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Spatial distribution of benthic foraminiferal stable isotopes and dinocyst assemblages in surface sediments of the Trondheimsfjord, central Norway

Authors

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

 

First Author affiliation

EPOC

 

Journal

Biogeosciences

10, 4433-4448, 2013

 

Abstract

Instrumental records from the Norwegian Sea and the Trondheimsfjord show evidence that changes of bottom water temperature and salinity in the fjord are linked to the salinity and temperature variability of the North Atlantic Current (NAC). Changes in primary productivity and salinity in the surface and intermediate water masses in the Trondheimsfjord as well as the fjord sedimentary budget are mainly driven by changes in riverine input. In this study we use 59 surface sediment samples that are evenly distributed in the fjord to examine whether dinocyst assemblages and stable isotope ratios of benthic foraminifera reflect the present-day hydrology and can be used as paleoceanographic proxies. In general, modern benthic δ18O and δ13C values decrease from the fjord entrance towards the fjord head with lowest values close to river inlets. This is essentially explained by gradients in the amounts of freshwater and terrigenous organic matter delivered from the hinterland. The distribution of benthic δ13C ratios across the fjord is controlled by both the origin (terrigenous vs. marine) of organic matter and local topography-induced variability in organic matter flux at the water-sediment interface. The dinocyst assemblages display the variations in hydrography with respect to the prevailing currents, the topography, and the freshwater and nutrient supply from rivers. The strength and depth of the pycnocline in the fjord strongly varies seasonally and thereby affects water mass characteristics as well as nutrient availability, temporally creating local conditions that explain the observed species distribution.

Our results prove that dinocyst assemblages and benthic foraminiferal isotopes reliably mirror the complex fjord hydrology and can therefore be used as proxies of Holocene climatic variability.

 

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