Ocean carbon cycling during the past 130 000 years – a pilot study on inverse palaeoclimate record modelling

Abstract. What role did changes in marine carbon cycle processes and calcareous organisms play in glacial–interglacial variation in atmospheric pCO₂? In order to answer this question, we explore results from an ocean biogeochemical general circulation model. We attempt to systematically reconcile model results with time-dependent sediment core data from the observations. For this purpose, we fit simulated sensitivities of oceanic tracer concentrations to changes in governing carbon cycle parameters to measured sediment core data. We assume that the time variation in the governing carbon cycle parameters follows the general pattern of the glacial–interglacial deuterium anomaly. Our analysis provides an independent estimate of a maximum mean sea surface temperature drawdown of about 5 °C and a maximum outgassing of the land biosphere by about 430 Pg C at the Last Glacial Maximum as compared to pre-industrial times. The overall fit of modelled palaeoclimate tracers to observations, however, remains quite weak, indicating the potential of more detailed modelling studies to fully exploit the information stored in the palaeoclimatic archive. This study confirms the hypothesis that a decline in ocean temperature and a more efficient biological carbon pump in combination with changes in ocean circulation are the key factors for explaining the glacial CO₂ drawdown. The analysis suggests that potential changes in the export rain ratio POC : CaCO₃ may not have a substantial imprint on the palaeoclimatic archive. The use of the last glacial as an inverted analogue to potential ocean acidification impacts thus may be quite limited. A strong decrease in CaCO₃ export production could potentially contribute to the glacial CO₂ decline in the atmosphere, but this remains hypothetical.