Origins and development of Holocene coral reefs: a revisited model based on reef boreholes in the Seychelles, Indian Ocean

Abstract

Until recently, concepts of coral reef growth and accumulation have been predominantly based on a Darwinian model. In this, the upwards and outwards growth of a reef core (a coral framework) takes place over a foreslope consisting of reef talus, with the simultaneous filling of the back-reef lagoon by reef-derived debris. The principal adaptations of this pattern relate to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity. Boreholes through the outer edge of a fringing reef in the Seychelles, western Indian Ocean, reveal a record of Holocene sediment accumulation first established approximately 8 ka ago. Faunal and floral associations show that growth of this body began in relatively deep water but that this shallowed to ~5 m within 1 ka. Subsequent accumulation was of "keep-up" style but, as the rate of sea-level rise slowed, shoaling became more frequent and aggradation was limited by reducing accommodation space. Constructional facies are characterised either by massive corals, including Leptastrea, Porites and faviids, or by branching corals, typically Acropora of the danairobusta group. Coral surfaces may be encrusted by red algae, foraminifera and vermetids, and are commonly bored by filamentous algae, clionids and molluscs. However, detrital facies are volumetrically dominant, and the paucity of a constructional framework requires a re-evaluation of models of reef accretion. New models relate the geometry of accretion to the interplay between extreme storm events and fairweather hydrodynamic conditions. These suggest that a contiguous framework forms in areas of moderate fairweather energy without extreme storm events. Severe storms destroy the continuity of reef structures and generate increasing volumes of coarse detritus. Low storm severity, coupled with low fairweather hydrodynamic energy, may promote the accumulation of fine-grained reef-derived sediments that inhibit framework growth. While ecology reflects year-by-year sea conditions, lithology and structure are controlled by exceptional storms, with the effects of changing sea level superimposed.

Retrograde Accretion of a Caribbean Fringing Reef Controlled by Hurricanes and Sea-level Rise

“…But in an increasing number of areas, coring has shown that not all reefs develop this way (Blanchon et al, 1997;Braithwaite et al, 2000;Meyer et al, 2003;Blanchon and Perry, 2004;Palmer et al, 2010;Ryan et al, 2016). Studies of fringing reefs in the tropical Western-Atlantic in particular, are beginning to show that hurricanes play a prominent role in controlling both the configuration and composition of these structures (Blanchon et al, 1997;Macintyre et al, 2001;Perry, 2001;Blanchon and Perry, 2004).…”

Spatial and temporal dynamics of Arabian Gulf coral assemblages quantified from remote-sensing and in situ monitoring data

“…Reef growth needs a long enough period of stability to allow 'catch-up' (van Woesik & Done 1997, Braithwaite et al 2000. If a critical disturbance-threshold is exceeded, no reefs can be built, even if the area in question is rich in corals (Riegl 2001).…”

New constraints on late Holocene eustatic sea-level changes from Mahé, Seychelles

“…Reefs are better developed on the east than the west coast where they are protected from the impact of waves generated by the NW monsoon (October -March). Bays around the island are partly filled by the 'plateau'; a discontinuous belt of carbonate sands up to 500 m wide that borders the present beach and reef flat and which has a surface relief that is up to a metre above highest tide level (Lewis 1969;Braithwaite et al 2000). We hypothesise that these sands are fossil beach features which formed on the fringing reef flat when Holocene RSL was close to or above present.…”

“…The main granitic islands, which are thought to be tectonically stable over Quaternary timescales (Israelson and Wohlfarth 1999), lie in the northeast of the archipelago, with the largest and principal island of Mahé at 4° 41'S, 55° 28'E. The geology of these northern islands is Precambrian granite with dolerite intrusions; they sit on the Seychelles Bank, a 43,000 km 3 area of shallow shelf between 44-65 m deep, once part of a micro-continent connected to India and 5 Madagascar (Braithwaite et al 2000;Torsvik et al 2001). Mahé sits at the centre of the Seychelles Bank, with a 50-100 km-wide shallow shelf surrounding the island ( Figure 1A).…”

“…Dated cores taken from a fringing coral reef at Anse aux Pins on Mahé (Figure 1) extend back to c. 10 ka BP (Braithwaite et al 2000;Camoin et al 2004). These authors interpret this reef to have been in "catch-up" mode through the Holocene, with a variable relationship between coral and water depth (Camoin et al 2004).…”