Arctic sea ice export as a driver of deglacial climate

Anthony J Joyce,Alan Condron,Raymond S Bradley

doi:10.1130/g47016.1

Anthony J Joyce, Alan Condron + Show 1 more

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https://doi.org/10.1130/g47016.1

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Abstract

Abstract A widespread theory in paleoclimatology suggests that changes in freshwater discharge to the Nordic (Greenland, Norwegian, and Icelandic) Seas from ice sheets and proglacial lakes over North America played a role in triggering episodes of abrupt climate change during deglaciation (21–8 ka) by slowing the strength of the Atlantic Meridional Overturning circulation (AMOC). Yet, proving this link has been problematic, as climate models are unable to produce centennial-to-millennial–length reductions in overturning from short-lived outburst floods, while periods of iceberg discharge during Heinrich Event 1 (ca. 16 ka) may have occurred after the climate had already begun to cool. Here, results from a series of numerical model experiments are presented to show that prior to deglaciation, sea ice could have become tens of meters thick over large parts of the Arctic Basin, forming an enormous reservoir of freshwater independent from terrestrial sources. Our model then shows that deglacial sea-level rise, changes in atmospheric circulation, and terrestrial outburst floods caused this ice to be exported through Fram Strait, where its subsequent melt freshened the Nordic Seas enough to weaken the AMOC. Given that both the volume of ice stored in the Arctic Basin and the magnitude of the simulated export events exceed estimates of the volumes and fluxes of meltwater periodically discharged from proglacial Lake Agassiz, our results show that non-terrestrial freshwater sources played an important role in causing past abrupt climate change.

Highlights

The climate of the last deglaciation is marked by a series of abrupt changes in temperature
Large pieces of thick sea ice were still being reported in the early 20th century, including one ∼25-km-long and 15-m-thick on which the Norwegian explorer, Storker Storkerson, spent 6 months adrift (Stefansson, 1921), while similar-sized floes were used as scientific research stations during the Cold War (Walker and Wadhams, 1979; Fig. 1)
Our results show that the thick ‘palaeocrystic ice’ Sir George Nares reported in the Arctic Ocean over 140 years ago during the British Arctic Expedition (Nares, 1878) would likely have covered much of the Arctic Basin prior to deglaciation, creating an enormous reservoir of freshwater, independent from terrestrial sources

Summary

Introduction

The climate of the last deglaciation is marked by a series of abrupt changes in temperature. A sustained switch in the meltwater drainage route to the ocean—in response to a change in the position of the southern margin of the Laurentide ice sheet (LIS)—is frequently hypothesized to have played a role in triggering deglacial cooling by altering the delivery of meltwater to sites of deep convection that regulate North Atlantic Deepwater (NADW) formation (e.g., Clark et al, 2001). As the YD is widely viewed as a time of glacial re-advance and reduced terrestrial meltwater discharge to the ocean, it is likely that freshwater forcing was less during this period (Abdul et al, 2016). A similar dichotomy surrounds the cooling during Heinrich Event 1: while this cold stadial was originally hypothesized to have been triggered by icebergs freshening the ocean (Broecker, 1994), recent findings suggest that the cooling might have begun prior to significant ice rafting, such that freshwater forcing (from ice sheets) played a relatively minor role (Barker et al, 2015)

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