Sedimentary processes in the middle Nazaré Canyon

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Abstract

Nazaré Canyon extends from a water depth of 50 m near the Portuguese coast to 5000 m at the edge of the Iberian Abyssal Plain. The system is not connected to a modern river and instead obtains its present day sediment input by capture of along-shelf sediment transport. Much of this sediment is deposited in the middle canyon between about 2700 and 3800 m. However, the middle canyon is a highly heterogeneous environment, with areas of both high and low sedimentation rates, exposed rock outcrop, erosion and stable and unstable slopes in close juxtaposition. This paper explores how the various sedimentary processes interact to create the observed heterogeneous canyon environment, which will influence benthic biodiversity in the canyon. Seafloor heterogeneity is investigated using a nested approach to data interpretation, using local high-resolution data to calibrate regional lower resolution data. Six different data types, ship and ROV-mounted swath bathymetry, 30 kHz sidescan sonar images, sediment cores, seafloor video/photographs and current metre/acoustic backscatter data, were incorporated into the analysis. The main morphological characteristic of the middle canyon is a narrow, steep-sided, axial channel flanked by gently sloping terraces. Small-scale landsliding, active at the present day, is the main process that exposes a variety of substrates, ranging from semi-consolidated Holocene sediments to rock of probable Mesozoic age, on the steep axial channel walls. The axial channel floor is characterised in part by large-scale sediment bedforms and in part by landslide debris, suggesting some reworking of landslide debris by currents within the channel. The terraces are interpreted as inner levees with high sedimentation rates. Cores show a dominantly muddy sequence interrupted by thin turbidite sands emplaced on decadal to centennial timescales. The fine-grained sedimentation is the product of continuous settling from fine-grained flows that range from gravity currents to lateral advection of nepheloid layers. The close proximity of areas of high sedimentation and erosion creates a highly heterogeneous seafloor, with the highest heterogeneity on the steepest slopes.

Introduction

Submarine canyons are a characteristic feature of continental margins worldwide. Their formation, morphology and development reflect a dynamic equilibrium between erosion and deposition, driven by tectonics, terrestrial erosion and sediment input, regional oceanography and sediment transport (e.g. Shepard, 1981). The broad-scale morphology and overall sedimentary processes of several canyon systems have been described extensively in the recent literature (Greene et al., 2002, Lastras et al., 2009, Mountjoy et al., 2009). However, due to the lack of detailed high-resolution information (metre-scale), such large-scale studies inevitably miss the true complexity of the deep-sea canyon terrain. In this way they overlook essential sedimentary processes that shape the microenvironments and small-scale sedimentary regimes that influence the distribution of benthic fauna, and together form the canyon system.

This paper documents the sedimentary features and processes that characterise the middle reaches of Nazaré Canyon, offshore Portugal, between 2700 and 3800 m water depth (Fig. 1, Fig. 2). This part of the canyon is a highly heterogeneous environment, with areas of high and low sedimentation rates, exposed rock outcrop, erosion, stable and unstable slopes in close juxtaposition. A unique approach, combining 5 datasets ranging in spatial scale from video observations to ship-board bathymetry, allows the identification of the role of local sedimentary processes within the regional morphological context. The main aim of the paper is to explore how the various sedimentary processes interact to create the observed canyon environment, and to demonstrate how, in combination, they may influence benthic biodiversity in the canyon through the creation of increased habitat heterogeneity.

Section snippets

Study area

Nazaré Canyon is the longest submarine canyon on the western Iberian margin, extending over 200 km from a water depth of about 50 m near the Portuguese coast to 5000 m at the edge of the Iberian Abyssal Plain (Fig. 1A). It is not connected to a modern drainage system on land and its location appears to be related to a fault crossing the margin (Vanney and Mougenot, 1990). Present day sediment input is dominated by capture of sediment transported along the continental shelf (Oliveira et al., 2007).

Material and methods

Understanding how a complex and heterogeneous environment works requires integration of local scale but high-resolution data that capture detail with regional scale but lower resolution data to give insights into the overall character of the system. This ‘nested’ approach uses localised data inputs, such as sediment cores or seafloor photographs, to calibrate more regional geophysical mapping datasets, such as swath bathymetry maps and sidescan sonar images (Masson et al., 2010). In this case,

Results

The most detailed study of canyon processes, based on a high-resolution swath bathymetry survey and several ROV video transects, was undertaken between 3500 and 3600 m water depth (Fig. 4). A number of distinct seafloor environments can be recognised in this topographically and sedimentologically heterogeneous area. These include the canyon axial channel with its steep slopes and relatively flat axial channel floor, plus a series of adjacent terraces.

Origin and maintenance of the axial channel

The axial channel walls expose strata ranging in age from probable Mesozoic rocks to semi-consolidated late Holocene mud and it seems likely that the canyon morphology is the product of multiple phases of erosion and deposition, with the present day axial channel and flanking terraces being the manifestation only of the most recent phase. The overall morphology of the middle canyon, with a narrow axial channel flanked by terraces (Fig. 2, Fig. 3), could suggest that the axial channel is a

Conclusions

A unique nested dataset of the middle Nazaré Canyon offshore Portugal provides new insights into the sedimentary environments and processes in submarine canyons, acting at spatial scales ranging from metres to tens of kilometres.

The middle Nazaré Canyon is a late Holocene sediment depocentre. The main morphological elements of the middle canyon are a narrow axial channel, up to 140 m deep, flanked by gently sloping terraces. The steep axial channel walls expose a variety of substrates ranging

Acknowledgments

Support by the HERMES (EC contract GOCE-CT-2005-511234) and HERMIONE (EC contract 226354) projects, funded by the European Commission, is gratefully acknowledged. Veit Hühnerbach is thanked for the processing of the TOBI sidescan sonar data. The Isis ROV Team and the officers and crew of the RRS James Cook are thanked for their support during data acquisition on cruise JC10. The officers and crew of the R.V. Pelagia are thanked for their support during TOBI surveys and deployment and recovery

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