Some chemical observations on post-glacial lake sediments

Abstract

This paper reports the results of a somewhat exploratory investigation of the chemical composition of cores taken from the sediments of a number of lakes of the English Lake District. All the lakes studied are of glacial, and therefore presumably of synchronous, origin. The depth of post-glacial sediment varies from lake to lake but is normally between 4 and 6 m. This material, with which the paper is mainly concerned, was laid down during the biologically active phase following the end of glaciation approximately 10000 years ago. Below the organic muds of the post-glacial period, the sediment consists of glacial clays, usually varved, which pass downward into silty clays and sands the total depth of which is at present unknown. The variation in composition of the sediments with depth and therefore with time presents a pattern of change which is observed to recur in most of the lake sediments studied. This pattern of change is most strikingly seen in the variation of carbon content with depth, but related sequences of change may be observed in the distribution of major inorganic components, for example sodium, potassium and magnesium. The observed changes in composition of the sediment can most easily be explained if the sediment is regarded as a sequence of soils derived from the drainage areas of the lakes. The composition of the residues eventually reaching the lake bed can then be accounted for in terms of the rate of erosion of the drainage basin rather than in terms of changing rates of organic productivity either on the drainage basin or in the lake waters. The composition of the sediments does not appear to be greatly influenced by events within the lakes themselves except in the case of elements which may migrate more readily under the reducing conditions which may arise in soils and in lake muds. Examples of these are iron and manganese, and the distribution of these elements in the sediment may be used to deduce the redox conditions in the soils of the drainage basin in past times, or in the muds and hypolimnetic waters of the lakes themselves. Biological activity within the lake waters may also influence significantly the distribution in the sediments of distinctly biophile elements, of which phosphorus and sulphur are examples. Even in the case of phosphorus, however, it seems likely that co-precipitation reactions may have more influence on deposition efficiency than does incorporation into biological tissues. The cause of the apparently synchronous changes in erosion intensity recorded in the sediments of the various lakes remains in doubt. Some evidence derived from the halogen and boron content of the sediments is presented which may suggest at least a synchronism between major climatic changes and the variations in erosional activity. Since halogens and perhaps boron are largely derived from oceanic air-streams, the rate of deposition of these elements may be expected to be related to the 'oceanicity' of the climate at the time of deposition of the sediment. A direct relationship is observed between the deduced intensity of erosional activity and the halogen and boron content of the sediment. The meaning of this relationship is rendered somewhat ambiguous however by consideration of possible changes in soil ionic balance attributable to progressive loss by leaching of soil-derived anions with the passage of time. Although the gross composition of the sediment is largely dependent on conditions in the drainage system rather than in the lake waters, some deductions may be made which indicate the availability of nutrients dissolved in the waters of the lakes in past times. If material is rapidly removed from the drainage basin by erosion, nutrient elements are lost to the sediment locked in the lattice of unleached mineral particles. If however the rate of erosive removal of soil from the land surface is reduced, the mineral particles are held in the soil column in a position which allows more efficient leaching of soluble components, which then become available to the living populations of the lake.