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 ±10Ka in the early Pleistocene (∼2∼2–1 Ma) and ±7Ka in the late Pleistocene (∼1Ma to the present). Twelve benthic and five planktic δ18O 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 ∼100Ka variability. This continuous obliquity pacing indicates that the glacial theory can be simplified. An explanation for the ∼100Ka 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.