The accuracy of the magnesium/calcium palaeotemperature proxy has been questioned, in particular because the ratio of magnesium to calcium in foraminiferal tests could be affected by local or global changes in carbonate ion concentrations in deep water. A related question regarding the technique is its problematic phase relationship to orbital eccentricity: Mg/Ca records of intermediate and deep waters typically show a phase lead with respect to orbital eccentricity. This calls into question either the validity of the Mg/Ca palaeotemperature proxy, or the assumption that orbital eccentricity is pacing the 100 kyr climate oscillations, or both. This paper addresses these questions, and suggests that a phase lead of the type observed at ODP 1123 is unlikely to be generated by the operation of the carbonate ion effect, and might be attributable to heat storage in the oceans during low eccentricity episodes
