Ann. Rev. Earth Planet. Sci. 1989. 17: 141-67 Copyright © 1989 by Annual Reviews Inc. All rights reserved

STRONTIUM ISOTOPES IN SEAWATER THROUGH TIME

Jim Veizer

Institut fur Geologie, Ruhr-UniversiHit, Postfach 1021 48, 4630 Bochum 1, Federal Republic of Germany, and Derry Laboratory, Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada

STRONTIUM ISOTOPE SYSTEMATICS

The element with atomic number 38-strontium-has four isotopes, 84, 86, 87, and 88, having the approximate proportions of 0 .56: 9.87: 7.04: 82.53. Geologically short-lived nuclides, such as 90Sr, are not con­sidered in this review. The above abundances are somewhat variable because 87Sr is a radiogenic isotope, generated by emission of a negative p-particle from 87Rb. Thus

nRb --USr+p-+v+Q, ( 1)

where p- is the p-particle, v is an antineutrino, and Q represents the decay energy (0.275 MeV). The recommended decay constant for 87Rb is 1.42 x 10-11 yr-I (Steiger & Jager 1977), and its half-life T is therefore 48.8 Gyr. On geological time scales, the isotopes 84, 86, and 88 are stable, in contrast to 87Sr, and their abundance ratios are therefore invariant.

The present-day quantity of isotope 87 (87Srp) depends on its initial stock (87Sr 0) and the amount of radiogenic Sr generated from decay of 87Rb over time t:

87Srp = 87Sro+87Rb (eAt_I).

This, normalized to the stable isotope 86Sr, transforms into

(87SrrSr)p = (87Sr/86Sr)o + (87RbrSr) (eAt -1).

(2)

(3)

The (87Srj86Sr) terms in Equation (3) are usually expressed directly as the ratios of the two isotopes ( '" 0.7). In recent times, the notations aSr

141 0084-6597/89/05 15-0 141$02.00

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(DePaolo & Wasserburg 1976) and b87Sr (Ikpeama et al 1974, Hess et al 1986, Elderfield 1986) or A87Sr (DePaolo & Ingram 1985) have gained popularity, particularly for sedimentary systems. The derivations of these notations are the following:

8sr = [{ (87Sr/86Sr)sample/(87Sr;S6Sr)uniform reservoir} - 1] X 104,

where the (chondritic) uniform reservoir, or present-day bUlk Earth ratio, is given as 0.7045. The derivation of t587Sr is analogous to that for the isotopes of 0, C, S, and H:

(j87Sr = [{(87SrrSr)sample/(87srjB6Sr)seawater}-1] x 103 (or 105),

where (87SrrSr)seawater equals 0.709241 ± 32 (Elderfie1d 1986). Note that modern mass spectrometers attain precisions into the fifth decimal place. However, interlaboratory comparisons must be normalized to a set of international standards, such as the Atlantic seawater, the Eimer and Amend SrC03, and the National Bureau of Standards SRM-987. Their accepted 87SrrSr ratios are 0.70920, 0.70800, and 0.71014, respectively.

The primordial Sr isotopic ratio, incorporated into the Earth at the time of its formation �4.5 Gyr ago, is likely similar to that of meteorites. The (�7SrrSr)o ratio has now been established with great precision, but for our purposes it is sufficient to know its approximate value of 0.699 (Wetherill et al 1973). Subsequent Sr isotopic evolution of distinct geological reser­voirs has been a function of their RblSr ratio [cf. Equation (3)]. In general terms, the differentiation of the Earth has been accomplished via fractional crystallization, culminating in the generation of granitic melts (and con­tinents). Fractional crystallization concentrates Sr, and even more so Rb, into the melt, resulting in high RblSr ratios in the continental crust and its progressive diminution in the residual mantle (Figure 1). As a result, the present-day mantle (and oceanic crust) has a depleted 87SrrSr ratio of � 0.703 ± 1, whereas the bulk of the continents is enriched in radiogenic 87Sr (> 0.7 10). This enrichment is higher for older continental segments. The isotopic dichotomy between continents and mantle is of fundamental importance for understanding the Sr isotopic evolution of seawater. Further details ofSr isotope geochemistry are available in Faure & Powell ( 1972) and Faure (1986).

87SRjB6SR OF PRESENT-DAY SEAWATER

High-precision measurements of water and of Holocene marine carbonates bracket the 87SrrSr ratio of present-day seawater at 0.709241 ± 32 and 0.70921 1 ± 37, respectively (Elderfield 1986). This isotopic composition appears to be homogeneous within the limits of analytical precision. While

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0.720

0710

0.700

STRONTIUM ISOTOPES IN SEAWATER 1 43

4 3 2 I present Time (billions of years)

Figure 1 Schematic presentation of isotopic evolution of terrestrial strontium. BABI is the basaltic achondrite best initial ratio. B represents Sr evolution for a mantle depleted in Rb, while the curvature to A approximates an evolution of the mantle with progressive depletion in Rb/Sr ratio. In geological terms, the two alternatives may signify instantaneous vs. progressive generation of continental crust. Modified from Faure ( 1986).

the leading laboratories may reach an internal reproducibility of perhaps ± 1 x lO-5, geological and interlaboratory constraints result in a realistic resolution of about ± 3 x 10-5.

The concentration of Sr in seawater varies slightly with depth and salinity, but the accepted average value is 7.85 ± 0.03 ppm (Bernat et al 1 972, Brass & Turekian 1 974). The reasons for the isotopic homogeneity of Sr will become apparent after a discussion of its inputs into modern oceans. These are the following:

1. Runoff, principally as river discharge (lRW); 2. Groundwater runout (hw); 3. Oceanic crust-seawater interaction (lod at (a) high temperatures at

mid-oceanic ridges (lOCH) and at (h) low temperatures on ridge flanks and within the cold oceanic crust (locd; and

4. Diagenetic reflux of Sr into the oceans from buried pore waters and from recrystallization of sediments (lDIA)'

At steady state, inputs (1)-(4) are counteracted by the following:

5. Removal of Sr via sedimentation (lSED); and 6. Exchange of radiogenic Sr in hydrothermal waters for that in basalts

( = lod· The magnitudes and Sr isotopic compositions of the above fluxes are now discussed sequentially.

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Sr Fluxes Into Oceans CONTINENTAL RUNOFF The present-day annual runoff from continents is estimated at 4.55 ± 0. 15 x 1019 cm3 of water (Lvovitch 1973, Korzun et al 1978). The Amazon alone accounts for '" 15%, and the top 30 rivers (Figure 2) for ",47%, of this total (Wadleigh et aI1985).

Owing to geological and climatic factors, the observed Sr concentrations in the dissolved load of rivers vary from '" 1 to '" 3000 ppb, and their 87Srj86Sr ratios range from 0.7045 to 0.943 (Faure et al 1967, Brass 1976, Brandt et a1 1976, Wadleigh et al 1985, Goldstein & Jacobsen 1987). The radiogenic 87Sr component originates from igneous and metamorphic rocks of the old shields, the nonradiogenic component derives from young volcanics (and related sources, such as volcanic ash, glass, tuff, and first­cycle sediments), and the intermediate values are typical of large rivers with extensive drainage basins (Faure 1986, Chap. II) (Figure 3). The latter usually contain some chemical sediments and cements, mostly as

AMAZON � .............................................. .. y;����r-----------------------------------�

O RINOCO PLATA -PARANA

YENISEI MISSISSIPPI

BRAHMAPUTRA LENA

ZAMBEZI MEKONG

ST. LAW RENCE IRRAWADDY

GANGES PEARL

08 SALWEEN

AMUR MACKENZIE

PARA -TOCANTINS f-::==========::::;::r MAGDALENA t-VOLGA YUKON INDUS

DANUBE NIGER �����������

COLUMBIA OGOOUE P���?�� 1----------0'

URUGUAY .......... ,--' FRASER .. DVINA '---_____ -'

" II 0.(

II I" I \.0 ("Ie)

I 11111 (0

Figure 2 Contributions of major rivers to world runoff. Rivers with measured 87Sr/B6Sr ratios in black. Modified from Wadleigh et at ( 1985). A recent paper by Palmer & Edmond (1 989), accepted for publication while this review was in press, gives an exhaustive list of new measurements for most of the important world rivers.

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o

0.72

STRONTIUM ISOTOPES IN SEAWATER 145

0.74

15 I 26

18 4 7 =-.. 32 II!I!I 39

���12 5 36 31 20 10

t::::::iiii�=�!:=:::i::3429 27 30 35 24 II 19 28 8 16 21 26

100 b 200 pp

C]c DD

Figure 3 '7Srt'6Sr and SrH in the dissolved load of Canadian rivers as a reflection of bedrock geology of the river basins. Note that the dissolved load actually reflects the composition of the surficial glacial tillites, but basinwide the latter represent an averaged bedrock geology. The river basins, in descending order of water discharge, are the following: I-St. Lawrence, 2-Mackenzie, 3-Fraser, 4-Columbia, 5-Churchill (Newfoundland), 6-Nelson, 7-Yukon, 8-Koksoak, 9-Saguenay, IO-Nottaway, ll-Rupert, 12-

Manicouagan, 13-St. lohn, 14-Skeena, 1 5-Nass, 1 6-LaGrande, 17-Stikine, 1 8 -Albany, 19-Eastmain, 20-Severn, 2 1-Aux Feuilles, 22-St. Maurice, 23-Moose, 24-

Ala Baleine, 25-Grande Riviere de la Baleine, 26-Petit Mecatina, 27-Churchill (Mani­toba), 28-Back, 29-Thelon, 30-Kazan, 3 1-Winisk, 32-Moisie, 33-Porcupine, 34-Aux Outardes, 35-Arnaud, 36-Hayes, 37-Natashquan, 38-Attawapiskat, 39-Har­ricanaw. The lithologic patterns, expressed as percentages in a given basin, are the following: (A) Phanerozoic sediments, (B) Phanerozoic igneous and volcanic rocks, (C) Proterozoic clastic sediments, (D) Proterozoic igneous and metamorphic rocks, (E) Archean igneous and metamorphic rocks, (F) others. Based on the data in Wadleigh ( 1 982) and Wadleigh et al (1985).

carbonates, in their geological makeup, and these-owing to their higher solubility-effectively buffer the 87SrrSr ratios in surface waters at near­marine values. Geographically, the case is well illustrated by North America (Figure 4), where 87Sr-enriched rivers are confined to the

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Canadian Shield, 87Sr-depleted ones to the western seaboard, and inter­mediate rivers draining the continental interior to the southeastern seaboard. Other things being equal, rivers draining the relatively leached and unreactive old shields contain an order of magnitude less total dis­solved solids (TDS), and Sr, than the "carbonate" and "young volcanic" streams (Figure 3). This generalization, however, is complicated by climatic considerations. Rivers of arid zones, such as the Avon and the Murchison of Western Australia, may carry up to 103 ppm of dissolved Sr (Goldstein & Jacobsen 1987), irrespective of their catchment-area geology. Nevertheless, these highly concentrated streams have mostly small water discharges and contribute therefore little to the global budget. A partial exception may have been the Colorado River, which prior to its diversion into Los Angeles water taps could have contributed '" 1 % of the global Sr flux, despite its putative (",0.05%) share of the global water discharge (cf. Goldstein & Jacobsen 1987).

If we take 60 ppb as the weighted mean value for Sr in continental runoff (Durum & Hafty 1963, Martin & Meybeck 1979), the annual flux of

87Sr/86Sr • 0.730 - 0.740 • 0.720 - 0.730 � 0.716 - 0.720 • 0.712 - 0.716 EJ 0.708 - 0.712 o 0.704 - 0.708

Figure 4 Geographic distribution of R7Srj86Sr ratios in the dissolved load of North American river basins. Based on the data of Wadleigh et al (1985) and Goldstein & Jacobsen (1987).

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STRONTIUM ISOTOPES IN SEAWATER 147

strontium into the oceans (JRW) is � 2.73 ± 0.09 x 1012 g. Its weighted mean 87SrrSr ratio is probably 0.7 101 ± 5 (Goldstein & Jacobsen 1987).

CONTINENTAL RUNOUT This term represents the discharge of groundwater directly into oceans and has been estimated at only 5% of the surficial runoff (Nace 1969). Nevertheless, since the Sr content of waters from deep aquifers may attain the parts-per-million range, run out may be potentially significant for the Sr budget. Chaudhuri & Clauer ( 1986) estimated the groundwater Sr flux (JGw) to be � 1.95 ± 0.5 x 1012 g yr-I and assumed its weighted mean 87SrrSr ratio to be similar to that of the runoff(0.7101 ± 5). Note, however, that these may be the least-constrained variables in the entire exogenic budget of Sr.

OCEANIC CRUST-SEAWATER INTERACTION The Sr content of hot water discharged from axial mid-oceanic vents (East Pacific Rise, Iceland) is � 1 1.4 ± 5.0 ppm, and the 87Sr/86Sr ratio is � 0.7035 ± 5 (Edmond et al 1979, A 1barede et a1 198 1, Elderfield & Grieves 1981, Michard et a1 1984, Piepgras & Wasserburg 1985). Based on an integrated 3He flux, the water flow through hydrothermal systems is '" 1.85 X 1017 g yr-I (Jenkins et al 1978, Craig & Lupton 1981), with � 20% related to axial circulation (Morton & Sleep 1985). Thus the annual hydrothermal flux of Sr (JOCH) is '" 0.42 ± 0 . 19 x 1012 g (see also Palmer & Elderfield 1985).

15180 and calcium concentrations of interstitial waters (Lawrence & Gieskes 1981) indicate that seawater alteration of oceanic crust persists for � 120 M yr. For most elements, and for crust older than � 13 ± 5 Myr, these fluxes are negligible (Staudigel & Hart 1985). For Sr, the flux is difficult to quantify. Since the continentally derived detrital components of marine sediments are relatively unreactive (Dasch 1969), diagenetic flux from noncarbonate sediments may originate mostly from volcanic sources in the underlying crust. If so, the low-temperature ( < 70°C) submarine weathering processes, or halmyrolysis, may result in a locc flux of Sr of �0.035 x 1012 g yr-I, with a 87SrrSr ratio of 0.7064 (Palmer & Elderfield 1985).

DIAGENETIC FLUX Diagenetic recrystallization of carbonate minerals leads to partitioning of Sr into pore fluids and subsequent diffusion and/or advection into seawater. This process, because of its importance for pre­servation of initial 87Sr/86Sr ratios, is discussed in detail in the subsequent text. The estimated diagenetic reflux of Sr from deep-sea sediments (JDIA) is � 0.26 X 1012 g yr-I, with a 87 Sr/86Sr ratio of � 0.7087 (Elderfield & Gieskes 1982, Palmer & Elderfield 1985). A reflux from diagenetic recrys­tallization of shallow-water carbonates (particularly those containing Sr­rieh aragonite) is unknown but may be considerable. Nevertheless, it is

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148 VEIZER

probably discharged into the oceans via groundwater runout (JGw) and thus encompassed in that term.

Seawater Isotopic Variations

The compound annual flux of Sr into the ocean is rv 5.4 X 1012 g, yielding a 87SrrSr ratio of 0.70987 (Table 1). This value differs somewhat from the measured ratio of 0.70924 ± 3, suggesting that either the runoff and/or runout terms overestimate, or the hydrothermal term underestimates, the natural fluxes by � 73% and 1 1 %, respectively. The existing data base does not permit any unique solution of the discrepancy. The modeling of the geological past poses similar problems and is compounded by the fact that not only the fluxes but also the 87Srj86Sr ratios of JRW and JGW are essentially unconstrained variables.

Table 1 also demonstrates that the residence time of Sr in seawater is 2: 4 Myr, which is long in comparison to the mixing rate of the oceans of � 103 yr (Broecker 1963, Goldberg 1963). This effective mixing results in the spatially homogeneous distribution of 87SrrSr in seawater at any instant of geological time. Even a large river discharge into marginal seas will not affect the 87Sr/86Sr of the latter, owing to the dilute nature of rivers (8 vs. � O.06 ppm Sr). For example, the Hudson Bay of Arctic Canada, with salinities down to < 16%0 ( '" 50% river water), has a 87Sr/86Sr ratio indistinguishable from that of the Atlantic Ocean (Faure et al 1967).

PRESERVATION AND EXTRACTION OF SEAWATER SIGNAL

In the absence of samples of ancient seawater, the Sr isotopic record of past oceans has been deciphered from measurements of authigenic marine

Table 1 Summary of Sr fluxes into seawater" and their impact on Sr isotopic ratio

Time (in 1 04 yr) Magnitude of Residence time required to alter

Sr flux (in (,) in seawater seawater 87Sr/""Sr Flux 1012 g ycl) 87Sr/86Sr (in Myr) by 1 x 10-5

JRW 2.73±0.09 0.710\ ±O.OOOS 4.0 4.6 Jaw 1 .9S ±O.OS 0 .7101 ±O.OOOS S.6 6.S

JOCH 0.42±0.19 0.7035 ± 0.0005 26.2 4.6

Jacc 0.035 0.7064 314.3 1 1 0.6

JorA 0.26 0.7087 42.3 78.3

L 5.395 0.70987

'Seawater: Sr content II x [0" g, average concentration 7.85 ppm, '7Sr/86Sr 0.70924.

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STRONTIUM ISOTOPES IN SEAWATER 149 minerals, such as carbonates, sulfates, phosphates, and some salts. The 87SrrSr ratio is incorporated into these minerals without isotopic frac­tionation, regardless of whether they precipitated chemically or bio­logically. The authigenic phases are usually metastable and prone to diagenetic stabilization via dissolution-reprecipitation in pore fluids. As a consequence, the 87Srj86Sr ratio of the successor phase(s) evolves. Further complications arise from constraints of stratigraphic resolution and from experimental procedures.

Diagenesis

In this section, only the diagenesis of carbonate rocks, which-owing to their ubiquity-are the main source of isotope data, is discussed. Similar considerations apply, however, to the other types of authigenic minerals.

DEEP SEA Carbonate sediments of this milieu are predominantly accumu­lations of foraminiferal tests. Despite their "stable" low-Mg calcitic min­eralogy, the tests are involved in dissolution-reprecipitation processes upon burial. Judging from the continuous diminution of Sr contents with depth (Figure 5), the process persists [or perhaps 100 Myr. A typical depth profile

00 .40 eo 120 Sediment age (my)

Figure 5 The Sr content of pelagic carbonates at Deep Sea Drilling Project (DSDP) sites 289, 167, 305,3 17, and 3 16 as a function of their age. Modified from M anghnani et al (1980).

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0.707 0.708 0.709 0.710 0

OOZE carbonate � ------87Sr/86Sr �#

� 200 �� CHALK E , +

� OOZE .c � 0. � Q.l %% ------a � 400 %% CHALK

o zoo 400 600 pM Figure 6 Dissolved Sr2+ and "Sr/86Sr at DSDP site 528 as functions of depth. Modified from Gieskes et al (1986).

for sediments and pore waters is illustrated in Figure 6. Owing to the partition coefficient D��lcilC < 1, the dissolution-reprecipitation of car­bonates results in net partitioning of Sr into pore waters (cf. Baker et al 1982). The maximal recrystallization rate, and hence the pore-water Sr peak, likely predates the ooze-chalk transition (Elderfield et al 1982, Gieskes et al 1986). The accumulated Sr diffuses through pore water upward into the overlying ocean and downward into noncarbonate sinks, such as volcanogenic sediments and/or basalts and Sr-rich diagenetic min­erals (celestite, strontianite). The overall gradients are essentially functions of the rates of recrystallization vs. the ionic diffusion rates, and of the partition coefficient(s). Note, however, that in contrast to the enclosing carbonates, the pore-water 87Sr/86Sr depth profile is almost linear. This is because the downgradient rate of Sr diffusion exceeds the rate of in situ recrystallization. As a result, the water above the Sr concentration maximum is depleted, and that below enriched, in �7Sr. If the �7SrrSr Cenozoic secular trend for seawater were that of continual decline rather than of increase, the pore-water isotopic trends would also be reversed. The in situ diagenetically precipitated carbonates inherit this pore-water Sr and shift the bulk-rock isotopic composition toward that of the ambient pore waters. The "smoothing" of the carbonate isotope signal is par­ticularly potent at times when the principal diagenetic front traverses intervals of rapid secular shifts and/or of pronounced higher order fluc­tuations. For example, modeling of Richter & DePaolo (1987) indicates a shift of � 5 x 10-5 in 87Sr/86Sr for bulk carbonates older than � 5 Ma. It follows that the Sr isotopic curve of paleoceans cannot be constrained

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STRONTIUM ISOTOPES IN SEAWATER 15 1

better than that dictated by diagenetic considerations, irrespective of the resolution capabilities of modern mass spectrometry. For very young deep­sea sediments ( < 5 Ma), the backstripping technique of Richter & DePaolo ( 1987) may possibly be of utility, provided that the model assumptions mimic nature and that the pore-water as well as the solid compositional profiles are well established. A more fruitful approach is that based on well-preserved fossils (Hess et al 1986), although care must be taken to exclude secondary diagenetic phases from their shells.

MARGINAL SEAS For most of the geological past, shallow seas have been the prime environment of carbonate deposition. Diagenesis of these sedi­ments is considerably more complex than that of their deep-sea counter­parts. The additional factors include (a) thc metastable mineralogy of carbonate minerals (aragonite, Mg-ca1cites), (b) the polycomponent nature of sediments, (c) the meteoric or mixed meteoric/marine origin of pore waters, and (d) the importance of fluid flow. A detailed discussion is beyond the scope of this review, and the reader is referred to Veizer ( 1983a,b) for further information. The complexity is somewhat mitigated by the fact that the stabilization of these sediments is accomplished in a much shorter time ('" 105 yr) (Gavish & Friedman 1969, Veizer 1978) and mostly via localized rock-dominated micro systems. As already pointed out, meteoric waters usually have low Sr contents (parts-per-billion range), whereas the sediments contain � 2000-9000 ppm Sr. During dissolution of metastable precursors, the microenvironment is usually swamped by the carbonate Sr, with the successor diagenetic calcite inheriting its isotopic composition. All this occurs despite the fact that the bulk of the released Sr is flushed away by fluid flow.

The above situation approximates only the stage of mineral inversion of original marine components (e.g. shells, oolites, cements). Subsequent precipitation of early meteoric cements usually proceeds in water-domi­nated systems (see the review of Choquette & James 1984). Since these waters represent various marine-meteoric mixtures and are frequently buffered by the confining carbonate, the inherited 87Sr/86Sr signal may be that of seawater. Nonetheless, displacements toward more radiogenic values, reflecting the meteoric component, are ubiquitous. In contrast, shifts toward less radiogenic values, while observed, are rare and usually reflect abundant young volcanogenic material in the upstream pathway of the aquifer.

The situation is more complex for dolostones because the dolomitization process is superimposed-sometimes with considerable time lag-on the above-mentioned diagenetic phenomena. Thc previously discussed con-

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152 YEIZER

siderations still apply, but the chance of preserving an original seawater Sr isotope value is remote. Usually the displacement is toward 87Sr addition, and a prudent approach (Veizer & Compston 1974) is to regard the least radiogenic Sr for a given sequence as only an upper limit for the broadly coeval seawater.

DEEP BURIAL Pressure solution under deep burial conditions results in generation of the late, usually ferroan, sparry calcite cements (Choquette & James 1987). These occlude the residual porosity in most carbonates, regardless of whether they are of shallow or deep-sea origin. Undcr favor­able conditions, the ambient pore waters (brines) may still be buffered by the enclosing carbonates with respect to their Sr isotopes. lfso, the cements may inherit a homogenized isotopic composition of the carbonate pre­cursors. As a norm, however, this is not the case. Deep brines usually acquire an excess of 87Sr (Chaudhuri 1978, Stueber et al 1984, Woronick & Land 1985) from interactions with silicate minerals, and this signature is imparted into late cements.

Many carbonate rocks contain an appreciable amount of "insoluble residue," composed predominantly of (alumino) silicate phases. The latter have mostly, although not exclusively, an excess of 87Sr inherited in their detrital components and/or derived from postdepositional decay of 87Rb. Such noncarbonate Sr may be sequestered into carbonate phases during dissolution-reprecipitation events, particularly at times of deep burial of the rocks.

In summary, diagenesis imposes limits on preservation of the original seawater 87Sr/86Sr ratio in marine sediments, particularly if measured in bulk rocks. A better-constrained secular curve can be derived from well­preserved low-Mg calcitic shells, such as the Cenozoic foraminifera, Meso­zoic belemnites, and Paleozoic brachiopods.

Nevertheless, even with this approach, the Phanerozoic Sr isotope curve for seawater will probably not even replicate the interlaboratory uncer­tainty of ± 3 x 10-5, and the band of this width should be considered as an effective resolution limit. The matter is further complicated by the problems of correlation, which are discussed below.

Stratigraphic Resolution The Sr isotope signal of ancient seawater, to be discussed in the subsequent text, is not a simple linear trend. Superimposed on the general 87Sr increase is a hierarchy of oscillations of progressively higher order (smaller) period­icities and wavelengths. This signature is recorded by sediments deposited at variable rates and interspersed with nondepositional intervals and with times of erosion. This is true even of the "continuous" deep-sea sedi-

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STRONTIUM ISOTOPES IN SEAWATER 153

mentation (cf. Moore & Heath 1977). Mathematically, this sedimentary record has fractal properties (see Mandelbrot 1983). With longer obser­vational intervals (poorer stratigraphic resolution), the combined time represented by nondepositional and erosional episodes expands faster than that accounting for depositional events. As a consequence, the apparent rates of sedimentation, and the completeness of the stratigraphic sections, are inversely proportional to stratigraphic resolution (cf. Sadler 1981, Gingerich 1983, Anders et aI 1987). No amount of stacking of such incom­plete sections will yield a continuous signal of higher order oscillations for Sr isotopes in seawater. At best, it can reveal their total range. As a norm, stratigraphic resolution decreases with the antiquity of geologic sections. Consequently, the 87Srj86Sr seawater curve must be a band of increasing width, encompassing oscillations of ever larger amplitudes and wave­lengths (lower hierarchies). This reasoning is as valid for the Cenozoic as it is for the Precambrian. Since the stratigraphic resolution for the former is much better, the width of the band is of course much smaller.

Nevertheless, the assumption that the growth of 87Sr for the last � 40 Myr has been more or less linear is valid only in terms of the average values for the "primordial" stratigraphic intervals of the original calibrating scale. To interpolate linearly between calibration points, with disregard for almost certain higher order oscillations, is a reasonable approximation but no more than that. The subsequent step of utilizing such a smoothed curve for "dating" and correlation at precisions well beyond the original calibrating stratigraphic capabilities may not always be warranted. As a justification, it is frequently pointed out that this approach yields results consistent with magnetostratigraphy. Yet, it is forgotten that the latter scale has been erected by a similar linear extrapolation between more or less identical calibrating points, and the purported consistency is therefore not surprising.

Analytical Problems

The extraction of Sr for isotopic measurements involves acid digestion of the carbonates. This procedure may result in partial digestion of the noncarbonate components, particularly if bulk rocks are involved. For time intervals where the availability and the quality of samples is limited (e.g. the Precambrian), those with a higher insoluble residue content cannot be always disregarded. Furthermore, less soluble samples-such as dolo­mites-require stronger acids or longer digestion times, thus enhancing the possibility of contamination by noncarbonate strontium. The sign of the shift in 87SrrSr of such samples depends on the nature of the con­tributing noncarbonate component(s), but an excess of 87Sr is the rule rather than an exception (cf. Veizer & Compston 1974).

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87SRj86SR OF PALEOSEAWATER

The preceding discussion, highlighting the problems of extraction of an unmodified signal for seawater at any instant of the geological past, was intended only as a warning and not a denial of the feasibility of the task. Bearing in mind these qualifications, we see that past effort yielded a well­constrained curve (or a band with a precision in the ;:S 10-4 range) for the Phanerozoic and at least the lower envelope for the considerably broader Precambrian scatter.

The original idea that seawater should reflect an increase in 87SrrSr with time, due to radioactive decay of 87Rb, was proposed by Wickman (1948). His prediction of a 30% increase in 87SrrSr was based on poorly known estimates of Rb and Sr in the crust and was not substantiated experimentally (Gast 1955, Ewald et al 1956, Gerling & Shukolyukov 1957). Subsequently, Hedge & Walthall ( 1963) and Hurley et al ( 1965) confirmed a I % increase, but it was believed that the increase has been more or less linear through time, a proposition not in accord with the Permian data of Brookins et al ( 1968). In a classical study, based on calcareous shells, Peterman et al (1970) demonstrated unequivocally that the strontium isotopic composition of Phanerozoic seawater fluctuated within the '" 0.7080 ± 12 range, with no clear unidirectional trend. Dasch & Biscaye (197 1) confirmed the Cretaceous-Recent portion of the Peter­man et al curve, and Veizer & Compston (1974, 1976), utilizing bulk rocks, further constrained it, particularly for the Paleozoic. The latter authors also extended the data base into the Precambrian. In the 1970s the increased sampling density, the improvements in stratigraphic resolution, and particularly the development of a new generation of mass spec­trometers stimulated vigorous research effort, which culminated in the publication of the Mobil curve (Burke et al 1982). The latter is presently an unchallenged reference for the 87SrrSr ratio of Phanerozoic seawater, despite the fact that its analytical documentation has been published only for the Cretaceous and the Cenozoic (Koepnick et al 1985). The notable advances of the 1980s have been the development of the high-resolution Cenozoic curve (DePaolo & Ingram 1985, Palmer & Elderfield 1985, DePaolo 1986, 1987, Hess et al 1986) and the emerging acceptance of the technique as a tool for solution of problems of stratigraphy, petrology, diagenesis, genesis of mineral deposits, and related applications.

In the subsequent text, I discuss the Sr isotopic variations of paleo­seawater in an ascending hierarchy of resolutions and causes.

First-Order 87Sr;S6Sr Trend at l09-Years Resolution The Sr isotopic curve for the last � 3.5 Gyr (Veizer & Compston 1976) is reproduced in Figure 7. Its most notable feature is the sudden increase in

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STRONTIUM ISOTOPES IN SEAWATER 155

I 0 0.715

87Sr 86Sr

river water

-"'"'--.,....----r---�---_+0.700 b.y 3 2 o

Figure 7 Variations of 87Srj86Sr of "seawater" on a l 09-yr time scale. Modified from Veizer & Compston (1976).

87Sr at '" 2.5 ± 0.3 Ga. The Archean samples are frequently in the range of the coeval mantle values, whereas the Proterozoic ones have distinctly more radiogenic 87Sr;S6Sr ratios. Gibbs et al ( 1986) asserted that the pre­ponderance of mantle like values in the Archean is a consequence of biased sampling (Archean greenstone belts vs. Proterozoic shelf sequences) and/or postdepositional alteration phenomena and not of seawater evolution. Veizer ( 1988a) disputed such an assessment. Indeed, if the data listed in his rebuttal were plotted against time, the overall picture of the curve would resemble that of Figure 7. This Archean-Proterozoic increase in 87Sr had first been ascribed (Veizer & Compston 1976) to a concomitant growth of continental crust and thus to an influx of radiogenic Sr from rivers draining these newly formed continents. Subsequent modeling (Brevart & Allegre 1977) hinted that this factor alone might not explain the magnitude of the observed trend. Following the realization. that ridge hydrothermal fiux (JOCH) may introduce nonradiogenic Sr into the oceans (Spooner 1976), and the suggestion of Perry et al ( 1978), Veizer et al ( 1982) modified their views. They proposed that the � 2.5 ± 0.3 Ga transition represents a crossover point of two coupled phenomena: the exponentially decreasing heat flux from the mantle, and the concomitant growth of continents. The first-order seawater Sr isotopic trend would therefore reflect a transition from the Archean "mantle"-buffered ocean into the "river"-buffered one in the Proterozoic and the Phanerozoic. Such a sce­nario could have consequences far beyond Sr isotopes (e.g. for the evolution of atmospheric Po 2; Veizer 1983c, 1988b) and for this reason has been vigorously challenged by Holland (1984, Chap. 6). Nevertheless, the Nd

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isotope data for banded iron formations (Jacobsen & Pimentel-Klose 1988a,b, Derry & Jacobsen 1988) support this interpretation, and Holland, in recent oral prcsentations, has also been perceptibly reversing his position.

Second-Order 87 Sr;S6Sr Oscillations at 108- Years Resolution Figure 7 denotes only the lower envelope of the Sr isotope trend because Veizer & Compston ( 1 976) did not possess the later-developed tcehniques (Veizer 1983a,b) for separation of higher order oscillations from 87Sr enhancements due to postdepositional phenomena. This precluded also the delineation of an upper envelope for the seawater trend. Subsequently, Veizer et al (1983) and Derry & Jacobsen (1988) demonstrated a second­order structure for the late Proterozoic, but its details are obscured by the poor stratigraphic and correlation capabilities. Nevertheless, the existing 1 Ga trend indicates second-order oscillations at � 500 ± 200 Myr frequency, reflected in the Phanerozoic record by the overall decline in 87Sr during the Paleozoic and its enhancement in the Cretaceous and Tertiary (Figure 8) (Albarede & Michard 1 987). The causes of these variations are as yet uncertain. Qualitatively, the overall Phanerozoic 87SrrSr trend resembles the b34S and b13C trends for marine carbonate and sulfate, respectively (Figure 8) (Brass 1976, Veizer et al 1982, Holser 1984, Veizer 1983b, Albarede & Michard 1986). The geological scenario is, however, difficult to interpret, and the oscillations at higher hierarchies (shorter time scales)

0

100

200 � ,. �300 ..

400

500

600 +10 +20 +30

%0

%0 +4 +2 0 -2 - 0+ 200 400m

0.707 o.70e 0.709 0.710

Figure 8 Variations of ,534S, ,513C, and 87Sr/86Sr of seawater and of "sea-level" stands during the Phanerozoic. Modified from Holscr (1984) and Veizcr (1988b).

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STRONTIUM ISOTOPES IN SEA WATER 157

diverge, reflecting variable response times of these isotopes to their specific causes. The hydrothermal flux of Sr from mid-oceanic ridges (JOCH) in the geological past is believed to have been proportional to the rate of seafloor spreading. Since fast spreading should have caused high-standing mid­oceanic ridges and thus displacement of seawater onto shelves, the times of fast-spreading, high-"sea-level" stands (Vail et aI1977) and of input of nonradiogenic strontium ( '" 0.703) should coincide. Such correlations have been claimed for the last", 100 Myr (Spooner 1976) but are not borne out by the preceding 500-Myr record (Figure 8). Overall, the Phanerozoic Sr isotope seawater curve does not show any clear correlation with "sea­level" stands (Chaudhuri & Clauer 1986). Alternatively, if the Cretaceous were accepted as an exception rather than a norm (Veizer 1985, 1988b), the correlation for the time resolution of '" 108 yr exists, but the Sr isotope response is opposite to that predicted by the above scenario (see Figure 8).

Third-Order 87Sr;S6Sr Fluctuations at l07-Years Resolution Correlation capabilities at � 107 years resolution are routinely available only for the Phanerozoic; its seawater Sr isotopic trend is reproduced in Figure 9. Superimposed on the previously discussed second-order trough-

y----,------,----,--,----r------,-,---,------,--,-----.--r-r-� 7100

PC. CAMBRIAN ORDOVICIAN ILURIAN DEVONIAN MISS. PENN PERMIAN RIASSIC JURASSI CRETACEOUS i TERTIARY 7080

600 soc 300 200 100 MILLIONS OF YEARS

Figure 9 Variations of 87Srj86Sr of seawater during the Phanerozoic. Modified from Burke et al ( 1 982). In addition to references quoted in the text, various portions of the curve have been confirmed also by Boger et al (1973), Tremba et al (1975), Faure et al (1978), Jorgensen & Larsen (1980), Kovach (1980), Clauer (1981), Shaw & Wasserburg (1985), and Grandjean et al (1987).

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like trend are third-order oscillations at 107 -yr frequencies. The causes of these fluctuations are as yet uncertain, although it is clear from Table 1 that only the runoff (JRW), runout (JGw), and hydrothermal flux (JOCH)­or their combination-have the capacity to cause the observed shifts.

The variations in intensity, and particularly in the 87Srj86Sr ratios of the runoff flux have been postulated to be the principal cause by Brass ( 1976), Brass & Turekian (1 977), and Palmer & Elderfield ( 1985). Specifically, the shifts in Sr isotopic composition of runoff have been ascribed to geographic and climatic phenomena, such as glaciations (Armstrong 1971, Palmer & Elderfield 1985, DePaolo & Ingram 1985), as well as to tectonic (Clauer 1976, Raymo et a1 1988) forcing factors. Yet, Albarede & Michard ( 1987) showed that the 87SrrSr ratio of rivers in southern France, as recorded by lacustrine limestones, remained remarkably constant at 0.7085 ± 12 from 65 Ma to the present, a time of rapid growth in marine 87Sr (Figure 9). The riverine isotopic composition has apparently been buffered by carbonates, despite the concurrent uplift of the Pyrenees and Massif Cen­tral that has resulted in the unroofing of their pre-Hercynian basement.

A primary role for the changing hydrothermal flux, due to variations in the seafloor spreading rates, has been advocated by Spooner ( 1976) and Albarede et al ( 1981), and the possible importance of runout has been pointed out by Chaudhuri & Clauer (1986). In my view, the problem involves at least four variables (JRW' JGW, JOCH, and 87Srr6Sr of the combined runoff and runout); none of these is tightly constrained at present and is even less so in the geological past. With four fluctuating and counteracting unknowns, a unique solution requires prior quantification of the Phanerozoic variations for n - 1 variables, a task beyond our present capabilities.

For the last 60 Myr, the onset of a particularly rapid enhancement of 87Sr in seawater apparently coincided with lower sea-level stands, onset of glaciations, decreasing bottom-water temperatures of the world oceans, deeper carbonate compensation depth, and overall cooling (DePaolo 1986, Ludwig et al 1988). If so, the negative correlation of 87SrrSr with "sea­level" stands is opposite to that of the lower (I OB-yr) hierarchy. This implies that one of the above Sr isotope/"sea-level" correlations may be spurious. Alternatively, the processes controlling the second- and third-order oscil­lations of marine 87Sr/86Sr differ and operate on variable time scales.

Fourth- and Higher-Order 87 Sr;S6Sr Fluctuations at :::;; 106- Years Resolution Theoretically, the controling fluxes are capable of generating measurable changes in 87Sr/86Sr of seawater in � 104 yr (Table 1). Indeed, Capo & DePaolo ( 1986) and Capo et al (1987) found � 5 x 10-5 differences in

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STRONTIUM ISOTOPES IN SEAWATER 159

87Sr/86Sr for Quaternary glacial and interglacial intervals. These differences may reflect transient fluxes associated with geologically rapid waning and waxing of ice sheets. Others invoked more exotic causes for generation of rapid Sr isotopic spikes at the � 106-yr hierarchy, such as acid rain phenomena triggered by bolide impact at the KIT boundary (Macdougall 1988).

APPLICATIONS AND FUTURE POTENTIAL

In recent times, the Sr isotope curve for seawater has been increasingly utilized as a geological reference, and examples of practical applications are listed below.

DATING OF CENOZOIC SEDIMENTS The Cenozoic portion of the curve, in particular its 0-40 Ma section, shows a unidirectional increase in 87Sr, although the rate of this enhancement is variable (Figure 10). For a given age, the Sr isotope band, collating the recent high-precision measurements, has a typical width of "-'2.0--3.5 x 10-5• An isotope curve based on the averaged 87SrrSr values may potentially yield a stratigraphic resolution of about 1 Myr over the segments of rapidly rising isotopic ratios (DePaolo 1986, Elderfield 1986). The correspondence between geologically assigned ages and Sr isotope ages for the last 80 Myr appears satisfactory, with the exception of the 40-60 Ma segment. This potential, if realized, may yield stratigraphic resolution superior to biostratigraphic zonation, and the tool

0.7090

0.7085 8 75, 86s,

0.7080

0. 7075

.. ' . :� ..

' . '

t� 0

.. .. . --; . . /:: . . , -0.50

-1.00 0875,(., .• )

-1.50

-2.00

-2.50

O. 7070 �_L--:!:_-'----:":-----L_..ll::--...J...._::'::--.l_--! - 3.00 100 80 60 40 20 0

AGE (myr.)

Figure 10 Variations in the 87Srj8·Sr of seawater for the 0-100 Ma interval based on high­precision measurements. Modified from Elderfield (1986).

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may be applicable also to sparsely fossiliferous sequences. For example, Ludwig et al (1988) were able to show that the sharp rises in 87SrrSr at Enewetak atoll correlate with disconformities caused by subaerial erosion (sea-level falls), whereas intervals with little change correspond to times of rapid accumulation of shallow-water carbonates. The resulting trend mirrors the deep-sea-based Sr isotope curve. A similar detailed stratig­raphy, albeit with lesser resolution, may be applied to other segments of the Phanerozoic trend, and an initial attempt has been published by Popp et al (1986) and Brookins (1988). An interesting innovation has been the work of Richardson et al ( 1980), who utilized the seawater Sr isotope curve to date the formation of vein calcites in basalts of the oceanic crust.

DOLOMITIZATION Attempts to constrain the timing and the mode of origin of Cenozoic dolomites by Sr isotopes have been published by Saller (1984), Aharon et al (1987), Swart et al (1987), and Vahrenkamp et al (1988). A similar approach to Jurassic and Devonian dolo stones has been utilized by Andrews et al (1987), Medford et al ( 1983), Morrow et al (1986), and Banner et al (1988).

ORIGIN OF DIAGENETIC CEMENTS The Sr isotope technique has been utilized extensively to trace the sources and to constrain the timing of the gen­eration of diagenetic cements, particularly of carbonate type (Stanley & Faure 1979, Medford et al 1983, Moore 1 985, Woronick & Land 1 985, Lundegard & Land 1986, Fisher & Land 1986, Wiggins 1986, Emery et al 1987, Smalley et al 1987, Szabo & Faure 1987). If cements were cogenetic (that is, generated in the same aquifer flow), their 87Srj86Sr ratios should be identical. Thus, the technique may also provide useful data concerning the continuity of ancient aquifers, information of potential value to hydro­carbon and mineral exploration.

MARINE AND NONMARINE SEDIMENTS Since continental watersheds, if com­pared with seawater, are characterized by elevated 87Sr/86Sr ratios, this parameter represents a potential environmental discriminant. The approach has been applied to ancient and Quaternary (bio)chemical sedi­ments by Clauer & Tardy (197 1), Faure & Barrett (1973), Neat et al (1979), Jones & Faure (1972, 1978), and Veizer et al (1982).

MINERAL DEPOSITS The origin, sources, and timing of gangue minerals for a plethora of ore deposits have been investigated using this tool by Kessen et al ( 198 1), Kesler & Jones (198 1), Kesler et al ( 1983, 1988), Lange et al (1983), Norman & Landis (1983), Barbieri et al ( 1987), and Ruiz et al (1988).

GROUNDWATERS, BRINES, AND GEOTHERMAL WATERS Oil-field and deep

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STRONTIUM ISOTOPES IN SEAWATER 1 6 1

subsurface brines (Sunwall & Pushkar 1969, Chaudhuri 1978, Stueber et al 1 984, McNutt et al 1984, Chaudhuri et al 1 987, Burtner 1987, Smalley et a1 1 988) usually contain an excess of 87Sr from interactions with silicates, although some brines (Starinsky et al 1 983b, Morton & Land 1 987) and artesian waters (Collerson et al 1988) may have attained less radiogenic ratios as a result of interactions with the Jurassic-Cretaceous carbonates or with young volcano-plutonic rocks. The uniformity of Sr isotopes in subsurface brines may be utilized to trace the continuity of the under­ground reservoirs, information of potential economic interest. Starin sky et al (1 980, 1 983a) exploited the Sr isotope approach to trace the nature of confining rocks in subterranean aquifers of Israel and to constrain the sources of Sinai aerosols, respectively. For geothermal waters (Doe et al 1 966, Stettler 1 977, Stettler & Allegre 1 978, Vinogradov & Bakin 1 983), the technique has been utilized to constrain the role of subsurface carbonates in buffering their chemistry.

OTHER APPLICATIONS An approach similar to that for mineral deposits is applicable to studies of vein mineral assemblages in plutons being con­sidered as repositories for nuclear waste (Bottomley 1988), to studies dealing with the origin of travertines (Barbieri et aI 1979), to quantification of weathering rates (Aberg & Jacks 1987), and to other questions con­cerning the source of solutes and the timing of their introduction.

Future trends and needs In the immediate future it is likely that the major impact of the seawater strontium isotopic curve will be in applications along the lines of those listed in this section. Applications for the Paleozoic will be somewhat retarded by the less precise delineation of the coeval 87Sr/86Sr seawater trend, but the problems are not insurmountable and will eventually be mastered. For the Precambrian, where the applications for chronological, paleoenvironmental, diagenetic, and genetic studies would be of greatest benefit, this potential is not easy to realize. Concerted effort is required to delineate at least the second- and third- ( 108_107 yr) order structure of the 87Sr/86Sr secular trend as the inevitable prelude to a wide­spread acceptance of the tool for use in solving applied problems in geology.

ACKNOWLEDGMENTS

I acknowledge financial support of the Natural Sciences and Engineering Research Council of Canada, preparation of figures by M. A. Wadleigh, W. Schmiedel, W. Malcherek, and M. ReB, and typing by J. Hayes and B. Kemper.

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Aberg, G., Jacks, G. 1 987. 8'Sr/86Sr as a tool in studies of weathering rates. In Geochemistry and Mineral Formation in the Earth Surface, ed. R. Rodriguez­Clemente, Y. Tardy, pp. 67-76. Madrid: CSICjCNRS

Aharon, P., Socki, R. A., Chan, L. 1 987. Dolomitization of atolls by sea water con­vection flow: test of hypothesis at Niue, South Pacific. J. Geol. 95: 1 87-203

Albarede, F., Michard, A. 1 986. Transfer of continental Mg, S, 0 and U to the mantle through hydrothermal alteration of the oceanic crust. Chem. Geol. 57: 1-1 5

Albarede, F., Michard, A. 1 987. Evidence for slowly changing 8'Sr/86Sr in runoff from freshwater limestones in southern France. Chern. Geol. 64: 55-65

Albarede, F., Michard, A., Minster, J. F., Michard, G. 1 98 1 . 8'Sr/86Sr ratios in hydrothermal waters and deposits from the East Pacific Rise at 2 1 oN. Earth Planet. Sci. Lett. 55: 229-36

Anders, M . H., Krueger, S. W., Sadler, P. M . 1 987. A new look at sedimentation rates and the completeness of the strati­graphic record. J. Geol. 95: 1-14

Andrews, J . W., Hamilton, P. J., Fallick, A. E. 1 987. The geochemistry of early dia­genetic dolo stones from a low-salinity Jurassic lagoon. J. Geol. Soc. London 144: 687-98

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