Analytical and sampling constraints in ²¹°Pb dating.

²¹°Pb 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 ²¹°Pb 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 idealized, theoretical ²¹°Pb 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.

Stable lead isotopic characterisation of the historical record of environmental lead contamination in dated freshwater lake sediment cores from northern and central Scotland.

Sediment cores from three Scottish freshwater lakes, Loch Ness in the remote north and Loch Lomond and the Lake of Menteith, much closer to the heavily populated and industrialised central belt were analysed for 210Pb, 137Cs, Pb and stable Pb isotopic composition (206Pb/207Pb). The radionuclide data were used to establish chronologies for the Loch Ness and Loch Lomond cores, but a chronology could not be developed for the Lake of Menteith core, in which the surface sediment had been subject to intense mixing. Although Pb concentrations generally started increasing during the mid-17th Century, a small peak occurred for Loch Ness in the early 16th Century, perhaps attributable to the influence of medieval mining and smelting in mainland Europe. Temporal trends in the pattern of Pb accumulation were similar for Loch Ness and both sites in Loch Lomond, with 40-50% of the anthropogenic Pb deposited prior to the 20th Century. Fluxes of anthropogenic Pb to the lake sediments peaked during the 1950s at all locations where chronologies could be established. The 5-fold increase in anthropogenic Pb inventory for the southern basin of Loch Lomond relative to Loch Ness reflected geographical proximity to the main polluting sources. The 206Pb/207Pb data for anthropogenic Pb in the sediments from Loch Ness and Loch Lomond exhibited largely similar trends related to five different time periods. Pre-1820, the 206Pb/207Pb ratio was close to that for coal (1.181). From 1820 to 1900, a fairly constant 206Pb/207Pb ratio of approximately 1.17 probably resulted from a combination of emissions from the smelting of indigenous Pb ore (1.170) and coal burning (1.181) in Scotland, and industrial activity to the south in England, where Australian Pb of characteristically low 206Pb/207Pb ratio (1.04) was already in use. From 1901 to 1930, the 206Pb/207Pb ratio declined by <0.01, due to the increasing influence of Australian Pb. From 1931 to 1975/1985, the 206Pb/207Pb ratio of anthropogenic Pb declined by a further 0.03 to 0.04, to minimum values from approximately 1975 to 1985, primarily a consequence of car-exhaust emissions of Pb arising from the introduction of alkyl Pb petrol additives (206Pb/207Pb approximately 1.06-1.09). From 1975/1985 to the mid-1990s, the 206Pb/207Pb ratio of anthropogenic Pb increased by up to 0.015, a consequence of a reduction in car-exhaust emissions of Pb, resulting from reductions in the maximum permitted concentration of Pb in petrol, and the introduction and increasing uptake of unleaded petrol. Source apportionment calculations, on the basis of 206Pb/207Pb values in surface sediment, suggested that the contribution of Pb emissions from the use of leaded petrol was 27-40% of the atmospheric burden by the mid-1990s, in line with estimates from rainwater 206Pb/207Pb data.

The changing nature of the 206Pb/207Pb isotopic ratio of lead in rainwater, atmospheric particulates, pine needles and leaded petrol in Scotland, 1982-1998.

The inductively coupled plasma-mass spectrometry (ICP-MS)-determined 206Pb/207Pb ratio of 145 samples of rainwater collected at 25 locations around Scotland during December 1997 and January 1998 and at three longterm monitoring stations in the northeast, central belt and southeast of the country from November 1997 to December 1998 averaged 1.144+/-0.017 (1 s). This represents a significant increase from the mean value of 1.120+/-0.016 recorded for the long-term sites in 1989 1991, only partly attributable to a concomitant increase in the 206Pb/207Pb ratio of leaded petrol from 1.075+/-0.013 to 1.088+/-0.007. The rainwater 206Pb/207Pb data for the late 1990s also contrast markedly with the lower 206Pb/207Pb ratios found for pine needle and atmospheric particulate samples from Scotland (e.g. Glasgow: 1.085+/-0.012 in 1985-1986, 1.099+/-0.007 in 1991-1992), England and Western Europe in this study for the period 1982-1992 when emissions of lead to the atmosphere from petrol-engined vehicles in the UK were approximately 2-9 times higher. The observed change in the lead isotopic signature of rainwater predominantly reflects the impact of measures, such as the introduction and growing uptake of unleaded petrol, to reduce car exhaust emissions of lead to the atmosphere in the UK. Based on the rainwater data, source apportionment calculations suggest a general decline in the contribution of leaded petrol to atmospheric lead in Scotland from 53-61% in 1989-1991 to 32-45% in 1997-1998, with a corresponding decline in the urban environment from 84-86% to 48-58%.

A point mutation in the Nul gene of bacteriophage lambda facilitates phage growth in Escherichia coli with himA and gyrB mutations.

A mutant of lambda was isolated that grows in the Escherichia coli himA delta/gyrB-him320(Ts) double mutant at 42 degrees C; conditions which are non-permissive for wild-type lambda growth. The responsible mutation, ohm1, alters the 40th codon of the Nul reading frame. The Nul and A gene products comprise the terminase protein which cleaves concatameric DNA into unit-length phage genomes during DNA packaging. The Nul-ohm1 gene product acts in trans to support lambda growth in the double himA/gyrB mutant, and lambda cos154 growth in the single himA mutant. The observation that an alteration in Nul suppresses the inhibition of growth in the double himA/gyrB mutant implicates DNA gyrase, as well as integration host factor, in the DNA:protein interactions that occur at the initiation of packaging.

A restriction enzyme cleavage map of the histidine utilization (hut) genes of Klebsiella aerogenes and deletions lacking regions of hut DNA.

The histidine utilization (hut) operons of Klebsiella aerogenes were cloned into pBR322. The hut genes are wholly contained on a 7.9 kilobase pair fragment bounded by HindIII restriction sites and expression of hut is independent of the orientation of the fragment with respect to pBR322. A restriction map locating the 27 cleavage sites within hut for the enzymes, HindIII, PvuII, SalI, BglII, KpnI, PstI, SmaI, AvaI, and BamHI was deduced. Several of the cleavage sites for the enzymes HaeIII and HinfI were also mapped. A set of deletion plasmids was isolated by removing various restriction fragments from the original plasmid. These deletions were characterized and were used to assist in mapping restriction sites. This physical characterization of hut DNA opens the way for genetic and molecular analysis of the regulation of hut gene expression in vitro as well as in vivo.

A nonsense mutation in the structural gene for glutamine synthetase leading to loss of nitrogen regulation in Klebsiella aerogenes.

An amber mutation (glnA3711), the first nonsense mutation isolated in Klebsiella aerogenes, is described. When amber suppressors were present, the mutant made active glutamine synthetase which was more thermolabile than wild type, showing that glnA3711 lies in the structural gene for glutamine synthetase. Strains carrying the glnA3711 allele were unable to express nitrogen regulation of genes coding for histidase, asparaginase, and glutamate dehydrogenase unless amber suppressors were also present. These results support a model that expression of gene(s) from the glnA promoter is required for nitrogen regulation in K. aerogenes.