Twentieth century proxy records of temperature and soil organic matter input in the Drammensfjord, southern Norway

The Drammensfjord, in southern Norway, is an enclosed anoxic basin with a laminated sedimentary record. The fjord is a transition area between the marine and terrestrial environments and thus its sedimentary archive contains information on both environments. In order to reconstruct recent environmental changes in southern Norway, two sediment cores covering the last 90 years were analyzed. We used the Branched versus Isoprenoid Tetraether (BIT) index as an indicator of terrestrial organic mater input and compared it to the stable carbon isotopic composition of total organic carbon (δ13CTOC). The δ13CTOC and BIT index show opposing trends, with δ13CTOC indicating an increase in terrestrial input while the BIT index suggests a decrease towards the present. This contradiction is probably due to the different sources reflected by δ13CTOC (vegetation, soil and anthropogenic input) and the BIT index (only soil organic matter input). In addition, an increase in the recycling of respired CO2 within the fjord’s water column could have depleted the δ13CTOC of the marine component towards more negative values, thus seemingly indicating an increase in terrestrial input, whilst the BIT index could have been affected by a reduction of riverine input and/or an increase in archaeal biomass production. The TetraEther indeX of lipids with 86 carbons (TEX86) was determined as a proxy for past water surface temperatures. While some of the signal could be coming from the chemocline, our results suggest that TEX86 indicates surface (upper 25 m) temperatures. Moreover, TEX86 temperature patterns seem to follow variations in the historical record of averaged spring to fall Oslo air temperature relatively closely, suggesting the signal is produced during this period. Therefore, TEX86 temperature records of fjord sediment cores can potentially be a useful tool in reconstructing past variations of coastal climates.