Conodont apatite δ88/86Sr and δ44/40Ca compositions and implications for the evolution of Palaeozoic to early Mesozoic seawater

We present high precision (TIMS double spike) stable isotope measurements of both δ44/40Ca and δ88/86Sr 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 δ44/40Ca 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 δ44/40Ca record (Farkaš et al., 2007a). We calculated a δ44/40Ca seawater-bio-apatite fractionation factor of about −1.9‰, which allowed us to reconstruct a palaeo-seawater δ44/40Ca record from bio-apatites. Despite a slightly positive offset of about +0.2 to +0.5‰, the δ44/40Ca record obtained from bio-apatites is consistent with the previously reported δ44/40Ca seawater record inferred from carbonates. We find that unlike the carbonate δ44/40Ca records, the δ88/86Sr 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 δ88/86Sr 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.