Iceberg discharges of the last glacial period driven by oceanic circulation changes

Abstract

Proxy data reveal the existence of episodes of increased deposition of ice-rafted detritus in the North Atlantic Ocean during the last glacial period interpreted as massive iceberg discharges from the Laurentide Ice Sheet. Although these have long been attributed to self-sustained ice sheet oscillations, growing evidence of the crucial role that the ocean plays both for past and future behavior of the cryosphere suggests a climatic control of these ice surges. Here, we present simulations of the last glacial period carried out with a hybrid ice sheet-ice shelf model forced by an oceanic warming index derived from proxy data that accounts for the impact of past ocean circulation changes on ocean temperatures. The model generates a time series of iceberg discharge that closely agrees with ice-rafted debris records over the past 80 ka, indicating that oceanic circulation variations were responsible for the enigmatic ice purges of the last ice age.glacial climate variability | climate modeling | abrupt changes C ompared with the present interglacial period, the last glacial period (LGP) (∼110-10 ka before the present), and almost certainly previous ones (1), were characterized by substantial climatic variability on millennial timescales. This variability is mainly manifested in two types of events. Dansgaard-Oeschger (D/O) events are most notable in Greenland ice core records and involve decadal-scale warming of more than 10 K (interstadials) followed by slow cooling lasting several centuries and a final more rapid fall to cold background (stadial) conditions (2). Heinrich (H) events consist of massive iceberg discharges from the Laurentide Ice Sheet at intervals of ∼7 ka during peak glacial conditions throughout the LGP (3). Both D/O and H events are associated with widespread centennial-to millennial-scale climatic changes, including a synchronous temperature response over the North Atlantic and an antiphase temperature relationship over Antarctica and most of the Southern Ocean, as revealed by a wealth of deep-sea sediments, ice core, and terrestrial records (4). The Atlantic meridional overturning circulation (AMOC) is thought to play a central role in these abrupt glacial climatic changes. Although the paleoceanographic evidence on this link is scarce and mostly restricted to a few highresolution deep-sea sediment records of the last deglaciation (5, 6), both modeling studies and reconstructions provide strong support for the hypothesis that D/O events were caused by reorganizations of the AMOC (7,8). H events, identified as enhanced ice-rafted detritus (IRD) in North Atlantic deep-sea sediments (3, 9), occur during climatic minima of the Northern Hemisphere. They have classically been attributed to internal oscillations of the Laurentide (10) and assumed to lead to important disruptions of the Atlantic Ocean circulation (11). However, paleoclimate data have revealed that most H events likely occurred about a thousand years after North Atlantic Deep Water (NADW) formation had already slowed down or largely coll...

Deglacial ice sheet meltdown: orbital pacemaking and CO<sub>2</sub> effects

“…However, the smaller heat transport in the Atlantic may have been compensated by an intensified heat transport in the Pacific (Okazaki et al, 2010). And the subsequent recovery of the Atlantic meridional overturning circulation (Liu et al, 2009) may have accelerated the deglaciation again, for example via the destabilization of grounded ice due to the collapse of shelf ice (Alvarez-Solas et al, 2013). In the accelerated experiments presented here, the ice sheets and the oceans do not exchange freshwater (Sect.…”

“…Ice shelves, which are the floating extensions of the grounded ice sheets, and the dynamics of the grounding line are not included. Note that therefore, the destabilization of grounded ice and acceleration of ice streams via the collapse of shelf ice, which is a process that was potentially important during the deglaciation (Alvarez-Solas et al, 2013), is not accounted for in this study. Ice sheet growth is restricted to a predefined geographic domain, which is kept fixed throughout the simulations (Fig.…”

Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

“…In contrast, instabilities from the Laurentide ice sheet occurred less frequently but were associated with much larger iceberg and freshwater discharges, leading to complete AMOC shutdown and larger SST and hydroclimate changes in the North Atlantic realm and beyond. Changes in sea level during Heinrich events and subsurface temperatures (Shaffer et al, 2004;Mignot et al, 2007;Alvarez-Solas et al, 2010;Marcott et al, 2011;Alvarez-Solas et al, 2013) (Fig. 10) may have subsequently triggered marine-ice-sheet instabilities, thus increasing the initial freshwater discharge.…”

“…Subsequently the IRD composite (orange) was calculated. Timmermann et al, 2003;Shaffer et al, 2004;Alvarez-Solas et al, 2010;Marcott et al, 2011;Alvarez-Solas et al, 2013).…”

From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model

“…Periodic oscillations of ice-sheet growth and surging have been suggested either to be of unforced type and thus originating from ice-internal mechanisms ("binge-purge" oscillations, see or to be driven externally, i.e., by climate forcing (e.g., Hulbe, 2004;Alvarez-Solas et al, 2013;Bassis et al, 2017). Numerical modeling studies that investigated ice-sheet-intrinsic surging include the demon-stration of creep instability (Clarke et al, 1977) and hydraulic runaway (Fowler and Johnson, 1995) as possible main feedbacks that drive unforced surging, the application to the Laurentide Ice Sheet to simulate its quasi-periodic surging (Marshall and Clarke, 1997;Calov et al, 2002Calov et al, , 2010Greve et al, 2006;Papa et al, 2006;Roberts et al, 2016), the simulation of (cyclic) ice streaming and stagnation reminiscent of the flow variability of the Siple Coast ice streams (Alley, 1990;Pattyn, 1996;Payne and Dongelmans, 1997;Fowler and Schiavi, 1998;Bougamont et al, 2011;Van Pelt and Oerlemans, 2012;Robel et al, 2013) and the investigation of ice-stream oscillations in interaction with bed topography under the influence of ice-shelf buttressing (Robel et al, 2016).…”