Carbonate mineralogy and stable isotope composition of cool-water bryozoans from sediments of ODP Leg 182 sites

The carbon and oxygen isotopic compositions of selected bryozoan skeletons from upper Pleistocene bryozoan mounds in the Great Australian Bight (Ocean Drilling Program Leg 182; Holes 1129C, 1131A, and 1132B) were determined. Cyclostome bryozoans, Idmidronea spp. and Nevianipora sp., have low to intermediate magnesian calcite skeletons (1.5-10.0 and 0.9-6.4 molar percentage [mol%] MgCO3, respectively), but a considerable number include marine cements. The cheilostome Adeonellopsis spp. are biminerallic, principally aragonite, with some high magnesian calcite (HMC) (6.6-12.1 mol% MgCO3). The HMC fraction of Adeonellopsis has lower d13C and similar d18O values compared with the aragonite fraction. Reexamination of modern bryozoan isotopic composition shows that skeletons of Adeonellopsis spp. and Nevianipora sp. form close to oxygen isotopic equilibrium with their ambient water. Therefore, changes in glacial-interglacial oceanographic conditions are preserved in the oxygen isotopic profiles. The bryozoan oxygen isotopic profiles are correlated well with marine isotope Stages 1-8 in Holes 1129C and 1132B and to Stages 1-4(?) in Hole 1131A. The horizons of the bryozoan mounds that yield skeletons with heavier oxygen isotopic values can be correlated with isotope Stages 2, 4(?), 6, and 8 in Hole 1129C; Stages 2 and 4(?) in Hole 1131A; and Stages 2, 4, 6, and 8 in Hole 1132B. These results provide supporting evidence for a model for bryozoan mound formation, in which the mounds were formed during intensified upwelling and increased trophic resources during glacial periods.

Benthic foraminifera and stable isotope record of ODP Leg 182 sites

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2-8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2-4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial-interglacial fluctuations is a distinct long-term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial-interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long-term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.