Behaviour of the major elements and minerals in sediments of four humic lakes in south-western Finland

Abstract

The average chemical compositions of Finnish clays, sands, and lake sediments are presented. Fourty‑four new total analyses of the ash of gyttjas and dys from four oligotrophic humic (dystrophic) lakes are given. The mineral and chemical composition of these reduzates is com­pared with that of hydrolyzates (clays) and of resistates (sands), and the relative mobilities of the major and some minor elements present are calculated.

The concentration of cations in sediments has decreased after separa­tion of the lakes from the Ancylus Lake about 8000 years ago. This decrease reflects the appearance of bogs and acid podzolic soils in which low polymerized organic acids have formed. These have leached the surrounding sediments, causing a decrease in the content of bases and an increase in silica. The amelioration of climate during the Atlantic stage is seen as increased calcium content, because during a warm period more than during a cool, calcium reacts chemically to form hu­mates with humic substances as they in increased degree are polymerized and disintegrated. Therefore calcium concentration in sediment char­acterizes climate and analysis of this element in humic sediment profiles at short intervals allows changes in climate to be studied.

Relative mobilities of major elements in reducing organic‑rich lake sediments were, from the fastest to the slowest, Na, K, Mn, Ca, Al, Fe, Si, Mg, P, and Ti.

The investigated area is typical of central Finland. The bedrock, consisting of granites, granodiorites, and quartz diorites, has a chemical composition about the same as the average for Finnish bedrock calcu­lated by Sederholm (1925). Generalized, therefore, the results of this study are pertinent to wide areas in Finland and elsewhere where the climate is humid and temperature low; the frost‑free period is short; Precambrian silicic rocks prevail; sediments, mostly glacigene, are young (usually less than 10 000 years); mires are present in the vicinity; and pod­zolic soils and coniferous forest prevail.