| 1. |
- Nilsson, Andreas, et al.
(författare)
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Holocene geocentric dipole tilt model constrained by sedimentary paleomagnetic data
- 2010
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 11, s. 08018-08018
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Tidskriftsartikel (refereegranskat)abstract
- Paleomagnetic data, although undeniably limited, have been used to constrain complex time-varying geomagnetic field models. Here we present four different versions of a time varying geocentric dipole model for the past approximately 9000 years based on a limited selection of sedimentary paleomagnetic data with good spatial coverage. The robustness of the modeling approach is evaluated using modern magnetic observation station data provided by the World Data Centre for Geomagnetism in Edinburgh and we discuss the magnitude of the error introduced by insufficient data coverage. The prediction in terms of directional variations is tested against paleomagnetic data from 63 different sedimentary sites and compared to results from recently published spherical harmonic geomagnetic field models. We find that the dipole estimate can explain the paleomagnetic data almost as well as the more complicated spherical harmonic models. Specific areas and records are identified where there are significant differences between the performance of the dipole estimate and the spherical harmonic models. Our results suggest that many of the differences between the dipole model predictions and the paleomagnetic data can be related to poor paleomagnetic quality and/or the lack of accurate and precise time control inherent to some of the records. These results agree with Valet et al. ( 2008) who suggested that the CALS7K. 2 model probably underestimates the dipole contribution in order to reduce the misfit between the inversion and the data. We conclude that better paleomagnetic data and associated time control are needed to improve global geomagnetic field models.
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| 2. |
- Striberger, Johan, et al.
(författare)
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Climate variability and glacial processes in eastern Iceland during the past 700 years based on varved lake sediments
- 2011
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Ingår i: Boreas. - : Wiley. - 1502-3885 .- 0300-9483. ; 40:1, s. 28-45
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Tidskriftsartikel (refereegranskat)abstract
- Properties of varved sediments from Lake Logurinn in eastern Iceland and their link to climate and glacial processes of Eyjabakkajokull, an outlet glacier of the Vatnajokull icecap, were examined. A varve chronology, which covers the period AD 1262-2005, was constructed from visual observations, high-resolution images, X-ray density and geochemical properties determined from X-radiography and X-ray fluorescence scanning. Independent dating provided by 137Cs analysis and eight historical tephras verify the varve chronology. The thickness of dark-coloured seasonal laminae, formed mainly of coarser suspended matter from the non-glacial river Grimsa, is positively correlated (r=0.70) with winter precipitation, and our 743-year-long varve series indicates that precipitation was higher and more varied during the later part of the Little Ice Age. Light-coloured laminae thickness, controlled mainly by the amount of finer suspended matter from the glacial river Jokulsa i Fljotsdal, increased significantly during the AD 1972 surge of Eyjabakkajokull. As a consequence of the surge, the ice-dammed Lake Haoldulon formed and recurrently drained and delivered significant amounts of rock flour to Lake Logurinn. Based on these observations, and the recurring cyclic pattern of periods of thicker light-coloured laminae in the sediment record, we suggest that Eyjabakkajokull has surged repeatedly during the past 743 years, but with an increased frequency during the later part of the Little Ice Age.
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| 3. |
- Striberger, Johan, et al.
(författare)
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Climatic control of the surge periodicity of an Icelandic outlet glacier
- 2011
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Ingår i: Journal of Quaternary Science. - : Wiley. - 1099-1417 .- 0267-8179. ; 26:6, s. 561-565
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Tidskriftsartikel (refereegranskat)abstract
- Surging outlet glaciers are important in draining large ice caps, but the mechanisms controlling surge periodicities are poorly known. We investigated a sediment sequence from the glacier-fed Lake Logurinn in eastern Iceland, and our unique annually resolved data, based on sedimentary varves, imply that Eyjabakkajokull, an outlet glacier of Vatnajokull, began surging about 2200 cal a BP (before 1950 AD). Approximately 1700 cal a BP, the glacier started to surge at a uniform 34- to 38-year periodicity that prevailed until the coldest part of the Little Ice Age when the periodicity almost halved to 21-23 years. Since the late 1800s the surge periodicity of Eyjabakkajokull has returned to a longer period of 35-40 years. We suggest that surge periodicities of Eyjabakkajokull are forced by climatically driven mass balance changes, which may be a common forcing factor for similar surge-type outlet glaciers. Copyright (C) 2011 John Wiley & Sons, Ltd.
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