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

Abstract

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 been affected by sea surface temperatures in the region, which appear to have been about 1°C warmer about 5000 years ago (Gagan, M.K., Ayliffe, L., Hopley, D., Cali, 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).

Introduction

Although tropical cyclones are extreme events, they are elements of the natural climate of northern Australia. Well dated coastal sequences of cyclonic deposits can be used to assess the frequency of past cyclones and assist long term climatic prediction. Partial records of cyclone activity during the last 4000 years in northeast Queensland were reported from Curacoa Island and Princess Charlotte Bay by Chappell et al. (1983), and from Lady Eliot Island by Chivas et al. (1986). These studies did not detect any statistically significant variation of depositional events, but stratigraphic sampling and dating were not exhaustive. Others have claimed that the frequency of ridge-building events in northern Australia varied in the last 6000 years (Rhodes, 1980, Lees and Clements, 1987), but Chappell and Thom (1986) argued that the variations may have been caused by changes of sediment supply.

The present study is based on the beach-ridge and shingle spit complex at Curacoa Island in the Palm Group, Central Queensland, first described by Hopley (1968). Radiocarbon dates of coral fragments collected from shallow pits along a transect crossing the beach ridges were reported by Chappell et al. (1983); however, their pits were irregularly spaced and had too few dates to assess possible variations of ridge building events. Here, we examine two new transects in detail. Radiocarbon ages of 22 morphostratigraphic units were determined with multiple dates from each unit, and the series of dated deposits was analysed for trends or changes in frequency of depositional events.