Distributions of biogenic silica and dissolved silicic acid ...
Ocean and sea-ice studies__________________
Distributions of biogenic silica and dissolved silicic acid in the surface waters of the Ross Sea, January and February 1990 DAVID M. NELSON
and Louis
I. GORDON
College of Oceanography Oregon State University Corvallis, Oregon 97330
Global mass balances indicate that approximately 75-85 percent of the modern accumulation of biogenic silica in marine sediments takes place south of the Antarctic Convergence
(DeMaster 1981; Ledford-Hoffman, DeMaster, and Nittrouer 1986). The antarctic and subantarctic are nowhere near this important quantitatively in the global deposition pattern of organic carbon (Lisitzin 1972; Holland 1978), and appear to account for no more than 5 percent of the global-scale photosynthetic production of organic carbon by phytoplankton (Smith and Nelson 1986). These mass-balance calculations combine to suggest that the cycles of siliceous and organic biogenic material in the southern ocean are decoupled to an extent that is not true in most other oceanic regions, and that this decoupling makes the Antarctic the main site of long-term removal of silica from the oceans. Data collected in the western Ross Sea in 1983 tended to support the decoupling hypothesis. The region was found to have an intense diatom bloom near the edge of the receding pack ice (Smith and Nelson 1985), within which the concentrations of biogenic particulate silica were frequently greater than 25 micromoles per liter and silica production rates averaged 38 millimoles per square meter per day (Nelson and Smith 1986). These silica production rates were comparable to those
160°W 1650 1700 1750 1800 1750 170°E 720S 73°
74°
75°
760
77°
78° S Figure 1. Locations of stations where nutrient and biogenic silica data were collected. Moored arrays of sediment traps, current meters and transmissometers were deployed at the sites marked A, B, and C. (m denotes meter.) 98
ANTARCTIC JOURNAL
observed in the highly productive, diatom-dominated waters of the Peruvian and northwest African upwelling zones, and the biogenic silica levels were the highest ever reported in surface seawater. Accumulation rates of silica in the seabed were also very high, averaging over 4 moles per square meter per day in deep basins (Ledford-Hoffman et al. 1986), but photosynthetic carbon productivity averaged only 0.96 grams of carbon per square meter per day (Wilson, Smith, and Nelson 1986), which is only moderately high and, in fact, very low for waters exhibiting such high levels of phytoplankton biomass. In January and early February 1990, we returned to the Ross Sea to conduct a more detailed study of the cycling of biogenic silica in the surface waters. Our stations were oriented primarily along east-west transect lines at 76°30' and 72°30' south (figure 1). We collected data on dissolved nutrient (nitrate, nitrite, ammonium, silicic acid, and phosphate) and particulate silica (both biogenic and lithogenic) concentrations and performed silicon-30 tracer experiments to measure the production and dissolution rates of biogenic silica. At this writing (June 1990), we have reasonably complete data sets on nutrient and biogenic silica distributions. The east-west distributions of dissolved and particulate silica in the western Ross Sea in the summer of 1990 (figure 2) were very similar to those observed in the same region in the sum-
mer of 1983 (Nelson and Smith 1986). That is, there is clear evidence that an ice-edge diatom bloom had produced exceptionally high (greater than 25 micromoles per liter) biogenic silica levels in the surface water (figure 2a) with corresponding depletion of dissolved silicic acid (figure 2b). The 1990 transect, however, extended considerably farther east than any that had been occupied before and showed two other (though less intense) regions of biogenic silica accumulation and silicic acid depletion to the east. While lower than those within the iceedge bloom, the biogenic silica levels in these more easterly maxima exceeded 8 micromoles per liter, which is very high in comparison with other ocean surface waters. Unlike the diatom bloom to the west, these eastern features had a more mixed algal assemblage, with diatoms and the prymnesiophyte Phaeocystis pouchetii both abundant. Carbon-14 primary productivity (Smith, Kelley, and Rich, Antarctic Journal, this issue) data indicate that both the diatom bloom to the west and the mixed diatomlPhaeocystis bloom to the east, were sites of elevated productivity, resulting in an integrated productivity for the entire southern transect of 1.37 grams of carbon per square meter per day which is 40 percent higher than was observed in the 1983 study (Wilson et al. 1986). It is thus evident from these observations that there are other sites of potentially significant silica production within the Ross Sea, in addition to the previously documented ice-edge diatom 1
TRANSECT 1 (76 0 30 S 1 13-16 Jan., 1990)
2.5
0 20 a-
: '
V.75Y
25 Biogenic Si02 • (mol ) S S S
80 0
S
S
40
060
S:
20 B __
I
S
' 'S
20 0
MEO
S
S
S S
•
•
5 5 ' • IS • S S • • S S • • I S • • • S
60
60[ (tmoIf) • I 80, 0 100 200 300 400 500 1
S I I
i $
i • i
D
XON
Distance from Coast (km) Figure 2. Sections of biogenic particulate silica (A) and dissolved silicic acid (B) along the east-west transect at 76 030' south, 13-16 January 1990. (m denotes meter. km denotes kilometer. pimol . I denotes micromoles per liter.) 1990 REVIEW
99
bloom. Thus, the annual mass balances of both biogenic silica and organic carbon in the water column of the Ross Sea are likely to be considerably more complex, and the spatially integrated production rates higher, than preliminary mass balances based largely on 1983 data (Jones, Nelson, and Treguer in press) indicate. We are grateful to Julie A. Ahern, Linda J. Herlihy, Joe C. Jennings, Jr., and Paul Treguer for their valuable assistance in this work, and to Captain Tor Arne Jacobson and the crew of the R/V Polar Duke for excellent ship support. David J. DeMaster (North Carolina State University), Amy Leventer (Ohio State University), Walker 0. Smith, Jr. (University of Tennessee), and their assistants worked with us on this collaborative cruise. This research was supported by National Science Foundation grant DPP 88-17441 to Oregon State University. References
Jones, E.P., D.M. Nelson, and P. Treguer. In press. Polar chemical oceanography. In W.O. Smith, Jr. (Ed.), Polar Oceanography. New York: Academic Press. Ledford-Hoffman, P.A., D.J. DeMaster, and C.A. Nittrouer. 1986. Biogenic silica accumulation in the Ross Sea and the importance of Antarctic shelf deposits in the marine silica budget. Geochimica et Cosmochimica Acta, 50, 2,099-2,110.
Lisitzin, A. P. 1972. Sedimentation in the world ocean. Society of Economic Mineralogists and Paleontologists, Special Publication number 17. Nelson, D.M., and W.O. Smith, Jr. 1986. Phytoplankton dynamics of the Ross Sea ice edge. II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Research, 33, 1,389-1,412.
Smith, W.O. Jr., H.P. Kelley, and J.R. Rich. 1990. Chlorophyll distribution and primary productivity in the Ross Sea, summer, 1990. Antarctic Journal of the U.S., 25(5).
Smith, W.O. Jr., and D.M. Nelson. 1985. Phytoplankton bloom produced by a receding ice edge in the Ross Sea: Spatial coherence with the density field. Science, 227, 163-166. Smith, W.O. Jr., and D.M. Nelson. 1986. The importance of ice-edge phyto-plankton production in the Southern Ocean. BioScience, 251257.
DeMaster, D.J. 1981. The supply and accumulation of silica in the marine environment. Geochimica et Cosinochimica Acta, 45, 1,715-1,732. Holland, H.D. 1978. The chemistry of the atmosphere and oceans. New York: Wiley.
Wilson, DL., W.O. Smith, Jr., and D.M. Nelson. 1986. Phytoplankton bloom dynamics of the western Ross Sea ice edge. I. Primary productivity and species-specific production. Deep-Sea Research, 33, 1,375-
Biogenic sediment fluxes in the western Ross Sea
margin, the extent to which photic-zone and mid-water-column processes control the flux of organic debris to the seafloor is not yet known. A major impediment to a more complete understanding of the fluxes of key elements like carbon, nitrogen, silicon, and phosphorus in the southern ocean water column has been the absence of year-round environmental monitoring and sampling. Our principal goal during the 19891990 field season was to install four sets of time-series sediment traps on winter-over moorings in the western Ross Sea. We also recovered winter-over moorings equipped with current meters and single-cup sediment traps and completed a sediment collection program in Granite Harbor. Our field work was conducted from the sea ice during November, 1989, and aboard the R/V Polar Duke during January and February, 1990. On Polar Duke, we deployed six timeseries (15 cups) sediment traps at three sites in the western Ross Sea. These deployments are part of an interdisciplinary, multi-institutional study of the biogeochemical cycles of silicon and carbon in the Ross Sea. The sediment traps will be deployed for 2 years and will provide the first view of sediment fluxes over a monthly time-scale throughout the austral winter and summer in the Ross Sea. These results will complement the water-column production and recycling studies of organic matter (W. Smith, University of Tennessee) and biogenic silica (D. Nelson, Oregon State University), and the shelf current and seafloor sediment studies of D. DeMaster (North Carolina State University) and C. Nittrouer (State University of New York at Stonybrook). The mooring sites (table 1) were located at the ends of sampling transects which traverse areas ranging from ice-free to heavily ice-covered during most of the austral summer. Mooring A is located in a region of highly biosiliceous seafloor sediment (greater than 40 percent opal; Dunbar et al. 1985) and in which a large ice-edge bloom was encountered
ROBERT B. DUNBAR, DAVE MUCCIARONE, and BILL JONES
Department of Geology and Geophysics Rice University Houston, Texas 77251 A. R. LEVENTER-REED
Byrd Polar Research Center Ohio State University Columbus, Ohio 43210
A. R. PYNE Antractic Research Center Victoria University Wellington, New Zealand C. MOSER
School of Oceanography Oregon State University Corvallis, Oregon 97331
Although it is clear that deep shelf currents play a major role in the redistribution of biogenic phases on the antarctic 100
1,387.
ANTARCTIC JOURNAL
Distributions of biogenic silica and dissolved silicic acid in the surface waters of the Ross Sea, January and February 1990 DAVID M. NELSON
and Louis
I. GORDON
College of Oceanography Oregon State University Corvallis, Oregon 97330
Global mass balances indicate that approximately 75-85 percent of the modern accumulation of biogenic silica in marine sediments takes place south of the Antarctic Convergence
(DeMaster 1981; Ledford-Hoffman, DeMaster, and Nittrouer 1986). The antarctic and subantarctic are nowhere near this important quantitatively in the global deposition pattern of organic carbon (Lisitzin 1972; Holland 1978), and appear to account for no more than 5 percent of the global-scale photosynthetic production of organic carbon by phytoplankton (Smith and Nelson 1986). These mass-balance calculations combine to suggest that the cycles of siliceous and organic biogenic material in the southern ocean are decoupled to an extent that is not true in most other oceanic regions, and that this decoupling makes the Antarctic the main site of long-term removal of silica from the oceans. Data collected in the western Ross Sea in 1983 tended to support the decoupling hypothesis. The region was found to have an intense diatom bloom near the edge of the receding pack ice (Smith and Nelson 1985), within which the concentrations of biogenic particulate silica were frequently greater than 25 micromoles per liter and silica production rates averaged 38 millimoles per square meter per day (Nelson and Smith 1986). These silica production rates were comparable to those
160°W 1650 1700 1750 1800 1750 170°E 720S 73°
74°
75°
760
77°
78° S Figure 1. Locations of stations where nutrient and biogenic silica data were collected. Moored arrays of sediment traps, current meters and transmissometers were deployed at the sites marked A, B, and C. (m denotes meter.) 98
ANTARCTIC JOURNAL
observed in the highly productive, diatom-dominated waters of the Peruvian and northwest African upwelling zones, and the biogenic silica levels were the highest ever reported in surface seawater. Accumulation rates of silica in the seabed were also very high, averaging over 4 moles per square meter per day in deep basins (Ledford-Hoffman et al. 1986), but photosynthetic carbon productivity averaged only 0.96 grams of carbon per square meter per day (Wilson, Smith, and Nelson 1986), which is only moderately high and, in fact, very low for waters exhibiting such high levels of phytoplankton biomass. In January and early February 1990, we returned to the Ross Sea to conduct a more detailed study of the cycling of biogenic silica in the surface waters. Our stations were oriented primarily along east-west transect lines at 76°30' and 72°30' south (figure 1). We collected data on dissolved nutrient (nitrate, nitrite, ammonium, silicic acid, and phosphate) and particulate silica (both biogenic and lithogenic) concentrations and performed silicon-30 tracer experiments to measure the production and dissolution rates of biogenic silica. At this writing (June 1990), we have reasonably complete data sets on nutrient and biogenic silica distributions. The east-west distributions of dissolved and particulate silica in the western Ross Sea in the summer of 1990 (figure 2) were very similar to those observed in the same region in the sum-
mer of 1983 (Nelson and Smith 1986). That is, there is clear evidence that an ice-edge diatom bloom had produced exceptionally high (greater than 25 micromoles per liter) biogenic silica levels in the surface water (figure 2a) with corresponding depletion of dissolved silicic acid (figure 2b). The 1990 transect, however, extended considerably farther east than any that had been occupied before and showed two other (though less intense) regions of biogenic silica accumulation and silicic acid depletion to the east. While lower than those within the iceedge bloom, the biogenic silica levels in these more easterly maxima exceeded 8 micromoles per liter, which is very high in comparison with other ocean surface waters. Unlike the diatom bloom to the west, these eastern features had a more mixed algal assemblage, with diatoms and the prymnesiophyte Phaeocystis pouchetii both abundant. Carbon-14 primary productivity (Smith, Kelley, and Rich, Antarctic Journal, this issue) data indicate that both the diatom bloom to the west and the mixed diatomlPhaeocystis bloom to the east, were sites of elevated productivity, resulting in an integrated productivity for the entire southern transect of 1.37 grams of carbon per square meter per day which is 40 percent higher than was observed in the 1983 study (Wilson et al. 1986). It is thus evident from these observations that there are other sites of potentially significant silica production within the Ross Sea, in addition to the previously documented ice-edge diatom 1
TRANSECT 1 (76 0 30 S 1 13-16 Jan., 1990)
2.5
0 20 a-
: '
V.75Y
25 Biogenic Si02 • (mol ) S S S
80 0
S
S
40
060
S:
20 B __
I
S
' 'S
20 0
MEO
S
S
S S
•
•
5 5 ' • IS • S S • • S S • • I S • • • S
60
60[ (tmoIf) • I 80, 0 100 200 300 400 500 1
S I I
i $
i • i
D
XON
Distance from Coast (km) Figure 2. Sections of biogenic particulate silica (A) and dissolved silicic acid (B) along the east-west transect at 76 030' south, 13-16 January 1990. (m denotes meter. km denotes kilometer. pimol . I denotes micromoles per liter.) 1990 REVIEW
99
bloom. Thus, the annual mass balances of both biogenic silica and organic carbon in the water column of the Ross Sea are likely to be considerably more complex, and the spatially integrated production rates higher, than preliminary mass balances based largely on 1983 data (Jones, Nelson, and Treguer in press) indicate. We are grateful to Julie A. Ahern, Linda J. Herlihy, Joe C. Jennings, Jr., and Paul Treguer for their valuable assistance in this work, and to Captain Tor Arne Jacobson and the crew of the R/V Polar Duke for excellent ship support. David J. DeMaster (North Carolina State University), Amy Leventer (Ohio State University), Walker 0. Smith, Jr. (University of Tennessee), and their assistants worked with us on this collaborative cruise. This research was supported by National Science Foundation grant DPP 88-17441 to Oregon State University. References
Jones, E.P., D.M. Nelson, and P. Treguer. In press. Polar chemical oceanography. In W.O. Smith, Jr. (Ed.), Polar Oceanography. New York: Academic Press. Ledford-Hoffman, P.A., D.J. DeMaster, and C.A. Nittrouer. 1986. Biogenic silica accumulation in the Ross Sea and the importance of Antarctic shelf deposits in the marine silica budget. Geochimica et Cosmochimica Acta, 50, 2,099-2,110.
Lisitzin, A. P. 1972. Sedimentation in the world ocean. Society of Economic Mineralogists and Paleontologists, Special Publication number 17. Nelson, D.M., and W.O. Smith, Jr. 1986. Phytoplankton dynamics of the Ross Sea ice edge. II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Research, 33, 1,389-1,412.
Smith, W.O. Jr., H.P. Kelley, and J.R. Rich. 1990. Chlorophyll distribution and primary productivity in the Ross Sea, summer, 1990. Antarctic Journal of the U.S., 25(5).
Smith, W.O. Jr., and D.M. Nelson. 1985. Phytoplankton bloom produced by a receding ice edge in the Ross Sea: Spatial coherence with the density field. Science, 227, 163-166. Smith, W.O. Jr., and D.M. Nelson. 1986. The importance of ice-edge phyto-plankton production in the Southern Ocean. BioScience, 251257.
DeMaster, D.J. 1981. The supply and accumulation of silica in the marine environment. Geochimica et Cosinochimica Acta, 45, 1,715-1,732. Holland, H.D. 1978. The chemistry of the atmosphere and oceans. New York: Wiley.
Wilson, DL., W.O. Smith, Jr., and D.M. Nelson. 1986. Phytoplankton bloom dynamics of the western Ross Sea ice edge. I. Primary productivity and species-specific production. Deep-Sea Research, 33, 1,375-
Biogenic sediment fluxes in the western Ross Sea
margin, the extent to which photic-zone and mid-water-column processes control the flux of organic debris to the seafloor is not yet known. A major impediment to a more complete understanding of the fluxes of key elements like carbon, nitrogen, silicon, and phosphorus in the southern ocean water column has been the absence of year-round environmental monitoring and sampling. Our principal goal during the 19891990 field season was to install four sets of time-series sediment traps on winter-over moorings in the western Ross Sea. We also recovered winter-over moorings equipped with current meters and single-cup sediment traps and completed a sediment collection program in Granite Harbor. Our field work was conducted from the sea ice during November, 1989, and aboard the R/V Polar Duke during January and February, 1990. On Polar Duke, we deployed six timeseries (15 cups) sediment traps at three sites in the western Ross Sea. These deployments are part of an interdisciplinary, multi-institutional study of the biogeochemical cycles of silicon and carbon in the Ross Sea. The sediment traps will be deployed for 2 years and will provide the first view of sediment fluxes over a monthly time-scale throughout the austral winter and summer in the Ross Sea. These results will complement the water-column production and recycling studies of organic matter (W. Smith, University of Tennessee) and biogenic silica (D. Nelson, Oregon State University), and the shelf current and seafloor sediment studies of D. DeMaster (North Carolina State University) and C. Nittrouer (State University of New York at Stonybrook). The mooring sites (table 1) were located at the ends of sampling transects which traverse areas ranging from ice-free to heavily ice-covered during most of the austral summer. Mooring A is located in a region of highly biosiliceous seafloor sediment (greater than 40 percent opal; Dunbar et al. 1985) and in which a large ice-edge bloom was encountered
ROBERT B. DUNBAR, DAVE MUCCIARONE, and BILL JONES
Department of Geology and Geophysics Rice University Houston, Texas 77251 A. R. LEVENTER-REED
Byrd Polar Research Center Ohio State University Columbus, Ohio 43210
A. R. PYNE Antractic Research Center Victoria University Wellington, New Zealand C. MOSER
School of Oceanography Oregon State University Corvallis, Oregon 97331
Although it is clear that deep shelf currents play a major role in the redistribution of biogenic phases on the antarctic 100
1,387.
ANTARCTIC JOURNAL
