An aspartate-rich peptide enhances magnesium uptake into calcite, a calcium carbonate mineral, a finding that raises questions about factors influencing estimates of ocean temperatures during prehistoric times (Science 2008, 322, 724). Levels of magnesium in fossil calcites such as those in seashells have fluctuated over geologic history. Because more magnesium goes into calcite at higher temperatures, scientists use the magnesium content of calcite as a "paleothermometer." But calcites of biological origin can contain levels of magnesium and other impurities that can't be accounted for by temperature alone. To better understand the factors in play, a multi-institution team led by Virginia Polytechnic Institiute & State University geoscientist Patricia M. Dove measured calcite growth within a chamber in an atomic force microscope and determined the magnesium content by mass spectrometry. In the presence of a carboxyl-rich peptide similar to ones associated with calcification in relevant marine organisms, calcite grew 25 to 50% faster and had up to 3 mol % higher magnesium content, which helps account for the discrepancies. A difference this large corresponds to an offset in temperature of 7 to 14 °C.