Dr Rachel Smedley

This research is focused on understanding the former dynamics of lake-terminating glaciers and how they have previously responded to past changes in precipitation related to shifts in the Southern Westerlies. It involves the reconstruction of former ice extents and palaeoenvironmental change in unstudied regions of NE Patagonia using geomorphological, sedimentological and geochronological (luminescence and cosmogenic nuclide dating) techniques.

Funded by the Royal Society; British Society for Geomorphology.
Collaborators: Alessa Geiger (Pontificia Universidad Católica, Chile), Richard Chiverrell (University of Liverpool).

Records of past ice extents are used to evaluate the accuracy of numerical models used for projecting future change. The aim of this research project is to use a novel approach to OSL-CN dating to reconstruct former ice extents of the Gran Campo Nevado in SW Chile. We embarked on an 14-day field expedition aboard the Mary Paz II Vessel (Punta Arenas, Chile) to access remote field sites in the fjords of SW Chile (~52-53 °S, 72-72 °W). The climatic setting of SW Chile is dominated by the southern westerly cyclone system leading to a hyper-humid environment with longitudinal and altitudinal differences in ice dynamics.

Funded by: Pontificia Universidad Católica; Royal Geographical Society; British Society for Geomorphology; Quaternary Research Association; University of Liverpool; Vango.
Collaborators: Alessa Geiger, Juan Garcia (Pontificia Universidad Católica, Chile), Gaston Herrera (BIMS, Chile), Lidia Ferari (IANIGLA-Mendoza, Argentina), Paulo Rodriguez (University of Magellan, Chile).

Luminescence dating of K-feldspar is a geochronological technique important for reconstructing past environments on Earth. However, the variability in the internal geochemistry of feldspar can still add significant uncertainty into ages determined, especially for single grains (Smedley et al. 2012, 2016; Smedley and Pearce, 2016; Buylaert et al. in 2019). Feldspar form a solid solution series from orthoclase (K-rich) to albite (Na-rich) to anorthite (Ca-rich), which includes perthites composed of different phases; thus, grains of feldspar can contain variable internal concentrations of K, Rb, U and Th. K-feldspar are isolated using density and/or magnetic separation and signals are detected in blue wavelengths (~400 nm), expected to be dominated by K-feldspar emission. However, the internal composition of grains emitting blue signals can vary from 0 – 14 % K (e.g. Smedley et al. 2012), and from 0 – 3 ppm U and 0 – 16 ppm Th (Smedley and Pearce 2016). This is because different feldspar can emit blue signals, even though the peak emission of Na-feldspar is yellow (~570 nm), and perthites have wide emission bands (Krbetschek et al. 1997). Our understanding of emission spectra from different feldspar is solely derived from museum specimens, rather than sand-sized feldspar grains from nature. No studies have yet investigated the relationship between the emission spectra of single-grain feldspar and their internal geochemistry, nor considered exploiting this to ensure that dating is only performed on K- feldspar. Also, initial measurements suggest that the IPRL signal is emitted strongest by K-rich feldspar (K. Thomsen Pers. Comms) and can even be imaged so we may identify different phases of feldspar within a grain (i.e. perthites). This research project proposed here aims to address these major uncertainties and improve the accuracy and precision of ages calculated by devising a new robust luminescence procedure to ensure that only K-rich feldspar grains are used for age calculation.

Funded by: Aarhus University.
Collaborators: Jan-Pieter Buylaert, Andrew Murray (DTU Nutech, Aarhus University).