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Victoria University Antarctic Research Expedition Science and Logistics Reports 1982-83: VUWAE 27
McMURDO SOUND SEDIMENT STUDIES (K5)
McMURDO SOUND SEDIMENT STUDIES (K5)
PART I - SEA ICE PROGRAMME
This season a relatively small bathymetry and seafloor sampling programme was designed to cover an area in the southwest corner of McMurdo Sound, which we had not previously sampled and also to start a detailed study in Granite Harbour. Samples were collected in front of the Blue Glacier and 10km east of Cape Chocolate near the Dailey Islands. In addition, bathymetry transects were done in this area and for 3km off Scott Base.
Granite Harbour can be considered a "closed basin" which collects sediment deposited from the Mackay Glacier Tongue. Study of this "sedimentation system" will enable a more precise interpretation of the marine glacial contribution to recent McMurdo Sound-Ross Sea sediments and the older Cenozoic sediments that will be cored in the next few years.
The other major part of the sea ice programme was in support of CIROS, and comprised seismic refraction studies for drill site location at Butter Pt., Dailey Islands, sea ice movement monitoring off Butter Pt and tide movements at Cape Roberts. The only project not completed was seismic refraction work east of Cape Roberts which was to be a co-operative project with Northern Illinois University. A spreading crack in the sea ice 3km east of Cape Roberts could not be crossed by bulldozer (D4) or the vehicles used by the seismic party.
Granite Harbour Sediment Studies (GHSS) - A. Macpherson.
The 1982-83 GHSS science programme can be considered successful not only due to the favourable results obtained this season but also as a result of the initiation of long-term monitoring, (glacial movement and sedimentation rates), where results will not be available for a year or more.
1. Pole network.
In an attempt to determine both rate of movement and amount of plastic deformation of the Mackay Glacier Tongue, a network of eight poles was established up and down glacier from the grounding line, and was subsequently surveyed by K26. Difficulty was experienced in identifying the poles using a theodolite alone, and it proved necessary to have people moving to each successive pole on the glacier at the time of surveying.
2. Bottom Sediment Sampling.
Collection of undisturbed bottom sediment enables detailed paleontological and sedimentological studies of various core to be related spatially
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and possibly temporally. Two gravity cores, one from a crack about the grounding line, and another from in front of the snout were retrieved using a sphincter corer developed and successfully operated during previous seasons by the Antarctic Research Centre. Core 82/1, approximately 30cm in length was taken from a depth of 303m, half a kilometre east of the grounding line. Core 83/2 was retrieved from a depth of 796m immediately east of the glacier snout.
In both cases, corer penetration appeared to be stopped by a coarse sand layer observed at the base of each core. It is unlikely that the addition of more weight would have increased the penetration. Distinct black mottling, thought to relate to reducing conditions at the time of deposition (and shortly after), was observed in both cores, and in addition, a general fining up sequence was recognised in core 83/2.
3. Sediment Trap Deployment.
Sediment traps enable the capture of settling sediment within a known area and hence are a quantitative method of estimating rates of sedimentation.
Two traps were deployed at depths of 400m and 750m respectively, immediately east of the Mackay Glacier Tongue. It was planned to retrieve these traps during February 1983. However, due to calving of the Mackay Glacier these traps were lost.
A sediment trap was later successfully deployed using the "USCGS GLACIER" (see Part II).
4. Current Monitoring.
Currents have the ability to distribute and rework sediment, but the importance of currents within a semi-enclosed basin such as Granite Harbour is not widely known.
A "savonius rotor" current meter which measures currents greater than 0.02m/sec−1 was used immediately east of the Mackay Glacier snout at 1/2 hour intervals, at depths of 5, 10, 20, 40, 60, 80, 100, 150, 200, 400, 650 metres. The programme was terminated when no measurements were recorded at any depths after 4 consecutive hours of monitoring.
Sea ice crack off Granite Harbour - A. Pyne.
A "spreading crack" formed in sea ice has been encountered east of Cape Roberts during the last two seasons (December 1981, November 1982). The crack has proved impassable by heavy surface transport (D4 and IH500 bulldozers) thereby preventing access further seaward. This season the crack was visited twice in November (6th and 18th) and detailed measurements were made both times at the same site.
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On November 6, 1982 the crack was located 2.91km due east of Cape Roberts. It was traced for 5km south and north, and remained undiminished over the total 10km. The orientation was northwest-southeast over this short distance but it is thought to curve towards Mt Bird further south based on previous seasons' sea ice breakout observations. North of Cape Roberts the crack cut close to a distinctive grounded iceberg, as it had done in December the previous season. While the location and orientation of the crack is not yet well known it has probably been the same in both the 1981-82 and 1982-83 seasons.
The spreading crack is filled with ice that has a ridged and terraced surface and thins towards the central crack (Fig. 2). On November 6 the distance between the edges of the first year ice where initial rifting began was 33m. Ice cores were taken from the western side of the crack to define the thickness profile of the infilling ice on November 6 and 18 (see Fig. 2). Two pegs straddling the central crack were fixed in the ice on November 6 and resurveyed on November 18 to determine a spreading rate for that period. The total spreading for the 12 days was 1.2m (10cm/day) and 24 "growth lines" that ranged in width from 2.5 - 10cm were counted in the new ice. The growth lines for this period appear to form daily probably at "night" when temperatures are cooler. The variation in width of the growth lines and terraced surface of the infilling ice indicate that the spreading rate is not constant and was probably greater earlier in the season.
Tide Monitoring - A. Pyne.
A tide gauge designed to automatically record tidal amplitude was established on the sea ice for 11 days at both Scott Base and Cape Roberts. The tide gauge is similar to that described in Pyne and Waghorn (1980) and records tidal amplitude by measuring the relative movement between the floating sea ice and a counterweighted line anchored to the seafloor.
At Scott Base recordings were made from 19-30 October in 80m of water approximately 200m off Scott Base. Unfortunately, good records were only obtained during the first five days (19-24 October), but showed a maximum tidal range of 0.75m for this period. Later recordings at Scott Base appeared to be dampened due to ice freezing on the steel anchoring wire immediately below the sea ice. The steel wire was replaced by monofilament nylon at Cape Roberts and no further dampening occurred.
Recording at Cape Roberts was made over the period 8-18 November with a recording break from 10-13 November, due to unwinding of the clockwork recording mechanism. Sufficient records were obtained, however, to show that the tidal cycle at Cape Roberts is similar to other parts of McMurdo Sound. The change from the "higher" amplitude single daily cycle to a "lower" amplitude double daily cycle was recorded. The maximum range measured was 0.76m and it is estimated that no tides would exceed 1m in the area.
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FIG. 2. Spreading crack in 2.5m thick sea ice off Cape Roberts. Profiles are of the west side of the crack, ice thickness was measured by coring (lower left). Lower right photo shows new ice growth lines on November 18.
Seismic Refraction Surveys at New Harbour and Dailey Islands - R.R. Dibble.
These surveys were made to aid the selection of drill sites for the Cenozoic Investigations in the Western Ross Sea, and extended the work begun by lies and Dibble in 1980. The team consisted of Bay Dibble and Ian Paintin (geophysicists), Alex Pyne and Tony Macpherson (geologists), and Malcolm McLeod (mechanic/field assistant). We used the SIE-RS44 and Nimbus 1210 seismographs belonging to the Institute of Geophysics. Each has 12 recording channels, and a 330m cable spread.
At New Harbour, both seismographs were operated in a heated Wannigan, and the Nimbus was used with an extension cable so that the two spreads along the seismic line were 330m apart. Two seismic lines, each 8.66km long with shot points at each end and at the centre, were laid out in the form of a cross with 6 Wannigan sites and 12 evenly spaced spreads on each line. (A 7th site was added in the centre of line B). Lines were laid out dead straight and flagged at 320, 330, 340m intervals by means of a precision odometer fitted to one of the 2 motor toboggans.
Two shooting parties, each with a shot instant tone encoder and Compak radio, fired charges of between 1kg at 5m depth and 8.8kg at 15m depth at all 3 shot points for each Wanigan site. Shot depths were chosen to avoid damaging the shot holes, rather than avoiding bubble pulses. At the Wannigan a Compak receiver and tone decoder started the Nimbus automatically at the shot instant, and the tone code was fully recorded by the RS44. At each Wanigan site a 30cm hole was drilled through the 2m sea ice for the echo sounder, and the water depth was measured.
At the Dailey Islands, Tim Stern and Peter Whiteford (DSIR geophysicists) and Brian Smith (field assistant) joined us with their Nimbus seismograph and Snotrac vehicle, enabling three shooting parties and two separate recording parties to be used. Here, sea bottom depth and dip along the seismic line were determined at each spread by stacking sledge hammer blows on the ice in the Nimbus digital seismograph.
Two 8.66km lines similar to those at New Harbour were laid out in the form of a "T". Four extra shot points were included on line A because a complex sea bottom was expected near the islands.
References
PYNE, A.R. and WAGHORN, D.B. 1980. Immediate Report of Victoria University of Wellington Antarctic Expedition No. 24.
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FIG. 3. Location of seismic refraction lines off Butter Point and the Dailey Islands.
PART II - SHIPBOARD PROGRAMME - P.J. Barrett
| 1. | Sediment in the deep water (about 850m) around Ross Island is mud deposited from suspension with a small ice-rafted component, mainly basaltic. Its variable firmness a few centimetres below the seafloor may be due to differences in sedimentation rate or composition. |
| 2. | Granite Harbour is a deep basin (890m) cut or cleaned out by an enlarged Mackay Glacier and little modified by sedimentation since that event. |
| 3. | Tillite pebbles from seafloor sediment off Granite Harbour suggest that the strata forming the south-trending ridge about 20km offshore include the Cenozoic glacial sequence cored at MSSTS. |
| 4. | The Western Shelf and Slope in McMurdo Sound have not been crossed by similar extensions of Taylor or Ferrar Glaciers, at least since the Early Cenozoic. The Slope is cut by at least two narrow valleys, but their origin is not yet understood. |
| 5. | The McMurdo Volcanics extend as scattered cones or cone complexes along the edge and possibly beneath the McMurdo Ice Shelf and for some distance offshore from Ross Island. |
| 1. | To take several deep water samples not accessible from the sea ice. |
| 2. | To sample off (and if possible in) Granite Harbour. |
| 3. | To obtain continuous depth records from previously uncharted areas in western McMurdo Sound. |
The ship's track and stations for bottom sampling are shown in Figure 4. Station data is given in Table 2.
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FIG. 4. Map of McMurdo Sound area, showing the track and bottom sample locations from USCGC GLACIER Cruise IV. shallow areas thought to be of volcanic origin are marked by a star. Depths in metres.
sand lenses. The sediment has been deposited largely from suspension with a small coarse component rafted by floating ice, probably some of it glacial and some shore ice. The sediment below 2cm is softest at Station 6 in the northern part of Bird Basin, becoming firmer to the south, though the floor of the basin remains at about the same depth (Fig. 5A). The variation in firmness may be due to differences in sedimentation rate or composition.Samples from slightly shallower depths (Stations 1, 2, 3A, 7, 8, 26; 547-755m) were also taken for foraminiferal studies by B.L. Ward to cover the range that includes the carbonate compensation depth. Sample 1, a few miles north of Scott Base, had pebbles encrusted with bryozoans, indicating their exposure above the seafloor and again a very low sedimentation rate.
With Granite Harbour virtually ice-free we first carried out a bathymetric survey, 250km of track in an area of about 400km2. The work in Granite Harbour also involved the setting of a sediment trap in 800m of water at the tip of the Mackay Glacier Tongue (to be retrieved in November 1983) and the recovery of 13 bottom samples. The latter were mainly grab samples, but included 2 cores 60cm long and 20cm across for detailed sedimentological study. A grab sample near the middle of the Harbour (Station 16, 849m) contained black mud, but a piston core attempt nearby (Station 16A, 880m) yielded only 25cm of core because the corer encountered stony till, showing that the pattern of sedimentation in Granite Harbour is not simple. The bathymetry is also quite complex for although the Harbour can be described in gross terms as a broad basin almost 900m deep, the floor is hummocky with a relief of 200-300m (Fig. 5B).
A bathymetry line was run east from Cape Roberts and two bottom samples were taken to help with CIROS site selection. The seafloor east of Cape Roberts is sandy or gravelly, even in 489m of water at Station 19, where a boulder of granite 30cm long was taken by the grab. Worm trails of cemented sand over part of the surface of the boulder show that it was partly buried in sand. Grabs from near Cape Roberts and the 108m deep shoal 18km offshore represented a gravel pavement, judging from the encrusting bryozoans and other organisms. Bathymetric data obtained off Granite Harbour can now be used to chart the North-South-trending ridge and West-facing scarp that is cut in the East-dipping sequence to be sampled by CIROS 3 and 4. Some idea of the sequence was obtained from pebbles in the till recovered at Station 8 which is on a broad, flat "delta" surface at 550m. Although most pebbles were granite and presumably from Granite Harbour, a number were sandy mudstone with scattered coarse sand grains, like the glacial mudstones in the Early-Mid Cenozoic MSSTS core. The west-facing scarp was also sampled by grab (Station 11) but recovery was poor and no new lithologies were seen. Nevertheless, we hope that processing the sand fraction may yield micro-fossils that indicate the age range of strata forming the scarp (Pig. 5C).
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FIG. 5. Topographic features in McMurdo Sound and Granite Harbour. Depths in metres. Horizontal scale bars 5km long.
| A. | Southern part of Bird Basin, contrasting the smooth flat floor with the rough topography beyond the margin. |
| B. | Cross-section near the head of Granite Harbour, with the glacially moulded topography still evident in the deepest parts. |
| C. | Bathymetry across the "scarp" off Granite Harbour. |
| D. | Submarine valleys glacially carved by the Debenham Glacier 10km south of Station 19. |
| E. | Narrow valley of unknown origin off New Harbour. |
| F. | Submarine peaks along the edge of the McMurdo Ice Shelf, and inferred to be volcanic cones. |
reports (including Barrett et al., 1974) show submarine valleys extending from the Dry Valleys into the "moat" around Ross Island. However, the Western Shelf is in fact a rather flat feature (average depth 180m) with two very narrow (about 1km) valleys running from its edge to near the foot of the slope. These features seem far too small and well-defined to be glacial in origin, especially when compared with the broad submarine extension of the valley now containing the Debenham Glacier (5km wide and 500m deep) or the 800m deep basin carved by the Mackay Glacier off Granite Harbour (Figs. 5D, E). The lack of broad glacial valleys in the Western Shelf east of the Dry Valleys, in contrast to those cut by the outlet glaciers north of 77 10′S, suggest that none of the Dry Valleys have served as outlet glaciers since the deposition of the near surface strata of the Western Shelf, which at MSSTS-1 are as old as Early Cenozoic (Webb, Leckie and Ward, in press).The bathymetric data also revealed another feature no doubt seen before, but not to our knowledge taken into account in discussions of McMurdo Sound geology. Tracks along the edge of the McMurdo Ice Shelf showed a hilly topography superimposed on the gentle slope of the Western Shelf (Fig. 5F). Because of the nearby Dailey Islands, scattered basaltic cones that rise to 132m above sea level, these hilly features are also thought to be of volcanic origin. Defense Mapping Agency Hydrographic Center Chart 29321 shows a number of unusually shallow soundings in deep water around Ross Island, and one of these is crossed by Track XXX of Northey et al. (1976), showing it to have a conical section and an apparent height of 165m above the floor of Bird Basin. These are also judged to be of volcanic origin, and may present a significant extension of the Erebus volcanic Province (Figure 1).
Results of the cruise will appear in several different places. Selected samples will be used by Ms B. Ward in her study of the distribution of modern foraminifera in McMurdo Sound, and the Granite Harbour samples and data will be used as part of a thesis on the Mackay Glacier-Granite Harbour sedimentary system by Mr. Macpherson. The bathymetry will be added to an earlier compilation to complete a bathymetric map of McMurdo Sound with the assistance of Geophysics Division and the Institute of Marine and Freshwater Science, DSIR. The samples and bathymetry will also provide the basis for papers on recent sedimentation in McMurdo Sound.
A cruise plan is being prepared for work in February 1984 mainly to carry out further site investigation for CIROS drilling. A shipboard survey has become necessary because sea ice conditions off Cape Roberts prevented the planned "land" based seismic survey off Cape Roberts. The survey will link with previous shipboard surveys in the area. The cruise plan also includes some time for gravity cores from Bird Basin (not collected in 1983 because of equipment problems and [unclear: lack] of time), and seafloor photography.
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TABLE 2. Grab/Core Stations from USCGC GLACIER in McMurdo Sound, 12-19 February, 1983.
References
BARRETT, P.J.; CHRISTOFFEL, D.A.; NORTHEY, D.J.; SISSONS, B.A. 1974. Seismic profiles across the extension of Wright Valley into McMurdo Sound. Antarctic Journal of the U.S., 9(4), 138-140.
BARRETT, P.J.; PYNE, A.R.; WARD, B.L. In press. Modern sedimentation in McMurdo Sound, Antarctica. Proc. Fourth International Symposium on Antarctic Earth Sciences, Adelaide, August 1982.
McGINNIS, L.D. 1973. McMurdo Sound - a key to the Cenozoic of Antarctica. Antarctic Journal of the U.S., 8(4), 166-169.
NORTHEY, D.J.; BROWN, C.; CHRISTOFFEL, D.A.; WONG, H.K.; BARRETT, P.J. 1975. Dry Valley Drilling Project Bull. No. 5, Northern Illinois University, De Kalb, 167-179.
WEBB, P.N.; LECKIE, M.; WARD, B.L. In press. Paleogene-Neogene Foraminifera from the MSSTS-1 drill hole, McMurdo Sound, Antarctica. In: P.J. Barrett (ed.). Report on MSSTS-1 and associated studies, western McMurdo Sound, Antarctica, Department of Scientific and Industrial Research, Wellington.