Climate “Noise” and the Cryosphere: New Constraints on the Evolution of Ice Sheets during the Cenozoic

Abstract:

The evolution of global climate during the past 34 million years is characterized by a transition from warm conditions with unipolar continental glaciation, to the bipolar ice sheets of the present. Much of our knowledge about the chronology of cryosphere development is based upon deep-sea benthic foraminiferal δ¹⁸O compilations that integrate global ice volume and regional deep ocean temperature. To deconvolve these mixed signals, the reconstruction of global ice volume typically requires the application of temperature proxy methods and/or global climate/ice-sheet models. These approaches have proven powerful, yet contribute their own uncertainty, particularly when the ice-volume signal is small. In this study we consider a novel means to constrain Cenozoic glaciation history through an assessment of the stochastic component of climate, using a new composite benthic d¹⁸O record, noise parameter estimation, and quantitative sedimentation models. We demonstrate that the relatively long climate response time diagnostic of continental ice sheets is imparted on observed stochastic benthic foraminiferal d¹⁸O variability, and is quantifiable using “noise response time” (NRT). Evaluation of changes in NRT using the new d¹⁸O composite identifies the proposed Cenozoic glaciation (“Oi” and “Mi”) events - as well as some new events - and provides new evidence for their magnitudes. Overall, assessment of the d¹⁸O composite in the context of stochastic variability provides unique insights into climate system history, complementing the evaluation of astronomical forcing/pacing, and yielding new quantitative constraints on previously ambiguous aspects of cryosphere development during the Cenozoic.