Reconstructing sea-level change from coral micro-atolls, Tongareva (Penhryn) Atoll, Northern Cook Islands

T. Spencer (with AW Tudhope and TP Scoffin (Department of Geology and Geophysics, University of Edinburgh) and JR French (Department of Geography, University College London))

In some parts of the world, oceanographic processes give rise to inter-annual fluctuations in sea level which are as great as the projected increase in global sea level over the next one hundred years. These sea level changes may be recorded in the surface topography of large, pancake-like coral colonies - micro-atolls - in intertidal reef environments. With vertical growth constrained by exposure at low spring tides, micro-atolls usually only grow laterally. However, if sea level changes the living coral rim responds in the vertical, either dying back downwards on sea level fall or growing upwards in response to sea level rise. Annual growth bands within the micro-atoll then allow retrospective dating of these sea level signals. At Tongareva Atoll, South Central Pacific Ocean, a micro-atoll based sea level record for the last 60 years has been constructed from i) detailed (2 cm spacing) transects of micro-atoll surface micro-topography and ii) X-ray photographs of annual skeletal density bands in coral slabs recovered from coherent coral wedges sawn from selected micro-atolls. The concentric ring structure of the micro-atoll surfaces shows not only a general rise in sea level over the life of the colonies but also substantial sea level variations on inter-annual timescales related to El Nino Southern Oscillation (ENSO)-driven sea level fluctuations.