Quaternary contourite drifts of the Western Spitsbergen margin
Graphical abstract
Introduction
The study of contourite drifts is valuable as a proxy for the climate history of the oceans since these sedimentary deposits typically form along the pathways of major bottom currents (Laberg et al., 2005, Rebesco et al., 2008). They have relatively high and continuous accumulation rates in contrast to adjacent condensed pelagic sequences (Knutz, 2008). Contourite drifts are well-known throughout the world oceans, occurring anywhere from the abyssal floor to outer shelf settings, and particularly along the continental slope where bottom currents are confined by the Coriolis effect (Faugères and Stow, 2008).
Although they show a large morphological variability, most drifts have an elongated, mounded shape and variable dimensions, ranging from small patch drifts (about 100 km2) to giant elongated drifts (>100,000 km2). Contourite drifts are tens to hundreds of km long and from 10 to more than 100 km wide, with sediment thicknesses from some tens to more than 1000 m. Their body is generally elongated, more or less parallel to the continental margin and hence to the contour-current flow direction. Erosion and non-deposition are dominant near the current axis, while deposition occurs further away where the velocity is lower (Faugères and Stow, 2008).
The Fram Strait (Fig. 1) is the only deep opening through which water masses are exchanged between the Nordic Seas and the Arctic Ocean. Atlantic water is advected northward through the eastern Fram Strait in the West Spitsbergen Current. This warm water keeps the large areas west and north of Svalbard (increasingly larger due to present warming) nearly ice-free in winter and thus has implications for the Arctic climate (IPCC, 2007). It is hence of climatologic interest to know how this flow has changed during geological time scales.
Bottom currents have been shown by Eiken and Hinz (1993) to have influenced the sedimentation in the Fram Strait since the Late Miocene. Their study was based on contourites identified west of the Svalbard Archipelago with multichannel seismic profiles from the seventies and eighties, and a correlation to DSDP site 344. More recently, the identification of mounded seismic patterns in the Early Pleistocene sediments off Bellsund Fan (Fig. 2) and on the Vestnesa Ridge (Fig. 1) led Amundsen et al. (2011), Sarkar et al. (2011) to infer that contour-controlled currents on the Svalbard margin were effectively influencing the sedimentation during that period.
In this paper we show two contourite drifts along the continental margin west of the Svalbard Archipelago between 76–78°N. We illustrate their geometry and facies on seismic reflection data directly correlated to ODP Site 986 and their location and extension on bathymetric data. The flow structure and properties of water masses in the area are shown based on oceanographic data including hydrographic sections and a 13 year long time series from moored instruments. The time series displays a local near-bottom velocity maximum in close proximity to the contourites. The aim of this paper is to explain the growth of these contourite drifts in the light of modern oceanography and discuss the palaeoceanographic changes that may be inferred from the seismostratigraphic evidence.
Section snippets
Methods
Acquisition of seismic and geologic data (Fig. 2) was performed during the EGLACOM cruise (R/V OGS-Explora, summer 2008) and the SVAIS cruise (R/V Hesperides, summer 2007) contributing to IPY Activity 367 NICE-STREAMS (Neogene ice streams and sedimentary processes on high-latitude continental margin). The seismic gear included a 1200 m long digital streamer with 96 channels (spaced 12.5 m) and a 160 cubic inches array of sleeve guns. Fold coverage was 24, shot interval 25 m and sampling rate 1 ms.
The West Spitsbergen current in Fram Strait
Previous investigations (e.g. Aagaard et al., 1973, Jonsson et al., 1992) have detailed a continuous inflow of warm and saline Atlantic-derived water to the Arctic on the eastern side of the Fram Strait (Fig. 3), steered by the continental slope as is common for large-scale ocean currents due to Earth's rotation (e.g. Gill, 1982). In the western Fram Strait, Arctic water and recirculating Atlantic Water are flowing southward. The currents in the eastern part have a strong barotropic component
Seismic geometry and morphologic expression
A series of seafloor undulations are visible on the seismic profiles crossing the mid continental slope west of Spitsbergen (Fig. 2). These undulations consist of a broad upward-convex body (mound) whereas the upper slope shallower than about 1200 m depth is characterized by reduced- or non-deposition (Fig. 7, Fig. 8, Fig. 9). The mounds extend for a few tens of km in the dip direction (perpendicular to the shelf edge). They are either without any further deeper depression (Fig. 7), with a
Interpretation of the sediment mounds
The seismic expression of the two sediment mounds identified on the mid continental slope west of Spitsbergen is common to that of many contourite drifts (Faugères et al., 1999, Rebesco and Stow, 2001, Stow et al., 2002, Rebesco, 2005, Rebesco et al., 2008, Hernández Molina et al., 2011a) including previously described drifts in the Norwegian-Greenland Sea (Laberg et al., 1999, Laberg et al., 2002). In fact, both mounds are distinctly upward convex and there is some morphologic evidence that at
Conclusions
Two sediment mounds on the continental slope between 1200 and 1800 m depth west of Spitsbergen are described on the basis of multichannel seismic, sub-bottom, bathymetric and lithologic data. On the basis of their mounded morphology, the along-slope elongation and their internal sigmoidal geometry they are interpreted as contourite drifts. The upper continental slope, swept by the robust and persistent flow of West Spitsbergen Current (averaged velocities of more than 20 cm/s), is characterized
Acknowledgements
This study has been supported by Italian project MELTSTORM (MELTwater plumites and recessional moraines at the STORfjorden trough Mouth during deglaciation of the Barents Sea Ice Sheet) funded by PNRA (Progetto Nazionale di Ricerche in Antartide) and by OGS IPY Project EGLACOM. Participation of RG Lucchi in this study was supported by the Spanish IPY projects NICE STREAMS (CTM2009-06370-E/ANT) and DEGLABAR (CTM2010-17386). The contribution of JS Laberg was supported by the Research Council of
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