Sediment fluxes to Marion Plateau (southern Great Barrier Reef province) over the last 130 ky: New constraints on ‘transgressive-shedding’ off northeastern Australia

Abstract

Generic models for sediment accumulation predict that fluxes of siliciclastic and neritic carbonate material to continental slopes of tropical mixed systems will be out-of-phase over cycles of sea level change. Siliciclastic and carbonate fluxes should be highest during lowstands and highstands, respectively, and condensed sections should form during transgressions. Recent studies have documented the opposite response for slopes along portions of the largest modern tropical mixed system, the northeast Australian margin: siliciclastic and carbonate fluxes are in-phase and highest during transgression. An outstanding issue, however, is the extent of this response because studies to date have mostly focused on sedimentation over the last 40 ky off the central GBR province, an area flanked by a narrow shelf and a wet tropical hinterland. Here we investigate mixed sediment deposition in a core from Marion Plateau, offshore a wide shelf and a dry tropical hinterland in the southern GBR province. Bulk carbonate content and carbonate mineralogy were determined downcore, while foraminiferal oxygen isotopes and radiocarbon ages were used to constrain ages and accumulation. Carbonate content and mineralogy vary predictably with changes in sea level over the last 130 ky; carbonate and aragonite are more abundant during highstands and less abundant during lowstands. However, component mass accumulation rates reveal that highest fluxes of all material (siliciclastic, low-Mg calcite, aragonite, and high-Mg calcite) occurred during transgressions. Sedimentation consistently diverges from conventional models in all cores examined to date from slopes of the northeast Australian margin, regardless of core location and modern differences in climate and physiography. The unexpected sediment response could result from changes in climate, particularly increased precipitation during deglaciations, although this does not explain elevated fluxes of low-Mg calcite. Alternatively, fluvial sediments and diagenetically altered carbonate aggraded on the shelf during lowstands, and were eroded and transported to the slope during transgressions.