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A Possible Late Pleistocene Impact Crater in Central North America and Its Relation to the Younger Dryas Stadial

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A Possible Late Pleistocene Impact Crater in Central North America and Its Relation to the Younger Dryas Stadial A POSSIBLE LATE PLEISTOCENE IMPACT CRATER IN CENTRAL NORTH AMERICA AND ITS RELATION TO THE YOUNGER DRYAS STADIAL SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA BY David Tovar Rodriguez IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Howard Mooers, Advisor August 2020 2020 David Tovar All Rights Reserved ACKNOWLEDGEMENTS I would like to thank my advisor Dr. Howard Mooers for his permanent support, my family, and my friends. i Abstract The causes that started the Younger Dryas (YD) event remain hotly debated. Studies indicate that the drainage of Lake Agassiz into the North Atlantic Ocean and south through the Mississippi River caused a considerable change in oceanic thermal currents, thus producing a decrease in global temperature. Other studies indicate that perhaps the impact of an extraterrestrial body (asteroid fragment) could have impacted the Earth 12.9 ky BP ago, triggering a series of events that caused global temperature drop. The presence of high concentrations of iridium, charcoal, fullerenes, and molten glass, considered by-products of extraterrestrial impacts, have been reported in sediments of the same age; however, there is no impact structure identified so far. In this work, the Roseau structure's geomorphological features are analyzed in detail to determine if impacted layers with plastic deformation located between hard rocks and a thin layer of water might explain the particular shape of the studied structure. Geophysical data of the study area do not show gravimetric anomalies related to a possible impact structure. One hypothesis developed on this works is related to the structure's shape might be explained by atmospheric explosions dynamics due to the disintegration of material when it comes into contact with the atmosphere. Relationship between structure's diameter (D) and deformed strata thickness (h) as well as the relationship between the diameter of the projectile (d) and the depth of the water column (H ), which is considered in this study due to the geographical considerations of the area 12.9 ky ago and BP, are consistent with an extraterrestrial event. Other hypotheses, such as lake processes and glacial processes, are difficult to reconcile with the reported observations, so the impact hypothesis and its relationship with the formation of the Roseau structure are viable. ii TABLE OF CONTENTS LIST OF ILLUSTRATIONS 1. INTRODUCTION 2. BACKGROUND 2.1. CONTEXTUALIZING THE YD EVENT: TIMING AND POSSIBLE TRIGGERS 2.2. THE IMPACT HYPOTHESIS AND THE CONTROVERSY 2.2.1. THE PHYSICAL AND GEOCHEMICAL EVIDENCE 2.2.2. ONSET OF THE YD AND TIMING OF CULTURAL TRANSITIONS AND EXTINCTIONS 2.2.3. THE GEOMORPHOLOGICAL EVIDENCE 2.3 ARGUMENTS AGAINST ASTEROID IMPACT HYPOTHESIS 2.4 GENERAL RIDICULE 3. GLACIAL AND POSTGLACIAL HISTORY WITH EMPHASIS ON LAKE AGASSIZ 3.1 GLACIAL HISTORY 3.2 A BRIEF HISTORY OF LAKE AGASSIZ AND THE YD 3.3 STAGES OF LAKE AGASSIZ 4. DESCRIPTION AND SETTING OF THE ANOMALOUS STRUCTURE 4.1 GLACIAL ORIGIN 4.2 LACUSTRINE ORIGIN 4.3 IMPACT ORIGIN 5. DISCUSSION 6. CONCLUSIONS REFERENCES iii LIST OF TABLES Table 1……………………………………………………………………………………8 Table 2……………………………………………………………………………………16 Table 3……………………………………………………………………………………33 Table 4……………………………………………………………………………………35 iv LIST OF FIGURES Figure 1. Lake Agazzis drainage routes………………………………….44 Figure 2. Lake Agazzis strandline point elevation data………………....45 Figure 3. Lake Agassiz lake-level hydrograph…………………………...46 Figure 4. Roseau Structure………………………………………………...47 Figure 5. Gravity data……………………………………………………….48 Figure 6. Carolina Bays……………………………………………………..49 v 1. Introduction Few climate events have received as much attention from the scientific community as the Younger Dryas. As climate was rapidly warming at the end of the last Ice Age, this climatic anomaly plunged the Earth back into glacial conditions from 12.9 - 11.7 cal ka BP. The abrupt end of the Younger Dryas marks the beginning of the Greenlandian Age and the transition from the Late Pleistocene to the Holocene. The causes of this climatic event have been the subject of heated debate, and hypotheses have evolved rapidly. Mangerud (1987) argues that an event of this magnitude requires a global trigger such as changes in ocean circulation. In addition, given the importance of the North Atlantic currents in terms of thermal equilibrium of climate, it is sensible to think that the YD event did not exclusively affect Europe; therefore it had an impact in other parts of the world, particularly in North America. This idea is reinforced by Broecker et al. (1989) suggests, through analysis of marine oxygen isotope records, that Lake Agassiz in North America had a flow of melted water to the north, which considerably reduced the salinity of the North Atlantic, resulting in a variation in its circulation pattern and consequently a variation in the climate of the northern hemisphere. Teller et al. (2002) show that not only large amounts of water were released from Lake Agassiz into the North Atlantic once Laurentide Ice Sheet (LIS) began its recession. Perhaps one of the most controversial hypotheses is that raised by Firestone et al. (2007a) in which it is stated that the start of the YD is due to the impact of an asteroid with the Earth's surface. This hypothesis is based on the observation of charcoals, nanodiamonds, fullerenes, spherules, iridium anomalies, among others, in the sediments associated with Lake Agassiz. The research reported by Firestone et al. (2007a) summarized evidence from 50+ Clovis- age archaeological sites in North America that is consistent with an extraterrestrial impact. 6 Common to all of these sites is an organic layer dating to 12.9 cal ka BP that corresponds to the extinction of Pleistocene megafauna and marks the end of Clovis culture. Contained within this layer are magnetic grains with elevated iridium concentrations, magnetic microspherules, charcoal, carbon spherules, nanodiamonds, and fullerenes with extraterrestrial (ET) helium signatures. They suggest that all of these lines of evidence are consistent with an ET impact and associated biomass burning (Firestone et al., 2007a). This assertion has been met with cautious support (Israde-Alcantara et al. 2012; Haynes, 2008; Kennett et al., 2009), arguments against (Paquay et al. 2009; Boslough et al. 2012; Buchanan et al. 2008; Wu et al. 2013; LeCompte et al. 2012), and general ridicule (Surovell et al. 2009; Pinter et al. 2011). We herein describe a circular feature in northern Minnesota that has characteristics of an impact crater. In addition, the location of this feature constrains its formation to within error of 12.9 cal ka BP, and we evaluate possible sets of conditions that would allow a feature of this size and characteristics to result from a bolide impact. The feature is located a few km north of the Campbell Beach of Lake Agassiz in Roseau County, Minnesota. Because of its location, the feature is hereafter referred to as The Roseau Structure, or simply the Structure. The Structure is a circular shallow depression with concentric rings on the NE side. The concentric rings have been wave washed during rising and falling stages of Glacial Lake Agassiz. The location of the Structure constrains its age to after ice retreat from this area but before the drainage of Lake Agassiz. Lake Agassiz occupied the Tintah Beach at an elevation of 359 meters until 12.9 cal ka BP at the beginning of the Younger Dryas. At that time, the lake level dropped as the outlet of Lake Agassiz switched from the southern outlet to a lower outlet to the east (Teller, 2013) or northwest (Fisher et al., 2009) or neither (Lowell et al., 2013). By 11.7 cal ka BP Lake Agassiz rose again to the Campbell Beach. 7 2. Background 2.1. Contextualizing the YD event: Timing and possible triggers The Last Glacial Maximum refers to the time of maximum extension of the ice sheets during the last glacial period, approximately 21,000 years BP (Table 1). Global climate was generally warming and environmental conditions in Western Europe began to resemble current conditions ca. 15,000 cal yr BP. The Younger Dryas (YD) was a brief phase (~1,300 ± 70 years long) of climate cooling in the late Pleistocene, between 12.9 and 11.7 cal ka BP (Broecker, 2010; Carlson, 2010; Lowell, 2013 ). In Blytt-Sernander climate theory, the Younger Dryas succeeds the Bölling / Allerød interstade, and precedes the Lower Holocene Preboreal period. This event takes its name from the alpine flower Dryas octopetala, an arctic-alpine flower plant in the Rosaceae family. Few other climate anomalies have been as widely studied as the YD, as this event marked the last stadial at the end of the most recent glacial period. Numerous investigations around the proposal of hypotheses that allow us to account for the event or events that could have contributed to the termination of the YD. Timing last glacial maximum Date (ka cal BP) Oldest Dryas (stadial) 18.5- 14.7 Bølling (interstadial) 14.7–14.1 Older Dryas (stadial) 14.1 – 13.9 Allerød (interstadial) 13.9 – 12.9 8 Younger Dryas (stadial) 12.9 – 11.7 Pre Boreal (interstadial) 10.2 - 8 Table 1. Timing of the main stadial and interstadial periods during the Last Glacial Period In order to contextualize the Young Dryas, here we outline the hypotheses put forward for this event. The YD has received great attention from the scientific community, and even within the press and broader public. The causes that produced this event are hotly debated and the hypotheses related to its origin are diverse and controversial. Late Glacial climate oscillations, including the Allerød and Younger Dryas, were identified in the Paleoenvironmental record as early as the investigation of Hartz and Milthers (1901), and Flohn (1979) explores the time scales and causes of abrupt paleoclimate events. In an effort to explain YD cooling, Mangerud (1987) summarized the possible causes of the Younger Dryas based on evidence collected in Norway, the North Sea, and Svalbard.
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