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Geology

From the beach to the abyss: A sand grain’s journey at La Réunion Island

Article:  Babonneau, N., Delacourt, C., Cancouët, R., Sisavath, E., Bachèlery, P., Mazuel, A., Jorry, S.J., Deschamps, A., Ammann, J., Villeneuve, N., 2013.  Direct sediment transfer from land to deep-sea: Insights into shallow multibeam bathymetry at La Réunion Island.  Marine Geology 346, 47-57.  http://dx.doi.org/10.1016/j.margeo.2013.08.006

La Réunion Island is a volcanic island located ~750km East of Madagascar in the Indian Ocean (Figure 1).  It is structurally and volcanically very similar to the big island of Hawaii, and is considered very active with frequent eruptions.  Volcanic islands, especially those in tropical climates, tend to have rapid rates of erosion due to the unconsolidated nature of volcanic materials.  La Réunion Island is no exception with plentiful sediment being transported by the island’s river systems.  A typical sand grain will be transported by a river system downslope and is eventually deposited in a river delta in the shallow coastal ocean.  Waves, tides and currents over the shallow continental shelf redistribute sediment grains, but largely act as a trap, preventing sediment from spilling into the deeper ocean.  However, a grain of sediment can escape this trap by hitching a ride on a turbidity current.

Figure_1bFigure_1a

Figure 1. La Réunion Island, ~750km east of Madagascar (left).  Satellite imagery of La Réunion Island (right).

Turbidity currents are dense currents of sediment mixed with water traveling rapidly through the ocean.  Estimation from the Grand Banks example, discussed in more detail below, indicates speeds of 42 MPH (19 m/s), (Piper et al., 1988).  The higher density of the turbidity current compared to the surrounding ocean confines the current to the ocean floor, in which the current travels downslope due to the force of gravity.  Check out these great laboratory examples of turbidity currents to help visualize this type of flow Experiment 1, Experiment 2.  Like a river spilling into the ocean and forming a delta, turbidity currents cut channels and canyons in the continental shelf, slope and rise, dumping sediment in the deep ocean into fan-shaped deposits (Figure 2).

Figure_2

Figure 2. Cross-section of a continental margin. 

Turbidity currents have been observed in nature, as well as through laboratory and computer modeling.  They are unpredictable in nature, and can be destructive.  The famous (well, famous to those studying continental margins, or perhaps underwater telecommunications) example would be the 1929 Grand Banks earthquake south of Newfoundland, Canada, which triggered a powerful turbidity current.  The submarine landslide on the continental slope generated a deadly tsunami and the turbidity current wiped out 12 transatlantic telegraph cables (Heezen and Ewing , 1952).

The study by Babonneau et al. focuses on the land-sea continuity between the Saint-Etienne River (land), Saint-Etienne and Pierrefonds Canyons (shallow to deep sea) (Figure 3, panel B) and the Cilaos Fan (deep sea fan deposit) (Figure 3, panel A) on the SW side of La Réunion Island.       Figure_3a Figure_3b

Figure 3. Location of the Cilaos Fan, SW of La Réunion Island (Panel A).  Saint-Etienne River; Saint-Etienne and Pierrefonds Canyons (Panel B). 

More specifically, the study uses submarine mapping techniques to help understand the turbidity current system feeding the Cilaos fan, one of the largest volcanic turbidity current systems in the world.  Mapping the ocean is very different from mapping land.  On land, aerial photography would do the job just fine.  Photography in the ocean, especially at depths would be very difficult due to light limitations.  Instead, mapping technology uses sound waves to illuminate the ocean floor, just as a dolphin uses echo-location to “see” distant fish.  Two acoustic mapping techniques were used during this study.  Multibeam sonar allows for mapping the bathymetry (depth) of the ocean floor by sending sound waves to the ocean floor and timing how long it takes to hear the sound return.  Acoustic backscatter is a similar technique, but rather than timing the return of the sound, it is a measure of how strongly the sound is reflected off the bottom.  A resulting map produced by multibeam bathymetry can be seen in Figure 4, panel B, where the color gradient corresponds to different depths.  Figure 4, panel D shows a resulting map from acoustic backscatter where lighter grey corresponds to highly reflective surfaces (hard, rough bottom or flat angle) and darker grey corresponds to low reflectivity (soft, smooth bottom or sloped angle).

 Figure_4

Figure 4. Example of multibeam bathymetry (Panel B).  Example of acoustic backscatter (Panel D).

The multibeam and backscatter data (Figure 5) for this study reveal new information about the canyons in this region.  Based on distinct geological features, the mapped region could be divided into sectors, each with their own unique characteristics.   Additionally, the mapping data can be used to interpret and explain how the canyons in each sector came to be, based off of the observed geological features – pretty powerful conclusions without collecting physical samples.Figure_5aFigure_5b

Figure 5. Three sectors defined by characteristic geology as observed in multibeam bathymetry data (left).  Two distinct units of sediment as observed in backscatter data (right).

1.) Saint Etienne sector canyons

The dominant processes are therefore suggested to be continuation of the processes that formed the river.  This sector features the deepest submarine valley suggesting it is the most active of the sectors.  The size of the bedforms (packages of sediment shaped by erosion and deposition) at the canyon axis are largest in this sector suggesting the highest activity of turbidity currents.

2.) Etang-Salé sector canyons

The canyons in this sector are narrow, shallow incisions through the coastal bar.  There is no sediment supplied from the Saint Etienne river, and the dominant process is from storm waves.  The more offshore reaches of these canyons are wider and deeper, interpreted to be further cut by turbidity currents.

3.) Pierrefonds sector canyons

The canyons in this sector begin at deeper depths than the other two sectors, and also appear have less sediment transport.  The lack of activity suggest that this canyon may have been active when the Saint Etienne river’s mouth was at this location in the past.

The study of the Cilaos turbidite system on La Réunion Island utilizes bathymetry and backscatter data to understand what dominant processes generate the submarine canyons in the region.  Furthermore, the study uses the data to interpret ongoing processes which make the Cilaos turbidite system one of the most active systems of its kind.  Understanding the continuation of sediment from land to the deep sea is important due to the inherent hazards as evidenced by the Grand Banks earthquake, tsunami and submarine debris flow.  Large-scale submarine debris flows and turbidity currents have the potential to trigger destructive tsunamis and damage offshore infrastructure.  Broadening scientific knowledge of continental margin geology can only benefit society by identifying and better understanding potential hazards.

References:

Heezen, B.C., Ewing, M., 1952.  Turbidity currents and submarine slumps and the 1929 Grand Banks earthquake.  American Journal of Science 250, 849-873.

Piper, D.J.W., Shor, A.N., 1988.  The 1929 “Grand Banks” earthquake, slump and turbidity current.  GSA Special Papers 229, 77-92.

Links:

Turbidity current lab experiment 1: http://www.youtube.com/watch?v=ZhDQnnONWl4

Turbidity current lab experiment 2: http://www.youtube.com/watch?v=tfNLI2JW7mg

 

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