Iron-Coated Quartz as a Provenance and Paleoclimatic Marker in the Rhone Delta, France
Keywords:Archaeology, climatic cycles, coastal processes, Costieres, Crau, distal sediment, floods, Gulf of Lion, Holocene, iron oxides, Pleistocene, proximal sediment, quartz coating, sediment dispersal, size-sorting, source terrains, weathering
Analyses of 430 surficial and core sediment samples in the Rhone delta and adjacent region in southern France record similar average proportions of iron-stained quartz (lSQ) on the delta plain and in the Rhone and Durance rivers (~30%). These two rivers, largely responsible for the formation of the Holocene delta, carried most of their sediment load from distal source terrains. The much higher ISQ values in sand on the delta's modern coast (to >40%) and on older sand strand deposits (56%) subaerially exposed in the central and southern delta plain are primarily the result of post-depositional in situ formation of iron coating on quartz particles. However, some subsurface delta plain core samples with high stained-grain content (>40%) have a different origin. These layers were derived from Pleistocene and older sediment in proximal terrains, characterized by very high ISQ values (67%), that were transported to the delta plain. Evidence indicating displacement of highly-stained material eroded from these more local source areas is quartz with high ISQ values (50%) in the Vidourle and Gardon rivers with headwaters in adjacent uplands. Strata with large ISQ values (>40%) in Holocene core sections are attributed primarily to periodically increased hydrological fluxes that resulted from marked paleoclimatic changes and, possibly, effects of human activity as seen in the archaeological record in southern France. Wetter climatic conditions increased the erosion and reworking of proximal ISQ-enriched terrigenous sediment to the delta. This study indicates that the stained-grain method can be used to help identify changes in sediment provenance and dispersal patterns related to Holocene paleoclimatic fluctuations and/or human activity in the Rhone delta region.