USING d18O VALUES OF APATITIC CONODONTS TO EVALUATE THE ORIGIN OF 3RD-ORDER PALEOZOIC SEA-LEVEL CHANGES

Third-order (1-5 My-scale) transgressive-regressive sequences are pervasive in Precambrian through Phanerozoic marine deposits. The origin of eustatic sea-level changes generating such sequences is not understood because there is no known glacio-eustatic climate driver occurring at these time scales and tectonically driven changes in mid-ocean ridge volumes are too slow to account for such sequences. To address the origin of persistent Paleozoic 3rd-order sea-level changes, we are analyzing the d¹⁸O values of apatitic conodonts from Middle Pennsylvanian (icehouse climate) and Middle Devonian (greenhouse climate) 3rd-order carbonate sequences to determine if there is a systematic relationship between transgressive-regressive facies and d¹⁸O values. In contrast to CaCO₃, phosphatic minerals are particularly resistant to diagenetic alteration and provide an excellent proxy for determining changes in ice volume and seawater temperatures. If the sequences were generated by climatically driven processes (glacio- or thermal-eustasy), then transgressive facies should record lower d¹⁸O values then the overlying regressive facies. No such relationship should exist if the sequences were generated by tectonic processes.

One of 4 globally correlative Middle Devonian 3rd-order sequences (~200 m) from the Great Basin (central Nevada) was sampled every 3-8 m. Conodont samples weighing between 300-600 mg (5-30 conodonts) were converted to Ag₃PO₄ to assure only the P-bound oxygen was analyzed. Preliminary results indicate that conodonts from transgressive and maximum flooding facies have systematically lower d¹⁸O values and those from regressive facies have higher d¹⁸O values suggestive of climatic forcing (controlling changes in glacial ice volume, seawater temperature, and/or evaporation). One of 5 regionally correlative Middle Pennsylvanian 3rd-order sequences (~80 m) in central New Mexico was sampled every 5-10 m and Ag₃PO₄-converted conodonts are presently being analyzed. This study highlights the use of apatitic conodonts for understanding the controls on My-scale infilling of marine basins as well as a powerful tool for Paleozoic-Triassic paleoclimatic investigations.