Strontium isotope age-dating of fossil shark tooth enameloid from the Upper Cretaceous Strata of Alabama and Mississippi, USA

•Strontium isotopes in most shark tooth enameloid show good numeric age correlation.•Samples from non-porous marine strata produce better results than sandy strata.•Strontium isotope ratios are similar between the different shark taxa analyzed.•Some strontium ages compare well with the absolute age of a bentonite studied.•This method may be useful for regional and global stratigraphic correlation.

Cretaceous strata in Alabama and Mississippi (USA) represent one of the most complete records of shallow marine deposition worldwide for the Upper Cretaceous. The age assignment of these strata in the eastern Gulf Coastal Plain is difficult due to the comparative lack of radiometrically datable beds and sometimes conflicting results of biostratigraphy using different taxonomic groups. Numerical age dating using strontium isotope ratios (87Sr/86Sr) preserved in diagenetically resistant fossil shark tooth enameloid had been proposed by previous researchers as a solution to dating some geologic units. Here we apply this methodology to the whole Upper Cretaceous, using teeth of two fossil shark genera (Scapanorhynchus and Squalicorax) collected from variable facies. Shark teeth collected from a bentonite mine in Monroe County, Mississippi, were also analyzed and compared with the radiometric date of the bentonite layer. Results indicate a strong correlation between stratigraphic position of the fossil teeth and numerical age determination based on 87Sr/86Sr content. Furthermore, this method is equally effective for both of the fossil shark genera analyzed in the study. Because of the nearly uniform distribution of strontium in ocean water, numerical age dating using strontium isotope ratios preserved in fossil shark tooth enameloid can be a useful method to employ in the correlation of marine geological strata on both regional and global scales.