"deforming till" from Upstream B, West Antarctica
Glacial geology
Microfossil assemblages in "deforming till" from Upstream B, West Antarctica: Implications for ice-stream flow models REED P. SCHERER
Byrd Polar Research Center
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
Sediments underlying grounded ice at the Upstream B camp in central West Antarctica (83°28'40"S 138°05'49"W) were collected by glaciologists from the California Institute of Technology and other institutions, during the 1988-1989 field season (figure). The ice sheet at the drill site is 1,030 meters thick and the glacier bed lies 644 meters below sea level. About 10 cubic centimeters of sediment was made available for preliminary microfossil study. This paper reports the initial findings of diatom analysis with respect to ice stream flow models. Paleoenvironmental interpretations of the microfossil assemblages are reported elsewhere (Scherer, Antarctic Journal, this issue).
East Antarctica West Antarctica '.' 4)
80 0 S 1800 .'..
O, vo
S.
Ross Sea
UpO
Ross Ice Shelf
Location of Upstream B (UpB), RISP site J-9, and Crary Ice Rise (CIR) samples with respect to ice streams A, B, and C (after Shabtaie and Bentley 1987).
54
It has been proposed that a water-saturated till, several meters thick, acts as a lubricating layer that aids ice stream flow (Alley et al. 1986, 1989; Blankenship et al. 1986). The "deforming till" layer has been identified by seismic reflection and appears to be widespread underneath the west antarctic ice streams (Rooney et al. 1987). Beneath the till layer lies a thick section of sediments (approximately 600 meters) which is actively being eroded at its top. Based on the low sonic velocities measured in the sediment column (generally less than 2.0 kilometers per second), Rooney et al. (in press) tentatively correlated this unit with Ross Sea sediments underlying the Neogene Ross Sea Unconformity. Previous diatom study of west antarctic sediments collected south of the Ross Ice Shelf barrier is limited to two regions in the Ross embayment: site J-9 of the Ross Ice Shelf Project (RISP) (Brady and Martin 1979; Kellogg and Kellogg 1986; Harwood et al. 1989) and Crary Ice Rise (Scherer et al. 1988). Sediment from site J-9 includes abundant and diverse diatoms, dominated by Early Miocene forms but including diatoms as young as mid-Late Miocene (Harwood, Scherer, and Webb in press). Sediment collected from Crary Ice Rise includes diatoms and silicoflagellates of latest Miocene age with rare reworked older microfossils (Scherer et al. 1988). Neither RISP nor Crary Ice Rise sediments include diatoms with stratigraphic appearances younger than Miocene, implying initial deposition prior to the Pliocene. The ice at Upstream B camp was penetrated by a Polar Ice Coring Office hot-water drill, and sediment was collected by stirring up the sediment with hot water jets and collecting suspended material in cups. This sampling method biased textural characteristics (grain size) of the till by removing coarse sand and large clasts and much of the clay-sized material, concentrating fine sand and silt-sized material. No stratigraphic information was preserved, and physical property measurements were not possible. Although the sample is size-sorted, the dominant grain size of the sediment collected includes most microfossil groups. Estimates of microfossil abundance are highly qualitative, due to the size sorting. The sample was processed for diatoms using both standard and enhanced diatom extraction methods. Micro-clasts of soft sediments were separated from the matrix sediments by sieving the sample at 63 microns and separating low-density sediment clasts from the greater than 63 micron fraction by floatation in Sodium Polytungstate (s.g. 2.4). Diatoms are relatively rare in Upstream B sediment. This is in marked contrast with RISP and Crary Ice Rise sediments, which are diatom rich. Although the concentration of diatoms is small relative to terrigenous mineral matter, the diatom assemblage from Upstream B is diverse. More than 50 species have been identified thus far. A variety of ages is represented with examples from Eocene/Oligocene, lower, middle and upper Miocene, and Pliocene. In addition to marine microfossils, ANTARCTIC JOURNAL
there is fossil evidence of terrestrial and lacustrine environ ments (Scherer, Antarctic Journal, this issue). The wide spread of ages and depositional environments reflects extensive mixing of west antarctic sediments beneath the grounded ice sheet. Some particles have probably been transported a great distance from within West Antarctica and possibly East Antarctica, whereas other particles are likely to be locally derived. The sediment differs greatly from RISP and Crary Ice Rise sediments in textural characteristics. RJSP and Crary Ice Rise sediments were originally deposited in richly productive glacial-marine environments in the Ross embayment. High diatom abundance and limited stratigraphic mixing suggests that these sediments have not experienced extensive glacial bed transport. By contrast, Upstream B sediment has characteristics more typical of basal till: diatoms are relatively rare among abundant angular quartz grains, and diatoms present represent a wide range of ages and depositional environments through the Cenozoic. Within the wide spread of biostratigraphic ages of microfossils at Upstream B, distinct subpopulations can be identified. These separate assemblages, which are mixed together in the matrix, provide certain insights with regard to the active till model of Alley et al. (1986, 1989). The relative abundance of diatoms of different ages provides qualitative information about the age and provenance of source beds. With more data than are currently available, it may be possible to backtrack particles to potential source beds. This information may be useful in defining constraints on glaciological models of bed deformation, erosion of the underlying sediments, particle transport and mechanics of ice-stream flow. M icropaleontological analysis of the Upstream B sample provides some specific observations that bear on interpretation of the regional setting of ice stream B and its underlying sedi ments. A distinctive Paleogene (probably Late Oligocene) marine diatom assemblage is present as reworked micro-clasts and individual diatom valves in the matrix sediment (Scherer, Antarctic Journal, this issue). Although Oligocene antarctic marine sediments are generally characterized by high diatom diversity, the marine Paleogene component of Upstream B consists of a nearly monospecific diatom assemblage. Most common, widespread Paleogene antarctic diatoms are absent. Monospecific marine diatom assemblages are atypical and are generally spatially and especially temporally restricted. It is unlikely that a stratigraphic occurrence containing a diatom assemblage such as this is very widespread or stratigraphically thick. The presence of this distinct, low-diversity diatom assemblage in Upstream B sediment suggests that this reworked component is derived from a single, relatively thin, source bed. This implies that Paleogene marine material found in the Upstream B sediment is most likely reworked only locally by grounded ice and is derived from a single sedimentary unit. The Alley et al. (1986, 1989) model suggests that the thin deforming till layer beneath the ice stream is actively eroding the underlying sedimentary succession and that these eroded sediments are incorporated into the till. The monospecific Paleogene diatom assemblage found in Upstream B matrix sediments is probably eroded from an in situ Paleogene marine
1989 REVIEW
deposit underlying the till at or near Upstream B camp. This interpretation suggests that the 600-meter, low-sonic-velocity sediment column underlying deforming till at Upstream B is Oligocene and older. The presence of Oligocene marine sediment underlying deforming till at Upstream B is not incon sistent with the predictions of Rooney et al. (in press), based on interpretation of seismic surveys under ice stream B. Future drilling and basal sediment collection on the west and east antarctic ice sheets, using new sampling techniques, will better establish the relative homogeneity and lateral continuity of deforming till and other basal tills, in terms of sediment texture, microfossil assemblages, and physical properties. These analyses will lead to a better understanding of ice-stream flow, ice-sheet stability, and glacial-bed processes. The fossils within these sediments also provide a fundamental source of paleoclimatic and paleoenvironmental information for vast regions obscured by the great ice sheets. Hermann Englehardt and Neil Humphrey graciously provided the sample for study. Peter Webb, Don Blankenship, Amy Leventer, and David Harwood contributed useful discussion and comments. Fieldwork was supported by National Science Foundation grants to Englehardt and others. Additional support came from National Science Foundation grant DPP 87-16261 to P. Webb.
References
Alley, R., D. Blankenship, S. Rooney, and C. Bentley. 1986. Deformation of till beneath ice stream B. Nature, 322, 57-59. Alley, R., D. Blankenship, S. Rooney and C. Bentley. 1989. Sedimentation beneath ice shelves—The view from Ice Stream B. Marine Geology, 85, 101-120.
Blankenship, D., C. Bentley, S. Rooney, and R. Alley. 1986. Seismic measurements reveal a saturated porous layer beneath an active Antarctic ice stream. Nature, 322, 54-57. Brady, H., and H. Martin. 1979, Ross Sea region in the Miocene: A glimpse of the past. Science, 203, 437-438. Harwood, D., R. Scherer, and P. Webb. In press. Multiple Miocene productivity events in West Antarctica as recorded in Upper Miocene sediments beneath the Ross Ice Shelf (Site J-9). Marine Micro paleontology.
Rooney, S., D. Blankenship, R. Alley, and C. Bentley. 1987. Till beneath ice stream B: 2. structure and continuity. Journal of Geophysical Research, 92(139), 8,913-8,920.
Rooney, S., D. Blankenship, R. Alley, and C. Bentley. In press. Seismic reflection profiling of a sediment-filled graben beneath ice stream B, West Antarctica. In Proceedings of the 5th International Symposium on Antarctic Earth Science, Cambridge: Cambridge University Press. Scherer, R. 1989. Paleoenvjronments of the West Antarctic interior:
microfossil study of sediments below Upstream B.
of the U. S., 24(5).
Antarctic Journal
Scherer, R., D. Harwood, S.Ishman, and P. Webb. 1988. Micropaleontological analysis of sediments from Crary Ice Rise, Ross Ice Shelf. Antarctic Journal of the U.S., 23(5), 34-36.
Shabtaie, S., and C. Bentley. 1987. West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance. Journal of Geophysical Research, 92(139), 1,311-1,336.
55
Microfossil assemblages in "deforming till" from Upstream B, West Antarctica: Implications for ice-stream flow models REED P. SCHERER
Byrd Polar Research Center
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
Sediments underlying grounded ice at the Upstream B camp in central West Antarctica (83°28'40"S 138°05'49"W) were collected by glaciologists from the California Institute of Technology and other institutions, during the 1988-1989 field season (figure). The ice sheet at the drill site is 1,030 meters thick and the glacier bed lies 644 meters below sea level. About 10 cubic centimeters of sediment was made available for preliminary microfossil study. This paper reports the initial findings of diatom analysis with respect to ice stream flow models. Paleoenvironmental interpretations of the microfossil assemblages are reported elsewhere (Scherer, Antarctic Journal, this issue).
East Antarctica West Antarctica '.' 4)
80 0 S 1800 .'..
O, vo
S.
Ross Sea
UpO
Ross Ice Shelf
Location of Upstream B (UpB), RISP site J-9, and Crary Ice Rise (CIR) samples with respect to ice streams A, B, and C (after Shabtaie and Bentley 1987).
54
It has been proposed that a water-saturated till, several meters thick, acts as a lubricating layer that aids ice stream flow (Alley et al. 1986, 1989; Blankenship et al. 1986). The "deforming till" layer has been identified by seismic reflection and appears to be widespread underneath the west antarctic ice streams (Rooney et al. 1987). Beneath the till layer lies a thick section of sediments (approximately 600 meters) which is actively being eroded at its top. Based on the low sonic velocities measured in the sediment column (generally less than 2.0 kilometers per second), Rooney et al. (in press) tentatively correlated this unit with Ross Sea sediments underlying the Neogene Ross Sea Unconformity. Previous diatom study of west antarctic sediments collected south of the Ross Ice Shelf barrier is limited to two regions in the Ross embayment: site J-9 of the Ross Ice Shelf Project (RISP) (Brady and Martin 1979; Kellogg and Kellogg 1986; Harwood et al. 1989) and Crary Ice Rise (Scherer et al. 1988). Sediment from site J-9 includes abundant and diverse diatoms, dominated by Early Miocene forms but including diatoms as young as mid-Late Miocene (Harwood, Scherer, and Webb in press). Sediment collected from Crary Ice Rise includes diatoms and silicoflagellates of latest Miocene age with rare reworked older microfossils (Scherer et al. 1988). Neither RISP nor Crary Ice Rise sediments include diatoms with stratigraphic appearances younger than Miocene, implying initial deposition prior to the Pliocene. The ice at Upstream B camp was penetrated by a Polar Ice Coring Office hot-water drill, and sediment was collected by stirring up the sediment with hot water jets and collecting suspended material in cups. This sampling method biased textural characteristics (grain size) of the till by removing coarse sand and large clasts and much of the clay-sized material, concentrating fine sand and silt-sized material. No stratigraphic information was preserved, and physical property measurements were not possible. Although the sample is size-sorted, the dominant grain size of the sediment collected includes most microfossil groups. Estimates of microfossil abundance are highly qualitative, due to the size sorting. The sample was processed for diatoms using both standard and enhanced diatom extraction methods. Micro-clasts of soft sediments were separated from the matrix sediments by sieving the sample at 63 microns and separating low-density sediment clasts from the greater than 63 micron fraction by floatation in Sodium Polytungstate (s.g. 2.4). Diatoms are relatively rare in Upstream B sediment. This is in marked contrast with RISP and Crary Ice Rise sediments, which are diatom rich. Although the concentration of diatoms is small relative to terrigenous mineral matter, the diatom assemblage from Upstream B is diverse. More than 50 species have been identified thus far. A variety of ages is represented with examples from Eocene/Oligocene, lower, middle and upper Miocene, and Pliocene. In addition to marine microfossils, ANTARCTIC JOURNAL
there is fossil evidence of terrestrial and lacustrine environ ments (Scherer, Antarctic Journal, this issue). The wide spread of ages and depositional environments reflects extensive mixing of west antarctic sediments beneath the grounded ice sheet. Some particles have probably been transported a great distance from within West Antarctica and possibly East Antarctica, whereas other particles are likely to be locally derived. The sediment differs greatly from RISP and Crary Ice Rise sediments in textural characteristics. RJSP and Crary Ice Rise sediments were originally deposited in richly productive glacial-marine environments in the Ross embayment. High diatom abundance and limited stratigraphic mixing suggests that these sediments have not experienced extensive glacial bed transport. By contrast, Upstream B sediment has characteristics more typical of basal till: diatoms are relatively rare among abundant angular quartz grains, and diatoms present represent a wide range of ages and depositional environments through the Cenozoic. Within the wide spread of biostratigraphic ages of microfossils at Upstream B, distinct subpopulations can be identified. These separate assemblages, which are mixed together in the matrix, provide certain insights with regard to the active till model of Alley et al. (1986, 1989). The relative abundance of diatoms of different ages provides qualitative information about the age and provenance of source beds. With more data than are currently available, it may be possible to backtrack particles to potential source beds. This information may be useful in defining constraints on glaciological models of bed deformation, erosion of the underlying sediments, particle transport and mechanics of ice-stream flow. M icropaleontological analysis of the Upstream B sample provides some specific observations that bear on interpretation of the regional setting of ice stream B and its underlying sedi ments. A distinctive Paleogene (probably Late Oligocene) marine diatom assemblage is present as reworked micro-clasts and individual diatom valves in the matrix sediment (Scherer, Antarctic Journal, this issue). Although Oligocene antarctic marine sediments are generally characterized by high diatom diversity, the marine Paleogene component of Upstream B consists of a nearly monospecific diatom assemblage. Most common, widespread Paleogene antarctic diatoms are absent. Monospecific marine diatom assemblages are atypical and are generally spatially and especially temporally restricted. It is unlikely that a stratigraphic occurrence containing a diatom assemblage such as this is very widespread or stratigraphically thick. The presence of this distinct, low-diversity diatom assemblage in Upstream B sediment suggests that this reworked component is derived from a single, relatively thin, source bed. This implies that Paleogene marine material found in the Upstream B sediment is most likely reworked only locally by grounded ice and is derived from a single sedimentary unit. The Alley et al. (1986, 1989) model suggests that the thin deforming till layer beneath the ice stream is actively eroding the underlying sedimentary succession and that these eroded sediments are incorporated into the till. The monospecific Paleogene diatom assemblage found in Upstream B matrix sediments is probably eroded from an in situ Paleogene marine
1989 REVIEW
deposit underlying the till at or near Upstream B camp. This interpretation suggests that the 600-meter, low-sonic-velocity sediment column underlying deforming till at Upstream B is Oligocene and older. The presence of Oligocene marine sediment underlying deforming till at Upstream B is not incon sistent with the predictions of Rooney et al. (in press), based on interpretation of seismic surveys under ice stream B. Future drilling and basal sediment collection on the west and east antarctic ice sheets, using new sampling techniques, will better establish the relative homogeneity and lateral continuity of deforming till and other basal tills, in terms of sediment texture, microfossil assemblages, and physical properties. These analyses will lead to a better understanding of ice-stream flow, ice-sheet stability, and glacial-bed processes. The fossils within these sediments also provide a fundamental source of paleoclimatic and paleoenvironmental information for vast regions obscured by the great ice sheets. Hermann Englehardt and Neil Humphrey graciously provided the sample for study. Peter Webb, Don Blankenship, Amy Leventer, and David Harwood contributed useful discussion and comments. Fieldwork was supported by National Science Foundation grants to Englehardt and others. Additional support came from National Science Foundation grant DPP 87-16261 to P. Webb.
References
Alley, R., D. Blankenship, S. Rooney, and C. Bentley. 1986. Deformation of till beneath ice stream B. Nature, 322, 57-59. Alley, R., D. Blankenship, S. Rooney and C. Bentley. 1989. Sedimentation beneath ice shelves—The view from Ice Stream B. Marine Geology, 85, 101-120.
Blankenship, D., C. Bentley, S. Rooney, and R. Alley. 1986. Seismic measurements reveal a saturated porous layer beneath an active Antarctic ice stream. Nature, 322, 54-57. Brady, H., and H. Martin. 1979, Ross Sea region in the Miocene: A glimpse of the past. Science, 203, 437-438. Harwood, D., R. Scherer, and P. Webb. In press. Multiple Miocene productivity events in West Antarctica as recorded in Upper Miocene sediments beneath the Ross Ice Shelf (Site J-9). Marine Micro paleontology.
Rooney, S., D. Blankenship, R. Alley, and C. Bentley. 1987. Till beneath ice stream B: 2. structure and continuity. Journal of Geophysical Research, 92(139), 8,913-8,920.
Rooney, S., D. Blankenship, R. Alley, and C. Bentley. In press. Seismic reflection profiling of a sediment-filled graben beneath ice stream B, West Antarctica. In Proceedings of the 5th International Symposium on Antarctic Earth Science, Cambridge: Cambridge University Press. Scherer, R. 1989. Paleoenvjronments of the West Antarctic interior:
microfossil study of sediments below Upstream B.
of the U. S., 24(5).
Antarctic Journal
Scherer, R., D. Harwood, S.Ishman, and P. Webb. 1988. Micropaleontological analysis of sediments from Crary Ice Rise, Ross Ice Shelf. Antarctic Journal of the U.S., 23(5), 34-36.
Shabtaie, S., and C. Bentley. 1987. West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance. Journal of Geophysical Research, 92(139), 1,311-1,336.
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