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Sökning: WFRF:(Hill Mimi J.)

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  • Greve, Annika, et al. (författare)
  • The geomagnetic field intensity in New Zealand : Palaeointensities from Holocene lava flows of the Tongariro Volcanic Centre
  • 2017
  • Ingår i: Geophysical Journal International. - Wiley-Blackwell. - 0956-540X. ; 211:2, s. 814-830
  • 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.
  • Nilsson, Andreas, et al. (författare)
  • Short-term magnetic field variations from the post-depositional remanence of lake sediments
  • 2018
  • Ingår i: Frontiers in Earth Science. - Frontiers Research Foundation. ; 6
  • 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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fritt online (1)
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tidskriftsartikel (2)
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refereegranskat (2)
Nilsson, Andreas (2)
Hill, Mimi J., (2)
Greve, Annika, (1)
Turner, Gillian M., (1)
Suttie, Neil (1)
Lunds universitet (2)
Engelska (2)
Forskningsämne (UKÄ/SCB)
Naturvetenskap (2)


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