Antarctic CRC Marine Geoscience, Vincennes Bay, Prydz Bay and Mac.Robertson Shelf : Post-cruise Report : AGSO Cruise 186, ANARE Voyage 5, 1996/97 (BRAD)

Brief description

This record is a summary of the preliminary results of the third AGSO / Antarctic Co-operative Research Centre/ ANARE marine geoscience program in East Antarctica since 1993. The voyage included visits to Vincennes Bay, Prydz Bay and the Mac. Robertson Shelf, Antarctica and to Heard Island on the Kerguelen Plateau. The purpose of the voyage was to conduct a marine science geophysical survey and sediment processes/sampling program at several key areas in the Australian Antarctic Territory (AAT) during the 1996-97 season, namely: (i) the deep shelf basin located in Vincennes Bay; (ii) Prydz Bay grounding line moraines and trough mouth fan deposits, and (iii) the Nielsen shelf valley off Mac.Robertson Land. The objectives for the cruise included: (1) to collect cores and seismic data from shelf basins that preserve a record of the timing of ice sheet retreat from the shelf break following the last glacial maximum (LGM); (2) to collect geochemical data from the water column and underlying sediments which will improve interpretation of short-term (Holocene) palaeoenvironmental records preserved in biogenic sediment cores; (3) to obtain seismic data in support of proposed ODP drilling on the Prydz trough mouth fan; and (4) to quantify modern sedimentary processes for the purpose of deriving facies models and analysis of facies successions in the cores.
In Vincennes Bay, eight cores were collected at six stations; 3 are located on an outer shelf bank and 3 are from a deep (up to 1,850 m water depth) glacial trough on the inner shelf. The inner shelf cores are interpreted as Holocene in age throughout and contain a diatom and possible organic carbon record that looks promising for correlation with the Law Dome ice core record. About 200km of seismic data were collected in Vincennes Bay which show the inner shelf to be glacially incised and deeply eroded, exposing crystalline basement, whilst the mid to outer shelf preserves much of its sedimentary cover, although glacial erosion is also exhibited.
A total of 19 gravity cores were collected in Prydz Bay. The cores indicate that the LGM was associated with grounding of a paleo-ice shelf only along the periphery of Prydz Channel and not to the shelf break. Most deposition in front of the grounding line was dominated by ice-rafted sedimentation, as opposed to sediment gravity flows. Beneath some LGM glacial marine deposits lies a key bed of reworked diatom ooze that is distinct in its size (species) sorting and abundance of Pliocene diatoms. This "interstadial" unit can be traced between cores across more than 15,000 km2 of the Prydz Channel. About 900 km of seismic and sidescan sonar data were collected from Prydz Bay and have provided an insight into the internal structure of several inner shelf moraines. A large (>80km long) iceberg was located on the slope off Prydz Bay and prevented site survey work on the proposed ODP sites. However, some outstanding sidescan sonar records were collected from inner Prydz Bay, which revealed iceberg gouges, flutes and some previously undescribed sedimentary bedforms in several areas. On the Mac.Robertson shelf, two current meters were successfully deployed, 320 km of seismic and sidescan sonar data and 7 cores were collected. One seismic line was completed from the shelf break to the inner part of the Nielsen Basin and three gravity cores were also collected along this seismic line. Further to the west in "Iceberg Alley", an additional 4 gravity cores were collected and a seismic line was shot along part of a cross-shelf transect.

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