Oxygen isotope variations in phosphate of biogenic apatites, III. Conodonts

The method for measurement of the isotopic composition of oxygen in phosphates has been improved and adapted for analysis of small quantities of apatite, down to 10 mg. This extension enables one to analyze hand-picked conodont samples with an analytical reproducibility better than ±0.5‰ (1σ). 46 samples of conodonts ranging from the Ordovician to the Pennsylvanian of North America were analysed. Some insoluble phosphatic residues, ichthyoliths and inarticulate brachiopods of the same time range were also measured. The range of the δ¹⁸O values of the analysed conodonts is between 15 and 19‰. It shows a general trend of decreasing d¹⁸O with increasing age, from an average value of about 19 in the Pennsylvanian to 17 in the Ordovician. This trend parallels that previously detected in marine phosphorites. For the time range between the Devonian and Pennsylvanian our data are in agreement with independent paleoclimatic information. Specifically, we detect maximum ¹⁸O enrichment at the end of the Pennsylvanian, and minimum enrichment at the end of the Devonian. The difference between these two extremes is equivalent to about 10°C (from about 40° to 30°C), assuming constant isotopic composition of ocean water. The success of oxygen isotopic analysis of conodonts raises the possibility of their use in Paleozoic paleo-oceanography in a similar way to foraminifera in the Mesozoic-Cenozoic.