al., 1993).

The above discussion is based on the assumption that the distribution and speci-ation of elements in the oceans is controlled by thermodynamic factors. As is wellknown, however, sorption reactions dominate in the marine environment (Turekian,1977) with sorption of trace elements on Mn (Balistrieri and Murray, 1986) andFe (Tessier et al., 1985; Liang and Morgan, 1990) oxyhydroxides being of par-ticular importance. As an example, the association of elements with the Mn andFe oxyhydroxide phases in marine manganese deposits is directly related to theelement speciation in seawater and the physico-chemical properties of the carrierphase (Koschinsky and Halbach, 1995). The adsorption of Co, Ni and Cu on Mnoxyhydroxides and of Co and As on Fe oxyhydroxides should therefore be bornein mind.

5. Conclusions

This study has shown that EH, pH diagrams calculated using the PHREEQC methodare useful tools in determining the speciation of trace elements in seawater. Thesediagrams have the potential to establish the speciation of other elements in seawa-ter. We believe that this approach can offer valuable insights into the marine geo-chemistry of these elements. However, it is necessary to calculate EH, pH diagramsfor each specific situation if meaningful results are to be obtained.

Acknowledgements

We would like to thank Martin Kölling for helpful discussions. We thank John W.Morse for the editorial handling of the manuscript and an anonymous reviewerfor helpful and constructive comments on the first draft of this manuscript. This


research was funded by the Deutsche Forschungsgemeinschaft (contribution no.268 of Special Research Project SFB 261 at the University of Bremen).

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