1 Introduction

Since the Last Glacial Maximum (LGM) around 23–21 thousand years ago (ka), the Earth underwent a major transition into the current interglacial period, during which the North American and Eurasian continents deglaciated entirely, and the Greenland and Antarctic ice sheets, as well as glaciers worldwide, retreated. In total, this produced around 130 m of global mean sea level rise (GMSLR) [Lambeck et al., 2014], which was sometimes contributed to by major episodes of accelerated ice melt. Meltwater Pulse 1a (MWP1a) is the largest of these, identified as a 14–18 m of GMSLR in less than 340 years at 14.6 ka [Deschamps et al., 2012] in coral reef records from Tahiti and Barbados, as well as other sea level proxies around the world. This event also occurred around the time of an abrupt Northern Hemisphere warming of 4–5°C that took place within a few decades to centuries [Buizert et al., 2014; Deschamps et al., 2012]. However, the link between this intense ice melt and warming remains elusive. This is partly because of the imprecise chronology of events, and also because the origin of the MWP1a is uncertain. Ice melt can significantly disturb ocean circulation producing widespread changes in surface climate, but this impact is different (and can be opposite) depending on whether the ice melt goes into the Arctic, North Atlantic, or Southern Ocean [Clark, 2001; Ivanovic et al., 2016; Menviel et al., 2011; Peltier et al., 2006]. It is therefore important to know how much the respective ice sheets each contributed to the event.

The source of MWP1a has previously been assessed through fingerprinting the pattern of sea level rise/fall caused by the change in gravitational pull exerted by an ice mass on oceans. Although initial studies rejected a major North American ice sheet (NAIS) source, recent work overturned this finding and suggested it could actually have contributed up to 10 m to the total sea level change in 340 years [Gomez et al., 2015]. However, the large uncertainties in sea level reconstructions make it impossible to discriminate between a major NAIS contribution and a 100% contribution from the Antarctic ice sheet [Gomez et al., 2015; Liu et al., 2016].

For the Antarctic ice sheet, direct constraints on changes in thickness or extent around the time of MWP1a are limited and debated. Southern Ocean records of iceberg‐rafted debris [Weber et al., 2014] show that the largest iceberg fluxes occur around the time of MWP1a (14.6 ka). Moreover, numerical ice sheet modeling [Golledge et al., 2014] suggests that Southern Ocean overturning triggered up to 2 m sea level equivalent of ice loss in Antarctica in 340 years, a significant but relatively small contribution to the 14 m GMSLR at MWP1a [Deschamps et al., 2012]. In contrast, North American ice sheet reconstructions show major ice sheet changes around the time of MWP1a [Dyke, 2004; Gowan et al., 2016; Peltier et al., 2015; Tarasov et al., 2012]. Moreover, Gregoire et al. [2012, hereafter “G12”] provided a mechanistic explanation for a major NAIS contribution to this event, showing that the Cordilleran‐Laurentide ice sheet separation caused accelerated ice melt due to a height‐mass balance feedback triggered by gradual climate forcing. This “saddle collapse” mechanism in North America produced 7 m of GMSLR in 350 years (10 m in 500 years).

From geological constraints, exactly when the separation of the two ice sheets took place is uncertain [Dyke, 2004]. It is clear from the synthesis of these data that the separation of the Cordilleran and Laurentide ice sheet, which caused the saddle collapse and accelerated ice melt, could not have occurred after 14 ka and thus could not have corresponded to Meltwater Pulse 1b (11.3 ka) [Abdul et al., 2016]. The separation of the two ice sheets likely occurred between 16 ka and 14 ka [Dyke, 2004], overlapping with the timing of MWP1a (~14.6 ka). Thus, G12 suggested that the Cordilleran‐Laurentide saddle collapse could have contributed around half of MWP1a.

Given their close timing, it is compelling to think that at least part of MWP1a could also have been linked to the abrupt Bølling warming in the Northern Hemisphere, through accelerated ice melt in North America and Europe. Carlson et al. [2012] suggested that this abrupt warming would have caused a total of 6.9 m GMSLR in 500 years in North America, but their result did not account for dynamical or elevation‐melt feedbacks, which are likely to have intensified the melt rates. In short, it remains unclear how the Bølling warming and the saddle collapse are related.

Here we provide a mechanistically based statistical assessment of the possible range of North American ice sheet contribution to MWP1a from both the saddle collapse mechanism and the Bølling warming.