From in-situ pore water and rock analyses of a lithified Holocene beach deposit in Jamaica (1,240 ± 50 yr B. P. at 22 cm depth, 670 ± 50 yr B. P. at 14 cm depth, 0 yr B. P. at surface), we propose a geochemical model for intertidal carbonate cementation. The beachrock unit is laterally and vertically discontinuous with unconsolidated beach sands surrounding it. The unit dips seaward at an angle of 10° and contains localized open orthogonal fractures that are oriented parallel and normal to the shore line. Three distinct cement types are found in Jamaican beachrock: (1) equant and bladed high-Mg calcite (12-29 mole % MgCO₃), (2) low-strontium fibrous aragonite (1,700-3,100 ppm SrO), and (3) micritic high-Mg calcite envelopes. These cements vary bo h laterally and with depth in the unit, and accurately reflect the changes in time and space of the chemistry of the interstitial water; the cements are produced in stoichiometric equilibrium with the pore-water chemistry. The high-Mg calcite cements are precipitated when CO₂ degases (P_CO2 = 10^-5.1) through agitation in the surf and consequently raises the pH to a maximum of 8.4 during the higher tides. During these times, the pore waters are saturated with respect to the precipitating Mg calcite containing 15-29 mole % MgCO₃. During low tide, when the agitation of the surf is minimal, the CO₂ does not degas, increasing the P_CO2 to a maximum of 10^-4.0. Continued precipitation aids in the in rease in CO₂ levels, the decrease in pH to a minimum of 7.9 and the lowering of saturation states of Mg calcites. Phreatic fresh water flows seaward during low tide, preferentially through the open fractures, lowering strontium levels and saturation states in the pore waters. Thus, at low tide, lower Mg calcites of 12-15 mole % MgCO₃ are precipitated where fresh water has not invaded (maximum Cl = 22^pmil). Models of Sr partitioning show low-strontium aragonite is produced from the neomorphism of high-Mg calcites near the open fractures in mixed meteoric and marine interstitial waters (Cl = 11.05-13.48^pmil). Our data suggest that P_CO2 is the master variable and that beachrock cements are not static but ever-changing in mineralogy and chemistry

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