GEOLOGY, September 2009 831 ABSTRACT Accelerated melting of Greenland ice has raised concern about the future impact of enhanced freshwater discharge on regional climate through its effect on ocean circulation. An abrupt cool- ing event ca. 8200 cal. yr B.P. has been linked to meltwater from the decaying North American ice sheet. Oxygen isotopic analyses of cellulose from subfossil Sphagnum mosses, isolated from a New- foundland peat core, reveal a pronounced anomaly ca. 8350 yr B.P. with a duration of ~150 years. The maximum estimated δ 18 O precipitation change, 4.53‰ ± 1.05‰ (Vienna standard mean ocean water), is the largest observed in the circum-North Atlantic region. The magni- tude of change exceeds that predicted by recent paleoclimate simu- lations. Comparisons with recent records of surface and deep ocean proxies in the Labrador Sea and the wider North Atlantic region suggest synchroneity. However, an ~200 year delay between the responses of the Labrador Sea region and the Greenland Ice Sheet to the effects of meltwater release remains to be explained. INTRODUCTION The largest climatic event recorded in the Northern Hemisphere dur- ing the past 10 ka (Alley et al., 1997; Klitgaard-Kristensen et al., 1998; von Grafenstein et al., 1998) has been attributed to an outburst of meltwater from glacial Lake Agassiz, 8470 ± 270 years ago (Barber et al., 1999), with a discharge of ~5.2 Sv (1 Sv = 10 6 m 3 s –1 ) over ~1 year (Teller et al., 2002; Clarke et al., 2004). In the western Labrador Sea, this megaflood is represented by a double layer of carbonate-rich turbidites, dated between ca. 8500 and ca. 8350 yr B.P. (Hillaire-Marcel et al., 2007). It was followed by a sustained increase in meltwater flux through the Hudson Strait (Teller et al., 2002; Clarke et al., 2004) that created a freshwater anomaly suffi- ciently large to slow the Meridional Overturning Circulation of the Atlantic Ocean (AMOC) (Renssen et al., 2001; Wiersma and Renssen, 2006), and resulted in an abrupt cooling of the circum-North Atlantic region (Klit- gaard-Kristensen et al., 1998; von Grafenstein et al., 1998; Rohling and Pälike, 2005; Thomas et al., 2007; Marshall et al., 2007). An increasing meltwater flux from the Greenland Ice Sheet (Chen et al., 2006) into the Nordic and Labrador Seas during the twenty-first century could poten- tially impede deep convection and northward heat transport by the AMOC (Intergovernmental Panel on Climate Change, 2007). The “8200 yr B.P.” event, therefore, provides a means of validating the response of numerical climate models to a large meltwater perturbation (Alley and Ágústsdót- tir, 2005; Wiersma and Renssen, 2006; LeGrande et al., 2006). While sig- nificant cooling at that time has been reported from Europe and Greenland (Alley et al., 1997; Klitgaard-Kristensen et al., 1998; von Grafenstein et al., 1998; Thomas et al., 2007; Marshall et al., 2007), quantitative estimates of change from northeastern North America are scarce. Paleoecological data from peat cores have traditionally been used to reconstruct past moisture balance (Barber et al., 1998; Hughes et al., 2006). Here we use a novel extraction method to derive estimates of variation in the oxygen isotopic composition of precipitation over northeastern North America from analy- ses of the cellulose fraction of subfossil Sphagnum leaves. These results challenge present understanding of the climatic sensitivity of this region to the postulated forcing mechanisms for the 8200 yr B.P. event. SITE SETTING AND THE ADVANTAGES OF SPHAGNUM Nordans Pond Bog is a precipitation-fed mire located 1.5 km inland from the present-day Atlantic coast of Newfoundland (49.150°N, 53.583°W; 60 m altitude) (Fig. 1). Excellent Sphagnum preservation throughout the 8700 year period captured in peat core NDN02/1, taken from the center of the mire, allowed us to construct a Sphagnum-specific time series of isotopic variations. Sphagnum moss is an excellent indicator of the isotopic composition of growing-season precipitation, owing to the simplicity of the pathway by which it incorporates meteoric water into cellulose. Given its lack of roots and functioning guard cells, all fractionation of the plant water is environmentally controlled prior to assimilation and cellulose synthesis (Ménot-Combes et al., 2002). Isotopic fractionation during cellulose syn- thesis is independent of temperature (DeNiro and Epstein, 1979; Sternberg et al., 1986). Previous studies have reported the preservation of an evapora- tive-enrichment signal in the cellulose of surface Sphagnum relative to peat pore waters, based on samples collected on a single day or month during summer months (Brenninkmeijer et al., 1982; Aravena and Warner, 1992). Subsequent isotopic analyses of surface samples of Sphagnum moss, mod- ern precipitation, and modern bog waters taken regularly throughout an Geology, September 2009; v. 37; no. 9; p. 831–834; doi: 10.1130/G30043A.1; 4 figures; Data Repository item 2009203. © 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Terrestrial climate signal of the “8200 yr B.P. cold event” in the Labrador Sea region Timothy J. Daley 1,2* , F. Alayne Street-Perrott 1 , Neil J. Loader 1 , Keith E. Barber 2 , Paul D.M. Hughes 2 , Elizabeth H. Fisher 3 , and James D. Marshall 3 1 School of the Environment and Society, Swansea University, Swansea SA2 8PP, UK 2 Palaeoecology Laboratory (PLUS), School of Geography, University of Southampton, Southampton SO17 1BJ, UK 3 Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK *E-mail: t.j.daley@swansea.ac.uk. 0 1500 km NGRIP1 GRIP Goose Bay Truro GREENLAND North Atlantic Ocean 40ºW 70ºN 30ºW 50ºW 60ºN 50ºN Nordans Pond Bog MD99- 2251 MD03-2665 E .G . C. E .G . C. I . C. N.A.D. G . S. L . C. L . C. Figure 1. Map of northwest Atlantic region showing modern ocean- surface currents, glacial Lake Agassiz outflow, and site locations. Also shown are core locations (solid circles), Global Network of Iso- topes in Precipitation (GNIP) (International Atomic Energy Agency– World Meteorological Organization, 2004) monitoring stations at Goose Bay, Labrador, and Truro, Nova Scotia (open circles), modern ocean-surface currents (dark lines—warm currents, gray lines—cold currents, L.C.—Labrador Current, G.S.—Gulf Stream, N.A.D.—North Atlantic Drift, E.G.C.—East Greenland Current, I.C.—Irminger Cur- rent), and routing of final drainage from glacial Lake Agassiz ca. 8470 years ago (Barber et al., 1999) (thick gray arrow). GRIP—Greenland Ice Core Project; NGRIP—North Greenland Ice Core Project.