Palaeomagnetism, rock magnetism and evolution of the Great Barrier Reef

Abstract

Palaeomagnetic and rock magnetic properties have been determined for sediment cores collected during ODP Leg 133 from the continental slope adjacent to the Great Barrier Reef. The magnetic remanence properties of the sediments are poor, with a pervasive viscous overprint that obscures most reversal boundaries. However, an inter-core magnetic susceptibility stratigraphy has been established that permits some refinement of the existing biostratigraphy. The quality of the palaeomagnetic remanence record relates to the residence time of magnetic particles in the corrosive, near-surface, sulphate-reducing zone. Magnetic susceptibility variations appear to reflect glacio-eustatic sea level cycles, and show a strong inverse correlation with both d18O and carbonate content. For the last several glacial maxima there are marked changes in magnetic properties, largely grain size-related, with sharp peaks in susceptibility and a reversal of the d18O-susceptibility phase relationship. One possible explanation can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop during times of low sea level and become reworked during transgressions. Changes in the pattern of susceptibility variation occur at approximately 0.8 Ma and 0.4 Ma. Boundaries at these ages are seen in seismic profiles, they occur in the d18O frequency spectrum, and there is a rise in sea surface temperature at ~0.4 Ma. We suggest that these boundaries relate through palaeoclimatic and palaeoceanographic changes to stages in evolution of the Great Barrier Reef.