Natural vs. human-induced facies changes in recent,shallow fjord sediments of the Store Lungegårdsvannet in Bergen (western Norway)

A basin-wide change from an older homogeneous bioturbated to a younger laminated facies occurs in sediments from the 26-m-deep, permanently anoxic Store Lungegårdsvannet fjord. The younger facies consists of alternating packages of light and dark layers, each containing millimetre-scale laminae. The composition of the dark layers differs from that of the light layers in the occurrence of predominantly marine benthic diatoms, larger grain size, lower loss-on-ignition (LOI) and lower trace metal (Cu, Pb, Zn) concentrations. Total trace metal concentrations correlate with the redox-sensitive trace elements Mo and V (r>0.8) and with LOI (r=0.6). The correlation of the minerogenic matter and the grain-size related elements Al and Li is poor (r=0.2). The shoaling of the sill depth and reduction of the fjord area resulting in the reduction of bottom water exchange accounted for the shift from bioturbated to laminated sediments during the end of the last century. The alternating changes within the laminated facies are the result of alternating wind forces. During years of calm wind conditions (1908-1919 and 1945-1961), undisturbed sedimentation of organic matter from primary production and an associated accumulation of trace metals prevail. Years of high wind force (1919-1945 and 1961-1991) trigger shore erosion and an increased influx of minerogenic matter.