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Sr concentrations and isotope ratios as tracers of ground-watercirculation in carbonate platforms: Examples fromSan Salvador Island and Long Island, Bahamas

Jonathan B. Martin⁎, Paul J. Moore

Department of Geological Sciences, 241 Williamson Hall, PO Box 112120, University of Florida, Gainesville, FL 32611-2120, United States

Received 20 June 2007; received in revised form 28 November 2007; accepted 29 November 2007

Editor: J. Fein

Abstract

The depth to which seawater and fresh water circulate through modern carbonate platforms may be estimated with 87Sr/86Srisotope ratios of dissolved Sr2+ that is enriched through carbonate mineral dissolution and recrystallization. In 23 water samplesfrom onshore San Salvador Island and Long Island, Bahamas, carbonate mineral dissolution and aragonite-to-calcite trans-formations elevate Sr2+ concentrations to twice seawater values in water with near seawater salinity, and to about 130 times theexpected value for seawater that has been mixed with fresh water. Carbonate mineral dissolution enriches Ca2+ concentrations toaround 30 times seawater concentrations only in the mixed waters; water with seawater salinity has approximately seawater Ca2+

concentrations. Assuming two end-member mixing between seawater Sr2+ and mineral-derived Sr2+, model estimates indicate thatmineral-derived Sr2+ of 19 samples have 87Sr/86Sr ratios equivalent to modern seawater within error of the measurement, indicatingalteration of shallow buried Late Pleistocene to Holocene carbonate minerals. Four samples have mineral-derived Sr2+ with87Sr/86Sr ratios lower than modern seawater value. These low ratios reflect alteration of carbonate minerals that were depositedaround 1 mybp, although the measured 87Sr/86Sr values could reflect ages as great as 4.6 Ma considering the analytical uncertaintyof the measurements. These estimated ages are likely to be minimum values because alteration of modern carbonate minerals at thesurface would provide an unknown, but probably large amount of Sr2+ with modern seawater isotope signatures, therebyoverprinting any low 87Sr/86Sr ratios of non-modern mineral-derived Sr2+. Three of the four samples with low 87Sr/86Sr ratios havehigh salinities and were collected from the interior of the islands. They reflect seawater flow paths at least tens of meters deep maylink the ocean to water several kilometers inland. The fourth sample is from the fresh-water lens below a Pleistocene beach ridge(~125 ka) only 100 m from the shore line. This sample suggests the lens may be thicker than expected based on estimates ofrecharge, hydraulic conductivity and size of the ridge.© 2007 Elsevier B.V. All rights reserved.

Keywords: Carbonate platforms; Sr isotope ratios; Hydrogeology; Bahamas; Aragonite; Calcite

1. Introduction

Both seawater and fresh water flow through mod-ern carbonate platforms. Mixtures of these waters and

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Chemical Geology 249 (2008) 52–65www.elsevier.com/locate/chemgeo

⁎ Corresponding author. Tel.: +1 352 392 2231; fax: +1 352 392 9294.E-mail addresses: [email protected] (J.B. Martin), [email protected]

(P.J. Moore).

0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2007.11.009


variations in their chemical compositions depend ontheir flow paths, and are critical to the major diageneticreactions in carbonate platform sediments, includingcarbonate mineral dissolution and transformation ofaragonite to calcite (Plummer et al., 1976; Budd, 1988;Anthony et al., 1989; Quinn, 1991; McClain et al.,1992; Whitaker and Smart, 1997a). The volume offresh water flow is limited by precipitation and evapo-transpiration, and its flow paths are controlled bypermeability distribution and topography (i.e., gravity-driven flow). In contrast, seawater has an essential-ly unlimited source from the surrounding ocean, butits flow paths depend on processes that draw thewater into the platform (e.g., Whitaker and Smart,1997c), as well as permeability distributions within theplatforms.

Seawater has been shown to flow through carbonateplatforms using a variety of observational techniques andmultiple processes have been proposed to drive this flow.Seawater Sr2+/Cl−, Ca2+/Cl−, and Mg2+/Cl− ratiosdemonstrate that modern seawater flows to depths ofaround 200 m below the sediment–water interface in thewestern Grand Bahama Bank (Swart et al., 2001b). Suchdownward flow may be related to density inversionsfrom evaporation on bank tops (e.g., Adams and Rhodes,1960). Flow across banks may occur when ocean cur-rents vary sea level on opposite sides of carbonatebanks (Drogue, 1989; Whitaker and Smart, 1990,1997b). Seawater flows upward from hundreds of metersfollowing geothermal heating of water in platform in-teriors (Kohout et al., 1977; Aahron et al., 1987). Up-ward flow from depths of 260m below sea level has beenobserved on eastern Grand Bahama Bank as reflectedin temperature and chemical compositions found in ablue hole offshore of North Andros Island (Whitakerand Smart, 1990). Elsewhere on the eastern GrandBahama Bank, pore waters have elemental concentra-tions and ratios that reflect extensive carbonate recrys-tallization, implying little flushing by modern seawaterand reflecting the heterogeneous distribution of flow(Kramer et al., 2000; Malone et al., 2001; Swart et al.,2001b).

In contrast to seawater circulation systems, meteoricwater that recharges island aquifers flows through shal-low fresh-water lenses beneath islands perched oncarbonate platforms (e.g., Plummer et al., 1976; Anthonyet al., 1989). In Bermuda, fresh water extends to around16 m below the water table (Vacher, 1974; Plummeret al., 1976), while on Laura Island inMajuro Atoll, freshwater extends to depths of around 12 m below modernsea level (Anthony et al., 1989). On both islands, theshape of the lens is influenced by the diagenetic history

and physical characteristics of the island limestones(e.g., Vacher, 1988). Topographic flow of fresh watertowards the coast initiates circulation of the underlyingsaline water, thereby drawing seawater from the sides ofthe banks (Reilly and Goodman, 1985). This mixing hasbeen suggested to drive much dissolution and may berelated to the formation of flank margin caves (Mylroieand Carew, 1990). At the fresh water-salt waterboundary, density differences have been found to beimportant for flow and mixing in coastal karst springs(Drogue, 1989; Fleury et al., 2007). The complexhydrology of these islands is shown by lakes and blueholes in the interior of the Bahamian islands that havesalinity similar to seawater although they are separatedfrom the ocean by fresh-water lenses (Davis andJohnson, 1989; Teeter, 1995).

Understanding the complex hydrology of carbonatebanks requires developing a universally applicable andquantitative method to determine depths of flow throughthe platforms. Flow typically is estimated from assess-ments of water-rock reactions along flow paths and theresulting changes in elemental concentrations and ratiosof those concentrations, particularly of Ca2+ and Sr2+,that are enriched through carbonate mineral diagenesis(e.g., Whitaker and Smart, 1990; Kramer et al., 2000;Malone et al., 2001; Swart et al., 2001b). Wehypothesize that complimentary information may bederived from 87Sr/86Sr ratios of Sr2+ derived fromdissolving carbonate minerals. This mineral-derivedSr2+ should have 87Sr/86Sr ratios identical to the mineralproviding the Sr2+ (e.g., Elderfield et al., 1993; Swartet al., 2001a), which would have recorded the seawater87Sr/86Sr ratios when they formed. Variations of sea-water 87Sr/86Sr ratios are well defined through time(e.g., Burke et al., 1982; Howarth and McArthur, 1997),and thus these 87Sr/86Sr ratios should provide informa-tion on the age of the dissolved mineral. Age could beconverted to depth of flow in carbonate platforms withknown subsidence rates. Consequently, in this paper wedevelop a mass balance model to separate seawater andmineral-derived Sr2+ and estimate the 87Sr/86Sr ratio ofthe mineral-derived Sr2+. We test this model in watercollected from various sites on San Salvador and LongIsland Bahamas including ground-water wells, cavepools, and interior lakes. Our results indicate that car-bonate minerals with Sr2+ older than modern seawatercontributes to the dissolved Sr2+, but that much of themineral-derived Sr2+ originates from the diagenesis ofmodern carbonate minerals. Nonetheless, the distribu-tion of non-modern Sr2+ reveals new information aboutflow in one fresh-water lens and circulation of seawaterat three sites on the islands.

53J.B. Martin, P.J. Moore / Chemical Geology 249 (2008) 52–65


2. Study area

The Bahamian Archipelago consists of numerousshallow-water platforms separated by deep-water chan-nels. It extends from 21° to 27° 30′ N and from 69°to 80° 30′ W, covering 300,000 km2 of which about136,000 km2 are shallow-water banks and 11,400 km2

are islands perched on the banks (Carew and Mylroie,1997). While debate continues on the nature of thebasement rocks below the carbonate platforms and howindividual platforms become separated, seismic strati-graphic studies indicate that the platforms are composedof prograding Upper Cretaceous and Tertiary sequences(Eberli and Ginsburg, 1987; Ladd and Sheridan, 1987;Eberli, 1991). These sequences are incised with ancient

deep-water channels that have been filled to withina few hundred meters of the top of the banks withmaterial shed off of the tops of the platforms (Eberli andGinsburg, 1987). These buried channels could disruptflow paths at depths greater than a few hundred metersdeep unless material filling the channels has simi-lar porosity and permeability to the surrounding bankdeposits.

San Salvador Island is located on a small isolatedbank in the southeastern edge of the archipelago, andLong Island is a narrow strip of land exposed on theeastern edge of Grand Bahama Bank (Fig. 1). LongIsland is around 100 km long and about 6.5 km wide atits widest point with the eastern coast a few hundredmeters from the shelf break. San Salvador Island is

Fig. 1. A. Map of the Bahamas showing locations of B. San Salvador Island and C. Long Island. Location points for all the sampling sites areindicated in panels B and C.

54 J.B. Martin, P.J. Moore / Chemical Geology 249 (2008) 52–65


about 20 km long and about 8 km wide and is onlyslightly smaller than its bank. San Salvador's sub-merged bank extends on average about 1000 m fromthe edge of the island and drops nearly vertically todepths of around 3000 m (Carew and Mylroie, 1997).During a dive with the Johnson Sea-Link on the flanksof San Salvador Island's platform, cave openings wereobserved at depths greater than 100 m below sea level,reflecting possible inlet points for seawater to theinterior of the island (Carew and Mylroie, 1987). Sur-face water on Long Island covers less than 5% of theland, but about half of San Salvador Island is coveredby lakes separated from each other and the ocean bynarrow ridges of lithified Holocene- and Pleistocene-dunes (Fig. 1). Salinity of the lakes on both islandsranges from marine to hypersaline. Lakes with nearmarine salinity show diurnal tidal fluctuations withperiodicities that are similar to, but out of phase with,ocean tides, suggesting they are connected to the oceansthrough conduit systems (Davis and Johnson, 1989;Teeter, 1995). No other hydrologic work has been doneon the San Salvador platform; most hydrology of theBahamian archipelago has focused on the much largerGrand Bahama Bank (Malone et al., Swart et al.,2001b; Whitaker and Smart, 1997b,c; Malone et al.,2001; Swart et al., 2001b). In addition to the lakes,

numerous water supply wells and caves provide accessto fresh and saline ground waters (Davis and Johnson,1989; Moore et al., 2006).

3. Sampling and analytical methods

Between June 2004 and June 2006, 23 water sampleswere collected for Sr isotope analyses from 14 sites onSan Salvador Island and two sites on Long Island(Fig. 1) during four different sampling trips. Some of thelocations were sampled multiple times, while othershave only a single sample. The San Salvador sites in-clude six samples from five lakes, nine samples fromthree cave pools, and one sample each from six differentwells. The Long Island sites include one sample eachfrom two cave pools. Easily accessed pools were sam-pled by grab sampling, but large water bodies, wells andcave pools were sampled through polyethylene tubingwith a hand-held vacuum pump or a 12-V peristalticpump. The cave pools had brackish water overlyingsaline water and several samples were collected throughthe mixing zone in individual pools (Moore et al., 2006).Salinity, relative conductivity, and temperature weremeasured at the time of sampling using field meters.Water was collected in 30 ml HDPE bottles withoutadding preservatives.

Table 1Sample sites, chemical analyses, and estimates of carbonate mineral 87Sr/86Sr ratios

Samples Date Salinity Cl (mM) SO4 (mM) Ca (mM) Sr (μM) 87Sr/86Sr fSrmin⁎ 87Sr/86SrCaCO3⁎⁎

Bat Nook—0 6/9/2006 27.4 366 18.3 8.0 88.4 0.709169 0.36 0.709158Bat Nook—1 6/6/2004 29.2 425 22.2 8.8 90.7 0.709182 0.27 0.709201Bat Nook—2 6/9/2006 30.3 403 20.2 8.4 91.4 0.709170 0.31 0.709159Bat Nook—4 6/6/2004 31.5 416 21.7 8.7 96.0 0.709178 0.33 0.709184Bat Nook—4 6/9/2006 31.5 416 20.9 8.7 115 0.709159 0.44 0.709123Bat Nook—6 6/9/2004 27.1 407 21.2 7.9 89.1 0.709178 0.29 0.709184Crescent Pond 6/9/2006 37.5 500 25.6 9.7 121 0.709177 0.36 0.709180Crescent Pond 4/6/2005 37.8 581 30.4 11.2 133 0.709174 0.32 0.709172Crescent Top Cave 6/9/2006 40.1 542 28.4 10.3 129 0.709173 0.35 0.709170Flamingo Pond Surface 6/6/2004 69.4 969 47.3 17.5 235 0.709167 0.36 0.709152Gerace Research Center Well 6/11/2006 0.5 7 0.4 2.0 147.7 0.709160 0.99 0.709159Ink Well Blue Hole 4/5/2005 5.2 78 3.7 3.3 50 0.709175 0.76 0.709175Majors Cave pool—5 4/5/2005 28.3 436 22.1 9.5 116 0.709171 0.41 0.709166Majors Cave pool—6 4/5/2005 27.8 426 21.6 9.0 116 0.709167 0.43 0.709156Museum Well 6/11/2006 0.9 16 0.6 1.8 161.2 0.709149 0.98 0.709148North Point Well 4/7/2005 1.9 30 0.2 4.9 255.6 0.709192 0.98 0.709192North Victoria Hill Well 6/1/2006 10.0 128 6.4 3.9 88 0.709173 0.77 0.709172Overbridge Well 1 4/7/2005 0.3 3 0.3 1.6 49.2 0.709174 0.99 0.709174Overbridge Well 2 4/7/2005 0.5 4 0.4 1.7 47.8 0.709166 0.99 0.709166Six Pack Pond 4/8/2005 51.7 781 38.7 15.3 232 0.709178 0.48 0.709181Watling's Blue Hole 4/4/2005 27.8 422 21.3 9.3 116 0.709160 0.43 0.709140Spanish Church Cave 12/14/2004 38.0 558 29.2 11.1 127 0.709162 0.31 0.709134Crevasse Collapse Cave 12/13/2004 20.2 299 14.2 6.3 126 0.709168 0.63 0.709164

⁎Fraction of Sr in the sample derived from carbonate mineral alteration (Eq. (3)).⁎⁎Estimated 87Sr/86Sr isotope ratio of carbonate mineral providing excess Sr to samples (Eq. (4)).



The samples were analyzed for Ca2+, SO42−, Cl−, and

Sr2+ concentrations, and 87Sr/86Sr ratios in the Depart-ment of Geological Sciences at the University of Florida.Calcium, SO4

2−, and Cl− concentrations were measuredwith an automated Dionex DX500 ion chromatograph.Replicate analyses of Ca2+, SO4

2−, and Cl− internal

standards reflect a precision of better than 3% of the valueof the measurement. Strontium concentrations were mea-sured using isotope dilution with a VG Micromass triplecollector Thermal Ionization Mass Spectrometer (TIMS)simultaneouslywithmeasurements of the 87Sr/86Sr ratios.Strontium isotope ratios were normalized to an 86Sr/88Srratio of 0.1194 during analysis. Five samples weremeasured in duplicate with an average difference of theduplicate measurements of 0.000017. Replicate analysesof the NIST standard NBS987 measured simultaneouslywith the samples averaged 0.710245 with a 2 σ range of0.000023. Reported values were not corrected to thereported value of 0.710250 for NBS987.

4. Results

4.1. Cl, Ca and Sr concentrations

Salinity of our samples ranges from 0.3 psu to69.4 psu (practical salinity unit), with many of the sam-ples clustering around seawater values (Table 1). Aver-age temperature of all samples was 25.3 °C±2.0. Therange of variations in Cl− concentrations is similar tothose of salinity, and Cl− concentrations correlate strong-ly with salinity (r2 =0.99), with the regression linepassing through the origin (Fig. 2). The good correlation

Fig. 2. Cl− concentration versus salinity.

Fig. 3. Ca2+ and Sr2+ concentrations versus Cl− concentrations. The solid line is a linear regression fit of the Ca2+ and Cl− concentrations. The dottedline represents a constant Sr2+/Cl− ratio for seawater. Measured Sr2+/Cl− ratios do not correlate well (r2=0.34, regression not shown), but all Sr2+

concentrations are enriched relative to the Sr2+/Cl− seawater ratio.



and lack of Cl-bearing minerals on San Salvador andLong Island suggest that Cl− behaves conservatively,with its concentration altered only by dilution fromprecipitation and concentration by evapotranspiration.Annual average evapotranspiration exceeds precipita-

tion in the southern Bahamas (Sealey, 1994), includingSan Salvador and Long Island, and thus the wide range insalinity and Cl− concentrations reflects extensive mixingbetween fresh-water lenses, seawater, and brines thatform in the lakes (Davis and Johnson, 1989; Teeter,

Fig. 4. Ca2+/Cl− and Sr2+/Cl− ratios versus salinity. A. All data. B. Ca2+/Cl− and Sr2+/Cl− ratios below 0.05 and 0.8 respectively. In panel B, the solidand dotted lines represent the Ca2+/Cl− and Sr2+/Cl− ratios of seawater, respectively.



1995). The conservative nature of Cl− allows it to beused to estimate the amount of reactive components thatare gained or lost to water during dissolution orprecipitation reactions (Plummer et al., 1976; Gebeleinet al., 1980; Budd, 1988; McClain et al., 1992).

Calcium and Cl− concentrations show a good linearcorrelation (r2 =0.99), with a relationship represented by

Ca2þ ¼ 0:017TCl� þ 1:7 ð1Þ(Fig. 3). Samples with salinity less than about 30 psushow Ca2+/Cl− ratios that are greater than seawatervalue, reflecting greatest enrichment of Ca2+ in thelowest salinity samples (Fig. 4). Samples with salinitynear or elevated above seawater values have Ca2+/Cl−

ratios close to seawater values. The highest measuredCa2+/Cl− ratio of 0.53 indicates a 30 times enrichmentof Ca2+ contributed to the fresh-water samples from theseawater fraction.

Strontium and Cl− concentrations do not correlatewell (r2 =0.34, regression line not shown on Fig. 3) Allsamples, regardless of their salinity, have Sr2+/Cl− ratioselevated above seawater values, reflecting an enrich-ment in Sr2+ (Fig. 4). Similar to the Ca2+ enrichments,the largest Sr2+ enrichments occur in the low salinitysamples, with the highest measured Sr2+/Cl− ratioof about 0.02, indicating about 130 times enrichmentof Sr2+ over seawater concentrations, or more than 4times the greatest Ca2+ enrichment. For samples close toor elevated above seawater salinity, the Sr2+/Cl− ratiosaverage about 50% higher than seawater Sr2+/Cl− ratio.Samples with elevated Sr2+/Cl− ratios also haveelevated Ca2+/Cl− ratios indicating the most enrichedsamples have elevated concentrations of both Sr2+ andCa2+.

4.2. Sr isotope ratios

Measured Sr isotope ratios range from 0.709149 to0.709192 (Table 1). Within uncertainty of the analyticaltechnique, 22 of the 23 samples have bulk 87Sr/86Srisotope ratios that are indistinguishable from modernseawater value, (0.709175, Howarth and McArthur,1997) (Fig. 5). Only the Museum Well, with a measured87Sr/86Sr ratio of 0.709149, is lower than modern sea-water value within the error of the measurement. Thesemeasured Sr isotope ratios reflect the combined value ofseawater Sr2+ isotope ratios and Sr2+ derived fromcarbonate mineral reactions. Mixing with modern sea-water Sr2+ will increase the measured ratio of themineral-derived 87Sr/86Sr ratios if they are lower thanmodern seawater values. Consequently, the amount of

Sr2+ derived from seawater and from carbonate mineralsmust be separated to determine the 87Sr/86Sr ratio of themineral-derived Sr.

Assuming simple two end-member mixing betweenseawater and carbonate mineral sources, the 87Sr/86Srratio of the water samples can be estimated from:

87Sr86Sr

� �sam

¼ fCaCO3T87Sr86Sr

� �CaCO3

þ 1� fCaCO3ð ÞT87Sr86Sr

� �sw

ð2Þ

where the subscripts sam, CaCO3, and sw represent thevalues in the sample, carbonate mineral, and seawater,

Fig. 5. Measured 87Sr/86Sr ratios versus salinity. A. All samples.B. Samples with salinity b6‰. All samples are identical to seawatervalues within the error of the measurement (0.000023) except for thewater sample collected from the Museum Well.



respectively, and fCaCO3 represents the molar fraction ofSr in the sample that originated from carbonate mineralalteration. The value for (87Sr/86Sr)sw is known and(87Sr/86Sr)sam has been measured (Table 1). Unknownvariables in Eq. (2) are fCaCO3 and the 87Sr/86SrCaCO3.

Determining the fraction of Sr2+ derived from car-bonate mineral alteration can be found based on com-parisons of Sr2+/Cl− ratios of seawater, (Sr2+/Cl−)sw, andthe Cl− concentrations in the sample, Clsam, (Budd,1988; McClain et al., 1992) by:

SrCaCO3 ¼ Srsam � Sr=Clð ÞswT Clsamð Þ ð3Þ

where Sr and Cl represent the molar concentrations ofSr2+ and Cl−. Eq. (3) assumes that concentrations ofseawater-derived Sr2+ and Cl− in the samples changeonly by dilution from precipitation and concentration byevapotranspiration. Although some Sr2+ will be incor-porated into diagenetic calcite, the loss of this Sr2+ willnot alter the Sr isotope ratios of the water and is notconsidered in this formulation. The value of fCaCO3 canbe estimated by dividing Eq. (3) by the molar concen-tration of Sr2+ in the sample:

fCaCO3 ¼SrCaCO3

Srsam¼ 1� Sr=Clð ÞswTClsam

Srsamð4Þ

Combining Eqs. (2) and (4) and rearranging yields anexpression for the 87Sr/86Sr isotope ratio of the mineralproviding Sr to the sample:

87Sr=86Sr� �

CaCO3

¼87Sr=86Sr

� �sam

� 87Sr=86Sr� �

sw

1� Sr=Clð ÞswTClsamSrsam

þ 87Sr=86Sr� �

sw

ð5ÞValues of 87Sr/86SrCaCO3 for each sample have beencalculated based on Eq. (5) and are tabulated in Table 1.

Eq. (5) can be used to generate a family of curves thatshows how the 87Sr/86Sr isotope ratio of water will varydepending on the fraction of seawater Sr2+ in the sampleand the 87Sr/86SrCaCO3 ratio (Fig. 6A). By convertingfrom 87Sr/86SrCaCO3 to age using the seawater Sr isotopecurve of Howarth and McArthur (1997), Eq. (5) can alsobe used to estimate the age of the source minerals(Fig. 6B). These curves graphically represent the sen-sitivity of measured 87Sr/86Sr ratios in water of car-bonate platforms to the isotope ratio and amount ofmineral-derived Sr2+. For example, water containingmineral-derived Sr2+ with an 87Sr/86Sr ratio of 0.709100

could have isotope ratios ranging from 0.709077 to0.709123, considering an analytical uncertainty of0.000023 (2 σ). If 50% of the Sr2+ originated fromcarbonate minerals, the estimated Sr isotope ratio of thecarbonate minerals would range from about 0.708782 to0.709860 (Fig. 6A), and the estimated ages for themineral-derived Sr would range from about 10.9 Ma to14.5 Ma (Fig. 6B).

The fraction of mineral-derived Sr2+ in the samples isthe primary factor controlling the sensitivity of thedetermination of the mineral-derived 87Sr/86Sr ratio, andsensitivity improves as the amount of mineral-derived

Fig. 6. 87Sr/86Sr isotope ratios expected in water samples derived fromcarbonate mineral Sr depending on A. the isotope ratio of the mineral,and B. the age of the carbonate mineral providing Sr. In panel B, theage has been converted from the 87Sr/86SrCaCO3 ratio based on theseawater Sr isotope curve of Howarth and McArthur (1997).



Sr2+ increases. Sensitivity for the conversion from87Sr/86Sr ratio to age of carbonate mineral also dependson the slope of the seawater curve. Times with rapidlychanging seawater Sr isotope ratios have the greatestsensitivity, for example, from the present to about 2 Maand between 5 and 6 Ma. Sensitivity of the estimatedages of the mineral-derived Sr2+ may either increase ordecrease depending on the slope of the seawater curve(Fig. 6).

5. Discussion

5.1. Sources of Ca2+ and Sr2+

Elevated Ca2+ concentrations occur exclusively inthe low salinity samples (Fig. 4) and thus net carbonatedissolution appears to originate primarily from interac-tions between the carbonate minerals and fresh water,with little influence from mixing with seawater. Baha-mian platforms are composed almost entirely of car-bonate minerals, with the minor exception of silicateminerals deposited as dust during low stands of sea level(Carew and Mylroie, 1997), and consequently, theobserved changes in compositions of Ca2+ and Sr2+ areunlike to be caused by dissolution of other Ca-bearingminerals such as gypsum or anhydrite. Furthermore,average SO4

2+/Cl− molar ratios of our water samplesaverages 0.053, while seawater ratio is 0.052 indicatingno excess SO4

2+ originates from gypsum or anhydritedissolution. Mixing with seawater either by incorpora-tion of sea spray into the rain or after deposition on landwould result in a Ca2+/Cl− ratio identical to seawatervalues, with a Ca2+−Cl− correlation that would passthrough the origin in Fig. 3. The positive intercept of1.7 mM in the Ca2+−Cl− correlation also reflects netdissolution, resulting in an enrichment of Ca2+ in thelow salinity water. In addition to the positive intercept,the slope of the Ca2+−Cl− correlation (0.017) is slightlylower than the value for seawater Ca2+/Cl− ratio (0.019).The slope becomes 0.02 (r2 =0.94), close to the Ca2+/Cl− ratio in seawater, if the regression is forced throughthe origin (Fig. 3). Although dissolution reactions arealso likely in water with seawater salinity (e.g. Budd,1988; McClain et al., 1992), their lack of Ca2+ en-richments reflect similar magnitudes of reprecipitation(Fig. 3 and Eq. (1)).

Dissolution of carbonate minerals by fresh watercommonly occurs from elevated CO2 concentrationscaused by equilibration of rainwater with atmosphericCO2, from CO2 derived from root respiration, and bymicrobial oxidation of organic matter (Plummer et al.,1976; Stoessell et al., 1989). Neither San Salvador nor

Long Island has surface streams, and thus any rainwaterthat is not transpired to the atmosphere would rechargefresh water lenses on the islands directly through thevadose zone (e.g., Vacher et al., 1990). Surface rockscommonly contain large amounts of aragonite, whichwhen dissolved by the recharged rainwater would enrichthe water in Ca2+. Surface seawater is slightly supersatu-rated with respect to calcite and thus calcite wouldbe expected to precipitate in water with near seawatercompositions, especially as Ca2+ concentrations in-creased during aragonite dissolution, i.e. the aragonite-to-calcite transformation (Budd, 1988, and Fig. 4).

In contrast to Ca2+, a source of Sr2+ to the water fromdissolution, as well as from the aragonite-to-calcitetransformation, is shown by elevated Sr2+/Cl− ratios,including samples with seawater salinity and higher(Fig. 4). The aragonite-to-calcite transformation in-creases Sr2+ concentrations because the greater concen-tration of Sr in aragonite than calcite results in a net fluxof Sr to the water (Budd, 1988). The poor correlationbetween Sr2+ and Cl− reflects the variable concentrationsof Sr in carbonate minerals dissolving to elevate Sr2+ andCa2+ concentrations, as well as variable amounts of Srincorporated during the reprecipitation of calcite duringthe aragonite-to-calcite transition. Similar to Ca2+, Sr2+

originates by mineral dissolution in low salinity water asshown by a positive y-intercept of the Sr2+−Cl− corre-lation (around 80 μM Sr2+, regression line not shown inFig. 3), and higher Sr2+/Cl− ratios at low rather than athigh salinity (Fig. 4). In addition to the excess Sr2+ at lowsalinity, a good inverse correlation (r2 =0.94 for samples

Fig. 7. Salinity versus fraction of Sr in the water derived from carbonatemineral dissolution.



with salinity b50 psu) occurs between salinity and thefraction of mineral-derived Sr2+ estimated from Eq. (4)(Fig. 7). This inverse correlation reflects the greateramount of dissolution and less reprecipitation of car-bonate minerals in fresh water than in saline water, withthe freshest samples containing nearly 100% mineral-derived Sr2+.

In summary, these results reflect water that has beeninvolved with extensive carbonate mineral diagenesisand associated increases in Ca2+ and Sr2+ concentrationsfrom water–rock reactions (Figs. 3 and 4). Enrichmentof Ca2+ and Sr2+ in low salinity water indicates thatmuch of the mineral dissolution has occurred since thefresh water recharged the island, and thus is probably a

Fig. 8. 87Sr/86Sr ratio of Sr2+ derived from carbonate mineral dissolution and/or the aragonite-to-calcite transition as estimated by Eq. (5). A. Allsamples. B. Those samples with 87Sr/86Sr ratios that are significantly below the value for modern seawater. Samples are labeled with their samplinglocation. The axis on the right converts 87Sr/86Sr ratio to age using the seawater Sr isotope curve of Howarth and McArthur (1997) without includingthe uncertainty in the measurement of the 87Sr/86Sr ratio. See text for further explanation.



recent near-surface phenomenon (e.g., Vacher et al.,1990). Elevated Sr2+/Cl− ratios in samples with nearseawater salinity suggest that some of the mineral-derived Sr may have originated at greater depths aswater flowed to sampling points on the surfaces of theislands. The following section explores if the 87Sr/86Srratio can be used to estimate the age of the mineralsproviding the excess Sr2+, and thus infer possible burialdepths where the reactions occurred.

5.2. Sr isotope ratios and possible ages of carbonatefraction

Only one sample, collected from the Museum Well,has a bulk 87Sr/86Sr ratio (0.709149) lower than seawaterratios, indicating that most of the dissolved Sr2+ in oursamples originates from modern seawater and dissolu-tion of Holocene or Late Pleistocene limestones thathave 87Sr/86Sr ratios identical to modern seawater(Fig. 5B). While only the sample from the MuseumWell has an 87Sr/86Sr ratio below modern seawatervalue, a total of four samples have 87Sr/86SrCaCO3 ratiosthat are lower than modern seawater values (Fig. 8B).These samples include on San Salvador Island: theMuseum Well, Watling's Blue Hole, and one of sixsamples collected from Bat Nook pool within MajorsCave, and on Long Island Spanish Church Cave. Thesamples from Bat Nook pool were collected during twodifferent sampling trips and are separated from eachother by about 1 m depth in the pool. Although no othersamples from the pool show the same low 87Sr/86Srisotope ratio, duplicate measurements of the sample areconsistent, indicating there is no laboratory contamina-tion. Contamination to lower values during sampling isunlikely, because the limestone that contains MajorsCave is Late Pleistocene in age and thus would have87Sr/86Sr ratios indistinguishable from modern seawater.Salinity of the MuseumWell is 0.9 psu, and consequent-ly much of its Sr2+ (98%) is derived from mineral dis-solution (Table 1). This sample is thus sensitive to theisotope ratio of the carbonate providing its Sr2+ (Fig. 6).The other three samples have near seawater salinity andmineral-derived Sr2+ ranges from 31 to 43% of the totalSr2+ and thus have less sensitivity to the mineral-derivedSr2+.

The values of the isotopic composition of the min-eral-derived Sr2+ range from 0.709123 for Bat Nook to0.709148 for Museum Well (Table 2). These valueshave analytical uncertainties that are compounded bythe calculation of fCaCO3 (Eq. (4)). The maximumpossible lower limit of the mineral-derived 87Sr/86Srratio can be estimated by subtracting 0.000023 (2 σ)

from the measured value for total Sr2+ and using thisvalue to calculate the value for 87Sr/86SrCaCO3 fromEq. (5). This calculation indicates the lower limit for87Sr/86SrCaCO3 could range from 0.709047 for the BatNook pool to 0.709125 for the Museum Well.

Using the 87Sr/86Sr seawater curve fromHowarth andMcArthur (1997), the 87Sr/86SrCaCO3 ratios were con-verted to estimated ages of the carbonate minerals pro-viding Sr2+ to each water sample. These ages range from0.85 to 1.26 Ma for the measured value of the Sr2+ andfrom 1.23 to 4.68 Ma for the lower limit considering theuncertainty of the measurement (Table 2). The modelused for these calculations (Eq. (5)) only estimates the87Sr/86Sr ratio of all mineral-derived Sr without separat-ing Sr2+ that originates from old buried carbonates andyoung exposed carbonates that would have modern sea-water 87Sr/86Sr ratios. Exposed carbonate minerals onLong Island and San Salvador Island include Holocene-and Pleistocene-aged carbonate minerals producedduring past interglacial highstands including MarineIsotope Stages (MIS) 5, 7, 9, and 11 (Carew andMylroie,1997). Strontium from these exposed minerals wouldincrease the estimated 87Sr/86SrCaCO3 ratio, similar tomixing with modern seawater (Eq. (5)). Estimated87Sr/86SrCaCO3 ratios are thus likely to represent mini-mum ages for these four water samples; other watersamples that have 87Sr/86Sr ratios similar within error tomodern seawater values may also contain Sr2+ fromcarbonate minerals older than Late Pleistocene, butis overprinted by dissolution or recrystallization ofyounger carbonate minerals. Because some mineral-derived 87Sr/86Sr ratios are less than modern seawatervalues, the water must have interacted with carbonateminerals older than the Late Pleistocene.

Table 287Sr/86Sr isotope ratios and age estimates for carbonate mineral Sr

Sample location 87Sr/86SrCaCO3⁎ Age (Ma)⁎⁎

Bat Nook Value 0.709123 1.26Minimum 0.709047 4.68

Watling's Blue Hole Value 0.709140 1.00Minimum 0.709087 2.04

Spanish Church Cave Value 0.709134 1.10Minimum 0.709061 3.52

Museum Well Value 0.709148 0.85Minimum 0.709125 1.23

⁎87Sr/86Sr ratio estimated for carbonate mineral fraction of dissolvedSr2+. The minimum value is estimated from the sample 87Sr/86Sr ratiominus 1 σ error (0.000023) and calculated for the 87Sr/86Sr ratio.⁎⁎Age converted from Howarth and McArthur (1997), but neglectinguncertainty in the Sr seawater curve. The uncertainty in the curve isroughly 0.04 for the youngest samples to 0.25my for the oldest samplesand thus much smaller than the uncertainty of 87Sr/86Sr measurement.



5.3. Assessment of depth of flow paths

The 87Sr/86SrCaCO3 values observed here reflectinteractions with young rocks, implying that flow occursat shallow depths (Fig. 5). Evidence for shallow flowbased on 87Sr/86Sr ratios is expected since models ofdeep flow through carbonate platforms suggest that it isunlikely that carbonate dissolution would occur at greatdepths (e.g. 100s of meters) (Kohout et al., 1977; ReillyandGoodman, 1985;Whitaker and Smart, 1990, 1997b).For example, geothermal heating driving Kohout con-vection (Kohout et al., 1977) would decrease the solu-bility of carbonate minerals. The elevated salinityassociated with reflux processes (Reilly and Goodman,1985) should saturate or supersaturate the water withrespect to calcite. In addition, most of the deeply-buriedcarbonate minerals in the platform would have stabilizedduring extensive diagenesis in fresh, marine, and mixedwaters during repeated sea level fluctuations of thePleistocene (e.g., Budd, 1988; Beach, 1995;Melim et al.,2004). The presence of even small amounts of Sr2+ with87Sr/86Sr ratios lower than seawater values reflect flowalteration of minerals within the platforms along the flowpaths, and indicate that 87Sr/86Sr ratios should be usefulto assess depths of flow (Fig. 8).

The depth of flow cannot be determined until exactages of the mineral-derived Sr can be separated. Theestimated age of around 1 Ma for the mineral-derivedSr2+ of four samples (Table 2) suggests that water hasinteracted with carbonate minerals that are older than theOwl's Hole Formation, which constitute the oldest rocksexposed on the surface (Carew and Mylroie, 1997). Theage of this formation is unknown, but stratigraphicrelationships indicate it formed during sea level highstands at most 440 kybp (MIS 9) and possibly duringmore recent high stands (e.g. MIS 7 or 5e). Strontiumisotopes from carbonate minerals from the Owl's HoleFormation would thus be indistinguishable from modernseawater values (Howarth and McArthur, 1997, andFig. 6). Considering an approximate and average subsi-dence rate of Bahamian platforms of between 10 and20 m per million years (Carew and Mylroie, 1995), andestimated ages for the mineral-derived Sr2+, water couldhave interacted with carbonate minerals at depths rang-ing to at least a few tens of meters below the surface.

Three of the water samples with low 87Sr/86Sr valuesand near seawater salinity (Bat Nook pool, SpanishChurch Cave, and Watling's Blue Hole) are located upto several kilometers inland from the coast. In contrast,the low salinity sample (Museum Well) is located in afresh-water lens of a Late Pleistocene beach ridge about100 m from the open ocean (Fig. 1). Bat Nook pool is

surrounded by shallow hypersaline lakes (Davis andJohnson, 1989; Moore et al., 2006), but contains waterwith salinity ranging from brackish to seawater values(Table 1). The pool's salinity suggests it is nothydrologically connected to the lakes. The fraction ofmineral-derived Sr2+ in these samples ranges from 0.31to 0.44 and thus the model has low sensitivity to the87Sr/86Sr ratio of the mineral-derived Sr2+ (Fig. 6).Mineral-derived Sr2+ thus may be older than estimatedby the model.

The low 87Sr/86Sr ratio of the low salinity samplefrom the Museum Well is particularly remarkable be-cause a significant amount of modern Sr2+ is likely tohave been released during alteration of the sedimentshosting the fresh-water lens thereby overprinting thelow 87Sr/86Sr ratio. This sample contains 98% of min-eral-derived Sr2+ and thus is highly sensitive to the87Sr/86Sr ratio of the carbonate minerals contributingSr2+. The dune ridge hosting the fresh-water lens wherethe Museum Well is located was deposited duringthe MIS 5e transgression and highstand (Carew andMylroie, 1997) and thus the 87Sr/86Sr ratio of the hostrock would be indistinguishable from modern seawater.The rock units directly below and to the east of the ridgeare undefined (Robinson and Davis, 1999), and couldhost older carbonate minerals carrying low 87Sr/86Srratios.

The dune ridge is about 250 m wide and about 1 kmlong, and thus approximates the infinite strip-islandassumptions for thickness of fresh water lenses ofVacher (1988). Lens thicknesses on strip islands dependon the amount of recharge and width of the lens, andinversely on the hydraulic conductivity of the aquiferrocks (Vacher, 1988; Budd and Vacher, 1991). On SanSalvador Island, recharge is estimated to be about300 mm/yr based on 25% of average annual rainfall of1200 mm (Cant and Weech, 1986), and a conservativeestimate of hydraulic conductivity for the Pleistocenedune ridge is 100 m/day (e.g., Vacher, 1988; Budd andVacher, 1991). Using the model proposed by Budd andVacher (1991), the root of the fresh-water lens wouldextend about 2.3 m below sea level. Considering thelocal stratigraphy, this depth is unlikely to be sufficientlydeep to supply the observed old 87Sr/86Sr isotope ratios.The discrepancy between Sr2+ that originated from pre-Owl's Hole rocks and the shallow estimated depth of thefresh-water lens suggests that the base of the fresh-waterlens may be deeper than estimated from the hydrologicmodel (e.g. Budd and Vacher, 1991). The size of thedune ridge and amount of recharge are fairly well con-strained suggesting that hydraulic conductivity may belower than estimated, thereby expanding the thickness



of the lens. The size of the lens has important implica-tions for water resources on these islands.

6. Conclusions

All water sampled on San Salvador Island and LongIsland Bahamas have Sr2+ concentrations enriched overvalues that would be provided from admixed seawater,reflecting a source of Sr2+ from carbonate mineral alter-ation. Comparison with the Ca2+ enrichments suggeststhat alteration includes both carbonate mineral dissolutionand transformation of aragonite to calcite. A model basedon variations in Cl− and Sr2+ concentrations providesestimates of the 87Sr/86Sr ratio of themineral-derived Sr2+

in the water. Four of the 23 samples measured, or roughly17%, have mineral-derived 87Sr/86Sr ratios that are lowerthan modern seawater value. Comparison of these87Sr/86Sr ratios with the seawater 87Sr/86Sr curve ofHowarth and McArthur (1997) indicates the maximumage of the Sr2+ is 4.68Ma, considering the analytical errorof the measurement, althoughmore likely ages are around1Ma. Carbonate minerals of this age could be buried by afew tens of meters based on subsidence rates of theBahamian platforms of 10 to 20 m/my. Three of thesamples with 87Sr/86Sr lower than modern seawater valueare located several kilometers from the coast, but haveapproximately seawater salinity, reflecting a connectionbetween the interior of the platform and the ocean.The fourth sample with an 87Sr/86Sr ratio lower thanmodern seawater value is located in the fresh-water lensof a Pleistocene dune ridge only about 100 m from theocean. The presence of non-modern Sr2+ at this locationindicates that the fresh-water lens may be thicker thanexpected based on estimates of recharge and hydraulicconductivity.

The model presented here illustrates the utility ofusing 87Sr/86Sr ratios to trace flow through carbonateterrains. The value of 87Sr/86Sr is particularly useful inmodern carbonate systems undergoing steady subsi-dence because the relationship between carbonatemineral 87Sr/86Sr ratio and the burial depth can beconstrained by subsidence rates. This technique shouldalso be applicable in ancient systems, although such anapplication would require measuring the 87Sr/86Sr ratiosof the rocks in the system. The primary limitation of themodel as applied in modern carbonate systems comesfrom the ubiquitous and rapid alteration of modern ara-gonite to calcite and/or dissolution of aragonite at thesurface. Alteration of modern carbonate minerals pro-vides a large source of Sr2+ with modern seawater87Sr/86Sr ratios that are included in the estimates of the87Sr/86Sr ratios of mineral-derived Sr2+. A mixture of

Sr2+ derived from modern and old carbonate mineralswould elevate the measured value of the 87Sr/86Sr ratio,making the age estimates minimum values. Better esti-mates of the age of Sr2+ derived from deeply-buriedcarbonate will require knowing the amount of Sr2+

derived from modern carbonate minerals.

Acknowledgements

Vince Voegeli and other staff at Gerace ResearchCenter provided logistical support for sampling on SanSalvador Island and John Mylroie provided logisticalsupport for sample collection on Long Island. KellyDeuerling provided valuable assistance in the labora-tory. JBM thanks personnel at La Maison des Sciencesde l'Eau, Université Montpellier II for their hospitalityduring the writing of the paper. We thank two anony-mous reviewers who greatly helped the focus of thepaper.

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