Strengthening Atlantic Inflow Across the Mid‐Pleistocene Transition

Stephen Barker,Sian Lordsmith,Lukas Jonkers,Xu Zhang,Stephen Conn,Gregor Knorr

doi:10.1029/2020pa004200

Abstract

AbstractThe development of larger and longer lasting northern hemisphere ice sheets during the mid‐Pleistocene Transition (MPT) coincided with global cooling. Here, we show that surface waters of the north‐eastern Atlantic actually warmed across this interval (∼1.2–0.8 Ma), which we argue reflects an increase in the north‐eastward transport of heat and moisture via the North Atlantic Current (NAC) into the Nordic Seas (the Atlantic Inflow). We suggest that simultaneous cooling and warming along the north‐western and south‐eastern margins (respectively) of the NAC during Marine Isotope Stage 28 (∼995 ka) reflected the increasing persistence of northern ice sheets as Atlantic Inflow increased. This resulted in a diachronous shift from ∼40 to ∼100 kyr cyclicity across the North East (NE) Atlantic as the growing influence of northern ice sheets spread southwards; to the north‐west of the NAC the first “100 kyr” cycle preceded Termination 12 (∼960 ka), while on the south‐eastern margin of the NAC the transition occurred ∼100 kyr later. Exploratory climate model simulations suggest that increasing Atlantic Inflow at this time could have accelerated ice sheet growth because pre‐existing moderately sized ice sheets allowed the positive effect of increasing precipitation to outpace melting. In addition, we propose that the dependence of post‐MPT ice sheets on moisture transport via the Atlantic Inflow may ultimately contribute to their apparent vulnerability to insolation forcing once a critical size threshold is crossed and high latitude ice sheets become starved of a vital moisture source.

Highlights

The Mid-Pleistocene Transition and First “Missed” Interglacial: “IG 28”The characteristic saw-tooth shape and long (∼100 kyr) duration of glacial cycles within the Late Pleistocene emerged sometime around 1 million years ago during the mid-Pleistocene Transition (MPT, Clark et al, 2006; McClymont et al, 2013) (Figure 1)
We have shown that the transition from ∼41 to ∼100 kyr periodicity was diachronous across the North East (NE) Atlantic, providing a longer timescale analogy to the time transgressive cooling observed both at orbital (Alonso-Garcia et al, 2011; Wright & Flower, 2002) and millennial-timescales (Barker et al, 2015) in this region
In the case of the MPT, we suggest that such diachoronicity reflected the increasing influence of expanding ice sheets superimposed on the increasing transport of heat via the North Atlantic Current (NAC)

Summary

Introduction

The characteristic saw-tooth shape and long (∼100 kyr) duration of glacial cycles within the Late Pleistocene emerged sometime around 1 million years ago during the mid-Pleistocene Transition (MPT, Clark et al, 2006; McClymont et al, 2013) (Figure 1). Glacial cycles were largely symmetric in form with a period of ∼41 kyr, more closely reflecting the influence of northern hemisphere summer insolation (Tzedakis et al, 2017). The MPT heralded the emergence of longer glacial cycles, and the appearance of much larger ice sheets (Elderfield et al, 2012), despite no apparent change in orbital forcing (Figure 1). At the same time (and somewhat counterintuitively), it should accommodate the apparent vulnerability of these new “super glacial” ice sheets to rather modest insolation forcing once they exceed some critical size (Raymo, 1997)

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Conclusion