Eccentricity Forcing and Preconditioning of the Super Interglacials

Abstract:

Global climate during Quaternary and Late Pliocene (present–3.1 Ma) is characterized by alternating glacial and interglacial conditions. Several proposed theories associate these cycles with variations in the Earth’s orbital configuration. There is geological evidence of obliquity forcing of the glacial-interglacial cycles during the Late Pliocene/Early Pleistocene (41 kyr world), which stands in sharp contrast to the primary cyclicity of insolation at precessional periods (23 kyr). Here we compare proposed theories of two separate studies that attempt to explain this anomaly (Huybers 2006 & Raymo et al. 2006). Huybers proposed that glacial cycles are driven by the total integrated summer insolation, which varies at the obliquity period and is thus consistent with the glacial records. Raymo proposed that glacial cycles are driven by summer insolation at precessional periods, but hemispheric responses are out-of-phase, thus global proxies do not record the precession signal.

Using a metric for the total integrated insolation, we study how obliquity and precession affects Northern and Southern hemisphere glaciation-deglaciation. We carried out a series of GCM simulations using a range of obliquity and precessional forcings. By repeating the experiment at different eccentricities, the obliquity and precessional response of both hemispheres was studied. Model results show obliquity response between hemispheres is in-phase at all eccentricities and precessional response is out-of-phase at high eccentricities. At low eccentricities (<0.015), the precessional response on the summer metric is negligible. We suggest that at eccentricities less that 0.015, glaciation-deglaciation processes occur concurrently in both hemispheres and are primarily forced by obliquity. This has important implications for the forcing of super interglacials (MIS 9, 11, 31 etc.). Indeed, it has been observed statistically that most super interglacials are forced by extreme low eccentricities, albeit with a lag of ~53ka. We explore how these low eccentricity conditions may have preconditioned the polar regions to produce a large response during some interglacials.