EL NIÑO INFLUENCE ON HOLOCENE REEF ACCRETION IN HAWAII

Observations of reef accretion from Hawaii show that accretion during early to middle Holocene time occurred in areas where today it is precluded by the wave regime, suggesting an increase in wave energy. Many coastal areas in the main Hawaiian Islands are periodically exposed to large waves, in particular from North Pacific swell and hurricanes. These are of sufficient intensity to prevent modern net accretion as evidenced by the antecedent nature of the seafloor. Only in areas sheltered from intense wave energy is active accretion observed. Analysis of reef cores reveals patterns of rapid early Holocene accretion in several locations that terminated by middle Holocene time, ca. 5,000 BP despite a continued rise in sea level until 3,000 to 4,000 BP. Previous analyses have suggested that changes in Holocene accretion were a result of reef growth "catching up" to sea level. New data and interpretations indicate that the end of reef accretion in the middle Holocene may be influenced by factors in addition to sea level. Reef accretion histories from Kauai, Oahu, and Molokai suggest that a change in wave energy contributed to the reduction or termination of Holocene accretion by 5,000 years ago. Evidence indicates that rapid accretion occurred at these sites in early Holocene time, and that no permanent accretion is occurring at these sites today. This pattern persists despite the availability of hard substrate suitable for colonization at a wide range of depths. We infer that forcing other than RSLR has altered the natural ability to support reef accretion on the Oahu shelf. The limiting factor in these areas today is wave energy. Numbers of both large North Pacific swell events and hurricanes in Hawaii are greater during El Niño years. We infer that if these major reef-limiting forces were suppressed, net accretion would occur in some areas in Hawaii that are now wave-limited. Studies have shown that El Niño /Southern Oscillation (ENSO) was significantly weakened during early-mid Holocene time, only attaining an intensity similar to present ca. 5,000 years ago. We speculate that this shift in ENSO may assist in explaining patterns of Holocene Hawaiian reef accretion that are different from those of the present and apparently not related to RSLR.