Tropical coastal environments: coral reefs and mangroves - Environmental Sedimentology - page 327

km 2

WNW ESE

+ 30

+ 20

Accretion

+ 10

0

- 10

Mana

- 20

Magnan

Iracoubo

Kourou
Malmanoury

Cayenne

- 30

Erosion

Sinnamary

Case Fig. 9.1B Areas of land-gain and loss at different sections along the French Guiana coast between 1979 and 1984.
(Adapted from Froidefond et al. 1988.)

Temporal data indicate that sediment mobilization is continual along this coast and that
mudbank migration rates average c . 1.2 km yr −1 . Different sectors, however, migrate at different
rates and displacement rates between 1979 and 1984 ranged from 320 m yr −1 (Approuague-
Behague bank) to 1220 m yr −1 (Organabo bank). Despite these spatial differences, there appears,
to be a net balance along the wider coastal sector in terms of the area of annual mudflat gain
(around 60 km 2 ) and erosion (around 58 km 2 ) (Case Fig. 9.1B). One consequence of this highly
mobile intertidal setting is to produce a continually changing mangrove environment. Rapid
mangrove colonization follows new substrate accretion along the leeward fringes, with the
forests continuing to mature until interrupted by later erosion along the windward margins.
Interestingly, this transitional community development is mirrored by changes in sediment
accumulation and sediment diagenetic processes, which change with age of the mudbanks.
Within the young (and frequently flooded) mangroves, organic matter in the sediment mainly
derives from algal-mat communities, and diagenetic processes are dominantly suboxic, leading
to rapid degradation of organic matter. As the forests mature, most of the organic matter is
derived from higher plant material and diagenetic processes shift to anoxic sulphate-reducing
phases, associated with which occurs deposition of pyrite framboids. There is some evidence
that these diagenetic characteristics may be preserved beneath areas of newly accreting mud-
banks and thus serve as an indicator of previous erosional and accretionary phases.

Relevant reading

Baltzer, F., Allison, M. & Fromard, F. (2004) Material exchange between the continental shelf and mangrove-fringed
coasts with special reference to the Amazon-Guianas coast. Marine Geology 208 , 115-26 (and references therein).
Blasco, F., Saenger, P. & Janodet, E. (1996) Mangroves as indicators of coastal change. Catena 27 , 167-78.
Debenay, J.P., Guiral, D. & Parra, M. (2002) Ecological factors acting on the microfauna in mangrove swamps.
The case of foraminifera assemblages in French Guiana. Estuarine, Coastal and Shelf Science 55 , 509 -33.
Froidefond, J.M., Pujos, M. & Andre, K. (1988) Migration of mud banks and changing coastline in French
Guiana. Marine Geology 84 , 19 -30.
Marchand, C., Lallier-Vergès, E. & Baltzer, F. (2003) The composition of sedimentary organic matter in relation
to the dynamic features of a mangrove-fringed coast in French Guiana. Estuarine, Coastal and Shelf Science
56 , 119 -30.