Fluctuations of isotopic composition of strontium in seawater during the Phanerozoic Eon

Chemical Geology (Isotope Geoscience Section), 59 (1986) 293-303 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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F~~CT~AT~O~S OF ISOTOPIC CO~POS~T~O~ OF STRO~T~~~ IN SEAWATER D~R~~~ THE P~A~EROZO~C EON

S. CHAUDHURI and N. CLAUER

Department of Geology, Kansas State University, Manhattan, KS 66506 (U.S.A.) Centre de Sddimentologie et Ggochimie de la Surface, F-67084 Strasbourg (France)

(Received June 25,1986; revised and accepted September 17,1986)

Abstract

Chaudhuri, S. and Clauer, N., 1986. Fiuctuations of isotopic composition of strontium in seawater during the Pha- nerozoic Eon. Chem. Geol. (Isot. Geosci. Sect.), 59: 293-303.

Inclusion of a runout Sr term in the equation for the isotopic budget of seawater provides a better understanding of the cause for the lack of correlation between sea-level changes and the isotopic variations of Sr in seawater during the Phanerozoic Eon. Continental unions and break-ups that change the length of coastlines can greatly alter the flux of runout Sr. The estimate for the present-day flux of runout Sr is (1.9-2.0) * 10”’ g a-‘. Taking the runout Sr as an additional component of continental Sr delivered to the oceans, the flux of Sr from ridge basalts at hydrothermal centers is estimated at - 1.27 - 10” g a-‘.

1. Introduction

Studies of marine fossils and carbonate rocks have demonstrated that the Sr isotopic composition of seawater varied considerably during the Phanerozoic Eon (Peterman et al., 1970; Dasch and Biscaye, 1971; Veizer and Comps- ton, 19’74; Tremba et al., 1975; Clauer, 1976; Kovach, 1980; Burke et al., 1982; DePaolo and Ingram, 1985; Palmer and Elder~eld, 1985; DePaolo, 1986; Hess et al., 1986). Fig. 1 illus- trates the mode and the magnitude of the Sr isotopic variations of seawater during the Pha- nerozoic, as given by Burke et al. (1982). To date, the Sr isotopic balance of the seawater has been ascribed to differing fluxes and isotopic compositions of Sr to the oceans from three sources that include: (a) basalts at centers of

hydrothermal activities along ocean ridges; (b ) runoff discharged to the sea; and ( c ) pore water associated with recrystallization of carbonate sediments in oceans. The flux of Sr released from recrystallization of carbonate sediments is considered to be about one-fifth of the flux of Sr from runoff, and its Sr isotopic composition appears to be similar to the isotopic composition of Sr in contemporaneous seawater. Thus, the temporal variations of the Sr isotopic composition of seawater during the Phanerozoic Eon have been commonly attributed to variations in the flux of Sr from runoff with varied but generally high 87Sr/86Sr and the flux of ridge-basalt Sr with low 87Sr/86Sr. To under- stand the cause for the long-term isotopic variations of Sr in seawater, the trend of the isotopic variation has been compared with the recently

0168-9622/~/$03.50 0 1986 Elsevier Science Publishers B.V.

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suggested sea-level changes by Vail et al. (1977)) because both sea-level changes and the rate of hydrothermal circulation of seawater through ocean ridges should respond to changes in volume of ocean-ridge systems so that the periods of low volume of ocean-ridge systems should correspond to periods of marine regression and high 8’Sr/86Sr of seawater, whereas periods of high volume of ocean-ridge systems coincide with periods of transgression and low 87Sr/86Sr of the seawater. Except for the last 80 Ma or so, the correlation between the sea-level changes and the Sr isotopic variations of the sea is extremely poor. Therefore, examination of other parameters that can also influence the Sr isotopic composition of the seawater is desirable. We think that at least one additional source of continental Sr to the sea that has so far not been considered is the flux of Sr from subsurface water delivered directly to the ocean, which is known as the runout. The purpose of our paper is to show that inclusion of the runout Sr term in the equation for the marine Sr isotopic budget offers a good explanation for the observed trend of variation of the isotopic composition of Sr in seawater during the Phanerozoic Eon.

2. Volume of runout and the flux of runout Sr to the oceans

2.1. Seawater-continental water interface

The concept of the hydrological cycle is based on the continuous movement of all forms of water and on the interdependence of the different components. Part of the precipitation which infiltrates through the surface of the ground may be held in the underground for several thousand years or longer. The groundwater component is eventually removed by evapora- tion and transpiration to the atmosphere via upward capillary movement to the soil surface and vegetation cover, by subsurface seepage and flow to surface streams, and by runoff to the oceans. Although some information is available about the global amount of groundwater that is presently stored, little is known about how much

water annually circulates through each of the groundwater removal domains.

The interface between seawater and continental water to some depths is commonly diffuse as a result of the combined effects of mechanical dispersion and chemical diffusion (Cooper, 1964; Kohout, 1964). Cooper (1964) predicted that where a zone of diffusion exists, the salt water flows continuously in a cycle from the floor of the sea to the zone of diffusion and then back to the sea. Kohout (1964) confirmed Cooper’s suggestion by providing field data from the Biscayne aquifer in Florida. He observed a flow velocity of more than 20 m day-’ for groundwater in the mixed zone in an aquifer with an effective porosity of 0.2. The transition zone is less than 30 m thick vertically and has a width of more than 600 m at its seaward limit. Cooper (1964) maintained that groundwater-salt water mixing and convective dispersion is a continuous process that creates the zone of diffusion by the reciprocative motion of the salt-water front as a result of ocean tides, storms, and the rise and fall of the water table. Like Cooper (1964)) Lusczynski and Swarzen- ski (1962) also observed a transition zone, 30-60 m thick, in the Magothy aquifer in southwestern Long Island, New York. Meisler (1981) reported a transition zone, 500 to more than 700 m thick and extending laterally to as much as 90 km, that underlies part of the Atlantic Coastal Plain and adjacent Continental Shelf. He also noted that F.T. Manheim and cowork- ers (unpublished data, 1980) recognized a transition zone with a vertical thickness of N 1000 m. Several investigators concluded that the thick transition zones along the Atlantic Coastal Plains and the Continental Shelf are a consequence partly of tidal fluctuations and partly of large-scale eustatic sea-level fluctuations that occurred primarily during the Qua- ternary and the Pliocene (Upson, 1966; Meisler et al., 1984).

2.2. Volume of runout

Nate (1969) was probably the first to give an estimate of annual discharge of groundwater to

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the sea, and subsequently many authors have quoted his figure. Based on approximate cal- culations for the conterminous U.S.A. that suggested that the runout is equal to -5% of streamflow, Nate (1969) estimated that the global runout volume is N 1.5. lo3 km3 a- ‘.

Alternatively, we may calculate the amount of runout to the oceans by using hydrologic information on the flow of water across a given plane in the coastal regions. Fairbridge (1977) estimated that the total length of coastlines of the continents is N 312,000 km. While considering flow of groundwater across planes in coastal regions, attention should be given to lithology of subsurface rocks in the coastal areas because crystalline rock sequences and some sedimentary strata will have such extremely low transmissivities that they will not contribute significantly to the volume of runout per unit time. We do not know the global amount of low- transmissivity rocks along the coastlines. But we assume that as much as 50% of the coastlines consist of rocks of extremely poor transmissivity, in which case the effective length of coastline for the groundwater flow may be assumed to be 156,000 km. In the preceding dis- cussion of the development of diffuse zones between groundwater and seawater, ocean tides and rise and fall of water table were considered to be partly the cause of the presence of diffuse zones that were tens of meters thick. Kohout (1964) reported flow rates of N 20 m day-’ in a diffuse zone that is less than 20 m thick. Meis- ler et al. (1984) determined hydraulic conduc- tivities of 3 to more than 30 m day-l in the Coastal Plain aquifers in New Jersey. Assum- ing an average flow rate of N 15 m day-l across a plane N 10 m wide and 156,000 km long with an effective porosity of 0.2, the estimated runout through the diffuse zone at shallow depth becomes 1.7~10~ km3 a-‘.

Although the water in the upper part of the transition zone is very mobile, the flow rate in much of the thick transition zone is extremely small, 0.1-0.3 mm day-’ (Manheim and Horn, 1968; Meisler et al., 1984). If we consider that

the rate of movement of the interface repre- sents the rate of input of the groundwater to the oceans, we can compute the amount of slow- moving groundwater that annually comes in contact with the oceans. Assuming a plane 156,000 km long and 3 km wide with an average porosity of 0.15 along the coastlines and a par- ticle-movement velocity of N 0.1 mm day-‘, the total amount of very slow moving runout across the plane is estimated at N 2.6 km3 a-l.

Clearly, the volume of runout through much of the interface between the seawater and the continental water is going to be overwhelm- ingly influenced by the flow through the very top 10 m or so of the interface. Despite its low value, the volume of the runout through the deeper segment of the interface cannot be dis- regarded while calculating the flux of runout Sr to the oceans because such runouts can have extremely high Sr contents.

2.3. Flux of the runout Sr

Not enough data exist to determine a reliable value for the average Sr content of groundwater at shallow depths in the coastal regions. Khar- aka et al. (1978) reported an average Sr content of little more than 1 ppm for some waters from a depth of N 10 m in the Texas Gulf Coast. Although Magaritz and Luzier (1985) did not report the Sr contents for their study of groundwaters in coastal sandy aquifers in Ore- gon, an average Sr content of N 0.8 ppm may be estimated from the Ca data they presented, assuming that the Sr/Ca ratio of the waters was similar to the average value of runoff. Sass and Starinsky (1979) reported an average Sr content of 130 ppm for two water samples from aquifers, 30-40 m deep, in the Mediterranean coastal plains of southern Israel. The high values of Sass and Starinsky are probably unusual for waters from such shallow aquifers. Recog- nizing the widely varied Sr contents for groundwaters in shallow aquifers in the coastal regions, we take a value of w 1 ppm as being the average Sr content of the shallow runouts to the oceans.

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We have previously noted an estimate of 1.7. lo3 km3 a-l for the shallow runout and thus the flux of this runout Sr is calculated to be 1.7*1O”g a-l.

The Sr contents of deep subsurface waters vary widely. Sass and Starinsky (1979) and Starinsky et al. (1983) found the Sr contents of waters from depths between 1300 and 3000 m in the subsurface in the Mediterranean coasts range between 50 and 1200 ppm. Manheim and Horn (1968) noted Sr contents between 7 and 1150 ppm for waters from depths ranging from 500 m to more than 3000 m along the North American Atlantic Coastal Plains. Schmidt (1973) reported Sr contents of 135-600 ppm for saline waters from llOO- to 2800-m-deep Mio- cene sandstone-shale sequence in the Gulf Coast area of southwestern Louisiana. Khar- aka et al. (1978) found that waters in normal- pressured reservoirs to depths between 1000 and 3000 m had Sr contents between 15 and 440 ppm. Taking an average Sr content of N 100 ppm for groundwaters to depths of -3000 m, we estimate the flux of runout Sr to the oceans from the slow-moving part of the transition zone to be N 0.26*1012 g a-‘. When the flux of runout Sr from the deep part of the transition zone is added to the flux of runout Sr from the shallow part of the diffuse zone, the total flux of runout Sr to the oceans is N (1.9-2.0) * 1012 g a-‘.

2.4. Isotopic composition of the runout Sr

The Sr isotopic composition of the runout flux should be known to assess quantitatively its impact on the Sr isotopic budget of the sea. We may expect that the average 87Sr/86Sr of the runout flux through the upper part of the mixed zone to be similar to the average value for the runoff, which at the present time is N 0.7111, as Wadleigh et al. (1985) suggested. As the flux of runout Sr from the deep zone constitutes only N 15% of the total flux of runout Sr, the deep- water component, even differing in Sr isotopic composition from the shallow-water component by as much as 0.2-0.3%, probably will not

substantially alter the overall isotopic composition of the runout Sr, which is dominated by the shallow groundwater component. We assume that the isotopic composition of the runout Sr from shallow zone is essentially the same as that of the surface runoff. Hence we take a value of 0.711 as the average Sr isotopic composition of the runout.

3. Hydrothermal input of Sr from ridge basalts

For a steady-state condition, the Sr isotopic balance of the modern seawater can be expressed as:

Sr, R, + Sr, R, + Sr,, Rdc + Sr,,, Rob =

( Sr, + Sr, + Srdc + Sr,,,,) R,

where Sr,=flux of runoff Sr (2.21~10’~ g a-‘; Elderfield and Greaves, 1981) ; R, = 87Sr/86Sr of the runoff (0.7111; Wadleigh et al., 1985); Srgw=flux of runout Sr (1.9*1012 g a-l; this study) ; R,= 87Sr/86Sr of the groundwater (0.711; this study); Sr,,=flux of Sr from diagenetic recrystallization of carbonate sediments (0.48. 1012 g a-‘; Elderfield and Gieskes, 1982) ; Rdc=87Sr/86Sr of the flux from carbonate sediment diagenesis (0.7084; Elderfield and Gieskes, 1982); Srob=flux of oceanic basalt Sr; R,,,h87Sr/./86Sr of the oceanic basalt (0.7029; Spooner et al., 1977) ; and R, =87Sr/86Sr of the seawater (0.7092; Burke et al., 1982).

Considering the invariant nature of Sr in seawater associated with the isotopic exchange during the seawater-basalt interaction at ocean ridges (Edmond et al., 1979)) the flux of Sr from the ridge basalts is estimated to be N (1.26-1.29) .1012 g a-l. This estimate is somewhat higher than previously reported values of 0.876 - 1012 g a-’ by Albarede et al. (1981), 0.91 - 1012 g a-’ by Elderfield and Greaves (1981)) and 0.65 * 1012 g a-’ by Wad- leigh et al. (1985)) but our figure is not unrea- sonable. Taking 5 km as being the maximum depth of penetration for the hydrothermal

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solutions along the 54 * 103-km length of the ridge and 6 cm aa1 as being an average rate of sea-floor spreading, the volume of new oceanic crust annually produced that is subject to potential alteration at the spreading centers is estimated to be 16.2 km3 a-‘. With a density of -3 g cmw3 and an average Sr content of 105 ppm, the amount of Sr in the new oceanic crust that becomes potentially available for hydrothermal Sr isotopic exchange with seawater is -5.1 * lOI g a-‘. Thus, our estimate of (1.26-1.29) * 1012 g a-’ for the flux of Sr from oceanic ridge basalts indicates an alteration of -25% of the new crust annually produced at the ridges. This figure compares favorably with the recent suggestion of Mottl (1983) that as much as 34% of the oceanic crust could be altered at the axial region of the ridges.

4. Runout flux and isotopic variations of Sr in seawater

The introduction of runout flux to the equation for the Sr isotopic balance of seawater gives a new perspective on the role of the flux of continental Sr in the long-term Sr isotopic variations of the sea. The new term in the equation provides additional rational explanations of the excursions of the Sr isotopic compositions of seawater through the Phanerozoic Eon. To assess the importance of the runout Sr to the temporal changes in the isotopic composition of seawater, we examine whether or not both the flux and the isotopic composition of the runout Sr to the oceans could have changed with time.

Leaving aside climatic variations, which undoubtedly would be important, a major factor that can substantially change the flux of runout Sr to the oceans is the variation of coastlines. Whereas continental break-ups and regressions of the sea will cause an increase in the coastlines, continental collisions or continent-arc collisions and transgressions of the sea can have the opposite effect. Transgression or regression of the sea will simultaneously affect

the runout and the runoff Sr fluxes, but continental unions or break-ups will produce little change on the runoff flux while they can have a strong impact on the runout flux.

4.1. Post-Late Cretaceous Sr isotopic variation

Several explanations have recently been given for the general increase in the s7Sr/86Sr of seawater since the Late Cretaceous. Spooner (1976) suggested that the long-term increase in the ratio has probably been the result of variations in the runoff flux produced by variations in land area. Palmer and Elderfield (1985) con- tended that the post-Late Cretaceous increase was due to long-term increase in the 87Sr/a6Sr of the runoff flux, whereas Hess et al. (1986) argued for an increase in the flux of Sr from rivers to the oceans. We suggest that the general trend of increasing 87Sr/86Sr of seawater since the Late Cretaceous was due to increase in both the flux and the 87Sr/86Sr of the continental Sr. The increase in the flux of runout Sr since the Late Cretaceous was at least in part due to increase in the runout from combined effects of the continued splitting of the supercontinent Pangea and the latest long-term regression of the sea.

To explain quantitatively the temporal variation of the Sr isotopic composition of seawater since the Late Cretaceous, some extreme assumptions have to be made as to the fluxes and the isotopic composition of Sr of different components that influence the seawater isotopic composition. The Late Cretaceous was a time of major transgression of the sea. Hallam (1977) and Pitman (1978) noted that as much as 35% of the present land mass was covered by the sea during the Late Cretaceous. If the runoff varies in proportion to the change in area of the land mass, as Garrels and MacKenzie (1971) suggested, then the flux of runoff Sr at the time of the Cretaceous transgression may be taken to be -35% less than the flux of Sr( w 2.24 . 1012 g a-‘) at the present time, or -1.46 * 1Ol2 g a-‘. The proposition that the

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flux of runoff Sr during the Late Cretaceous was less than that during any subsequent times may not be valid. Berner et al. (1983) suggested that the carbon dioxide level in the Cretaceous atmosphere was higher than that in the Recent atmosphere and, consequently, both weathering rate and continental runoff were higher in the Cretaceous than today. Taking the view that the flux of runoff Sr increased in proportion to the increase in landmass and applying the same proportionality factor to the change in the flux of runout Sr, we assume that the flux of runout Sr during the Late Cretaceous was - 35% less than that in Recent time. But the flux of runout Sr had to be even lower than that, because the length of coastlines in the Cretaceous was smaller than today as the Pangean break-ups continued through the Tertiary. We assume that an additional lo- 15% coastlines were created since the Late Cretaceous due to continental break-ups. Thus, the flux of runout Sr during the Late Cretaceous could have been N 50% of the value of -2.0 * 1012 g a-’ at the present time, or - 1.0 * 1012 g a-l. The flux of Sr from diagenetic recrystallization of submarine carbonate deposits certainly changed as a result of change in the carbonate compensation depth, but we take the flux during the Late Cretaceous to be nearly the same as the flux today. The Sr isotopic compositions of all sources of Sr to the sea are taken to be nearly constant over the last 80 Ma, although the possibility exists that the isotopic composition of the continental fluxes changed during this time interval, as Palmer and Elderfield (1985) suggested. Taking a steady- state condition, the flux of Sr from ridge basalts at the time of the Late Cretaceous is estimated to be -2.3 * 1012 g a-l. A similar calculation may be made for the flux of Sr from ridge basalts 65 Ma ago, giving an estimate of - 1.9 * 1012 g@, which is - 50% higher than the flux today. Pitman (1978) estimated that the global sea- floor spreading rate 65 Ma ago was -60% higher than the rate today. Assuming that the rate of hydrothermal cycling of the seawater through ridge basalts is the same as the rate of

sea-floor spreading, our estimate of the basaltic Sr flux from ridge systems 65 Ma ago being 50% higher than the flux today is in reasonable agreement with the figure of the sea-floor spreading rate given by Pitman (1978).

The continued rifting of Pangea, which caused enhanced flux of runout Sr, could have increased not only the flux of continental Sr to the oceans but also the average 87Sr/86Sr of the continental Sr flux. The many rifts of Gond- wanaland occurred apparently along some very old mobile belts of earlier periods of instability. The newly created coastlines bordered by some old continental rocks could have greatly increase the average 87Sr/86Sr of the flux of continental Sr to the sea. Our suggestion of the increase in s7Sr/86Sr is not new, as Palmer and Elderfield (1985) previously also made a similar suggestion. But our rationale behind it is different from that of Palmer and Elderfield who believed that the post-Cretaceous increase in the isotopic ratio of the flux of runoff Sr could have occurred due to accelerated erosion of continental rocks from uplifted land masses following erogenic activities, at least during the Tertiary. But we are not fully convinced of their explanation of the increase in the 87Sr/ssSr as a result of accelerated erosion following an orogeny. Reasons for our doubts about the causal relationship between the two are that several orogenies during the Paleozoic and the Mesozoic eras are apparently marked by prominent isotopic minima and the increases in the isotopic ratio following many past orogenies are not necessarily related to increased weathering of high 87Sr-bearing silicate minerals and rocks. We believe that, in many instances, after the climax of an orogeny the erosion of the uplifted mass continues to contribute Sr with overall low 87Sr/86Sr due to the dominant influence of young crustal materials in the uplifted mass. Only at a much later period, when the relief is considerably reduced, can the influence of old sialic materials be demonstrably sufficient to account for the increase in the 87Sr/86Sr of the continental Sr flux to the oceans. Thus, we pre-

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fer to emphasize that the continental splits might have been more influential than accelerated erosion for a possible increase in the isotopic composition of the flux of continental Sr to the oceans since the Late Cretaceous. We therefore suggest that any numerical modeling explaining the increase in the 87Sr/86Sr of seawater since the Late Cretaceous should consider increases both in the flux and the isotopic composition of continental Sr to the sea.

4.2. Mesozoic marine transgression and the Sr isotopic composition

As illustrated in Fig. 1, the Sr isotopic value of seawater at the time of the Late Cretaceous transgression lies on the segment of the seawater isotopic variation curve that broadly indicates gradual increase in the 87Sr/86Sr of seawater since the Late Jurassic. If correlation between changes in sea level and changes in volume of ocean-ridge systems is valid, as Vail et al. (1977) suggested, then the isotopic minimum at Late Jurassic indicating a very intense hydrothermal activity appears anomalous to what the data on sea-level changes would suggest. We consider that the very low Sr isotopic value during the Late Jurassic is not anomalous. The isotopic minimum can still be explained within the premise of the relationship between changes in sea level and changes in volume of ocean-ridge systems. A major rea- son for the low 87Sr/86Sr of seawater in the Late Jurassic compared to in the Late Cretaceous is that the flux of runout Sr was much lower in the Jurassic than in the Cretaceous because of the lesser fragmentation of Pangea in the earlier period. A lower 87Sr/86Sr of the flux of continental Sr during the Jurassic, as compared to the ratio during the Late Cretaceous, can also partly account for the Jurassic seawater having lower 87Sr/8sSr than the Late Cretaceous seawater.

The effect of the reduction of the runout Sr flux during Late Jurassic may be quantitatively demonstrated by considering some approxi-

I gw F 0 0 g s

6 k? 0

5:

8 Lo

IST- ORDER CYCLES

RELATIVE CHANGES OF SEA LEVEL

cRlSlNG _ Fl

f

PRESENT SEA LEVEL

I

PERIODS

TERTIARY

CRETACEOUS

JURASSIC

PERMIAN

‘ENNSYLVANIAN

MISSISSIPPIAN

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

PRECAMBRIAN

Fig. 1. Variations of isotopic composition of Sr in Phane- rozoic sea (after Burke et al., 1982) and changes of sea level during the Phanerozoic (after Vail et al., 1977).

mate figures for the fluxes and the isotopic compositions of Sr of different components that influence the isotopic composition of sea water. Hallam (1977) noted that N 25% of the present land mass was flooded during the Jurassic. We may expect the Jurassic flooding to have caused proportional decreases in the fluxes of runout and runoff Sr. But the flux of runout Sr had to be even lower than that because the continents were less fragmented during the Jurassic than at any other subsequent time. Assuming that fragmentation alone has caused about a 50% increase in the flux of runout Sr since the Jur- assic, and taking an additional 25% increase in

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the flux since the Jurassic as a result of the increased land mass, the flux of runout Sr during the Late Jurassic may have been N 75% less than the present flux of runout Sr of w 2.0 * 1012 g a-‘. The flux of Sr from pore waters associated with diagenetic recrystallization of carbonate sediments is assumed to be nearly the same as today. Keeping the isotopic compositions of different sources of Sr to the oceans to be approximately similar as today, the steady- state Sr isotopic balance of 0.7068 for the Jur- assic sea could have been maintained with a flux of ridge-basalt Sr of N 1.5 * 10” g a-l, which is N 15% more than the flux of Sr from ridge basalts of recent time, but N 65% less than that of the Late Cretaceous. The computation shown here is not intended to provide a unique solution to the question of the flux of Sr from ridge basalts during the Jurassic; but it demonstrates that changes in the flux of runout Sr as a result of continental fragmentation can cause a considerable decrease in the isotopic composition of seawater despite low hydrothermal activities at the ocean ridges. Thus, we conclude that the lowest 8’Sr/8sSr of the seawater since the Pan- gean break-up is not an indicator of the maximum hydrothermal activity at the ocean ridges, but a reflection of the very low flux of runout Sr as a result of the formation of the supercontinent.

4.3. Sr isotopic trend during the Paleozoic

A major enigma in the history of the isotopic evolution of seawater Sr during the Phanero- zoic Eon is that the “Sr/“Sr of the seawater generally decreased, with the exception of the points of some sharp declines which appear to coincide with several orogenies, while the sea apparently changed from a major transgression in Late Cambrian-Early Devonian to a major regression in Permo-Triassic time (Fig. 1). Whereas long-term eustatic fluctuations of sea level have often been viewed as changes in volume of ocean-ridge systems, the long-term change from transgression to regression of the

sea that apparently occurred during Paleozoic time should have produced a trend of increasing 87Sr/86Sr of seawater, provided a simple correlation existed between changes in the isotopic composition and changes in the volume of ocean-ridge systems. The apparent anomalous relationship between the sea-level changes and the Sr isotopic variations of seawater during the Paleozoic has led some to conclude that either the transgression-regression of the sea or the Sr isotopic changes of sea water or both are unrelated to changes in volume of ocean-ridge systems.

We believe that changes in the flux of runout Sr can partly explain the broad trend of isotopic variation of seawater through Paleozoic time. The Paleozoic continental evolutionary history is highlighted by periods of several major orogenies, culminating in the development of the Pangean supercontinent. The consequence of progressive welding of the continents and continental accretions was a gradual reduction in the coastlines and hence a reduction in the flux of runout Sr. But the reduction in the flux of runout Sr associated with the decrease in the coastlines had to be counteracted by increases both in the runout and runoff fluxes due to accompanying progressive regression of the sea. Although the simultaneous effects of the two processes of the decrease of coastlines and the regression could amount to a net reduction in the flux of continental Sr to the oceans, this reduction alone may not be sufficient to explain the Sr isotopic difference between the Late Cambrian-Early Devonian and the Permian seas, especially when the reduction of the flux of continental Sr has to be coupled with possible attenuation of the input of Sr from ridge basalts as dictated by the long-term sea-level drop connected with a decrease in volume of ocean-ridge systems. We suggest that a progressive decrease in the 87Sr/86Sr of the flux of continental Sr may have accentuated the effect of decreased flux of the runout Sr to produce the general decrease in the 8’Sr/86Sr of seawater beginning early Paleozoic. Our postulate of

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the progressive decrease in the 87Sr/86Sr of the continental Sr flux seems reasonable because the evolution of continents during the Paleo- zoic is characterized by gradual accretion of young materials with low 87Sr/86Sr as a result of several continent-arc and continent-continent collisional orogenies. The weathering and erosion of the low 87Sr/86Sr- bearing siliciclastic materials from the uplifted land masses may have caused an overall decrease in the isotopic composition of the flux of continental Sr. Concrete evidence for the support of our postulate of progressive changes in the isotopic composition of the flux of runout Sr, and hence also of the flux of runoff Sr, is yet to be found.

4.4. Orogenies and isotopic minima

Several prominent isotopic minima are superimposed on the general trend of a progressive decrease of 87Sr/86Sr of seawater during the Paleozoic. The timing of these isotopic minima appears to coincide with several major orogenies. This relationship between orogeny and prominent isotopic minimum was apparently not duplicated at all times; for example, a Sr isotopic shift can hardly be recognized during the Tertiary Himalayan orogeny. If a decrease of Sr isotopic values of seawater occurs with all major orogenies, then the influence of the Himalayan orogeny toward creating a low Sr isotopic value for the seawater may have been mitigated by a concurrent increase elsewhere in the flux of runout Sr with a high isotopic value as part of the process of continued break-ups of the Pangean supercontinent. Though changes in the flux of runout Sr can at least partly explain the long-term Sr isotopic variations of seawater, such changes cannot account for the relatively rapid decrease followed by an equally rapid increase in the Sr isotopic values of seawater that is coincident with an orogeny. The very low 87Sr/86Sr of the seawater during an erogenic phase is undoubtedly due to increased influence of input of “basaltic” Sr from either

oceanic ridges or from the collisional zone or both. Should the collisional zones become the major contributors of low 87Sr/86Sr, then the isotopic minima may be related to the nature of erogenic developments. The causal relationship between the isotopic response of seawater and orogeny remains a subject of considerable challenge to geochemists.

5. Conclusions

The flux of runout Sr is an important component of the continental Sr annually delivered to the oceans. An estimate of the present-day annual flux of runout Sr is given as (1.9-2.0) * 10” g. Taking a steady-state condition for the isotopic composition of Sr in seawater, the present-day annual flux of Sr from ocean-ridge basalts appears to be N 1.3 * 1012 g, which amounts to alteration of -25% of the newly created oceanic crust at the ridge areas.

Progressive continental unions or accretions during the Paleozoic produced a gradual decrease in the flux of runout Sr, whereas continued break-ups of the supercontinent Pangea caused a gradual increase in the flux of runout Sr. The lack of correspondence between the trend of isotopic variations of Sr in seawater and the trend of long-term changes of sea level during much of the Phanerozoic Eon may be attributed to variations in the flux of runout Sr and, conceivably, to associated long-term changes in the isotopic composition of continental Sr delivered to oceans in response to the continental unions and break-ups.

Acknowledgments

We thank G. Faure, J. Graf, J. Veizer, Y.K. Kharaka and C. Oviatt for their helpful sugges- tions and thoughtful reviews.

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Documents

Fluctuations of isotopic composition of strontium in seawater during the Phanerozoic Eon