Detection of significant climatic precession variability in early Pleistocene glacial cycles

Despite having a large influence on summer insolation, climatic precession is thought to account for little variance in early Pleistocene proxies of ice volume and deep-water temperature. Various mechanisms have been suggested to account for the dearth of precession variability, including meridional insolation gradients, interhemispheric cancellation of ice-volume changes, and antiphasing between the duration and intensity of summer insolation. We employ a method termed Empirical Nonlinear Orbital Fitting (ENOF) to estimate the amplitudes of obliquity and precession forcing in early Pleistocene proxies and their respective leads or lags relative to the timing of orbital variations. Analysis of a high-resolution North Atlantic benthic delta O-18 record, comprising data from IODP sites U1308 and U1313, indicates a larger precession contribution than previously recognized, with an average precession-to-obliquity amplitude ratio of 0.51 (0.30-0.76 95% confidence interval) in the rate-of-change of delta O-18 between 3 and 1 Ma. Averaged when eccentricity exceeds 0.05, this ratio rises to 1.18 (0.84-1.53). Additional support for precession's importance in the early Pleistocene comes from its estimated amplitude covarying with eccentricity, analyses of other benthic delta O-18 records yielding similar orbital amplitude ratios, and use of an orbitally-independent timescale also showing significant precession. Precession in phase with Northern Hemisphere summer intensity steadily intensifies throughout the Pleistocene, in agreement with its more common identification during the late Pleistocene. A Northern Hemisphere ice sheet and energy balance model run over the early Pleistocene predicts orbital amplitudes consistent with observations when a cooling commensurate with North Atlantic sea surface temperatures is imposed. These results provide strong evidence that glaciation is influenced by climatic precession during the late Pliocene and early Pleistocene, and are consistent with hypotheses that glaciation is controlled by Northern Hemisphere summer insolation. (C) 2020 Elsevier B.V. All rights reserved.