Importance of freshwater injections into the Arctic Ocean in triggering the cooling

James T. Teller1 Department of Geological Sciences, University of , , MB, Canada R3T 2N2

he cause of past climate change has been the focus of many T studies in recent years. Various explanations have been advanced to explain the record of Quaternary cli- mate change identified in sediments on continents, in oceans, and in the ice caps of Greenland and . Important in this is the scale of change—both temporal and spatial—and our best records, espe- cially in terms of chronological resolution, come from the past several hundred thousand years. A number of climate fluctuations, some of them abrupt and some of them short, have been linked to changes in the flux of freshwater to the oceans that, if large enough, might have impacted on ocean circulation and, in Fig. 1. Boulder concentration in gravel pit in the Valley that connects the Agassiz turn, have resulted in a change in climate. and basins, near Fort McMurray, AB, Canada, attributed to overflow from Lake Agassiz Broecker et al. (1) were among the first to during the YD by Teller et al. (12) and Murton et al. (18). propose that an injection of freshwater from was responsible for that of much of the western interior of Overturning Circulation (AMOC), which the anomalous Younger Dryas (YD) North America—really east through the is similar to THC. The authors assume cooling, ∼12.9 to 11.5 ka, known from Great /St. Lawrence during the YD that the LIS was the main source of the Europe and other circum-Atlantic conti- (11–13)? Second, was the contribution freshwater, and use a flux greater than nental regions. In their hypothesis, an from Lake Agassiz overflow enough to that known to have been released outburst of water from Agassiz trigger a change in THC and sustain it during the YD from Lake Agassiz to (once the largest lake in the world) for >1,000 y (3, 14, 15)? Third, was an perturb the ocean. They choose a eastward through the and increasing outflow of meltwater from the “modern-day” ocean, rather than make St. Lawrence Valley to the North Atlantic, melting LIS itself—rather than an abrupt assumptions about what ocean conditions which coincided in time with the YD, was injection of rerouted overflow from a large were like during late-glacial time or fi identi ed as the mechanism for slowing —enough to bring about “pre-condition” THC by other meltwater thermohaline circulation (THC) and, in a large change in ocean circulation, per- additions, as Fanning and Weaver (8) turn, for triggering the YD cooling. haps after crossing some threshold, at the and others suggested might make the Subsequent field research led others to specific times known from records in fl ocean more sensitive. In fact, precondition- conclude that changes in over ow routing Greenland ice cores and stratigraphic se- ing may have allowed a flux smaller than from this giant lake were responsible quences in North America, Europe, and the 5 Sv used by Condron and Winsor for the Preboreal oscillation and 8.2-ka elsewhere (16)? (10) to perturb AMOC and cause the event (2–5). Coupled ocean/atmosphere Condron and Winsor (10) incorporate fi YD cooling. modeling seemed to con rm these inter- these questions into their research by us- In the Condron and Winsor (10) model, pretations (6–9), although the coarse ing a high-resolution (1/6°, 18 km) global the Mackenzie River route (see Fig. 1) has nature of the models, which injected coupled ocean sea-ice circulation model two scenarios for the interconnectivity of freshwater uniformly over large areas, to assess the impact of a large (5 Sv), the Arctic Ocean to the North Atlantic was recognized as a limitation. In PNAS, abrupt, 1-y injection of freshwater from Ocean: (i) Mackenzie River open to the Condron and Winsor (10) present a North America through two different Labrador Sea and (ii) Mackenzie River high-resolution model that brings impor- routes, namely (i) to the North Atlantic closed to the Labrador Sea but open to the tant insight into the role of freshwater Ocean through the St. Lawrence Valley ii North Atlantic north of Greenland to the injections into the North Atlantic during and ( ) to the Arctic Ocean through the – – the YD. Mackenzie River in northern Canada. The Fram Strait (Greenland Iceland Norwegian Since the initial description of the hy- authors state that “this model captures the Sea). As noted by Condron and Winsor pothesis that freshwater outbursts from circulation of the ocean and sea ice at (10), the eastward routing of runoff from Lake Agassiz triggered a change in THC Lake Agassiz and its has been 10 to 15 times higher resolution than – and the YD cooling (1), research on the previous models attempting to understand questioned (11 13, 17, 18), and recent late-glacial melting of the Laurentide Ice how meltwater acts to trigger the YD” Sheet (LIS) and routing of its meltwater to (10). The goal of Condron and Winsor the oceans during the YD (including that (10) is to understand the influence of Author contributions: J.T.T. wrote the paper. stored in and released from Lake Agassiz) geographically different discharge loca- The author declares no conflict of interest. has raised some questions. First, was the tions on deep convection in the ocean and See companion article on page 19928. routing of Lake Agassiz overflow—and on the strength of Atlantic Meridional 1E-mail: [email protected].

19880–19881 | PNAS | December 4, 2012 | vol. 109 | no. 49 www.pnas.org/cgi/doi/10.1073/pnas.1218344109 Downloaded by guest on September 29, 2021 COMMENTARY

evidence from sediment in the Arctic a 1-y slug of water into the Arctic Although the fluxes chosen by Condron Ocean basin (19, 20), as well as modeling Ocean and then to the North Atlantic and Winsor (10) may be argued, and of the impact of freshwater discharge to via the Greenland–Iceland–Norwegian they could have added a range of lower the Arctic Ocean from the LIS (16) have Sea “provides a mechanism unique to fluxes so that smaller water sources than strengthened arguments for a northwest the melting LIS might be considered as route to the Arctic Ocean. Modeling of This research adds triggers for changes in THC (e.g., Lake the impact of freshwater additions to the Agassiz and other ice-marginal lakes), this Arctic Ocean, however, has been limited important insight into research adds important insight into the and at coarse scales (17, 21). The three routing of much of North America’s runoff geographic routings modeled by Condron the routing of much of during the YD. Because continental and and Winsor (10) led them to conclude ocean records have been used to argue for that “only meltwater released from the North America’s runoff both an eastern and a northwestern routing Mackenzie Valley creates a classic ‘fresh- ’ of meltwater from North America and the water cap that inhibits open ocean deep during the YD. 2 convection. The water column of the 2-million-km Lake Agassiz drainage basin, central Labrador and Greenland Seas is the Condron and Winsor high-resolution unaltered by meltwater discharge from the Arctic that is capable of turning a modeling (10) is an especially valuable the St. Lawrence Valley” (10) because, short-duration (∼1 y), high-magnitude addition in terms of resolving the question, when 5 Sv of meltwater was injected meltwater discharge event into a signifi- and illustrates the importance of fine-scale into the North Atlantic at the mouth of cantly larger...meltwater event” (10) that modeling whereby freshwater is injected the St. Lawrence River, it turned south reduces ocean deep convectionby 32% (vs. at specific points in the ocean rather than along the coast, as outflow does today, 9% if the slug of water arrives via the St. being spread out over large regions. As well, rather than move offshore into the con- Lawrence Valley) and the northward it provides support for the hypothesis that vection region. In contrast, routing of transport of heat by 29%. freshwater triggered the YD cooling.

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