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Publication and Communication

Hydrological and Climatological Changes in the Trondheimsfjord/Norway during the late Holocene inferred from Benthic Stable Isotopes and Dinocyst Assemblages

Authors

G. Milzer, J.Giraudeau, S. Schmidt, J. Faust, J. Knies, F. Eynaud, C. Rühlemann


First Authors affiliation

EPOC


Event

PAGES 2nd Young Scientists Meeting and 4th Open Science Meeting

Goa, India

11-16 February 2013


POSTER


Abstract

Fjords are semi-enclosed basins surrounded by continental bedrock and are characterized by high sedimentation rates of several mm/yr. The hydrology of Norwegian fjords is linked to the North Atlantic Current (NAC) and the Norwegian Coastal Current (NCC), two major northward flowing sea surface/intermediate depth currents. The comparison of instrumental records from the Norwegian Sea and the Trondheimsfjord suggests that changes of bottom water temperature and salinity in the fjord are related to the NAC variability. Variations in primary productivity and salinity of the surface and intermediate water masses as well as the sedimentary budget in the fjord are driven to a high extent by variabilities in river input and precipitation in the hinterland. We test the use of dinocyst assemblages and stable isotope ratios of benthic foraminifera as proxies of surface/intermediate and bottom water conditions in the Trondheimsfjord, respectively. The calibration of these two proxies against modern and recent (past 60 years) hydrological conditions is based on 60 surface sediment samples that are evenly distributed in the fjord and three multi-cores recovered from locations along the fjord’s axis. The chronology of the multi-cores is based on 210Pb and 137Cs measurements. Regardless of the locations of the surface sediment samples with respect to the river mouths, the modern benthic δ18O ratios and dinocyst assemblages show continuous gradients from the fjord’s entrance toward the innermost basin. Our multi-core time series suggest that the relative influence of the bottom water temperature and salinity on the oxygen isotope signature varies according to the distance of the core location to the fjord entrance and stratification patterns. The dinocyst assemblages clearly record changes of the surface water characteristics and nutrient delivery due to river input. Since the benthic δ13C ratios across the fjord vary according to the fjord’s topography and the associated changes in flow speed of bottom waters (winnowing effect), we assume that temporal variations in the carbon isotope ratios at a given location are mainly recording changes in the flux of marine organic matter at the water-sediment interface as well as variable inputs of terrigenous organic matter through rivers. We use this information to reconstruct the paleohydrology and paleoenvironmental conditions in the Trondheimsfjord from a piston-core which covers the last 3175 years. This late Holocene record shows an abrupt shift from lighter to heavier δ18O ratios at 1200 years BP and high amplitude variations from 1000 to 2100 cal. yr BP. This variability is discussed in view of other evidences for changes in surface water physico-chemical and productivity changes as indicated by the down-core distribution of benthic δ13C ratios and dinocyst assemblages.

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