Leg 182 drilling in the Great Australian Bight was designed around six fundamental scientific topics to be addressed:
1. The paleoceanographic history of a carbonate-dominated, mid-latitude continental margin and adjacent basin during evolution of the Southern Ocean. Because the Southern Ocean is one of the major controlling influences on global circulation and climate, it is imperative that the oceanic history of this region be refined as much as possible. However, the paleoceanographic development of this area is not nearly as well known at present as that of the high-latitude North Atlantic (Kennett and Barron, 1992). Although there are numerous paleoceanographic problems that can be answered by the Leg 182 drilling transect, four stand out as critical:
(a) The relationship between circulation patterns in the deep ocean and on the shelf during times of warm vs. cold ocean conditions. The stratigraphic record in the Southern Ocean is punctuated with numerous breaks in sedimentation that are attributed to erosive periods related to increased circulation during initiation of the Circum-Antarctic Current (Miller et al., 1987; Kennett and Barker, 1990). Although such hiatuses are thought to develop on deep margins during times of lower sea level and to correlate with unconformities on the continental shelf, there are apparently continuous onshore sequences of Oligocene shelf carbonates that were deposited concurrently with erosion or nondeposition of the entire Oligocene succession on the adjacent ocean floor.
(b) The precise timing and nature of the opening of the Tasman Gateway. Subsidence of the Tasman Rise, which permitted initiation of the cold Circum-Antarctic circulation and thermal isolation of Antarctica, is one of the most important developments in Cenozoic paleoceanography (Kennett, 1982). The history of this event is poorly constrained because so much of the oceanic record is missing as a result of seafloor erosion. The Leg 182 shelf-to-basin transect is sufficiently proximal to the Tasman Rise that it should contain an excellent record of the paleoceanographic development of this seaway.
(c) The evolution and effect of the Leeuwin Current. The first evidence for the existence of the Leeuwin Current occurred in middle Eocene time, when currents from the Indian Ocean were deflected into the elongate proto-Great Australian Bight embayment. In support of this, the record of warm-water intervals is more common in the west than it is in the east, implying that the source of the water is from the west. Studies of Quaternary cores from the Great Australian Bight suggest a complex interplay between the Leeuwin Current and the West Wind Drift. This interplay appears to have had dramatic effects on primary productivity, as the Leeuwin Current is a source of warm oligotrophic waters, whereas the West Wind Drift causes upwelling of cooler eutrophic waters.
(d) The relationship between primary productivity and cool-water carbonate development. The Leg 182 shelf-to-basin transect should contain an important record of paleoproductivity linked to upwelling. Such periods should be recorded in the biota by low species diversity, high numbers of individuals, increased sedimentation rates, and distinctive changes in stable-isotopic and trace-element compositions.
2. The formulation of models for carbonate sedimentation on continental margins bathed predominantly by cool oceanic waters. The deposition and accumulation of platform (neritic) carbonate sediments under cool-water (~<20°C) conditions is poorly understood compared to warm-water carbonates, primarily because the database is so small (Nelson, 1988; James and Kendall, 1992). Yet because of their dominantly skeletal composition, nutrient-dependent biology, and low diagenetic potential, cool-water carbonates record the history of oceanic change in ways that are profoundly different from tropical carbonates. In hydrodynamic terms, cool-water carbonate shelves are hybrids, possessing some of the characteristics of both terrigenous clastic shelves and warm-water carbonate shelves. Sediments are produced on the shelf, in contrast to terrigenous clastic shelves where sediment is transported onto the shelf from the hinterland. Without the elevated rim that typifies warm-water carbonate shelves, however, the sediments are subject to the full sweep of oceanic waves and swells, because they are on terrigenous clastic shelves. Cenozoic exposures of inner-shelf facies in Australia suggest that storm- and wave dominated processes tend to control deposition. By contrast, many contemporaneous deposits in New Zealand are clearly tide dominated. Are the models of wave-dominated shelf deposition developed onshore applicable throughout the Cenozoic? All seismic profiles across the southern margin of Australia indicate that a large proportion of the youngest part of the succession is made up of prograding clinoforms (James and von der Borch, 1991; Feary and James, 1998). Such clinoforms seem to be a signature of cool water platforms and ramps and are postulated to be a product of accumulation dynamics (Boreen and James, 1993). There is little information regarding the composition of these deposits. Specifically, are they produced by in-place, enhanced bioproduction along the shelf edge, or are they made up of finer grained material produced on the shelf and swept offshore to accumulate below the wavebase?
3. Determination of the Southern Ocean basin sea-level record, and the effect of sea-level fluctuations on stratigraphic packaging and early diagenesis of cool-water carbonates. The Eucla margin is rich in biogenic carbonate sediments that respond in a sensitive way to variations in sea level and contain vital geochemical information needed for linking sea level changes to paleoceanography. This information can be utilized to address two major questions of global and temporal significance.
(a) What is the detailed sea-level history of the Southern Ocean basin, and can it be linked to paleoceanographic variations? The southern Australian neritic shelf record, derived largely from onshore successions in which the marine record is preserved only in highstand systems tracts, appears to be at odds with the global model (Haq et al., 1987) during several critical periods. Is this because the sediments were deposited in cool water? Through the use of a combination of physical stratigraphy and proxy paleoenvironmental parameters in a much more expanded section than exists onshore, the well-preserved EoceneOligocene and early to middle Miocene successions will allow a thorough testing of this part of the sea-level curve and resolution of specific eustatic events. The late MiocenePliocene sequence is unknown onshore except for the early Pliocene highstand, so this will be the first clear record of this component of the sea-level record in the region.
(b) How do cool-water carbonate platforms respond to changes in sea level? Carbonate platforms, with their chemically metastable sediments born largely in place, are particularly responsive to changes in seawater temperature and chemistry and variations in sea state and sea level. To date, most information on carbonate platforms comes from rimmed, warm-water platforms (Kendall and Schlager, 1981; Sarg, 1988). There is almost no information on the manner in which cool-water carbonate platforms respond to changes in these critical parameters at a variety of different time scales. Specifically, we require information to describe how different segments of the shelf react during different parts of the sea-level cycle and to determine whether cold- and warm-water carbonate platforms have basically different depositional geometries as a result of the different ways the carbonate factory responds to sea-level changes.
4. The circulation patterns of shallow subsurface fluids in an area of low hydraulic gradient and minimal recharge. The Eucla margin is one of the few modern shelves where the onshore recharge zone is an areally vast, flat-lying karst (the Nullarbor Plain). The high primary depositional permeability of winnowed grainstones of the Eucla Shelf and the lack of early cementation suggest that significant ground-water circulation may occur, at least at shallow depths. The drive for such a circulation may come from temperature contrasts between cool ocean waters and ground waters warmed by geothermal heat flux (and possibly volcanics) within the shelf, concentrating waters on the shelf margin (Simms, 1984). Alternatively, despite inland aridity, recharge occurring over the vast continental hinterland may drive brackish to saline waters southward to discharge through the flooded shelf. Such a circulation has been recognized by James (1992) and is associated with cave development on the Nullarbor Plain (James et al., 1989). In contrast to the long-lived nature of the above systems, differences in sea-surface elevation, on and off the shelf, associated with regional wave buildup (Feary, 1995), current flow (Rockford, 1986), and atmospheric pressure system changes may cause pumping of marine waters into and out of the platform (Marshall, 1986).
5. Early seafloor and shallow burial diagenesis and dolomitization of calcite-dominated sediments. Cool-water carbonates exhibit a radically different pattern of diagenesis from that of tropical aragonitic carbonates. Slow sedimentation permits seafloor lithification by intermediate Mg-calcite cements, but these appear to be volumetrically limited and localized to omission surfaces and hardgrounds, which are ubiquitous in the inner platform. Indeed, both shallow-marine and meteoric cements appear to be very sparse, with magnesium being lost from high-Mg calcite to low-Mg calcite during grain recrystallization. Sparse calcium-rich dolomites may be present (Reeckmann, 1988; Bone et al., 1992), and at some locations replacement can be pervasive (James et al., 1993), although the fine subtidal evaporation-related dolomites typical of tropical platforms are absent. It is not known whether dolomitization is episodic, as recognized in other present-day platforms (Vahrenkamp, et al. 1991; McKenzie et al., 1993), or occurred over extended time periods. The Eucla margin carbonates will provide an opportunity to determine the present-day associations among ground-water circulation, fluid geochemistry, and diagenetic products, and by inference from the temporal and spatial distribution of ancient diagenetic components, those that occurred under different conditions in the past. This has the potential to provide fundamental insights into the diagenesis of cool-water, open-shelf carbonates, which are direct analogs for the comparable carbonate platforms that were ubiquitous during Paleozoic and other times.
6. The pace and style of evolution of mid-latitude oceanic and neritic biotas. The Leg 182 drilling transect offers the opportunity for pioneering analysis of the Cenozoic evolution of cool-water calcareous biota, with direct applicability to studies of ancient carbonate platforms presently lacking modern analogs. Linked information from the neritic and oceanic high- to mid-latitude carbonate realm should produce an unmatched record of paleobiological information. Specifically, the patterns and modes of speciation and diversification of coeval shallow- and deep-water benthic organisms as well as contemporaneous planktonic biota should be revealed. By comparing these results with those from Antarctica and the northeast Australian shelf, the geography of such processes and their relationship to physiochemical factors should be discernible. Paragraph ONE.
To 182 Synthesis and Results
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