Multidisciplinary study to highlight interaction between anthropogenic disturbance and natural coastal sedimentary processes. Complementarity of geochemical and granulometric methods and proposal of a new spatial geostatistical approach
Résumé
Geochemistry and sedimentary studies allow to highlight surface processes. Traditional geochemistry studies use mapping techniques (GIS) and multivariate analyzes (PCA and HAC) to highlight the spatial distribution of concentrations of one or several chemical parameters (Delgado et al., 2010; Coura Borges et al., 2014; El-Amier et al., 2017; Hou et al., 2017; Bodrud-Doza et al., 2019; Jin et al., 2019). Granulometric parameters are used to study sedimentary transport, especially through the Grain Size Trend Analysis (GSTA) approach, which integrates three parameters: mean, sorting and skewness (MacLaren, 1982; Gao and Collins, 1994, Poizot et al., 2006). In the context of the increasing pressure of human activities on marine coastal ecosystems, the study of sedimentary disturbances remains a challenge. The eastern part of the Bay of Seine (France) is very anthropized by the presence of harbour activities. The dredging sediments of The Havre Harbour are deposited on subtidal area. To understand sedimentary mechanism in this mixed area, a geochemical and sedimentological study was conducted at the beginning of 2016 (Baux et al., 2019). A sampling of 179 subtidal stations and 16 stations in the harbour basins and in the Seine estuary were studied. Geochemical elemental composition (18 calibrated elements) was obtained through X-Ray Fluorescence spectrometry and infrared spectrometry (TOC). On this dataset, multi-treatments were applied and the results compared:-Mapping of the spatial distribution of the concentrations of each chemical element.-PCA analyzes and selection of the information-bearing parameters and mapping of the PCA results.-Application of the "classical" GSTA approach on the three granulometric parameters.-"Innovative" integration of the chemical parameters and PCA results in the GSTA approach. The results show the necessity of multidisciplinary research and the interest of geochemical tracers to study coastal sediment dynamics and to highlight the impact generated by anthropogenic activities. The geochemical GSTA approach confirms the sedimentary dynamics observed by Baux et al. (2019) and appears more robust than the methods conventionally used. To generalize this approach, an adaptation of the concepts governing the GSTA approach will be developed.