Holocene reef accretion: Southwest Molokai, Hawai'i

Abstract

Two reef systems off south Molokai, Hale O Lono and Hikauhi, show differences in modern ecosystem structure and Holocene accretion history that reflect the influence of wave-induced near-bed shear stresses on reef development in Hawaii. Both sites are exposed to similar impacts from southern, Kona, and trade wind swell, but differing wave impacts result from relative exposure to north swell. Analysis and dating of 10 cores from Hale O Lono reveal a back stepping reef ranging from ~8,100 cal yr BP (offshore) to ~4,800 cal yr BP (nearshore). A depauperate community of modern coral diminishes shoreward and seaward of~15 m depth due to wave energy, disrupted recruitment activities and physical abrasion. Evidence suggests a change from conditions conducive to accretion during the early Holocene to conditions detrimental to accretion in the late Holocene. Reef structure at Hikauhi, reconstructed from 14 cores, reveals a thick, rapidly accreting and young (maximum age ~900 cal yr BP) prograding reef. Modem live coral cover on this reef increases seaward with distance from the reef crest but terminates at a depth of ~20m where the reef ends in a large sand field. The primary limitation on vertical reef growth is accommodation space under wave base, not recruitment activities or energy conditions. This study suggests modem reef growth on the southwest comer of Molokai is controlled by wave-induced near-bed shear stress related to refracted North Pacific swell. Holocene accretion patterns here also reflect the influence of wave-induced near-bed shear stress from north swell. Other factors controlling Holocene reef accretion are relative sea-level and wave sheltering by Laau Point. Habitat suitable for reef accretion on the southwest shore of Molokai has shrunk throughout the Holocene.