Victoria University Antarctic Research Expedition Science and Logistics Reports 2002-03: VUWAE 47
OCEANOGRAPHIC AND SEA FLOOR MEASUREMENTS
OCEANOGRAPHIC AND SEA FLOOR MEASUREMENTS
The first measurements at Site 1 were to establish water depth and nature of the sea floor. A 3.5 kHz transducer was lowered and set 2 m below the base of the ice shelf to obtain a high quality acoustic record of the sea floor and subsurface sediment. Water depth was determined to be 855 m below the base of the ice shelf (908 m bsl, 926 m below the ice shelf surface, a convenient reference point for all subsequent oceanographic measurements). The sea floor reflector was sharp and stratification recognized down to a depth of over 300 m. This indicates that the sediment for this interval is largely fine-grained and unconsolidated. The water depth was determined by weighted line to be 938 m, 12 m deeper than the acoustic estimation. This depth was used for operational purposes on oceanographic casts. A similar procedure was followed for site 2, and an outline of the scientific data gathered for both sites is shown in table 3.
Fig. 4. Seismic record through Site 1 (MIS-1 line, Bannister et al., 2002), with image from 3.5kHz profiler inserted. Reflectors can be seen down to ~300 m below the sea floor.
|DEVICE||MEASUREMENTS OR SAMPLES TAKEN||HWD03-1||HWD03-2|
|3.5 kHz sounder||Water depth and sub-seafloor stratigraphy||Penetration to ~300 m||Ice shelf too thick for cable|
|S4||Current speed and direction profiles through the water column||Casts 1 to 13 from 1400 Jan 14 to 1700 Jan 16||Casts 1 to 10 from 1110 Jan 28 to 1240 Jan 29.|
|CTD||Conductivity and temperature profiles through the water column|
|NIO bottles (1 litre)||Water samples for chemistry and suspended particulate matter||~20 samples taken at 6 levels||~20 samples taken at 6 levels|
|ADCP array||Current speed and direction measured simultaneously through the water column for at least 48 hours||Deployed for 87 hours on Jan 1822. See Fig. 3||Deployed for 47 hours on Jan 31 to Feb 1|
|Gravity cores||48 mm diameter sediment cores – at least 3 from each site more than 50 cm long||Cores 7, 11, 13, 50, 60 and 61 cm||Cotes 29, 42, 61 and 63 cm|
|Grab||Top 3-5 cm from sea floor.||Grabs 1 and 2 empty. Grab 3 30% full.||Grabs 1 & 2 empty. Grab 3 full|
Cast procedure and water/particulate chemistry samples: After a trial cast with the S4 1 m above the weighted end of the line and the CTD 5 m above that, a second cast was run with 1 litre NIO bottles at depths at 6 levels through the water column for water sampling. Several bottles proved difficult to bring up through the hole because currents in the water column deflected the rope, and were damaged. It was then decided to attach only two bottles 5 m apart and 5 m above the CTD on each cast but to trip them each cast at successively higher levels to cover the 6 planned sampling levels. This worked well, and the 14 casts were completed late on January 16. Filtration was done on samples from each level for 3 casts with a fourth close to the sea floor, and 3 further samples from water in the gravity corer just above the sediment water interface. Material could be seen on all filters, with those close to the sea floor showing an obvious brown coating. Samples were also taken for water chemistry.
CTD measurements: Oceanographic data were collected at both sites primarily with a Conductivity-Temperature-Depth profiler during water sampling casts, and later by a moored array of Acoustic Doppler Current Profilers (ADCPs). The setup and some results are shown in figure 5.
|Zone||Depth range [m]||Salinity [psu]||Temperature [°C]|
|A||53 - 150||34.38 to 34.46||−1.915 to −1.90|
|B||111- 275||34.59 to 34.63||− 1.93 to −1.936|
|C||260 - 660||34.65 to 34.67||−1.918 to −1.914|
|D||660 - 922||34.70 to 34.71||− 1.911|
Zones B, C and D resemble High Salinity Shelf Water (HSSW) and Deep Ice Shelf Water (DISW) of Jacobs et al. (1985), whereas Zone A resembles Shallow Ice Shelf Water (SISW). Site J9, 450 km south of the Ross Ice shelf, has a similar Temperature/Salinity structure suggesting continuity of water masses with Windless Bight. However, the shallowest zones are much colder at J9 and lack the relatively warm intrusion (Zone A) seen in the Bight.page 7
Current velocity measurements: Current measurements from the ADCP arrays are heavily influenced by the diurnal tides (Fig 5). Currents are relatively slow in the floor of the basin at both sub-ice shelf sites (averaging around 7 cm/s with a maximum of 17 cms), but are considerably faster (up to 60 cm/s) at the ice shelf edge off Hut Point Peninsula where the water is shallower (~600 m). Flood tides flow to the E and swing S and W below the McMurdo Ice shelf. This is also the direction of the mean flow, suggesting that, like the western and central Ross Ice Shelf edge, McMurdo Sound may also be a point of sub-ice shelf inflow.
Sea floor sediments: Initial gravity coring attempts yielded cores only a few cm long but by applying grease to the inside of the core liner cores in excess of 50 cm were consistently recovered page 8 from both sites. Grab samples of the top 2-3 cm were also recovered from both sites, and showed that the sea floor sediment at each comprises a thin (5 mm) layer of sandy mud with scattered basaltic pebbles up to 15 mm across. The sediments beneath are diatomaceous sandy mud with diverse diatoms, occasional foraminifera and rare shells. The mud gives way at 31 cm at Site 1 and 60 cm at Site 2 to a pebbly sandy mud or diamicton beneath (Fig. 6). Cores from Site 1 have from 23 to 30 cm an unusual laminated well sorted sand with a sharp base and top, tentatively interpreted as a sediment gravity flow. The diamicton in the lower part of each core was firm but not over-consolidated, indicating that the basal ice that deposited the sediment did not load or erode the sea floor.
The sand mineralogy is basaltic glass and rock fragments throughout both cores, but they also have a significant proportion of quartz, some of it rounded like grains in the Beacon sandstone from the Transantarctic Mountains 100 km to the west. Smear slides also show a trend of increasing biogenic silica up the core. While further work is needed these results are support the view that the cores represent a period of glacial retreat from a time of more extensive grounded ice, effectively recording ice retreat since the Last Glacial Maximum, a pattern consistent with cores elsewhere in the Ross Sea. A combination of field evidence and ice sheet modelling indicates that the last stage of the retreat to the present position of the ice margin on either side of Ross Island took place between 8000 and 4000 years ago (Kellogg et al., 1996)