Abstract
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 palaeoceanographic 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 fresh water and terrigenous organic matter delivered from the hinterland. The distribution of benthic δ13C ratios across the fjord is controlled by the origin (terrigenous vs. marine) of organic matter, local topography-induced variability in organic matter flux at the water–sediment interface, and organic matter degradation. 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 vary seasonally and thereby affect 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 be used as proxies of Holocene climatic variability.
Highlights
Earth System Sciences mainly driven by changes in riverine input
We assume that the spatial distribution of benthic foraminiferal δ18O primarily reflects the decrease in salinity and δ18O of the water (δ18Ow) with increasing distance from the fjord entrance as a consequence of increasing brackish water supply towards the fjord head
Based on an extensive and evenly distributed set of surface sediment samples we show that the potential proxy indicators analysed in this study reliably record the complex modern hydrological and environmental setting of the Trondheimsfjord
Summary
Earth System Sciences mainly driven by changes in riverine input. In this study we use 59 surface sediment samples that are evenly dis- The North Atlantic Current (NAC) is the prolongation of the tributed 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 palaeoceanographic proxies. Gulf Stream and transports warm and saline water northward along the NorweOgicanecaonntiSnecntiael nmcaregin (Ganachaud and Wunsch, 2000) This poleward NAC flow essentially contributes to the deep water formation in the Nordic Seas and strongly affects the northern European climate Many fjords have is controlled by the origin (terrigenous vs marine) of or- a bedrock threshold at the entrance causing a complex hyganic matter, local topography-induced variability in organic drology linked to the ocean currents in the adjacent sea, as matter flux at the water–sediment interface, and organic mat- well as their salinity and temperature structure, wind dynamter degradation. Our results prove that dinocyst assemblages and ben- of the temporal variations in the physico-chemical characteristics of the NAC and NCC as well as the regional climate
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