Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Althoughthe intervalcorresponding tothe LGM (19,000to 23,000 yearsago) does notcorrespond toan abrupt climatechange, a large bodyof evidence pointsto a significantlydifferent AMOC atthat time.confidence in our inferences based on themincreases when there is consistency betweenmore than one independent line of evidence.4.3 Evidence for State of the AMOCDuring the Last Glacial MaximumAlthough the interval corresponding to theLGM (19,000 to 23,000 years ago) does notcorrespond to an abrupt climate change, alarge body of evidence points to a significantlydifferent AMOC at that time (Lynch-Stieglitzet al., 2007), providing an important target forcoupled climate model simulations that areused to predict future changes. Among theseindicators of a different AMOC, the geographicdistribution of different species of surfacedwelling(planktonic) organisms can be usedto suggest latitudinal shifts in sites of deepwater formation. Accordingly, while warm currentsextend far into the North Atlantic today,compensating the export of deep waters fromthe polar seas, during the LGM, planktonicspecies indicate that the North Atlantic wasmarked by a strong east-west trending polarfront separating the warm subtropical watersfrom the cold waters which dominated theNorth Atlantic during glacial times, suggestinga southward displacement of deep water formation(CLIMAP, 1981; Ruddiman and McIntyre,1981; Paul and Schafer-Neth, 2003; Kucera etal., 2005).The chemical and isotopic compositions of benthicorganisms suggest that low-nutrient NADWdominates the modern deep North Atlantic(Fig. 4.8). During the LGM, however, theseproxies indicate that the deep water massesbelow 2 kilometers (km) depth appear to beolder (Keigwin, 2004) and more nutrient rich(Duplessy et al., 1988; Sarnthein et al., 1994;Bickert and Mackensen, 2004; Curry and Oppo,2005; Marchitto and Broecker, 2006) than thewaters above 2 km, suggesting a northwardexpansion of AABW and corresponding shoalingof NADW to form Glacial North AtlanticIntermediate Water (GNAIW) (Fig. 4.8). Finally,pore-water chloride data from deep-seasediments in the Southern Ocean indicate thatthe north-south salinity gradient in the deepAtlantic was reversed relative to today, with thedeep Southern Ocean being much saltier thanthe North Atlantic (Adkins et al., 2002).The accumulation of the decay products ofuranium in ocean sediments (Pa/Th ratio) isconsistent with an overall residence time ofdeep waters in the Atlantic that was slightlylonger than today (Yu et al., 1996; Marchal etal., 2000; McManus et al., 2004). Reconstructionsof seawater density based on the isotopiccomposition of benthic shells suggest a reduceddensity contrast across the South Atlantic basin,implying a weakened AMOC in the upper 2 kmof the South Atlantic (Lynch-Stieglitz et al.,2006). Inverse modeling (Winguth et al., 1999)of the carbon isotope data is also consistent witha slightly weaker AMOC during the LGM.4.4 Evidence for Changes in the AMOCDuring the Last DeglaciationMultiple proxies indicate that the AMOC underwentseveral large and abrupt changes duringthe last deglaciation (11,500 to 19,000 yearsago). Proxies of temperature and precipitationsuggest corresponding changes in climate(Fig. 4.7) that can be attributed to these changesin the AMOC and its attendant feedbacks(Broecker et al., 1985; Clark et al., 2002a; Alley,2007). Many of the AMOC proxy records frommarine sediments show that the changes in deepwater properties and flow were quite abrupt, butdue to slow sedimentation rates and mixing ofthe sediments at the sea floor, these records canonly provide an upper bound on the transitiontime between one circulation state and another.Radiocarbon data from fossil deep-sea corals,however, show that deep water properties canchange substantially in a matter of decades (Adkinset al., 1998). Several possible freshwaterforcing mechanisms have been identified thatmay explain this variability, although there arestill large uncertainties in understanding therelation between these mechanisms and changesin the AMOC (Box 4.3).Early in the deglaciation, starting at ~19,000years ago, water mass tracers ( 14 C and δ 13 C)suggest that low-nutrient, radiocarbon-enrichedGNAIW began to contract and shoal from itsLGM distribution so that by ~17.5 ka, a significantfraction of the North Atlantic basin wasfilled with high-nutrient, radiocarbon-depletedAABW (Fig. 4.9) (Sarnthein et al., 1994; Zahnet al., 1997; Curry et al., 1999; Willamowskiand Zahn, 2000; Rickaby and Elderfield, 2005;Robinson et al., 2005). Dynamic tracers of the138