AN ABSTRACT OF THE THESIS OF

AN ABSTRACT OF THE THESIS OF

David J. Leydet for the degree of Master of Science in Geography presented on May

3, 2016.

Title: Eastward Routing of Glacial Lake Agassiz Runoff caused the Younger Dryas

Cold Event

Abstract approved:

______________________________________________________

Anders Carlson

The purpose of this thesis is to analyze an abrupt case of climate change in the past as a means to understand the mechanisms that force climate change. By looking to past analogs of climate change, we hopefully will gain an understanding of these events, which could be used to further our understanding of future climate change.

In this light, I analyze the case of the Younger Dryas (YD), an abrupt cooling event that occurred from ~12.9 ka to ~11.7 ka. We investigate several hypotheses regarding the cause of the YD and attempt to determine the forcing mechanism for this abrupt cooling event. I use 10 Be surface exposure dating as our method for dating retreat of the Laurentide Ice Sheet from the eastern outlets of glacial Lake Agassiz, a large pro-glacial lake that formed during the last deglaciation whose drainage into the

North Atlantic is hypothesized to have caused the YD via a slowing of ocean overturning circulation.

I find that the eastern outlets of glacial Lake Agassiz begin to deglaciate at 14.0 ±

0.3 ka with ice retreating from the key Lake Kaministikwia outlet at 13.0 ± 0.3 ka,

concurrent with the onset of the YD. I also date retreat from the Steep Rock moraine at 13.8 ± 0.2 ka and retreat from the Marks moraine by 11.0 ± 0.4 ka.

I use our chronology along with other terrestrial and marine proxies to reconstruct the meltwater routing history of Lake Agassiz. Specifically, the Gulf of St. Lawrence isotopic record indicates meltwater routing through Eastern Outlets, peaking at ~12.6 ka. Subsequently, the isotopic record of the Arctic Ocean near the mouth of the

Mackenzie River indicates meltwater routing beginning at ~12.4 ka and peaking at

12.2 ka. I argue that the timing of these meltwater pathways support the hypotheses that the YD was caused by freshwater forcing, weakening the Atlantic Meriodional

Overturning Circulation, and thus cooling the climate of the Northern Hemisphere.

The results demonstrate the importance of meltwater routing on the climate system and will be important in understanding the implications of future ice sheet-oceanclimate interactions in a climatically changing world.

©Copyright by David J. Leydet

May 3, 2016

All Rights Reserved

Eastward Routing of Glacial Lake Agassiz Runoff caused the Younger Dryas Cold

Event by

David J. Leydet

A THESIS submitted to

Oregon State University in partial fulfillment of the requirements for the degree of

Master of Science

Presented May 3, 2016

Commencement June 2016

Master of Science thesis of David J. Leydet presented on May 3, 2016

APPROVED:

Major Professor, representing Geography

Dean of the College of Earth, Ocean, and Atmospheric Sciences

Dean of the Graduate School

I understand that my thesis will become part of the permanent collection of Oregon

State University libraries. My signature below authorizes release of my thesis to any reader upon request.

David J. Leydet, Author

ACKNOWLEDGEMENTS

Many endeavors, scientific or otherwise, are often the collective efforts of a team of individuals who dedicate their time, expertise, and effort for a collective goal. I have been extremely fortunate to have an outstanding team to work with while here at

Oregon State University. I cannot fully express in a short acknowledgements section my heartfelt gratitude to each and every person who has contributed to the work presented here, but I want all of you to know what a profound impact you have had on me personally and professionally.

First and foremost I must thank Dr. Anders Carlson for agreeing to take me on as one of his students. I know it must have been challenging to accept someone into the program and the paleoclimate group who had almost no formal training or education in the geosciences. You have a tremendous depth of knowledge and I thank you for your patience as I developed as a geoscientist. I appreciate your candor, support, and positive attitude you had with me during my time here. All of my accomplishments here would not have been possible without you!

The community of students at Oregon State University is tremendous. I have been fortunate to engage in several meaningful and productive dialogues with many of my peers here. Whether you think so or not, each of you has had a direct influence on this work, but more importantly, each of you has had a direct influence on my personal and professional development. I would be remiss if I didn’t publically acknowledge the following individuals for their impact on this work: Aaron Barth - thank you for your willingness to teach me how to process cosmogenic samples.

Most people won’t truly understand the amount of time you dedicated to help make

this project possible. You have one of the best attitudes towards life that I have encountered. I know you will go on to do great things and I am lucky to have learned from you. Gaylen Sinclair – You are one of the most gifted and intelligent people I have ever met. You were instrumental in helping me understand much of the key concepts in paleoclimate and geography as well as vigorously debating pop culture topics in Burt Tower. Thank you for being such a great officemate and even better friend. Sam Hooper – Work life balance was an important part of my mantra while here at grad school. Thank you for the endless hours we spent driving to and from our ski touring, mountaineering, and climbing adventures. I have definitely learned a great deal from you and your experiences. I hope we continue to climb in the future!

Hopefully, I am not missing anyone, but I also need to thank: Nilo Bill, Jeremy

Hoffman, Andrea Balbas, Brendan Reilly, Jon Edwards and the rest of the paleoclimate crew for their discussions and friendships; Anna Glueder and Heather

Bervid for being awesome officemates and friends; Kate Jones, Melissa McCracken,

Melva Treviño, and Dan Stephen – for being the best geography cohort a guy could ask for!

I also would like to express my thanks and appreciation for the faculty members for their support and mentorship specifically, Peter Clark, Robert Kennedy, Larry

Becker, Aaron Wolf, Anne Nolin, Ed Brook, Roy Haggerty, and Julia Jones. It was great to have individuals who were invested in my growth not only as a geographer and scientist, but as a person as well. Also, I must thank Randy Rosenberger for serving as my graduate council representative.

I must thank my co-authors on this project for all of their extensive work and dedication to this project. Jim Teller – You are a titan in the paleoclimate world, especially with the Younger Dryas and Lake Agassiz. Thank you for your input and your down-to-earth nature! Andy Breckenridge – Thank you for all of your work on lake level reconstructions and your in-depth knowledge of the Lake Agassiz outlets.

Aaron Barth – Thank you again for your help in the lab. Dave Ullman – Thank you for your help on the columns as well as the statistical analysis for cosmogenic ages. I also appreciate our talks on a wide range of subjects! Gaylen Sinclair – thank you for all of your help with samples and running analysis on the ages. Glenn Milne – for providing uplift correction data. Marc Caffee – Thank you for all of the smooth interactions with Purdue’s Rare Isotope Measurement Laboratory.

Thank you to the United States Army who gave me opportunity to broaden myself as a leader. The credit goes to all of the Soldiers I have been privilege to serve along side for the past eight years. I wouldn’t be here without all of them. COL Mark

O’Donnell, COL Stephen Michael, COL Keith McKinley, COL Patrick Michaelis,

COL Andrew Lohman, LTC Dan Heape, MAJ James Browning, MAJ Rick

Montcalm, and MAJ Paul Tanghe are leaders of mine who supported and fostered my goal to complete graduate school and teach at the United States Military Academy.

Thank you all for your support, mentorship, and friendship. I also must thank the

National Science Foundation who funded the research portion of this thesis.

Finally, and most importantly, I would like to express my love and appreciation to my family for their enduring love and support during my graduate studies. To my parents, Dennis and Theresa Leydet thank you for everything you have done for me

and the girls while we have been here at Oregon. To Rick Van Wickler, Sarah

Hoskins, Mike and Joann Jardine, thank you for your endless support. Lastly to my wife Stacey and my daughters Abby and Ellie, you cannot fathom how much my love and appreciation grows for you everyday. Stacey, you are the best decision I have ever made and I thank you for supporting my dreams and passions. Abby and Ellie, you are both the apple of my eye, and I cherish the time I get to spend with you. It is my hope that we will leave this earth to you and future generations in better condition than we found it. I love you both.

CONTRIBUTIONS OF AUTHORS

Anders Carlson and I collected samples from the study site. I performed the sample processing with assistance from Aaron Barth and David Ullman. Anders

Carlson, David Ullman, Gaylen Sinclair and I performed calculations and statistical analysis on the 10 Be data. Glenn Milne provided uplift correction data. Marc Caffee oversaw AMS measurements at PRIME Lab. Anders Carlson was responsible for primary idea development. Anders Carlson and I co-wrote the manuscript for

Chapter 2 with input from the co-authors.

TABLE OF CONTENTS

Page

1 Introduction...……………………………………………………………………… 1

2 Eastern routing of glacial Lake Agassiz runoff caused the YD cold event…………6

2.1 Abstract …………………………………………………………………...7

2.2 Introduction ……………………………………………………………….7

2.3 Methods ………………………………………………………...................8

2.4 Results……………………………………………………………………..9

2.5 Discussion ……………………………………………………………….10

2.6 Conclusion.................................................................................................12

2.7 References………………………………………………………………..13

2.8 Figures……………………………………………………………………19

2.9 Supplementary Information……………………………………………...21

3 Conclusion ………………………………………………………………………...32

Bibliography…………………………………………………………………………36

Appendix A: Procedure for preparing and processing 10 Be cosmogenic samples …..43

LIST OF FIGURES

Figure Page

3-1. Overview Map, Map of Deglacial Ages, Eastern Outlet Map……...…………...19

3-2. Comparison of deglacial chronology, marine records, and terrestrial record......20

Extended Data Figure 3-1. Individual 10 Be Ages with outliers identified……....…..24

A-1. Photo of a typical sample boulder (FR-8-15)…………………………………..57

A-2. Photo of a typical sample boulder (LN-10-15)…………………………………58

LIST OF TABLES

Table Page

3.1 Cosmogenic Surface Exposure Ages for the Eastern Outlet by sample site…25

3.2 Cosmogenic Surface Exposure Ages by production rate and scaling………..26

A.1 Parent Regeants……………………………………………………………... 56

A.2 Working Solutions……………………………..…………………………… 56

1

CHAPTER 1: Introduction

Global climate change has been at the forefront of public policy and science.

Anthropogenic green house gas emissions have a major impact on the climate system and subsequently the interactions of the coupled human-natural system 1 . As we enter an unprecedented period of Earth’s climatic history, understanding forcing and response mechanisms to changing climate will be critical for evaluating our current environmental policy.

Predictions and projections of future climate scenarios must be based on our ability to understand the climate system. In that sense, we rely on modern observations and paleoclimate records to inform our understanding of an intricate and dynamic climate system. Here we examine one example of an abrupt climate change event during the last deglaciation, furthering our understanding of the climate system.

The Younger Dryas was a brief return to a cooler climate during the last deglaciation, named for the plant Dryas octopetala, a cold tolerant plant that returned to northern Europe following the Ållerød warm interval from 14.0 – 12.9 ka 2 .

Records from ice cores in Greenland confirmed this cool period, assigning an age of

12.9 -11.7 ka 3 . As such, the Younger Dryas was a period of cooler temperatures and lower precipitation rates in the Northern Hemisphere 4 . This brief return to cooler climate conflicted with the orbital forcing of deglaciation, as summer insolation at

65° N neared its peak during this time 2 . Therefore, some other mechanism must have been responsible for forcing the climate during this time period.

2

Freshwater from ice-sheet sources is a key forcing mechanism for global climate change and is the leading hypothesis used to explain the Younger Dryas 5,6,7 .

Perturbations to the formation of North Atlantic Deep Water (NADW) slowed the

Atlantic Meriodional Overturning Circulation (AMOC), which in turn limits heat transport to the Northern Hemisphere 8 . This reduction in strength would cause cooling in the Northern Hemisphere similar to that of the Younger Dryas 1,5,6,9 .

The objective of this thesis is to examine and directly date the deglaciation of the eastern outlets of glacial Lake Agassiz, a large proglacial lake that formed as the

Laurentide Ice Sheet retreated during the last deglaciation, which stored a large volume of freshwater. The deglacial chronology of the eastern outlets of Lake

Agassiz is constrained by a variety of dating methods, which include predominantly radiocarbon ages that conclude that the eastern outlets were not ice-free at the start of the Younger Dryas 9,10,11 . However these are minimum-limiting ages, with some older ages still suggesting that the eastern outlets were ice-free at the beginning of the

Younger Dryas 7,12 . I seek to answer the question: Did the eastern outlet of Lake

Agassiz open at the beginning or prior to the start of the Younger Dryas? I aim to answer this question using 10 Be cosmogenic surface exposure dating to directly date the deglaciation of the Laurentide Ice Sheet from the eastern outlets. The answer to this question would help resolve the meltwater routing history of Lake Agassiz and subsequently support or refute the hypothesis that meltwater routing through the eastern outlets caused the Younger Dryas.

Additionally, my dating efforts add to the existing Laurentide ice-sheet deglacial chronology. This chronology can be incorporated into models that examine glacial-

3 ocean-atmosphere climate interactions. This could assist in the way we understand the global climate system and predict its response. This is particularly useful given that our current climate is warming causing mass loss to most of the world’s glaciers and ice sheets 1 . The Greenland Ice Sheet is located proximal to the location of

NADW formation and could contribute ~7.5 meters of sea-level rise to this sensitive part of the ocean 13 . Consequently, the results of this work that document past sensitivity of NADW formation to ice-sheet meltwater are important to future broader projections of the Earth’s climate, especially in a warming world.

4

References

1.

IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of

Working Groups I, II and III to the Fifth Assessment Report of the

Intergovernmental Panel on Climate Change [Core Writing Team, R.K.

Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

2.

Carlson, A.E. Global Younger Dryas. Encyclopedia of Quaternary Science 3 ,

126-134 (2013).

3.

Rasmussen, S.O. et al.

A new Greenland ice core chronology for the last glacial termination. J. Geophys. Res.

111 , doi: 10.1029/2005JD006079

(2006).

4.

Alley, R.B. The Younger Dryas cold interval as viewed from central

Greenland. Quat. Sci. Rev.

19 , 213-226 (2000).

5.

Rooth C. Hydrology and ocean circulation. Progress in Oceanography 11 ,

131–149 (1982).

6.

Broecker, W.S. et al. Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode. Nature, 341, 318-321 (1989).

7.

Carlson, A.E., Clark, P.U. Ice-sheet sources of sea-level rise and freshwater discharge during the last deglaciation. Rev. Geophys . 50 , doi:

10.1029/2011RG000371 (2012).

8.

Clark, P.U. et al. Freshwater forcing of abrupt climate change during the last deglaciation. Science 293 , 283-287 (2001).

5

9.

Teller, J. T. et al. Alternative routing of Lake Agassiz overflow during the

Younger Dryas: new dates, paleotopography, and a re-evaluation. Quat. Sci.

Rev.

24 , 1890-1905 (2005).

10.

Lowell, T.S. et al. Testing the Lake Aggassiz Meltwater Trigger for the

Younger Dryas. EOS, 86, 365-373 (2005).

11.

Lowell T.V. et al. Radiocarbon deglaciation chronology of the Thunder Bay,

Ontario area and implications for ice sheet retreat patterns. Quat. Sci. Rev.

28 ,

1597–1607 (2009).

12.

Carlson, A.E., Clark, P.U., Hostetler, S.W. Comment: Radiocarbon deglaciation chronology of the Thunder Bay, Ontario area and implications for ice sheet retreat patterns. Quat. Sci. Rev. 28 , 2546-2547 (2009).

13.

Alley, R.B., Clark, P.U., Huybrechts, P., Joughlin, I. Ice-Sheet and Sea-Level

Changes. Science, 310 , 456-460 (2005).

6

CHAPTER 2:

Eastward routing of glacial Lake Agassiz runoff caused the Younger Dryas cold event

David J. Leydet 1,* , Anders E. Carlson 1 , James T. Teller 2 , Andrew Breckenridge 3 ,

Aaron Barth 1 , David J. Ullman 1 , Gaylen Sinclair 1 , Glenn A. Milne 4 Marc Caffee 5

1 College of Earth, Ocean, and Atmospheric Science, Oregon State University,

Corvallis, OR, USA

2 Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada

3 Natural Sciences Department, University of Wisconsin-Superior, Superior, WI, USA

4 Department of Earth and Environmental Sciences, University of Ottawa, Ottawa,

ON, Canada

5 Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA

*corresponding author: david.j.leydet.mil@mail.mil

This manuscript has been prepared for submission to the journal Nature . The formatting reflects the requirements of the journal.

7

The Younger Dryas (YD, ~12.9-11.7 ka) was a return to cold conditions in parts of the North Hemisphere during the last deglaciation 1 . The YD was originally hypothesized to be caused by routing of glacial Lake Agassiz basin runoff from the Gulf of Mexico to the North Atlantic due to the opening of the Eastern

Outlets of Lake Agassiz from Laurentide ice-sheet (LIS) retreat out of Lake

Superior 2-4 . This runoff routing would have caused a reduction in Atlantic

Meriodional Overturning Circulation (AMOC) strength and the YD 1-4 . Recent arguments for the cause of the YD favor northward routing of Lake Agassiz runoff to the Arctic Ocean (Northern Outlet) 5,6 , based on minimum-limiting 14 C dates from the Eastern Outlets that could indicate that these Outlets were not ice-free at the start of YD 5,7 . Here we present new 10 Be surface exposure ages from the Eastern Outlets that show the area was ice-free at the YD onset. We date opening of the main Eastern Outlet at 13.0±0.3 ka, concurrent with increased salinity in the Gulf of Mexico 8 , and decreased salinity in the lower

Great Lakes, St. Lawrence lowlands and Gulf of St. Lawrence 9-15 . Salinity records from the Gulf of St. Lawrence 13,14,16 and isostatic rebound modeling suggest Eastern Outlet abandonment ~12.2 ka 17,18 , with northward routing of runoff until the end of the YD 6,13,19 . Our results confirm that eastward routing of

Lake Agassiz runoff to the North Atlantic caused the YD while later northward routing sustained the cold event.

The Southern Outlet of glacial Lake Agassiz, which routed runoff down the

Mississippi River to the Gulf of Mexico, was abandoned at the start of the YD 4,8,19

(Fig. 1a). Following the Southern Outlet abandonment, Lake Agassiz levels fell,

8 reaching a low, called the Moorhead low, by ~12.4 ka 17,20 , but it is unclear where

Lake Agassiz basin runoff was routed during and after this lake-level fall.

Determining the cause of the YD is critical to understanding AMOC sensitivity to freshwater discharge and the general causes of abrupt climate events as the YD is viewed as the canonical abrupt climate event. In addition to the minimum-limiting

14 C ages (Fig. 3-2a), the eastward routing hypothesis as the cause of the YD has been questioned due to the lack of channelization or depositional evidence in the Eastern

Outlets (Fig. 3-1a) from an outburst flood from Lake Agassiz as its lake level fell to the Moorhead low, if this is assumed to occur in a year or less 5,7,21 . However, such short-lived floods fail to produce a millennia-long reduction in AMOC strength in climate models, questioning the necessity for a flood in causing the YD 4,19,22 . The evidence for northward Lake Agassiz basin routing at the start of the YD is also mainly based on minimum-limiting 14 C ages from the Northern Outlet 6,23 (Fig. 3-1a).

Although flood deposits have been identified at the mouth of the Mackenzie River 6 , these were more likely deposited near the end of the YD rather than at the start of the

YD as the flood removed age control on its onset and the age on its cessation from conformable fluvial sands is ~11.9 ka 19 . High-resolution ocean models have been interpreted as suggesting only northward Lake Agassiz routing can slow the

AMOC 24 . However, these decadal-length simulations used fixed modern wind fields, making them inapplicable to the last deglaciation 19 when the LIS greatly altered wind fields that shifted the subpolar North Atlantic gyre southward 25 increasing AMOC sensitivity to eastern LIS discharge. Likewise, the Younger Dryas is a millennial, not

9 decadal, timescale event. What has confounded confirmation of the forcing of the YD is a direct deglaciation chronology for the Eastern Outlets of Lake Agassiz.

Here, we use 10 Be surface exposure ages from glacial erratic boulders to directly date

LIS retreat from the Eastern Outlets. We collected boulder samples from three

Eastern Outlets of Lake Agassiz 5,17 : North Lake, Flatrock Lake, and the Lake

Kaministikwia (Kam) outlets (Fig. 3-1b, c) (Methods). We also collected samples from behind the Dog Lake/Marks Moraines (Fig. 3-1b) that were deposited during

LIS readvance at the end of the YD 5,26 . After excluding five outliers (Extended Data

Fig. 3-1), we calculated the error weighted mean and uncertainty for each site (Fig. 3-

1b) (Extended Data Table 3-1, 3-2).

The southernmost site, the North Lake outlet, has a mean deglacial age of 14.0±0.3 ka

(n=3, 1 outlier). The next site northward, Flatrock Lake outlet, has a mean age of

13.8±0.2 ka (n=4, 2 outliers). Further north, the Lake Kam outlets became ice free at

13.0±0.3 ka (n=3, 2 outliers). Our northernmost site dates LIS retreat from the Dog

Lake/Marks Moraines by 11.0±0.4 ka (n=2, 1 outlier).

Our chronology indicates that northward LIS retreat from the Eastern Outlets began

~14.0 ka when the North Lake outlet opened, followed by opening of the Flatrock

Lake outlet ~13.8 ka (Fig. 3-2a). Isostatic rebound modeling of Lake Agassiz shorelines shows that both of these outlets could route Lake Agassiz basin runoff eastward when the lake occupied the high shorelines before the Moorhead low 5,17 .

While these two outlets were ice free before YD onset, 10 Be ages from the Gogebic

Mountains in northern Wisconsin (Fig. 3-1b, 3-2a) indicate the LIS still covered Lake

10

Superior until 13.2±0.4 ka 27 , preventing eastward routing of Lake Agassiz basin runoff.

Our Lake Kam 10 Be ages indicate that these outlets deglaciated at 13.0±0.3 ka (Fig. 3-

2a) when the LIS margin had retreated into Lake Superior 27 . The opening of the Lake

Kam outlets is coincident with abandonment of the Southern Outlet 4,19 , decreased meltwater discharge to the Gulf of Mexico indicated by increased δ 18 O of seawater

(sw) 8 (Fig. 3-2c), and the start of the YD at 13.0-12.8 ka 28 (Fig. 3-2d). The Lake Kam region encompasses multiple sub-outlets of the Shebandowen, North, Seine, and

Oskondaga Rivers (Fig. 3-1c). These sub-outlets could route Lake Agassiz basin runoff prior to the Moorhead low lake level and during the early Moorhead low lake level 5,17 that was reached by ~12.4 ka 20 (Fig. 3-2a).

Our direct chronology for eastward Lake Agassiz basin routing at the start of the YD is supported by downstream evidence for freshening of the lower Great Lakes and the

St. Lawrence lowlands at the start of the YD 8-12 , with Sr-isotopic data indicating the enhanced meltwater originated from the Lake Agassiz basin 8 . Gulf of St. Lawrence planktonic δ 18 O decreases at the start of the YD, which is further decreased after accounting for the effects of YD cooling (i.e., δ 18 O sw

) 4,13,14,16,19 (Fig. 1b). The micropaleontology of the Gulf of St. Lawrence also indicates decreased salinity at the start of the YD 15 . Planktonic foraminifera Sr-isotopes, U/Ca and Mg/Ca trace the increased runoff to be from the Lake Agassiz basin 13,19 . Thus, there is clear evidence for the opening of the Eastern Outlets of Lake Agassiz at the start of the YD and its runoff routing to the North Atlantic where it could cause the AMOC reduction and the YD.

11

Following deglaciation of the Eastern Outlets, isostatic rebound would close these

Outlets, with the late Moorhead low lake level not being capable of eastward routing 17 . Gulf of St. Lawrence records show decreased meltwater discharge at ~12.2 ka 13,15,16 (Fig. 3-2b) in excellent agreement with the rebound-estimated Eastern Outlet abandonment at ~12.2 ka 17 . The only viable late Moorhead low outlet of Lake

Agassiz is the Northern Outlet 5,17 . While not directly dated, the Northern Outlet of

Lake Agassiz (Fig. 3-1a) was deglaciated prior to ~11 ka based on minimum-limiting

14 C dates 6,23 (Fig. 3-2a). However, the opening of the Northern Outlet cannot be pushed back to the start of the YD, as glacial isostatic modeling of Lake Agassiz shorelines requires the LIS to be positioned southwest of the Northern Outlet until after the start of the YD 18 . Optically stimulated luminescence ages on fluvial sands conformable with an underlying flood deposit from the mouth of the Mackenzie River

(Fig. 3-1a) indicate a flood ended ~11.9 ka 6,19 (Fig. 3-2a), with an adjacent record in the Arctic Ocean (Fig. 1a) showing decreased planktonic δ 18 O ~12.2-11.9 ka 29 (Fig.

3-2b). Consequently, we suggest that Lake Agassiz basin runoff was routed to the

Arctic Ocean midway through the YD 13 with this routing ceasing at the end of the YD

11.8-11.6 ka 28 .

Abandonment of the Northern Outlet at the end of the YD raises the question as to where Lake Agassiz basin runoff was subsequently routed. 14 C dates from the Lake

Gribben Forest bed (Fig. 3-1b) document LIS readvance to southern Lake Superior at the end of the YD 26 (Fig. 3-2a), likely coincident with the deposition of the Dog Lake and Marks Moraines west of Lake Superior 5 (Fig. 3-1b). Our southern Lake Nipigon

10 Be ages from within the Dog Lake/Marks Moraines (Fig. 3-1b) date LIS retreat

12 from these moraines by ~11.0 ka (Fig. 3-2a). Thus Lake Agassiz basin runoff could not be rerouted to the Eastern Outlets at the end of the YD. There is, however, a decrease in Gulf of Mexico δ 18 O sw

at the end of the YD 8 (Fig. 3-2c) that may indicate reoccupation of the Southern Outlet.

Our new 10 Be chronology for the Eastern Outlets of Lake Agassiz confirms that eastward routing of the Lake’s basin runoff to the North Atlantic occurred at the start of the YD, and thus was the likely trigger of the canonical abrupt climate event 1 .

However, subsequent northward routing of Lake Agassiz basin runoff half way through the YD allowed the YD to persist for another ~0.5 ka until basin runoff was rerouted to the Southern Outlet, allowing the AMOC to strengthen 4 . Here we have reconciled numerous YD-related terrestrial and marine records, showing that this abrupt Northern Hemisphere cold event was caused by a more nuanced forcing than just eastward 2-4 or northward 5,6,24 freshwater forcing, with AMOC and climate sensitivities to small changes in both the northern North Atlantic and Arctic hydrologic budgets as the changes in associated freshwater discharge are only ~0.1

Sverdrups (10 6 m 3 s -1 ) or less 4,13 .

13

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Carlson, A.E., Clark, P.U., Rayburn, J.A., Leydet, D.J. Comment on “The deglaciation over the Laurentide Fan: History of diatoms, IRD, ice and fresh water”. Quat. Sci. Rev.

doi: 10.1016/j.quascirev.2015.12.001 (2016).

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Levac, E., et al.

Evidence for meltwater drainage via the St. Lawrence River

Valley in marine cores from the Laurentian Channel at the time of the

Younger Dryas. Global Planet. Change , 130, 47-65 (2015).

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Gil, I.M., Keigwin, L.D., Abrantes, F. The deglaciation over Laurentian Fan:

History of diatoms, IRD, ice and fresh water. Quat. Sci. Rev. 129, 57-67

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Breckenridge, A. The Tintah-Campbell gap and implications for glacial Lake

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124-134 (2015).

18.

Gowan, E.J. et al. A model of the western Laurentide Ice Sheet, using observations of glacial isostatic adjustment. Quat. Sci. Rev.

139, 1-16 (2016).

19.

Carlson, A.E., Clark, P.U. Ice-sheet sources of sea-level rise and freshwater discharge during the last deglaciation. Rev. Geophys . 50, doi:

10.1029/2011RG000371 (2012).

20.

Fisher, T.G. et al. The chronology, climate, and confusion of the Moorhead

Phase of glacial Lake Agassiz: new results from the Ojata Beach, North

Dakota, USA. Quat. Sci. Rev. 27, 1124-1135 (2008).

21.

Voytek, E.B. et al. Thunder Bay, Ontario, was not a pathway for catastrophic floods from Glacial Lake Agassiz. Quat. Int. 260, 98-105 (2012).

22.

Meissner, K.J., Clark, P.U. Impact of floods versus routing events on the thermhaline circulation. Geophys. Res. Lett. 33, doi: 10.1029/2006GL026705.

23.

Fisher, T. G., Waterson, N., Lowell, T.V., Hajdas, I. Deglaciation ages and meltwater routing in the Fort McMurray region, northeastern Alberta and northwestern Saskatchewan, Canada. Quat. Sci. Rev.

28, 1608-1624 (2009).

24.

Condron, A., Winsor, P. Meltwater routing and the Younger Dryas. Proceed.

Nat. Acad. Sci. doi: 10.1073/pnas.1207381109 (2012).

25.

Ullman, D.J. et al. Assessing the impact of Laurentide Ice Sheet topography on glacial climate. Clim. Past 10, 487-507 (2014).

16

26.

Lowell T.V., Larson G.J., Hughes J.D., Denton G.H. Age verification of the

Lake Gribben forest bed and the Younger Dryas advance of the Laurentide Ice

Sheet. Can J. Earth Sci.

36, 383–393 (1999).

27.

Ullman, D.J. et al. Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation. Geology 43, 23-26 (2015).

28.



111, doi: 10.1029/2005JD006079

(2006).

29.

Andrews, J.T., Dunhill, G. Early to mid-Holocene Atlantic water influx and deglacial meltwater events, Beaufort Sea slope, Arctic Ocean. Quat. Res.

61,

14-21 (2004).

30.

Dyke, A.S. An outline of North American Deglaciation with emphasis on central and northern Canada. Quaternary Glaciations-Extant and Chronology,

Part II 2b, 373-424 (2004).

Acknowledgements: Research was funded by United States National Science

Foundation award EAR-1449946. Comments from and discussions with Peter U.

Clark improved an earlier version of this manuscript.

Author Contributions: DJL and AEC collected samples from the study site. DJL performed the sample processing with assistance from AB and DJU. DJL, AEC,

DJU, and GS performed calculations and statistical analysis on the 10 Be data. MC oversaw AMS measurements at PRIME Lab. AEC was responsible for primary idea development. DJL and AEC co-wrote the manuscript with input from co-authors.

17

Fig. 3-1: Map of study area. (a) LIS extent at ~14 ka and ~11 ka 30 with study area noted by the red box. Lake Agassiz basin runoff routes 4,5 and locations of records in

Fig. 3-2 are noted. (b) Digital elevation model of western Lake Superior and moraines 7,26 , with locations of sample sites (red circles): NL=North Lake,

FR=Flatrock Lake, KM=Lake Kaministikwia, and LN=Lake Nipigon. Number of samples, and the error-weighted mean age for each site are shown. Outlier ages are transparent/light gray. The red star indicates surface exposure ages (n=7) from the

Gogebic Mountains 27 . Location of the Lake Gribben Forest and the Porcupine Mt.

Moraine deposited at the end of the YD are also shown 26 . (c) Eastern Outlets are indicated by blue triangles with runoff paths shown by blue arrows 5,17 .

Fig. 3-2: Records of Lake Agassiz basin routing. (a) Outlet ages (all means are error weighted; all uncertainties are 1 σ ). Blue diamonds are our new 10 Be ages; blue circle is the 10 Be age Gogebic Mountains deglaciation 27 ; light blue bars are associated oldest

Eastern Outlet 14 C ages 7,30 that show a significant lag in vegetation arrival following deglaciation; red bars are 14 C ages from the Moorhead (MH) low 20 ; black bars are

Lake Gribben Forest (GF) bed 14 C ages 26 ; green bars are 14 C ages from the Athabasca

Valley (AV; Northern Outlet) 23 ; pink triangle is the mean of two optically stimulated luminescence (OSL) ages from the Mackenzie River (MR) mouth 6 . (b) δ 18 O sw

from the Gulf of St. Lawrence (Eastern Outlet; purple) 13,16 and ice-volume free (ivf) δ 18 O from off of the Mackenzie River in the Arctic Ocean (Northern Outlet; orange) 29 . (c)

Gulf of Mexico δ 18 O sw

that records discharge down the Mississippi River (Southern

Outlet; green) 8 , with a five-point running mean. (d) North Greenland Ice Core Project

(NGRIP) δ 18 O (ref. 28). Gray bars are the uncertainty in YD onset and end 28 . Yellow

18 shading denotes the period of eastward Lake Agassiz routing followed by northward routing followed by northward routing (blue shading) at ~12.2 ka (vertical line).

19

Figure 3-1

20

Figure 3-2

21

Methods

Geomorphic setting and field methods.

The overall study site encompasses the area surrounding Thunder Bay, Ontario. Four sites were selected based on work that previously defined the Eastern Outlets of the glacial Lake Agassiz drainage basin based on paleotopography 6,17 . We sampled the North Lake outlet, Flat Rock outlet, and Lake Kaministikwia (Kam) outlets (Fig. 1b, c). The Lake Kam region encompasses several sub-outlets that all route through our sampling are (Fig. 1c). The

Steep Rock Moraine lies between the North Lake and Flatrock Lake outlets, with our

Flatrock Lake ages thus dating ice retreat from the moraine. Likewise, the Brule

Moraine is between the Flatrock Lake and Lake Kam outlets, with the Lake Kam ages dating ice retreat from this moraine. We also sampled south of Lake Nipigon (Fig.

1b). We purposely sampled in higher elevation areas located above potential outlet channels to avoid areas that may have been disturbed by meltwater. In the case of

Lake Kam, we made sure to sample above this proglacial lake that formed at the end of the YD when the LIS deposited the Marks Moraine blocking eastward Lake

Agassiz basin routing into Lake Superior 5 . Erratic boulders were selected from bedrock topographic highs with no topographic shielding. Two to four kg of sample were removed using a hammer and chisel on the top surface of the boulder. Latitude, longitude, and elevation were recorded for each sample (Extended Data Table 1).

Sample processing and analytical techniques.

All samples were prepared at

Oregon State University’s Cosmogenic Nuclide Laboratory. In order to physically isolate quartz, bulk rock samples were crushed, pulverized, and sieved down to a 250-

22

500 µm fraction. Physical separation continued with magnetic separation of magnetic and non-magnetic minerals. Chemical separation of quartz was performed by frothing the sample using a laurel amine, compressed CO

2

, and deionized water solution, followed by etching in diluted HF/HNO

3

. Quartz purity was tested at the University of Colorado-Boulder. A known concentration and amount of the low10 Be OSU-Blue

Be carrier was added to each sample 31 . Samples were converted to BeO through dissolution, anion and cation exchange, precipitation, and oxidizing steps. 10 Be/ 9 Be ratios were measured by accelerator mass spectrometry (AMS) at Purdue University’s

Rare Isotope Measurement Laboratory (PRIME) against the 07KNSTD standard.

Blanks averaged ~9.09

× 10 -16 10 Be/ 9 Be (n=2).

Exposure age calculation. Uplift corrections were made to each sample as our study site has undergone significant isostatic rebound since the retreat of the LIS 6,17,

18,33 . Using the ICE-5G model 33 , we calculate for each site its elevation change since exposure of the sample 34 . We then estimate the average elevation of the site since exposure and use this elevation for our age calculations (Extended Data Table 1).

Samples ages were calculated using the CRONUS-Earth Calculator version 2.2 with the Northeast North America production rate 32 (Extended Data Table 1). We analyze our data based on the Lal/Stone time dependent scheme along with the internal uncertainty (Extended Data Table 1). Utilizing other scaling schemes does not alter our conclusions as the ages are within the internal uncertainty of our reported ages

(Extended Data Table 2).

Removal of outliers and calculation of site averages . We find six outliers out of our 18 samples (Extended Data Fig. 1). North Lake’s outlier (n=1) is a young

23

10.5±0.5 ka age (NL-4-15) that disagrees with the minimum-limiting 14 C age of deglaciation of 12.5±0.1 ka 7 . This sample was the smallest boulder we sampled. We interpret this young age as reflecting post-depositional movement or exhumation.

Flatrock Lake’s outliers (n=2) are out of stratigraphic order and ~2.4 ka younger than the other four samples, suggesting these outliers reflect either post-depositional movement or exhumation. Likewise, they are younger than the minimum-limiting 14 C age of deglaciation of 12.3±0.2 ka 7 . The Lake Kam outliers (n=2) are either too old or too young based on the most basic understanding of LIS deglaciation in this region 5,7,30 . Specifically, the old age is out of stratigraphic order with even the North

Lake ages. The young age disagrees with the minimum-limiting 14 C age of deglaciation of 12.0±0.2 ka 7 . Lake Nipigon’s one old outlier is rejected based on prior stratigraphic work on the Marks Moraine, which was deposited at the end of the YD following LIS margin advance during the YD 5 . Other boulder samples further north but within the Marks/Dog Lake Moraines also date final deglaciation of the region after the YD 35 . We calculated an error-weighted mean and uncertainty for each of our study sites as the best estimate for the timing of LIS retreat from the outlet. Use of a straight mean and standard error would not change our conclusions, but would result in a smaller uncertainty of the mean.

Extended Data Fig. 3-1: Individual 10 Be samples by region with outliers denoted by red diamonds; ages used to calculate deglaciation are the blue circles. Error bars are 1

σ . Vertical gray bars denote the existing 14 C minimum-limiting constraints on deglaciation with bar width indicating 1 σ uncertainty 7 .

Extended Data Figure 3-1

24

25

Extended Data Table 3-1

26

Extended Data Table 3-2

27

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32

CHAPTER 3: Summary and Conclusions

This thesis aimed to answer the question: Did the eastern outlet of Lake Agassiz open at the beginning or prior to the start of the Younger Dryas? In larger context, I attempted to definitively identify the triggering mechanism and resolve a long withstanding debate for the cause of the Younger Dryas cooling event. This chapter summarizes the major results from this work.

The new 10 Be chronology presented in this work indicate that the Laurentide Ice

Sheet began its retreat from the eastern outlets at 14.0 ± 0.3 ka from North Lake and

13.8 ± 0.2 ka from the Flatrock Lake. The key drainage pathway, the Lake

Kaministikwia (Kam) outlet, deglaciated at 13.0 ± 0.3 ka right at the beginning of the

Younger Dryas, which confirms eastern routing of glacial Lake Agassiz. Such routing of meltwater to the North Atlantic would slow the AMOC resulting in cooling of the Northern Hemisphere 1,2,3,4 .

Additionally, isotopic evidence from the St. Lawrence lowlands and the

Laurentian Fan support freshwater routing to the eastern outlets during the start of the

Younger Dryas 5-9 as we observe a negative excursion of δ 18 O sw contemporaneous with the onset of the Younger Dryas 10-12 . The isotopic composition, Sr-isotopes,

U/Ca, and Mg/Ca, of the micropaleontology of the Laurentian Fan trace the source of this freshwater back to glacial Lake Agassiz, as the bedrock of the Canadian Plains strongly influences the geochemistry of Lake Agassiz runoff 10 .

Additionally, I argue that there is evidence of a two-phased meltwater forcing for the Younger Dryas. Based on terrestrial and marine records, I suggest that the eastern

33 outlets were the initial meltwater pathways, triggering the Younger Dryas at approximately 13.0 ka from the Lake Kam outlet. Subsequently, the eastern outlets become unable to route freshwater eastward due to isostatic rebound at ~12.2 ka 13 .

This shifts the initial drainage of Lake Agassiz by routing meltwater to the northern outlet at ~12.2 ka, contemporaneous with a negative excursion of δ 18 O ivf

in the Arctic

Ocean 14 . This meltwater routing to the north continues to perturb the AMOC 15 allowing the Younger Dryas to persist until ~11.9 ka.

The case of the Younger Dryas demonstrates the complex glacial-meltwater-ocean interactions and the effects on the climate system. The role of freshwater routing is a critical forcing mechanism of past climate change and will continue to be an important mechanism for future climate change as both the Greenland Ice Sheet and

Antarctic Ice Sheet could contribute up to approximately 70 meters of sea-level rise as they melt 16 . This addition of freshwater, specifically in the North Atlantic, could likely perturb the AMOC triggering another cold episode. As climate change continues to be an important issue that we face as a modern society, hopefully we can use these lessons from the past to guide our decision-making moving forward.

34

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, 24 , 1463-1478

(2005).

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43

APPENDIX A: Procedure for preparing and processing 10 Be cosmogenic samples

Introduction

Cosmogenic surface exposure dating of terrestrial in-situ cosmogenic nuclides has had far ranging impacts on the study of landscapes evolution 1 . Cosmogenic dating using the 10 Be isotope has been widely used to constrain the deglacial chronology of many areas worldwide 2,3,4,5 . Using this method of dating ice-sheet retreat has provided more accurate deglacial ages than other methods of dating such as radiocarbon or optically stimulated luminescence dating. As such, I chose to utilize the techniques and procedures outlined in previous works for sampling and processing sample material for 10 Be 1,2,3 . Measurements of 10 Be/ 9 Be were made by accelerator mass spectrometry at Purdue’s Rare Isotope Measurement Laboratory

(PRIME). Ages were calculated using the CRONUS-Earth calculator with the

Northeast North America Production 6 . Here I present ages using the Lal/Stone time dependent production rate and scaling scheme.

Study Site

The overall study site encompasses the area surrounding Thunder Bay, Ontario.

Thunder Bay is a declining shipping hub centered around manufacturing and forest products. The area surrounding Thunder Bay is typical of Canada’s dominant boreal forest landscape. Four sites were selected in the Thunder Bay, Ontario area based on previous work that refined the sub-outlets that compose the eastern outlets of glacial

Lake Agassiz based on 14 C dates and paleotopography 7 . I selected to sample the

North Lake outlet, Flatrock Lake outlet, Lake Kam outlets (also referred to as the

44

Savanne Outlet), and the Lake Nipigon outlet (Figure 3-1). I purposely sampled in higher elevation areas located outside potential outlet channels as to avoid areas that may have been disturbed or covered by the Marquette re-advance of the Laurentide

Ice Sheet.

Field Methods

Medium to large sized erratic boulders from each site were selected to sample from high elevation areas. Boulders were selected on a number of criteria to include: quartz content, no signs of erosion or chemical weathering, and no signs of postdepositional movement. The use of boulders rather than bedrock helped reduce the possibility of cosmogenic nuclide inheritance that is more common in bedrock 8,9 .

Samples were taken from high elevation areas in order to negate any topographic shielding issues. 2-4 kg of sample was removed using a hammer and chisel on the top surface of the rock. Latitude, longitude, and elevation were recorded from a

Garmin eTrex 20 handheld GPS unit. Short descriptions, diagrams, and sample thickness for each sample were recorded in the field (Table 3.1).

Laboratory Methods

The laboratory procedure for processing 10 Be target material is outlined in the

Oregon State University Cosmogenic Nuclide Laboratory Notebook 3,10 . Adjustments to the original procedures have been included in this appendix.

Physical Separation of Quartz Grains

The initial steps of the laboratory procedure call for physically isolating quartz grains from other accessory minerals within the rock. Large bulk rock samples are crushed, pulverized, and sieved down to a 250 – 500 µm fraction. This was

45 accomplished using a jaw crusher and pulverizer (disk mill). The 250 – 500 µm fraction was captured and labeled appropriately in a clean plastic container. The fraction <250 µm was saved in a separate container, but was not used in subsequent steps. The physical separation process continued with magnetic separation of magnetic and non-magnetic minerals. The Frantz magnetic separator was the electromagnetic mineral separator that was used to perform the separation. The

Frantz operated at a forward tilt of 15°, side tilt of 10°, and an amperage of 0.5. The magnetic and non-magnetic fractions were collected and stored in separate, clean, plastic containers.

Frothing and Leaching the Sample

Following the physical separation of minerals from bulk rock, initial chemical separation was performed to isolate and clean quartz from other organic, mafic, or other materials left over from the physical separation steps. The samples were initially leached in a 2 L Nalgene HDPE bottle filled with approximately 1800 mL of deionized water, 40 mL of concentrated HNO

3

, and 40 mL of concentrated HF. The bottles were capped and placed on a hotdog roller in order to agitate the grains for approximately 1-2 hours. After the initial leaching was complete, the samples were decanted into a chemical waste container and placed in a chemical hood. The samples were frothed, which removed a large amount of accessary minerals and left a majority of the quartz minerals. The froth machine is a pump connected to a CO

2 canister and a carboy filled with 10 L of deionized water and 2 mL of lauryl amine.

The CO

2 and fluid mixture is pumped into the machine where it can then be dispensed with a small hose and nozzle. The samples were individually placed in a large clean

46 mixing bowl. The frothing mixture was dispensed into the bowl with care taken to not spill grains out of the bowl. Following the introduction of the mixture into the bowl the accessory materials rose to the top of the liquid while the quartz grains remained settled on the bottom. The sample was carefully decanted to remove the accessory materials while leaving the quartz undisturbed. This process was repeated several times until a majority of the accessory materials were removed.

Individual samples were placed back into separate 2 L Nalgene HDPE bottle filled with approximately 1800 mL of deionized water, 40 mL of concentrated HNO

3

, and

40 mL of concentrated HF and placed back onto the hotdog roller. After approximately 24 hours, the sample was decanted into the waste container and rinsed with mili-q ultrapure water 3 times. The sample bottled was filled again with the same 1800 mL of deionized water, 40 mL of concentrated HNO

3

, and 40 mL of concentrated HF and placed back onto the hotdog roller for another 24 hours. This process was repeated until the samples were clean and consisted of nearly all quartz.

The goal was to have approximately 20-40 g of pure quartz at the end of the chemical separation.

Quartz Purity Testing

A small aliquot of the pure quartz sample was taken in order to measure Be, Al,

Fe, Ti, B, K, and Ca concentrations using inductively coupled plasma atomic emission spectrometry (ICP-AES). On average, 0.25-0.35 g of the pure quartz sample was dissolved in 5 mL of concentrated HF acid in a covered10 mL savillex jar. The jar was placed on a hotplate set at approximately 150°C in order to accelerate dissolution. Following dissolution the samples were uncapped and left on

47 the hotplate to allow the HF to evaporate. Once the samples were dry and cool, 1 mL of HNO

3 was added to re-dissolve the sample, which was then transferred to a 15 mL centrifuge tube. Approximately 3 mL of mili-q water was added to the jar and transferred to the centrifuge tube in order to insure the entire sample was transferred over. Mili-q water was added to the tube in order to bring the total volume to 10 mL.

ICP-AES measurements were made by Dr. Fred Luiszer at the University of

Colorado- Boulder. These measurements were used in order to determine if the samples had been cleaned sufficiently from the prior leaching steps.

Initial Dissolution, Evaporation, HClO

4

& HCl Drydowns, Conversion to Chloride

Form

Samples were processed in batches, typically 9 plus 1 blank, for the extraction of

10 Be from pure quartz. The blank sample was prepared in the same fashion as the other samples with the exception of the sample-weighing step. The samples were placed in clean 360 mL savillex jars and weighed. On average, 40 g of pure quartz sample were processed. Approximately 850 µL of the 9 Be OSU Blue carrier solution was added to the sample. Both the sample and carrier weight are recorded in the sample log. 200 mL of HF was added to the sample, capped, and placed on a hotplate set at approximately 150°C in order to fully dissolve it. Once the sample was fully dissolved the cap was removed to allow for full evaporation. A small residue was left following the dissolution and evaporation of HF.

Concentrated TM grade HClO

4 and HNO

3

were used to remove fluorides from the sample. 2 mL of HNO

3

and 1 mL of HClO

4

were added to each sample and placed back on a hotplate set at approximately 150°C to evaporate. This step was repeated

48 with only 2 mL HClO

4

. The final evaporation was conducted using only 1 mL of

HClO

4

.

6N HCl was used to convert the samples to chloride form in preparation for anion column exchange. 2 mL of 6N HCl was added to each sample and placed on a hotplate set at approximately 150°C to evaporate. This step is repeated two more times for a total of three evaporations. These evaporations ensured that all of the fluorides were removed. Fe, Ti, Al, Be, and other ions were left as chloride salts.

The final solution is a yellow-green color from FeCl

3

. The samples were subsequently transferred to 15 mL centrifuge tubes. 2 mL of concentrated TM grade

HCl was used to dissolve the sample and pour into the tube. A second addition of 2 mL of concentrated TM grade HCl is added to the savillex jar and poured into the centrifuge tube in order to ensure full transfer of the sample.

Anion Column Exchange

Anion exchange columns are filled with approximately 7 mL of anion exchange resin. The anion resin volume will shrink and expand with the addition of various concentrations of HCl and mili-q water. The resin was changed and preconditioned with one full anion procedure without sample. Once the columns are set up and preconditioned, they were used indefinitely, unless there was a question of contamination.

Anion exchange was performed to remove Fe and some Ti from the samples both of which hinder the ability to measure Be accurately. The columns were opened and drained of the mili-q water used to store the column when not in use. 30 mL of 1N

HCl was run through the column as a cleaning measure. 30 mL of concentrated TM

49 grade HCl was run through the column in order to condition the column for the sample. The eluate from this conditioning measure was discarded. The sample was centrifuged for 10-15 minutes to remove insoluble residues from the sample solution.

A 250 mL Teflon beaker that was previously cleaned with 10% HNO

3 was placed under the column. The sample was then poured directly from the centrifuge tube into the column, ensuring none of the insoluble residue is poured into the column. Once the sample was drawn into the resin, 10 mL of concentrated TM grade HCl was added to the column. The eluate was captured in the Teflon beaker. A second addition of

10 mL concentrated TM grade HCl was added to the column. The eluate was again captured in the Teflon beaker. Once this second addition was captured, the Teflon beaker was moved to a hotplate, set at approximately 120°C, and evaporated to dryness.

A 60 mL bottle, previously cleaned with 10% HNO

3

, was placed under the column. The column was cleaned by adding 60 mL of 1N HCl in two separate 30 mL additions. The eluate bottle was labeled and saved. 50 mL of mili-q water was added to the column. The eluate was then discarded. 20-30 mL of mili-q water was then added to the column for storage in order to prevent the resin from drying out.

Once the samples evaporated following the anion exchange, they were dried down

3 times using 2 mL of 6N HCl. Following the third dry down, the sample was transferred to a centrifuge tube. The transfer was accomplished by adding 2 mL of

0.6N HCl to the Teflon beaker containing the sample and swirled to ensure all the residue had been dissolved. The solution was then poured into a centrifuge tube. An addition of 1 mL of 0.6N HCl was then added to the Teflon beaker and swirled again.

50

The solution was poured into the centrifuge tube bringing the total volume up to 3 mL.

Cation Exchange

Cation exchange was performed using cation exchange columns in order to separate Ti, Al, and Be. Cation columns were filled with exactly 10 mL of cation exchange resin. Columns were preconditioned with 30 mL of 0.6N HCl followed by portions of 50 mL of 6N HCl until the eluate runs clear. Once the column runs clear,

50 mL of 1N HCl was added to the column to allow gradual expansion of the resin.

Following these preconditioning steps, one full cation procedure was performed without the sample. After this was completed, the column was used for actual sample processing. Once the columns are set up and preconditioned, they were used indefinitely, unless there was a question of contamination.

The cation exchange procedure requires one Teflon beaker (cleaned from the previous anion exchange procedure), one 125 mL eluate bottle and one 60 mL eluate bottle cleaned with 10% HNO

3

. The 125 mL bottle was labeled with the sample name and “Cation Eluate”. The 60 mL bottle was labeled with the sample name and

“Al Fraction”.

The cation columns were drained of approximately 50 mL mili-q water as a preliminary cleaning measure. The eluate was discarded. 30 mL of 0.6N HCl was added to condition the column before adding the sample. The eluate was discarded.

The sample, which was still in the centrifuge tube, is centrifuged for 10-15 minutes to remove any insoluble precipitate. The “Cation Eluate” bottle is placed under the column. The sample was loaded using a 1 mL disposable pipette, with care taken not

51 to draw any precipitate into the pipette. After the sample was completed transferred to the column, 130 mL of 0.6N HCl was added to the column. The eluate was collected in the “Cation Eluate”. The “Cation Eluate” bottle was then removed and replaced with the clean Teflon beaker. 100 mL of 0.6N HCl was added to the column and collected in the Teflon beaker. Once the eluate was collected the beaker was placed on a hotplate set at approximately 120°C and evaporated to dryness.

50 mL of 6N HCl was added to the column and collected in the “Al Fraction” bottle. 50 mL of 6N HCl was added to clean the column. The eluate was discarded.

50 mL of 1N HCl was added to the column and the eluate was discarded. The addition of weak acid allows for gradual expansion of the resin, this ensured homogenized pore spaces. 100 mL of mili-q water was added to the column and the eluate was discarded. 20-30 mL of mili-q water was added to store the columns and prevent the resin from drying out.

The final product of the cation exchange column was transferred to 15 mL centrifuge tubes. The transfer was accomplished by adding 2 mL of 0.6N HCl to the

Teflon beaker containing the dry-downed sample. The sample was swirled in order to ensure the entire sample residue was dissolved. The sample is carefully poured into the centrifuge tube. Another 1 mL of 0.6N HCl was added to the beaker, swirled, and transferred to the centrifuge tube; this step is repeated with 1 mL of 0.6N HCl. The solution is brought up to 5 mL by adding 1 mL of mili-q water. The entire tube is mixed thoroughly on a vortex mixer.

52

Removal of Ti

The next step of the process called for removing Ti from the sample.

Approximately 200-300 µL of concentrated TM grade NH

4

OH was added to the sample solution in order to increase the pH to ~5. The sample was then mixed well on the vortex mixer. The sample pH was checked by removing 5 µL of sample with a pipette and applying it to pH paper. If the sample pH was greater than 5, the acidity was adjusted by adding drops of 1:1 HNO

3

:H

2

O to the sample solution. The sample was again mixed well on the vortex mixer. The sample pH was then retested. Once the sample solution reached a pH of ~5, the sample was centrifuged for 10-15 minutes. If the sample contained Ti, a white precipitant would form. The supernatant is poured into a clean centrifuge tube labeled with the sample number and “Be”. The old tube is labeled with the sample number and “Ti”, capped, and saved.

Once the Ti was removed from the sample, the pH was adjusted to ~8, in order to precipitate Be, by adding ~50 µL of NH

4

OH. The sample was then mixed on a vortex mixer and tested for pH. Be(OH)

2

could also be visually identified by looking carefully at the solution. The solution should have a “cloudy” appearance, although it could be challenging to identify. Once the solution reached a pH of ~8, the sample was centrifuged for 10-15 minutes. Following the centrifuge, the Be(OH)

2

forms a small gel-like lens on the bottom of the centrifuge tube. The supernatant is poured into a 30mL plastic bottle labeled with the sample number and “supernatant fraction”.

This bottle is used to collect all of the supernatant fractions from this point forward.

100 µL of 1:1 HNO

3

:H

2

O was added to the sample and mixed on a vortex mixer in order to re-dissolve the precipitant. The solution is brought up to 5 mL by adding

53 mili-q water to the centrifuge tube. Be(OH)

2

is re-precipitated by adding ~200 µL of

NH

4

OH to the solution. The solution is mixed on a vortex mixer and centrifuged for

10-15 minutes. Once the centrifuge was complete the supernatant was poured into the plastic bottle labeled with “supernatant fraction”.

Boron Rinses

The sample was brought up to a 5 mL solution by adding pH 8 water to the centrifuge tube. The sample was centrifuged for 10-15 minutes and the supernatant was then poured off. This process was repeated two more times for a total of 3 rinses.

The successive pH 8 rinses ensured that any possible boron contamination was removed from the sample. Following the last rinse, as much liquid as possible is removed from the sample and prepared for the oxidation of beryllium hydroxide.

Oxidation of Beryllium Hydroxide

A set of quartz crucibles was thoroughly cleaned by immersing them into a solution of 10% HNO

3 for approximately 24 hours. The weight of a single quartz crucible (to the 0.0001 g) was recorded. The crucible was labeled with the sample number and its position within the quartz crucible holder was recorded as well. 400

µL of 10% HNO

3 was added to the final product in the centrifuge tube. The tube was mixed well on a vortex mixer to ensure the sample was completely dissolved. The solution was carefully transferred from the centrifuge tube to a quartz crucible. 100

µL of HNO

3 was added to the centrifuge tube and mixed on the vortex mixer. The solution was again transferred to the quartz crucible. If it was possible, another 100

µL was added to the centrifuge tube, mixed, and transferred. This was done to ensure

54 total transfer of sample to the quartz crucible. These steps were completed in a clean air hood in order to avoid as much contamination as possible.

Once the samples were transferred to the quartz crucibles, the crucibles were placed on a hotplate set at 100°C to evaporate to dryness. The low temperature setting ensured minimal boiling and splattering of sample. After the initial evaporation, the hotplate temperature was increased to 425°C for 30 minutes.

Following this, the crucibles were capped and placed in a rapid mineralizer set at

999°C for one hour to convert all material to BeO. Once the crucibles were cool, the caps were removed, the crucibles were re-weighed, and the weights were recorded.

Packing Accelerator Mass Spectrometry Targets for PRIME Lab

After recording the weight, the samples are transferred to accelerator mass spectrometry targets provided by PRIME Lab. The transfer was accomplished by adding one small scoop of niobium powder to the quartz crucible. The powder was mixed manually with a small clean metal stem in order to bind the BeO. The sample is carefully transferred to an AMS target and compacted. Once the target was packed, the target number was recorded, it was sealed in a plastic tube, labeled, and placed in a Ziploc bag. All tools and work surfaces were cleaned with ethanol between loading samples. AMS targets were mailed to Kasey Howe, PRIME Lab, 525 Northwestern

Avenue, West Lafayette, IN, 47907.

All Teflon beakers and savillex jars were cleaned using mili-q water and lab soap. Following this initial cleaning, the beakers and jars were immersed in 10%

HNO

3 for at least 24 hours before being used again. Centrifuge tubes, quartz crucibles and caps, plastic bottles, and pipette tips were immersed in 10% HNO

3 for

55 at least 24 hours before being used. Quartz crucibles and pipette tips were used only once before being discarded.

Exposure Age Calculation

Uplift corrections were made to each sample as our study site has undergone significant isostatic rebound since the retreat of the Laurentide Ice Sheet 11,12,13,14 .

Using the ICE-5G model, I calculated the elevation change for each site since exposure of the sample 15 . The uplift corrections are important factor in calculating exposure ages as they directly determine the atmospheric depth at the site, which is a key factor in determining production rates 16 .

Samples ages were calculated using the

CRONUS-Earth Calculator version 2.2 with the Northeast North America production rate 6 . Samples were calculated with a standard atmosphere, shielding correction of 1

(no shielding), erosion rate of 0, and sample density of 2.65. Measurements at

PRIME Lab were made against the 07KNSTD (Table 3-1). We reported and analyzed our data based on the Lal/Stone time dependent scheme along with the internal uncertainty (Extended Data Table 3-1). Utilizing other scaling schemes does not alter our conclusions as the ages are within the internal uncertainty of our reported ages (Extended Data Table 3-2).

Potential Sources of Error

Potential sources of error for calculating exposure ages include inadequate shielding correction for topographic shielding or vegetation and snow cover shielding. Using a conservative approach, we estimate that the shielding factor could account for approximately a 5% difference from actual age 1 . Applying this correction would shift individual ages older to account for shielding effects. Another potential

56 source would include an inadequate correction for erosion. Assuming an erosion rate of 5mm/ka, the ratio of apparent age/true age would be approximately 0.95

1 , again shifting the individual ages older to account erosion. These scenarios of error are worst case and would not alter our conclusions regarding the deglacial chronology for our study site.

Table A-1: Parent Regeants

Table A-2: Working Solutions

57

Figure A-1: Photo of a typical sample boulder (FR-8-15)

58

Figure A-2: Photo of a typical sample boulder (LN-10-15

59

References

1.

Gosse, J.C. Phillips, F.M. Terrestrial in situ cosmogenic nuclides:

Theory and application. Quat. Sci. Rev., 20 , 1475-1560 (2001).

2.

Balco, G. Contributions and unrealized potential contributions of cosmogenic-nuclide exposure dating to glacier chronology. Quat. Sci.

Rev.

, 30 , 3-27 (2011).

3.

Licciardi, J.M. Alpine Glacier and Pluvial Lake Records of Late

Pleistocene Climate Variability in the Western United States. Ph.D.

Dissertation, Oregon State University, 1-170 (2001).

4.


5.

Winsor, K., Carlson, A.E., Rood, D.H. 10 Be dating of the Narsarsuaq moraine in southernmost Greenland: evidence for a late-Holocene ice advance exceeding the Little Ice Age maximum. Quat. Sci. Rev.

, doi:

10.1016/j.quascirev.2014.04.026.

6.

Balco, G. et al. Regional beryllium-10 production rate calibration for northeastern North America. Quaternary Geochronology 4 , 93-107

(2009).

7.

Teller, J. T. et al. Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a re-evaluation.

Quat. Sci. Rev.

24 , 1890-1905 (2005).

8.

Colgan, P.M., Bierman, P.R., Mickelson, D.M., Caffee, M. Variation in glacial erosion near the southern margin of the Laurentide Ice Sheet,

60 south-central Wisconsin, USA: Implications for cosmogenic dating of glacial terrains. GSA Bulletin , 114 , 1581-1591 (2002).

9.

Goehring, B.M., Brook, E.J., Linge, H., Raisbeck, G.M., Yiou, F.

Beryllium-10 exposure ages of erratic boulders in southern Norway and implications for the history of the Fennoscandian Ice Sheet. Quat.

Sci. Rev. 27 , 320-336 (2008).

10.

Rinterknecht, V.R. Cosmogenic 10 Be chronology for the last deglaciation of the southern Scandinavian Ice sheet: Ph.D.

Dissertation, Oregon State University, 99 pp (2003).

11.

Peltier, W.R. Global glacial isostasy and the surface of the ice-age earth: The ICE-5G (VM2) model and GRACE. Annual Review of

Earth and Planetary Sciences , 32 , 111-149 (2004).

12.

Murton J.B., et al. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature 464 , 740–743 (2010).

13.

Breckenridge, A. The Tintah-Campbell gap and implications for glacial Lake Agassiz drainage during the Younger Dryas cold interval.

Quat. Sci. Rev.

117 , 124-134 (2015).

14.


139 , 1-16

(2016).

15.

Mitrovica, J.X., Davis, J.L., Shapiro, I.I. A Spectral Formalism for

Computing 3-Dimensial Deformations Due to Surface Loads: 1.

Theory. Journal of Geophysical Research , 99 , 7057-7073 (1994).

61

16.

Balco, G., Stone, J.O., Lifton, N.A., Dunai, T.J. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10 Be and 26 Al measurements. Quat. Geochron . 3 , 174-195

(2008).

AN ABSTRACT OF THE THESIS OF

AN ABSTRACT OF THE THESIS OF

ACKNOWLEDGEMENTS

CONTRIBUTIONS OF AUTHORS

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

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AN ABSTRACT OF THE THESIS OF

AN ABSTRACT OF THE THESIS OF

ACKNOWLEDGEMENTS

CONTRIBUTIONS OF AUTHORS

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

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