Assessing the reliability of foraminiferal Mg/Ca thermometry by comparing field-samples and culture experiments: a review

Trace elements incorporated into foraminiferal test carbonate are commonly used as palaeoproxies. For instance, benthic and planktonic Mg/Ca ratios are frequently used for reconstructing bottom and sea surface temperature (SST) changes, respectively. However, over the past few decades it has been shown that the incorporation of Mg2+ into foraminiferal calcite is controlled by more than one environmental parameter, with significant variations of their sensitivities between culture and field-based studies. Opinions differ as to whether the laboratory conditions during culturing experiments or the natural conditions during field sampling (core-tops, sediment traps and plankton tows) better trace the wealth of information with improved accuracy. Laboratory culture experiments that isolate the effects of individual environmental parameters have been used to identify secondary controls on Mg uptake into planktonic foraminifer tests. However, field-based data (core-top sediments and plankton tows) from high salinity super-saturated settings have shown the additional presence of high-Mg inorganic precipitates leading to significant salinity (S) biases on the Mg/Ca palaeothermometer. Testing such synergistic effects between temperature, salinity and calcite saturation state would require an experimental design where all these parameters are varied systematically, but such experiments have yet to be conducted. Since the synergistic effects cannot presently be ruled out through culturing experiments, it is imperative to initially confirm the amplitude and geographic distribution of the detectable diagenetic precipitations (Scanning Electron Microscopy analysis) through field-based work, further quantify their importance (discrimination of distinct diagenetic stages and quantification of the diagenetic imprint) and finally estimate its potential effect on Mg/Ca-T calibration (e.g., overgrowth-corrected species-specific calibration equations). The example of the marginal high-salinity settings, among others, clearly highlights that the optimal use of Mg/Ca as a palaeotemperature proxy urgently requires the complementarity of both culture- and field-based data. To this end, we here present advantages and disadvantages to each approach. These insights reinforce the potential of the combined use of culture- and field-based foraminiferal studies, where possible, in order to minimize the observed inconsistencies, and to advance Mg/Ca thermometry by both providing a framework for better understanding the nature of Mg/Ca dependence on seawater temperature, and the effects of complicating factors.