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A Late Holocene High Resolution Study of Paleoceanographic Changes in the North-East Atlantic based on Stable Isotope Data in the Trondheimsfjorden (Norway)


G. Milzer1, J. Giraudeau1, J. Faust2, J. Knies2, S. Schmidt1, C. Rühlemann3

Authors Affiliation

1. EPOC, University of Bordeaux – CNRS, Talence/ France
2. Geological Survey of Norway (NGU), Trondheim/ Norway
3. Federal Institute for Geosciences and Natural Resources (BGR), Hannover/ Germany


AGU Fall Meeting 2011

San Francisco, USA

5-8 December 2011




Important climate-sensitive parameters such as water exchange, freshwater discharge and sedimentary patterns can be quantified at very high resolution in fjord environments as sedimentation rates exceed several mm/yr. The Trondheimsfjorden is located at the west coast of Mid-Norway and is characterized by local environmental and hydrological changes that are tightly reflecting regional oceanographic and atmospheric processes in the Norwegian Sea. The North Atlantic Current (NAC) and the Norwegian Coastal Current (NCC), two major northward flowing sea surface/intermediate currents, strongly contribute to the oceanography of the Norwegian Sea and thus to the hydrological settings of the Trondheimsfjorden. Previous  investigation (Rühlemann et al., 2008; EGU Fall Meeting) suggested that oxygen isotope ratios in shells of the benthic foraminifera Melonis Barleanum of the Trondheimsfjorden record past changes in bottom water density, directly linked to the intensity of the NAC and NCC during the late Holocene. In order to verify this assumption, 60 surface sediment samples, evenly distributed across the Trondheimfjorden, have been obtained and analysed for oxygen isotope ratios of Melonis Barleanum.  We use this “modern” database as well as a composite (multi-core and piston core) sedimentary archive recovered from the center of the fjord in order (1) to test the validity of the oxygen isotope ratio as a proxy of bottom water density, and (2) to provide detailed information on hydrographic changes in the research area throughout the past 3,175 years BP. The stratigraphy of the composite core is based on 10 14C dates as well as on 210Pb and 137Cs measurements, leading to sedimentation rates of 2.5 - 4 mm/yr. The oxygen isotope ratios derived from Melonis Barleanum in the surface sediment samples clearly indicate shifts according to the sample locations in the fjord and can be mainly related to local factors such as distance to the fjord entrance, current patterns as well as river inlets. The late Holocene downcore record shows a trend to heavier d18O values from ca. 2,000 years BP, with high amplitudes changes, followed by a decrease in the stable isotope signal within the last 300 years. We interpret these drastic changes in bottom water density as a reflection of variable dynamics of Atlantic-derived water in the Norwegian Sea, which directly affect the ventilation of the Trondheimsfjorden.

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