Calcifying organisms secrete skeletons consisting of calcium carbonate (CaCO3). The two principle CaCO3 polymorphs used by marine calcifiers are aragonite and calcite. In the abiotic precipitation of CaCO3, the Mg/Ca ratio and the temperature are established controls of polymorph formation, but the importance of other factors, for example salinity, remains uncertain. The first objective of this thesis was to clarify the role of salinity in abiotic CaCO3 precipitation (Chapter 2). Experiments at salinities of 20, 35 and 50 and a Mg/Ca ratio of 2 resulted in co-precipitation of aragonite and calcite in all experiments, and no significant salinity influence on overall polymorph formation could be deduced. The average growth of individual calcite crystals was reduced, however, in the low-salinity setup. Although the skeletal mineralogy of calcifiers is generally tied to their phylogenetic history, the skeletal mineralogy, growth rates and skeletal production of many calcifying taxa are affected by the ambient Mg/Ca ratio, temperature and salinity. To test whether environmental factors shape the distribution of modern calcifiers with regards to their skeletal mineralogy, epifaunal bivalve occurrences from the Ocean Biodiversity Information System (OBIS) were related local temperatures and salinities (Chapter 3). Bimineralic bivalve species had higher temperature ranges, on average, than purely aragonitic bivalves, and bimineralic bivalves occurred more often in regions of high annual temperature variability. By adjusting the relative amount of aragonite and calcite secretion, bimineralic bivalves may be able to endure a greater range of environmental conditions. Additionally, bimineralic occurrences were associated with lower temperatures and lower salinities, matching with observations of increased calcite proportions in Mytilus shells under these conditions. Recent improvements in the understanding of how the Mg/Ca ratio and temperature combine to control CaCO3 polymorph formation have allowed to create an Ordovician – Pleistocene curve of aragonite or calcite favouring conditions, termed “aragonite sea intensity” (ASI, Chapter 4). In the Ordovician – Middle Jurassic, ASI was correlated with the relative success of aragonitic calcifiers from the Paleobiology Database (PBDB), as well as with the relative production of aragonite in reefs from the Paleoreefs Database (PARED). From the Late Jurassic onwards, temporal trends in marine skeletal mineralogy appeared unaffected by changes in ASI, demonstrating a decoupling of aragonite-calcite sea conditions and the evolutionary success of marine calcifiers. The onset of the modern carbon cycle and the resulting ocean buffering, along with rising metabolic rates may explain why marine calcifiers were less susceptible to changes in the abiotic environment after the mid-Mesozoic (Chapter 5). However, latitudinal and depth trends in skeletal mineralogy within stages persisted after the Jurassic (Chapter 4), indicating, together with the results of Chapter 3, that aragonite-calcite sea conditions still influence the distribution of modern marine calcifiers.
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