Laurentide Ice Sheet Retreat During the Younger Dryas: Central Upper Peninsula of Michigan, USA

Laurentide Ice Sheet Retreat during the Younger Dryas: Central Upper Peninsula of Michigan, USA

A thesis submitted to the

Graduate school

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree of

Masters of Science

In the Department of Geology of the

McMicken College of Arts and Sciences

Kent Andrew Walters

B.A. Grand Valley State University

April, 2011

Committee Chair: Thomas V. Lowell, Ph.D

Committee Members: David B. Nash, Ph.D

Timothy G. Fisher, Ph.D

Abstract

The response of ice sheets to climate change is of concern because of meltwater introduction to the oceans raising sea level. Yet it is uncertain how the Laurentide Ice Sheet (LIS) responded to the widely studied Younger Dryas (YD) cold interval (~12.9- 11.6 cal ka BP). In northwestern Wisconsin and the Upper Peninsula of Michigan, two sites record advances of the LIS before and near the end of the YD. The Two Creeks (~13.7 cal ka BP) and Lake Gribben (~11.6 cal ka BP) forest beds indicate that the ice sheet margin was between those sites during the YD. New geomorphic mapping and synthesis of existing and new chronology for the ice margin, document activity of the ice sheet during the YD. Combined, these data indicate that the ice sheet margin had at least six stationary positions during retreat through the YD interval and a readvance after the YD. When this pattern is compared to the isotopic record from the Greenland ice core, an apparent conflict arises. The ice sheet is retreating at a time generally thought to be cold and correlating with glacial advances. This may imply that seasonality and the corresponding warmer summers during the YD controlled the ice sheet response. Alternatively they may imply caution is needed when interpreting the ice core record.

iii

Acknowledgements:

This study would like to thank the support of the Comer Science and Education Foundation’s grant to Aaron Putnam for providing this study with funding for radiocarbon dates. Putnam provided thoughtful comments on the progress and presentation of this study. We would also like to thank Robert Regis for his thoughts and information on the study region. We are grateful to Colby Smith and Paul Wilcox for their help with the sediment core collection process. Finally, I would like to thank my committee members Dr. David Nash and Dr. Timothy Fisher for their insightful comments and motivation on this project.

Table of Contents

1. Introduction……………………………………………………………………………..1 2. Regional Setting…………………………………………………………………………2 2.1 Study Region………………………………………………………………………2 2.2 Geology…………………………………………………………………………….3 2.3 Prior Surficial Mapping…………………………………………………………...3 2.4 Existing Glacial Chronology……………………………………………………...4 3. Methods………………………………………………………………………………….5 4. Results…………………………………………………………………………………...8 4.1 Mapping…………………………………………………………………………….8 4.1.1 Subglacial Forms………………………………………………………….8 4.1.2 Ice Margin Forms………………………………………………………….9 4.1.2.1 Moraines…………………………………………………………9 4.1.2.2 Ice Contact Moraines…………………………………………..9 4.1.2.3 Moraine Spacing………………………………………………..9 4.1.3 Correlation of Refinement of Moraines………………………………...10 4.1.3.1 Late Mountain and Early Athelstane Moraine………………10 4.1.3.2 Denmark/Late Athelstane Moraine……………………………10 4.1.3.3 Intermediate Moraines………………………………………….11 4.1.3.4 Marquette Moraine………………………………………………12 4.1.4 Proglacial Deposits………………………………………………………..12 4.1.5 Meltwater Channels……………………………………………………….12 4.1.6 Non-glacial Forms…………………………………………………………13 4.1.7 Anthropogenic Forms……………………………………………………..13 4.2 Chronology………………………………………………………………………….13 4.2.1 Sagola/Saint Johns and Late Sagola/Republic Moraines…………….14 4.2.2 Denmark/Late Athelstane/Green Hills/Ishpeming Moraine…………...14 4.2.2.1 Maximum Ages………………………………………………….14 4.2.2.2 Minimum Ages…………………………………………………..15

4.2.3 Pembine/Hay Lake, Felch/Pike Lake and Foster City/Shag Lake Moraines…………………………………………………………………….16 4.2.4 Gwinn Moraine……………………………………………………………...16 4.2.5 Gladstone and Rapid River Moraines……………………………………17 4.2.6 Marquette Moraine…………………………………………………………17 4.2.6.1 Maximum Ages……………………………………………………17 4.2.6.2 Minimum Ages…………………………………………………....17 4.3 Timing of the Two Creeks and Marquette Moraines Refined…………………19 4.3.1 Summing Probabilities…………………………………………………….19 4.3.2 Refinement of the Green Hills/Ishpeming Moraine Formation……….20 4.3.3 Refinement of the Marquette Moraine Formation……………………...20 4.4 Interpreted Sequence of Events………………………………………………….20 4.5 Time-Distance Diagram……………………………………………………………22 4.6 Dune Sands and Lake Bottom Inorganic Sediments…………………………..23 5. Discussion……………………………………………………………………………....23 5.1 Young Minimum Ages……………………………………………………………..23 5.2 Green Hills/Ishpeming Age Assignment…………………………………………25 5.3 Laurentide Ice Sheet Retreat during the Younger Dryas………………………25 5.3.1 Step-Wise Retreat………………………………………………………...25 5.3.2 LIS Deglaciation…………………………………………………………..25 5.3.3 LIS Retreat Before or During the Younger Dryas?...... 26 5.3.3.1 Ice Margin Formation…………………………………………..26 5.3.3.2 Calving Retreat………………………………………………….27 5.3.3.3 Terrestrial Retreat………………………………………………28 5.3.4 Regional Ice Sheet Comparison………………………………………...28 5.4 Marquette Advance During an Interglacial Climate…………………………….29 5.5 Contradicting Messages…………………………………………………………..30 5.5.1 Seasonality………………………………………………………………….30 5.5.2 Scandinavian Ice Sheet……………………………………………………31 6. Conclusions……………………………………………………………………………..32

1. Introduction

Modern day observations of glaciers and ice sheets indicate they are sensitive, and respond quickly to changes in climate (Oerlemans, 1986; Oerlemans, 1990;

Oerlemans and Fortuin, 1992; Fabre et al., 1995; Lowell, 2000). Changes in ice margin position can be reconstructed from geomorphic evidence, and therefore can be used to study paleoclimatic events of rapid climate change such as the Younger Dryas cold period (YD; 12.9-11.6 cal ka BP). Much effort has been put into examining how small glaciers were affected by this rapid cooling event (Rodbell and Seltzer, 2000; Barrows et al., 2007; Ackert et al., 2008; Kaplan et al., 2010; Palmer et al., 2010; MacLeod et al.,

2011; Rinterknecht et al., 2012; Young et al., 2012), but little effort has been put into the response of large ice sheets. This is especially true for the Laurentide Ice Sheet (LIS).

The behavior of the LIS during the YD is poorly understood. Clayton and Moran

(1986) suggested that the LIS retreated to the northern shores of Lake Superior before the onset of the YD to allow for drainage of glacial Lake Agassiz. The age and position of the Marquette Moraine in the Upper Peninsula of Michigan suggests that the LIS readvanced almost 200 km across Lake Superior during the YD. Lowell et al. (1999) refined the age of the moraine and also attributed LIS expansion to the YD cooling event. This idea was later supported and accepted after a widespread database of radiocarbon dates constraining LIS retreat was produced (Dyke, 2004). Most recently

Lowell et al. (2009) dated recessional moraines refining LIS retreat near Thunder Bay,

Ontario showing a step wise retreat pattern with small readvances through the Late

Glacial and into the Holocene. This suggests 200 km of retreat and readvance across the Lake Superior basin was unlikely.

This study was undertaken to: (i) refine the pattern of LIS recession using glacial mapping and radiocarbon dates in the Upper Peninsula of Michigan; and, (ii) determine how the LIS responded during the Younger Dryas event.

2. Regional Setting

2.1 Study Region

This study focuses on glacial landforms and LIS deglaciation chronology in central Upper Peninsula of Michigan (Figure 1). The study region ranges from northeast Wisconsin near Lake Winnebago into the Upper Peninsula of Michigan near

Marquette. The primary study site lies between the cities of Au Train, Big Bay and Iron

Mountain, MI, focusing on the glacial features southwest of Marquette, MI (red box

Figure 1).

Within the study region are two well-known and dated forests buried by the LIS.

The first forest was buried by the LIS at 13.7 cal ka BP and is known as the Two Creeks event (Figure 1). Here, the Lake Michigan sublobe of the LIS advanced over the Two

Creeks spruce forest burying it in till (Figure 1; Black, 1970). The adjacent lobe of ice known as the Green Bay lobe advanced in phase, incorporating tree remains of Two

Creeks age into its sediments. A second recognized forest bed in the northern Upper

Peninsula of Michigan is known as the Lake Gribben Forest, and was buried by glacial outwash during the Marquette readvance (Figure 1; Hughs and Merry, 1978; Lowell et al., 1999). It is dated to 11.6 cal ka BP (Table 1). These ages indicate that the Two

Creeks and Marquette LIS advances occurred before and after YD time. Thus the behavior of the LIS can be extracted by studying the glacial features between these two

2 locations which must incorporate retreat northward from the Two Creeks site to a position north of the Marquette Moraine before readvancing to the Marquette Moraine.

2.2 Geology

Two major groups of bedrock underlie the study area. In the southeast lies the

Michigan basin where the bedrock consists of various limestones, Cambrian aged sandstones and few shales (Farrand, 1982). The western and northwestern landscape is controlled by Archean-aged granites, gneisses and various other volcanic or igneous rocks (Farrand, 1982).

2.3 Prior Surficial Mapping

The Upper Peninsula of Michigan has had a long history of glacial investigations.

Leverett (1929) interpreted the glacial history using the matrix color of tills. Much later,

Black (1969) studied the glacial history using striations, drumlins, and end moraines to suggest a different glacial history of several ice lobes occupying the region.

Subsequently, investigations focused on the chronology of events (Saarnisto, 1974;

Drexler et al., 1983; Clayton and Moran, 1986). Two hypothesis emerged; one of slow retreat through the Upper Peninsula of Michigan (Saarnisto, 1974), and another of rapid deglaciation with a later readvance into the region (Drexler et al., 1983; Clayton and

Moran, 1986). The latter was accepted as the general pattern of ice sheet recession and expansion by virtue of its inclusion in Dyke (2004).

New digital elevation model data led to mapping of glacial landforms. Regis

(1997) focused on the suite of interlobate moraines located southwest of Marquette, MI.

Regis mapped moraines and completed a detailed study of the landscape providing a

3 glacial history of the interlobate region. Regis (1997) concluded that the LIS stabilized or had small readvances to at least eight positions throughout the region.

The glacial geology in Wisconsin has been thoroughly described over the decades by multiple workers with the earliest work completed by Chamberlin (1877,

1880, 1882, 1883) who published four volumes of Wisconsin geology. More detailed surficial mapping completed by various workers (Clayton, 1986; Attig and Ham, 1990;

Colgan, 1996; Hooyer et al., 2004; Mickelson and Socha, 2004; Hooyer and Mode,

2008) provides an excellent history of past glaciations in Wisconsin. However, most of these mapping efforts stop at the border between Wisconsin and the Upper Peninsula of Michigan.

Few surficial maps exist for the Upper Peninsula of Michigan. Farrand (1982) produced a Quaternary geology map of upper and lower Michigan, but many of the surficial features needed to interpret the glacial history are not included on the map.

The only detailed surficial mapping was completed by Regis (1997).

2.4 Existing Glacial Chronology

The Two Creeks forest bed has been extensively dated (Table 1; Libby, 1955;

Preston et al., 1955; Crane, 1956; Shutler and Damon, 1959; Suess, 1954; Rubin and

Alexander, 1960; Broecker and Farrand, 1963; Leavett and Kalin, 1992; Kaiser, 1993;

Rech et al., 2012). At least 37 individual analysis clusters around 13.7 cal ka BP.

These ages provide maximum ages for when the LIS advanced to the Two Creeks site

(Figure 1). Additional maximum ages are found in the greater Lake Winnebago, WI region where 23 samples of wood are found in till deposited by the Two Creeks

4 advance (Table 1; Flint, 1956; Black and Rubin, 1968; Black, 1976; McCartney and

Mickelson, 1982; Schneider and Hansel, 1990; Kaiser, 1994; Maher and Mickelson,

1996)

Other than five paleoecology studies providing some constraints, few radiocarbon dates record the deglaciation after the Two Creeks forest bed was buried

(Table 1; West, 1961; Brubaker, 1975; Goodwin, 1976; Davis, 1978; Woods and Davis,

1989). Three of these studies dated bulk basal sediment which has associated large errors due to hardwater effects which can cause artificially old ages (MacDonald et al.,

1991). An investigation in the eastern section of our study region produced a transect of minimum ages recording deglaciation (Dueroin et al., 2007). These minimum ages were all younger than minimum ages defining the Marquette Moraine.

The Lake Gribben forest is an in situ white spruce forest buried in outwash from the Marquette readvance. The forest bed is well dated with nearly 20 radiocarbon dates to an age of ~11.6 cal ka BP (Table 1; Hughes and Merry, 1978; Lowell et al., 1999;

Pregitzer et al., 2000). Derouin et al. (2007) sampled Ackerman Lake located on the proximal side of the Marquette Moraine producing an age of ~10.8 cal ka BP, which is considered a minimum age of deglaciation from the Marquette Moraine (Table 1).

3. Methods

Geomorphic mapping was completed using a 10 m digital elevation map (DEM) obtained from http://datagateway.nrcs.usda.gov/. ArcMAP GIS and Global Mapper 12 were utilized to view the topography, digitize landforms and plot sample locations.

To establish a chronology for deglaciation, lake-bottom sediments were collected and analyzed. Lakes were selected for coring based on their location relative to glacial landforms, lake depth and accessibility. Smaller lakes were preferred as they usually preserve organic material better than larger, deeper lakes. Bathymetry of the lakes was determined from, http://www.michigan.gov/dnr/0,1607,7-153-30301_31431_32340---

,00.html.

Eleven, lake-bottom sediment cores were collected in 2008, processed and stored in refrigerated storage. In 2012, lake-bottom sediment cores were collected from five additional lakes in mid-March. Coring sites were determined with GPS. All sediment cores were collected using a Livingston square rod coring device in one meter segments. Collected sediment was extruded in the field and stratigraphy briefly described. Recovered cores focused on the contact between organic and inorganic material. All cores were taken to The University of Cincinnati for further processing.

Full cores were split lengthwise into two halves, described, photographed and logged into a core database. Magnetic susceptibility was collected from each core at 2 cm intervals. Each core was sampled at 2 cm intervals in order to determine percent organic and carbonate as per procedures outlined in Dean (1974).

Macro organic material for dating was collected from as close to the inorganic/organic contact as possible. In the absence of macro organics, 2 cm sections of sediment were collected from the working half of the core. The sediment was soaked in a 10% KOH solution until it dispersed, then run through a 355 μm sieve and washed with deionized water to separate clastic material from organic material. Terrestrial and

6 aquatic organic material was classified and separated into separate containers then dried overnight. Samples were weighed and stored in separate containers to be sent for radiocarbon dating at the Woods Hole Oceanographic Institution. The radiocarbon analyses were conducted according to their standard procedures available at http://www.whoi.edu/nosams/home. Radiocarbon ages were converted into calibrated years before present using the Reimer et al. (2009) calibration set using CALIB 6.1.

The ages of glacial margins were determined by summing radiocarbon probabilities. Radiocarbon ages representing the same event were calibrated and summed into one probability plot using the CALIB 6.1 program. This provides a range for the age of a particular event.

The age assignment for specific moraines was constrained using summed radiocarbon dates representing maximum and minimum ages. A statistical program developed by Patrick Applegate (per comm.) was employed to analyze the area constrained by the minimum and maximum plots. This program analyzes the area between minimum and maximum summed probability plots to produce a probability curve representing a range of ages which best represents the age of the specific moraine.

Grain size analysis was completed on the inorganic sediments in lakes located on the distal side of the Marquette moraine. The first 2-10 cm of inorganic material underneath organic material was sampled in 2 cm intervals then analyzed for its properties. Grain size analysis was completed using a Beckman Coulter LS230 series laser particle analyzer. The results were then processed through the GRADISTAT

(Blott and Pye, 2001) program to recover grain size distribution, sorting and skewness values.

4. Results

This study produced two major results. Geomorphic mapping was first completed to map recessional moraines and correlate to them prior work. Deglaciation chronology was later added to provide chronology for the recession of the LIS across the area.

4.1 Mapping

Mapping was focused on correlating moraines mapped in Michigan to the moraines in

Wisconsin. This reveals the ice marginal behavior of the LIS through the region.

4.1.1 Subglacial Forms. A large drumlin field occupies the south central region of our study region (Figure 3). The drumlins become increasingly elongated to the south reaching lengths of 2.5 km. In the northern part of the drumlin field, two orientations of drumlin formation suggest different episodes of drumlin formation (Figure 3). Near the interlobate region between the Green Bay and Michigamme lobes, smaller drumlins define ice flow retreat from the Green Hills Moraine (Figure 3). Included in a field dominated by smaller drumlins exist longer drumlins orientated to the southwest. Regis

(1997) explains there is no evidence for readvance over the sets of moraines indicating these longer drumlins must be older than the Green Hills Moraine.

Eskers occur throughout the study area. Eskers are better developed in the south and generally are found in clusters where bedrock landscape is not present

(Figure 3). Tunnel channels (described by Kehew et al., 1999; Fisher et al., 2005) in this study are at a maximum of 18 km long, 2 km wide and 30 m in depth, and are found on the right lateral side of the Green Bay Lobe parallel to ice flow (Figure 3).

4.1.2 Ice Margin Forms.

4.1.2.1 Moraines. Moraines are found marking the right lateral side of the Green Bay lobe and the terminal position of the Michigamme lobe (Figure 3). These moraines are defined by ridges of discontinuous hummocky terrain with a relief of 5-10 meters and a width of ~0.5 km. The outermost Sagola/Saint Johns and Late Sagola/Republic moraines are the only moraines defined by both the Michigamme and Green Bay lobes

(Figure 3). The next seven inset ice margins are only defined by moraines from the

Green Bay lobe (Figure 3). As the moraines trend south towards Wisconsin they become fragmented, less well defined, and with minimal relief. Commonly associated with these moraines are meltwater channels parallel to ice margins.

4.1.2.2 Ice Contact Moraines. At the interlobate zone between the Green Bay lobe and

Michigamme lobe across an 80˚ arc is deposits of outwash. The ice margins are defined by the ice contact slopes. Here the moraines consist of outwash forming a gently sloping surface on the distal side with a sharp and steep contact on the proximal side. The deposition of outwash deposits produced sets of terraces defining each ice margin. Outwash terrace deposits are stepped with each postceeding ice contact moraine being 25 m below the next. The Pembine/Hay Lake, Felch/Pike Lake and

Foster City/Shag Lake moraines are only defined by ice contact slopes while the other moraines elsewhere are defined with a combination of ridges and outwash fans.

4.1.2.3 Moraine Spacing. The Green Bay lobe right lateral recessional moraines undergo a spatial transformation from its terminus near Lake Winnebago into the study region. At the south end of Figure 2, recessional moraines are spaced approximately

10-30 km apart. Further north across the state line into Michigan the distance between them decreases to approximately 2-5 km (Figure 2).

4.1.3 Correlation and Refinement of Moraines

Moraines in Wisconsin have been correlated to the newly refined moraines in

Michigan. The moraines in our study region are assigned two names because of the convergence of the Green Bay and Michigamme lobes. The moraine marking the right lateral position of the Green Bay lobe is reported first followed by the name of the corresponding moraine defined by the Michigamme lobe. Similarly, moraines in

Wisconsin are given two names, but represent different areas of the same ice margin.

4.1.3.1. Late Mountain and Early Athelstane Moraine. Refinement of previous mapping led to the correlation of Wisconsin moraines into Michigan. The two oldest ice margins in our study region, the Sagola/Saint Johns and the Late Sagola/Republic moraines, correlate to the Late Mountain and Early Athelstane moraines in Wisconsin (Figure 2).

4.1.3.2 Denmark/Late Athelstane Moraine. A key goal of this project was to establish the moraine equivalent in Michigan to the moraine recording the LIS advance over the

Two Creeks forest bed. The southern moraine is first mapped south of the Two Creeks forest bed on the right lateral side of the Lake Michigan Lobe of the LIS (Figure 2). The moraine trends to the north before sharply turning to the southwest where it marks the ice marginal position of the Green Bay Lobe known as the Denmark Moraine (Figure 2).

The moraine continues south west to Lake Winnebago before gradually trending northward where it becomes the right lateral indicator of the Green Bay lobe known as the Late Athelstane Moraine (Figure 2). Mapping revealed the Green Hills/Ishpeming

Moraine in the study region corresponds to the Late Athelstane/Denmark Moraine in

Wisconsin (Figure 2).

Additional evidence the Green Hills Moraine was formed in phase with the burial of the Two Creeks forest is the spatial relationship of the interlobate moraine positions.

The Late Sagola/Saint Johns and Sagola/Republic interlobate position is offset to the south from the remaining six interlobate ice marginal positions (Figures 2 and 3). In addition, a noticeable bend in the Sagola/Saint Johns and Late Sagola/Republic moraines is absent in the five subsequent ice margins (Figure 2). Finally, the appearance of drumlins and tunnel channels forming at the Green Hills/Ishpeming

Moraine and the presence of outwash deposits at each recessional margin further supports this correlation (Figure 3). The formation of drumlins, tunnel channels and a different ice margin geometry starting at the Green Hills/Ishpeming Moraine may indicate ice advance to this location.

4.1.3.3 Intermediate Moraines. A suite of six ice margins were mapped between the

Green Hills/Ishpeming and Marquette margins (Figure 2). The names of Pembine/Hay

Lake, Felch/Pike Lake and Foster City/Shag Lake moraines have been assigned to the first three previously unnamed margins immediately east of the Green Hills/Ishpeming

Moraine (Figure 2). A new moraine not previously mapped by Regis (1997) has been named the Gwinn Moraine. Two additional unnamed moraines which lay inside the

Gwinn Moraine have been assigned the Gladstone and Rapid River moraines (Figure

2).

The lack of moraines mapped inside the Late Athelstane Moraine as well as drumlins and lowlands cut by meltwater channels does not allow correlation to any other

11 moraines. Moraine fragments are preserved on areas of higher topography which trend towards Lake Michigan.

4.1.3.4 Marquette Moraine. The Marquette Moraine was interpreted to have formed by a readvance, and has been investigated in multiple studies (Figure 2; Leverett, 1929;

Black, 1969; Saarnisto, 1974; Regis, 1997). Higher resolution DEMs in this study permitted refining the position of the Marquette Moraine from Au Train, across the large outwash deposits and through the bedrock terrain west of Marquette to a large outwash deposit south of Big Bay known as the Yellow Dog sand plains (Figure 2 and 3).

Meltwater channels were used to define the ice margin in bedrock terrain where glacial depositional features are obscured.

4.1.4 Proglacial Deposits. The most laterally expansive glacial deposit in central and eastern Upper Peninsula of Michigan is outwash plains (Figure 3). These landforms are characterized by sandy, gently sloping, generally featureless terrains which originate from ice margins at ice contact slopes. Outwash fans can be found forming to the boundaries of the interlobate ice margins radiating outwards and sloping away from the ice margin (Figure 3). East of Au Train, the landscape is composed of kettled outwash forming a heterogeneous landscape with a high density of lakes throughout (Figure 3).

4.1.5 Meltwater Channels. As the LIS was retreating it left behind many meltwater channels (Figure 3). Glacial meltwater channels are identified as deeply incised channels which abruptly start at or are parallel to ice margins. The probable flow of meltwater is indicated by the direction of the blue arrows on the map (Figure 3).

4.1.6 Non-glacial Forms. The landscape west and east of Marquette is primarily controlled by the underlying bedrock structure. This terrain is distinguished by a blocky jointed landscape which masks the presence of glacial landforms. Eolian dunes in the southeast of the map area are found in patchy groupings across the outwash (Loope et al., 2012). Relict shorelines were mapped primarily using aerial photography. These are defined by series of multiple ridges forming parallel to the modern shoreline (Figure

3). Modern alluvial channels typically have a dendritic pattern and progressively flow into larger channels if not occupying a glacial meltwater channel.

4.1.7 Anthropogenic Forms. Multiple mining operations exist across the study region leaving scars across the landscape. These are indicated so as not to be taken as depositional features.

4.2 Chronology

This study uses a combination of minimum and maximum ages to track LIS deglaciation through the region. Since it is known when the LIS buried the Two Creeks forest bed, the addition of minimum ages following retreat brackets the timing of deglaciation from the Green Hills/Ishpeming Moraines. Similarly, existing maximum ages at the Lake Gribben forest bed limit when the LIS was actively constructing the

Marquette Moraine and can be supplemented with deglacial ages from further north.

The addition of minimum ages to the existing database of maximum dates refines LIS deglaciation chronology.

It was later discovered through communication with Catherine Yansa of Michigan

State University, samples using Chara species and drupes from the Potamogeton

13 species for radiocarbon dates produce artificially old ages due to the hardwater effect

(MacDonald et al., 1991). These samples are noted in Table 1 and are not included in the analysis.

4.2.1 Sagola/Saint Johns and Late Sagola/Republic Moraines. Glidden Lake, site 12, lies outside of the Sagola Moraine by 12 km from which two minimum ages were collected (Figure 2). The first minimum date was collected in the sandy gyttja layer between two sand layers in which woody material was dated to an age of 13.6 cal ka

BP (OS-97747; Figure 4). The next date was collected immediately above the previous one, and yielded an age of 14.0 cal ka BP (OS-100914; Figure 4).

The Late Sagola/Republic Moraine is constrained by one minimum age. Site 11 is Perch Lake which lies immediately on the distal side of the Republic Moraine (Figure

2). A radiocarbon age from plants and woody debris gives an age of 10.9 cal ka BP

(OS-97750) at the contact between gyttja and underlying sand (Figure 4).

4.2.2 Denmark/Late Athelstane/Green Hills/Ishpeming Moraine.

4.2.2.1 Maximum Ages. At least 37 ages from 9 separate investigations date pieces of wood from the type locality of the Two Creeks forest bed (Table 1). The most recent radiocarbon ages cluster around 13.7 cal ka BP while early investigations have large error and cluster around 13.1 cal ka BP.

The greater Lake Winnebago, WI region hosts at least 23 Two Creeks forest bed aged pieces of wood dated in seven different studies (Table 1). Various pieces of wood were collected from till deposited by the glacial advance which buried the Two Creeks

14 forest. These ages cluster around 13.7 cal ka BP confirming they are Two Creeks aged.

4.2.2.2 Minimum Ages. This study adds seven minimum ages on the Green

Hills/Ishpeming Moraine (Table 1). Site 10 is Frenchie Lake which lies on the moraine in which two ages were collected from one core (Figure 2). The first sample was collected in the sandy organic mixture underlain by sand where aquatic seed pods were dated to an age of 14.4 cal ka BP (OS-100598; Figure 4). This date was later determined to be artificially old due to the hardwater effect (per comm. Catherine

Yansa). The second date was collected 13 cm above the sand contact in which a small piece of wood was dated to an age of 11.8 cal ka BP (OS-100597; Figure 4). Site 9 is

Johnson Lake located 0.5 km on the distal side of the Green Hills Moraine where three ages were collected from three cores (Figure 2). In Johnson Lake BT1 woody material was collected at the contact between gyttja and sand and dated to an age of 12.8 cal ka

BP (OS-97748; Figure 4). Johnson Lake CT2 dated small sticks at the gyttja and sand contact producing an age of 11.6 cal ka BP (OS-100596; Figure 4). The final core,

Johnson Lake BT1 dated aquatic plant material at a gyttja and gravel contact giving an age of 13.2 cal ka BP (OS-100692; Figure 4). This age was also determined to be artificially old due to the hardwater effect (per comm. Catherine Yansa). Solberg Lake, site 8, is 0.5 km on the proximal side of the Green Hills margin where two ages were produced (Figure 2). The first age of 15.75 cal ka BP (OS-100599) is from aquatic plant material in a grey silt layer underlain by sand (Figure 4). This was confirmed to be artificially old due to the hardwater effect (per comm. Catherine Yansa). The second age from the grey silt and gyttja contact is from a pine needle revealing an age of 11.2

15 cal ka BP (OS-97749; Figure 4). The oldest minimum age constraining retreat from the

Two Creeks advance is from Johnson Lake with an age of 12.8 cal ka BP.

4.2.3 Pembine/Hay Lake, Felch/Pike Lake and Foster City/Shag Lake Moraines. No new chronological data was produced for this suite of moraines. Based on geomorphic evidence these moraines must be younger than the Green Hills/Ishpeming Moraine and older than the Gwinn Moraine.

4.2.4 Gwinn Moraine. The Gwinn Moraine is the eastern most interlobate ice margin in the study area from which five minimum ages were collected adjacent to it (Figure 2).

Site 6 is Anderson Lake which is located on the proximal side of the Gwinn Moraine

(Figure 2). Seed pods were collected at a gyttja and sand contact producing an age of

10.7 cal ka BP (OS-97746; Figure 4). Little Shag Lake, site 7, is 1 km on the distal side of Gwinn Moraine where four radiocarbon dates were collected from two separate cores

(Figure 2). Several small sticks produced an age of 11.6 cal ka BP (OS-97745) at a contact between gyttja and underlying sand (Figure 4). The other age collected 2 cm above the previous date is 11.8 cal ka BP (OS-97744; Figure 4). Another core at the opposite side of the lake provided two ages from a contact between gyttja and underlying sand. Ages of 8.9 cal ka BP (OS-100594) and 9.1 cal ka BP (OS-100595) were collected from woody debris and aquatic material (Figure 4). Because the Lake

Gribben forest lies a few kilometers on the proximal side of the Gwinn Moraine, its oldest ages can be used as minimum age for the Gwinn Moraine. The Gwinn Moraine can thus be assigned a minimum age of ~12.1 cal ka BP (Table 1).

4.2.5 Gladstone and Rapid River Moraines. The Gladstone and Rapid River are the youngest moraines mapped in the recessive sequence. Their geomorphic position determines that these moraines must be younger than the Gwinn Moraine and older than the Marquette Moraine. Absolute chronologic data is not available.

4.2.6 Marquette Moraine.

4.2.6.1 Maximum Ages. The Lake Gribben forest bed was buried in outwash by the

Marquette advance of the LIS and for which 19 radiocarbon dates are available (Table

1). The original study dating the forest resulted in five ages with large errors clustering around 11.35 cal ka BP (Hughes and Merry, 1978). The next study produced 10 ages clustering around 11.6 cal ka BP (Lowell et. al. 1999). The most recent study provided four more ages which average to 11.35 cal ka BP (Pregitzer et. al. 2000). The suite of

19 radiocarbon ages range from 10.9-12.15 cal ka BP (Table 1).

4.2.6.2 Minimum Ages. This study provides ten new radiocarbon dates for deglaciation from the Marquette Moraine. Site 3, Enchantment Lake, is a lake within a bedrock meltwater channel with a minimum age recording the abandonment of glacial meltwater

(Figure 2). An age of 10.15 cal ka BP (OS-97742) was collected from terrestrial plant material at the contact between gyttja and underlying sand (Figure 4). Site 4 is Big

Trout Lake located 1 km inboard of the Marquette ice margin (Figure 2). Aquatic pods collected at a contact between interbedded sand and silt with grey silt produced an age of 12.96 cal ka BP (OS-97743; Figure 4). This was later found to be artificially old (per comm. Catherine Yansa). Site 2 is Engman Lake where two ages were collected from two cores. A stick in core BT2 at a contact between diamicton and sandy gyttja yielded an age of 10.9 cal ka BP (OS-97739; Figure 4). Woody debris collected in core CT1 at

17 the contact of gyttja and sand gave an age of 10.9 cal ka BP (OS-97738; Figure 4). Site

1 is Ackerman Lake located approximately 3 km inboard of the Marquette Moraine

(Figure 2). Two ages from separate cores showing the same stratigraphy were collected from this lake. In core CT1 a large stick 8 cm above the contact between gyttja and sand was dated to 10.6 cal ka BP (OS-97578; Figure 4). The second age from core CT1 is from a stick 11 cm above the gyttja and sand contact producing an age of 10.35 cal ka BP (OS-97579; Figure 4). Site 5, Goose Lake, located approximately 5 km on the distal side of the Marquette Moraine (Figure 2). Goose Lake lies on top of the outwash which buried the Lake Gribben forest bed and can therefore be used as a minimum age as it indicates when outwash deposition ceased therefore indicating ice recession from the Marquette Moraine. Four minimum ages were collected from three separate cores. Core BT1 has two dates from a sandy gyttja mixture underlain by sand. The lowest wood gave an age of 11.5 cal ka BP (OS-97741) and wood a few cm higher in the core produced an age of 11.55 cal ka BP (OS-100615;

Figure 4). Woody material in core ET1 at the gyttja and sand contact produced an age of 10.4 cal ka BP (OS-97740; Figure 4). In core CT1 one age was recovered from the contact between gyttja and sand giving an age of 10.05 cal ka BP (OS-101361; Figure

4).

Two minimum ages from core BT1 from Goose Lake agree in age with the burial of the Lake Gribben Forest bed. Both of these samples were sampled in sandy gyttja with less than 5% organics (Figure 4). With the close proximity of Goose Lake to the buried Lake Gribben forest, it is interpreted that fragments of wood from the Lake

Gribben forest were incorporated into the outwash. These two ages were not included

18 in the minimum age data set for the Marquette Moraine. The oldest minimum age collected representing ice recession from the Marquette Moraine is 10.9 cal ka BP. This age best represents the timing of deglaciation from this margin.

4.3 Timing of the Two Creeks and Marquette Moraines Refined

The timing of glacier advance to the Green Hills/Ishpeming and Marquette moraines is respectively 13.7 and 11.6 cal ka BP (Table 1). The addition of minimum ages constrains the timing of these moraines giving insight to LIS deglaciation chronology.

4.3.1 Summing Probabilities. When constraining the timing of a moraine using minimum and maximum ages there are two methods which can be applied. One is using the raw radiocarbon ages in which the youngest maximum and oldest minimum are used to constrain the timing of the moraine. The other method calibrates radiocarbon ages representing the same event, and then sums their probabilities into one plot. Both methods yield similar results yet this study prefers summing the probabilities because it provides a more comprehensive display of the data used to represent the event.

Constraining the timing of a moraine by summing the probabilities of the maximum and minimum ages leaves a range of values between which could represent the actual age of the moraine. To address this problem, a statistical program which analyzes the space between the minimum and maximum summed probabilities and the ages themselves was employed (per comm. Patrick Applegate). The program produces a probability curve between the maximum and minimum dates which represents a range of the most probable ages of the moraine. Instead of arbitrarily choosing a point

19 between the constrained ages, the program statistically produces a “best” age for the moraine.

4.3.2 Refinement of the Green Hills/Ishpeming Moraine Formation. Only one prior minimum age exists to constrain the timing of retreat from the Green Hills/Ishpeming

Moraine. This study adds four additional minimum ages. Because the original minimum age of 11,450 ± 1,200 14C ka BP (Table 1) contains a large error, it was not included when producing a summed probability plot of the minimum ages. A summed maximum age was calculated to have a most probable age of 13.75 cal ka BP while the minimum ages are best represented by an age of 12.85 cal ka BP (Figure 5). With summed minimum and maximum ages constraining the timing of LIS advance and subsequent retreat a statistical program providing the most probable age was applied

(per comm. Patrick Applegate). The program calculated a most probable age of the

Green Hills/Ishpeming Moraine to 12.75 ± 0.2 cal ka BP (Figure 5).

4.3.3 Refinement of the Marquette Moraine Formation. It is similarly well known when ice advanced to the Marquette moraine yet only one prior age constrains the timing of retreat. This study adds 7 minimum ages. Summed maximum ages produced a most probable age of 11.25 cal ka BP while minimum ages yield 11.05 cal ka BP (Figure 6).

With the timing of advance and retreat well constrained, a most probable age calculated by the statistical program (per comm. Patrick Applegate) gave an age of 11.06 ± 0.15 cal ka BP. This represents the most probable time of the Marquette moraine (Figure 6).

4.4 Interpreted Sequence of Events. The Sagola/Saint Johns and Late Sagola/Republic moraines formed before the burial of the Two Creeks forest bed (Table 2; Figure 10). It is likely one of these moraines is equivalent to what is known as the Port Huron

20 readvance dated to approximately 15 cal ka BP (Dreimanis and Karrow, 1972; Gravenor and Stupavsky, 1976; Blewett et al., 1993). Meltwater from the Winegar and Saint

Johns moraine flows towards the Sagola margin where it is then routed south following the ice margin into glacial Lake Oshkosh (Table 2; Figure 10).

The Green Hills/Ishpeming Moraine is equivalent to the moraine formed after the

LIS advanced over the Two Creeks forest bed. Maximum and minimum dates indicate that the most probable age of this moraine is 12.75 ± 0.2 cal ka BP. Ice readvance repositioned the interlobate region between the Green Bay lobe and the Michigamme lobe 6 km north of the previous two moraines (Figure 10). Drumlins and tunnels channels first begin to form at this position (Table 2). Meltwater is routed south along the ice margin as indicated by large parallel meltwater channels into glacial Lake

Oshkosh (Table 2; Figure 10). Minimum ages indicate LIS retreat was underway at approximately 12.7 cal ka BP.

From the Green Hills/Ishpeming position ice retreated in a step-wise pattern forming six ice margins between 12.7-11.2 cal ka BP (Table 2; Figure 10). At each of the first four ice margins large outwash deposits and outwash fans are formed (Table 2).

Drumlin density increases reaching its highest density at the Gwinn moraine. At the

Pembine/Hay Lake Moraine meltwater is routed along the ice margin into a diminishing glacial Lake Oshkosh (Table 2; Figure 10). As ice retreated to the Felch/Pike Lake,

Foster City/Shag Lake and Gwinn moraines meltwater was able to cut across to the south east through what is the modern day Menominee River and into Green Bay

(Table 2, Figure 10). Drumlin formation ceases at the Gladstone and Rapid River moraines with meltwater routed through modern day Escanaba, MI. These moraines

21 were the last moraines formed before ice retreated north of the Marquette Moraine allowing the Lake Gribben forest bed to grow.

Prior work dating wood buried in till inside the Marquette Moraine indicates that the LIS readvanced to the Marquette margin (Hack, 1965; Black, 1976; Peterson, 1982).

Minimum and Maximum ages indicate ice occupied the Marquette margin at 11.06 ±

0.15 cal ka BP (Table 2). During this time large outwash deposits were formed in central and eastern sections of the study area, and the Lake Gribben forest bed was buried by outwash (Table 2). The ice margin is defined by ice-contact slopes near

Marquette while well-defined moraines mark the ice margin near Au Train before transitioning back to ice contact slopes. Meltwater was either routed through the modern day Escanaba River flowing through Escanaba, MI or flowing directly south from the ice margin eventually into Lake Michigan through the Au Train channel, and past Rapid River, MI (Table 2; Figure 10). Ice then retreated north of the Marquette

Moraine and did not return to the area.

4.5 Time-Distance Diagram. Calibrated radiocarbon curves are plotted against time and distance along a transect shown on Figure 3. The nature of the radiocarbon curves represents organic material collecting at that site indicating deglaciation has happened by that time. The age of the Green Hills/Ishpeming and Marquette moraines are well established by this study yet the chronology of the intermediate moraines remain unknown.

Two Transects are created to calculate retreat rates on the long axis of the

Green Bay lobe (A-A’) and short axis (B-B’) shown in Figure 1. Because the growth of

22 the Lake Gribben forest bed indicates deglaciation, the oldest age can be used as a minimum age controlling the retreat rate of ice (Figure 7). Retreat rates calculated for the long axis of Green Bay lobe retreat yields ~360 m/y (Figure 7) with a short axis yielding ~80 m/y of retreat (Figure 10).

4.6 Dune Sands and Lake Bottom Inorganic Sediments. Sand samples were collected from three separate dunes located near site 40 (Figure 2). Grain size distribution between the dunes was very consistent with mean grain size of medium sand.

Approximately 75% of the dune samples were moderately well sorted or well sorted, with most samples having symmetrical skewness (Figure 8).

No sedimentary structures were present in the sand units below the organic sediment. Over 75% of the samples consist of fine sand or smaller, with only one sample of well-sorted sand and the remainder being moderately-sorted or poorly-sorted sand (Figure 8). Over 75% of the samples were fine skewed and very-fine skewed sand (Figure 8).

5. Discussion

5.1 Young Minimum Ages

Minimum ages outside of the Marquette ice margin appear to be too young, some younger than the minimum ages constraining the recession for the Marquette

Moraine (Table 1, Figure 2). One explanation might be the delayed formation of lakes or preservation of organic material after deglaciation.

A common suspect delaying the age of minimum ages is kettle lakes. Here ice persists in lakes after deglaciation delaying the preservation of organic material. For this to be plausible, ice in kettle lakes would have to persist in some cases over 3000 years to delay preservation of organic material long enough to achieve some ages this study collected. This is not probable.

Loope et al. (2012) studied large relict dune fields in mainland central and eastern Upper Peninsula of Michigan. This study used 65 optically stimulated luminescence ages on quartz to determine the timing of dune activity of the large dune fields. The dunes were stabilizing between 10-8 cal ka BP, about 1000 years after ice left the Marquette ice margin. It is therefore plausible arid climate and aeolian dune activity delayed lake formation producing young minimum ages.

Analysis of lake-bottom sands show minimal similarities to eolian sand stabilizing at 10-8 cal ka BP (Figure 8). Anderson Lake (site 6) was the most similar with a comparable grain size distribution and skewness (Figure 8). Overall, lake bottoms sands are not as well sorted and finer grained than the aeolian sand (Figure 8). This evidence indicates that dune formation prior to lake formation is not a likely cause delaying lake formation.

The simplest explanation for the young minimum ages is climate was too arid for lake formation. Arid conditions lowering the water table in combination with sandy outwash deposits may have delayed formation of lakes after deglaciation. This in turn delays the preservation of organic material in lake bottom sediments, thus recording ages much younger than deglaciation.

5.2 Green Hills/Ishpeming Age Assignment. Through analysis, the Green

Hills/Ishpeming moraine was assigned an age of ~12.75 cal ka BP. This is an age younger than the minimum age from Johnson Lake (site 9) constraining deglaciation.

The lack of minimum ages well constraining deglaciation and five young maximum ages from the Two Creeks forest bed causes overlap between the maximum and minimum summed probability plots (Figure 5). The program used to calculate the probability curves accounts for the overlap producing an age younger than the minimum age constraining deglaciation. Because only one age well constrains deglaciation, more weight is given to amount of young maximum ages skewing the age assignment younger. Future efforts to better constrain the timing of the Green Hills/Ishpeming moraine must focus on additional minimum ages recording deglaciation.

5.3 Laurentide Ice Sheet Retreat during the Younger Dryas

5.3.1 Step-wise Retreat. At least six ice marginal positions exist between the Green

Hills/Ishpeming and Marquette moraines (Figure 2). The presence of moraines or ice contact slopes suggest a period of ice sheet stability long enough to establish a moraine or ice contact slope. At least six ice margins suggest step-wise retreat. This would agree with the previously rejected hypothesis Saarnisto (1974) proposed of gradual LIS retreat across the Upper Peninsula of Michigan, and rejects the former accepted hypothesis (Drexler et al., 1983; Clayton and Moran, 1986), of rapid continuous retreat to the northern shores of Lake Superior and thereafter readvance to the Marquette

Moraine.

5.3.2 LIS Deglaciation. Through the use of minimum and maximum dates this study has refined the age of advance and subsequent retreat from both the Green Hill/Ishpeming

25 and Marquette moraines. Plotting the summed probabilities permits constructing a time frame for LIS deglaciation in the study area (Figure 11).

Minimum ages on the Green Hills/Ishpeming Moraine provide timing of LIS deglaciation initiation, while ages on the Lake Gribben forest bed and Marquette

Moraine limit the age of the Marquette Moraine (Figure 11). Although the timing of the

Marquette Moraine formation is tightly constrained, the oldest minimum age for the

Green Hills/Ishpeming moraine is ~12.8 cal ka BP. Since moraine formation must be older than this age and younger than 13.7 cal ka BP it leaves an approximate 800 year gap of potential ice sheet activity before the onset of the YD.

5.3.3 LIS Retreat Before or During the Younger Dryas? The burial of the Two Creeks forest bed and formation of the Green Hills/Ishpeming Moraine occurred at a maximum age of 13.7 cal ka BP. This leaves an approximately 800 years before the start of the

YD period. This raises the possibility of two options for the style of ice sheet retreat.

One where retreat and formation of the ice contact slopes happens very quickly, occurring before the onset of the YD, or another where they are formed gradually, with the lobe retreating in steps during the 800 year gap and the YD.

5.3.3.1 Ice Margin Formation. Between the Green Hills/Ishpeming and Marquette moraines, the LIS must produce six ice margins during deglaciation. This suggests the

LIS paused long enough at these six positions to establish an ice margin. In order for deglaciation before the YD to occur all six margins must be produced in the 800 year period before YD initiation. While it is difficult to determine the duration of moraine production, at a maximum, it must produce one ice contact slope or moraine every 125

26 years. If the LIS retreats over the duration of the YD, moraines are produced over a greater length of time at about one every 250 years.

5.3.3.2 Calving Retreat. Because there is a component of calving during LIS retreat due to the presence of a glacial lake in the Lake Michigan basin, calving retreat rates are considered. Calving retreat rates are calculated as a function of water depth using an equation discussed in Benn et al. (2007). The deepest section of the Lake Michigan basin is approximately 250 m in depth. Additionally, lake levels were approximately 13 m higher than current during LIS advance over the Two Creeks forest bed (Colman et al., 1994). Therefore, a water depth of 263 m was used to estimate calving controlled retreat rates.

LIS retreat from the Denmark/Late Athelstane moraine controlled by calving yields a retreat rate of approximately 622 m/y. Over the 800 year period before the YD this accounts for approximately 500 km of retreat but does not include the time needed build and deglaciate six ice contact slopes or moraines. This rate permits the ice margin on the northern shores of Lake Superior. However, this value estimates if the entire ice margin was only controlled by calving at a water depth of 250m. Moraines in the study region show retreat was controlled by a combination of terrestrial and lacustrine retreat.

Additionally, ice recession was completely controlled by terrestrial retreat as it retreated across the Upper Peninsula of Michigan. It is therefore unlikely ice recession occurred at that rate. This exercise again highlights how this magnitude of retreat proposed by early researchers is not probable (Drexler et al., 1983; Clayton and Moran, 1986).

5.3.3.3 Terrestrial Retreat. Four prior studies provide retreat rates of different sections of the LIS through the Late Glacial (Ridge et al., 1999; Lepper et al., 2007; Fisher et al.,

2009; Lowell et al., 2009). In order to cover all possible values of retreat, this study considers the lowest (20 m/y) and highest (250 m/y) retreat rates proposed by these studies during YD time.

The lowest retreat rate values calculated by prior studies are approximately 20 m/yr. Over an 800 year period this is equivalent to roughly 16 km of retreat. Retreat on this scale places the ice margin somewhere between the Green Hills/Ishpeming and

Pembine/Hay Lake moraines. Ice at this position during the onset of the YD shows that the LIS must retreat in steps to the remaining six moraines during the YD indicating stepwise retreat through the YD chronozone.

The highest rates of retreat suggested by prior work reach values of approximately 250 m/yr. Retreat at this rate over an 800 year period is equal to approximately 200 km of retreat. From the terminus of the Green Bay lobe marked by the Denmark/Late Athelstane Moraine, 200 km of retreat places the ice margin 10 km south of the Gladstone Moraine (Figure 2). This implies that after the start of the YD, at least two moraines must be formed. This indicates retreat in a step wise pattern during the YD noting that this does not account for the time needed to construct the four moraines deposited before the Gladstone Moraine.

5.3.4 Regional Ice Sheet Comparison. Two studies mapped moraines and refined LIS retreat chronology through the late glacial in Thunder Bay, Ontario and Fort McMurray,

Alberta (Lowell et al., 2009; Fisher et al., 2009). Similar to this study, both studies used

28 minimum ages to constrain the timing of moraines creating a deglaciation chronology.

In Thunder Bay, a set of three moraines were defined. Minimum ages refined the ages of these moraines and indicate step-wise retreat with small readvances during 13.9-11.0 cal ka BP (Figure 10; Lowell et al., 2009). A set of four moraines were mapped and the timing refined in Fort McMurray. Similarly, deglaciation chronology also indicates stepwise retreat from approximately 12.4-10.8 cal ka BP (Figure 10; Fisher et al., 2009).

Both studies produced retreat rates during the YD chronozone of approximately 40-135 m/y compared to 360 m/y of retreat for the Green bay lobe during the same interval

(Figure 10). This evidence suggests step wise retreat of the LIS was not just a regional behavior, but likely the whole ice sheet was responding similarly during the Late Glacial.

5.4 Marquette Advance During an Interglacial Climate

Pieces of wood buried in the Marquette moraine to the west of our study region dated to an age of approximately 11.2-11.6 cal ka BP indicate that LIS advanced to the

Marquette Moraine position (Hack, 1965; Black, 1976; Peterson, 1982), but it remains unknown how far north of the Marquette Moraine the ice retreated before readvancing.

The timing of advance to this moraine was refined and assigned the age of 11.6 cal ka

BP (Lowell et al., 1999). This study has further refined the age of the Marquette advance to 11.06 ± 0.150 cal ka BP (Figure 5). This is approximately 500 years after the termination of the Younger Dryas event suggesting that the LIS advanced during the early Holocene, a much warmer interglacial period. This analysis suggests an alternate variable controlled the ice margin during this time and that there is a disconnect between the ice core temperature reconstruction and LIS response.

5.5 Contradicting Messages.

The goal of this study was to determine how the LIS was affected during the YD. Both geomorphic and chronological evidence suggest that the LIS was retreating in steps through the YD period. This suggests that during a reported near glacial temperature interval, this section of the LIS was melting and retreating across the Upper Peninsula of Michigan. Since modern day observations of glaciers and ice sheets indicate they are sensitive and respond quickly to changes in climate (Oerlemans, 1986; Oerlemans,

1990; Oerlemans and Fortuin, 1992; Fabre et al., 1995; Lowell, 2000), this study investigates the oxygen isotope temperature reconstruction for explanation of our results.

5.5.1 Seasonality. The GISP and GRIP Greenland ice cores and their temperature reconstructions using oxygen isotopes have been the basis for paleoclimatic studies since their collection (Cuffey et al., 1995; Dahl-Jensen et al., 1998). These records have been interpreted to represent mean-annual air temperature with large dips in the

δO18 record indicating periods of colder climate (Cuffey et al., 1995; Dahl-Jensen et al.,

1998). Because of this it has been widely used to study periods of rapid climate change.

Large dips in the δO18 record indicating colder climates favor glacial expansion as the ablation season, which is known to control the ice margin, is shortened

(Oerlemans, 2001). The YD, a 1300 year long interval decreasing annual temperatures by approximately 15 ˚C (Cuffey et al., 1995), should promote glacial expansion.

It was noticed in East Greenland that YD cooling should have lowered snowlines by 2000 m where modern day snowlines range from 1200-1500 m (Denton et al., 2005)

This would place snowlines below sea level during the YD (Denton et al., 2005).

Evidence for an expansion during the YD to that extent was not seen in the region

(Denton et al., 2005). In addition, prior work was used to reconstruct snowlines from dated moraines which reveal summer temperatures approximately 4.3-6.1 ˚C cooler than present, significantly less than 15 ˚C as recorded by the ice cores (Denton et al.,

2005). A proposed explanation for the apparent mismatch between the ice core records and glacial response is seasonality (Denton et al., 2005). Extremely cold winters with moderate summers would decrease the annual average, as recorded by the Greenland ice cores, to lower colder temperatures (Denton et al., 2005). This allows for warmer summers to exist during a recorded cold interval.

Furthermore, a detailed study investigating seasonality in East Greenland dated moraines using exposure dates to reconstruct past glacial extent and snowline (Kelly et al., 2008). Investigating seasonally in East Greenland provides a direct check of the temperature reconstruction as the Greenland Ice cores are only a few hundred kilometers away. Glacial reconstruction revealed summer temperatures in East

Greenland only decreased 3.9-6.6 ˚C, again, much less than the records during the YD

(Kelly et al., 2008). This study also supports warmer summers during the YD.

Seasonality in Greenland suggests other areas also experienced warmer summers during the YD. The findings of this study support greater seasonality during the YD as the geomorphic and chronologic evidence indicate the LIS was retreating in steps during this time, indicating warmer summers.

5.5.2 The Scandinavian Ice Sheet. Two studies interpret the age of the Salpausselkä moraines I and II deposited by the eastern section of the Scandinavian Ice Sheet (SIS;

Saarnisto and Saarinen, 2001; Donner, 2010). Varve chronology revealed both moraines were deposited during the YD interval. This suggests that this portion of the

SIS was in a period of step-wise retreat during the YD. These results from the SIS for the same time duration support the observation from this study, that recession of the

Green Bay lobe from the Denmark Moraine was in a step-wise manner.

6. Conclusions

An investigation of glacial landforms in central Upper Peninsula of Michigan resulted in the refinement of previously mapped moraines and ice sheet chronology.

The Two Creeks (13.7 cal ka BP) and Lake Gribben (11.1 cal ka BP) sites were both buried by a readvance of the LIS indicating that it was in this region during the YD.

Between the Green Hills/Ishpeming (Two Creeks equivalent) and Marquette (Lake

Gribben equivalent) moraines are at least six recessional moraines. The moraines indicate that the LIS retreated in steps between 13.7 and 11.1 cal ka BP. In addition, 25 minimum ages were used to refine LIS deglaciation, specifically the Marquette and

Green Hills/Ishpeming moraines. The Green Hills/Ishpeming Moraine has been assigned an age of 12.75 ± 0.2 cal ka BP and the Marquette Moraine 11.06 ± 0.15 cal ka BP. Retreat rates of all values from other portions of the LIS suggested by former studies were applied to the Green Bay lobe. These results indicate that the LIS must have undergone step wise retreat during at least some of the YD. Retreat rates calculated for the Green Bay lobe during YD time yield 360 m/y.

The Greenland ice core temperature reconstruction indicates that the YD brought climate back to near full glacial conditions. Glacial conditions would promote glacial advance, yet evidence instead points towards glacial retreat. Seasonality and warmer

32 summers shown to have occurred in Greenland suggest that other glaciated regions during that time may also have been affected. There is strong evidence in our study region that shows the LIS retreated in steps, which supports seasonality of the Late

Glacial climate. It can therefore be concluded that using the Greenland ice core records for paleoclimatic reconstruction should only be attempted with caution.

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Geographic locations of the sediment cores analyzed.

Appendix B-B7: Images of the sediment cores collected and used for radiocarbon samples. The red arrows point to specific samples used for radiocarbon dating. The red boxes represent areas where sediment was sieved in order to locate macro organic material for radiocarbon dating. Codes refer to radiocarbon ages in Table 1 in text. The green arrows indicate the locations of grain size analysis shown in appendix D-D5.

Imagery of the radiocarbon samples dated. The code refers to Table 1 in text.

Appendix D-D5: All grain size data collected in the sediment cores analyzed. Locations of grain size analysis are indicated by the green arrows in Appendix B-B7.

D 2

D 3