The Severn Estuary on the west coast of Britain is large, macrotidal and well mixed, receiving fine sediment from many sources. Within the last few thousand years, at least four discrete lithostratigraphic units, predominantly of sandy to silty clay, have accumulated along the shores of the estuary in the upper intertidal zone. The three youngest are continuing to be deposited, each beneath a distinctive geomorphic surface reached by a proportion of the tides. These surfaces form a stair-like succession on the salt marshes and high mud flats, the most elevated and outermost of the surfaces overlying the formation that, of the three, began to accumulate earliest. We here describe and name these linked geomorphic and lithostratigraphic features, and outline on a reconnaissance basis their distribution within the estuary. The (upper) Wentlooge Formation consists of pale green estuarine silty clays that began to accumulate 2500-3000 years ago and ceased to form in the Roman period or soon after. Reclamation during the Roman period isolated the Wentlooge Surface on large areas of tidal wetland in the lower estuary. The soil that developed on this surface is recognized as the Wentlooge palaeosol in those places where post-Roman breaching of the Roman sea defences led to a resumption of tidal sedimentation. The Rumney Surface is the most elevated of the geomorphic surfaces on the salt marshes of the estuary. It overlies thick largely pink sandy to silty clays, termed the Rumney Formation, that began to form at times ranging from the early mediaeval to the early modern periods. Mediaeval reclamation of wetlands led to the isolation of the Oldbury Surface during the early stages in the accumulation of the Rumney beds. Locally, the breaching of the mediaeval seabanks led to renewed tidal sedimentation on the Oldbury Surface. Wherever the Wentlooge and Rumney formations are seen in contact, the base of the latter so far proves to be sharp and erosional. Locally, the Rumney Formation is found to abut against and smother a low embayed cliff cut into the upper Wentlooge beds. The Awre Surface forms the intermediate level on the salt marshes and overlies pink to grey sandy to silty clays (Awre Formation) that bank against a low cliff and gently shelving platform cut into older deposits. This unit began to accumulate probably in the 19th century. The lowest step on the salt marshes is formed by the Northwick Surface, underlain by the grey sandy to silty clays of the Northwick Formation. Like the Rumney and Awre beds, the Northwick formation abuts against a cliff and shelving platform eroded into older sediments, chiefly the Rumney and Awre formations. The erosion surface at the base of the Northwick Formation dates from the 19th century and the beds themselves from the early 20th century. The reconnaissance shows that these linked formations and geomorphic surfaces are represented in most parts of the Severn Estuary. The evidence available for dating — archaeological, historical, radiocarbon, and geochemical — points to the broad synchroneity of each formation throughout this extensive area. The geochemistry of the tidal sediments, reflecting the pollution history of the estuary, is particularly useful in the recognition and correlation of the Northwick Formation. These associated geomorphic and lithostratigraphic features point to the instability of the shores of the estuary and to important horizontal movements of the strand over the last 2000 years. Intrinsic as well as external factors may have controlled these oscillations, but which factor, or factors, was responsible for the movements recorded cannot as yet be decided.

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The rias of NW Spain are coastal ecosystems of high biological productivity and great economic importance. They are intensively exploited by man for fish and shellfish. There are a number of important centres of population and industrial activity along their margins, which serve as sources of contamination. In this context, it is desirable to achieve the best possible understanding of the physico‐chemical processes that control spatial and temporal variations in the geochemical, mineralogical and sedimentological characteristics of near‐surface sediments in the Ria de Pontevedra and, in particular, the distribution and mobility of heavy metal contaminants. Thus, adequate environmental planning can be achieved for this site and understanding gained for comparable contexts. Core samples were examined from the inner, middle and outer parts of the ria. Grain‐size distributions reflect the presence of two main populations, one dominated by silt and clay, derived mainly from terrestrial sources, and the other by fine sand to coarse silt, which is derived mainly from continental shelf and ria mouth sources. Mineralogical analysis shows an abundance of terrestrial intensive‐weathering products near the ria head, a dominance of shelf‐derived sediment towards the mouth and the presence of several diagenetic minerals whose nature varies with location within the ria. In the inner ria, the near‐surface sediments are slightly enriched in Pb, Cu and Zn from anthropogenic sources. These sediments are fine grained and have a high organic content; hence, they have a higher potential to sorb contaminants than the coarser grained, less organic‐rich sediments of the mid and outer ria. The estimated sedimentation rates for the fine‐grained organic‐rich sediments from the inner part of the ria are about 1 mm year–1. The dominant authigenic minerals in the inner ria are iron sulphides, whereas in the mid and outer ria, iron silicates and oxyhydroxides are more important. These differences in authigenic iron mineralogy are clearly reflected by the magnetic properties of the sediments.

The relative importance of suspended (>0.45 µm) and organic colloidal (<0.45 µm) matter in the transport of pesticide through and across a soil was studied by monitoring suspended and colloidal matter from lysimeters over a winter (1994–1995) and relating it to the leaching of pesticides. The major results were: (i) The organic colloidal matter and suspended sediment appeared at different times in the leachate. (ii) The suspended sediment was enriched in total organic carbon (TOC). (iii) The leaching of pesticide occurred largely in first few samples following rainfall, coinciding with the occurrence of the colloidal matter. (iv) Modelling the distribution of pesticide between the colloids, suspended matter, and pure solution suggests that negligible amounts of isoproturon are carried on suspended sediment and the organic colloids are more important. The importance of organic colloids may be even more significant for compounds more hydrophobic than those studied here: this has implications for the management of soils and pesticides in relation to the release of organic colloids into solution.

Many ancient early diagenetic siderite concretions are thought to have been precipitated by microbially‐mediated reactions, and their trace element chemistry has been used to infer palaeoenvironments of formation (e.g. marine versus freshwater). In this study, pure cultures of the microorganism Geobacter metallireducens were used to precipitate siderite in the laboratory at a range of temperatures (18–40°C). Magnesium and calcium, in three different ratios (3:1, as in seawater, 1:1, and 1:3 as in freshwater), or manganese were added to some cultures at each incubation temperature to study trace element incorporation. The siderite produced exhibited the rhombohedral crystal form typical of concretionary siderite. However, this microbial siderite did not simply retain the trace element chemistry of the water from which it precipitated, as is assumed in palaeoenvironmental interpretations. Instead, manganese and calcium incorporation were found to be inversely proportional to the rate of microbial activity, which itself is dependent upon the nutritional status of the microorganisms. Magnesium incorporation was found to be rate‐independent, and at high Mg/Ca ratios, it caused inhibition of calcium incorporation. In short, the influence of microorganisms on the trace element composition of early diagenetic siderite must be taken into account in order to produce valid palaeoenvironmental interpretations.