Many biogeochemistry models assume all material reaching the seafloor is remineralized and returned to solution, which is sufficient for studies on short-term climate change. Under long-term climate change, the carbon storage in sediments slows down carbon cycling and influences feedbacks in the atmosphere–ocean–sediment system. This paper describes the coupling of a sediment model to an ocean biogeochemistry model and presents results under the pre-industrial climate and under CO₂ perturbation.

Using a carbon cycle model I here show that the 405-kyr periodicity found in marine δ¹³C during the last 5 million years and the offset in atmospheric δ¹³CO₂ between the last and the penultimate glacial maximum are probably related to each other. They can be explained by variations in the δ¹³C signature of weathered carbonate rock or of volcanically degassed CO₂ which vary mainly with obliquity (41-kyr) suggesting that northern hemispheric land ice sheets are their ultimate drivers.

We reconstruct changes in mean ocean temperature (ΔMOT) over the last 4.5 Myr. We find that the ratio of ΔMOT to changes in global mean sea surface temperature was around 0.5 before the Middle Pleistocene Transition but was 1 thereafter. We subtract our ΔMOT reconstruction from the global δ¹⁸O record to derive the δ¹⁸O of seawater. Finally, we develop a theoretical understanding of why the ratio of ΔMOT/ΔGMSST changed over the Plio-Pleistocene.

We constructed 160 kyr long mono-specific stacks of δ¹³C and of δ¹⁸O from the wider tropics from the planktic foraminifera G. ruber and/or T. sacculifer and compared them with carbon cycle simulations using the BICYCLE-SE model. In our stacks and our model-based interpretation, we cannot detect a species-specific isotopic fractionation during hard-shell formation as a function of carbonate chemistry in the surrounding seawater, something which is called a carbonate ion effect.

In this paper we describe the implementation of the carbon isotopes 13C and 14C into the marine biogeochemistry model FESOM2.1-REcoM3 and present results of long-term test simulations. Our model results are largely consistent with marine carbon isotope reconstructions for the pre-anthropogenic period, but also exhibit some discrepancies.