Laser ablation and silver phosphate procedures were used to measure the oxygen isotope composition in organophosphatic shells of 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, between localities, and between modern and fossil specimens. Specimens included modern lingulids from several patches in the northern Gulf of California (Mexico) and the Gulf of Nicoya (Costa Rica), and fossil specimens from the lower Triassic Dinwoody Formation. All specimens display a high degree of intrashell variability, frequently exceeding 4‰. This variability is not symmetrical within the shell, does not appear to reflect growth bands, and is not consistent with a published lingulid phosphate-oxygen isotope thermometer. Shells analyzed using silver phosphate preparations have variability similar (>3.5‰) to that of shells analyzed using the laser ablation technique, ruling out the influence of organic carbon contamination. We interpret this variation as primary, representing a vital effect, possibly the result of enzymatic fractionation near mantle canals and muscle scars or in vivo mineralogical changes in shell composition. In contrast, oxygen isotope analysis of carbonate from these shells is repeatable and appears to represent equilibrium values. Although oxygen isotope analyses from laser ablation and silver phosphate methods indicate that the phosphate in lingulid valves is an unreliable recorder of oxygen isotope ratios in seawater, it may be possible to derive paleoclimate data from the carbonate fraction.
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