DECODING THE GEOLOGIC RECORD OF HOLOCENE SEA-LEVEL CHANGE IN THE HAWAIIAN ISLANDS

Coastal systems in Hawaii are subject to high energy levels related to large swell, tsunami, tropical cyclones, extreme tides, and periods of intense rainfall. These are modulated by climatic factors such as El Niño, the Pacific Decadal Oscillation, and stochastic shifts in North Pacific currents and SST's. Although carbonate accretion (i.e., eolianites, reef framework, coral growth, beachrock, and intertidal notches) can record past sea-level positions, in general these are not sensitive records and are contaminated by environmental noise. For example, typically only 3 to 5 species reflect over 75% of living coral cover on a fringing reef and these vary in proportion and depth to reflect a continuum of seasonal wave stress, decadal disturbance, and ecological succession. From one reef to another the same species exhibit high plasticity such that they change growth morphology in response to bed shear stress. Interpreting cored sequences of these assemblages for past sea-level positions can be challenging. Nonetheless, the Hawaiian carbonate system has revealed several notable patterns that characterize Holocene sea-level change: 1) accelerated rise ca. 8.1 to 7.9 kyrs; 2) widespread end of fringing reef accretion ca. 5 kyrs related to strengthened El Niño and onset of extreme interannual swell; 3) culmination of sea-level rise ca. 3 kyrs; 4) late Holocene coastal plain development until the historic period; 5) modern sea-level rise characterized by extreme tides and shoreline recession. We will discuss the evidence and geologic framework for these Holocene events as well as the 1 m rise scenario for urban locations.