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

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