Modern shelf carbonates are produced and deposited in tropical environments and in nontropical settings. Since Chave (1967) pointed out that carbonates could form at all latitudes, many investigations have been conducted on sedimentology of modern nontropical carbonates (Nelson, 1988; James, 1997). The comparative sedimentology showed that the carbonates produced in mid to high latitudes on high-energy open shelves may be good analogues for many ancient shelf limestones (James and Bone, 1989, 1991; James and von der Borch, 1991; James et al., 1992). These nontropical shelves have been the sites of extensive carbonate production and accumulation through the Phanerozoic.
The modern southern Australian continental margin constitutes the largest cool-water carbonate platform on Earth (Connolly and von der Borch, 1967; Wass et al., 1970; Marshall and Davies, 1978; Bone et al., 1992; James et al., 1992; Bone and James, 1993; Feary and James, 1995, 1998; James et al., 2001). The modern shelf sedimentation is controlled dominantly by swells and storms. The shelf is "shaved" under such a condition, with most skeletal fragments transported seaward to the shelf edge and slope or moved landward (James et al., 1994). The sediments consist exclusively of heterozoan skeletal constituents containing abundant bryozoans (cf. James, 1997).
Seismic images indicate that the platform, composed predominantly of successions of cool-water carbonate sediments, includes numerous biogenic buildups throughout the Cenozoic (Feary and James, 1995, 1998). Drilling into these structures revealed that they are bryozoan reef mounds constructed by a diverse suite of bryozoans (James et al., 2000). The bryozoan mounds were considered to have grown at paleowater depths of 80-200 m during glacial lowstands, when upwelling was intensified and consequently nutrient supply increased along the southern Australian continental margin (James et al., 2000). This nutrient-rich marine environment was caused by a combination of changes in oceanographic conditions during glacial periods (e.g., the subtropical convergence moved northward, there were strong offshore westerlies, and the Leeuwin Current weakened considerably or ceased flowing) (Wells and Wells, 1994; Wells and Okada, 1996; Okada and Wells, 1997).
Bryozoans are the principal constituents of deeper cool-water carbonates. In spite of this potential availability of bryozoan skeletons for paleoenvironmental reconstruction (Bone and James, 1997; Crowley and Taylor, 2000), the isotopic records have not been used for paleoceanographic determinations. This research aims to examine the downhole isotopic records of bryozoan skeletons collected from the Great Australian Bight (Ocean Drilling Program [ODP] Leg 182; Sites 1129, 1131, and 1132) and to discuss the late Pleistocene bryozoan mound developments. Also presented is an evaluation of the validity of bryozoan isotopic records as a paleoceanographic proxy.