Carbon isotopic composition of particulate organic carbon in Ross Sea ...
generally lower than arctic ice of comparable thickness. The
the highest values. We suggest, however, that the fast freezing of the frazil accumulation is responsible here for the high salinity observed at the surface. Subsequent brine drainage also appears to be related to structural features, warranting additional study of the relationship of ice structure and salinity, especially in newly forming ice. We thank Chris Fritsen, Bruce Elder, and Dave Bell for their assistance in the coring and salinity analysis programs. The support of our colleagues on ISW- 1, field technicians and logisticians of Antarctic Support Associates, and the crew of the R/V Nathaniel B. Palmer during these expeditions is also appreciated.
Palmer cores, reflecting the mix of first-year and second-year
ice sampled, correspond to either the first- or second-year ice sampled at ISW- 1. Some Palmer samples, presumed also firstyear, correspond in average salinity to arctic values. The site J cores of first-year ice show behavior at variance with arctic ice of similar age primarily because of their anomalously high near-surface salinities. This results from an initial thick layer of frazil ice and fast freezing of the resulting ice slurry that apparently both contribute to the high surface salinity. Regional examples as shown by the Palmer cores are found of both the site I and arctic behavior for the first-year ice. For the second-year ice at 15W- 1, the mean salinity falls below that of the arctic winter ice, reflecting some transformation during the summer warming. These values are still above the values found for arctic first-year ice during the summer season (Weeks and Ackley 1986). Generally, colder conditions prevail in the summer in the Weddell Sea, compared to the Arctic, retarding the brine flushing by surface melt, as also indicated by the relatively intact snow cover we observed at ISW- 1 at the end of summer. The snow cover typically disappears on arctic pack ice during the summer. Comparison with Eicken's (1992) analysis indicates that the variability in salinity seen here is typical of the range of Weddell Sea values previously observed and is a manifestation of the complexity of processes observed here relative to some arctic regions. Although Eicken (1992) showed that a salinity maximum was obtainable by either upward expulsion or by surface flooding, the flooding mechanism accounted for
References Ackley, S.F., and V.I. Lytle. 1992. Sea-ice investigations on Ice Station Weddell #1, II. Ice thermodynamics. Antarctic Journal of the U.S., 27(5),109-111. Ackley, S.F., and C.W. Sullivan. In press. Physical controls on the development and characteristics of antarctic sea ice biological communities-A review and synthesis. Deep-Sea Research. Eicken H. 1992. Salinity profiles of antarctic sea ice: Field data and model results. Journal of Geophysical Research, 97(C10), 15545-15557. Gordon, A. 1993 Weddell Sea exploration from ice station. EOS, Transactions of the American Geophysical Union, 74(11), 121 and 124-126. Gow, A.J., S.F. Ackley, V.I. Lytle, and D. Bell. 1992. Ice core studies in the western Weddell Sea (Nathaniel B. Palmer 92-2). Antarctic Journal of the U.S., 27(5), 91-93. Weeks, W.F., and S.F. Ackley. 1986. The growth structure and properties of sea ice. In N. Untersteiner (Ed.), Geophysics of sea ice. New York: Plenum Press.
Carbon isotopic composition of particulate organic carbon in Ross Sea surface waters during austral summer JENNIFER C. ROGERS and ROBERT B. DUNBAR, Department of Geology and Geophysics, Rice University, Houston, Texas 77251-1892
arine organic matter isotopic carbon-13 (8 13 C) is M increasingly used in studies of the global carbon cycle. 8 13C of particulate organic carbon (POC) typically increases from values of -19 to -22%o at the equator to values of -26% to -31%o in polar regions (Sackett et al. 1965; Fontagne and Duplessy 1978; Rau et al. 1991b). Rau et al. (1989) and others have suggested that this latitudinal trend is caused by an increase in aqueous carbon dioxide (CO 2) concentration in cold polar waters, leading to proposals that sedimentary organic matter 8 13C can be used to reconstruct past oceanic and atmospheric particulate CO 2 levels (Jasper and Hayes 1990; Rau et al. 1991a). Such reconstructions involve several assumptions, including the following: • there is a CO 2 equilibrium between ocean and atmosphere;
• the influence of past temperature variations on aqueous CO2 levels can be independently resolved; and • sedimentary and diagenetic effects in the water column and at the seafloor do not overprint the original isotopic signature. As part of the Ross Sea flux experiment, we began a systematic survey of 8 13C in total dissolved CO 2 (CO2) as well as sinking, suspended, sea-ice, and seafloor organic matter in the Ross Sea to assess the degree of uniformity of 13C depletion in a polar "end-member" setting. We expected low and highly uniform 813C values because Ross Sea water temperatures range from -2°C to 0°C and the input of terrestrial carbon is negligible. We have previously reported the existence of a large range in Ross Sea marine POC 813C, from -8%o to -34%o, and suggested that open water and sea-ice phytoplankton blooms uti-
ANTARCTIC JOURNAL - REVIEW 1993 81
Figure 1. A. Concentration of particulate organic carbon (POC) in surface waters of the Ross Sea during February and March 1992. Base map shows bathymetry of the Ross Sea. Black ovals show location of surface-water sampling sites. Sites A, B, and C were the locations of major time series stations during the Ross Sea flux experiment (DeMaster et at. 1993). Samples were collected over a 5-week period beginning 3 February 1992. B. Distribution of 6 13 C in surfacewater POC.
A:
180' + Particulate Organic Carbon
J/\
>400 - 300-400 t/( k\ 250-300 200-250
P ) 1J _j///
J so I land ROSS
108 Shelf 180' -
iso' S 7W
+
B
c1iLi
1?o°W
Particulate Organic 8C (POB) -25 to -26 -27.5 to 28
' ::j } ?
oss Island ITS
Site B
Ross lce Shelf 180' 17O
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Particulate Organic Carbon (mg)
ANTARCTIC JOURNAL - REVIEW 1993 82
Figure 2. Particulate organic carbon (POC) vs. POC 6 1 C in a suite of samples collected between 6 February 1992 and 9 08 09 February 1992 along a west-toeast transect at 760S.
lize carbon from CO 2 reservoirs out of equilibrium with atmospheric CO 2 (Dunbar and Leventer 1992; Dunbar et al. 1993). Here we report on the areal distribution of PUG standing stock and PUG 8 13 G in ice-free areas of the Ross Sea during the 1991-1992 austral summer. If large, rapidly developing phytoplankton blooms deplete surface-water G0 2 levels and are the dominant source of POG, we expect a positive correlation between PUG concentration and PUG 813C Surface-water samples were collected by hydrocasting and shipboard pumping from the R/V Polar Duke and USGGG Polar Sea between 3 February and 12 March 1992. One liter of sea water was filtered onto glass-fiber filters, dried, and combusted in a Carlo-Erba NA 1500 GHNS analyzer modified for collection of gas for stable isotopic analysis (Mucciarone and Dunbar 1992). The resultant G0 2 gas was analyzed for 8 13 C using a VG Micromass mass spectrometer at Rice University. During February 1992, high surface-water PUG levels were observed in the western Ross Sea along the Victoria Land coast. PUG levels were low between 172° and 180°E and increased again to the east of 180° (figure 1). The highest PUG 13 G values correspond to areas with the highest concentrations of PUG (near site A), and the low 8 13 G values to the northeast of Ross Island are associated with low PUG levels. We also observed significant 13 G depletion in areas of moderate PUG concentration (near site B), however, and 13 G enrichment in areas with low PUG (east of site G). Transit samples collected from Polar Sea between 6 February 1992 and 9 February 1992 along the southern sampling transect show a significant positive correlation between 813G and PUG (figure 2). The data include the following: • a cluster of low PUG and 8 13 G values from low productivity areas of the transect; • a series of intermediate PUG and 8 13 G values from the margins of active blooms; and • high PUG values [greater than 0.5 milligrams per liter (mg L- 1 )], all with 6 13 G values of about -21.5%o from bloom regions. During the cruises, we measured depletions of G0 2 up to 15 percent and enrichment of 2:CO2 8 13 G up to 4%o in high PUG surface waters. Our results are consistent with G0 2 drawdown during phytoplankton blooms leading to 13 G enrichment of PUG from the influence of both reduced aqueous G02 levels and Rayleigh fractionation ofGU 2 . It is likely that large amounts of organic matter in the Ross Sea are produced within surface waters out of G0 2 equilibrium with the atmosphere. Within a single strong bloom event over a timescale of days, PUC concentration and POG 813C are positively correlated.
The reason for an apparent 13 G enrichment limit (at about 22%o) in open-water bloom products is not yet known but may reflect an equilibrium between rates of G0 2 uptake and replacement (from the atmosphere and deep water). Over longer timescales and larger areas, the relationship between PUG standing stock and PUG 8 13 G is complex. Phytoplankton blooms developing slowly or in areas of significant air/sea mixing or upwelling could produce high standing stocks of PUG with low 8 13 G. Conversely, zooplankton grazing of a phytoplankton bloom may result in low levels of PUG with high 8 13 G. Our results suggest the need for caution in the interpretation of sedimentary 8 13 G as an atmospheric particulate G02 indicator. Furthermore, within the southern oceans, PUG ö'C may prove useful as a tracer of organic matter provenance. This research was supported by National Science Foundation grant OPP 88-18136. We thank David Mucciarone and William Jones for shipboard analytical assistance.
References DeMaster, D.J., R.B. Dunbar, L.I. Gordon, A.R. Leventer, J.M. Morrison, D.M. Nelson, C.A. Nittrouer, and W.O. Smith. 1993. The cycling and accumulation of organic matter and biogenic silica in high latitude environments: The Ross Sea. Oceanography, 5, 146-153. Dunbar, R.B., and A.R. Leventer. 1992. Seasonal variation in carbon isotopic composition of antarctic sea ice and open-water plankton communities. Antarctic Journal of the U. S., 27(5). 79-81. Dunbar, R.B., A.R. Leventer, J.C. Rogers, and D.A. Mucciarone. 1993. CO 2 disequilibrium in antarctic sea ice and surface water: Evidence from 13C/12C ratios in organic C. 23rd Annual Arctic Workshop, Ohio State University, Byrd Polar Research Center. (Miscellaneous series M-322.) Columbus: Ohio State University. Fontagne, M., and 1.-C. Duplessy. 1978. Carbon isotope ratios of marine plankton related to surface water masses. Earth and Planetary Science Letters, 41, 365-371. Jasper, J.P., and J.M. Hayes. 1990. A carbon isotope record of levels during the late Quaternary. Nature, 347, 462-464. Mucciarone, D.A., and R.B. Dunbar. 1992. Collection of CO 2 for 13 C/ 12 C measurements and simultaneous C, H, N, and S analysis using an elemental analyzer. Journal Sedimentary Petrology, 62, 731-733. Rau, G.H., P.N. Froelich, T. Takahashi, and D.J. Des Marais. 1991a. Does sedimentary organic 8 13 C record variations in Quaternary ocean ICO 2 (aq)]? Paleoceanography, 6(3), 335-347. Rau, G.H., T. Takahashi, and D.J. Des Marais. 1989. Latitudinal variations in plankton 8 13 C: Implications for CO 2 and productivity in past oceans. Nature, 341, 516-518. Rau, G.H., T. Takahashi, D.J. Des Marais, and C.W. Sullivan. 1991b. Particulate organic matter 8 13 C variations across the drake passage. Journal of Geophysical Research, 96, 15131-15135. Sackett, W.M., W.R. Eckelmann, M.L. Bender, and A.W.H. Be. 1965. Temperature dependence of carbon isotope composition in marine plankton and sediments. Science, 148, 236-237.
ANTARCTIC JOURNAL - REVIEW 1993 83
the highest values. We suggest, however, that the fast freezing of the frazil accumulation is responsible here for the high salinity observed at the surface. Subsequent brine drainage also appears to be related to structural features, warranting additional study of the relationship of ice structure and salinity, especially in newly forming ice. We thank Chris Fritsen, Bruce Elder, and Dave Bell for their assistance in the coring and salinity analysis programs. The support of our colleagues on ISW- 1, field technicians and logisticians of Antarctic Support Associates, and the crew of the R/V Nathaniel B. Palmer during these expeditions is also appreciated.
Palmer cores, reflecting the mix of first-year and second-year
ice sampled, correspond to either the first- or second-year ice sampled at ISW- 1. Some Palmer samples, presumed also firstyear, correspond in average salinity to arctic values. The site J cores of first-year ice show behavior at variance with arctic ice of similar age primarily because of their anomalously high near-surface salinities. This results from an initial thick layer of frazil ice and fast freezing of the resulting ice slurry that apparently both contribute to the high surface salinity. Regional examples as shown by the Palmer cores are found of both the site I and arctic behavior for the first-year ice. For the second-year ice at 15W- 1, the mean salinity falls below that of the arctic winter ice, reflecting some transformation during the summer warming. These values are still above the values found for arctic first-year ice during the summer season (Weeks and Ackley 1986). Generally, colder conditions prevail in the summer in the Weddell Sea, compared to the Arctic, retarding the brine flushing by surface melt, as also indicated by the relatively intact snow cover we observed at ISW- 1 at the end of summer. The snow cover typically disappears on arctic pack ice during the summer. Comparison with Eicken's (1992) analysis indicates that the variability in salinity seen here is typical of the range of Weddell Sea values previously observed and is a manifestation of the complexity of processes observed here relative to some arctic regions. Although Eicken (1992) showed that a salinity maximum was obtainable by either upward expulsion or by surface flooding, the flooding mechanism accounted for
References Ackley, S.F., and V.I. Lytle. 1992. Sea-ice investigations on Ice Station Weddell #1, II. Ice thermodynamics. Antarctic Journal of the U.S., 27(5),109-111. Ackley, S.F., and C.W. Sullivan. In press. Physical controls on the development and characteristics of antarctic sea ice biological communities-A review and synthesis. Deep-Sea Research. Eicken H. 1992. Salinity profiles of antarctic sea ice: Field data and model results. Journal of Geophysical Research, 97(C10), 15545-15557. Gordon, A. 1993 Weddell Sea exploration from ice station. EOS, Transactions of the American Geophysical Union, 74(11), 121 and 124-126. Gow, A.J., S.F. Ackley, V.I. Lytle, and D. Bell. 1992. Ice core studies in the western Weddell Sea (Nathaniel B. Palmer 92-2). Antarctic Journal of the U.S., 27(5), 91-93. Weeks, W.F., and S.F. Ackley. 1986. The growth structure and properties of sea ice. In N. Untersteiner (Ed.), Geophysics of sea ice. New York: Plenum Press.
Carbon isotopic composition of particulate organic carbon in Ross Sea surface waters during austral summer JENNIFER C. ROGERS and ROBERT B. DUNBAR, Department of Geology and Geophysics, Rice University, Houston, Texas 77251-1892
arine organic matter isotopic carbon-13 (8 13 C) is M increasingly used in studies of the global carbon cycle. 8 13C of particulate organic carbon (POC) typically increases from values of -19 to -22%o at the equator to values of -26% to -31%o in polar regions (Sackett et al. 1965; Fontagne and Duplessy 1978; Rau et al. 1991b). Rau et al. (1989) and others have suggested that this latitudinal trend is caused by an increase in aqueous carbon dioxide (CO 2) concentration in cold polar waters, leading to proposals that sedimentary organic matter 8 13C can be used to reconstruct past oceanic and atmospheric particulate CO 2 levels (Jasper and Hayes 1990; Rau et al. 1991a). Such reconstructions involve several assumptions, including the following: • there is a CO 2 equilibrium between ocean and atmosphere;
• the influence of past temperature variations on aqueous CO2 levels can be independently resolved; and • sedimentary and diagenetic effects in the water column and at the seafloor do not overprint the original isotopic signature. As part of the Ross Sea flux experiment, we began a systematic survey of 8 13C in total dissolved CO 2 (CO2) as well as sinking, suspended, sea-ice, and seafloor organic matter in the Ross Sea to assess the degree of uniformity of 13C depletion in a polar "end-member" setting. We expected low and highly uniform 813C values because Ross Sea water temperatures range from -2°C to 0°C and the input of terrestrial carbon is negligible. We have previously reported the existence of a large range in Ross Sea marine POC 813C, from -8%o to -34%o, and suggested that open water and sea-ice phytoplankton blooms uti-
ANTARCTIC JOURNAL - REVIEW 1993 81
Figure 1. A. Concentration of particulate organic carbon (POC) in surface waters of the Ross Sea during February and March 1992. Base map shows bathymetry of the Ross Sea. Black ovals show location of surface-water sampling sites. Sites A, B, and C were the locations of major time series stations during the Ross Sea flux experiment (DeMaster et at. 1993). Samples were collected over a 5-week period beginning 3 February 1992. B. Distribution of 6 13 C in surfacewater POC.
A:
180' + Particulate Organic Carbon
J/\
>400 - 300-400 t/( k\ 250-300 200-250
P ) 1J _j///
J so I land ROSS
108 Shelf 180' -
iso' S 7W
+
B
c1iLi
1?o°W
Particulate Organic 8C (POB) -25 to -26 -27.5 to 28
' ::j } ?
oss Island ITS
Site B
Ross lce Shelf 180' 17O
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Particulate Organic Carbon (mg)
ANTARCTIC JOURNAL - REVIEW 1993 82
Figure 2. Particulate organic carbon (POC) vs. POC 6 1 C in a suite of samples collected between 6 February 1992 and 9 08 09 February 1992 along a west-toeast transect at 760S.
lize carbon from CO 2 reservoirs out of equilibrium with atmospheric CO 2 (Dunbar and Leventer 1992; Dunbar et al. 1993). Here we report on the areal distribution of PUG standing stock and PUG 8 13 G in ice-free areas of the Ross Sea during the 1991-1992 austral summer. If large, rapidly developing phytoplankton blooms deplete surface-water G0 2 levels and are the dominant source of POG, we expect a positive correlation between PUG concentration and PUG 813C Surface-water samples were collected by hydrocasting and shipboard pumping from the R/V Polar Duke and USGGG Polar Sea between 3 February and 12 March 1992. One liter of sea water was filtered onto glass-fiber filters, dried, and combusted in a Carlo-Erba NA 1500 GHNS analyzer modified for collection of gas for stable isotopic analysis (Mucciarone and Dunbar 1992). The resultant G0 2 gas was analyzed for 8 13 C using a VG Micromass mass spectrometer at Rice University. During February 1992, high surface-water PUG levels were observed in the western Ross Sea along the Victoria Land coast. PUG levels were low between 172° and 180°E and increased again to the east of 180° (figure 1). The highest PUG 13 G values correspond to areas with the highest concentrations of PUG (near site A), and the low 8 13 G values to the northeast of Ross Island are associated with low PUG levels. We also observed significant 13 G depletion in areas of moderate PUG concentration (near site B), however, and 13 G enrichment in areas with low PUG (east of site G). Transit samples collected from Polar Sea between 6 February 1992 and 9 February 1992 along the southern sampling transect show a significant positive correlation between 813G and PUG (figure 2). The data include the following: • a cluster of low PUG and 8 13 G values from low productivity areas of the transect; • a series of intermediate PUG and 8 13 G values from the margins of active blooms; and • high PUG values [greater than 0.5 milligrams per liter (mg L- 1 )], all with 6 13 G values of about -21.5%o from bloom regions. During the cruises, we measured depletions of G0 2 up to 15 percent and enrichment of 2:CO2 8 13 G up to 4%o in high PUG surface waters. Our results are consistent with G0 2 drawdown during phytoplankton blooms leading to 13 G enrichment of PUG from the influence of both reduced aqueous G02 levels and Rayleigh fractionation ofGU 2 . It is likely that large amounts of organic matter in the Ross Sea are produced within surface waters out of G0 2 equilibrium with the atmosphere. Within a single strong bloom event over a timescale of days, PUC concentration and POG 813C are positively correlated.
The reason for an apparent 13 G enrichment limit (at about 22%o) in open-water bloom products is not yet known but may reflect an equilibrium between rates of G0 2 uptake and replacement (from the atmosphere and deep water). Over longer timescales and larger areas, the relationship between PUG standing stock and PUG 8 13 G is complex. Phytoplankton blooms developing slowly or in areas of significant air/sea mixing or upwelling could produce high standing stocks of PUG with low 8 13 G. Conversely, zooplankton grazing of a phytoplankton bloom may result in low levels of PUG with high 8 13 G. Our results suggest the need for caution in the interpretation of sedimentary 8 13 G as an atmospheric particulate G02 indicator. Furthermore, within the southern oceans, PUG ö'C may prove useful as a tracer of organic matter provenance. This research was supported by National Science Foundation grant OPP 88-18136. We thank David Mucciarone and William Jones for shipboard analytical assistance.
References DeMaster, D.J., R.B. Dunbar, L.I. Gordon, A.R. Leventer, J.M. Morrison, D.M. Nelson, C.A. Nittrouer, and W.O. Smith. 1993. The cycling and accumulation of organic matter and biogenic silica in high latitude environments: The Ross Sea. Oceanography, 5, 146-153. Dunbar, R.B., and A.R. Leventer. 1992. Seasonal variation in carbon isotopic composition of antarctic sea ice and open-water plankton communities. Antarctic Journal of the U. S., 27(5). 79-81. Dunbar, R.B., A.R. Leventer, J.C. Rogers, and D.A. Mucciarone. 1993. CO 2 disequilibrium in antarctic sea ice and surface water: Evidence from 13C/12C ratios in organic C. 23rd Annual Arctic Workshop, Ohio State University, Byrd Polar Research Center. (Miscellaneous series M-322.) Columbus: Ohio State University. Fontagne, M., and 1.-C. Duplessy. 1978. Carbon isotope ratios of marine plankton related to surface water masses. Earth and Planetary Science Letters, 41, 365-371. Jasper, J.P., and J.M. Hayes. 1990. A carbon isotope record of levels during the late Quaternary. Nature, 347, 462-464. Mucciarone, D.A., and R.B. Dunbar. 1992. Collection of CO 2 for 13 C/ 12 C measurements and simultaneous C, H, N, and S analysis using an elemental analyzer. Journal Sedimentary Petrology, 62, 731-733. Rau, G.H., P.N. Froelich, T. Takahashi, and D.J. Des Marais. 1991a. Does sedimentary organic 8 13 C record variations in Quaternary ocean ICO 2 (aq)]? Paleoceanography, 6(3), 335-347. Rau, G.H., T. Takahashi, and D.J. Des Marais. 1989. Latitudinal variations in plankton 8 13 C: Implications for CO 2 and productivity in past oceans. Nature, 341, 516-518. Rau, G.H., T. Takahashi, D.J. Des Marais, and C.W. Sullivan. 1991b. Particulate organic matter 8 13 C variations across the drake passage. Journal of Geophysical Research, 96, 15131-15135. Sackett, W.M., W.R. Eckelmann, M.L. Bender, and A.W.H. Be. 1965. Temperature dependence of carbon isotope composition in marine plankton and sediments. Science, 148, 236-237.
ANTARCTIC JOURNAL - REVIEW 1993 83
