Coral records of relative sea-level changes

Abstract

Coral reefs provide substantial evidence of Quaternary sea-level positions because of their geological preservation and suitability for dating, using both radiocarbon and U-series techniques. Interpretation of this evidence requires an understanding of reef geomorphology, modern reef organism distributions, and the environmental factors influencing them. Fossil reef terraces, formed during the Last Interglacial, marine oxygen isotope (MIS) substage 5e (~128–116ka), occur at elevations of at least 2m above present sea level on many tropical shorelines. Along plate margins experiencing rapid tectonic uplift there are flights of reef terraces, and radiometric dating of corals within these provided some of the first evidence in support of the orbital fluctuations of sea level, related to Milankovitch cycles. Observations of corals at elevations up to 8–9m above present on apparently stable shorelines continue to contribute to ongoing debate about the height reached by sea level during the Last Interglacial. Elsewhere Last Interglacial reefs are found below sea level. Oscillation of sea level during MIS 5e has been interpreted from several sites, with some studies inferring rapid rise of several meters during the interglacial. Older reef terraces are apparent on uplifted margins, but ages become increasingly unreliable before MIS7. Drilling of coral reefs, initially in Barbados and on the Huon Peninsula in Papua New Guinea, but augmented by more recent Integrated Ocean Drilling Program (IODP) expeditions to Tahiti, and the Great Barrier Reef, has provided insights into Last Glacial Maximum (LGM) and deglacial sea level changes. Dating of corals indicates a stepped fall to the LGM followed by several phases of rapid sea-level rise, attributed to pulses of meltwater input to the oceans, resulting in drowning and demise of the reefs. The magnitude and timing of these remains poorly resolved and is a function of the extent to which fossil corals may have been in growth position and the paleowater depth in which they grew. Studies of Holocene coral reefs have indicated three different modes of reef growth; some reefs kept up with sea level, others caught up when sea level decelerated, whereas some were drowned, or back-stepped with establishment of a new reef higher and landward of the former reef structures. Holocene sea level appears to have experienced a gradual rise up to present across the Caribbean, providing accommodation space for reefs to accrete vertically; whereas in the Indo-Pacific sea level has been near its present level for the past 6–7000 years, with many reef flats emergent following a slight fall of sea level caused by ocean siphoning. Microatolls, large flat-topped intertidal corals on reef flats, provide the clearest evidence of past sea-level position, but, in their absence, novel biological or other sea-level indicators, such as thick crusts of specific coralline algae and vermetids, are required to better constrain paleowater depths. The upper surface of living microatolls can preserve a subdued indication of water level changes that have occurred during the decades of its lifetime. The sensitivity of corals to exposure by the lowest tides means that long-lived intertidal microatolls can contain a filtered record of low water levels, with the prospect of extrapolating this across recent centuries to millennia using preserved fossil colonies. Some living intertidal corals show upward growth in response to the increased accommodation space as a result of ongoing sea-level rise.