Aragonite-To-Calcite Transformation During Fresh-Water Diagenesis of Carbonates: Insights from Pore-Water Chemistry

Aragonite-to-calcite transformation during fresh-water diagenesis of carbonates: Insights from pore-water chemistry

DAVID A. BUDD Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309

ABSTRACT 1968; Back and Hanshaw, 1970; Plummer and others, 1976), dolomitization (Gebelein and others, 1980; Patterson and Kinsman, 1982), and Dissolved strontium and calcium concentrations in fresh-water burial diagenesis (Prezbindowski, 1981). lenses (FWL) and associated mixing zones (MZ) on two small, Holo- Pore-water chemistry is used in this study to examine the diagenetic cene ooid-sand islands in the Schooner Cays, Bahamas, were moni- alteration of Holocene ooid sands in thin, transient fresh-water lenses on tored during a 1-yr period to quantitatively analyze the transformation two small 700-yr-old islands in the Schooner Cays, Bahamas (Fig. 1). The of aragonite to calcite. The observed characteristics of this mass dissolution of aragonite and precipitation of calcite on these islands have transfer are functions of climate and hydrology. been established petrographically (Budd, 1984). The objective of this study Aragonite-to-calcite transformation in all hydrologic zones is is to quantify the aragonite-to-calcite transformation, determine its distri- primarily associated with meteoric recharge. The transformation oc- bution in time and space, and examine the effects of climate and hydrology curs throughout the FWL and in the MZ to relative salinities of 19% on transformation rates and mechanisms. This is accomplished by using and 36% sea water on the two islands. Rates of transformation are aqueous calcium, strontium, and chlorine concentrations to determine the rapid in all zones and are greatest in the FWL. A limestone composed magnitude, rate, and efficiency of the aragonite-to-calcite transformation, of 100% calcite should form from an aragonite precursor within where the transformation occurs within the hydrologic system, when it 4,700 to 15,600 yr in the FWL, and within 8,700 to 60,000 yr in the occurs with respect to the hydrologic cycle, and what drives it. upper MZ. Efficiencies of transformation can vary between hydrologic zones HYDROGEOLOGIC FRAMEWORK due to PCO2 effects; yet, the efficiency of the entire system (FWL +

MZ) is high (87%). This indicates that most CaC03 derived from Geology aragonite dissolution is reprecipitated as calcite somewhere in the fresh-water system or upper mixing zone. The two islands studied, Wood and Water Cays, are situated in the CO2 effects, fresh-water-sea-water mixing, and the differing sol- southeastern portion of the Schooner Cays oolitic tidal bar belt (Fig. 1). ubilities between aragonite and calcite all drive the mass transfer. The The islands are about 625 m and 200 m long, respectively, and both are latter is the most significant, accounting for up to nine times more about 150 m wide. Elevations average 0.5 to 1.5 m above sea level on the

mass transfer than C02 effects and at least ten times more mass interior of each island, and each island is bordered by a 1.0- to 2.5-m-high transfer than fresh-water-sea-water mixing. Differing solubilities dune ridge. Seismic refraction profiles indicate an average of 10.7 m of should also cause mass transfer to occur throughout the hydrologic Holocene sediment below each island (Sagasta, 1984), and vibracores cycle, but it apparently becomes ineffective after the rainy season, reveal that at least the upper 5.8 m of this sediment is oolitic sand (Curtis, possibly due to the inhibition of calcite precipitation. 1985). Island surfaces are sparsely vegetated, and no true soil zone exists on either island. Mineralogic and petrographic analyses of these sediments INTRODUCTION reveal an average depositional mineralogy of greater than 95% aragonite (Budd, 1984). Calcite-cemented rocks contain 55%-95% aragonite and Hydrochemical studies of pore water in sediments and sedimentary 45%-5% low-Mg calcite. The simple mineralogy of these sediments allows rocks can be used to monitor and model on-going diagenetic processes. a precise quantification of the aragonite-to-calcite transformation that is Examining diagenetic processes in this manner yields insights into those not easily done in mineralogically more complex aquifers. processes that are not available from studying diagenetic products alone. This is because pore fluids record diagenetic reactions occurring during Hydrology their residence time in the system, whereas solid phases exhibit the cumulative effects of diagenesis. Pore-water studies have been successfully applied Ground water from Wood and Water Cays was monitored periodi- to carbonate diagenetic systems in all types of hydrochemical settings, cally for a year between July, 1981, and August, 1982, utilizing a series of including marine diagenesis of sediments (Berner, 1966; Baker and others, observation wells. The hydrology is summarized here; see Budd (1984) for 1982; Morse and others, 1985), meteoric diagenesis (Harris and Matthews, a more complete discussion.

Additional material for this article (Table A) may be secured free of charge by requesting Supplementary Data 8819 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 100, p. 1260-1270, 13 figs., 2 tables, August 1988.

1260

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Figure 1. Location of the study area and iow-altitude aerial photograph of Wood Cay. Long dimension of the island is 625 m; view is to the north.

Pore-water samples were collected after first emptying the pipe of I OOLITIC TIDAL standing water. A downhole bailer device or suction applied to tygon ' BAR BELT tubing was used to collect the water. Each sample represents a mixture of ISLANDS water from around the base of the pipe. No impervious seals, such as cement, were placed around the pipes, therefore, some minor mixing of water down the length of the pipes may have occurred. Fresh-water lenses on Wood and Water Cays are transient due to The pH was measured in the field using a Sargent-Welch 401 porta- the seasonal distribution of rainfall (Fig. 2). The thickness, volume, and ble pH meter with a 0-14 scale and combination electrode. No corrections areal extent of the fresh-water lenses are greatest immediately after the were applied to pH values. Total alkalinity was measured in the field by rainy season which ends by November (Fig. 2). Maximum observed the electrometric acid tritration technique of Barnes (1964). Repetitive thickness, as defined by the 1.0 ppt isohaline (2.9% sea water), was 1.25 m analyses indicate a precision of ±2% for alkalinity. Temperature was meas- on Water Cay and 1.0 m on Wood Cay. The sizes of the fresh-water lenses ured downhole (±0.1 °C). Acidified and unacidified filtered samples were on both islands decrease drastically during the dry season that extends prepared for cation and chloride analyses, respectively, as well as a filtered from December through early summer. sample treated with zinc acetate for sulfate analysis. The fresh-water-sea-water mixing zones (2.9%-90% sea water) on Chloride concentrations were determined by titration with silver ni- each island range in thickness from 2 to 3 m and are always much thicker trate (Skougstad and others, 1979). Sulfate was analyzed indirectly by the than the overlying fresh-water lenses. The thickness of the mixing zones titration technique of Howarth (1978). Concentrations of major cations does not vary seasonally as greatly as does the thickness of the fresh-water (Ca++, Na+, Mg++, K+, and Sr++) were determined by atomic adsorption lenses. High permeabilities (4 to 180 darcies), large tidal ranges spectrometry. Solutions and standards for calcium determinations were (10-20 cm at water table in center of islands; adjacent marine averages spiked with 1% lanthium chloride solutions to eliminate interferences 90 cm) and low hydraulic heads (3-18 cm) on both islands result in (Skougstad and others, 1979). Strontium analyses were done by the meth- reduced fresh-water flow, greater tidal dispersion, and relatively thick od of addition (Volborth, 1969). Analytical precision, as determined mixing zones. against standards, was ±2% for CI", Na+, K+, and Mg++; ±3% for Ca++; ++ ±5% for Sr ; and ±10% for S04=. PORE-WATER CHEMISTRY The distribution and activities of all aqueous species, the partial pres- sure of CO2, ion-activity products, and saturation states (log IAP/K) of Methods carbonate minerals were calculated using the aqueous equilibrium model PHREEQE (Parkhurst and others, 1980). The thermodynamic equilib- Water wells were established on Wood and Water Cays by drilling rium constants of Plummer and Busenberg (1982) were used for calcite, 2-in. holes with a portable drill and then inserting open-ended PVC pipes. aragonite, and dolomite. All other thermodynamic data were from the A well site consisted of two to four such pipes set to different depths and MINTEQ data base (Felmy and others, 1984). Activity coefficients were open to the aquifer only at their base. A total of 42 pipes were emplaced: calculated using the Truesdell and Jones (1973) extension to the Debye- 29 on Wood Cay in 15 well sites, and 13 on Water Cay in 6 well sites. Huckel equation for high ionic strengths. Considering uncertainties in the After drilling, but prior to sampling, the ground water around each analyses and equilibrium calculations, uncertainties in saturation indices well was given at least 12 days to re-establish a hydrochemical equilib- and PCO2 values are about ±0.1 and ±0.05 log units, respectively. rium. Consistent conductivity readings were recorded within 5 to 6 days of Chloride, calcium, and strontium concentrations, calculated PCO2 drilling. Water samples were collected in July and November, 1981, and values, and the log of aragonite saturation indices are given in Table A.1 in April and August, 1982. This sampling spanned a complete hydrologic cycle covering the fall recharge season, the winter and spring dry season, 'Table A may be secured free of charge by requesting Supplementary Data and the hot summer months (Fig. 3). 8819 from the GSA Documents Secretary.

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(A) WOOD CAY (C) WATER CAY

NOVEMBER, 1981 AUGUST, 1982 NOVEMBER, 1981 AUGUST, 1982 A c ^ D.

• FRESH WATER X WELL SITES

0 100 (B) (meters) (D)

NOVEMBER NOVEMBER A S12 S10 S,9

' 50 --

AUGUST AUGUST B S10 S9 S8 S6 S1 S2 S4 B'

-50-

—so— PERCENT SEA WATER 0 50 100 . GROUND-WATER SAMPLE (meters) • FRESH WATER

Figure 2. Areal and cross-sectional distribution of the fresh-water lenses on Wood Cay (A, B) and Water Cay (C, D) at the end of the rainy season (November 1981) and at the end of the dry season (August 1982).

Concentrations of all other dissolved species and calculated saturation aragonite dissolution. As seen in Figure 6, however, most pore water states are available from the author upon request. examined in this study has a composition which falls above such a line, indicating a strontium build-up relative to calcium. The build-up in stron- Results tium results from calcite precipitation because less strontium is incorporated into calcite than is derived from aragonite. Pore water on Wood and Water Cays can be subdivided into two basic types, a calcium-bicarbonate water and a diluted sea water, with a ARAGONITE-TO-CALCITE TRANSFORMATION well-defined mix between the two end-member solutions (Fig. 4). The concentrations of sodium, potassium, and magnesium are conservative Calculations with respect to chloride (Figs. 5A, 5B, 5C), indicating that the mixing of rain water and sea water controls the concentrations of these species. The cumulative amount of aragonite dissolution and calcite precipita- Enrichment in sulfate with respect to chloride in a few fresh-water tion in a pore-water sample can be calculated by considering the flux of samples (Fig. 5D) can be attributed to sea-salt aerosols in rain water both strontium and calcium in the rock-water system (Harris and Mat- (Matthess and Harvey, 1982). Sulfate concentrations are reduced in other thews, 1968; Plummer and others, 1976; Gebelein and others, 1980). The samples with respect to rain-water-sea-water mixing concentrations, in- total amount of strontium measured in a pore-water sample (MSr++) dicating sulfate reduction has occurred in those samples. Calcium and equals the net amount of strontium derived from rock-water reactions strontium concentrations are increased in all samples relative to rain- (RSr++) as that water sample moved through the various hydrologic zones, water-sea-water mixing concentrations (Fig. 5E, 5F), indicating dissolu- plus the strontium derived from intermixed sea water (Sr++/Cl~ ratio in tion of aragonite. Sulfate reduction and aragonite dissolution both cause an sea water times the measured chlorinity of the sample): increase in total alkalinity relative to the mixing line (Fig. SG). Dissolved strontium and calcium concentrations, when considered MSr++ = RSr++ + (Sr++/Cl" x MCI"). (1) together, also indicate that calcite precipitation has occurred. Aragonite dissolution adds strontium and calcium to the pore water in proportion to RSr++ equals the amount of strontium derived from aragonite dissolution the molar ratio of strontium and calcium in aragonite, which is 0.011 in less the strontium which is incorporated into precipitated calcite cements: the oolitic sediment of Wood and Water Cays (Budd, 1984). After correcting for strontium and calcium derived from fresh-water-sea-water RSr++ = y(0.011) - z(0.0017) (2) mixing, the molar ratio of the remaining strontium to calcium should plot on a line with a slope of 0.011 if the waters have experienced only where y and z are the moles of aragonite dissolved (DISS) and calcite

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precipitated (PPT), respectively, and 0.011 and 0.0017 are average molar waters and ±9% to ±25% in lower mixing-zone waters. The higher uncer- fractions of strontium in the aragonite sediments and calcite cements, tainty in the latter case is due to the very low values of DISS and PPT in respectively, as determined by electron microprobe analyses (Budd, 1984). such waters. The flux of calcium in the rock-water system can be modeled in a similar manner. The resultant equations are Amount and Location

++ ++ ++ MCa = RCa + (Ca /Cr * MCI") (3) The cumulative amounts of aragonite dissolution and calcite precipi- RCa~ = y(0.9890) - z(0.9803) (4) tation as functions of relative salinity throughout the fresh-water lenses and mixing zones on Wood and Water Cays are shown in Figure 7 and where 0.989 and 0.9803 are molar fractions of calcium in the aragonite summarized in Table 1. Two trends occur on Water Cay, and one trend sediments and calcite cements, respectively. Solution of all four characterizes the data from Wood Cay. Each trend exhibits an increasing mass-balance equations for the cumulative amounts of dissolution (y) and cumulative amount of dissolution and precipitation through the fresh- precipitation (x) gives water lens and into the upper mixing zone with maxima between 15% and 40% sea water. The cumulative amounts of dissolution and precipitation y = (107.7)MSr++ - (0.19)MCa++ - .015 MCI" = on all three trends decrease with increasing salinity after the maxima are DISS (mmoles/1) (5) reached. This decrease represents the cessation of dissolution and precipita- ++ ++ z = (108.7)MSr++ - (1.21)MCa++ + .0037 MCI" = tion and an independent dilution effect on RSr and RCa by fresh- PPT (mmoles/1) (6) water-sea-water mixing. Because of this dilution effect and the cumulative nature of the DISS and PPT values, it is not readily apparent in Figure 7 The amount of aragonite dissolved (DISS) and calcite precipitated (PPT) where the transformation reactions actually occur. Incremental changes in during the chemical evolution of each pore-water sample are given in the amount of dissolution and precipitation with increasing salinity were Table A. The uncertainty in DISS and PPT values due to analytical thus calculated and are shown in Figure 8 for a better approximation of precision varies, depending on Sr++, Ca++, and Cl~ concentrations. In where the transformation reactions occur on each trend. general, uncertainties are ±6% to ±8% in fresh and upper mixing-zone Three observations are evident from Figures 7 and 8. First is that trends 1 and 2 on Water Cay differ only in the initial amount of transformation at salinities <5% sea water; thereafter, the two trends are quite similar. Trend 3 (Wood Cay), however, is quite different, exhibiting lesser amounts of dissolution and precipitation at any particular salinity than trends 1 and 2. Trend 3 also exhibits transformation reactions farther into 40- the mixing zone than do trends 1 and 2. The second observation is that most of the transformation reactions on all three trends occur in the upper mixing zones (Fig. 7); however, the incremental changes in the amount of dissolution and precipitation with increasing salinity are greatest in the fresh-water lenses (Fig. 8). These two relationships are not in conflict; rather they result from the hydrology of

30- these settings. Although the amount of transformation per unit volume of 1980-81 1981-82 HYDROLOGIC CYCLE HYDROLOGIC CYCLE E o

10-

Lilsd

J'A'S'O'N'DIJ'F'M'A'M'J'J'ASO'N'DU'F'M'A'M J J A S' O' N 1 1980 1981 1982 TIME

Figure 3. Average monthly rainfall for the period 1980 to 1982 at Harbour Island, Eleuthera. Seasonality of rainfall defines annual hy- Figure 4. Piper diagram of pore-water compositions. Composi- drologic cycles. Sampling intervals for this study are shown by vertical tions range from calcium-bicarbonate waters (fresh water) to sodium- arrows. chloride waters (sea water).

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200 k

0 5 10 15 20 0 5 10 15 20 5 10 15 20 3 3 3 CI" (PPM X 10 ) CI- (PPM x 10 ) CI- (PPM x 10 )

Figure 5. Cross plots of major cations and anions versus chloride concentrations in all samples. The line on each plot is drawn between normal sea water and rain water. Representative uncertainties due to analytical precision are shown by crosses.

.6-1 o E E Figure 6. The amount of strontium in excess of fresh-water-sea-water mixing versus the amount of calcium in excess of fresh- o .4 water-sea-water mixing (that is, amount of s strontium and calcium above mixing lines in Figs. 5E and 5F). Waters whose compositions plot on the line with a slope of 0.011 have experienced only aragonite dissolution. Waters whose compositions plot above the .2- same line have also experienced calcite precipitation. Representative uncertainties due to w analytical precision are shown by crosses. S

6 8

1 MCa" -(ciI —- x MCI- ) (mmoles/l)

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TABLE 1. AMOUNT OF ARAGONITE DISSOLUTION AND CALCÏTE PRECIPITATION BY (A) HYDROLOGIC ZONE ON WOOD AND WATER CAYS

WATER CAY

DISSOLUTION (mmoles/I)

Fresh-water system 15.5 34.0 6.1 Upper mixing zone 23.5 36.0 16.9

PRECIPITATION (mmoles/l)

Fresh-water system 14.0 30.5 1.8 Upper mixing zone 20.0 28.5 19.7

salinities at which the aragonite-to-calcite transformation stops for trends 1,2, and 3, respectively. .5 1 PERCENT SEA WATER Timing

X TREND 11 X X WATERCAY The general trends of aragonite dissolution and caicite precipitation o°o TREND 2J TREND 3> WOOD CAY defined in Figure 7 are not time dependent. Each trend contains samples collected throughout the hydrologic cycle that began with meteoric re-

(B) + ANALYTICAL UNCERTAINTY charge in the fall of 1981 and ended in August 1982. For example, all samples from well site 3 on Water Cay lie on trend 1, regardless of when they were collected. Separate trends for each sampling period could not be defined. This means that most of the dissolution and precipitation reactions occurred at the beginning of the hydrologic cycle. Thereafter, changes in the amount of dissolution and precipitation observed at any particular observation well (Table A) were primarily the result of increased salinity as the fresh-water lenses shrank in size and the mixing zones migrated upward toward the water table. Any additional aragonite-to-calcite transformation that occurred after recharge had to be of lesser magnitude. The scatter of data in Figure 7 about any one general trend may be an indica- tion that some alterations did indeed occur after recharge; however, such additional transformations are impossible to resolve with this data set.

Reaction-Driving Mechanisms

The driving mechanisms behind aragonite dissolution and caicite precipitation can be divided into two categories (James and Choquette, 1984), one involving equilibration between water and carbonate minerals in general (water controlled), and a second involving the differing solubili- 1 5 ties of caicite and aragonite (mineral controlled). Equilibration of recharg- PERCENT SEA WATER ing fresh water to the carbonate minerals and soil-zone CO2 (CO2 effects), and fresh-water-sea-water mixing are the two water-controlled mecha- Figure 7. Amount of aragonite dissolution (A) and caicite precipi- nisms affecting the aragonite-to-calcite transformation on Wood and tation (B) as functions of relative salinity throughout the fresh-water Water Cays. The greater solubility of aragonite relative to caicite is the lenses and mixing zones of Wood and Water Cays. Lines through the mineral-controlled mechanism. data are best-fit approximations. CO2 Effects. The effect of CO2 on aragonite dissolution and caicite precipitation in Wood and Water Cay pore water can be evaluated by water is greatest in the fresh-water lenses, the larger total volume of the examining only fresh-water samples in which sulfate behaves conserva- upper mixing zones relative to the lenses means that more transformation tively, thus eliminating the effects of sea-water-fresh-water mixing and occurs in the mixing zones as a whole. The data in Table 1 support this sulfate reduction. The amount of aragonite that will dissolve in such waters

latter point, indicating that from 48% to 92% of the aragonite-to-calcite due to C02 effects is shown in Figure 9, a plot of total dissolved calcium in transformation reactions occur in the mixing zones. fresh-water samples as a function of calculated PCO2. All of the dots in The third observation is that the decreasing incremental changes in Figure 9 are in approximate equilibrium with aragonite, meaning that the the amount of dissolution and precipitation shown in Figure 8 infer where amount of calcium in solution is equivalent to the amount of aragonite that the transformation reactions cease in the upper mixing zones. The salinities had to dissolve in order to obtain that equilibrium given each sample's at which the incremental changes become less than zero are the points at PCO2. Figure 9 indicates that from 1.1 to 2.6 mmoles/l of aragonite which the rate of RSr++ and RCa++ dilution by fresh-water-sea-water should dissolve due to CO2 effects during the equilibration of recharging mixing exceeds the rate of dissolution and precipitation. The dramatic shift rain water with aragonite. This is true regardless of whether the pore water from positive to negative incremental changes in the amount of mass equilibrated in a system open or closed to CO2. transfer suggests that the dissolution and precipitation reactions actually The 1.1 to 2.6 mmoles/l of aragonite dissolution that can be attrib- cease, and the values of 18%, 19%, and 36% sea water are the approximate uted to CO2 effects represents less than 50% of the total amount of arago-

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12 -| Figure 8. Incremental changes in aragonite dissolution 10 - (A) and calcite precipitation (B) with increasing salinity. Lines are the first derivatives of the best-fit curves shown in Figure 7.

6 -

TREND 1 • : WATER CAY TREND 2 ' TREND 3-WOOD CAY

2 - PERCENT SEA WATER

*N 50 100 —o— \ / PERCENT SEA WATER

19 - nite that dissolved in the majority of these fresh-water samples (dots versus X's, Fig. 9). The additional dissolution must be independent of the CO2 reservoir and must have occurred after saturation with respect to aragonite was attained. Although CO2 effects during recharge do cause aragonite 17- dissolution, they are not the dominant mechanism behind the aragonite-to- calcite transformation in fresh water on Wood Cay and Water Cay. Fresh-Water-Sea-Water Mixing. The second possible water- X*X controlled driving mechanism of the aragonite-to-calcite transformation is 15 - fresh-water-sea-water mixing. The mixing of waters saturated or supersat- urated with respect to calcium carbonate at differing CO2 pressures, temperatures, and ionic strengths can produce fluids undersaturated with respect to calcium carbonate and thus promote dissolution (Thrailkill, 13 - 1968; Runnels, 1969; Plummer, 1975; Wigley and Plummer, 1976). Saturation indices of aragonite and calcite were calculated as a function of fresh-water-sea-water mixing to test whether the mixing of these

two water masses can explain the extensive aragonite dissolution and ° I 11 • calcite precipitation observed in upper mixing-zone pore water. The aqueous equilibrium model PHREEQE (Parkhurst and others, 1980) was used to simulate the mixing and to calculate calcite-saturation states in a system I< !_i

closed to CO2 (thermodynamic data and calculation methods were the « 9 mm

same as those given in Methods section). The average chemical composi- 3 HI tion of fresh-water samples on each island at each sampling period was 35 used as one component. The average composition of sea water (Martin, So o< 1970), normalized to observed chlorinities of local Schooner Cay sea

-aj Figure 9. Total dissolved calcium in fresh-water samples as a . S function of calculated PCO2 (dots) and the amount of aragonite disso- 1 - 0 lution indicated in the same samples versus PCOj (X's). More aragonite has dissolved (X's) than necessary to merely attain equilibrium I r- —r~ -1.5 with respect to aragonite (dots). Representative uncertainties due to -3.5 -2.5 -0.5 analytical precision are shown by crosses. LOG PCO,

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containing from 8% to 18% sea water on Water Cay. Figure 8 indicates 1974). This is not the case in Wood Cay and Water Cay pore water, that dissolution in upper mixing-zone water occurs to salinities of about however. The trends defined in Figure 7 were independent of the sampling 18.5% sea water on Water Cay and 36% sea water on Wood Cay, meaning period, indicating that most transformation reactions occurred in associa- a good correlation between the model results and observation. The tion with recharge. Calculated saturation indices of pore water from Wood

broader range of undersaturation on Wood Cay is due to the larger PC02 and Water Cays indicate saturation with respect to aragonite in the winter, values (Table A) observed in pore water from that island (Plummer, spring, and summer (Table A), suggesting that the calcite precipitation 1975). reaction is being inhibited, and this is the reason the mineral-controlled No undersaturation with respect to aragonite or calcite was calcu- reaction mechanism slows or ceases to function after meteoric recharge. lated for either island in mixtures involving sea water and fresh water Surface poisoning of the growing calcite crystal is proposed as the collected in July 1981 or April 1982. Aragonite saturation states in the most likely cause of kinetic interferences on the calcite precipitation reac- April mixtures approached equilibrium over the same range as November tion. At natural pH and PCO2 levels, such as occur in Wood Cay and mixtures (Fig. 10); thus, fresh-water-sea-water mixing only drives the Water Cay pore water, surface-controlled phenomena are more likely to aragonite-to-calcite transformation during meteoric recharge. affect calcite-precipitation kinetics than transport-controlled phenomena The amount of aragonite dissolution necessary to raise the aragonite (Carlson, 1983; Morse, 1983). The rate at which the poisoning agents saturation index from the greatest amount of undersaturation in the No- accumulate after meteoric recharge would vary on both a microscopic and vember mixtures to slight oversaturation (+0.1 units) at the same point in a megascopic scale, and thus some variation in the amount of mineral- the mixtures is a reasonable estimate of the amount of dissolution that can controlled transformation would be expected between well sites, and is be attributed to fresh-water-sea-water mixing during meteoric recharge. indeed observed (Fig. 9). PHREEQE was used to calculate this estimate. About 0.06 mmoles/1 of The nature of the possible surface-poisoning agent is unknown, but aragonite should dissolve on Water Cay, and 0.18 mmoles/1 should dis- certain ions are known to affect the rate of calcium carbonate precipita- solve on Wood Cay. These estimates are one-half to two orders of magni- tion. These include Mg++ (Berner, 1975; Mucci and Morse, 1983), SO4- tude less than the actual incremental changes observed in upper (Walter, 1986), and PO4- (DeKanel and Morse, 1978; Walter, 1986; mixing-zone pore water (Fig. 8). An additional driving mechanism of the Mucci, 1986). Perhaps more important, the adsorption of lipid-rich hy- aragonite-to-calcite transformation must therefore operate in the upper drophobic algal matter on crystal surfaces could also inhibit calcite precipi- mixing zone, and this additional mechanism must be the dominant process tation (Mitterer and Cunningham, 1985). This type of organic matter is responsible for the amount of mass transfer attributed to the upper mixing very abundant in these ooid sands (Feguson and Ibe, 1982). Adsorption of zone. dissolved organic matter is known to occur on all carbonate substrates Mineral-Controlled Reactions. Mineral-controlled reactions, related (Seuss, 1970, 1973) and has long been known to hinder precipitation of to the great solubility of aragonite relative to calcite, are interpreted to be calcium carbonate from sea water (Chave, 1965; Chave and Seuss, 1970). the driving mechanism behind the additional aragonite dissolution and subsequent calcite precipitation in both the fresh-water system and upper Efficiency of Dissolution and Precipitation mixing zone. The saturation state of CaC03 in the pore waters will be a weighted average of the individual saturation concentrations of each The percentage of calcium carbonate dissolved as aragonite and re- CaCC>3 polymorph (Palciauskas and Domenico, 1976). The weighted precipitated as calcite within the rock-water system is a measure of the average will occur between aragonite and calcite saturations in the bimin- efficiency of the transformation process (Harris and Matthews, 1968; eralic systems on Wood and Water Cays. A thermodynamic drive thus Kinsman, 1969). The aragonite-to-calcite transformation is very efficient exists for aragonite to dissolve and for the CaCC>3 derived from the addi- on Water Cay, where the fresh-water system averages 88%-94% efficiency, tional dissolution to reprecipitate as calcite (Matthews, 1968, 1974). As and the upper mixing zone averages 83%-92% efficiency (Fig. 11). The indicated in Figure 9, this mechanism causes more CaCC>3 transformation efficiency on Wood Cay varies from 10%-25% in the fresh-water system in fresh-water samples than CC>2 effects. and then systematically increases in the upper mixing zone to about 98% at The mineral-controlled reaction mechanism should operate continu- 25% sea water (Fig. 11). At salinities greater than 30% sea water, the ously as long as aragonite and calcite coexist in the rock (Matthews, 1968, efficiency on Wood Cay steadily decreases (Fig. 11).

NOVEMBER, 1981 APRIL, 1982

oUJ z z po < Œ P —

2 o o< <Œ

PERCENT SEA WATER WATER

Figure 10. Calculated calcite and aragonite saturation indices due to the mixing of fresh water and sea water. Shaded region indicates undersaturation with respect to aragonite.

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The differences in efficiencies between hydrologic zones and between 87% the two islands are interpreted to be the result of PCO2 differences. In 20% general, higher PCO2 values mean that greater amounts of calcium car- / 40% 60% bonate must remain in solution. The efficiency is therefore low at high / / / 80% 70- / PCO2 because the difference between DISS and PPT values is large. / / / 100% / / / Conversely, at low PCO2, the efficiency is high because the difference / / /' / / / / s / between DISS and PPT values is small. The average PCO2 of the fresh- 60 " / water samples on Wood Cay is about 2.5 times greater than the average / / 'a / / PCO2 of fresh-water samples from Water Cay (Table A), thus explaining o / ; / E 50- the low efficiency of the aragonite-to-calcite transformation in the fresh- E / water system of Wood Cay. PCO2 also decreases as fresh water mixes w I ' / / V/ w with sea water (Runnels, 1969; Plummer, 1975; Wigley and Plummer, I B 40" I / / 'V'' 1976), which reduces the amount of dissolved calcium carbonate required z I to maintain saturation with respect to CaCC>3, thus causing efficiencies o + in the upper mixing zone on Wood Cay to exceed those in the fresh- 30 • , ; / /•// water lens. o 1 r that the transformation process on these islands is basically conservative I / A* / 'As with respect to calcium carbonate. Figure 12 indicates that the variations z in efficiency observed between hydrologic zones tend to compensate each o <3 10- k other such that the average efficiency for all ground water on both islands < cc. is 87%. Only an average of 2.5 mmoles/1 of calcium carbonate is trans- < ported through the entire system and then discharged into the sea. This 2.5 T T T T T mmoles/1 is the average amount of excess calcium carbonate necessary to 0 10 20 30 40 50 60 70 maintain saturation with respect to aragonite in each liter of pore water in CALCITE PRECIPITATION (PPT, mmoles/l) the system. The high efficiency of the entire hydrologic system and the small amount of dissolved calcium carbonate lost from the rock-water Figure 12. Bivariant plot of the amount of calcite precipitation system indicate that most of the calcium carbonate derived from aragonite (PPT) and aragonite dissolution (DISS) for all pore-water samples dissolution is reprecipitated as calcite cement in either the fresh-water from both islands. Efficiency of the mass-transfer process is contoured system or the upper mixing zone on these islands. in 20% intervals (dashed lines). The solid linear regression line through the data represents an efficiency of 87% with an intercept of 2.5 Rate of Transformation mmoles/1. Representative uncertainties due to analytical precision are shown by crosses. Calcite crystal growth is generally regarded as the rate-limiting step of the aragonite-to-calcite transformation in natural environments (Fyfe and where k is the observed rate of calcite precipitation, expressed as cubic Bischoff, 1965; Matthews, 1968,1974; Carlson, 1983). Supersaturation of centimeters of calcite precipitated per year in a cubic meter of sediment. fresh water with respect to calcite supports this conclusion (Plummer and The amount of calcite is in similar units. others, 1976), as does the experimental work of Walter (1986). The amount of aragonite, A, remaining after t years is related to C by If calcite-crystal growth is the rate-limiting step, and if calcite super- the efficiency of the transformation: saturation is maintained, then to a first approximation the rate of transformation is constant (Plummer and others, 1976). The amount of calcite, C, A = Aq - (C H- efficiency) (8) present after t years of steady-state diagenesis is given by: where AQ (6 x 105 cm3/m3) is the original amount of aragonite in a cubic C = kt (7) meter of sediment with a porosity of 40%. The percent calcite by volume present after t years can then be calculated from equations 7 and 8. Values for k are derived from the incremental changes in precipitation with increasing salinity shown in Figure 8. These incremental changes, expressed as mmoles/1 per percent sea water, are converted to cm3/m3-yr by assuming an average porosity of 40% and a residence time of 1 yr (one hydrologic cycle) in the fresh-water system and another year in the upper mixing zone. Such residence times are supported by field observations (Budd, 1984) and the modeling of Vacher and Bengtsson (1987; and Figure 11. Efficiency of the L. Vacher, 1987, personal commun.). The relative salinities of interest would aragonite-to-calcite transforma- be the top and bottom of the fresh-water lens (0% and 2.9% sea water), and tion as a function of relative sa- the maximum salinity in the upper mixing zone where the transformation O 40- linity. Trends correspond to actively proceeds, yet DISS and PPT values are unaffected by sea-water best-fit curves in Figure 7. dilution (6%, 5%, and 20% sea water for trends 1, 2, and 3, respectively; 20- Fig. 8). The efficiencies and rates of transformation per year at these TREND 1 • WATER CAY TREND 2 - salinities are shown in Table 2. By definition, the rates and efficiencies are TREND 3 WOOD CAY applicable to only one salinity, and thus one particular point below the water table. The range in rates between the different trends, however, 1 5 10 50 PERCENT SEA WATER overlaps with the range in rates within any one trend (Fig. 8); thus all

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points in the fresh-water lenses and upper mixing zones can be discussed in terms of the rates and efficiencies given in Table 2. The amount of calcite expected in a rock sample from Wood Cay or Water Cay with time is shown in Figure 13. From 4,700 to 15,600 yr is apparently necessary to convert the sediment in the fresh-water lenses to 100% calcite, and a range of 8,700 to 60,000 is indicated for the upper mixing zone. The faster rates of diagenesis in both hydrologic zones proba- bly occur where the mineral-controlled driving mechanism is most produc- tive because of the limitations on the amount of aragonite-to-calcite transformation from water-controlled alterations. This means that the faster rates of diagenesis (103 yr) would be more applicable than the slower rates (104 yr) if recharge were not seasonal.

DISCUSSION AND CONCLUSIONS

Five aspects of the aragonite-to-calcite transformation on Wood Cay TIME (yrs) and Water Cay are of particular significance. 1. The transformation is primarily associated with the wet season. Figure 13. Percent calcite expected in a rock sample from Wood The implication is that the transformation might proceed continuously in a and Water Cays with time (shaded area). Average rates for specific more humid climate with year-round rainM. Higher rates of diagenesis hydrologic zones are shown by lines. would be expected in such a setting because the water-controlled driving mechanisms would not be seasonal, and PCO2 effects would be slightly greater due to more vegetation and a greater soil-zone PCO2. Indeed, 100% calcite. If exposed to the appropriate hydrologic zones, a single variations in the nature of diagenetic products and their abundance over genetic depositional package could be completely lithified before the depo- s 10 yr have been noted between climatically different areas by Matthews sition of the subsequent package. (1968), Ward (1973), Harrison (1975), and James and Choquette (1984). 4. Significant amounts of aragonite-to-calcite mass transfer occur in 2. The efficiency of the aragonite-to-calcite transformation is very the upper mixing zone. Aragonite dissolution, but not calcite precipitation,

high. Excluding the small amount of CaC03 that remains in solution to is indicated in the mixing-zone hydrochemistry of many other carbonate maintain a steady-state condition, all of the CaC03 derived from aragonite systems, such as Holocene coralgal sands of Engebi Island, Enewetak dissolution is reprecipitated as calcite within the over-all system. That is, (Goff, 1979), and Pleistocene rocks on Jamaica (O'Neil, 1974). Only on between the air-sediment interface and the middle of the mixing zone, all Barbados is there also evidence for aragonite dissolution and calcite precip- of the cement is autochthonous. If examined on a finer scale, however, itation in upper mixing-zone water (Harris, 1971). Petrographically, there there is evidence on Wood Cay for export of CaC03 from the fresh-water is abundant evidence for dissolution in mixing zones (Land, 1973; Han- zone to the mixing zone, indicating an internal allochthonous source of shaw and Back, 1980), but only minor amounts of calcite cementation in

cement. The implication is that CaC03 and porosity are, in general, both mixing zones have been reported (Moore, 1973; Steinen, 1974,1982). being conserved within the over-all system, but they may be redistributed The apparent uniqueness of calcite precipitation in the Wood Cay between the various hydrologic zones within the system. and Water Cay upper mixing zones may, in part, be due to the difficulty in 3. The rate of the aragonite-to-calcite mass transfer is very rapid. The quantifying the amount of precipitation in other mixing-zone pore water. 4,700 to 60,000 yr necessary to produce a rock of 100% calcite on Wood Mineralogic complexities (Mg-calcite + aragonite + calcite) in most other and Water Cays is similar to the 10,000 to 40,000 yr determined by Halley Quaternary carbonate aquifers make it difficult to decipher from the pore- and Harris (1979) for the Holocene ooid sands of Joulter's Cay. This water chemistry how much material may have dissolved or precipitated. means that during the time span of Milankovich sea-level cycles (~20,40, Only the potential for each reaction can be addressed. Thus, a Mure to and 100 thousand years), the entire sediment pile affected by fresh-water- recognize calcite precipitation in the pore-water chemistry does not neces- phreatic and upper mixing zone diagenesis could be converted to a rock of sarily mean that it is not occurring. For example, Gebelein and others (1980) did not find any evidence for calcite precipitation in the chemistry TABLE 2. YEARLY RATES OF CALCITE PRECIPITATION IN WOOD CAY AND WATER of mixing-zone water from Andros Island tidal flats in the Bahamas, yet CAY PORE WATER Steinen (1982) has described mixing-zone calcite cements from that

Percent Rate (mmoles/l-yr) k (cm3/m3 -yr) Efficiency locality. sea water (Fig. 9) (cq. 8) (by vol.) The difficulty in distinguishing upper mixing-zone calcite cements TREND I, WATER CAY from lower fresh-water zone cements may also be a reason for the sparsity

0.1% 6.7 106.8 94% of petrographically documented upper mixing-zone calcites. For example, 2.9% 2.3 36.7 93% Halley and Harris (1979) reported no significant amounts of dissolution or 6.0% 2.0 31.9 89% cementation in the upper mixing zone of Joulter's Cay, which is also a TREND 2, WATER CAY Holocene ooid sand shoal in the Bahamas. Halley and Harris based their 0.1% 7.S 119.6 98% 2.9% 3.8 60.6 92% conclusion on a decreasing amount of cementation and dissolution in their 5.0% 3.3 52.6 88% core samples with depth. The base of their moderately cemented zone, how- TREND 3, WOOD CAY ever, corresponded to a water sample with 7%-8% sea water, suggesting

0.1% 0.3 4.8 12.5% that upper mixing-zone cementation may actually occur on that island. 2.9% 1.2 19.1 26% 20.0% 0.5 8.0 83% 5. The thermodynamic drive for aragonite to dissolve at slight undersaturation and calcite to precipitate from the accompanying oversaturation Sou: rates are calculated Tor the top of the fresh-water lens (0.1% sea water), base of the lens (2.9% sea water), and in is by far the major driving mechanism of the aragonite-to-calcite transfor- the upper mixing zones. Molar efficiencies from Figure 12 have been converted to volumetric efficiencies. mation. PCO2 effects below soil zones, at the water table, and in fresh-

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Hanshaw, B. B., and Back, W., 1980, Chemical mass-wasting of the northern Yucatan Peninsula by groundwater water lenses had previously been established as a driving mechanism for dissolution: Geology, v. 8, p. 222-224. aragonite dissolution and calcite precipitation (Back and Hanshaw, 1970; Harris, W. H., 1971, Groundwater—Carbonate rock chemical interactions, Barbados, W. I. [Ph.D. dissert.]-. Providence, Rhode Island, Brown University, 348 p. Matthews, 1974; Plummer and others, 1976; Hanor, 1978), as was the Harris, W. H., and Matthews, R. K., 1968, Subaerial diagenesis of carbonate sediments: Efficiency of the solution- precipitation process: Science, v. 160, p. 77-79. potential of fresh-water-sea-water mixing (Back and Hanshaw, 1970; Harrison, R. S., 1975, Porosity in Pleistocene grainstones from Barbados: Some preliminary observations: Bulletin of O'Neil, 1974; Goff, 1979; Hanshaw and Back, 1980). The inability to Canadian Petroleum Geology, v. 23, p. 383-392. Howarth, R. W., 1978, A rapid and precise method for determining sulfate in seawater, estuarine waters and sediment quantify the amount of transformation due to complex mineralogies, how- pore waters: Limnology and Oceanography, v. 23, p. 1066-1069. James, N. P., and Choquette, P. W., 1984, Limestones—The meteoric diagenetic environment: Geosciences Canada, ever, may mean that the mineral-controlled thermodynamic drive has v. 11, p. 161-194. often been minimized. Matthews (1968), Steinen (1974), and Harrison Kinsman, DJ.J., 1969, Interpretation of Sr*2 concentrations in carbonate minerals and rocks: Journal of Sedimentary Petrology, v. 39, p. 486-508. (1975) recognized this as a locally significant mechanism in fresh- Land, L. S., 1973, Holocene meteoric dolomitization of Pleistocene limestones, North Jamaica: Sedimentology, v. 20, p. 411-424. water-phreatic zone rocks from Barbados, and Plummer and others Martin, D. F., 1970, Marine chemistry, Volume 2: Theory and application: New York, Marcel Dikker Inc., 451 p. (1976) documented this as a driving mechanism in Bermuda ground Matthess, G., and Harvey, J. C., 1982, The properties of Ground-water: New York, John Wiley & Sons, 406 p. Matthews, R. K., 1968, Carbonate diagenesis: Equilibrium of sedimentary mineralogy to the subaerial environment; coral water. cap of Barbados, W.I.: Journal of Sedimentary Petrology, v. 38, p. 1110-1119. 1974, A process approach to diagenesis of reefs and reef associated limestones, in Laporte, L. F., ed., Reefs in time and space: Society of Economic Paleontologists and Mineralogists Special Publication 18, p. 234-256. Mitterer, R. M., and Cunningham, R., Jr., 1985, The interaction of natural organic matter with grain surfaces: Implications ACKNOWLEDGMENTS for calcium carbonate precipitation, in Scbneidermann, N., and Harris, P. M., eds., Carbonate cements: Society of Economic Paleontologists and Mineralogists Special Publication 36, p. 17-30. Moore, C. H., 1973, Intertidal carbonate cementation, Grand Cayman, West Indies: Journal of Sedimentary Petrology, This research is part of the author's Ph.D. dissertation completed at v. 43, p. 591-602. Morse, J. W., 1983, The kinetics of calcium carbonate dissolution and precipitation, in Reeder, R. J., ed., Carbonate: the University of Texas at Austin and supported by National Science Mineralogy and chemistry: Mineralogical Society of America, Reviews in Mineralogy, Volume 11, p. 227-264. Foundation Grant no. EAR-8001650 to L. S. Land. Additional financial Morse, J. W., Zullig, J. J., Bernstein, L. D., Millero, F. J., Milne, P., Mucci, A., and Choppin, G. R., 1985, Chemistry of calcium carbonate-rich shallow water sediments in the Bahamas: American Journal of Science, v. 285, support was obtained from the Geology Foundation, University of Texas p. 147-185. Mucci, A., 1986, Growth kinetics and composition of magnesium calcite overgrowths precipitated from seawater at Austin; and the Applied Carbonate Research Program, Department of Quantitative influence of orthophosphate ions: Geochimica et Cosmochimica Acta, v. 50, p. 2255-2266. Geology, Louisiana State University. ARCO Exploration and Production Mucci, A., and Morse, J. W., 1983, The incorporation of Mg*2 and Sr*2 into calcite overgrowths: Influences of growth rate and solution composition: Geochimica et Cosmochimica Acta, v. 47, p. 217-233. Research provided assistance in preparation of the manuscript, line draw- O'Neil, T. J., 1974, Chemical interactions due to mixing of meteoric and marine waters in a Pleistocene reef complex, Rio Bueno, Jamaica (M.S. thesis]: Baton Rouge, Louisiana, Louisiana State University, 186 p. ings, and photomicrographs. K. Hoops provided instruction in analytical Palciauskas, V. V., and Domenico, P. A., 1976, Solution chemistry, mass transfer, and the approach to chemical techniques and performed the Ca++ and Sr++ analyses. D. D. Runnels equilibrium in porous carbonate rocks and sediments: Geological Society of America Bulletin, v. 87, p. 207-214. Parkhurst, D. L, Thorstenson, D. C., and Plummer, L. N., 1980, PHREEQE—A computer program for geochemical provided assistance with PHREEQE. A. H. Sailer, P. W. Choquette, calculations: U.S. Geological Survey Computer Center, National Technology Information Report PB81-167801, 210 p. L. S. Land, and L. M. Walters critically reviewed early drafts of the manu- Patterson, R. J., and Kinsman, D.J.J., 1982, Formation of diagenetic dolomite in coastal Sabkhas along the Arabian script. The comments of reviewers J. Morse, R. M. Owens, B. H. Wilkin- (Persian) Gulf: American Association of Petroleum Geologists Bulletin, v. 66, p. 28-44. Plummer, L. 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