ARE CA ISOTOPES A RELIABLE MONITOR OF TROPHIC LEVEL IN ANCIENT MARINE ECOSYSTEMS?

Recent research has shown that calcium isotopes are fractionated by metabolic processes, leading to a decrease in ⁴⁴Ca/⁴⁰Ca ratio with increasing trophic level. If so, calcium isotopes could provide information on trophic relationships within food webs millions of years older than what we have been able to study thus far with alternative methods (i.e., nitrogen isotopes (d¹⁵N), Sr/Ca). To explore whether d⁴⁴Ca values provided marine trophic level information, we measured the d⁴⁴Ca composition of tooth enamel and bone from modern marine mammals representing a 2.5 order range in tropic level. Marine mammal enamel d⁴⁴Ca values clustered into two groups - mammals foraging on vegetation or invertebrates exhibited higher d⁴⁴Ca 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 d⁴⁴Ca values of specimens followed the same pattern as observed in modern faunas, but the mean d⁴⁴Ca values were significantly different from modern d⁴⁴Ca values for mammals of similar trophic level. We conclude that the relative spacing of d⁴⁴Ca 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.