HR: 15:10h
AN: U23A-07    [Abstracts]
TI: Obliquity Pacing of the 40 and 100 kilo-year Modes of Glacial Variability
AU: * Huybers, P
EM: phuybers@mit.edu
AF: Woods Hole Oceanographic Institution, Geology and Geophysics Department, Mail Stop 22, Woods Hole, MA 02543-1050
AB: The Mid-Pleistocene Transition (MPT), $\sim$700ky ago, marks the switch from a climate with maximum variability at 40ky periods to one with maximum variability at $\sim$100ky periods. There is consensus that variations in Earth's obliquity control the 40ky variability, but there are conflicting hypotheses regarding the more recent $\sim$100ky glacial/interglacial cycles---variously attributing control to Earth orbital variations or mechanisms internal to the climate system. A formal test is made of the $\sim$100ky orbital hypotheses, distinguishing eccentricity, precessional and obliquity control of glacial cycle timing. The null hypothesis that glacial cycles are independent of obliquity is rejected at the 5% significance level. In contrast, the corresponding null-hypotheses for eccentricity and precession cannot be rejected. The simplest inference, consistent with the observations, is that the MPT represents a mode switch from ice-sheets terminating every obliquity cycle to a mode where ice-sheets terminate every second (80ky) or third (120ky) obliquity cycle (averaging $\sim$100ky). A modified version of \textit{Imbrie and Imbrie}'s [1980] simple glacial model is used to describe the obliquity pacing of the glacial cycles. The model reproduces the correct timing for each termination as well as many of the linear and nonlinear features identified in marine $\delta^{18}O$ records. Furthermore, under a wide range of parameterizations the model's response to obliquity period forcing is chaotic and has two distinct modes. One mode lies near an unstable fixed point of the model and gives successive ablation events which are relatively small, of nearly equal magnitude, and periodic at 40ky; in the other mode ice accumulates over two or three forcing cycles before rapidly ablating---in agreement with the hypothesis test results. The model chaotically switches between these 40ky and $\sim$100ky modes of glacial variability, suggesting that the MPT may be a spontaneous event independent of shifts in the background climate state.
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 3240 Chaos
DE: 1863 Snow and ice (1827)
DE: 1620 Climate dynamics (3309)
SC: Union [U]
MN: 2004 AGU Fall Meeting