Oxygen stable isotopic values of bryozoans from the upper Pleistocene sediments in the Great Australian Bight (ODP Leg 182, Holes 1129C, 1131A, and 1132B) suggest that the oxygen isotope signals can be potentially useful as a paleoceanographic proxy, providing they have not been diagenetically altered. However, in our study, the bryozoan 18O signals are not as sensitive as those of planktonic and benthic foraminifers.
This study led to the following conclusions:
- The cyclostomes Idmidronea spp. and Nevianipora sp. have skeletons with LMC to IMC (1.5-10.0 and 0.9-6.4 mol% MgCO3, respectively), but some skeletons may be enriched in Mg because of marine diagenesis (i.e., early marine cement). The cheilostome Adeonellopsis spp. have a mainly aragonitic skeleton, although some specimens have associated IMC (6.6 to 12.1 mol% MgCO3). The latter have lower
13C and similar
18O values compared with the totally aragonitic specimens. Marine cements occur more frequently within intraskeletal spaces with increasing depth (>40 mbsf).
- Reexamination of modern bryozoan data shows that (1) Adeonellopsis and Nevianipora form their skeletons at close to oxygen isotopic equilibrium with ambient seawater and (2) in contrast, the carbon isotopic composition reflects vital effects.
- Glacial-intergalcial cycles are detected from the oxygen isotope profiles. The profiles can be correlated with isotope Stages 1-8 in Holes 1129C and 1132B and with Stages 1-4(?) in Hole 1131A.
- The bryozoan mounds generally coincide with heavier oxygen isotopic values during 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. Our results support a model in which the mounds were formed by intensified upwelling and increased nutrient levels and trophic resources during glacial periods when the warm nutrient-depleted Leeuwin Current was inactive.