Control ofseawater composition bymixingofriverwaters

Fluid-Mineral Interactions: A Tribute to H. P. Eugster© The Geochemical Society, Special Publication No.2, 1990

Editors: R. J. Spencer and l-Ming Chou

Control of seawater composition by mixing of river watersand mid-ocean ridge hydrothermal brines

RONALD J, SPENCERDepartment of Geology and Geophysics, The University of Calgary, Calgary, Alberta T2N lN4, Canada

and

LAWRENCEA. HARDIEDepartment of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.

Abstract-The two major input streams to seawater are Na-HCOrS04-rich river waters and Ca-Cl-rich mid-ocean ridge hydrothermal brines. A model is presented which accounts for the major elementcomposition of modern seawater by mixing of river water with mid-ocean ridge hydrothermal brinesand the precipitation of calcium carbonate. A steady-state composition for the major ionic speciesin seawater, Na", Ca2+,Mg2+,Cl", SO~- and HCO), can be obtained within the range of publishedvolume fluxes for river water and mid-ocean ridge hydrothermal brines using reasonable estimatesof river water and mid-ocean ridge hydrothermal brine compositions. A steady-state concentrationof K+ in modern seawater requires additional sinks for potassium.

The steady-state concentrations of the major ionic species in seawater are very sensitive to changesin the volume ratios of river water and mid-ocean ridge hydrothermal brines input to the oceans. Itis likely that the ratio of these two major input streams to seawater has changed through geologictime, and that the major ion concentrations in seawater have changed. A smaller mid-ocean ridgehydrothermal brine flux results in lower concentrations of Ca?" in seawater and higher concentrationsof Na", Mg2+,SO~- and HCO), Larger mid-ocean ridge hydrothermal brine fluxes result in higherconcentrations of Ca2+ in seawater and lower concentrations of Na", Mg2+,SO~- and HCO). Thecompositional variations in seawater that result from even modest changes of a few percent in theratio of river water and mid-ocean ridge hydrothermal brines are significant because of the potentialinfluence on chemical sediments produced from seawater. Smaller mid-ocean ridge hydrothermalbrine volume fluxes are likely to favor aragonite precipitation rather than calcite and result in marineevaporites which contain magnesium sulphate minerals. Larger mid-ocean ridge hydrothermal brinevolume fluxes favor calcite precipitation and result in marine evaporites containing potash salts(sylvite and carnallite) without magnesium sulphate minerals.

INTRODUCTION: THE PROBLEM OF THECHEMICAL COMPOSITION OF SEAWATER

composition for the last 1-2 billion years (GARRELSand MACKENZIE, 1971; HOLLAND, 1972). Based onthese assumptions, then, the principal question thata number of workers have addressed is how thechemistry of the oceans is buffered against the mas-sive inflow of river waters which have compositionsso widely different from that of modern seawater(Table 1).

SINCETHECLASSICPAPEROF RUBEY (1951) on theproblem of the history of seawater, a number ofworkers, using heterogeneous equilibrium modelsor mass balance methods, have attempted to ac-count for the chemical composition of the presentoceans (e.g., SILLEN, 1961, 1967; GARRELS andTHOMPSON, 1962; MACKENZIE and GARRELS,1966; HELGESONand MACKENZIE, 1970; GARRELSand MACKENZIE, 1971; HOLLAND, 1972, 1984;LAFON and MACKENZIE, 1974; MAYNARD, 1976;DREYER, 1982). There is wide agreement amongthese workers that:

SILLEN(1961, 1967) first suggested that seawateris in heterogeneous equilibrium with the silicate andcarbonate sediments on the ocean bottom. In thisapproach, "reverse weathering" reactions such asdetrital kaolinite forming authigenic illite-smectitewere called on to control the ratios of the major

1) the basic salinity and chlorinity, and perhaps cations at or near those of modern seawater (seealso the essential major ion composition of the also MACKENZIE and GARRELS, 1966; HELGESONoceans were, inherited from the primordial reaction and MACKENZIE, 1970). Recently, however, it hasbetween crustal igneous rocks and outgassed acid been argued that authigenic silicates are too sparselyvolatiles such as HCl and SOz at the time the oceans distributed in the ocean, and that reactions involv-were formed (RUBEY, 1951; GARRELS and MACK- ing detrital marine clays are too slow, to buffer sea-ENZIE, 1971; LAFON and MACKENZIE, 1974), water composition against the river input (DREYER,

2) seawater has remained close to its present 1974, 1982; MAYNARD, 1976; HOLLAND, 1978).409

410 R. J. Spencer and L. A. Hardie

This, together with the discovery of extensive hy-drothermal alteration of seawater at mid-oceanridges (see THOMPSON,1983, for a summary), hasgiven rise to steady-state models involving the bal-ancing of sources and sinks (e.g., MAYNARD,1976;DREYER,1982; HOLLAND,1978), In these modelsthe present day river water fluxes of ions are com-pared to the estimated rates of ion uptake fromocean waters by:

1) deposition of marine carbonates, cherts andevaporites,

2) storage of seawater in the pores of marinesediments,

3) low temperature interaction between seawaterand ocean floor basalts,

4) high temperature interaction between sea-water and basalt at mid-ocean ridges,

5) ion-exchange by marine clays,6) precipitation of authigenic silicates and sul-

fides.

If the chemical composition of the oceans is to re-main constant, then the input fluxes must remainin dynamic balance with the output fluxes through-out geologic time, that is, for each ion:

dAJdt = L fir (inputs) - L fir (outputs) = 0

where Ai is the concentration of ion i, t is time, andfir is the flux of ion i from reservoir r.

It is the view of HOLLAND(1978, p. 5) that "earthreservoirs are not at steady state" and that excur-sions in seawater composition through time arelikely (see also GARRELSand MACKENZIE,1971, p.

297) given that global tectonic cycles operate overtime spans that are longer than the residence timesof all the major ions in the oceans. In addition,climatic changes such as occurred during the majorice ageswould have been capable of influencing thefluxes of those components with relatively shortresidence times, These components include bicar-bonate-carbonate, silica,water and perhaps calcium,HOLLAND(1972, 1984)has attempted to determinethe limits of these chemical excursions by inversemodelling using the mineralogy of marine evapo-rites deposited over the last 900 million years. Hehas concluded that while "the concentration of themajor ions in seawater has not varied a great dealduring the Phanerozoic, , . the imposed limits onexcursions. , , are. , , fairly broad" (HOLLAND,1984,p. 536). In particular, concentrations of Mg'",ea2+ and SO~- could have been two or three timeslarger or smaller than the present day ocean values.Such changes, coupled with variations in biologi-cally-controlled or biologically-moderated com-ponents (oxygen, carbon dioxide, silica, phosphate,nitrate, trace elements, stable isotopes, etc.), mayhave considerable effects on the nature and com-position of chemical and biochemical sedimentarydeposits through geologic time,

With all this in mind we have explored the con-sequences of a simple model that assumes that thecomposition of seawater is mainly controlled now,and has been controlled through geologic time, bythe mixing of river waters and mid-ocean ridge(M,O,R,) hydrothermal brines, Using this modelwe have attempted to predict the direction and pos-

Table I. Major ion composition of seawater, mid-ocean ridge hydrothermal brines and average world river waterin ppm

2 3 4 5 6 7 8 9

Na+ 10800 9610 10444 11725 9932 11243 11794 6.3 5.15K+ 407 1348 1382 1009 907 1896 1450 2.3 1.3Ca2+ 413 1530 1812 805 625 1162 1663 15, 13.4Mg2+ 1296 16 8 0 0 0 0 4.1 3.35

Cl- 19010 19260 20745 20530 17338 21310 22586 7.8 5.75SO~- 2717 31 72 0 0 0 0 11.2 8.25HCO) 137 1926* 2650* 0 0 647 396 58.4 52.

I. Seawater, RILEY and CHESTER (1971),2. Hydrothermal brine, Borehole #8, Reykjanes, Iceland, BJORNSSON et al. (1972),3. Hydrothermal brine, Borehole #2, Reykjanes, Iceland, BJORNSSON et a/. (1972),4. Hydrothermal brine NGS, 210 North, VON DAMM et al. (1985a).5, Hydrothermal brine OBS, 210 North, VON DAMM et a/. (1985a),6. Hydrothermal brine #1, Guaymas Basin, VON DAMM et al. (1985b).7. Hydrothermal brine #3, Guaymas Basin, VON DAMM et al. (l985b).8. Average world river water, LIVINGSTON (1963).9. Natural average world river water, MEYBECK (1979).* Reported as total CO2,

Control of seawater composition

sible magnitude of the changes in the major ioncomposition of seawater that would result fromlikely secular changes in the relative magnitudes ofthe fluxes of river water and M,O.R. hydrothermalbrines. In turn, we have used the predicted seawatercomposition variations to determine the nature ofvariations likely to have occurred in the primarymineralogy of ancient marine carbonates and ma-rine evaporites,

SEAWATER AS A MIXTURE OF RIVERWATER AND M.O,R, HYDROTHERMAL

BRINES: A WORKING MODEL

We present in this paper a very simple modelthat explores how the chemical composition of sea-water is influenced by the two major input streamsof fluid to the oceans, that is, river waters andM,O,R, hydrothermal brines, To explain the evo-lution of brine compositions in non-marine salinelakes HARDIE and EUGSTER (1970) developed theidea of "chemical divides" in which early precipi-tation of the relatively insoluble minerals calciteand gypsum determined the ultimate chemical sig-nature of the brines. The success of this simple con-cept in accounting for the composition oflacustrinebrines, where mixing of several source waters is thenorm, has encouraged us to apply it to the evolutionof seawater.

The concept of chemical divides can be illustratedon a simple ea-He03-S04 ternary phase diagram(Fig. 1). Two chemical divides (one from eae03 toS04 and one from eae03 to eaS04) separate threefields (Na-He03-S04, el-S04 and Ca-Cl waters) onthe phase diagram, The body of the phase diagramis the primary stability field for calcite; the stabilityfields for gypsum and anhydrite are located alongthe ea-S04join, Waters within the body of the phasediagram precipitate eae03 solid phases and the so-lution compositions evolve directly away from theeae03 composition point. In this scheme modernseawater is a el-S04 type water (Fig, 1), The mole-equivalent concentration of ea2+ in seawater isgreater than the equivalents of He03", but is lessthan the combined mole-equivalent concentrationof He03" and SO~-, Therefore, after the precipi-tation of calcite and gypsum, evolved seawaterbrines become enriched in both Cl" and SO~- , anddepleted in ea2+,

Evolved continental el-S04 brines, similar incomposition to modern seawater, that have resultedfrom mixing ofNa-He03-S04 river waters and Ca-Cl spring waters, have been documented for theGreat Salt Lake, Utah (SPENCER et al., 1984, 1985)and several lakes on the Qarhan salt plain, QaidamBasin, China (LOWENSTEIN et al., 1989; SPENCER

411

Ca 2+M.O.R.

HYDROTHERMALBRINES

Ca-CI

GYPSUMANHYDRITE

WORLD RIVERWATER

FIG, I. Ternary Ca2+-S0~--HCO) phase diagram, inmole-equivalents, The body of the diagram is the primarystability field for calcite; gypsum and anhydrite stabilityfields are along the Ca2+-S0~- join. Lines from calcite togypsum/anhydrite and from calcite to SO~- are chemicaldivides for the system which separate Ca-Cl, Cl-SO. andNa-HCOrSO. type waters, Average world river water hasa Na-HC03-SO. composition, modern seawater is a Cl-SO. water and M.O.R. hydrothermal brines have a Ca-Clcomposition.

et al., 1990), The compositions of these non-marinebrines can be explained by our simple model. Whatis particularly relevant here is that the basic chemicalsignatures of the two major inflow waters to thesenon-marine basins are similar to the two major in-flow waters to the oceans. In this sense the GreatSalt Lake and Qaidam Basin mixed inflow systemsare valuable analogues for the modern ocean sys-tem, and provide ample support for the basic con-cept behind our simple model.

In the chemical divide scheme, biological andchemical precipitation of calcium carbonate phasesis the most important process in the early stages ofbrine evolution, Since seawater is a relatively dilutebrine that falls into the calcium carbonate precip-itation regime, we have restricted our modelling ofseawater to simple mixing of Na-He03-S04 riverwater and Ca-Cl M,O,R. hydrothermal brine com-bined with precipitation of calcium carbonate. Wehave not included more complex mechanisms suchas cation exchange by river-borne clays, authigenicsilicate mineral precipitation, or sulphate reductionby microbes, although their exclusion undoubtedlyintroduces some error (see estimations by MAY-NARD, 1976, of the contributions of several pro-cesses to the chemical mass balance in the presentday oceans), However, our experience in non-ma-rine systems indicates that these reactions do notplay more than minor roles in the chemical evo-


lution of brines. We have not considered passivesinks such as evaporite precipitation and connateseawater storage that do not change the compositionof seawater.

THE MIXING MODEL AND THECOMPOSITION OF THE MODERN OCEAN

The basic data needed to calculate the compo-sition of seawater with this simple model are:

1) the chemical composrtion of average riverwater and M,O.R. hydrothermal brine,

2) the annual inflow volumes of river water andM.O.R. hydrothermal brine,

3) information on the anthropogenic input ofsolutes to the oceans.

River water composition

The average composition of modern rivers flow-ing into the oceans has been estimated by LIY-INGSTON(1963) and MEYBECK(1979) (Table 1).This average world river water has a Na-He03-S04composition, as illustrated in Fig. 1, The concen-tration, in mole-equivalents, of He03" is greaterthan that of ea2+ in this average world river waterand therefore, on evaporation and precipitation ofalkaline earth carbonates the resulting brines wouldbe enriched in He03" and SO~- and depleted inCa"', With continued evaporative concentrationthese river water-derived brines will eventually pre-cipitate NaHe03 and NaS04 salts and become al-kaline brines (HARDIEand EUGSTER,1970). Sig-nificant uncertainty in the river inflow compositionis introduced by the uncertainties in the amountsof anthropogenic sources of solutes in modern riverwaters. MEYBECK(1979) estimates the anthropo-genic input of sulphate to be 25% of the total riversulphate while HOLLAND(1978) suggests a highervalue around 40%, For bicarbonate or carbon inmodern river waters MEYBECK(1979, 1982) esti-mates 2 to 9% of the total is anthropogenic,

M,o.R. hydrothermal brine composition

For the M,O,R, hydrothermal brine input wehave used the chemical analyses of BJORNSSONetal. (1972), EDMONDet al. (1979) and YONDAMMet al. (1985a,b) (Table 1), M,O,R, hydrothermalbrines belong to the Ca-Cl composition type (Fig.1), In these brines the mole-equivalent concentra-tion of ea2+ exceeds the combined mole-equivalentconcentrations of He03" and SO~- and therefore,on evaporation and precipitation of alkaline earth

carbonates and calcium sulphate minerals, the re-sidual brines will become depleted in He03" andSO~- and enriched in ea2+ and cr.

Volume estimates

Volume estimates of river water inflow into theoceans today range from 3.0 X 1016 to 5.0 X 1016

litres per year (1/yr) (see HOLLAND,1978, p. 65),LIYINGSTON(1963), for example, estimates 3.25X 10161/yrwhile MEYBECK(1979) gives 3.74 X 1016

l/yr, The equation of PROBSTand TARDY(1989,p. 276) yields runoff values from 3,90 X 1016 to4,02 X 10161/yr. PROBSTand TARDY(1989) suggestthat anthropogenic sources have led to an increasein discharge with time. For the model calculationswe have selected a value of 3,75 X 10161/yr for thetotal flux of river waters to the oceans. As a guideto the flux ofM,O.R, hydrothermal brine we haveused the estimates based on heat flowmeasurements(see, for example, HOLLAND,1978). These valuesrange from 6,0 X 1013to 9. X 10141/yr.

A constant composition modern ocean model

A constant composition modern ocean modelrequires that the net flux of each species be zero, Inour model the net flux for each of the major ionicspecies in seawater is examined for various com-binations of river water and M,O,R, hydrothermalbrine volumes within the limits given above. Wehave made minor alterations in the composition ofriver inflow in order to simplify the model.

The starting point for our model calculation isthe average world river water composition, forwhich we have used the estimate of MEYBECK(1979) (Table 2, line 1), The composition of thisaverage river water is adjusted to zero chloride bysubtracting ions in proportion to the compositionof modern seawater (Table 2, line 2) on the as-sumption that the chloride in river waters is recycledfrom seawater as salt aerosols dissolved in rainwater,The chloride-adjusted composition of river wateris used along with the estimate for the volume ofinflow to arrive at an initial value of the total fluxof solutes to the ocean from rivers.

The analysis ofM.O,R, hydrothermal brine fromborehole #8, Reykjanes, Iceland, given by BJORNS-SONet al. (1972) (Table 1) is used to represent thecomposition of M,O,R, hydrothermal brines, Thebicarbonate alkalinity is zero, by charge balance(Table 2, line 5), Seawater is assumed to be theoriginal source water which interacted with oceanicbasalts at greenschist facies temperatures to producethe hydrothermal brine (for details see THOMPSON,

Control of seawater composition 413

Table 2. Composition of seawater inflows (rneq/l)

Na+ K+ Ca2+ Mg2+ CI- SO~- HCO)--

River

I 0.224 0.036 0.669 0.275 0.162 0.172 0.8522 0.085 0.033 0.663 0.244 0.0 0.155 0.8523 0.083 0,032 0.644 0.237 0,0 0.126 0.8524 0.089 -0.062 0,713 0,237 0.0 0.126 0.852

M.O.R.

5 418.0 37.3 76.3 1.3 532.3 0.6 0.06 430.2 38.4 78.5 1.3 547.9 0.7 0.07 469.7 10.3 20.6 106,6 547.9 56.6 2.28 -39.4 28.1 57.9 -105.3 0.0 -55.9 -2.2

I-natural river input (MEYBECK,1978),2-adjust for CI- = O.3-adjust for mass balance on SO~-.4-adjust for mass balance with constant M.O.R. hydrothermal flux.5-M.O.R. hydrothermal brine.6-M.O.R. hydrothermal brine at seawater cr.7-seawater.8-net M.O.R. flux.

1983;HARDIE,1983). Thus, all solutes are adjustedproportionately to modern seawater chlorinity (Ta-ble 2, line 6) and the difference between the adjustedcomposition and the average composition of mod-ern seawater (from RILEYand eHESTER, 1971) asgiven in Table 2 (line 7) is used to estimate the netaddition or subtraction of each solute fromthe M.O.R, hydrothermal brine system (Table 2,line 8).

The net flux of Cl" is zero because we adjustedboth of the inputs to be zero. The dissolved solutefluxes of all other river water components are pos-itive, The fluxes of K+ and ea2+ from the M.O,R.hydrothermal brine source are positive, while thoseof Na", Mg2+,SO~- and He03" are negative. In or-der to maintain a constant composition ocean inour simple model, the net flux of Na", K+, Mg2+and SO~- must be zero and the net flux of ea2+must equal that of He03" .

One test of whether this simple model can ac-count for the composition of modern seawater isto calculate the residual net flux of each solute aftermixing the adjusted compositions of river water andM,O,R. hydrothermal brine in proportion to theirestimated volume inflow fluxes, The mixing of riverwater with M.O.R. hydrothermal brine leads to su-persaturation with respect to calcite (and aragonite)and thus in the model calculations, appropriateamounts of ea2+ and He03" must be subtracted. Ifafter this the net solute fluxes are zero, or withinthe uncertainty of the input data, then the modelis a viable one, Unfortunately, the range of estimates

of the volume flux of M.O.R. hydrothermal brineis large enough that an unequivocal answer doesnot result from this approach, The range of mixturescalculated from the volume estimates are shown onthe Ca-He03-S04 ternary phase diagram (Fig, 2),

A test of the model that avoids the uncertainties

FIG. 2. Ternary phase diagram (see Fig. I) showing themixing line for M.O.R, hydrothermal brines and averageworld river water, heavy portion of the line is the rangeof mixtures obtained using published values of river waterand M,O.R. hydrothermal volume fluxes to the oceans,Mixtures ofM.O.R. hydrothermal brines and river watersare supersaturated with respect to calcite. Arrows indicatedirection of brine evolution during calcium carbonateprecipitation and range of compositions obtained frommixtures.


in the present day M.O.R. hydrothermal brine vol-ume flux is to make this flux a dependent variable,In this approach, the volume flux of M.O.R. hy-drothermal brine required to achieve a net zero fluxfor each major ion species is calculated. The cal-culated fluxes are compared with the published es-timates of M.O.R. hydrothermal brine and witheach other. This comparative approach will revealwhether or not our model is able to account for thecomposition of modern seawater within the rangeof uncertainty of the estimates of the volume fluxfor M.O.R. hydrothermal brine and whether or nota net zero flux for each ionic species can be obtainedfrom the same M.O.R, hydrothermal brine volumeflux.

With the initial estimates given in Table 2 (lines2 and 8) the volume fluxes for M,O,R. hydrothermalbrines required for net zero fluxes of the major ionspecies are given in Table 3. Zero net fluxes forNa", Mg2+ and SO~- are obtained for very similarM.O.R. hydrothermal brine volume fluxes, indi-cating the ability of the mixing model to explainthe proportions of these species in modern seawater.An improvement in the balance of these solutescan be obtained if more sulphate is attributed toanthropogenic sources, closer to HOLLAND'S (1978)estimate of close to 40% of the total river bornesulphate (as per Table 2, line 3). Larger M.O.R.hydrothermal brine inflow requires additionalsources for these species, while a lesser inflow re-quires additional sinks,

The ea2+ and K+ fluxes for both the river andM.O.R. sources are positive. Excess ea2+ is removedby "precipitating" an amount equal to the totalmole-equivalent concentration of He03" to simu-late biogenic uptake by organisms and chemicalprecipitation ofeae03, The M.O.R. hydrothermalbrine volume flux required to balance the river-borne He03" is higher than the fluxes required forNa2+, Mg2+ and SO~- (Table 3), although an in-crease in river-borne ea2+, or decrease in He03" ofless than 10% will yield volume fluxes for M,O,R.hydrothermal brines identical to those for Mg2+ andSO~-. There is a net positive flux of'K" that cannotbe accounted for without introducing additionalsinks such as the uptake of K+ by terrestrial claysbrought down to the sea by rivers or perhaps "re-verse weathering" of oceanic basalts,

Overall, apart from the K+ problem (see alsoMAYNARD, 1976), our model seems to be able toaccount for the present composition of seawaterwithin the limits of the input data available in theliterature, A river water composition with K+ ad-justed to a negative value to simulate exchange ofCa?" on river clays for seawater K+ as the clays

Table 3. M.O.R. hydrothermal brine volume fluxes re-quired for zero net flux of species using river volume of3.75 X 1016l/yr and river water compositions from lines

2 and 3, Table 2, volumes in 1013l/yr

Line Line2 3

Na+ 8.09 7.90Mg2+ 8.69 8.44SO~- 10.40 8.44Ca2+-HCO) 11.60 12.98

enter the ocean which yields a net zero flux for allspecies using a M.O.R. hydrothermal brine flux of8.45 X 1013 l/yr is given in Table 2, line 4. Themodel described allows for a constant compositionocean based on mixing of Na-He03-S04 river wa-ters and Ca-Cl M,O.R. hydrothermal brines, cou-pled with the precipitation of calcium carbonateand exchange of ea2+ on river-borne clays for sea-water K+. Several reactions, such as sulphate re-duction or Mg2+ uptake on clays, that other workershave suggested might have an influence on seawatercomposition have not been taken into account inour modeL The acceptable balance achieved by ourmodel suggests that these reactions do not playamajor role in determining the composition of sea-water, or that they have the same net effect as theprocesses underlying our modeL

VARIATIONS IN SEAWATER COMPOSITIONIN RESPONSE TO SECULAR VARIATIONS

IN RIVER WATER AND M.O,R,HYDROTHERMAL BRINE FLUXES

Variations in the fluxes of river water and M,O,R.hydrothermal brine are almost certain to have oc-curred back through geologic time as a function ofvariations in the rates of sea floor spreading, moun-tain building, continental accretion, global climateand so on. Therefore, if simple mixing of river wa-ters and M.O,R. hydrothermal brines controls thecomposition of seawater, then it follows that anychanges in the river water/M.O.R. hydrothermalbrine flux ratios should result in changes in seawatermajor ion chemistry, If these changes in seawatercomposition are large enough they could affect themineralogy of marine carbonates and marine evap-orites. Such changes might explain, for example,the problem of "aragonite seas" vs. "calcite seas"(SANDBERG, 1983) or the restriction ofMgS04-richevaporites to the Permian and Neogene (HARDIE,1990).

Our simple model is used to test the sensitivityof seawater composition to variations in the ratio


of river water and M.O.R. hydrothermal brinefluxes, We use the composition of river water fromline 4, Table 2, and hold the volume flux of riverwaters constant at 3,75 X 1016 l/yr, Steady-stateseawater compositions are calculated assuming thatthe removal of Na+, Mg2+ and SO~- are propor-tional to the M.O,R. hydrothermal volume flux,and that the addition of K+ and Ca2+is controlledby charge balance. For instance, Mg2+is removednearly quantitatively from seawater at the mid-ocean ridge (compare lines 6 and 7, Table 2), sothat if the M.O.R. hydrothermal fluxwere to double,then to balance the river input a seawater Mg2+concentration one-half of the modern concentrationis required. Our calculations indicate that steady-state concentration values for Mg2+and SO~- areapproached rapidly (on the order of a few millionto a few tens of millions of years), but that muchlonger times (several tens of millions of years) arerequired to reach steady-state values for Na+ (be-cause only a small proportion of Na" is removedfrom a given volume of seawater at the mid-oceanridge). The sum of the change in K+ and ea2+ con-centrations are calculated in order to balance thecharge removed or gained in obtaining the new Na+,Mg2+and SO~- steady-state concentrations. Distri-butions ofK" and ea2+ are calculated in proportionto the flux ratios for these species determined forthe modern M.O,R. hydrothermal flux, Results ofour calculations of steady-state seawater composi-tions for various river waterjM,O.R. hydrothermalbrine volume flux ratios are given in Table 4.

Modest changes in the ratio of river water inflowand M,O,R. hydrothermal brine inflow result inwhat we consider to be very significant changes inthe major ion composition of seawater, significantbecause of the potential effect on chemical and bio-chemical sediments produced from these waters, AM.O.R. hydrothermal brine volume flux of between95 and 96% of the modern flux results in a steady-state seawater composition along the CI-S04 and

Table 4, Steady-state composition of major ions in seawater(rneq/l) as a function ofM.O.R. hydrothermal brine flux

Flux* Na+ K+ Ca2+ Mg2+ Cl- SO~- HCO)

0.95 495 2.0 0.5 112 548 59.6 3.00.96 490 3.7 4.8 III 548 59.0 2.71.00 470 10,2 20.3 106 548 56.6 2.21.05 448 17.6 36.2 101 548 53.9 0.61.10 427 24.3 51.8 97 548 51.4 0.41.25 376 41.2 91.4 85 548 45.3 0,3

* Ratio ofM.O.R. hydrothermal brine flux relative tomodern,

415

Na-He03-S04 chemical divide (see Figs, 1 and 2),a lesser M.O.R. hydrothermal brine volume fluxleads to alkaline Na-He03-S04 brines, An increaseof 10%in the relative proportion ofM.O.R. hydro-thermal brine results in a steady-state seawatercomposition along the el-S04 and Ca-Cl chemicaldivide. Higher relative proportions of M,O,R. hy-drothermal brine inflow result in Ca-Cl brines. Ourcalculations indicate that a 25% increase in theM,O,R. hydrothermal brine influx yields a seawatercomposition with ea2+ rather than Mg2+as the thirdmost abundant ionic species in seawater (Table 4).

SOME IMPLICATIONS FOR CHANGES INCHEMICAL SEDIMENTS THROUGH TIME

Calcium carbonate precipitation

Although calcite is the more stable polymorphof calcium carbonate at earth surface conditions,aragonite, rather than calcite, is the dominant formin the present ocean. The dominant polymorphprecipitated from seawater may have changedthrough the Phanerozoic (see SANDBERG,1983).FuCHTBAUERand HARDIE(1980) precipitated ei-ther calcite (with varying Mg) or aragonite or bothfrom Na+-Mg2+-ea2+-eO~-solutions depending onthe Mg2+jea2+ in solution, For solutions with Mg2+jea2+ less than two (mole ratio) only calcites formed,and for low ionic strength solutions with Mg2+/ea2+greater than five only aragonite formed, bothformed from solutions of intermediate ratios,Comparison of our model ratios with the valuesfound by FuCHTBAUERand HARDIE(1980) to con-trol the polymorph of calcium carbonate precipi-tated indicates that aragonite is favored to precip-itate from seawater with a relatively low M,O.R.hydrothermal brine volume flux (high Mg2+/ea2+).Calcite precipitation is favored from seawater withrelatively higher M.O.R. hydrothermal brine vol-ume fluxes (low Mg2+/ea2+), It may be possible touse the presence of "aragonite seas" as indicatorsoflow M.O.R. hydrothermal brine volume flux and"calcite seas" as indicators of high M.O,R. hydro-thermal brine volume flux.

Composition of evaporites through time

Evaporation paths and equilibrium mineral pre-cipitation sequences are calculated using the ther-mochemical model ofHARVIEet al. (1984), Mineralprecipitation sequences for the seawater composi-tions presented in Table 4 are given in Table 5. Themineral precipitation sequences vary significantly,Waters with a el-S04 composition (including mod-ern seawater) all end their crystallization with a final

416 R. J, Spencer and L. A. Hardie

f-<..: f-< +U ..:< f-<

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invariant assemblage of calcite-anhydrite-halite-kieserite-carnallite-bischofite. The paths taken inorder to reach this assemblage vary; glauberite ap-pears earlier from waters with a lower M.O,R. hy-drothermal brine input (lower ea2+ and higherSO~-). The mineral kainite also appears in thesesequences, but not in the sequence for modern sea-water.

Slight increases in the M.O.R, hydrothermalbrine input to seawater result in the disappearanceof glauberite as a replacement of anhydrite and theprecipitation of magnesium sulphate-bearing phases(polyhalite, epsomite, hexahydrite and kieserite) atlater stages in the evaporation sequence. Progressiveincrease in the proportion ofM.O.R. hydrothermalbrine to the mixture (between 1.00 and 1.05 timesour estimated modern flux) results in carnalliteprecipitation prior to magnesium sulphate phases(epsomite, hexahydrite and kieserite), and even-tually to sylvite precipitation prior to carnallite, Stillhigher input ofM.O.R. hydrothermal brines resultsin Ca-Cl type waters, which do not precipitateMgS04-bearing phases (flux ratios greater than 1,10,Tables 4 and 5), All Ca-Cl brines follow similarmineral precipitation sequences during evaporationand vary significantly from that of modern seawater.The final invariant assemblages include calcite-an-hydrite-halite-carnallite-tachyhydrite, with eitherbischofite or antarcticite,

Ancient evaporites have been used to evaluatechanges in the composition of surface waters, in-cluding seawater, through time (for example see:HOLSER, 1983; HARDIE, 1984; HOLLAND, 1984;DAS et al., 1990). Evaporites have the potential togive a great deal of information about the compo-sition of ancient surface waters, however, there areproblems in using evaporites to estimate seawatercomposition. The most important problem is thedetermination of the purely marine origin of a givenevaporite (HARDIE, 1984). Our models show thatrelatively minor changes in the input of Ca-Cl brinesgreatly influence the composition and mineral pre-cipitation sequence of evaporating seawater, Mix-tures of Ca-Cl hydrothermal brines with river waterscan produce brine compositions and mineral pre-cipitation sequences similar to or very different fromthose of modern seawater, This is demonstrated forthe entirely non-marine evaporites of the QaidamBasin, China, reported by LOWENSTEINet a!. (1989)and SPENCER et a!. (1990). The same types ofchanges we expect for seawater as a result of changesin M.O.R. hydrothermal brine volume fluxes canbe produced on a local or regional scale by mixturesof Ca-Cl brines and river waters with or without amarine component. Reading the composition of

417

ancient seawater from ancient evaporites is notstraightforward and is unlikely to yield unique so-lutions, Examples of Permian evaporites are usedto illustrate the problems.

HARYIE et al. (1980) compare the evaporationsequence predicted from modern seawater with thatof the Permian Stassfurt Series of the GermanZechstein II. They find a remarkable agreement inthe mineral assemblages, precipitation sequence andeven in the relative proportions of various minerals.This sequence of minerals includes glauberite as areplacement of gypsum or anhydrite, as well aspolyhalite as a replacement of gypsum or anhydriteat relatively early stages in the evaporation sequence.These minerals are not found in our evaporationsequences with increased M.O.R. hydrothermal in-put. Further, glauberite is reported from the Perm-ian Salado Formation, and polyhalite is reportedto replace glauberite, gypsum and anhydrite (Low-ENSTEIN, 1983). Magnesium sulphate salts are alsopresent in the Salado Formation (JONES, 1972;LOWENSTEIN, 1983), The presence of these "keyminerals," which indicate compositional ratios forthe major elements close to that of modern seawater,appear to give strong evidence for a similar com-position of Permian and modern seawater.

However, glauberite and polyhalite have not beenreported from a number of other Permian evapo-rites, including those found stratigraphically aboveand below, and time equivalents of the StassfurtSeries of the German Zechstein II (see HARDIE,1990). Many of these deposits lack these "key min-erals" which form at relatively early stages duringthe evaporation of seawater, yet contain potassiumchloride salts (sylvite or carnallite), which are foundmuch later in evaporation sequences. The mineralassemblages for these Permian evaporites are similarto those formed during the evaporation of our cal-culated seawater compositions with a relatively highM,O.R. hydrothermal brine input, compositionswhich vary greatly from that of modern seawater,

If we assume that the Stassfurt Series of theZechstein II and the Salado Formation evaporitesresulted from the evaporation of Permian seawater,then Permian seawater must have been similar incomposition to modern seawater. The M.O.R. hy-drothermal brine volume flux during the Permianwould have been similar, possibly slightly lower,than the modern flux, Mineral sequences found inthe other Permian evaporites may have resultedfrom local or regional input of Ca-Cl brines whichaltered the evaporation sequence. However, if thePermian evaporites which lack the "key minerals"glauberite and polyhalite and contain potassiumchloride salts, such as sylvite and carnallite, but do

418 R. J, Spencer and L. A. Hardie

not contain MgS04 minerals, are the result of theevaporation of Permian seawater, then Permianseawater must have been compositionally very dif-ferent from modern seawater. The minerals indicatea higher M,O.R. hydrothermal brine volume flux,In this case the Stassfurt series of the Zechstein IIand the Salado Formation evaporites may have re-ceived a significant input of river water, The workreported by LOWENSTEINet a!. (1989) and SPENCERet a!. (1990) on modern salt deposits of the QaidamBasin, China, demonstrates that alteration of evap-oration sequences by changes in the relative inputof river water and Ca-Cl brines does occur,

CONCLUSIONS

The major ion composition of modern seawatercan be explained by mixing of river water andM,O,R. hydrothermal brine, A steady-state com-position for the major ions in seawater can be ob-tained by removal of river-borne Na", Mg2+andSO~- during seawater circulation through mid-ocean ridges, without significant additional sourcesor sinks. The mole-equivalent concentration ofbi-carbonate input by river waters to the oceans is inexcess of Ca'". Additional ea2+ added to the oceansat mid-ocean ridges is precipitated along with river-borne ea2+ and He03" as calcium carbonate. Ad-ditional sinks for K+ are required, as the dissolvedpotassium fluxes from river waters and fromM.O,R. hydrothermal brines are positive,

The major element composition of seawater isvery sensitive to changes in the M.O,R. hydrother-mal brine volume flux, This flux is likely to havechanged during geologic time. Therefore, we expectthat the major element composition of seawater hasalso changed through time, The compositionalvariations in seawater that result from even modestchanges of a few percent in the ratio of river waterand mid-ocean ridge hydrothermal brines are sig-nificant because of the potential influence onchemical sediments produced from seawater,Smaller mid-ocean ridge hydrothermal brine fluxesresult in lower concentrations of ea2+ in seawaterand higher concentrations of Na", Mg2+,SO~- andHe03", These compositional variations are likelyto favor aragonite precipitation rather than calciteas the dominant marine carbonate mineral, and re-sult in marine evaporites which contain magnesiumsulphate minerals, Larger mid-ocean ridge hydro-thermal brine volume fluxes result in higher con-centrations of ea2+ in seawater and lower concen-trations of Na+, Mg2+, SO~- and He03". Thesecompositional variations favor calcite as the dom-inant marine carbonate mineral. Marine evaporites

formed from such calcium-rich waters contain po-tash salts (sylviteand carnallite) without magnesiumsulphate minerals.

Acknowledgments-Our ideas of brine evolution have beeninfluenced by discussions with many individuals, we par-ticularly thank Hans Eugster, Blair Jones and Tim Low-enstein for many discussions on the origin of brines.

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Control ofseawater composition bymixingofriverwaters