RECONSTRUCTING MID-LATE HOLOCENE CYCLONE AND EL NIÑO VARIABILITY IN THE CENTRAL PACIFIC USING SEDIMENTARY RECORDS FROM TAHAA, FRENCH POLYNESIA

We lack an understanding of the geographic and temporal controls on South Pacific cyclone activity. Overwash records from back-barrier, coastal wetlands and shallow lagoons have been used to reconstruct tropical cyclone strikes in the North Atlantic basin. However, a lack of back-barrier environments has limited Central Pacific cyclone records to the period of modern observation. This instrumental record suggests a weak correlation with the El Niño-Southern Oscillation (ENSO) but longer records are necessary to constrain this relationship over geologic timescales.

Back-reef lagoons surrounding high volcanic islands in the Central Pacific may offer yet unexplored records of both Holocene ENSO and tropical cyclone variability. Flooding and overwash of these reef systems during extreme storm events results in deposition of coarse material in the lagoon, yielding a long-term archive of cyclone activity. Likewise, drowned paleo-river valleys incised into the volcanic upland preserve terrestrial sediments likely shed by nearby slopes during periods of increased precipitation, recording changes in El Niño activity.

Using sediment cores taken from a back-reef lagoon surrounding Tahaa, an island in French Polynesia, we reconstruct 4000 years of ENSO-tropical cyclone interaction. Coarse grained layers are most likely related to overwash, moving bioclastic sediments on the reef flat into the lagoon during storm events. Comparison of these observed changes in sediment size to a nearby record of El Niño activity based on terrestrial runoff into the lagoon indicates a weak correlation between storminess and ENSO. Both tropical cyclone and El Niño activity show significant intensification since the mid-late Holocene. Together, these records provide the first insights into the long-term cyclone-ENSO interaction in the South Pacific as well as a template for developing a network of overwash records from coral islands.