Strontium isotopes in biogenic phosphates from a neogene marine formation: implications for palaeoseawater studies

Abstract

Neogene marine deposits of NW France (“Faluns”) are rich in a variable array of well preserved phosphatic vertebrate fragments. Many of these are of the same age as the host sediments and consist of the remains of fish, mammals and reptiles whilst others are clearly Mesozoic (Turonian) in age. We have determined the Sr isotopic composition of 24 phosphatic samples in order to study the efficacy of using the method to date the Neogene materials and to investigate the effects of diagenesis and seawater alteration on the terrestrial material. The Sr isotope results obtained on the ichthyoliths and the other Neogene marine vertebrates suggest an age of 11 Ma for the base of the host sediments (Tortonian), which corresponds precisely with the palaeontological data. In contrast, the initial isotopic composition of the contemporaneous terrestrial mammals and reptiles, which should record higher (continental) Sr ratios are close to the value of their marine counterparts. The same trend is observed for a sample of shark's tooth of Mesozoic age where the Sr isotopic composition approaches a Neogene value. The results demonstrate that alteration by seawater or interstitial waters is able to introduce significant quantities of Sr into the phosphate. Leaching experiments carried out on the materials does not remove this secondary strontium and we were unable to establish the original values of the terrestrial material. Our conclusion is that the use of Sr isotopes to characterise phosphatic materials must be viewed with caution particularly if the materials are reworked.