The cation content of Swedish Post-Glacial sediments as a criterion of palaeosalinity

Abstract

The present investigation attempts to ascertain the possibility of establishing former salinity conditions from analyses of the extractable cation content of sediments. The influence of factors other than palaeosalinity, such as the cation-exchange capacity of the sediment and post-depositional changes, is stressed.

Three sites, each with different characteristics, have been studied. The first site, Kyrkviken, comprises sediment deposited during the Ancylus and Littorina periods. The sediment sequence is relatively homogeneous with respect to its physical characteristics. The second site, Rässan, consists of an isolation sequence with a transition from clay-gyttja to a gyttja with a very low minerogenic content. The physical properties of the sediment consequently varies considerably through the profile. The third site, Angskär, is a sea inlet which has yet to be isolated, where the sediment also changes from clay-gyttja to gyttja.

Wet density, dry matter, carbon content, exchangeable Mg, Ca, Na and K analyses were carried out on the sediments from each site. Grain-size analyses were carried out on the Kyrkviken profile and the total minerogenic content of a number of selected samples from the Rässan and Ängskär profiles was determined.

It is suggested that the Mg and in some cases the Ca contents show very good agreement with the former salinity conditions inferred from the diatom evidence. The Na content is shown to fluctuate most and is consequently not regarded as a suitable criterion of palaeosalinity. The K content of each series of analyses is thought to be associated with the minerogenic content of the sediment, and shows no correspondence with the presupposed salinity changes, especially in the Kyrkviken profile.

The ion ratios are not thought to show any clear relationship with the palaeosalinity, due to differences in the exchanging power of the different ions and due to the varying strengths with which the ions are bound to the mineral particles and to organic material.

It is, however, of great importance that the structural characteristics of the sediment should be known in detail, in order to ensure a correct assessment of the results.