Benthic stable isotope variability in the Trondheimsfjord during the last 50 years: Proxy records of mixing dynamics related to NAO

Abstract

Benthic stable isotopes (δ18Oc and δ13Cc) in the Trondheimsfjord reliably reflect the prevailing hydrological conditions with regard to the distance to the fjord entrance and the local topography. At the fjord entrance the δ18Oc and δ13Cc variabilities record changes in the temperature of the North Atlantic Current (NAC), and in the delivery of terrigenous organic matter by the Norwegian Coastal Current (NCC) and the fjord estuarine circulation, respectively. In the inner fjord basins, the benthic stable isotope compositions are predominantly controlled by hydrological characteristics linked to internal mixing and the prevailing wind pattern. The fjord deep basins are relatively sheltered from erosion and both land and marine-derived sediments accumulate at very high rates indicating past regional climate and associated driving parameters. In the present study we discuss the recent variability (last 55 years) of the stable isotope compositions (oxygen and carbon) measured in calcite shells of the benthic foraminiferan Melonis barleeanus with regard to the changing influence of marine versus fresh water inputs and of the prevailing wind patterns. The relative influence of marine versus continental waters were analysed as well as the complex topography and the related hydrology of the Trondheimsfjord using sediment cores collected in three different settings along the fjord axis. The variability the of stable isotope compositions obtained from the sedimentary archives is compared with in situ instrumental measurements over the investigated time interval.