Glacial variability over the last two million years: an extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression

An agemodel not relying upon orbital assumptions is estimated over the last 2 Ma using depth in marine sediment cores as a proxy for time. Agemodel uncertainty averages +/- 10 Ka in the early Pleistocene (similar to 2-1 Ma) and +/- 7 Ka in the late Pleistocene (similar to 1 Ma to the present). Twelve benthic and five planktic delta O-18 records are pinned to the agemodel and averaged together to provide a record of glacial variability. Major deglaciation features are identified over the last 2 Ma and a remarkable 33 out of 36 occur when Earth's obliquity is anomalously large. During the early Pleistocene deglaciations occur nearly every obliquity cycle giving a 40 Ka timescale, while late Pleistocene deglaciations more often skip one or two obliquity beats, corresponding to 80 or 120 Ka glacial cycles which, on average, give the similar to 100 Ka variability. This continuous obliquity pacing indicates that the glacial theory can be simplified. An explanation for the similar to 100 Ka glacial cycles only requires a change in the likelihood of skipping an obliquity cycle, rather than new sources of long-period variability. Furthermore, changes in glacial variability are not marked by any single transition so much as they exhibit a steady progression over the entire Pleistocene. The mean, variance, skewness, and timescale associated with the glacial cycles all exhibit an approximately linear trend over the last 2 Ma. A simple model having an obliquity modulated threshold and only three adjustable parameters is shown to reproduce the trends, timing, and spectral evolution associated with the Pleistocene glacial variability. (c) 2006 Elsevier Ltd. All rights reserved.