Are calcium isotopes a reliable monitor of trophic level in marine settings?

Recent research has shown that calcium isotopes are fractionated by metabolic processes, leading to a decrease in 44 Ca/ 40 Ca ratio with increasing trophic level. If so, calcium isotopes could provide information on trophic relationships within foodwebs millions of years older than what we have been able to study thus far with alternative methods (i.e., nitrogen isotopes (δ 15 N), Sr/Ca). To explore whether δ 44 Ca values provided marine trophic level information, we measured the δ 44 Ca composition of tooth enamel and bone from modern marine mammals representing a 2.5 order range in trophic level. Marine mammal enamel δ 44 Ca values clustered into two groups—mammals foraging on vegetation or invertebrates exhibited higher δ 44 Ca values than those foraging on fish or other marine mammals. We next examined whether this correlation was preserved in the fossil record by examining a 15 Ma marine fauna from southern California and observed that the relationship between δ 44 Ca values of specimens followed the same pattern as observed in modern faunas, but the mean δ 44 Ca values were significantly different from modern δ 44 Ca values for mammals of similar trophic level. We conclude that the relative spacing of δ 44 Ca values amongst fossil taxa can serve as a valuable tool for defining trophic level of extinct organisms and can provide critical information on relationships within ancient foodwebs. Copyright  2003 John Wiley & Sons, Ltd.