High-resolution seismic survey of the Ross Sea ...
High-resolution seismic survey of the Ross Sea continental shelf: Implications for ice-sheet retreat behavior STEPHANIE S. SHIPP
and JOHN B. ANDERSON, Department of Geology and Geophysics, Rice University, Houston, Texas 77005
understanding of the past activity of the west antarctic observed in Ross Sea tentatively has been correlated with that X-c e sheet is critical to evaluate future behavior of the ice of Karl, Reimnitz, and Edwards (1987, pp. 77-92). Paleo-icesheet and its potential impact on sea level. The primary stream boundaries and bank tops are erosional (figure 3). The objectives during R/V Nathaniel B. Palmer cruise 94-1 includboundaries are deeply cut into older strata and capped by ed determining the extent of ice advance during the last more recent units. In contrast, eastern Ross Sea is characterglacial maximum and determining the mechanisms, ice-sheet ized by a less organized depositional pattern. Paleo-iceconfiguration, and timing of glacial retreat. Toward these stream boundaries tend to be depositional; the thickest goals, approximately 4,100 kilometers of high-resolution seisdeposits of the region constitute these banks. In some regions mic data and 45 piston cores and grab samples were collected of the troughs, the sediment cover is thin to absent; but a on the Ross Sea continental shelf (figure 1). A 50-cubic-inch 20-40-millisecond thick sediment package covers the majoribubble-free air gun served as the seismic source. This source ty of the area. Intrastream bank tops occur at greater water was selected because it offered vertical resolution of features depths and do not exhibit flat, eroded tops. on the order of 5 to 7 meters, sufficient to image glacial depoRoss Sea typifies a polar glacial setting; it is devoid of meltsitional features in detail. water-derived features. A tentative deglaciation scenario is that Ongoing debate focuses on the extent of ice and how the interaction between sea-level fluctuations and substrate conice sheet retreated during the last deglaciation. The majority ditions controlled the retreat of the marine-based ice sheet. of the west antarctic ice sheet lies below sea level, and extenDuring the last glacial maximum, ice extended to the continensive regions overlie sedimentary basins. This, in conjunction tal shelf edge. A series of ice streams was active at some point with the foredeepened nature of the Ross Sea continental during deglaciation, evinced by a distinct topographic signashelf, means that during deglaciation, increased sea level ture and continuous petrographic provinces across the conticould buoy the ice sheet, causing rapid grounding-line retreat nental shelf (Jahns, Antarctic Journal, in this issue). The trough and draw-down of interior ice (e.g., Hollin 1962; Thomas and axes formed sites of erosion or limited deposition, but thick Bentley 1978). Alternatively, thinning of the ice sheet could packages of material were deposited on the sides of the occur by enhanced ice flow across a deforming substrate (e.g., troughs. The bank tops in western Ross Sea probably were the Alley et al. 1989), without being triggered by rising sea level. sites of thinner, perhaps stagnant ice. In eastern Ross Sea, ice Preliminary analysis of seismic data reveals that the probably did not pin on the banks dividing ice streams. Due to majority of the continental shelf has thin to absent late Pleistocene sedimentary cover. 165' 170 175 180' 185 190' The sediment thickens on the outer shelf, -72' -72 within the troughs, to a maximum of 120 milliseconds. At the shelf break and across the -73. slope, thicknesses are greater. A single seismic facies constitutes the sedimentary cover. It is acoustically massive internally with rare clino- 74 -74. forms and commonly exhibits a hummocky surface relief and hyperbolic reflectors. The surface relief rarely exceeds 30 milliseconds -75, -75, (figure 2). The hummocky nature is most apparent on lines perpendicular to the glacial troughs and may indicate furrows aligned -76 -76' with the direction of ice movement. The seismic facies overlies an erosional unconformity. This unit is interpreted as till. -77, -77. Distinct differences occur between eastern and western Ross Sea. Western Ross Sea has more deeply carved troughs on the inner continental shelf. The troughs are filled with -78' -78' 160' 165' 170' 175 150' 185' 190' 20-60 milliseconds of sediment on the outer shelf, but thick sedimentary packages occur Figure 1. Locations of high-resolution seismic data, piston cores, and grab samples along the bank edges. The depositional unit collected during RN Nathaniel B. Palmer cruise 1994-1.
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Figure 2. Example of the upper depositional unit in Ross Sea. Note the presence of the glacial erosional unconformity. Location of profile is indicated on figure 1.
0.4 Cl) V C
0 C.)
0.5
a)
Cl)
a)
E
I-
0.6
I-
I> 0
0.7
0.8
Figure 3. Erosional paleo-ice-stream boundary in western Ross Sea. Location of seismic line is shown on figure 1. its relatively shallower nature, ice in eastern Ross Sea would have remained grounded longer in the event of sea-level rise. Ongoing analyses should constrain the timing of deglaciation and determine whether or not deglaciation occurred in pulses. This work was supported by the National Science Foundation grant OPP 91-19683. Appreciation is extended to the crew of the R/V Nathaniel B. Palmer for a highly productive and pleasant cruise.
Sedimentation beneath ice shelves-The view from ice stream B.
Marine Geology, 85,101-120. Hollin, J.T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4, 173-195. Jahns, E. 1994. Evidence for a fluidized till deposit on the Ross Sea continental shelf. Antarctic Journal of the U.S., 29(5). Karl, HA, E. Reimnitz, and B.D. Edwards. 1987. Extent and nature of Ross Sea unconformity in the western Ross Sea, Antarctica. In A.K.
Cooper and F.J. Davey (Eds.), The antarctic continental margin: Geology and geophysics of the western Ross Sea (Earth Science Series, Vol. SB). Houston: Circum-Pacific Council for Energy and Mineral Resources. Thomas, R.H., and C.R. Bentley. 1978. A model for Holocene retreat of the west antarctic ice sheet. Quaternary Research, 10, 150-170.
References Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1989.
ANTARCTIC JOURNAL - REVIEW 1994 138
and JOHN B. ANDERSON, Department of Geology and Geophysics, Rice University, Houston, Texas 77005
understanding of the past activity of the west antarctic observed in Ross Sea tentatively has been correlated with that X-c e sheet is critical to evaluate future behavior of the ice of Karl, Reimnitz, and Edwards (1987, pp. 77-92). Paleo-icesheet and its potential impact on sea level. The primary stream boundaries and bank tops are erosional (figure 3). The objectives during R/V Nathaniel B. Palmer cruise 94-1 includboundaries are deeply cut into older strata and capped by ed determining the extent of ice advance during the last more recent units. In contrast, eastern Ross Sea is characterglacial maximum and determining the mechanisms, ice-sheet ized by a less organized depositional pattern. Paleo-iceconfiguration, and timing of glacial retreat. Toward these stream boundaries tend to be depositional; the thickest goals, approximately 4,100 kilometers of high-resolution seisdeposits of the region constitute these banks. In some regions mic data and 45 piston cores and grab samples were collected of the troughs, the sediment cover is thin to absent; but a on the Ross Sea continental shelf (figure 1). A 50-cubic-inch 20-40-millisecond thick sediment package covers the majoribubble-free air gun served as the seismic source. This source ty of the area. Intrastream bank tops occur at greater water was selected because it offered vertical resolution of features depths and do not exhibit flat, eroded tops. on the order of 5 to 7 meters, sufficient to image glacial depoRoss Sea typifies a polar glacial setting; it is devoid of meltsitional features in detail. water-derived features. A tentative deglaciation scenario is that Ongoing debate focuses on the extent of ice and how the interaction between sea-level fluctuations and substrate conice sheet retreated during the last deglaciation. The majority ditions controlled the retreat of the marine-based ice sheet. of the west antarctic ice sheet lies below sea level, and extenDuring the last glacial maximum, ice extended to the continensive regions overlie sedimentary basins. This, in conjunction tal shelf edge. A series of ice streams was active at some point with the foredeepened nature of the Ross Sea continental during deglaciation, evinced by a distinct topographic signashelf, means that during deglaciation, increased sea level ture and continuous petrographic provinces across the conticould buoy the ice sheet, causing rapid grounding-line retreat nental shelf (Jahns, Antarctic Journal, in this issue). The trough and draw-down of interior ice (e.g., Hollin 1962; Thomas and axes formed sites of erosion or limited deposition, but thick Bentley 1978). Alternatively, thinning of the ice sheet could packages of material were deposited on the sides of the occur by enhanced ice flow across a deforming substrate (e.g., troughs. The bank tops in western Ross Sea probably were the Alley et al. 1989), without being triggered by rising sea level. sites of thinner, perhaps stagnant ice. In eastern Ross Sea, ice Preliminary analysis of seismic data reveals that the probably did not pin on the banks dividing ice streams. Due to majority of the continental shelf has thin to absent late Pleistocene sedimentary cover. 165' 170 175 180' 185 190' The sediment thickens on the outer shelf, -72' -72 within the troughs, to a maximum of 120 milliseconds. At the shelf break and across the -73. slope, thicknesses are greater. A single seismic facies constitutes the sedimentary cover. It is acoustically massive internally with rare clino- 74 -74. forms and commonly exhibits a hummocky surface relief and hyperbolic reflectors. The surface relief rarely exceeds 30 milliseconds -75, -75, (figure 2). The hummocky nature is most apparent on lines perpendicular to the glacial troughs and may indicate furrows aligned -76 -76' with the direction of ice movement. The seismic facies overlies an erosional unconformity. This unit is interpreted as till. -77, -77. Distinct differences occur between eastern and western Ross Sea. Western Ross Sea has more deeply carved troughs on the inner continental shelf. The troughs are filled with -78' -78' 160' 165' 170' 175 150' 185' 190' 20-60 milliseconds of sediment on the outer shelf, but thick sedimentary packages occur Figure 1. Locations of high-resolution seismic data, piston cores, and grab samples along the bank edges. The depositional unit collected during RN Nathaniel B. Palmer cruise 1994-1.
ANTARCTIC JOURNAL - REVIEW 1994
137
Figure 2. Example of the upper depositional unit in Ross Sea. Note the presence of the glacial erosional unconformity. Location of profile is indicated on figure 1.
0.4 Cl) V C
0 C.)
0.5
a)
Cl)
a)
E
I-
0.6
I-
I> 0
0.7
0.8
Figure 3. Erosional paleo-ice-stream boundary in western Ross Sea. Location of seismic line is shown on figure 1. its relatively shallower nature, ice in eastern Ross Sea would have remained grounded longer in the event of sea-level rise. Ongoing analyses should constrain the timing of deglaciation and determine whether or not deglaciation occurred in pulses. This work was supported by the National Science Foundation grant OPP 91-19683. Appreciation is extended to the crew of the R/V Nathaniel B. Palmer for a highly productive and pleasant cruise.
Sedimentation beneath ice shelves-The view from ice stream B.
Marine Geology, 85,101-120. Hollin, J.T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4, 173-195. Jahns, E. 1994. Evidence for a fluidized till deposit on the Ross Sea continental shelf. Antarctic Journal of the U.S., 29(5). Karl, HA, E. Reimnitz, and B.D. Edwards. 1987. Extent and nature of Ross Sea unconformity in the western Ross Sea, Antarctica. In A.K.
Cooper and F.J. Davey (Eds.), The antarctic continental margin: Geology and geophysics of the western Ross Sea (Earth Science Series, Vol. SB). Houston: Circum-Pacific Council for Energy and Mineral Resources. Thomas, R.H., and C.R. Bentley. 1978. A model for Holocene retreat of the west antarctic ice sheet. Quaternary Research, 10, 150-170.
References Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1989.
ANTARCTIC JOURNAL - REVIEW 1994 138
