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On Réunion Island, the eroding volcano feeds the deep ocean sediments

Réunion Island experiences one of the most active erosion rates in the world. Sediments removed from the volcano are transported by rivers and accumulate in huge fans spread on the bottom of the ocean. Studying these deposits shows that their transport towards the deep ocean is controlled by the activity cycles of the volcano.

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The sediments covering the sea floor are made of calcareous or siliceous muds, whose particles are elements of planktonic organisms or minerals removed from land erosion. The latter are transported in very sudden events of large submarine avalanches generating "turbidity currents" heavily loaded with sediments and flowing along the steep slopes; the materials are finally deposited on the abyssal plain, sometimes very far from their source. These deposits are called "turbidites" and form very large sedimentary fans.

The two factors which generally influence the activity and the intensity of these processes during the Quaternary are sea-level variations and climate (mainly rainfall). When it comes to volcanic islands, the debate about their respective importance is complicated by a third factor, the eruptive activity. New insights about this question are brought by the present study of turbidites discovered recently around Réunion Island.

The island is the emerged part of a volcanic edifice more then 7,000 m high and made of two volcanoes, Piton des Neiges and Piton de La Fournaise. Its submarine flanks are mainly made of blocks and debris coming from catastrophic collapses of the volcanoes, particularly during the major eruptions. Piton des Neiges has been inactive for at least 12,000 years; three major erosional depressions opened in the heart of the volcano are the "cirques" of Cilaos, Mafate and Salazie. Very abundant rainfall, steep slopes and friable rocks make erosion in La Réunion one of the most actives in the world. Materials removed from the volcano are transported along valleys then submarine canyons until the settle on the ocean floor within five large sedimentary fans facing the mouths of the main rivers. The largest one is the Cilaos fan which lies over 250 km at about 4,500 m deep, on 15,000 km²; it is fed by Saint-Etienne river, which flows from the Cirque of Cilaos and brings to the sea 0.5 to 2 million m3 of sediments per year.


Bathymetric and topographic chart showing Réunion Island and the sedimentary fan of Cilaos (red and yellow). The red line in the lower right panel shows the Saint Etienne river watershed, which provides sediments to Cilaos deep-sea fan.



During three oceanographic cruises, five sediment cores were extracted from Cilaos fan, measuring 3.40 to 6.70 m in length. Analyses of sedimentology (description, grain-size distribution,...), physics and chemistry (calcium, oxygen isotopes,…) , as well as 14C dating, were conducted on samples taken from these cores.

Three of the five cores showed the same sediment sequence:
- at their base, over about 3 m, 30 to 50 cm-thick alternating layers of sand coming from erosion (turbidites), separated by clay layers (U1);
- at their top, over about the same height, clay layers alternatively light and dark according to their calcareous (planktonic) or siliceous (mineral) origin (U2).
In a fourth core drilled in the Cilaos fan, a sandy deposit (U3) is also found. These three units are recognizable on the sediment echosounder profiles.


Lithological log correlated with the corresponding echosounder profile, grain‐size curve and X-ray image of core KERO09.

The results on oxygen isotopes, radiocarbon dating and presence or absence of some planktnic species show that the sedimentary history covered by these cores extends over 130,000 years (130 ka, the approximate duration of a Quaternary climatic cycle).

During the course of this history, the Cirque of Cilaos went through three phases of sedimentation, corresponding to the sedimentary units identified in the cores. At the base, U1 (-186 to -128 ka) was deposited by large turbidity currents extending over the whole fan. The second phase (-128 to -26 ka) then left the thick layer of planktonic-origin clay U2, which is present in all cores, without signs of turbidity currents. And finally, during the most recent period, (since -30 ka), U3 is the result of the activity of large turbidity currents in the least remote part of the fan.

All these results make it possible to better understand the respective role of the three main factors controlling the input of sediment to the ocean and their offshore transport in volcanic islands.

Sea level never played an important role in Réunion Island as turbiditic activity took place at different levels. As this activity does not seem to have occurred during periods of maximum rainfall, climatic variations could also be discarded. But the two main periods of turbiditic activity coincided with periods of low effusive volcanic activity of the Piton des Neiges. The decrease of volcanic production enabled the formation of the Cirques through erosion, thence the increase of the input of sediments to the sea. Conversely, the interruption of turbiditic activity between -127 and -30 ka coincided with a resumption of the effusive and explosive activity of the Piton des Neiges volcano, with large lava flows that filled the cirques and their drainage valleys, thereby reducing both erosion and sedimentary input to the sea.

As a result, the volcanic activity appears to be a major controlling factor influencing turbidite development of the Cilaos deep-sea fan.


The three phases of volcanic and turbiditic activity in the Cilaos area: low volcanic activity thence erosion and transport to the sea before 127 ka (1) and after -30 ka (3), high volcanic activity thence limited erosion and transport to the sea between -127 and -30 ka.


The paper

Sisavath E., Mazuel A., Jorry S.J., Babonneau N., Bachèlery P., de Voogd B., Salpin M., Emmanuel L., Beaufort L., Toucanne S., 2012. Processes controlling a volcaniclastic turbiditic system during the last climatic cycle: Example of the Cilaos deep-sea fan, offshore La Réunion Island. Sedimentary Geology 281 : 180–193.
See the first page


The authors

This work was conducted by researchers from Domaines Océaniques laboratory of IUEM in collaboration with their colleagues of Ifremer (Laboratoire Environnements Sédimentaires) and of five other French laboratories (Univ. La Réunion, Univ. Clermont-Ferrand, Univ. Pau, IPGP, Univ. Aix-Marseille)


The journal

Sedimentary Geology is an international journal of fundamental and applied sedimentology, published by Elsevier. It covers all aspects of research on sediments and sedimentary rocks, at all scales of space and time. Sedimentary geology is considered in all its aspects, from the analytical techniques of sediments to the evolution of sedimentary basins.



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