A 400-year record of environmental change in an euxinic fjord as revealed by the sedimentary biomarker record

A 400-year sedimentary record of the euxinic Kyllaren Fjord was investigated using organic biogeochemical, geochemical and sedimentological approaches. Accumulation rates of a suite of biomarkers, and in some cases their stable carbon isotopic composition, were determined. Biomarkers were separated into three different groups: terrestrial derived, general aquatic/marine derived, and aquatic/marine derived but related to high primary productivity. Variations in bulk sediment parameters and biomarker accumulation rates with depth were used to interpret the sedimentary record. We infer that in the first half of the 19th century an environmental change occurred, caused by a change in either the precipitation regime, vegetation and/or land use. The data also show that in the last century, and especially in the last decades, a distinct increase in primary productivity occurred. Most likely this is caused by a ‘natural’ eutrophication due to the transformation of the fjord from an open to a confined system by the building of a partially open dam in 1954, possibly combined with anthropogenic eutrophication. A concurrent shoaling of the fjord chemocline may have had a mediating effect in the change towards a higher productivity by enhancing nutrient and carbon recycling. Enhanced carbon recycling is reflected by decreased δ13C values of several biomarkers.