Suspended particulate matter, sedimentation and early diagenetic processes in the Kalix River estuary, northern Sweden

Sammanfattning: Many important reactions controlling the transfer of elements from the continents to coastal waters and the oceans are taking place in estuaries. For many elements, estuaries can act as filters, capable of reducing the river load of dissolved and particulate elements to the oceans. Although sedimentation is an important process in estuaries, early diagenetic processes in sediments may be responsible for the remobilization of redox-sensitive elements back into estuarine waters. This thesis focuses on the behaviour of suspended and settling particulate Fe - Mn oxides and organic matter in the stratified, low-salinity (< 3‰) Kalix River estuary. In addition, the early diagenetic cycling of Fe, Mn, organic C and trace metals was also studied. Samples of the dissolved (< 0.45 ?m) and suspended phase were collected along vertical profiles at three stations in the estuary. Settling particulate matter was collected by the use of sediment traps. Pore-water and solid phase sediment data were used to study early diagenetic processes. The estuary appears to be relatively inefficient as a trap for river transported, particulate, oxide- and organic-associated elements. Suspended, slow-settling particles rich in Fe - Mn oxides and organic matter to a large extent are flushed through the estuary in the seaward flowing surface water. Organic matter is an important carrier of Cu, which appears to be due in part to biological uptake of Cu. Flocculation of dissolved Fe, Si and Al is of little or no significance, while Mn is desorbed from suspended particles early in the estuarine mixing of freshwater and brackish water. A study of the varved, undisturbed sediments deposited in the central part of the estuary shows that the deposition of trace metals has decreased since the late 1970's. Despite active post-depositional redox cycling of Fe and Mn, efficient oxidation processes in the surface sediment prevent diffusion of dissolved Fe and Mn back into the water column. Reduction/ dissolution of Fe oxides is a slow process, resulting in burial of Fe oxides in permanently deposited, anoxic sediments. Post-depositional mobilization of trace metals in the bottom sediment is controlled by the type of carrier phase and the redox conditions in the sediment. The post-depositional mobility of trace metals is low - moderate, except for As, which is associated with redox-sensitive Fe oxides. Copper is associated with with labile organic matter, and early diagenetic remobilization of Cu appears to be entirely controlled by oxic decomposition of organic matter close to the sediment - water interface.