Modelling the oxygen isotope distribution of ancient seawater using a coupled ocean–atmosphere GCM: Implications for reconstructing early Eocene climate

One of the motivations for studying warm climates of the past such as the early Eocene, is the enhanced understanding this brings of possible future greenhouse conditions. Traditionally, climate information deduced from biological or chemical proxies have been used to “test” computer model simulations of past climatic conditions and hence establish some of the uncertainties associated with model-based predictions. However, extracting climate information from proxies is itself an interpretative process and discrepancies between climate information inferred from different types of proxy undermines the assumption that model-data conflicts automatically mean that the model is inherently flawed. A new approach which both acknowledges and reduces the uncertainties associated with both model and data is required.Although the oxygen isotopic ratio (δ18O) preserved in calcareous marine fossils has been used to reconstruct past seawater temperature for several decades, significant uncertainties associated with this method persist. These include assumptions about past seawater δ18O for which no proxy exists and which is a key control on the temperature inferred from fossil carbonate. Here we present the results of an early Eocene simulation made using a state-of-the-art General Circulation Model (GCM; HadCM3) with CO2 set at six times pre-industrial values and which has oxygen isotopes incorporated into the full hydrological cycle and hence simulates the δ18O of past seawater. This allows us to explore the implications of the different seawater δ18O correction factors commonly used for δ18O-based temperature reconstruction. It also allows us to focus model-data comparison on δ18O rather than interpret ocean temperature, an approach that reduces uncertainties in model-data comparison since the effects of both the temperature and the isotopic composition of ocean water on δ18O of carbonate are accounted for. The good agreement between model and data for both modern and well-preserved early Eocene carbonate increases confidence in climate reconstructions of this time.