Factors influencing conodont apatite δ~(18)O variability in the Ordovician: a case study from New South Wales, Australia

摘要

Oxygen isotopic ratios of conodont apatite can be a robust proxy for sea surface temperatures and provide important constraints on global climate. The oxygen isotopic composition of seawater is a function of local, regional, and global processes. To determine the relative importance of regional and local influences (and therefore make global inferences), it is necessary to document high-resolution δ~(18)O records from a variety of paleogeographic and environmental settings. Available conodont δ~(18)O_(phos) records for the Ordovician, however, are biased towards North American samples, often come from multiple discontinuous sections, and tend to focus only on specific intervals (e.g., the Katian Stage). To extend the paleogeographic range and test the generality of Ordovician δ~(18)O_(phos) trends, we measured δ~(18)O_(PhOS) values from species-specific conodont assemblages from New South Wales, Australia. Our results from this region show that Early Ordovician δ~(18)O_(phos) averages are ~2o lower than those in the Late Ordovician and are consistently ~2.5o lower relative to values reported from North America and central Australia. This offset suggests that regional environmental conditions influenced the δ~(18)O_(phoS) record from New South Wales. Despite evidence suggesting a departure from open ocean conditions, our results still show increasing δ~(18)O_(phos) values through the Early Ordovician in agreement with documented trends from other regions. This similarity suggests that the δ~(18)O_(phos) increase (documented on separate continental blocks and different environmental settings) reflects global cooling rather than a change in the oxygen isotopie composition of the Ordovician oceans and strengthens the argument that biodiversification and climate change in the first half of the Ordovician were related.