Holocene environmental change and the accumulation–erosion balance of sheltered river-mouth sediments, Göteborg, SW Sweden

Holocene sedimentological change was investigated in 15 sediment piston cores (250 cm long) from the mouth of the Göta älv River, outside of Göteborg, which is sheltered by an archipelago. The main objective was to interpret recent, natural and human-induced influences upon the accumulation–erosion balance and the sedimentary environment from Holocene sediment units. The five main units are: consolidated glaciomarine clay (Facies gC) lowermost, consolidated estuarine clay (Facies dC), soft sub-recent sediment (Facies omC and Facies C) uppermost and dumped sediment (Facies D) in some parts of the study area. The sediment facies are divided by two major hiatuses: (a) the early Holocene hiatus between Facies gC and dC involves a time gap of at least 7000 years, ending at ca. 4000 BP and (b) the late Holocene hiatus between the consolidated clays (Facies gC and dC) and Facies omC spans 1000 years and ends ca. 50 BP (i.e. 100 years ago). Both hiatuses probably relate to the effects of isostatic land uplift. Surface sediments consist mainly of an organic-matter-rich clay facies, suggesting changes in the trophic state of the estuary during the last 100 years. Together with human-induced increased river erosion and loss of accommodation space in the estuary, this caused the onset of sedimentation in the estuary. The recent sediments are contaminated with heavy metals (Cu, Zn, Pb, Hg), with highest concentrations in the lower part of Facies omC.