3.7 Consensus Dansgaard–Oeschger cycle theory and challenges

Fig. 2.15 The timing of Pleistocene glaciations as a function of summer energy, ice-volume and eccentricity

a) LR04 benthic stack as an ice proxy (multi-colored line; after Lisiecki & Raymo 2005) for the past 341 kyr. The line was colored in 40–41 Kyr segments with orange-red tones for low ice segments, green tones for intermediate ice segments and blue tones for high ice segments. Dots of the same color mark the start of each segment. Background color defines four different states, light orange for interglacial, light green for mild-glacial, light blue for full-glacial, and cyan for deep-glacial. Black curve is summer energy at 65°N with a 275 W/m2 threshold (Huybers 2006), lagged by 6000 years to compensate the delay between forcing and effect. Thick grey curve is eccentricity (without scale; after Laskar et al. 2004). b) Plot of the multi-colored ice proxy curve versus summer energy. It is evident that the icevolume at the start of the orbital 41-kyr oscillation in summer energy determines the subsequent ice-volume evolution during the oscillation and the possibility of an interglacial taking place in that oscillation. c) Simple excitation/relaxation multi-state model explains the timing of Pleistocene glaciations. Under Early Pleistocene or high eccentricity conditions climate operates as a simple oscillatory system represented by dashed lines, reversibly transitioning at 41-kyr frequency between mild-glacial (D) and cool interglacial (D'). The MPT when eccentricity is not high introduced an ice-volume requirement for excitation out of glacial conditions, represented by the downward arrow (A), and at the same time resulted in warmer interglacials (A'), as the upward arrow indicates. Depending on the speed of icevolume accumulation, that is inversely correlated to eccentricity, the system must transition through usually one or exceptionally two oscillations (two represented, B' & C') in the relaxation process to reach the excitable state (A). Low eccentricity favors high ice accumulation accelerating the relaxation (only one oscillation required). Medium eccentricity delays the relaxation as ice accumulates more slowly (two oscillations required). High eccentricity bypasses the ice requirement, returning the system to Early Pleistocene conditions as it happened in the C & D transition 245 kyr BP when due to high eccentricity MIS 7e was produced despite low ice-volume and being very late in the summer energy oscillation. The system transitioned back to Mid-Pleistocene conditions after MIS 7a–c (D & C). The double dependency on ice-volume and insolation to produce interglacials at obliquity maxima (Fig. 2.14), results in the absence of a regular pattern. Interglacials are produced at 41, 82, or 123-kyr intervals. However, the ice-volume dependency on eccentricity results in a 100-kyr cycle on ice accumulation that is clearly appreciable in ice proxies.

transition towards more extreme conditions until icevolume would be so high as to cause high ice-sheet instability (A). This ice-sheet instability is reflected in massive iceberg discharge when perturbed, resulting in Heinrich events. Also, the rebound effect from ice-sheet instability causes warmer interglacials (A'). Mid and Late Pleistocene are characterized by bigger temperature swings between deep glacial and warm interglacial.

When the interglacial ends, the system must relax back to the initial (A) state, but the ice-volume required is so high that it must transition through one or two oscillations (two shown in Fig. 2.15c) during which little ice is melted during the summer energy increase (B & B', C & C'), but considerable ice-sheet growth takes place during the summer energy decrease (B' & C, C' & A). This causes some glacial periods to last one or exceptionally two complete obliquity oscillations.