Late Quaternary Mixed Carbonate-Siliciclastic Sediment Slope Accumulation: Unexpected Responses of Australia and Papua New Guinea Reefs to Glacio-Eustatic Sea Level Fluctuations
Harper, Brandon Babbel
Droxler, Andre W
Doctor of Philosophy
Overtime, the north Queensland (Australia) and Gulf of Papua (Papua New Guinea) margin has been constructed by the biogenic carbonate growth and development of large barrier reefs on the mid-to-outer shelves and offshore platform/atoll tops, in conjunction with latitudinally variable riverine delivery of terrigenous siliciclastic material to the coast and inner shelf. This association forms Earth’s longest tropical mixed carbonate-siliciclastic system. Spatial and temporal variations of neritic carbonate (chiefly aragonite) production and siliciclastic sediment supply are controlled by late Quaternary high amplitude sea level fluctuations and climate changes. Sediment accumulation on the upper slopes adjacent to mixed margins is an important indicator of reef establishment/demise and siliciclastic shelf bypass during a time when glacio-eustatic sea level fluctuations are well understood and sediment dating techniques well developed. Sedimentation variability since the last deglaciation and, to some degree, during the last glacial cycle, is well understood and described by the transgressive and highstand shedding, reciprocal and coeval sedimentation concepts; based on low resolution records. My Ph.D. research results enhance the understanding of reef initiation, accretion, and demise along the Great Barrier Reef and Gulf of Papua margin by adding new cores and elemental (Sr, Ca, Si, K, Al) data from high resolution x-ray fluorescence core scanning. Sea level related reef evolution along this margin is more complex than previously thought. Contrary to established models, the central Great Barrier Reef exited the photic zone during peak interglacial, Marine Isotope Stage (MIS) 5e, resulting in the drowning of the reef and extremely low aragonite mass accumulation on the upper slopes. When sea level fell, reef-tops re-entered the photic zone increased production and export of aragonite to the upper slopes. The precise timing and elevation of sea level during MIS-5a is defined by the presence and absence of aragonite within slope sediments from the flooding and exposure of reef-tops. Additionally, along the Papuan Peninsula, a barrier reef established on a glacial shelf edge delta during re-flooding at 19 ka, followed by the subsequent drowning and back-stepping of the reef, synchronous to Termination I melt water pulses, away from siliciclastics.