Cover Page

Contents

PREFACE

Abrupt Climate Change Revisited

1. INTRODUCTION

2. KEY QUESTIONS FOR THE ACC CONFERENCE

3. DISCUSSION OF MAJOR FINDINGS

4. RECOMMENDATIONS

A Review of Abrupt Climate Change Events in the Northeastern Atlantic Ocean (Iberian Margin): Latitudinal, Longitudinal, and Vertical Gradients

1. INTRODUCTION

2. MODERN HYDROGRAPHIC SETTING

3. MATERIAL AND METHODS

4. CHRONOSTRATIGRAPHIES

5. SURFACE WATER GRADIENTS AND IMPLICATIONS FOR THE POLAR FRONT POSITION

6. IMPACTS ON THE GLACIAL UPPER WATER COLUMN

7. IMPRINTS THROUGHOUT THE WHOLE WATER COLUMN

8. CONCLUSIONS

Laurentide Ice Sheet Meltwater and the Atlantic Meridional Overturning Circulation During the Last Glacial Cycle: A View From the Gulf of Mexico

1. INTRODUCTION

2. PREVIOUS STUDIES OF DEGLACIAL MELTWATER IN THE GULF OF MEXICO

3. ORCA BASIN CLIMATE RECORDS

4. CESSATION OF MELTWATER INPUT AND THE YOUNGER DRYAS EVENT

5. ISOLATING δ18OSW USING PAIRED δ18O AND MG/CA DATA

6. TESTING THE MELTWATER ROUTING HYPOTHESIS DURING MIS 3

7. ICE SHEET MELTWATER AND THE ATLANTIC OCEAN DEEP CIRCULATION

8. THE “MELTWATER CAPACITOR” HYPOTHESIS AND SEASONALITY IN THE NORTH ATLANTIC

9. CONCLUSIONS

Modeling Abrupt Climate Change as the Interaction Between Sea Ice Extent and Mean Ocean Temperature Under Orbital Insolation Forcing

1. INTRODUCTION

2. BISTABLE CLIMATES

3. SEA ICE EXTENT AND OCEAN TEMPERATURE DURING ABRUPT CLIMATE CHANGE

4. THE LANGEVIN AND VAN DER POL MODELS

5. RELATION BETWEEN SURFACE AND DEEP OCEAN TEMPERATURE HISTORIES

6. DISCUSSION

APPENDIX A: MODELS

Simulated Two-Stage Recovery of Atlantic Meridional Overturning Circulation During the Last Deglaciation

1. INTRODUCTION

2. MODEL AND EXPERIMENTAL SETUP

3. TWO-STAGE FEATURE OF AMOC RECOVERY

4. CAUSES OF TWO-STAGE AMOC RECOVERY

5. DISCUSSIONS AND CONCLUSIONS

The Role of Hudson Strait Outlet in Younger Dryas Sedimentation in the Labrador Sea

1. INTRODUCTION

2. ICE MARGINS, ICE EXTENT DURING EARLIER HEINRICH EVENTS, AND ICE EXTENT AT YD

3. MATERIALS AND METHODS

4. RESULTS

5. DISCUSSION

6. CONCLUSIONS

Challenges in the Use of Cosmogenic Exposure Dating of Moraine Boulders to Trace the Geographic Extents of Abrupt Climate Changes: The Younger Dryas Example

1. INTRODUCTION

2. PRIOR WORK

3. SELECTED DATA SETS

4. IMPLICATIONS FOR FIELD SAMPLING CRITERIA

5. IMPLICATIONS FOR DETERMINING NUCLIDE PRODUCTION RATES

6. DISCUSSION

Hypothesized Link Between Glacial/Interglacial Atmospheric CO2 Cycles and Storage/Release of CO2-Rich Fluids From Deep-Sea Sediments

1. INTRODUCTION

2. PREVIOUS HYPOTHESES

3. THE Δ14C RECORD AND THE TIMING OF ATMOSPHERIC CO2 CHANGE

4. ARE THERE OTHER SOURCES OF Δ14C-DEPLETED CARBON IN THE OCEAN?

5. AN OCEAN CO2 CAPACITOR?

6. STORAGE AND PRODUCTION OF CO2 DURING GLACIATIONS

7. A MECHANISM FOR CO2 RELEASE DURING DEGLACIATION

8. SUMMARY

The Impact of the Final Lake Agassiz Flood Recorded in Northeast Newfoundland and Northern Scotian Shelves Based on Century-Scale Palynological Data

1. INTRODUCTION

2. REGIONAL SETTING

3. METHODS

4. CORE STRATIGRAPHY

5. RESULTS

6. DISCUSSION

7. CONCLUSIONS

The 1500 Year Quasiperiodicity During the Holocene

1. INTRODUCTION

2. DATA SELECTION

3. PALEOTEMPERATURE VARIABILITY IN ATLANTIC

4. SUBPOLAR ATLANTIC VARIABILITY OF PALEOTEMPERATURE AND SALINITY

5. ON THE ORIGIN OF THE 1500 YEAR OSCILLATION

6. DISCUSSION

Abrupt Climate Changes During the Holocene Across North America From Pollen and Paleolimnological Records

1. INTRODUCTION

2. DATA AND METHODS

3. RECORDS OF ABRUPT CLIMATE CHANGE IN NORTH AMERICA DURING THE HOLOCENE

4. DISCUSSION

Abrupt Holocene Climatic Change in Northwestern India: Disappearance of the Sarasvati River and the End of Vedic Civilization

1. INTRODUCTION

2. PHYSIOGRAPHIC CONDITIONS

3. FOSSIL GROUNDWATER IN THE REGION

4. NEOTECTONIC DISTURBANCES

5. INDIAN MYTHOLOGY AND THE SARASVATI RIVER

6. ARCHEOLOGICAL FINDINGS IN THE REGION

7. HOLOCENE CLIMATIC CHANGES

8. CONCLUSIONS

Evidence for Climate Teleconnections Between Greenland and the Sierra Nevada of California During the Holocene, Including the 8200 and 5200 Climate Events

1. INTRODUCTION

2. STUDY AREA

3. METHODS

4. RESULTS

5. DISCUSSION

6. CONCLUSIONS

Abrupt Climate Change: A Paleoclimate Perspective From the World’s Highest Mountains

1. INTRODUCTION

2. ABRUPT CLIMATE EVENTS IN THE HOLOCENE

3. CLIMATE CHANGE IN THE PERUVIAN ANDES AND THE RISE AND FALL OF CIVILIZATIONS

4. CLIMATE CHANGE AND ITS CURRENT EFFECTS ON THE CRYOSPHERE

5. CONCLUSIONS

AGU Category Index

Index

Geophysical Monograph Series

158 The Nordic Seas: An Integrated Perspective Helge Drange, Trond Dokken, Tore Furevik, Rüdiger Gerdes, and Wolfgang Berger (Eds.)
159 Inner Magnetosphere Interactions: New Perspectives From Imaging James Burch, Michael Schulz, and Harlan Spence (Eds.)
160 Earth’s Deep Mantle: Structure, Composition, and Evolution Robert D. van der Hilst, Jay D. Bass, Jan Matas, and Jeannot Trampert (Eds.)
161 Circulation in the Gulf of Mexico: Observations and Models Wilton Sturges and Alexis Lugo-Fernandez (Eds.)
162 Dynamics of Fluids and Transport Through Fractured Rock Boris Faybishenko, Paul A. Witherspoon, and John Gale (Eds.)
163 Remote Sensing of Northern Hydrology: Measuring Environmental Change Claude R. Duguay and Alain Pietroniro (Eds.)
164 Archean Geodynamics and Environments Keith Benn, Jean-Claude Mareschal, and Kent C. Condie (Eds.)
165 Solar Eruptions and Energetic Particles Natchimuthukonar Gopalswamy, Richard Mewaldt, and Jarmo Torsti (Eds.)
166 Back-Arc Spreading Systems: Geological, Biological, Chemical, and Physical Interactions David M. Christie, Charles Fisher, Sang-Mook Lee, and Sharon Givens (Eds.)
167 Recurrent Magnetic Storms: Corotating Solar Wind Streams Bruce Tsurutani, Robert McPherron, Walter Gonzalez, Gang Lu, José H. A. Sobral, and Natchimuthukonar Gopalswamy (Eds.)
168 Earth’s Deep Water Cycle Steven D. Jacobsen and Suzan van der Lee (Eds.)
169 Magnetospheric ULF Waves: Synthesis and New Directions Kazue Takahashi, Peter J. Chi, Richard E. Denton, and Robert L. Lysal (Eds.)
170 Earthquakes: Radiated Energy and the Physics of Faulting Rachel Abercrombie, Art McGarr, Hiroo Kanamori, and Giulio Di Toro (Eds.)
171 Subsurface Hydrology: Data Integration for Properties and Processes David W. Hyndman, Frederick D. Day-Lewis, and Kamini Singha (Eds.)
172 Volcanism and Subduction: The Kamchatka Region John Eichelberger, Evgenii Gordeev, Minoru Kasahara, Pavel Izbekov, and Johnathan Lees (Eds.)
173 Ocean Circulation: Mechanisms and Impacts—Past and Future Changes of Meridional Overturning Andreas Schmittner, John C. H. Chiang, and Sidney R. Hemming (Eds.)
174 Post-Perovskite: The Last Mantle Phase Transition Kei Hirose, John Brodholt, Thorne Lay, and David Yuen (Eds.)
175 A Continental Plate Boundary: Tectonics at South Island, New Zealand David Okaya, Tim Stem, and Fred Davey (Eds.)
176 Exploring Venus as a Terrestrial Planet Larry W. Esposito, Ellen R. Stofan, and Thomas E. Cravens (Eds.)
177 Ocean Modeling in an Eddying Regime Matthew Hecht and Hiroyasu Hasumi (Eds.)
178 Magma to Microbe: Modeling Hydrothermal Processes at Oceanic Spreading Centers Robert P. Lowell, Jeffrey S. Seewald, Anna Metaxas, and Michael R. Perfit (Eds.)
179 Active Tectonics and Seismic Potential of Alaska Jeffrey T. Freymueller, Peter J. Haeussler, Robert L. Wesson, and Göran Ekström (Eds.)
180 Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications Eric T. DeWeaver, Cecilia M. Bitz, and L.-Bruno Tremblay (Eds.)
181 Midlatitude Ionospheric Dynamics and Disturbances Paul M. Kintner, Jr., Anthea J. Coster, Tim Fuller-Rowell, Anthony J. Mannucci, Michael Mendillo, and Roderick Heelis (Eds.)
182 The Stromboli Volcano: An Integrated Study of the 2002–2003 Eruption Sonia Calvari, Salvatore Inguaggiato, Giuseppe Puglisi, Maurizio Ripepe, and Mauro Rosi (Eds.)
183 Carbon Sequestration and Its Role in the Global Carbon Cycle Brian J. McPherson and Eric T. Sundquist (Eds.)
184 Carbon Cycling in Northern Peatlands Andrew J. Baird, Lisa R. Belyea, Xavier Comas, A. S. Reeve, and Lee D. Slater (Eds.)
185 Indian Ocean Biogeochemical Processes and Ecological Variability Jerry D. Wiggert, Raleigh R. Hood, S. Wajih A. Naqvi, Kenneth H. Brink, and Sharon L. Smith (Eds.)
186 Amazonia and Global Change Michael Keller, Mercedes Bustamante, John Gash, and Pedro Silva Dias (Eds.)
187 Surface Ocean–Lower Atmosphere Processes Corinne Le Quèrè and Eric S. Saltzman (Eds.)
188 Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges Peter A. Rona, Colin W. Devey, Jérôme Dyment, and Bramley J. Murton (Eds.)
189 Climate Dynamics: Why Does Climate Vary? De-Zheng Sun and Frank Bryan (Eds.)
190 The Stratosphere: Dynamics, Transport, and Chemistry L. M. Polvani, A. H. Sobel, and D. W. Waugh (Eds.)
191 Rainfall: State of the Science Firat Y. Testik and Mekonnen Gebremichael (Eds.)
192 Antarctic Subglacial Aquatic Environments Martin J. Siegert, Mahlon C. Kennicutt II, and Robert A. Bindschadler (Eds.)

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PREFACE

When the first book on abrupt climate change was published in 1987, the idea of millennial climatic oscillations now known as the Dansgaard-Oeschger events, named after seminal researchers Willi Dansgaard and Hans Oeschger, was slowly taking shape, while periodic collapse and rebuilding of the Northern Hemisphere ice sheets at millennial timescales, recognized as the Heinrich (named for Hartmut Heinrich) iceberg-rafting events, were not yet known. Using marine sediment data from the northeast Atlantic deep-sea cores, Gerard Bond and Wallace Broecker, and colleagues in 1992 and 1993 placed Heinrich’s work into a wider climate context; climatologists began to consider mechanisms of reorganization of the global ocean-atmospheric system at millennial and finer timescales. Several collections of papers and reports published since that time reflect an impressive development in our understanding of the history and mechanisms of abrupt climate events: Abrupt Climatic Change: Evidence and Implications (NATO-ASI series published in 1987); the 1999 AGU Geophysical Monograph Mechanisms of Global Climate Change at Millennial Scale Time Scales; Abrupt Climate Change: Inevitable Surprises, published by the National Academy Press in 2002; and “Abrupt Climate Change,” a 2008 report by the U.S. Climate Change Science Program and Subcommittee on Global Change Research.

This volume arises from contributions to the 2009 American Geophysical Union Chapman Conference on Abrupt Climate Change that addressed the progress made in understanding the mechanisms of abrupt climate events in the decade following the previous Chapman Conference on this topic (Mechanisms of Global Climate Change at Millennial Scale Time Scales and Abrupt Climate Change: Inevitable Surprises). The 2009 conference was held at the Byrd Polar Research Center of the Ohio State University, Columbus, Ohio, on 15–19 June 2009. A total of 105 scientists from 24 countries working in disciplines ranging from paleoclimatology, paleoceanography, and atmospheric and marine chemistry to paleoclimate model-data comparison to archaeology attended and presented their cutting-edge research at the weeklong conference. The basic purpose of the 2009 conference was to understand the spatiotemporal extent of abrupt climate change and the relevant forcings. This monograph covers a breadth of global paleoclimate research discussed at the conference and provides a list of critical topics that need to be resolved to better understand abrupt climate changes and thus advance our knowledge and the tools required to project future climate.

We would like to express our deep appreciation to participants and presenters for their cutting-edge research both at the conference and in this monograph. The conference was sponsored by the Office of Research and the Climate, Water and Carbon Program (CWC) of the Ohio State University, the Consortium for Ocean Leadership, and the National Science Foundation (OCE-0928601). As a result, we were able to provide travel support to all of the participating graduate students, postdoctoral researchers, and a few young investigators and most of the invited speakers. We acknowledge numerous reviewers for their critical assessment of the papers and help in streamlining the manuscripts. We would also like to acknowledge invaluable assistance provided by the staff of the American Geophysical Union.

Harunur Rashid
Byrd Polar Research Center, Ohio State University

Leonid Polyak
Byrd Polar Research Center, Ohio State University

Ellen Mosley-Thompson
Byrd Polar Research Center, Ohio State University
Department of Geography, Ohio State University

Abrupt Climate Change Revisited

Harunur Rashid and Leonid Polyak

Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA

Ellen Mosley-Thompson

Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA

Department of Geography, Ohio State University, Columbus, Ohio, USA

This geophysical monograph volume contains a collection of papers presented at the 2009 AGU Chapman Conference on Abrupt Climate Change. Paleoclimate records derived from ice cores, lakes, and marine sedimentary archives illustrate rapid changes in the atmosphere-cryosphere-ocean system. Several proxy records and two data-model comparison studies simulate Atlantic meridional overturning circulation during the last deglaciation and millennial-scale temperature oscillations during the last glacial cycle and thereby provide new perspectives on the mechanisms controlling abrupt climate changes. Two hypotheses are presented to explain deep Southern Ocean carbon storage, the rapid increase of the atmospheric carbon dioxide, and retreat of sea ice in the Antarctic Ocean during the last deglaciation. A synthesis of two Holocene climate events at approximately 5.2 ka and 4.2 ka highlights the potential of a rapid response to climate forcing in tropical systems through the hydrological cycle. Both events appear contemporaneous with the collapse of ancient civilizations in lowlatitude regions where roughly half of Earth’s population lives today.

1. INTRODUCTION

Remarkable, well-dated evidence of high-magnitude, abrupt climate changes occurring during the last glacial period between ∼80 and 10 ka have been documented in Greenland ice cores and marine and terrestrial records in the Northern Hemisphere. These climate perturbations are known as the Dansgaard-Oeschger (D-O) warming and cooling cycles of 2–3 kyr duration. These are bundled into Bond cycles of 5–10 kyr and are terminated by Heinrich events consisting of massive iceberg rafting from the late Pleistocene Laurentide Ice Sheet (LIS). Since the discovery of the D-O cycles in ice cores and their counterparts in marine sediments of the North Atlantic, the search for abrupt, millennial- or finer-scale events has intensified across the globe (see Voelker et al. [2002] and Clement and Peterson [2008] for an overview). In recent years, an increasing number of paleoclimatic records, mostly in the Northern Hemisphere show teleconnections with the D-O cycles recorded in Greenland. The most commonly inferred link between these rapid climate events is related to the release of cold fresh water by iceberg melting. The introduction of this fresh water is assumed to slow or shut down the formation of the North Atlantic Deep Water (NADW), thus preventing the penetration of the North Atlantic Drift (the northern branch of the Gulf Stream) into high latitudes. The climatic importance of the Gulf Stream stems from the enormous quantity of heat it transports to northwestern Europe and its facilitation of the exchange of moisture between the ocean and atmosphere.

Marine and terrestrial paleoclimate records from the Southern Hemisphere (SH) are sparse and lack sufficient temporal resolution to characterize the timescales relevant for high-frequency climate variability. Although the evidence for abrupt climate change in the SH is not clear, a one-to-one correlation has been established between the new Eastern Droning Maud Land (EDML) Antarctic ice core record and that from Greenland [EPICA Community Members et al., 2006]. The EDML ice core was recovered from a location facing the South Atlantic and is assumed to record climate changes in the Atlantic. Paleoclimatic records from the northern tropics and subtropics mainly show changes concordant with those in the North Atlantic, while asynchronous or even anticorrelated events are exhibited in records from the southern tropics and high latitudes of the SH. For example, the Indian and East Asian monsoon histories seem to correlate with the North Atlantic climate, whereas South American monsoon records are anticorrelated with the Greenland records [Wang et al., 2006]. Furthermore, paleoclimatic proxy records from the equatorial Pacific are characterized by a complex pattern of abrupt climate change that borrows elements from both the Northern Hemisphere and Southern Hemisphere end members, suggesting that the tropical Pacific may have played a significant role in mediating climate teleconnections between the hemispheres [Clement et al., 2004; Li et al., 2010].

The Chapman Conference on Abrupt Climate Change (ACC) held at the Ohio State University in June 2009 brought together a diverse group of researchers dealing with various paleoclimatic proxy records (such as ice cores, corals, marine sediments, lakes, and speleothems) and coupled ocean-atmosphere climate models to discuss advances in understanding the ACC history and mechanisms. Special attention was given to the three most commonly invoked ACC mechanisms: (1) freshwater forcing, in which meltwater from the circum–North Atlantic ice sheets may have disrupted the meridional overturning circulation (MOC) by preventing or slowing down the formation of NADW in the Labrador and Nordic Seas; (2) changes in sea ice extent, which affect the ocean-atmosphere heat exchange, moisture supply, and salt content; and (3) tropical forcing that calls for a combination of Earth’s orbital configuration, El Niño–Southern Oscillation, and sea surface temperature conditions. Several contributions dealing with various aspects of these topics are presented in this volume.

2. KEY QUESTIONS FOR THE ACC CONFERENCE

Without providing an exhaustive list of topics raised at the conference, we list a number of pressing scientific questions that were discussed:

1. Do paleoproxies suggest a one-to-one proxy-based relationship between circum–North Atlantic freshwater pulses and the strength of the MOC, as well as the determination of meltwater sources?
2. Are kinematic and nutrient proxies for the strength of the MOC (Pa/Th, Cd/Ca, and δ13C) congruent across abrupt climate changes that occurred during the Younger Dryas (YD) or Heinrich events? How do these proxies differ between periods of stable and transient climate?
3. Do current general circulation models capture abrupt strengthening and gradual weakening of thermohaline circulation, consistent with the rapid warming and gradual cooling of D-O events? If not, what other factors must be considered?
4. Is there robust evidence for sea ice in the North Atlantic during the last glacial cycle? How much has sea ice extent fluctuated on millennial timescales? How have the fluctuations influenced the surface salinity and thus water mass stratification?
5. With the exception of the tropical Atlantic, most tropical paleorecords show a clear lack of D-O cooling events. Does this indicate that various parts of the tropics respond differently to the North Atlantic freshwater forcing because of local hydrological and temperature variability?
6. Given the dramatic changes in Arctic sea ice and circulation, how did the Arctic freshwater budget affect the MOC in the North Atlantic?
7. Why do Antarctic temperatures show more gradual and less pronounced warmings and coolings compared to the D-O events in Greenland? Does this suggest that the deep ocean circulation is modulating the abrupt climate change?
8. In light of the current concern about instabilities of the West Antarctic and Greenland ice sheets, how can paleoceanographic records be used to decipher past ice sheet dynamics?
9. What is the link between sea ice extent and ice sheet dynamics? How does ocean heat transport influence the ice sheet margin? Might coastal ice shelves be slaves to ocean currents?
10. Was there a relationship between the demise of past civilizations and climatic deterioration? What are the climate tipping points that have driven past civilizations to collapse?

One of the important points raised at the conference is that a close examination of paleoclimatic data and modeling results does not show adequate support for many of the widely accepted explanations for abrupt climate change. For example, it is almost taken for granted that fresh water released from circum–North Atlantic ice sheets during Heinrich events perturbed the Atlantic meridional overturning circulation (AMOC), which caused abrupt changes recorded around the globe. However, with the exception of the Younger Dryas, there is no paleoproxy evidence from deep waters that would indicate a shutdown or slowdown of the AMOC during Heinrich events [Lynch-Stieglitz et al., 2007]. This situation does not necessarily mean that changes in paleobottom water composition did not occur but simply shows that adequate supporting data are lacking. Even less certainty can be applied to paleorecords older than the last glacial cycle, when these abrupt climate changes were a recurrent phenomenon. Whether such recurrent events occurred during previous glacial cycles is not well documented because of the scarcity of long paleoclimatic records with the requisite spatial and temporal resolution.

Several new areas of inquiry were discussed during the meeting including (1) development of a new chronostratigraphy for Antarctic ice cores based on local insolation and independent from bias-prone orbital tuning [Kawamura et al., 2007; Laepple et al., 2011], (2) phasing between the deep ocean and surface water warming during terminations as derived from oxygen isotope records on benthic and planktonic foraminifers [Rashid et al., 2009], (3) indication of monsoon failure from atmospheric oxygen isotopes and deep ocean temperature change from inert gases [Severinghaus et al., 2009], (4) timing of elevated subantarctic opal fluxes and deep ocean carbon dioxide release to the atmosphere and phasing between these features and the position of the westerlies [Anderson et al., 2009], (5) use of dynamic circulation proxies (Pa/Th, Nd isotopes, etc.) and models of freshwater forcing in assessing the strength of MOC, and (6) role of the Antarctic Intermediate Water in distributing heat and transporting old carbon around the ocean [Marchitto et al., 2007; Basak et al., 2010].

Members of the paleoclimate modeling community stressed the need to improve the temporal resolution and age constraints on paleoclimatic records. In addition, it was suggested to further explore “the meaning of proxies,” resolve the leads and lags in important paleorecords, and provide benchmark tests for climate sensitivity analysis. For example, oxygen isotopes in speleothems could indicate the amount of precipitation, changes in seasonality, or changes in source region of moisture. More research is needed to clarify the relationship of oxygen isotopes with these variables. Modelers also asked questions regarding the sources of carbon dioxide increase during the Antarctic isotope maximum (AIM) warming events and factors that could be attributed to the deep ocean warming during Heinrich events.

3. DISCUSSION OF MAJOR FINDINGS

3.1. Last Glacial-Interglacial Climate Cycle

After four decades of intense research, there are not that many high-resolution (millennial to submillennial scale) paleoceanographic records available from the North Atlantic, the critical area for understanding changes in the MOC. Voelker and de Abreu [this volume] provide an overview of the last four glacial cycles from high-accumulation-rate sites on the Iberian margin based mostly on data from Martarat et al. [2007], Voelker et al. [2009], and Salgueiro et al. [2010]. Latitudinal (43.20° to 35.89°N) and longitudinal (10.39° to 7.53°W) gradients in sea surface temperature and density are reconstructed in this chapter using a foraminiferal assemblage–based transfer function (SIMMAX [Pflaumann et al., 1996]) and δ18O from 11 sediment cores from the northeastern Atlantic. In addition to sea surface conditions, δ18O and δ13C compositions in various planktonic foraminifers covering the depth range of 50 to 400 m indicate changes in calcification depth during glacial intervals. Voelker and de Abreu also evaluated changes in seasonality based on the difference in δ18O between Globigerina bulloides and Globorotalia inflata [Ganssen and Kroon, 2000]. As a result, migration of subpolar and subtropical boundaries and hydrographic fronts during abrupt climate events were identified. Voelker and de Abreu suggest that nutrient levels and thus ventilation of the upper 400 m of the water column were not driven by millennial-scale events. In contrast, millennial-scale oscillations in ventilation were recorded in the intermediate to bottom waters, indicating the status of the overturning circulation either in the North Atlantic or in the Mediterranean Sea that admixed deep flowing Mediterranean Overflow Water to the Glacial North Atlantic Intermediate Water. One of the important aspects of Voelker and de Abreu’s contribution is the detailed documentation of the upper water structure during the penultimate glacial (marine isotope stage (MIS) 6) that has been characterized by only scarce data thus far.

Flower et al. [this volume] review planktonic Mg/Ca-δ18O evidence from the Gulf of Mexico (GOM) to investigate the role of meltwater input from the LIS in abrupt climate change during MIS 3 and the last deglaciation. The chapter provides an important summary of the current understanding of the ACC in the GOM by synthesizing data mostly presented by Flower et al. [2004], Hill et al. [2006], and Williams et al. [2010]. These data show that the ice volume–corrected seawater δ18O in the GOM matches the East Antarctic ice core δ18O [EPICA Community Members et al., 2006]. Flower et al. [this volume] conclude that (1) LIS meltwater pulses started during Heinrich stadials and lasted through the subsequent D-O events; (2) LIS meltwater pulses appear to coincide with the major AIM events; and (3) LIS meltwater discharge is associated with distinct changes in deep ocean circulation in the North Atlantic during H events. These observations lead the authors to propose a direct link between GOM meltwater events and the weakening of the AMOC as modulated by the Antarctic climate. Furthermore, Flower et al. hypothesize that LIS melting is linked to the Antarctic climate, so that it was the AMOC reduction (via the bipolar seesaw and Antarctic warming) that drove increased LIS meltwater input to the GOM and not vice versa. This causality, however, may have been limited as LIS meltwater input to the GOM continued throughout D-O 8 and Bølling/Allerød events despite a lack of evidence for AMOC reduction during these intervals [Rahmstorf, 2002].

Since the D-O cycles were first discovered in Greenland ice cores and then later confirmed in deep-sea sediments of the Atlantic, a large number of modeling efforts were directed toward understanding the origin and significance of these paleoclimatic events. Rial and Saha [this volume] simulate the D-O cycles using a conceptual model, based on simple stochastic differential equations, termed the sea ice oscillator (SIO), which borrowed elements from the Saltzman et al. [1981] concept. Simulation results show that sea ice extent and mean ocean temperature can be driven by changes in orbital insolation, which play a dominant role in controlling atmospheric temperature variability. The SIO model has two internal parameters controlling the abruptness of temperature change: the free frequency of the oscillator and the intensity of positive feedbacks. The model best reproduces the D-O cycles if the free oscillation period is set around 1.5 kyr, as originally proposed by Bond et al. [1997]. The performance of the SIO model is tested against two of the best resolved paleoclimate records: the Greenland ice core δ18O [North Greenland Ice Core Project members, 2004] and planktonic and benthic foraminiferal δ18O records of sea surface and deep-sea temperature from the Portuguese margin (core MD95-2042 [Shackleton et al., 2000]). Shackleton et al. [2000] have shown that the δ18O of planktonic foraminifer exhibits changes similar to those in the Greenland ice core, whereas the δ18O record of benthic foraminifer (water depth below 2200 m) varies in a manner similar to the Antarctic air temperature. At any rate, Rial and Saha conclude that the time integral of the surface ocean proxy history is proportional to that of the deep ocean temperature, and the latter is shifted by π/2 with respect to sea ice extent. Surprisingly, a similar relationship was found between methane-synchronized temperature proxies from Greenland and East Antarctic ice core records [Blunier and Brook, 2001]. Rial and Saha’s model output is reproduced using ECBilt-Clio, another well regarded model [Goosse et al., 2002].

To summarize up-to-date results on abrupt climate events of the last glacial cycle, Figures 1 and 2 show records representative of the global climate during this period. These records were selected based on the geographic location and temporal resolution adequate to assess millennial- or finerscale climate events. Regardless of the nature of climate archives, the climatic expression of the D-O cycles is evident from the southwest Pacific Ocean to northeastern Siberia.

3.2. Deglaciation Period

One of the most studied periods of the last glacial cycle is the time between 10 and 25 ka, commonly known as the last deglaciation. This period contains four very well studied abrupt climate events: the Younger Dryas (also known as H0 event, 11.6–12.9 ka); Bølling-Allerød (B-A) (14.6 ka); the so-called “Mystery Interval” (14.5–17.5 ka); and the last glacial maximum (LGM) (19–26.5 ka). The rationales behind focusing on the last deglaciation interval include the availability of paleoclimate records with high temporal resolution and a robust age control constrained by 14C–accelerator mass spectrometry and U-Th dating. It is not surprising that 5 out of 14 chapters in this volume focus on climate events from this interval. In addition, we outline several new hypotheses (relative to the deglaciation events) that have emerged since the 2009 conference.

Simulating the impact of freshwater discharge into the North Atlantic during the Heinrich and other meltwater events has gained a significant momentum after the work of Stouffer et al. [2006] and Liu et al. [2009]. Numerous modeling studies, where freshwater discharge was artificially added to the North Atlantic, have shown large climate impacts associated with abrupt AMOC changes. These results highlight the need to better understand the mechanisms of the AMOC variability under the past, present, and future climate conditions. The mechanisms controlling the recovery of the AMOC are, however, difficult to investigate as they require long simulations with models that can capture very different internal variabilities. Cheng et al. [this volume] describe the AMOC recovery stages from a model simulation of the last deglaciation built on the work of Liu et al. [2009]. These authors perform a remarkably long simulation using an atmosphere-ocean general circulation model [Liu et al., 2009], which allows for a clear identification of transient states of the recovery process including the AMOC overshoot phenomenon [Renold et al., 2010]. This 5000 year simulation time covers the last deglaciation period using insolation and fresh water estimated from paleoproxies as forcings. In particular, the simulation offers an explanation for the B-A warm period by a recovery of the AMOC in less than 400 years. An in-depth analysis of the AMOC recovery suggests that the two convection sites in the North Atlantic simulated in the model do not recover at the same time: the first stage of the recovery occurred in the Labrador Sea and was then followed by convection recovery in the Greenland-Iceland-Norwegian (GIN) Seas. The study suggests that reinitiation of convection in the Labrador Sea is related to the reduction of freshwater forcing, leading to an increase in salinity. Convection in the GIN Seas is then forced by a (partial) recovery of the AMOC and associated salinity transport. Overall, Cheng et al. provide a convincing mechanism, in which both local and remote salinity feedbacks play a role in the AMOC recovery.

Figure 1. Geographic distribution of paleoclimate proxy records for the last glacial cycle (stars) and the last deglaciation (circles) as shown on Figures 2 and 3.

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Figure 2. Comparison of various proxy records that demonstrate abrupt climate changes during the last glacial cycle. (a) Global stack of benthic foraminiferal oxygen isotopes (δ18O) [Lisiecki and Raymo, 2005]. (b) The δ18O in planktonic foraminifera Globigerina bulloides from the North Atlantic Integrated Ocean Drilling Program Site 1313 (H. Rashid, unpublished data, 2011). (c and d) Carbon isotopes (δ13C) in benthic foraminifera Cibicidoides wuellerstorfi from the South Atlantic Ocean Drilling Program Site 1089 [Charles et al., 2010] and core MD97-2120 from the SW Pacific Ocean [Pahnke and Zahn, 2005], respectively. (e and f) Alkenone (UK37) derived sea surface temperatures from the NE Atlantic core MD01-2443 [Martarat et al., 2007; Voelker and de Abreu, this volume, Figure 5] and core MD01-2412 from the NW Pacific Ocean [Harada et al., 2008], respectively. (g) Magnetic susceptibility record from Lake El’gygytgyn, NE Siberia [Nowaczyk et al., 2007]. (h) The δ18O in the Shanbao speleothems, NE China [Wang et al., 2008]. (i) Antarctic temperature reconstructed from the deuterium content ((ΔT) elevation corrected) in the Eastern Droning Maud Land facing the South Atlantic [Stenni et al., 2010]. (j) The δ18O in the North Greenland Ice Core Project ice core with a revised chronology [Svensson et al., 2008]. (k) June insolation at 65°N [Berger and Loutre, 1991]. All records are plotted according to their independent age models. Note that the vertical grey bar indicates the duration of marine isotope stage 4 and the two vertical discontinuous lines indicate the 20 and 130 ka time horizons.

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New data from four sediment cores in addition to the synthesis of earlier results from the Labrador margin [Rashid et al., this volume] shed light on the development of the YD event. There has been a long-standing debate about the origin of YD event [e.g., Mercer, 1969; Johnson and McClure, 1976; Ruddiman and McIntyre, 1981; Teller and Thorleifdson, 1983; Teller et al., 2005; Broecker et al., 1989; Andrews et al., 1995; Lowell et al., 2005]. Most of these studies propose freshwater flooding at sites of deepwater formation in the North Atlantic; however, a freshwater signature has not been found in proxy records near these sites thus far. Rashid et al. show that a δ18O depletion in planktonic foraminifers indicates a YD freshwater signature in the Hudson Strait but not in the more distal cores despite the presence of a H0 highcarbonate bed in these cores. Rashid et al. hypothesize that if the fresh water discharged through Hudson Strait was admixed with fine-grained detrital carbonates, it would form a hyperpycnal flow transported through the deep Labrador Sea Northwest Atlantic Mid-Ocean Channel to distal sites of the NW Atlantic Ocean such as the Sohm Abyssal Plain. With the release of entrained sediment, fresh water from the hyperpycnal flow would buoyantly rise and lose its signature by mixing with the ambient sea water. This mechanism explains a lack of YD δ18O depletion in sediment cores retrieved from the NW Atlantic. Thus, the long-sought smoking gun for the signature of YD (H0) freshwater flood [e.g., Broecker et al., 1989] remains elusive.

Applegate and Alley [this volume] evaluate the potential of using cosmogenic radionuclides (CRN) to date glacial moraine boulders to trace the geographic expression of abrupt climate change. The chapter provides a synthesis of the current state of the knowledge using CRN to determine the extent and retreat of glaciers in a terrestrial setting. Problems highlighted deal with selecting samples for exposure dating and with the calculation of exposure dates from nuclide concentrations. Failure to address these issues will yield too-young exposure dates on moraines that have lost material from their crests over time. In addition, geomorphic processes are likely to introduce errors into the calibration of nuclide production rates. The authors point to a conclusion from a recent study by Vacco et al. [2009] that the ages of true YD moraines should cluster around the end of YD, however the recent modeling study complicated this simplistic assumption. Applegate and Alley demonstrate their points using beryllium-10 exposure dates from two “not so primed” moraines of inner Titcomb Lakes (Wind River Range, Wyoming, United States) and Waiho Loop of New Zealand. The sampling strategies prescribed may help in minimizing problems with defining true age of the moraines. The chapter includes a guide for determining the minimum number of samples that must be collected to answer a particular paleoclimate question.

During the termination of the last ice age, atmospheric CO2 rapidly increased in two steps [Monnin et al., 2001], and atmospheric Δ14C (Δ14Catm) decreased by ∼190% between 17.5 and 14.5 ka [Reimer et al., 2009] (Figure 3), coined the “Mystery Interval” by Denton et al. [2006]. From two eastern Pacific sediment cores off Baja California, Marchitto et al. [2007] have documented two strong negative excursions of Δ14C, which were corroborated by another eastern Pacific record of Stott et al. [2009, this volume] and a record from the western Arabian Sea [Bryan et al., 2010]. One of those negative excursions of Δ14C corresponds to the Mystery Interval and the other to the YD. The emerging understanding is that during the LGM, there was a hydrographic divide between the upper 2 km of the water column and the deep Southern Ocean, where dense and salty deep waters hosted the depleted Δ14C [Adkins et al., 2002]. Accordingly, these depleted Δ14C waters were termed the “Mystery Reservoir.” The origin of the “Mystery Reservoir” has been linked to freshwater release into the North Atlantic during H1 and YD (H0) [Toggweiler, 2009]. It has been further inferred that the resulting AMOC weakening initiated a chain of events reaching to the tropics and Southern Hemisphere [Anderson et al., 2009]. According to this interpretation, as the northward heat flow slowed, the heat accumulated in the tropics and warmed the Southern Ocean, resulting in the reduction of sea ice extent around Antarctica. This sea ice retreat shifted the Southern Hemisphere westerlies poleward through a mechanism that remains unknown, allowing the ventilation of the deep ocean that stored CO2 and some other chemical components such as silica. As a result, a rise in atmospheric CO2, depleted Δ14Catm, and a dramatic increase in the accumulation of siliceous sediments were observed in the Southern Ocean [Anderson et al., 2009].

Broecker and Barker [2007], Broecker [2009], and Broecker and Clark [2010] launched an extensive search for the “Mystery Reservoir” in the Pacific Ocean and did not find any evidence for it. Stott and Timmermann [this volume] put forward a provocative “CO2 capacitor” hypothesis that resembles the “clathrate gun” hypothesis of Kennett et al. [2003] and offers a way to account for a decrease in the depleted Δ14Catm and increase in the atmospheric CO2. The main point of the “CO2 capacitor” hypothesis is the presumed existence of an unspecified source of CO2 that can explain both the glacial to interglacial CO2 change and the signal of old radiocarbon in the atmosphere during the deglaciation. This source is inferred to be a combination of liquid and hydrate CO2 in subduction zones and volcanic centers other than on mid-ocean ridges. During glacial times, CO2 hydrates in the cold intermediate ocean are stable and able to trap liquid CO2 bubbling up from below the seafloor. During deglaciation, a swath of these hydrates becomes unstable and releases radiocarbon-dead CO2 to the ocean-atmosphere system, thus increasing atmospheric CO2 and decreasing Δ14Catm. This hypothesis is likely to stimulate wide interest in the scientific community; however, it requires vigorous testing through experiments, observations, and modeling.

Figure 3. Paleoclimatic proxy records of the last deglaciation. (a) Biogenic opal flux in the Southern Ocean, interpreted as a proxy for changes in upwelling south of the Antarctic Polar Front [Anderson et al., 2009]. (b) Atmospheric CO2 from Antarctic Dome C [Monnin et al., 2001] placed on the Greenland Ice Sheet Project 2 (GISP2) timescale. (c) Baja California intermediate water Δ14C [Marchitto et al., 2007]. (d) The 231Th/230Th ratios from the Bermuda Rise (increasing values reflect reduced Atlantic overturning circulation) [McManus et al., 2004]. (e) The εNd of fossil fish tooth/debris record suggesting variations of water mass at Baja California [Basak et al., 2010]. (f) Ti/Ca ratios in bulk sediment from western equatorial Atlantic, interpreted as a proxy for high Amazon River runoff [Jaeschke et al., 2007]. (g) TEX86-derived surface temperature of Lake Tanganyika [Tierney et al., 2008]. (h) The δ18O in Shanbao speleothems [Wang et al., 2008]. (i and j) GISP2 methane and oxygen isotope ratios (δ18O) [Stuiver and Grootes, 2000; Blunier and Brook, 2001], respectively. All records are plotted according to their independent age models.

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In a related study that takes into account the relationship between the rise in atmospheric CO2 and temperature in low to high latitudes, Russell et al. [2009] reconstruct precipitation and temperature histories from southeast African Lake Tanganyika based on compound-specific hydrogen isotopes and paleotemperature biomarker index TEX86 in terrestrial leaf waxes. It has been shown that Lake Tanganyika paleotemperature followed the temperature rise in Antarctica at 20 ka [Monnin et al., 2001] as well as the Northern Hemisphere summer insolation at 30°N [Tierney et al., 2008]. Furthermore, temperatures in Lake Tanganyika began to increase ∼3000 years before the rise of atmospheric CO2 concentrations during the last ice age termination (Figure 3). This is a significant discovery that needs to be replicated from similar climate settings in other regions of the world. An extensive statistical testing is also needed to confirm whether the temperature record from Lake Tanganyika can be applied more generally throughout the tropics or whether it represents only a regional warming. If the reconstructed temperature history from Lake Tanganyika survives the scrutiny of proxy validation, it would strongly suggest that the primary driver for glacial-interglacial termination, at least for the last ice age, lies in the tropics rather than in high latitudes.

3.3. Holocene Climate

The conventional view based primarily on Greenland ice core δ18O records suggests that climate in the Holocene (current interglacial) was rather uniform and remarkably stable in comparison to the preceding glacial and interstadial periods (MIS 2–3). However, as more new paleoclimatic records emerge, such as palynological and paleolimnological data from North America [Viau et al., 2006; Gajewski and Viau, this volume] and tropical climate records reconstructed from high-elevation ice cores [Thompson, this volume], it appears that Holocene climate variability may have been greater than has been assumed based on prior data sets.

In the early Holocene, large climatic fluctuations were related to freshwater pulses from the disintegrating LIS into the western North Atlantic [e.g., de Vernal et al., 2000]. Using palynological data (terrestrial palynomorphs and marine dinocysts) from the Newfoundland and northern Scotian Shelf sediment cores, Levac et al. [this volume] assess the impact of fresh water on the circulation over the Labrador margin and related climate change during the final demise of the proglacial Lake Agassiz. The authors identify a circa 8.7 ka detrital carbonate bed accompanied by two meltwater pulses that lowered sea surface salinity (SSS) in the coastal water of Newfoundland. In contrast, data from the Scotian Shelf do not show changes in the SSS at this time. The divergent impact of freshwater pulses at these two sites is explained by the absence of a diluting effect of the North Atlantic Current at the Newfoundland margin as opposed to the warmer Scotian Shelf. Keigwin et al. [2005] have documented a cooling event around 8.5 ka, likely correlative to that identified by Levac et al., as far south as Cape Hatteras, suggesting a large geographic impact of the Lake Agassiz drainage. Surprisingly, such a pronounced cooling event has not been found in the northern Labrador Sea and Baffin Bay, possibly because of a lack of high-resolution records due to the stronger winnowing conditions of the Labrador coastal current [Chapman, 2000; Rashid et al., 2011].

Ruzmaikin and Feynman [this volume] investigate a quasiperiodic 1500 year climate oscillation during the Holocene and its relation to the AMOC. They employed a simple conceptual model to simulate the forcing mechanism for this oscillation. The analysis of six paleoclimatic records from the Atlantic Ocean and sunspot numbers using wavelet and empirical mode decomposition methods allowed the authors to overcome the leaks from one mode to another, a common problem in conventional band-pass filtering. Ruzmaikin and Feynman conclude that solar forcing does not drive the 1500 year climate cycle “directly,” contrary to the inference by Bond et al. [2001]. Instead, they suggest a simple model of excitation of this oscillation in a nonlinear dynamical system with two equilibrium states. They reason that the transitions between the two states are caused by the noise, ocean, and solar variability, and the implication is that the beats between the centennial ocean variability and ∼90 year solar cycles produce the 1500 year oscillation in the noisy system. This conceptual framework for the 1500 year oscillation requires more rigorous testing by more sophisticated, coupled ocean-atmosphere models.

Gajewski and Viau [this volume] suggest that the Holocene in North America can be divided into four general periods with abrupt transitions at ∼8, 6, and 3 ka, generally consistent with the hypothesis of Ruzmaikin and Feynman