Conodont apatite δ^88/86Sr and δ^44/40Ca compositions and implications for the evolution of Palaeozoic to early Mesozoic seawater

We present high precision (TIMS double spike) stable isotope measurements of both δ^44/40Ca and δ^88/86Sr together with radiogenic ⁸⁷Sr/⁸⁶Sr ratios determined from conodont apatite. These data represent five intervals ranging from the early Ordovician to late Triassic. The conodont δ^44/40Ca values (relative to NIST 915a) range from − 0.47‰ to + 0.15‰, with an apparent shift to more positive values between the early Silurian and late Devonian/early Carboniferous, similar to the brachiopod-based marine δ^44/40Ca record (Farkaš et al., 2007a). We calculated a δ^44/40Ca seawater-bio-apatite fractionation factor of about − 1.9‰, which allowed us to reconstruct a palaeo-seawater δ^44/40Ca record from bio-apatites. Despite a slightly positive offset of about + 0.2 to + 0.5‰, the δ^44/40Ca record obtained from bio-apatites is consistent with the previously reported δ^44/40Ca seawater record inferred from carbonates. We find that unlike the carbonate δ^44/40Ca records, the δ^88/86Sr measurements from conodont apatite show unexpectedly large variations (up to ~ 1‰), with ratios ranging from − 0.6‰ to 0.3‰. These reconnaissance data reveal a reasonable correlation between δ^88/86Sr and radiogenic ⁸⁷Sr/⁸⁶Sr* (r² = 0.60, n = 13, p = 0.002), suggesting that the controls from differential weathering regimes and/or continental crustal compositions buffered ancient seawater compositions.