Lagroix, France, 2004, Contributions from rock magnetism to central Alaskan loess deposits and titanium rich compositions of the hematite-ilmenite solid solution series: University of Minnesota, Minneapolis, Ph.D. dissertation, 255 p.
This dissertation is a compilation of contributions from rock magnetism aimed at increasing our understanding of loess deposits in Central Alaska, and of titanium rich compositions of the hematite-ilmenite solid solution series. Contributions herein were motivated by a common goal, to reconstruct the paleoenvironment and paleoclimate of Central Alaska since the Late Pleistocene from loess and paleosol deposits. Through the analysis of the anisotropy of magnetic susceptibility, primary (undisturbed since initial aeolian deposition) and secondary (reworked or redeposited since the initial deposition) loess intervals are identified. Secondary magnetic fabrics suggest permafrost played a dominant role in inducing post-depositional deformations. Permafrost loess behaved (1) as rigid bodies undergoing rotations and lateral translations, and (2) as impermeable layers focusing and channeling flow. Primary magnetic fabrics act as a proxy for past transport directions. Temporal variations of inferred paleowind directions track paleoclimate glacial--interglacial cycles. Temporally coeval average paleowind directions at both sites confirm a regional (spatial) consistency of past air circulation. Numerous tephra beds found in loess deposits across central Alaska have been indispensable for regional stratigraphic correlations. The Old Crow tephra is by far the most prominent and wide-spread. Through multiple high- and low-temperature magnetization experiment three phases of the magnetite-ulvöspinel solid solution series, Fe 3-x Ti x O4 , x = 0, 0.1 and 0.3 along with one phase of the ilmenite-hematite solid solution series, Fe2- y Tiy O3 , y = 0.83 are identified. Furthermore, low-temperature magnetism locates a 1 mm thick tephra bed dispersed within 10 cm depth of loess by identifying very low concentrations of a titanohematite phase with y = 0.9 and shows the potential as a tool for regional correlation of sedimentary deposits. Finally, the mechanism of reversed thermoremanent magnetization (rTRM) is revisited and clarified by investigating the magnetic behavior of well characterized synthetic titanohematite (y = 0.7). The data unambiguously support a less than perfect ferrimagnetism--antiferromagnetism exchange interaction as the fundamental source of the rTRM. However, we conclude that favorable conditions for the acquisition of rTRM are not only dependent on the strength of the exchange anisotropy but that a crucial role is played by the size of the cation ordered domains.
Theses and Dissertations