Immediate Report of Victoria University Of Wellington Antarctic Expedition 1987-88: VUWAE 32
PART I - Scientific Programmes. McMURDO SOUND SEDIMENT STUDIES (K042)
PART I - Scientific Programmes. McMURDO SOUND SEDIMENT STUDIES (K042)
Sea floor sediment samples were obtained from seven shore-normal transects on the southwestern Ross Sea coast from Blue Glacier to Tripp Bay, plus one transect off Cape Armitage, on Ross Island. The transects were designed to sample "near shore" sea floor sediment and biota to 100 m water depth. Samples were taken with modified Shipek grab. Current measurements were made at most of the 100 m site with an InterOcean S4 electromagnetic current meter.
- Sea floor slopes, up to 17°, for example, Tripp Island and Gregory Island, are generally devoid of sediment and bedrock is exposed at the sea floor. On low angle slopes, averaging 3°, sediment fined offshore from gravelly sand muddy sand and mud.
- Sea ice scouring and anchor ice prevents sediment accumulation on bedrock least down to 5 or 10 metres, especially at exposed coastal locations.
- Current activity which modifies or prevents sediment accumulation even moderate depths, for example Gregory Island.
- Entrapment and protection of sediment, especially by sponge mat, occurs below 50 metres, even at sites with moderate to high current regimes, such as Cape Armitage.
Current flow is approximately parallel to the southwestern Ross Sea coast and shows reversal of direction within the tidal period. Current speeds up to 25 to 30 cm/sec were measured at Gregory Island and decrease to the south at Blue Glacier where a maximum of 12 cm/sec was measured. At Cape Armitage, current speeds of up to 75 cm/sec were measured, and were accompanied by a marked directional change within the tidal period.
The macro biota from most sites have consistent substrate preference and depth zonations. Red spiny echinoids and small (up to 5 cm high) red-brown algae prefer exposed bedrock and coarse gravel generally to 20 to 30 metres depth. Pectens commonly were recovered in shallow depths less than 30 metres, usually on sandy sediment and occasionally bedrock. Sponges and sponge mat were recovered from depths greater than 50 metres on substrate from gravel to muddy sand.
Surveys of the sea floor of McMurdo Sound and Granite Harbour have shown that sediment texture there is, in broad terms, bathymetrically controlled with mud in the basins, muddy sand on the slopes and sand on the shelves (Barrett et al. 1983). Foraminiferal studies also show some bathymetric control, though in this case the controlling factor is the carbonate compensation depth at 620 m offshore and 230 m in the harbours (Ward et al. 1987). The main purpose of this project is to document the relationship between sediment texture, micro-organisms (diatoms and foraminifera) and water depth from the shoreline to the 100 m contour along the Victoria Land coast. This areally limited zone has come to be of particular interest because studies of the MSSTS-1 drill core (Barrett 1986) show how variations in species diversity and sediment texture may be used to follow sea level changes in cored sequences. Changes to be expected are an increase in mud content and a decrease in mean size seaward due to declining wave power (cf. Jago & Barusseau 1981; Barrett 1986), an increase in species diversity (Webb et al. 1986) and in the case of diatoms an increase in benthic/planktic ratio (Harwood 1986). Data from the modern shoreline are needed for comparison with older shallow marine polar sequences like that cored in MSSTS-1.
Scientific Endeavours and Achievements
This programme involved a five-person party working from the fast ice in the southwestern Ross Sea. Sea floor samples were taken along eight shore-normal transects from Blue Glacier to Tripp Bay and at Cape Armitage (Figure 1). The transects represent a range of coastal types (exposed to embayed) with a variety of substrates, including bedrock, gravel, sand, mud and sponge mat.
We travelled on the fast sea ice by D-3 tractor pulling three Cantago sledges and used a Grizzly toboggan for bathymetry surveys and route finding. The first sledge was set up as a sea ice drilling platform, with hydrographic winch, drill and drill mast, grab, fuel and tools. The second sledge carried the NZ-1 wannigan, which is fitted with bunks, table, desk and small kitchen. This was used as a laboratory for mixing preservative for the sediment samples, as a dry lab for the IBM PC, used for programming and interrogating the S4 current meter, and as a kitchen and working area. The third sledge carried the remaining cargo, such as tents, personal baggage, the Grizzly when not in use, further fuel and miscellaneous cargo. A VUW ski trailer for use with the Grizzly was towed last.page 4
Sea floor sediment sampling
At each of the eight sampling transects, a bathymetric survey was carried out to determine the slope and topography of the sea floor (Figure 2). This was carried out by drilling 4-inch holes in the sea ice and using an echo sounder to measure the water depth by passing the 200 kHz transducer through the hole below the sea ice.
The sampling strategy was designed so that sediment was recovered from similar depths at each transect. The depth zones sampled included a sea ice scoured zone, from 0 to 5-10 m, a possible wave influenced zone, from 5 to 20 m, and a non-wave influenced zone, from 20 to 100 m. The effects of bottom currents, which are depth and site specific, were also considered in the sampling strategy.
The sediment samples were recovered with a modified Shipek grab designed and built at Victoria University. The grab has two interchangeable 180×180 mm hemicylindrical buckets that take a 90 mm deep scoop of the sea floor. These samples were first photographed and described intact, then split into portions and preserved in formalyn. A subsample of each grab will be preserved as an archive specimen, and another split will be used for foraminiferal work and grain size analysis.
Several areas of exposed bedrock were identified during the sampling programme. These sites had very little sediment cover, and are generally in shallow water. They include exposed capes and steeply dipping (17°) sea floor. Tripp Island and Gregory Island exhibit this sort of submarine topography (Figure 2). The bedrock surfaces extend deeper than sea ice influences, so other factors must be responsible for the lack of sediment. These include glacial ice scouring, non-deposition at the present time and strong current activity.
Intermediately sloping sea floor localities include Cape Roberts, Dunlop Island and Cape Bernacchi (Figure 2). These have exposed bedrock from 0 m to 10 or 20 m, then gravel which grades to muddy sand at about 100 m.
Beach-type coastal topography is found at Explorers Cove (New Harbour, Figure 2) and Blue Glacier. These areas have gravelly sand at 0 m with sediment fining into the deeper water. Blue Glacier (0 to 40 m depth) is a moraine remnant, presumably ice cored, with a slope of 15°. Further offshore the slope lessens to 3°.
Cape Armitage, on Ross Island (Figure 2) has a substrate of volcanic scoria. We found this transect to be strongly influenced by sea floor anchor ice formation in water to 11 m depth, and a very strong current (75 cm/sec) regime.page 6
Figure 2. Bathymetry and sediment distribution along each of the eight sampling transects. Spot depths and sample sites (arrows) are shown for each transect. The major types of biota recovered are shown for each sample at the left side of each transect.
Invertebrate animals and some plant materials were collected during the grab sampling programme. Different species of animals and plants have certain substrate preferences. Echinoids and some pectens were found on the bedrock surfaces (Figure 2). Pectens were also found in shallow water on sandy mud bottoms. Live sponges and sponge mat (disaggregated dead siliceous sponges) were found from 50 to 100 m and probably continue deeper in certain areas off shore (Ward 1984). Large (5 cm high) reddish algae was found only in the northern areas, as isolated specimens, the algae occurred in water less than 50 m deep, on bedrock or coarse sediment. Its distribution could be influenced by proximity to more open water and Ross Sea circulation patterns.
Diatoms, often comprising biogenic mud of an olive-green colour, were present entrapped in sponge mat and in deeper water basins, especially in areas of low current activity, such as Tripp Bay. Specimens of diatom ooze were collected for S216.
Water current measurements were made this season at the 100 m site of most of our coastal transects using a newly purchased InterOcean S4 electromagnetic current meter. This current meter is housed in a 250 mm diameter sphere, has no moving parts, records data internally and is ideal for deployment through our 300 mm diameter access holes in the sea ice. The instrument was programmed and interrogated with an IBM PC operated in the warm environment of the wannigan (NZ-1).
Two modes of deployment were used at the 100 m deep sites. Water column profiling measurements were made while either raising or lowering the current meter with our new hydraulically controlled hydrographic winch. Short term (12-24 hours) fixed-mooring measurements were also made by suspending the current meter on the winch wire 1.5 m ± 0.5 m above the sea floor. The height of the instrument above the sea floor could not be fixed precisely using this method of deployment because of the approximately 1 m tidal rise and fall of the sea ice platform.
Profiles and short term deployments were also made in central Granite Harbour near the MacKay Glacier Tongue and in central New Harbour (S216 Trap Site). This data will be used for planning a longer term deployment (2 months) proposed for the 1988-1989 season.
Figure 3. Current measurements at Gregory Island (98 m site).
|A.||Profile showing consistent speed and direction through most of the water column.|
|B.||Stationary mooring 1-2 m above the sea floor (13 hour 48 min record). Note the increase in speed as high tide is approached and progressive change in direction from southward flow to a northward flow. The strongest flow is to the north.|
Figure 4. Current measurements at Cape Armitage (90 m site).
|A.||26 hour deployment 1-2 m above the sea floor showing currents up to 75 cm/sec were recorded flowing to 150° true (SSE). Lower velocity currents up to 25 cm/sec flowing to 250° true (WSW).|
|B.||Profile about 2 hours after the end of record above (A). A steady flow of 75 cm/sec to 150° T is shown for most of the water column.|
Water samples for oxygen and carbon isotope analyses by Dr Enriqueta Barrera, Ohio State University, were taken at the 100 m site of each of the eight transects. Samples were collected using a small Niskin bottle deployed 10 m below the sea ice and 5 m above the sea floor at each site. Subsamples were measured for pH within 2 hours of collection. We had hoped to collect water samples for this study from previous 1981 sea floor sample sites, but the lack of sea ice cover in the central part of McMurdo Sound made this impossible this season.
Tripp Bay Bathymetry
A spot depth bathymetry survey was begun this season in Tripp Bay in association with Dr Robert Dunbar of Rice University, Texas (S216). The aim of the survey was to compare Tripp Bay with Granite Harbour and identify any deep basins which could be accumulating diatom-rich sediment as in Granite Harbour. We were also interested in this area as a site which is isolated from the influence of McMurdo Sound oceanographic circulation.
The maximum depth recorded this season was 550 m in the area south of Tripp Bay Glacier Tongue (Figure 1). Further measurements are still required to the east and north of the glacier tongue to complete the survey. The New Zealand survey party, K191, surveyed all flagged bathymetric locations so an accurate map of the sea floor in this area can eventually be constructed.
The three main investigators involved in this project, Dr Peter Barrett, Mr Alex Pyne and Dr Barbara Ward, intend to publish this joint study relating the modern near shore sediment, current measurements and foraminiferal diversity along the coast in a periodical such as the NZ Journal of Marine and Freshwater Research.
The purchase and successful deployment of the S4 current meter this season now gives us the ability to quantitatively study water circulation. In the 1988-1989 season we intend to deploy the current meter at the MacKay Glacier Tongue for two months to record bottom current activity to determine whether glacier-generated mud-carrying density currents occur in this polar marine glacial setting.
Sea floor coring in Granite Harbour by Victoria University (Macpherson 1987) and Dunbar (S216) this season indicates that glacially deposited sediment may exist beneath the 1 m thick cover of diatom-rich mud. The glacial sediments were probably deposited during Holocene expansion of the MacKay Glacier, which is fed from the polar plateau. Granite Harbour therefore may contain accessible glacial marine sediments which could record Holocene changes in polar ice volume for this area of Antarctica.
Management of Science in the Ross Dependency
Granite Harbour continues to be an area of interest for polar marine glacial study because it can be logistically supported from McMurdo-Scott Base, yet is sufficiently far away from McMurdo Sound to be a good model for the deep basins along the Ross Sea coast that have been influenced by plateau ice. We suggest that a small summer science base could be established at Cape Roberts to support marine glacial, oceanographic and biological studies in Granite Harbour and this area of the Ross Sea coast. Such a summer base could also be the nucleus of a drilling camp if further offshore drilling were to go ahead near Cape Roberts.
This season we were joined by Lt Fernando Zurita, a guest scientist (geologist) from Ecuador, whose scientific interest was close to our oceanographic studies. He was only able to spend a short time with us in the field before leaving to organise the Ecuador Antarctic Oceanic Cruise beginning in early December. I think Lt Zurita's visit to Scott Base and involvement in our programme was successful because of our compatible scientific interests, but unfortunately his time with us in the field was rather short, and he was not able to observe the full range of our operations.
We wish to thank the University Grants Committee and the VUW Internal Research Committee for financial support. Our thanks also to Antarctic Division, DSIR, for logistic and field support, to OIC, Scott Base and summer support staff, particularly Clayton Ross, summer mechanic, for advice and help while we were in the field.
Special thanks to Geoffrey Blake, our DSIR field assistant, for his enthusiasm and mechanical expertise, and to Lt Fernando Zurita, for his help and cooperation at Scott Base and during the short time he was in the field.
We are grateful for the technical expertise and willing assistance of the Kiwi surveyors, Brian Anderson and Garth Falloon, K191. We also appreciated the assistance and cooperation of S216, from the U.S. programme.
VUW Mechanical Workshop provided practical assistance and expertise with the hydraulic winch, grab and other equipment, and their efforts are greatly appreciated.
Barrett, P.J., Pyne, A.R. and Ward, B.L. 1983. Modern sedimentation in McMurdo Sound, Antarctica. In Oliver et al. (eds), Antarctic Earth Science, Australian Academy of Science, Canberra.
Barrett, P.J. 1986. Sediment texture. In Barrett, P.J. (ed.), Antarctic Cenozoic history from the MSSTS-1 drillhole, McMurdo Sound. DSIR Miscellaneous Bulletin 237.
Harwood, D.M. 1986. Diatoms. In Barrett, P.J. (ed.), ibid.
Jago, C.F. and Barisseau, J.P. 1981. Sediment Entrainment on a Wave-graded Shelf, Roussillon, France. Marine Geology 42: 279-299.
Macpherson, A.J. The Mackay Glacier/Granite Harbour System (Ross Dependency, Antarctica) - A Study in Nearshore Glacial Marine Sedimentation. PhD Thesis, VUW Library.
Ward, B.L. 1984. Distribution of Modern Benthic Foraminifera of McMurdo Sound. Antarctica. PhD Thesis, VUW Library.
Ward, B.L., Barrett, P.J. and Vella, P. 1987. Distribution and Ecology of Benthic Foraminifera from McMurdo Sound, Antarctica. Paleogeography, Paleoclimatology, Paleoecology, 58: 139-153.
Webb, P.N. 1986. Foraminifera (Late Oligocene). In Barrett, P.J. (ed.) ibid.
Webb, P.N., Leckie, R.M., and Ward, B.L. 1986. Foraminifera (Late Oligocene). In Barrett, P.J. (ed.), Antarctic Cenozoic history from the MSSTS-1 drillhole, McMurdo Sound. DSIR Miscellaneous Bulletin 237.