| 1. |
- Allington, Megan L., et al.
(författare)
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Obtaining archaeointensity data from British Neolithic pottery : A feasibility study
- 2021
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Ingår i: Journal of Archaeological Science: Reports. - : Elsevier BV. - 2352-409X. ; 37
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Tidskriftsartikel (refereegranskat)abstract
- There is a significant lack of geomagnetic field strength (archaeointensity) measurements for many archaeological time periods in the United Kingdom (UK). This not only makes past geomagnetic secular variation difficult to model but also limits the development of archaeointensity dating. This paper presents the first archaeointensity study on UK Neolithic material. In this study, twenty-five sherds of Neolithic Grooved Ware pottery from the Ness of Brodgar, Orkney, UK, some with direct radiocarbon dates, were subjected to a full archaeomagnetic investigation with the aim of increasing the amount of archaeointensity data for the UK. Both thermal Thellier and microwave palaeointensity experiments were used to determine which technique would be most suitable for British Neolithic pottery. Three successful archaeointensity results between 35 and 40μT were obtained using thermal Thellier method, which is consistent with the limited data available within a 15° radius and geomagnetic field model predictions from the same time. We separated the results into four different types with an intention of explaining the behaviours that determine the likelihood of achieving an acceptable archaeointensity estimate. The feasibility of obtaining geomagnetic field strength information during the UK Neolithic from ceramics has been demonstrated and the results provide a solid basis for improving our knowledge of geomagnetic secular variation during archaeological time in Britain.
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| 2. |
- Allington, Megan L., et al.
(författare)
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Constraining the eruption history of Rangitoto volcano, New Zealand, using palaeomagnetic data
- 2023
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Ingår i: Quaternary Geochronology. - : Elsevier. - 1871-1014 .- 1878-0350. ; 78
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Tidskriftsartikel (refereegranskat)abstract
- New Zealand's largest city, Auckland, is situated within the basaltic Auckland Volcanic Field (AVF). Therefore, understanding the eruption history of the local volcanoes in the field is of great importance in order to assess future hazards that they may pose. Rangitoto is the youngest and largest volcano in the AVF, although the timing of the first eruption and the duration of the volcanic activity are still uncertain. Here, we use palaeomagnetic methods to provide additional constraints to the duration of the main shield-building phase of Rangitoto, previously estimated to have lasted either less than 100 years or approximately 1000 years. Lava flow samples from an -140 m length vertically oriented drill core produced 203 palaeoinclinations and 74 palaeointensity estimates. Our results show significant variation in both inclination (up to 30 degrees) and intensities (which fall between 25 and 60 & mu;T; present day field values for Auckland are -55 & mu;T). Potential non-geomagnetic explanations for these variations, including thermochemical processes and rheological deformation are discussed. A statistical model was created to determine the minimum duration for the construction of Rangitoto that is compatible with the variations in the palaeomagnetic data, using prior information about the rate of change of the geomagnetic field. We conclude that the palaeomagnetic data are incompatible with the previously suggested short duration (under 100 years) of the shield-building phase and suggest instead a range of 150-450 years, when also accounting for the available radiocarbon ages. Given these results, this has implications for the timing, and thus impact of possible future eruptions in the AVF.
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| 3. |
- Greve, Annika, et al.
(författare)
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The geomagnetic field intensity in New Zealand : Palaeointensities from Holocene lava flows of the Tongariro Volcanic Centre
- 2017
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Ingår i: Geophysical Journal International. - 0956-540X. ; 211:2, s. 814-830
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Tidskriftsartikel (refereegranskat)abstract
- Very few absolute palaeointensity data exist from Holocene-aged rocks in New Zealand. Here we present a new suite of high-quality palaeointensities, supported by detailed rock magnetic investigations. Samples from 23 sites representing 10 distinct eruptive units of the Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand, were studied. Both traditional double heating and microwave palaeointensity methods were employed. The reliability of the palaeointensity data varies with rock magnetic properties of the samples, corresponding, in particular, to their positions within the lava flows. The highest success rates are from samples obtained from near the flow tops where a significant proportion of the remanence unblocked at intermediate temperatures (200-350 °C). By contrast, samples from flow centres, particularly the parts showing platey fracturing, have the lowest success rates. Reliable, high-quality palaeointensity results ranging from 32.4 ± 5.1 μT to 72.1 ± 4.7 μT were obtained from six flows with ages between c. 12 800 yr BP and the present. These correspond to virtual dipole moments that increase from 52 ± 10 ZAm2 in the early Holocene and peak at 112 ± 14 ZAm2 about 300 yr ago. The data agree well with calibrated relative palaeointensities from New Zealand lake sediments. The volcanic and sedimentary data together yield a Holocene virtual axial dipole moment curve that fits the global average variation well in the early Holocene, but which differs significantly in recent millennia. This difference is associated with recent migration of the southern high latitude core-mantle boundary flux lobe towards New Zealand, as is seen in global field models.
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| 4. |
- Nilsson, Andreas, et al.
(författare)
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Short-term magnetic field variations from the post-depositional remanence of lake sediments
- 2018
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Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Paleomagnetic records obtained from lake sediments provide important constraints on geomagnetic field behavior. Secular variation recorded in sediments is used in global geomagnetic field models, particularly over longer timescales when archeomagnetic data are sparse. In addition, by matching distinctive secular variation features, lake sediment paleomagnetic records have proven useful for dating sediments on various time scales. If there is a delay between deposition of the sediment and acquisition of magnetic remanence (usually described as a post-depositional remanent magnetization, pDRM) the magnetic signal is smoothed and offset in time. This so-called lock-in masks short-term field variations that are of key importance both for geomagnetic field reconstructions and in dating applications. Understanding the nature of lock-in is crucial if such models are to describe correctly the evolution of the field and for making meaningful correlations among records. An accurate age-depth model, accounting for changes in sedimentation rate, is a further prerequisite if high fidelity paleomagnetic records are to be recovered. Here we present a new method, which takes advantage of the stratigraphic information of sedimentary data and existing geomagnetic field models, to account for both of these unknowns. We apply the new method to two sedimentary records from lakes Kälksjön and Gyltigesjön where 14 C wiggle-match dating floating varve chronologies provide an independent test of the method. By using a reference magnetic field model built from thermoremanent magnetization data, we are able to demonstrate clearly the effect of post-depositional lock-in and obtain an age-depth model consistent with other dating methods. The method has the potential to improve the resolution of sedimentary records of environmental proxies and to increase the fidelity of geomagnetic field models. Furthermore, it is an important step toward fully explaining the acquisition of post-depositional remanence, which is presently poorly understood.
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