Browsing by Author "Sun, Wei-wei"

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    The rock magnetic fingerprint of chemical remagnetization in midcontinental Paleozoic carbonates
    (Geophysical Research Letters (American Geophysical Union), 1992) Jackson, Mike; Sun, Wei-wei; Craddock, John
    Results of a paleomagnetic and rock magnetic survey of Paleozoic carbonates from 39 sites in the midcontinental U.S. show that many of these sites carry a stable remanence of apparent Permian age. Many of these remagnetized sites also have relatively high anhysteretic susceptibilities, and higher saturation remanence than most of the sites where the late Paleozoic remanence is absent. However the correlation between late Paleozoic remanence and high anhysteretic susceptibility or high saturation remanence is imperfect. The most diagnostic rock magnetic parameter for recognizing remagnetized sites is a ratio of anhysteretic remanence/saturation remanence exceeding 10%. We have found high ratios in almost all remagnetized sites, but in very few sites where the Late Paleozoic remanence is absent. The high ratios reflect the presence of a significant fraction of extremely fine-grained magnetite (a few tens of nanometers), spanning the superparamagnetic-single domain threshhold.
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    Rock-magnetic proxies of climate change in the loess-palaeosol sequences of the western Loess Plateau of China
    (Geophysical Journal International (Oxford University Press), 1995) Hunt, Christopher; Banerjee, Subir; Han, J.=M.; Solheid, Peter; Oches, E.A.; Sun, Wei-wei; Liu, T.-S.
    Results of the first detailed study of the climate proxy record in the loess-palaeosol sequence at Xining—one of the few palaeoclimate sites in the currently arid western Loess Plateau of China—illustrate the importance of making many types of rock-magnetic measurements other than susceptibility. A multiparameter approach yielded confirmation that here, as elsewhere in the Loess Plateau, the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite. Nevertheless, magnetic enhancement was caused not exclusively by changes in relative grain size, but also by variations in concentration and mineralogy of the magnetic fraction. The effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence, respectively. The resulting signal was ascribed more confidently to variation in magnetic grain size, which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility. Variations in magnetic mineralogy were also determined to constrain interpretations further. The data were then used to discuss climate history at Xining. Finally, results from Xining were compared with other western sites and contrasted with eastern sites. In summary: (1) data is presented from a new Loess Plateau site which also appears to yield a global climate signal; (2) a demonstration is made of a more rock-magnetically robust way to separate concentration, composition and grain-size controls on susceptibility and other magnetic parameters; and (3) models are provided for inter-regional comparisons of palaeoclimate proxy records.
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    Scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early Paleozoic carbonates from Missouri
    (Journal of Geophysical Research (American Geophysical Union), 1994) Sun, Wei-wei; Jackson, Mike
    Magnetic carriers in remagnetized early Paleozoic carbonates of the North American midcontinent were studied in extracts and in situ, using rock magnetism and scanning electron microscopy (SEM). SEM observation of extracts and in situ samples shows that the dominant magnetic particles are large irregular grains and framboids (10–20 μm in diameter) and individual spherical and euhedral particles (0.2–1 μm). Comparing the saturation remanence of the bulk rock and extracts indicates that the extracts only account for ∼10% of the saturation remanence of the bulk rock. Most of the remanence carriers were lost during the dissolution and separation procedure, especially the finer-grained carriers. The framboids and the individual submicron euhedral magnetic particles observed by SEM thus make a fairly minor contribution to the magnetic remanence of the remagnetized carbonates. Hysteresis loops of the extracts are very similar to those reported by Suk et al (1993), with saturation remanence to saturation magnetization (Jr/Js) ratios from 0.1 to 0.13 and remanent to bulk coercivity (Hcr/Hc) ratios ∼4. However, the loops for the “nonmagnetic” residue are strongly “wasp-waisted”, and Hcr/Hc ratios are very high, similar to whole rock specimens of remagnetized carbonates (Jackson, 1990). This suggests that bulk rock and “nonmagnetic” residue contain very fine (superparamagnetic (SP) and single domain (SD)) magnetic particles which were not recovered by the magnetic separation procedure. Low-temperature experiments support the conclusion that the principal remanence carriers in the remagnetized carbonates are SD magnetite and that SP magnetite plays an important part in the unusual bulk magnetic properties of these rocks. Owing to their dissolution, the actual natural remanent magnetization bearing particles cannot be observed petrographically.