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

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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), dolomitiza- tion (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 cumula- tive 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/8/1260/3380428/i0016-7606-100-8-1260.pdf by guest on 29 September 2021 ARAGONITE-TO-CALCITE TRANSFORMATION 1261 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.
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