Study of grounding-line sedimentation processes at the ...Iceberg scours were commonly observed. A...
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Study of grounding-line sedimentation processes at the Mackay Glacier tongue using a remotely operated vehicle Observations on the seafloor epibenthic community MICHAEL DAwBER and Ross D. POwELL, Department of Geology, Northern Illinois University, De Kalb, Illinois 60115 T he condition of grounding lines of polar ice sheets appears to be one of the critical factors governing ice- sheet stability. Sedimentation processes at the grounding line of the Mackay Glacier are being documented using a sub- mersible remotely operated vehicle (ROV). The Mackay Glaci- er (76°58'S 162°24'E), an outlet glacier from the east antarctic ice sheet, flows through the Royal Society Range of the Transantarctic Mountains to the Ross Sea. The RO y, which is equipped with black-and-white and color video cameras, a stereo still camera, a forward-looking sonar, a sampling arm for collecting bottom sediment samples, and a side-scan sonar, can continuously measure and record depth, salinity, temperature, water velocity, dissolved oxygen, and optical backscatter for sediment concentration. In addition, its loca- tion from the dive-site control center is continuously deter- mined. The primary purposes of this study are to determine the physical processes by which terrigenous clastic sediments move from antarctic glaciers into marine environments and to estimate the volume contributed annually at the grounding line. A complement to these goals is an evaluation of epiben- thic communities that may aid in the understanding of the physical and chemical characteristics of the environment near the glacier tongue and may also be a useful additional aid to interpretation of sediment cores taken from ice-proxi- mal facies. The seafloor beneath and adjacent to the glacier supports an epibenthic community that shows considerable variation in species composition and community structure. The com- munity has some similarities with that at Cape Armitage, McMurdo Sound (cf. Dayton, Robillard, and Paine 1970, pp. 244-258; Dayton et al. 1974). In this article, preliminary obser- vations of the epibenthic community (specifically, that por- tion of the epibenthos that can be discerned on color and black-and-white video recordings, i.e., the "megabenthos") and possible links between these variations in the community and the deposition and distribution of sediments and some other factors are discussed. In the 1993 field season, eight dives were made with the submersible ROV from three holes in the sea ice, two of which were located in wedge chasms on the southern margin of the glacier tongue and the third at the front of the glacier tongue. Despite attempts being made on each of the dives, the grounding line was not reached. In the preliminary 1991 sea- son, on dives made from the northern edge of the glacier tongue, the grounding line was reached on several occasions. The seafloor beneath the Mackay Glacier tongue is com- posed of a mixture of the following: •stiff but not over-compacted subglacial till deposited when the grounding line was in a more advanced position than at present; • glacimarine diamict (shelfstone diamict) deposited on top of the subglacial till by meltout of basal debris since the retreat of the grounding line; • fine (sand-sized particles and finer) sediment probably consisting of a mix of fine sediment winnowed from the shelfstone diamict by bottom currents (Macpherson 1986) and biogenic sediment; and • bedrock outcrops, occurring at several localities as smooth, rounded, linear ridges several tens of meters long, up to 2 meters (m) high and up to 10 in and at other localities as less continuous features often with vertical or overhanging sides, up to 3 in and several meters across. The distribution of sediments on the seafloor varies greatly through abrupt transitions from areas dominated by larger clasts (e.g., 60 percent boulders and cobbles to areas of virtually 100 percent fine sediment) to relatively smooth gra- dations from one substrate type to another. This great areal variation in the characteristics of bottom sediments is the result of random deposition of shelfstone diamicts onto exposed subglacial till and the winnowing and subsequent redeposition of fine particles by bottom currents. Iceberg scours were commonly observed. A scour is formed when a grounded iceberg is moved, either by pushing from the glacier or by winds or ocean currents causing the iceberg keel to be dragged through sediment on the seafloor. These scours have a vertical relief of up to 2 m, a width of up to 3 m, and a variable orientation with respect to the glacier tongue. Some recent scours have obviously disturbed established epibenthic communities, the scour itself lacking sessile epi- fauna in contrast to an established community in the sur- rounding area. The least colonized scours are likely the most recent, however, rate of colonization will vary with location. For example, those scours nearest the grounding line where sedimentation rates are high may require a longer period for colonization to occur than those scours farther away from the grounding line, where lower rates of sedimentation are more conducive to colonization by sessile-filter feeding animals. Several of the scours have been occupied by large (up to 1-rn- high) Hexactinellid sponges indicating a possible age of sever- al hundred years (Dayton et al. 1970, pp. 244-258). The grounding line was reached on several occasions on dives in 1991. Much recently deposited shelfstone diarnict is present on the seafloor, and the sediment concentration in ANTARCTIC JOURNAL - REVIEW 1994 77
Transcript of Study of grounding-line sedimentation processes at the ...Iceberg scours were commonly observed. A...
Study of grounding-line sedimentation processes at theMackay Glacier tongue using a remotely operated vehicle
Observations on the seafloor epibenthic communityMICHAEL DAwBER and Ross D. POwELL, Department of Geology, Northern Illinois University, De Kalb, Illinois 60115
The condition of grounding lines of polar ice sheetsappears to be one of the critical factors governing ice-
sheet stability. Sedimentation processes at the grounding lineof the Mackay Glacier are being documented using a sub-mersible remotely operated vehicle (ROV). The Mackay Glaci-er (76°58'S 162°24'E), an outlet glacier from the east antarcticice sheet, flows through the Royal Society Range of theTransantarctic Mountains to the Ross Sea. The ROy, which isequipped with black-and-white and color video cameras, astereo still camera, a forward-looking sonar, a sampling armfor collecting bottom sediment samples, and a side-scansonar, can continuously measure and record depth, salinity,temperature, water velocity, dissolved oxygen, and opticalbackscatter for sediment concentration. In addition, its loca-tion from the dive-site control center is continuously deter-mined.
The primary purposes of this study are to determine thephysical processes by which terrigenous clastic sedimentsmove from antarctic glaciers into marine environments andto estimate the volume contributed annually at the groundingline. A complement to these goals is an evaluation of epiben-thic communities that may aid in the understanding of thephysical and chemical characteristics of the environmentnear the glacier tongue and may also be a useful additionalaid to interpretation of sediment cores taken from ice-proxi-mal facies.
The seafloor beneath and adjacent to the glacier supportsan epibenthic community that shows considerable variationin species composition and community structure. The com-munity has some similarities with that at Cape Armitage,McMurdo Sound (cf. Dayton, Robillard, and Paine 1970, pp.244-258; Dayton et al. 1974). In this article, preliminary obser-vations of the epibenthic community (specifically, that por-tion of the epibenthos that can be discerned on color andblack-and-white video recordings, i.e., the "megabenthos")and possible links between these variations in the communityand the deposition and distribution of sediments and someother factors are discussed.
In the 1993 field season, eight dives were made with thesubmersible ROV from three holes in the sea ice, two of whichwere located in wedge chasms on the southern margin of theglacier tongue and the third at the front of the glacier tongue.Despite attempts being made on each of the dives, thegrounding line was not reached. In the preliminary 1991 sea-son, on dives made from the northern edge of the glaciertongue, the grounding line was reached on several occasions.
The seafloor beneath the Mackay Glacier tongue is com-posed of a mixture of the following:
•stiff but not over-compacted subglacial till deposited whenthe grounding line was in a more advanced position thanat present;
• glacimarine diamict (shelfstone diamict) deposited on topof the subglacial till by meltout of basal debris since theretreat of the grounding line;
• fine (sand-sized particles and finer) sediment probablyconsisting of a mix of fine sediment winnowed from theshelfstone diamict by bottom currents (Macpherson 1986)and biogenic sediment; and
• bedrock outcrops, occurring at several localities assmooth, rounded, linear ridges several tens of meters long,up to 2 meters (m) high and up to 10 in and at otherlocalities as less continuous features often with vertical oroverhanging sides, up to 3 in and several metersacross.
The distribution of sediments on the seafloor variesgreatly through abrupt transitions from areas dominated bylarger clasts (e.g., 60 percent boulders and cobbles to areas ofvirtually 100 percent fine sediment) to relatively smooth gra-dations from one substrate type to another. This great arealvariation in the characteristics of bottom sediments is theresult of random deposition of shelfstone diamicts ontoexposed subglacial till and the winnowing and subsequentredeposition of fine particles by bottom currents.
Iceberg scours were commonly observed. A scour isformed when a grounded iceberg is moved, either by pushingfrom the glacier or by winds or ocean currents causing theiceberg keel to be dragged through sediment on the seafloor.These scours have a vertical relief of up to 2 m, a width of upto 3 m, and a variable orientation with respect to the glaciertongue.
Some recent scours have obviously disturbed establishedepibenthic communities, the scour itself lacking sessile epi-fauna in contrast to an established community in the sur-rounding area. The least colonized scours are likely the mostrecent, however, rate of colonization will vary with location.For example, those scours nearest the grounding line wheresedimentation rates are high may require a longer period forcolonization to occur than those scours farther away from thegrounding line, where lower rates of sedimentation are moreconducive to colonization by sessile-filter feeding animals.Several of the scours have been occupied by large (up to 1-rn-high) Hexactinellid sponges indicating a possible age of sever-al hundred years (Dayton et al. 1970, pp. 244-258).
The grounding line was reached on several occasions ondives in 1991. Much recently deposited shelfstone diarnict ispresent on the seafloor, and the sediment concentration in
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the water column tends to be high. Many boulders are drapedwith fine sediment up to several centimeters thick. As thegrounding line is approached, abundance and diversity of themegabenthos is reduced, and within a few tens of meters ofthe grounding line, virtually no megabenthic animals are pre-sent. Very occasionally, bryozoans/hydroids (or both, identifi-cation is not yet conclusive) grow on the edges of boulders,and asteroids, ophiuroids, echinoids, and tubeworms are alsooccasionally seen. Antarctic scallops (Adamussium colbecki)occur at the grounding line itself in several areas.
In contrast, the 1993 dive sites, located on the southernmargin of the glacier, were found to be out of reach of thegrounding line with the 500-rn umbilical. Water column sedi-ment concentrations and amounts of recent shelfstonediamict on the seafloor are generally lower than thoseobserved within the proximity of the grounding line in 1991.On areas of the seafloor south of the southern margin of theglacier tongue, boulders were covered with a dense associa-tion of animals including bryozoans, hydrozoans, sponges,tubeworms, alcyonarians, and ascidians. The antarctic scallopA. colbecki occurred in great numbers in some areas; individ-uals covered much of the surface of a bedrock ridge 100-200m long at a depth of 50-90 m. Highest densities were a mini-mum of 100 per square meter (cf. Berkman 1990). In general,densities of the megabenthos beneath the glacier tongue at itssouthern margin were lower than they were south of the glaci-er tongue. Densities and numbers of species beneath thetongue at its southern margin, however, were not as low asthose encountered near the grounding line on the north sideof the tongue in 1991.
Some influence of sedimentation is to be expected, giventhat many members of the megabenthos are sessile-filterfeeders whose feeding ability maybe reduced in turbid condi-tions (Pearse et al. 1992). The high frequency of deposition ofshelfstone diamicts near the grounding line and the high con-centration of fine sediment in the water column must severelylimit development of the community in this area. Furtheranalyses to quantify these aspects are continuing.
The 1993 dives also revealed a patchiness in the densityof the megabenthos over a range of distances from tens ofmeters to less than a meter. Some of this patchiness is relatedto the availability of hard substrate. Densities of themegabenthos tend to be lower in areas of the seafloor domi-nated by finer sediments compared with those dominated bylarger clasts (cf. Grassle et al. 1975) in the shelfstone diamictsand protruding from the subglacial till.
The development of the epibenthic community on alltypes of substrate by the megabenthos must, to some degree,
be a reflection of the age of the substrate, but many other fac-tors—including, among others, sedimentation rate, waterdepth, food supply, influence of currents, and biologicalinteractions—must play a role. Most of the seafloor beneathand within the vicinity of the glacier tongue is at a depthgreater than 40 m; thus, disturbance of the benthic communi-ty by anchor ice formation (cf. Dayton et al. 1970, pp.244-258) is not significant.
It is hoped that further, more quantitative analyses of the1991 and 1993 data and of additional data that will be collect-ed in the 1994 season may reveal important links betweencommunity patterns and physical parameters reflectingaspects of glacier behavior and may also provide an aid to theinterpretation of past glacier behavior from sedimentarysequences in cores.
This work was supported by National Science Founda-tion grant OPP 92-19048 to Ross D. Powell. Thanks to TheExplorer's Club, New York for financial assistance to MichaelDawber. Thanks to Lewis Hunter and Tom Hooyer for theirhelp as members of the field team; to Diane Carney and MarcSlattery for biological assistance; to the New Zealand Antarc-tic Programme (Alex Pyne and Peter Barrett) especially forcollaboration in collecting the 1991 data; and to National Sci-ence Foundation, Antarctic Support Associates, and U.S. Navypersonnel for their valuable support.
References
Berkman, P.A. 1990. The population biology of the antarctic scallop,Adamussium colbecki (Smith 1902) at New Harbor, Ross Sea. InK.R. Kerry and G. Hempel (Eds.) Antarctic ecosystems. Ecologicalchange and conservation. Berlin/ Heidelberg: Springer-Verlag.
Dayton, P.K., G.A. Robillard, and R.T. Paine. 1970. Benthic faunalzonation as a result of anchor ice formation at McMurdo Sound,Antarctica. In M.W. Hoidgate (Ed.), Antarctic Ecology (Vol. 1). Lon-don: Academic Press.
Dayton, P.K., G.A. Robillard, R.T. Paine, and L.B. Dayton. 1974. Bio-logical accommodation in the benthic community at McMurdoSound, Antarctica. Ecological Monographs, 44(1), 105-128.
Grassle, J.F., H.L. Sanders, R.R. Hessler, G.T. Rowe, and T. McLellan.1975. Pattern and zonations: A study of the bathyl megafaunausing the research submersible Alvin. Deep-Sea Research, 22(5),457-481.
Macpherson, A.J. 1986. The Mackay Glacier/Granite Harbor System(Ross Dependency, Antarctica)—A study in nearshore glacialmarine sedimentation. (Ph.D. thesis, Victoria University ofWellington, New Zealand)
Pearse, V., J. Pearse, M. Buchsbaum, and R. Buchsbaum. 1992. Livinginvertebrates. Boston: Blackwell Scientific Publications; PacificGrove, California: The Boxwood Press.
Slattery, M. 1994. Personal communication.
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