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Publication and Communication

A high resolution study of paleoceanographic changes in the North Atlantic realm during the late Holocene based on benthic stable isotopes and dinocyst assemblages from sediments of the Trondheimsfjord (Mid-Norway)

Authors

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

 

First Author Affiliation

EPOC, France

 

Event

GV and SEDIMENT meeting

Hamburg, Germany

23-28 September 2012

 

Abstract

Due to high sedimentation rates of several mm/yr fjords are excellent locations for high resolution studies of important climatesensitive parameters such as fjord ventilation patterns, freshwater discharge and sedimentary patterns. The Trondheimsfjord is located at the west coast of Mid-Norway. The North Atlantic Current (NAC) and the Norwegian Coastal Current (NCC), two major northward fl owing sea surface/intermediate currents, strongly contribute to the oceanography of the Norwegian Sea and thus, to the hydrological settings of the Trondheimsfjord. Instrumental records show evidence that the renewal of the bottom water of the fjord by Atlantic-derived water masses occurs twice a year and that bottom water temperature and salinity changes can be related to the NAC variability. We present an examination of modern stable isotope data from benthic foraminifera and dinocyst assemblages deriving from 60 surface samples as well as from three multi-cores distributed along the axis of the Trondheimsfjord. These sedimentary archives cover the past 60 years based on sedimentation rates from 2.5 to 7 mm/yr according to 210Pb and 137Cs measurements. The benthic stable isotope ratios refl ect water mass characteristics linked to circulation and stratifi cation patterns in the fjord which are linked to the topography. The benthic δ18O variability on a decadal scale shows a strong link to temperature and salinity variability of the bottom water in the fjord, measured at different mooring stations along the fjord axis. On a multi-decadal scale, benthic δ18O variability varies according to each core location and its particular hydrological settings. Dinocyst assemblages and benthic δ13C on the same sedimentary archives refl ect primary productivity in the fjord and are tracing NAC characteristics. Thus, these associated records allow for a differentiation of either changes in the ventilation of the fjord by nutrient-rich Atlantic water and thus, the NAC strength variability, or changes of water mass characteristics. In order to reconstruct late Holocene paleoclimate variability we applied our proxies on a high resolution sedimentary archive from the centre of the fjord throughout the past 3 175 years BP. The stratigraphy of this piston core is based on ten 14C dates leading to sedimentation rates of 2.5 - 4 mm/yr. The late Holocene record shows high oxygen isotope variability with maximum amplitudes from 1 500 to 2 100 cal. yr BP, followed by a decrease of the oxygen isotope ratio during the last 300 years. The δ13C ratios and the dinocyst assemblages in this sedimentary archive imply a decreasing strength of the NAC towards recent times with short term variations that can be linked to notable European climate events such as the Little Ice Age and the Medieval Warm Period.

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