Bispectra of climate cycles show how ice ages are fuelled

Abstract

Abstract. The increasingly nonlinear response of the climate–cryosphere system to insolation forcing during the Pliocene and Pleistocene, as recorded in benthic foraminiferal stable oxygen isotope ratios (δ18O), is marked by a distinct evolution in ice-age cycle frequency, amplitude, phase, and geometry. To date, very few studies have thoroughly investigated the non-sinusoidal shape of these climate cycles, leaving precious information unused to further unravel the complex dynamics of the Earth's system. Here, we present higher-order spectral analyses of the LR04 δ18O stack that describe coupling and energy exchanges among astronomically paced climate cycles. These advanced bispectral computations show how energy is passed from precession-paced to obliquity-paced climate cycles during the Early Pleistocene (from ∼2500 to ∼750 ka) and ultimately to eccentricity-paced climate cycles during the Middle and Late Pleistocene (from ∼750 ka onward). They also show how energy is transferred among many periodicities that have no primary astronomical origin. We hypothesise that the change of obliquity-paced climate cycles during the mid-Pleistocene transition (from ∼1200 to ∼600 ka), from being a net sink into a net source of energy, is indicative of the passing of a land-ice mass loading threshold in the Northern Hemisphere (NH), after which cycles of crustal depression and rebound started to resonate with the ∼110 kyr eccentricity modulation of precession. However, precession-paced climate cycles remain persistent energy providers throughout the Late Pliocene and Pleistocene, which is supportive of a dominant and continuous fuelling of the NH ice ages by insolation in the (sub)tropical zones, and the control it exerts on meridional heat and moisture transport through atmospheric and oceanic circulation.