Author’s Accepted Manuscript

Nearshore intertidal topography and topographicforcing mechanisms of an Amazon-derived mud bank in French Guiana Edward J.Anthony, Franck Dolique,Antoine Gardel, Nicolas Gratiot, Christophe Proisy, Laurent Polidori PII: DOI: Reference:

S0278-4343(08)00008-3 doi:10.1016/j.csr.2008.01.003 CSR 1713

To appear in:

Continental Shelf Research

Received date: Revised date: Accepted date:

www.elsevier.com/locate/csr

24 July 2007 29 October 2007 7 January 2008

Cite this article as: Edward J. Anthony, Franck Dolique, Antoine Gardel, Nicolas Gratiot, Christophe Proisy and Laurent Polidori, Nearshore intertidal topography and topographicforcing mechanisms of an Amazon-derived mud bank in French Guiana, Continental Shelf Research (2008), doi:10.1016/j.csr.2008.01.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nearshore intertidal topography and topographic-forcing mechanisms of an Amazon-derived mud bank in French Guiana

Edward J. Anthony*, Franck Dolique*1, Antoine Gardel*, Nicolas Gratiot², Christophe Proisy3, Laurent Polidori4, *Coastal Geomorphology and Shoreline Management Unit, EA 3599, Université du Littoral Côte d’Opale, MREI 2, Avenue Maurice Schumann, 59140 Dunkerque, France. 1 IRD Guyane, US 140 Espace, Laboratoire de télédétection, BP 165, Route de Montabo, 97323 Cayenne Cédex, France. ² IRD, LTHE 1025, rue de la piscine, BP 53, 38041 Grenoble Cédex 9, France. 3 IRD, UMR AMAP, Montpellier F-34000 France. 4 Ecole Supérieure des Géomètres et Topographes, 1, boulevard Pythagore, Campus Universitaire, 72000 Le Mans, France.

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Corresponding author Email address: [email protected]

Abstract

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The intertidal topography in the vicinity of the contact zone between a longshoremigrating Amazon-derived mud bank and the muddy terrestrial shoreline in French Guiana was defined from a combination of satellite-based SPOT images, airborne lidar data and high-

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resolution total station ground surveying of a 75,000 m² plot. The three approaches, at different

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scales, were carried out at different periods. Digital elevation models generated from these three techniques however converge in highlighting the topographic micro-scale (centimetre-scale) variability of the mud bank surface while showing meso- to macro-scale features that reflect the dominance of wave activity in mud bank mobilisation and attachment to the terrestrial shoreline. These features are bar-like longshore forms that develop in the intertidal zone from the shoreward drift of gel-like mud that accompanies wave damping. The features progressively become consolidated through mud drying out associated with the formation of cracks that are

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important in mangrove colonisation and ecological changes. Fluid-mud accumulations formed from high concentrations of mud trapped in the troughs behind these linear bar forms generate flat featureless surfaces that tend to mask topographic heterogeneity of the mud bank surface. Dewatering of these lower zones by progressive mud consolidation complements tidal water discharge in providing a mechanism for the formation of the numerous channels that dissect the linear bar features, especially in the upper intertidal contact zone with the terrestrial shoreline.

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This dissection in the upper intertidal zone generates an intricate topography that replaces the

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original linear bar forms. The innermost bar forms a ‘suture’ zone with the terrestrial shoreline. Reworking of this bar by high-energy waves may lead to mud dispersal over old terrestrial

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mangrove substrates, resulting in stifling of mangrove pneumatophores. Mud reworking at the

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narrow trailing edge of the mud bank in the subtidal and lower intertidal zones leaves behind a flat bed that will eventually be completely eroded by waves in the course of mud bank migration.

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Keywords: mud bank, Amazon-influenced coast, topography, fluid mud, mud bars, wave-mud interaction, mangroves.

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1. Introduction

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The 1600 km-long coast of South America between the mouths of the Amazon and the Orinoco (Fig. 1) is the world’s muddiest. The highly dynamic mangrove-colonized shorelines may fluctuate in the cross-shore direction in the short term (months to years) at rates of several metres to several kilometres depending on the attachment to the terrestrial shoreline, and eventual subsequent erosion, of Amazon-derived mud banks. These mud banks are spaced at

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intervals of 15 to 25 km, are up to 5 m thick, 10 to 60 km long and 20 to 30 km wide, and migrate at velocities ranging from 1 to more than 5 km yr-1, driven essentially by waves, aided by tidal and wind-induced geostrophic currents (Augustinus, 1978; Wells and Coleman, 1978, 1981; Froidefond et al., 1988; Eisma et al., 1991; Gardel and Gratiot, 2005; Gratiot et al., 2007; Winterwerp et al., 2007). The waxing and waning of mud-bank activity may be characterized by ‘bank’, ‘inter-bank’ and transitional phases (Anthony and Dolique, 2004). Allison et al. (2000)

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have shown that the mud banks migrate over a relict shelf mud surface in shallow inshore water depths of 5 to 20 m. Using 7Be, 137Cs, and

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Pb signatures in sediment cores from inner (