Sr /Mg ratios of modern marine calcite: Empirical indicators of ocean chemistry and precipitation rate Scorn

Holocene biotic and abiotic marine calcite have a similar range of Mg contents (0 to 22 and 4 to 2 1 mol% MgC03, respectively), yet biotic calcite has Sr*+ concentrations that are consistently 1250 ppm higher than those of abiotic calcite. As in laboratory experiments, a positive linear relation is observed between Dsr and calcite Mg content. This produces two distinct linear trends on a plot of Sr2+ vs. Mg2+ concentrations. Principal axes of variation for both trends have similar slopes, yet distinctly different Sr’+ concentration intercepts. (Biotic: y = 0.024x + 1298, r2 = 0.70; Abiotic: y = 0.027x + 47, r2 = 0.77). The similar slopes of these trends reflect the constancy of Mg/Ca and Sr/Ca ratios of modem seawater. Equations describing the dependence of Ds* on calcite Mg content are derived from both trends (Biotic: Ds,,, = 3.16 X lo-@’ (ppm Mg) + 0.169; Abiotic: Ds* = 3.52 X 10e6 (ppm Mg) + 0.0062). Characterization of Sr-Mg trends for Holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater Mg/Ca and Sr/Ca ratios. The relatively high Sr contents of biotic calcite result from rapid p~~pi~tion rates associated with shell accretion in marine organisms. Calcites precipitated from seawater in laboratory experiments have Dsr values that are similar to those of biotic marine calcite, suggesting that both precipitate at approximately the same rate. Our estimates of surface area-normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded, pH-stat experiments. We conclude that the Dsl values for biotic and experimental marine calcite are kinetically controlled, whereas the lower precipitation rates of abiotic marine calcite yield Dsl values that approximate equilibrium conditions. Experimentally derived equations describing the relation between Dsr and calcite precipitation rate indicate that the offset in Sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates. However, observations of naturally occurring marine cements suggest that the five-order-of-magnitude offset best represents natural system processes. FOLLOWING THE EARLY CHEMICAL surveys by CLARK and WHEELER (1917, 1922) and VINOGRAWV (1953), various workers have analyzed marine carbonates for either Mg or Sr contents (e.g., KuLp et al., 1952; CHAVE, 1954; THOMP~~N and CHOW, 1955; ODUM, 1957). These studies provided the first minor element data for a wide variety of sediment-producing, calcareous marine organisms. The Sr and/or Mg contents of marine calcite were later examined using experimental (BERNER, 1975, 1978; MUCCI and MORSE, 1983; MUCCI, 1987; BURTON and WALTER, 1987, 1991) and empirical (GOLDSMITH et al., 1955; TUREKIAN and ARMSTRONG, 1960; ~WENSTAM, 1961; BUCKER and VALENTINE, 1961; DODD, 1965, 1967; LERMAN, 1965; GUNATILAKA, 1975; OHDE and KITANO, 1984, BRAND et al., 1987; CARPENTER et al., 1991) approaches. In our present study, results from laboratory precipitation experiments are used to interpret the Sr-Mg relations of Holocene abiotic and biotic marine calcite. Nonthermodynamic homogeneous distribution or partition coefficients (i.e., DM, and Ds,) are used in this study (e.g., HENDERSON and KRACEK, 1927 ) . Here DMe = ( mMe / mCaIsalid/( mMe/mCa),i,i,,, where &, is the distribution coefficient of a minor element (Me), and mMe/mCa is the molar concentration ratio of the minor element relative to calcium in the solid or the solution. A detailed review of the various types of ~st~bution coefficients is found in MUCCI and MORSE ( 1990 ) . Laboratory precipitation experiments provide a basic understanding of the factors that control Sr and Mg incorporation into calcite from seawater and related fluids (e.g., MORSE and BENDER, 1990). It has been shown that the Dws value increases with increasing temperature and decreases with increasing FCOz and [SO;] (e.g., KATZ, 1973; BURTON and WALER, 1987,199l; MUCCI, 1987; Mucc~ et al., 1989), and increases exponentially with decreasing Mg/Ca ratios (e.g., MUCCI and MORSE, 1983 ) . The inlluence of saturation state and precipitation rate on Dgg has been highly debated (e.g., LAHANN and SIEBERT, 1982; MACKENZIE et al., 1983; GIVEN and WILKINSON, 1985a,b; MORSE, 1985; MUCCI and MORSE, 1983; BURTON and WALTER, 1987; MORSE and BENDER, 1990 ), yet there are no experimental data which suggest that DMg is precipitation rate dependent. In contrast, the DQ value increases with increasing precipitation rate ( LORENS, 198 1; MUCCI, 1986) and Mg content of the calcite (Muccr and MORSE, 1983; see MORSE and BENDER, 1990, for summary). Although these studies have added significantly to our understanding of physicochemical conditions controlling cation incorporation into calcite, elevated temperatures and/or low prJo2 values must be employed in laboratory experiments to attain the Mg contents of naturally occurring high Mgcalcite (HMC) marine cements (e.g., GIVEN and WILKIN~N,