Morphodynamic Features of Palaeo Ice Streams Offshore N-Iceland Based on Multibeam Bathymetric and High Resolution Chirp Seismic Data

Morphodynamic Features of Palaeo Ice Streams offshore N-Iceland based on Multibeam Bathymetric and High Resolution Chirp Seismic Data

Patricia Höfer

Faculty of Earth Sciences University of Iceland 2019

Morphodynamic Features of Palaeo Ice Streams offshore N-Iceland based on Multibeam Bathymetric and High Resolution Chirp Seismic Data

Patricia Höfer

60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Geology

MS Committee Bryndís Brandsdóttir Ásta Rut Hjartardóttir

Master’s Examiner Ívar Örn Benediktsson

Faculty of Earth Science School of Engineering and Natural Sciences University of Iceland Reykjavik, October 2019

Morphodynamic Features of Palaeo Ice Streams offshore N-Iceland based on Multibeam Bathymetric and High Resolution Chirp Seismic Data

Palaeo Ice Streams, offshore N-Iceland

60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Geology

Faculty of Earth Sciences School of Engineering and Natural Sciences University of Iceland Sturlugata 7 101 Reykjavík Iceland

Telephone: 525 4000

Bibliographic information: Patricia Höfer, 2019, Morphodynamic Features of Palaeo Ice Streams offshore N-Iceland based on Multibeam Bathymetric and High Resolution Chirp Seismic Data, Master’s thesis, Faculty of Earth Sciences, University of Iceland, pp. 71.

Printing: 04.10.2019 Reykjavik, Iceland, October 2019

Abstract

Glacial landforms on the N-Iceland shelf associated with the last deglaciation demonstrate that the ice streams of the Last Glacial Maximum extended further north than previously assumed. Multibeam bathymetric data used in this study was acquired during four mapping surveys in 2002-2004. Glacial landforms represent the ice extent of the major ice streams and their associated velocity: Skagafjörður-Skagafjarðargrunn, Eyjafjörður-Eyjafjarðaráll, Bárðardalur-Skjálfandi-Skjálfandadjúp, and Öxarfjörður. Crag and tails, drumlins and high parallelisms of mega-scale glacial lineations not only show the trend of the expected palaeo ice flow, they also show the fast velocity of the ice streams. Asymmetrical ridges interpreted as grounding zone wedges suggest that the deglaciation was episodic, with periods of standstill. A moraine in the northern part of Kolbeinsey Ridge with more than 150 km distance to the present coastline and a 100 km remote grounding zone wedge between Eyjafjarðaráll and Skjálfandadjúp shows a longer extent of Last Glacial Maximum than previously thought. These ice streams extended at least 110 km north of Grímsey during the Last Glacial Maximum. Based on tephrachronological data from sediment cores, the shelf segment from Kolbeinsey to Grímsey was deglaciated before 14000 years b2k (Borrobol or BAS-1 event) and has not been subjected to grounded ice cover since that time.

Útdráttur

Ummerki ísstrauma ísaldar á landgrunni Norðurlands, kortlagning með fjölgeisla- og hátíðni-endurkastsmælingum.

Fjölgeislamælingar sýna að ísstraumar síðustu ísaldar á landgrunni Norðurlands, náðu mun lengra en áður hefur verið talið. Kortlagning hafsbotnsins á þessu svæði á árunum 2002- 2004, hefur leitt í ljós markvísleg ummerki meginísstrauma frá Skagafirði út í Skagafjarðardjúp, frá Eyjafirði og út Eyjafjarðarál, frá Bárðardal, Skjálfanda og Öxarfirði út Skjálfandadjúp. Jökulöldur (drumlins, drymbilurðir), skafurðir (crag and tails), og risakembur (mega-scale glacial lineations) endurspegla skriðstefnur og skriðhraða meginstrauma ísaldarjökulsins, m.a. hvernig stærri ísstraumur Skagafjarðar sveigir ísstraum Eyjarfjarðaráls til norðausturs. Óreglulegir hryggir (grounding zone wedges) afmarka svæði þar sem skriðjöklar hafa, gengið fram aftur, staðið í stað, eða keflt í sjó. Jökulgarðar samsíða suðurhluta Kolbeinseyjarhryggjar, í meira en 150 km fjarlægð frá landi og stór jökulhryggur á milli Eyjafjarðaráls og Skjálfandadjúps um 100 km frá landi gefa til kynna að ísstraumar hafi legið að minnsta kosti 110 km til norðurs frá Grímsey í hámarki síðasta jökulskeiðs. Öskulagarannsóknir úr setkjörnum af landgrunni Norðurland gefa til kynna að ísaldarjöklar hafi horfið af svæðinu fyrir meira en 14 þúsund árum, þ.e. áður en Borrolaskjan fell. Frekari kortlagningar er þörf til þess að afmarka hámarksútbreiðslu ísaldarjökla og nánari hörfunarsögu þeirra á þessu svæði.

Table of Contents

List of Figures ...... x

List of Tables ...... xiii

Abbreviations ...... xiv

Acknowledgements ...... xv

1 Introduction ...... 1 1.1 Geological Setting of Iceland ...... 1 1.2 Glacial History of Iceland ...... 2 1.3 Study Area ...... 5 1.4 Definition of Palaeo Ice streams and the Dynamic of Ice sheets ...... 7

2 Methods and Data ...... 9 2.1 Principle of Multibeam Bathymetry ...... 10 2.2 Multibeam Bathymetric Data ...... 11

3 Results: Description and Interpretation ...... 13 3.1 Processes within the Subglacial Environment...... 13 3.1.1 Crag-and-Tails ...... 13 3.1.2 Mega-Scale Glacial Lineation...... 19 3.1.3 Cross-Shelf Troughs ...... 26 3.1.4 Drumlins ...... 28 3.1.5 Eskers ...... 29 3.1.6 Meltwater Channels/ Canyons/ Gullies ...... 30 3.2 Processes within the Ice-marginal Environment ...... 31 3.2.1 Grounding-Zone Wedges ...... 31 3.2.2 Moraines ...... 34 3.3 Processes within the Glacimarine Environment ...... 36 3.3.1 Iceberg Scours/ Ploughmarks ...... 36

4 Discussion ...... 39

5 Conclusions ...... 47

References...... 48

List of Figures

Fig. 1: The multibeam map with a tectonic map of Iceland (Einarsson and Sæmundsson, 1987; Sigmudsson et al., 2018). The divergent plate boundary is made up of central volcanoes (red circles), and transecting fissure swarms (yellow shaded areas). Some have developed calderas (dark circles within red zones). The Neovolcanic zone is divided into the Northern Volcanic Zone (NVZ), Eastern Volcanic Zone (EVZ), Western Volcanic Zone (WVZ), and Reykjanes Peninsula Oblique Rift Zone (RP). The South Iceland Seismic Zone (SISZ) and the Tjörnes Fracture Zone (TFZ) are transform zones. The multibeam map shows the Kolbeinsey Ridge (K) connecting to the Eyjafjarðaráll Basin (E) and the Grímsey Oblique Rift (GOR) which has four volcanic systems, Mánáreyjar (M), Nafir (N), Hóllinn (H) and Stóragrunn (St). Other offshore basins within the TFZ are Skjálfandadjúp (Sk) and Öxarfjörður (Ö)...... 2

Fig. 2.: Glacial geomorphology of the Iceland continental shelf from Spagnolo and Clark (2009) and Patton et al. (2017), corroborated and supplemented with additional landform mapping from Landsat satellite imagery (onshore) and the EMODnet DTM (composite bathymetric dataset) (offshore) (A). Bathymetric profile over a prominent moraine ridge close to the northern shelf edge (B) (from Patton et al., 2017)...... 5

Fig. 3: Multibeam map of the study area. The Kolbeinsey Ridge (K) connects to the Eyjafjarðaráll Basin (E) and the Grímsey Oblique Rift (GOR) which has four volcanic systems, Mánáreyjar (M), Nafir (N), Hóllinn (H) and Stóragrunn (St). Other offshore basins within the TFZ are Skjálfandadjúp (Sk) and Öxarfjörður (Ö). The locations of sediment cores relevant to this study are marked by red (Hakon Mosby cores) and yellow (Marion Dufresne cores) stars. Red boxes show the locations of examined glacier landforms...... 6

Fig. 4: Principle of multibeam echosounder sonar system (Hogg et al., 2010)...... 11

Fig. 5: Crag and tail features next to Nafír a) Overview of the area marks by crag and tails, iceberg scours, GZW and MSGL b) Closer examination of crag and tails next to Nafír with the location of the profiles c) Length profiles over two crag and tail structures from N-S ...... 14

Continuation of Fig. 5 d) Transects (1-3) over one crag and tail landform from SW- NE (Box 1 in Figure 3)...... 15

x Fig. 6: Crag and tails in the eastern part of Skjálfandadjúp Basin. a) Location of Transects 1-5 from SW-NE b) Transects over crag and tail field 1-5 ...... 16

Continuation of Fig. 6: c) Location of length profiles over two striking features d) Profiles x-x’ and y–y’, along the length of the two most prominent crag and tail structures, (Box 2 in Figure 3)...... 17

Fig. 7: The Hóllinn crag and tail in south-central Skjálfandi Bay (Box 3 in Figure 3). The profile x-x’ along the length, shows the crag and tail structure...... 18

Fig. 8: Crag and tail in the Grimsey Volcanic Zone. The profile x-x’, along the length of the feature shows the crag and tail structure and the profile y–y’ shows the width of the crag (Box 4 in Figure 3)...... 19

Fig. 9: MSGL and GZW with iceberg scours in Grímsey Volcanic Zone. a) Overview of the area marks by crag and tails, iceberg scours, GZW and MSGL b) Location of W-E profiles in red ( 1-7) and blue (1-6) (Box 5 in Figure 3)...... 21

Continuation of Fig. 9: c) E-W profiles across the MSGL and GZWwith iceberg scours (1-7) ...... 212

Continuation of Fig. 9: d) A closeup of the MSGL on the eastern side (1-6) ...... 213 Continuation of Fig. 9: e) Overview of the added data with locations of the 13 km long transects over the MSGL and GZW. The black arrows show the direction of the ice flows, and the red truncated line is the limit of the GZW. Profiles 1 and 2 show the interaction between the GZW and the MSGLs (2005 Poseidon data) ...... 214 Fig. 10: Moraine overprinted by MSGL in Eyjafjarðaráll. The Profiles x–x’, y–y’ and z–z’ shows W-E transaction over the moraine where the profile z–z’ shows the part of the MSGL (Box 6 in Figure 3)...... 25

Fig. 11: Cross-shelf trough (red line) and GZW (green line) in southeastern Eyjafjarðaráll Basin with four transects over the cross-shelf trough from SW-NE (Box 7 in Figure 3)...... 27

Fig. 12: Drumlins in the western part of Skjálfandadjúp Basin. a) Overview of the area with location of profiles b) Transect over the Drumlins field c) Lengthprofile (green) and Widthprofile (red) of one Drumlin. (Box 8 in Figure 3)...... 29

Fig. 13: Eskers in Eyjafjarðaráll Basin. The black line in the middle of the Figure is a large normal fault, with a vertical drop of up to 50 m. The profiles created in W-E direction show the hight of the eskers (Box 9 in Figure 3)...... 30

Fig. 14: Flood channels. a) small channels next to Mánáreyjar (10a) b) channel with sediment fill on the western flank of the Eyjafjarðaráll Basin (10b). Small orange arrows indicate the location of diameter measurements (Box 10a, 10b in Figure 3)...... 31

xi Fig. 15: GZW in Grímsey Volcanic Zone with numbered red transactions over the features and profiles 1-8 of this GZW (SE to NW) (Box 11 in Figure 3)...... 33

Fig. 16: Lateral Moraine in Eyjafjarðaráll with W-E Profiles (x-x’, y-y’, z-z’) over a length of 550 meters (Box 12 in Figure 3)...... 35

Fig. 17: Iceberg scours in Grímsey Volcanic Zone close by Nafír. This is a closer example of many iceberg scours in the study area. The profiles x–x’ and y–y’ shows the depth and width of the scours. The location of iceberg scours can be seen in Figure 21, where they marked by blue crosses...... 37

Fig. 18: Chirp data from Skjálfandadjúp Basin. a) Location of transects in Skjálfandadjúp Basin ...... 40

Continuation of Fig. 18: b) Chirp data transects: transect 19 (NE of Grímsey), transect 17 (straight east of Grímsey), transect 14 (SW of Grímsey). transect 18 (S to N file along the eastern margin of Grimsey)...... 41

Fig. 19: High-resolution Chirp seismic data across the faulted (F) Eyjafjarðaráll Basin reveals hardened glacially derived sediments (dark black surface), partially covered by postglacial sediments (lighter colored sections). Prominent glacial features are mega-scale glacial lineations (MSGL), grounding zone wedges (GZW), grounding zone moraines (GZM), V-shaped ridges (v) and an esker (E). Not all these features are labeled. a) Location of Chirp seismic data...... 42

Continuation of Fig. 19: b) High-resolution Chirp seismic data with prominent glacial features are mega-scale glacial lineations (MSGL), grounding zone wedges (GZW), grounding zone moraines (GZM), V-shaped ridges (v) and an esker (E)...... 43

Fig. 20: Overview map summarizing glacial landforms and ice flow directions (black arrows) offshore N-Iceland. Multibeam data combined in GIS with a hillshade elevation map from the National Land Survey of Iceland ...... 46

xii List of Tables

Table 1: Overview of research surveys offshore northern Iceland……………………....…10

Table 2: Overview of identified Tephra layers with constrained age (e.g. Guðmundsdóttir et al., 2011; Magnúsdottír et al., 2015)….44

xiii Abbreviations

b2k: before 2000

E: Eyjafjarðaráll Basin

EVZ: Eastern Volcanic Zone

GIS: Geographical Information System

GOR: Grímsey Oblique Rift

GZM: Grounding Zone Moraine

GZW: Grounding Zone Wedges

H: Hóllinn

HFF: Húsavík-Flatey Fault

K: Kolbeinsey Ridge

LGM: Last Glacial Maximum

M: Mánáreyjar

MB: Multibeam Bathymetric

MSGL: Mega-Scale Glacial Lineation

N: Nafír

NVZ: Northern Volcanic Zone

Ö: Öxarfjörður

RP: Reykjanes Peninsula Oblique Rift Zone

SISZ: South Iceland Seismic Zone

Sk: Skjálfandadjúp

St: Stóragrúnn

TFZ: Tjörnes Fracture Zone

WVZ: Western Volcanic Zone

xiv Acknowledgements

First, I thank the captains and crews of the R/V Árni Fridriksson, R/V Bjarni Sæmundsson R/V Baldur, and R/V Poseidon for skillfully maneuvering their ships during the multibeam and reflection surveys. The multibeam data were collected jointly by the Iceland GeoSurvey, the Marine Research Institute and the Science Institute, University of Iceland. This project was funded by the US National Science Foundation (NSF-OCE-0114357), the Icelandic Science Council (RANNÍS, grants 010680001 and 016120001), the Icelandic Ministry of Industry, the National Energy Authority and the University of Iceland Research Foundation.

Furthermore, I would like to express my gratitude to my supervisor Bryndís Brandsdóttir for the useful weekly meetings, comments, and remarks. For her engagement through the process of this master thesis, her open-door whenever I ran into trouble and the good and nice cooperation.

Then I would like to thank my second supervisor Ásta Rut Hjartardóttir for introducing me to the program ArcGIS, for her helpful instructions as well as for the support on the way.

Finally, I would love to express my very profound gratitude to my parents, my sisters, my family, my friends and my boyfriend who have supported me throughout the entire process, both by keeping me harmonious and helping me putting pieces together. This accomplishment would not have been possible without them. Thank you.

1 Introduction

When glaciers grow and decay, they leave certain fingerprints in the form of moraines, drumlins, lineations and grounding zone wedges as evidence for their presence. Through multibeam echo sounding seafloor mapping, the submarine glacial landforms can be projected, analysed and the palaeo ice streams can be reconstructed. The N-Iceland shelf has been shaped by the interaction of tectonics during repeated Pleistocene glaciations. The shelf topography is characterized by deep basins carved by major ice streams that drain ice sheets into the sea, similar to present major ice streams at the Greenland ice sheet and Antarctica ice sheet. The major ice stream, Eyjafjarðaráll, Skjálfandi-Skjálfandadjúp, and Öxarfjörður basins within the Tjörnes Fracture Zone have all been subjected to tectonic activity during postglacial time (e.g. Norðdahl, 1991; Patton et al., 2017). The Icelandic insular margin contains key evidence for the ways ice streams interact with topography. The aim of this thesis is to analyse submarine glacial landforms from major ice streams on the N-Iceland shelf based on multibeam bathymetric data and to use this landforms to answer the research questions “How far north did palaeo ice streams extend during the Last Glacial Maximum?”. Our mapping is based on this data that was collected during four research surveys offshore northern Iceland: in 2002 and 2004 from the R/V Árni Friðriksson, in 2003 from R/V Baldur, and in 2005 from R/V Poseidon (Table 1). These research projects were designed to investigate the tectonics of the Tjörnes Fracture Zone (TFZ) and the origin of fluid expulsion features discovered during Chirp seismic reflection surveys in 2001 and 2003 in the Skjálfandi Bay off Húsavík (Brandsdóttir et al., 2003).

The first section of this thesis provides an introduction of the project’s study area and the topic. The second section provides the reader with an overview of the data and the methodology used for this project. A description and interpretation of the submarine glacial landforms within this area is provided in the third section followed by the discussion of the glacial landforms regarding to the palaeo ice streams, in the fourth section. A summary of the key aspects of this study can be found in the last section of the thesis.

1.1 Geological Setting of Iceland

Iceland is a ridge-centered hotspot located in the North Atlantic Ocean between Greenland and Norway at 63°23´N to 66°30’N. Iceland is the subaerial part of the Mid-Atlantic Ridge and the Greenland-Iceland-Faeroes Ridge, a band of thickened basaltic crust, stretching across the North Atlantic that represents the product of hotspot volcanism from the Early- Tertiary to present (Figure 1). However, Iceland is geologically very young. The oldest rocks of ca. 15 Myr occur in the easternmost and westernmost parts of the island and the youngest along the rifting zone (Geirsdóttir et al., 2007). The geology structure of Iceland is controlled by the divergent plate boundary of the Eurasian and North America plates. The Neovolcanic Zone of Iceland represents the subaerial part of the plate boundary and is subdivided into three main rift zones, the Western Volcanic Zone (WVZ), the Eastern Volcanic Zone (EVZ) and the Northern Volcanic Zone (NVZ) (Sæmundsson, 1979). Rifting within these zones is mostly confined to fissure swarms associated with central volcanoes.

1 The Tjörnes Fracture Zone (TFZ) links the Northern Volcanic Zone with the Kolbeinsey Ridge (K), the segment of the Mid-Atlantic Ridge immediately north of Iceland. The bedrock of Iceland is covered by interglacial and subglacial volcanic formations as well as glacier deposits, mainly from the last glaciation period.

Fig. 1: The multibeam map north of Iceland (blue color), with a tectonic map of Iceland (Einarsson and Sæmundsson, 1987; Sigmudsson et al., 2018). The divergent plate boundary is made up of central volcanoes (red circles), and transecting fissure swarms (yellow shaded areas). Some have developed calderas (dark circles within red zones). The Neovolcanic zone is divided into the Northern Volcanic Zone (NVZ), Eastern Volcanic Zone (EVZ), Western Volcanic Zone (WVZ), and Reykjanes Peninsula Oblique Rift Zone (RP). The South Iceland Seismic Zone (SISZ) and the Tjörnes Fracture Zone (TFZ) are transformed zones. The multibeam map shows the Kolbeinsey Ridge (K) connecting to the Eyjafjarðaráll Basin (E) and the Grímsey Oblique Rift (GOR) which has four volcanic systems, Mánáreyjar (M), Nafir (N), Hóllinn (H) and Stóragrunn (St). Other offshore basins within the TFZ are Skjálfandadjúp (Sk) and Öxarfjörður (Ö).

1.2 Glacial History of Iceland

The earliest glacier deposits within the deeply eroded eastern Pliocene rock sequences found in the eastern and western Iceland rock suggest initial glaciation of Iceland occurred as early

2 as 5 Ma with increased intensity within the last 3 Ma (Geirsdóttir and Eiríksson, 1994, 1996; Helgason and Duncan, 2001). Stratigraphic studies and sedimentological studies by ∂18O records from deep-sea sediments imply that Iceland has experienced over 20 glaciations of different intensities during the last 4-5 Myr (Geirsdóttir et al., 2007). Local glacier activity is identified in a 4.6-4.7 Myr old glacier deposit in the Skaftafell area at the southern margin of Vatnajökull (Helgason and Duncan, 2001) and in separate glacial deposits within ~3-4 Myr old lava flows in Fljótsdalur, eastern Iceland (Geirsdóttir and Eiríksson, 1994, 1996). Based on a correlation between glacial deposits found in two valleys in eastern Iceland, the ﬁrst indication of a progressively advancing ice sheet in Iceland dates to ~2.9 Ma (Geirsdóttir et al., 2007). The oldest glacial deposit was found in the Tjörnes sediment, N- Iceland indicates a northward and hence more regional expansion of the Iceland ice sheet around 2.5 Ma (Geirsdóttir and Eiríksson, 1994). The glacial stratigraphy from 1.5 Ma to the last deglaciation is fragmentary apart from the Tjörnes section which contains nine stratigraphically separated glacial deposits in the last 1.5 Myr and is the only site known in Iceland with a complete glacial stratigraphy for the last 0.5 Myr (Eiríksson, 1985). The period of the last maximum glacier extension, i.e., the Last Glacial Maximum (LGM), occurred between 28.100 – 22.800 years (Patton et al., 2017).

Although there is now a general consensus that during the LGM, the Iceland ice sheet extended out towards the shelf break around Iceland (Geirsdóttir et al., 2007;2009; Norðdahl et al., 2008), the size of the Iceland ice sheet during the LGM has been delineated and modeled with variable extent. Endmoraines, which are on the outer shelf of Breiðafjörður, west Iceland, and are approximately 130 km from land at 150-250 m depth (Ólafsdóttir, 1975), are generally considered to represent the maximum LMG extent on the Iceland shelf. Vogt et al., (1980) mapped the edge of the shelf glacially incised troughs. He defined the 200 m depth contour, as the grounding line of the Weichselian ice sheet at the LGM. This grounding line is filled up by glacial sediments (Norðdahl, 1991). Meltwater channels, lineations, and drumlins have been found on Grímsey ~40 km offshore northern Iceland, which confirms that the ice sheet covered the highest parts of the island (>100 m) (Keith and Jones, 1935; Hoppe, 1968). As the bathymetry of the Iceland shelf has not been mapped in detail, the extent of the Icelandic ice sheet during the LGM in N-Iceland is mostly based on relatively few sedimentary cores with few radiocarbon (C14) age determinations (Andrews, 2000; Hubbard et al., 2006; Geirsdóttir et al., 2007; Norðdahl et al., 2008; Patton et al., 2017). An overview of the extent from the LGM is given by Patton et al., 2017 where they consolidate and extend the previous mapping offshore from Spagnolo and Clark, 2009 with prominent features observed from Landsat imagery onshore as well as new Landsat features observed from previous data gaps offshore (Figure 2).

Based on ∂18O records from the NGRIP, Greenland ice core, the transition from the last glacial (oldest Dryas) into the Bølling interstadial is dated to 14.692 b2k. The Younger Dryas to Holocene (Preboreal) transition occurred 11.703 b2k (Rasmussen et al., 2006; 2014). The abrupt global warming at the end of the last glacial 14.700 years ago caused the extremely rapid retreat of the immense Pleistocene ice sheets. Several ice sheet re-advances and retreats occurred in the Dryas period. Based on the ∂18O ice core records, the most pronounced Pleistocene cooling (with ice sheet re-advancement) took place between 13.954-14.075 b2k (GI-1d, Older Dryas), 13.660-13.600 b2k (GI-1c2), 13.099-13.311 b2k (GI-1b, Inter-Allerød) and 12.896-11.703 b2k (GS-1, Younger Dryas) (Rasmussen et al., 2014). The Younger Dryas is thus the longest and coldest of several very abrupt climatic changes that took place near the end of Pleistocene.

3 Marine sedimentary records indicate that the deglaciation of Iceland began ~15.000 years ago with rapid ice retreat interrupted by periods of readvancement (e.g. Norðdahl, 1990; Ingólfsson, 1991; Ingolfsson and Norðdahl, 1994; Ingólfsson et al., 1997; Syvitski et al., 1999; Andrews et al., 2000; Geirsdóttir et al., 2002; 2009; Andrews and Helgadóttir, 2003). A marine sediment core north of Grímsey indicates that Atlantic water was present on the northern shelf >16.500 years ago (Eiríksson et al., 2000). The rapidly rising of the sea-level supported the deglaciation between 15.000 and 13.000 years ago (Ingólfsson et al., 1997; Eiríksson et al., 2000; Ingólfsson and Norðdahl, 2001; Lambeck et al., 2014). For regulating the change in the sea level, the ice stream dynamics, and the associated ice sheet mass balance changed (Anderson et al., 2002; Stokes et al., 2016). The ice sheet expanded during the Younger Dryas (Norðdahl and Pétursson, 2005). In northern Iceland the ice sheet extended offshore into fjords (Norðdahl and Haflidason, 1992; Geirsdóttir et al., 2000; Principato et al., 2006; Brynjólfsson et al., 2015) and onto the shelf at least half-way out narrow Eyjafjörður during the Younger Dryas (Norðdahl and Haﬂidason, 1992; Geirsdóttir et al., 2000; Hardardóttir et al., 2001). The prominent phase of deglaciation was between 11.500 to 10.100 years ago where the main ice sheet had disappeared (Hjartarson and Ingólfsson, 1988; Geirsdóttir et al., 1997, 2000).

Fig. 2: Glacial geomorphology of the Iceland continental shelf from Spagnolo and Clark (2009) and Patton et al. (2017), corroborated and supplemented with additional landform mapping from Landsat satellite imagery (onshore) and the EMODnet DTM (composite bathymetric dataset) (offshore) (A). Bathymetric profile over a prominent moraine ridge close to the northern shelf edge (B) (from Patton et al., 2017).

1.3 Study Area

The study area is located offshore NE-Iceland at the Tjörnes Fracture Zone (TFZ), a transform fault connecting the rift zone in north Iceland and the Kolbeinsey Ridge (Figure 3). The TFZ is a complex oceanic transform zone with trans-tensional rifting and shear distributed within three main segments, the Grímsey Oblique Rift (GOR), Dalvík Lineament and the Húsavík-Flatey Faults, which are superimposed on the NS-trending rift basins of Eyjafjarðaráll, Skjálfandadjúp, Skjálfandi Bay and Öxarfjörður (Sæmundsson, 1979; Einarsson, 1976; 1991; 2008). The Skjálfandadjúp and Öxarfjörður basins are transected by the Grímsey Oblique Rift (GOR), representing the offshore extension of the NVZ, which includes the volcanic systems of Mánáreyjar, Nafir, Hóllinn and Stóragrunn (Magnúsdóttir et al., 2015). The basins of Eyjafjarðaráll and Öxarfjörður mark the western and eastern boundary of the Tjörnes Fracture Zone (Magnúsdóttir et al., 2015). The active rift basin Eyjafjarðaráll is the southern continuation of the Kolbeinsey Ridge, and Öxarfjörður is the northern continuation of the Northern Volcanic Zone (Sæmundsson, 1974; Einarsson, 1991; 2008). The Skjálfandadjúp Basin has been created by rifting within the Grímsey Volcanic Zone and is located as well as Skjálfandi Basin between Eyjafjarðaráll and Öxarfjörður basins. Skjálfandi was formed in relation to rifting within the Tjörnes Fracture Zone and transtensional shear along the Húsavík-Flatey Fault System (Magnúsdóttir et al., 2015). The

5 rift basins of Eyjafjarðaráll, Skjálfandadjúp, Skjálfandi Bay and Öxarfjörður thus also form the basis of the way of the four major ice stream: Skagafjörður-Skagafjarðargrunn, Eyjafjörður-Eyjafjarðaráll, Bárðardalur-Skjálfandi-Skjálfandadjúp, and Öxarfjörður. These major ice streams were active during the LGM along the insular shelf of northern Iceland. Sediment cores within this area (Figure 3) marked by red (Hakon Mosby cores) and yellow (Marion Dufresne cores) stars are relevant this study and will discuss in part four.

6 1.4 Definition of Palaeo Ice streams and the Dynamic of Ice sheets

Palaeo ice stream research can provide useful information about the behavior of modern- day ice sheets and the ice sheet dynamics. The term palaeo ice stream is used to describe ice streams that no longer exist as a result of deglaciation. The term “ice stream” is defined as a region in a grounded ice sheet in which the ice flows much faster than in adjacent regions (Paterson, 1994). Glacier movement is attributed to: plastic deformation of the ice, sliding of ice over the bed and deformation of the bed itself (Cuffey and Paterson, 2010). The motion of an ice stream is dominated by basal sliding, the sliding over the glacier bed due to meltwater, and from accumulation areas towards ablation areas. To analyse the ice flow, it is important to understand the relation between basal velocity, shear stress and the general characteristics of the bed such as water pressure, volume, bedrock, topography and sediment properties (Cuffey and Paterson, 2010). The velocity direction and general dynamics of the ice sheet flow depend on internal parameters such as the mechanical and thermal properties of the ice as well as external boundary conditions (Bourgeois et al., 2000). The external conditions include the topography, size, and slope of the responsible glacier, the nature of the bed and the bed roughness, the distribution of accumulation at the surface, ablation and temperature at the surface as well as the geothermal heat flux at the base (Paterson, 1994). Active rift zones and volcanic activity also play a role in affecting the basal ice flow (Bourgeois et al., 2000), which is important to consider in the tectonically and volcanically active study area. The associated geothermal heat flux can affect glacial flow as a result of the flow towards geothermal anomalies because the ablation rate in these regions is increased by intense basal melting (Jónsson et al., 1998). Meltwater production above geothermal anomalies can favor saturation and deformation of the subglacial till, thus creating channels of preferential flow for the ice (Blankenship et al., 1993).

2 Methods and Data

The bathymetric data used in this study was acquired during mapping surveys in 2002-2004, two on board the new Marine Research Institute vessel Árni Friðriksson and one on the Icelandic Coast Guard Hydrographic Survey vessel Baldur. In addition to multibeam bathymetric mapping, high-resolution seismic reflection (Chirp), gravity coring, submarine photography and sediment sampling were conducted on the North Iceland shelf in 2001 and 2003, in order to investigate the tectonics of the Tjörnes Fracture Zone (TFZ). Mapping programs were conducted from the R/V Bjarni Sæmundsson in addition to displaying various tectonic and volcanic features the data provided valuable new information on glacial features and postglacial sedimentation within this area. The focus and goal for this study is the identification and detailed mapping of the various glacial landforms on the N-Iceland shelf, associated with the last deglaciation.

The bathymetric data were processed using the open-source MB-System software package (Caress and Chayes, 1995) and converted by using ERDAS Imagine prior to integration using the Geographical Information System (GIS) software ArcMap 10.6 (Brandsdóttir et al., 2003). The coordinate system (WGS_1984_UTM_Zone_27N; WKID: 32627 Authority: EPSG) for the northern hemisphere between the equator and 84°N, was used both on land and offshore. Hillshade maps were generated using the DEM data from the multibeam bathymetry. Hillshades are a 3D representation of a surface using a hypothetical light source to provide shading and depth to the image to visualize the terrain as shaded relief. The GIS software allowed integration of the various data, as well as providing and processing multiple displays from the bathymetric data by the use of surface analysis tools. One of the most important tools for the interpretation of some features is the interpolate line, which is created by interpolating heights from selected surfaces to create a profile. With the knowledge of length and width, important conclusions can be drawn on the features. ArcScene, a 3D visualization application, was used to view the GIS data in three dimensions. The features are placed in 3D by providing height information from the DEM Data. The program allows the features to get a better view in relation to their size and height and gives a good colored overview of the heights of the area.

9 Table 1: Overview of research surveys offshore northern Iceland

Date Vessel Research

16-29 July 2001 Bjarni Sæmundsson Chirp and Multi-channel seismic

2-16 July 2002 Árni Friðriksson Multibeam mapping

21 July – 1 August 2003 Bjarni Sæmundsson Chirp survey

5-10 August 2003 Baldur Multibeam mapping

6-16 September 2004 Árni Friðriksson Multibeam mapping

14-22 August 2005 Poseidon Multibeam mapping

2.1 Principle of Multibeam Bathymetry

The multibeam bathymetric instrument is a multibeam echosounder, a type of sonar that is used to map the seabed. Acoustic mapping has become an efficient method to map and survey the seabed. The bathymetric derived from measuring water depth with different devices (Saracin and Calin, 2014). The multibeam echosounders use a wide range of frequencies, often employing more than a hundred beams transmitted at different angles from the same transducer unit (Figure 4) (Hogg et al., 2010). The principle of mapping the seafloor with multibeam echosounders is that the echosounders create a large number of beams for each ping. The beams itself are focused in different directions and span a fan cross-track of the vehicle (Saracin and Calin, 2014). The fan travels perpendicular to the direction of the survey vessel, and the angle is the swath angle (Hogg et al., 2010). The range to the seafloor is estimated along each beam and direction based time delay of the signal. This value gives the relative depth of the seafloor relative to the vehicle and is transformed into a map of the seafloor along with the fan. Consecutive pings give a continuous map of the area during the forward movement of the vehicle. The resolution of this map is determined by the 2D beam width and the range resolution (Saracin and Calin, 2014). The result is influenced by the frequency and length of the transducer array. Higher frequency and longer transducer arrays will result in narrow beam widths and smaller footprints on the seafloor (Jakobsson et al., 2016b).

Fig. 4: Principle of multibeam echosounder sonar system (Hogg et al., 2010).

2.2 Multibeam Bathymetric Data

The 70 m RV. Árni Friðriksson is equipped with a 30 kHz Simrad EM300 multibeam echo sounder whereas the 12 m RV. Baldur is equipped with a 240 kHz RESON Seabat8101 system. Both systems are hull-mounted. The multibeam system was integrated with a differential GPS system providing positions accurate to ~0.3 m. The lower frequency EM300 system transmits 135, two degree beams at a maximum rate of 10 profiles per second whereas the higher frequency RESON system transmits 101, 1.5 degree beams at a maximum rate of 30 profiles per second. Having a maximum depth range of 300 m the RESON system is designed for high-resolution imaging at shallow depths whereas the EM300 system is designed for deeper penetration, up to 5000 m. The angular coverage sector and beam pointing angles of the EM300 vary automatically with depth, maximizing the number of usable beams. The swath of seafloor covered on each survey line was typically three to five times the water depth. Surveying speeds were typically 9-10 kts. By using an automated beam editing program to edit the multibeam data, the data can be produced and display gridded data which delayed only a few minutes from the end of each line. The corrected grids have a 1 m and 5 m grid interval. In total, approximately 8600 km2 were surveyed during the four cruises (Brandsdóttir et al., 2003). The bathymetric map presented here demonstrates the necessity of collecting high-resolution data as a prerequisite to addressing a wide range of research topics (Figure 3).

3 Results: Description and Interpretation

The bathymetric map shows that the area is made up of normal and transformed faults draped by subglacial landforms, large volcanic complexes smoothed by glacial erosion and irregular postglacial volcanic features on the seafloor. Subglacial landforms are produced at the base and margin of past ice sheets. Protected from subaerial erosion and beneath wave-base and tidal currents in the water, they may be preserved over long periods on the geological record (Dowdeswell and Bamber, 2007). The focus of this thesis results is the variety of submarine landforms produced in glacial and glacimarine environments which include subglacial, ice- marginal, and glacimarine processed environments (Dowdeswell et al., 2016). All these processes are related to processes that involve the action of glaciers and palaeo ice streams in the sea. In general, the submarine landscape of this area is dominated by streamlined sedimentary features that are linked to the presence of fast-flowing ice streams. The results provide some answers to questions about the extent of the North Iceland ice cap and the main ice streams during the LGM. The following subchapter includes a description and interpretation of the results of the features.

3.1 Processes within the Subglacial Environment

Landforms produced at the ice-bed interface by subglacial processes are mostly streamlined and elongate in shape, with their long axes oriented in the direction of ice flow (Dowdeswell et al., 2016). We can use the subglacial landforms to reconstruct the flowlines within the past ice sheets. In this chapter, I present the subglacial features within the study area.

3.1.1 Crag-and-Tails

Crag-and–tails are elongate hills or ridges in which a resistant mass of bedrock (crag) has withstood the passage of an ice sheet or glacier, thereby either protecting or causing the deposition in a lee-side cavity of an elongate ridge (tail) and of more easily eroded sedimentary debris, often till, on its ice distal side (Bell et al., 2016).

The length of the two features measured by Nafír (Figure 5) is ~8 km (red) and ~5 km (blue). The streamline sediment ridges observed in the lee of the resistant bedrock which most likely represents older volcanic seamounts are ~6 km in length (red) and ~2.5 km in length (blue). The red-lined seamount is 2 km in length and ~1400 m in width, with a maximum height of 120 m, and ~ 30-40 m high sediment trail (green transects 1, 2, Figure 5d).

The blue-lined seamount is 2.5 km in length and 500 m in width with a height up to 70 m. Based on their morphology the elongate streamlined ridges are interpreted as tails, in the lee side of a resistant bedrock, the crag. The elongate sediment tails form behind from their

13 associated bedrock crags and lie in the direction of the ice stream. These crag and tails are part of an area in Skjálfandadjúp Basin that stands out for its very distinct contiguous glacial landforms (Figure 5). It should be noted that the figure shows other glacial landscapes besides crag and tails: Iceberg scours, GZW and MSGL. These landscape forms will be explained later in this Chapter. 17°54’W 17°48‘W a b

Iceberg scours 18°0’W 17°50‘W

GZW 1

66°46‘N Crag and Tail 2

MSGL

66°44‘N

66°42’N

0 1 2 0 1 2 Km

14 1 d

Continuation of Fig. 5 d) Transects (1-3) over one crag and tail landform from SW-NE (Box 1 in Figure 3).

Another series of streamlined crag and tail features are prominent on the eastern flank of the Skjálfandadjúp Basin (Figure 6). The area is characterized by an irregular seafloor with parallel streamlined features up to 2.5 km in length and up to 80 m in height. On a length profile, they are highest in the south and tail off in a northwesterly direction. Crag and tail landforms can be differentiated from other glacially derived elongate features by their overall size and elongation ratios (Clark et al., 2009). The shape of the elongation is not typical for drumlins because it is rather steep at the southern margin. The elongation ratios of these features have been suggested to reflect past ice velocity, with higher elongation ratios being characteristic of faster ice flow (Clark, 1993).

17°36’W 17°30‘W a )

66°36‘N

66°32’N

b 0 1 2

Fig. 6: Crag and tails in the eastern part of the Skjálfandadjúp Basin. a) Location of Transects 1-5 from SW-NE b) Transects over crag and tail field 1-5.

17°36’W 17°30‘W

N c

66°34‘

x x

’N

66°32 x‘

y‘

d 0 1 2

The elongated hill (Hóllinn), located in the central Skjálfandi Bay (Figure 7), reaches a length of 8 km with a width of 2 km and up to about 60 m in height (green) in southernmost bedrock, which is resistant to erosion. Hóllinn is located between the Húsavík and Flatey fault section and is flanked by seafloor sediments, isolated from other seabed structures. The structure looks similar to a crag and tail, but due to the location, it could also be partly formed as a push-up between two strands of the HFF.

17°40’W 17°30`W x

66°8‘N

x‘

0 1 2 Km

x-x‘

Fig. 7: The Hóllinn crag and tail in south-central Skjálfandi Bay (Box 3 in Figure 3). The profile x-x’ along the length, shows the crag and tail structure.

An elongated landform with a length of 2.6 km and a height of over 20 m and a width of 300 m at the resistant frontal bedrock (x’, Figure 8), has a gentle slope towards the northwest and is interpreted based on morphology and elongation ratios as a crag and tail.

18°0’W 17°50‘W x

66°52‘N y y‘ x‘

0 1 2 Km

x-x‘/y-y‘

3.1.2 Mega-Scale Glacial Lineation

Mega-scale glacial lineation (MSGL) is defined as “elongate landforms, produced typically in subglacial sediments, which reflect fast ice flow of an ice sheet and thought to be indicative of ice streaming.” (Bell, et al., 2016).

MSGL are typically defined by their morphology, characterized by parallel ridges that are kilometres long, a hundred meters wide, and only a few meters high (Clark, 1993; Spagnolo et al., 2014). The high parallelisms of their limitations suggest that they are formed beneath the ice stream by the forward motion of a fast-flowing large mass of ice (e.g., Clark, 1993; Bourgeois et al., 2000; Stokes and Clark, 2001; 2002). It is suggested that MSGL may be formed by the plowing action of basal ice keels through soft sediment (Tulaczyk et al., 2001; Clark et al., 2003). They are visually apparent because they have been constructed by erosion and/or deposition on an initially smooth topography (Spagnolo et al., 2017). Similar features have been identified in many glaciated regions in the world. MSGLs have generally been assumed to comprise unlithified, soft sediment (principally till) that has been modified subglacially into very long drumlins and megaridges, but how they form is still not fully understood (e.g., Boyce and Eyles, 1991; Clark et al., 2003; Stokes et al., 2013; Spagnolo et al., 2014).

19 Several prominent linear ridges are present in the area east of the Grímsey Volcanic Zone (Figure 9). Their trend is parallel to the expected palaeo ice flow direction. These ridges show some variations in relief along their length and width, but overall all linear features within this area have the same direction. A younger bank, marked by iceberg grooves and scours, overlies the linear ridges which partly disappear underneath. The height of the ridges varies between 2-8 m. The length also varies, some ridges that are up to 7 km long but the majority is about 4-5 km. These elongate ridges are interpreted as mega-scale glacial lineations (MSGLs) based on their dimensions. The sedimentary wedge with an average length of 3 km is interpreted as a GZW which has been affected by the grounding of iceberg keels (Figure 9 e). The GZW with the MSGL in Grímsey Volcanic Zone is similar to submarine glacial landforms identified elsewhere on the Norwegian-Svalbard margin (Ottesen et al., 2005) and in the Kveithola Trough, Barents Sea (Bjarnadóttir et al., 2012; Rebesco et al., 2016). The interaction between MSGL and the GZW can be seen in Figure 9 e) where a new part was added which shows the further course of the MSGL on the eastern side of the dataset.

20 17°54’W 17°48‘W

a Iceberg scours

GZW

Crag and Tail

MSGL MSGL

66°42’N 66°46‘N

0 1 2

17°50‘W b

’

66°48

66°44’N

0 1 2 Km Fig. 9: MSGL and GZW with iceberg scours in Grímsey Volcanic Zone. a) Overview of the area marks by crag and tails, iceberg scours, GZW and MSGL b) Location of W-E profiles in red ( 1-7) and blue (1-6) (Box 5 in Figure 3).

21 c )

Continuation of Fig. 9: c) E-W profiles across the MSGLs and GZW with iceberg scours (1-7).

Continuation of Fig 9: d) A closeup of the MSGL on the eastern side (1-6).

18°0’W 17°40‘W e

66°50‘N

66°40’N

0 1 2

GZW MSGL MSGL

GZW MSGL Tail MSGL

Continuation of Fig 9: e) Overview of the added data with locations of the 13 km long transects over the MSGL and GZW. The black arrows show the direction of the ice flows, and the red truncated line is the limit of the GZW. Profiles 1 and 2 show the interaction between the GZW and the MSGLs (2005 Poseidon data).

A landform, on the western flank of the southern Kolbeinsey Ridge, consists of a 14 km long sedimentary ridge overprinted in the southern end by smaller ridges (z-z’, Figure 10). The height of the long ridge varies between 6-10 m over its length (Figure 10). The height of the overprinted ridges is between 4-10 m. The ridge, parallel to the ice flow direction, is interpreted as a medial moraine, i.e. a moraine between two adjacent ice streams. The flank ridge is overprinted by MSGL, a relic of a fast NNE flowing ice stream. Both flanks of the southern Kolbeinsey Ridge show signs of glacial erosion (Figure 3), indicating that during the Last Glacial Maximum an ice stream flowed northwards along the ridge axis.

18°50’W 18°40‘W

x x‘

y y‘

z 67°20’N67°24‘N z‘

0 2.5 5 Km

x-x‘

y-y‘

z-z‘

Fig. 10: Moraine overprinted by MSGL in Eyjafjarðaráll. The Profiles x–x’, y–y’ and z–z’ shows W-E transaction over the moraine where the profile z–z’ shows the part of the MSGL (Box 6 in Figure 3).

25 3.1.3 Cross-Shelf Troughs

Cross-shelf troughs are the largest and most prominent geomorphological features on the Icelandic continental shelf. They extend from major onshore valley systems and widen towards the continental shelf edge (Patton et al., 2017; Spagnolo and Clark, 2009). Compared to other cross-shelf troughs in high-latitude settings, they seemed to be more shallow and draped with a thin layer of deglacial sediments, indicative of moderate rates of glacial erosion (Syvitski et al., 1987; Andrews et al., 2000).

Based on Olex data, most of these troughs appear a short distance offshore or on the outer shelf. Compared to relatively short, narrow and straight troughs in the eastern and southern parts of Iceland, the troughs in the northern part are deep, long and sinuous (Clark and Spagnolo, 2016).

The Eyjafjarðaráll Basin has been generated by combined tectonic and glacial erosional processes. The northern part of Eyjafjarðaráll is a 500-700 m deep asymmetric rift, marked by normal faults of 20-25 m vertical displacement on each side. The southern Eyjafjarðaráll Basin merges with the seismically active Húsavík Flatey fault system (HFF) at the mouth of the Eyjafjörður Bay, forming an entwined NW-striking pull-apart basin and a cross-shelf glacially carved trough. The southeastern margin of the Eyjafjarðaráll Basin has a vertical drop of 40 to 140 m (Figure 11), parallel to the HFF.

A 6 km long wedge which drops between 35 to 80 meters on its steeper side, could be a GZW which marks a still-stand of the Eyjafjörður ice stream (Figure 11, green line). Both features have similar cross-sections making it difficult to distinguish between tectonic and erosional relief in this region.

26 18°32‘W

66°20‘N

0 2

Fig. 11: Cross-shelf trough (red line) and GZW (green line) in southeastern Eyjafjarðaráll Basin with four transects over the cross-shelf trough from SW-NE (Box 7 in Figure 3).

27 3.1.4 Drumlins

Drumlins are defined as low, smoothly rounded and streamlined oval hills or mounds, commonly elongate parallel to the former ice-flow direction with a blunter up-ice face, composed of till or stratified drift and sometimes having a bedrock core; formed by either erosion or deposition beneath an actively flowing glacier (Bell, et al., 2016).

A field of small elongated hills in the direction of the ice flow is located in Skjálfandadjúp (Figure 12). The hills vary in shape and size. The shape is oval, some of them are steeper than others and remind to a crag and tail form (green profile Figure 12). But the other elongate hills in this field are more oval with a shorter length of only a few hundred meters and a height of up to 30 meters. The morphometry of these landforms here is consistent with drumlins. However, some of their length profiles are similar to crag and tail features (Figure 12 c). As no general geometry exists for drumlins, it is hard to differentiate them from crag and tail structures. Drumlins often merge together to form larger complexes, where several drumlins are seemingly streamlined into the same underlying body of sediment (Jakobsson et al. 2016a). The two structures can be distinguished more easily if we know the composition and the material from which the features are formed. Crag and tails have rock deposits especially in the front area of the crag. In contrast, drumlins mostly consist only of till or composition of rock and till. Chirp transects across similar formations as in Figure 18, show hardened glacially derived sediments indicating that these are rock drumlins (hard drumlins).

18°0‘W

66°38‘N

0 1 2

Fig. 12: Drumlins in the western part of Skjálfandadjúp Basin. a) Overview of the area with location of profiles b) Transect over the Drumlins field c) Length profile (green) and width profile (red) of one Drumlin. (Box 8 in Figure 3).

3.1.5 Eskers

Eskers are defined as long, narrow, sinuous ridges of stratified glacifluvial material deposited by a stream normally flowing beneath a stagnant or retreating glacier in an ice tunnel or subglacial stream bed, and left behind after the disappearance of the glacier (Bell et al., 2016).

A 6 km long, elongated feature, with a width of 200 m and up to 9 m in height located on the western flank of the Eyjafjarðaráll Basin (Figure 13) is interpreted as an esker. Another smaller esker ridge (green marked in Figure 13) is 4,5 km in length, seemingly partly eroded. The width of the smaller esker is ~100 m, with variable height along its length, on average around 6 m. Between these two significant eskers are more streamlined features as well as one 2.5 km ridge, similar to the other two eskers. The eskers occur at various water depths

29 with an orientation generally in the direction of the palaeo ice stream flow. Compared to other streamlined landforms that mark palaeo ice streams, the eskers represent efficient subglacial drainage (Shreve, 1985). The small gullies on the ridge flank can indicate possible sediment remobilization (Dowdeswell and Ottesen, 2016b). The streamlined features between the eskers interpreted as MSGLs which show the way from the palaeo ice stream from the southwest. 19°6‘W

66°32‘N

66°30’N

0 1 2

3.1.6 Meltwater Channels/ Canyons/ Gullies

Glacial meltwater channels, canyons, and gullies are present west of Eyjafjarðaráll and near Mánáreyjar (Figure 3). In a subglacial channel, pressurized water may flow upslope as well as downslope, producing an undulating channel long-profile (Bell et al., 2016). Meltwater channels are rarely show up in our dataset, implying that they may be destroyed by till

30 deformation or that water was moving as part of the till itself (Dowdeswell et al., 2016). Some channels may also have been filled by later sediment transport, i.e. early postglacial jökulhlaups or more steady sediment transport with glacial rivers from land.

The small channels next to Mánáreyjar (Figure 14 a) are generally about 100 m in width. Their depth is 20 to 25 meters. The channel in the western Eyjafjarðaráll Basin (Figure 14 b) is located at the margin of a broad, sedimentary filled valley. The visible section of the channel is approximately 6 km long with a depth of 60 meters and a width of 100 meters in narrower sections and 350 m in the middle section.

a b Fig. 14: Flood channels. a) small channels next to Mánáreyjar (10a) b) channel with sediment fill on the western flank of the Eyjafjarðaráll Basin (10b). Small orange arrows indicate the location of diameter measurements (Box 10a, 10b in Figure 3).

3.2 Processes within the Ice-marginal Environment

Within this environment, the landforms are more line-sourced ridges and produced at grounded tidewater glacier margins or at the grounding zone of floating ice shelves (Dowdeswell et al., 2016). The important difference to the subglacial environment is the transverse orientation to the direction of the ice flow of some line-sourced ridges. The line source ridges created by sediment are the length of the marine-terminating ice mass (Ottesen and Dowdeswell, 2009). Following the features from the study area created in the ice- marginal process environment.

3.2.1 Grounding-Zone Wedges

A Grounding Zone Wedge (GZW) is defined as a “sedimentary depocentre formed at the grounding zone of an ice-sheet/ice-shelf system, formed of dipping diamicton beds overlain by horizontal sheets of diamicton, mainly subglacial till. Till emerging from beneath the

31 glacier along a line-source is redistributed by the subaqueous debris flow, producing diamicton beds that dip away from the margin: They are usually asymmetrical in long- profile, steeper in the ice-distal direction” (Bell et al., 2016). They are often identified by the asymmetrical shape of the depocentre along the axes of the cross-shelf troughs, and the size is depending on the rate of supply of deforming subglacial sediments to the grounding zone (Dowdeswell et al., 2016).

An asymmetrical ridge with a sharp drop toward the ice-flow direction is present northeast of Skjálfandadjúp. This more than 6 km long feature lies transverse to the ice flow direction with its steeper face towards the ice flow direction. The vertical drop is between 25-40 m. Superimposed across the ridge are smaller, linear streamlined landforms (Figure 15). While the steep slope of the ridge is similar to a normal fault, its location east of the Grímsey Volcanic Zone and NE-SW strike direction does not comply with other faults within the TFZ. The large asymmetrical sedimentary ridge is interpreted to be a GZW, similar to those in Vestfjorden, North Norway (Dowdeswell and Ottesen, 2016a) which are considered to have taken decades to form. The size of the GZW may reflect a sedimentary basin source (McMullen et al., 2016). The parallel, elongate ridges at the surface of the GZW as well as at the surrounded area are interpreted as MSGL. The MSGL overlie the GZW, implying that a fast ice stream flow continued during the formation (Dowdeswell et al., 2016) and it also suggests that the deglaciation was episodic, with periods of standstill and GZW deposition punctuating more rapid ice retreat (Dowdeswell et al., 2008).

32 19°6‘W

67°0‘N

0 1 2Km

Fig. 15: GZW in Grímsey Volcanic Zone with numbered red transactions over the features and red profiles 1-8 (SE to NW) (Box 11 in Figure 3).

33 3.2.2 Moraines

The definition of moraines says that they are “a mound, ridge or other distinct accumulation of generally unsorted, unstratified glaciogenic sediment, predominantly till, deposited chiefly by direct contact with glacier ice, commonly subglacial” (Bell et al., 2016).

A significant ridge is located in the central, eastern Eyjafjarðaráll Basin. It is 9-10 km in length, 150-300 m in width and a variable relief up to 16 meters (Figure 16). It looks like some parts are already eroded. The ridge is located within a zone of small, V-shaped sedimentary ridges, which open towards the inferred flow direction of the Eyjafjarðaráll palaeo ice stream. The V-shaped ridges are up to 1 km long and 300-500 m wide and increase in volume down glacier,

The V-shaped ridges are interpreted as small push moraines are formed by the “bulldozing” of sediment by an advancing ice front. This leads to a glaciotectonic deformation, a process where subglacial sediment is disrupted by ice flow (Bell et al., 2016). The size depends on the rate of sediment supply from the parent glacier (Dowdeswell et al., 2016).

The V-shaped ridges strike NNE-SSW, parallel to the significant long ridge which is interpreted as a lateral moraine, which most likely separated the two main outlet glaciers in this area, Skagafjörður-Skagafjarðaráll and Eyjafjörður-Eyjafjarðaráll. Lateral moraines are parallel ridges of debris deposited along the sides of a glacier or ice stream. Ice streams are fast-flowing zones of an ice sheet bounded to either side by a slower-flowing or even stagnant ice (Bentley, 1987). This lateral moraine is likely formed at the interface of two outlet glaciers, i.e. within a shear zone between fast and slower-flowing ice streams, where the ridge is produced from the deformation of soft subglacial sediments (Stokes and Clark, 2002; Hindmarsh and Stokes, 2008; Batchelor and Dowdeswell, 2016). Based on changes in the direction of palaeo ice stream flow, marked by glacial features along the Eyjafjarðaráll Basin, it is clear that the Skagafjarðaráll ice stream deflected the Eyjafjarðaráll ice stream to the northeast north of Tröllaskagi (Figures 2 and 18).

34 18°50‘W

x x‘

66°35‘N

y y‘

z z‘

66°32’N 0 1 2

x-x‘

y-y‘

z-z‘

Fig. 16: Lateral Moraine in Eyjafjarðaráll with W-E Profiles (x-x’, y-y’, z-z’) over a length of 550 meters (Box 12 in Figure 3).

35 3.3 Processes within the Glacimarine Environment

Landforms within the glacimarine process environment produced directly by ice by ploughing action of drifting icebergs (Dowdeswell et al., 2016). The following section gives a short description of these features in the study area.

3.3.1 Iceberg Scours/ Ploughmarks

Iceberg scours and ploughmarks are defined as “grooves or furrows caused by the impact and movement of grounded icebergs along the sea or lake floor” (Bell et al., 2016).

The data show scours in different areas all over the data set. These features occur especially in relatively shallow and flat areas at about 200 meters deep except in the Grímsey Volcanic Zone they are also visible in deeper areas up to 350 m. The scours verify in size, but most of them are less than 4 m in depth (Figure 17). These scours are interpreted as iceberg scours and ploughmarks, and they are among to the most characteristic submarine landforms produced directly by ice beyond the margins of marine ice sheets. They give a good example of landforms that are produced almost instantaneously or over hours to days (Dowdeswell et al., 2016). The highest density of scours and ploughmarks in this area is around the Grímsey Volcanic Zone, and Eyjafjarðaráll Basin coincides with the location of the drifts. The scours are made by icebergs from the Iceland icecap during the last glacial period as well as postglacial icebergs.

36 x x‘ y y‘ ‘

y-y‘

x-x‘

38 4 Discussion

Streamlined landforms with no major changes in the flow path direction are an indicator of high ice flow velocities (Stokes and Clark, 2002). Elongated streamlined landforms, in the form of MSGLs, represent fast ice flow beneath former ice sheets (e.g., Clark, 1993; Bourgeois et al., 2000; Stokes and Clark, 2001; 2002; Krabbendam et al., 2016). MSGLs on the insular shelf of central-northern Iceland represent the retreat of the Pleistocene ice sheet into separate ice streams within the insular fjords. Four major palaeo ice streams, drained the ice sheet on the central northern shelf of Iceland during late Pleistocene: Skagafjörður- Skagafjarðargrunn, Eyjafjörður-Eyjafjarðaráll, Bárðardalur-Skjálfandi-Skjálfandadjúp, and Öxarfjörður. The reconstruction of these palaeo ice streams relies on the mapping of MSGLs and drumlins composed of hardened glacially derived sediments (Figure 20). Using multibeam bathymetric data it was possible to examine both ice stream extent, velocity and internal flow structures. Previously, glacial features along the northern coast of Iceland and on Grímsey Island, located 40 km to the north, have been used to constrain the glacial coverage (Hoppe, 1982; Norðdahl, 1991). The multibeam map has revealed MSGLs and other glacially formed bedrock features up to 150 km offshore (Figure 20). Present-day ice streams represent fast-flowing ice, with typical velocities from 200 to 1000 m a-1 (Bentley, 1987; Clarke, 1987). Glaciers that terminate in the ocean can be tidewater glaciers, ice-shelf tributary glaciers, or grounded outlet glaciers. A fast-flowing, well-grounded ice stream will scour the bedrock whereas tidewater (floating) ice stream will drop its load onto the seafloor (Dowdeswell et al., 2016). The parallel conformity of glacial landforms offshore N-Iceland, drumlins, mega-scale glacial lineations (MSGL), and crag and tails, suggests fast ice flow during their formation, in agreement with calculated velocity of 140-400 m a-1 estimated for the Bárðardalur-Skjálfandi ice stream by Bourgeois et al., (2000) comparable to velocities measured in Greenland and West Antarctic ice streams (Clarke 1987 and references therein). Some of the MSGLs are overprinted by grounding zone wedges (GZWs) indicating where the edge of the glacier edge was located when it became a tidewater or floating ice stream.

During the last glacial period, the palaeo ice stream of Skagafjörður-Skagafjarðargrunn merged with the ice stream from Eyjafjörður-Eyjafjarðaráll. The lateral moraine at the eastern flank of Eyjafjarðaráll (Figure 16) was formed at the marginal zone of these two ice streams. Based on various glacial features on the seafloor, the Skagafjörður- Skagafjarðargrunn ice stream deflected the Eyjafjarðaráll ice stream northeastwards. The sedimentary depocentre, interpreted as a GZW, in the Eyjafjörður-Eyjafjarðaráll ice flow direction marks a standstill in the ice sheet retreat. The edge of the Eyjafjörður-Eyjafjarðaráll ice flow is marked by a significant cross-shelf trough. Most of the glacial features in the area are the result of the main ice flows (Figure 20). Only a few smaller ice flows came from the uplifted Tröllaskagi area and Flateyjarskagi. The flow through the valleys of these two peninsulas was controlled by the mass balance of independent ice domes (Norðdahl, 1991). The valley system itself is part of a glacially eroded system where the pattern indicates that it was also formed by glaciers originating within Tröllaskagi itself (Norðdahl, 1991).

The ice stream Bárðardalur-Skjálfandi-Skjálfandadjúp followed one primary flow path with streamlined, SE-NW oriented landforms. Tracing the ice flow, Bárðardalur-Skjálfandi-

39 Skjálfandadjúp reaches the end of the data sets in a GZW overwritten with MSGL. That's an indication of a fast ice stream flow that continued during the formation (Dowdeswell et al., 2016). However, it is unclear whether it is the GZW of the Bárðardalur-Skjálfandi- Skjálfandadjúp ice stream or a result of the ice flow from the direction of Öxarfjörður. The MSGL in this area is very parallel arranged. The salient part of the MSGL is superimposed by a large transverse sedimentary wedge, marked with strong structures of iceberg scours, is interpreted as a GZW that was formed by a standstill of grounded ice in the trough. The morphology suggests that the MSGL formed by deposition of unconsolidated, subglacial till layers during a temporary standstill in the grounding-zone position of the ice stream. The superimposition of the GZW on the MSGL, implying that these landforms were produced mostly through a recessional trend during the deglaciation phase (Rebesco et al., 2016). The Chirp data shows hard underground with now sediment at the top, but a hummocky structure which most likely represents glacial drumlins are visible (Figure 18 c, Line 18). Line 19 crosses the inner, west-dipping boundary fault with 45 m thick sediment sequences formed within the deepest part of the rift (Figure 18, Line 19) (Magnúsdóttir et a., 2015). Transects over the deepest part of Skjálfandadjúp Basin, shows that the sediment here is more than 50 m thick (Figure 18 Line 17) (Magnúsdóttir et al., 2015). Compare to this the Transect SW of Grímsey shows a decrease in sediment thickness with less than 11 m (Figure 18, Line 14) (Magnúsdóttir et al., 2015).

Fig. 18: Chirp data from the Skjálfandadjúp Basin. a) Location of Transects in Skjálfandadjúp Basin.

40 b

Continuation of Fig. 18: b) Chirp data transects: transect 19 (NE of Grímsey), transect 17 (straight east of Grímsey), transect 14 (SW of Grímsey), transect 18 ( S to N file along the eastern margin of Grimsey).

41 MSGLs have generally been assumed to comprise unlithified, soft sediment (principally till) that has been modified subglacially into very long drumlins and megaridges. How they form is still not fully understood (e.g., Boyce and Eyles, 1991; Clark et al., 2003; Stokes et al., 2013; Spagnolo et al., 2014). High-resolution Chirp seismic data across the MSGLs in Eyjafjarðaráll and Skjálfandadjúp shows them to be hard instead of soft (Figure 19)

42 b

Continuation of Fig.19: b) High-resolution Chirp seismic data with prominent glacial features are mega-scale glacial lineations (MSGL), grounding zone wedges (GZW), grounding zone moraines (GZM), V-shaped ridges (v) and an esker (E).

Based on the ∂18O ice core records, the most pronounced Pleistocene cooling (with ice sheet re-advancement) took place between 13.954-14.075 b2k (GI-1d, Older Dryas), 13.660- 13.600 b2k (GI-1c2), 13.099-13.311 b2k (GI-1b, Inter-Allerød) and 12.896-11.703 b2k (GS- 1, Younger Dryas) (Rasmussen et al., 2014), as in Iceland (Geirsdóttir et al., 2007; 2009; Norðdahl et al., 2008). Younger Dryas moraines in Norway are located just within the present-day coastline (Bondevik and Mangerud, 2002). In northern Iceland, the ice sheet extended at least half-way out the narrow Eyjafjörður during the Younger Dryas (Geirsdóttir et al., 2007, and references therein).

The Younger-Dryas-Vedde and Holocene-Saksunarvatn tephra layers have been used as prominent markers in the Icelandic tephrachronology. At least another seven potential tephrochronostratigraphical marker horizons have been identiﬁed in sediment cores offshore N-Iceland (Figure 3, Table 2). With tephra layers, it is possible to more accurately determine the age of the offshore glacial landforms. The 390 cm long sediment core HM 5 which is located close by the GZW overlying the MSGL at the eastern part of the Kolbeinsey Ridge, about 90 km offshore contains the tephra layers Saksunarvatn, Vedde and Borrobol

43 (Eiríksson et al., 2000). The presence of Borrobol tephra at a depth level of 382-379 cm (Eiríksson et al., 2000) suggests that the GZW has not been glaciated since this event where the exact date is still disputed (13.700 years b2k – 14600 years b2k) (e. g. Guðmundsdóttir et al., 2011). The age of this tephra layer has been estimated first to ~12.300 b2k in lake sediments on the Scottish mainland (Lowe et al., 1999) but it gives a lower age than the present rhyolitic tephra in the marine core ( Eiríksson et al., 2000) for the further discussion the estimated age of ~14.000 b2k is used. The core MD72 located next to the GZW further north from the location of HM 5 contains diamicton, interpreted as the youngest glacial deposit in the core, predating the Vedde ash (Knudsen and Eiríksson, 2002). The GZW northeast of Skjálfandadjúp thus represents an earlier standstill event. The GZW may thus have been formed by a tidewater glacier along the Northern Tjörnesgrunn shoal. Tidewater glaciers represent an efficient system for delivering large volumes of sediment to the marine environment (Forbes, 2019). The HM4 core is located on a northern trending trough on the western margin of the Kolbeinsey Ridge, about 120 km offshore. This core contains Hekla 4, Saksunarvatn and Vedde ash over a depth of 390 cm (Eiríksson et al., 2000). The 30 km away moraine overprinted by MSGL must, therefore, be older than the Vedde ash event (12.100 years b2k). The core MD75 lies in the Skjálfandadjup trough, about 50 km offshore and consists Vedde tephra layer (Knudsen and Eiríksson, 2002), as well as the prominent ash layer Borrobol (or BAS-1), has also been identified in sediment core MD75 in Skjálfandadjúp (Guðmundsdóttir et al., 2011). HM3 is located on relatively small step faulted platforms to the west of Eyjafjardaráll and consists of the Hekla tephra layer 3 and 4 (Knudsen and Eiríksson, 2002). HM1 is a neighboring gravity core from HM3 and shows as well the Hekla tephra layers 3 and 4 (Knudsen and Eiríksson, 2002). The presence of the BAS-1 tephra in marine sediments offshore N-Iceland (Eiríksson et al., 2000; Guðmundsdóttir et al., 2011) indicates that the Younger Dryas palaeo ice streams had retreated towards the coastline ~14000 years b2k and the glacial features further north from Grímsey have to be older than 14000 years b2k.

Table 2: Overview of identified Tephra layers with constrained age (e.g., Guðmundsdóttir et al., 2011; Magnúsdottír et al., 2015).

Tephra Layer Age ( years b2k)

Borrobol or BAS-1 13.700 years b2k – 14600 years b2k

Veidivötn-Bárdarbungs 13.000-14.000 years b2k

Vedde (Katla) 12.100 years b2k

Tjörnes Fracture Zone and/or Kolbeinsey 10.300 and 10.600 years b2k Ridge (TFZ/K)

Saksunarvatn (Grímsvötn) 10.300 years b2k

Hekla 3/ Hekla 4/ Hekla 5 3000/ 4200/ 7200

In conclusion, the Iceland ice sheet is considered to have broken up catastrophically around 15 ka as the polar front migrated northward and sea level rose (Geirsdóttir et al., 2009). Our

44 data shows that they were individual palaeo ice sheets with an extension of at least 150 km from the present coastline at the LGM. Individual retreat phases are marked by the GZWs shows that the deglaciation phase was more episodic with periods of standstill. Based on tephrachronological data from sediment cores, the shelf segment from Kolbeinsey to Grímsey was deglaciated before 14000 years b2k (Borrobol or BAS-1 event in core MD75) and has not been subjected to grounded ice cover since that time (Eiríksson et al., 2000).

46 5 Conclusions

A detailed and continuous geomorphological analysis of glacial landforms on the Icelandic shelf is necessary for assessing the ice extent and the style of shelf glaciation, and it can be used for planning future coring and dating campaigns. To accurately reconstruct former ice sheets history, we need to have more coherent data and coring samples to determine the age of the sediments. Since our understanding of the ice stream geomorphology is limited, the locations are mostly hypothesized and also somewhat conflictive in the literature (e. g. Norðdahl, 1991, Andrews, et al., 2000, Norðdahl and Pétursson, 2005, Patton et al., 2017). This thesis has been discussed based on the four major ice streams in this area. The data conducted through the multibeam bathymetric mapping along the north Iceland shelf in 2002 and 2005 gives a good opportunity to increase our understanding of the glacial landforms and processes in glacimarine environments. We were able to examine features of these ice stream flows which represent both ice stream extent and velocity as well as internal flow structures.

Nevertheless, it is difficult, due to the lack of data, to draw coherent conclusions on the ice streams and connect them to the onshore area. Primarily the result of this thesis supports the models, investigations, and interpretations of previous work. The map with the results is given a good overview of the glacial features within this area. I propose that many of the main glacial features on the seafloor have been identified and marked on the map. The interpretation of the ice flow direction is based on the features and past literary work.

The main finding of this work is that the LGM was more extensive offshore N-Iceland than it has been suggested. The Iceland ice sheet extended at least 110 km north of Grímsey during the Last Glacial Maximum. Based on tephrachronological data from sediment cores, the shelf segment from Kolbeinsey to Grímsey was deglaciated before 14000 years b2k (Borrobol or BAS-1 event in core MD75) and has not been subjected to grounded ice cover since that time (Eiríksson et al., 2000).

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