Past Climate Variability and Change in the Arctic and at High Latitudes

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180 The U.S. Climate Science Program A somewhat different history of ice extent in the Holocene emerges from the northern north AtlAntic and nordic SeAS, exemplified by the icelAnd margin. A 12,000 year record of quartz content in shelf sediment, which is used in this area as a proxy for the presence of drift ice (Eiriksson et al., 2000), has been produced for a core (MD99-2269) from the northern icelAnd shelf. The record has a resolution of 30 years per sample (Moros et al., 2006); these results are consistent with data obtained from 16 cores across the northwestern icelAnd shelf (Andrews, 2007). These data show a minimum in quartz and, thus, ice cover at the end of deglaciation, whereas the early Holocene area of ice increased and then reached another minimum around 6 ka, after which the content of quartz steadily rose (Figure 6.12). The lagged Holocene optimum in the north AtlAntic in comparison with high Arctic records can be explained by the nature of oceanic controls on ice distribution. In particular, the discharge of glacial meltwater from the remains of the lAurentide ice Sheet slowed the warming in the north AtlAntic region in the early Holocene (Kaufman et al., 2004). Additionally, oceanic circulation seesawed between the eastern and western regions of the nordic SeAS throughout much of the Holocene. For example, in the norweGiAn SeA the Holocene ice-rafting peaked in the mid-Holocene, 6.5–3.7 ka (Risebrobakken et al., 2003), and changes in Earth’s orbit forced decreasing summer temperatures and decreased seasonality (Moros et al., 2004). By contrast, the middle Holocene is a relatively warm period off East GreenlAnd, and it received a strong subsurface current of Atlantic Water around 6.5–4 ka, while ice-rafted debris was low (Jennings et al., 2002). These patterns are consistent with modern marine and atmospheric temperatures that commonly change in opposite directions on the eastern and western side of the north AtlAntic (“seesaw effect” of van Loon and Rogers, 1978). The Neoglacial cooling of the last few thousand years is considered overall to be related to decreasing summer insolation (Koç and Jansen, 1994). However, high-resolution climate records reveal greater complexity in the system— changes in seasonality and links with conditions in low latitudes and southern high latitudes (e.g., Moros et al., 2004). Variations in the volumes of ice-rafted debris indicate several cooling and warming intervals during Neoglacial Chapter 6 time, similar to the so-called “Little Ice Age” and “Medieval Climate Anomaly” cycles of greater and lesser areas of sea ice (Jennings and Weiner, 1996; Bond et al., 1997; Jennings et al., 2002; Moros et al., 2006). Polar Water excursions have been reconstructed as multi-century to decadal-scale variations superimposed on the Neoglacial cooling at several sites in the subarctic north AtlAntic (Jennings et al., 2002; Andersen et al., 2004; Giraudeau et al., 2004). In contrast, a decrease in drift ice during the Neoglacial is documented for areas influenced by the north AtlAntic Current, possibly indicating a warming in the eastern nordic SeAS (Moros et al., 2006). A seesaw climate pattern has been evident between seas adjacent to West GreenlAnd and Europe. For instance, warm periods in Europe around 800–100 BC and 800–1300 AD (Roman and the Medieval Climate Anomalies) were cold periods on West GreenlAnd because little warm Atlantic Water fed into the West GreenlAnd Current. Moreover, a cooling interval in western Europe (during the Dark Ages) correlated with increased meltwater—and thus warming—on West GreenlAnd (Seidenkrantz et al., 2007). Bond et al. (1997, 2001) suggested that cool periods manifested as past expansions of drift ice and ice-rafted debris (most notably, hematite-stained quartz grains) in the north AtlAntic punctuated deglacial and Holocene records at intervals of about 1,500 years and that these drift ice events were a result of climates that cycled independently of glacial influence. Bond et al. (2001) concluded that peak volumes of Holocene drift ice resulted from southward expansions of polar waters that correlated with times of reduced solar output. This conclusion suggests that variations in the Sun’s output is linked to centennial- to millennial-scale variations in Holocene climate through effects on production of north AtlAntic deep water. However, continued investigation of the drift ice signal indicates that although the variations reported by Bond et al. (2001) may record a solar influence on climate, they likely do not pertain to a simple index of drift ice (Andrews et al., 2006). In addition, those cooling events prior to the Neoglacial interval may stem from deglacial meltwater forcing rather than from southward drift of Arctic ice (Jennings et al., 2002; Giraudeau et al., 2004). In an effort to test the idea of solar forcing of 1,500-year cycles in
QUARTZ WEIGHT, IN PERCENT 6 5 4 3 2 1 0 0 2,000 4,000 6,000 Past Climate Variability and Change in the Arctic and at High Latitudes MD99-2269 North Iceland Cal yrs BP 8,000 1 10 4 1.2 10 4 Figure 6.12. Variations in the percentage of quartz (a proxy for drift ice) in Holocene sediments from the northern Iceland shelf (from Moros et al., 2006). BP, before present. [Copyright 2006 American Geophysical Union, reproduced by permission of American Geophysical Union.] Holocene climate change, Turney et al. (2005) compared Irish tree-ring-derived chronologies and radiocarbon activity, a proxy for solar activity, with the Holocene drift-ice sequence of Bond et al. (2001). They found a dominant 800-year cycle in moisture, reflecting atmospheric circulation changes during the Holocene but no link with solar activity. Despite many records from the Arctic margins indicating considerably reduced ice covering the early Holocene, no evidence of the decline of perennial ice cover has been found in sediment cores from the central Arctic oceAn. Arctic oceAn sediments contain some ice-rafted debris interpreted to arrive from distant shelves requiring more than 1 year of ice drift (Darby and Bischof, 2004). One explanation is that the true record of low-ice conditions has not yet been found because of low sedimentation rates and stratigraphic uncertainties. Additional investigation of cores by use of many proxies with highest possible resolution is needed to verify the distribution of ice in the Arctic during the warmest phase of the current interglacial. 181
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Past Climate Variability and Change
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Past Climate Variability and Change
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AUTHOR TEAMS FOR THIS REPORT Execut
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ACKNOWLEDGMENTS Many individuals ma
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TABLE OF CONTENTS Executive Summary
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2 The U.S. Climate Change Science P
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4 The U.S. Climate Change Science P
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6 The U.S. Climate Science Program
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100 The U.S. Climate Science Progra
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The U.S. Climate Science Program Ch
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Page 169 and 170: CHAPTER 6 ABSTRACT Past Climate Var
Page 171 and 172: second, and the Transpolar Drift, t
Page 173 and 174: EXTENT, IN MILLION SQUARE KILOMETER
Page 175 and 176: Until recently, and for logistical
Page 177 and 178: 1990; Fyles et al., 1994), the nort
Page 179 and 180: FREQUENCY 50 45 40 35 30 25 20 15 1
Page 181 and 182: Seas around icelAnd provide a rare
Page 183 and 184: at least 14 Ma (Darby, 2008). Sever
Page 185 and 186: T. quinqueloba, IN SPECIMENS PER GR
Page 187 and 188: 7 6 5 4 3 2 1 Past Climate Variabil
Page 189: SEA-ICE COVERAGE, IN NUMBER OF MONT
Page 193: 6.5 SUMMARY Geological data indicat
Page 201 and 202: GLOSSARY GLOSSARY 8 ka cold event A
Page 203 and 204: CAPE Project Circum-Arctic PaleoEnv
Page 205 and 206: Eccentricity Out of roundness (elli
Page 207 and 208: Innuitian sector The ice sheet that
Page 209 and 210: Moraine Landforms (typically ridges
Page 211 and 212: Positive feedback In climate studie
Page 213: Trough-mouth fans Undersea deposits
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