Inorganic geochemical analysis of sediment has played a central role in palaeolimnology since its establishment as a research field. This chapter outlines methods that can be applied, explores the issues affecting reliability of interpretations, and refers readers to the appropriate literature. A distinction is made between inorganic geochemical palaeolimnology and inorganic geochemistry in general. While sharing many techniques and information sources, the two subjects differ fundamentally in purpose. Inorganic geochemistry aims to understand the chemical properties of the natural world, and the behaviour of chemical substances within it. Geochemical palaeolimnology uses such information to describe and quantify the environment. Furthermore, whereas palaeolimnological analysis is about lake, catchment or even landscape scale processes, geochemistry often considers minute scales. This contrast in scale has lead to conflicting interpretations. Geochemists working with laboratory experiments, and studying speciation and small-scale mobility, emphasize complexity. Palaeolimnologists, on the other hand, tend to look at large scale phenomena, and often find consistent patterns, even where a full geochemical understanding is lacking. The two disciplines should not be seen as separate. Inorganic geochemistry feeds into geochemical palaeolimnology because it is important to have a good understanding of the behaviour of the elements being measured. Conversely, palaeolimnological data can help to constrain geochemical models. Nevertheless, the contrasts in purpose and scale must be kept firmly in mind. A further distinction is made between organic and inorganic methods. For many inorganic elements, behaviour is highly dependent upon organic substances, and the two topics cannot be treated in isolation. Rather, inorganic methods must include a basic evaluation of organic components.

Inorganic Geochemical Methods in Palaeolimnology

This paper reviews the major sources and transport characteristics of heavy metals in the hydrological cycle. It is demonstrated that heavy metal releases to the environment have changed from 19th and early 20th century production-related activities to consumption-oriented factors in more recent times. The relative roles of particle size, sorption and desorption processes, partitioning and the chemical speciation of heavy metals on fine sediments are identified to understand the likely fate of heavy metals released into fluvial systems. It is argued that the spatial and temporal distribution of heavy metals in the river corridor depends not only on an understanding of metal solubility and speciation, but also on an understanding of sediment dynamics which control, for example, floodplain alluviation and the accumulation of metals in the bottom sediments of contaminated rivers, lakes and reservoirs. Existing long- and short-term records are examined to identify the current state of knowledge about the factors which affect heavy metal releases into aquatic environments. With limited exceptions, it is shown that few long-term studies of trends in heavy metal transport are available although, for some major rivers, limited data on trends in metal concentration exists. Palaeolimnological reconstruction techniques, based on an analysis of lake and reservoir sediments, are identified as a possible means of supplementing monitored records of heavy metal transport. Although numerous studies have suggested that trends in atmospheric contamination, mining and urbanization may be identified in the bottom sediment record, other research has shown that the radionuclide-based chronology and the heavy metal distribution within the sediment are more likely to be a function of post-depositional remobilization than the history of metal loading to the basin. Despite these limitations, it is shown that the incorporation of reservoir bottom sediment analysis into a heavy metal research programme, based in river corridors of Midland England, provides an opportunity to identify and quantify the relative contribution of point and non-point contributions to the heavy metal budget and to relate trends in metal contamination to specific periods of catchment disturbance.

Heavy metals in the hydrological cycle: Trends and explanation

Diatom microfossils preserved in lake sediments can be used to provide evidence for lake acidification. Unlike documentary methods of historical reconstruction, the sediment record is potentially useful at all sites, it is usually continuous and it can be extended backwards in time as desired. Cores can be taken with little disturbance, by freezing in situ if necessary, and sediments deposited since about 1800 A.D. can be dated using $^{210}$ Pb analysis. The strong relationship between diatom occurrence and water pH allows pH to be reconstructed with a standard error of between ca. 0.25 and 0.5 pH unit using either an index system or multiple regression analysis. Although there is evidence that long term acidification is a natural process for lakes in areas of resistant base-poor bedrock, diatom analyses from NW European and North American sites shows that rapid acidification has taken place within the last 150 years. The first major sign of acidification is the loss of planktonic diatoms at pH values between about 5.5 and 5.8. Acidification below pH 5.5 leads to the decline of species typical of circumneutral water, such as Achnanthes microcephala Kutz. Cymbella gracilis (Rabh). Cleve and Anomoeoneis vitrea (Grun). Ross, and the expansion of acidophilous taxa such as Tabellaria flocculosa (Roth.) Kutz. and Eunotia veneris (Kutz.) O. Muller. Acidobiontic species become common at pH values less than 5.5 and as the pH drops to 4.5 these taxa begin to replace acidophilous taxa in the assemblage. Tabellaria binalis (Ehr.) Grun. is probably the most faithful member of this group. Diatom analysis for four acid lakes in Galloway, SW Scotland show that the beginning of acidification has varied from 1840 (Loch Enoch) to 1925 (Loch Grannoch) and that pH has declined by between ca. 0.5 units (Loch Dee) and ca. 1.2 pH units (Loch Grannoch) in these lakes. Since lakes without afforested catchments have been acidified and lakes with afforested catchments were acidified before afforestation it can be concluded that afforestation is not responsible for acidification in this region.

Diatom analysis and the acidification of lakes

The distribution of fallout 137cs and 210pb in undisturbed and cultivated soils

241Am derived from decay of fallout 241Pu is now frequently detected in analyses of lake sediments by low-background gamma assay, and offers an alternative to weapons test 137Cs in dating recent sediments at those sites where the 137Cs record has been degraded by post-depositional mobility or obliterated by Chernobyl fallout Calculations of the in-growth of 241Am from 241Pu indicate a nominal distribution broadly similar to that of 137Cs, with the maximum 241Am activity occuring in fallout dating from 1963 Results from a number of sites suggest that 241Am is significantly less mobile in lake sediments than 137Cs, and that its distribution in cores reflects more closely the fallout record Since further decay of existing weapons debris will increase 241Am concentrations by about 24% over the next 40 years, 241Am is likely to play an increasingly important role in assessing the validity of 210Pb dates at sites with varying sediment accumulation rates

Kyle D. Hoagland

Papers

137Cs and 210Pb dating of sediments from soft-water lakes in New England (U.S.A.) and Scandinavia, a failure of 137Cs dating