The Oceanographic Influence of Sedimentation on the Continental Shelf: A Numerical Comparison Between Tropical and Antarctic Environments.

Sedimentation of two contrasting continental shelf environments have been investigated. Numerical ocean models were used to assess the oceanic processes which dominate Torres Strait and the Gulf of Papua, Northern Australia, and then the ocean cavity beneath the Amery Ice Shelf, East Antarctica. A three-dimensional, numerical ocean model (MECO) was applied to the Torres Strait/Gulf of Papua region at 0:05 degree resolution. Validation of the model was carried out against current meter and sea-level observational data. Dispersal pathways of sediments, derived from the Fly River, Papua New Guinea, and from a resuspension event on the Northern Great Barrier Reef, into Torres Strait were investigated via the introduction of passive tracers into the model. Sediment input into Torres Strait is found to be greater during the Trade season by approximately 10%. Wave data was also obtained, and together with hydrodynamic model output, sediment mobility due to currents, waves and wave-current interactions was considered for both the Trade and Monsoon seasons. Sediment mobility in the Gulf of Papua is dominated by wave motion, whereas Torres Strait is a mixed environment of waves and tidal currents. Two numerical ocean models were applied to the Amery Ice Shelf/Prydz Bay region to determine the oceanic processes responsible for the distribution of marine sediments beneath the ice shelf. The MECO model was used to determine the sub-ice-shelf tidal environment. A modified version of the Princeton Ocean Model was applied to determine the baroclinic circulation beneath the ice shelf. Validation of the tides within each model was carried out against available current meter data and sea-level records. Spring tidal currents (up to 10-15 cm/sec) are approximately two to three times the magnitude of the maximum density-driven flows (5-6 cm/sec). However, the influence of tides on the mean sub-ice-shelf melt-rates, and the mean density-driven currents, was shown to be insignificant. Thermohaline flows dominate the predicted mean sub-ice-shelf circulation and indicate a depth-integrated mass transport of 0.2 Sv, of the same order of magnitude as the overturning circulation. Combined tidal and density-driven maximum bottom currents are too small (up to 16 cm/sec)to remobilise sediments. Sediments were collected from beneath, and directly in front of, the Amery Ice Shelf. The observed sub-ice-shelf surface sediment distribution supports the dispersal pathways of diatoms beneath the ice shelf, predicted using the baroclinic model. The sediment core collected from beneath the eastern side of the Amery Ice Shelf in a region of predicted inflow (site AM02) contains a Holocene-age, siliceous mud and ooze layer, providing evidence that landward transport of hemipelagic sediments occurs beneath the Amery Ice Shelf. An imposed increase in the ocean temperature in the baroclinic model predicts increased melt rates at the ice-ocean interface, and a strengthened sub-ice-shelf circulation. An increase in sea-ice associated diatom deposition during the mid-Holocene is interpreted from AM02 down-core changes in the diatom assemblage. Results of the hydrodynamic model suggest the diatom signal may be a response to increased temperatures which may have occurred during the mid-Holocene climatic optimum. The dominant processes acting in the two continental shelf environments are able to be distinguished from each other allowing separate classification; the Torres Strait/Gulf of Papua environment is classified as a wave- and tide-dominated shelf, and the Prydz Bay/sub-Amery Ice Shelf environment is classified as a density-driven current-dominated continental shelf, and is probably typical of Antarctic shelf environments.