Spatial Distribution of Benthic Foraminiferal
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EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Biogeosciences, 10, 4433–4448, 2013 Open Access www.biogeosciences.net/10/4433/2013/ Biogeosciences doi:10.5194/bg-10-4433-2013 Biogeosciences Discussions © Author(s) 2013. CC Attribution 3.0 License. Open Access Open Access Climate Climate of the Past of the Past Discussions Spatial distribution of benthic foraminiferal stable isotopes and Open Access Open Access dinocyst assemblages in surface sediments of the Trondheimsfjord,Earth System Earth System central Norway Dynamics Dynamics Discussions G. Milzer1, J. Giraudeau1, J. Faust2, J. Knies2, F. Eynaud1, and C. Rühlemann3 Open Access 1Université de Bordeaux, CNRS, UMR5805 EPOC, Talence, France Geoscientific Geoscientific Open Access 2Geological Survey of Norway (NGU), Trondheim, Norway Instrumentation Instrumentation 3 Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany Methods and Methods and Correspondence to: G. Milzer ([email protected]) Data Systems Data Systems Discussions Open Access Received: 4 February 2013 – Published in Biogeosciences Discuss.: 26 March 2013 Open Access Revised: 22 May 2013 – Accepted: 23 May 2013 – Published: 1 July 2013 Geoscientific Geoscientific Model Development Model Development Abstract. Instrumental records from the Norwegian Sea and thic foraminiferal isotopes reliably mirror the complex fjord Discussions the Trondheimsfjord show evidence that changes of bot- hydrology and can be used as proxies of Holocene climatic Open Access tom water temperature and salinity in the fjord are linked variability. Open Access to the salinity and temperature variability of the North At- Hydrology and Hydrology and lantic Current (NAC). Changes in primary productivity and Earth System Earth System salinity in the surface and intermediate water masses in the Trondheimsfjord as well as the fjord sedimentary budget are 1 Introduction Sciences Sciences mainly driven by changes in riverine input. In this study Discussions Open Access we use 59 surface sediment samples that are evenly dis- The North Atlantic Current (NAC) is the prolongationOpen Access of the tributed in the fjord to examine whether dinocyst assem- Gulf Stream and transports warm and saline water north- Ocean Science blages and stable isotope ratios of benthic foraminifera re- ward along the NorwegianOcean continental Science margin (Ganachaud flect the present-day hydrology and can be used as palaeo- and Wunsch, 2000). This poleward NAC flow essentially Discussions ceanographic proxies. In general, modern benthic δ18O and contributes to the deep water formation in the Nordic Seas δ13C values decrease from the fjord entrance towards the and strongly affects the northern European climate (Vellinga Open Access fjord head with lowest values close to river inlets. This is es- and Wood, 2002). The hydrology of NorwegianOpen Access fjords, ad- sentially explained by gradients in the amounts of fresh wa- joining the Norwegian Sea, is equally linked to the NAC and Solid Earth ter and terrigenous organic matter delivered from the hinter- the Norwegian Coastal CurrentSolid (NCC) Earth (Sætre, 1999; Eilert- land. The distribution of benthic δ13C ratios across the fjord sen and Skarðhamar, 2006; Sætre, 2007). Many fjords have Discussions is controlled by the origin (terrigenous vs. marine) of or- a bedrock threshold at the entrance causing a complex hy- ganic 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,Open Access wind dynam- Open Access ter degradation. The dinocyst assemblages display the varia- ics, and riverine input (Inall and Gillibrand, 2010 in Howe et The Cryosphere tions in hydrography with respect to the prevailing currents, al., 2010). Additionally,The fjords Cryosphere are semi-enclosed basins and Discussions the topography, and the freshwater and nutrient supply from act as natural sediment traps with high accumulation of ma- rivers. The strength and depth of the pycnocline in the fjord rine and terrigenous sediments. As a consequence fjord sed- strongly vary seasonally and thereby affect water mass char- iments bear a combined signature of marine, continental and acteristics as well as nutrient availability, temporally creating atmospheric environments and processes. Norwegian fjords local conditions that explain the observed species distribu- are therefore excellent locations for high resolution studies tion. Our results prove that dinocyst assemblages and ben- of the temporal variations in the physico-chemical character- istics of the NAC and NCC as well as the regional climate Published by Copernicus Publications on behalf of the European Geosciences Union. 4434 G. Milzer et al.: Benthic foraminiferal stable isotopes and dinocyst assemblages (Cage and Austin, 2008; Alve et al., 2011). The objective of ever, more sensitive to post-depositional effects than others, this study is to examine if micropaleontological proxies de- and bottom and pore water oxygenation may alter the origi- riving from sediments from the Trondheimsfjord reflect the nal cyst assemblage (Zonneveld et al., 1997, 2007). main elements of present-day oceanography and hydrology. Hence, in the following we will investigate benthic stable The motivation for this examination implies the question to isotopes and dinocyst assemblages across this particular fjord which extent these proxies from fjord sedimentary archives environment with regard to the hydrological conditions and can be used to determine changes in oceanographic regimes sediment characteristics. in the recent past (Holocene). In order to grasp the complex hydrological and environmental patterns of the bottom and surface waters in the Trondheimsfjord we analysed the stable 2 Physical settings and oceanography isotope composition of the benthic foraminifera Melonis bar- leeanus and organic-walled dinoflagellate cyst assemblages The following oceanographic and hydrological overview is from 59 surface sediment samples evenly distributed along mainly based on Jacobson (1983) which contains a detailed the fjord’s axis. description of the geomorphology and the main circulation Stable isotope ratios were analysed to determine bottom processes affecting the hydrology of the Trondheimsfjord. water characteristics and sediment dynamics in the fjord. Moreover, we used monthly hydrographical data and mea- Oxygen isotopes in calcite tests are conventional tools for surements from three mooring stations as well as CTD mea- the reconstruction of the past climate since the calcification surements conducted in April 2011 (Fig. 2) to further illus- processes are considered to occur in close equilibrium with trate the fjord hydrological context. the surrounding sea water (e.g. Urey, 1946; and Shackleton The Trondheimsfjord is located at the coast of mid Norway and Opdyke, 1973). The foraminiferal 18O/16O isotope ratio extending from 63◦400 N, 09◦450 E at Ørland to 64◦450 N, (δ18O) denotes the isotopic composition of the water linked 11◦300 E at the fjord head at Verdal (Bøe et al., 2003) (Fig. 1). to the salinity and temperature, in which the foraminifera has The Beitstadfjord adjoins the Trondheimsfjord at Skarnsund been ultimately living (e. g. Urey, 1946; Epstein et al., 1953; 63◦500 N, 11◦040 E and extends until Steinkjer at 63◦000 N, Shackleton and Opdyke, 1973; and Bemis and Spero, 1998). 11◦280 E. The Trondheimsfjord is divided into two basins, In coastal environments, however, the relative impact of the the Seaward Basin and the Middle Fjord basin, which are to- freshwater δ18O related to the source increases with increas- pographically separated from the Norwegian Sea and from ing proximity to the shore (Ingram et al., 1996; Scheurle each other by bedrock thresholds (Figs. 1, 2a). Stjørnfjord and Hebbeln, 2003). The carbon isotope ratio in benthic and the Seaward Basin are separated from the Norwegian foraminifera is mainly linked to the influence of deep wa- Sea by a broad and shallow sill at the fjord entrance rising ter ventilation, to the origin (marine vs. terrigenous) and the to 195 m water depth (Fig. 2a). The Tautra Sill with a mini- amount of organic matter deposited at the sediment surface, mum water depth of 98 m separates the Seaward Basin from and to remineralization processes in the sediment (e.g. Car- the Middle Fjord (Fig. 2a). The innermost fjord, the Beitstad- alp 1989; Schmiedl et al., 2004; and Fontanier et al., 2008). fjord, is separated from the Trondheimsfjord by the Skarn- The stable isotope ratio incorporated in the calcite shell may sund Sill with a minimum water depth of 140 m (Fig. 2a). thus considerably deviate from the isotopic ratio of the bot- The deepest part in the fjord with a maximum water depth tom water due to fractionation effects, the microhabitat in of 620 m is located in the narrow passage between the fjord which the foraminifera lives and calcifies, and the geochem- entrance and Røberg in the Seaward Basin. In the follow- ical composition