Multiple instabilities and modes of glacial rhythmicity in the plio-Pleistocene: a general theory of late Cenozoic climatic change

Abstract

It has been noted that several distinct modes of glacial oscillation have existed during the past few million years, ranging from low-amplitude, high-frequency oscillations in the early Pliocene, through relatively high amplitude, predominantly near-40 ky period, oscillations in the late Pliocene and early Pleistocene, to the major near-100 ky period oscillations of the late Pleistocene. In addition to other plausible mechanisms suggested previously to explain aspects of this multirhythmic phenomenon, we now illustrate another possible contributor to this type of behavior based on the hypothesis that the slow-response climatic system is bistable and that two kinds of internal instability may be operative along with externally imposed forcing due to earth-orbital (Milankovitch) radiation changes and slow, tectonically-induced changes in atmospheric carbon dioxide. These two instabilities have been discussed previously: one is due to positive feedback in the global carbon cycle leading to near-100 ky free oscillations of the ice sheets, and the other is due to the potential for ice-calving catastrophes associated with bedrock variations that can lead to oscillations of a period near 40 ky, independent of obliquity forcing. Within the framework of a dynamical model containing the possibility for these two instabilities, as well as for stable modes, we show (1) how Milankovitch radiative changes or stochastic forcing influencing ice sheets can induce aperiodic (chaotic) transitions between the possible stable and unstable modes, and more significantly, (2) how progressive, long-term, tectonically-induced, changes in carbon dioxide, acting in concert with earth-orbital radiative variations in high Northern Hemisphere latitudes, can force systematic transitions between the modes. Such systematic changes can result in an ice mass chronology for the past 5 My that is qualitatively similar to the observed record of global ice mass. In essence, we have constructed a minimum dynamical model of the late Cenozoic climatic changes, containing what are believed to be the main physical factors determining these changes: ice mass, bedrock depression, atmospheric carbon dioxide concentration, deep ocean thermohaline state, Milankovitch radiation forcing, and slow tectonically-induced carbon dioxide forcing. This model forms the basis for a coherent theory for the complex climatic events of this long period.