HIGH RESOLUTION ANALYSIS OF STABLE OXYGEN ISOTOPE RATIOS IN LINGULID BRACHIOPOD SHELLS

Recently developed laser ablation techniques provide an unprecedented opportunity for high-resolution isotopic analysis of biogenic phosphates, which typically require relatively large samples (20-30 mg) when conventional techniques are applied. Both laser ablation (Sharp and Cerling, 1996, 1998) and silver phosphate (O'Neil et al., 1994) procedures were used to make over 400 d¹⁸O measurements in modern and fossil lingulids. We examined oxygen isotope variations in lingulid brachiopods at a variety of scales: within valves, between valves from the same individual, between individuals collected at the same location at the same time, and between localities. Specimens included modern lingulids from several patches in the northern Gulf of California and the Gulf of Nicoya in Costa Rica, and fossil specimens from the Lower Triassic Dinwoody Formation. After correction for drift against two separate standards, both modern and fossil lingulid valves display a high degree of intrashell variability, frequently exceeding 4‰. Parallel profiles show that this variability is not symmetrical within the shell, and thus oxygen isotopic ratios in the biogenic phosphate of the skeleton do not reflect equilibrium values. We do not believe organic carbon contamination is responsible, as data from the silver phosphate preparations agree with mean values computed per shell from laser ablation data, with whole valve d¹⁸O measurements varying by more than 4‰. In contrast, carbonate extractions performed on these shells are highly repeatable and appear to represent equilibrium values; mass balance calculations indicate that the carbonate component can not account for the intrashell variations detected. Thus, ironically, while high resolution laser ablation analysis indicates that the phosphate phase of lingulid apatite is not a reliable recorder of oxygen isotope ratios in seawater, it is possible to derive usable data from the carbonate fraction.