Reconstructing Weathering and Climate Trends on Loess Deposits in NW France Using XRF Analysis

Abstract: Analyzing climate changes by loess sediment deposits formed after the last glacial maximum gives us information about how the environment was dynamically before humans physically could record climate changes. Sampling and analyzing loess from different places in the world give us a more reliable view of when events and changes in climate took place. Loess is a last step eolian transported, fine grained sediment and can be a good record as a Quaternary climate archive (Muhs 2013; Stevens et al. 2020). Loess-paleosol sequences can be used as a relative record for climate change. The paleosols in the sequence corresponds to soil which can indicate warmer interglacial/interstadial periods when chemical weathering was active at site. Loess in these sequences reflects on cold and dry climate during glacial periods (Buggle et al. 2008; Muhs 2013). By analyzing geochemical composition of bulk-samples from chosen site one can get information about if paleosols or loess deposits are present at certain depths of profile. Also, if a glacial or interglacial period was present then (Buggle et al. 2011; Johansson 2017). In this study X-ray fluorescence Spectroscopy (XRF), will be used to analyze elemental composition of loess. This to see how soluble and mobile elements relate to immobile and non-soluble ones. This can indirectly tell how paleosols and loess are distributed over the site and if shifts attempt by depth. The samples of loess in this study were taken from north-western France at a site called Primel-Tregastel. Soil weathering indexes will be used in this study to calculate weathering intensities for deposit. Na/Al-based indices called CIA, CIW, CPA, PIA, Index B and Sr-based ratios Ba/Sr and Rb/Sr was used (Buggle et al. 2011). The purpose of the study is to see if weathering has been active at site. And if, to reconstruct at which depths and to make a paleoclimatic interpretation of the site. Na/Al-based indices CIA, CIW, CPA, PIA and Index B shows indications of active weathering at site. Sr-based ratios Ba/Sr and Rb/Sr do also show same indications but with more dramatically variations. The content of mobile and immobile elements does also follow the principle that when mobile elements decrease in percentage, immobile elements get enriched in bulk-sample. The trends shown are a long time of cold climate due to low weathering intensities followed by one warmer and more humid period. Increasing weathering intensities, indicating an interglacial/interstadial period happens as a first development. Continuously a decrease happens in weathering intensity which would indicate a start of a colder period. Topmost layer has been improved and re-worked by human activity and will be misleading to use in a geochemical analysis and paleoenvironmental interpretation.