SCIENTIFIC OBJECTIVES

The scientific themes of Leg 194 include the following:

1. Magnitude of the second-order eustatic sea level fall during calcareous nannofossil Zones N12-N14: The Marion Plateau provides ideal targets to address the magnitudes and effects of sea level change on continental margin sediments. Although one of the fundamental controls on the nature and geometry of continental margin deposition is sea level change, much of the information on the relationship between sea level and depositional facies is qualitative. Leg 194 drilling will determine the magnitude of the Zone N12-N14 sea level fall to help calibrate the Phanerozoic sea level curve.
2. Development of carbonate platforms in a current-dominated environment: Slope sedimentation adjacent to most carbonate platforms is controlled by the prevailing wind direction with the windward side of the platform being relatively sediment starved and the leeward side having higher depositional rates. On the other hand, carbonate platforms and slope sediments of the Marion Plateau are controlled by strong seafloor currents. These currents determine the morphology and growth potential of the platforms as well as the location and amount of sediment transported from the platform top. The results of Leg 194 drilling, along with available seismic data, will enable the characterization of these current dominated carbonate platform systems.
3. Development of subtropical carbonate platforms: The carbonate platforms of the Marion Plateau are dominated by subtropical and cool subtropical carbonates. The results of Leg 194 coring will bridge the gap between the cool-water carbonates sampled during Leg 182 (Great Australian Bight: Cenozoic cool-water carbonates) and the tropical to subtropical carbonates recovered during Legs 133 (Northeast Australian Margin) and 166 (Bahamas Transect). Stable isotopic data from Leg 133, Site 811, on the more northerly Queensland Plateau, showed that during the late Miocene, regional SSTs were cool (~20°-22°C), as were global SSTs (Isern et al., 1996). Given the more southerly location of the SMP with respect to Site 811 (Queensland Plateau), temperatures were probably similar to, if not cooler than, those over the Queensland Plateau. SSTs at or below 20°C would not prevent tropical coral growth but would make it more likely that the platforms were constructed of a "cooler," more subtropical bioassemblage. Documenting the transition of cooler water biota into warmer water forms will be an important outcome of Leg 194.
4. Facies change and development of sequence stratigraphic units controlled by sea level changes in a mixed carbonate and siliciclastic sediment system: Leg 194 coring recovered a detailed record of carbonate and siliciclastic sediment facies variations resulting from the mixing of sediment from carbonate banks on the Marion Plateau and the continental margin of Australia. In general, carbonate sediment export increases during sea level highstands as carbonate banks have additional accommodation space for increased growth of carbonate-producing organisms. On the other hand, terrigenous sedimentation generally decreases during highstands because of the elevated erosional base level. Detailed analysis of the cumulative result of these different sedimentological responses to sea level forcing will be an important result of Leg 194 drilling.
5. Oligocene-Pliocene third-order sea level fluctuations: The Oligocene-Pliocene sea level record preserved in the carbonate platform growth phases of the Marion Plateau includes a third-order event stratigraphy within the second-order sea level falls that dominate the sequence stratigraphic framework. Analyses of these variations, and the higher order fluctuations that are contained within them, provide information on the timing and influence of sea level on the carbonate growth phases and sedimentation of the Marion Plateau.
6. Mechanisms and causes of fluid flow within pure carbonate and mixed siliciclastic/carbonate depositional environments: Determining the mechanism and rate of fluid transport through carbonate platforms and reef structures is critical to understanding diagenetic processes (Buddemeier and Oberdorfer, 1986) and the geochemical cycling of many elements. Fluid movement has the ability to chemically alter the mineralogic composition of the sediment by hastening the conversion of metastable minerals such as high-Mg calcite and aragonite to more stable calcite and dolomite (Mullins et al., 1984; Simms, 1984). Alteration of carbonate sediments to dolomite has been significant in both the Bahamas (Varenkamp, 1991) and the carbonate platforms of northeast Australia (McKenzie et al., 1993; Davies, McKenzie, Palmer-Julson, et al., 1991). Studies using ⁸⁷Sr/⁸⁶Sr isotopic ratios have shown that carbonate sediments off northeast Australia were dolomitized by multigenerational fluids flowing through the platforms (McKenzie et al., 1993). Fluid flow can also alter sedimentary structure, permeability, and porosity of a carbonate deposit, thus having important effects on flow pathways and reservoir potential. The existence of fluid flow has been described in tropical carbonate platforms such as the Great Bahama Bank and the Queensland Plateau (Eberli, Swart, Malone, et al., 1997; Elderfield et al., 1993) and also in temperate-water carbonates (Feary, Hine, Malone, et al., 2000). However, the mechanisms causing this flow are neither well documented nor understood.
7. Role of climatic and paleoceanographic change in the subtropical South Pacific and its influence on carbonate platform development: In addition to sea level fluctuations, paleoceanographic variations in the western Coral Sea have significantly affected the development of carbonate platforms and reefs off northeast Australia. Paleocirculation has been modified both by the movement of continental fragments resulting from local rifting events and by the northward movement of the Indo-Australian plate. Northward movement of the Indo-Australian plate also resulted in significant variations in climate caused by movement across climatic boundaries. These changes, in addition to global climatic variations, influenced the depositional environments in the Coral Sea, which today are dominated by tropical carbonates.