Isotopic and elemental systematics of Sr and Nd in 454 Ma biogenic apatites: implications for paleoseawater studies

Pristine conodonts (CAI ≤ 1.5), inarticulate brachiopods, and conulariids, all from a single hand sample of Ordovician limestone, define a co-varying trend of 87Sr86Sr and Sr and Sr concentration. Most of the apatitic fossils have 87Sr86Sr ratios that are more radiogenic than the enclosing whole-rock limestone, indicating a general susceptibility of biogenic apatites to post-depositional Sr exchange. The largest isotopic shifts were measured in inarticulate brachiopods and conulariids, and deduced for conodont basal body material. Conodont crown material exhibits the smallest effects. The Sr exchange effects are strongly dependent on differences in apatite composition, as revealed by contrasting CaP ratios. Although conodont crown material (with low CaP ratios) is less prone to isotopic disturbance relative to other types of coexisting apatite fossils, high resolution X-ray mapping reveals that even conodont crowns exchange Sr, as is shown by a gradient of decreasing Sr concentration from crown rim to core. trast to Sr, all coexisting fossil apatites have identical initial 143Nd144Nd ratios over a wide range of Nd concentration. No relationship between 87Sr86Sr and 143Nd144Nd was observed despite a pronounced antithetic pattern of Sr and Nd distribution both between the fossil types, and within individual conodonts containing preserved basal body material. In agreement with earlier studies, it is concluded that the bulk of the Nd in fossil apatites is from seawater that originally overlay the depositional site.