| 1. |
- Alexanderson, Helena, et al.
(författare)
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An Arctic perspective on dating Mid-Late Pleistocene environmental history
- 2014
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 92, s. 9-31
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Forskningsöversikt (refereegranskat)abstract
- To better understand Pleistocene climatic changes in the Arctic, integrated palaeoenvironmental andpalaeoclimatic signals from a variety of marine and terrestrial geological records as well as geochronologicage control are required, not least for correlation to extra-Arctic records. In this paper we discuss,from an Arctic perspective, methods and correlation tools that are commonly used to date ArcticPleistocene marine and terrestrial events. We review the state of the art of Arctic geochronology, withfocus on factors that affect the possibility and quality of dating, and support this overview by examples ofapplication of modern dating methods to Arctic terrestrial and marine sequences.Event stratigraphy and numerical ages are important tools used in the Arctic to correlate fragmentedterrestrial records and to establish regional stratigraphic schemes. Age control is commonly provided byradiocarbon, luminescence or cosmogenic exposure ages. Arctic Ocean deep-sea sediment successionscan be correlated over large distances based on geochemical and physical property proxies for sedimentcomposition, patterns in palaeomagnetic records and, increasingly, biostratigraphic data. Many of theseproxies reveal cyclical patterns that provide a basis for astronomical tuning.Recent advances in dating technology, calibration and age modelling allow for measuring smallerquantities of material and to more precisely date previously undatable material (i.e. foraminifera for 14C,and single-grain luminescence). However, for much of the Pleistocene there are still limits to the resolutionof most dating methods. Consequently improving the accuracy and precision (analytical andgeological uncertainty) of dating methods through technological advances and better understanding ofprocesses are important tasks for the future. Another challenge is to better integrate marine andterrestrial records, which could be aided by targeting continental shelf and lake records, exploringproxies that occur in both settings, and by creating joint research networks that promote collaborationbetween marine and terrestrial geologists and modellers.
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| 2. |
- Alexanderson, Helena, et al.
(författare)
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An interglacial polar bear and an early Weichselian glaciation at Poolepynten, western Svalbard
- 2013
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Ingår i: Boreas. - : Wiley. - 1502-3885 .- 0300-9483. ; 42:3, s. 532-543
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Tidskriftsartikel (refereegranskat)abstract
- The recent discovery of a subfossil polar bear (Ursus maritimus) jawbone in the Poolepynten coastal cliff sequence, western Svalbard, and its implications for the natural history of the polar bear motivated an effort to better constrain the environmental history and age envelope of the Poolepynten sediment sequence. The focus of the present study is on the lithostratigraphy of the coastal cliffs and on re-dating the sequence using the Optically Stimulated Luminescence (OSL) dating technique. We report a revised lithostratigraphy and nine new OSL ages. It is concluded that the Poolepynten sequence contains evidence of four regional glaciation events, recorded in the strata as erosional unconformities or glacial deposits followed by shallow-marine deposition signifying transgressions and subsequent glacio-isostatic rebound and regression. Our OSL ages refine previous age determinations (14C and IRSL) and support the interpretation that the subfossil polar bear jawbone is probably of last interglacial (Eemian) age.
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| 3. |
- Cornell, David H., 1948, et al.
(författare)
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Evidence from Dwyka tillite cobbles of Archean basement beneath the Kalahari sands of southern Africa
- 2011
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Ingår i: Lithos. - 0024-4937. ; 125, s. 482-502
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Tidskriftsartikel (refereegranskat)abstract
- We have found a new source of information about what lies beneath the Kalahari sands. The Kheis and Rehoboth Provinces of southern Africa were thought to be underlain by either an ~1800 Ma orogenic belt, or a northern branch of the ~1200 Ma Namaqua–Natal Province. Glacial diamictites of the Permocarboniferous Dwyka Group exposed at Rietfontein west of the Kalahari sands carry cobbles plucked from the bedrock by the ice sheet which covered the Gondwana supercontinent about 300 Ma ago. Despite altered mineralogy, the cobbles are not severely altered geochemically. Their normative mineral compositions give classifications as trondhjemites and granites, supported by rare earth element diagrams. Microbeam U–Pb zircon dating of the granitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200 Ma. Rather they testify to the presence of 2500 to 2900 Ma Archaean trondhjemitic and granitic crust beneath the Kalahari, with a lesser ~2050 Ma granite component. The pebble assemblages from the diamictites we sampled lack the diagnostic banded iron formation (BIF), stromatolitic limestone and other supracrustal pebbles which characterise diamictites derived from the Kaapvaal Craton, thus we envisage shorter transport distances and derivation from the region now beneath the Kalahari sands. Three of the Archaean granite cobbles have unusual less-than-mantle zircon oxygen isotope values around +3 (δ18O VSMOW), which may reflect interaction of their source with high-temperature, originally meteoric water before melting to produce the granites. The mafic cobbles described in a companion paper are much younger and are related to intrusions of the 1.1 Ga Umkondo Large Igneous Province, probably located on the Kalahari Line or Rehoboth Province. Five trondhjemitic granites from the westernmost outcrops of the Kaapvaal Craton were dated, the oldest being 3061±9 Ma and four others between 2882±7 Ma and 2854±7 Ma, reflecting the cratonisation of the Kimberley Terrane. Four of the Archaean Dwyka cobbles we dated are younger than the 2.7 Ga Kaapvaal cover sequence and are thus too young to be derived from the craton. All the Dwyka cobbles described here are most likely derived from either the Rehoboth Province or the Kalahari Line with origins from the Kheis Province, Kaapvaal Craton, or further afield considered unlikely. We envisage the Rehoboth Province to consist of an Archaean core supplemented by Palaeoproterozoic granitoids, which was joined to the Kaapvaal Craton at an early stage of crustal development and played an important role during later tectonic events. This has important implications not only for the tectonic framework and assembly of Southern Africa, but also for exploration for diamonds and other ore deposits.
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| 4. |
- Holmgren, Sofia, 1975, et al.
(författare)
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The Holocene-Anthropocene transition in lakes of western Spitsbergen, Svalbard (Norwegian High Arctic): climate change and nitrogen deposition
- 2010
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Ingår i: JOURNAL OF PALEOLIMNOLOGY. - : Springer Science and Business Media LLC. - 0921-2728 .- 1573-0417. ; 43:2, s. 393-412
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Tidskriftsartikel (refereegranskat)abstract
- Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High Arctic) preserve biostratigraphic and isotopic evidence for a complex suite of twentieth century environmental changes. At Lake Skardtjørna and Lake Tjørnskardet on Nordenskio¨ldkysten, there is a marked diatom floristic change coupled to increased diatom concentrations beginning around 1920. At Lake Istjørna and Lake Istjørnelva, 25 km southwest of Longyearbyen, both diatom total valve and chrysophyte stomatocyst concentrations have increased dramatically since the beginning of the 1900s. The early twentieth century changes are probably related to climate warming after the Little Ice Age. However, the most pronounced changes in diatom assemblages seem to have occurred in the last few decades. At the same time, nitrogen stable isotopes in sediment organic matter in two of the lakes became progressively depleted by*2%, which is consistent with diffuse atmospheric inputs from anthropogenic sources and attendant fertilization. These data suggest that climate change and nitrogen deposition may be acting together in driving these lakes towards new ecological states that are unique in the context of the Holocene.
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| 5. |
- Jakobsson, Martin, et al.
(författare)
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Arctic Ocean glacial history
- 2014
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 92, s. 40-67
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Forskningsöversikt (refereegranskat)abstract
- While there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the Arctic Ocean, a holistic view on the Northern Hemisphere's late Quaternary ice-sheet extent and their impact on ocean and sea-ice dynamics remains to be established. Here we aim to provide a step in this direction by presenting an overview of Arctic Ocean glacial history, based on the present state-of-the-art knowledge gained from field work and chronological studies, and with a specific focus on ice-sheet extent and environmental conditions during the Last Glacial Maximum (LGM). The maximum Quaternary extension of ice sheets is discussed and compared to LGM. We bring together recent results from the circum-Arctic continental margins and the deep central basin; extent of ice sheets and ice streams bordering the Arctic Ocean as well as evidence for ice shelves extending into the central deep basin. Discrepancies between new results and published LGM ice-sheet reconstructions in the high Arctic are highlighted and outstanding questions are identified. Finally, we address the ability to simulate the Arctic Ocean ice sheet complexes and their dynamics, including ice streams and ice shelves, using presently available ice-sheet models. Our review shows that while we are able to firmly reject some of the earlier hypotheses formulated to describe Arctic Ocean glacial conditions, we still lack information from key areas to compile the holistic Arctic Ocean glacial history.
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| 6. |
- Jónsson, Sverrir Aðalsteinn, et al.
(författare)
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The drumlin field and the geomorphology of the Múlajökull surge-type glacier, central Iceland
- 2014
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Ingår i: Geomorphology. - : Elsevier BV. - 0169-555X .- 1872-695X. ; 207, s. 213-220
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Tidskriftsartikel (refereegranskat)abstract
- Here we present a new geomorphological map of the active drumlin field and the forefield of Múlajökull, a surge-type outlet glacier, Iceland. The map is based on aerial photographs taken in 1995 and LiDAR data recorded in 2008. Mapping was done using ArcGIS 10 software on orthorectified imagery, LiDAR data and digital elevation models. The mapped landforms were initially identified on the aerial imagery and LiDAR and then ground-checked in the field. We mapped subglacial, supraglacial, ice-marginal, periglacial, and glaciofluvial landforms. The geomorphology of the Múlajökull forefield is similar to that of the forefields of other surge-type glaciers in Iceland: with a highly streamlined forefield, crevasse-fill ridges, and series of glaciotectonic end moraines. However, the large number (i.e., 110) of drumlins forming the drumlin field is unique for modern Icelandic surge-type glaciers and, as yet, unique for contemporary glaciers in general. Also apparent is that the drumlins are wider and shorter in the distal part of the drumlin field and narrower and longer in the proximal part. Hence, the mapping reveals a development of the drumlins toward a more streamlined shape of the proximal landforms that have experienced more surges. The drumlins in the drumlin field are active, i.e., they form during the modern surges of Múlajökull.
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| 7. |
- Jonsson, Sverrir, et al.
(författare)
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THE MÚLAJÖKULL DRUMLIN FIELD - SEDIMENTOLOGY AND GEOMORPHOLOGY
- 2011
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Ingår i: Geological Society of America abstracts with programs Minneapolis 2011.
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Konferensbidrag (refereegranskat)abstract
- The drumlin field in front of Múlajökull, a surge-style, outlet glacier from Hofsjökull in Iceland, is the only known active drumlin field (Johnson et al., 2010). The aim of this study is to further explore the formation of drumlins in a modern glacial environment. We use data from geological sections, DEMs, aerial imagery and field mapping. Here we present preliminary results from section logging and geomorphological mapping in the summer of 2011. Geomorphological mapping of the drumlin field both with DEMs and ground proofing has revealed over 100 drumlins and a number of drumlinized ridges. The drumlins furthest from the present ice margin appear broader and have lower relief than those closer to the ice. We suggest that this reflects an evolution of the drumlin form during recurrent surging. The drumlins farther away from the ice have experienced fewer surges than those that have just been uncovered due to retreat of the ice margin. During successive surges, the drumlins become narrower and develop higher relief. In one section close to the present ice margin, we identified at least 9 till beds in the crest of a drumlin, each likely the product of a surge, representing approximately 1/3 of the drumlin relief. The top till bed parallels the drumlin form and truncates the older tills. The older units also dip parallel to the drumlin form, but at a slightly lower angle. We believe that this represents an earlier, broader shape of the drumlin prior to the more recent surges, implying an evolution of form similar to that seen in the evolution in form in the drumlin field. The Múlajökull drumlins have thus grown during surging by erosion on the proximal end and sides of the drumlin followed by accretion of till sheets concentrically on the resulting form.
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| 8. |
- Landvik, Jon Y., et al.
(författare)
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Landscape imprints of changing glacial regimes during ice-sheet build-up and decay: a conceptual model from Svalbard
- 2014
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 92, s. 258-268
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Tidskriftsartikel (refereegranskat)abstract
- The behaviour of ice sheets and their geologic imprints in fjord regions are often multifaceted. Fjords, which were temporarily occupied by fast flowing ice-streams during major glaciations, and inter-fjord areas, which were covered by less active ice, show different signatures of past glaciations. The land and marine records of glaciations over the western Svalbard fjord region have been extensively studied during the last few decades. We have re-examined ice-flow records from stratigraphic and geomorphic settings, and propose a succession of ice-flow events that occurred repeatedly over glacial cycles: the maximum, the transitional, and the local flow style. The differently topographically constrained segments of the ice-sheet switched behaviour as glacial dynamics developed through each glacial cycle. These segments, as well as the different flow styles, are reflected differently in the offshore stratigraphic record. We propose that the glacial geomorphological signatures in the inter ice-stream areas mostly developed under warm-based conditions during a late phase of the glaciations, and that the overall glacial imprints in the landscape are strongly biased towards the youngest events. (C) 2013 Elsevier Ltd. All rights reserved.
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| 9. |
- Schomacker, Anders, et al.
(författare)
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Late Holocene and modern glacier changes in the marginal zone of Solheimajokull, South Iceland
- 2012
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Ingår i: Jökull: Journal of the Glaciological and Geological Societies of Iceland. - 0449-0576. ; 62, s. 111-130
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Tidskriftsartikel (refereegranskat)abstract
- The forefield of the Solheimajokull outlet glacier, South Iceland, has a variety of glacial landforms and sediments that are products of late Holocene and modern glacier oscillations. Several sets of moraine ridges reflect past ice front positions and river-cut sedimentary sections provide information about past environments. Here, we describe sediments and landforms deposited during the late Holocene. Chronology is obtained by C-14 dating and cosmogenic exposure dating. The age determinations suggest that Solheimajokull had major advances in the late Holocene prior to the Little Ice Age, and more restricted advances during the Little Ice Age, after AD 1539. Oscillations of the Solheimajokull ice margin between 1938 and 2010 are documented by aerial photographs. Digital elevation models were produced from selected years in order to quantify ice thickness changes at the glacier margin over the last 50 years. The glacier margin thickened 70-100 m front 1960 to 1996 and then thinned 120-150 m between 1996 and 2010. In 2010, the glacier snout was 20-40 in thinner than in 1960. Additionally, the DEM time-series detect areas of erosion and deposition in the forefield.
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| 10. |
- Schomacker, Anders, et al.
(författare)
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THE ACTIVE DRUMLIN FIELD AT THE MÚLAJÖKULL SURGE-TYPE GLACIER, ICELAND – GEOMORPHOLOGY AND SEDIMENTOLOGY
- 2011
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Ingår i: Geological Society of America abstracts with programs Minnespolis 2011.
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Konferensbidrag (refereegranskat)abstract
- Recent marginal retreat of Múlajökull, a surge-type, outlet glacier of the Hofsjökull ice cap, central Iceland, has revealed a drumlin field consisting of over 50 drumlins. The drumlins are 90–320 m long, 30–105 m wide, 5–10 m in relief, and composed of multiple beds of till deposited by lodgement and bed deformation. The youngest till layer truncates the older units with an erosion surface that parallels the drumlin form. Thus, the drumlins are built up and formed by a combination of subglacial depositional and erosional processes. Field evidence suggests each till bed to be associated with individual, recent surges. We consider the drumlin field to be active in the sense that the drumlins are shaped by the current glacial regime. To our knowledge, the Múlajökull field is the only known active drumlin field and is, therefore, a unique analogue to Pleistocene drumlin fields.
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