Mercury concentration and solid phase speciation changes in the course of early diagenesis in marine coastal sediments (Southern Baltic Sea)

On reaching the sea, mercury discharged into the environment is deposited on and buried in marine sediments. Diagenetic processes in sediments lead to changes in both mercury concentration and speciation; they may enhance mercury bioavailability and/or lead to mercury remobilisation. Total concentrations and speciation of mercury were measured in dated sediment samples from sites covering a range of different environmental conditions and mercury concentrations. In the course of 200 year-long diagenetic processes, the dominant changes in mercury speciation have involved diffusion of labile mercury to the overlying water, transformation of organic-bound mercury to HgS under anoxic conditions or to insoluble humin-bound Hg under oxic conditions, and dismutation of HgS into soluble polysulfides under hyperanoxic conditions. Rate constants of labile-to-stable mercury species transformations and the return flux of mercury were calculated on the basis of sediment core geochronology obtained by the Pb-210 method. Although the study was performed in a specific area, we believe that the observed processes are valid for other shallow, coastal seas and that the results enable mercury stability in sediments to be assessed in similar areas worldwide.