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On the mid-Pleistocene transition to 100-kyr glacial cycles

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On the mid-Pleistocene transition to 100-kyr glacial cycles and theasymmetry between glaciation and deglaciation timesEli Tziperman and Hezi Gildor1Environmental Sciences, Weizmann Institute, Rehovot, IsraelReceived 30 January 2001; revised 26 June 2002; accepted 25 July 2002; published 9 January 2003.[1] A mechanism is proposed for the mid-Pleistocene transition from a dominant periodicity of 41 kyr to 100kyr in glacial oscillations. The same mechanism is shown to also explain the asymmetry between the longglaciation and short deglaciation phases of each cycle since that transition versus the symmetry of the 41-kyroscillations prior to the transition. These features arise naturally within the framework of the sea-ice switchglacial cycle mechanism of Gildor and Tziperman [2000] as a result of the gradual cooling of the deep oceanduring the Pleistocene. This cooling results in a change of the relation between atmospheric temperature and therates of accumulation and ablation of continentals ice sheets. It is this latter change that leads to the activation ofthe sea-ice switch and therefore to the initiation of the 100-kyr oscillations. The gradual glaciation and rapiddeglaciations during these oscillations occur because the mean value of the ice sheet ablation is not far from themaximum rate of snow accumulation during warm periods. This proximity of mean ablation and maximumaccumulation rates is shown to be also a consequence of the mid-Pleistocene gradual cooling of the deepocean.INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 1620 Global Change: Climate dynamics(3309); 1640 Global Change: Remote sensing; 4267 Oceanography: General: Paleoceanography; KEYWORDS: climate dynamics, glacialcycles, mid-Pleistocene transition, 100 kyr, 41 kyrCitation: Tziperman, E., and H. Gildor, On the mid-Pleistocene transition to 100-kyr glacial cycles and the asymmetry betweenglaciation and deglaciation times, Paleoceanography, 18(1), 1001, doi:10.1029/2001PA000627, 2003.1. Introduction[2] A new mechanism for the glacial-interglacial cycleshas been recently introduced by Gildor and Tziperman[2000, 2001b] (hereafter GT). The mechanism relies onthe rapid growth and melting of extensive sea-ice cover. Thesea-ice cover, through its effects on the atmospheric energybalance and air-sea fluxes, is able to switch the climatesystem from a growing land-ice mode (glaciation) to aretreating land-ice mode (deglaciation). GT dubbed thisthe sea-ice switch (SIS) mechanism of glacial cycles. GThave also proposed that the mid-Pleistocene transition froma 41-kyr glacial cycle to a 100-kyr cycle some 800 kyr agomay have been due to a general climate cooling that allowedextensive sea-ice cover to develop and resulted in theactivation of the SIS mechanism at that time. However,no specific explanation of how the switch activationoccurred was given. The SIS mechanism also results in asimple heuristic explanation for the timescale of the oscil-lation. Yet, the heuristic explanation in GT did not accountexplicitly for the fact that the glaciation stage takes signifi-cantly longer than the deglaciation stage. The asymmetryobtained in their model was thus a result of the (reasonable)choice of the model parameters, rather than of an explicitphysical argument.[3] The objective of this paper is to provide a detailedexplanation for two features of the glacial oscillations: thetransition from a dominant period of 41 kyr to one of 100kyr in glacial cycles, and the asymmetry between thetimescale of the glaciation and deglaciation. We show thatboth features arise naturally within the framework of the SISglacial cycle mechanism (GT) as a possible result of theclimate transition (or equivalently, bifurcation) whichoccurred about 800 kyr ago. In the mechanism proposedhere, the transition occurs due to a cooling of the deep oceanwhich is known to have been warmer than the modernocean prior to the mid-Pleistocene transition [Billups et al.,1998; McIntyre et al., 1999; Ruddiman et al., 1989; Marlowet al., 2000]. We do not specify the reason for the deepwater cooling, but one may invoke reasons such as a changein the level of atmospheric CO2[Raymo, 1998; Maasch andSaltzman, 1990] or a gradual increase in ice volume [Ghiland Ch ildress, 1987]. As a secondary objective of thispaper, we introduce a highly simplified model of GT’sSIS mechanism governed by few coupled ordinary differ-ential equations. This simple model is used to demonstratehow both the 100-kyr timescale and the asymmetric glacialoscillation structure arise via the mid Pleistocene climatebifurcation, and is also useful in demonstrating the SISmechanism in the simplest possible framework.[4] There have been some alternative explanations for theasymmetry of the glaciation and deglaciation phases. Le-Treut and Ghil [1983] argued that the load-accumulationfeedback between ice sheet mass balance and the bedrockisostatic rebound played a decisive role. Pollard [1983] andPALEOCEANOGRAPHY, VOL. 18, NO. 1, 1001, doi:10.1029/2001PA000627, 20031Now at Lamont-Doherty Earth Observatory of Columbia University,Palisades, New York, USA.Copyright 2003 by the American Geophysical Union.0883-8305/03/2001PA000627$12.001 - 1Watts and Hayder [1984] have explained the rapid termi-nation as the results of an ice sheet instabilities or enhancedcalving. Galle´e et al. [1992] found that a specified decreasein the albedo of aging snow during deglaciation inducedrapid glacial termination in their model. Similarly, Peltierand Marshall [1995] attributed the rapid deglaciation toalbedo variations due to dust loading together with marine-based ice sheet instability. It is fair to state, though, that asatisfactory detailed mechanism of the contrast betweenslow glaciation and rapid deglaciation is still missing.[5] Before the 100-kyr glacial cycles of the past 800 kyror so, glacial variability was dominated by a 41-kyr cyclewhich seemed very well co rrelated with Milankovitchforcing [Ruddiman and Raymo, 1988]. The transition to100-kyr cycles may have been related to tectonic inducedchanges in CO2or to ice volume change as mentionedabove [Raymo, 1998; Maasch and Saltzman, 1990; Bergeret al., 1999; Saltzman and Sutera, 1987; Ghil and Childr-ess, 1987; Paillard, 1998], although alternative mechanismswere propose such as the interaction between ice sheets andthe underlying bed [Clark et al., 1999; Clark and Pollard,1998], or other processes [Mudelsee and Schulz, 1997;Deblonde and


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