Underlying causes for long-term global ocean δ¹³C fluctuations over the last 1.20 Myr

Pleistocene stable carbon isotope (δ¹³C) records from surface and deep dwelling foraminifera in all major ocean basins show two distinct long-term carbon isotope fluctuations since 1.00 Ma. The first started around 1.00 Ma and was characterised by a 0.35‰ decrease in δ¹³C values until 0.90 Ma, followed by an increase of 0.60‰ lasting until 0.50 Ma. The subsequent fluctuation started with a 0.40‰ decrease between 0.50 and 0.25 Ma, followed by an increase of 0.30‰ between 0.25 and 0.10 Ma. Here, we evaluate existing evidence and various hypotheses for these global Pleistocene δ¹³C fluctuations and present an interpretation, where the fluctuations most likely resulted from concomitant changes in the burial fluxes of organic and inorganic carbon due to ventilation changes and/or changes in the production and export ratio. Our model indicates that to satisfy the long-term 'stability' of the Pleistocene lysocline, the ratio between the amounts of change in the organic and inorganic carbon burial fluxes would have to be close to a 1:1 ratio, as deviations from this ratio would lead to sizable variations in the depth of the lysocline. It is then apparent that the mid-Pleistocene climate transition, which, apart from the glacial cycles, represents the most fundamental change in the Pleistocene climate, was likely not associated with a fundamental change in atmospheric pCO₂. While recognising that high frequency glacial/interglacial cycles are associated with relatively large (100 ppmv) changes in pCO₂, our model scenario (with burial changes close to a 1:1 ratio) produces a maximum long-term variability of only 20 ppmv over the fluctuation between 1.00 and 0.50 Ma.