Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4The cause of thereduced AMOCduring the YoungerDryas has commonlybeen attributedto the routing ofNorth Americanrunoff with aresulting increasein freshwater fluxdraining eastwardthrough the St.Lawrence, which issupported by recentpaleoceanographicevidence.The cause of the reduced AMOC during theYounger Dryas has commonly been attributedto the routing of North American runoff witha resulting increase in freshwater flux drainingeastward through the St. Lawrence River(Johnson and McClure, 1976; Rooth, 1982;Broecker et al., 1989), which is supported byrecent paleoceanographic evidence (Flower etal., 2004; Carlson et al., 2007) (Box 4.3).4.5 Evidence for Changes in the AMOCDuring Stage 3Marine isotope stage 3 (30,000–65,000 yearsago) was a period of intermediate ice volumethat occurred prior to the LGM. This periodof time is characterized by the Dansgaard-Oeschger (D-O) oscillations, which were firstidentified from Greenland ice-core records(Johnsen et al., 1992; Grootes et al., 1993)(Fig. 4.7). These oscillations are similar to theabrupt climate changes during the last deglaciationand are characterized by alternatingwarm (interstadial) and cold (stadial) stateslasting for millennia, with abrupt transitionsbetween states of up to 16 °C occurring overdecades or less (Cuffey and Clow, 1997; Huberet al., 2006). Bond et al. (1993) recognizedthat several successive D-O oscillations ofdecreasing amplitude represented a longerterm (~7,000-yr) climate oscillation whichculminates in a massive release of icebergsfrom the Laurentide Ice Sheet, known as aHeinrich event (Fig. 4.7) (Box 4.3). The D-Osignal seems largely confined to the NorthernHemisphere, while the Southern Hemisphereoften exhibits less abrupt, smaller amplitudemillennial climate changes (Clark et al., 2007),best represented by A-events seen in Antarcticice core records (Fig. 4.7). Synchronizationof Greenland and Antarctic ice core records(Bender et al., 1994, 1999; Sowers and Bender,1995; Blunier et al., 1998; Blunier and Brook,2001; EPICA Community Members, 2006)suggests an out-of-phase “seesaw” relationshipbetween temperatures of the Northern andSouthern Hemispheres, and that the thermalcontrast between hemispheres is greatest at thetime of Heinrich events (Fig. 4.7).By comparison to the deglaciation, there arefewer proxy records constraining millennialscalechanges in the AMOC during stage 3.Most inferences of these changes are based onδ 13 C as a proxy for water-mass nutrient content.A depth transect of well-correlated δ 13 C recordsis required in order to capture temporal changesin the vertical distribution of any given watermass, since the δ 13 C values at any given depthmay not change significantly if the core siteremains within the same water mass.Figure 4.10 illustrates one such time-depthtransect of δ 13 C records from the easternNorth Atlantic that represent changes in thedepth and volume (but not rate) of the AMOCduring an interval (35–48 ka) of pronouncedmillennial-scale climate variability (Fig. 4.7).We emphasize this interval only because itencompasses a highly resolved and well-datedarray of δ 13 C records. The distinguishing featureof these records is a minimum in δ 13 C at thesame time as Heinrich events 4 and 5, indicatingthe near-complete replacement of nutrient-poor,high δ 13 C NADW with nutrient-rich, low δ 13 CAABW in this part of the Atlantic basin. Theinference of a much reduced rate of the AMOCfrom these data is supported by the proxyrecords during the last deglaciation (Fig. 4.9),which indicate a similar distribution of δ 13 C ata time when Pa/Th ratios suggest the AMOChad nearly collapsed by the time of Heinrichevent 1 (see above). Insofar as we understandthe interhemispheric seesaw relationship establishedby ice core records (Fig. 4.7) to reflectchanges in the AMOC and corresponding oceanheat transport (Broecker, 1998; Stocker andJohnsen, 2003), the fact that Heinrich eventsduring stage 3 only occur at times of maximumthermal contrast between hemispheres (coldnorth, warm south) further indicates that someother mechanism was responsible for causingthe large reduction in the AMOC by the time aHeinrich event occurred.While many of the Heinrich stadials show upclearly in these and other δ 13 C records, there isoften no clear distinction between D-O interstadialsand non-Heinrich D-O stadials (Fig. 4.10)(Boyle, 2000; Shackleton et al., 2000; Elliot etal., 2002). While some δ 13 C and Nd records doshow millennial-scale variability not associatedwith the Heinrich events (Charles et al., 1996;Curry et al., 1999; Hagen and Keigwin, 2002;Piotrowski et al., 2005), there are many challengesthat have impeded the ability to firmlyestablish the presence or absence of coherent142