Coccolith strontium to calcium ratios in Emiliania huxleyi: The dependence on seawater strontium and calcium concentrations

Abstract

In recent studies the Sr/Ca ratio of coccolithophore calcite was used as a proxy for past coccolithophore growth and calcification rates. Since Sr and Ca concentrations in seawater have not remained constant through time, interpretation of Sr/Ca data from the coccolith-dominated sedimentary record requires knowledge about the incorporation of seawater Sr into coccolith calcite during coccolithogenesis. Here we show that Sr/Ca of Emiliania huxleyi coccoliths is linearly related to seawater Sr/Ca, meaning that the Sr exchange coefficient does not change with changing seawater Sr/Ca. The exchange coefficient for Sr in this study, 0.39, is close to values presented in the literature and is high compared with values obtained by inorganic precipitation experiments. This suggests a strong effect of cell physiology on biogenic calcite precipitation in coccolithophores. We present a conceptual model, based on the transmembrane transport of Sr and Ca, which explains the offset.

Calcite crystal growth orientation: implications for trace metal uptake into coccoliths

“…The results show that calcite crystals precipitated in the presence of large malonic acid concentrations show significant elongation along the c axis, and suggest that increasing malonate concentrations corresponded with increasing D Sr . Experiments with 10 À1 M malonic acid caused elevated D Sr comparable to that predicted for E. huxleyi coccolith calcite (Langer et al, 2006). …”

“…Carboxylic acids can affect calcite growth because of the similarity of the carboxyl (COO À ) group to the carbonate group (CO 3 2À ), allowing it to interact with Ca ions at the calcite surface through electrostatic attraction. The bidentate binding of the carboxyl groups of dicarboxylic acids to the calcite surface can effectively block acute kink sites (Mann et al, 1990) Langer et al (2006), inorganic calcite D Sr as expected from Tesoriero and Pankow (1996).…”

Dependence of calcite growth rate and Sr partitioning on solution stoichiometry: Non-Kossel crystal growth

“…This, however, may no longer be the case for biologically mediated calcite precipitation. Organisms such as marine calcifying algae actively regulate Ca 2+ transport from the surrounding seawater to the site of calcification through cell physiological processes (Langer et al, 2006). In the highly controlled cell environment, r may differ significantly from the value in the bulk medium.…”

Barium partitioning in coccoliths of Emiliania huxleyi

“…The rate dependence of the Sr exchange coefficient as determined from inorganic calcite precipitation experiments was used to explain the apparent Sr exchange coefficients of biogenic calcite (Carpenter and Lohmann, 1992). The trace metal exchange coefficient of biogenic calcite is termed apparent exchange coefficient, because the trace metal to calcium ratio of the solution from which the crystal is precipitated (at the site of calcification within the organism) is unknown (Langer et al, 2006). A subsequent study, however, showed that kinetic effects are not appropriate to explain Sr partitioning in coccolithophores (Stoll et al, 2002).…”