EVALUATING CO2 AS A PRIMARY DRIVER OF PALEOZOIC-MESOZOIC CLIMATE CHANGE

There is a general consensus that changes in atmospheric carbon dioxide are a major driver of global temperature and climate change. Many studies have shown that changes in atmospheric carbon dioxide and temperature over geologic time are linked. Crucially, however, some studies of Mesozoic temperature change, derived from isotopic analyses of marine calcitic fossils, have shown a negative relationship between estimates of temperature and past atmospheric carbon dioxide, with evidence of falling temperatures but rising carbon dioxide levels. This apparent mismatch between temperature and carbon dioxide has been used as a central argument by those sceptical of current global warming and anthropogenic greenhouse gas issues. The isotope data (used as a temperature proxy) from which interpretations and conclusions have been drawn has been derived from the analysis of a range of different fossil groups. Brachiopods dominate the Paleozoic portion of the isotope curve, whereas Mesozoic data are derived largely from belemnites which potentially poorly reflect marine isotopes and temperatures. In this study we test whether this may have led to the apparent mismatch. We present new oxygen isotope data derived from well-preserved brachiopods combined with published data from the late Paleozoic. These data, if interpreted in terms of temperature, confirm the positive link between temperature and carbon dioxide levels. In addition to our new Paleozoic data we also examined the isotopic composition of early Mesozoic brachiopods in order to constrain temperature. Although severely affected by the Late Permian extinction event brachiopods are frequently common and locally abundant during the early Mesozoic. Our new data, combined with the few published data that exist, provide critical input into the debate on the main drivers of climate change. Significantly, our new Paleozoic-Mesozoic brachiopod isotope curve is consistent with carbon dioxide curves, whereby high but declining temperatures are recorded for the Permian-Triassic interval but appreciably warm temperatures are maintained throughout the remainder of the Mesozoic. We suggest that the apparent mismatch between atmospheric carbon dioxide and temperature is an artefact due to the use of an inappropriate paleotemperature proxy in the early Mesozoic.