Pleistocene Glaciation: The 100 ky Problem

Figure 1. Gain in the precession and obiliquity bands as a function of time for the past 400 ky. The amplitude of incoming radiation at 65 N is taken as the forcing and the amplitude of the ¹⁸O signal in the same bands is taken as the response.

The response in the 100 ky band has a substantially diferrent character that remains difficult to explain.

1. The forcing generated by eccentricity variations is too weak (Figure 2) to produce the responses observed in the climate proxies. We can can compare the radiative forcing to the ice volume in the three principal frequency bands:

   

   Figure 2. Partitioning of incoming 65 N June radiation and ¹⁸O (ice volume) record into precession, tilt, and eccentricity bands. Note the striking discrepancy between the amplitudes of the radiation and ice volume time series in the 100 ky band.

   The amplitude in the eccentricity band is ~2 W/m², producing a response with amplitude ~1 o/oo. This corresponds to a gain of 0.5 (o/oo)/(W/m²), about 50x as great as the responses at higher frequency.

2. The eccentricity record has power at periods of both 100 and 400 ky, whereas the ice volume record only shows 100 ky power. At 100 ky, there is fair agreement between the two signals (Figure 3).

   

   Figure 3. Eccentricity showing 100 ky and 400 ky cycles during the past 500 ky. After removing the 400 ky cycle, the amplitudes of the eccentricity and 100 ky ¹⁸O records are quite similar.

3. The evolution through time of the spectra of the ice volume and the eccentricity over the past 2 my (Figure 4) are very different. The amplitude of the 100 ky peak in the eccentricity spectrum is decreasing whereas the amplitude of the same peak in the ice volume record is increasing.

   

   Figure 4. Benthic ¹⁸O record for the past 2 my showing the increasing importance of the 100 ky frequency band and the absence of a 400 ky response. In contrast, the amplitude of the 100 ky eccentricity signal declines over the same period.

So we are left to explain the 100 ky response:

Figure 5. Residual ¹⁸O signal after removing the linear responses to precession and obliquity.

The SPECMAP group suggested that the ice volume in the 100 ky band could be explained as a non-linear response to the forcing.

The similarity of the phase lag groupings of proxies in all three frequency bands suggests that the climate system operates similarly at all three periods, although with a longer delay between early responses and ice volume at 100 ky. The similarity of the ice volume and eccentricity records in the 100 ky band is compatible with a resonance phenomenon in which "pacing" of the ice volume record is phase locked with the eccentricity. The sequence of proxy phase leads and lags relative to ice volume is the same for the 100 ky band as at higher frequency, but the time delay between the early response group and ice volume (¹⁸O) is 15 ky rather than about 3 ky (Lecture 22).

Figure 6. Imbrie et al. model for the 100 ky climate cycle compared to the 23/41 ky model (Lecture 22). Note the longer response time for S3.

This led Imbrie et al. to suggest that the 100 ky climatic signal results from non-linear amplification of the incoming radiation signal (Figure 7) once an internal critical threshold is exceeded.

Figure 7. Conceptual model for climate where the response is linear until a threshold is crossed, after which the response is disproportionately amplified.

They suggest that this threshold is set by feedbacks (albedo, atmospheric circulation) that develop when northern hemisphere ice caps exceed a critical size which interferes with the re-establishment of the Nordic heat pump by radiation increases at the higher frequencies.

The system is paced by eccentricity because of its influence on the amplitude of the incoming radiation signal in the 23 ky band. The key appears to lie in the extension of the icecaps on to isostatically depressed continental shelves as the ice masses approach maximum size. Once this happens, even a small sea level rise (due to peak radiation at the precession frequency, for example) results in destabilization of the ice caps with rapid melting that leads to further destabilization. Such rapid collapses produce the "Terminations" which are such a prominent feature of the late Pleistocene ice volume record.

Alternatively, others have suggested that low 65 N summer radiation peaks that occur every fourth or fifth precessional cycle lead to a build-up of "excess" ice on the icecaps, followed by rapid deglaciation during the next precessional peak. Time series analysis of a mixed 4+5 precessional cycle yields a 100ky spectral peak that resembles the SPECMAP ¹⁸O peak. Because the low summer radiation precession peaks result from modulation by eccentricity, such a 100 ky spectral peak will be in phase with eccentricity.

A third alternative involves models of ice cap development that exhibit oscillations with periods in the 70-130 ky range due to various feedback mechanisms.

Still unexplained is the geologically rapid evolution of the 100 ky spectral peak in ¹⁸O records compared to the 23 and particularly the 41 ky records which have been relatively stable for the past two million years (Figure 29-9).

Hypotheses to explain the increasing importance of the 100 ky cycle focus on modifications to the ice covered regions by a succession of glaciations. Examples include:

• the gradual stripping of the regolith (surface soil and weathered rock layer) which is thought to act as a lubricant that prevents the build-up of very thick ice, and
• downcutting of the continental shelf by ice streams so that the toes of the streams are well below modern sea level and are, therefore, vulnerable to collapse as a result of small rises in sea level due to radiation increases at 65 N resulting from Milankovitch forcing.