A record of the Mg/Ca ratio of ancient seawater is essential to understand variations in the major geological processes that control ocean chemistry and for estimating past ocean temperatures. A recent contribution by Broecker and Yu (2011) regarding past seawater Mg/Ca provides an unbalanced assessment of the uncertainties and key assumptions of the methodologies considered. It misrepresents aspects of a new method to estimate past ocean Mg/Ca proposed by Coggon et al. (2010); here we provide clarification of that approach. We estimate the accuracy and precision of seawater Mg/Ca estimates derived from planktonic foraminifera test calcite, the preferred approach of Broecker and Yu. These are shown to be consistent with other records that suggest major changes in seawater cation chemistry. An explanation of how major changes in the Mg/Ca of seawater might occur is beyond the scope of this comment.