Evaporative fractionation of H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O in the polar ocean and the invisibility of large changes of ice volume and sea level in the Saalian δ&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O proxy records