Radium determinations have been made for shells from Swedish waters, the Mediterranean Sea and the Pacific Ocean. Their Ra/Ca ratio has been estimated and compared to the Ra/Ca ratio in sea water. It was found that, in general, the Ra/Ca ratio is proportional to the Sr/Ca ratio found by Thompson and Chow....
Koczy, F. F., and Heinz Titze. 1958. "Radium content of carbonate shells." Journal of Marine Research 17, (1). https://elischolar.library.yale.edu/journal_of_marine_research/921