Geochemistry of coastal Baltic sediments : processes and sampling procedures

Abstract: Quantitative and experimental studies on Recent sedimentary processes, with particular reference to element dynamics and mass transport, were conducted in a coastal area of the north-western Baltic proper. A five-year geochemical study reports sediment trap catches, and estimates the flux dynamics of resuspended sediments and primary settling matter. Considerable variations within and between years, seasons and nearby stations were revealed for gross sedimentation rate, and for the ratio between primary settling matter and resuspended sediment. This pattern also applied to carbon. Resuspended matter was found to be a large item in the annual budget for all years and stations. In fact, the resuspended portion commonly exceeded 50 %, and periods without resuspension seem rarely or never to occur in the investigated area. Dominance of primary settling matter was recorded mainly in spring. These findings illustrate problems that may be caused by the use of coastal marine data onsettling flux uncorrected for the resuspended portion. These conclusions are based on analyses of petrogenic elements, and their use as markers, to derive quantitative estimates of the origin of the intercepted matter. A second geochemical paper also deals with petrogenic elements, and illustrates the importance of considering natural geochemical variation in sediments when assessing changes in metal loading. Conclusions drawn in these two papers may have implications for much geochemical, ecological and environmental research in coastal areas. The credibility of these geochemical conclusions depend critically on the use of reliable and dependable sedimentological field sampling techniques. The methods used were selected after extensive evaluations of available methods and instruments. These evaluations include an insitu experiment on the dependence of the orifice area of the collecting tubes on sediment trapping, and studies on soft bottom sediment sampling, and the biases caused by coreshortening, redistribution, resuspension and loss of enclosed sediments. Newly designed instruments for acquisition of sediments and a new sediment trap system are described. The methodological evaluations also involve two review papers, one on the sediment trap technique and one on quantitative sampling of soft-bottom sediments.