Behavior of the James Lobe, South Dakota during Termination I
A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
in the Department of Geology
of the McMicken College of Arts and Sciences
by
Stephanie L. Heath
MSc., University of Maine
BSc., University of Maine
July 18, 2019
Dissertation Committee:
Dr. Thomas V. Lowell
Dr. Aaron Diefendorf
Dr. Aaron Putnam
Dr. Dylan Ward
i
ABSTRACT
The Laurentide Ice Sheet was the largest ice sheet of the last glacial period that terminated in an extensive terrestrial margin. This dissertation aims to assess the possible linkages between the behavior of the southern Laurentide margin and sea surface temperature in the adjacent North Atlantic Ocean.
Toward this end, a new chronology for the westernmost lobe of the Southern Laurentide is developed and compared to the existing paradigm of southern Laurentide behavior during the last glacial period.
Heath et al., (2018) address the question of whether the terrestrial lobes of the southern
Laurentide Ice Sheet margin advanced during periods of decreased sea surface temperature in the North
Atlantic. This study establishes the pattern of asynchronous behavior between eastern and western sectors of the southern Laurentide margin and identifies a chronologic gap in the western sector. This is the first comprehensive review of the southern Laurentide margin since Denton and Hughes (1981) and
Mickelson and Colgan (2003).
The results of Heath et al., (2018) also revealed the lack of chronologic data from the Lobe, South
Dakota, the westernmost lobe of the southern Laurentide margin. To address this problem, surface exposure age dating is applied to James Lobe deposits within the Pierre Sublobe, a westward-flowing sublobe of the western James Lobe. A new geomorphic map of the Pierre Sublobe is produced to provide geologic context for 10Be surface exposure age dating efforts. Results of this study offer insights into possible problems associated with site selection in this region and yields some insight into the behavior of the James Lobe during Termination I. The history of the main trunk of the James Lobe is expanded upon using radiocarbon ages compiled from South Dakota Geological Survey reports. Three stratigraphic cross sections of the James Lobe are assembled to provide stratigraphic context for these radiocarbon ages
Results of this study bear on the timing of maximum extent of the James Lobe, as well as advance and retreat patterns of the lobe during late-glacial time. The James Lobe is compared to the neighboring Des
Moines Lobe in Iowa and the existing paradigm of surging and streaming is evaluated.
ii Finally, the behavior of the James and Des Moines Lobes is compared to the rest of the southern
Laurentide margin for three time slices to evaluate the asynchronous relationship established in Chapter 2
(Heath et al., 2018). Results of this study bear on differences between extent and timing of the lobes, and the mechanisms that may have controlled southern Laurentide margin position during the last glacial period and termination. This dissertation affords insight into the patterns of advance and retreat of the southern Laurentide Ice Sheet during the last glacial period and Termination. In reconstructing these behaviors and producing a new chronology for the westernmost Laurentide lobe, this work offers new perspective for the mechanisms that controlled southern Laurentide margin position during the
Termination.
iii
iv
Acknowledgements
My Ph.D. program and research activities were funded by the Department of Geology at the University of Cincinnati and by the Comer Science and Education Foundation.
I would like to extend my sincere thanks to my primary advisory Professor Tom Lowell for his invaluable instruction, patience and support through this process. I would like to thank my committee members, Dr. Dylan Ward, Dr. Aaron Diefendorf and Dr. Aaron Putnam for their expertise and thoughtful suggestions. I owe a great deal of gratitude to Dr. Brenda Hall for her continued support and to Jill Pelto, Peter Strand, Lizzie Orr, and Sarah Hammer for processing my samples.
I am eternally grateful for UC’s Counseling and Psychological Services, who helped me to find a way forward during the most difficult moments of the last five years. Thank you to my family in Rhode Island and Maine for their love and support. I thank Lizzie Orr for her unwavering patience and friendship. I would like to thank the students, staff and faculty of the Department of Geology for their support and kindness.
I dedicate this dissertation research to my husband Adam Heath, who held me up and believed in me when I needed it the most.
v Table of Contents
ABSTRACT...... ii
Acknowledgements ...... v
Chapter 1: Introduction ...... 1
References ...... 3
Chapter 2: Pattern of southern Laurentide Ice Sheet margin position changes during
Heinrich Stadials 2 and 1
ABSTRACT ...... 4
1. Introduction ...... 5
2. Data and methods ...... 6
2.1. Establishment of Heinrich Stadial timing ...... 6
2.2. Chronologic data of Laurentide Ice Sheet history ...... 7
2.3. Analysis ...... 9
3. Results of chronologic data compilation ...... 10
3.1. New England Region ...... 10
3.2. Finger Lakes Region ...... 11
3.3. Lake Huron-Erie Lobe ...... 12
3.4. Lake Michigan Lobe ...... 14
3.4.1. Lake Michigan Lobe chronology ...... 14
3.4.2. Harvard Sublobe chronology ...... 16
3.5. Green Bay Lobe ...... 17
3.6. Chippewa Lobe ...... 19
3.7. Des Moines Lobe ...... 19
vi 4. Summary ...... 21
5. Discussion ...... 22
5.1. Did the terrestrial margin of the southern Laurentide Ice Sheet advance during cold
phases of sea surface temperature (Heinrich Stadials 2 and 1) in the adjacent North
Atlantic? ...... 22
5.2. Comparison with other Northern Hemisphere ice sheets ...... 23
5.2.1. Cordilleran Ice Sheet ...... 23
5.2.2. British-Irish and Scandinavian Ice Sheets ...... 24
5.3. Hypothesis for differences in Heinrich Stadials ...... 25
6. Conclusions ...... 27
Acknowledgements ...... 27
Tables ...... 28
Figures ...... 29
References ...... 40
Chapter 3: Assessing the utility of surface exposure age dating of fluctuations of the James Lobe,
South Dakota
ABSTRACT ...... 48
1. Introduction ...... 48
1.1. Regional setting ...... 50
2. Methods and materials ...... 51
2.1. Geomorphic mapping ...... 51
2.2. Sample collection for cosmogenic surface exposure age dating ...... 51
2.3. “Tazewell” sites ...... 52
2.3.1. Black Dog ...... 52
2.3.2. West Sully ...... 52
vii 2.4. “Cary” sites ...... 53
2.4.1. Sutton Rodeo ...... 53
2.4.2. Bieber ...... 53
2.5. “Mankato” sites ...... 53
2.5.1. Eagle Pass South ...... 53
2.5.2. Eagle Pass North ...... 54
2.6. 10Be surface exposure age analysis ...... 54
3. Results ...... 55
3.1. Geomorphic map ...... 55
3.2. Geomorphic context of sample collection sites ...... 56
3.3. 10Be surface exposure ages ...... 57
3.3.1. Black Dog ...... 57
3.3.2. West Sully ...... 57
3.3.3. Sutton Rodeo ...... 57
3.3.4. Bieber ...... 57
3.3.5. Eagle Pass South ...... 57
3.3.6. Eagle Pass North ...... 58
3.4. Summary of mapping and 10Be surface exposure age dating results ...... 58
3.5. Testing the effects of inheritance and moraine degradation ...... 58
4. Discussion ...... 60
4.1. Evaluation and interpretations of 10Be surface exposure ages ...... 60
4.2. Is 10Be surface exposure age dating suitable for dating James Lobe glacial deposits? .. 61
4.3. Implications for the glacial chronology of the Pierre Sublobe ...... 62
5. Conclusions ...... 63
Acknowledgements ...... 64
Tables ...... 65
viii Figures ...... 66
References ...... 76
Chapter 4: Behavior of the James Lobe during the last glacial period and termination
ABSTRACT ...... 78
1. Introduction ...... 79
2. Methods ...... 80
2.1. Regional setting ...... 80
2.2. Stratigraphy ...... 80
2.3. Acquisition and reduction of radiocarbon ages ...... 81
3. Results ...... 82
3.1. Stratigraphy ...... 82
3.1.1. A-A’ ...... 82
3.1.2. B-B’ ...... 83
3.1.3. C-C’ ...... 83
3.1.4. Summary of stratigraphy ...... 83
3.2. Radiocarbon chronology ...... 84
3.2.1. Summary of chronology...... 85
4. Discussion ...... 85
4.1. Evaluation of chronologic data ...... 85
4.2. Glacial chronology of the James Lobe ...... 85
4.3. Comparison with the Des Moines Lobe, Iowa ...... 87
4.4. The role of climate in the fluctuations of the western lobes...... 88
5. Conclusions ...... 88
Tables ...... 90
Figures...... 92
ix References ...... 98
Chapter 5: Implications of new James Lobe Chronology
ABSTRACT ...... 100
1. Introduction ...... 100
2. Methods ...... 101
2.1. Chronologic data from James Lobe glacial deposits ...... 101
3. Discussion ...... 102
3.1. Summary of James Lobe behavior ...... 102
3.2. Comparison with the Des Moines Lobe: Did both western lobes advance and
retreat at the same time? ...... 104
3.3. Extent versus timing ...... 105
3.3.1. Extent...... 106
3.3.2. Timing ...... 106
3.3.3. Summary ...... 107
3.4. Existing paradigms to explain patterns of advance and retreat ...... 107
3.4.1. Streaming and surging of the western sector lobes ...... 108
3.4.2. Multiple Laurentide domes ...... 109
3.5. Hypothesis for climate conditions as a control for timing...... 109
4. Conclusions ...... 110
Figures ...... 112
References ...... 115
Chapter 6: Conclusions ...... 117
x
xi Chapter 1
Introduction
With present concerns over rising global temperatures and ice-sheet stability, there arises the need for better understanding of the mechanisms that control ice sheet extent and behavior. Former ice sheets, such as the Laurentide Ice Sheet, the largest of the last glacial period (Denton and Hughes, 1981), afford insights into the scale and influence of these mechanisms. During the last glacial period, the southern
Laurentide Ice Sheet culminated in seven distinct lobes that spanned ~3000 km across central North
America from South Dakota to Massachusetts (Flint et al., 1959; Mickelson and Colgan, 2003). The dynamics of this ice sheet remain incompletely understood – as it appears to have experienced asynchronous behavior during the last glacial period and termination (Clayton and Moran, 1982; Hallberg and Kemmis, 1986; Mickelson and Colgan, 2003).
Decreased SST in the North Atlantic is linked to depression of the Intertropical Convergence Zone
(ITCZ) and a southward shift in the Northern Hemisphere westerlies, especially during HS1 (Denton et al., 2010). This shift had observable impacts on Northern Hemisphere hydrology, including weakened monsoons in China (Wang et al., 2001), pluvial lake highstands in the western United States (e.g. McGee et al., 2012; Munroe and Laabs, 2013) and increased precipitation in the southwestern United States as recorded in speleothems (Wagner et al., 2010; Asmerom et al., 2010). Thus, we evaluate in Chapter 2 whether the southern Laurentide margin advanced in response to North Atlantic cooling associated with
Heinrich Stadials 2 and 1. The advance and retreat history of the southern Laurentide margin is constructed using available chronologic data from along the former ice margin and compared to the timing of Heinrich Stadials 2 and 1. The goal of this chapter is to evaluate North Atlantic SST as a possible forcer for ice margin fluctuations.
1 Chapter 3 focuses in on the westernmost James Lobe of the southern Laurentide margin in South Dakota, which remains poorly dated. The most recent comprehensive study of the James Lobe deposits and history was by R.F. Flint in 1955. Flint (1955) describes the subsurface stratigraphy of eastern South
Dakota as well as surface landforms based on field observations. Subsequent county-scale reports published by the South Dakota Geological Survey have improved on this work on a local scale. These reports, however, largely lack chronology, preventing comparison of the James Lobe to the rest of the margin. Further, this lack of chronologic data prevents full understanding of the southern Laurentide during Heinrich Stadials. Thus, we assess the utility of surface exposure age dating on James Lobe deposits by establishing the chronology of a westward-flowing sublobe of the James Lobe.
Present reconstructions of the James Lobe are based largely on correlations of moraines and other surficial features with the Des Moines Lobe of Iowa and Minnesota (e.g. Hallberg and Kemmis, 1981;
Clayton and Moran, 1982). Despite the lack of chronologic data, both lobes are thought to have exhibited similar behavior based on these correlations (e.g. Hallberg and Kemmis, 1981). The present paradigm for western lobes behavior invokes surging or streaming (Clayton et al., 1985; Patterson, 1997; Patterson,
1998; Jennings, 2006). In Chapter 4 this paradigm is evaluated by shifting the focus from the Pierre
Sublobe to the James Lobe as a whole. We constructed three stratigraphic cross sections of the James
Lobe footprint, which provide context for the existing radiocarbon ages assembled from various South
Dakota Geological Survey publications. The timing of James Lobe fluctuations is also compared to that of the Des Moines Lobe in Iowa to determine whether they did in fact behave synchronously.
In Chapter 5, the discussion returns to the problem of mechanisms that controlled southern Laurentide margin position during the last glacial period and Termination. The extent and timing of James Lobe and
Des Moines Lobe margin changes are once again compared to the rest of the southern Laurentide margin, to help understand the mechanisms that controlled southern Laurentide behavior during the last glacial period and Termination.
2 References Asmerom, Y., Polyak, V.J., Burns, S.J., 2010. Variable winter moisture in the south- western United States linked to rapid glacial climate shifts. Nature Geoscience 3, 114-117. Clayton, L., Teller, J.T., Attig, J.W., 1985. Surging of the southwestern part of the Laurentide Ice Sheet. Boreas 14 (3), 235-241. Denton, G.H., Hughes, T.J., 1981. The Last Great Ice Sheets. John Wiley and Sons, New York, p. 484. Denton, G.H., Anderson, R.F., Toggweiler, J.R., Edwards, R.L., Schaefer, J.M., Putnam, A.E., 2010. The last glacial termination. Science 328, 1652-1656. Flint, R.F., 1955. Pleistocene Geology of Eastern South Dakota. Geological Survey Professional Paper 262, p. 173. Flint, R.F., Colton, R.B., Goldthwait, R.P., Willman, H.B., 1959. Glacial Map of the United States East of the Rocky Mountains. Geological Society of America, New York. Hallberg, G.R., Kemmis, T.J., 1986. Stratigraphy and correlation of the glacial deposits of the Des Moines and James lobes and adjacent areas in North Dakota, South Dakota, Minnesota, and Iowa. In: Sibrava, V., Bowen, D.Q., Richmond, G.M. (Eds.), Quaternary Glaciation in the Northern Hemisphere. Pergamon, Elmsford, N.Y. Quaternary Science Reviews 5. Heath, S.L., Loope, H.M., Curry, B.B., Lowell, T.V., 2018. Pattern of southern Laurentide Ice Sheet margin position changes during Heinrich Stadials 2 and 1. Quaternary Science Reviews 201, 362- 379. Lowell, T.V., Hayward, R.K., Denton, G.H., 1999. Role of climate oscillations in determining ice-margin position: Hypothesis, examples, and implications. Geological Society of America Special Papers 337, 193-203. McGee, D., Quade, J., Edwards, R.L., Broecker, W.S., Cheng, H., Reiners, P.W., Evenson, N., 2012. Lacustrine cave carbonates: Novel archives of paleohydro- logic change in the Bonneville Basin (Utah, USA). Earth and Planetary Science Letters. 351-352, 182-194. Munroe, J.S., Laabs, B.J.C., 2013. Temporal correspondence between pluvial lake highstands in the southwestern US and Heinrich Event 1. Journal of Quaternary Science 28 (1), 49-58. Mickelson, D.M., Colgan, P.M., 2003. The southern Laurentide Ice Sheet. Developments in Quaternary Science 1, 1-16. Jennings, C.E., 2006. Terrestrial ice streams — a view from the lobe. Geomorphology 75, 100-124. Patterson, C.J., 1997. Southern Laurentide ice lobes were created by ice streams: Des Moines Lobe in Minnesota, USA. Sedimentary Geology 111, 249-261. Patterson, C.J., 1998. Laurentide glacial landscapes: The role of ice streams. Geology 26 (7), 643-646. Wagner, J.D.M., Cole, J.E., Beck, J.W., Patchett, P.J., Henderson, G.M., Barnett, H.R., 2010. Moisture variability in the southwestern United States linked to abrupt glacial climate change. Nature Geoscience 3, 110-113. Wang, Y.J., Cheng, H., Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.-C., Dorale, J.A., 2001. A high- resolution absolute-dated late Pleistocene monsoon record from Hulu Cave, China. Science 294, 2345-2348.
3 Chapter 2
Pattern of southern Laurentide Ice Sheet margin position changes during Heinrich Stadials 2 and
11
ABSTRACT
With concerns over rising global temperatures, ice sheet stability and implications for sea level rise, the responses of former ice sheets to past climate change provide useful insights into linkages between the atmosphere, cryosphere and oceans. One example is the behavior of the Laurentide Ice Sheet (LIS) during
Heinrich Stadials, times of cooler sea surface temperature (SST) in the North Atlantic. Using existing cosmogenic surface exposure ages, varve chronologies, radiocarbon ages, and optically stimulated luminescence ages from the southern LIS margin, this report tests the hypothesis that the southern side of the LIS advanced during times of surface cooling of the North Atlantic. This test reconstructs the timing of local glacial maxima and the times immediately pro- and preceding them, from multiple areas and compares them to the timing of Heinrich Stadial 2 (HS2; 26.6e23.6 ka) and to the timing of Heinrich
Stadial 1 (HS1; 19.3e15.3 ka).
During HS2 southern sector lobes (New England region, Huron-Erie Lobe, Lake Michigan Lobe, Green
Bay Lobe and the Chippewa Lobe) advanced towards their maximum position, except for the Des Moines
Lobe for which there is no applicable data. During HS1 these same lobes experienced substantial retreat, but the Des Moines Lobe advanced. Thus, the southern Laurentide margin behaved differently during the two Heinrich Stadials. This pattern may be attributed to differing east-west temperature gradients across
North America and associated changes in atmospheric circulation.
1 Heath, S.L, Loope, H.M., Curry, B.B., Lowell, T.V., 2018. Pattern of southern Laurentide Ice Sheet margin position changes during Heinrich Stadials 2 and 1. Quaternary Science Reviews 201, 362-379.
4 1. Introduction
The connections between ice sheets and ocean systems remain incompletely understood. A specific example of this problem is the relationship between the North Atlantic and the Laurentide Ice Sheet during the late Pleistocene. Did the terrestrial margin of the southern Laurentide Ice Sheet advance during cold phases of sea surface temperature (SST) in the adjacent North Atlantic? This question has yet to be explored in the context of millennial-scale sea surface temperatures (SST) changes, although it bears on the direct linkages between ice-sheet and ocean systems. Here the timing of the two most recent intervals of cold SST in the North Atlantic, known as Heinrich Stadials (HS) 2 and 1 is compared to the advance and retreat history of the southern Laurentide Ice Sheet (LIS).
Heinrich Stadials refer to the periods of cool sea surface temperatures during which Heinrich
Events occur (Bond et al. 1992; Bond and Lotti 1995; Bard et al. 2000). Heinrich Stadials are detected in
K marine sediment cores records as changes in planktonic foraminifera assemblages, Mg/Ca ratios or U 37 alkenones (Waelbroeck et al. 2001, Naughton et al. 2007, Salguiero et al. 2014, Schönfeld et al. 2003, de
Abreu et al. 2003, Voelker et al. 2009, Bard et al. 2001, Chapman et al. 2000). To date, six distinct
Heinrich Stadials have been identified in the last 100,000 years, and they are numbered sequentially from youngest to oldest (1 to 6). Heinrich Stadials are different than “Heinrich Events”, which are sediment layers rich in ice-rafted debris in North Atlantic marine sediment cores first described by Heinrich (1988).
Heinrich Event layers are interpreted to result from massive iceberg discharges from the Laurentide Ice
Sheet (e.g. Hemming 2004, Hodell et al. 2016). There is an important difference between the terms
Heinrich Event and Heinrich Stadial. Rather than a specific event, Heinrich Stadials are 1-2 kyr spans of persistently cold North Atlantic SST.
The Laurentide Ice Sheet was the largest ice sheet of the last glacial period (Denton and Hughes
1981) and covered much of North America (Figure 2.1). Its southern margin spanned nearly 3000 km across the central continent and culminated in multiple distinct lobes (Flint 1959, Mickelson and Colgan
2003).
5 2. Data and Methods
2.1. Establishment of Heinrich Stadial timing
This study is focused on the two youngest Heinrich Stadials 2 and 1 (HS2 and HS1) for two reasons. First, they are the best dated of the Heinrich Stadials, and second, they correspond to time intervals with the richest data set and the best preservation of morphological evidence of ice-sheet behavior. Here the term “stadial” refers to a time of relatively cold SST in the North Atlantic, and the term “interstadial” refers to a time of relatively warm SST in the North Atlantic. Bard et al. (2000)
K reconstructed the SST conditions around the time of Heinrich events using U 37 alkenones, for a sediment core off the Iberian Peninsula and reported a 2-3°C decrease in SST at minimum, at several locations in the North Atlantic during both Heinrich Stadials 2 and 1. Subsequent studies of North Atlantic marine sediment cores have confirmed this cooling during Heinrich Stadial times (e.g. Rasmussen et al. 2003a,
Hodell et al. 2017), and additional studies (e.g. McManus et al. 2004, Bradtmiller et al. 2014) suggest reduced meridional overturning in the North Atlantic during Heinrich Stadials.
For this purpose of this study, the timing of Heinrich Stadials was established using dated marine cores (Figure 2.2; Table 2.1). The motivation behind this method was to develop an independent age range for the stadials using only SST proxies. Marine sediment cores from the North Atlantic that were selected for this study each have an independent 14C-based age model, which have not been tuned to ice core records or other sediment cores. In addition, each core record yields a sea surface temperature
K record derived from one or more of the three following proxies: U 37 alkenones, planktonic foraminifera assemblages and Mg/Ca ratios.
The goal here is to make a direct chronological comparison between a proposed forcer of SST and resulting response of ice margin changes on a common time scale. Therefore, the radiocarbon chronology associated with each individual sediment core was recalibrated using the IntCAL13 dataset
(Reimer et al. 2013) and the age-depth model for each core was reconstructed using data from the original sources. The start and end timing for Heinrich Stadials 2 and 1 were determined using the original core
6 records and compared to the newly calibrated age model for each core. These start and end depths and times are compiled in Table 1.
Since the age ranges for each Heinrich Stadial varied between the individual studies, the most probable age range of the existing data was determined using the following method. Using a 10-year bin from 30.0 to 14.0 ka, a “1” was assigned for each bin if the reported age range from each marine core fell within a Heinrich Stadial, or “0” otherwise. The occurrences for each bin were then summed and the relative probability was computed with the area under the resulting histogram equal to one. The results
(Figure 2.2) are plotted numerically as area under the curve and visually as a relative gray scale. The start and end time for each stadial was determined at both the 68% (1-sigma) and the 95% (2-sigma) confidence levels (Table 2.1). For the purposes of this study, the latter is used in both cases. Heinrich
Stadial 2 is reflected as two “peaks” in the probability analysis, however due to the uneven distribution as reflected in Figure 2.2, these have been combined as one phase. This analysis indicates cold phases HS2
(26.6-23.6 ka) and HS1 (19.3-15.3 ka) punctuated by a period of warmer sea surface temperature in the
North Atlantic, termed here the Heinrich Interstadial (HI; 23.6-19.3 ka). We return to this point in the discussion.
2.2. Chronologic data of Laurentide Ice Sheet history
The ice margin position of the southern Laurentide Ice Sheet over time is tracked by stratigraphic correlations and geomorphological and sedimentological features (moraines, till, outwash, etc.). Many of these features have been assigned ages using cosmogenic exposure, varve counting, or radiocarbon dating techniques. The reported context of the geomorphic features and varves, as well as sedimentological and stratigraphic relationships were used to determine if the ice sheet was advancing or retreating and then use the chronology to assign an age for that behavior. The maximum extent of the LIS during the last glacial period has been mapped extensively in most places, most notably by Flint et al. (1959), and in subsequent
US Geological Survey 4 x 6°, 1:1,000,000 scale quadrangles (Goebel et al. 1983, Lineback et al. 1983,
Farrand et al. 1984, Dyke and Prest 1987, Fullerton et al. 1991, Gray et al. 1991, Hallberg et al. 1991,
7 Hartshorn et al. 1991, Howard et al. 1991, Fullerton et al. 1991, Whitfield et al. 1993). Most stratigraphic and chronologic data comes from near the maximum limit of the ice sheet. Therefore, the focus was on all chronologic data with known stratigraphic context (sediment unit or feature) within ~200 km of the outermost moraine for each of the six individual lobes (Figure 2.3). Any changes in ice margin position would be reflected in the outermost 200 km of each lobe. This paper deals with changes in the ice sheet specifically during Heinrich Stadials 2 and 1, therefore only chronologic control between 28.0 and 15.0 ka was considered. This review includes all known age analyses in the literature relevant to this question.
Limited unpublished data from the authors is also included here.
The following is a brief description of each dating method and individual calibration method as well as the interpretation strategy.
Cosmogenic radionuclide exposure ages represent the amount of time since the boulder sampled was uncovered by ice and exposed to cosmic rays and is taken here as a minimum age for the formation and stabilization of the moraine. All surface exposure ages here were calculated using the CRONUS-
Earth online exposure age calculator version 2.2 (Balco et al. 2008), using the northeastern North
American production rate developed by Balco et al. (2009), and the scaling scheme described by Stone
(2000) following Lal (1991). The erosion rates and sample densities input came from the original literature source (Table S1). Sample details and analytical results, as well as estimated ages, are provided in Table S1.
Varves are distinct layers of sediment that record variations in seasonal melt water input. Varves occur as seasonal couplets with summer varves that can vary in thickness due to proximity to the ice margin position; the closer to the ice margin, the thicker the varve. These varve sequences, if present in multiple sediment cores, such as along the Connecticut River Valley in New England (Ridge et al. 2012), can be correlated using stratigraphy as well as radiocarbon ages of organics within the sediment. Ridge et al. (2012) used organic materials within the varves to assign the Connecticut River Valley varve records an absolute age. This sequence tracks the New England region ice sheet margin.
Radiocarbon ages represent the amount of time that has passed since the dated organic material was
8 alive. Thus, they offer either maximum or minimum constraints on changes in glacier margin position. A radiocarbon age of material from within or below a glacial till provides a maximum age for advance, because the age represents organic material living before being incorporated by advancing ice or buried by proglacial deposits. Radiocarbon ages of organic material found in outwash deposits, kettle lakes, ice- walled lake plains and other basins are interpreted as minimum ages for glacial advance. Only radiocarbon ages with certain stratigraphic context were considered in this study. All 14C ages were recalibrated using CALIB 7.1 software, with the IntCal13 calibration curve (Reimer et al. 2013).
Radiocarbon ages are presented in the text and figures as the mean calibrated age (abbreviated as ka, thousands of years before 1950). The radiocarbon ages utilized here are presented in Supplementary
Table S2.
Optically stimulated luminescence (OSL) ages represent the time elapsed since sediment was buried and no longer exposed to light. In this study, these ages come from proglacial-lake sediments.
These must be considered in context as outwash can reflect both advance and retreat of the ice sheet edge.
Analysis results and sample information for all OSL ages are compiled in Table S3. In total, this study utilizes 82 10Be surface exposure ages, 6 varve-derived ice margin positions, 430 radiocarbon ages, and 9
OSL ages to reconstruct the behavior of the ice sheet.
2.3. Analysis
For each lobe, a time-versus-distance plot for each lobe is constructed to represent the advance and retreat pattern. Each chart features individual ages plotted along a flow line drawn parallel to the long axis of each lobe. The stratigraphic context of each sample was considered (maximum versus minimum age for ice sheet position) in order to “track” each lobe’s position over time. An interpretation of whether the lobe was advancing, retreating or if the behavior is unknown is included in the discussion section.
9 3. Results of Chronologic Data Compilation
3.1. New England Region
The ice sheet margin in the New England Region reached as far south as 41˚N, a position marked by a distinct moraine at present-day Long Island, Martha’s Vineyard and Cape Cod (Figure 2.3). Sirkin and Stuckenrath (1980) report maximum radiocarbon ages for organic material collected from within and below till exposed at sites near the New England region, such as the Harbor Hill moraine on Long Island.
Some of these ages fall numerically within HS2, however due to multiple age reversals within the stratigraphy, these ages have been excluded from this analysis.
In southeastern Massachusetts, three 10Be exposure ages of 26.3 ± 1.0, 25.5 ± 0.7, and 24.9 ± 0.7 ka come from boulders collected from the terminal moraine at Martha’s Vineyard (Site 1; Table S2; Balco et al. 2002). Consistent with the original authors (Balco et al. 2002), two outliers with relatively young ages of 22.8 ± 0.9 ka and 17.8 ± 0.7 ka were excluded. Four samples with exposure ages greater than 28.0 ka were excluded for this analysis because they fall outside the age range criteria outlined in the methods above. The three ages from Site 1 provide an average age of 25.5 ± 0.4 ka for the timing of moraine abandonment following the ice margin’s most extensive advance in New England. This scenario requires that the ice sheet was advancing before this time. Balco et al. (2002) also present ten 10Be exposure ages from the Buzzards Bay recessional moraine on Cape Cod, MA (Site 2), which lies about 15 km north of the terminal moraine, that range from 22.8 ± 0.8 to 19.1 ± 0.8 ka with an average of 21.3 ± 0.4 ka (Balco et al. 2002). Balco and Schaefer (2006) present fourteen 10Be ages from boulders atop recessional moraines in southern Connecticut, which range from 21.8 ± 0.7 to 19.7 ± 0.5 ka (Ledyard Moraine, Site
3) and 22.2 ± 0.6 to 19.9 ± 0.5 ka (Old Saybrook moraine, Site 4; Balco and Schaefer 2006, Balco et al.
2009). The average age for these two sites are 20.7 ± 0.3 ka and 21.1 ± 0.4 ka, respectively. Taken together, these ages suggest that advance of the Laurentide ice margin in the New England region occurred before 25.5 ± 0.4 ka, which is derived from the age of the Martha’s Vineyard moraine, which is the outboard location of the LIS local maximum in this region. The Buzzards Bay moraine was uncovered ~21.3 ± 0.4 ka, which suggests that retreat was underway by that time. Retreat continued over
10 the next several thousands of years, uncovering the Ledyard and Old Saybrook moraines in southern
Connecticut by ~20.7 ± 0.3 ka.
The present framework for New England’s subsequent deglacial history is based on the varve records presented by Ridge et al. (2012). This record is compiled from sediment outcrops and cores across the Connecticut River Valley and the age model is based on radiocarbon ages from these cores and supported by surface exposure ages discussed above. Between ~18.6 ± 0.2 and 17.3 ± 0.2 ka, the margin underwent retreat from its position at the Ledyard and Old Saybrook moraines in southern Connecticut
(Ridge et al. 2012; J. Ridge, personal communication). The pattern of retreat was interrupted by two minor readvances, the first being a ~30 km readvance ~17.3 ± 0.2 ka in southern Massachusetts (named the Chicopee Readvance). Ridge et al. (2012) also acknowledge a second readvance of unknown magnitude ~16.0 ± 0.2 ka in northern Massachusetts, termed the Hatfield Readvance.
In summary, the ice margin of the New England region advanced during the entirety of HS2
(Figure 2.4b). The ice remained near its maximum position until ~23 ka and continued retreating during
HS1, with the exception of two minor readvances ~17.3 and ~16.0 ka (Figure 2.4c).
3.2. Finger Lakes Region
Early work on the timing of glaciation in New York (Muller and Calkin, 1993) concluded that the maximum extent was reached between ~21.8 and ~18.6 ka, which is based on morphostratigraphy and correlation with nearby moraine systems. Recent work in the Finger Lakes region of New York, ~150 km north of the terminal moraine, yield minimum ages for retreat of the margin (Kozlowski et al. 2018).
These sites lie just north of the Outer Valley Heads moraine, which is interpreted as the product of a readvance ~17.0 ka (Muller and Culkin 1993). These new minimum ages range from 15.0 ± 0.1 to 13.7 ±
0.1 ka and suggest that the margin had retreated more than 150 km from its terminal position by ~15.0 ka.
This is consistent with the history of the New England region to the east. Although these ages fall strictly outside of our criteria, they can be taken to suggest ice sheet retreat in this sector during HS1.
11 3.3. Lake Huron-Erie Lobe
The Lake Huron-Erie Lobe advanced southward across Ohio from the Lake Huron and Erie basins and reached its southernmost extent at ~39° latitude, in southwestern Ohio (Figure 2.2). The lobe also extended westward into eastern Indiana and eastward into northeastern Ohio, northeastern Ohio (Figures
2.3, 2.5). The maximum timing for advance is constrained using radiocarbon ages of organics recovered both from within or beneath the glacial till.
Maximum radiocarbon ages for the initial advance of the Lake Huron Erie Lobe are from the
Garfield Heights site in northeastern Ohio (Fullerton 1972, Sumodi 1974), which range from 28.9 ± 1.5 to
26.4 ± 0.3 ka (Figure 2.5a, Site 26). Five radiocarbon ages from the Sidney RR Cut (Site 64; Forsyth
1965, Lowell et al. 2018) that range from 27.7 ± 0.2 and 26.9 ± 0.3 ka are consistent with those from the
Garfield Heights site. These ages imply that advance was underway prior to the start of HS2 (Figure
2.5b). The ice margin advanced over the Oxford, OH area, where seven radiocarbon ages of wood from the lowermost till exposed at the Oxford East site (Site 71a) range from 26.6 ± 0.7 to 24.7 ± 0.6 ka, which are maximum ages for the ice margin advance. This till is overlain by a distinct gastropod-rich silt unit, which is exposed at several sites including Oxford East (Site 71b) in Ohio. The presence of this silt unit, as well as the gastropods within, indicate a retreat of the ice margin and suggests temporarily warmer conditions. Radiocarbon ages from this unit provide a minimum bracketing age for this retreat, and a maximum age for the overlying till and associated advance. At Oxford East (Site 71b), ages range from
25.7 ± 1.4 to 24.2 ± 1.3 ka (Pigati et al. 2010). New radiocarbon ages from the silt layer underlying till, as observed in a sediment core from the Brush Creek Site in Indiana (Site 45), range from 24.8 ± 0.4 to
23.9 ± 0.3 ka. Based on stratigraphic correlation as well as the radiocarbon ages, the interstadial silt and upper till were both deposited during the second phase of HS2.
Radiocarbon ages of wood from till that occur stratigraphically higher than the Oxford East silt unit provide minimum ages for the end of this brief interstadial, and also provide maximum age constraint for a second advance. A till unit exposed at the Oxford West site (Site 70) and Oxford Middle site (Site 69) overlies the gastropod-rich interstadial silt and contains organics with radiocarbon ages that range from
12 25.7 ± 0.4 to 23.6 ± 1.5 ka (Ogden and Hay 1973, Suess 1954, Ekberg et al. 1993, Lowell and Brockman
1994). Consistent with these ages are seven ages from Milford Cemetery (Site 72) that range from 24.5 ±
0.5 to 22.5 ± 0.2 ka, and three ages from Doty’s Highbank (Site 67) which range from 26.5 ± 0.5 to 24.7
± 0.4 ka. Finally, seven radiocarbon ages from logs broken by the ice during an advance, recovered by the nearby Reily Site range from 27.3 ± 0.4 to 22.7 ± 0.3 ka (Site 73; Ekberg et al. 1993, Lowell and
Brockman 1994).
Lowell et al. (1999) summarize radiocarbon ages from four sites near Cincinnati, Ohio (Sites 68,
75-78; Figure 2.5c), which are interpreted as maximum ages for a second HS2-age advance based on their stratigraphic position (Goldthwait 1958, Lowell et al. 1990, Lowell and Brockman 1994, Lowell et al.
1999). These thirty-one ages range from 26.8 ± 0.6 to 23.1 ± 0.4 ka and are from wood either within or below the till unit at each site. Younger ages from south central Ohio come from the Chillicothe sites
(Sites 38, 39, 40), which range from 22.7 ± 0.7 to 21.2 ± 0.6 ka (Goldthwait 1958, Ogden and Hay 1964,
Goldthwait et al. 1981, Lowell et al. 1999). Similar radiocarbon ages from the nearby Hamilton (Site 74) range from 23.8 ± 0.7 to 22.6 ± 0.7 ka (Rubin and Alexander 1960, Olson and Broecker 1961). The
Snyder Farm site in Indiana (Site 66) also lies just within the late Wisconsin limit. Radiocarbon ages of wood collected there range from 24.5 ± 0.6 to 22.0 ± 0.5 ka (Gooding 1963, Hall et al. 1991, Hall and
Anderson 2000). Taken together, the chronology of the western Huron-Erie Lobe suggests two advances during HS2, separated by a minor retreat and interstadial conditions.
Glover et al. (2011) present a deglacial chronology for Ohio and Indiana, which includes thirty radiocarbon ages from post-glacial organic sediments deposited following retreat of the East, White,
Miami and Scioto sublobes across eastern Indiana and central and western Ohio (Figure 2.5). These ages provide a minimum constraint for retreat and range from 19.8 ± 0.4 to 14.2 ± 0.7 ka. Retreat was underway in some parts of Ohio before 19.8 ± 0.4 ka, during early HI (23.5-17.5 ka). This is based on the oldest ages which come from Mechanicsburg (Sites 15, 16), Murphy’s Bog (Site 13), Farm Pond (Site
12).
13 3.4. Lake Michigan Lobe
The Lake Michigan Lobe flowed through the present Lake Michigan basin to its terminal position in central Illinois (Figure 2.2). The lobe also fed the Harvard Sublobe to the north, which flowed westward from the Lake Michigan Basin into northern Illinois and southern Wisconsin. Minimum and maximum radiocarbon ages of organics constrain the timing of advance and retreat during the Heinrich
Stadials and HI. To track the glacier margin only, we exclude radiocarbon ages from intermediate till, loess, glaciolacustrine, and outwash units in the Lake Michigan Lobe. This means radiocarbon ages from three units were used here - ages from two loess units that underlie the entire sequence as maximums for advance, and the ice-walled lake plain deposits at the surface as minimums for retreat.
3.4.1. Lake Michigan Lobe chronology
The Robein Member of the Roxana Silt and the Morton Tongue of the Peoria Silt directly underlie the Wedron Group of glacial tills. Therefore, radiocarbon ages of organics from these units provide a maximum constraint for advance. Sites located near the outer moraine belts of the Lake Michigan Lobe in central Illinois (Sites 102-108, 110-112, 114-123, 125, 126, 129; Figure 2.6) yield 37 radiocarbon ages that range from 29.0 ± 1.9 to 23.3 ± 0.5 ka (Rubin and Alexander, 1958; Kim, 1970; Kempton and Gross,
1971; Johnson et al., 1971; Coleman, 1973; Liu and Coleman 1981; Johnson and Hansel, 1990; Hansel and Johnson 1996; Grimley et al. 2016), with one anomalously young age of 21.7 ± 0.5 ka (Site 118; this study). These ages provide a maximum constraint for advance of the Lake Michigan Lobe into south central Illinois. Ages from the same silt units exposed in northeastern Illinois (Sites 91, 93, 95, 101 and
113) range from 29.0 ± 0.5 to 27.2 ± 2.0 ka (Kempton and Hackett, 1968; Curry, 1989; Hansel and
Johnson, 1996; Johnson and Hansel, 1990; Curry et al. 1999; Curry, 2015).
Two noteworthy readvance events in Lake Michigan Lobe history are recorded in units overlying that Peoria and Roxana Silts. Evidence from northern Illinois and southern Wisconsin suggests a substantial retreat (> 100 km) after ~23.0 ka, based on sediments associated with glacial Lake Milwaukee.
Additional radiocarbon ages from some of the intertonguing tills of Lake Michigan Lobe stratigraphy
14 suggest a substantial readvance by ~18.7 ka during HS1, known locally as the Crown Point Phase (Curry et al. 2018; B.B. Curry, personal communication).
Minimum ages for retreat of the Lake Michigan Lobe in Illinois come primarily from ice-walled lake plain deposits (Table S1, Figures 2.6, 2.7) as noted below. These former lakes formed in depressions on dead-ice permafrost (Curry 2008, Clayton et al. 2008, Curry et al. 2010). Named DeKalb mounds after their type location in DeKalb County, Illinois, their stratigraphy commonly consists of a basal sand and gravel layer overlying till. This sequence is then overlain by organic-rich laminated lake sediment, another sand and gravel, and capped by loess. During deglaciation, as the ice stagnated and melted, these sediments were eventually let down, leading to inverted topography (Curry 2008, Clayton et al. 2008,
Curry et al. 2010). Radiocarbon ages of leaves and stems of tundra plants, such as Dryas integrifolia
(Curry and Yansa, 2004) yield minimum ages for the formation of the lake on the dead-ice permafrost surface, and therefore the stagnation and down-wasting of the ice itself. These ages are close minimum brackets because sedimentation occurs directly in contact with the ice (Curry et al. 2010). Ages on organics from ice-walled lake plains typically are from 500 to 5,000 yrs older than ages of organics from the basal “trash zone” of kettle lake successions located on or adjacent to the same moraine (Curry et al.,
2018).
We will discuss the chronologic data from IWLP sites starting at the outermost moraine belt, and progress northeastward, towards the youngest moraine belt near the present shore of Lake Michigan. The type DeKalb mounds in northern Illinois are dated by radiocarbon ages from three sites: Burlington,
Hampshire and Schnucks (Sites 88, 87, and 94, respectively). The Burlington site yields eight ages, which range from 22.3 ± 0.2 to 18.4 ± 0.2 ka (Curry 2008). The Hampshire site yields two ages of 21.3 ±
0.7 and 20.2 ± 0.3 ka (Curry 2008), and the Schnucks site yields just one age of 26.0 ± 0.3 ka. The nearby Paw Paw Moraine has two IWLP sites that yield four radiocarbon ages that range from 25.4 ± 0.3 to 21.1 ± 0.3 ka (Sites 96 and 98). The Gilman moraine in the central LML footprint has two IWLP sites that yield three radiocarbon ages that range from 22.1 ± 0.3 to 21.6 ± 0.3 ka (Sites 127 and 128).
Similarly, two IWLP sites along the Marseilles moraine complex, which lies ~50 km west of the terminal
15 moraine (Sites 99 and 100; Curry and Petras 2011), yield 17 radiocarbon ages that range from 22.2 ± 0.2 to 21.1 ± 0.2 ka. One site from Minooka Moraine IWLP (Site 96) yields two ages of 25.4 ± 0.3 and 21.1
± 0.3 ka. Finally, the Valparaiso Moraine, which wraps around the southern end of Lake Michigan, yields two IWLP sites: Crabtree Nature Center and South Barrington (Sites 89 and 92, respectively). The
Crabtree Nature Center site yields two ages of 18.1 ± 0.2 and 18.0 ± 0.2 ka, and the South Barrington site yields two ages of 18.1 ± 0.2 and 16.8 ± 0.2 ka (Curry, 2015). Taken together, these radiocarbon ages from IWLP record the timing of stagnation of the Lake Michigan Lobe (Figure 2.6, 2.7). With the exception of the ages from the outermost moraine belt (Sites 88, 87 and 94), which range from 26.0 ± 0.3 to 18.4 ± 0.2 ka, the ages tend to decrease with more distance from the outermost moraine (Figure 2.6b).
This suggests a steady retreat of the margin over the course of HI (23.5-19.3 ka) and HS1 (19.3-15.3 ka).
3.4.2. Harvard Sublobe chronology
The Harvard Sublobe moraines are discontinuous with and cross-cut the other major moraine belts of the Lake Michigan Lobe (Curry and Yansa, 2004). This flow line is significantly shorter than that of the Peoria Sublobe (Figure 2.6). The sites are described from west to east. Maximum limiting radiocarbon ages for the Harvard Sublobe advance also come from the Robein Member of the Roxana Silt and the Morton Tongue of the Peoria Silt. These three ages range from 28.8 ± 0.8 to 27.1 ± 0.7 (Sites 79,
80, 86; Curry and Pavich 1994).
Minimum radiocarbon ages associated with the west-ward flowing Harvard Sublobe come from ice-walled lake plain deposits with radiocarbon dated organic material in northeastern Illinois. These sites are discussed from west to east. Two sites from IWLPs along the Woodstock moraine IWLPs (Sites
82 and 83, Figure 2.7) yield five radiocarbon ages of that range from 20.2 ± 0.2 to 17.1 ± 0.2 ka (Curry,
2015; this study). To the east, IWLP deposits on the Barlina Moraine yield sixteen radiocarbon ages from the Bryn Mawr Site (Site 90; Figure 2.7), which range from 19.1 ± 0.2 to 17.6 ± 0.2 ka (Curry et al.
2018). The third site, Mill Valley, yields three ages that represent the Tinley Moraine IWLP, and range from 17.2 ± 0.2 to 17.1 ± 0.2 ka (Site 84, Curry 2015). Finally, three ages from the Wadsworth Town
16 Hall Site (Site 85, Curry and Petras 2011) range from 16.8 ± 0.2 to 16.5 ± 0.2 ka. These radiocarbon ages are taken as minimum ages for the timing of retreat of the Lake Michigan Lobe.
Taken together, these minimum radiocarbon ages from the Lake Michigan Lobe suggest that by
HS1, the lobe had undergone significant stagnation and retreat. It is apparent from the Harvard Sublobe transect (Figure 2.7b) that ice stagnation and subsequent IWLP formation within this sector of the Lake
Michigan Lobe continued through HS1.
3.5. Green Bay Lobe
The Green Bay Lobe was fed by ice flowing through the Lake Superior basin and western Lake
Michigan basin into eastern Wisconsin, where it deposited belts of both north-south and east-west trending moraines (Figure 2.8). These moraines are traced into northern Wisconsin, where they converge with east-west trending moraines that mark the outermost extent of the Chippewa lobe, discussed in the next section. An extensive intermorainic area divides the Green Bay and Lake Michigan Lobes, known locally as the “Kettle Moraine”. The ages of the Green Bay Lobe moraines are dated using surface exposure ages from three locations along the outermost moraine. Colgan et al. (2002) present six 10Be exposure ages from the Waterloo site along the southern Green Bay Lobe margin (site 142). These ages range from 25.4 ± 2.3 to 19.9 ± 1.4 ka. Ullman et al. (2014) present additional 10Be exposure ages from two sites in the same region (Sites 130 and 131). The outermost site (Site 130) yields samples with exposure ages that range from 26.7 ± 1.3 to 16.2 ± 1.3 ka and produce an average age of 22.1 ± 1.4 ka.
The innermost site (Site 131) yields boulder samples with exposure ages of 19.4 ± 0.9 ka and 19.3 ± 1.1 ka and a mean age of 19.4 ± 0.1 ka (Ullman et al. 2014).
Along the northern Green Bay Lobe terminal moraine, Colgan et al (2002) reports four exposure ages from the Cactus Rock site that range from 19.9 ± 1.3 to 18.5 ± 1.7 ka (Site 143). A nearby site (Site
140) yields boulder samples with surface exposure ages that range from 27.0 ± 1.0 to 19.7 ± 0.9 ka with an average of 23.4 ± 0.8 (Ullman et al. 2014). Another site just 15 km north of this moraine yields four samples with ages that range from 19.9 ± 0.9 to 16.5 ± 0.9 ka (Site 141; Ullman et al. 2014) which
17 produce an average age of 18.4 ± 0.8 ka.
The retreat history of the Green Bay Lobe is also constrained by both OSL and radiocarbon ages from former proglacial lake environments. The OSL ages come from two sites high in the Baraboo Hills:
Feltz Basin and South Bluff Basin (Sites 133 and 134, respectively; Attig et al. 2011). The Baraboo Hills mark the outermost extent of the Green Bay Lobe in central Wisconsin. Both of these sites are ideal for capturing the retreat history of the Green Bay Lobe because they can only contain lakes when dammed by ice on all sides. Therefore, any lake sediment recovered from these two sites was deposited while the
Green Bay Lobe occupied the Johnstown Moraine position around the Baraboo Hills. The OSL ages from these sites come from the top of the lake sediment package, meaning they represent a maximum timing for GBL retreat from its outermost position. The uppermost age from each site is taken as the constraining age. These ages are 20.2 ± 2.3 ka and 18.5 ± 1.7 ka (Sites 133 and 134, respectively; Attig et al. 2011) and suggest that the lobe remained at this location until after 18.5 ± 1.7 ka. Carson et al. (2012) present a similar set of OSL ages from the southern Green Bay Lobe margin, however these sites lie outside the maximum glacial limit and therefore are excluded from this analysis.
The retreat history of the Green Bay Lobe is also constrained by 14C ages of proglacial lake-bottom sediments. One such site, called Hook Lake (Site 137), is a post-glacial lake deposit within 10 km outermost moraine of the southern Green Bay Lobe. Radiocarbon ages of 19.2 ± 0.4 and 14.6 ± 0.5 ka from this lake core record also provide a minimum age constraint for the retreat of the lobe (Steventon and Kutzbach 1985). Five additional sites that lie further inboard of the outermost moraines (10-20 km) yield nine additional minimum radiocarbon ages (Sites 132, 135, 136, 138 and 139). These ages range from 17.7 ± 0.2 to 15.0 ± 0.5 (Bender et al. 1965, Black 1976, Winkler et al. 1986, Maher and Mickelson
1996, Maher et al. 1998). The Devil’s Lake site in the same vicinity (Maher 1982) was excluded due to uncertain stratigraphic context – it is unclear whether that lake sediments were deposited during advance or retreat. Maher (1982), however, suggested that a basal radiocarbon age of 14.7 ka represents a minimum age for the retreat of the GBL.
Taken together, the exposure age chronology of the Green Bay Lobe suggests that the ice margin
18 underwent advance prior to ~23.4 ka, which is based on the average age of the terminal northern Green
Bay Lobe moraine at site 140. Ages from sites that lie inboard of the terminal moraine (Sites 131 and
141) suggest that retreat of the Green Bay Lobe was underway by 19.2 ± 0.4 ka. The available OSL ages and minimum radiocarbon ages are in agreement with this timing of retreat initiation, suggesting that retreat started by 19.2 ± 0.4 ka.
3.6. Chippewa Lobe
The Chippewa Lobe flowed south through the western Lake Superior basin and terminated in northern Wisconsin. This lobe is plotted with the nearby Green Bay Lobe in Figure 2.8 to highlight their close proximity and continuous moraine systems. Ullman et al. (2014) report six 10Be exposure ages from a site along the terminal Chippewa Lobe moraine (Site 144), which range from 25.6 ± 1.1 to 20.6 ± 0.7 ka, with an average of 22.8 ± 0.9 ka (Figure 2.8a). These ages represent the timing of moraine abandonment and suggest that lobe advanced to and reached its maximum position prior to ~22.8 ka, which falls during HS2. Ullman et al. (2014) also present two 10Be ages of 20.5 ± 0.9 and 19.5 ± 0.7 ka from a recessional moraine which lies about 10 km north of the terminal moraine (Site 145). This site has an average age of 20.0 ± 0.5 ka, which suggests that retreat was underway by this time.
Ullman et al. (2014) report seven ages from a recessional moraine in the Gogebic Mountains, which lie ~100 km north of the terminal moraine in northern Wisconsin (Site 146; Figure 2.8a). These ages range from 14.4 ± 0.8 to 11.9 ± 0.6 ka, which produce an average age of 13.2 ± 0.4 ka. With the exception of one outlier of 17.2 ± 3.2 ka, these ages are all younger than HS1. In order for the ice margin to reach this position by this time, it requires a combined retreat of ~100 km through HI (23.5-19.3 ka) and HS1.
3.7. Des Moines Lobe
The Des Moines Lobe (DML) flowed southward through Minnesota and terminated in central
19 Iowa. Several workers (Flint 1955, Ruhe 1969, Kemmis et al. 1981, Clayton and Moran 1982, Bettis et al. 1996) have made significant contributions to understanding this region’s glacial history. The timing of
DML advance is constrained by maximum ages from a glacial till unit (Dows Formation) and the underlying alluvium (Ruhe 1969, Kemmis et al. 1981, Hallberg and Kemmis 1986), and minimum- limiting radiocarbon ages of organics from overlying outwash. The oldest radiocarbon ages of 24.1 ± 1.7 ka (Site 171; Ruhe 1969) comes from a till unit known locally as the Tazewell till, which underlies the
Dows Formation and alluvium. This radiocarbon age is from the outer moraine of the Des Moines Lobe and suggests an advance during HS2.
Benn and Bettis (1981), Bettis and Hoyer (1986) and Lowell et al. (1999) report radiocarbon ages on wood recovered from an alluvium unit that underlies the Dows Formation exposed at Saylorville
Spillway (Site 168; Table S2). These ages range from 20.5 ± 0.4 to 15.7 ± 0.4 ka, and are maximum ages for the advance of the lobe to its southernmost extent. Ruhe (1969) and Bettis et al. (1996) report additional radiocarbon ages from below the Dows Formation at the National Gypsum site (Site 156), which range from 18.5 ± 0.4 to 16.2 ± 0.7 ka. An additional 14 ages come from a unit underlying the
Dows Formation exposed at 10 sites (Sites 155, 160-167, 169) range from 20.6 ± 1.2 to 14.4 ± 1.4 ka
(Ruhe 1969, Bettis and Hoyer 1986). These ages represent organic material in alluvium that formed prior to the expansion of the Des Moines Lobe and the associated deposition of the Dows Formation till.
Finally, radiocarbon ages of organics collected from within the Dows Formation at seven additional sites also afford a maximum age for timing of Des Moines Lobe expansion. These eight ages range from 17.5
± 0.5 to 14.8 ± 0.8 ka (Sites 149, 150, 154, 157, 158, 159; Ruhe 1969, Kemmis et al. 1981, Bettis et al.
1996).
Minimum ages that constrain the timing of Des Moines Lobe advance come from post-glacial lake cores and radiocarbon ages of organics collected from outwash sediments. Each of these sites is discussed in order of stratigraphic position, from oldest to youngest. A radiocarbon age of 17.6 ± 0.9 ka from the
McCulloch Bog Site (Site 153; Figure 2.10a; Ruhe 1969) and one age of 17.0 ± 0.4 ka from Little Millers
Bay (Site 147; Bettis et al. 1996) were both collected from material that overlies the Dows Formation
20 directly, although both are much closer in age to the maximum ages collected from within the actual till.
Other minimum ages from the DML include two ages of 15.5 ± 0.8 and 15.3 ± 0.9 ka from outwash exposed at Britt (Site 151, Ruhe 1969), an age of 16.7 ± 0.8 ka from Colo Bog (Site 170; Ruhe 1969), and one age of 14.6 ± 0.6 ka from the Zuehl Farm pollen site (Site 152, Bettis et al. 1996). One additional sample from this outwash unit that overlies the Dows Formation exposed at Saylorville Spillway yields an age of 14.0 ± 0.2 ka (Site 168; Benn and Bettis 1981). Finally, two radiocarbon ages of 14.0 ± 0.5 and
13.8 ± 0.3 ka come from peat that overlies this outwash unit at Whittemore (Site 148, Bettis et al. 1996).
Taken together, there is substantial overlap between the maximum and minimum ages from the Des
Moines Lobe. The maximum ages range from 24.1 ± 1.7 to 14.4 ± 1.4 ka, and the minimum ages range from 17.6 ± 0.9 to 13.8 ± 0.3 ka. The oldest maximum age of 24.1 ± 1.7 ka suggests an advance during
HS2. The presence of the overlying alluvium unit and associated radiocarbon ages suggests a possible retreat of up to 100 km after 24.1 ± 1.7 ka at the start of HI. This is followed by a readvance to the Des
Moines Lobe’s maximum position by 16.2 ± 0.3 ka, based on the youngest maximum age at Saylorville
Spillway (Site 168). This advance culminated at the end of HS1 and was followed by a rapid retreat.
This is an intriguing difference from the history of the rest of the LIS margin explored above. During
HS1, the Des Moines Lobe underwent advance, and did not retreat until after HS1.
4. Summary
The ice sheet margin in the New England region reached its maximum extent (Site 1) by ~25.5 ±
0.4 ka, during the first phase of HS2. The margin remained relatively stable through the second phase of
HS2, and underwent retreat starting by ~21.3 ± 0.4 ka. The Lake Huron-Erie Lobe advanced through
HS2, underwent minor retreat between 25.2-25.0 ka, and then readvanced to its maximum extent. The
Lake Michigan Lobe in Illinois advanced through both phases of HS2 and reached its maximum extent during the first phase of HS2 and retreat generally coincided with the end of HS2. The Green Bay Lobe advanced to its maximum position by 23.4 ± 0.8 ka. Limited radiocarbon ages from the Green Bay Lobe suggest that retreat may have been underway by the end of the second phase of HS2. Available
21 cosmogenic surface exposure ages suggest that the Chippewa Lobe advanced through Heinrich Stadial 2 and reached its maximum position mid-stadial.
By the start of HS1 ~17.5 ka, the ice sheet margin in the New England region had retreated about
100 km from its maximum position. Retreat continued through HS1, with the exception of two minor readvances in central Massachusetts at ~17.3 and ~16.0 ka (Ridge et al. 2012), the second of which is coincident with the advance of the Des Moines Lobe. Both the Lake Huron-Erie Lobe and Lake
Michigan Lobe exhibited similar behavior, having retreated nearly 150 km by that start of HS1. The
Green Bay Lobe’s behavior during HS1 is unclear due to the large range of ages and there is limited age control directly during the HS1 interval. The Chippewa Lobe likely retreated through HS1, based on the data available. The only lobe along the southern Laurentide margin that did not undergo retreat was the
Des Moines Lobe, which advanced during HS1, reaching its maximum position ~16 ka, near the end of
HS1. The lobe retreated rapidly just after this maximum.
For convenience, the behavior of each lobe is summarized in Figure 2.11 as either a blue bar for advance or a red bar for retreat.
5. Discussion
The following sections include a discussion of the overarching patterns of LIS behavior during
Heinrich Stadials. First, the original research question is revisited. Second a comparison of the behavior of the LIS during HS2 and HS1 is made. Third, the behavior of the LIS is compared with that of other
Northern Hemisphere ice sheet systems. Finally, a hypothesis is introduced to explain the observed differences in ice sheet behavior between HS2 and HS1 (Figure 2.11), as well as the results of the
Northern Hemisphere ice sheet comparison.
22 5.1. Did the terrestrial margin of the southern Laurentide Ice Sheet advance during cold phases of sea surface temperature (Heinrich Stadials 2 and 1) in the adjacent North Atlantic?
In brief, the Laurentide Ice Sheet exhibited inconsistent behavior during Heinrich Stadials 2 and
1 in the North Atlantic. HS2 (time of cold SST) is coincident with advances of the New England region,
Lake Huron-Erie Lobe, Lake Michigan Lobe, Green Bay Lobe and Chippewa Lobe. The end of HS2 is marked by the initiation of retreat for these same five lobes, and their retreat continued through the following HI. During HS1, the New England region, Huron-Erie Lobe, Lake Michigan Lobe, Green Bay
Lobe and Chippewa Lobe underwent overall continued retreat. The exception being minor readvances of the New England region and Lake Michigan Lobe. In contrast, HS1 is the time of maximum extent of the
Des Moines Lobe, which underwent advance only at the end of HS1, followed by very rapid retreat.
Overall, HS2 is characterized by advance of the southern Laurentide margin, but following HS2, the ice sheet retreated and that retreat continued through HS1. This pattern is inconsistent with modulation of the ice sheet by SST, however it reveals a pattern of asynchronous behavior across the terrestrial LIS margin.
5.2. Comparison with other Northern Hemisphere ice sheets
In this section, the glacial history of the southern Laurentide margin is compared with that of the nearby Cordilleran Ice Sheet of western Canada and the European Ice Sheets, respectively.
5.2.1. Cordilleran Ice Sheet
Radiocarbon ages from the southernmost lobe of the Cordilleran Ice Sheet (CIS) suggest a relatively late timing of maximum ice extent. The Puget Lobe of the Cordilleran Ice Sheet flowed southward, terminating in the southern Puget lowland in Washington State. The lobe reached its maximum extent during HS1, ~17.0 ka based on maximum radiocarbon ages (Porter and Swanson 1998).
Minimum radiocarbon ages from surficial outwash sediments suggest that retreat was underway by mid-
HS1 time. A time distance curve reconstructed for the lobe suggests that, like the Des Moines Lobe, the
Puget Lobe only remained at its maximum position for about ~100 years. This is consistent with the Des
23 Moines Lobe record, which suggests that the lobe reached its maximum during HS1 and underwent a very rapid retreat. This implies that the same climate conditions affecting the Des Moines Lobe and causing a relatively late advance, are also affecting the southern Cordilleran Ice Sheet.
5.2.2. British-Irish and Scandinavian Ice Sheets
The British-Irish and Scandinavian Ice Sheets (BIIS and SIS, respectively; Figure 2.1) are important in this discussion because of their proximity to the North Atlantic, with the possibility that they were susceptible to downstream effects associated with changes in ocean temperature. Hughes et al.
(2016) summarize the timing of maximum extent for these two individual ice sheets, as well as for the entire Eurasian Ice Sheet complex. Overall, these ice sheets had all reached their maximum extent by
~21.0 ka, with some reaching their maximum extent as early as ~27.0 ka. Toucanne et al. (2015) explore the retreat history of the BIIS and SIS using mass accumulation rates (MAS) and isotopic analysis from sediment cores in the Bay of Biscay and the North Atlantic. These accumulation rates allow for the reconstruction of meltwater discharge via the Channel River over the last glacial period and termination
(Toucanne et al. 2009). Toucanne et al. (2015) identify meltwater output peaks between 22.5 ± 0.2 and
21.3 ± 0.2 ka, between 20.3 ± 0.2 and 18.7 ± 0.3 ka, and between 18.2 ± 0.2 and 16.7 ± 0.2 ka. These three peaks occurred during Heinrich Interstadial and through the beginning of HS1. This suggests that the BIIS and SIS underwent retreat during the HI, which implies that the pattern of HS1-time retreat spans the North Atlantic.
The Scandinavian Ice Sheet (SIS; Figure 2.1) in particular is of interest because like the
Laurentide, it had a terrestrial southern margin that advanced and retreated during the last glacial period and deglaciation. Stroeven et al. (2016) provide a review of radiocarbon, surface exposure and OSL ages for the SIS and summarize the chronologic history as distinct moraine-building phases. Advance towards the southernmost margin initiated by ~25.0 ka (Ehlers et al. 2011) and the maximum extent (known locally as the Brandenburg-Lezno phase) was reached by ~22.0 ka. The ice margin then underwent a minor retreat, named the Frankfurt/Poznań phase, at ~20-19 ka. The margin retreated again, eventually
24 stabilizing between ~18.0-17.0 ka to build moraines associated with the Pomeranian phase. Finally, the
Mecklenburgian phase at ~16.0 ka marks the last period of moraine building event before the ice margin retreated into the modern Baltic Sea. Other studies (i.e. Rinterknecht et al. 2014, Marks 2012) suggest slightly different ages for each of these phases, but they vary by at most ~1-2 ka. Overall, chronologic data from the southern SIS in Germany and Poland suggest that maximum extent was reached at the end of HS2 ~22 ka and retreated through the HI and HS1 (Houmark-Nielsen et al. 2003). This overall retreat was interrupted by advances or brief stabilizations of the ice margin, allowing for moraine building throughout HS1. However, this retreat is coterminous with meltwater output into the Bay of Biscay through the first part of HS1 (Toucanne et al. 2015), which originated from both the BIIS and SIS and indicates a pattern of overall retreat through this time.
This pattern is remarkably similar to that of the eastern lobes of the Southern Laurentide. It appears that HS2 was a time of overall advance in both ice sheets, and that HS1 was a time of retreat. Not only does this suggest the possibility of similar climatic controls on the terrestrial margin of both ice sheets, but also highlights the differences between HS2 and HS1 in terms of ice sheet behavior.
5.3. Hypothesis for differences in Heinrich Stadials
Modeling studies (e.g. Bromwich et al. 2004) have suggested that during the Last Glacial
Maximum (LGM), coincident with HS2 time, the winter Northern Hemisphere jet stream split around the
Laurentide Ice Sheet, occupying a strong southerly track along the southern LIS margin, and a weaker northerly track across the Canadian Arctic (Bromwich et al. 2004). This southern track is associated with moisture advection from the Gulf of Mexico and the northern track is associated with cold air advection over the accumulation zone of the LIS (Bromwich et al. 2004). Southerly displacement of the jet stream across central North America during this time may be reflected in pluvial lake levels in the southwestern
United States (Munroe and Laabs 2012).
Changes in jet stream position and associated precipitation patterns, however, were even stronger during Heinrich Stadial 1. During HS1, the Intertropical Convergence Zone (ITCZ) in the North Atlantic
25 shifted southwards in response to sea ice expansion (Denton et al. 2010). The effects of this shift are observed in paleoclimate records across the Northern Hemisphere. Evidence includes a weakened Asian monsoon (Wang et al. 2001), increased precipitation in the southwestern United States as recorded by speleothems (Wagner et al. 2010, Asmerom et al. 2010) and pluvial lake highstands in the western United
States (McGee et al. 2012; Munroe and Laabs 2013). These precipitation changes in this region may have been enhanced by a southward displacement of the westerlies, leading to a southwest-northeast precipitation gradient that strengthened during HS1 (Oster et al. 2015).
Based on the patterns of advance and retreat during HS1, one might also suggest an east-west trending summer temperature or precipitation gradient across north-central North America. Coupled with increased precipitation in the western United States, cooler temperatures in the west and warmer temperatures in the east would allow for such different behavior of the western and eastern sectors of the
LIS margin. This is a rough approximation of direction of course, because there are very few present reconstructions of the equilibrium line altitude of the Laurentide Ice Sheet domes during the last glacial period. However, in order for there to be advance in the western sector and retreat in the eastern sector, summer temperatures were probably warmer in the east than in the west, leading to enhanced melting
(retreat) in the east and reduced summer melting (advance) in the west.
This hypothesis is supported by paleoclimate records from the central United States. Pollen records (Liu et al. 2013, Jones et al. 2017) from Missouri, Kentucky and Tennessee (just south of the
Lake Michigan and Huron-Erie Lobes) indicate a decline in conifer pollen and increase in oak during HI
(23.5-17.5 ka) ~19 ka, indicative of warmer and wetter conditions. In order to further test this hypothesis, paleotemperature and paleoprecipitation records from near the western margin of the Laurentide Ice Sheet in Iowa and South Dakota are needed.
26
6. Conclusions
1. All but one lobe of the southern Laurentide Ice Sheet underwent advance during HS2, and retreat
during HI and HS1. The Des Moines Lobe of the western Laurentide margin is the outlier, reaching
its maximum extent during HS1;
2. Significant decrease in sea surface temperature does not directly affect the behavior of adjacent
terrestrial ice sheet margins. However, an emergent hypothesis developed from these observations
suggests that a temperature or precipitation gradient extended roughly east-west across the southern
Laurentide Ice Sheet during HS1, caused by southerly suppression of the westerly wind belts over
North America. In this scenario, a ridge-trough system with meridional flow (rather than zonal)
could explain drastic temperature differences over North America. This is supported by
paleotemperature reconstructions derived from pollen records from the mid-continent;
3. Paleoclimate records from the western United States, close to the former Laurentide margin, are
necessary to determine whether either a temperature or precipitation gradient existed across central
North America during Heinrich Stadials 2 and 1.
Acknowledgements: The authors appreciate comments by C. Smith, two anonymous reviewers and
Editor N. Glasser, all of which greatly improved this manuscript. G. Balco, J. Ridge, P. Colgan, and P.
Bierman provided helpful assistance with data acquisition and clarification. The Gary C. Comer Science and Education Foundation provided financial support for field work and for a portion of the radiocarbon analyses reported here.
27
Tables
Table 2.1. Heinrich Stadial timing raw data. Compilation of marine sediment cores and proxies used to determine timing of Heinrich Stadials 2 and 1. The core ID, location and reference are provided. The start and end depth columns refer to the core depths at which temperature change was interpreted as transitions in and out of Heinrich Stadials 2 and 1. These depths were used to determine stadial timing.
28 Figures
Figure 2.1. Map of the Northern Hemisphere ice sheets during the last glacial period (LIS =
Laurentide Ice Sheet, GIS = Greenland Ice Sheet, CIS = Cordilleran Ice Sheet, BIIS = British-Irish Ice
Sheet, SIS = Scandinavian Ice Sheet). Black squares represent locations of North Atlantic marine cores used in reconstructing Heinrich Stadial timing (Table 2.1). Box over central North America represents area detailed in Figure 2.3. Modified from Dyke (2004).
29
Figure 2.2. Analysis of Heinrich Stadial timing based on results from SST records compiled in
Table 1. The first graph on the left is the relative probability of a given time slice being included in
Heinrich Stadials 2 or 1. This probability is represented in shades of grey on the right. Dashed boxes outline the interpreted age range for HS2 (26.5 to 23.5 ka) and HS1 (17.5-15.5 ka). Heinrich Interstadial
(HI) is also indicated from 23.5 to 17.5.5 ka.
30
Figure 2.3. Map of Southern Laurentide Ice Sheet margin. Lobes of the southern Laurentide Ice
Sheet. The bold black line represents the outermost moraine of each lobe (after Flint 1955, Mickelson and
Colgan 2003). Dashed black lines represent the approximate boundaries between Lake Michigan
Lobe/Huron Erie Lobes and Chippewa/Green Bay Lobes. Solid blue boxes indicate locations of following figures.
31
Figure 2.4. New England Margin Chronologic Data. a) Map featuring a digital elevation model
(DEM) of the New England region in Massachusetts and Connecticut. The DEM was generated from
NASA’s Shuttle Radar Topography Mission (SRTM), with a sampling of 3 arc-seconds or 90 m. Ages are reported in thousands of years before present (ka) and correspond to sites pictured on map and presented in Supplementary Table 1S. b) Time/distance plot of the New England region using data presented on map. Transect line for ice margin position interpretation is the dotted line. Blue bars represent Heinrich Stadials 2 and 1, while red bar represents the intervening warmer period here called
Heinrich Interstadial. HS2 has been divided to provide visual representation of possible warming during period.
32
Figure 2.5. Lake Huron-Erie Lobe Chronologic Data a) Map featuring radiocarbon ages of the Lake
Huron-Erie Lobe in Ohio and Indiana. Ages are reported in calibrated years before present (ka). Upright blue triangles represent maximum radiocarbon ages, while downward-pointing red triangles represent minimum radiocarbon ages. b) Time/distance plot with Huron-Erie Lobe ice margin position interpretation superimposed as a dotted line. Blue bars represent Heinrich Stadials 2 and 1 cold periods, and red shaded horizontal bar represents Heinrich Interstadial warm period.
33
Figure 2.6. Southern Lake Michigan Lobe Chronologic Data a) Map featuring DEM of the southern
Lake Michigan Lobe in Illinois. b) Interpretation of time/distance plot with Lake Michigan Lobe ice margin position as dashed line. Symbols correspond to key in figures 2.5 and 2.6a. Symbols and figure format is the same as in Figure 2.5.
34
Figure 2.7. Northern Lake Michigan Lobe Chronologic Data. a) Map featuring a DEM of the northern
Lake Michigan Lobe in Illinois and Wisconsin. This map includes the Harvard Sublobe as well as some ages included in Figure 2.6 (southern Lake Michigan Lobe time-distance). b) Time/distance plot with
Harvard Sublobe ice margin position interpretation superimposed as a dotted line. Symbols and figure format are the same as Figure 2.5.
35
Figure 2.8. Green Bay and Chippewa Lobe Chronologic Data. a) Map featuring DEM of Green Bay
Lobe and Chippewa Lobe glacial deposits in Wisconsin. c) Time/distance plot of Green Bay Lobe with ice margin position interpretation superimposed as a dotted line. Symbols and formatting are the same as in Figures 2.4 and 2.5. Black dots represent OSL ages.
36
Figure 2.9. Chippewa Lobe Time/distance plot. Time/distance plot of the chronologic from the
Chippewa Lobe in Wisconsin using data presented in the map of Figure 2.8a with ice margin position interpretation superimposed as a dotted line. Symbols and formatting is the same as in Figure 2.4.
37
Figure 2.10. Des Moines Lobe Chronologic Data. a) Map featuring DEM of Des Moines Lobe deposits in Iowa. b) Time/distance plot with ice margin position interpretation superimposed as a dotted line. Symbols and format are the same as in Figure 2.6.
38
Figure 2.11. Advance and retreat history for the individual lobes of the southern LIS margin.
Summary diagram for the six lobes of the southern terrestrial margin of the Laurentide Ice Sheet. Vertical blue bars represent advance, and vertical red bars represent retreat. The horizontal blue shaded bars represent times of cold North Atlantic SST (Heinrich Stadials), and the horizontal red shaded bars represent times of warm North Atlantic SST. These patterns were derived from the discussion of chronologic data from each lobe.
39
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47 Chapter 3
Assessing the utility of surface exposure age dating of fluctuations of the James Lobe, South
Dakota2
ABSTRACT
The Laurentide Ice Sheet was the largest ice sheet of the last glacial period that culminated in an extensive terrestrial margin. This margin is thought to have exhibited asynchronous behavior between the eastern and western sectors during the last glacial period. However, the westernmost James Lobe remains poorly dated, preventing full understanding of this possible asynchronous behavior. In this report, we assess the utility of surface exposure age dating in this key location by establishing the age of glacial deposits of a westward flowing sublobe of the James Lobe (~44° latitude), here called the Pierre Sublobe.
We produced a new geomorphic map of the Pierre Sublobe and identified three different glacial drifts.
We applied 10Be surface exposure age dating to boulders on moraine ridges associated with each drift.
The oldest drift abuts the Missouri River trench and presents inconsistent 10Be surface exposure ages that span 10,000 years. Thus, we suggest that surface exposure age dating is not suitable for dating glacial deposits directly adjacent to the Missouri River valley, due to potential moraine degradation and subsequent boulder exhumation. Youngest drift of the easternmost Pierre Sublobe appears more stable and provides internally consistent, tightly clustered sets of 10Be surface exposure ages.
1. Introduction
During the last glacial period, the southern Laurentide Ice Sheet culminated in the most extensive terrestrial glacial margin of its time (Figure 3.1). The southern Laurentide margin spanned over 3000 km
2 Heath, S.L., Hall, B.L., Lowell, T.V., 2019. Assessing the utility of surface exposure age dating of fluctuations of the James Lobe, South Dakota: Some preliminary results. Manuscript submitted for publication in Quaternary Research.
48 from South Dakota to Massachusetts and culminated in seven distinct lobes that terminated in central
North America, reaching as far south as 38° latitude. The dynamics of this ice sheet margin remain incompletely understood - as it appears to have experienced asynchronous behavior across its southern lobes during the last glacial period and termination (Clayton and Moran, 1982; Hallberg and Kemmis,
1986; Mickelson and Colgan, 2003). The easternmost five lobes underwent relatively early advance to their maximum position before ~20.0 ka (Heath et al., 2018), while the western Des Moines Lobe in Iowa advanced to its maximum extent after ~16.2 ka (Lowell et al., 1999). This offset pattern between the east and the west has important implications for the understanding of ice sheet dynamics – truly asynchronous behavior of the ice sheet margin implies internal dynamical control of the individual lobe. Here we consider the James Lobe of South Dakota is one place to further explore the pattern of a late maximum advance for these Laurentide Lobes. Both the Des Moines and James Lobes are thought to have had the same ice stream source during the last glacial period (Flint, 1955; Clayton and Moran, 1982).
The most recent comprehensive study of the James Lobe deposits and history was by R.F. Flint in
1955. Flint (1955) describes the subsurface stratigraphy of eastern South Dakota, as well as the surface landforms, based on field observations. Subsequent county-scale geologic reports published by the
South Dakota Geological Survey have improved on this work on a local scale. These reports, available in about half of the counties covered by the James Lobe footprint, provide stratigraphic cross sections and more detailed geomorphic maps, but largely lack chronology.
Cosmogenic surface exposure age dating – specifically using 10Be, has been used successfully in establishing glacial chronology for the southern Laurentide Ice Sheet in New England (e.g. Balco et al.,
2002; Balco and Schaefer, 2006; Balco et al., 2009; Bromley et al., 2015; Bierman et al., 2015; Davis et al., 2015; Hall et al. 2017; Koester et al., 2017; Corbett et al., 2019). This method works well in areas with high concentrations of transported crystalline boulders on relatively stable substrate. However, 10Be exposure age dating has had less success farther west along the Laurentide margin, resulting in large age ranges (~10,000 years at a given site), the reasons for which remain unclear (e.g. Ullman et al. 2011).
Our objective is to test the utility of 10Be exposure age dating at a new region along the southern
49 Laurentide margin, where transported crystalline boulders lie on weak and inherently unstable Pierre
Shale bedrock. Toward this end, we ask two questions: 1. Does the method work in this area? And 2. If it does, what does it tell us about the behavior of the James Lobe?
1.1. Regional setting
During the late Pleistocene, the James Lobe occupied the James River Lowland of eastern South
Dakota and was flanked by the Missouri Coteau to the west and the Prairie Coteau to the east (Flint 1955,
Clayton and Moran 1982, Mickelson and Colgan 2002). Its southernmost margin terminated at the
Missouri River, based on till and moraine correlations by Flint (1955) and extensive mapping efforts by local state geologic survey workers (Christensen and Stephens, 1967; Christensen, 1974; McCormick and
Hammond, 2004; Johnson and McCormick, 2005).
The presence of the elevated Missouri Coteau along the James Lobe’s western flank allowed for the preservation of glacial deposits associated with several ice margin positions through the Pleistocene
(Flint 1955). The James Lobe advanced westward onto the Missouri Coteau in several places, one example being to the Missouri River near Pierre, SD which we refer here to as Pierre Sublobe (Figure
3.2). This lobe deposited a series of distinct, arcuate moraine ridges separated by extensive plains of ground moraine and stagnation deposits. Scattered, rounded igneous boulders on the western side of the
Missouri River suggest an earlier expansion beyond the current river route (“Iowan” glaciation; Flint,
1955), however we focus on the more recent glacial period. Based on the well-preserved, extensive moraines east of the Missouri River, Flint (1955) proposed three separate phases of advance during the last glacial period. These phases were named “Tazewell”, “Cary”, and “Mankato”, with suggested ages of >20.0 ka, 20.0 ka, and 15.0 ka, respectively (Flint, 1955; Clayton and Moran, 1982).
50 2. Methods and materials
2.1. Geomorphic mapping
The purpose of producing a new geomorphic map of the Pierre Sublobe was to establish a relative age sequence and geologic context for the 10Be surface exposure-age dating efforts. Geomorphic mapping was completed in two scales. The first scale, which covered the entire Pierre Sublobe, utilized remote mapping using Shuttle Radar Topography Mission (SRTM) imagery as a basemap for field mapping and verification. The second scale involved site-specific, drone-derived, digital elevation models for each sampling site. We discuss each method in turn below.
SRTM 1-arc-second resolution digital elevation model (DEM) files were downloaded from the
United States Geological Survey’s EarthExplorer tool (earthexplorer.usgs.gov). These DEM files were then uploaded into QGIS open-access geographic information system (GIS) software version 2.8.1.
Using QGIS, hillshade layers were created using the DEM files. Surface features were classified based on their orientation, size, shape and relief and divided into the following interpretive categories: ground moraine, hummocky moraine, moraine ridges, outwash, eskers, kames, kettles, scarps, bedrock, bedrock- cored ridges, and alluvium (Figure 3.3). In some cases, lithologic logs provided by the South Dakota
Geological Survey were used to differentiate between bedrock ridges and moraine ridges. Local county reports produced by the South Dakota Geological Survey (Crandell, 1958; Duchoissis, 1993; White,
1960) were also consulted for subsurface information and for map comparison.
In order to develop site-scale geomorphic maps, a drone flightpath was created for each site that covered the entire sample area. Post-flight processing was completed using the Structure from Motion
(SfM; Carrivick et al., 2016) algorithm employed by PhotoScanPro 1.4.2. by Agisoft (agisoft.com).
Hillshade layers were created using QGIS version 2.8.1 software.
2.2. Sample collection for cosmogenic surface exposure age dating
10Be surface exposure age dating was chosen as a pilot geochronology tool for this region due to the lack of sediment exposures and organic material suitable for radiocarbon dating and the presence of
51 boulders on the surface drift The working hypothesis was that glaciation of the Pierre Sublobe occurred in three phases as suggested by Flint (1955) and that surface exposure ages would produce different absolute ages for the three phases. We identified sampling sites via reconnaissance work and satellite imagery based on Flint’s (1955) three drift sheets (from oldest to youngest: Tazewell, Cary and Mankato)
(Figure 3.2). In May 2016 we collected 32 boulder and cobble samples from six sites across the Pierre
Sublobe. Our presumed “Tazewell Sites” were West Sully and Black Dog, our “Cary” sites were Sutton
Rodeo, Bieber, and Eagle Pass North, and our “Mankato” site was Eagle Pass South. All sites were active grazing pastures, but as far as we know, have never been plowed. In some places, however, air photos reveal boulder piles in the corners of some pastures near to sample sites.
2.3. “Tazewell” sites
2.3.1. Black Dog
Black Dog site lies on a large bedrock-cored feature known as the Sully Buttes (Bolin and
Wilson, 1950) that wraps around Bakers Gulch, a large gully system that drains to the west into the
Missouri River (44.70016°N, -100.48513°W; Figure 3.2). The Virgin Creek Member of the Pierre Shale forms the core of Sully Buttes (Bolin and Wilson, 1950). The roughly north-south trending, 200-m long sinuous ridge exhibits up to 45 m of relief relative to the gently rolling till plains to the east. Outboard of the Black Dog Ridge lies a 0.8 km swath of hummocky ground moraine that extends westward to the
Missouri River trench. The Black Dog ridge appears to be affected by gullying largely to the west, although the 50 m wide ridge crest from which boulders were collected appears stable.
2.3.2. West Sully
The West Sully site lies on a northeast-southwest trending bedrock-cored ridge in western Sully
County, north of Pierre, SD (44.75233°N, -100.43952°W). The ridge has ~23 m relief with a broad, flat crest. This crest is ~500 m wide at its southwestern point and gradually narrows to ~100 m wide at its northeastern end. Its northwestern slope is affected by active gullying which extends down the Missouri
52 River trench, and its southern slope gradually gives way to extensive muted topography. Sparse outwash deposits that directly overlie the Pierre Formation shale (Stevens et al., 1949) lie outboard of the ridge.
The plain to the south of the ridge is composed of as much as 40 m of interbedded till and alluvium and is interpreted as ground moraine (Stevens et al., 1949). Similar to the Black Dog Site, this ridge is a bedrock-cored butte covered with a relatively thin layer of till.
2.4. “Cary” sites
2.4.1. Sutton Rodeo
The Sutton Rodeo site lies on a broad, hummocky ridge in central Sully County (44.7127°N, -
100.01636°W). This site lies roughly in the center of the Pierre Sublobe footprint. This northeast- southwest trending ridge is ~40 km long, as much as 15 km wide and has ~50 m of relief. The ridge is bounded to the northwest and southeast by abandoned channels with abundant kettles (1.0-1.5 km diameter) and ground moraine deposits. Its surface is characterized by abundant raised, ~0.5 km diameter circular terraces with ~5.0 m relief. The Sutton Rodeo site is atop one such flat-topped raised surface that measures ~0.6 km in diameter with ~2.0 km relief. Flint (1959) interpreted this feature as an end moraine.
2.4.2. Bieber
The Bieber site lies along the same northeast-southwest trending hummocky ridge as Sutton
Rodeo. The site is split between two ~100-meter diameter hummocks and lies adjacent to a roughly north-south trending sinuous, abandoned stream channel that dissects the moraine feature (Figure 3.2).
2.5. “Mankato” Sites
2.5.1. Eagle Pass South
Eagle Pass South (EPS) is located on an east-west trending hummocky ridge perched on the
Missouri Coteau. This ridge is mapped within Flint’s (1955) “Mankato” limit. The site is on a ~4.0 km long ridge segment near the junction of two moraines. To the south of EPS site lies the eastern limit of a
53 large stagnation moraine, traceable for ~62 km to the west. The stagnation moraine is characterized by these small ridge segments, as well as large raised hummocks up to 1.0 km in diameter. To the north of
EPS is the southernmost extent of a large arcuate moraine on which Eagle Pass North lies.
2.5.2. Eagle Pass North
The Eagle Pass North (EPN) Site lies on an east-west trending ridge segment that is part of the large continuous arcuate moraine mapped by Flint (1955) as the extent of the “Mankato”, or the latest phase of glacial advance. The ridge lies parallel to, and north of the Eagle Pass South ridge. North of the site is the gradual slope down into the broader Pierre Sublobe footprint (Figure 3.2).
2.6. 10Be surface exposure age analysis
Thirty-two samples were collected from the six sites discussed above (Figure 3.2). We targeted relatively flat-topped boulders embedded in the ground surface. We sought coarse-grained granitic boulders for sampling. Each sample was collected following methods of Kelly (2003), using a battery powered hammer drill and explosive caps to remove the upper ~5 cm of the boulder surface. Samples were transported to the University of Cincinnati for initial processing, crushing and sieving. The remainder of processing was completed at the University of Maine cosmogenic isotope laboratory.
Quartz isolation and purification of the 250-500-micron grain size fraction was completed using surfactants and weak hydrofluoric acid (Kohl and Nishiizumi, 1992). Beryllium was isolated using ion exchange chemistry protocols after Ditchburn and Whitehead (1994). The samples were precipitated as beryllium hydroxide, combusted, mixed with niobium, and packed into cathodes. Cathodes were analyzed by Lawrence Livermore National Laboratory and are normalized to 07KNSTD standard
(Nishiizumi et al. 2007).
10Be ages were calculated using the CRONUS-Earth online exposure age calculator version 2.2
(Balco et al., 2008), using the north-eastern North American production rate developed by Balco et al.
(2009), and the scaling scheme described by Stone (2000) following Lal (1991). Blank-corrected 10Be
54 concentrations and all other sample parameters used to calculate ages are presented in Table 1, including both internal and external errors.
3. Results
In the following sections, we first present the results of geomorphic mapping efforts in the Pierre
Sublobe with respect to the sampling sites described above. Second, we present 10Be surface-exposure dating results for each site. The details of individual samples, as well as analytical results can be found in
Table 1. 10Be surface exposure ages are plotted on DEMs of each individual site in Figures 3.4-3.9.
3.1. Geomorphic map
We separate the mapping into three distinct regions, based on landform morphology.
The western region of the Pierre Sublobe is characterized by large ridges (>40 m relief) that wrap around massive gully systems that drain into the river trench. These features are surrounded by broad ground moraine with < 1.0 m relief, as well as abandoned channels. These features were previously interpreted as terminal moraines due to their relief and the presence of boulders (Flint, 1955). However, subsequent mapping efforts and drill cores have revealed that ground moraine deposits extend well past these buttes to the edge of the Missouri River trench where they have been eroded, exposing Pierre Shale.
Further, one lithologic log (core location 111N79W17ADDD; South Dakota Geological Survey
Lithologic Log Database http://cf.sddenr.net/lithdb/) from Snake Butte (Figure 3.2) suggests that these buttes are bedrock-cored overlain by a relatively thin (~10 m) layer of till. This lithologic log also indicates that these features consist of a continuous, in situ bedrock sequence with no evidence of tectonization.
The central Pierre Sublobe is characterized by two dominant types of surficial features. The first consists of broad, flat till plains with large hummocks as much as ~1.0 km in diameter, which are interpreted as ground moraine. The second type of feature consists of very broad, hummocky ridges as much as 40 km long, consisting of round, flat-topped mounds as much as ~1.0 km in diameter. One such
55 feature underlies the Sutton Rodeo and Bieber sites. Due to their orientation, hummocky surface, and lack of ridges or other clear structures, they are interpreted here as stagnation moraine, formed by the abandonment and subsequent melting of glacial ice following glacier retreat. Lateral stream channels, both active and abandoned, are present along several stagnation moraine features.
The eastern Pierre Sublobe is characterized by a large arcuate moraine complex with abundant ridge segments and large abandoned channel systems. This moraine complex is younger than the stagnation moraine features to the west. The Eagle Pass sites lie on two ridge segments along the southern edge of the moraine complex. Abandoned lateral channels lie adjacent to the moraine complex, and in many cases, hold kettle lakes ~1.0 km in diameter.
Overall, we found that determining ice-margin positions based solely on geomorphology is difficult in the Pierre Sublobe, because of the absence of continuous moraine ridges. Many of the moraines are products of ice stagnation, rather than ice-margin stabilization.
3.2. Geomorphic context of sample collection sites
Based on the geomorphic map, the six 10Be surface exposure sampling sites occupy features of very different origin. Black Dog and West Sully both occupy buttes along the Missouri River. These features are interpreted here as ground moraine that covers a bedrock ridge. Boulders at these sites hence were deposited when the Pierre Sublobe was distal to the buttes and these ridges were overrun by ice.
The surface exposure ages will represent how long the boulders have been uncovered by ice and provide minimum age constraint for ice margin retreat. The Sutton Rodeo and Bieber sites are located atop stagnation moraine in the central Pierre Sublobe. Boulders at these two sites were most likely let down as ice stagnated and represent the retreat or disintegration of the Pierre Sublobe. Therefore, these surface exposure ages will be minimum ages for ice stagnation. The Eagle Pass North and South sites both occupy moraine ridge segments that are part of a complex made up of distinct ridges. The Eagle Pass ridge segments represent minor fluctuations of the ice margin.
56 3.3. 10Be surface exposure ages
3.3.1. Black Dog
Seven 10Be surface-exposure ages from the Black Dog Site range from 19.6 ± 0.6 to 7.0 ± 0.2 ka
(Table 1; Figure 3.4). The youngest age of 7.0 ± 0.2 ka (JL-16-26) lies below the 1.5 inter-quartile range and excluded as an outlier.
3.3.2. West Sully
Nine boulder samples were analyzed for 10Be surface-exposure dating, and the ages range from
19.8 ± 0.4 to 11.0 ± 0.3 ka (Figure 3.5). The youngest age of 11.0 ± 0.3 ka (JL-16-33) falls below the 1.5 inter-quartile range. The remaining data are similar to those from Black Dog (Figures 3.6a and 3.6b), in that both sites exhibit age distributions with peaks ~19.0 ka and ~14.0 ka. The West Sully sites exhibits an additional peak ~17.0 ka.
3.3.3. Sutton Rodeo
Two boulder samples from Sutton Rodeo yield surface exposure ages of 20.6 ± 0.7 ka (JL-16-03) and 14.7 ± 0.5 ka (JL-6-02). These samples were all collected from near the edge of the same large hummock (Figure 3.6).
3.3.4. Bieber
Three samples from the Bieber site yield surface exposure ages of 15.7 ± 0.2 ka (JL-16-41), 14.2
± 0.5 (JL-16-39) and 12.8 ± 0.2 ka (JL-16-40). These were collected from two different hummocks along the same stagnation moraine (Figure 3.7).
3.3.5. Eagle Pass South
Six samples from Eagle Pass South yield ages that range from 16.2 ± 0.7 to 11.7 ± 0.3 ka (Figure
3.8). The youngest age of 11.7 ± 0.3 k (JL-16-58) falls just below the 1.5 inter-quartile range and is
57 therefore excluded as an outlier. The remaining ages exhibit a normal distribution and produce a mean age of 15.8 ± 0.4 ka.
3.3.6. Eagle Pass North
Five samples from Eagle Pass North yield 10Be exposure ages that range from 16.7 ± 0.4 ka to
14.8 ± 0.3 ka (Figure 3.9). These ages produce a mean of 16.0 ± 0.7 ka.
3.4. Summary of mapping and 10Be surface exposure age dating results
• The two sites that represent “Tazewell” drift, Black Dog and West Sully, produce 10Be surface exposure
age data sets that show a wide range of ages with a non-simple distribution (Figures 3.10a, 3.10b);
• The two sites that represent the “Cary” drift, Sutton Rodeo and Bieber provide minimum ages of 20.6 ±
0.7 ka and 15.7 ± 0.2 ka, respectively, for exposure of associated landforms, in keeping with their
geologic context on stagnation moraine;
• The two sites that represent “Mankato” drift, Eagle Pass North and Eagle Pass South exhibit a tight
distribution between 16.7 ± 0.4 ka and 14.8 ± 0.3 ka.
3.5. Testing the effects of inheritance and moraine degradation
Surface exposure ages from West Sully and Black Dog exhibit the largest scatter in ages, of all the sites sampled for this project. Two specific processes are generally thought to explain non-normal age distributions: inheritance and boulder exhumation. Boulders may contain inherited 10Be, which accumulated in the quartz crystals during prior episodes of exposure, and thus may yield anomalously old ages. Alternatively, boulders may have been exhumed well after the glacier retreated due to moraine degradation, resulting in anomalously young ages (Applegate et al. 2012).
In order to test whether either of these processes has contributed the distribution of exposure ages at the West Sully and Black Dog sites, we employed the two theoretical models developed by Applegate et al. (2012). Even though neither of these sites are on moraine ridges, both features are covered by
58 glacial drift that could have undergone degradation since deposition. Both the inheritance and degradation models generate a hypothetical set of surface exposure ages based on user input, such as moraine age, initial moraine height, initial moraine slope, and density of overlying material. The output is a histogram of theoretical ages, which can then be compared to the actual data set.
For the inheritance model, we ran several dozen simulations for the West Sully and Black Dog sites separately, adjusting the drift age for each site each time. We tested assumed moraine ages from
20.0 ka to 14.0 ka at 2.0 ka intervals. The maximum time that any boulder was able to acquire inherited nuclides was set at the default of 30.0 ka, and the maximum boulder depth default remained at 3.0 m. All simulations predicted that the maximum age would be older than ~40 ka. However, the maximum exposure age at West Sully is 19.8 ± 0.4 ka and the maximum age at Black Dog is 19.6 ± 0.6 ka. If inheritance was a problem at these sites, regardless of where the true age falls in the tested range of 20.0 and 14.0 ka, the surface exposure ages would exhibit a much larger range than observed and would be skewed to the right (older), and there would at least be a small population of exposure ages between 50.0 and 40.0 ka. This is not observed at either the Black Dog or West Sully site. Therefore, we do not favor inheritance to explain the distributions at these sites.
The diffusion model of Applegate et al., (2012) was tested using similar moraine age inputs.
This model also requires input values for initial moraine height and moraine slope. Because the Black
Dog and West Sully features are not technically moraines, we only considered the till layer in approximating these values. We tested three different initial till depth (“initial moraine height”) values of
10m, 20m, and 30m. We used a value of 10° for initial moraine slope based on the maximum slope observed at either the West Sully or Black Dog sites. Other parameters were set as follows: 0.5 m for boulder height, 2.7 g/cm3 for boulder material density, and 2.0 g/cm3 for till density. Overall, the hypothetical data sets produced by the model were very similar to the actual surface exposure age data sets in terms of range, minimum age and maximum age. However, the diffusion model was unable to replicate the multi-peak distribution observed at each of these sites. Rather, in every scenario tested, the diffusion model produced J-shaped distributions, representing steady diffusion of the landform over time.
59 These results are not entirely consistent with the observed surface exposure age distributions. The general shape is similar — J-shaped skewed towards the older ages, leading us to suggest that landform diffusion or degradation occurred at these two sites. However, the multi-modal distribution suggests that the diffusion may have been episodic and erosion at the two sites may be more complex than steady diffusion modeled of Applegate et al. (2012).
4. Discussion
In the following discussion, we first evaluate the 10Be exposure age data set from the Pierre
Sublobe and determine whether this dating method is suitable for building a glacial chronology for this region. For four sites with suitable distributions we can extract information on the glacial chronology of the Pierre Sublobe.
4.1. Evaluation and interpretation of 10Be surface exposure ages
Of the six sample sites, Black Dog and West Sully provide the most problematic sets of surface exposure ages, with very large age ranges and tri-modal distributions that cannot be explained by inheritance or diffusion models designed by Applegate et al. (2012). The Black Dog and West Sully sites lie on bedrock-cored ridges along the Missouri River, and both yield data sets that span more than 10,000 years. One might suggest that more samples might smooth out the distribution. However, these two sites, with the same geomorphic context, exhibit the same temporal pattern. Combining ages from both sites, effectively doubling the data set (n=17), still results in a trimodal distribution.
Taken at face value, the surface exposure ages from the Black Dog and West Sully sites do not exhibit any clear pattern or appear to give any information about the glacial history. However, it is curious that there are no ages greater than ~19.0 ka at either site. One simple interpretation for this cluster of ages at ~19.0 ka is that they represent roughly the timing for the earliest till and boulder exposure at these two sites. This would serve as a minimum age for ice incursion in the western Pierre Sublobe. This
60 ~19.0 ka cluster may also be an upper age bracket for the start of some geologic process, such as boulder exhumation, in order to expose those three boulders at different times. If we reconsider the exposure age of 20.6 ± 0.7 ka from the Sutton Rodeo site, the concept of glacial retreat or erosion ~19.0 ka may be possible. In this model, the younger ages from these sites may represent the timing of subsequent exposure from erosion events. Support for this scenario comes from the trimodal distribution at both the
Black Dog and West Sully sites - the probability peaks ~ 17 ka and ~14 ka may represent specific episodes of erosion and subsequent boulder exhumation and exposure. This is only one possible scenario, though it is clear based on the distribution of ages at both river sites and Sutton Rodeo that at least in the western Pierre Sublobe, this is not a simple case of boulder emplacement and ice retreat, nor is it is not simple constant diffusion as modeled by Applegate et al. (2012).
Surface exposure ages from the Eagle Pass North and South sites provide a contrast to the scattered data sets produced at the two river sites. The Eagle Pass sites provide the only normal distributions of 10Be surface exposure ages of the six sites sampled around the Pierre Sublobe (Figure
3.10c, 3.10d). At Eagle Pass North, the ages range from 16.7 ± 0.4 to 14.8 ± 0.3 ka, to produce an average of 16.0 ± 0.7 ka. At Eagle Pass South, the ages (excluding one outlier) range from 16.2 ± 0.7 to
15.4 ± 0.6 ka to produce an average age of 15.8 ± 0.8 ka. All ages from both sites lie within one standard deviation of one another. Geomorphic mapping shows that the sites lie on separate ridge segments but are part of the same massive arcuate moraine complex interpreted by Flint (1955) as the Mankato advance.
We interpreted these features as minor moraines, the result of ice margin stabilization. Based on the moraines’ proximity and geomorphic context, the surface exposure ages from both sites provide acceptable age distributions that provide chronologic control on glacial activity in the Pierre Sublobe.
4.2. Is 10Be surface exposure age dating suitable for dating James Lobe glacial deposits?
In short, this dating method is unsuitable for bedrock buttes that abut the Missouri River valley in central South Dakota - specifically the Black Dog and West Sully sites. The dating method does appear
61 suitable for moraine ridges further east of the Missouri River trench - specifically the Eagle Pass North and Eagle Pass South sites.
We suggest that the scatter in ages at Black Dog and West Sully is in part the result of their geomorphic context. These two river sites are atop bedrock ridges cored by weak Pierre Shale and topped by a relatively thin (~10m) ground moraine. Both Black Dog and West Sully exhibit extensive gullying on the western and northwestern ridge slope, where Pierre Shale is exposed at the surface. Crandell
(1958) infers that these buttes are remnants of an ancient divide that pre-dates the modern Missouri River trench (as old as mid-Pliocene). If correct, these features have then been susceptible to erosion through multiple glaciations.
We suggest that future dating efforts using 10Be exposure age dating would benefit greatly from detailed mapping of potential sites. Drone mapping and LiDAR (where available) can help identify potential stability issues prior to sampling.
4.3. Implications for the glacial chronology of the Pierre Sublobe
If correct, the older cluster of ages ~19 ka from Black Dog and West Sully may suggest that ice advanced to the Missouri River trench and deposited boulders at these sites at or prior to ~19 ka.
However, the timing of ice incursion at these two sites is speculative at best.
At the Sutton Rodeo in the central Pierre Sublobe, based on the interpretation of no inheritance, the oldest surface exposure age of 20.6 ± 0.7 ka may provide minimum age constraint for the stagnation of ice in this region, as the boulders were likely uncovered as ice melted and the moraine was formed.
However, this age is older than those at the outboard sites along the Missouri River and its interpretation remains uncertain.
We suggest that the arcuate moraine complex that characterizes the eastern region of the Pierre
Sublobe is the product of ice margin stabilization, based on geomorphology and lack of cross cutting relationships with abutting stagnation features. We further suggest that the moraine ridge segments that underlie the Eagle Pass North and Eagle Pass South sites were deposited either during this stabilization or
62 its subsequent retreat, and that the surface exposure ages of boulders at these sites represent the timing of ice retreat from the features. The boulders at Eagle Pass South were uncovered by Pierre Sublobe ice by
15.8 ± 0.8 ka. The ice retreated from Eagle Pass North by 16.0 ± 0.7 ka. If this interpretation is correct it implies that the “Mankato” drift of Flint (1955) represents stabilization of the James Lobe before ~16 ka.
If so, Pierre Sublobe behavior may be similar to that of the nearby Des Moines Lobe in Iowa, which underwent extensive advance by ~16.2 ka based on radiocarbon ages from till (Lowell et al., 1999; Heath et al., 2018). Although the glacial chronology of these two lobes is based on two different dating methods, there is nothing to preclude these two lobes as correlative. This suggests a relatively late episode of stabilization or advance in the western sector of the southern Laurentide margin, during which the eastern sector underwent extensive retreat. The mechanisms behind the difference in southern
Laurentide behavior at this time have yet to be explored. A better understanding of these mechanisms would provide invaluable insight into ice sheet dynamics during a glacial termination.
5. Conclusions
- Black Dog and West Sully sites along the Missouri River produce large ranges of 10Be surface exposure
ages that have a trimodal distribution;
- Cluster of ages ~19.0 at these river sites suggest possible initial uncovering due to ice retreat ~19.0 ka,
followed by episodic erosion for ~10 kyr;
- The Pierre Sublobe stabilized and deposited the arcuate moraine complex in the eastern Pierre Sublobe
and retreated from Eagle Pass South at 15.8 ± 0.8 ka, at Eagle Pass North at 16.0 ± 0.7 ka.
- 10Be surface exposure dating unsuitable for dating glacial deposits near the Missouri river valley and
more generally unstable sites due to erosion and is best suited for features located further away from
Missouri River trench.
63 Acknowledgements
The Gary C. Comer Science and Education Foundation provided financial support for this research. We thank the various landowners in central South Dakota for allowing access to their land for mapping and sample collection. We are grateful to Jill Pelto, Peter Strand, Elizabeth Orr and Sarah Hammer for lab assistance.
64 Tables
Sample 10Be Sample Latitude Elevation Thickness Shielding 10Be Error Exposure age Site ID (°) Longitude (°) (m.a.s.l.) (cm) Correction (atoms/g) (atoms/g) (yr) Black JL-16-21 44.70016 -100.48513 626 1.35 0.998 1.34E+05 3.90E+03 19590 ± 577 Dog JL-16-22 44.69931 -100.48566 635 2.01 1.000 7.42E+04 2.19E+03 10852 ± 322 JL-16-23 44.6993 -100.48554 634 1.62 0.984 9.02E+04 2.56E+03 13377 ± 382 JL-16-24 44.69906 -100.48527 632 3.77 1.000 9.58E+04 2.06E+03 14256 ± 308 JL-16-25 44.69894 -100.48506 634 1.15 1.000 8.57E+04 1.78E+03 12466 ± 260 JL-16-26 44.69625 -100.48389 625 2.50 0.993 4.71E+04 1.63E+03 7005 ± 244 JL-16-27 44.69716 -100.48441 628 1.36 0.998 9.41E+04 1.86E+03 13812 ± 274 JL-16-28 44.70198 -100.48624 625 1.70 1.000 1.30E+05 3.45E+03 19057 ± 512 West JL-16-29 44.75233 -100.43952 610 2.01 1.000 1.33E+05 2.54E+03 19779 ± 383 Sully JL-16-30 44.75252 -100.4389 612 2.11 1.000 1.10E+05 2.10E+03 16382 ± 315 JL-16-31 44.75274 -100.43949 611 2.55 1.000 9.65E+04 2.86E+03 14461 ± 431 JL-16-32 44.75324 -100.43708 611 2.26 1.000 1.15E+05 2.32E+03 17149 ± 349 JL-16-33 44.75323 -100.43722 605 3.30 1.000 7.28E+04 1.68E+03 11029 ± 256 JL-16-34 44.75347 -100.43672 612 1.33 1.000 1.19E+05 2.33E+03 17590 ± 348 JL-16-35 44.7536 -100.43632 612 1.98 0.996 9.61E+04 1.96E+03 14374 ± 295 JL-16-36 44.75362 -100.43612 615 2.15 0.999 1.01E+05 1.81E+03 15042 ± 271 JL-16-37 44.7537 -100.43594 612 1.49 1.000 1.18E+05 2.27E+03 17467 ± 339 Sutton JL-16-02 44.71288 -100.01944 582 2.58 1.000 9.59E+04 3.50E+03 14730 ± 541 Rodeo JL-16-03 44.71292 -100.01934 584 2.45 1.000 1.35E+05 4.40E+03 20583 ± 680 Bieber JL-16-39 44.83749 -99.73464 579 2.00 0.996 9.22E+04 3.29E+03 14177 ± 508 JL-16-40 44.83739 -99.73467 579 2.80 0.997 8.31E+04 1.51E+03 12847 ± 234 JL-16-41 44.83743 -99.73984 583 1.40 0.982 1.02E+05 1.54E+03 15745 ± 239 EPN JL-16-42 44.48734 -99.26794 591 2.99 0.999 1.09E+05 2.30E+03 16685 ± 355 JL-16-44 44.48771 -99.26947 593 2.45 1.000 9.72E+04 1.97E+03 14803 ± 302 JL-16-46 44.48787 -99.2723 593 2.30 1.000 1.06E+05 2.00E+03 16103 ± 306 JL-16-47 44.48833 -99.27395 592 1.98 1.000 1.08E+05 2.37E+03 16373 ± 362 JL-16-48 44.48803 -99.27473 589 2.12 0.995 1.03E+05 3.10E+03 15762 ± 478 EPS JL-16-51 44.48021 -99.26767 609 2.97 1.000 1.02E+05 3.47E+03 15385 ± 527 JL-16-53 44.481 -99.26978 611 1.70 1.000 1.08E+05 2.10E+03 16080 ± 315 JL-16-54 44.48135 -99.26991 611 1.81 1.000 1.08E+05 1.84E+03 16094 ± 277 JL-16-55 44.4813 -99.27017 610 1.49 1.000 1.09E+05 4.81E+03 16213 ± 722 JL-16-57 44.48052 -99.2707 611 2.11 1.000 1.03E+05 4.02E+03 15398 ± 605 JL-16-58 44.48014 -99.27147 617 2.69 1.000 7.80E+04 1.71E+03 11664 ± 257
Table 3.1. Sample details and 10Be surface exposure ages from six sites within the Pierre Sublobe.
All samples were calculated using the value of 2.7 g/cm3 for sample density, and zero for erosion.
65 Figures
Figure 3.1. Map of the outermost extent of the Laurentide Ice Sheet during the last glacial period.
The two domes of the Laurentide Ice Sheet are noted. Thin black lines represent approximate flowlines from the domes, after Denton and Hughes (1981). The James Lobe is outlined in a red box.
66
Figure 3.2. Digital elevation model of the Pierre Sublobe. The dotted black lines represent drift limits estimated by Flint (1955). Yellow dots mark boulder sample sites for 10Be surface exposure age dating.
Surface exposure ages for each site are presented for each individual site in Figures 3.4-3.9.
67
Figure 3.3. Surface geomorphic map of the Pierre Sublobe. Yellow dots represent 10Be surface exposure sampling sites.
68
Figure 3.4. Local DEM of Black Dog. Yellow dots represent individual samples. Higher elevation is represented by warmer color tones. Note that the samples were collected as close to the crest as possible, away from active gullying to the east and west. The site of sample JL-16-26 was not imaged, as the flight session was cut short due to high winds.
69
Figure 3.5. Local DEM of the West Sully Site. Color coding is the same as Figure 3.4. Note gullying to the north and northeast, and the proximity of sample sites to the ridge crest.
70
Figure 3.6. Local DEM of Sutton Rodeo. Color coding is the same as in Figure 3.4. The north-south trending line on the left side of image is a dirt road and ditches. Other visible features are cattle dugouts, fence lines and tire tracks in grass.
71
Figure 3.7. Local DEM of Bieber site. Color coding is the same as in Figure 3.4. The hummock underlying JL-16-41 was not captured due to high winds.
72
Figure 3.8. Local DEM of Eagle Pass South. Color coding and symbols are the same as Figure 3.4.
There was no evidence of active gullying at this site, though samples were still collected as close to the ridge crest as possible. This site lies ~1.0 km south of the Eagle Pass North site
73
Figure 3.9. Local DEM of Eagle Pass North. Color coding and symbols are the same as Figure 3.4.
Samples were collected on the crests of the very subtly different segments and mounds that made up this ridge.
74
Figure 3.10. Relative probability distributions for each a) Black Dog, b) West Sully, c) Eagle Pass
South and e) Eagle Pass North. Each individual surface exposure age is plotted as a black dot, with the black bar representing internal error. The Y-axis is time, in thousands of years before present (ka). The blue line represents the summed probability of all of the samples. The highest probability age is generally taken as the most likely indication of the age of the landform, assuming there are no other geologic processes at work. Note the trimodal distribution at Black Dog and West Sully sites. The Sutton Rodeo and Bieber sites were excluded from this analysis due to a limited number of samples.
75 REFERENCES
Applegate, P.J., Urban, N.M., Keller, K., Lowell, T.V., Laabs, B.J.C., Kelly, M.A., Alley, R.B., 2012. Quaternary Research 77, 293-304. Balco, G., Stone, J.O.H., Porter, S.C., Caffee, M.W., 2002. Cosmogenic-nuclide ages for New England coastal moraines, Martha’s Vineyard and Cape Cod, Massachusetts, USA. Quaternary Science Reviews 21, 2127-2135. Balco, G., Schaefer, J.M., 2006. Cosmogenic-nuclide and varve chronologies for the deglaciation of southern New England. Quaternary Geochronology 1, 15-28. Balco, G., Stone, J.O., Lifton, N.A., Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology 3, 174-195. Balco, G., Briner, J., Finkel, R.C., Rayburn, J.A., Ridge, J.C., Schaefer, J.M., 2009. Regional beryllium- 10 production rate calibration for late-glacial northeastern North America. Quaternary Geochronology 4, 93-107. Bierman, P.R., Davis, D.T., Corbett, L.B., Lifton, N.A., Finkel, R.C., 2015. Cold-based Laurentide ice covered New England’s highest summits during the Last Glacial Maximum. Geology 43 (12), 1059-1062. Bromley, G.R.M., Hall, B.L., Thompson, W.B., Kaplan, M.R., Garcia, J.L., Schaefer, J.M., 2015. Late glacial fluctuations of the Laurentide Ice Sheet in the White Mountains of Maine and New Hampshire, U.S.A. Quaternary Research 83, 522-530. Bolin, E.J., Wilson, R.C., 1951. Areal Geology of the Okobojo Quadrangle. South Dakota State Geological Survey, Scale 1:62,500. Carrivick, J.L., Smith, M.W., Quincey, D.J., 2016. Structure from Motion in the Geosciences. Wiley- Blackwell. 208 pages. Christensen, C.M., 1974. Geology and water resources of Bon Homme County South Dakota, Part 1: Geology. South Dakota Geological Survey Bulletin 21, 48 pp. Christensen, C.M., Stephens, J.C., 1967. Geology and Hydrology of Clay County South Dakota. South Dakota Geological Survey Bulletin 19, 95 pp. Clayton, L., Moran, S.R., 1982. Chronology of late Wisconsinan glaciation in middle North America. Quaternary Science Reviews 1, 55-82. Corbett, L.B., Bierman, P.R., Wright, S.F., Shakun, J.D., Davis, P.T., Goehring, B.M., Halsted, C.T., Koester, A.J., Caffee, M.W., Zimmerman, S.R., 2019. Analysis of multiple cosmogenic nuclides constrains Laurentide Ice Sheet history and process on Mt. Mansfield, Vermont’s highest peak. Quaternary Science Reviews 205, 234-246. Crandell, D.R., 1958. Geology of the Pierre Area, South Dakota. Geological Survey Professional Paper 307, 88 pp. Davis, P.T., Bierman, P.R., Corbett, L.B., Finkel, R.C., 2015. Cosmogenic exposure age evidence for rapid Laurentide deglaciation of the Katahdin area, west-central Maine, USA, 16 to 15ka. Quaternary Science Reviews 116, 95-105. Ditchburn, R.G., Whitehead, N.E., 1994. The separation of 10Be from silicates. G. Hancock, P. Wallbrink (Eds.), Third Workshop of the South Pacific Radioactivity Association, Australian National University, Canberra (1994), pp. 4-7. Duchoissois G.E., 1993. Geology of Hughes County, South Dakota. South Dakota Geological Survey Bulletin 36, 42 pp. Flint, R.F., 1955. Pleistocene Geology of Eastern South Dakota. Geological Survey Professional Paper 262, p. 173. Hall, B.L., Borns Jr., H.W., Bromley, G.R.M., Lowell, T.V., 2017. Age of the Pineo Ridge system: Implications for behavior of the Laurentide Ice Sheet in eastern Maine, U.S.A., during the last deglaciation. Quaternary Science Reviews 169, 344-356.
76 Hallberg, G.R., Kemmis, T.J., 1986. Stratigraphy and correlation of the glacial deposits of the Des Moines and James lobes and adjacent areas in North Dakota, South Dakota, Minnesota, and Iowa. Quaternary Science Reviews 5, 65-68. Heath S.L., Loope H.M, Curry B.B., Lowell T.V., 2018. Pattern of southern Laurentide Ice Sheet margin position changes during Heinrich Stadials 2 and 1. Quaternary Science Reviews 201, 362-379. Johnson, G.D., McCormick, K.A., 2005. Geology of Yankton County, South Dakota. South Dakota State Geological Survey Bulletin 34, 89 pp. Kelly, M.A., 2003. The late Wurmian Age in the western Swiss Alps – last glacial maximum (LGM) ice- surface reconstruction and 10Be dating of late-glacial features. Ph.D. dissertation, University of Bern, 105pp. Koester, A.J., Shakun, J.D., Bierman, P.R., Davis, P.T., Corbett, L.B., Braun, D., Zimmerman, S.R., 2017. Rapid thinning of the Laurentide Ice Sheet in coastal Maine, USA, during late Heinrich Stadial 1. Quaternary Science Reviews 163, 180-192. Kohl, C., Nishiizumi, K., 1992. Chemical isolation of quartz for measurement of in situ-produced cosmogenic nuclides. Geochimica et Cosmochimica Acta 56, 3586-3587. Lal, D., 1991. Cosmic ray labeling of erosion surfaces: in-situ nuclide production rates and erosion models. Earth and Planetary Science Letters 104, 424-439. Lepper, K., Fisher, T.G., Hajdas, I., Lowell, T.V., 2007. Ages for the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz: Implications for deglaciation chronology. Geology 35 (7), 667- 670. Lowell, T.V., Hayward, R.K., Denton, G.H., 1999. Role of climate oscillations in determining ice-margin position: Hypothesis, examples, and implications, in Mickelson, D.M., and Attig, J.W., eds., Glacial Processes Past and Present: Boulder, Colorado, Geological Society of America Special Paper 337. McCormick, K.A., Hammond, R.H., 2004. Geology of Lincoln and Union Counties, South Dakota. Department of Environment and Natural Resources/Geological Survey Bulletin 39, 43 pp. Mickelson, D.M., Colgan, P.M., 2003. The southern Laurentide Ice Sheet. Developments in Quaternary Science 1, 1-16. Nishiizumi, K., Imamura, M., Caffee, M.W., Southon, J.R., Finkel, R.C., McAninch, J., 2007. Absolute calibration of 10Be AMS standards. Nuclear Instruments and Methods in Physics Research Section B, 258, 403-413. Ruhe, R.V., 1969. Quaternary Landscapes in Iowa. Iowa State University Press, Ames, Iowa, p. 255. Stevens E.H., Bogan R.J., Burge F.H., Fenske P.R., 1952. Areal geology of the Artichoke Butte quadrangle. South Dakota State Geological Survey, 1:62:500. Stone, J.O., 2000. Air pressure and cosmogenic isotope production. Journal of Geophysical Research 105, 23753-23759. Ullman, D.J., Carlson, A.E., LeGrande, A.N., Anslow, F.S., Moore, A.K., Caffee, M., Syverson, K.M., Licciardi, J.M., 2015. Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation. Geology 43 (1), 23-26. White, E.M., 1960. Some surface glacial deposits in Hand County, South Dakota. State Geological Survey Miscellaneous Investigations No. 1, 19 pp.
77 Chapter 4
Behavior of the James Lobe during the last glacial period and termination
ABSTRACT
During the last glacial period, the southern Laurentide ice Sheet margin may have reached its maximum extent up to 5000 years later in the western lobes (South Dakota and Iowa) than in the eastern lobes (Wisconsin to Massachusetts). This pattern is not well established, however, as the James Lobe in
South Dakota remains poorly dated. Heath et al., (submitted; Chapter 3) determined that surface exposure ages from sites that abut the Missouri River trench may be affected by erosional processes. There is an underutilized resource in the form of subsurface logs and radiocarbon ages recovered from organic materials from the subsurface. Radiocarbon ages from South Dakota county geological reports, as well as three stratigraphic cross sections of James Lobe glacial deposits are analyzed in order to develop age constraints for the advance and retreat patterns of the James Lobe during the last glacial period. The resulting reconstruction of the James Lobe’s behavior allows for comparison of the James Lobe to the neighboring Des Moines Lobe, and to the rest of the southern Laurentide margin.
Lithologic logs identify two glacial till units associated with James Lobe advance during the last glacial period. The stratigraphy suggests two advances of the James Lobe during the last glacial period, which is supported by radiocarbon ages. Separating these two till units in the central James Lobe footprint is a distinct weathering horizon and loess or lacustrine deposits. We take this as evidence of interstadial conditions between glacial advances. Radiocarbon ages from the lowermost till suggest that the James Lobe covered the Prairie Coteau to its east after ~27 ka. Radiocarbon ages from within and below the uppermost till suggest that the James Lobe advanced to its southernmost maximum extent at the Missouri River after 13 ka. This timing of maximum extent is similar to the nearby Des Moines Lobe, which reached its maximum extent after 16.2 ka. We find that the James Lobe and Des Moines Lobe
78 exhibit similar timing of advance, which has implications for climate conditions along the western sector margin of the Southern Laurentide.
1. Introduction
The Laurentide ice Sheet margin was the largest ice sheet of the last glacial period and extended over 3000 km across central North America, culminating in seven distinct lobes that reached as far south as 38° latitude (Figure 4.1). Understanding of the southern Laurentide margin’s behavior remains incomplete yet is suggested to exhibit asynchronous behavior during the last glacial period and termination (Clayton and Moran, 1982, Hallberg and Kemmis, 1986; Mickelson and Colgan, 2003). The eastern lobes advanced to their maximum extent before ~20.0 ka, while the Des Moines Lobe in Iowa advanced to its maximum extent after ~16.2 ka.
Previous workers suggest that many of the southern Laurentide Lobes were fed by ice streams - which flowed faster than the ice around them (Figure 4.1; Flint, 1955; Denton and Hughes, 1981; Clayton and Moran, 1982; Patterson, 1997; Jennings, 2006; Margold et al., 2015). Because the James Lobe and
Des Moines Lobe were thought to have been sourced from the same ice stream, originating from the
Keewatin Dome of the northwestern Laurentide Ice sheet (Figure 4.1; Denton and Hughes, 1981), the
James Lobe in South Dakota is a logical place to investigate the timing of southern Laurentide fluctuations.
The most recent study of the James Lobe tested the use of 10Be surface exposure age dating on glacial deposits of the Pierre Sublobe, a westward flowing sublobe along the western James Lobe margin.
Heath et al. (submitted; Chapter 2) found that the Pierre Sublobe terminated at the Missouri River during the last glacial period, and may have undergone retreat before ~19.0 ka, although the surface exposure ages exhibit large ranges at the two sites adjacent to the Missouri River. The Pierre Sublobe stabilized at an inboard moraine complex >15.9 ka. What remains unknown, however, is whether this reflects the timing of the James Lobe behavior in the James River Lowland. The objective of this study is to
79 reconstruct the behavior of the main trunk of the James River Lowland and compare its advance and retreat history to the nearby Des Moines Lobe and the rest of the southern Laurentide margin.
2. Methods
2.1. Regional setting
During the late Pleistocene, the James Lobe occupied the James River Lowland of eastern South
Dakota. The lobe was flanked by the Missouri Coteau to the west, and by the Prairie Coteau to the east
(Flint 1955, Clayton and Moran 1982, Mickelson and Colgan 2002). The Prairie Coteau is a triangular- shaped raised feature made up of glacial till (Flint 1955, Gilbertson, 1990). The James Lobe terminated at the Missouri River at both its western and western margins (Flint, 1955; Christensen and Stephens, 1967;
Christensen, 1974; McCormick and Hammond, 2004; Johnson and McCormick, 2005).
2.2. Stratigraphy
Stratigraphic cross sections were constructed for three transects across the James River lowland in order to establish geologic and stratigraphic context for the radiocarbon chronology (Figure 4.2).
Lithologic logs from exploratory drill holes and well installations were downloaded from the South
Dakota Geologic Survey Lithologic Log database (http://cf.sddenr.net/lithdb/). These were organized into three transects reflecting the geometry of the lobe.
Transect A-A’ is 220 km long and trends West-East along the northern James River Lowland.
Eleven lithologic logs at a spacing of 20 km were used to construct the cross section. These logs varied in depth from 10-150m. Transect B-B’ is 110 km long and trends west-east across the central James River
Lowland. Eleven lithologic logs from drill holes that ranged in depth from ~70-100m were acquired at a spacing of 10 km for Transect B-B’. Transect C-C’ is 380 km long and trends north-south from the North
Dakota/South Dakota border to the Missouri River. A total of 20 lithologic logs were acquired at a spacing of 20 km. These lithologic logs ranged in depth from ~30-100m. The geographic information for all lithologic logs was downloaded and plotted using QGIS software version 2.8.1.
80 2.3. Acquisition and reduction of radiocarbon ages
Radiocarbon ages were acquired from county reports and bulletins published by the South Dakota
Geological Survey (SDGS), and through personal communication with L. Shultz of the SDGS. All of the material used for radiocarbon dating was recovered from the subsurface. Criteria used for quality control of the radiocarbon age data set were as follows:
1. The original source or citation was available for every sample number, and the radiocarbon age
and stratigraphic context could be verified;
2. Each sample had clear and definite geologic context and stratigraphic position;
3. Each sample had location data (i.e. latitude, longitude, depth);
4. Each sample was from organic material that was in place and showed no sign of transport (i.e.
glaciotectonic disturbance).
In total, twenty-four radiocarbon ages were retained, and eight were excluded based on the above criteria (Table 1). All radiocarbon ages were recalibrated using CALIB 7.1 software, which utilizes the
IntCal13 calibration curve (Reimer et al., 2013). Radiocarbon age are presented in the text and figures as the mean calibrated age (abbreviated as ka, thousands of years before 1950). The basemap was derived from SRTM 1-arc-second resolution DEM files, which were downloaded from the United States
Geological Survey’s EarthExplorer tool (earthexplorer.usgs.gov). Hillshade layers were created in QGIS using the DEM files. The raster layer of South Dakota county boundaries was downloaded from the
South Dakota GIS Data site (http://opendata2017-09-18t192802468z-sdbit.opendata.arcgis.com).
Each radiocarbon age offers either a maximum or minimum age constraint for a change in glacier margin change. In this case, we consider the stratigraphic unit from which each sample was collected in order to determine whether it is a maximum or minimum constraining age. If the sample was collected from an outwash unit, then the radiocarbon age would be considered to be a minimum constraining age for the presence of the James Lobe at that location. If the sample was collected from a till unit, then it would be interpreted as a maximum age for the enclosing till and for all overlying stratigraphic units. If a sample was collected from interstadial sediments between till units, then the radiocarbon age would be a
81 maximum for the overlying till, and a minimum age for the underlying till. To establish context for each of the radiocarbon ages used in this study, we use the sediment descriptions and interpretations of the sample collectors and sediment loggers.
3. Results
3.1. Stratigraphy
3.1.1. Transect A-A’
Transect A-A’ stretches across the northern James Lobe from the Missouri Coteau to the Prairie
Coteau. The Pierre Shale is overlain in most places by a unit of unoxidized sand, gravel and clay which varies in thickness from 5m thick on the Missouri Coteau to 40 m thick in the central James Lobe valley to 60 m thick on the Prairie Coteau. This unit is interpreted as glacial till. It is difficult to differentiate between these interbedded layers of clayey sand and gravel, sandy gravel, etc. because many of them are discontinuous, so there may be more than one glacial till unit here representing more than one glacial episode. For the purposes of this discussion, we label this till unit “Till B”. In the central portion of transect A-A’, Till B is overlain by up to 20 m of lacustrine deposits. To the east and west Till B is absent and is replaced by lenses of fine gravel. Overlying till B is a unit of oxidized sand, gravel and clay unit that is continuous from the Missouri Coteau to the Prairie Coteau and varies in thickness from
~5 m to 10 m. This unit is interpreted as a separate glacial till unit based on oxidation and underlying lacustrine deposits and is labeled “Till A”. Till A is exposed at the surface and is sometimes overlain by
< 1.0 m of soil, sand or lacustrine sediment.
Overall, the stratigraphy of transect A-A’ suggests two separate advances of the James Lobe, which deposited Till B and Till A separated by an interstadial, during which a proglacial lake formed, and lacustrine sediments were deposited.
82 3.1.2. Transect B-B’
Overlying the Niobrara Formation of the Pierre Shale is a thick (up to 20 meters) unit of unoxidized sand, gravel and clay interbedded with lenses of fine sand with rare gravel and rocks. This unit is interpreted as Till B. At sites 11-14 (Figure 4.2) the upper three meters of this unit is oxidized and weathered and exhibits some soil development in some places (Hedges, 2001). This weathered contact is overlain by loess or lacustrine silt (Hedges, 2001). The overlying unit, identified as Till A, consists of oxidized sand gravel and clay that ranges in thickness from 5 to 20 meters. Till A is stratified in some places and contains lenses of find sand (Figure 4.3).
Similar to transect A-A’, the stratigraphy of transect B-B’ suggests two advances of the James
Lobe, separated by an interstadial during which Till B was weathered, and either lacustrine sediments or loess was deposited.
3.1.3. Transect C-C’
Transect C-C’ extends from the South Dakota- North Dakota border in the northern James Lobe to the South Dakota-Nebraska border at the Missouri River. The transect is not a straight line - its path follows that of the main trunk of the James Lobe (Figure 4.2). The bedrock is overlain by Till B - a unit of grey, unoxidized clayey sand and gravel interbedded with thin (<1m) layers of sand and gravel and fine to medium grained sand. Till B varies in thickness from 30-70m with common laminated silt and sand lenses (<1m thick) in the central portion of the transect (Figure 4.5). Till B is overlain by Till A — a five-meter thick unit of oxidized clay, silt and sand. This unit is exposed at the surface and in several places is covered by less than one meter of soil or lacustrine sediment.
3.1.4. Summary of stratigraphy
Overall, we identify two till units that overlie that Pierre Shale bedrock in the James River
Lowland. Till B is a grey, unoxidized massive unit of clayey sand and gravel with interbedded sections of sand and gravel, lenses of fine to medium sand, and lenses of laminated silt. Reworked pieces of Pierre
83 Shale are common near the lower contact of Till B. Overlying Till B in the northern James River lowland is a thick package of lacustrine sediment (Figure 4.3), indicative of interstadial conditions following the deposition of Till B. Interstadial lacustrine deposits or loess is also present along Transect B-B’ (Figure
4.4), near sample sites 11-14 (Figure 4.2).
Overlying these interstadial deposits is Till A, which is exposed at the surface across much of the
James Lobe and occurs as yellow, oxidized clayey sand and fine gravel. Till A is distinguished from Till
B based on oxidation. Where the interstadial sediments are absent, the contact between the two till units is sharp. Gravel stringers are common throughout Till A, and in some locations the till is stratified. In the northern James Lobe and the Pierre Sublobe, Till A is especially silty and pebbly. We name this unit
“Till A” for discussion purposes.
3.2. Radiocarbon chronology
In the James Lobe, there are two sets of maximum ages based on stratigraphic context. The first set comes from within or below Till B, and the second set comes from within or below Till A. Each set of maximum radiocarbon ages provides maximum age constraint for the timing of deposition of the overlying or enclosing till. In the following sections, maximum limiting radiocarbon ages from within and beneath both Till B and Till A are reviewed (Table 1). Radiocarbon ages excluded from analysis are presented in Table 2.
A total of three maximum ages come from within Till B on the Prairie Coteau. The oldest age of
33.7 ± 2.3 ka (GX-14675) comes from a wood sample collected from a river bank section in Grant
County. The two remaining ages of 30.1 ± 6.1 ka (GX-2864) and 27.0 ± 2.0 ka (GX-3439) come from drill holes in Hamlin County (Beissel and Gilbertson, 1987; Gilbertson, 1990). According to available lithologic logs, both of these samples come from the same stratigraphic position as GX-14675.
A total of eleven maximum ages come from the base of or within Till A in the main James Lobe footprint (Figure 4.2). These ages range from 15.4 ± 0.6 to 14.3 ± 1.0 ka (Beta-30559, W-1372, W-987,
W-801, W-1756, GX-5611, W-1189, I-11976, I-11975, Y-452, I-6361, Y-925). One older age of 16.9 ±
84 1.3 ka (W-1373; Site 10), and one younger age of 14.1 ± 1.0 (W-1033; Site 13) from this stratigraphic position were excluded as outliers.
Minimum radiocarbon ages come from surface sediments overlying Till A that are interpreted as post-glacial outwash or lacustrine sediments. A total of twelve minimum ages help constrain the timing of James Lobe margin changes. Four minimum radiocarbon ages come from lake cores collected from
Medicine Lake and Pickerel Lake on the Prairie Coteau. Two ages of 14.7 ± 0.5 and 12.8 ± 0.2 ka (WIS-
1225 and WIS-1227, respectively; Bender et al., 1982) come from Medicine Lake in Codington County,
South Dakota. Two ages of 13.9 ± 0.1 ka (WW-2112; Dean and Schwalb, 2000) and 12.4 ± 0.3 ka (Y-
1361; Watts and Bright, 1968) come from sediment cores from Pickerel Lake in Day County.
A total of four samples (W-1530, Y-595, W-2305, and W-1033) come from outwash deposits in the main James Lobe footprint and range in age from 15.1 ± 0.4 to 10.6 ± 0.5 ka (Deevey et al., 1959;
Ives et al., 1964; Ives et al., 1967; Spiker et al., 1978). One age of 13.9 ± 1.2 ka (W-1755) comes from lacustrine deposits exposed at the land surface (Steece, 1966).
3.2.1 Summary of chronology
Two maximum bracketing radiocarbon ages for Till B of 33.7 ± 2.3 to 27.0 ± 2.0 ka come from the Prairie Coteau (Figure 4.2) and produce an average age of 30.3 ± 3.4 ka. Maximum radiocarbon ages for the deposition of Till A are available from radiocarbon ages of organics collected at the contact between Tills A and B. These ages range from 15.4 ± 0.6 to 14.3 ± 1.0 ka. These ages provide a minimum age of ~16 ka for Till B and a maximum age of ~13 ka for Till A based on the summed probability plot in Figure 4.6.
4. Discussion
4.1. Evaluation of chronologic data
Clayton et al. (1980) suggested that many of the maximum radiocarbon ages from wood below and within Till A were anomalous. The authors suggest that the wood was buried by mass movement
85 during melting of buried stagnant glacial ice, and that the wood was deposited during an earlier advance.
This idea was based on the assumed age for the formation of Lake Agassiz in eastern North Dakota and western Minnesota that indicated that the James Lobe must have been fully retreated by ~14 ka (Clayton and Moran, 1982). This notion was subsequently reevaluated based on updated chronology of Lake
Agassiz and now suggests that because the wood samples come from several meters depth, they are free of contamination or the effects of post-depositional disturbance.
4.2. Glacial chronology of the James Lobe
In earlier sections we identified two glacial till units in the James River lowland: the lower unoxidized till (Till B) and the upper oxidized till (Till A). The youngest maximum age from Till B indicates that the unit was deposited after 27.0 ± 2.0 ka during an extensive advance. Till B is present in all three transects discussed above, which suggests an extensive advance that covered the entirety of the
James River Lowland, covered the Prairie Coteau, and impinged on the Missouri Coteau.
The weathering horizon at the upper contact of Till B observed in the southern James River
Lowland suggests that the ice margin retreated, and the till surface was exposed at the land surface. Till
B was then covered by either a proglacial lake or wind-blown loess, depositing the overlying silt unit observed in the southern James River Lowland (Levin et al., 1965; Hedges, 2001). Radiocarbon ages of wood and shells from this contact also provide minimum age constraint for the deposition of Till B.
These ages range from 15.4 ± 0.6 to 14.3 ± 1.0 ka (Beta-30559, W-1371, W-987, Y-925, W-801, W-
1756, GX-5611, Y-452, W-1189, I-11976, I-11975; Sites 8, 11-13, 15-17, 19, 21, 23, and 24, respectively). Taken together, both maximum and minimum radiocarbon ages suggest Till B was deposited between ~27 and ~16 ka.
Till A represents a readvance of the James Lobe in the southern James River Lowland.
Maximum radiocarbon ages from below and within Till A indicate the margin readvanced ~13 ka. The spatial extent of Till A and associated maximum radiocarbon ages indicate that the James Lobe occupied the James River lowland and reached the Missouri River trench to the south during this advance.
86 The stratigraphy of Transects A-A’ and B-B’ indicate that the ice may have expanded slightly onto the Missouri and Prairie Coteaus. However, the ice margin did not reach the Missouri River to the west (Heath et al., in prep.; Chapter 2). By this time, the higher elevation areas of the Missouri Coteau were occupied by stagnant ice which was covered by superglacial till as active ice occupied the James
River Lowland (Christensen, 1977). Two radiocarbon ages from superglacial till provide minimum ages for stagnation of ice at high elevations on the Missouri Coteau 17.2 ± 0.6 and 10.4 ± 0.8 ka. One additional minimum age of 13.5 ± 0.4 ka comes from a kettle lake surrounded by stagnation moraine within the Pierre Sublobe (Figure 4.2; Site 6). This represents the beginning of sediment deposition in
Cottonwood Lake (Site 6) and provides minimum age constraint for the melting of stagnant ice in this part of the Missouri Coteau. This minimum age also indicates the absence of ice in the Pierre Sublobe by
13.5 ± 0.4 ka. Basal radiocarbon ages from kettle lakes on the Prairie Coteau suggest that sedimentation within the lakes had begun by ~14.8 ka.
4.3. Comparison with the Des Moines Lobe, Iowa
In past reconstructions of southern Laurentide behavior, some researchers have suggested that the
Des Moines Lobe and James Lobe had the same source – the Keewatin Dome of the Laurentide Ice Sheet
(e.g. Denton and Hughes, 1981; Dyke and Prest, 1987). These are thought to have been the only two lobes of the southern Laurentide margin fed by this dome. Thus, we compare the pattern of James Lobe advance and retreat to that of the Des Moines Lobe in Iowa to determine if the two lobes exhibited the same timing and extent.
The James Lobe underwent its first advance of the last glacial period after ~27.1 ka. This advance correlates with that of the Des Moines Lobe after ~24.1 ka, although this estimate is based on one radiocarbon age from the Tazewell till (O-1325; Ruhe, 1969). The upper contacts of the Tazewell till of the Des Moines Lobe and Till B of the James Lobe exhibit weathering (Hedges, 2001; Hallberg and
Kemmis, 1986), which suggest that both lobes retreated following this initial advance.
87 The period of slowed or paused retreat of the Pierre Sublobe of the western James Lobe margin
(Figure 4.2) before ~15.7 ka correlates with an advance of the Des Moines Lobe after ~16.2 ka (Chapter
1; Lowell et al., 1999; Heath et al., 2018). The James Lobe underwent its final advance after ~13 ka, which may correlate with the “Algona” readvance of the Des Moines Lobe after ~13.6 ka (DIC-1361;
Hallberg and Kemmis, 1981). Two minimum ages atop the Algona Moraine of ~13.4 ka (I-3145 and I-
1416, respectively; Hallberg and Kemmis, 1981) suggest that the margin retreated within centuries of reaching the Algona position. The overlap of minimum and maximum radiocarbon ages from the James
River Lowland suggest that the James Lobe also underwent rapid retreat.
4.4. The role of climate in the fluctuations of the western lobes
We advance an alternative hypothesis for the mechanism responsible for the relatively late, synchronous behavior of the western lobes during the last glacial period and termination. Summer temperatures along the southern James Lobe and Des Moines Lobe margin may have experienced episodic decrease during the termination, leading to decreased ablation and similar timing of either stabilization or advance.
5. Conclusions
1. The James Lobe advanced after ~27 ka, depositing Till B across the James River Lowland, the
Prairie Coteau and the Missouri Coteau. The lobe then retreated to at least 44.3° latitude,
exposing Till B; James Lobe readvanced after ~13.3 ka, depositing Till A and reaching the
Missouri River.
2. The James Lobe and Des Moines Lobe exhibited synchronous behavior during the last glacial
period and Termination. The James lobe advanced after ~27 ka, stabilized before ~15.9 ka in the
Pierre Sublobe, and advanced after ~13 ka. The Des Moines Lobe advanced after ~24.3 ka, after
16.2 ka, and after ~13.6 ka.
88 3. The existing paradigm for western lobe behavior invokes surging, which does not consider
synchronous behavior of James Lobe and Des Moines Lobe.
4. An alternate hypothesis invokes summer temperature along the margin as the primary control on
ice margin position during Termination.
89 Tables
14C Lower cal. Upper cal. Average Site ID Sample ID Latitude Longitude Material 14C age Error Context Reference error range Range age
1 W-2305 45.852500 -99.204167 Pelecypod shells 9220 300 9610 11198 10404 794 Min. age - MC Christensen, 1977
2 I-6361 45.824444 -99.259722 Organic material 14190 220 16595 17850 17223 628 Min. age - MC Christensen, 1977
3 WW-2112 45.5333 -97.28333 Wood 12070 40 13772 14062 13917 145 Min. age - PC Dean and Schwalb, 2000
3 Y-1361 45.533333 -97.283333 Picea Wood 10670 140 12131 12808 12470 339 Min. age - PC Watts and Bright, 1968
4 W-2201 45.476667 -97.589167 Pelecypod shells 10880 320 11939 13430 12685 746 Min. age - PC Sullivan et al. 1970
5 WIS-1225 44.980556 -97.356944 Gyttia 12610 120 14287 15297 14792 505 Min. age - PC Bender et al. 1982
5 WIS-1227 44.980556 -97.356944 Wood 10940 140 12647 13088 12868 221 Min. age - PC Bender et al. 1982
6 WIS-1626 44.83333 -99.9 Picea needles 11690 180 13152 13872 13512 360 Min. age - MC Barnoskey et al., 1987
7 GX-2864 44.751389 -96.929167 Wood 26150 3000 23941 36296 30119 6178 Max. age - lowermost till Beissel and Gilbertson, 1987
8 Beta-30559 44.666667 -98.966667 Picea Wood 12220 150 13747 14855 14301 554 Max. age - uppermost till Martin and Klukas, 1989
9 GX-3439 44.645000 -96.925556 Wood 22900 1000 25066 29069 27068 2002 Max. age - lowermost till Beissel and Gilbertson, 1987
10 W-1373 44.484444 -98.0175 Pelecypod shells 14000 500 15577 18261 16919 1342 Max. age - uppermost till Levin et al., 1965
11 W-1372 44.194722 -98.450556 Wood 12200 400 13282 15514 14398 1116 Max. age - uppermost till Levin et al., 1965
12 W-987 44.135000 -98.353611 Wood Fragments 12530 350 13732 15842 14787 1055 Max. age - uppermost till Ives et al., 1964
13 Y-925 44.013611 -98.346111 Wood 12520 100 14240 15140 14690 450 Max. age - uppermost till Stuiver, 1969
14 W-1033 44.098611 -98.258333 Shells 10060 300 10768 12565 11667 899 Max. age - uppermost till Ives et al., 1964
15 W-801 43.946389 -97.749167 Wood 12200 400 13282 15514 14398 1116 Max. age - uppermost till Rubin and Alexander 1960
16 W-1756 43.695833 -97.975 Wood 12340 300 13566 15374 14470 904 Max. age - uppermost till Ives et al., 1967
17 GX-5611 43.542500 -98.674444 Wood 12180 360 13308 15325 14317 1009 Max. age - uppermost till Hedges, 2001
18 W-1755 43.408333 -96.645833 Lymnaea shells 11770 500 12701 15233 13967 1266 Min. age Steece, 1966
19 Y-452 43.372778 -97.120833 Spruce Wood 12330 180 13823 15082 14453 630 Max. age - uppermost till Barendsen et al., 1957
20 Y-595 43.320556 -97.096944 Picea Wood 12760 120 14713 15654 15184 471 Min. age Deevey et al., 1959
21 W-1189 43.240278 -97.596111 Wood 12050 400 13115 15269 14192 1077 Max. age - uppermost till Ives et al., 1964
22 W-1530 43.186111 -97.658333 Wood 9300 200 10133 11178 10656 523 Min. age Ives et al., 1967
23 I-11976 43.083333 -97.385278 Wood 12880 170 14770 15945 15358 588 Max. age - uppermost till SDGS Lithologic log
24 I-11975 42.954167 -97.524722 Wood 12540 170 14111 15303 14707 596 Max. age - uppermost till SDGS Lithologic log Table 4.1. Radiocarbon age sample information. Site IDs correspond with Figure 4.2. In context column, MC = Missouri Coteau, PC = Prairie Coteau
90
Sample Latitude Longitude Material Reference Notes ID Unable to find GX- 43.458889 -96.43889 Wood SDGS Lithologic log documentation of 14C 13776 age Litho log/ Radiocarbon Wood Ives et al. 1964; Redrilled hole. Same W-983 44.135000 -98.35361 Fragments Aurora/Jerauld County log as another sample? Report Radiocarbon Deevey et Y-571 44.716667 -101.2167 Wood Possible contamination al. 1959 Radiocarbon Ives et al. Not in place - W-1757 43.995833 -98.31 Wood 1967 transported Unable to find original GX-5614 45.529167 -97.15556 Wood Unknown source Unable to find original GX-2565 45.222778 -96.94583 Mullusk shells Unknown source Clayton and Moran Collected from beneath 1982; Gilbertson 1990 GX-2741 45.308333 -96.475 Wood a glacially-disturbed thesis; NE SD block of till guidebook McCormick and Uncertain stratigraphic I-6561 42.877778 -96.72917 Wood Hammond, 2004 context
Table 4.2. Radiocarbon ages excluded from analysis due to uncertain stratigraphic context. Notes column includes reasoning behind exclusion.
91 Figures
Figure 4.1. Maximum extent of the Laurentide ice Sheet during the last glacial period. Dashed lines represent approximate flow lines after (Denton and Hughes, 1981). The James Lobe is outlined by a black box and is detailed in Figure 4.2.
92
Figure 4.2. Digital elevation model of the James Lobe in eastern South Dakota. Blue triangles represent locations of maximum radiocarbon ages. Red upside-down triangles represent locations of minimum radiocarbon ages. Each site is numbered and refers to the list of calibrated radiocarbon ages at left. Radiocarbon ages are presented as thousands of years before present (ka). Transects A-A’, B-B’ and
C-C’ are represented by black lines across the James River Lowland.
93
Figure 4.3. Stratigraphic cross section of transect A-A’. The starting point is 45.828°N, -99.266°W, and end point at 44.414°N, -97.353°W. b) Key for stratigraphic units in Figures 4.3-4.5. Format follows
South Dakota Geological Survey County Bulletins.
94
Figure 4.4. Stratigraphic cross section of transect B-B’. The starting point is 44.064°N, -98.586°W; and end point at 44.404°N, -98.576°W. Unit abbreviations are described in Figure 4.4b. Blue triangles represent maximum radiocarbon ages listed in Table 4.1. Site numbers correspond to Table 4.1 and
Figure 4.2.
95
Figure 4.5. Stratigraphic cross section of transect C-C’. The start point is 45.993°N, -98.262°W, with a bend at 45.191°N, -98.579°W. The end point is 42.947°N, -97.305°W. Blue triangles represent maximum radiocarbon ages. Red triangles represent minimum radiocarbon ages. Site numbers correspond to Table 4.1 and Figure 4.2.
96
Figure 4.6. Summed probability distribution of maximum radiocarbon ages from contact between
Tills B and A. Red diamonds represent individual calibrated radiocarbon ages. Red lines represent errors. Blue line represents summed probability of ages.
97
References
Barendsen, G.W., Deevey, E.S., Gralenski, L.J., 1957. Yale natural radiocarbon measurements III. Science 126 (3279), 908-919. Bamosky, C. W., Grimm, E. C., and Wright, H. E., Jr. 1987. Towards a postglacial history of the northern Great Plains: A review of the paleoecologic problems. Annals of the Carnegie Museum 56, 259- 273. Biessel, D.R., Gilbertson, J.P., 1987. Geology and water resources of Deuel and Hamlin Counties, South Dakota, Part 1: Geology. South Dakota Geological Survey Bulletin 27, 41 pp. Christensen, C.M., Stephens, J.C., 1967. Geology and hydrology of Clay County South Dakota. South Dakota Geological Survey Bulletin 19, 86 pp. Christensen, C.M., 1974. Geology and water resources of Bon Homme County, South Dakota, Part I: Geology. South Dakota Geological Survey Bulletin 21, 48 pp. Christensen, C.M., 1977. Geology and water resources of McPherson, Edmunds and Faulk counties, South Dakota, Part I: Geology. South Dakota Geological Survey Bulletin 26, 58 pp. Clark, P. U., 1992, Surface form of the southern Lau- rentide Ice Sheet and its implications to ice-sheet dynamics: Geological Society of America Bul- letin, v. 104, p. 595–605. Clayton, L., Moran, S.R., 1982. Chronology of late Wisconsinan glaciation in middle North America. Quat. Sci. Rev. 1, 55-82. Dean, W.E., Schwalb, A., 2000. Holocene environmental and climatic change in the northern great plains as recorded in the geochrmistry of sediments in Pickerel Lake, South Dakota. Quaternary International 67, 5-20. Deevey, E.S., Gralenski, L.J., Hoffren, V., 1959. Yale natural radiocarbon measurements IV. American Journal of Science Radiocarbon Supplement 1, 144-172. Denton, G.H., Hughes, T., 1981. The Last Great Ice Sheets. John Wiley and Sons, New York, p. 484. Dyke, A.S., Prest, V.K., 1987. Paleogeography of northern North America, 18000-5000 years ago. Geological Survey of Canada, Map 1703A, scale 1:12,500,00. Flint, R.F., 1955. Pleistocene Geology of Eastern South Dakota. Geological Survey Professional Paper 262, p. 173. Gilbertson, J.P., 1990. Quaternary geology along the eastern flank of the Coteau des Prairies, Grant County, South Dakota. Master’s Thesis, University of Minnesota, 155 pp. Hallberg, G.R., Kemmis, T.J., 1986. Stratigraphy and correlation of the glacial deposits of the Des Moines and James lobes and adjacent areas in North Dakota, South Dakota, Minnesota, and Iowa. In: Sibrava, V., Bowen, D.Q., Richmond, G.M. (Eds.), Quaternary Glaciation in the Northern Hemisphere. Pergamon, Elmsford, N.Y. Quaternary Science Reviews 5. Hedges, L.S., 2001. Geology of Aurora and Jerauld counties, South Dakota. South Dakota Geological Survey Bulletin 32, 31 pp. Ives, P.C., Levin, B., Robinson, R.D., Rubin, M., 1964. U.S. Geological Survey radiocarbon dates VII. Radiocarbon 6, 37-76. Ives, P.C., Levin, B., Oman, C.L., Rubin, M., 1967. U.S. Geological Survey radiocarbon dates IX. Radiocarbon 9, 505-529. Johnson, G.D., McCormick, K.A., 2005. Geology of Yankton County, South Dakota. South Dakota Geological Survey Bulletin 34. 30 pp. Levin, B., Ives, P.C., Oman, C.L., Rubin, M., 1965. U.S. Geological Survey radiocarbon dates VIII. Radiocarbon 7, 372-398. Martin, J.E., Klukas, R.W., 1989. New locality for late Pleistocene fossils from Edmunds County, South Dakota. Proc. S.D. Acad. Sci., 68, 57-58. McCormick, K.A., Hammond, R.H., 2004. Geology of Lincoln and Union Counties, South Dakota. Department of Environment and Natural Resources/Geological Survey Bulletin 39, 43 pp.
98 Mickelson, D.M., Colgan, P.M., 2003. The southern Laurentide Ice Sheet. Developments in Quaternary Science. 1, 1-16. Patterson, C. J., 1995, Surficial geologic map, in Setter- holm, D. R., project manager, Regional hydro- geologic assessment: Quaternary geology— Southwestern Minnesota: Minnesota Geological Survey Regional Hydrogeologic Assessment Series RHA-2, Part A, plate 1, scale 1:200 000. Patterson, C. J., 1997a, Southern Laurentide ice lobes created by ice streams: Des Moines lobe in Min- nesota, U.S.A.: Sedimentary Geology, v. 111, p. 249–261. Patterson, C. J., 1997b, Quaternary geology of south- western Minnesota, in Patterson, C. J., ed., Contributions to the geology of southwestern Minnesota: Minnesota Geological Survey Report of Investigation 47, p. 1–45. Reimer, P.J., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Ramsey, C.B., Buck, C.E., Cheng, H., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Haflidason,H.,Hajdas,I.,Hatte,C.,Heaton,T.J.,Hoffmann,D.L.,Hogg,A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., Manning, S.W., Niu, M., Reimer, R.W., Richards, D.A., Scott, E.M., Southon, J.R., Staff, R.A., Turney, C.S.M., van der Plicht, J., 2013. IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP. Radiocarbon 55, 1869-1887. Rubin, M., Alexander, C., 1960. U.S. Geological Survey radiocarbon dates V. American Journal of Science Radiocarbon Supplement 2, 129-185. Spiker, E., Kelley, L., Rubin, M., 1978. U.S. Geological Survey radiocarbon dates XIII. Radiocarbon 20 (1), 139-156. Steece, F.V., 1966, Late Wisconsin ice-perched lake deposit, Lincoln County, South Dakota. Proceedings of the South Dakota Academy of Science, v. 45, p. 67-73. Stuiver, M., 1969. Yale natural radiocarbon measurements IX. Radiocarbon 11 (2), 545-658. Sullivan, B.M., Spiker, E., Rubin, M., 1970. U.S. Geological Survey radiocarbon dates XI. Radiocarbon 12 (1), 319-334. Watts, W.A., Bright, R.C., 1968. Pollen, seed and mollusk analysis of a sediment core from Pickerel Lake, northeastern South Dakota. Geological Society of America Bulletin 79, 855-876. Wright, H. E., Jr., 1980, Surge moraines of the Klutlan glacier, Yukon Territory, Canada: Origin, wastage, vegetation succession, lake development, and application to the late-glacial of Minnesota: Quaternary Research, v. 14, p. 2–18.
99
Chapter 5
Implications of new James Lobe chronology
ABSTRACT
The Laurentide Ice Sheet was the largest ice sheet of the last glacial period that culminated in an extensive terrestrial margin. The southern Laurentide lobes are thought to have behaved asynchronously between the eastern and western sectors during Heinrich Stadials 2 and 1 (HS2 and HS1, respectively).
New chronologic data from the James Lobe, South Dakota bears on the timing of fluctuations of the western sector. In this report we synthesize chronologic data from the James Lobe and Des Moines Lobe to establish the advance and retreat patterns of the western lobes during the last glacial period and termination. We compare these patterns to those of the eastern lobes and propose possible mechanisms for each time slice. Between ~27 and 19 ka, all of the southern Laurentide lobes underwent advance, in agreement with the existing hypothesis of Northern Hemisphere cooling and subsequent ice sheet expansion associated with decreased northern insolation. At ~16 ka, the New England margin, Lake
Michigan Lobe, Des Moines Lobe, James Lobe of the southern Laurentide margin, and the Puget Lobe of the Cordilleran Ice Sheet either stabilized or advanced. At ~13 ka, the New England Margin, Green Bay
Lobe, Des Moines Lobe and James Lobe either stabilized or advanced. We find that the lobes of the southern Laurentide exhibit similar timing but different extents of advance during the last glacial period and Termination. We attribute widespread advance during the Termination to cooler summers and reduced ablation along the southern Laurentide margin. We suggest that the mechanisms behind the advance and retreat patterns of the southern Laurentide margin are time-dependent.
1. Introduction
In an effort to determine whether terrestrial ice sheet margins advance in response to cool sea surface temperature (SST), Heath et al., (2018) compared the timing of southern Laurentide margin
100 change to periods of cool SST in the North Atlantic Ocean. The behavior of the southern Laurentide Ice
Sheet appears to be different during Heinrich Stadials 2 and 1. During HS2, southern Laurentide Lobes
(New England Region, Lake Huron-Erie Lobe, Lake Michigan Lobe, Green Bay Lobe and Chippewa
Lobe) advanced towards their maximum position, except for the Des Moines Lobe in Iowa, for which there is no applicable data. During HS1, those lobes experienced substantial retreat, while the Des
Moines Lobe advanced to its maximum position. Until recently the Des Moines Lobe was the only lobe that underwent advance during HS1, and its behavior seemed anomalous. However, there remained a gap in the chronology of the southern Laurentide margin – the James Lobe of South Dakota, which is thought to have shared the same ice stream source as the Des Moines Lobe (Clayton and Moran, 1982; Hallberg and Kemmis, 1986; Margold et al., 2015). The James Lobe was addressed in Chapters 3 and 4, in which a new surface exposure chronology was developed, and existing subsurface radiocarbon ages were reanalyzed. The objective of this report is to step back and understand the James Lobe glacial chronology in the context of the entire southern Laurentide margin.
2. Methods
The following sections detail the methodology used to establish the glacial chronology of the
James Lobe. The surface exposure ages from the Pierre Sublobe (Chapter 3) and radiocarbon ages from the main James Lobe footprint (Chapter 4) are revisited and summarized.
2.1. Chronologic data from James Lobe glacial deposits
In Chapter 4, two till units associated with the James Lobe are identified in the James River lowland — Till B and Till A. Till B underlies Till A and exhibits a ~3 m deep weathering horizon at its upper contact (Hedges, 2001). The two tills are separated in some places by lacustrine silts or loess
(Hedges, 2001). Radiocarbon ages of organics collected at the contact of Tills B and A provide age constraint for both till units. These ages provide a minimum age of ~16 ka for the deposition of Till B
(Chapter 4) and a maximum age of ~13 ka for till A (Chapter 4). Minimum radiocarbon ages for Till A
101 come from surficial outwash and lacustrine deposits (Figure 5.2; Sites 5, 10, 13, 15) and range in age from 15.1 ± 0.4 to 13.9 ± 1.2 ka (Deevey et al., 1959; Ives et al., 1964; Ives et al., 1967).
10Be surface exposure ages come from four sites within the Pierre Sublobe, a westward flowing sublobe along the James Lobe’s western margin (Chapter 3). These ages represent record the time elapsed since the boulders (and moraine surface) was uncovered by ice, therefore are minimums for retreat. The two sites that abut the Missouri River trench (Sites 1 and 2; Figure 5.3) produce surface exposure ages that exhibit a range of 10,000 years. However, if correct, the oldest cluster of ages ~19.0 ka can be taken as a minimum age for retreat. Sites 3 and 4 lie on the southern end of a large arcuate moraine complex in the easternmost Pierre Sublobe (Figure 5.3) which is the product of ice margin stabilization. These two sites produce ten internally consistent, tightly clustered ages that provide a minimum age of ~15.7 ka for the retreat of the ice margin from Sites 3 and 4 (Chapter 4).
3. Discussion
Here we first summarize the timing of glacier cover in the James Lobe. We then compare that working chronology with the adjacent Des Moines lobe to the east to develop a working correlation. To further understand the control of these lobs, we discuss the difference between correlations based on ice extent and on timing. Using this framework, we consider Laurentide Ice Sheets lobes located further east.
After these comparisons we examine current paradigms for the ice sheet and discuss the implications of these findings for those models.
3.1. Summary of James Lobe behavior
Maximum radiocarbon ages from the Prairie Coteau indicate that the James Lobe was present in easternmost South Dakota after ~27.1 ka. New surface exposure ages from the westernmost Pierre
Sublobe, a westward flowing sublobe of the western James Lobe margin, indicate that the ice margin may have extended to the Missouri River trench to its west. These surface exposure ages indicate that for the western margin of the James Lobe retreat from the Missouri River was underway by ~19 ka. This age
102 provides a minimum age for the presence of ice in the Pierre Sublobe (Heath et al., submitted; Chapter 3).
The period of ice cover between ~27 and 19 ka corresponds to “Phase D” of Clayton and Moran (1982), which is based on the radiocarbon ages from the Prairie Coteau.
The next event in the history of James Lobe margin change is characterized by two key changes.
After ~19 ka, the Pierre Sublobe retreated ~100 km and stabilized in the eastern Pierre Sublobe where it deposited a large arcuate moraine complex. Surface exposure ages from two moraine segments along the southernmost segment of this moraine complex indicate that the retreat of the Pierre Sublobe from this position was underway before ~15.7 ka. (Heath et al., submitted; Chapter 3). The second change is the
~200 km retreat of the main trunk of the James Lobe. Evidence for this retreat comes from the stratigraphic record. Underlying the surface till is a unit of silt, which has been interpreted as either lacustrine or eolian (Hedges, 2001) but in either case indicates interstadial conditions during subaerial exposure in the central James River Lowland. In some places there is evidence of a buried forest rooted in the silt. Radiocarbon ages from wood collected from this layer provides a minimum age of ~16 ka for retreat and onset of interstadial conditions. The timing of this event corresponds roughly with “Phases F,
G, and H” of Clayton and Moran (1982), although in their reconstruction the James Lobe extended to the
Missouri River to the south.
The final advance of the James Lobe occurs after ~13.3 ka, when the lobe advanced ~230 km to its southernmost maximum extent at the Missouri River. Based on the absence of terminal moraines, the lobe likely terminated int eh Missouri River. During this advance, the ice overrode the interstadial silt unit discussed above (Chapter 4) and deposited the surface till. The extent of this advance is in good agreement with “Phase I” of Clayton and Moran (1982).
The final event of James Lobe margin change was substantial retreat from its southernmost maximum position at the Missouri River. Three minimum radiocarbon ages from the James River
Lowland range from 15.1± 0.4 to 13.9 ± 1.2 ka and overlap with maximum radiocarbon ages of subsurface wood. If we take the youngest age as minimum constraint for retreat of the main trunk of the
James Lobe, this suggests retreat from the southernmost James River lowland was underway by ~13.9 ka.
103
3.2. Comparison with the Des Moines Lobe: Did both western lobes advance and retreat at the same time?
Heath et al., (2018) revealed a complex pattern of advance and retreat across the southern
Laurentide margin during Heinrich Stadials 2 and 1. Specifically, the eastern and western sectors of the margin may have advanced and retreated at different times. In order to examine this pattern further, we first compare the two western sector lobes (the James Lobe to the Des Moines Lobe) to determine whether they exhibit the same pattern of advance and retreat. Toward this end, we use existing radiocarbon chronologies from both lobes (Heath et al., 2018; Chapter 4) and new surface exposure ages from the James Lobe (Heath et al., submitted; Chapter 3).
The initial advance of the James Lobe culminated after ~27.1 ka, during which the ice covered the Prairie Coteau. The behavior of the Des Moines Lobe during this time is constrained by two radiocarbon ages from Iowa — 29.4 ± 2.7 ka and 24.1 ± 1.7 ka (Beta-1764 and O-1325, respectively;
Ruhe, 1969), and one radiocarbon age of 24.6 ± 3.2 ka from southwestern Minnesota (GX-2741; Matsch,
1972). These three ages come from within or below the lowermost till unit associated with the Des
Moines Lobe. Therefore, they provide maximum age constraint for the deposition of that till. Based on their stratigraphic position. While the data set from the Des Moines Lobe is more limited, it is not inconsistent with the possibility both lobes on either side Prairie Coteau of the were in extended positions at the same time.
After ~19.1 ka, the Pierre Sublobe underwent retreat, based on surface exposure ages. Retreat of the Pierre Sublobe was interrupted by a period of ice margin stabilization before ~15.7 ka, during which the large arcuate moraine complex was deposited. The main trunk of the James Lobe also retreated before ~16 ka. After ~16.2 ka the Des Moines Lobe advanced to its maximum extent (Chapter 1; Heath et al., 2018). Given the differences in stratigraphy context afforded by these two dating techniques, we suggest the possibility that both the Des Moines and James Lobes advanced ~16 ka. However, we point
104 out that the James Lobe retreated prior to the moraine formation, whereas the Des Moines Lobes extended to its furthest south position at this time.
After ~13.3 ka, the James Lobe advanced to its southernmost maximum extent and terminated in the Missouri River. The Des Moines Lobe was retreating at this time, though some suggest that the lobe stabilized or readvanced to the Algona Moraine, which lies ~100 km north of the Des Moines Lobe terminal moraine ~13.6 ka (Ruhe, 1969; Kemmis et al., 1981; Clayton and Moran, 1982). Though the extends differed greatly, the advance of the James Lobe and the readvance of the Des Moines Lobe may correlate.
Overall, the timing of advance and retreat of the James Lobe and Des Moines Lobe is similar.
Both underwent advance after ~27 ka, and both lobes either advanced or stabilized ~16 ka and ~13 ka.
However, the extent or magnitude of these changes were very different. At ~16 ka the Des Moines Lobe advanced to its maximum extent, while the James Lobe stabilized following substantial retreat. The opposite is true for the next stage of advance ~13 ka. In the following section we discuss the difference between timing and extent of glacier margin change and its importance.
3.3. Extent versus timing
The behavior of a given glacier is typically influenced by both climate and individual physical characteristics of that glacier (e.g. Huybers and Roe, 2009). The extent refers to the geographic extent or magnitude of ice margin position change. Extent and magnitude of advance is controlled largely by many physical characteristics of the glacier and surrounding landscape, including topography, substrate, subglacial water pressure, bed slope and others (e.g. Clark, 1992; Lowell et al., 1999). The timing of ice margin change is controlled by climate – glacial advance or retreat typically coincides temporally with changes in climate conditions along the ice margin (Lowell et al., 1999). Thus, the extent of an ice margin advance and the timing of advance are controlled in many cases by different mechanisms. In the case of a terrestrial ice sheet margin, examining extent and timing of advance separately can help determine
105 dominant mechanisms from lobe to lobe, hence we compare the James and Des Moines package with other Laurentide margins in turn below.
3.3.1. Extent
The extent of each lobe’s advance is different. During the first phase of James Lobe ice cover
~27-19 ka, the eastern sector lobes reached their maximum extents. The extent of the western sector lobes is not well constrained. During the second phase of James Lobe advance, much further west, in the
Cordilleran Ice Sheet, the Puget Lobe advanced ~100 km to its maximum position ~16 ka (Porter and
Swanson, 1998) at the same time as the Des Moines Lobe advanced to its maximum extent (Ruhe, 1969;
Lowell et al., 1999). In between, the James Lobe stabilized at a retracted position ~100 km north of its maximum position. The New England margin underwent a readvance of less than 10 km in southern New
England called the Chicopee readvance ~17.0 ka (Ridge et al., 2012) and advanced to Pineo Ridge in southeastern Maine ~15.3 ka (Hall et al., 2017). The Lake Michigan Lobe also readvanced ~110 km
(Curry et al., 2018). However, both the New England margin and Lake Michigan Lobe were in a retracted position up to 300 km from their terminal positions. During the final phase of James Lobe ice cover, the James Lobe again advanced 120 km to its maximum position after ~13.3 ka. The New
England Margin advanced in the White Mountains of Northern New England ~13.2 ka (Bromley et al.,
2015) and the Green Bay Lobe advanced at Two Creeks ~13.7 ka (Broecker and Farrand, 1963). Finally, the Des Moines Lobe readvanced to the Algona Moraine ~13.6 ka (Ruhe, 1969; Kemmis et al., 1981;
Clayton and Moran, 1982). All of these lobes underwent minor readvances at retracted positions some
100-200 km behind their maximum extent.
3.3.2. Timing
The presence of James Lobe ice between ~27 and 19 ka coincides with widespread advance of all southern Laurentide lobes between ~29 and 20 ka (Chapter 2; Heath et al., 2018). When the James Lobe retreated and stabilized in the Pierre Sublobe before ~15.7 ka (Chapter 3; Heath et al., submitted), the Des
106 Moines Lobe advanced to its maximum position ~16.2 ka (Lowell et al., 1999). Further east, the Lake
Michigan Lobe readvanced ~18.5 - 16.5 ka (Curry et al., 2018) and the New England margin readvanced at ~17.3 ka and stabilized ~16.0 ka (Ridge et al., 2012). Advance during this time was not restricted to the southern Laurentide margin — the Puget Lobe of the southernmost Cordilleran Ice Sheet in
Washington advanced to its maximum position ~16 ka (Porter and Swanson, 1998). Finally, the James
Lobe advanced to its maximum position after ~14.7 ka (Chapter 3; Heath et al., submitted) and the Des
Moines Lobe readvanced to the Algona moraine (Ruhe, 1969; Kemmis et al., 1981; Clayton and Moran,
1982). This correlates with readvance of the New England margin at Pineo Ridge ~15.3 ± 0.2 ka (Hall et al., 2017) based on surface exposure ages. The Green Bay Lobe also readvanced ~13.7 ka, overrunning the Two Creeks forest bed (Broecker and Farrand, 1963).
3.3.3. Summary
As a working hypothesis, we suggest that the timing of glacier advance or ice margin stabilization across the southern Laurentide Lobes during the last glacial period and Termination is the same. The extent of these advances, however, is different from lobe to lobe. The most apparent difference in extent is the offset in timing of maximum extent. The eastern sector lobes reached their maximum extent earlier than the western lobes. In the following sections, we explore hypotheses that may explain the pattern of similar timing and differing extent.
3.4. Existing paradigms to explain patterns of advance and retreat
In this section, we discuss and evaluate the existing hypotheses for the patterns explored above.
The purpose is to determine whether these paradigms are sufficient in explaining the similarity of timing of southern Laurentide margin change and the disparity in extent or magnitude of change across the lobes.
107 3.4.1. Streaming and surging of the western sector lobes
The repeated advances of the western lobes during the Termination have led some researchers to invoke surging as the underlying mechanism (Clayton et al., 1985). This hypothesis attributes the extensive advances of the western lobes to surging or streaming due to clay-rich substrate that underlies both lobes (Clayton et al., 1989; Patterson, 1997, 1998; Jennings, 2006; Carlson et al., 2007). This clay- rich sediment would allow for high subglacial water pressure, high ice flow velocity and even decoupling of the lobe from its bed (Hooyer and Iverson, 2002). Some suggest that this was a cyclic process, appearing in the geologic record as multiple advances of a glacier in a short amount of time (Clayton et al., 1985; Patterson, 1997; Jennings, 2006).
The surging hypothesis is invoked to explain both the timing and extent of western lobe advances during the last glacial period (Clayton et al., 1985; Day, 2014). The timing of surges is not in tune with changes in climate or mass balance, rather it corresponds to when a subglacial water pressure threshold is reached (e.g. Clayton et al., 1985; Day, 2014). During episodes of surging, the Des Moines Lobe expanded longitudinally, extending its maximum extent beyond that supported by positive mass balance
(Cline, 2011). If correct, this suggests that the relatively late maximum extent does not reflect changes in mass balance due to climate, rather subglacial conditions of that specific lobe.
With emerging chronologic data from the rest of the southern Laurentide Ice Sheet margin, it is apparent that advance during the Termination time was not restricted to the western sector lobes. It is unlikely that surging tuned to the subglacial hydrologic conditions of the lobes was the mechanism that controlled timing of glacier advance, as the timing seems to be similar across the entire southern
Laurentide margin. Based on the number of southern Laurentide Lobes advancing during the
Termination, in addition to the Puget Lobe of the southern Cordilleran Ice sheet (Porter and Swanson,
1998), it is more likely that some other mechanism is controlling the timing of advance.
We are cautious to dismiss subglacial hydrology as a dominant mechanism controlling extent of western lobe advance during the Termination. The factors cited as evidence for surging may have indeed impacted magnitude of advance - high subglacial hydrology supported by a clay-rich substrate may have
108 been responsible on controlling the extent. However, it is likely that climate still played a role in the timing of advance and retreat of the western lobes.
3.4.2. Multiple Laurentide domes
On a larger scale, the dynamics of the southern Laurentide margin may have been controlled by different ice domes. In this model, the James and Des Moines Lobe flowed from the Keewatin Dome over northwestern Canada and the eastern sector lobes flowed from the Hudson Dome near Hudson Bay
(Denton and Hughes, 1981; Dyke and Prest, 1986). The two domes may have controlled extent and may explain the relatively late maximum extents of the western lobes. The eastern lobes reached their maximum extents before ~20 ka, and during their subsequent retreat, the Des Moines Lobe and James
Lobe advanced to their maximum extents, after ~16.2 and ~13.3 ka, respectively. If the Laurentide Ice
Sheet split from one to two different domes during deglaciation, the two domes may have operated as independent systems– due to either differences in accumulation or subglacial geomorphology or hydrology. This model is supported by our findings and may explain the differing extents reached by the southern Laurentide lobes through the Termination – specifically the late maximum extents of the western sector lobes relative to the eastern sector.
3.5. Hypothesis for climate conditions as a control for timing
The two sectors of the southern Laurentide margin experienced synchronous advance at least three times during the last glacial period and termination. However, the extent of advance varied from lobe to lobe across the margin during these times. The timing of changes in ice margin position is thought to reflect changes in climate (e.g. Lowell et al., 1999; Lowell, 2000; Clark et al., 2009). Thus, the similar timing of advance across the southern Laurentide margin indicates similar climate conditions at those times. Laurentide Ice Sheet expansion > 20 ka is attributed to widespread cooling across the
Northern Hemisphere due to decreased insolation (e.g. Hays et al., 1976; Saltzman et al., 1984; Clark et
109 al., 2009; Lisiecki, 2010; Huybers, 2011). Cool air temperatures led to increased accumulation over the
Laurentide Ice Sheet domes and advance of the entire margin.
Later advances of the much of the southern Laurentide margin lobes occurred during the
Termination (< 20 ka), a time of increasing Northern Hemisphere insolation (Hays et al., 1976; Lisiecki and Raymo, 2005; Roe, 2006; Kawamura et al., 2007; Cheng et al., 2009; Denton et al., 2010). After
~16.2 ka, the James Lobe stabilized, the Des Moines Lobe and Puget Lobe reached their maximum extents and multiple eastern sector lobes readvanced. After ~14.7 ka, the James Lobe reached its maximum position and the Des Moines Lobe and eastern sector lobes readvanced. These two episodes may have been caused by a temporary decrease in summer temperature and associated decrease in melting at the ice margin. This phase correlates with Heinrich Stadial 1 (Chapter 2; Heath et al., 2018), during which the eastern lobes of the southern Laurentide underwent overall retreat, which was interrupted by brief periods of readvance or stabilization. At this time, Northern Hemisphere westerly wind belts shifted southward in response to expanded sea ice in the North Atlantic Ocean and southward depression of the Intertropical Convergence Zone (Denton et al., 2010). This shift may have led to brief periods of cooler summers across central North America leading to synchronous stabilization and advance of southern Laurentide lobes.
4. Conclusions
1. The James Lobe experienced three phases of advance or ice margin stabilization during last
glacial period and termination.
2. The James Lobe and Des Moines exhibited similar timing of advance between ~27 and 19 ka, at
~16 ka and at ~13 ka.
3. The entire southern Laurentide margin underwent advance between ~27 and 19 ka, which is
attributed to Northern Hemisphere cooling and ice sheet expansion due to decreased summer
insolation.
110 4. At ~16 ka, the New England margin, Lake Michigan Lobe, Des Moines Lobe, James Lobe and
the Puget Lobe of the Cordilleran Ice Sheet underwent advance.
5. At ~13 ka, the New England margin, Green Bay Lobe, Des Moines Lobe, and James Lobe
underwent advance.
6. Overall the lobes of the southern Laurentide exhibit similar timing of advance, but vastly
different extents of advance. This may be the result of the transition from a single-dome system
to a multi-dome system.
111 Figures
Figure 5.1. Maximum extent of the Laurentide Ice Sheet during the last glacial period. Dotted lines represent approximate flow lines from the Hudson and Keewatin Domes, after Denton and Hughes
(1981). The southern lobes are outlined by a black box and detailed in Figure 5.2.
112
Figure 5.2. The lobes of the southern Laurentide Ice Sheet. The black line represents the outermost extent of each lobe. From Heath et al. (2018).
113
Figure 5.3. Radiocarbon ages and cosmogenic surface exposure ages from glacial deposits associated with the James Lobe. Blue triangles represent maximum radiocarbon ages, red triangles represent minimum radiocarbon ages, and green circles represent cosmogenic surface exposure ages.
114
References
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115 Huybers, P., 2011. Combined obliquity and precession pacing of late Pleistocene deglaciations. Nature, 480, 229-232. Huybers, K., Roe, G.H., 2009. Spatial patterns of glaciers in response to spatial patterns in regional climate. Journal of Climate, 22, 4606-4620. Jennings, C.E., 2006. Terrestrial ice streams — a view from the lobe. Geomorphology 75, 100-124. Kaiser, K.F., 1994. Two Creeks interstade dated through dendrochronology and AMS. Quaternary Research 42, 288-298. Kawamura, K., Parrenin, F., Lisiecki, L., Uemura, R., Vimeux, F., Severinghaus, J.P., Hutterli, M.A., Nakazawa, T., Aoki, S., Jouzel, J., Raymo, M.E., Matsumoto, K., Nakata, H., Motoyama, H., Fujita, S., Goto-Azuma, K., Fujii, Y., Watanabe, O., 2007. Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years. Nature, 448 (3), 912-917. Kemmis, T.J., Hallberg, G.R., Lutenegger, A.J., 1981. Depositional environments of glacial sediments and landforms on the Des Moines Lobe, Iowa; Field trip guidebook. Iowa Geological Survey Series Number 6, 132 pp. Lisieck, L.E., 2010. Links between eccentricity forcing and the 100,000-year glacial cycle, Nature Geoscience, 3, 349-352. Lisieck, L.E., Raymo, M.E., 2005. A Pliocene-Pleistocene stack of 57 globally distributed benthic 18O records. Paleoceanography, 20, PA1003. Lowell, T.V., Hayward, R.K., Denton, G.H., 1999. Role of climate oscillations in determining ice-margin position: Hypothesis, examples, and implications. Geological Society of America Special Papers 337, 193e203. Lowell, T.V., 2000. As climate changes, so do glaciers. Proceedings of the National Academy of Science, 97 (4), 1351-1354. Margold, M., Stokes, C.R., Clark, C.D., 2015. Ice streams in the Laurentide Ice Sheet: Identification, characteristics and comparison to modern ice sheets. Earth-Science Reviews 143, 117-146. Patterson, C.J., 1997. Southern Laurentide ice lobes were created by ice streams: Des Moines Lobe in Minnesota, USA. Sedimentary Geology 111, 249-261. Patterson, C.J., 1998. Laurentide glacial landscapes: The role of ice streams. Geology 26 (7), 643-646. Porter, S.C., Swanson, T.W., 1998. Radiocarbon age constraints on rates of advance and retreat of the Puget Lobe of the Cordilleran Ice Sheet during the last glaciation. Quaternary Research 50, 205- 213. Ridge, J.C., Balco, G., Bayless, R.L., Beck, C.C., Carter, L.B., Dean, J.L., Voytek, E.B., Wei, J.H., 2012. The new North American varve chronology: A precise record of southeastern Laurentide Ice Sheet deglaciation and climate, 18.2-12.5 kyr BP, and correlations with Greenland ice core records. American Journal of Science, 312, 685-722. Roe, G., 2006. In defense of Milankovitch. Geophysical Research Letters, 33, L23703, 1-5. Ruhe, R.V., 1969. Quaternary Landscapes in Iowa. Iowa State University Press, Ames, Iowa, p. 255. Saltzman, B., Hansen, A.R., Maasch, K., 1984. The Late Quaternary glaciations as the response of a three-component feedback system to Earth-Orbital forcing. Journal of the Atmospheric Sciences, 41(23), 3380-3389.
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Chapter 6
Conclusions
1. The lobes of the southern Laurentide Ice Sheet exhibited asynchronous behavior between
Heinrich Stadials 2 and 1.
2. Surface exposure ages from two sites in the Pierre Sublobe of the western James Lobe (JL), South
Dakota suggest that the JL margin stabilized before ~15.7 ka, during HS1. 10Be surface exposure
age dating appears to be suitable for stable moraine ridges not affected by gullying or moraine
degradation. This method is not suitable for till-covered bedrock buttes adjacent to the Missouri
River trench.
3. Radiocarbon ages from the JL footprint and adjacent Prairie Coteau suggest that the lobe
underwent initial advance after ~27 ka, underwent a retreat of unknown magnitude before ~16 ka
and advanced to its maximum extent after ~13 ka.
4. The JL and DML exhibited similar behavior during the last glacial period and termination. The
JL and DML underwent initial advance after ~27 ka and ~24 ka. The JL margin stabilized in the
Pierre Sublobe before ~15.7 ka, while the DML underwent advance to its maximum extent after
~16.2 ka, during HS1 time. Both the JL and DML underwent advance after ~13 ka. The James
Lobe reached its maximum extent during this later advance.
5. When compared to the rest of the southern Laurentide margin, the behavior of the JL and DML
seems less anomalous. Between ~27 and 19 ka, all of the southern Laurentide lobes underwent
advance. This synchronous advance is attributed to cooler temperatures in the Northern
Hemisphere due to decreased northern insolation. At ~16 ka, the New England margin, Lake
Michigan Lobe, DML and JL of the southern Laurentide and the Puget Lobe of the Cordilleran
Ice Sheet either advanced or stabilized. At ~13 ka the New England margin, Green Bay Lobe,
DML and JL either advanced or stabilized.
117 6. All of the southern Laurentide Ice Sheet lobes exhibit similar timing through the last glacial
period and Termination but reached different extents during each phase of advance. This pattern
has implications for mechanisms affecting the advance and retreat patterns. The extent of each
lobe is dependent on physical characteristics of that region and lobe. The disparity in extents
supports the multi-dome hypothesis We suggest that climate was the dominant mechanism and
the timing of advance of the southern Laurentide lobes reflect changes in climate conditions.
7. The existing paradigm of surging and streaming of the western lobes may not be the dominant
mechanism controlling advance and retreat of the western lobes. Regional variations in summer
temperatures along the southern Laurentide margin during the Termination may explain the
pattern of stabilization or advance in some lobes, and retreat of others. Cooler summers along the
ice margin lead to decreased ablation and either advance or stabilization of the margin.
118