STABILITY OF MERCURY AND METHYL-MERCURY DURING EARLY DIAGENESIS IN LAKE SEDIMENTS

Sedimentary records are used in paleolimnology to infer past environmental conditions. An important premise for this use is that the environmental signals of interest are unaffected over time. In this study we have used a unique series of 8 sediment freeze cores taken in different years from 1979 to 2007 from a lake with varved (annually laminated) sediment. This series of freeze cores allows us to study in situ, in a way, whether different types of environmental information are preserved in the sediment, or whether diagenetic processes in some way affect this information. With this sample collection we can follow the sediment material deposited in a single year; for example, we can analyze the newly deposited material in the 1979-core (material deposited in 1978) and compare these results to those for the same sediment layer in the 1985-core (after 6 years) and onwards to the 2007-core (after 28 years). Here we focus our interest on the fate of mercury and methyl-mercury, as well as the closely linked carbon and sulphur. Mercury continues to be a serious environmental pollutant, with many questions remaining regarding its fate and distribution in the environment.

During the first 20 years after deposition about 20-25 % of the carbon is lost from newly deposited sediment due to diagenetic processes, during this process the C/N atomic ratio also changes from ~9.5-10 to ~11-13. Although carbon is lost total sulphur and mercury concentrations are virtually unaffected. The unchanged sulphur concentrations and the changing C/N-ratio support the idea that the loss of carbon is due to degradation of autochthonous algae material (low C/N), and that old organic material, i.e. humic substances from the catchment (high C/N) to which the main part of the mercury is bound, is unaffected during early diagenesis.

In contrast to total mercury, methyl-mercury decrease with time in the sediment, i.e. de-methylation occurs. The declining methyl-mercury concentrations can be due to either mineralization, i.e. transformation of methyl-mercury to inorganic mercury which is retained in the sediment, or a result of methyl-mercury loss from the sediment to the water column. Since the 1 ng g^-1 change in methyl-mercury concentration is within the analytical error of the total mercury analysis we can not say whether methyl-mercury is mineralized or lost.