Shelf mud fields formation within historical times: examples from offshore the Gironde estuary, France

Abstract

The Gironde shelf mud fields are distinctive morpho-sedimentary units that overlap the sands and gravels covering the North Aquitaine continental shelf (France). The muddy deposits are located offshore of the two main channels of the Gironde mouth, the West-Gironde mud patch (WGMP) and South-Gironde mud patch (SGMP). Fine-grained sediments originate in the Gironde, especially during river floods, and a smaller part of silty and shelly fine sand has its origin in the sandy covering of the inner shelf, during periods of high-energy (i.e. storms). The stratigraphy of these muddy deposits has been revised, using 14C dates and palynological analyses from cores. The results do not agree with previous estimates, of 10,000 or 7000 BP, for the age of the deposits. The deposition of the Aquitaine shelf mud fields is not linked to the postglacial transgression. The substrate has formed between 2000 and 5000 BP. Some shells from the lower part of the muddy sediments have been dated at 1000 or 1100 BP (SGMP) and 500–600 BP (WGMP). Pollen analyses confirm they formed after 2000 BP. The oldest muds were deposited in the Subatlantic period, and evidence of human activities (such as clearance and agriculture) are conspicuous. Some palynological markers provide evidence of Roman colonization at about 2000 BP. Causes of deferred settling of the muds have been sought through variations in the rate of escape of Gironde suspended matter. This depends on the infilling and the morphological changes within the estuary since the post-Holocene stillstand and the increase of sediment supply resulting from soil erosion within the catchment area. The latest link to climatic fluctuations, and intensive human activities that have occurred during the last thousand years, are suspected to have been influential. However, the various events were concomitant and are difficult to separate. Comparison between cores suggests that at first there was a concentration of sedimentation at the site of the SGMP and in the inner zone of the WGMP. However, later, recent fine-grained sedimentation was concentrated on the outer zone of the WGMP, as testified by the preponderance of pollen that records the afforestation of Les Landes (mid-19th century). The sedimentation rates are variable and range between 1 and 4 mm y −1.