Study of the Chinese loess and Siberian flood basalts: new global scale insights to the paleoclimate and geomagnetic field changes

• Author / Creator

  Anwar, Taslima

• Paleomagnetism, in conjunction with other earth science methods, studies the behavior of the Earth's magnetic field, the drift of tectonic plate, climate and environmental changes. This thesis implements different applications of paleomagnetism to develop a technique to evaluate age for the Chinese red clay sequence, to reconstruct the regional climate and environment change in the Holocene recorded in the Chinese Loess, and to reveal absolute paleointensity variations for the gigantic magmatic events to fulfill the gaps in paleointensity data during the Permo-Triassic boundary and the late Devonian as the data for these periods are either contradictory or just absent. A magnetostratigraphic age model for the Shilou red clay sequence on the eastern China is tested by cyclostratigraphy matching the magnetic susceptibility and grain size variations with the known orbital parameters of the Earth. This approach provides an age of 5.2 Ma for the Shilou profile instead of previously found age of 11 Ma with magnetostratigraphy alone. Wavelet analysis reveals the well-preserved 400 kyr and possible 100 kyr eccentricity cycles on the eastern Chinese Loess Plateau. Further, the paleomonsoon evolution during 2.58–5.2 Ma is reconstructed and divided into three intervals (2.58–3.6 Ma, 3.6–4.5 Ma, and 4.5–5.2 Ma). The use of cyclostratigraphy along with magnetostratigraphy proves to be an effective method of dating the red clay sequences, and it implies that many presently published age models for the red clay deposits based on magnetostratigraphy alone, perhaps, need to be re-evaluated. A high-resolution petromagnetic analysis is conducted on the Holocene Chinese loess sequence. It reveals that three warm-humid intervals (~8.4–3.7 ka, ~2.4–1.2 ka, and ~0.81–0.48 ka) and two cold-dry intervals (~3.7–2.4 ka and ~1.2–0.81 ka) occurred during the Holocene. A significant paleosol development from ~8.4 to 3.7 ka along with the higher values of proxy parameters indicates a generally strong warm-humid phase in the mid-Holocene which can be attributed as a Holocene optimum in the studied regions. A complete Holocene climate record is constructed and that correlates well with the other regional and global climate records. The study demonstrates that the Holocene climatic optimum took place at the same time interval all over the northern hemisphere. A microwave paleointensity study for the Permo-Triassic boundary, considering both the northern extrusive and the southeastern intrusive parts of the Siberian trap basalt, is carried out for the first time. The accepted microwave paleointensity results from this study are combined with thermal Thellier results from previously published studies to obtain a grand mean paleomagnetic dipole moment for the Permo-Triassic boundary. The results suggest that multiple localities need to be considered to obtain an accurate paleomagnetic dipole moment for this time period. The grand mean paleointensity during the Permo-Triassic boundary is 19.5 ± 10.5 µT which corresponds to a mean virtual dipole moment of 3.2 ± 1.5 × 1022 Am2. The study indicates that the magnetic field intensity during this period is significantly lower (by approximately 50%) than the present geomagnetic field intensity, implying that the Mesozoic dipole low might have begun at the Permo-Triassic boundary. A microwave study, complemented by thermal Thellier one, for the late Devonian-aged volcanics and instrusives from the Viluy traps is performed. The Arai plots show a distinct two-slope concave-up shape, but pseudo-Thellier experiments support that it occurs due to a strong overprint component rather than alteration or multi-domain effects. The site mean paleointensities, range from 4.6 µT to 16.5 µT which correspond to a virtual axial dipole moments of (0.81–2.97) ×1022 Am2, suggest that the field was extremely weak during the late Devonian. It provides the evidence for the field being weak ~50 Ma before the onset of the superchron state, between ~310 and 265 Ma. Considering low dipole moment an indicator of high reversal frequency (as appears to be the case in the mid-Jurassic), this study demonstrates that rapid transitions between reversal hyperactivity and superchron states are a recurring feature in the paleomagnetic records, indicating the influence of mantle convection changing heat flow across the Core Mantle Boundary.

• Subjects / Keywords

  • Devonian
  • Cyclostratigraphy
  • Holocene
  • Paleoclimate
  • Permo Triassic
  • Siberian Traps
  • Paleointensity
  • Chinese Loess

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R35D8NS74

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.