Leg 182 drilling in the Great Australian Bight will allow 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. As 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, at present the paleoceanographic development of this area is not nearly as well known 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.
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, as 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, in press). 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 then be utilized to address two major questions of global and
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 then be utilized to address two major questions of global and temporal significance.
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 groundwater circulation may occur, at least at shallow depths. The drive for such a circulation may come from temperature contrasts between cool ocean waters and groundwaters 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 groundwater 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 analogues 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 analogues. 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.
To 182 Drilling Strategy
To 182 Table of Contents