Carbonate Mineralogy and Seawater through Time

Stanley, Steven M.¹ (1) University of Hawaii, Honolulu, HI

Through study of the mineralogy of oolites and early marine cements in the 1980's, Philip Sandberg introduced the concept of "aragonite seas" and "calcite seas". During Phanerozoic time, there have been three intervals of aragonite seas separated by two intervals of calcite seas, when lime mud consisted of calcite rhombs. Lawrence Hardie has shown that the Mg/Ca ratio of seawater, governed largely by global rates of spreading along midocean ridges, determines the mineralogy of nonskeletal carbonates in the ocean: at 28°C, low-Mg calcite forms when the Mg/Ca molar ratio is <1, high-Mg calcite when the ratio is 1-2, and high-Mg calcite and/or aragonite when it is >2. The Mg content of calcite increases with increasing temperature. The fossil record reveals that biocalcifying organisms have tended to conform to this pattern, in that throughout Phanerozoic time the mineralogies of simple organisms functioning as major reef-builders or sediment producers have tended correspond to those of nonskeletal precipitates. Experiments show that skeletal mineralogies of particular taxa are phenotypically labile. In artificial Cretaceous seawater (Mg/Ca = ~1), organisms that produce high-Mg calcite in modern seawater produce low-Mg calcite, and some organisms that produce aragonite today, produce a substantial percentage of calcite (corals 30% and Halimeda 45%). Coccolithophores fertilize their photosynthesis with carbon dioxide produced by their calcification. Therefore, because they secrete calcite more readily in artificial Cretaceous seawater than in modern seawater, their population growth rates are much higher. This explains the widespread occurrence of massive Cretaceous chalk deposits.