BODY SIZE AS A BAROMETER OF OCEAN OXYGENATION IN EARTH HISTORY (Invited Presentation)

The oxygenation of the ocean has varied over Earth’s history, with major impacts on marine ecosystems. However, estimates of the magnitude of O₂ changes, typically derived from geochemical proxies or ecological correlations, are imprecise and/or limited to very low O₂ thresholds. Here we use a trait-based physiological model to attribute the climatic drivers of observed body size changes in two major microfossil taxa, ostracods and foraminifera, during two climate epochs, gradual Cenozoic cooling and abrupt warming of the PETM. The body size of both fossil taxa is correlated to temperature in both cooling and warming climates. However, the slopes of the relationships are too large to be explained by the direct effect of temperature, as observed in laboratory experiments and reproduced by our mechanistic model. Ascribing the residual body size changes to organismal O₂ supply yields quantitative vertically-resolved estimates of changes in pO₂, which show that deep ocean pO₂ rises with cooling and falls with warming, except in the tropical thermocline where rapid warming elevates pO₂. These patterns are quantitatively consistent with equilibrium model simulations and with observations over the past century of anthropogenic warming. Our model highlights the potential for mechanistic organismal models to be used as a paleo-barometer for the evolution of oceanic O₂ in Earth’s history.