Summer-winter comparison of Weddell Sea surface water and its ...
Underlying the surface water all locations was the Weddell winter water. This layer reflected cooling and freezing which occurred during the preceding winter and had temperatures between about -1.5° and -1.7°C and a salinity of about 34.5 parts per thousand. The winter water extended from just beneath the surface water to about 100 meters and was separated from the upper waters by a region of very strong vertical gradients in temperature, salinity, and density. A weak (0.1°-0.2°C) temperature maximum layer 10-20 meters thick was often present between the surface and winter water layers. This thermal feature was a remnant due to absorption of incoming solar radiation during the preceding summer. The deepest water mass observed was the Weddell warm water, which extended from the bottom of the winter water at about 100 meters to more than the 1,500 meters maximum cast depths. Temperatures in this layer varied between 0°C and 0.5°C, and salinities were 34.65 parts per thousand at the upper boundary of the layer increasing to 34.7 parts per thousand at the greatest observed depths. The greatest observed temperatures in this layer occurred at about 500 meters depth. The region between this temperature maximum and the top of the layer at about 100 meters was characterized, over much of the study region, by large (more than 100 meters in vertical extent) temperature and salinity steps. These features have also been noted by Middleton and Foster (1980). There is as yet no satisfactory explanation for their presence, though they may play a significant role in vertical heat fluxes. Dynamic heights relative to the 1,500-decibar level were computed to estimate whether appreciable baroclinic circulation was present over the study region. The total dynamic relief was of the order of only a few dynamic centimeters over the entire area, indicating that the baroclinic circulation was negligible. During the field program, two separate "rapid transects" were occupied by both vessels. There were attempts to sample along transects over time intervals which were short relative to the periods over which changes might be expected to occur in the water column. No significant variations were detected be-
tween occupations of these transects, suggesting that the physical system was reasonably steady-state during the 30-day duration of the cruise. Finally, no strong horizontal gradients, or frontal systems, were observed. This was in contrast to the AMERIEZ I program, which sampled in November 1983 along the strong frontal system separating the outflowing Weddell Sea water from the waters of the Eastern Scotia Sea. The region sampled during the March 1986 program was characterized only by very gradual horizontal variation—primarily in the upper layers, in moving from west to east away from the sea-ice edge. The northwestern Weddell Sea MIZ region was intensively sampled, revealing three water masses: the Weddell surface water, the Weddell winter water, and the Weddell warm water. The dominant upper layer feature was the widespread surface water layer, which was underlain by very strong vertical gradients. Such gradients imply that little vertical mixing is occurring through the bottom of the upper layer. The dominant deep feature was the presence of vertical temperature steps which may be related to region-wide vertical heat transfer. This work was supported in part by National Science Foundation grants DPP 84-20646 (to Muensch) and DPP 85-13098 (to Husby), by ONR contract N00014-82-C-0064 with Science Applications International Corporation (to Muensch) and by the National Marine Fisheries Service/National Oceanic and Atmospheric Administration (to Husby).
Summer-winter comparison of Weddell Sea surface water and its productivity
tigation for the first time of the cumulative winter effects on the distribution of chemical properties in the Weddell Sea surface water. Temperature, salinity, oxygen, and nutrient data all indicate that the temperature-minimum layer found in summer is the remnant winter surface water. In summer, solar heating and melting of sea ice transform the top layer of the winter surface water into a warmer, less saline water than the winter surface water which in summer is partially preserved beneath the top layer. Subsequently, biological activity reduces the nutrient concentration in the summer surface layer. The oxygen concentration in the surface layer, however, increases in summer because the oxygen-depleted winter surface water picks up oxygen from the atmosphere after sea ice is melted. In summary, the winter Weddell Sea surface water is colder, saltier, contains more nutrients but less oxygen than the summer surface water (Chen 1984; Gordon, Chen, and Metcalf 1984; Jennings, Gordon, and Nelson 1984). Comparison of WEPOLEX and GEOSECS (Geochemical Ocean Section Studies) normalized total carbon dioxide (NTCO 2 = TCO2 x 35/salinity, TCO 2 is the total amount of carbonate spe-
C.-T.A. CHEN* Institute of Marine Geology National Sun Yat-Sen University Kaohsiung, Taiwan Republic of China
The U.S.-U.S.S.R. Weddell Polynya Expedition (WEPOLEX) (Chen 1982; Gordon 1982; Jennings, Nelson, and Gordon 1982) generated chemical data near the outflow region of the Weddell Sea in the late winter/early spring. These data permitted inves* On leave from College of Oceanography, Oregon State University, Corvallis, Oregon 97331.
128
References Carmack, E.C. 1977. Water characteristics of the Southern Ocean south of the Polar Front. In M. Angel (Ed.), A voyage of discovery. New York: Pergamon Press. Middleton, J.H., and T.D. Foster. 1980. Fine structure measurements in a temperature-compensated halocline. Journal of Geophysical Research, 85(C2), 1107-1122.
ANTARCTIC JOURNAL
20 WEPOLEX
GEOSECS Summer Surface
L)
°.. 0 CD
-10
Winter Surface 20 2200 2250
1/
2300
2
Remnant Winter Surface Water 2200
2250
2300
NTCO , )JmoI kg-'
Potential temperature (0) plotted vs. normalized total carbon dioxide (NTCO2 ; "ii. mol/kg" denotes "micromoles per kilogram") for the WEPOLEX data (all stations) and the GEOSECS data (stations 79, 82,85, and 89).
cies dissolved in sea water) data (Takahashi etal. 1980; Huber et al. 1983; Chen 1984) also indicate that the winter NTCO7 values in the surface layer agree with the GEOSECS summer values in the temperature-minimum layer (figure). The summer surface NTCO2 and normalized alkalinity values are lower than the winter values by about 50 micromoles per kilogram and 10 microequivalents per kilogram, respectively. These data indicate a production of 5 micromoles per kilogram of calcium carbonate and 45 micromoles per kilogram of soft tissue, a ratio somewhat lower than an average ratio of 1-to-4 for the world oceans (Broecker and Peng 1982). The lower ratio is expected because siliceous rather than calcareous organisms dominate in the cold waters of the southern oceans. The average amount of soft tissue production in the surface layer above the temperature-minimum layer is about 22.5 micromoles per kilogram. This value translates to a productivity of 300-450 milligrams per square meter per day between late winter WEPOLEX and summer (cEosEcs) if we assume average surface layer to be 100 meters thick. The average calcium carbonate productivity is 33-50 milligrams per square meter per day. Although the effect of meltwater in summer would decrease the estimated productivity by about 2 percent, the effect of air-tosea transport of carbon dioxide would probably offset the meltwater effect. Primary productivity in the southern oceans shows much spatial and temporal variability (Balech et al. 1968; KoblentzMishke et al. 1970; El-Sayed 1970; Lisitzin 1970; Goodell 1973). For instance, Lisitzin (1970) reports the diatom concentration in seawater to be between 0.5 x 10 6 and 1.0 x 10 per cubic meter. Such a large variability may explain partially why some direct productivity measurements have yielded low values when the general assumption is that the productivity is high in the southern oceans (El-Sayed and Turner 1977; Jennings et al. 1984). Our estimates were obtained following the method of Jennings et al. (1984) based on nutrient data. This method gives a temporally and spatially averaged result. Our estimated productivity of 300-450 milligrams per square meter per day agrees very well with the results of Jennings et al. (220-420 milligrams per square meter per day) and supports the notion that the
1986 REVIEW
productivity is indeed high in the southern oceans. Even if we assume that the average yearly productivity is reduced by half as a result of the winter ice cover, we still have a high organic carbon productivity of 55-82 grams per square meter per year and calcium carbonate productivity of 6-9 grams per square meter per year. These values agree very well with the result, 54 and 7.6 grams per square meter per year, respectively, obtained from the recent sediment trap experiments in the southern oceans (Noriki, Harada, and Tsunogai 1985). I acknowledge support from the U.S. Department of Energy (subcontract 19X-89608 C under Martin Marietta Energy Systems, Inc. contract DE-ACO5-84 OR 21400 with Department of Energy), and the National Science Council of the Republic of China (NSC 76-0209-M110-03). References Balech, E., S.Z. El-Sayed, C. Hasle, M. Neushul, and J.S. Zaneveld. 1968. Primary productivity and benthic marine algae of the Antarctic and Subantarctic. (Antarctic Map Folio Series, 10.) New York: American Geophysical Society. Broecker, W.S., and T.H. Peng. 1982. Tracers in the sea. Palisades, N.Y.: Eldigio Press. Chen, C.T. 1982. Carbonate chemistry during WEPOLEX-81, Antarctic Journal of the U.S., 17(5), 102-103. Chen, C.T. 1984. Carbonate chemistry of the Weddell Sea. (U.S. Department of Energy Technical Report, DOE/EV/10611-4.) Washington, D.C.: U.S. Government Printing Office. El-Sayed, S.Z. 1970. On the productivity of the Southern Ocean. In M.W. Holdgate (Ed.), Antarctic ecology. New York: Academic Press. El-Sayed, S.Z., and J.T. Turner. 1977. Productivity of the Antarctic and tropical/subtropical regions: A comparative study. In M.J. Dunbar (Ed.), Polar oceans. Calgary, Alberta: Arctic Institute of North America. Goodell, H.G. 1973. Marine sediments of the Southern oceans, the sediments. (Antarctic Map Folio Series, 17.) New York: American Geophysical Society. Gordon, A.L. 1982. The U.S.-U.S.S.R. Weddell Polynya Expedition, Antarctic Journal of the U.S., 17(5), 96-98. Gordon, A.L., C.T.A. Chen, and W.G. Metcalf. 1984. Winter mixed layer entrainment of Weddell Deep Water. Journal of Geophysical Research, 89, 637-640. Huber, BA., J. Jennings, C.T. Chen, J . Marra, S. Rennie, P. Mele, and A. Gordon. 1983. Reports of the U.S.-U.S.S.R. Weddell Polynya Expedition. (Hydrographical data, LDGO 83-1.) Palisades, N.Y.: Columbia University Press. Jennings, J.C., L. Gordon, and D.M. Nelson. 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature, 308, 51-54. Jennings, J . , D. Nelson, and L.I. Gordon. 1982. Nutrient chemistry program during the U.S.-U.S.S.R. Weddell Polynya Expedition, Antarctic Journal of the U.S., 17(5), 101. Koblentz-Mishke, O.J., V.V. Volkovinsky, and J.G. Kabanova. 1970. Plankton primary production of the world ocean. In W.S. Wooster (Ed.) Scientific Exploration of the South Pacific. Washington, D.C.: National Academy of Sciences. Lisitzin, A.P. 1970. Sedimentation and geochemical considerations. In W.S. Wooster (Ed.), Scientific Exploration of the South Pacific. Washington, D.C.: National Academy of Sciences. Noriki, S., K. 1-larada, and S. Tsunogai. 1985. Sediment trap experiments in the Antarctic Ocean. In A.C. Sigleo and A. Hattori (Eds.), Marine and Estuarine Geochemistry. Chelsea, Mich.: Lewis Publishers,
Inc. Takahashi, T., W.S. Broecker, A.E. Bainbridge, and R.F. Weiss. 1980.
Carbonate chemistry of the Atlantic, Pacific and Indian Oceans: The results of the GEOSECS expeditions, 1972-1978. (Lamont-Doherty Geological Ob-
servatory Technical Report No. 1, CV1-80.) Palisades, N.Y.: Columbia University Press.
129
tween occupations of these transects, suggesting that the physical system was reasonably steady-state during the 30-day duration of the cruise. Finally, no strong horizontal gradients, or frontal systems, were observed. This was in contrast to the AMERIEZ I program, which sampled in November 1983 along the strong frontal system separating the outflowing Weddell Sea water from the waters of the Eastern Scotia Sea. The region sampled during the March 1986 program was characterized only by very gradual horizontal variation—primarily in the upper layers, in moving from west to east away from the sea-ice edge. The northwestern Weddell Sea MIZ region was intensively sampled, revealing three water masses: the Weddell surface water, the Weddell winter water, and the Weddell warm water. The dominant upper layer feature was the widespread surface water layer, which was underlain by very strong vertical gradients. Such gradients imply that little vertical mixing is occurring through the bottom of the upper layer. The dominant deep feature was the presence of vertical temperature steps which may be related to region-wide vertical heat transfer. This work was supported in part by National Science Foundation grants DPP 84-20646 (to Muensch) and DPP 85-13098 (to Husby), by ONR contract N00014-82-C-0064 with Science Applications International Corporation (to Muensch) and by the National Marine Fisheries Service/National Oceanic and Atmospheric Administration (to Husby).
Summer-winter comparison of Weddell Sea surface water and its productivity
tigation for the first time of the cumulative winter effects on the distribution of chemical properties in the Weddell Sea surface water. Temperature, salinity, oxygen, and nutrient data all indicate that the temperature-minimum layer found in summer is the remnant winter surface water. In summer, solar heating and melting of sea ice transform the top layer of the winter surface water into a warmer, less saline water than the winter surface water which in summer is partially preserved beneath the top layer. Subsequently, biological activity reduces the nutrient concentration in the summer surface layer. The oxygen concentration in the surface layer, however, increases in summer because the oxygen-depleted winter surface water picks up oxygen from the atmosphere after sea ice is melted. In summary, the winter Weddell Sea surface water is colder, saltier, contains more nutrients but less oxygen than the summer surface water (Chen 1984; Gordon, Chen, and Metcalf 1984; Jennings, Gordon, and Nelson 1984). Comparison of WEPOLEX and GEOSECS (Geochemical Ocean Section Studies) normalized total carbon dioxide (NTCO 2 = TCO2 x 35/salinity, TCO 2 is the total amount of carbonate spe-
C.-T.A. CHEN* Institute of Marine Geology National Sun Yat-Sen University Kaohsiung, Taiwan Republic of China
The U.S.-U.S.S.R. Weddell Polynya Expedition (WEPOLEX) (Chen 1982; Gordon 1982; Jennings, Nelson, and Gordon 1982) generated chemical data near the outflow region of the Weddell Sea in the late winter/early spring. These data permitted inves* On leave from College of Oceanography, Oregon State University, Corvallis, Oregon 97331.
128
References Carmack, E.C. 1977. Water characteristics of the Southern Ocean south of the Polar Front. In M. Angel (Ed.), A voyage of discovery. New York: Pergamon Press. Middleton, J.H., and T.D. Foster. 1980. Fine structure measurements in a temperature-compensated halocline. Journal of Geophysical Research, 85(C2), 1107-1122.
ANTARCTIC JOURNAL
20 WEPOLEX
GEOSECS Summer Surface
L)
°.. 0 CD
-10
Winter Surface 20 2200 2250
1/
2300
2
Remnant Winter Surface Water 2200
2250
2300
NTCO , )JmoI kg-'
Potential temperature (0) plotted vs. normalized total carbon dioxide (NTCO2 ; "ii. mol/kg" denotes "micromoles per kilogram") for the WEPOLEX data (all stations) and the GEOSECS data (stations 79, 82,85, and 89).
cies dissolved in sea water) data (Takahashi etal. 1980; Huber et al. 1983; Chen 1984) also indicate that the winter NTCO7 values in the surface layer agree with the GEOSECS summer values in the temperature-minimum layer (figure). The summer surface NTCO2 and normalized alkalinity values are lower than the winter values by about 50 micromoles per kilogram and 10 microequivalents per kilogram, respectively. These data indicate a production of 5 micromoles per kilogram of calcium carbonate and 45 micromoles per kilogram of soft tissue, a ratio somewhat lower than an average ratio of 1-to-4 for the world oceans (Broecker and Peng 1982). The lower ratio is expected because siliceous rather than calcareous organisms dominate in the cold waters of the southern oceans. The average amount of soft tissue production in the surface layer above the temperature-minimum layer is about 22.5 micromoles per kilogram. This value translates to a productivity of 300-450 milligrams per square meter per day between late winter WEPOLEX and summer (cEosEcs) if we assume average surface layer to be 100 meters thick. The average calcium carbonate productivity is 33-50 milligrams per square meter per day. Although the effect of meltwater in summer would decrease the estimated productivity by about 2 percent, the effect of air-tosea transport of carbon dioxide would probably offset the meltwater effect. Primary productivity in the southern oceans shows much spatial and temporal variability (Balech et al. 1968; KoblentzMishke et al. 1970; El-Sayed 1970; Lisitzin 1970; Goodell 1973). For instance, Lisitzin (1970) reports the diatom concentration in seawater to be between 0.5 x 10 6 and 1.0 x 10 per cubic meter. Such a large variability may explain partially why some direct productivity measurements have yielded low values when the general assumption is that the productivity is high in the southern oceans (El-Sayed and Turner 1977; Jennings et al. 1984). Our estimates were obtained following the method of Jennings et al. (1984) based on nutrient data. This method gives a temporally and spatially averaged result. Our estimated productivity of 300-450 milligrams per square meter per day agrees very well with the results of Jennings et al. (220-420 milligrams per square meter per day) and supports the notion that the
1986 REVIEW
productivity is indeed high in the southern oceans. Even if we assume that the average yearly productivity is reduced by half as a result of the winter ice cover, we still have a high organic carbon productivity of 55-82 grams per square meter per year and calcium carbonate productivity of 6-9 grams per square meter per year. These values agree very well with the result, 54 and 7.6 grams per square meter per year, respectively, obtained from the recent sediment trap experiments in the southern oceans (Noriki, Harada, and Tsunogai 1985). I acknowledge support from the U.S. Department of Energy (subcontract 19X-89608 C under Martin Marietta Energy Systems, Inc. contract DE-ACO5-84 OR 21400 with Department of Energy), and the National Science Council of the Republic of China (NSC 76-0209-M110-03). References Balech, E., S.Z. El-Sayed, C. Hasle, M. Neushul, and J.S. Zaneveld. 1968. Primary productivity and benthic marine algae of the Antarctic and Subantarctic. (Antarctic Map Folio Series, 10.) New York: American Geophysical Society. Broecker, W.S., and T.H. Peng. 1982. Tracers in the sea. Palisades, N.Y.: Eldigio Press. Chen, C.T. 1982. Carbonate chemistry during WEPOLEX-81, Antarctic Journal of the U.S., 17(5), 102-103. Chen, C.T. 1984. Carbonate chemistry of the Weddell Sea. (U.S. Department of Energy Technical Report, DOE/EV/10611-4.) Washington, D.C.: U.S. Government Printing Office. El-Sayed, S.Z. 1970. On the productivity of the Southern Ocean. In M.W. Holdgate (Ed.), Antarctic ecology. New York: Academic Press. El-Sayed, S.Z., and J.T. Turner. 1977. Productivity of the Antarctic and tropical/subtropical regions: A comparative study. In M.J. Dunbar (Ed.), Polar oceans. Calgary, Alberta: Arctic Institute of North America. Goodell, H.G. 1973. Marine sediments of the Southern oceans, the sediments. (Antarctic Map Folio Series, 17.) New York: American Geophysical Society. Gordon, A.L. 1982. The U.S.-U.S.S.R. Weddell Polynya Expedition, Antarctic Journal of the U.S., 17(5), 96-98. Gordon, A.L., C.T.A. Chen, and W.G. Metcalf. 1984. Winter mixed layer entrainment of Weddell Deep Water. Journal of Geophysical Research, 89, 637-640. Huber, BA., J. Jennings, C.T. Chen, J . Marra, S. Rennie, P. Mele, and A. Gordon. 1983. Reports of the U.S.-U.S.S.R. Weddell Polynya Expedition. (Hydrographical data, LDGO 83-1.) Palisades, N.Y.: Columbia University Press. Jennings, J.C., L. Gordon, and D.M. Nelson. 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature, 308, 51-54. Jennings, J . , D. Nelson, and L.I. Gordon. 1982. Nutrient chemistry program during the U.S.-U.S.S.R. Weddell Polynya Expedition, Antarctic Journal of the U.S., 17(5), 101. Koblentz-Mishke, O.J., V.V. Volkovinsky, and J.G. Kabanova. 1970. Plankton primary production of the world ocean. In W.S. Wooster (Ed.) Scientific Exploration of the South Pacific. Washington, D.C.: National Academy of Sciences. Lisitzin, A.P. 1970. Sedimentation and geochemical considerations. In W.S. Wooster (Ed.), Scientific Exploration of the South Pacific. Washington, D.C.: National Academy of Sciences. Noriki, S., K. 1-larada, and S. Tsunogai. 1985. Sediment trap experiments in the Antarctic Ocean. In A.C. Sigleo and A. Hattori (Eds.), Marine and Estuarine Geochemistry. Chelsea, Mich.: Lewis Publishers,
Inc. Takahashi, T., W.S. Broecker, A.E. Bainbridge, and R.F. Weiss. 1980.
Carbonate chemistry of the Atlantic, Pacific and Indian Oceans: The results of the GEOSECS expeditions, 1972-1978. (Lamont-Doherty Geological Ob-
servatory Technical Report No. 1, CV1-80.) Palisades, N.Y.: Columbia University Press.
129
