The geochemistry of Recent sediments of the Bristol Channel—Severn Estuary System

Abstract

Multi-element distributions are described for sediments and particulate materials suspended in the water column which are representative of the Bristol Channel—Severn Estuary System, U.K. The overall geochemistry of bottom-deposited sediments is very similar to that found for mean crustal rocks and reflects the locally available sediments which contain large amounts of well-mixed glacio-fluvial deposits of Pleistocene age. A significant enrichment in some elements, for example Li, B, C, N, F, Zn, Br, I and Pb, was observed for suspended particulates, while an enrichment in other elements, such as As and Sn, may also occur. The highest levels of enrichment were found in materials containing a large proportion of ferruginous floc enclosing finely divided clay debris. The observed enrichments may be partly due to natural processes which give rise to sedimentary debris having very large surface areas suitable for the absorption of elements, but also because of the proximity of an extensive coal field which is a source of a considerable amount of mineral debris, together with waste products from various industrial processes common to the area. The upper reaches of the system contain extensive silt and mud banks and about 5 cm below the surface of these sediments anaerobic conditions exist. As a result of tidal movements, supplemented by storm disturbances, anaerobic pore liquids are released providing a significant flux of elements to the overlying water column. Elements such as iron and zinc, released from pore liquids can be detected in near-bottom waters of the system. Following release of elements and compounds from bottom waters, subsequent removal through flocculation processes occurs of which the iron system is of particular importance. An examination of the aeolian flux, for some selected elements, is discussed and it is shown that relative to inputs of Pb, Cr, Ni and Cu from rivers and sewage dumping, aerial inputs may be significant. The exceptionally high tidal energy of the system would appear to demand thorough mixing of constituents in the water column, yet evidence is presented to show some degree of chemical stratification which may also be relevant to suspended materials.