USING OXYGEN ISOTOPE VALUES FROM MIDDLE PENNSYLVANIAN APATITIC CONODONTS FOR ESTIMATING CHANGES IN GLACIAL ICE VOLUMES

Oxygen isotope values from foraminiferal calcite have conventionally been used to determine Cenozoic seawater temperatures and/or isotope compositions related to glacial-ice volumes. Pre-Cenozoic carbonates are commonly diagenetically altered making paleoclimate interpretations from their d¹⁸O values less reliable. In contrast to calcite, phosphate minerals are particularly resistant to diagenetic alteration, and thus provide and excellent potential for answering a wide range of pre-Cenozoic paleoclimatic questions. This study focuses on using apatitic conodonts (extinct Paleozoic-Triassic marine microfossils) to estimate changes in Middle Pennsylvanian glacial-ice volumes and seawater temperatures.

The Middle Pennsylvanian (Desmoinesian) Gray Mesa Formation in central New Mexico is characterized by »77 upward-shallowing, mixed carbonate-siliciclastic cycles. Cycles are composed of deeper subtidal calcareous mudstones or fine skeletal wackestones overlain by shallow subtidal skeletal wacke-pack-grainstones; cycle tops record common meteoric diagenetic and pedogenic features (depleted d¹³C values, rhizoliths, calcretes). To strengthen our interpretations that the cycles developed in response to high-frequency, glacio-eustasy and to estimate changes in Pennsylvanian glacial-ice volumes, we collected conodonts through two separate cycles for oxygen isotope analysis.

The conodonts were analyzed for their d¹⁸O values using the laser ablation technique of Sharp and Cerling (1996), which permits analysis of single conodonts. Between 5-20 conodonts were analyzed from each stratigraphic horizon. Both cycles record a »1 to 4‰ positive shift in d¹⁸O values associated with the upward-shallowing facies trends supporting a glacio-eustatic cycle origin; these values also include the effects of any paleotropical seawater temperature changes associated with glacial-interglacial climate phases. The magnitudes of intracycle d¹⁸O shifts are minimum values because of nondeposition/subaerial exposure during sea-level fall and lowstand. These Pennsylvanian d¹⁸O shifts are comparable to values observed for Pleistocene glacial-cycles suggesting similar changes in glacial-ice volumes during parts of the Desmoinesian.