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Since the discovery of ice ages in the 19th century, a central\udquestion of climate science has been to understand the respective role of the\udastronomical forcing and of greenhouse gases, in particular changes in the\udatmospheric concentration of carbon dioxide. Glacial–interglacial cycles have\udbeen shown to be paced by the astronomy with a dominant periodicity of\ud100 ka over the last million years, and a periodicity of 41 ka between\udroughly 1 and 3 million years before present (Myr BP). But the role and\uddynamics of the carbon cycle over the last 4 million years remain poorly\udunderstood. In particular, the transition into the Pleistocene about\ud2.8 Myr BP or the transition towards larger glaciations about 0.8 Myr BP\ud(sometimes referred to as the mid-Pleistocene transition, or MPT) are not easily\udexplained as direct consequences of the astronomical forcing. Some recent\udatmospheric CO2 reconstructions suggest slightly higher pCO2\udlevels before 1 Myr BP and a slow decrease over the last few million years\ud(Bartoli et al., 2011; Seki et al., 2010). But the dynamics and the climatic\udrole of the carbon cycle during the Plio-Pleistocene period remain unclear.\udInterestingly, the δ13C marine records provide some critical\udinformation on the evolution of sources and sinks of carbon. In particular, a\udclear 400 kyr oscillation has been found at many different time periods and\udappears to be a robust feature of the carbon cycle throughout at least the\udlast 100 Myr (e.g. Paillard and Donnadieu, 2014). This oscillation is also\udvisible over the last 4 Myr but its relationship with the eccentricity\udappears less obvious, with the occurrence of longer cycles at the end of the\udrecord, and a periodicity which therefore appears shifted towards 500 kyr\ud(see Wang et al., 2004). In the following we present a simple dynamical model\udthat provides an explanation for these carbon cycle variations, and how they\udrelate to the climatic evolution over the last 4 Myr. It also gives an\udexplanation for the lowest pCO2 values observed in the Antarctic ice\udcore around 600–700 kyr BP. More generally, the model predicts a two-step\uddecrease in pCO2 levels associated with the 2.4 Myr modulation of the\udeccentricity forcing. These two steps occur respectively at the\udPlio-Pleistocene transition and at the MPT, which strongly suggests that\udthese transitions are astronomically forced through the dynamics of the carbon\udcycle

The Plio-Pleistocene climatic evolution as a consequence of orbital forcing on the carbon cycle