CONODONT APATITE AS A PALEOTEMPERATURE PROXY: RECENT DEVELOPMENTS

Conodont apatite has rarely been used for oxygen isotope analysis due to the small size of conodont elements and the number of elements required by conventional analysis. With new analytical techniques, isotope analysis can now be performed on a relatively small number of elements. Since apatite has a high potential to preserve its primary oxygen isotope composition, isotope ratios measured on conodonts may serve to decipher the paleotemperature history of Paleozoic and early Mesozoic seas. In addition, paleotemperatures calculated from the oxygen isotope ratios can help to reconstruct the life habitat of different conodont genera.

Currently available oxygen isotope data indicate that thermal overprint does not affect the oxygen isotope ratio of apatite phosphate up to a conodont colour alteration index of 5. Analysis of Devonian and Carboniferous shallow-water and pelagic conodonts from several time periods reveal no prominent difference in their oxygen isotope ratios. This suggests that pelagic conodonts were not thriving in deeper but surface waters and that oxygen isotope ratios of shallow-water as well as pelagic conodonts can be used to reconstruct surface water temperatures. This finding facilitates isotope analysis of conodonts since analyses have not to be performed on monospecific samples. However, further tests have to be performed on conodonts from other time intervals.

Short-term as well as long-term changes in the oxygen isotope ratios of Ordovician to Carboniferous conodonts are interpreted as the consequence of climatic changes and the waning and waxing of high-latitude ice sheets. However, conodonts do not show a prominent secular decrease in the oxygen isotope ratios as indicated by the oxygen isotope record of brachiopod calcite. Consequently, the conodont oxygen isotope record does not support the hypothesis that the oxygen isotope composition of the Paleozoic oceans was significantly different from the modern oceans.