Mineralogical fingerprinting to characterize spatial distribution of coastal and riverine sediments in southern Japan

Abstract

Knowledge of the provenance and transport processes of coastal sediments is central to safeguarding against coastal erosion. However, to date coastal sediments have been poorly investigated without considering complex artificial and natural attributes such as coastal structures and watershed geology of potential source rivers. In this study, mineralogical fingerprinting technique was used to track the sources of sediments along a coastline in southwest Japan with reference to sediments of outflowing rivers. The sediment samples were collected from 65 beach sites along the Miyazaki Coast and its surrounding coastal area, and five river sites nearby. To determine the size distribution and geochemical and mineralogical compositions of the sediments, grain size, X-ray fluorescence, and X-ray diffraction (XRD) analyses were performed. Subsequently, cluster analysis was used to classify mineralogically similar groups in terms of XRD peak intensity ratio. The coastal sediment clusters corresponded well to nearby riverine clusters, suggesting that the coastal sediments were primarily transported from these river watersheds, although uncertainty as contributions of oceanic flows might be involved. The concordance ascribes to the major geological classifications in the watersheds of major rivers, as well as smaller rivers flowing in the northern area (e.g., the Shimanto Supergroup and Osuzuyama Acid Rocks). In addition to the major tendency, we also observed potential effects of manmade structures on the coastal sediment prevalence; the mineralogical similarities decreased across jetties in the Hitotsuse River mouth. The vertical mineralogical distribution of the core sediment samples observed on the left and right banks of the Hitotsuse River mouth supported the general trend of uniform cluster patterns in each core sample with horizontally different clusters across the river mouth. Mineralogical analysis targeting both coastal and riverine sediments is effective for understanding the characteristics of sediment distribution, which in turn is useful for efforts devoted to protection against coastal erosion.