13. Carbon and Oxygen Isotopes of Cool-Water Bryozoans from the Great Australian Bight and Their Paleoenvironmental Significance¹

Hideaki Machiyama,² Tsutomu Yamada,³ Naotomo Kaneko,⁴ Yasufumi Iryu,³ Kei Odawara,³ Ryuji Asami,³ Hiroki Matsuda,⁵ Shunsuke F. Mawatari,⁶ Yvonne Bone,⁷ and Noel P. James⁸

ABSTRACT

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 ¹³C and similar ¹⁸O 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.

¹Machiyama, H., Yamada, T., Kaneko, N., Iryu, Y., Odawara, K., Asami, R., Matsuda, H., Mawatari, S.F., Bone, Y., and James, N.P., 2002. Carbon and oxygen isotopes of cool-water bryozoans from the Great Australian Bight and their paleoenvironmental significance. In Hine, A.C., Feary, D.A., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 182 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/182_SR/007/007.htm>. [Cited YYYY-MM-DD]

²Deep Sea Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. bucci@jamstec.go.jp

³Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aobayama, Sendai 980-8578, Japan.

⁴Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan.

⁵Department of Earth Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan.

⁶Division of Biological Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan.

⁷Department of Geology and Geophysics, The University of Adelaide, SA 5005, Australia.

⁸Department of Geological Sciences, Queen's University, Kingston ON K7L 3N6, Canada.

Initial receipt: 1 April 2001
Acceptance: 30 May 2002
Web publication: 7 January 2003
Ms 182SR-007