POST-GLACIAL SEA LEVEL CHANGE IN THE FAR FIELD: HAWAII ISLANDS

The record of sea level (SL) changes in Hawaii is largely preserved in the form of carbonate reef sediments and corals. However, due to high environmental energy and plasticity of coral communities, the SL record lacks precision. For instance, only 3 to 5 species reflect the majority of coral cover on reefs and these vary in abundance to reflect a continuum of seasonal wave stress, decadal disturbance, and ecological succession. Coral species also exhibit high plasticity (they change growth morphology) in response to bed shear stress and light attenuation with depth. Uncertainties in SL position are compounded by climatic variables such as the ENSO and PDO processes. In this context, there are 4 primary "pinning points" for developing a post-glacial SL history in Hawaii: 1) 17-14.8 ka - A well-preserved, fossil coral tract W of Molokai observed and sampled with the Pisces V submersible in 165 to 125 m water depth. The assemblage is dominated by large (10-15 m tall) Porites head structures at depth, transitioning to smaller heads and then into smaller encrusting morphologies at the shallowest elevations. U-series ages of pristine skeletal material from 164 m to 133 m water depth indicate a mean relative sea level (RSL) change of 0.0080±.001 m/yr from 18.5 to 14.7 ka, i.e., in the time interval before MWP-1a. 2) 8.1 - 7.9 ka - A drowning event, recovered in cores of fossil reef offshore of Molokai, and corresponding to a drowned intertidal notch found on Molokai and Oahu. The cored sequence shows a brief coral growth hiatus with branching coral framestone (lower reef facies) abruptly overlain by encrusting coral-algal bindstone and unconsolidated rudstone (shallow facies). The sequence indicates rapid drowning (>1-5 m) ca. 8.1 - 7.9 ka. This event corresponds to the "8.1 ka event" climate episode seen in other coastal records. 3) 3.5 ka - A late mid-Holocene sea level highstand approximately 2 +/-0.35 m above present mean sea level. This high stand is consistent with "equatorial oceanic siphoning" proposed by Mitrovica and Peltier (1991) across the far field equatorial Pacific. 4) Modern - The tide gauge record showing a century of sea level rise and thus indicating a late Holocene lowstand of sea level at some point following the fall of the late mid-Holocene highstand (ca. 1.5 - 0.5 ka?).