Pleistocene stable carbon isotope (d13C) 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 per mil decrease in d13C values until 0.90 Ma, followed by an increase of 0.60 per mil lasting until 0.50 Ma. The subsequent fluctuation started with a 0.40 per mil decrease between 0.50 and 0.25 Ma, followed by an increase of 0.30 per mil between 0.25 and 0.10 Ma. Here, we evaluate existing evidence and various hypotheses for these global Pleistocene d13C 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.

The 1.2 Ma stacked benthic foraminiferal carbon isotope record is created from averaging five 60,000 year smoothed benthic foraminiferal carbon isotope records. We used benthic foraminiferal carbon isotope records of DSDP/ODP Sites 68-502, 108-659, 121-758, 138-849 and 184-1143 from Oppo et al. (1995), Tiedemann et al. (1994), Chen et al. (1995), Mix et al. (1995), and Wang et al. (2004).The smoothed (and stacked) record show two distinct long-term carbon isotope fluctuations since 1.0 Ma.

Supplement to: Hoogakker, Babette A A; Rohling, Eelco J; Palmer, Martin R; Tyrrell, Toby; Rothwell, Robin Guy (2006): Underlying causes for long-term global ocean d13C fluctuations over the last 1.20 Myr. Earth and Planetary Science Letters, 248(1-2), 15-29