SALINITY VARIATIONS IN THE CAMPANIAN WESTERN INTERIOR SEAWAY: EVIDENCE FROM STRONTIUM, OXYGEN AND CARBON ISOTOPES

The oxygen isotope composition of calcium carbonate shells of fossil cephalopods is principally a function of the water temperature at the time of deposition, although it may also reflect the salinity. The aim of the present study was to assess seasonal fluctuations in water temperature, marine chemistry and habitat recorded in the oxygen, carbon and strontium isotopic composition of shells of the Cretaceous nautilid Eutrephoceras dekayi (Morton 1834). We sampled the nacreous shell material from 198 septa of eight specimens of E. dekayi from the Pierre Shale (Late Campanian) and Fox Hills Formation (Maastrichtian)of South Dakota. Samples were examined by scanning electron microscopy and X-ray diffraction to confirm that the original aragonite had not been diagenetically altered. Values of d¹⁸O and d¹³C vary throughout the ontogeny of each specimen by as much as 6.0 and 9.0 per mil respectively; in three specimens, the d¹⁸O and d¹³C values are positively correlated. If the variations in the d¹⁸O signal are attributable exclusively to changes in water temperature, these data suggest that E. dekayi may have experienced sustained temperature variations greater than 20°C throughout its ontogeny. However, the d¹⁸O signal may also reflect the isotopic composition of the water, which, in turn, would have been correlated with water salinity. To assess salinity variations recorded in the shell of E. dekayi , we measured the strontium isotopic composition of the septal aragonite of these specimens. We report that the ⁸⁷Sr/⁸⁶Sr values vary from 0.707097±0.000013 to 0.707955±0.000030 in these specimens; furthermore, ⁸⁷Sr/⁸⁶Sr values correlate positively with both d¹⁸O and d¹³C. These data strongly suggest that the observed variations in d¹³C and d¹⁸O are principally the result of changes in water salinity, not temperature. We propose that E. dekayi migrated between coastal and off-shore habitats, or that the flux of fresh water into the Seaway, either through rivers or groundwater, may have fluctuated seasonally or regionally. The cephalopods would have experienced isotopically light water in near-shore or estuarine habitats, producing septa that are isotopically lighter in both carbon and oxygen and have lower ⁸⁷Sr/⁸⁶Sr values relative to those septa deposited in offshore habitats.