Abstract

Abstract. ​​​​​​​Abrupt climate changes are defined as sudden climate changes that took place over tens to hundreds of years or recurred at millennial timescales; they are thought to involve processes that are internal to the climate system. By contrast, astronomically forced climate changes involve processes that are external to the climate system and whose multi-millennial quasi-periodic variations are well known from astronomical theory. In this paper, we re-examine the main climate variations determined from the U1308 North Atlantic marine record, which yields a detailed calving history of the Northern Hemisphere ice sheets over the past 3.2 Myr. The magnitude and periodicity of the ice-rafted debris (IRD) events observed in the U1308 record allow one to determine the timing of several abrupt climate changes, the larger ones corresponding to the massive iceberg discharges labeled Heinrich events (HEs). In parallel, abrupt warmings, called Dansgaard–Oeschger (DO) events, have been identified in the Greenland records of the last glaciation cycle. Combining the HE and DO observations, we study a complex mechanism giving rise to the observed millennial-scale variability that subsumes the abrupt climate changes of last 0.9 Myr. This process is characterized by the presence of Bond cycles, which group DO events and the associated Greenland stadials into a trend of increased cooling, with IRD events embedded into every stadial, the latest of these being an HE. These Bond cycles may have occurred during the last 0.9 Ma when Northern Hemisphere ice sheets reached their maximum extent and volume, thus becoming a major player in this time interval's climate dynamics. Since the waxing and waning of ice sheets during the Quaternary period are orbitally paced, we conclude that the abrupt climate changes observed during the Middle Pleistocene and Upper Pleistocene are therewith indirectly linked to the astronomical theory of climate.

Highlights

  • – Present investigations – including both recurrence plot (RP) analysis and Kolmogorov–Smirnov (KS) methodology – point to internal mechanisms being responsible for these millennial-scale events and for the associated abrupt changes

  • – The Bond cycles are linked to the dynamics of the Northern Hemisphere ice sheets, to variations in their spatial extent and their elevation

  • The classical Bond cycles end with massive iceberg discharges into the North Atlantic Ocean mainly from the Laurentide ice sheet and from the Fennoscandian, Greenland, Iceland, and British ice sheets

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Summary

Introduction

Well-dated geological data indicate that the Earth experienced orbitally paced climate changes from at least the late Precambrian, i.e., 1.4 billion years ago during the Proterozoic Eon (Benn et al, 2015; Zhang et al, 2015; Hoffman et al, 2017; Meyers and Malinverno, 2018), and all along the Phanerozoic (Lisiecki and Raymo, 2005; Liebrand et al, 2011; Miller et al, 2011; Kent et al, 2017, 2018; Olsen et al, 2019; Drury et al, 2021; Westerhold et al, 2020). As the Southern Hemisphere receives less solar radiation per year than the Northern Hemisphere (Adhemar was relying on the number of the nights at the boreal pole versus the number of nights at the austral pole), this contributed to keeping temperatures cold enough to allow ice sheets to build up It was 5 decades later that Croll (1890) presented his theory of the ice ages being driven by the changing distance between the Earth and the Sun as measured on 21 December, which is due to the eccentricity of the Earth’s orbit and the precession of the equinoxes. The δ18O record best describes the more or less regular recurrence of cold and warm events that we are analyzing; see Fig. 1c

The records
The methods
Millennial-scale variability
DO events and bond cycles
Concluding remarks
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