Abstract
The global LR04 δ18O, the tropical ODP Site 846 sea surface temperature (SST), and the global ΔSST stack records were investigated using the advanced method for time-series decomposition singular spectrum analysis to outline the quantitative role of orbital forcings and to investigate the nonlinear dynamics of the Pliocene and Pleistocene climate system. For the first time, a detailed quantitative evaluation is provided of the δ18O and SST variance paced by long-period orbital modulation, short eccentricity, obliquity, precession, and half-precession cycles. New insights into the nonlinear dynamic of the orbital components suggest considering astronomical signals as composite feedback lagged responses paced by orbitals and damped (Early Pliocene) or amplified (Mid-Late Pleistocene) in a range of − 100 to + 400% the forcing. The Early Pliocene asymptotic decay of the δ18O and SST response sensitivity up to − 100% observed for the first time in all orbital responses is interpreted as damping effect of a wide global forest cover along with a possible high ocean primary productivity, through the CO2-related negative feedbacks during time of global greenhouse. An anomalous post-Mid-Pleistocene Transition (MPT) sharply declines to near-zero in obliquity response sensitivity observed in both global δ18O and tropical SST, suggesting an attenuation mechanism of the obliquity driving force and a reduction of the related feedback amplification processes. It is hypothesized the post-MPT obliquity damping has contributed to the strengthening of the short eccentricity response by mitigating the obliquity “ice killing”, favoring a long-life ice sheet sensitive to a synergistic ~ 100-kyr amplification of positive feedback processes during the time of a global icy state. The global δ18O, the tropical SST, and the global ΔSST trend components, all explaining ~ 76% of the Plio-Pleistocene variance and significantly modifying the mean climate state, appear to be related to the long-term pCO2 proxies, supposedly controlled by plate tectonics through the global carbon cycle (CO2 outgassing, explosive volcanism, orography and erosion, paleogeography, oceanic paleocirculation, and ocean fertilization). Finally, singular spectrum analysis provides a valuable tool in cyclostratigraphy with the remarkable advantage of separating full-resolution time series by variance strength.
Highlights
The Milankovitch theory elegantly describes the collective effects of changes in the Earth’s movements on its climate
The main conclusions of this study can be summarized as follows: 1) A detailed evaluation is provided of the variance paced by long-period orbital modulation, short eccentricity, obliquity, precession, and half-precession cycles, estimated for the δ18O at approximately 4.2%, 6.5%, 9.9%, 2.0%, and 0.6%, respectively, and for the tropical sea surface temperature (SST) at about 10.3%, 5.1%, 5.5%, 1.6%, and 0.5%, respectively, both resulting in total orbital contributions of ~ 23.0%
The asymmetry of the Response sensitivity (Rs) suggests an amplified orbital response during times of a global icehouse state (Mid-Late Pleistocene, with the exception of obliquity), which can reach a magnitude of + 200 to + 400% global δ18O forcing, or + 100 to + 170% the tropical SST forcing, and a minimum up to − 100% the damped response for all signals during time of a global greenhouse state (Early Pliocene)
Summary
The Milankovitch theory elegantly describes the collective effects of changes in the Earth’s movements on its climate. A lot of studies have analyzed a variety of records finding the Milankovitch frequencies, but few have quantitatively investigated the hypothesis that most of the energy conveyed in paleoclimate records is not related to orbital forcing (Kominz and Pisias 1979; Raymo 1994; Wunsch 2004; Mudelsee and Raymo 2005; Hansen et al 2013). The aim of this study is to investigate these topics by decomposing two selected δ18O and sea surface temperature (SST) records to outline the quantitative role of astronomical forcings and the nonlinear behavior in the orbital responses and to assess the Earth’s long-term controlling factors as a “stand-alone” climate system and its interaction with the astronomical one. A link among the atmospheric pCO2, the sea surface temperature, and the δ18O long-term trend components is provided to support the hypothesis of a dominant non-astronomical control on the Earth’s climate
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