Timing of the descent into the last Ice Age determined by the bipolar seesaw

Stephen Barker,Paula Diz

doi:10.1002/2014pa002623

Abstract

We present planktonic foraminiferal fauna and isotope records from the SE Atlantic that highlight the nature of millennial-scale variability over the last 100 kyr. We derive an hypothesis-driven age model for our records based on the empirical link between variations in Greenland temperature, ocean circulation and carbonate preservation in the deep SE Atlantic. Our results extend earlier findings of an anti-phase (seesaw) relationship between north and south for the largest abrupt events of Marine Isotope Stage (MIS) 3-2 and the last deglaciation. In particular we find that Heinrich Stadials were paralleled by inferred southward shifts of the thermal Subtropical Front. These were followed by pronounced rebounds of the front with the return to interstadial conditions in the north. Our results also shed light on the mechanism of glaciation. In contrast to the last deglaciation, which was a globally symmetric change superposed by interhemispheric asynchronicity, we find that the descent into full glacial conditions at the onset of MIS 4 (~70 ka) displayed interhemispheric synchrony. We suggest that this globally synchronous descent into glacial MIS 4 was preconditioned by orbital changes but that the timing was ultimately determined by abrupt changes in ocean / atmosphere circulation patterns i.e. the bipolar seesaw.

Highlights

It is commonly argued that Earth’s glacial cycles are driven by changes in orbital geometry [Imbrie et al, 1993], but it is acknowledged that nonlinearities and feedbacks within the climate system are required in order to explain the precise timing and magnitude of climatic change that is observed [Broecker and Denton, 1989; Imbrie et al, 1993]
We called on a latitudinal shift in the position of the thermal Subtropical Front (STF) to explain the abrupt assemblage changes we observed during Termination 1 (T1)
Given the large latitudinal gradients in sea surface temperature associated with the frontal zones in this region [Deacon, 1982; Orsi et al, 1995] (Figure 4a), we feel that this is more reasonable than calling on large temperature changes of specific water masses, which would be contrary to modeling studies [Vellinga and Wood, 2002; Timmermann et al, 2007]

Summary

Introduction

It is commonly argued that Earth’s glacial cycles are driven by changes in orbital geometry [Imbrie et al, 1993], but it is acknowledged that nonlinearities and feedbacks within the climate system are required in order to explain the precise timing and magnitude of climatic change that is observed [Broecker and Denton, 1989; Imbrie et al, 1993]. The fact that interglacial to glacial (IG-G) transitions are associated with enhanced millennial-scale climate variability [Sima et al, 2004; Barker et al, 2011] begs the question as to whether or not such variability might play an active role in glacial development. We present new records from the SE Atlantic that we argue provide evidence for an active role of the bipolar seesaw in the most recent IG-G transition.

Methods

Results

Conclusion