Landscape response to Quaternary climate change. An investigation of two antipodal orogens using uranium isotopes

The depth of erosion occurring in river catchments may be influenced by prevailing climatic conditions. In order to understand how future climate may influence erosion and soil availability, it is paramount to investigate to what extent erosion has responded to climate in the past. Marine sediments derived from continental catchments document conditions of erosion depth while also recording the history of chemical weathering and system energy. This study incorporates marine sediments from antipodal catchments, the Var River Catchment (southeast France) and the Hokitika River Catchment (western New Zealand South Island), with ages of up to 75ka and 120ka, respectively. Geochemical and mineralogical techniques were utilized to establish an understanding of the response of catchment-wide erosion and the extent of chemical weathering to climatic variability. Mineralogical analysis was performed via X-ray diffraction, and geochemical techniques included U isotope analysis and major and trace element quantification. Grain size distribution of sediments was analysed to act as a potential proxy for fluvial system energy and to constrain the results of U isotope analysis. Brunauer–Emmett–Teller analysis was performed to measure the surface area of sieved and leached samples, and to subsequently derive comminution and residence ages of sediments. Analyses indicated a reversal of erosion and weathering responses in the two antipodal catchments, and an inability to determine erosion response to short-lived climatic variability. In the Var, The Last Glacial Maximum and subsequent deglaciation phase coincides with deep and fast erosion and subsequent transition into the Holocene correlates with the onset of increased chemical weathering, shallowing in erosion and increase residence times of sediments. Conversely, in the Hokitika, glacial periods correlate with slightly shallower and slow erosion relative to interglacial periods, though erosion is consistently deep and fast during the past 120 ka due to the prevailing precipitation trends