The empirical relationship between the Mg/Ca of foraminifer shells and temperature is widely used as a seawater palaeo-thermometer. However, foraminiferal Mg/Ca is also sensitive to various aspects of seawater chemistry, including pH and carbon chemistry, and the concentrations of [Mg] and [Ca] in seawater. This raises significant problems for the application of the Mg/Ca palaeo-thermometer across carbon system perturbations, and in pre-Pleistocene oceans, when ocean [Ca] and [Mg] were significantly different than modern. These influences must be quantified if the Mg/Ca proxy is to be applied to anthropogenic climate change analogues like the Paleocene-Eocene Thermal Maximum (PETM), or to investigate ocean temperatures in deep time.
To quantify the influence of seawater composition on the Mg/Ca thermometer, I have grown the planktic foraminiferOrbulina universa in a wide range of controlled seawater compositions, independently varying temperature, [Mg], [Ca], [C] and pH. This matrix of conditions allows me to identify the influence of individual parameters on foraminiferal Mg/Ca, and explore interactions between the parameters, to develop a comprehensive understanding of the Mg/Ca palaeo-thermometer. My results show that the seawater Mg/Ca ratio, Ca concentration and dissolved inorganic carbon concentration need to be considered to determine past seawater temperatures accurately. To highlight the importance of these factors, I apply this model to re-interpret foraminiferal Mg/Ca records across the PETM.