Contourites of the Nova Scotian continental rise and the HEBBLE area

Abstract

The Nova Scotian continental rise is swept by a Deep Western Boundary Current system comprising layers of Labrador Sea Water overlying a core of Norwegian Sea Overflow Water at depths of 3100-3900 m, and below about 4600 m a cold stream of southern-source water. Seismic-reflection data show that the rise contains sediments transported downslope in channels and debris lobes, but there is also evidence of current-controlled deposition and erosion in the post-Eocene sequence. The rise is now mantled by Holocene contourites that have accumulated at a rate of c. 6 cm ka -1 and are <1 m thick. Bottom photographs show a zonation in current effects and bedform types, with longitudinal ripples and strong currents prevalent at 4800-5000 m, smaller bedforms and progressively weaker currents up to c. 4000 m, and mostly tranquil seafloor above 4000 m. Bedform scales and orientations also suggest significant short-term (hours to weeks) variability in current velocity but a mean contour-following flow to the southwest at longer time scales (months to years). These structures are not preserved in the sediment because of pervasive bioturbation and the uppermost layers have negligible preservation potential. The sediments display clear current controlled effects in their grain-size structure involving both percentage of (foraminiferal) sand, and size and percentage in the 10-63 pm range, the 'sortable silt'. There is a sand-rich zone at 4800-4900 m and below 5000 m, and a decreasing silt/clay ratio from 5100 m up to 4000 m. Although much of the sedimentary sequence probably has been emplaced by downslope processes, it has been significantly modified by the Deep Western Boundary Current. Particularly strong and variable currents which rework sediments below c. 4800 m probably are engendered by interaction of Gulf Stream eddies with the DWBC. Although strong currents and upstream input from turbidites and debris flows might be thought to favour a coarse-grained deposit, the facies at the HEBBLE (High Energy Benthic Boundary Layer Experiment) site is muddy contourite with < 12% sand.This was to have been Charley Hollister's paper. He led many cruises to the HEBBLE area between 1979 and 1986, and insights gained from studies of the area are an important part of his scientific legacy. We might have expected to have to help out with production of this paper via some last minute cajoling, but we certainly did not expect the sad task of doing it without him. So this is the best we can do, without his insight, and we dedicate it to Charley's memory.

Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes

“…Similarly, deepwater deposits can be subject to energetic physical conditions (e.g., western boundary current regions; McCave et al, 2002), sediment reworking, and seasonal input of surface water-derived organic inputs, although physical disturbance is far less common in deep than shallow water. Seasonal increases of organic matter supply or distinct pulses of organic matter to the seabed are expressed in temporarily enhanced production of metabolic products, such as the production of CO 2 (Figure 12).…”

Geophysical evidence for widespread Cenozoic bottom current activity from the continental margin of Nova Scotia, Canada

“…Two regional structural features influence Cenozoic depositional patterns along the outer Scotian margin (McCave et al, 2002) . The first is the Jurassic Abenaki carbonate bank that flanks the outer LaHave Platform (Fig.…”

“…Schmitz and McCartney (1993) propose a second anticlockwise deep gyre north of the Gulf Stream and south of the continental slope that would serve to enhance a DWBC along the Scotian margin. Periodic crossing of the HEBBLE site by warm-cored Gulf Stream rings (clockwise meso-gyres, 200 km across) caused flow reversals and enhancements, and were implicated as the cause of benthic storms in the area (Hollister and McCave, 1984;Weatherly and Kelley, 1985;McCave et al, 2002). Pickart et al (1999) describe the slope water system, a narrow filament of east flowing water resulting from the minor bifurcation of the Gulf Stream south of Nova Scotia, which acts as a coupled system with the westward flowing DWBC adjacent to the continental margin.…”

“…However, Piper et al (1987); Hughes Clarke et al (1992); Mosher et al (2004) among others demonstrate that the Pleistocene geological record is dominated by down-slope processes. Additionally, a number of studies report that pre-Quaternary sediment drifts are absent or of limited geographical extent compared to the continental margin to the south and north, although erosional features are recognized on the continental rise (Swift, 1987;McCave and Tucholke, 1986;Ebinger and Tucholke, 1988;Faugères et al, 1993;Faugères et al, 1999;McCave et al, 2002;Faugères and Stow, 2008) (Fig. 1a).…”

The economic importance of contourites

“…The gyres can develop independently of the water masses involved (Fig. 4) and at any water depth, as demonstrated by the results of the HEBBLE project (McCave 1976;Nowell et al 1982McCave et al 2002), and also recently exemplified by the break-up of both the Atlantic Deep Western Boundary Current and the North Brazil Current into several eddies along their path in the eastern and equatorial South America margins (Dengler et al 2004). Thus, alternation between current acceleration and deceleration along its path is expected, as well as short-term reversals in its sense of flow.…”