RADIOCARBON ESTIMATES OF TIME-AVERAGING ON HOLOCENE FOSSIL REEFS AND IMPLICATIONS FOR QUATERNARY SEA LEVEL CURVES

The accuracy of global eustatic Quaternary sea level curves measured from raised Quaternary reefs, using radiometric ages of corals at known heights, may be limited by time-averaging, which affects the variation in coral age at a given height. We assessed time-averaging from uplifted Holocene reef sequences from the Huon Peninsula, Papua New Guinea, using extensive radiocarbon dating of coral skeletons in both horizontal transects and vertical sequences. Calibrated 2s age ranges varied from 800 to 1060 years along horizontal transects, but weighted mean ages calculated from 15-18 dates per horizon were accurate to within 154-214 years (2s, calibrated ages). Approximately 40% of the variability in age estimate resulted from internal variability inherent to ¹⁴C estimates, and 60% was due to time-averaging, and not dependent on dating method. There was no consistent relationship between reef accretion rate and degree of time averaging or age range of corals in horizontal transects. Because most corals dated were whole and upright, most time-averaging was dominantly ecological time-averaging, resulting from mixed age of recruitment on topographically irregular surfaces, rather than sedimentological time-averaging resulting from vertical mixing of corals in sediment of the taphonomically active zone. Rare corals that were older than all other corals in a horizontal transect did not have a distinct taphonomic signature indicating long exposure on the sea floor. Paleoecological reconstructions of reef communities based on horizontal transects or bedding plane exposures may include several generations of corals, of which most but not all co-occurred in life. Age estimates of sea level change in studies using single dated corals as proxies for sea level are probably accurate to within 1000 years of reported sea level peaks, but can be resolved to £200 years if supported by dates from analysis of a statistical population of corals at each stratigraphic interval.