A Geomorphological reconnaissance of Tromelin Island, Indian Ocean

www.cerf-jcr.org

A Geomorphological Reconnaissance of Tromelin Island,Indian Ocean

Nick Marriner{, Paolo A. Pirazzoli{*, Michel Fontugne1, Max Guerout{{, Mireille M.M.Guillaume{{11, and Jean-Louis Reyss1

{Centre Europeen de Recherche etd’Enseignement des Geosciences del’Environnement CNRS UMR6635

Europole de l’Arbois, BP 8013545 Aix-en-Provence cedex 04, France

{Laboratoire de Geographie Physique(CNRS-UMR 8591)

1 Place Aristide Briand92190 Meudon, [email protected]

1Laboratoire des Sciences du Climat et del’Environnement (UMR 8212 CNRS-CEA-UVSQ)

Avenue de la Terrasse91198 Gif-sur-Yvette, France

{{GRAN4 rue Antoine Condorcet34500 Beziers, France

{{Departement Milieux et PeuplementsAquatiques (UMR CNRS-MNHN-UPMC-IRD BOrEA)

Museum National d’Histoire Naturelle61 rue Buffon75005 Paris, France

11Laboratoire ECOMARUniversite de la ReunionBP 715197715 Saint-Denis Messag cedex 9, France

ABSTRACT

Marriner, N.; Pirazzoli, P.A.; Fontugne, M.; Geerout, M.; Guillaume, M.M.M., and Reyss, J.L., 2012. A geomorphologicalreconnaissance of Tromelin Island, Indian Ocean. Journal of Coastal Research, 28(6), 1606–1616. Coconut Creek(Florida), ISSN 0749-0208.

Tromelin is a small coral reef island (,1 km2) located in the Indian Ocean, approximately 440 km E of Madagascar and580 km N of La Reunion. Despite the presence of a permanent Meteo France weather station on Tromelin since the1950s, a detailed geomorphological study of the island has never been undertaken. In this paper, we describe thegeomorphology of the atoll, detailing seven geomorphological zones from the outer reef to the atoll crown. We also discussHolocene relative sea-level changes in the light of new radiometric datings. Uranium–thorium dating of a remnant mid-Holocene reef-flat yielded an age of 5700 6 400 years and could be interpreted as a relative sea level approximately 1.2 mabove present. Subsequent possible sea-level positions have been inferred from coral conglomerates left by high-energywaves. These conglomerates might be interpreted as a beachrock, cemented mainly in the former intertidal zone whensea level was still approximately 1.2 m above present. The radiocarbon dating of three coral fragment samples collectedfrom these conglomerates has yielded ages spanning 4200 to 2900 calibrated years BP.

ADDITIONAL INDEX WORDS: Coral reef, patch reef, cay, Holocene sea level, geomorphology, cyclone, storm deposits,Scleractinian coral.

INTRODUCTION

Located about 580 km NW of La Reunion (54u319 E, 15u539 S),

Tromelin is a small coral island about 1750 m long by 700 m

wide and bordered by a reef system exposed at low tide

(Figures 1 and 2). Tromelin belongs to the French Eparses

Islands and, together with the islands of Glorieuses, Juan de

Nova, Bassas da India, and Europa, located in the Mozambique

Channel, constitutes the fifth district of the Terres Australes et

Antarctiques Francaises (TAAF) since February 2007.

Discovered on 10 August 1722, by la Diane, a vessel of the

French East India Company, the island is best known for the

tragic events that followed the sinking of l’Utile (Guerout, 2007;

Guerout and Romon, 2007, 2008). On 31 July 1761, 120 crew

members and 60 slaves were marooned on the island following

the wrecking of l’Utile, en route from Madagascar to the Ile de

France (present-day Mauritius). From the wreckage, the

survivors built a second boat, la Providence, and on 27

September of the same year the French mariners set sail for

Madagascar, leaving the Madagascan castaways behind. Only

eight survivors, seven women and a young child, were rescued

on 29 November 1776 (Guerout and Romon, 2010).

In 1953, the Direction des Services Meteorologiques decided

to locate a weather station on Tromelin Island. The runway was

completed on 20 June 1954 and the first liaison with La

Reunion was completed on 9 July of the same year. The runway

is oriented SSE–NNW and is around 1100 m long. The weather

station buildings are situated on the western extremity of

Tromelin, the highest point on the island (Marriner, Guerout,

and Romon, 2010). This infrastructure includes the weather

station and service buildings, an electric generator, and an

aeolian. Four Meteo France workers, who undertake daily

meteorological measurements, permanently occupy the island.

Although described and visited by numerous travelers since

the 18th century (Guerout and Romon, 2008), Tromelin has

DOI: 10.2112/JCOASTRES-D-11-00029.1 received 13 February 2011;accepted in revision 21 March 2011.* Corresponding author.’ Coastal Education & Research Foundation 2012

Journal of Coastal Research 28 6 1606–1616 Coconut Creek, Florida November 2012

never been the subject of an extensive geographical or

geological study, research thus far having focused upon the

reef’s zoology and botany (Bouchon and Faure, 1979; Brygoo,

1955; Paulian, 1955; Seguy, 1955; Staub, 1970). To fill this

knowledge gap, and to better understand Tromelin’s geomor-

phological context, coral taxonomy, and sea-level indicators,

two expeditions were conducted in 2008 (GRAN/INRAP, M.

Guerout and Romon) and 2009. Here, we report a summary of

these findings.

GEOLOGICAL ORIGIN AND CLIMATICSITUATION OF TROMELIN

Tromelin, which rises steeply from the ocean floor in the

middle of the Mascarene Basin, is a coral cay whose maximum

height does not exceed 8 m. In its present configuration, the

island might have formed when eustatic sea level stabilized

during the mid-Holocene, leading to the accretion of a

sandbank atop shallow Quaternary reefs (Barry, Cowell, and

Woodroffe, 2007; Guilcher, 1988; Montaggioni, 2005). The reef

system that flanks the island is the main sediment source.

Before our surveys, however, no radiometric dating was

available from Tromelin. From about 6000 BP, when global

sea level achieved broad stability (Bard et al., 1996; Camoin

et al., 1997; Camoin, Montaggioni, and Braithwaite, 2004;

Pirazzoli, 1996), bioclastic reef material eroded on the reef

complex probably helped extend the island’s surface.

Three main forcing agents have affected the geomorphology

of Tromelin. First, swell generated by the trade winds typically

blows from the SE. These winds are consistently above 8 m/s in

austral winter (from May to October) (Meteo France, Tromelin,

unpublished data). Second, long swell is independent of local

conditions. Such waves are common during the austral winter,

resulting from distant storms in the fetch area (i.e., .6500 km

to the east). These waves contribute to the active erosion of

Tromelin’s coastline and act as motors of the S–N longshore

drift. The final forcing agent consists of cyclones. Tromelin is

located in the cyclone belt of the SW Indian Ocean, which

extends from the African coast to the meridian 90u east

(MTOTEC, 2011). On average, nine tropical depressions form

in the region each year, four of which reach the stage of tropical

cyclones, while the five others do not evolve beyond the storm

stage. Considerable interannual variability is nonetheless

observed, as data series for the last 40 years attest (Mayoka,

1998). Most depressions in the basin form between 10u and

15u S in two geographical zones: (i) the SW of the Chagos

archipelago and (ii) the area between Saint-Brandon and north

Madagascar. Because of its low topography (,8 m), Tromelin

Island is particularly vulnerable to cyclone hazards. The

austral winter season, from May to October, is characterized

by trade winds from S to E (Mancel, 1991). In austral summer,

from November to April, the southern displacement of low

equatorial pressure systems shifts the flow from E to NE and

rainfall is more frequent (Figure 3). Average air temperature

varies between 23 and 28uC, and the temperature is between 24

and 28uC for seawater. Trade winds affecting Tromelin

frequently attain speeds of 20 km/h. Because of its hot spot

bathymetry, Tromelin seems relatively unaffected by tsunami

events, and even the December 2004 tsunami did not have

significant local impacts.

METHODS

Topographical measurements were made using a total

station (Leica TPS800). We established a reference point

(9000) on the roof of the Meteo France building with the

coordinates 1000 (x), 1000 (y), and 100 (z). A total of 20

measurement stations (9000–9001, 9003–9014, and 9016–

9021) were positioned around the island to optimize spatial

Figure 2. Digital elevation model of Tromelin’s bathymetry and wind rose

(Meteo France, Tromelin).

Figure 1. Location map of Tromelin, Indian Ocean.

Geomorphology of Tromelin Island 1607

Journal of Coastal Research, Vol. 28, No. 6, 2012

coverage and facilitate measurements. Topographical maps

were drawn in ArcGIS in collaboration with P. Dussoulliez of

the Centre Europeen de Recherche et d’Enseignement des

Geosciences de l’Environnement. The cartographical work was

supplemented by detailed geomorphological surveys of the

island. Results of radiocarbon dating are presented in Table 1

as conventional 14C ages and calibrated dates, calculated using

the Calib 6.0 program (Stuiver and Reimer, 1993). The

calibrated dates were corrected for marine reservoir effects

using a DR correction of 140 6 25 years proposed by Southon

et al. (2002) for the SW tropical Indian Ocean.

RESULTS

Seven geomorphological units on Tromelin can be described

from the external submarine slope to the island crown

(Figure 4).

External Submarine Slope and Coral-ReefGeomorphology

Tromelin’s bathymetry is characterized by a steep outer

slope that reaches a depth of about 1000 m approximately

2.5 km from the shoreline (Figures 2–5). This bathymetry

recalls that of volcanic hot spots (Menard, 1986), intraplate

volcanism found mainly on the oceanic lithosphere. Tromelin is

located in the Mascarene Basin, a rift formed 83 million BP

when Madagascar was separated from India. This rift was

abandoned approximately 60 million BP when the extension

moved to a new area, the Central Indian Ridge, between the

Seychelles and India (Lenat, Merle, and Lespagnol, 2009). To

date, no study of this offshore zone has been conducted.

Tromelin Island is at present surrounded by coral reefs

whose geomorphology attests to high-energy environments.

Altogether, the reef size has been estimated by Andrefouet

et al. (2008) at 5.42 km2 for the bank barrier, 0.85 km2 for the

bank barrier land, and 0.63 km2 for the drowned bank. An

almost horizontal or gently sloping shallow reef platform, 20 to

30 m wide, is exposed at low tide and terminates seaward by a

1- to 2-m drop, the reef front where waves break. The upper

part of the outer reef slope is gradual but interrupted by pits at

a 5- to 6-m depth, aligned parallel to the front; they are

attributed to the interaction between waves and lateral

currents. The deeper reef slope is reported to be steeper.

Narrow and shallow grooves, perpendicular to the shore,

indent the reef flat and the upper reef slope, giving rise to the

ragged appearance of the reef flat on the NE part of the island

(Bouchon and Faure, 1979). A series of spurs and grooves are

noted on aerial photographs; these comb structures run

perpendicular to the reef flat (Battistini et al., 1975) and where

mechanical erosion yields the bulk of biogenic blocks. Grooves

are U-shaped, are 1 to 2 m deep, and extend to the outer slope.

They are spaced 15 to 20 m apart on the SE coast but are closer

together on the NE coast. The upper part of the outer slope is 50

to 100 m wide down to a depth of about 5 m, where it is

interrupted, on the SW coast, by a depression 10 to 20 m wide

and 3 to 5 m deep. On both sides of this depression, the outer

slope is similar. Below a depth of 15 m, the outer slope increases

gradually, reaching 20u to 30u. Coral colonies exhibit growth

forms adapted to the island’s high-energy context. Hard coral

communities are poorly diversified, containing 16 genera in

1977 (Bouchon and Faure, 1979) and 18 in 2009 (Guillaume,

Denis, and Bruggemann, 2009). Indeed, the southern reef flat

is subject to strong mechanical erosion (breaking waves and rip

currents) and biological attack by perforating algae.

Coral Reef PlatformA reef system only exposed at low tide surrounds the

supratidal crown of Tromelin. The tidal range on Tromelin is

very low, not exceeding 1 m. According to French Hydrographic

and Oceanographic Service (SHOM) calculations (B. Roump,

personal communication), the lowest tide is approximately

0.00 m, spring low tide is approximately 0.17 m, mean tide is

approximately 0.50 m, mean spring high water is approxi-

mately 0.84 m, and highest tide is approximately 1.02 m. This

Figure 3. Average rainfall at Tromelin.

Table 1. Results of AMS radiocarbon dating carried out on samples collected at Tromelin Island.

Field Reference Latitude S Longitude E Material

Elevation Above

Present Low

Tide (m) Lab. No. q13C*

Conventional

Radiocarbon Age

(BP 6 s)

Calibrated Median

Probability

(and 2s range)

9TR02 15u539.85 54u319.50 Coral from

beachrock

1.65 SacA 15690/

Gif-12410

24.2 4140 6 30 4009 (3884–4134)

9TR03 15u539.85 54u319.50 Coral from

beachrock

1.45 SacA 15691/

Gif-12411

22.5 4180 6 30 4062 (3933–4202)

9TR04 15u539.80 54u319.64 Coral from

beachrock

1.65 SacA 15692/

Gif-12412

22.4 3375 6 30 3057 (2999–3195)

* The q13C values used for conventional age calculation are obtained from accelerator measurements and include carbon isotope fractionation because of

graphitization and ion beams during measurements. They have no palaeoenvironmental significance.

1608 Marriner et al.


substrate, of which some remains are presently found about 1 m

above current sea level, has been extensively eroded by wave

action since the stabilization of relative sea level around 6000

BP; the reef yields the majority of bioclastic material to the

island crown. We also studied sediment transfers, particularly

the movement of coral blocks, gravels, and sands from the S to

the N of the island. The coral reef yields the bulk of the

sediment budget to the island’s supratidal formations. Accord-

ingly, with the exception of some pumice stones of undefined

origin, the island’s sediments are bioclastic. The sedimentary

deposits, studied through a series of sampling stations, show a

clear S–N grain-size differentiation. These variations are

mainly the result of the dominant meteomarine forcing agents.

A band of raised reef fringing the beach, mapped in the SW

part of the island by Staub (1970) as ‘‘upraised coral reef rocks,’’

is the only reference to possible local sea-level changes that

could be found in the literature before our fieldwork. These

features appear as highly eroded remnants of former reef

structures now emerging at low tide from the present reef flat a

few meters from the contemporary beach (Figure 6). These

remnants, which are not located on the windward side of the

reef, form lines parallel to the reef margin or beach and are

scattered, with decreasing height, at variable distances from

the shoreline. They could be dismantled remnants of a former

rampart-rock platform (Scoffin and McLean, 1978), and their

lithification is inferred to have taken place in the intertidal

zone. From these remnants, a coral sample (9TR01) of Favites

peresi was collected 1.2 m above spring low tide at 15u539.82 S,

54u319.46 E (near topographical station 9008). This species

belongs to the present coral communities (Bouchon and Faure,

1979; Guillaume, Denis, and Bruggemann, 2009).

In the field, it was difficult to ascertain whether this was

organically cemented reef rock (i.e., whether the coral sample

was in growth position, therefore testifying to a former mean

sea level, or MSL, at least 1.2 m above present) or whether

Staub’s (1970) description was incorrect and the sample

collected was just a reef block deposited within a rampart-rock

(i.e., consistent with a sea level similar to present). Because

sample 9TR01 could be older than the Holocene a priori,

uranium–thorium (U/Th) dating was undertaken. The arago-

nitic coral (calcite ,0.5%) yielded an age of 5.7 6 0.4 ka

(Table 2).

Figure 4. Digital elevation model of Tromelin.



Absence of watercourses on the island provides little

terrestrial sediment to feed longshore drift. Fourteen repre-

sentative sampling stations were studied at strategic locations

around the island (Figure 5). We find that the main axis of the

largest block decreases from about 75 cm for southern stations

(e.g., station 9010) to less than 20 cm for northern stations.

Some large blocks (.1 m at the longest axis) lie on the edge of

the southern reef, just above MSL. These blocks correspond to

wave-fractured sections of the reef from which they have been

displaced just a few meters. Despite Tromelin’s high-energy

context, we found no evidence for megablocks on Tromelin.

Beachrock FormationsThe southern fringe of the island is characterized by outcrops

of beachrock, particularly in the south of the island. These

formations show stratification in slabs, with a seaward dip that

follows the alignment of the present beach clasts. Beachrocks

are lithified conglomerates of coastal sediments, formed by

carbonate cement precipitation, mainly calcite, aragonite, and

magnesium (Vousdoukas, Velegrakis, and Plomaritis, 2007).

The outcrops studied correspond to the beachrock definition

given by Scoffin and Stoddart (1987) ‘‘consolidated deposits that

result from lithification by calcium carbonate of sediments in

Figure 5. Location map of topographical stations and cartography of coral blocks around Tromelin.



the intertidal and spray zones of mainly tropical coasts.’’

Lithification normally occurs in the intertidal zone, some-

times over very short decadal timescales. Mechanisms of

cementation may include physicochemical precipitation of

high-Mg calcite and aragonite from seawater as a result of

high temperatures, CaCO3 supersaturation, evaporation, or

a combination of these; physicochemical precipitation of low-

Mg calcite and aragonite by mixing of meteoric water with

seawater; physicochemical precipitation of high-Mg calcite

and aragonite by degassing of CO2 from beach sediment pore

water; and finally precipitation of micritic calcium carbonate

as a byproduct of microbiological activity (Turner, 2005). At

Tromelin, the beachrock formation disappears beneath the

boulder ramparts, comprising coral blocks and slabs of

detached beachrock (Figure 7A).

Some former beachrock-type outcrops, developed higher

than the present intertidal range, up to 1.7 m above present

sea level, have been investigated in greater detail. At the foot of

the boulder rampart, near topographical station 9009, a

beachrock is developed between about 0.8 m and 1.6 to 1.7 m

above low tide, with a slightly inclined morphology (Figure 7B).

Its surface, frequently washed by waves in this exposed part of

the island, is devoid of a cover of unconsolidated beach

sediments. The conglomerate slab consists of a single unit

apparently representing a single episode of deposition and

cementation. A sample of coral fragments (sample 9TR04)

collected from the surface of the slab has been radiocarbon

dated at the Saclay Artemis accelerator mass spectrometry

(AMS) facilities to 3375 6 30 BP (Table 1).

In the same area, the surface of the conglomerate is quite

regular, consistent with a heterometric sediment composition.

In some instances, a similar beachrock is found forming

irregular mounds (again between about 0.8 m and about 1.6 m

above low tide), probably attesting to chaotic deposition and

high-energy wave action during and after the projection of the

storm material. The surface of the formation is in this case very

irregular (Figure 7C). The sediments forming the conglome-

rate were deposited by very high-energy waves in a coral reef

island context. The great variety in the conglomerate’s size and

material was confirmed in an exposed section near topograph-

ical station 9011 (Figure 7D), which contained a 0.5-m-wide

coral boulder, coral fragments of various sizes, and even some

thin coral sand, all lightly cemented together. The section is

about 0.8 m high, i.e., equivalent to the local spring tidal range.

These deposits, which were found at just a few locations in the

southernmost part of the island, seem to have been left by a

strong storm and subsequently consolidated, possibly in the

intertidal zone of a sea level that could have been approxi-

mately 1.2 m above present. In the present intertidal range,

there is often no evidence of beachrock or even of unconsol-

idated beach deposits in the same area of the island.

Hopley (1986) notes that, for beachrock interpretation,

problems may arise with dating material from outcrops,

because a maximum age may be obtained from constituent

organic material and there can be chronological offsets between

the death of the organism and the cementation of the

beachrock. In the present case, dated material comprises small

coral fragments of various sizes that, in such an exposed

environment, could have been reworked from the reef directly

or shortly after the coral death. The three dated beachrocks

therefore probably comprise sediments eroded from the reef

between 3300 and 4200 BP. The vertical range of lithification,

in the three cases examined, corresponds to the tidal range of a

sea level about 1.2 m higher, which suggests that the beachrock

may be consistent with a higher sea level.

As an indicator of former sea levels, Hopley (1986) recom-

mends that, where possible, beachrock should be used in

conjunction with other types of evidence. In Tromelin, however,

no other type of evidence could be found. Kelletat (2006)

remarks on the possibility, in extremely exposed locations, of

supratidal genesis of beachrock, in which the carbonates

necessary for cementation may be delivered by sea spray. On

the southern coast of Tromelin, exposed to frequent tropical

cyclones, it is not possible to exclude the possibility that the

Figure 6. ‘‘Upraised coral reef rock’’ (Staub, 1970) reaching up to 1.2 m

above spring low tide level. An in situ coral sample (9TR01) of Favites

peresi, collected from the top, has been dated by U/Th to 5.7 6 0.4 ka. One

meter gives scale.

Table 2. Results of Th/U analysis carried out on samples collected at Tromelin Island.

Field

Reference Latitude S

Longitude

E Material

Elevation

Above

Present

Low Tide

(m) 238U (ppm) 234/238U (ppm) 230/234U (ppm) Age (ka) (234/238U)t0

232Th

(ppb)

230/232Th

(ppb)

9TR01 15u539.82 54u319.46 Favites

peresi

,1.0 2.416 6 0.042;

2.380 6 0.050

1.140 6 0.016;

1.144 6 0.017

0.051 6 0.016 5.7 ± 0.4 1.142 6 0.02;

1.144

2 6 11 270



beachrock was lithified by spray from an undetermined sea

level even higher than the present one.

The limestone substratum comprises the geological base upon

which the Holocene deposits have accreted; no precoralline out-

crops are exposed on the island. A large amount of volcanic

pumice is observed in supratidal storm deposits, especially on the

SE flank of the island. At present, the origin of the pumice

remains unclear, although we suggest that they are linked to the

volcanic base of the island; the possibility of a more distant origin

must not, however, be discarded. Mechanical erosion of the

beachrock formations by strong wave action has led to a fracturing

of the outcrops, which are undergoing active erosion at present.

The Beach SystemTromelin’s beach system is divided into three units: (i) the

storm beaches to the south of the island, comprising coral blocks

(15u539.562 S, 54u319.242 E and 15u539.341 S, 54u319.484 E);

(ii) mixed sediment beaches to the SE and SW (15u539.464 S,

54u319.164 E to 15u 539.660 S, 54u319.313 E and 15u539.303 S,

54u319.423 E to 15u539.391 S, 54u319.571 E); and (iii) the sandy

beaches to the north. Wave diffraction and S–N sediment

transfers have formed a prominent sand spit (,125 3 ,225 m),

whose morphology varies seasonally in response to the

changing meteomarine contexts. This sandbank comprises

gravels and coarse-grained sands and is located in a sheltered

downwind position behind the island.

The southern beaches are around 20 to 35 m wide and

constitute coral blocks eroded during storm episodes and

deposited in the supratidal zone; these deposits form a

staircase of storm berms parallel to the shoreline (Otvos,

2000). Several generations of storm ridges are present,

although at present there is no chronological control for these

elements. The most recent ridge lies just above MSL and

constitutes white coral blocks that contrast with the darker

shades, because of Cyanobacteria, of older ridges. Tromelin’s

reef flat is relatively wide (300–400 m, depending on location),

with gentle slopes that promote the transport of clastic

Figure 7. Beachrock and storm tract deposits. (A) Near topographical station 9010, other beachrock slabs, higher and possibly older, extend the beachrock

formation in the foreground below storm deposits. (B) Beachrock slabs are developed as irregular slabs between approximately 0.8 m and 1.6 to 1.7 m above

low tide, where the first boulders of the rampart appear. A sample of coral fragments (sample 9TR04) collected from the surface of the slab at 1.65 m (close to

the hammer) has been dated 3375 6 30 BP. (C) Beachrock-type material is found forming irregular mounds, probably attesting to chaotic deposition and high-

energy wave action during and after the projection of the beachrock material. The surface of the formation is in this case very irregular, suggesting the

presence of heterometric material. A sample (9TR03) of coral fragments collected 1.45 m above low tide (left of the hammer) has been dated 4180 6 30 BP.

(D) A section of the beachrock shows that the height of the formation is about 0.8 m (i.e., of a range similar to that of the tide). The size of the material

contained in the beachrock is extremely heterometric, from boulders several decimeters wide to small coral fragments and coral sand. A sample (TR02)

collected near the top of the section, 1.65 m above low tide (left of the hammer), has been dated 4140 6 30 BP. White and blackened layers of the boulder

rampart can be seen in the background.



material to the shoreline. The exposure of the southern coast to

the SE winds and swell evokes the reworking of older storm

ridges during powerful cyclone events (Nott and Hayne, 2001;

Nott et al., 2009; Scoffin, 1993).

A transitional zone is observed between the southern storm

beaches and the sandy northern beaches that is linked to

source to sink sediment transfers. This area is characterized by

coarse bioclastic sands and coral-block storm deposits.

The northern beaches reach a maximum width of about 50 to

75 m and are characterized by white biogenic sands, reworked

from the south by the dominant longshore drift from the SE.

Locally, the clastic sediments have been eroded to expose

beachrock outcrops. A few coral blocks are present, generally

less than 20 cm in diameter.

DunesThe best-developed dunes are located in the N, NE, and NW

parts of Tromelin, usually associated with Tournefortia

argentea, a shrub about 1 to 1.5 m high. This dune system

reaches a maximum width of 200 m and lies about 8 m above

MSL. Dune heights vary from 10 cm for the southern

microdunes to 250 cm in the NE of the island. The dunes are

Figure 8. Simplified geomorphological map of Tromelin, depicting the distribution of the storm tract and dune systems.



parabolic, oriented SE–NW (Pye and Tsoar, 1990). To the SW of

Tromelin, this dune system is less well developed (Figure 8).

The presence of a coral block ridge in the upper part of the

crown inhibits sediment supply to the dune field. The

vegetation cover comprises herbaceous plants, mainly Boerha-

via diffusa (Paulian, 1955), which are well adapted to the sandy

matrix with very low nutrient content.

Two factors explain the absence of dunes in the south of the

island. First, salt spray is not conducive to the establishment

of an important vegetation cover. The relative absence of

vegetation therefore means that fine-grained sediment is not

readily trapped. Second, the southern storm ridges block fine-

grained sediments, starving sediment supply and its transport

by aeolian processes in this area.

Storm TractThe southern facade of Tromelin is characterized by a series

of boulder ramparts, juxtaposed parallel to the shoreline from

MSL up to the atoll crown. We measured four ramparts at 1, 3,

4.5, and 7 m above MSL. The height of these ramparts may

reach as high as 6.5 m above high tide level between stations

9012 and 9013 (Figure 9), which is exceptional for a coral reef

(Pirazzoli, 2010) and has principally resulted from the strength

of tropical storms and the long fetch area (.6500 km from the

east). The blocks constituting this embankment are generally

rounded and shaped by abrasion and saltation processes

during their reworking from the intertidal zone.

Exceptional storm waves have breached the upper part of

the boulder rampart to form a storm tract up to 250 m

inland. The sediments are mostly of hydrodynamic origin,

deposited by high-energy waves associated with intense

cyclones. These storm deposits comprise coral blocks (usually

,30 cm) and are principally visible in the south of the island

because of the low vegetation cover and low sediment

budgets. This unit also covers the geomorphological dune

area between 54u319.300 E, 15u539.400 S and 54u319.200 E,

15u539.400 S. The largest blocks (long axis 5 40–30 cm) are

located close to the southern beach berm. Their presence

behind the storm beaches implies that they were transported

by high-energy storm waves and deposited on the supratidal

crown of the island.

Figure 9. Boulder rampart attested all around the southern part of

Tromelin. White boulders correspond to deposits left by recent storms. In

the upper part of the rampart, which may reach up to 6.5 m above high tide

level, the boulders have been blackened by cyanophycean algae, indicating

less recent storms. A folded double meter gives scale.

Figure 10. N–S transect of Tromelin.



Southern DepressionThe southern depression covers an area of 40,000 m2. Its

lowest area lies a mere 1.2 m above MSL. A thin layer of well-

sorted aeolian sand characterizes this zone; surface sediments

have undergone an active aeolian deflation that has shaped

small fields of microdunes, which accrete slowly because of low

sediment supply and high wind speeds in this part of the island.

Clastic sediments are rapidly lithified to form a conglomerate

about 15 cm below the ground surface. This low topography has

resulted from low sediment supply and the strong accretion of

flanking areas during storm events.

DID THE HOLOCENE SEA LEVEL EXCEED THEPRESENT ONE AT TROMELIN?

Sea-level changes in the Mascarene Basin are generally

considered not to have reached a level higher than present

during the Holocene (Camoin et al., 1997; Camoin, Montag-

gioni, and Braithwaite, 2004; Montaggioni, 1979). This absence

of higher sea level, which has been ascribed to the specific

response of volcanic islands to hydroisostatic processes,

contrasts to the higher sea levels reported from the coasts of

Madagascar (Battistini, 1970; Battistini, Delibrias, and La-

borel, 1976; Laborel and Delibrias, 1976), from Farquhar atoll,

south of the Seychelles Archipelago (Battistini and Jouannic,

1979; Pirazzoli, Kaplin, and Montaggioni, 1990), and from the

eastern (Cocos Islands) and central (Chagos) Indian Ocean

(Woodroffe, 2005; Woodroffe et al., 1990). Particular attention

was therefore paid to possible former sea-level indicators.

In the absence of drilling possibilities below the reef surface,

a careful survey was undertaken along the coast and on the

surface of Tromelin in search of fossil sea-level indicators

exposed in growth position. None, however, were found.

The elevated ‘‘reef rock’’ mentioned by Staub (1970), which

we dated to 5.7 6 0.4 ka (as explained earlier), cannot be

considered as a reliable sea-level indicator. In a less exposed

context, the beachrock-like conglomerates developed higher

than the present intertidal range and could attest to a former

sea-level position about 1.2 m higher than present about 3300

to 4200 BP. Nonetheless, because of the frequency of cyclonic

events in Tromelin, with the possibility of supratidal genesis of

cementation by sea spray (Kelletat, 2006), further data are

required from other types of evidence.

CONCLUSION

Topographical and geomorphological data have allowed seven

geomorphological units to be defined on Tromelin, from the

outer slope to the cay’s crown. Sediment transport takes place

via two pathways, S–N longshore drift and aeolian transport,

driven by the prevailing winds and swell from the SW. Cyclones

have played an important role in the island’s morphogenesis

and the distribution of its sedimentary formations, as shown by

a N–S transect of the island (Figure 10). Indeed, the geomor-

phology of the island’s windward facade is in stark contrast to

the leeward flank. Southern Tromelin comprises high-energy

boulder ramparts mobilized during storm and heavy swell

episodes. Because of the strength of aeolian processes (.8 m/s

20% of the time) and the trapping of fine-grained sediments by

the boulder ramparts, accretion of the southern island crown is

relatively moderate. The northern part of Tromelin is charac-

terized by important accretion of bioclastic sediments, essen-

tially rounded gravels and coarse to medium sands. This is the

highest and best-sheltered point on Tromelin, an environmental

potentiality that was exploited by the castaways during the 18th

century to build their camp. The northern spit’s morphology

varies according to seasonal weather and oceanic conditions.

Concerning Holocene changes in sea level, while some

beachrock evidence may suggest that Tromelin’s sea level

could have been about 1.2 m higher than present during the

mid-Holocene, the occurrence of a higher sea level remains

uncertain in the absence of other data.

ACKNOWLEDGMENTS

We thank S. Fuma, J. Guesnon, L. Hoarau, J. Mouret, J.-F.

Rebeyrotte, and T. Romon for their assistance during the

2008 expedition. We also wish to thank the following

institutions: the Prefecture des Terres Australes et Antarc-

tiques Francaises, the Commandement des Forces Armees

dans la Zone Sud de l’Ocean Indien, Meteo France La

Reunion, the Fondation du Patrimoine, the Conseil Regional

de La Reunion, the Direction Regionale des Affaires Cul-

turelles de la Reunion, and the Fondation d’Entreprise

Groupe Banque Populaire.

The 2009 fieldwork has benefited from financial support by

the CNRS (UMR-8591), the Agence Nationale de la Recher-

che ANR-06-BDIV-002-BIOTAS Program, and the DRRT-

Reunion. It would not have been possible without the logistic

support of the TAAF, which organized the 2009 scientific

expedition aboard the Marion Dufresne, and the logistic help

of C. Marteau and T. Perillo. We thank J.P. Dumoulin and C.

Moreau for AMS measurements and B. Roump (SHOM) for

hourly tide predictions.

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A Geomorphological reconnaissance of Tromelin Island, Indian Ocean

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