Before the MPT at around 900 ka BP, the 100 kyr cycle was essentially absent in 818O deep -sea core records, and most of the spectral power was at the orbital obliquity period of ~ 41 kyr, with little at the precessional period ~ 23 kyr (Mudelsee and Schulz, 1997). A number of models that explicitly involve NH ice sheets have attempted to understand this abrupt change. Most rely on a special mechanism that allows 100 kyr cycles in the model only when the maximum ice sheet size is large, and not before ~ 1 Ma when the ice sheets never reached the required size. This size evolution is accomplished in turn by an assumed long-term trend in some external model parameter such as declining atmospheric CO2 (Deblonde and Peltier, 1991; Mudelsee and Schulz, 1997; Berger et al., 1999). Clark and Pollard (1998) achieved the same result not by a prescribed long-term trend, but by adding a deforming sediment component underneath the ice sheet. Starting with a 50 m regolith at 3 Ma, the early ice sheets were kept thin by the greater basal sliding due to the sediment. After the sediment is eroded by repeated glacial growth and retreat, by about 1 Ma enough hard bedrock is exposed to support the greater basal shear stresses of thick ice sheets, allowing the 100 kyr mechanism in that model (proglacial calving) to operate. However, the true cause(s) of the MPT remain unknown. Other proposed mechanisms are noted below in section 11.6.4.
None of these models address the dominance of ~41 kyr spectral power (driven by obliquity) compared to ~ 23 kyr (driven by precession) before the MPT. One recent theory (Raymo et al., 2006) hypothesises that before the MPT, the EAIS was somewhat smaller than at present and much more variable, with terrestrial margins and ablation (summer-melting) zones that responded significantly to orbitally driven summer-temperature variations. If so, precession-driven ice volume variations would have been out of phase between the hemispheres, and obliquity-driven variations in phase, resulting in a cancelling between northern and southern ice volume variations at 23 kyr, and dominant 41 kyr power. This hypothesis is testable by establishing the character and variability of the Antarctic Ice Sheet during the Pliocene up to the MPT (see below).
Another recent hypothesis notes that total melt season insolation is affected more strongly by obliquity than by precession (Huybers, 2006); this is testable by models with seasonal cycles and positive-degree-day parametrisations of ice melt.
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