Geochemistry of suspended material and sediments in boreal lakes

Abstract: The geochemistry of suspended material and sediment has been studied in four boreal lakes: Lakes Kutsasjärvi, Sakajärvi, and Ala Lombolo in northern Sweden and Lake Imandra on the Kola Peninsula, Russia. Suspended material and filtered water have been sampled in time series and depth profiles. The suspended material is collected on filters, which are dissolved and analyzed. Sediment cores from all lakes are taken. Major and trace element analyses are mainly performed by ICP–QMS and ICP–AES. The main theme in this thesis is the relation between the suspended material and the sediment, and the geochemical cycling of elements within the lakes. In this context, processes like biological production and decomposition, and precipitation and dissolution of inorganic phases are important. Special attention is paid to redox processes of Fe and Mn, and to the implications of these processes. A thorough discussion of diatom Si in sediments and suspended material is also included. The high concentrations of organic material and diatom Si in the water column of Lake Kutsasjärvi during summer dilutes most other components, rendering these concentrations lower in the suspended material than in the sediment. The non-detrital fractions (i.e., the fraction that is not related to primary mineral particles) of all major elements are, however, higher in the suspended material, partly as a consequence of higher dissolution rates of diatom Si and non-detrital Fe and Mn in the sediment, but also because no sampling of suspended material was performed during spring-flood, when the inflow of detrital particles (i.e., minerogenic particles deriving from the mechanical weathering of rocks) is probably highest. Like the major elements, most trace elements show higher concentrations in the sediment than in the suspended matter. The enrichment of trace elements in the suspended material, compared with detrital particles, is, however, higher. During winter, when Lake Kutsasjärvi is ice-covered, the redox cycle of Fe is a dominant process in the bottom water. There, particulate Ca, Mg, and P co-vary with particulate Fe. Similarly, several trace elements appear to be associated with non-detrital Fe, e.g., As, Mo, U, and V. Barium and Co, that have often been observed to co-vary with Mn, are correlated with both Fe and Mn in Lake Kutsasjärvi, probably as a consequence of the strong dominance of Fe over Mn in the bottom water suspended matter. For, e.g, Ba, Cr, Cu, and Ni, detrital particles are the most important source to the sediment. Besides non-detrital Fe and Mn and detrital particles, organic matter may be an important source of trace metals to the sediment. Further, anthropogenic aerosols are suggested to be important for the flux of Cd, Pb, and Zn to the sediment. The normalization method for estimating diatom Si is evaluated by comparing the calculated non-detrital Si concentrations in sediments and suspended material with the concentration of diatom frustules, as counted under a microscope. The results show a fairly good agreement between the methods, where deviations can be attributed to, e.g., greatly varying diatom cell sizes. Concentration variations of non-detrital Si in three sediment profiles are examined. It is suggested that controlling factors behind the variations may include varying degrees of dilution by detrital material; increasing dissolution rate with depth of the diatom remains, possibly linked to a decreasing redox potential which affects protective coatings; and fluctuations of the primary diatom production. The highly polluted (with regard to, e.g., Ni, S, and P) Lake Imandra presents an interesting object for studying how natural geochemical processes work in disturbed systems. In the nearly anoxic bottom water of the ice- covered Lake Imandra, the redox cycle of Mn plays an important role. It controls the distribution of Ni and some other trace elements between the dissolved and particulate phases. A substantial sulphide formation in the sediment, probably a consequence of the increased SO42- loading into the lake, enables an efficient fixation of metals.