During the Phanerozoic， the mineralogies of nonskeletal marine cements and oolites oscillated between aragonite Mgrich calcite(aragonite sea)and Mgpoor calcite(calcite sea). Oscillations in the carbonate mineralogy of dominant reefbuilding and sediment producing organisms are in harmony with the oscillations for nonskeletal carbonates. These oscillations can be explained by secular variation in the Mg/Ca ratio of the seawater. Evidence from fluid inclusions， isotopes and trace elements also shows that very significant changes occurred in seawater chemistry. Though the relative importance of biologically induced biomineralization and biologically controlled biomineralization has been a topic of much controversy， evidence from the paleontological records and experiment shows that throughout the Phanerozoic the seawater chemical evolution  had significantly influenced biomineralization. Selection of aragonite versus calcite was largely dependent on seawater chemistry at the time of first appearance of carbonate skeletons in animals. Seawater Mg/Ca ratios controlled polymorph mineralogy of skeleton， which supplied a new proxy for calcitearagonite seas in the Precambrian， reconstruction of paleoclimate and paleoenvironment， isotope stratigraphy， carbonate sedimentation and diagenesis.
seawater chemistry|biomineralization|biomineral|aragonite sea|calcite sea
 Liu Xiting， born in 1983， is a master candidate of marine geology in China University of Geosciences(Wuhan)， and is engaged in carbonate sedimentology. E-mail:liu1999@126.com.