Rapid climate change and Arctic Ocean freshening: COMMENT and REPLY: REPLY

The Research Focus article by Peltier (2007), which is a distillation of Peltier et al. (2006), discusses the possible effects of Arctic freshwater forcing on the strength of Atlantic meridional overturning circulation (AMOC), with emphasis on the Younger Dryas cold event (YD).The cause of the YD was originally associated with northward retreat of the Laurentide Ice Sheet out of Lake Superior, rerouting continental drainage from the Mississippi to the St. Lawrence River, with the attendant increase in eastward-fl owing freshwater perturbing North Atlantic climate (Johnson and McClure, 1976; Rooth, 1982).More recent work empha sizes the opening of the eastward outlet through Lake Superior that caused much of Lake Agassiz to drain rapidly, with a 1 yr fl ood triggering reduced AMOC (Teller et al., 2002).Lowell et al. (2005) argued that the lack of an identifi able spillway called the YD fl ood-trigger hypoth esis into question.Peltier cites this work as the motivation for Tarasov and Peltier ( 2005), who concluded that freshwater routing occurred from the south to the north into the Arctic Ocean.Peltier fails to cite two recent studies that shed additional light on this question.Meissner and Clark (2006) used the University of Victoria Climate Model to evaluate the response of AMOC to a 1 yr freshwater fl ood of 0.3 Sv (1 Sv = 10 6 m 3 s -1 ), a 0.074 Sv base discharge increase from the eastward freshwater routing for the duration of the YD, and the combination of the two.The modeled AMOC response to the 1 yr fl ood was negligible; only with inclusion of the base discharge increase did the model simulate a reduced AMOC for the duration of the YD.Thus, any fl ood that Lake Agassiz may have generated at the start of the YD was incapable of affecting AMOC, and the lack of evidence for a fl ood does not preclude the routing of western Canada freshwater to the St. Lawrence River.Carlson et al. (2007) demonstrated that there is a clear low-salinity signal present in planktonic δ 18 O during the YD in the St. Lawrence Estuary after accounting for decreased sea-surface temperature.Four independent geochemical tracers showed that this freshening was from an increased freshwater fl ux of 0.06 ± 0.02 Sv from western Canada to the St. Lawrence Estuary at the start of the YD.Peltier refers to "direct paleoceanographic evidence" in support of Arctic freshwater forcing.Though he provides no references for that evidence, they are presumably listed in Peltier et al. (2006).These do not show an Arctic source of freshwater at the start of the YD ~11.0 14 C k.y.B.P. Light planktonic δ 18 O anomalies are observed in the Arctic Ocean ~12.5-11.8 14C k.y.B.P. and ~10 14 C k.y.B.P. (Poore et al., 1999; Andrews and Dunhill, 2004; Hall and Chan, 2004) with heavier planktonic δ 18 O during the YD arguing against an Arctic freshwater forcing (Hillaire-Marcel et al., 2004).A light planktonic δ 18 O anomaly in the Laptev Sea off of northeastern Russia may or may not have occurred at the start of the YD (Spielhagen et al., 2005).During the last deglaciation, the largest light planktonic δ 18 O anomaly in Fram Strait occurred ~14.5 14 C k.y.B.P. due to the disintegration of the Barents ice sheet (Koç and Jansen, 1994).Peltier's model uses Arctic freshwater forcings of 0.3 and 1.0 Sv for 100 yr rather than the Tarasov and Peltier (2005) values of 0.12-0.22Sv 10.