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.

Submergence and uplift associated with the giant 1833 Sumatran subduction earthquake: Evidence from coral microatolls

“…We interpret this history of vertical tectonic displacement to be the result of the interseismic accumulation of strain prior to the 1833 earthquake and its relief by slip along the subduction interface during the giant earthquake. Variations in water temperature, rainfall, sediment influx, and other factors cause the density of the skeletal material in these coral "heads" to vary seasonally in such a way that the concentric shells appear as annual growth bands [Scoffin and Stoddart, 1978;Stoddart and Scoffin, 1979]. These annual bands are similar to tree rings in that they provide a yearly record of coral growth throughout the life of the coral [Taylor et al, 1987;Zachariasen, 1998].…”

“…The upward growth of coral microatolls is limited by low water levels because exposure above water for prolonged periods, especially in sunlight, is fatal to the living corallites on the outer surface of the head [Scoffin and Stoddart, 1978 What is important about HLS for paleoseismic studies is that the microatolls retain a record of HLS in their skeletal morphology. Microatolls grow hemispherically, outward and upward, until they reach the HLS (Figure 2a).…”

“…Work of this kind began about two decades ago [Scoffin and Stoddart, 1978;Stoddart and Scoffin, 1979;Taylor et al, 1987]. These earlier studies showed that upward growth of microatolls is limited by lowest water levels.…”

A 15 year slow‐slip event on the Sunda megathrust offshore Sumatra

“…Microatolls are coral colonies that grow upward to a limit near mean low water springs, and their upper surfaces can track changes in relative sea level (RSL) over time [Scoffin et al, 1978;Taylor et al, 1987;Zachariasen et al, 2000;Meltzner et al, 2010]. Microatoll morphologies form because prolonged subaerial exposure at times of extreme low water limits the highest level to which the coral colonies can grow [Briggs et al, 2006;Meltzner et al, 2010;Meltzner and Woodroffe, 2015].…”

Reproduction by Fragmentation in Corals

“…At many Indoflat, more or less circular shape. At Enewetak, miPacific locations, the massive coral Porites lutea forms croatolls detached from the reef due to their relatively microatolls on intertidal reef flats (Dana, 1853; Scoffin small basal attachment and the activity of boring and Stoddart, 1978;Highsmith, 1980aHighsmith, , 1981. Low tides organisms, are transported by currents across the prevent upward growth of microatolls so live tissue is smooth reef flat to the edge of the lagoon where t h e~r restricted to the periphery of the colonies resulting in a growth is less restricted by low tides (Highsmith.…”