Arctic Sea Ice Export As a Driver of Deglacial Climate Alan Condron1*, Anthony J
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https://doi.org/10.1130/G47016.1 Manuscript received 17 September 2019 Revised manuscript received 9 December 2019 Manuscript accepted 18 December 2019 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Arctic sea ice export as a driver of deglacial climate Alan Condron1*, Anthony J. Joyce2 and Raymond S. Bradley2 1 Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 2 Geosciences, University of Massachusetts at Amherst, Amherst, Massachusetts 01003, USA ABSTRACT nial-to-millennial–length cooling (Meissner and A widespread theory in paleoclimatology suggests that changes in freshwater discharge Clark, 2006; Renssen et al., 2015). A sustained to the Nordic (Greenland, Norwegian, and Icelandic) Seas from ice sheets and proglacial switch in the meltwater drainage route to the lakes over North America played a role in triggering episodes of abrupt climate change dur- ocean—in response to a change in the position ing deglaciation (21–8 ka) by slowing the strength of the Atlantic Meridional Overturning of the southern margin of the Laurentide ice circulation (AMOC). Yet, proving this link has been problematic, as climate models are un- sheet (LIS)—is also frequently hypothesized to able to produce centennial-to-millennial–length reductions in overturning from short-lived have played a role in triggering deglacial cool- outburst floods, while periods of iceberg discharge during Heinrich Event 1 (ca. 16 ka) may ing by altering the delivery of meltwater to sites have occurred after the climate had already begun to cool. Here, results from a series of of deep convection that regulate North Atlan- numerical model experiments are presented to show that prior to deglaciation, sea ice could tic Deepwater (NADW) formation (e.g., Clark have become tens of meters thick over large parts of the Arctic Basin, forming an enormous et al., 2001). Indeed, numerical model experi- reservoir of freshwater independent from terrestrial sources. Our model then shows that ments show that changing the main meltwater deglacial sea-level rise, changes in atmospheric circulation, and terrestrial outburst floods drainage route of the LIS from the Mississippi caused this ice to be exported through Fram Strait, where its subsequent melt freshened River to either the northwestern Atlantic Ocean the Nordic Seas enough to weaken the AMOC. Given that both the volume of ice stored in or the Arctic Ocean results in a significant reduc- the Arctic Basin and the magnitude of the simulated export events exceed estimates of the tion in the AMOC (Maier-Reimer and Mikolaje- volumes and fluxes of meltwater periodically discharged from proglacial Lake Agassiz, our wicz, 1989; Manabe and Stouffer, 1995; Clark results show that non-terrestrial freshwater sources played an important role in causing past et al., 2001; Peltier et al., 2006). However, as abrupt climate change. the YD is widely viewed as a time of glacial re-advance and reduced terrestrial meltwater INTRODUCTION is thought to have routed meltwater from the discharge to the ocean, it is likely that freshwa- The climate of the last deglaciation is Mississippi River to the St. Lawrence and/or ter forcing was less during this period (Abdul marked by a series of abrupt changes in tem- Mackenzie Rivers so that it entered the ocean et al., 2016). A similar dichotomy surrounds perature. Of particular note is the Younger closer to sites of deep water formation that the cooling during Heinrich Event 1: while this Dryas (YD) episode at ca. 12.9 ka that is often modulate the strength of the Atlantic Meridi- cold stadial was originally hypothesized to have described as a millennial-length rapid return to onal Overturning circulation (AMOC) and its been triggered by icebergs freshening the ocean glacial-like conditions over much of the North associated northward heat transport (Kennett (Broecker, 1994), recent findings suggest that Atlantic region (Alley, 2000). While a variety and Shackleton, 1975; Manabe and Stouffer, the cooling might have begun prior to significant of mechanisms have been proposed to explain 1995; Murton et al., 2010). In addition, fresh- ice rafting, such that freshwater forcing (from this cooling, including a volcanic eruption, water discharge from the Fennoscandian Ice ice sheets) played a relatively minor role (Barker meteorite impact, and changes in atmospheric Sheet and/or Baltic Ice Lake may have played et al., 2015). circulation (Wunsch, 2006; Firestone et al., a role in triggering the initial onset of cooling Here, we investigate whether changes in the 2007; Eisenman et al., 2009; Baldini et al., (Muschitiello et al., 2016). storage and export of Arctic sea ice to the sub- 2018), considerable research has postulated Nevertheless, unraveling the link between polar North Atlantic played a role in modulat- that the YD, and other periods of abrupt cool- times of increased meltwater input and cool- ing deglacial climate by periodically weakening ing such as the Preboreal Oscillation (PBO; ing remains difficult (Broecker, 2006; Renssen the overturning cell. While this mechanism was ca. 11.3 ka) and the Older Dryas (ca. 14 ka), et al., 2015). While the AMOC in most climate previously proposed by Bradley and England were triggered by variations in terrestrial melt- models is sensitive to small changes in fresh- (2008) as trigger for the YD, it has never been water discharge to the ocean (Broecker et al. water forcing (∼0.1 Sv; Sv = 106 m3/s; Stouffer explicitly tested. Marine proxies do, however, 1989; Clark et al., 2001; Teller et al., 2002). et al., 2006), these models indicate that short- support the idea that the YD was both a time of For the YD, a major switch in North Ameri- duration outburst floods—like the ones thought increased sea-ice drift and ice export to the sub- can glacial drainage patterns around this time to have occurred as new drainage outlets periodi- polar North Atlantic (Not and Hillaire-Marcel, cally opened—would not have led to any long- 2012; Müller and Stein, 2014; Müller, 2016). We term weakening of the large-scale AMOC and, suspect that additional sea-ice export events also *E-mail: [email protected] as such, were unlikely to have caused centen- played a role in triggering, or enhancing, other CITATION: Condron, A., Joyce, A.J., and Bradley, R.S., 2020, Arctic sea ice export as a driver of deglacial climate: Geology, v. 48, p. XXX–XXX, https://doi.org/10.1130/G47016.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47016.1/4937459/g47016.pdf by guest on 21 February 2020 periods of climate cooling during deglaciation. RESULTS which the Norwegian explorer, Storker Storker- For example, the drainage of meltwater into the Many of the diaries and journals written in son, spent 6 months adrift (Stefansson, 1921), Arctic Ocean as the Cordilleran-Laurentide ice the 19th and early 20th centuries provide direct while similar-sized floes were used as scientific saddle collapsed at ca. 14.5 ka likely created a evidence that vast areas of the Arctic Basin were research stations during the Cold War (Walker large ice export event to the North Atlantic that once covered by ice considerably thicker than and Wadhams, 1979; Fig. 1). These regions of could have enhanced the Older Dryas cooling observed over the past 30–40 years. For instance, thick ice had “long, prairie-like swells” (Peary, previously proposed to have been triggered by in 1875, Sir George Nares (Nares, 1878) intro- 1907, p. 181) typical of modern-day ice shelves, this meltwater event (Gregoire et al., 2012; Iva- duced the term “palaeocrystic ice,” to describe but unlike ice shelves around the Antarctic, this novic et al., 2017). A similar ice export mecha- the unique and exceptionally old and thick ice ice was not fed by glaciers. Instead, these re- nism at ca. 11.3 ka was also proposed to have his expedition encountered. This “sea of ancient gions of ice were composed of sea ice repeat- amplified the climatic cooling during the PBO ice,” as he named it, was found to extend along edly thickened by the accumulation of snow and (Fisher et al., 2002). the north coast of Arctic Canada for >450 km superimposed ice on the surface, as well as the More recently, Thornalley et al. (2018) pro- (Markham, 1878). In other accounts, Cook freezing of seawater onto the underside of the posed that a similar mechanism might explain (1911, p. 266) noted, “… from the 87th to the 88th ice (Dowdeswell and Jeffries, 2017). the pre-industrial slowdown of the AMOC parallel we passed for 2 days over old ice without Looking farther back in time, marine sedi- whereby the enhanced export of sea ice to the pressure lines or hummocks…, but the ice had ment records from the central Arctic suggest Nordic Seas formed during the Little Ice Age the hard, wavering surface of glacial ice with sea ice continuously covered the Arctic Basin had suppressed the sinking limb of the over- only superficial crevasses.” At the end of the 19th during glacial periods. For example, zero, or turning cell. Indeed, modern-day observations century, this region of thick Arctic sea ice was near-zero, biomarker concentrations and a hiatus and modeling show that changes in Arctic sea- estimated to have occupied an area of ∼8900 km2 in sediment deposition indicate that during the ice export play an important role in modulating (Vincent et al., 2001), while direct measurements Last Glacial Maximum (LGM; ca. 18 ka) the the AMOC (Liu et al., 2019). For example, the showed that the ice was at least 35–50 m thick central Arctic Ocean (north of 84°N) was cov- Great Salinity Anomaly in the subpolar North (Crary, 1958).