Plates 3 / 1-5 The diversity of New Caledonia coral reef geomorphology and genetic processes : a synthesis from optical remote sensing , coring and acoustic multibeam observations

The New Caledonia (NC) exclusive economic zone (EEZ) includes a large variety of occanic and continental coral reef formations. This geomorphological diversity provides a rich framework which itself supports a large diversity of shallow modem habitats and communities. As a background to the description of the marine check-lists published in this volume, we describe here for each of the main coral reef complexes (bank, atolls. uplifted reefs. drowned reefs, fringing reefs, barrier reefs, patch reefs), a review of their geomorphological diversity and genetic processes. There is a long history of scientific research on the formations of NC coral reefs. First. datations from corals from a variety of locations in uplifted and subtidal positions provided a general idea of the intensity and timing of the dynamic processes that resulted in the modern morphology (Dubois et al. 1974: Collot et al., 1975, Coudray 1976; Debenay 1986: Carriere 1987). With the generalization of drilling programs and the possibility to assess the vertical structures of a reef (succession of episodes of reef growth), genetic proccsses have been locally refined (Coudray. 1976; Cabioch, 1988: Degaugue-Michalsky 1993: Castellaro: 1993), especially around Grande-Terre (Cabioch 2001, 2003). Drilling provided a huge mass of information relevant to establish models of recf seulement and development during the last interglacial ages in the Indo-Pacific region (Montaggioni. 2005). However, drilling provides only point data, spatially limited, and inferences must be made for larger spatial sCOlIes. Synoptic data come with remote sensing observations. which include shipbome, airbome and spaceborne data. Using Landsat satellite images. Grande-Terre, Ile des Pins, d'Entrecasteaux. LoyaIty and Chesterfield recfs were recently mapped using the geomorphological typology proposed by Andréfouët et al. (2006). The resulting atlas provides a quasi-exhaustive detailed two-dimension (2D) view of modern NC reefs (Andréfouët & Torres-Pulliza, 2004) (Fig. l,plate 3/1). Finally, multibeam acoustic data were collected between -20/-40 to -1000 m along the New Caledonia and the Loyalty Ridges using the EM 1002 echosounder of the RlV Alis (Pelletier et al .• 2004). Data were processed using the software CARAIBES TD (© IFREMER) (Flamand. 2006). Bathymetrie multibeam data provided a 2D/3D vision of the outer slopes of several NC reef complexes (Flamand et al., 2(04). The internaI structure provided by coring is not accessible, but marine terraces and faulting became visible with the fine-resolution bathymetric data. These features helped drawing the evolution of the Grande-Terre barrier reef system (Flamand. 2(06). In this review. we use the synoptic exhaustive description provided by optical satellite images as the primary guideline to describe the main NC l'eef complexes. ln the Background section (next) we also briefly describe the main forcing oceanographie. tectonic and eustatic processes occurring in the l'egion. Then, for each l'eef complexe. when the information is available by datations and coring, we provide the cUITent interpretation of the local dynamics across times. As much as possible, given the existing data. we consider the 125 ka (1 ka = 1000 years)-Present time-period. This period includes the last interglacial period (125 ka, mean sea level 6 meters above present level), crosses the last glacial maximum (at 23-20 ka, lower sea stands at around -120 m and beginning of the end of aerial exposure for interglacial reefs). and eventually crosses the postglacial periods of rising (-20-6 ka). quickly falling (-6-5.5 ka) and stable (-5.5 ka-Present) sea levels. E E / J e l e J e e / Je e e e ll ll