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210Pb dating provides a valuable, widely used means of establishing recent chronologies for sediments and other accumulating natural deposits. The Constant Rate of Supply (CRS) model is the most versatile and widely used method for establishing 210Pb chronologies but, when using this model, care must be taken to account for limitations imposed by sampling and analytical factors. In particular, incompatibility of finite values for empirical data, which are constrained by detection limit and core length, with terms in the age calculation, which represent integrations to infinity, can generate erroneously old ages for deeper sections of cores. The bias in calculated ages increases with poorer limit of detection and the magnitude of the disparity increases with age. The origin and magnitude of this effect are considered below, firstly for an idealised, theoretical 210Pb profile and secondly for a freshwater lake sediment core. A brief consideration is presented of the implications of this potential artefact for sampling and analysis.

Results are presented for a study of manmade and natural radionuclides in north east Atlantic continental shelf and slope sediments to the west of Scotland. The data are interpreted in the context of sediment mixing and accumulation processes and are used to establish the westward extent of contamination of the sediment system. Offshore shelf and slope sediments were found to have post-glacial sedimentation rates of the order of 1 cm ky(-1) but nearshore sediments had much higher accumulation rates of the order of 0.1 cm y(-1). Surface mixed layer depths of up to 6 cm were observed and non-local mixing affected most of the slope sediments, resulting in advective transport of surface sediment to depths of up to 10 cm. Biodiffusion coefficients for offshore shelf and slope sediments were dominantly in the range 10(-8) to 10(-9) cm2 s(-1). The study confirmed that seawater contaminated with Sellafield waste radionuclides is dominantly entrained to the east of 7 degrees W and, consistent with this, higher levels of Sellafield derived radionuclides were confined to nearshore sediments, with lower levels to the west of 7 degrees W. 238Pu/(239,240)Pu data indicated that Sellafield contributed 75-91% of the total plutonium in coastal sediment but only about 4-8% of the total in slope sediments. By analogy, it can be concluded that a similar situation will apply to other contaminants in seawater entering the north east Atlantic via the North Channel.

As part of a wider investigation of the biogeochemistry and fate of Pb deposited from the atmosphere at Glensaugh, a rural upland catchment in N.E. Scotland, the concentration and isotopic composition of Pb were determined in four thinly sectioned monolith cores (25 cm) of peat collected at altitudes of 426--434 m from different faces of Thorter Hill and in a series of 21 10-cm unsectioned cores of peat and organic-rich soil along a transect from near the top (434 m) to the bottom (224 m) of the catchment. Depth profiles of Pb concentration and (206)Pb/(207)Pb ratio were similar for the longer cores. Subsurface Pb maxima (238--489 mg kg(-1)) typically occurred below (206)Pb/(207)Pb minima (1.123-1.134). One core was (210)Pb-dated and had a fairly constant (206)Pb/(207)Pb value of 1.170 from mid-19th century to ca. 1930, followed by a decline (attributable to the increasing influence of Australian Pb of much lower (206)Pb/(207)Pb ratio) to 1.134 by the early 1990s, and then a rapid increase to 1.160 by 2002, after the phased withdrawal of leaded petrol. The fluxes of Pb increased from 15 mg m(-2) year(-1) in the late 19th century to a peak of 60 mg m(-2) year(-1) ca. 1960, before declining steadily to 3.6 mg m(-2) year(-1) by the beginning of the 21st century. Some 40% of the anthropogenic Pb in the core had been deposited prior to 1900. The mean anthropogenic Pb inventory of the four longer cores was 7.4+/-1.5 g m(-2), of which approximately 70% occurred in the top 10 cm, in good agreement with the inventories of the shorter cores collected above 400 m. These inventories are higher than those of the industrial central belt of Scotland, probably because of enhanced deposition at altitude. This is consistent with the derived average (210)Pb flux of 198+/-11 Bq m(-2) year(-1), which is twice that of typical UK (210)Pb deposition and the rainfall for the site. The past deposition of Pb at Glensaugh, including that from sources (e.g., smelting, coal combustion) other than leaded petrol, has clearly been considerable. Even since the introduction of leaded petrol ca. 1930, car-exhaust emissions may have accounted for no more than 35% of the Pb deposited.

The use of concentration profiles of lead and other contaminant metals in 210 Pb-dated ombrotrophic peat cores for reconstruction of historical trends in atmospheric deposition has become relatively well established. However, uncertainty remains over the validity of the assumption of post-depositional immobility of lead in peats. In particular, a number of studies have suggested that in saturated peat systems, lead is subject to diagenetic remobilisation and redistribution, with the result that 210 Pb and lead profiles do not provide a historical record of deposition. Results are presented here for lead concentrations, inventories and stable isotope ratios and for 210 Pb activities in two peat cores from locations close to the Glasgow industrial area in west-central Scotland. Contrasting biogeochemical conditions prevailed at the two sampling sites, with one being unsaturated and ombrotrophic while the other was saturated and minerotrophic. The results for the ombrotrophic peat core were compatible with information from other peat and lake sediment core studies, data for archived herbage samples and with known historical trends both in industrial activity and in the use of lead as an additive to petrol, giving a high degree of confidence in the 210 Pb chronology and implied record of deposition. This provides strong support for the suggestion that lead is immobile in such systems. In contrast, the lead concentration profile and stable isotope data for the minerotrophic peat provided apparent temporal variations that were inconsistent with other studies and known historical trends, confirming that the saturated peat core did not provide a record of atmospheric deposition. This observation is consistent with the suggestion that lead is subject to diagenetic remobilisation and redistribution in saturated peats.

An ombrotrophic peat core from west-central Scotland was subjected to multi-element analysis and 210Pb dating in order to estimate historical trends in atmospheric deposition of contaminants. The inferred depositional flux of ash (total inorganic material) exhibited small, transient increases from as early as the mid 18th century, followed by a larger increase from the mid 19th century to a pronounced maximum approx. 1960. A significant decrease was observed from the peak ash deposition of 29.2 g m−2 year−1 to the contemporary level of 11 g m−2 year−1, although this is still an order of magnitude greater than the pre-industrial value of 1 g m−2 year−1. Depositional fluxes of a wide range of elements (scandium, copper, arsenic, antimony, samarium, cobalt, bromine, lanthanum and lead) also exhibited major increases from the mid 19th century and peak values in the mid 20th century (approx. 1940–1950). Aluminium, cerium and bromine also showed evidence of transiently increased deposition as early as the mid 18th century. Normalisation of the concentrations of individual elements to that of aluminium confirmed that the peak depositional fluxes in the mid 20th century were not due to increased re-deposition of soil minerals from the atmosphere, but were attributable to other sources of contamination including heavy industry, coal burning and vehicle exhaust emission. Nevertheless, re-deposited soil minerals were estimated to constitute an important fraction of the ash, with peak input at the start of the 20th century and subsequent deposition at a relatively constant level, about half that of the maximum flux. Excellent agreement was observed between the inferred trends in contaminant deposition and the known history of industrial development of the area.

The reconstruction of historical trends in atmospheric deposition of contaminant metals by use of their concentration profiles in accumulating, ombrotrophic peat deposits, in conjunction with radionuclide-derived chronologies, is subject to uncertainty because of potential post-depositional mobility both of the radionuclides and of the contaminant metals. Results are presented here for a study of the distribution of 210Pb, Pb, stable Pb isotopes and radiocaesium in the unsaturated, surface zone of ombrotrophic peat deposits in three contrasting locations in Scotland. 210Pb chronologies are derived and the implied historical variations in atmospheric depositional fluxes and atom ratios of contaminant Pb are considered, along with total inventories, relative to known historical trends in input and proximity to heavily populated, industrial areas. Similarly, radiocaesium concentration profiles, activity ratios and inventories are considered in the context of the 210Pb chronologies and known inputs. It is concluded that the historical trends in the depositional flux and stable Pb atom ratio of contaminant Pb derived on the basis of the 210Pb chronologies are in good agreement with known temporal variations in these parameters, consistent with post-depositional immobility of Pb under the conditions investigated here. The study confirms that the high degree of mobility of radiocaesium in organic soils renders it of little value in providing chronological information, but suggests that the presence of small amounts of mineral matter may restrict this mobility.