Sediment drifts and cold-water coral reefs in the Patagonian upper and middle continental slope
Highlights
► A multidisciplinary study of Patagonian margin is presented. ► We found two generations of sediment drifts, Pleistocene–Holocene in age on the middle continental slope off Patagonia. ► Cold-water coral reefs with relief from 1 to 10 m are located between 300 and 1400 m depth. ► Extensive presence of cold-water corals are scattered over most of the study area. ► Deep water digital photographs document for the first time marine vulnerable habitats.
Introduction
Sediment drifts are formed by contour-following currents generated by thermohaline circulation (Faugères and Stow, 1993, 2008; Viana, 2001). Frequently associated with the sediment drifts, erosional features as terraces, abraded surfaces, channels, moats and furrows are observed in areas where the currents are unusually strong (Hernández-Molina et al., 2010).
We follow in this paper the definition by Faugères et al. (1999; see also Rebesco and Stow, 2001; Faugères and Stow, 2008) who recognized seven types of drift geometry. They are: 1. Sheeted drifts characterized by low relief and large lateral extent; 2. Mounded elongate drifts characterized by moderate to high relief and variable extent and detached elongate to irregular small-scale patch mounds; 3. Channel-drifts that occur within channels/moats, fans at channel mouths and channel levee drifts; 4. Confined drifts in actively subsiding basins or troughs; 5. Infill drifts with moderate relief and relatively small extent infilling the head of slump scars and the margins and toes of slumps; 6. Mixed drifts that include turbidite-sediment drift, debrite-sediment drift, hemipelagic-sediment drift, glaciogenic-sediment drift systems; 7. Fault-controlled drifts generated in response to fault generated basement relief and subsequent syn-depositional fault reactivation.
Current-generated deposits on the Patagonian Argentine continental margin occur from shore to the deep sea. Viana et al. (1998) classified current-generated deposits in depth of less than 300 m as shallow-water bottom-current sands and deeper than 300 m as sediment drifts. The deposits on the 320–500 km wide shelf are the result of a broad northeast flow of cold water that intensifies toward the shelf edge where it merges with the northeast flowing Falkland/Malvinas Current. Sedimentation on the upper and middle continental slope is under the influence of the northeast flowing Falkland/Malvinas Current.
Also associated with areas of accelerated deep-water currents on the Patagonian middle continental slope are cold-water coral reefs. Cold-water coral ecosystems are among the richest biodiversity hotspots in the deep sea, providing shelter and food for hundreds of associated species, including commercial fish and shellfish as in Sula Ridge in Norwegian waters (Fosså et al., 2000), and Gully, off Nova Scotia, Canada (Harrison and Fenton, 1998). In this paper we use the term reef, as defined by Davies et al. (2008), as biogenic structures that influence sediment deposition, provide complex structural habitats and are subject to the process of growth and (bio) erosion. These reefs can be over 40 m high, have lengths of several kilometers and occur at depths of less than 200 m to over 2800 m; mean depths of 468 and 480 m (Friewald et al., 2004; Roberts et al., 2006; Davies et al., 2008).
The first objective of this paper is to describe the internal geometry and potential controlling factors of sediment drifts. The second objective of the paper is to describe the occurrence and distribution of cold-water corals and cold-water coral reefs associated with the Falkland/Malvinas Current.
Section snippets
Seafloor morphology
Hernández-Molina et al. (2010) ascribe the giant, elongated, and mounded sediment drift on the lower slope to a loop of the Antarctic Bottom Deep Water from the Eocene–Oligocene boundary to the middle Miocene during the opening of the Drake Passage. A change in the oceanographic conditions in mid-to late Miocene led to the fossilization of the drifts. Extension of North Atlantic Deep Water circulation into the southern Hemisphere and deepening of Antarctic Bottom Water circulation at that time
Methods
The morphology of the Argentine Patagonian margin was mapped aboard the R/V Miguel Oliver, of the Spanish Secretariat General of the Sea (SGM), in 2007 and 2008. During the cruises Patagonia-1107, Patagonia-1207, Patagonia-0108, Patagonia-0208, Atlantis-0308, Patagonia-1008, Patagonia-1108 and Patagonia-1208, lasting 179 days, 33,984 km2 of the seafloor off Patagonia were mapped from 44°40′S to 47°51′S and 58°52′W to 61°05′W (Fig. 1A). Navigation during the investigation of the Patagonian
Results
The segment of the Argentine Patagonian continental slope investigated during the present study is made up of three topographic elements, an upper, a middle and lower slope. The upper continental slope dips 2° and spans depths of 128/200 m to 250–750 m with its lower part being scarred by iceberg ploughmarks. The middle slope has an inclination of less than 1° and a depth of 250/750 m to about 1600 m, consists of two terraces: the Nágera Terrace extending from the base of the upper slope to a
Spatial distribution and controlling parameters of sediment drifts
Sediment making up the drifts on mid-water depths is generally transported to the site, where it undergoes reworking by bottom currents, off-shelf spillover by wave and tidal processes, slump-slides, debris flows, ice-tongues and turbidity currents (Stow and Mayall, 2000). None of these processes are important today in the Patagonian margin. Faugères and Stow (2008) state that sand drifts in mid-water form extensive sheets on a variety of slope, bank and channel settings. Only the drifts north
Conclusion
Large expanses of the Perito Moreno Terrace on the Patagonian middle slope at a depth of 1200–1600 m are mantled by sediment drifts sculptured out of early Holocene hemipelagics, late Pleistocene ice rafted clastics and fluvial sediments and Neogene/reworked Eocene sediments by the north flowing Falkland/Malvinas Current.
Scattered over the middle and upper slope are cold-water corals forming reefs of less than a meter to several meters in relief. The cold-water corals structures flourished as a
Acknowledgments
We are indebted to the Captain, officers and crew of R/V Miguel Oliver (Secretariat General of the Sea, SGM) for their efforts during the cruises to the Patagonian margin. We also wish to thank SGM, who provided the ship time and technical support and to the members of the Spanish Oceanographic Institute (IEO) who participated in the cruises. Gratitude also is expressed to the Cartography Group SGM-TRAGSA who participated in the different cruises and processed the multi-beam data.
We acknowledge
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Atlantis Group: S. Iglesias, J. Portela, J.L del Río, S. Parra, M. Sacau, R. Vilela, T. Patrocinio, B. Almón, E. Elvira, P. Jiménez, A. Fontán, C. Alcalá and V. López.