Cyclone frequency during the last 5000 years at Curacoa Island, north Queensland, Australia

A series of storm deposits at Curacoa Island, a steep bedrock island in the Palm Group, north Queensland, record the passage of cyclonic events. Storm ridge accretion occurs when cyclonic waves and surge combine to transport coarse sediment, mostly broken coral, onto the shore and deposit it above normal tidal levels. Narrow fringing reefs at Curacoa act as a rapidly replenished sediment source, augmented by lithic gravel transported longshore. Beachface, stormberm, ridge platform and washover deposits can accrete during a single event. A series of storm ridges were examined via stratigraphic pits and trenches on two traverses; deposits from 22 events were identified and their ages were determined using 68 radiocarbon dates from coral clasts. Most dates from each storm deposit formed statistically accordant groups. Only 15% of the dates were older than the group ages and were interpreted as reworked specimens. Accordant ages within each group were combined and converted to calibrated radiocarbon ages. Ridge ages decrease uniformly with distance from the oldest ridge, indicating uniform depositional processes over the last 5000 years. A time series of storm deposit ages indicates that cyclone frequency was statistically constant over the last 5000 years. This suggests that cyclone frequency may not have bean affected by sea surface temperatures in the region, which appear to have been about 1 degreesC warmer about 5000 years ago (Gagan, M.K., Ayliffe, L., Hopley, D., Call, J., Mortimer, G., Chappell, J., McCulloch, M., Head, M.J., 1998. Temperature and surface water balance of the mid-Holocene tropical western Pacific. Science 279, 1014-1018). (C) 2001 Elsevier Science B.V. All rights reserved.