The nature and significance of microatolls

Abstract

Microatolls, those coral colonies with dead, flat tops and living perimeters, result from a restriction of upward growth by the air/water interface. The principal growth direction is horizontal and is recorded in the internal structure, though fluctuations in water depth can influence the surface morphology producing a terraced effect. The morphology of the basal surface of the colony is controlled by the sand/water interface such that the thickness of the coral records the depth of water in which it lived. In open water at the margin of reefs in the Northern Province of the Great Barrier Reef, tall-sided uneven-topped microatolls live, whereas, on the reef flats in rampart-bounded moats and ponds, thin flat-topped and terraced microatolls are abundant. Because water in moats can be ponded to levels as high as high water neaps (1.6 m above datum at Cairns) and still have daily water replenishment, microatolls on reef flats can grow to levels 1.1 m higher than open-water microatolls (which grow up to a maximum elevation of low water springs, i.e. 0.5 m above datum). This imposes a major constraint on the use of microatolls in establishing sea level history. The two factors controlling pond height during one sea stand (relative to the reef) are tidal range (which governs the height of high water neaps) and wave energy (which governs the height of ramparts which enclose moats). Dating and levelling fossil microatolls exposed on the reefs show that 4000 years (a) B.P., high water neaps was at least 0.7 m higher than it is at present.