Hiroki Matsuda

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

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.

Sand and rhodolith-gravel entrainment on the mid- to outer-shelf under a western boundary current: Fraser Island continental shelf, eastern Australia

Abstract Carbonate sediments on the east Australian continental shelf off Fraser Island represent a transition from the warm-water, tropical, carbonate deposits in the north (i.e. the Great Barrier Reef) to cool-water, temperate, carbonate deposits in the south. On the outer shelf in 40–140 m water depth, these deposits are characterised by pebble- to cobble-sized rhodoliths. Current meter data obtained from this area demonstrates that near-bed, southward-flowing currents of up to 130 cm/s occur episodically to depths of over 70 m. These current events may last for several days and are probably related to intrusions of the East Australian Current, a western boundary current, onto the shelf. Flume experiments on different size classes of rhodoliths suggests that the threshold speed referenced to 100 cm above the bed ( U 100 cr ) for 10 mm diameter rhodoliths is of the order of 45 cm/s, whereas U 100 cr for 50 mm diameter rhodoliths is of the order of 80 cm/s. Therefore, the shelf currents measured are competent to initiate rhodolith movement. Percentage exceedence of these threshold speeds ranges from 1 to 10%. Surface waves also generate nearbed oscillatory flows which, when combined with the steady current, are considered to be important in initiating movement of the rhodoliths.