The paleoenvironmental history indicated by the LBF assemblages is even more fascinating. Much remains to be deciphered, though poor core recovery and dolomitization will limit what can be learned. One lesson that is evident in the Leg 194 story is that different photozoan assemblages respond differently to the coralgal "reef" turnon–turnoff gradient (e.g., Buddemeier and Hopley, 1988; Hallock 2001). At Site 1193, LBF occur abundantly in environmental conditions that favor bryozoan dominance of benthic carbonate production. At Site 1196, LBF occur at least as abundantly in coralgal reef facies. The environmental factors that likely controlled Miocene benthic community structure and carbonate sedimentation on the Marion Plateau include temperature, ocean currents, parameters associated with fluvial input, and ocean chemistry.
The cool–subtropical climate was critical to the overall history of the plateau, resulting in environmental conditions that were marginal for coralgal reef development throughout the Miocene. However, temperature differences were not likely to have driven sedimentation differences between the northern and southern platforms. If latitude was involved, the southern platform should have been cooler and therefore bryozoan dominated. Although differences in current regimes around the platforms are poorly known, currents certainly played a key role in the platform history (e.g., John and Mutti, 2005), and not only in transporting the vast drifts of fine terrigenous and calcareous sediments. Moreover, fluvial input, in conjunction with ocean currents and paleochemistry, very likely played the critical role in inhibiting reef development on the northern platform while promoting it on the southern platform.
Hallock (2001) reviewed and discussed the sensitivity of carbonate sedimentation, particularly reef development, to nutrients and carbonate saturation. The benthic communities that produced bryozoan-dominated carbonate facies at Site 1193 likely included abundant fleshy macroalgae and especially filamentous microalgae, which limited recruitment and growth of coralline red algae and stony corals. Pulses of nutrients in terrestrial runoff likely supported the algae and also plankton that provided food for the bryozoans. Nevertheless, the LBF assemblages attest to relatively clear waters with at least cool subtropical (>20°C) paleotemperatures, possibly similar to modern conditions on the west Australian shelf (see James et al., 1999). Conditions on the southern platform (Site 1196) probably included slightly lower fluxes of nutrients and stronger currents, both of which would have increased the competitive advantage of coralline red algae relative to the filamentous algae.
An additional factor that likely influenced overall carbonate production was ocean chemistry, specifically carbonate saturation. The west Australian shelf provides a modern example where photozoan calcite production (i.e., calcite calcification by coralline red algae and LBF) generally dominates over photozoan aragonite production (i.e., aragonite calcification by stony corals and calcareous green algae) (James et al., 1999), indicating that carbonate saturation is below the threshold required for prolific aragonite production (e.g., Kleypas et al., 2001). Where shallow-water conditions allow water to warm and thus increase supersaturation with respect to aragonite (e.g., the Abrolhos Islands off west Australia), carbonate production by calcareous algae and corals can be prolific (Collins et al., 1993, 1997).
The following scenario is proposed to explain platform development and growth on the southern platform of the Marian Plateau. Strong, warm, southward-flowing currents favored coralline algal dominance. Limited upwelling mixed higher-alkalinity waters into surface waters around the platform, thereby promoting calcification. Behind the rapidly accreting coralline algal rim, coralline algae, LBF, and some stony corals thrived. As noted by Hallock (2001), carbonate production rates approach maximum as the reef turn-off threshold is approached. The carbonate factory on the southern platform kept pace with sea level pulses throughout the Miocene, most dramatically during the early middle Miocene when LBF in lithologic Subunit IIA indicate 180 m of shallow (<12 m) platform sedimentation. Higher carbonate saturation on the platform is indicated through this interval by the prevalence of miliolid foraminifers that produced shells of high-Mg calcite.
Leg 194 provided a unique opportunity to examine the Miocene histories of two contemporaneous platforms with very different dominant biota and therefore different accretion potential. Analyses of LBF assemblages recovered during Leg 194 provided crucial biostratigraphic, paleodepth, and paleoceanographic information integral to the success of the project. Determining the seismic and biostratigraphic contexts for the LBF assemblages within the platforms and in associated slope facies has demonstrated that the northern platform accreted in the early and middle Miocene while growth of the southern platform occurred intermittently throughout the Miocene. The abundances of larger benthic foraminifers in both red algal and bryozoan-dominated facies is critical to understanding the nature of Marion Plateau carbonates that might otherwise be interpreted as temperate, based upon limited abundances of zooxanthellate corals and Halimeda (e.g., James, 1997).