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Nature Geoscience 1, 497–498 (1 August 2008) | doi:10.1038/ngeo273

Palaeoceanography: Seeing changes in a changing sea

Justin B. Ries

Fluctuations in the ratio of magnesium to calcium in ancient sea water have fascinated geoscientists for nearly half a century. This is because this ratio plays a critical role in determining which form of abiotic calcium carbonate — aragonite or calcite — will precipitate out of sea water.