Reef growth and carbonate sedimentation at the southernmost Pacific reefs

Abstract

Lord Howe Island and Balls Pyramid are located at the latitudinal limit of coral reef formation in the Tasman Sea, with the fringing reef on the western margin of Lord Howe Island considered to be the southernmost coral reef in the Pacific. An extensive submerged fossil reef occurs on the shelf around Lord Howe Island. Mapping of the shelf surrounding Balls Pyramid, 24 km south of Lord Howe Island, has revealed a comparable submerged mesophotic reef system in 30–50 m water depth supporting scattered live coral colonies. Radiocarbon dating of coral extracted from limestone cores on the fossil reef reveals that accretion occurred during the early Holocene (10,100–8800 cal. Years BP), concurrent with the first phase of Holocene accretion around Lord Howe Island. The timing of fossil coral growth on the shelves of the two islands indicates that the reefs have been drowned by rising sea level. The fossil reef around Balls Pyramid is a give-up reef that was unable to keep pace with sea level. After its demise, carbonate sediment production continued across the shelf with sands and gravels that have accumulated in basins and channels, between the submerged fossil reef structures. Vibrocores record a fining upward sequence of sands overlying rhodoliths and angular carbonate gravels. The coral reef around Lord Howe Island persisted a little longer (∼7000 cal. Years BP) and has accreted into shallower water (∼25 m). The reef here was able to backstep to form the modern fringing reef. Although similar benthic habitats characterise both shelves, availability of suitable accommodation space on Lord Howe Island enabled deposition of sequences of eolianite during successive Late Quaternary highstands, and the development of a modern fringing reef and lagoon over eroded basalt, and late Pleistocene reef and eolianite. Antecedent topography has provided a first-order control on establishment and persistence of reef habitats during post-glacial sea-level rise, and the relative extent of past reef structures and eroded bedrock outcrops has constrained coral reef accretion at this latitudinal limit.