Sedimentation history of the Terra Nova Bay region ...
References Birkenmajer, K. 1982a. Structural evolution of the Melville Peak volcano, King George Island (South Shetland Islands, West Antarctica). Bulletin de L'Academie Poloniase des Sciences, Série des sciences de la terre, 0 0 Z
Si02 wt%
Figure 2. Silica (S10 2) versus alkali (Na 20 + K20): a plot of Bransfield Strait and South Shetland Islands samples. Squares and triangles are dredge samples. Diamonds are King George Island. Octagons are Penguin Island. Asterisks are Deception Island. lB line is the alkali/tholeiite line from Irvine and Baragar (1971).
Sedimentation history of the Terra Nova Bay region, Ross Sea, Antarctica LAWRENCE A KRISSEK
Byrd Polar Research Center
and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210 A suite of 41 piston cores was obtained from the Terra Nova Bay region of the western Ross Sea during the austral summer 1979-1980 cruise of the U.S. Coast Guard icebreaker Glacier (figure 1). Because this region contains a consistent area of open water surrounded by ice (a polynya), these cores have been used to examine the characteristics of polynya-influenced sedimentation. The polynya itself is kept ice-free by two factors: the strong and persistent katabatic winds, which blow seaward from the glacial drainages along the coast, and the presence of the Drygalski ice tongue, which blocks the northward flow of sea ice into Terra Nova Bay (Bromwich and Kurtz 1984; Kurtz and Bromwich 1983, 1985). Hughes and Krissek (1985) and Hughes (1986) examined core-top samples from sub-ice and sub-polynya environments of Terra Nova Bay to identify distinctive compositional signatures for these environments. They showed that the ratio of terrigenous to biogenous (TIB) sediment components (the T/B ratio), determined from smear slides, is a consistent indicator of polynya- vs. ice-influenced environments, with high T/B ratios beneath the polynya and low T/B ratios in sub-ice settings (figure 2). The variation in component abundances probably reflects an increased input of land-derived eolian material through the open water of the polynya but may also
104
29(4), 341-351. Birkenmajer, K. 1982b. The Penguin Island volcano, South Shetland Islands (Antarctica): Its structure and succession. Studia Geologica Polonica, 74, 155-173. Irvine, T.N., and W.R.A. Baragar. 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8, 523-548. Keller, R.A., and M.R. Fisk. 1987. Magmatism associated with the initial stages of back-arc rifting, Bransfield Strait, Antarctica. Eos, 68(44), 1553. Suess, E., M. Fisk, and D. Kadko. 1987. Thermal interaction between back-arc volcanism and basin sediments in the Bransfield Strait, Antarctica. Antarctic Journal of the U.S., 22(5), 46-49. White, W.M., M. Cheatham, and M. Fisk. 1987. Geochemistry of backarc basin volcanics from Bransfield Strait, Antarctic Peninsula. Eos, 68(44), 1520.
reflect decreased biological productivity because of extremely rapid mixing (Dunbar et al. 1985). A subsequent study has examined downcore variations in the T/B ratio along two east-west transects (cores 105/107/110 and cores 111-117), which extend from the modern polynya to the ice-covered region to the east. The objective of the downcore study is to trace the position and extent of the polynya through time, thereby identifying past conditions of the katabatic windfield and past extent of the Drygalski ice tongue. The results of this study are summarized in figure 3, where T/B downcore profiles are plotted by geographic position within the transects. Cores 105, 115, 116, and 117 are located beneath the modern polynya, and T/B ratios in these cores remain high throughout the recovered intervals. The high T/B ratio suggests that these sites have experienced polynya-influenced deposition throughout the time represented. Cores 110, 111, and 112 are located beneath the modern pack ice, and T/B ratios in these cores remain low throughout the recovered intervals. The low T/B ratio suggests that these sites have experienced sub-ice deposition throughout the time represented. Cores 107, 113, and 114 are located in the transition zone between these two modern environments (Kurtz and Bromwich 1985), and all show at least one major increase in T/B ratio downcore. These changes are interpreted to record the past existence of a significantly larger polynya, with the polynya/ice boundary lying at least 90 kilometers offshore along the northern profile. Establishing the synchroneity of the compositional changes and, therefore, dating this expanded polynya, is problematic. Diatoms are present in these samples, but the effects of reworking are significant, and biostratigraphic resolution is limited (Harwood personal communication). Radiocarbon ages were measured for 14 samples from these cores (analyzed by Krueger Enterprises, Geochron Laboratories). The calculated ages range from 10,825 ± 640 years (core 114, 8-38 centimeters) to >32,000 years (core 113, 83-110 centimeters) and are all anomalously old, as has been observed for previous radiocarbon measurements of Ross Sea sediments (Dunbar perANTARCTIC JOURNAL
74°S MT. MELBOURNE. 125 *• .126
MT. MELBOURNE'.
.123 •122 ' J.. •.;;/ • 119 0127 1180 9120121 REEVES 126 GLACIER • • • •• • 117 116 115 114 113 112 111 750S S • • • . . 105 106 107 108 109 110 103 102 130 • DRYGALSKI 100 ICE TONGUE 0131 994 95 99 7 132 93 •96 98
E:.
0
I
74°S
1 WN0,19
• 25
s. .y .. REEVES ... 75°S GLACIER :
DRYGALSRI ICE TONGUE TERRIGENOUS S COMPONENT S ABUNDANCE (%)
7 .•.
X.
I I
1650E
1680E
Figure 1. Location map of austral summer 1979-1980 piston cores in Terra Nova Bay.
sonal communication). Although the radiocarbon ages are anomalously old in an absolute sense, differences in the ages of three samples from core 114 and two samples from core 107 define sedimentation rates of 0.05 to 0.20 millimeter per year. These rates are approximately 10 to 50 times slower than shortterm sedimentation rates determined by Ledford-Hoffman, DeMaster, and Nittrouer (1986), who employed lead-210 and plutonium-239/plutonium-240 techniques on box cores from the southwestern Ross Sea. While the expanded polynya cannot be dated accurately, it can be estimated to have existed during the Late Quaternary, and perhaps as recently as 6,000 years ago. This research was supported by a grant from the Office of Research and Graduate Studies, Ohio State University. The Byrd Polar Research Center, Ohio State University, provided partial support of radiocarbon analyses. References Bromwich, D.H., and D.D. Kurtz. 1984. Katabatic wind forcing of the Terra Nova Bay polynya. Journal of Geophysical Research, 89(C3), 35613572. Dunbar, R.B. 1988. Personal communication.
1988 REVIEW
46 60
:v:
F EWI
76°S ..' I 162 0 E
S
76°S . . 162 0 E
Q SI)
I I I
1650E
50-75% 25-50%fl
Si02 wt%
Figure 2. Silica (S10 2) versus alkali (Na 20 + K20): a plot of Bransfield Strait and South Shetland Islands samples. Squares and triangles are dredge samples. Diamonds are King George Island. Octagons are Penguin Island. Asterisks are Deception Island. lB line is the alkali/tholeiite line from Irvine and Baragar (1971).
Sedimentation history of the Terra Nova Bay region, Ross Sea, Antarctica LAWRENCE A KRISSEK
Byrd Polar Research Center
and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210 A suite of 41 piston cores was obtained from the Terra Nova Bay region of the western Ross Sea during the austral summer 1979-1980 cruise of the U.S. Coast Guard icebreaker Glacier (figure 1). Because this region contains a consistent area of open water surrounded by ice (a polynya), these cores have been used to examine the characteristics of polynya-influenced sedimentation. The polynya itself is kept ice-free by two factors: the strong and persistent katabatic winds, which blow seaward from the glacial drainages along the coast, and the presence of the Drygalski ice tongue, which blocks the northward flow of sea ice into Terra Nova Bay (Bromwich and Kurtz 1984; Kurtz and Bromwich 1983, 1985). Hughes and Krissek (1985) and Hughes (1986) examined core-top samples from sub-ice and sub-polynya environments of Terra Nova Bay to identify distinctive compositional signatures for these environments. They showed that the ratio of terrigenous to biogenous (TIB) sediment components (the T/B ratio), determined from smear slides, is a consistent indicator of polynya- vs. ice-influenced environments, with high T/B ratios beneath the polynya and low T/B ratios in sub-ice settings (figure 2). The variation in component abundances probably reflects an increased input of land-derived eolian material through the open water of the polynya but may also
104
29(4), 341-351. Birkenmajer, K. 1982b. The Penguin Island volcano, South Shetland Islands (Antarctica): Its structure and succession. Studia Geologica Polonica, 74, 155-173. Irvine, T.N., and W.R.A. Baragar. 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 8, 523-548. Keller, R.A., and M.R. Fisk. 1987. Magmatism associated with the initial stages of back-arc rifting, Bransfield Strait, Antarctica. Eos, 68(44), 1553. Suess, E., M. Fisk, and D. Kadko. 1987. Thermal interaction between back-arc volcanism and basin sediments in the Bransfield Strait, Antarctica. Antarctic Journal of the U.S., 22(5), 46-49. White, W.M., M. Cheatham, and M. Fisk. 1987. Geochemistry of backarc basin volcanics from Bransfield Strait, Antarctic Peninsula. Eos, 68(44), 1520.
reflect decreased biological productivity because of extremely rapid mixing (Dunbar et al. 1985). A subsequent study has examined downcore variations in the T/B ratio along two east-west transects (cores 105/107/110 and cores 111-117), which extend from the modern polynya to the ice-covered region to the east. The objective of the downcore study is to trace the position and extent of the polynya through time, thereby identifying past conditions of the katabatic windfield and past extent of the Drygalski ice tongue. The results of this study are summarized in figure 3, where T/B downcore profiles are plotted by geographic position within the transects. Cores 105, 115, 116, and 117 are located beneath the modern polynya, and T/B ratios in these cores remain high throughout the recovered intervals. The high T/B ratio suggests that these sites have experienced polynya-influenced deposition throughout the time represented. Cores 110, 111, and 112 are located beneath the modern pack ice, and T/B ratios in these cores remain low throughout the recovered intervals. The low T/B ratio suggests that these sites have experienced sub-ice deposition throughout the time represented. Cores 107, 113, and 114 are located in the transition zone between these two modern environments (Kurtz and Bromwich 1985), and all show at least one major increase in T/B ratio downcore. These changes are interpreted to record the past existence of a significantly larger polynya, with the polynya/ice boundary lying at least 90 kilometers offshore along the northern profile. Establishing the synchroneity of the compositional changes and, therefore, dating this expanded polynya, is problematic. Diatoms are present in these samples, but the effects of reworking are significant, and biostratigraphic resolution is limited (Harwood personal communication). Radiocarbon ages were measured for 14 samples from these cores (analyzed by Krueger Enterprises, Geochron Laboratories). The calculated ages range from 10,825 ± 640 years (core 114, 8-38 centimeters) to >32,000 years (core 113, 83-110 centimeters) and are all anomalously old, as has been observed for previous radiocarbon measurements of Ross Sea sediments (Dunbar perANTARCTIC JOURNAL
74°S MT. MELBOURNE. 125 *• .126
MT. MELBOURNE'.
.123 •122 ' J.. •.;;/ • 119 0127 1180 9120121 REEVES 126 GLACIER • • • •• • 117 116 115 114 113 112 111 750S S • • • . . 105 106 107 108 109 110 103 102 130 • DRYGALSKI 100 ICE TONGUE 0131 994 95 99 7 132 93 •96 98
E:.
0
I
74°S
1 WN0,19
• 25
s. .y .. REEVES ... 75°S GLACIER :
DRYGALSRI ICE TONGUE TERRIGENOUS S COMPONENT S ABUNDANCE (%)
7 .•.
X.
I I
1650E
1680E
Figure 1. Location map of austral summer 1979-1980 piston cores in Terra Nova Bay.
sonal communication). Although the radiocarbon ages are anomalously old in an absolute sense, differences in the ages of three samples from core 114 and two samples from core 107 define sedimentation rates of 0.05 to 0.20 millimeter per year. These rates are approximately 10 to 50 times slower than shortterm sedimentation rates determined by Ledford-Hoffman, DeMaster, and Nittrouer (1986), who employed lead-210 and plutonium-239/plutonium-240 techniques on box cores from the southwestern Ross Sea. While the expanded polynya cannot be dated accurately, it can be estimated to have existed during the Late Quaternary, and perhaps as recently as 6,000 years ago. This research was supported by a grant from the Office of Research and Graduate Studies, Ohio State University. The Byrd Polar Research Center, Ohio State University, provided partial support of radiocarbon analyses. References Bromwich, D.H., and D.D. Kurtz. 1984. Katabatic wind forcing of the Terra Nova Bay polynya. Journal of Geophysical Research, 89(C3), 35613572. Dunbar, R.B. 1988. Personal communication.
1988 REVIEW
46 60
:v:
F EWI
76°S ..' I 162 0 E
S
76°S . . 162 0 E
Q SI)
I I I
1650E
50-75% 25-50%fl
