Palaeoclimate in Western Europe during the Last Glacial Period : Understanding the Mixed Magnetic Signals in Lœss-Palaeosol Deposits
Start: 01 October 2011
End: 24 February 2015
Doctoral Dissertation - Samuel Taylor
Loess-palaeosol deposits are important terrestrial archives of palaeoclimate and palaeoenvironmental change. A major tool used to extract this information is rock and mineral magnetism, an area of research that has significantly advanced over the past 30 years. The Nussloch (Germany) loess-palaeosol deposits provide one of the key records of palaeoclimate in Western Europe. They have been shown to record millennial-scale climate variations that infer links between the N. Atlantic climate and the European continent. The main objectives of this thesis are to first evaluate the mineral magnetic response of the Nussloch deposits, a set of well-studied Pleniglacial sequences, to environmental and climatic changes occurring in Western Europe. This includes the magnetic response of loess to waterlogging-induced redoxomorphic processes, which have received little attention in loess literature. Second, to constrain their depositional and post-depositional history, which is fundamentally important for any sedimentary succession and will give a better interpretation of both magnetic and non-magnetic climate proxies obtained from Nussloch. How does the development of tundra gley horizons affect the deposited mineral magnetic fabric, and can prevailing palaeowind directions be recovered from pristine loess? Third, to further quantify variations in individual mineral components by decomposing the bulk magnetic assemblage via low temperature remanence experiments. This will provide insight into which magnetic minerals influence the bulk magnetic signal and the links between their relative concentrations and palaeoclimate interpretations.
Tundra gley horizons mark short periods of climate amelioration, where there is degradation of the permafrost and either enhanced snow melt or increased precipitation. Within these horizons, redoxomorphic processes induced by waterlogging (gleying) are shown to have secondary control on the mineral magnetic assemblage. If these alterations are not considered, interpretations of magnetic susceptibility records following the expected variations from the magnetic enhancement or wind vigour models could be erroneous. Anisotropy of magnetic susceptibility (AMS) revealed several layers showing post-depositional grain remobilisation associated with tundra gley horizons. Other AMS identified reworked intervals are associated with laminated loess. In both cases the cause of secondary magnetic fabric formation is linked to permafrost dynamics. Permafrost loess becomes impermeable resulting in percolating waters to accumulate at or flow along the upper permafrost boundary enabling particle remobilisation overprinting the primary magnetic fabric acquired initially through aeolian deposition. Intervals having preserved primary magnetic fabrics are found concentrated in the Upper Pleniglacial section. These have, in general, poorly developed magnetic lineations that are overpowered by a strong magnetic foliation. In addition ferrimagnetic particles are present in extremely low concentrations, less than 0.03 wt%. Taken together, weak magnetic lineation and low ferrimagnetic concentration, limit the confidence level with which palaeowind directions may be inferred.
Through low temperature remanence and demagnetisation experiments, it was possible to extract and quantify variations in the major magnetic components that comprise the mineral magnetic assemblage. Magnetite contributes approximately 70% to the acquired remanence and is calculated to make up 0.003-0.077 wt% of the total bulk sample. An oxidised form of magnetite is present in small amounts (0.016 wt%) but still contributes around 20% to total remanence. The final component identified is goethite, which contributes the least to remanence (<10%) due to its weak magnetisation, but has the greatest presence with a weight percent of 0.27-0.57.
- Taylor, S.N. and Lagroix, F., 2015. Magnetic anisotropy reveals the depositional and postdepositional history of a loess-paleosol sequence at Nussloch (Germany). Journal of Geophysical Research B: Solid Earth. doi: 10.1002/2014JB011803 HAL
- Taylor, S.N., Lagroix, F., Rousseau, D.D. and Antoine, P., 2014. Mineral magnetic characterization of the upper pleniglacial nussloch loess sequence (Germany): An insight into local environmental processes. Geophysical Journal International, 199(3): 1463-1480. doi: 10.1093/gji/ggu331 HAL