New constraints on the origin of the Australian Great Barrier Reef: Results from an international project of deep coring

Two new boreholes provide the first direct evidence of the age of the Austr alian Great Barrier Reef. An inner shelf sequence (total depth, 86 m; basal age = 210 +/- 40 ha) comprises a dominantly silicielastic unit (thickness similar to 52-86 m), overlain by four carbonate units (total thickness 0-34 m). A shelf-edge and slope sequence (total depth 210 m) reveals three majo r sections: (1) a lower section of resedimented flows deposited on a lower slope, (2) a mid-section including intervals of corals, rhodoliths, and cal carenites with low-angle graded laminae, and (3) an upper section of four s helf-margin coral-reef units separated by karst surfaces bearing paleosols, Sr isotope and magnetostratigraphic data indicate that the central Great B arrier Reef is relatively young (post Bruhnes-Matuyama boundary time), and our best estimate for the onset of reef growth on the outer barrier system is ca. 600 +/- 280 ka. This date suggests that reef initiation may have bee n related to the onset of full eccentricity-dominated glacio-eustatic sea-l evel oscillation as inferred from large-amplitude "saw-tooth" 100 k.y. delt a O-18 cycles (after marine isotope stage 17), rather than to some regional environmental parameter. A major question raised by our study is whether r eef margins globally display a similar growth history. The possibility of a global reef initiation event has important implications for basin to shelf partitioning of CaCO3, atmospheric carbon dioxide levels, and global tempe rature change during Quaternary time.