Effect of mineralogy, salinity, and temperature on Li/Ca and Li isotope composition of calcium carbonate

Li/Ca and Li isotope ratios in marine and freshwater calcium carbonates are potential tracers of past environmental variables such as weathering intensity and temperature. To make use of their potential requires full understanding of the fractionation of Li/Ca and Li-7/Li-6 during growth of calcium carbonate. This study improves such understanding with two sets of new measurements: on inorganic calcites and aragonites precipitated over a range of salinity and on the test of benthic foraminifera formed at a range of temperatures. Li/Ca in inorganic calcite increases by a factor of four as salinity increases from 10parts per thousand to 50parts per thousand. Inorganic aragonite demonstrates no significant change in Li/Ca over the same salinity range. The difference in behaviour is probably due to the incorporation mechanism of Li. Substitution of Li+ in the Ca2+ site in aragonite makes growth-solution Li/Ca the key variable, while interstitial incorporation of Li+ in calcite means that the growth-solution Li concentration is more important. Compared to the growth solution, Li-7/Li-6 is approximate to 3parts per thousand lower in calcite and approximate to11parts per thousand lower in aragonite, with no relationship to salinity for either mineral. Similar isotope fractionation to these inorganic experiments are observed for biogenically produced calcite (foraminifera) and aragonite (corals). This suggests little biological control during Li incorporation into biogenic calcium carbonates. The difference in fractionation between calcite and aragonite is consistent with an equilibrium isotope fractionation reflecting different incorporation mechanisms for Li into the two minerals. Four samples of the benthic foraminifera Uvigerina, precipitated over a temperature range of 14 degreesC, have Li/Ca ranging from 18.5 to 13.1 mumol/mol. The ratio is inversely related to temperature with a sensitivity of 2.5 +/- 0.8% per degreesC. This sensitivity is similar to that observed in a previous study of foraminiferal Li/Ca, but about half that observed during inorganic calcite growth. The large variability in both Li/Ca and temperature sensitivity exhibited between various foraminifera. species suggests an additional control on calcite Li/Ca apart from those of salinity and temperature observed in this study. (C) 2004 Elsevier B.V. All rights reserved.