THE BONNEVILLE LAKE BASIN SHORELINE RECORDS OF
LARGE LAKE AND ABRUPT CLIMATE
CHANGE EVENTS
by
Shizuo Nishizawa
A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
Department of Geography
The University of Utah
August 2010
Copyright © Shizuo Nishizawa 2010
All Rights Reserved
The University of Utah Graduate School
STATEMENT OF DISSERTATION APPROVAL
The dissertation of Shizuo Nishizawa has been approved by the following supervisory committee members:
Andrea R. Brunelle , Chair 5/13/2010 Date Approved
Richard R. Forster , Member 5/13/2010 Date Approved
Mitchell J. Power , Member 5/13/2010 Date Approved
Dorothy Sack , Member 5/13/2010 Date Approved
David E. Wilkins , Member 5/13/2010 Date Approved
and by Harvey J. Miller , Chair of the Department of Geography and by Charles A. Wight, Dean of The Graduate School.
ABSTRACT
This study examines lacustrine shoreline sediment records of the Bonneville lake basin in North America. One of the most straightforward, robust, and useful characteristics of lacustrine shorelines is that shoreline features always form at water’s edge to mark where the lake surface level is. Hence, mapping shoreline elevations enables researchers to reconstruct such physical dimensions of a paleolake as surface area, depth, and volume through geographic surveying, geomorphology, stratigraphy, sedimentology, tephrochronology, and radiocarbon dating. Good age control also makes it possible to put together lake size data from different time periods to chronicle details of when and how the lake changed its size in the past. Based on 200 radiocarbon dated shoreline data, this study presents a lake size change chronology of the Bonneville basin during the ice ages from 43,000 to 10,000 calendar years before present. Comparing the
Bonneville basin lake record with the well resolved, high resolution δ18O records from the Greenland ice sheet, Venezuelan varved ocean sediments, and Chinese cave stalagmites reveals that timing, magnitude, and duration of all major lake change events in the Bonneville basin are in phase with large-scale climate change events. Close synchroneity of Bonneville lake events with Asian and East Asian monsoon intensity fluctuations during the last deglaciation also illustrates that the Bonneville basin lake system was highly sensitive or responsive to hemispheric scale, abrupt climate change events. CONTENTS
ABSTRACT ...... iii
PREFACE ...... vi
INTRODUCTION ...... 1
CHAPTER
1 LAKE BONNEVILLE AND ABRUPT CLIMATE CHANGE EVENTS DURING THE LAST DEGLACIATION ...... 4
Abstract...... 4 Introduction ...... 5 Lacustrine chronology ...... 15 Lake Bonneville and severe environmental changes...... 19 References ...... 20
2 SHORELINE RECORDS OF LARGE LAKE EVENTS IN THE BONNEVILLE BASIN, NORTH AMERICA, IN RESPONSE TOABRUPT CLIMATE CHANGE EVENTS ...... 25
Abstract...... 25 Introduction ...... 25 Methods ...... 28 MIS 16 Large Lake ...... 42 MIS 3 Large Lakes ...... 47 Early MIS 2 Large Lake: Early Lake Bonneville ...... 59 Lake Events in the Bonneville Basin and Hemispheric Climate Change ...... 68 References ...... 75
3 LAKE BONNEVILLE SHORELINE RECORDS DURING THE LAST GLACIAL-INTERGLACIAL TRANSITION ...... 82
Abstract...... 82 Introduction ...... 83 Methods ...... 85 Evidence of Seven Lake Intervals ...... 91
Discussion of the Early Holocene Bonneville Basin Lacustrine Environments ...... 122 Bonneville Basin Lake in the Context of the Last Glacial-Interglacial Transition ...... 124 References ...... 128
EPILOGUE ...... 135
v
PREFACE
The geographical journey of this study starts from a point: an exposure of an ancient shoreline such as a beach barrier, delta, spit, or tombolo in the Bonneville lake basin, west-central North America. This point is then connected to other points at the same elevation with the same stratigraphic sequence to assume a shape of the lake that disappeared long ago (sort of like “connect dots”). It is a snapshot of the ancient (paleo) lake at one particular moment. After many such snapshots of the lake from different periods of time are put together, a history of the lake emerges to show how and when the lake size changed in the past. Then, the Bonneville lake history (or chronology) can be compared with paleo-lake records from other hydrologic basins in the North American
Great Basin to show if there is a coherent regional trend. Once the quality of the
Bonneville basin lake history is determined sufficient in the context of climate change, it may be possible to find a connection between the Bonneville shoreline record and different paleo-environmental records like ocean sediment or ice sheet core data, obtained far from North America. At this point, the geographical scope of the study can be expanded further to become: continental, hemispheric, global, and perhaps even interplanetary. Linking one ancient lake shoreline exposure in today’s desert landscape to the earth’s orbital change patterns or solar insolation regime variations, is quite a journey in both space and time. The following three chapters are presented as three separate manuscripts. The organization into chapters follows the chronological order of when they were completed:
Chapter 1, 2006, Chapter 2, 2009, and Chapter 3, 2010. Whereas a transition from
Chapters 2 to 3 is relatively seamless because they were divided into two by the time periods studied, Chapter 1 is quite different, reflecting my changing thoughts over the past five years.
Chapter 1 is intended for a much broader audience than, for example, the Great
Basin or Bonneville paleo-environment research communities. A main goal of the chapter is to tell the general history of Lake Bonneville in the context of large scale abrupt climate change events when the last ice age was coming to an end. It was originally written to fit format requirements of the journals, Nature, Science, and
Geology (<2,000 word limit). The depth of discussion, therefore, may look insufficient for those who are active in the Great Basin or Bonneville paleo-environment research communities. Chapter 1 helps readers, who are not familiar with the basic Lake
Bonneville chronology, to follow relatively easily the major Bonneville events in the context of the last deglacial climate change events. Detailed information regarding research methods and results are amply provided in Chapters 2 and 3. In this sense,
Chapter 1 prepares and guides readers to Chapters 2 and 3.
Chapter 2 presents the shoreline evidence of the 670,000, 43,000-28,000, and
27,000-24,000 calendar years before present (cal yr BP) large lake events (Marine
Isotope Stages 16, 3, and 2, respectively). The Marine Isotope Stages 3 and 2 lake events are also compared with the Greenland ice sheet δ18O records in terms of timing, duration, and magnitude to see if the Bonneville basin record correlates with Dansgaard-
vii Oeschger cycles. Lake events covered in Chapter 3 pick up where Chapter 2 ends and
extends to the early Holocene (24,000-10,000 cal yr BP). These lake events occurred
during the last deglaciation, which was filled with large-scale, abrupt climate change
events. The Bonneville lake chronology in Chapter 3 indicates its much closer
teleconnection to the changing intensity of the Asian and East Asian monsoons than to
the Greenland ice sheet temperature variability. Since the original manuscript for
Chapter 1 was finished in 2006, the author’s interpretation and understanding of some
events have changed and got refined in Chapter 3. I felt that presenting these two
chapters as they are in the same volume would give readers a good perspective on how
research can sometimes evolve in a relatively short period of time.
Don Currey introduced me to the Bonneville research. From 2000 to 2004, I did
my best to follow his guidance and learn as much as possible from him in the field and at
school. I was always excited to go out in the field with Don. This was probably why I was able to keep up with the quite insane research schedule we had: we drove hundreds of miles off pavement to collect data every weekend. His sudden death in 2004 was devastating. I lost my mentor and best friend. The way some people reacted to Don’s death also broke my heart. Less than a month after Don’s death, emails started circulating among those who were involved in two NSF projects with Don, discussing how to plan and publish “Don’s hydrograph and 14C table.” They openly talked about
whether somebody had already “secured” Don’s data at Don’s house…. From that point
on, there was constant pressure for graduate students to publish “Don’s data” and include certain individuals as coauthors. Feeling overwhelmed, discouraged, and disgusted, I
was very close to quitting. I simply didn’t want to have anything to do with these people.
viii Nevertheless, I was incredibly fortunate to be surrounded by so many good, caring people, who encouraged and supported me to keep going. Eventually, I convinced
myself that sorting the unpublished data Don compiled and presenting them in a coherent
and understandable manner were my mission to show how much I respected, and, most of
all, liked him as a person. Meanwhile, I have to admit that working on his unfinished
data always made me feel like a scavenger… Hence, I want to make sure that all credits for the data presented in this dissertation are properly acknowledged here once for all. I especially want to make it clear who are responsible for preparing the data presented in this study and paying for 14C dates.
This dissertation presents 200 14C dated shoreline units and their geographical,
sedimentological, and stratigraphic data. Don compiled the data on: 141 previously
published 14C dated shoreline units and 44 unpublished 14C dated units. Among these
unpublished 44 14C dates that were prepared by Don, 4 dates were paid by Don, 6 by the
University of Utah Geography Department, 6 by the University of Utah Anthropology
Department, and 28 by the NSF (SBR-9817777 and EAR-9809241) grants. The rest of the unpublished data on 15 14C dated shoreline units were collected and compiled by me.
Don paid for 3 14C dates, and my parents paid for 12 14C dates. I also modified Don’s
four figures and used them in this dissertation: Figure 2.2, 2.7, 3.1 and 3.2. Don’s
enthusiasm to link the Bonneville lake records with the North Atlantic abrupt climate
change events such as Heinrich and other ice rafted debris events also inspired me to
formulate some of the initial research questions and objectives for this dissertation. I
wish he were here to see how his idea evolved into something quite different from what
he envisioned. He is dearly missed. Thank you for everything, Don.
ix Words cannot describe how thankful I am to Andrea Brunelle. Andrea really rescued me and salvaged my program. She pulled me up from the ugly mess in which I was trapped after Don’s death, and gently supported me so that I could stand upright.
With her compassion and genuine concern for me, she always patiently waited for me to make one small step forward at a time. She never pushed me to go faster… In the beginning, I had a gut feeling that she could be trusted, and even protect me. That turned out to be true. When a buzz over how to publish Don’s data became more than a disruptive noise to me, Andrea effectively sealed me off from it. Even when a public question period of my dissertation oral defense was hijacked by two Geology Department professors and about to go out of control, Andrea ended in a very professional, civilized, but decisive manner. I stood there by the podium and kept thinking I was so glad and fortunate that she was my advisor and mentor. I was in awe for a while. I think I learned a lot there too. Watching Andrea work over the years also kept my hope for academia alive. It was great to see somebody with integrity and compassion succeed. I am so honored to be her first Ph.D. student. She is my awesome advisor, mentor, and trusted friend. What a spectacular job she did for me!
I was very fortunate to have Dorothy Sack on my committee. As one of Don’s star disciples, Dorothy offered me many suggestions and advices that often reminded me so much of what Don would have said if he were alive. Her knowledge and experience in the Bonneville research and geomorphology greatly improved this dissertation. She also flew from Athens, Ohio, to attend my dissertation oral defense, and spent the next few days to go over my dissertation with me. I really treasure her support and friendship. I am eternally grateful to her.
x David Wilkins also flew from Boise to attend my dissertation defense. I have
known him since the time he was finishing up his Ph.D. under Don’s guidance in the late
1990s. David provided me helpful comments and suggestions that bore very distinct
signatures of the Currey school of thoughts. When David and Dorothy were in the same
room, asking me questions during my dissertation defense, it was almost like Don was
there. It was so nice to have him as a member of my “Rescue Team.” I sincerely thank
him for being available and accessible to me in his very busy schedule.
Mitch Power became my committee member in the very last minutes of my
program. I am so glad that I was able to get his feedback on my dissertation. Ever since
he came to this department for a job interview, I have been very impressed with him.
After we spent some time together in Andrea’s RED Lab reading/discussion group
sessions, I fantasized about having him on my committee. Then, it came true! It was
truly great to have him on my committee.
I always appreciated Rick Forster’s calm demeanor, which created peaceful atmosphere in some of the potentially stressful situations we had. His section in my comprehensive exams was intellectually stimulating and fun. Although he could not see
my defense because he was in Greenland, he was always available to help me.
Three committee members had to excuse themselves from my committee under
various circumstances. The intellectual intensity of Jack Oviatt reminded me of Don
quite a bit. Although I was intimidated by his thorough criticism on my dissertation
proposal, I sincerely appreciate his professional comments and suggestions. I learned a great deal about good teaching when I was a Geography 1000 teaching assistant for
Katrina Moser. She spent a lot of time to help me go through the dissertation proposal
xi processes. I still feel bad that I never got interested in diatoms so much to work with her.
I kind of pushed Tom Kontuly to be on my committee after Katrina left the department.
His section of my comprehensive exams covered the philosophy of science. I not only learned a lot and enjoyed it, but also I became unafraid of being criticized for not being scientific. Whenever that happens, I can just tell myself to think of Paul Feyerabend! I
thank Jack, Katrina, and Tom for their services.
I would like to thank my Geography Department for supporting me financially
and psychologically. It has been very comfortable for me to study and hang out in this
department. I especially thank Harvey Miller for recruiting me in 1996. Administrative
staff was always wonderful. Great thanks to: Lisa, MaryAnn, Pam, Jill, and Susan.
Many thanks to Don’s brother, Stanley Currey, who generously let me access to
Don’s research collection before it was donated to the university. He and his wife,
Carole, were always concerned about my progress, and encouraged me to finish.
I thank the former Long Range Desert Geo-Limnology Group (LRDGLG)
members for their support: Peter Ainsworth, Nancy Caruthers, Molly Hanson, Elliott Lips,
Duncan Metcalf, Jackie Rabb, Corrine Springer, and Brad Thein. We all had way too
much fun in the field with Don. I also acknowledge people who assisted and helped Don
in the field and various lab analyses: Genevieve Atwood, Jay Banta, Gary Christensen,
Franklin “Nick” Foit, Holly Godsey, Marilyn Isgreen, Sue Lutz, Shela Patrickson, Mike
Perkins, Rachel Quist, and Alisa and Ian Schofield.
My 1995 Jeep Grand Cherokee performed wonderfully off pavement. I always
felt safe driving this green boxy vehicle on the rough terrains. Although it broke down a
few times in the field, I was never stranded, and able to come home somehow. Of course,
xii it swallowed thousands of dollars for repairs, but still runs great.
My girlfriend, Shawn Blissett, has always been there to support, encourage, console me and enjoy life with me. She made me realize the importance of taking time to think, ask questions, or just look around to notice something. Now I enjoy my life with
Shawn, her cats, Andy and Sunny, and my cat, Sumi (a.k.a. Dizzy) so much more than when I lived alone for a long time. They all did a great job of keeping me sane when I was writing this dissertation.
My brother, Takashi, has been nothing but supportive for the last ten years. He not only volunteered to go out in the field with me and helped me collect samples, but also took a good care of our house and yards even after he got married and got another house to take care! His wife Lisa has also been very supportive. It is great to have them live across the street. I hope I can be of some help for Takashi to finish his Ph.D. in economics in the very near future.
My father, Katsuo, and mother, Koko, have supported me financially and psychologically for decades. Their belief and faith in the value of higher education are, in fact, much stronger than mine. They encouraged me to pursue graduate degrees outside of Japan. Although they supported me patiently all these years, they recently confided to me that they thought I was going to quit after Don died, and were gravely concerned about me for a long time. I cannot thank them enough for what they did for me. They are even proud of themselves to pay for radiocarbon dates because they knew well that they were contributing to advancing our understanding of the world we live in.
I am so fortunate to have such understanding and smart parents.
xiii INTRODUCTION
This is the study of lacustrine shoreline deposits in the Bonneville lake basin. The term, “lake basin,” is used here to mean a basin in which a lake occupied some time in
the past and left shorelines. It differs from “the Bonneville drainage basin,” which is
bounded by drainage divides, encompassing the Bonneville lake basin and numerous high
altitude alpine lake basins in various sizes such as those in the Uintah Mountains. Hence,
the geographic scope of shorelines in this study is limited to an elevation range from
1551m, the highest shoreline of Lake Bonneville, to >1280 m, the historic average level
of Great Salt Lake.
Among different proxy records contained in shoreline deposits, this study focuses
on acquiring the information concerning: (1) elevation of shoreline depositional units to
estimate the lake level and area at the time of shoreline formation, (2) stratigraphic
context to determine if the lake level was going up or down at the time, and (3) age of the
sediment to estimate when the shoreline was formed. The obtained data on elevation, stratigraphic context, and age of each shoreline indicate magnitude and timing of a
shoreline forming event, referred to herein as a “lake event.” In this study, a lake event
provides a snapshot of the paleo-lake at a particular moment. The lake event concept allows us to examine one lake event independently or individually without assuming its being a part of oscillation or cycle. This is a major advantage since some of, especially the older, Bonneville shoreline events were so isolated from others and scattered 2
over the long period of time without temporal continuity among themselves that finding
such a definite pattern as cycle or oscillation is often impossible. Only after sufficient numbers of lake events data are gathered for the specific time period, a group of lake
events may emerge as a “lake interval” with an identifiable duration or possible
oscillatory nature. Whereas many of the sedimentary packages of lake events examined
for this study are bounded by unconformities, some lake event sedimentary sequences do
not show observable unconformities. Therefore, a “lake event” in this study is not necessarily defined by unconformities to comprise an allostratigraphic unit (North
American Commission on Stratigraphic Nomenclature, 1983).
For this study, the concept of lake event also facilitated presenting the Bonneville
shoreline chronology in bar graphs, instead of the traditional hydrograph curve (Currey,
1990; Oviatt, 1997). As shown in the following chapters, each lake event is represented
by a bar with its height indicating the lake level or area and its width corresponding to the
range of radiocarbon or calendar age. In this approach, a gap in the information is left to
be filled by future studies, eliminating an element of conjecture to connect distant and
few points to form a curve. Combining the lake event concept and bar graph approach
seems logical and practical for efficient, visually effective, and straightforward
representation of the Bonneville shoreline chronology.
The Bonneville shoreline chronology presented in this study is compared with the
Greenland ice sheet, Cariaco Basin varved marine sediment, and Hulu and Dongge cave stalagmite records. These records were chosen for comparison because: (1) they are by
far the most well established and reliable late Quaternary high resolution environmental
records; (2) all major abrupt climate change events from the late Pleistocene to the 3
Pleistocene/Holocene boundary are clearly recorded in these records; (3) they are three different proxy records from three different latitudes—ice at high latitude showing temperature fluctuations, cave deposits at middle and low latitudes recording precipitation changes, and marine sediment at low latitude indicating temperature fluctuations. It would be of great interest, especially to the climate modeling community, to see how the middle latitude, continental, and hydrologically closed, Bonneville lake basin shoreline records are compared with these records in the context of abrupt climate change events.
The primary research objectives of this study are to: (1) identify, survey, and describe shoreline features previously not reported; (2) estimate lake levels and surface areas from surveyed elevations of shoreline features; (3) obtain age controls by radiocarbon dating and tephrochronology; (4) combine newly obtained information for this study with the previously reported shoreline data to create the comprehensive
Bonneville shoreline database; (5) present the Bonneville lake level/size chronology in a visually coherent style; and (6) examine whether the Bonneville basin shoreline deposits are sensitive enough to record any signatures of large-scale abrupt climate change events.
References
Currey, D.R., 1990. Quaternary paleolakes in the evolution of semidesert basins, with special emphasis on Lake Bonneville and the Great Basin, U.S.A. Palaeogeography, Palaeoclimatology, Palaeoecology 76, 189-214.
North American Commission on Stratigraphic Nomenclature, 1983. North American stratigraphic code. American Association of Petroleum Geologists Bulletin 67, 841-875.
Oviatt, C., 1997. Lake Bonneville fluctuations and global climate change. Geology 25, 155-158.
CHAPTER 1
LAKE BONNEVILLE AND ABRUPT CLIMATE CHANGE EVENTS
DURING THE LAST DEGLACIATION
Abstract
Lake Bonneville in west-central North America fluctuated its level and size
widely throughout the last deglaciation. The pattern of lake fluctuation appears to have
followed closely the several abrupt climate change events in which a temperature regime
suddenly switched from cold to warm or vice versa on a hemispheric to global scale. The
extensive lake events exceeding the basin threshold occurred synchronously with
Heinrich Event 1, the Bølling/Allerød warm period, and the onset of the Holocene epoch.
Compared with these lake events, the extent of the lake event during the Younger Dryas
cold period, on the other hand, was marginal. Contrary to the previously accepted model,
the large lake events in the Bonneville basin occurred in both the stades and interstades
during the last deglaciation. Two of the notable aspects of the lake fluctuations in the
Bonneville basin during this period are speed and magnitude of the changes. The lake
level change up to 150 m with corresponding surface area change from 4000 km2 to
37,000 km2 (and vice versa) occurred repeatedly within the period of 200-300 radiocarbon years or less. These drastic changes in the Bonneville basin surface hydrologic system closely reflect the extreme nature of the large-scale environmental instability that characterized the last deglaciation. 5
Introduction
The Bonneville drainage basin is the largest and deepest hydrologic sub-basin in
the Great Basin of North America: 130,000 km2, and 1300-4000 m above sea level
elevation range from the basin floor to drainage divide (Figure 1.1). Despite its immense size, the hydrologically closed Bonneville basin is highly sensitive to climate change.
Dimensions of the perennial water body in the extensive lowest part of the basin result directly from the net balance between precipitation and evaporation, and closely reflect a
prevailing regional climate regime (Currey, 1990). Such hydrologic/climatic sensitivity,
great size, and mid-latitude, continental geographical location make the Bonneville lake
basin exceptional for recording details of large-scale climate change events in its lake
sediments. A series of distinct paleo-shorelines, some of which stand more than 100-200
m above today’s hyper-saline Great Salt Lake, contain a wealth of information on the size
of the paleo-lake, its depositional environments, and the prevailing climate on regional to
continental scales at the time of shoreline formation (Oviatt, 1997).
For this study, 89 radiocarbon dated paleo-shoreline features in the Bonneville
lake basin were examined for their geomorphic, sedimentological, and stratigraphic
contexts (Table 1.1). Then, the reconstructed details of timing and magnitude of each
lake level/size change event were compiled (Table 1.1), graphically presented, and
compared with well established climate records from the Greenland Ice Sheet and the
Cariaco Basin marine sediments (Figures. 1.2 and 1.3 B). This study’s lake level change
chronology reveals that rapid and significant lake expansion and contraction in the
Bonneville basin during the last deglaciation (20.0-9.0 ka, thousand calendar years before
present, Alley et al., 2002) occurred synchronously with the abrupt climate change 6
Figure 1.1. Location of the Bonneville basin and the changing extent of the lake at three levels (after Currey, 1990): Dark shade–Pre-Bonneville flood maximum extent of Lake Bonneville (area = 50,500 km2), Medium gray shade– Post-Bonneville flood maximum extent (area = 74% of 50,500 km2), Light gray–Great Salt Lake at its historic average (area = 8.7% of 50,500 km2). Locations of (A)–the basin threshold, (B)–Cariaco basin, (C)–GISP2 site.
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'" " I ~ >? -- dJ . l 11 ants itial correction while lly snail shells and aquatic pl e CALIB 5.0 calibration program to offset their in represents each calendar year range. . ) . 1.3 B g mer et al., 2004) using th contamination of carbon (especia lotted in Fi p as ( e g ear ran mum and minimum ages at the 1 sigma level y ker and Kaufman (1965), 500 years were added calendar years (IntCal04) (Rei onate dates reported before 1977. d/or inconsistency, (2) possible oint of each calendar p oint is the mid p 350 years were added to all the carb 350 years were Data Denoted by ‡ and/or stratigraphic (1) of: because and 1.3 in the text) (Figs.1.2 included in the hydrographs data were not 10 A total of sedimentological uncertainty an Radiocarbon Age For those carbonate dates reported in Broc Calendar Year Range converted to All radiocarbon years were A mid from Public Shooting Grounds. (Stuiver et al., 2005). A range between maxi 12 Table 1.1 Sediment Type This describes the dominant sediment in which radiocarbon dated material was found — bio/T: tufa and silt, bio/X: tufa and volcanic, gS: pebbly sand, kS: calcareous sand, M: mud, mO: muddy organic matter, mS: muddy sand, msG: muddy sandy gravel, O: organic, oG: organic mud, oM: organic mud, oS: organic sand, osM: organic sandy mud, oT/T: organic silt and silt, R: marl, S: sand, S/G: sand and gravel, sG: sandy gravel, sM: sandy mud, T: silt, T/G: silt and gravel, T/X: silt and volcanic, unkn: unknown. Shore Zone (sz) This describes the shore zone environment at the time of the deposition of the dated unit — sz00: zone not specified, sz01: lagoon (closed to partially closed, typically muddy), sz02: beach backshore face (typically sandy and gravelly), sz03: beach crest (typically gravelly), sz04: beach foreshore face (typically gravelly), sz05: beach foreshore toe (typically sandy), sz06: tributary stream (terminal reach), sz07: delta top, sz08: delta front, sz09: overflow (basin- or sub basin-spill) stream, sz10: lake-edge marsh, sz11: lowland (elevation typically < 30 m above lake), sz12: offshore (depth typically >10 m). LakeSurface Area Index (LSAI) The maximum surface area of Lake Bonneville (50,500 km2) is set as 1.00 (100%). Sources 1-Light (1996), 2-Scott et al. (1983), 3-Broecker and Kaufman (1965), 4-Burr and Currey (1992), 5-Kaufman (2003), 6-Scott (1988), 7-Godsey et al. (2005), 8-Oviatt et al. (1994), 9-Rubin and Alexander (1958), 10-Oviatt (1991), 11-Bright (1963), 12-Utah Geological Survey Files, 13-Marsters et al. (1969), 14-Trautman and Willis (1966), 15-Isgreen (1986), 16-Oviatt (1989), 17-Ives et al. (1967), 18-Bright (1966), 19-Murchison (1989), 20-Oviatt et al.(2003), 21-Jennings (1957), 22-Miller (1980), 23-Zachary (2001), 24- Currey et al. (1983), 25-Everitt (1991) 13 Figure 1.2. Magnitude and timing of the lake level/size changes in the Bonneville basin during the last deglaciation. The shades of gray and width of each bar represent the calibrated radiocarbon age (Reimer et al., 2004; Stuiver et al., 2005), which is grouped into: <200, 200–399, 400–599, 600–799, >800 IntCal04-range years. Bars with younger and/or smaller ranges were laid over those with older and/or larger ranges. The height of each bar represents the size of the lake in terms of lake level elevation (adjusted for the hydro-isostatic rebound) (Table DR1 and Figure DR1; see footnote 1) and lake surface area (Table DR1 and Figure DR2). (A) Pre-Bonneville flood threshold (maximum surface area) at 1550 m (Lake Surface Area Index, LSAI, = 1.00). (B) Post-Bonneville flood threshold at 1444 m (LSAI = 0.74). (C) Historic average level of Great Salt Lake at 1280 m (LSAI = 0.087). 14 Figure 1.3. Comparison of the last deglaciation records. (A) GISP2 ice-core δ18O record, measured at the 20-yr interval (Stuiver and Grootes, 2000). (B) The Bonneville basin lake level/size change profile, represented here by connecting the midpoints of all calibrated shoreline-age ranges (Table 1.1) (Reimer et al., 2004; Stuiver et al., 2005). (C) The Cariaco basin varve gray-scale record (Hughen et al., 1998a, 1998b). (D) The Northern Hemisphere insolation values at 0°, 30°N, and 60°N (Berger and Loutre, 1991). 15 events—Heinrich Event 1, the Bølling/Allerød warming period, the Younger Dryas cold period, and the onset of the Holocene Epoch. Lacustrine chronology One of the largest lakes known to have existed in the Bonneville basin is Lake Bonneville. Lake Bonneville is believed to have existed throughout the last ice age, and attained the 50,500 km2 surface area, 300 m depth, and 10,000 km3 volume at its maximum stage (Figure 1.1). In comparison, today’s Great Salt Lake over the past 150 years in the arid climate is on average 4400 km2 in area and <10 m deep (Arnow and Stephens, 1990) (Figure 1.1). This contrast in size implies that a larger lake in the Bonneville basin reflects, on a regional scale, the cold and wet climate of glacial periods, whereas a smaller or non-existent lake is linked to the warm and dry climate of interglacial periods (Antevs, 1938, 1948). Oviatt et al. (1999) suggested that larger lake events in the Bonneville basin during the late Pleistocene coincided with some of the most extensive glaciations (Marine Oxygen Isotope Stages 2, 6, 12, and 16). This proposed link is thought to reflect the changing mean position of the polar jet stream path, which was largely influenced by the expansion and contraction of the Cordilleran and Laurentide ice sheets in North America (Antevs, 1948). The timing of Lake Bonneville reaching its maximum extent in the last ice age seems to be consistent with this interpretation. Approximately 18.5 ka, Lake Bonneville began expanding significantly (Figure1.2). This large-lake event, during which the lake exceeded its threshold, coincided with the period of massive iceberg discharge from the circum-North Atlantic ice sheets, Heinrich Event 1 (HE1) (Elliot et al., 1998, 2002; Rashid et al., 2003a, 2003b) 16 (Figure 1.2). This lake event during HE1 is the longest and most pronounced known in the Bonneville basin lacustrine record for the past 45,000 years, though a limited number of dates (Table 1.1) indicate that the preceding event, coinciding with the ice-rafted debris event (IRD-a) of the North Atlantic (19.5 ka) (Bond and Lotti, 1995), was also significant in its magnitude and timing (Figure 1.2). The data analyzed for this investigation revealed that Lake Bonneville reached its maximum stage and began to spill over its threshold 200-300 years after the HE1 onset (Figures 1.1 and 1.2 A). The threshold controlled oscillating lake-level condition lasted for 700-800 years until an alluvial fan at the threshold failed catastrophically 17.5 ka. The resulting flood is estimated to have emptied half the volume of water from the lake over approximately eight weeks, draining into the Snake River basin and eventually into the Pacific Ocean (Jarrett and Malde, 1987). By removing material down to the bedrock sill, the flood lowered the threshold of the Bonneville lake basin by 106 m (Figure 1.2 A to 1.2 B) (Currey, 1990). As a result, the largest possible lake surface area in the Bonneville basin since the flood is approximately 37,100 km2 (74% of Lake Bonneville's maximum extent during HE1) (Figure 1.1). After the flood, the lake level stayed mostly at the new threshold for another 1000 years (Figures 1.2 B and 1.3 B). This post-flood and the preceding maximum stages produced some of the most pronounced shoreline- geomorphic features in the Bonneville lake basin, including, for example, beach ridges, spits, tombolos, and deltas (Burr and Currey, 1988; Sack, 1999; Godsey et al., 2005). As HE1 weakened, Lake Bonneville rapidly lost its water, indicating a drastic precipitation decrease and/or evaporation increase, possibly caused by a major northward retreat of the mean position of the polar jet stream path. The most extensive and longest (3000-year 17 long) large-lake event in the Bonneville basin in the last ice age ended abruptly around 15.5 ka. For the next few hundred years, the Bonneville basin was likely drier than today, maintaining a lake smaller than today’s Great Salt Lake (Figures 1.2 and 1.3 B). The beginning of the next large-lake event coincided with the onset of the Bølling/Allerød (BA) warm period around 14.8-14.9 ka (Björck et al., 1998) (Figures 1.2 and 1.3 B). Because of the inferred significant precipitation increase and/or evaporation decrease in the Bonneville lake basin was likely rapid, it was probably caused by an abrupt, large-scale change in atmospheric circulation. Approximately 200-300 years into the event, the rapidly enlarging lake regained almost 70% of Lake Bonneville's maximum surface area (Figures 1.1 B and 1.2). It is also likely that the lake exceeded its threshold more than once during the 1000 year-long episode (Figure 1.2). Considering the retreating ice sheets in North America at the time, it is unlikely that a southward migration of the mean position of the polar jet stream path enhanced winter moisture in the Bonneville basin as it had during the preceding large-lake events. Instead, significantly increased summer precipitation in the Bonneville basin might have resulted from a strong monsoon-like system in south-western North America, itself enhanced by increasing summer insolation in the Northern Hemisphere at the time (Berger and Loutre, 1991) (Figure 1.3 D). Under such conditions, the Bonneville basin could have been frequented by less stormy events with weaker wave energy than those resulting from the southward migration of the mean position of the Polar Jet Stream path during the Last Glacial Maximum or HE1. This may partially explain why most of shoreline- geomorphic features from this large-lake event are much less pronounced than those of the preceding episodes (Morrison, 1965). 18 The large-lake event coinciding with the BA started waning rapidly approximately 200-300 years before the onset of the Younger Dryas (YD) (12.9 ka) (Björck et al., 1998) (Figures 1.2 and 1.3 B). The effective moisture in the Bonneville basin seems to have decreased suddenly. The abrupt shift from deglacial warm to cold on the hemispheric to global-scale at the time might have weakened the monsoon-like system in western North America, leaving the Bonneville basin with much drier summers. During the YD, the lake in the Bonneville basin had a surface area 25%-27% of Lake Bonneville maximum (Figure 1.2). Since this lake was approximately three times as large as today’s Great Salt Lake, the Bonneville basin was still effectively wetter than today. Compared with the HE1 large-lake event, however, the magnitude of the YD lake event is marginal, implying that full-glacial hydrologic conditions were not reestablished in the Bonneville basin during the YD. Immediately after the abrupt end of the YD (11.6 ka) (Southon, 2004), a large lake suddenly re-emerged in the Bonneville basin (Figures 1.2 and 1.3 B). Similar to the beginning of the BA, the data indicate that within 200-300 years after the YD termination the lake rapidly grew to cover an area more than 70% of Lake Bonneville at its maximum. This large-lake event also lasted for >1000 years (11.5-10.3 ka), possibly exceeding the threshold at least once (Figures. 1.2 and 1.3 B). The summer insolation maxima in the Northern Hemisphere at the time (11.0 ka) (Berger and Loutre, 1991) is believed to have created a condition in which a strengthening monsoonal system in southwestern North America might have once again brought significantly increased summer precipitation to the Bonneville basin. All shoreline-geomorphic features identified and dated for this period are deltas (Table 1.1), indicating an overall low wave- 19 energy environment resulting from less stormy monsoonal-summer precipitation events than the frontal storm events during the full-glacial period (Morrison, 1965). Lake Bonneville and the severe environmental changes The speed and magnitude of lake fluctuations in the Bonneville basin presented here indicate that the large-scale environmental changes during the last deglaciation were rapid and severe as also implied by an extinction of a large number of Pleistocene mammals in the Great Basin and elsewhere in North America (Grayson, 2006). The Bonneville records of the last deglaciation also imply a large spatial perspective: hemispheric or even global. The magnitude and timing of abrupt climate/environmental changes during the last deglacial warming are well documented in the GISP 2 ice cores (high latitude, ice sheet) (Alley et al., 1993; Stuiver and Grootes, 2000) and the Cariaco basin marine sediment records (low latitude, ocean) (Hughen et al., 1996; Lea et al., 2000). These records show how different earth systems rapidly reorganized as the last ice age was waning throughout the circum-North Atlantic regions. The profile of hydrologic events in the Bonneville basin (mid-latitude, continental) after HE1 closely parallels those of the GISP 2 and Cariaco basin records (Figure 1.3). This apparent syncroneity seems to imply much more than the close connection between the Bonneville basin and the North Atlantic. The Bonneville basin is under the direct influence of atmospheric and oceanic circulation systems in the Pacific Ocean. Hence, this close parallel among the three records suggests that causal mechanisms, spatial scales, timing, and magnitude of abrupt climate change events during the last deglaciation were hemispheric, and perhaps global. 20 References Alley, R.B., and 10 others, 1993, Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event: Nature, v. 362, p. 527-529. Alley, R.B., Brook, E.J. and Anandakrishnan, S., 2002, A northern lead in the orbital band: north–south phasing of ice-age events: Quaternary Science Reviews, v. 21, p. 431– 441. Antevs, E., 1938, Climatic variations during the last glaciation in North America: Bulletin of the American Meteorological Society, v. 19, 172–176. Antevs, E., 1948, Climatic changes and pre-White man, in The Great Basin, with emphasis on glacial and postglacial times: Bulletin of the University of Utah, v. 38, p. 167–191. Arnow, T., and Stephens, D., 1990, Hydrologic Characteristics of the Great Salt Lake, Utah: 1847-1986: U.S. Geological Survey Water-Supply Paper 2332, 32 p. Berger, A., and Loutre, M.F., 1991, Insolation values for the climate of the last 10 million years: Quaternary Science Reviews, v. 10, p. 297–317, ftp://ftp.ncdc.noaa.gov/pub/data/paleo/insolation/readme_insolation.txt. Björck, S., Walker, M.J.C., Cwynar, L., Johnsen, S.J., Knudsen, K-L., Lowe, J.J., Wohlfarth, B., and INTIMATE Members, 1998, An event stratigraphy for the last termination in the North Atlantic Region based on the Greenland ice-core record: A proposal by the INTIMATE group: Journal of Quaternary Science, v. 13, p. 283-292. Bond, G., and Lotti, R., 1995, Iceberg discharges into the North Atlantic on millennial time scales during the last glaciation: Science, v. 267, p. 1005–1010. Bright, R.C., 1963, Pleistocene Lake Thatcher and Bonneville, southeastern Idaho [Ph.D. thesis]: Minneapolis, University of Minnesota, 292 p. Bright, R.C., 1966, Pollen and seed stratigraphy of Swan Lake, southeastern Idaho: its relation to regional vegetation history and to Lake Bonneville history: Tebiwa, v. 9, p. 1– 48. Broecker, W.S., and Kaufman, A., 1965, Radiocarbon chronology of Lake Lahontan and Lake Bonneville II, Great Basin: Geological Society of America Bulletin, v. 76, p. 537– 566. Burr, T.N., and Currey, D.R., 1988, The Stockton Bar: Utah Geological Survey Miscellaneous Publication 88-1, p. 66–73. 21 Burr, T., and Currey, D.R., 1992, Hydrographic modeling at the Stockton Bar: Utah Geological Survey Miscellaneous Publication 92–3, p. 207–219. Currey, D.R., 1990, Quaternary paleolakes in the evolution of semidesert basins, with special emphasis on Lake Bonneville and the Great Basin, U.S.A.: Paleogeography, Paleoclimatology, Paleoecology, v. 76, p. 189–214. Currey, D.R., Oviatt, C.G., and Plyler, G.B., 1983, Lake Bonneville stratigraphy, geomorphology, and isostatic deformation in west-central Utah: Utah Geological and Mineral Survey Special Study 62, 63 p. Elliot, M., Labeyrie, L., Bond, G., Cortijo, E., Turon, J.-L., Tisnerat, N., and Duplessy, J.-C., 1998, Millennial-scale iceberg discharges in the Irminger Basin during the last glacial period: Relationship with the Heinrich events and environmental settings: Paleoceanography, v. 13, p. 433-446. Elliot, M., Labeyrie, L., and Duplessy, J.-C., 2002, Changes in North Atlantic deep-water formation associated with Dansgaard-Oeschger temperature oscillations (60–10 ka): Quaternary Science Reviews, v. 21, p. 1153–1165. 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Kaufman, D.S., 2003, Amino acid paleothermometry of Quaternary ostracodes from the Bonneville basin, Utah: Quaternary Science Reviews, v. 22, p. 899–914. Lea, D.W., Pak, D.K., Peterson, L.C., and Hughen, K.A., 2000, Synchroneity of tropical and high-latitude Atlantic temperatures over the Last Glacial Termination: Science, v. 301, p. 1361–1364. Light, A., 1996, Amino acid paleotemperature reconstruction and radiocarbon shoreline chronology of the Lake Bonneville basin, U.S.A. [M.A. thesis]: Boulder, University of Colorado, 142 p. Marsters, B., Spiker, E., and Rubin, M., 1969, U.S. Geological Survey radiocarbon dates X: Radiocarbon, v. 11, p. 210–227. Miller, R.D., 1980, Surficial geologic map along part of the Wasatch Front, Salt Lake Valley, Utah: U.S. Geological Survey Map MF-1198. Morrison, R.B., 1965, Lake Bonneville: Quaternary Stratigraphy of Eastern Jordan Valley, South of Salt Lake City, Utah: U.S. Geological Survey Professional Paper 477, 79 p. Murchison, S.B., 1989, Fluctuation History of Great Salt Lake, Utah, during the last 13,000 years [Ph.D. thesis], Salt Lake City, University of Utah, 136 p. Oviatt, C.G., 1989, Quaternary geology of part of the Sevier Desert, Millard County, Utah: Utah Geological and Mineral Survey Special Study 70, 41 p. Oviatt, C.G., 1991, Quaternary geology of the Black Rock Desert, Millard County, Utah: Utah Geological and Mineral Survey Special Study 73, 23 p. Oviatt, C.G., 1997, Lake Bonneville fluctuations and global climate change: Geology, v. 25, p. 155–158. Oviatt, C.G., Sack, D., and Felger, T.J., 1994, Quaternary geologic map of the Old River Bed and vicinity, Millard, Juab, and Tooele Counties, Utah: Utah Geological Survey Map 161, 24 p. 23 Oviatt, C.G., Thompson, R.S., Kaufman, D.S., and Bright, J., 1999, Reinterpretation of the Burmester core, Bonneville basin, Utah: Quaternary Research, v. 52, p. 180–184. 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Scott, W.E., 1988, Transgressive and high-shore deposits of the Bonneville cycle near North Salt Lake, Utah: Utah Geological and Mineral Survey Miscellaneous Publication 88–1, p. 38–42. Scott, W.E., McCoy, W.D., Shroba, R.R., and Rubin, M., 1983, Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, western United States: Quaternary Research, v. 20, p. 261–285. Southon, J. A., 2004, Radiocarbon perspective on Greenland ice-core chronologies: Can we use ice cores for 14C calibration?: Radiocarbon, v. 46, p. 1239–1259. Stuiver, M., and Grootes, P.M., 2000, GISP2 oxygen isotope ratios: Quaternary Research, v. 53, p. 277–284, http://depts.washington.edu/qil/datasets/gisp2_20yr.txt. Stuiver, M., Reimer, P. J., and Reimer, R. W., 2005, CALIB 5.0: http://radiocarbon.pa.qub.ac.uk/calib/download/. Trautman, M.A., and Willis, E.H., 1966, Isotopes Inc. radiocarbon measurements V: Radiocarbon, v. 8, p. 161–203. Utah Geological Survey, Files, Salt Lake City, Utah. 24 Zachary, C., 2001, Late Pleistocene-early Holocene paleoenvironment of the northeast Bonneville basin, Utah [M.S. thesis], Manhattan, Kansas State University, 94 p. CHAPTER 2 SHORELINE RECORDS OF LARGE LAKE EVENTS IN THE BONNEVILLE BASIN, NORTH AMERICA, IN RESPONSE TO ABRUPT CLIMATE CHANGE EVENTS Abstract Lacustrine shoreline geomorphic and depositional features in the Bonneville lake basin, western North America, were investigated to present a chronology of lake expansion and contraction in response to climate change. Based on stratigraphy, sedimentology, geomorphology, radiocarbon dating, and tephra identification, this study identifies eight intervals of large to intermediate lake: one in Marine Isotope Stage (MIS) 16, five in MIS 3, and two in MIS 2. Timing, duration, and magnitude of the Bonneville basin lake size change events are compared with Dansgaard-Oeschger cycles. A pattern of lake events suggests that the amount of precipitation received by surface water systems in the Bonneville basin increased significantly when an abrupt shift from cold to warm air surface temperatures occurred in Greenland and the circum North Atlantic region. Introduction The Bonneville basin is the easternmost catchment in the North American Great Basin, and covers an area of approximately 135 000 km2 between latitudes 37.5º-42.5ºN and longitudes 111º-114ºW. Having its primary drainage sources on the west slope of 26 the Rocky Mountains and Colorado Plateau, the Bonneville drainage basin ranges in elevation from 4000 m mountaintops down to a basin floor slightly below 1300 m, making it the deepest and largest hydrologically closed basin in arid North America (Figure 2.1). Lake size in the Bonneville basin is known to have varied widely over the past two million years (Oviatt et al., 1999). Today, Great Salt Lake is the largest water body in the basin, and has sustained an average surface area of 4400 km2 for the last 150 years in semiarid climate (Arnow and Stephens, 1990). The surface area of this hypersaline lake, however, would comprise only 9% of 50 500 km2 fresh water Lake Bonneville at its maximum extent during late Marine Isotope Stage (MIS) 2 (Figure 2.1). Thus, the amplitude of lake size change in the Bonneville basin can be large, and closely reflect the scale of regional hydrologic and climate change events. When a lake level is below the 1444 m basin threshold (Red Rock Pass, Figure 2.1 locality RP), the Bonneville lake basin is hydrologically closed with no surface streams flowing out. As in other closed basins, climate plays the most important role in determining lake size as a result of the net balance between precipitation and evapotranspiration. Reconstructing past lake levels or lake sizes in the basin, therefore, has been considered a valuable source of information on how effective moisture changed in the past (Antevs, 1948; Hostetler et al., 1994; Oviatt, 1997; Kaufman, 2003). Antevs (1948), for example, suggested that pluvial lake level changes during ice ages in the Great Basin, including the Bonneville basin, were mainly caused by a shift in the polar jet stream’s mean path, which was influenced by the size of the Cordilleran and Laurentide ice sheets. This connection between pluvial lakes, the mean path of the polar jet stream, and continental ice sheets still seems relevant when interpreting the Great Basin 27 Great 2 (100% LB-max) at 2 PC-Pilot Creek Valley; PC-Pilot Creek Valley; rmester sediment core; DC-Deep Creek core; sediment rmester area (after Currey, 1990): 50 500 km life Refuge; LV-Little Valley; bury Gulch; SO-Shutoff; USC-Utah State Capitol. USC-Utah State Capitol. SO-Shutoff; bury Gulch; at the Red Rock Pass threshold (medium gray), and 4400 km at the Red Rock 2 Localities discussed in the text: BC-Bu in the Localities discussed ville basin and lake surface light gray), 38 000 km Salt Lake (9% LB-max) dark gray). LB-max) dark gray). Salt Lake (9% FSN-Fish Springs National Wild dam; FSF-Fish Springs Flat; PH-Promontory Hollow; RP-Red Rock Pass; SG-Stans known maximum extent ( Figure 2.1. Location of the Bonne 28 paleolake records (Benson et al., 1990; Benson, 2004). In line with Antevs’ idea, a study of the Bonneville basin long sediment core (Oviatt et al., 1999) indicated that a large, deep lake occurred only during some of the most extensive glaciations: MIS 16, 12, 6, and 2. The possible correlation between Lake Bonneville and the circum North Atlantic (including the Laurentide) ice sheets was also presented in the context of millennial scale hemispheric climate change events by Oviatt (1997). According to his chronology, terminations of the several Lake Bonneville highstands and those of North Atlantic Heinrich events 1 and 2, and ice rafted debris events a, b, and c, seem to have taken place synchronously. This study presents shoreline evidence of a large to intermediate lake in the Bonneville basin during MIS 16, 3, and 2, up to Greenland Interstadial (GI ) 2 (Björck et al., 1998). Six of the lake events have not been previously reported (Table 2.1). In addition, a revised Bonneville basin lake chronology is compared with the Greenland ice sheet records to illustrate whether timing, duration, and magnitude of the lake size change events were in phase with those of Dansgaard-Oeschger cycles. Methods Shoreline Approach This study analyzes lacustrine shoreline geomorphic features and their sediments in the Bonneville basin to estimate: (1) lake level at the time of sediment deposition, (2) lake size derived from lake level, and (3) timing and magnitude of each lake event. An essential principle in shoreline study is that lacustrine shoreline geomorphic features, such as barriers, spits, tombolos, and deltas, all develop at the edge of a lake. This is important, because their elevations approximate where the lake surface water was at the 29 Table 2.1 List of lake events discussed in this study. *large: >30 300 km2, Intermediate: 19 400-30 300 km2, Small: <19 400 km2 ______ Lake Event Marine Isotope Stage Size Previously Known 670 ka 16 large no 38-36 14C ka 3 large no 32 14C ka 3 large no 28-26 14C ka 3 large no 24.5 14C ka 3 intermediate yes 24-23 14C ka 3 intermediate no 22.5-22.0 14C ka 2 intermediate no 21.8-19.8 14C ka 2 large yes 30 time of their formation. In the Bonneville basin, once lake surface elevation is determined, lake surface area can be estimated because of its semi-linear relationship with lake surface elevation (Figure 2.2). With proper age control, it then becomes possible to reconstruct actual lake dimensions through time (Figure 2.3). Well exposed shoreline stratigraphy can indicate in detail how and when a lake level went up or down (or lake expanded or contracted) during a particular lake event. On the other hand, each shoreline depositional feature can typically provide the information on only one lake event, even though finding records of multiple lake events at one stratigraphic exposure in the Bonneville basin is not uncommon (as discussed below in this study). Compared to lake sediment core-based studies (Oviatt, 1999; Balch et al., 2005), a chronology derived from a shoreline-based approach tends to (1) have a relatively limited time span, and (2) lack temporal continuity if the number of studied sites is small. As a result, more or less fragmented information was presented in the shoreline-based Bonneville chronologies in the past (Currey, 1990; Oviatt et al., 1992; Oviatt, 1997). As illustrated in this study, however, the shoreline approach can become robust by simply expanding a database to make it more continuous and improving age control. In general, estimating how much, how often, and when a significant change in lake size occurred relies largely on (1) surveying a shoreline elevation of each geomorphic feature, and (2) acquiring good age control. Shoreline Elevation and Lake Surface Area Elevations at each measured stratigraphic section were determined by a combination of 1:24 000 topographic maps, a geodetic total station, benchmarks, and 31 Figure 2.2. Lake surface area curve for the Bonneville lake basin (after Currey and Oviatt, 1985; Currey, 1990). Y-axis is shoreline elevation in meters above sea level. (A): Zenda threshold, at this level Lake Bonneville was at its maximum size. (B): Red Rock Pass threshold, post-Bonneville flood and current Bonneville basin threshold. (C): This threshold separates a larger northern part of the Bonneville basin from smaller southern Sevier River tributary basin. (D): Threshold for Great Salt Lake basin. Lower x-axis represents lake surface area in square kilometers. Upper x-axis is the lake surface area ratio in which a lake surface area is divided by the maximum Lake Bonneville surface area (set to 100). 32 d ± 1 C ka. Height of each bar 14 75% LB-max) at the 1444 m shold. The lake levels an shold. The Known maximum lake extent of each bar represents analytical error ( each bar represents of e basin from 43 to 19.5 e basin from 43 to 19.5 ille flood maximum extent ( C dates were graphed. (A) 14 threshold. (B) Post-Bonnev tributary basin threshold. (D ) Great Salt Lake thre ce area and lake surface elevation. Width ce area and lake surface elevation. are light colored bars. C age estimate. A total of 37 published and 35 new 14 Figure 2.3. Magnitude, timing, and duration of lake episodes in the Bonnevill timing, and duration Figure 2.3. Magnitude, represents lake size in terms of surfa represents (100% LB-max) at the pre-Bonneville flood marl dates lacustrine area of Red Rock Pass threshold. (C) Sevier River sigma) on a 33 global positioning system. All surveyed elevations are accurate within 1 m. In addition, all lacustrine depositional features except for marl facies analyzed in this study were interpreted to be in the shore zone from foreshore to backshore/lagoon. Therefore, lake surface level at the time of deposition was assumed to be within 5 m of the surveyed stratigraphic unit (Godsey et al., 2005). The surveyed elevations in this study are reported without being adjusted for hydroisostatic rebound. The current models for the Bonneville lake basin hydroisostatic rebound are based on the chronology in which Lake Bonneville started expanding around 26,000 calendar years ago (cal yr BP), and reached its maximum stage around 18,000 cal yr BP (Oviatt, 1997; Bills et al., 2002). Because these hydroisostatic rebound models do not assume the existence of large and intermediate lake before 26,000 cal yr BP, several, previously unidentified, large, and intermediate lake events reported in this study would make it impossible to adjust the surveyed shoreline elevations by using the existing Bonneville hydroisostatic rebound models. Further studies for the Bonneville lake basin hydroisostatic rebound events prior to the main transgressive phase of Lake Bonneville during Greenland Stadial (GS) 2 are needed. From the elevation of the surveyed shoreline feature, lake surface area was estimated by using a lake surface area rating curve (Figure 2.2). The derived lake surface area was then divided by the Lake Bonneville maximum surface area during GS 2 (“LB- max,” 50 500 km2) (Figure 2.1 light gray), and multiplied by 100 for a lake surface area ratio (Figure 2.2). For example, the lake surface area of 38 000 km2 at the current basin threshold is 75% of LB-max (Figure 2.1 medium gray) while the historic average area of Great Salt Lake, 4400 km2 (Arnow and Stephens, 1990), is 9% of LB-max (Figure 2.1 34 dark gray). Three subbasin thresholds were used to group lake events according to their sizes for this study (Figure 2.2 B, C, D). Lakes below the Great Salt Lake basin threshold (Figure 2.2 D) are defined as “small” (<38.5% LB-max area or <19 400 km2) while those between the Great Salt Lake basin and Sevier River basin (Figure 2.2 C) thresholds are “intermediate” (38.5-60% LB-max area or 19 400-30 300 km2). Lakes above the Sevier River basin threshold (>60% LB-max area or >30 300 km2) are “large.” By following this classification scheme, we limited our focus to the evidence of “intermediate” to “large” lake events. Eight localities that sit above the Sevier River basin threshold were thereby selected as study sites (Figure 2.1). The topography and negligible flux of ground water in the Bonneville basin make lake surface area a more sensitive indicator of changing lake size than volume, as discussed in detail by Benson and Paillet (1989) and Benson et al. (1990). In this study, the magnitude, timing, and duration of large lake episodes in the Bonneville basin are expressed in terms of changing lake surface area. Age Control Age control is one of the most important elements for spatially correlating sets of lacustrine shoreline stratigraphy observed at different localities. In addition to 37 published 14C dates, 35 new 14C dates were obtained in this study to provide age control for the shore zone facies analyses (Table 2.2). Analyses of a tephra unit also provided crucial age control. As shown in Table 2, mollusk shells provided most of the new 35 " study Study Study Study Study Study Study Study Study 2 7 3 5 6 , , !J $ is , ~ his his his his his his his ~ Th T T T T T T T This (f o'f' ~ :§ 3 9 . . c1J _ " . of ~ 8! 32 73.4 72.5 72.5 72.0 12 44.5 84.1 60.4 60.4 60.4 32.3 71.8 82.0 :"r ! '< I'! >E' 00 f' ~ q ~ 6500 .$ :§ 16300 16300 22500 36300 37100 36600 36600 36400 30500 30500 30500 42500 414 '! 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O: S c, :g 0 . ~ found " m ,f & ~ " , • 60.6 74.6 73.0 60.8 63.2 62.5 40 68.1 61.1 69.0 60.8 60.8 60.8 ~ s oM: d .. , t!J a n d , ~ ?" w ~ ~ d sa 'b $ f u volcani ~ 20200 lJ700 30600 34400 34900 37700 30700 30700 36900 30800 319)(1 30700 31600 !,' mu al 0' m S: J' ri >? 14 , 4 M: d! ic and 1382 1384 1314 1418 1442 1383 1383 1435 1383 1393 1383 1300 1 ~ rl n S , lt a i mate s ga m r #" ~ and ,j' o ed 39 Table 2.2 Shore Zone (sz): This describes the shore zone environment at the time of the deposition of the dated unit —sz00: zone not specified, sz01: lagoon (closed to partially closed, typically muddy), sz02: beach backshore face (typically sandy and gravelly), sz03: beach crest (typically gravelly), sz04: beach foreshore face (typically gravelly), sz05: beach foreshore toe (typically sandy), sz06: tributary stream (terminal reach), sz07: delta top, sz08: delta front, sz09: overflow (basin- or subbasin-spill) stream, sz10: lake- edge marsh, sz11: lowland (elevation typically < 30 m above lake), sz12: offshore (depth typically > 10 m). Lake Surface Elevation: Except for sz00, 09, 10, 11, and 12, each radiocarbon dated shore zone material listed here is considered to have been deposited within 5 m of the lake surface. Modern day elevations were surveyed by the geodetic total station (unadjusted elevations). Lake Surface Area: Lake surface area was calculated from the surveyed elevation by using Fig. 2. Area Ratio (%): The maximum surface area of Lake Bonneville (50,500 km2) is set to 100%. References: 1-Rubin and Alexander (1958); 2-Light (1996); 3-Oviatt et al. (1992); 4- Everitt (1991); 5-Broecker and Kaufman (1965); 6-Oviatt (1986); 7-Benson et al. (1990); 8-Mehringer (1977); 9-Scott et al. (1983); 10-Oviatt et al. (2003); 11-Currey et al. (1983); 12-Scott (1988); 13-Trautman and Willis (1966); 14-Sack (1999); 15-Oviatt et al. (1990); 16-Oviatt (1989). 40 radiocarbon dates. Magnitudes of radiocarbon reservoir effects, mineral carbon, and hard water errors on mollusk shells in the Bonneville basin are not well understood. While this study follows the previous studies’ examples (Benson et al., 1990; Oviatt et al., 1992; Benson, 1993; Licciardi, 2001; Bacon et al., 2006), and reports all 14C dates uncorrected for the potential effects and errors, further studies are needed to resolve the 14C dating uncertainties in the Bonneville lake basin. Replacement of original aragonite with secondary calcite can indicate contamination of mollusk shells with young carbon (Oviatt et al., 1992; Licciardi, 2001). For this study, X-ray diffraction (XRD) analyses were conducted on most of the radiocarbon dated shells older than 25 14C ka. As shown in the following discussion, the analyses showed zero to negligible calcite content. The likelihood of young carbon contamination was considered minimal in most cases, even though young carbon contamination below the detection capabilities of XRD is still possible (Pigati et al., 2009). Another source of uncertainty stems from potential post-depositional transport of shells. In an effort to establish in situ deposition of the shells, faunal assemblage, size distributions, and preservation condition of mollusk shell samples were carefully examined. A practice of collecting and dating a single shell was avoided. The evidence of uncertainties in radiocarbon dating of mollusk shells is acknowledged. The integrity of radiocarbon-dated materials in this study, however, is interpreted to be sufficient as some studies have demonstrated in the past (Benson et al., 1990; Oviatt et al., 1992; Licciardi, 2001; Bacon et al., 2006). 41 Presenting Chronology While age control still poses a challenge for both sediment core and shoreline outcrop based studies in the Bonneville basin (Oviatt, 1997; Oviatt, 1999; Balch et al., 2005), numerous studies have presented some type of hydrograph to show the lake level change chronologies (e.g., Currey, 1990; Oviatt, 1997). In many of these hydrographs, however, a limited number of radiocarbon dated lake levels are connected by a curve, which tends to smooth out periods with no available data. As a result, these hydrographs appear to show more or less complete and continuous lake level change chronologies in the Bonneville basin despite the amount of missing information (Currey, 1990; Oviatt, 1997). This study adopted a bar graph (Figure 2.3) to present the lake size change chronology. On each bar, width represents radiocarbon or calendar age range while height represents lake size (Figure 2.3). Thus, those periods with missing information are simply presented as blanks. In addition, as the number of dated lake size change events, i.e. bars, increases in the future, patterns in lake size change in a bar graph should become more observable and coherent. Lake “event” in this study is defined as a 14C-dated lacustrine shoreline sedimentary package, often, but not necessarily, bounded by unconformities observed in the field. Since precision and accuracy of calibration are steadily improving, it will be beneficial and convenient for future studies to reference the following lake events in original 14C ages. Calendar years are used in a section, titled “Lake Events in the Bonneville Basin and Hemispheric Climate Change” for discussing a connection between the Bonneville basin lake size change and abrupt climate change events. 42 MIS 16 Large Lake Previous Studies The possible existence of a deep lake in the Bonneville basin during MIS 16 (680- 620 ka) was previously suggested by Morrison (1965), Eardley et al. (1973), Scott et al. (1982), McCoy (1987), Oviatt et al. (1999), and Kaufman (2003). In these studies, the regressive phase of the MIS 16 lake was thought to have preceded the deposition of the 640,000 year-old Lava Creek B tephra (Lanphere et al., 2002). Morrison (1965), Scott et al. (1982), and McCoy (1987) located the subaerially deposited Lava Creek B tephra overlying lacustrine sediment in the Little Valley gravel pit near Promontory Point, Utah (Figure 2.1 locality LV). Oviatt et al. (1999) also identified Lava Creek B tephra directly overlying a major lacustrine unit in the Burmester sediment core (Figure 2.1 locality BC). In addition, a number of amino acid analyses on mollusk and ostracode shells from the lacustrine sediment preceding the Lava Creek B tephra deposition broadly implied the MIS 16 age of this major lake episode (McCoy, 1987; Oviatt et al., 1999; and Kaufman, 2003). All these studies agree that a lake of unspecified magnitude (‘Lava Creek lake” of Oviatt et al., 1999) existed in the Bonneville basin just before the Lava Creek B tephra deposition in MIS 16. 670 ka Shoreline Stratigraphy Shoreline evidence of the MIS 16 large lake event was located where a major paleostream (ancestor of modern day Deep Creek) flowed into Lake Bonneville during the last ice age (Figure 2.1 locality DC). Around the last glacial maximum, this stream carried meltwater from most of the 19 alpine glaciers in the Deep Creek Range, and formed numerous deltas as it flowed into Lake Bonneville (Currey et al., 1983). The 43 1979 excavation and construction of the right abutment (Figure 2.4 C) of the Deep Creek Dam on Bureau of Land Management leased land (40°18.528’N, 113°56.172’W, 1420- 1438 m a.s.l.) exposed stratigraphy resulting from a series of lacustrine shoreline depositional events that centered on 670 000 years ago. The base of the exposure is a >5 m thick marl unit (Figure 2.4 C unit 11) that contains moderately abundant ostracode shells (nonfinite 14C age of >43 120 14C yr BP, Beta-169982). Unconformably overlying this marl unit is a >3 m thick greenish tan, clayey, laminated deltaic mud with abundant large ostracode shells (Figure 2.4 C unit 10). On the wall facing southwest (Figure 2.4 A), this greenish mud is overlain by a 35 cm thick layer of pebbles, which fine upwards, and range from angular at the bottom to subrounded at the top of the unit (Figure 2.4 A unit 9). Overlying these pebbles is a 9 cm thick thinly bedded to laminated tephra, which is interbedded with five to seven fine sand layers (Figure 2.4 A unit 8) (1435 m at the measured section). The contacts between these three units (8, 9, and 10) are sharp, but conformable. When the tephra was laid down in the shallow water, it draped over these beach pebbles (unit 9). The water-laid tephra was identified as the Rye Patch Dam (RPD) tephra (F. Foit, personal communication, 2002; M. Perkins, personal communication, 2002) (Table 2.3). It is believed to have erupted in an event that produced the Desert Spring ash-flow in the Cascade Range around 670 000 years ago (Hill, 1985; Rieck et al., 1992; Reheis, 1999; Reheis et al., 2002). This tephra was also previously identified in the Bonneville basin in the sediment core S208 (0.5 cm thick at a depth of 154.1 m) by Williams (1993, 1994). Above the 9 cm thick RPD tephra unit, a reworked tephra gradually transitions upward into ostracodal silty mud (20-25 cm thick Figure 2.4 A unit 7). A unit of 20 cm 44 is thwest facing walls. All numbered e same unit as 10 in B and C). Unit 8 exposures with southwest facing and nor Figure 2.4. Deep Creek Dam right abutment A, B, and C (i.e., unit 10 in panel is th in panels same units are stratigraphic 45 Table 2.3 Results of electron microbeam analyses on the RPD tephra ______ SiO2 Al2O3 Fe2O3 TiO3 Na2O K2O MgO CaO Cl Analyses normalized to 100 weight % 71.66 14.47 3.32 0.45 4.53 3.49 0.39 1.53 0.13 Standard deviations of the analyses 0.38 0.13 0.12 0.05 0.16 0.13 0.05 0.14 0.02 Total number of shards analyzed 26 Similarity coefficient 0.96 (Analyses by Franklin “Nick” Foit) 46 thick oxidized subangular pebbles (Figure 2.4 A unit 3) separates the underlying MIS 16 stratigraphic units from an overlying 5 cm thick, fine lacustrine sand of MIS 2 age that contains moderately abundant Stagnicola shells (20 230 ± 120 14C yr BP, Beta-189275) (Figure 2.4 A unit 2). Despite the significant length of time separating the MIS 16 and MIS 2 units, no buried soils were observed. A thinly bedded 30 cm thick sandy marl with abundant ostracode shells uncomformably overlies the 20.2 14C ka sand unit 2 to top the exposure (Figure 2.4 A unit 1). A northwest facing exposure (Figure 2.4 B) at the same site lies across a gully and approximately 20 m to the southwest from the southwest facing wall (Figure 2.4 A). The greenish tan clayey mud (unit 10) and the RPD tephra, including both the laminated (unit 8) and reworked (unit 7) units, are present at 1427-31 m (unit 8 slopes up 2-4° from southwest to northeast, Figure 2.4 C). Here, however, the layer of beach pebbles (Figure 2.4 A unit 9) between the greenish mud and tephra is absent (Figure 2.4 B). Instead, the RPD tephra directly overlies the greenish mud. The contact between these two units (8 and 10) is sharp, but conformable. The RPD unit here is also 9 cm thick and interbedded with fine sand layers (Figure 2.4 B unit 8). Above unit 8, a reworked tephra gradually transitions upward into 20-25 cm thick ostracodal silty mud (Figure 2.4 B unit 7). Unconformably overlying these units is a 25 cm thick unit of gravelly mud (Figure 2.4 B unit 6). The gravel consists of mostly angular pebbles, which originated from local rhyolitic and dacitic flows of unknown age. A 20 cm thick mud uncomformably overlies unit 6 (Figure 2.4 B unit 5). Above this mud is a 40 cm thick unit of angular pebbles in a muddy matrix (Figure 2.4 B unit 4), which grades upward into beach boulders that crop out at the modern day surface. 47 670 ka Lake Event Units 7, 8, 9, and 10 are not separated by any observable unconformities. This stratigraphy suggests that a shallow to deep lacustrine environment at this site persisted around the early to middle part of MIS 16. Laminations of the tephra unit and its subsequent reworking indicate that the air-fall tephra was very likely deposited in shallow water above wave base. Fining up beach pebbles of unit 9 also suggests that the lake level was rising around the time of the RPD tephra deposition 670 ka. Considering the fact that the Lava Creek B tephra appears to mark the end of a large lake as reported in the previous studies (Oviatt et al., 1999), it is likely that a MIS 16 lake of substantial size was expanding around 670 ka, and then shrinking around 640 ka in the Bonneville basin. Because details of basin tectonics and numerous hydroisostatic rebound events since MIS 16 are unknown, estimating an exact size of the 670 ka lake through the observed shoreline elevations alone is problematic. With the limited age control, this is also true for estimating duration of this lake event. Nevertheless, it is still useful to approximate the magnitude of the 670 ka lake based solely on the observed shoreline elevations. The observed maximum elevation of the 670 ka shoreline sediment, 1435 m, could mean a possible minimum lake surface elevation of 1440 m, 160 m above the 1280 m-historic average of Great Salt Lake (Arnow and Stephens, 1990). At this elevation, the lake could have covered an area of approximately 37 400 km2 (≈ 74% LB-max). MIS 3 Large Lakes The period of MIS 3 discussed in this study is defined by Voelker et al. (2002), Shackleton et al. (2004), and Svensson et al. (2006) as an interval between the transition from Heinrich event 6 to GIS 17 and the transition from GS 4 to GI 3. The exact 48 duration, however, differs between these studies due to the ongoing improvement in Greenland ice sheet chronologies. Voelker et al. (2002) and Shackleton et al. (2004) define MIS 3 from 59 to 29 calendar (cal) ka or roughly 56 to 24 14C ka. Meanwhile, based on more recent and improved Greenland ice sheet chronologies, Svensson et al. (2006) set the end of MIS 3 at 27.8 cal ka. Hence, in this study, MIS 3 is considered as a period between 59 to 27.8 cal ka. Previous Studies There has been no reported MIS 3 large or intermediate lake event in the Bonneville basin. Although the Cutler Dam alloformation (Oviatt et al., 1987) might have been deposited around the MIS 3 and MIS 4 boundary, its timing or duration has not been well established. Based on amino acid analyses of mollusk shells and one nonfinite radiocarbon date on wood (>36 14C ka), Oviatt et al. (1987) suggested that the probable age of the Cutler Dam alloformation was between 80 and 40 ka (considered MIS 4 at the time), with a lake surface area of 22 000 km2. A later study by Kaufman et al. (2001) used multiple dating methods to refine the age estimate of the Cutler Dam alloformation. Their infrared stimulated luminescence (IRSL) analyses on marsh mud (“marginal lacustrine” in Oviatt et al., 1987) yielded an age of 59 000 ± 5000 yr, placing the approximate timing of this lake episode at the boundary between MIS 4 and 3. In addition, their amino acid analyses on ostracode shells indicated an age of 46 000 ± 12 000 yr. They also interpreted a radiocarbon date of 39 500 ± 1250 14C yr BP on one mollusk shell in the same facies (Light, 1996) as a minimum age by assuming a small amount of isotopic exchange. According to Kaufman et al. (2001), the age range estimated by their amino acid 49 analyses overlaps those of both the IRSL and 39.5 14C ka dates. Based on these dates, they concluded that the lake existed during the late MIS 4 or the early MIS 3. Further studies with finer temporal constraints are needed. As discussed below, however, a large lake appears to have occurred approximately 2000 years after the 39.5 14C ka date (Light, 1996) of the Cutler Dam alloformation. 38-36 14C ka Large Lake Shoreline Stratigraphy On the left abutment of the Deep Creek Dam (40º18.560’N, 113º56.310’W, Figure 2.1 locality DC), 120 m west of the aforementioned right abutment, a section of the backshore side of a relatively small gravel spit was exposed by the 1979 dam construction (BLM files, Salt Lake City office, 1978-1982) (Figure 2.5 B). Stratigraphy, sedimentology, and radiocarbon dates of the excavated section reveal that the gravel spit resulted from at least four different large lake events, two of which occurred during MIS 3 (Figure 2.5 C). The oldest event appears to have taken place around 38-36 14C ka. The facies of this episode consists of tufa-capped boulders (>2 m thick) and up to 20 cm of fine yellow sand, which contains abundant Pyrgulopsis shells (Figure 2.5 D). The fine yellow sand drapes over the slope from east to west on the washover side of the spit, at an elevation ranging from 1432 to 1437 m (Figure 2.5 D). The lower to middle part of the fine sand lies between boulders capped with dendritic to lithoid tufa. In its upper section, a 5-10 cm thick layer of angular pebbles overlies the fine yellow sand, which is underlain by the tufa-capped boulders (Figure 2.5 D). The contact between the pebbles and sand is unconformable. More than 200 Pyrgulopsis shells found in this sand unit yielded three radiocarbon dates: 36 560 ± 570 (Beta-169221), 37 250 ± 500 14C yr BP (Beta-170326), 50 . tigraphic units ), showing stra eek Dam/Reservoir looking north. (B) A ) Cross-section (a—a’) of spit (C epCreek Dam/Reservoir. (A) Deep Cr . (C) Close-up of the spit. (D C dates were obtained. 14 gravel spit on the left abutment the left on gravel spit Figure 2.5. Shoreline complex at the De Points 1-6 indicate where 51 both from the lower part of the sand unit at 1433 m (Figure 2.5 D6), and 37 270 ± 300 14C yr BP (Beta-186152) from the upper part of the sand unit at 1437 m (Figure 2.5 D5). An x-ray diffraction analysis of these mollusk shells showed 100% aragonite content, indicating that weathering since their deposition was likely minimal (S. Lutz, personal communication, 2002). Tufa-capped boulders of different sizes overlie the lower to middle part of the fine yellow sand, mostly obscuring the stratigraphic boundary (Figure 2.5 D). Therefore, it is difficult to determine if the contact between the sand and overlying tufa unit is comformable and indicative of a regressive phase. This shore zone stratigraphic sequence is interpreted as a stillstand, likely during a regression of the 38-36 14C ka large lake, which might have had a minimum surface area of >37 000 km2 (73% LB-max). 32 14C ka Large Lake Shoreline Stratigraphy The sedimentary structure of the most well exposed east face of the spit at the Deep Creek Dam left abutment is predominantly clast supported (Figure 2.5 C and left side of D). The lowest 80-100 cm of the exposure consists of gravel, which fines upward, indicating a decrease in wave energy. This lower energy depositional environment probably allowed some molluskan fauna to survive. From the fine sandy matrix of the upper 15 cm of this gravel unit (Figure 2.5 D4), 120 intact Pyrgulopsis shells (2-5 mm) were collected. The radiocarbon date on the shells indicates that a stillstand phase (at 1431 m) of this lake event might have occurred around 32 090 ± 410 14C yr BP (Beta-232004), covering an area of 36 400 km2 (72% LB-max). A 20 cm thick layer of subangular to subrounded small pebbles overlies the gravel unit and points to the further decrease in wave energy. The change in wave energy appears to have occurred a 52 few more times as the upward coarsening and fining of gravel alternate over the next 50- 60 cm interval. A regressive to terminal phase of this 32 14C ka large lake is suggested by dendritic tufa (Figure 2.5 C). A radiocarbon date on the tufa yielded the age of 43 660 ± 1780 14C yr BP (Beta 171329) (Figure 2.5 D3), stratigraphically inconsistent with five other radiocarbon dates obtained at this site. Since the tufa had been the top unit of these beach gravel packages for an unknown period of time (possibly >10 000 14C years), it might have been subjected to surface weathering and contamination. In any case, the tufa marks the end of this large lake. Muddy gravel, 50 cm to 80 cm thick, unconformably overlies the 32 14C ka gravel units (Figure 2.5 D). As the unit coarsens upward, it becomes less muddy in its upper 20 cm. Although no direct date on this unit is available, its stratigraphic position relative to unit 2 (20.2 14C ka) in Figure 2.4 A at 1436-1437 m suggests its possible age of 20.5-20 14C ka or older. Mostly angular gravel unconformably overlies this muddy gravel unit with an observable thickness varying from 30 to 80 cm and the clast size of the unit increasing upward (Figure 2.5 D). A radiocarbon date on 57 Pyrgulopsis shells, found in the sandy matrix of this unit at 1434 m is 19 460 ± 90 14C yr BP (Beta-169981) (Figure 2.5 B2). Another date of 19 340 ± 100 14C yr BP (Beta-186153) was obtained from 15 Stagnicola shells in fine sand overlying tufa-capped boulders on a bayhead barrier 250 m southeast of the Deep Creek Dam right abutment at 1437 m (40º18.480’N, 113º56.040’W). The youngest radiocarbon-dated shore zone facies of the Deep Creek Dam spit is a medium sand unit up to 3 cm thick, which directly overlies the 19.4-19.3 14C ka gravel 53 package (Figure 2.5 B2) at 1438 m (Figure 2.5 B1). The contact between the sand and gravel is sharp, but appears conformable. Abundant Stagnicola shells found in this sand have yielded a radiocarbon age of 18 690 ± 240 yr 14C yr BP (Beta-186151) (Figure 2.5 B1). Overlying the sand is up to 60 cm of boulders capped with dendritic tufa, which uncomformably underlies white marl covering the crest of the spit (Figure 2.5 D). 28-26 14C ka Large Lake Shoreline Stratigraphy Oviatt (1991, locality M, p. 13) examined in detail the geomorphology, sedimentology, and stratigraphy of a gravel barrier at the south end of Fish Springs Flat (Figure 2.1 locality FSF). The lower stratigraphic sequence of the barrier consists of lagoonal marl overlying a transgressive phase gravelly sand (Oviatt, 1991, Figure 17, p.13). Valvata and Gyraulus shells from the lagoonal marl unit yielded a nonfinite radiocarbon age of >38,510 14C yr BP, which was rejected by Oviatt as being possibly contaminated by the local 14C depleted ground water. Because of the possibility of this unit being linked to the 38-36 or 32 14C ka lake events, these mollusk shells were reexamined and redated for this study. Shells from three genera, Valvata, Gyraulus, and Stagnicola were collected from the same marl unit and dated separately by accelerator mass spectrometry (AMS), yielding ages of 27 720 ± 40 (Beta-170252), 27 660 ± 250 (Beta-170371), and 27 210 ± 240 14C yr BP (Beta-170372), respectively. An x-ray diffraction analysis of Valvata shells showed 100% aragonite content, indicating minimum weathering since their deposition (S. Lutz, personal communication, 2002). As discussed by Oviatt (1991), a stratigraphic contact between the lagoonal marl and overlying carbonate-coated sand does not seem to be unequivocally unconformable with overlying MIS 2 Lake Bonneville packages (Oviatt 1991, Figure 17, p. 13). 54 Nevertheless, the three radiocarbon dates indicate an unconformity. If the ages are reliable, this 28-27 14C ka lake event appears to have reached a level of 1381 m with a 30 500 km2 surface area (60% LB-max). Fluviolacustrine evidence for a later phase of this intermediate lake is located at a site called the Shutoff along the Old River Bed (Currey et al., 1983, Figure 8, Stop C) (Figure 2.1E). This well studied area (e.g., Gilbert, 1890; Currey et al., 1983; Oviatt, 1984, 1987) is where the paleo-Sevier River flowed into Lake Bonneville forming shoreline geomorphic features such as deltas and spits at various levels. Two radiocarbon dates for this study were obtained from the “Yellow Clay” (Gilbert, 1890), which consists of deltaic sediments, varying in size from fine sand to silty clay (Oviatt, 1987). Previously, Currey et al. (1983, Figure 8, p. 74) radiocarbon dated Sphaerium shells from the Yellow Clay, yielding an age of 23 190 ± 1310 14C yr BP. From the same unit in the Yellow Clay at the same location (1378 m), Sphaerium, Gyraulus, and Valvata shells were collected for this study. An AMS analysis on articulated Sphaerium shells has yielded an age of 26 920 ± 40 14C yr BP (Beta-170251). Another radiocarbon date on Valvata shells from the Yellow Clay was obtained from >50 cm thick mud (at 1380m) overlain by a 40 cm thick, thinly bedded, fine sand at the mouth of Slow Elk Wash, approximately 350 m south of the re-dated Sphaerium site. These Valvata shells have yielded an age of 26 220 ± 240 14C yr BP (Beta-169798). As suggested by Currey et al. (1983) and Oviatt (1987), the Yellow Clay was deposited in deltas formed by the paleo-Sevier River that originated from the Sevier Desert basin and flowed northward into a shallow lake in the Old River Bed area at the time. The timing of the deposition indicated by this study, however, differs from their 55 proposed time frame, which was during the Stansbury Oscillation between 22.5 and 21.5 14C ka (Currey and Oviatt, 1985; Oviatt, 1987). The stratigraphic correlation and consistency of the radiocarbon dates between the Fish Springs Flat and Old River Bed sites obtained for this study instead suggest the large lake with a surface area ranging from 30 100 to 30 500 km2 occurred during 28-26 14C ka. 24.5 14C ka Intermediate Lake Shoreline Stratigraphy A transgressive facies of lagoonal sediments is located at a bayhead barrier in the Pilot Creek Valley, Nevada (40º50.160’N, 114º10.260’W) (Figure 2.1 locality PCV). A 50 cm thick massive fine sand lies at the bottom of a 3 m deep exposure of a lagoon. Overlying it, a 40 cm thick unit alternates between medium sand and fine sand at a 2 to 5 cm interval, with occasional crossbeds. The top 10 cm of this unit contains Gyraulus shells and charophytes (at 1371 m), indicating a shallow lacustrine environment at the time of deposition. An AMS analysis of the Gyraulus shells has yielded an age of 24 600 ± 150 14C yr BP (Beta-169220). The transition from this alternating sand into overlying >2 m thick marl is gradual. During this apparent transgressive phase, the lake would have covered an area of at least 26,500 km2 (52% LB-max). One published radiocarbon date (24 870 ± 410 14C yr BP, Beta-8343) on lacustrine marl (Oviatt et al., 1992) at Stansbury Gulch (Figure 2.1 locality SG) supports this 24.6 14C ka lake event. 24-23 and 22.5-22 14C ka Intermediate Lake Shoreline Stratigraphy Shore-zone evidence of intermediate size lake oscillations between 24 and 22 14C ka was observed in an exposure at Jay Banta gravel pit (Figure 2.6 A and B, 39º50.460’N, 113º24.000’W), 100 m northwest of the Fish Springs National Wildlife 56 Figure 2.6. Photos of shoreline-near shore carbonate deposits. (A) Locations B and C relative to the Fish Springs National Wildlife Refuge (FSNWR) headquarters. (B) Charophyte/marl facies at Jay Banta gravel pit near the FSNWR headquarters. (C) Marl and gravel facies at Trapper Jim gravel pit. All numbered stratigraphic units are different between panels B and C. 57 Refuge headquarters (Figures 2.1 locality FSN and 2.6 A). A measured section of 4 m consists of lacustrine carbonates, almost entirely charophytes and marl, with its top and bottom bounded by lacustrine beach gravel (Figure 2.6 B). While charophyte taxa were not identified for this study, abundant occurrence of charophytes was interpreted as an indication of shallow water (<10 m) in low to moderate wave energy (Coletta et al., 2001; Andrews et al., 2004). Future studies of taxa, stable carbon, and oxygen isotopes of charophytes at this site may reveal further insights into paleowater chemistry and temperature. The bottom of the measured section is composed of beach gravel (>1m, Figure 2.6 B unit 7). Overlying this gravel unit is 50 cm of sandy gray marl interbedded with silty white marl (Figure 2.6 B unit 6). The sandy marl contains moderately abundant charophytes, which yielded radiocarbon ages of 23 220 ± 140 (Beta-151450) and 23 400 ± 180 (Beta-179077) 14C yr BP (both samples at 1334 m, immediately above the beach gravel). The sandy marl grades upward into 50 cm thick silty white marl weakly interbedded with sandy marl, indicating the deepening of water. Overlying the silty marl is 20 cm of alternating sandy and silty marl beds, bounded by an unconformity at its top. Sandy marl, 50 cm thick and interbedded with silty marl, sits on top of the unconformity, indicating shallowing water environments. The upper 10 cm of this unit contains abundant charophytes, which yielded a radiocarbon age of 22 320 ± 140 14C yr BP (Beta- 124352) at 1336 m (Figure 2.6 B unit 5). A weak unconformity separates the charophyte-rich sandy marl and overlying 20 cm thick massive white marl. The white marl (Figure 2.6 B unit 4), indicative of substantial deepening of the lake, was also radiocarbon dated: 22 250 ± 140 14C yr BP (Beta-158144). Unconformably overlying the 58 white marl is 1 m thick sandy marl with abundant charophytes. Thus, a shallow water environment appears to have returned, but was replaced by another deep lake event as indicated by a massive white marl (25 cm) unconformably overlying the charophyte-rich sandy marl. This white marl has yielded a radiocarbon age of 20 330 ± 120 14C yr BP (Beta-158143) at 1338 m (Figure 2.6 B unit 3). Yet another shrinking lake event is represented by 30 cm of beach gravel (Figure 2.6 B unit 2) above the white marl. Overlying it, 30 cm of eolian silt forms the modern surface (Figure 2.6 B unit 1). Based on the shoreline stratigraphy and radiocarbon dates of this measured section, a lake appears to have started expanding around 23.4-23.2 14C ka with a minimum estimated surface area of 22 400 km2 (44% LB-max). A radiocarbon date from charophytes (23 630 ± 240 14C yr BP, Beta-151447) at Stansbury Gulch (Figure 2.1 locality SG, 40°47.460’N, 112°31.020’W, 1353 m ) suggests that it might have reached the size of 24 500 km2 (49% LB-max) during this interval. After the lake size apparently fluctuated numerous times over the following 1000 14C yr, the lake eventually attained a deep water phase around 22.3 14C ka depositing the massive white marl unit at the site (Figure 2.6 B, 1336 m). Two published radiocarbon dates of 22 500 ± 300 (Scott et al., 1983) (1379 m) and 22 300 ± 400 14C yr BP (Scott, 1988) (1380 m) from wood found in lagoon sediments at two localities near Salt Lake City (Table 2.2) indicate that the maximum surface area of this lake interval might have been 30 400 km2 (60% LB-max). As the 1 m thick charophyte-rich sandy marl facies at the Fish Springs gravel pit implies, the deep lake phase was followed by a regressive phase that terminated the 24-22 14C ka lake oscillations. The overlying youngest (20.3 14C ka) deep lake marl in the measured 59 section is likely to have been deposited during the following large lake from 21.8 to 19.8 14C ka. Early MIS 2 Large Lake: Early Lake Bonneville 21.8-21.2 14C ka Transgressive Lake Shoreline Stratigraphy Between 21.8 and 19.8 14C ka, the lake might have attained its maximum size of 39 200 km2 (78% LB-max). Shoreline stratigraphic records of the onset of this expansion to a large lake were found at three localities. An exposure at Trapper Jim gravel pit (Figure 2.6 C, 39º49.860’N, 113º23.460’W) in the Fish Springs National Wildlife Refuge shows 2 m of continuous laminated marl facies directly overlying beach gravel (Figure 2.6 C unit 4). The contact between these two facies is sharp, but appears conformable. While the gravel layer probably indicates the beginning of a transgressive phase, the immediately overlying laminated marl (1312 m) would mark a transition from a foreshore to near/offshore depositional environment. The marl has yielded a radiocarbon age of 21 650 ± 140 14C yr BP (Beta-151449) (Figure 2.6 C unit 3). In addition, the radiocarbon age from the marl at the top of the exposure (1314 m, Figure 2.6 C unit 2), 19 980 ± 240 14C yr BP (Beta-158142), indicates the prolonged duration of this deep lake phase. The top of the laminated marl is unconformably overlain by the beach gravel (>35 cm, Figure 2.6 B unit 1) of what appears to be a spit of unknown age. Although radiocarbon dating lacustrine marl is often considered unreliable (Benson et al., 1990; Oviatt et al., 1992), this 21.8- 21.5 14C deep lake date was supported by the additional shoreline dates from two other localities. 60 The second shoreline stratigraphic evidence of the 21.8-21.5 14C deep lake was exposed during construction at the Utah State Capitol east parking lot (40º46.680’N, 111º53.160’W) (Figure 2.1 locality USC) in the summer 2002. The 3.4 m measured section is dominated by deltaic facies. At the base of the section, alluvial gravel of unknown thickness is overlain by 10 cm thick tufa. Fine sand, <5 cm conformably overlies tufa, and contains abundant mollusk shells. Pyrgulopsis shells found in this fine sand yielded a radiocarbon age of 21 420 ± 160 14C yr BP (Beta-169537) at 1380 m. The fine sand is overlain successively by laminated silt (45 cm), massive very fine sand (45 cm), and weakly bedded-laminated fine sand (210 cm). No unconformities were observed. These three upper stratigraphic units resulted from deltaic environments during the early phase of the 21.8-19.8 14C ka large lake, also indicating the continuous expansion of the lake. The third locality is a gravel bayhead barrier (crest at 1414 m) (40º49.380’N, 114º8.880’W) (Figure 2.1 locality PC) in the Pilot Creek Valley, Nevada. Pyrgulopsis shells were located in the fine sand of transgressive facies in a lagoon, yielding a radiocarbon age of 21 310 ± 140 14C yr BP (Beta-156854). The elevation of the dated unit, 1402 m, indicates that the lake might have covered an area of 33 000 km2 (65% LB- max). These three radiocarbon dates and their corresponding lake sizes indicate a rapid rate of lake expansion. The estimated minimum surface area of the lake grew from 20 000 km2 around 21.7 14C ka, to 30,400 km2 around 21.4 14C ka, and then to 33 000 km2 around 21.3 14C ka. Therefore, the lake might have expanded its area by 65% within a few hundred 14C years or less (Figure 2.3). Radiocarbon dates reported in previous 61 studies suggest that this transgressive phase reached a highstand that probably centered around 20.8 14C ka and covered an area of 39 200 km2 (Scott et al., 1983; Sack, 1999) (Table 2.2). After this highstand phase, however, the lake size seems to have fluctuated widely between 32 600 and 38 600 km2, until 20.1 14C ka when a lowstand started (Figure 2.3). 20.8-19.8 14C ka Lake Oscillation Stratigraphy Shoreline evidence of the 20.8-19.8 14C ka lake oscillations was found in Promontory Hollow, 5 km southwest of the Golden Spike National Monument in Utah (Figure 2.1 locality PH). A unit of thinly bedded very fine lacustrine sand, ranging in thickness from 1 m to >2.5 m, was traced along a 3 km stretch of arroyo exposures, which included the stratigraphy of six bayhead gravel barriers with elevations ranging from 1393 to 1450 m (Figure 2.7). The lower portion of this sand unit is light yellow and ranges from 60 to 150 cm in thickness (Figure 2.8 unit 5). At four of the bayhead barriers (Figure 2.7 barriers B, C, D, and F), this yellow sand subunit conformably overlies beach gravel (Figure 2.8 unit 6), indicating a trangressive phase. The yellow sand is overlain by a light gray band of fine sand, up to 10 cm in thickness (Figure 2.8 unit 4). Overlying this light gray sand is 30-100 cm of reddish brown (7.5YR7/2), pebbly fine sand, which appears oxidized, possibly indicating its exposure to surface conditions immediately following the drop of lake level (Figure 2.8 unit 3). Some of these upper red sand beds contain ostracode shells. All contacts between these three subunits are sharp, but conformable (Figure 2.8). At all six bayhead barriers, the upper red sand subunit is unconformably overlain by a <5 cm thick, pebbly medium sand, containing mollusks and tufa-coated gravel (Figure 2.8 A, B, C, unit 2). This pebbly medium sand grades upward 62 .' Utah 50 BP ...... 1 yr ± Beta-1721 ( Northeast C . 630 3 ''' ~ 20 72665) 1 ) 90 - ± Beta ( 68996 480 , 1 0 eta-172664) BP B ( (Beta-172173) yr (Beta- c BP BP " 68082) 1 (kilometers) BP yr yr c c yr " " c (Beta- " 90 ~ deformed slumping 150 ± Road BP ± 110 yr by ± c " Beds 20,440 780 Jetty 20,600 130 1.6 Beta-172172) ± ( ory Hollow bayhead barrier staircase. BP Spiral yr ~0,490 (Beta-172171) c of .0 " C dated at all six bayhead barriers from A to F. BP sand 14 17 yr 150 X NE c fine ± " ollow H er ri 120 0.8 ± bar LU,b4U bedded l Exaggeration Distance hinly Promontory thalweg T Grave EXPLANATIONS " ... , Vertical , ~ c::::J Thinly bedded fine sand was Figure 2.7. Cross-section of the Promont ~ > Q) Q. o ro :!:' (f) 0:: --, "0 .. Southwest ~ ~1400 OJ c gJ1450 o E 2 ~ w - ~ 63 2.7) in Promontory Hollow. All numbered e same unit as 3 in B, and C). xt) at bayhead barrier C and D (Figure stratigraphic units are same in panels A, B, and C (i.e., unit 3 in panel is th in panels same units are stratigraphic Figure 2.8. Stratigraphic units 1-6 (see te 64 into >80 cm of thinly bedded to laminated light gray/yellow fine sand, deposited during a later lake event of unknown age (Figure 2.8 A, B, C, unit 1). Eight radiocarbon dates were obtained from Pyrgulopsis and Stagnicola shells found in the yellow sand subunit (Figure 2.8 unit 5) at all six bayhead barriers (Figure 2.7). These radiocarbon ages range from the oldest 20 780 ± 110 (Beta-168996) to the youngest 20 130 ± 110 (Beta-172171) 14C yr BP (Figure 2.7; Table 2.2). The shells were abundant at all sites mapped, but only present in a <10 cm section of the yellow sand subunit, indicating the limited duration of an optimum living environment for these mollusks. As Figure 2.7 shows, the spatial distribution of the eight radiocarbon dates and corresponding bayhead barrier elevations implies rapid oscillations of the lake. The largest amplitude over the shortest period of time occurred between 20 640 ± 150 14C yr BP (Beta-172172) (Figure 2.7 barrier B at 1399 m) and 20 630 ± 150 14C yr BP (Beta- 172174) (Figure 2.7 barrier F at 1450 m). This suggests that a 51 m change in lake depth (≈ 6,000 km2 in lake surface area) might have taken place over the period of <300 14C yr (considering the 14C dates errors). Likewise, a 31 m change in lake depth might have occurred between 20 490 ± 130 14C yr BP (Beta-168082) (Figure 2.7 barrrier C at 1417 m) and 20 480 ± 90 14C yr BP (Beta-172665) (Figure 2.7 barrier E at 1448 m). It can be argued that these apparent oscillations might have resulted from reworked mollusk shells or artifacts of standard errors of radiocarbon dating. This is not believed to be the case for three reasons. First, none of the mollusk shells radiocarbon dated bore apparent signs of being transported. Second, the six bayhead barriers in Promontory Hollow are distributed in a staircase fashion, covering over 2.5 km of distance and 67 m of elevation (Figure 2.7). This morphology does not seem to favor 65 reworking of mollusk shells. Third, two published radiocarbon dates (SI-4121, Scott, 1988; Beta-52614, Sack, 1999) (Table 2.2) on shoreline features at other localities in the Bonneville basin are consistent with the oscillation interpretation. Light (1996) reported five radiocarbon dates between 20.4 and 20.2 14C ka, and suggested that the corresponding lake levels might have fluctuated between 1418 m and 1399 m. This chronology broadly agrees with our radiocarbon age of 20 230 ± 120 14C yr BP (Beta-189275) on Stagnicola shells found in the lacustrine fine sand at 1435 m, at the Deep Creek Dam site (40°18.543’N, 113°56.208’W) (Figure 2.1 locality DC). Thereafter, from 20.1 to 19.8 14C ka the lake appears to have shrunk rapidly and stayed at levels lower than those of the preceding oscillations. At the lowest level bayhead barrier (1393 m, Figure 2.7 barrier A) in Promontory Hollow, Pyrgulopsis shells found in the aforementioned yellow fine sand (Figure 2.8 A unit 5) yielded a radiocarbon age of 20 130 ± 120 14C yr BP (Beta-172171). Pyrgulopsis shells were also found in a 50 cm thick fine sand overlying rusty colored gravel in a lagoon (39°37.920’N, 113°18.660’W) behind a gravel beach barrier at the south end of Fish Springs Flat (locality M, Oviatt, 1991) (Figure 2.1 locality FSF). These shells yielded a radiocarbon age of 20 140 ± 90 14C yr BP (Beta-170986) with a corresponding elevation of 1382 m. Oviatt (1989) and Light (1996) also reported radiocarbon dates of 19 920 ± 230 (Beta-18271) and 20 000 ± 150 (AA-19054) on mollusk shells found in shoreline features at 1390 and 1399 m at other localities (Table 2.2). Based on these dates and corresponding lake levels, the lake seems to have shrunk to 30 600 km2 (61% LB-max) during 20.1-19.8 14C ka. A possible further drop in lake level is implied by the two beach gravel units at Fish Springs National Wildlife Refuge (Figure 2.6 B unit 1 and 66 C unit 1). As discussed earlier, both units directly overlie 21.8-19.8 14C ka sedimentary packages and may indicate that the lake level had actually dropped to 1338 m (22 800 km2, 45% LB-max) or 1314 m (20 200 km2, 40% LB-max) between 20.1 to 19.8 14C ka. This lowstand phase likely corresponds to what was previously identified as the Stansbury Oscillation minimum (Oviatt et al., 1990). Reexamination of the Stansbury Oscillation Type Localities The currently accepted chronological model of the Stansbury Oscillation is derived mainly from three type localities: Stansbury Gulch on Stansbury Island, the Old River Bed, and Crater Island (Currey et al., 1983; Oviatt, 1987; Green and Currey, 1988; Oviatt et al., 1990; Oviatt et al., 1994b). In general, geomorphic, stratigraphic and sedimentological interpretations presented in these previous studies are highly consistent with each other. Their age controls, on the other hand, are somewhat tenuous due to too few dates and large standard errors. Because these published radiocarbon dates came mostly from Stansbury Gulch and the Old River Bed (Table 2.2), the two localities were reexamined for this study. At Stansbury Gulch (40°47.520’N, 112°30.960’W, 1365 m) (Figure 2.1 locality SG), Pyrgulopsis shells in a pebbly medium to coarse sand underlying sandy marl (Section 3, Figure 3, Currey et al., 1983) yielded a radiocarbon age of 20 990 ± 150 14C yr BP (Beta-156850). Approximately 6 m lower and <20 m downstream, Pyrgulopsis shells from the same pebbly sand unit (Section 2, Figure 3, Currey et al., 1983) (1359 m) yielded a radiocarbon age of 20 710 ± 310 14C yr BP (Beta-5566) (Currey et al., 1983). With its relatively large standard error, this date can be considered approximately the same as 20.99 14C ka. When compared with this study’s chronology, these two dates fall 67 in the period following the 21.8-21.2 14C ka lake expansion, implying that a marked lowstand (25 100 km2, 50% LB-max) separated the 21.8-21.2 14C ka transgressive phase from the 21.0-20.8 14C ka highstand (Figure 2.3). Although these Stansbury Gulch mollusk shells dates may indicate the 21.2-21.0 ka lowstand, the unconformity that would correspond to this lowstand was not observed in the shoreline facies at the Utah State Capitol or Pilot Creek valley sites for this study. Another possibility is that these mollusk shells were reworked from higher elevations during the Stansbury Oscillation minimum between 20.1 to 19.8 14C ka. One of the most salient characteristics of Stansbury Gulch is its steep slope ranging from 5° to 7° in the vicinity of these two radiocarbon date sites. Combined with relatively restricted shorezone accommodation space (both vertically and horizontally), the steep terrain would have resulted in favorable conditions for reworking of older materials, particularly during the time when high wave energy dominated. At the Old River Bed type locality (Figure 2.1 locality SO), a contact between the “Yellow Clay” and “White Marl” marks a significant change in depositional environments, and has been interpreted to represent the Stansbury Oscillation (Currey et al., 1983; Oviatt, 1987; Oviatt et al., 1990; Oviatt et al., 1994b). This contact was located at a site (39°55.740’N, 112°51.300’W) near a south-trending gravel spit at the Shutoff, which was discussed in detail by Currey et al. (1983). It consists of (from base to top), 5 cm of gray mud, 5 cm of orange sand, 5 cm of orange gravel, 5 cm of muddy gravel, and 2 cm of light gray marl. The orange sand contains abundant Pyrgulopsis shells, which yielded a radiocarbon age of 20 400 ± 80 14C yr BP (Beta-171197). At an elevation of 1384 m, this date may reflect the lowstand during the 20.8-20.2 14C ka oscillations with a possible lake surface area of 30 800 km2 (61% LB-max). From the same sand, Broecker 68 and Kaufman (1965) reported a radiocarbon age of gastropod shells to be 20 150 ± 400 14C yr BP (L-672J, with their reservoir correction removed and adjusted for 13C/12C). Because of its large standard errors, this date is considered equivalent to 20.4 14C ka. Approximately 5 km northwest of the 20.4 14C ka site near the Pony Express Road commemorative obelisk (39°58.140’N, 112°53.220’W), a 10 cm thick unit of medium sand at 1377 m, directly underlying a sandy marl, contains Pyrgulopsis shells, which yielded a radiocarbon age of 20 570 ± 80 14C yr BP (Beta-173280). The stratigraphic and chronological context of this date seems consistent with that of the 20.4 14C ka date, indicating the beginning of one of the lake expansion phases (30 000-30 800 km2, 59-61% LB-max) during the 20.8-20.2 14C ka oscillations (Figure 2.3). Lake Events in the Bonneville Basin and Hemispheric Climate Change MIS 16 Lake Episode and Climate Change On a regional scale, the timing of the MIS 16 lake event in the Bonneville basin is consistent with a proposed major pluvial event in the western part of the Great Basin, including the Lahontan basin (Reheis, 1999; Reheis et al, 2002). MIS 16, on the hemispheric to global scale, left one of the most pronounced δ18O profiles in the Quaternary deep ocean sediment records (Shackleton et al., 1990; Raymo, 1994). MIS 16 marks the beginning of glaciations, driven by the 100 000 year orbital cycle, with longer duration and more ice volume than those driven by the preceding 41 000 year orbital cycle (Ruddiman, 2001). As suggested by Oviatt et al. (1999), the first identifiable large lake in the Bonneville basin appears to have coincided with MIS 16— the first long and significant ice age in the Quaternary. 69 MIS 3 Lake Events and Climate Change An argument against the shoreline evidence for the Bonneville basin MIS 3 large and intermediate lake events presented in this study is in a study on sediment core GSL- 004 (Balch et al., 2005). Neither MIS 3 large nor intermediate lake event was observed in sediment core GSL-004 (Balch et al., 2005). Paleoecological records from core GSL- 004 suggest that the lake oscillated between shallow saline and hypersaline conditions throughout MIS 3. Nevertheless, the GSL-004 results are not necessarily contradictory to this study’s large to intermediate lake events. The GSL-004 chronology likely reflects the MIS 3 lowstands, which were not detected well by the shoreline study of large to intermediate lake events reported here. Although age control for the MIS 3 portion of GSL-004 is coarse, absence of the large or intermediate lake records in the core may indicate rapid transitions between, and relatively short durations of, the recurring stades and interstades of MIS 3. During the periods of extreme lowstand events, especially those of desiccation accompanied by deflation, preceding lake deposits could have been removed from the central portion of the basin where core GSL-004 was collected. Studies in the western part of the Great Basin seem to have established the correlation between the MIS 3 lake fluctuations and Dansgaard-Oeschger (D-O) cycles. Evidence of hemispheric scale climate change events has been reported in a number of lake sediment core-based studies. From analytically high resolution records of isothermal remanent magnetization (IRM), δ18O, total organic carbon (TOC), total inorganic carbon (TIC), tephrochronology, and 14C dates adjusted for reservoir effect, these studies revealed that changes in the hydrologic balance at Summer Lake, Mono Lake, Pyramid Lake, and Owens Lake were in phase with the GISP 2 chronology based D-O cycles 70 during MIS 3 (Benson, 1999, 2004; Benson et al., 1996, 2003a, 2003b; Zic et al., 2002). Highstands of these lakes in the western Great Basin appear to have occurred during the relatively warm and wet climate of the Greenland interstades between 46 and 27 GISP 2 ka (Benson et al., 2003a). The study by Benson et al. (2003a) showed that the abrupt shift in the air temperature regime of the North Atlantic during MIS 3 indeed resulted in almost synchronous changes in the hydrologic balance of lakes in the western Great Basin. A question here is whether such a correlation can be observed between the Bonneville basin lake events (eastern Great Basin) and D-O cycles. As cautioned by Shackleton et al. (2004), the MIS 3 time scale is replete with rapid cold to warm transitions, which require precise age determinations (Figure 2.9 NorthGRIP GICC05). As Figures 2.3 and 2.9 show, the shoreline records of the Bonneville basin during this time frame, especially a part older than 34 cal ka, are still too sparse for an exact comparison with the high resolution Greenland ice sheet chronology like NorthGRIP GICC05 (Andersen et al., 2006; Svensson et al., 2006). A number of uncertainties that are associated with radiocarbon dating, such as precision, accuracy, contamination, and calibration to calendar ages, also make comparing the Bonneville basin chronology with D-O cycles during MIS 3 challenging. Despite these, there is an observable, coherent pattern in occurrences of the MIS 3 large and intermediate lake events in the Bonneville basin in the context of D-O cycles. For calibration, the Fairbanks0107 marine calibration curve was used to constrain each lake event for this study (http://radiocarbon.ldeo.columbia.edu/cgi- bin/radcarbcal.htm, accessed 20 February 2009; Fairbanks et al., 2005). By calibrating the oldest and youngest 14C dates of each lake event (Table 2.4), all lake events are 71 Figure 2.9. δ18O profile in NorthGRIP GICC05 (Svensson et al., 2006). Numbers indicate Greenland Stadials (GS) and Interstadials (GI). Gray band shows estimated timing and duration of each Bonneville lake event (14C dates with >1000 yr analytical errors in Figure 2.3 were excluded here) in calendar age. Arrows and letters indicate O: oscillations, R: regression, S: stillstand, T:transgression of lake episodes. Lake level curve by Oviatt (1997) in a white dashed line for comparison. 72 Table 2.4 Calibrating 14C dated lake events into calendar years Radiocarbon age Calendar age Calibration mean 1std dev mean 1std dev max min version 37270 300 42368 311 42679 42057 Fairbanks0107 36560 570 41751 526 42277 41225 Fairbanks0107 32090 410 37478 444 37922 37034 Fairbanks0107 27660 250 33007 308 33315 32699 Fairbanks0107 27720 40 33071 159 33230 32912 Fairbanks0107 27210 240 32530 301 32831 32229 Fairbanks0107 26920 40 32221 161 32382 32060 Fairbanks0107 26220 240 31469 294 31763 31175 Fairbanks0107 24600 150 29425 229 29654 29196 Fairbanks0107 23630 240 28318 303 28621 28015 Fairbanks0107 23220 140 27856 210 28066 27646 Fairbanks0107 22320 170 26834 226 27060 26608 Fairbanks0107 22250 140 26754 198 26952 26556 Fairbanks0107 21650 140 26044 200 26244 25844 Fairbanks0107 20130 120 24038 144 24182 23894 Fairbanks0107 20025 155 23921 190 24111 23731 Fairbanks0107 19920 230 23794 281 24075 23513 Fairbanks0107 73 constrained by calendar age ranges: the 38-36 14C ka ≈ 42.7-41.2 cal ka; 32 14C ka ≈ 37.9-37 cal ka; 28-26 14C ≈ 33.3-31.2 cal ka; 24.5 14C ≈ 29.7-29.2 cal ka; 24-23 14C ka ≈ 28.6-27.7 cal ka; 22.5-22 14C ka ≈ 27-26.6 cal ka; 21.8-19.8 14C ka ≈ 26.2-23.7 cal ka. The oldest MIS 3 lake event, 42.7-41.2 cal ka stillstand/regression, coincided with GS 11 and GI 10 (Figure 2.9). Then, the 37.9-37 cal ka regressive large lake occurred during GI 8 while its highstand probably coincided with the abrupt transition between GS 9 and GI 8. A large lake was also expanding during GS 6 and GI 5, and possibly sustained its size through GI 5 and GS 5 transition. At a transition from GS 5 to GI 4, the 29.7-29.2 cal ka intermediate lake was expanding. The latest MIS 3 lake event between 28.6 and 27.7 cal ka occurred during GS 4 and GI 3 as oscillations. Most parts of at least four, and probably five, lake events in the Bonneville basin occurring during MIS 3 GI supports the results of western Great Basin lake studies (Benson et al., 2003a; Benson, 2004). On the other hand, all five Bonneville lake events appear to have started in and lasted from the preceding GS. In the Bonneville basin, large and intermediate lake events coincided not only with GI, but also with the preceding GS. In addition, the most notable is that in all five MIS 3 lake events a highstand centered on an abrupt shift from GS to GI (Figure 2.9). During the last half of MIS 3, effective moisture in the Bonneville basin was significantly and rapidly enhanced when the North Atlantic air temperature regime suddenly changed from cold to warm. This pattern is also observable during MIS 2. Early MIS 2 Lake Events and Climate Change An early MIS 2 lake chronology reveals in more detail than that of MIS 3 how the Bonneville basin surface water system responded to the last glacial maximum (GS 3) (Svensson et al., 2006). An intermediate lake oscillated during the first half of GS 3 from 74 27-25 cal ka (Figure 2.9). Then, the lake rapidly expanded to become large after 25 cal ka. Throughout the last half of GS 3, the lake oscillated, but nonetheless sustained a highstand toward the end of GS 3. The timing, duration, and magnitude of this large lake closely resemble those of the North Atlantic Heinrich event 2 records. Heinrich event 2 is estimated to have lasted from 22.1 to 20.4 14C ka (average of 7 cores, Elliot et al., 2002) or from 22.0 to 19.5 14C ka (top and bottom dates of 3 cores, Rashid et al., 2003). At the termination of Heinrich event 2 (the end of GS 3), the lake in the Bonneville basin underwent a lowstand (Figures 2.3 and 2.9). The timing of this lowstand, from 20.1 to 19.8 14C ka (24.2-23.7 cal ka), however, did not coincide with the timing of GI 2 (23.3 cal ka, Andersen et al., 2006). Instead, the beginning of the Lake Bonneville transgressive phase (Oviatt, 1997) was in phase with an onset of GI 2 in a similar pattern observed during MIS 3 (Figure 2.9). Large Lake Events and Abrupt Climate Change During the late MIS 3 and early MIS 2, a lake in the Bonneville basin expanded rapidly when an abrupt shift from cold to warm air surface temperatures occurred in Greenland and the circum North Atlantic region. Even in the late MIS 2, this appears to have happened to Lake Bonneville at the GS 2/GI 1 boundary (Godsey et al., 2005). As proposed by Benson et al. (2003), the amount of precipitation received by surface water systems in the Bonneville basin and Great Basin increased significantly and rapidly when the polar jet stream’s mean path was migrating from south to north (35°-43°N). An exact mechanism of such a close connection between the North Atlantic D-O cycles and spatial migration of the polar jet stream path in western North America still remains to be explained. Nevertheless, the apparent synchroneity of MIS 16, 3, and 2 large and 75 intermediate lake events in the Bonneville basin with the Northern Hemisphere climate change events seems to strengthen the case for a previously proposed linkage between fluctuating lakes in the Great Basin, migrating mean positions of the polar jet stream, and the expanding/contracting ice sheets (Antevs, 1948; Benson and Thompson, 1987; Oviatt, 1997; Benson, 2004). References Andersen, K.K., Svensson, A., Johnsen, S.J., Rasmussen, S.O., Bigler, M., Röthlisberger, R., Ruth, U., Siggaard-Andersen, M.-L., Steffensen, J.P., Dahl-Jensen, D., Vinther, B.M., Clausen, H.B., 2006. The Greenland Ice Core Chronology 2005, 15-42 ka. Part 1: constructing the time scale. Quaternary Science Reviews 25, 3246-3257. Andrews, J., Coletta, P., Pentecost, A., Riding, R., Dennis, S., Dennis, P., Spiro, B., 2004. 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Guidebook for the 1982 Friends of the Pleistocene, Rocky Mountain Cell, Field trip to Little Valley and Jordan Valley, Utah. U.S. Geological Survey Open-File Report 82-845. Scott, W., McCoy, W., Shroba, R., Rubin, M., 1983. Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, western United States. Quaternary Research 20, 261-285. Shackleton, N., Berger, A., Peltier, W., 1990. An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Transactions of the Royal Society of Edinburgh: Earth Sciences 81, 251-261. Shackleton, N., Fairbanks, R., Tzu-chien Chiu, Parrenin, F., 2004. Absolute calibration of the Greenland time scale: implications for Antarctic time scales and Δ14C. Quaternary Science Reviews 23, 1513-1522. Svensson, A., Andersen, K.K., Bigler, M., Clausen, H.B., Dahl-Jensen, D., Davies, S.M., Johnsen, S.J., Muscheler, R., Rasmussen, S.O., Röthlisberger, R., Steffensen, J.P., Vinther, B.M., 2006. 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CHAPTER 3 LAKE BONNEVILLE SHORELINE RECORDS DURING THE LAST GLACIAL-INTERGLACIAL TRANSITION Abstract Lacustrine shoreline geomorphic features of Lake Bonneville from 24 000 to 10 000 cal yr BP were investigated to examine how the Bonneville basin hydrologic system in western North America responded to abrupt climate change events during the late glacial and the last glacial-interglacial transition. Based on analyses of stratigraphy, sedimentology, geomorphology, and radiocarbon dates of shoreline depositional features, seven lake intervals were identified with their timing, magnitude, and duration resolved. An overall pattern of Lake Bonneville size change seems to be in phase with the changing strength of the East Asian and Asian Monsoons. All lake intervals large enough to exceed the basin threshold occurred when the summer monsoon was greatly enhanced in Asia, suggesting the great spatial extent and significance of the transport of heat and moisture from the warm tropical ocean to higher latitudes. A close teleconnection between the North American Great Basin and Western Pacific-Asian climate systems seems to have existed during the last deglaciation. 83 Introduction The 135 000 km2 Bonneville drainage basin is the largest, deepest, and the easternmost catchment in the hydrologically closed Great Basin of North America. Since the middle Quaternary, a large fresh water lake occupied the basin during Marine Isotope Stages (MIS) 16, 12, 6, 3, and 2, coinciding with some of the most extensive northern hemisphere glaciations (Oviatt et al., 1999: Nishizawa et al., in review). A series of distinct shorelines resulting from these lake intervals sit 100-200 m above the surface of today’s Great Salt Lake in an arid landscape. Among these paleoshorelines, those formed by the MIS 2 age Lake Bonneville have received perhaps the most attention and emphasis in numerous paleoenvironmental reconstruction studies since the seminal work by G. K. Gilbert in 1890 (see Sack, 1988; Oviatt, 2002). Focusing on this time period mainly results from the fact that the today’s most visible, preserved, and accessible shoreline features were formed when the lake was expanding to its 50 500 km2 maximum extent and undergoing the subsequent catastrophic flood from 23.3 to 14.7 cal ka during Greenland Stadial (GS) 2 (Björck et al., 1998). Studies on the late glacial Lake Bonneville chronology are, therefore, more prevalent than those on any other time periods (Scott et al., 1983; Currey, 1990; Oviatt et al., 1992; Oviatt, 1997; Sack, 1999; Godsey et al., 2005). On a regional scale, the currently accepted GS-2 Lake Bonneville chronology agrees broadly with the western Great Basin paleolake records (Benson et al., 1990). After the end of GS-2, however, the pattern of lake level change chronology of the eastern (Bonneville) Great Basin diverges from that of the western Great Basin. A number of recent studies presented evidence of highstands of Pyramid Lake, Lake 84 Chewaucan, and Owens Lake in the northwestern Great Basin during Greenland Interstadial (GI) 1 and the early Holocene (Licciardi, 2001; Benson et al., 2002; Benson, 2004; Bacon et al., 2006). By contrast, the lake in the Bonneville basin during both periods was reported at the level close to the historic average of Great Salt Lake (1280 m/4400 km2, Arnow and Stephens, 1990), with a minor GS-1 (Younger Dryas) expansion in between (Oviatt, 1997; Oviatt et al., 2003, 2005). Licciardi (2001) suggested that this disagreement might have resulted from spatial variability in response to regional climate change over <1000 km longitudinal distance between the easternmost and westernmost boundaries of the Great Basin. Recent studies, however, revealed that the previous Bonneville shoreline chronology for these periods had an insufficient age control (Light, 1996; Godsey et al., 2005). By adding 25 new radiocarbon (14C) dates to the existing Bonneville shoreline chronology, Godsey et al. (2005) proposed that a large lake also occupied and possibly overflowed the Bonneville basin during GI-1, contradicting the previously accepted models (Currey, 1990; Oviatt, 1997). Based on shoreline stratigraphy, sedimentology, geomorphology, and 14C data, this study presents a chronology of seven lake intervals in the Bonneville basin from 19,700 to 9200 14C yr BP (19.7-9.2 14C ka; 23.7-10.3 cal ka), covering GS-2, GI-1, GS-1, and the onset of the Holocene. In addition, a new Lake Bonneville chronology is compared with the Greenland ice sheet, Hulu Cave, and Dongge Cave δ18O records to examine whether timing, duration, and magnitude of the lake episodes indicate any sensitivity of the Bonneville hydrologic system to abrupt climate change events in the North Atlantic and Pacific Oceans during the last glacial-interglacial transition. 85 Methods Shoreline Approach Based on an assumption that lacustrine shoreline geomorphic features such as barriers, spits, tombolos, and deltas, all develop at the water’s edge to mark a lake surface level at the time of their formation, this study estimated: (1) lake level at the time of sediment deposition, (2) lake surface area derived from lake level, and (3) timing, duration, and magnitude of each lake episode. In the field, elevations of the measured stratigraphic sections were determined by using a combination of 1:24 000 topographic maps, a geodetic total station, benchmarks, and global positioning system. Unless noted, all surveyed elevations in this study were adjusted for hydroisostatic rebound and are accurate to ±1 m (Figure 3.1, modified after Currey and Burr, 1988). All lacustrine depositional features analyzed in this study were also interpreted to be in the shore zone from foreshore to backshore/lagoon (see Currey and Sack, 2009). Lake surface level at the time of deposition was assumed to be ±5 m of the surveyed stratigraphic unit (Godsey et al., 2005). From the hydroisostatic rebound-adjusted elevation of the surveyed shoreline feature, a lake surface area was estimated by a lake surface area rating curve (Figure 3.2, modified after Currey and Oviatt, 1985). As shown in Figure 3.2, lake surface area in the Bonneville basin has a semi-linear relationship with lake surface elevation. The derived lake surface area was then divided by the 50 500 km2 Lake Bonneville maximum surface area (“LB-max”), and multiplied by 100 for a lake surface area ratio (Figure 3.2). For example, the 38 000 km2 lake surface area at the current basin threshold is 75% of LB- max while the 4400 km2 historic average area of Great Salt Lake (Arnow and Stephens, 86 Figure 3.1. The linear approximation of Lake Bonneville hydroisostatic rebound adjusted shoreline elevations (after Currey and Burr, 1988). An adjusted paleoelevation is obtained by plotting a modern elevation of the local shoreline against a modern elevation of the highest local Bonneville shoreline in proximity. For example, if the modern elevation of the local shoreline is 1524 m (A), and the modern elevation of the highest local Bonneville shoreline is 1615 m (B), where these two elevations intersect (C) is the hydro-isostatic rebound adjusted paleoelevation of the shoreline, 1478 m. 87 Figure 3.2. Lake surface area curve for the Bonneville basin (after Currey and Oviatt, 1985). A: Zenda threshold, Lake Bonneville at its maximum size. B: Red Rock Pass threshold, post-Bonniville flood and current Bonneville basin threshold. C: Threshold between a larger northern part of the Bonneville basin and the smaller southern Sevier River tributary basin. D: Great Salt Lake basin threshold. Upper x- axis: lake surface area ratio (see text). 88 Figure 3.3. Localities discussed in the text (after Currey, 1990): BE–Bonneville Estates Rockshelter; BS–Browns Spring; CB–Camel Back Cave; CF–City Creek fan; DA– Danger Cave; DC/VV–Dry Creek/Valle di Villa; DD–Deep Creek Dam; DG–Dugway Proving Ground; DP–Dugway Pass; EB–Erda Borrow; FS–Fish Springs National Wildlife Refuge; FV–Faust Valley; GC–Government Creek; HC–Homestead Cave; LS– Lakeside; MS–Magna Spit; PC–Pilot Creek valley; PH–Promontory Hollow; PG–Public Shooting Grounds; PM–Point of the Mountain; TD–Tooele Army Depot; WD–Weber River delta; ZT–Zenda threshold. Bonneville basin lake surface area (after Currey, 1990): 50 500 km2 (100% LB-max) at maximum extent (dark shade), 38 000 km2 (75% LB-max) at the Red Rock Pass threshold (light shade), and 4400 km2 Great Salt Lake (9% LB-max) (white). 89 1990), is 9% of LB-max (Figures 3.2 and 3.3). As discussed in detail by Benson and Paillet (1989) and Benson et al. (1990), the topography and negligible flux of ground water in the Bonneville basin make lake surface area more sensitive indicator of changing lake size than volume. Hence, in this study, the magnitude, timing, and duration of lake episodes in the Bonneville basin are expressed in terms of changing lake surface area. Age Control Age control is crucial for spatially correlating sets of the lacustrine shoreline stratigraphy observed at different localities. In addition to 112 published 14C dates, this study obtained 23 new 14C dates for the shore zone facies analyses (Table 3.1). Mollusk shells provided most of new 14C dates. This study followed Oviatt et al. (1992) and Benson (1993), and radiocarbon-dates were not corrected for reservoir effects, mineral carbon, and hard water errors on mollusk shells. Magnitudes of radiocarbon reservoir effects, mineral carbon, and hard water errors on mollusk shells in the Bonneville basin are not well understood. Further studies are needed to resolve the 14C dating uncertainties in the Bonneville lake basin. Transportation of shells after their deposition is another source of uncertainty. Faunal assemblage, size distributions, and preservation condition of mollusk shell samples were carefully examined to establish their in situ deposition. Dating a single shell was avoided. Presenting Chronology As seen in Figure 3.4, this study uses bar graphs to present the lake size change chronology. On each bar, width represents a 14C or calendar age range while height 90 ±1 C ka. Height of each bar (in A, B, and C) . 14 of each bar represents analytical error ( each bar represents of is in a dashed line ville basin from 19.7-9.2 re grouped. RPT–Red Rock Pass threshold, post-Bonneville ce area and lake surface elevation. Width ce area and lake surface elevation. ) at 1444 m. Oviatt’s lake level curve (1997) C age estimate. See text for how A, B, C, and D we 14 flood maximum extent (75% LB-max represents lake size in terms of surfa represents Figure 3.4. Magnitude, timing, and duration of lake intervals in the Bonne of lake timing, and duration Figure 3.4. Magnitude, sigma) on a 91 marks lake size (Figure 3.4). Thus, those periods with missing information are simply presented as blanks. In addition, as the number of dated lake events (i.e. bars) increases in the future, patterns of lake size change in a bar graph should become more observable and coherent. Figure 3.4 illustrates these advantages. In Figure 3.4, the shoreline data were divided into three groups according to their 14C standard errors: (A) ≤±150 years; (B) ±151-299 years; (C) ≥±300 years. Group A consists mostly of those with AMS dates while B dates were dated from the mid 1980s to 2000. Group C data were mostly dated before the early 1980s. Despite large differences in 14C precision, Figure 3.4 shows an observable pattern of lake size change to be remarkably consistent among three groups. Those lake chronologies proposed in the 1960s (e.g., Broecker and Kaufman, 1965), represented in group C, appear not very precise, but still accurate, even compared with group A. Figure 3.4 also shows where or in which time period current gaps in information exist: 15.5-16.5, 10.5-11.3, 9-8.5 14C ka. Thus, by using this graphical representation, researchers can easily identify time periods to which data need to be added. Groups A, B, and C were overlaid in order from larger to smaller 14C standard errors to generate Figure 3.4 D, which may visually aid readers to follow the sequence of events discussed in the section below. Evidence of Seven Lake Intervals In this study, each radiocarbon-dated shoreline sedimentary package is considered to represent a “lake event.” A “lake interval” consists of multiple lake events, and is bounded by either unconformities observed in the field or the gaps in the shoreline data. In this section, we report all dates in 14C years. Table 3.1 lists the detailed information related to all 14C dates, including lab numbers reported here. 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III _ - 74 59 39 74 72 70 72.494 72.494 73.427 72.494 48 46 29 26 26 66 . 62 33.64116989 69 68 35.940 ----5 ~ 0 ... E ~ 'O't 8 '342 1431 1437 1433 1425 1294 1433 1433 1290 1354 _ 4 4 67143572.96036849 :sW :> ~:o:>. _ '0';:' .! __ w 4 '369 '468 1291 1291 1442 1442 1460 1293 1293 129 137 1455 1390 1378 1468 1465 1436 1414 1408 1314 1314 1290 1290 1290 1417 1389 1433 1422 1430 1417 1443 1443 .850 .0821353134446.4982348 Q1 01 C 0- ~ -'6'~ ~ . 'gE$.'g,;!: 112.587 111.989 112.369 112.057 112.044 112.038 112.386 112.848 112.292 112.582 113.186 113.092 111.833 114.172 114.018 112.307 112.035 112.607 111.834 113.1831 112.734 170 594 III '0 .a __ -iF~j!~ ~ ;:: ...J...J 2.258 0.479 0.708 0.709 40.507 41.584 4 41.628 41. 41 41.583 41.593 4 40.532 40.815 40.749 4 41.619 41.317 4 41.561 40.533 41.810112 40.567111 39.479 39.929 39.186 end sand) f Roy ditch f trench arroyo arroyo arroyo 3.1 al trench north Idaho e Grounds Grounds II 148 160 <5 E II! , " ~ l __ ~ Zi = .8 :2 ~ tombalo ...... ns nsnorthend flow i i Shuto (beach Wash Depot delta lot lot bluff Hollow Hollow Hollow (bh) sewer drainage , Nevada Tab lla lla Bed i i basalt Spring Flat V V Cave lory lory spil spil Canal Army Mounla River Creek Shoaling Shooling Dam Lake di di on on Valley River Valle Valle Willard Weber Willow Tooele Taylors Swan Old Cutler Magna Magna Browns Prom Pilot Promontory Danger Public HogupMounta Hogup Public Deseret Prom 0 o ! U) sz05 sz05 sz05 sz04 sz02 sz02 sz04 sz02 sz05 sz07 sz03 szOI sz05 szOI sz09 sz05 sz02 sz02 sz05 sz04 sz07 sz05 T S S S S S S S S S S S M _!! sG sM gS oS oM oM oM CIIN E SIG c . biolX __ Q.~ >..c ~ rad rad rad rad rad rad rad rad rad rad rad rad rad rad rad rad rad AMS AMS AMSunknszOO AMS AMS AMS ,Q) 10 =It ....J D geta-18272 gela-156660 geta-169098 gela-190889 8ela-169097 8ela-156849 8ela-156853 8ela-159195 Bela-13996 1-696 AA-19064 W-1824 W-1338 WW-3681 WW-2760 geta-171287 8ela-191294 C-610 GX-6949 l-774D l-774Q Beta-168997 L-672L ! ls ls ls l l i plan plant shells she shells ~ Q) co 10 - .! . - C 'U gastropod chips (slick) delrilus Stagnicola Stagnicola Stagnicola Stagnicola Stagnicola wood wood wood ostracodes gas!ropodshells gastropod gaslropod gastropod coated carbonized carbonized PyrguJopsis PyrguJopsis peat plant t: ... o:::!: W 50gaslropodshe 50 35 70 70 70 70 70 40 60 80 0 .. 100 130 130 110Sla9nioo/a 200 275 2OOgasiropodshe 310 300 570 300 350 30 4 >. O 0500 ; 10300 10570 12450 12 12320 12250 12130 11910 11910 11810 11550 11350 11150 1 12150 12090 10440 11940 10300 12290 12440 12410 12350 ... 97 rrection while 350 years were ce the factors of hard water effects hard water effects ce the factors of ussed in Godsey et al., 2005). For those carbonate dded to offset their initial co ocarbon dates were adjusted here sin been overestimated (as disc dates reported before 1977. Some of the previously reported radi dates reported in Broecker and Kaufman (1965), 500 years were a dates reported in Broecker and Kaufman (1965), 500 years were added to all the carbonate and younger carbon contamination seem to have Radiocarbon Age : 98 Table 3.1 Sediment Type: describes the dominant sediment in which radiocarbon dated material was found — bio/T: tufa and silt, bio/X: tufa and volcanic, G: gravel, gS: pebbly sand, kS: calcareous sand, M: mud, mO: muddy organic matter, mS: muddy sand, msG: muddy sandy gravel, O: organic, oG: organic mud, oM: organic mud, oS: organic sand, osM: organic sandy mud, oT/T: organic silt and silt, R: marl, S: sand, S/G: sand and gravel, sG: sandy gravel, sM: sandy mud, sR: sandy marl, T: silt, T/G: silt and gravel, T/X: silt and volcanic, unkn: unknown. Shore Zone (sz): describes the shore zone environment at the time of the deposition of the dated unit — sz00: zone not specified, sz01: lagoon (closed to partially closed, typically muddy), sz02: beach backshore face (typically sandy and gravelly), sz03: beach crest (typically gravelly), sz04: beach foreshore face (typically gravelly), sz05: beach foreshore toe (typically sandy), sz06: tributary stream (terminal reach), sz07: delta top, sz08: delta front, sz09: overflow (basin- or sub basin-spill) stream, sz10: lake-edge marsh, sz11: lowland (elevation typically <30 m above lake), sz12: offshore (depth typically >10 m). Sources: 1-Scott et al., 1983; 2-Sack, 1999; 3-Oviatt, 1991b; 4-Bright, 1963; 5-Light, 1996; 6-Scott, 1988; 7-Oviatt et al., 1992; 8-Broecker and Kaufman, 1965); 9-Oviatt et al., 1994; 10-Kaufman, 2003; 11-Burr and Currey, 1992; 12-Godsey et al., 2005; 13- Rubin and Alexander, 1958; 14-Oviatt, 1991a; 15-Utah Geological Survey; 16-Marsters et al., 1969; 17-Trautman and Willis, 1966; 18-Isgreen, 1986; 19-Oviatt, 1989; 20-Ives et al., 1967; 21-Bright, 1966; 22-Jennings, 1957; 23-Oviatt et al., 2005; 24-Zachary, 2001; 25-Currey et al., 1983; 26-Oviatt et al., 2003; 27-Everitt, 1991. 99 accuracy of calibration are steadily improving, it will be beneficial and convenient for future studies to reference the following lake events an intervals in original 14C ages. All lake radiocarbon-ages are calibrated later in a section, titled “Bonneville Basin Lake in the Context of the Last Glacial-Interglacial Transition,” for discussing a connection between the Bonneville basin lake size change and abrupt climate change events. Interval I, 19.7-18.7 14C (23.7-22.2 cal) ka The chronology from 19.7 to18.7 14C presented here complements the currently accepted models (Scott et al., 1983; Scott, 1988; Oviatt et al., 1992; Oviatt, 1997; Sack, 1999). The lake seems to have rapidly increased its size at the beginning of this lake Interval I (an early part of the major transgressive phase of Lake Bonneville) and reached its maximum around 19.1-18.9 14C ka with the 39 400-40 000 km2 surface area (≈ 78- 79% LB-max). The end of the episode is marked by a brief regressive period. From 19.7 to 19.2 14C ka, the lake increased its size from 33 900 to 37 400 km2 as indicated by seven published 14C dates on shoreline features (Scott et al., 1983; Scott, 1988; Sack, 1999) and two dates, obtained for this study from a gravel spit and bayhead barrier at the Deep Creek Dam/Reservoir (40º18.560’N, 113º56.310’W) (Figure 3.3 DD). Mollusk shells (57 Pyrgulopsis) were collected from the sandy matrix of a gravel facies on the washover side of the southern tip of the spit (40º18.532’N, 113º56.372’W, 1417 m). They dated 19 460 ± 90 14C yr BP. Another 14C date, 19 340 ± 100 14C yr BP, on 15 Stagnicola shells, was obtained from a fine sand overlying tufa-capped boulders on a transgressive bayhead barrier, 250 m southeast of the right abutment of the dam (40º18.480’N, 113º56.040’W, 1420 m). In addition, another transgressive facies of a bayhead barrier was located at the Pilot Creek valley (40º47.760’N, 114º09.540’W, 1431 100 m) (Figure 3.3 PC). A gravelly sand unit contained abundant mollusk shells (mainly Stagnicola), which dated 19 320 ± 120 14C yr BP. The stratigraphic context of the19.32 and 19.34 14C ka dates suggests the continuous lake expansion and deepening. At a lagoon behind a gravel barrier beach near the Toole Army Depot (40º30.760’N, 112º20.820’W, 1457 m) (Figure 3.3 TD), a fine sand contained abundant Stagnicola shells, which dated 19 090 ± 110 14C yr BP for an apparent highstand. This 14C date and six other published 14C dates at the proximity of the 1450 m elevation indicate the highstand from 19.1 to 18.9 14C ka (Broecker and Kaufman, 1965; Scott et al., 1983; Scott, 1988; Oviatt et al., 1992; Sack, 1999). A brief regressive phase followed the 19.1 to 18.9 14C ka highstand, and lowered the lake level possibly to <1390 m (Oviatt et al., 1992). This regression was recorded in a stillstand-regressive facies, observed on top of the 19.4-19.3 14C ka gravel package of the aforementioned spit at the Deep Creek Dam/Reservoir (40º18.561’N, 113º56.352’W, 1422 m) (Figure 3.3 DD). Directly overlying the gravel is a <3 cm thick medium sand, which contains abundant mollusk shells. This sand is overlain by >60 cm of boulders with a dendritic tufa, which is unconformably overlain a white marl that covers a crest of the spit. Stagnicola shells from the medium sand dated 18 690 ± 240 14C yr BP, indicating the timing of the stillstand-regression. Despite the difference in elevation, this date is consistent with the previously reported 18 760 ± 200 14C yr BP shoreline at 1390 m (Oviatt et al., 1992), suggesting regressive event U1 of Oviatt (1997). Interval II, 18.6-15.7 14C (22.1-18.9 cal) ka A period from18.6 to15.7 14C ka was previously described as the late trangressive phase that preceded the maximum stage of Lake Bonneville (Oviatt et al., 1992). Among 101 five 14C dated shoreline features investigated for this study, three suggest an expanding lake while other two indicate a brief, but substantial shrinking lake within the same interval. From 18.6 to 18.3 14C ka, the lake was likely >38 700 km2 (77% LB-max). A gravelly sand unit on the washover side of a gravel barrier near the Dugway Pass (39°53.160’ N, 113°08.460’ W, 1451 m, Figure 3.3 DP) contained abundant Stagnicola shells, which dated 18 270 ± 150 14C yr BP. Because the gravelly sand grades upward into gravel, the lake was likely shrinking at the time. Despite large standard errors, two other published 14C dates and corresponding lake levels (Broecker and Kaufman, 1965) also indicate the 38 700 km2 lake extent. Another date on wood in the lagoonal sediment, meanwhile, suggests much larger, 42 400 km2 lake (84% LB-max) 18 600 ± 150 14C yr BP (Scott et al., 1983). An exposure at a lagoon behind a baymouth barrier (41°42.900’N, 112°24.480’W) in Faust Valley (Figure 3.3 FV) shows a thinly bedded to laminated fine sand of 4 m, which is unconformably overlain by a >2 m laminated marl. The top 15 cm of the fine sand contained moderately abundant Stagnicola shells, which dated 18 050 ± 70 14C yr BP. Whereas the fine sand unit’s 1402 m elevation differs from the other published 14C dated lake levels ranging 1446-1500 m, its stratigraphic context seems to support the 18.2 14C ka lake shrinking event, reported from the Old River Bed locality (1366 m) by Oviatt et al. (1994). Additional evidence of the shrinking lake was located at a double tombolo at Browns Spring (40°28.740’N, 113°05.460’W) (Figure 3.3 BS). An excavated section at the base of the tombolo shows a 1.7 m massive medium to fine sand overlying a 2 m calcareous very fine sand, which overlies a sandy marl at the base. 102 Both stratigraphy and sedimentology indicate a regressive sequence. The medium to fine sand unit also underlies a sandy marl of unknown thickness. Approximately 50 Stagnicola shells were collected from the bottom 20 cm of the medium to fine sand at 1355 m, and dated 17 840 ± 70 14C yr BP. If these three dates and corresponding lake levels are valid, the 38 000 km2 lake had decreased to 31 800 km2, then to 24 700 km2 (49% LB-max) from 18.2 to17.9 14C ka, making an amplitude of the lake contraction comparable to that of the Stansbury Oscillation (Oviatt et al., 1990). This regressive phase contrasts sharply with the existing transgressive chronologies, illustrating some of the problems in the current Bonneville shoreline age control: large standard errors (>200 years) of the many published 14C dates (12 in this period), and interpreting 14C dates on wood. A14C date on wood in a lagoonal sediment (40°37.680’N, 112°27.600’W, Figure 3.3 EB) was obtained for this study. Its age, 18 000 ± 120 14C yr BP, and 1478 m lake level are consistent with the large expanding lake, but not with the small shrinking lake hypotheses. Nevertheless, as discussed by Oviatt et al. (1992), a 14C date on wood indicates when that tree died, but not necessarily when the burial of that wood occurred in the context of lacustrine shore zone deposition. Furthermore, it is extremely difficult to evaluate reworking of wood in the paleoshore zone environments. Between 17.8 and 17.5 14C ka, all the published 14C dated lake levels and one from this study (17 730 ± 120 14C yr BP at 1495 m) indicate a large, deep lake phase. Over a relatively brief period, the lake appears to have rapidly expanded its surface area from 37 100 to 45 700 km2 (90% LB-max). Then, from 17.5 to 17.0 14C ka, no lacustrine shoreline 14C dates in the Bonneville basin have been reported. With one 14C date (non- 103 shoreline), 17 100 ± 60 14C yr BP from sediment core NFM5, Oviatt (1997) proposed that the low stand U2 occurred during this period. In addition to the aforementioned 18.2- 17.9 14C ka low stand U1, this U2 would raise the possibility of multiple abrupt oscillations of substantial amplitudes punctuating the main transgressive phase of Lake Bonneville. From 16.9 to 15.7 14C ka, no new data were obtained for this study. Among eight published 14C dated lake levels, only three (two from Light, 1996; one from Burr and Currey, 1992) seem to provide precise enough information on changing lake size over such a relatively short period. All others have too large standard errors, >300 yrs, for their 14C dates (Broecker and Kaufman, 1965). Nonetheless, the 41 000-43 300 km2 (81-86% LB-max) lake likely existed during the 1000 14C yr long period. Interval III, 15.6-14.5 14C (18.8-17.3 cal) ka Between 15.6 and 14.5 14C ka, the expanding lake probably reached the basin threshold at Zenda, Idaho (Figure 3.3 ZT) and started overflowing into the Snake River drainage basin. This phase then culminated in the catastrophic failure at the threshold, causing the Bonneville Flood. An exposure at a gravel pit near the Dugway Proving Ground shows a transgressive facies on the washover side of a gravel beach barrier (40°04.980’N, 112°40.500’W) (Figure 3.3 GC). Silty gravel dominates the lower 4 m of the exposed section while alternating upward coarsening and fining. The silty gravel grades upward into a coarse silt of 2 m, which reaches to the modern surface. A <30 cm transition between the gravel and coarse silt units contained abundant mollusk shells, almost exclusively Stagnicola, which dated 15 610 ± 80 14C yr BP at 1532 m. Another transgressive stratigraphy was located at the Point of Mountain gravel pit (40°27.720’N, 104 111°54.120’W) (Figure 3.3 PM). In an exposed section of a >50 cm lagoon washover deposit on the north side of a bayhead barrier (a west projecting spit), a fine sand of unknown thickness grades upward into a 30 cm thinly bedded to laminated coarse silt, which reaches to the modern surface. The coarse silt contained moderately abundant Pyrgulopsis shells, which dated 15 570 ± 50 14C yr BP. The 1533 m lake level at the 14C dated unit is almost identical to 1532 m of the 15.6 14C ka shells at the Dugway Proving Ground site, indicating a >48 300 km2 lake surface area (96% LB-max). The lake also likely started spilling over the threshold at the time. Despite being 300 14C yr earlier than the Currey and Burr model (1988), this maximum phase onset appears to be B1 shoreline of Burr and Currey (1988, 1992). Kaufman (2003) reported two 14C dated low lake levels immediately following the 15.6 14C ka lake expansion: 15 450 ± 80 14C yr BP at 1423 m, and 15 150 ± 65 14C yr BP at 1410 m, inferring a rapid and substantial loss of water from the lake. These data suggest that the lake lost 12 800 km2 lake surface area (level going down by 110 m) over <100 14C yr with its magnitude almost equaling or exceeding the Bonneville Flood. Although its timing, but not magnitude, agrees with the U3 low stand of Oviatt (1997), these 14C dated shells could have been transported down from much higher lake level during the Bonneville Flood because of: (1) almost identical magnitude of lake level drop to that of the flood; (2) a fact that each 14C date was obtained from a single mollusk shell. As discussed by Godsey et al. (2005), the lake kept expanding and overflowing at the threshold while the basin crust was responding to increasing load of water from 15.1 to 14.9 14C ka. Lake Bonneville likely reached its 50 500 km2 maximum extent around 14.9 14C ka, forming B5 shoreline of Burr and Currey (1988, 1992). Currey et al. (1983) 105 and Burr and Currey (1988, 1992) proposed a brief low stand, Keg Mountain Oscillation, immediately after the B5 shoreline formation. Despite the insufficient evidence pointed out by Oviatt et al. (1992, 1994) and Oviatt (1997), one 14C dated lake level implies that the lake might have shrunk by 2800 km2 (>20 m lake level drop) around 14 830 ± 160 14C yr BP (Oviatt et al., 1994). This study, however, was not able to obtain any new evidence of the Keg Mountain Oscillation. The published 14C dated lake levels broadly suggest that the Bonneville Flood occurred between 14.7 and 14.3 14C ka (constrained by eight 14C dates, Table 3.1). Although the current consensus on the timing of the flood seems to be 14.5 14C ka (Godsey et al., 2005), its precise timing has not yet been resolved. Interval IV, 14.5-12.9 14C (17.3-15.1 cal) ka The magnitude of the Bonneville Flood was estimated by Jarrett and Malde (1987). According to their model, the flood might have emptied a half the volume of water from the lake in eight weeks. Triggered by the failure of a plug, possibly consisting of alluvium and landslide mass at the Zenda threshold, the flood also lowered the basin threshold by 106 m (Currey, 1990; Sack, 1999). Thereafter, a bedrock sill at Red Rock Pass (near Zenda) (Figure 3.3 ZT) became the Bonneville basin threshold at 1444 m, making a possible maximum lake surface area in the basin after the flood 38 000 km2 (75% LB-max). The recent study by Godsey et al. (2005) indicated that the lake might have stayed at the threshold level for >1200 14C yr after the flood (14.5-13.3 14C ka). Because this period was a continuation of maximum lake extent (the basin was still full), the Lake Bonneville maximum period can be considered to have lasted for >2300 14C yr (15.6-13.3 14C ka). This relatively long episode of a large, deep lake in the 106 Bonneville basin was likely driven by large-scale climate change events, like Heinrich events, even though the lake expansion was repeatedly checked by the threshold control and isostatic response of the basin crust to the increasing load of water. Similar to the large Bonneville basin lake 21.7-19.8 14C ka coinciding with Heinrich event 2 (Nishizawa et al., in review), the duration and timing of the 15.6-13.3 14C ka lake episode seem to be in a close synch with those of the North Atlantic Heinrich event 1: e.g., 15.1-13.4 14C ka by Elliot et al. (2002) or 15.1-13.2 14C ka by Rashid et al. (2003). A major regressive phase appears to have begun by 13.3 14C ka (Godsey et al., 2005). Stratigraphy, geomorphology, and 14C dates of regressive shore zone facies observed at two localities for this study provide the evidence of the shrinking lake from 13.3 to12.9 14C ka. A 10 m-exposure in a gravel pit, 2.5 km north of the Jordan Narrows (40°28.080’N, 111°55.740’W) (Figure 3.3 PM), shows a lagoon depositional sequence behind a bayhead barrier. An upper 3 m and lower 3 m of the measured section consist of well-sorted and well-stratified pebbly gravel. While the upper gravel has the Holocene soil developed in its top 30 cm, it overlies conformably a well-stratified, pebbly fine to medium sand unit with its thickness varying 0-4 m. Stagnicola shells were collected from the upper 1 m of the fine to medium sand, and dated 13 100 ± 40 14C yr BP. A 1385 m lake level indicates that the lake had shrunk to 31 000 km2 (61% LB-max) by then. In Promontory Hollow (Figure 3.3 PH), two exposed sections at a bayhead barrier (Figure 3.5) also show regressive sequences. A 1.3 m fine laminated sand grades upward into a 30 cm well-sorted gravel on a north facing arroyo wall at the barrier (Figure 3.5 A, 41°33.595’N, 112°36.400’W). The gravel is overlain by a massive coarse silt unit, which becomes soil in its top 20 cm to the surface (Figure 3.5 A). Moderately abundant 107 ) walls showing cross-sectional th facing (A) and south (B views of a bayhead barrier. Figure 3.5. Promontory Hollow arroyo nor 108 Stagnicola and Pyrgulopsis shells were collected from the fine sand transitioning upward into the gravel at 1398 m, and dated 13 010 ± 70 14C yr BP. On a south-facing arroyo wall (41°33.663’N, 112°36.455’W), 200 m north of the 13.0 14C ka section, a 1-1.5 m bayhead barrier crest is exposed (Figure 3.5 B). The lowest exposed section shows a ripple laminated fine sand grading upward into a 50 cm thinly bedded medium sand. With a sharp contact separating, the medium sand is overlain by a 50 cm well-sorted and well-stratified pebbly gravel. The gravel increases its clast size upward and underlies 20- 30cm of soil (Figure 3.5 B). At the contact between the medium sand and gravel, a moderate number of Pyrgulopsis shells were collected, and dated 12 910 ± 80 14C yr BP. A 1396 m lake level for this date makes the 13.0 14C ka and 12.9 14C ka units coeval. An inferred lake surface area at the time is 32 300-32 500 km2 (64% LB-max). After the 13.3-12.9 14C ka regressive period, there is no available shoreline information until 12.6 14C ka, except for one 14C date that indicates a drastically shrinking lake (Godsey et al., 2005). A 14C age of 12 880 ± 80 14C yr BP was obtained from a total organic carbon of a medium density tufa at Lakeside (41°13.380’N, 112°51.120’W) (Figure 3.3 LS). The 14C dated 10 cm layer is the second unit from the top in an exposed 3 m bioherm. The unit is massive to cavernous with wavy laminar fabric, containing abundant molds of brine shrimp eggs. Fillings of molds and caverns also contain foliose and filamentous septae. The 14C dated unit elevation of 1279 m would indicate the lake size close to today’s Great Salt Lake. Whereas this event fits the existing age models for the terminal phase of Lake Bonneville (Oviatt et al., 1992; Oviatt, 1997), it is not consistent with the aforementioned 13.3-12.9 14C ka lake chronology. It should be noted that a total inorganic carbon of the same material dated 12 420 ± 80 14C yr BP (Godsey et 109 al., 2005). In addition to the presence of Paleozoic carbonate bedrock at the site, uncertainties of old carbon effects, postdepositional contamination, and reservoir effects in the lake size comparable to today’s Great Salt Lake (see Spencer et al., 1984), make these two 14C dates suspect. Interval V, 12.6-11.4 14C (15.0-13.2 cal) ka Although Godsey et al. (2005) suggested that the 14.5-12.9 14C ka lake interval (Provo stage) continued for another 700 14C yr until 11.9 14C ka, the 12.6-11.4 14C ka lake is interpreted as separate because of: (1) the 13.3-12.9 14C ka regressive shore zone facies observed at the Jordan Narrows and Promontory Hollow sites; (2) stratigraphy, geomorphology, and 14C dates of a bayhead barrier near the Draper Spit (Figure 3.6 A site 16); and (3) the 11.4 14C ka regressive sequence observed in a lagoonal sediment in the Pilot Creek valley. During the 2004 spring Valle di Villa subdivision construction, a foreshore to crest section of a bayhead barrier was excavated approximately 1 km east of the Draper Spit (Figure 3.3 VV; Figure 3.6 A site 15)). A measured 5m exposure consists of: a base 80-100 cm coarse sand fining upward with a black fine sand interbedded in its upper 20 cm (units 1, 2); a 20 cm fine sand (unit 3); a 180-240 cm coarse silt (unit 4); a <5 cm calcareous coarse sand (unit 5); unconformity (UC); a 100 cm medium to coarse sand coarsening upward, then becoming pebbly with its top 30 cm turning into soil (units 6, 7) (Figure 6 C). The lower 4 m of the measured section showed a relatively complete lake episode from an onset of the transgression to its termination. The coarse silt unit 4 at 1422 m contained abundant Pyrgulopsis shells (40°31.980’N, 111°50.040’W), which dated 12 440 ± 40 14C yr BP. The similar 14C age on mollusk shells at 1417 m (12 350 ± 110 A Explanations for B B 14~ ~ cross bedded/§laminatedl maSSive ...,. (47 0) I ?a laminated a fine sand fine "' 1448 -laminated ± 60 (4750) gravel -'"Q) I silt I .2! ~ (\~~~) I -~ "'Q) I I 9160±40 ~ 4 9760t I I -Q) I I E ± I I 9590 40 .. ' :~ I I -§ (4714~~ ) , I I 3ETl I 5 I .. 2 I , I , I I I > I -,-I I , I I I '"Q) 1 I / I w (~~~8) - \ I I , , I / / , I I I \ "0 I , , I / / , I I Q) I I I I >- ,, I / / , I I Q) , I I I I I 1414 I I 2: (4640) t ' , J :, ::J + i- •• J t CfJ • • • 0.5 1.0 1.5 2.0 Horizontal Di stance (k ilometers) (Vertical Exaggeration: x 38) bJ Figure 3.6. Study area of the early Holocene shoreline deposits (A) Map-view of study sites along Dry Creek and near the Draper Spit. (B) Cross section and observed stratigraphy from sites 1 to 14. (C) Stratigraphic colum at sites 15 and 16. See text for the numbered units. 111 200 14C yr BP) was reported from a site along the Willow Creek Wash, 400 m northwest of Valle di Villa (Broecker and Kaufman, 1965, 40°34.020’N, 111°30.000’W). At the Valle di Villa site (Fig. 6 site 16), Pyrgulopsis shells were also collected from the top 50 cm of the coarse silt unit 4 at 1425 m, and dated 11 550 ± 80 14C yr BP. The 35 700 km2 lake (71% LB-max) 11.6 14C ka had not been reported previously. In fact, this large lake conflicts with at least three published 14C dates from three studies, which indicate a much smaller lake at the time (Murchison, 1989; Oviatt et al., 2003; Godsey et al., 2005). Although the stratigraphic, geomorphic, and sedimentological analyses and interpretations of these previous studies seem consistent, their 14C ages for the terminal lake Interval V are problematic. All three dates were obtained from the materials that were living in the water that had originated from spring discharge: Dugway Proving Ground (Figure 3.3 DG) (Oviatt et al., 2003), the Public Shooting Grounds (Figure 3.3 PG) (Murchison, 1989), and Fish Springs (Figure 3.3 FS) (Godsey et al., 2005). The case of such material yielding much older 14C ages was illustrated by Oviatt et al. (2005). A 14C age of modern shell in a wetland at the Public Shooting Grounds, where Murchison’s date was also obtained, showed >0.6 14C ka reservoir or old carbon effect. Based on this experiment, Oviatt et al. (2005) rejected all 14C dates on mollusk shells in their study. Nonetheless, they accepted all dates on aquatic plants that had also lived in the spring discharge water, showing that effects of spring-fed water on 14C ages have been underestimated in the Bonneville basin research when interpreting 14C ages on organisms that had lived in wetland or marshy environments. An archeological/paleoecological chronology at Dugway Proving Ground by Oviatt et al. (2003) also relies heavily on a large number of dates that were obtained from materials in sand channel deposits. 112 Recharged ground water, but not the paleo-Sevier River, was the likely source of these sand channels (Oviatt et al., 2003). In the Bonneville basin, 14C dates, which were obtained from materials in wetland or marsh deposits to which spring or ground discharge had been the main source of water, should be taken with caution. At the Valle di Villa site, the coarse silt unit 4 was unconformably overlain by a >1 m medium to coarse, well-sorted, lacustrine beach sand unit 6. It appears that sometime after the 12.4-11.6 14C ka lake episode, the lake had retuned to this level. As discussed below, the early Holocene was likely when the large lake reappeared. Throughout 12.4-11.6 14C ka, the lake seems to have sustained a >34 700-35 700 km2 surface area with few oscillations. The 22 700 km2 small lake 11.8 14C ka reported by Godsey et al. (2005), however, could indicate a major oscillation, which was not observed at the Valle di Villa site. A regression of the 12.4-11.6 14C ka lake episode likely began after the deposition of a top lagoonal fine sand behind a bayhead barrier (40°48.900’N, 114°10.320’W) in the Pilot Creek Valley, Nevada (Figure 3.3 PC). A moderate number of Stagnicola shells from this fine sand dated 11 350 ± 70 14C yr BP. Its 1408 m elevation indicates a 33 700 km2 lake (67% LB-max). From 11.3 to 10.614C ka, no reliable lacustrine shoreline records have been reported. Although at least six 14C dates were reported in the context of lake chronology from 11.3 to10.6 14C ka, they were all dated on the materials that existed in the water originating from springs (Murchison, 1989; Currey, 1990; Zachary, 2001; Oviatt et al., 2003). None of these dates were found in the shoreline sediment. Considering the more than three decades of effort and time spent on researching the GS-1 lake episode in the 113 Bonneville basin, the 700 14C yr gap of lacustrine shoreline information suggests the possibility of the GS-1 lake being smaller than today’s Great Salt Lake. Interval VI 10.6-10.0 14C (12.7-11.4 cal) ka Studies at only two type localities have provided reliable information on the lake size change from 10.6 to 10.0 14C ka in the Bonneville basin. In lagoonal sediments of the Magna Spit (Figure 3.3 MS), a 2 m section was excavated during a 2001 sewer trench construction (40°42.480’N, 112°03.420’W). The bottom of the measured section consists of a >15 cm thinly bedded coarse silt, which is overlain by an 85 cm massive, well-sorted fine sand. A 35 cm coarse silt overlies the fine sand with an irregular pebbly layer overlying the silt. Above the pebbly layer, a 65 cm lagoon colluvium of coarse silt sits at the top of the section. All contacts are sharp, but comformable. In the massive sand unit, a 5 cm weakly bedded, black organic mud (gyttja) lies at the bottom and reappears 20 cm above as a laminated, lighter colored muck. The gyttja dated 10 570 ± 60 14C yr BP. An estimated lake surface area is 14,000 km2 (based on the 1293 m elevation). Currey et al. (1983) also reported two 14C dates of 10 300 ± 310 and 10 280 ± 270 14C yr BP on mollusk shells at 1294 m from another site in close proximity (40°42.540’N, 112°2.640’W). The shoreline chronology obtained from the Magna Spit seems consistent with that of the Public Shooting Grounds (Oviatt et al., 2005). Zachary (2001) and Oviatt et al. (2005) have reported five 14C dated lake levels from the Public Shooting Grounds (Figure 3.3 PG). Carbonized plants, which they 14C dated, were collected from a ripple-laminated, medium to fine lacustrine sand unit, instead of more problematic wetland sediments. The five 14C ages range from 10.5 to 10.0 14C ka with the 12 700-13 300 km2 surface area (25-26% LB-max, 1289-1291 m 114 elevations). Oviatt et al. (2005) traced the ripple-laminated sand unit to the Gilbert shoreline (1296 m) and proposed that the unit was deposited during the transgression to the Gilbert shoreline. According to this chronology, the Gilbert shoreline highstand occurred after the sand unit. Therefore, the youngest three 14C dates of the sand unit (10.2-10.0 14C ka, two from the base) would suggest the timing of the Gilbert highstand to be younger than 10.0 14C ka, likely the early Holocene. From 10.6 to 10.0 14C ka, the lake in the Bonneville basin appears to have sustained a 12 700-14 400 km2 surface area: three times larger than the 4400 km2 historic average surface area of Great Salt Lake (Arnow and Stephens, 1990). As suggested by Oviatt et al. (2003, 2005), the Bonneville basin was effectively wetter than today. Nonetheless, the lake was only 25-29% LB-max, indicating that the GS-1 hemispheric cold reversal had not resulted in the return of the GS-2 hydrologic and climatic conditions in the Bonneville basin. A rather uneventful, stable small Bonneville lake coincided with the last Greenland stade during 10.6-10.0 14C ka. Interval VII, 9.8-9.2 14C (11.2-10.3 cal) ka Morrison (1965) analyzed stratigraphy and sedimentology of the lacustrine- fluviolacustrine deposits in the Little Cottonwood Creek and Dry Creek areas at the foot of the Wasatch Mountains (Figure 3.3 DC), and proposed that a large lake comparable to the 14.5-12.9 14C ka Interval IV lake had occupied the Bonneville basin shortly after 10.0 14C ka. The sedimentary package of this lake interval was named “the Draper Formation” with three phases (members). This chronology seemed to be supported by two 14C dates on peat (10.3-9.9 14C ka) (Rubin and Alexander, 1958) and three 14C dates on a plant matter (10.1-9.5 14C ka) (Broecker and Kaufman, 1965), all from a unit 115 directly underlying a laminated lacustrine marl at the Weber River delta (Figure 3.3 WD). In addition, a charcoal in the alluvium underlying the early phase (“lower tongue” of Morrison, 1965) of the Draper Formation in the Dry Creek valley (Figure 3.6) was dated 9800 ± 300 14C yr BP, providing further evidence for the early Holocene large lake (Broecker and Kaufman, 1965). Two decades later, however, Morrison’s interpretation of the Draper Formation was dismissed by the majority of the Bonneville researchers (e.g., Scott et al., 1983; Oviatt et al., 1992). Scott (1979) and Scott et al. (1983) argued that the early phase of the Draper Formation observed along Dry Creek was not lacustrine in origin, but Holocene valley fill of alluvium and colluvium. They also reported a 3520 ± 80 14C yr BP charcoal date from the Draper Formation to support the argument. Scott (1979) questioned the integrity of the 14C dates from the Weber River delta, citing inconsistent 14C dates, contamination of samples, and altered stratigraphic context by extensive landslides. Rubin and Alexander (1958) reported an age of the laminated marl, which overlies the unit of the aforementioned five Holocene 14C dates at the Weber River delta, to be 12 960 ± 350 14C yr BP. The reversal of dates, however, could result from reworking of the 14.5-12.9 14C ka marl by the early Holocene large lake. Nevertheless, both the Draper Formation and the early Holocene large lake episode are now considered inconsistent with the evidence of a much smaller lake by many researchers (e.g., Oviatt et al., 1992, 2003, and 2005) after most of Morrison’s study sites have disappeared under 40 years of urban growth. Even more than 40 years after Morrison’s study, the Draper Formation type localities can be still observed in the Salt Lake County Dimple Dell Regional Park. 116 Fourteen exposures in the 2.5 km stretch along Dry Creek were mapped between 2003 and 2007 for this study (Figure 3.6 Sites 1-14). During the five-year investigation, conditions of some exposures changed rapidly and greatly. In six months, site 12 increased its exposure by >5 m due to a slump, while site 4 gradually lost >2 m of exposure to overgrown vegetation and slope failure from 2004 to 2007 (Figure 3.7 Site 4 A and B). These constantly changing conditions of the exposures result from steep angles of bluff, nature of the sediment, and widely fluctuating annual precipitation and temperature regimes in the region. It is, therefore, likely that some of the stratigraphy and sedimentology presented in this investigation might not have been observable when Morrison (1965) or Scott et al. (1983) investigated. All exposures observed for this study result from downcutting by Dry Creek. A sedimentary package of the early phase Draper Formation was observed at the 1422-1443 m elevation range (Figure 3.6 Sites 3-14). At Site 1 (40°33.679’N, 111°50.982’W) and 2 (40°33.572’N, 111°50.636’W), measured sections were dominated by a very thinly bedded to laminated silt of >5m, which appears to indicate pro-delta environment (at 1408-1413 m). A 3 m measured section at Site 3 (40°33.587’N, 111°50.598’W) consists of a 2 m massive fine sand, overlain by a 1 m laminated very fine sand to coarse silt (at 1419-1422 m). Lamina of the very fine sand to coarse silt are curved downward (parallel to each other) with planar lamina at the bottom 5 cm (Figure 3.6 B). At Site 4, the bottom 10 m from the stream bed was completely covered by a slope of the weathered face (Figure 3.7 Site 4A). An overlying exposed unit is a 2 m laminated coarse silt, which unconformably underlies a 40 cm thick pebbly gravel unit. At the bottom of the laminated coarse silt unit (40°33.599’N, 111°50.460’W, at 1422 m, Figure 3.7 Site 4B), six Stagnicola shells were 117 Site 4 A (2007 photo) '1\ Bt--~ I \ {..: - I \ ~~7~;:t~_~± 40 " e yr BP I -- , 97215) \ \ -- \ \ \ \ \ . ~ .. 9760 ± 50 ,.c yr BP 9160 ± 40 14e yr BP (Bota-185636) (Bola-197216) Site 8 - ripple Figure 3.7. Photos of the stratigraphy at Dry Creek Sites 4 and 8. 118 collected, and dated 9760 ± 50 14C yr BP. Similarly, thirty two Stagnicola shells were collected at the top of the same laminated coarse silt unit (at 1424 m, Figure 3.7 Site 4A), and dated 9590 ± 40 14C yr BP. The pebbly gravel unit is overlain by a 1.5-2 m laminated very fine sand to coarse silt, which unconformably underlies 1m colluvium and the Midvale soil (Morrison, 1965). From the upper 30 cm of the laminated very fine sand to coarse silt unit (at 1426 m, Figure 3.7 Site 4B), twelve Stagnicola shells were collected for 14C dating. This laminated very fine sand to coarse silt, indicative of top-delta sediment, was deposited around 9160 ± 40 14C yr BP. Unlike Site 1, 2, and 3 where no gravel layer was observed, the pebbly gravel unit at Site 4 pinches out downstream while continuing and enlarging upstream to Site 14 (Figure 3.6). All the laminated coarse silt, pebbly gravel, and laminated very fine sand to coarse silt units, observed at Site 4, are traceable upstream for 1.9 km to Site 14 (Figure 3.6). Further upstream from Site 4, the Draper Formation below the gravel unit shows river dominated deltaic environments at Site 8 (40°33.582’N, 111°50.155’W, Figure 3.7) and 9 (40°33.590’N, 111°50.097’W, Figure 3.8). A 1.5-2 m thick gravel unit at Site 8 consists mostly of cobbles with few boulders of >30 cm (at 1425 m). It uncomformably overlies alternating plane lamina of coarse silt and ripple cross-laminated fine sand of >2 m (Figure 3.7 Site 8B). Ripple cross lamination indicates a rapid sedimentation of these materials from suspension in a deltaic environment. The bottom of the alternating lamina grades downward into a 10 cm blocky coarse silt, which overlies a thinly bedded coarse sand (Figure 3.7 B). Above the gravel unit, a 2 m thick thinly bedded to lamintaed very fine sand to coarse silt unit grades upward into the Midvale Soil (Morrison, 1965). At Site 9, a 1 m gravel unit consists mostly of boulders of <40 cm, fining upwards (at 1426 119 Site 9 Site 12 , .~ / / '. " / Site 1,::.0__ _ . ,- .' .~ .. . Figure 3.8. Photos of the stratigraphy at Dry Creek Sites 9,12, and 10. 120 m). A very fine sand of 2 m, uncomformably underlying the gravel, shows trough cross laminations in places (Figure 3.8 Site 9B). An upper measured section above the gravel shows a weakly bedded very fine sand and varies its thickness from 0 to 1 m (Fig. 8 Site 9A). An entire sequence of the Draper Formation’s early phase/lower tongue member is best exposed at Site 12 (40°33.540’N, 111°50.598’W, Figure 3.8 Site 12A and B). A bottom of the 10 m measured section is a 50 cm thinly bedded, light colored coarse sand (unit 1), which unconformably underlies a 30 cm thinly bedded coarse silt with a convoluted flame structure (unit 2). A thinly bedded pebbly coarse sand of 1.5 m (unit 3) unconformably overlies the thinly bedded coarse silt unit 2. The coarse sand is overlain by a 50 cm gray blocky silt (unit 4), which grades upwards into a thinly bedded coarse silt with convolute beddings (unit 5) (Figure 3.8 Site 12D). Because the thinly bedded coarse silt unit 2 is offset 1.0 m by an apparent fault, these convolute beddings were likely caused by early Holocene earthquakes (Figure 3.8 Site 12B-D). A 30 cm gravel unit (unit 6), which fines upward, unconfomably overlies the coarse silt unit 5. In places, the gravel forms cross beds, and is interbedded with a 15-20 cm thinly bedded fine sand (Figure 3.8 Site 12C), indicating alternations of river dominating and lake dominating depositional environments. A 6 m thinly bedded very fine to fine sand (unit 7) comformably overlies the gravel, and grades upwards into the Midvale Soil (unit 8). From the fine sand unit 7 at 1437 m, fifteen Stagnicola shells were collected, and dated 9260 ± 60 14C yr BP. This age is consistent with the 9.2 14C ka top delta sediment at Site 4. The fine sand unit 7 was traced farther upstream to Site 14 with a corresponding elevation of 1443 m (Figure 3.6 B), confirming Morrison’s original report (1965). The 121 lake was likely at the 1444 m Red Rock Pass threshold, spilling over to the Snake River Basin around 9.3 14C ka. Conditions of exposures at Site 10 and 11 may provide some insight into the non- lacustrine interpretation of Scott (1979) and Scott et al. (1983). Being the closest to and most visible from the Dimple Dell Road, Site 10 was probably the most visited and well known Draper Formation type locality since 1965 (Figure 3.8 Site 10A). Its exposure, however, is less optimal for study after having been weathered over the years by processes like solifluction, creep, slope wash, and slumping (Morrison, 1965). The Draper Formation in the 100 m stretch from Site 10 to 11 appears to be poorly sorted and poorly bedded colluvium (Figure 3.8 Site 10B), as described by Scott et al. (1983). Nevertheless, the sections where recent slumps removed a more than 1.5-2 m veneer of colluvium at Site 10 show that well-sorted, thinly bedded fine sand of the Draper Formation lies behind/below the colluvium (Figure 3.8 Site 10C). The deltaic facies observed at the Dimple Dell Regional Park supports Morrison’s interpretation (1965). It is very likely that a brief, but substantial, early Holocene lake in the Bonneville basin existed around 9.8-9.6 14C ka with a >35 600 km2 surface area, and again around 9.3-9.2 14C ka, possibly spilling over the threshold (38 000 km2, 75% LB- max). The gravel unit, which separates these two phases, may also indicate an oscillation of unknown magnitude. Based on the relatively small amount of sediments and lack of pronounced geomorphic features of the Draper Formation, Morrison (1965) postulated that the wave energy during the early Holocene lake episode was much weaker than the preceding episodes (e.g., during GS-2). This may be why locating the geomorphic evidence of this lake interval in the Bonneville basin has been difficult. Besides the Dry 122 Creek area, the Weber River delta seems to be the only type locality that has provided 14C dates in the lacustrine shoreline stratigraphic context to support the early Holocene large lake episode. If any exists, those paleo-deltas along the Wasatch Mountains that have survived urban development need to be studied in the near future. Discussion of the Early Holocene Bonneville Basin Lacustrine Environments The evidence of the early Holocene large lake interval presented in this study contradicts the previously reported much smaller 9.8 -9.2 14C ka lake, based on wetland chronologies (Oviatt et al., 2003, 2005). As mentioned earlier, twelve 14C dates were obtained by Oviatt et al. (2003, 2005) on carbonized aquatic plants and shells thatwere living in the water derived from springs with old carbon effects. Oviatt et al. (2003) also reported two 14C dates on wood from forest floor mats, which were interbedded with alluvial sand and gravel on the City Creek fan—a fan delta, consisting of lacustrine, fluviolacustrine, fluvial, and debris flow deposits (40°45.800’N, 111°54.150’W) (Figure 3.3 CF). Oviatt et al. (2003) interpreted that gallery forests were present at 1282 m 9670 ± 80 14C yr and 9360 ± 60 14C yr BP. If the climate at the time was not too different from today, it is not clear whether gallery forests could be present at the elevation only 2 m above the 1280 m historic average level of Great Salt Lake. This chronology is also inconsistent with the wetter than today regional climate and hydrology suggested by the study at Dugway Proving Ground (Oviatt et al., 2003) since the lake level had to be much higher. Records from some of the most well studied archeological caves in the Bonneville basin show a uniform hiatus from 10.0 to 8.6 14C ka. Recent studies (Rhode et al., 2005, 123 2006) refined the early Holocene chronology at Danger Cave (Figure 3.3 DA) by rigorous 14C dating. According to the new 14C dates, pickleweed processing and consumption by humans at the site began only after 8.6 14C ka, not 10.0-9.0 14C ka as previously suggested (Rhode et al., 2005, 2006). A substantial hiatus of human occupation from 10.0 to 8.6 14C ka was also inferred as a boundary between two strata (F31, F16, Rhode et al., 2005). A 14C chronology at Homestead Cave (Figure 3.3 HC) has a similar break from 10.2 to 8.8 14C ka at the boundary between two strata (Strata I, II, Madsen, 2000). At Camel Back Cave near Dugway Proving Ground (Figure 3.3 CB), no cultural material was present until 7.5 14C ka (Schmitt et al., 2002; Schmitt and Madsen, 2005). A coyote scat resting at a boundary between two strata (I and IIa) dated 9560 ± 40 14C yr BP. With a 1390 m elevation (not adjusted for isostatic rebound) of the cave, this date may be consistent with the 9.8-9.2 14C ka large lake Episode VII, coinciding with a brief pause in the episode, indicated by the aforementioned unconformity and gravel unit in the Draper Formation. Records from the Bonneville Estates Rockshelter at 1580 m (not adjusted for isostatic rebound) (Rhode et al., 2005) (Figure 3.3 BE), indicate that the human occupation took place at the site from 10.8 to 9.4 14C ka, and ceased until 7.5 14C ka. This pronounced, uniform, basin-wide early Holocene hiatus in the archeological records has been considered as the indication of significant decrease in effective moisture in the Bonneville basin (e.g., Rhode et al., 2005). Instead, it is proposed here that the hiatus resulted from the large lake drowning Danger (1305 m), Homestead (1406 m), and Camel Back Caves (1390 m) (all elevations not adjusted for isostatic rebound; Madsen 2000; Rhode et al., 2005; Schmitt and Madsen, 2005). 124 Lacustrine records of Pyramid Lake, Lake Chewaucan, and Owens Lake in the northwestern Great Basin consistently indicate the 12.0 14C ka and early Holocene highstands (Licciardi, 2001; Benson et al., 2002; Benson, 2004; Bacon et al., 2006). In light of the new evidence of large lake intervals in the Bonneville basin during 12.6-11.4 and 9.8-9.2 14C ka, it appears that responses of the expanding lakes to the regional to continental-scale climate change events were synchronous and coherent throughout the Great Basin during these time periods. Bonneville Basin Lake in the Context of the Last Glacial-Interglacial Transition Most studies consider that the GS-2 large lakes in the Great Basin, including the Bonneville basin, resulted from greatly enhanced effective moisture in the region that was caused mainly by a southward displacement of the mean path of the polar jet stream (e.g., Antevs, 1948; Benson and Thompson, 1987; Spaulding, 1991; Benson, 2004). It is widely believed that throughout ice ages the changing sizes of the Cordilleran and Laurentide ice sheets forced the mean position of the polar jet stream to migrate, thereby changing western North America’s regional climate regime. Hence, in theory, a climate change event of continental or hemispheric scale would have resulted in a substantial change in size of the Cordilleran and Laurentide ice sheets, then a change in the polar jet stream path, regional storm tracks, and eventually a change in the Great Basin lakes’ sizes, leaving distinct signatures in lacustrine sediments (Benson, 2004). By following this model, more recent studies have focused on the sensitivity of the Great Basin lakes in response to abrupt climate change events, such as the North Atlantic ice-rafted debris or Heinrich events (Oviatt, 1997; Benson, 1999), or Dansgaard-Oeschger cycles (Benson et 125 al., 2003). These studies suggest varying degrees of phasing of the Great Basin lake events/intervals with the North Atlantic climate change events (see Benson, 2004). In Figure 3.9, a pattern of timing, magnitude, and duration of the Bonneville basin lake events and intervals from 24.0 to 10.0 cal ka is compared with the δ18O records of the Greenland NorthGRIP GICC05 (Andersen et al., 2006; Svensson et al., 2006) to examine how the Bonneville basin hydrologic system responded to abrupt climate change events of the late glacial to the last glacial-interglacial transition. In Figure 3.9, all lake event 14C ages, except for those with >±300 standard error, were calibrated in calendar years by IntCal04.14C dataset (CALIB5.1.0Beta; Reimer et al., 2004). Those 14C ages with >±300 standard error were deemed too imprecise to be calibrated and included in a graph to present an observable pattern in Figure 3.9. Uncertainties of calibration are reported here at the 68% confidence level (1σ). When compared with the NorthGRIP GICC05 δ18O profile in Figure 3.9, the Bonneville basin lake Intervals I-IV, which encompass Lake Bonneville’s transgression, highstand, and flood, all occurred during GS-2 (23.3-14.7 cal ka, Andersen et al., 2006). The onset of Interval I, meanwhile, coincided with GI-2 (23.3 cal ka, Andersen et al., 2006). Intervals V and VII, in which the lake likely exceeded the basin threshold, were in phase with GI-1 and the beginning of Holocene, respectively. Meanwhile, much smaller Interval VI lake attained only 13 000 km2 area during GS-1. An emerging relationship between the Bonneville basin lake record and the Greenland/North Atlantic δ18O based temperature records is not straightforward. Whereas the lake was the largest during GS-2, the GS-1 Bonneville basin had the lake only <30% of GS-2 maximum. In addition to the Interval I lake expansion at GI-2, two abrupt transitions from Greenland 126 I -". I I 0 I I -a I I ., 90 I I I I IY'" I I I I N""E 80 I I I I I '"0 - +- ---1+- -- -+-1 - 0 " I 0 I c ~ a - 400 ~ '"~ « W u'" 50 i'! ~ ~ ~ CfJ ~ CfJ '"'" r:-130a 2! -''" j " LAKE BONNEV1LLE I " (37-43°N: 111-1 4°W) OJ " I I 0 I I "- "I I I > ~"I I I I .; I I I I I C I I 0 I I • I I <> I I DONPGE CliVE (25° 1ITN , 108°05'E) " -8 I I II OJ -0 I " "- I I " > I -6 :HULU CAVE I -J 1(32°30'N, O'E) -0 I I •<> I -4 I I -o I .. NOR~HGRIP o- (75°06'N 42°18'Wl •<> I' I Age (Cal k yr BP) Figure 3.9. Comparing the Bonneville basin lake events and intervals with the Asian and East Asian Monsoon records from Dongge Cave and Hulu Cave δ18O records (Yuan et al., 2004; Wang et al., 2001), and GICC05 Greenland Ice Sheet δ18O record (Andersen et al., 2006). 127 stades to interstades (GS-2 to GI-1, GS-1 to the Holocene) were followed by substantially large lake Intervals V and VII in the Bonneville basin. It appears that effective moisture in the Bonneville basin and Great Basin was greatly enhanced during the Greenland interstades (Benson et al., 2003; Nishizawa et al., in review). As suggested by Hostetler et al. (1994), lake-generated precipitation (lake-effect) might have played a large role in sustaining and possibly expanding Lake Bonneville during GS-2. The Bonneville Intervals III and IV, which resulted in the maximum lake extent and threshold failure, were also in close phase with the North Atlantic Heinrich event 1 (18.5-15.0 cal ka, Elliot et al., 2002; Rashid et al., 2003), even though this event itself is generally not recorded well in the Greenland ice δ18O profiles. As the net summer insolation of the Northern Hemisphere was steadily increasing to the early Holocene maxima, greatly enhanced ocean-land surface pressure gradients had resulted in intensification of monsoonal circulations throughout the Northern Hemisphere during the last deglaciation (e.g., North Africa, India, Southeast Asia, and North America) (Berger and Loutre, 1991; Spaulding, 1991). In this context, the Bonneville lake events and intervals are compared with the records of the Asian and East Asian Monsoons to see if there were any teleconnection between them. The δ18O records of stalagmites at Dongge Cave (Yuan et al., 2004) and Hulu Cave (Wang et al., 2001) in China provide detailed chronologies of changing intensity of the Asian Monsoon and East Asian Monsoon. As shown in Figure 3.9, their profiles closely resemble that of the Bonneville basin lake change record from 24.0 to 10.0 cal ka. Large lake Intervals V and VII occurred when the both Asian and East Asian Monsoons greatly intensified, resulting in enhanced summer precipitation in Asia. Meanwhile, small lake Interval VI coincided 128 with the time when the strength of the Asian and East Asian Monsoons greatly decreased. The Bonneville lake Intervals I-IV were also synchronous with the time of the prolonged East Asian Monsoon intensification from 23.5 to 17.5 cal ka. Even the Bonneville Interval IV’s regressive phase appears to have roughly coincided with the steady decline and then abrupt drop in the East Asian Monsoon intensity from 17.0 to 16.0 cal ka. Overall, the pattern of timing, magnitude, and duration of the Bonneville basin lake intervals from 24.0 to 10.0 cal ka is much more consistent with the Asian and East Asian Monsoon strength variability than with the high latitude North Atlantic temperature changes. 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As shown in this study, this number is still quite insufficient to encompass more than 45,000 year time frame. Hopefully, the number will double or triple in the next decade. Currently, there is definite urgency to speed up the Bonneville shoreline research. Rapid urban growth has been efficiently obliterating shoreline features and sediments over the years. Many of the shoreline features became premium locations for residential housing, big businesses, university campuses, and military facilities while easily accessible shoreline gravel was almost completely hauled away for road construction along the Wasatch Mountains. In this sense, it was rather miraculous that the 1965 Roger Morrison’s Draper Formation study sites escaped such a fate. Instead of turning the Dry Creek area into a golf course, Sandy/Draper horse riders were able to preserve a long stretch along Dry Creek as an equestrian park. Without their effort, the information on the early Holocene lake presented in this study would not have been available. 136 Unfortunately, many deltaic features, created by major paleo-streams coming out at the base of the Wasatch Mountains, did not have such luck and already disappeared for being prime real estate locations. Finally, I would really like to encourage future researchers to be brave and bold. Potentials in the Bonneville basin are limitless. It is a complete misconception that whatever can be done in the Bonneville basin has already been done—my initial impression when I barely started studying Lake Bonneville. Now I firmly believe that there is still so much more to explore in this big basin. The more new data come out, the more unexpected possibilities seem to open up. I believe that that is the way it should be. Accepted models and hypotheses are there so that you can challenge, support, refine, or refute them with your new data and interpretations. Ideally, the chronologies proposed in this study will become obsolete and corrected very soon. That will then show how much vigor this research community has. I wish my best and good luck to your future studies.