Rapid communication: Middle Pleistocene Transition as a phenomenon of orbitally enabled sensitivity to initial values

Abstract. The Middle Pleistocene Transition (MPT), i.e., the “fast” transition from ∼ 41 to ∼ 100 kyr rhythmicity that occurred about 1 Myr ago, remains one of the most intriguing phenomena of the past climate. The cause of this period shift is generally thought to be a change within the Earth System, since the orbital insolation forcing does not change its pattern through the MPT. Using a dynamical model rooted in ocean chemistry, we advance several novel concepts here: (i) the MPT could be a dominant-period relaxation process that may be dependent on the initial state of the system, (ii) this sensitivity to the initial state is enabled by the orbital forcing, (iii) depending on the amplitude of the orbital forcing and initial values, the MPT could have been not just of the 40–80 kyr type, as we observe in the available data, but also of a 20–40, 40–120, or even 80–40 kyr type, (iv) when the orbital forcing of the global glaciation-climate model is accompanied by the alkalinity (CO2) forcing containing a dominant-period shift from 41 to 80 kyr, this ice-climate system produces a 40-to-100 kyr glacial rhythmicity transition resembling the MPT LR04 data, and (v) when the glaciation-climate model is forced by an alkalinity (CO2) forcing containing a periodicity transition from 20 to 42 kyr, a non-linear interplay of the orbital forcing and of ∼ 40 kyr periods of the alkalinity forcing may produce glaciation periods of ∼ 100 kyr that are also consistent with the LR04 data.