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Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

¹ Queen's University, Kingston, Ontario K7L 3N6, Canada
² Australian Geological Survey Organization, Canberra ACT 2601, Australia
³ University of Copenhagen, 1350 Copenhagen, Denmark
⁴ University of Wisconsin, Madison, Wisconsin 53706, USA
⁵ Johann Wolfgang Goethe-Universität, Frankfurt-am-Main 60054, Germany
⁶ Kiel University, Kiel D-24118, Germany
⁷ University of Adelaide, Adelaide, South Australia 5005, Australia
⁸ Kumamoto University, Kumamoto 860, Japan
⁹ Japan Marine Science and Technology Center, Kanagawa 237-0061, Japan
¹⁰ Eckerd College, St. Petersburg, Florida 33711, USA
¹¹ ETH-Zentrum, 8092 Zurich, Switzerland
¹² University of South Florida, St. Petersburg, Florida 33701, USA
¹³ Ocean Drilling Program, Texas A&M University, College Station, Texas 77845, USA
¹⁴ No affiliation available

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The ¹⁴C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

Key Words: Australia • carbonates • bryozoan mounds • Quaternary

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