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Cycling and Accumulation of Biogenic Silica and Organic Matter in High-Latitude Environments: the Ross Sea

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Cycling and Accumulation of Biogenic Silica and Organic Matter in High-Latitude Environments: the Ross Sea FEATURE CYCLING AND ACCUMULATION OF BIOGENIC SILICA AND ORGANIC MATTER IN HIGH-LATITUDE ENVIRONMENTS: THE ROSS SEA By David J. DeMaster, Robert B. Dunbar, Louis I. Gordon, Amy R. Leventer, John M. Morrison, David M. Nelson, Charles A. Nittrouer and Walker O. Smith, Jr. SOUTHERN OCEAN SEDIMENTS are a ma- tively high rates of biogenic silica accumulation jor sink for biogenic silica (50-75% of the oceanic in Southern Ocean sediments relative to organic total of 4.5 X 1014 g SiOe y-l; DeMaster, 1981) carbon (~60:1 on a weight basis) must result from but only a minor sink for organic carbon (<1% the preferential preservation of biogenic silica in of the oceanic 2 × 1014 (g organic C) y-l; Ro- the Antarctic water column and/or seabed. mankevich, 1"984). The high rates ofbiogenic silica The focus of this high-latitude study has been accumulation in the Southern Ocean (predomi- to quantify the cycles of Si and organic C in the nantly beneath the Polar Front) initially were water column and seabed and to identify the pri- thought to result from high rates of primary pro- mary location of the fractionation between fluxes duction; however, direct measurements of pri- of biogenic silica and organic matter. Measure- mary production made during the past two de- ments of uptake and regeneration rates were made cades (e.g., E1-Sayed et al., 1983; Holm-Hansen for organic and siliceous phases throughout the et al., 1977) typically indicate that the rates in the upper 150 m of the water column. Particle traps Southern Ocean are low to moderate. These data were used to characterize changes in the nature suggest that the preservation efficiency for biogenic of the biogenic particles within the water column •.. to identify the silica in the Southern Ocean plays a key role in at depths of 231 and 500-750 m below the ocean the processes contributing to the massive silica surface. Box cores and kasten cores were collected primary location of the accumulation. Primary production in the waters from the seabed to evaluate regeneration of bio- fractionation between south of the Polar Front is not controlled by major genic phases below the sediment-water interface nutrient (nitrate, phosphate, or silicate) availabil- as well as accumulation rates of biogenic material fluxes of biogenic silica ity (see Kamykowski and Zentara, 1989, for re- in the sediment column. and organic matter. view) but may be restricted by low levels of irra- The Ross Sea was chosen because it typifies diance coupled with deep mixed layers (e.g., many Antarctic settings and is characterized by Mitchell et al., 1991; Nelson and Smith, 1991) or 1) significant biogenic production and seabed ac- by limited amounts of micronutrients such as iron cumulation (Ledford-Hoffman et al., 1986; Nel- (Martin et al., 1990). Regardless of the mechanism son and Smith, 1986), 2) relatively confined that limits biological production, Southern Ocean boundaries, 3) water circulation that is reasonably phytoplankton typically have a biogenic-silica-to- well understood (as compared with the complex organic-carbon weight ratio ranging from 1 to 3 dynamics of the Polar Front), and 4) close prox- (Nelson and Smith, 1986). Therefore, the rela- imity to a major Antarctic support facility (McMurdo Station). A two-year time-series study has been conducted in an effort to minimize po- D.J. DeMaster and J.M. Morrison, Department of Marine, tential biases from short-term blooms and episodic Earth, and Atmospheric Sciences, North Carolina State Uni- changes in circulation and sedimentation. This versity, Raleigh, NC 27695-8208, USA. R.B. Dunbar, De- partment of Geology and Geophysics, Rice University, Hous- paper describes several unique aspects of nutrient ton, TX 77251-1892, USA. L.I. Gordon and D.M. Nelson, and carbon cycling in the high-latitude environ- College of Oceanography, Oregon State University, Corvallis, ment of the Ross Sea. OR 97331, USA. A.R. Leventer, Byrd Polar Research Center, Ohio State University, Columbus, OH 43214, USA. C.A. Nit- Ross Sea Field Programs trouer, Marine Sciences Research Center, State University of New York, Stony Brook, NY 11794-5000, USA. W.O. Smith, In January of 1990 the initial field program in Jr., Botany Department and Program in Ecology, University the Ross Sea occupied 181 stations and deployed of Tennessee, Knoxville, TN 37996, USA. three moorings (Fig. 1A) from the research vessel 146 OCEANOGRAPHYoVol. 5, No, 3.1992 (A) 1990 Ross Sea Field Season 1992 Ross Sea Field Season (B) 160 E 170 E 180 170 W 160 E 170 E 180 170 W , , Victoria Land ~ ~_OooX~. ~ ~ -72 72- Victoria Land .~!:..... ,~-:'!i" ® < - 74 74- 74 .'~' ,-~ aoo ii ® ii £ i))!,~ V (~.nff ~ " Hydrography i i i i i i i 160 E 170 E 180 170 W 160 E 170 E 180 Fig. 1." Station locations from the 1990 and 1992 field cruises to the Ross Sea. (R/V) POLAR DUKE. Two east/west transects then redeployed after being out for a period of 23 were conducted during the cruise; one at 76°30'S and 14 days, respectively. One year later (January and the other 72°30'S. A conductivity-tempera- 1991) the Coast Guard Icebreaker POLAR SEA ture-depth probe (CTD) equipped with a Sea Tech was used to retrieve and redeploy the moorings fluorometer and 10-cm-path-length Sea Tech at sites A and B. transmissometer was deployed in the water col- During February 1992, the final Ross Sea umn at every station. At every other station along cruises aboard the POLAR SEA and the R/V PO- the two transects, water samples for nutrients (ni- LAR DUKE completed the time series and re- •.. three mooring sites trate, nitrite, ammonium, silicate, and phosphate), trieved the three moorings. One hundred forty- contrasted areas rate measurements (C, N, and Si), and suspended- six stations (Fig. 1B) were occupied with a protocol solid analyses (chlorophyll, organic C and N, and similar to that of the first cruise. A third east/west exhibiting differing biogenic silica) were collected. Sediment cores transect was added at 75°S to resolve differences were obtained from 65 stations, with porewater in water-column and seabed characteristics in the sea-i~ conditions... analyses and flux-core measurements (subcores northern and southern Ross Sea. incubated at in situ temperatures and sampled for changes in nutrient content in water overlying the subcore) used to evaluate the intensity of regen- Hydrography and Currents eration in the seabed. Water circulation in the Ross Sea generally is Each of the three moorings consisted of two cyclonic (clockwise), with gentle southward flow particle traps (0.5-m 2 cross-sectional area, rotating in the central and eastern sections and intensified 15-cup design from Oregon State University); one northward flow in the west (Klepikov and Gri- 231 m below the ocean surface and the other ~40 gor'yev, 1966; Jacobs et al., 1970; Pillsbury and m above the seabed (500-750 m below the sur- Jacobs, 1985). Flow in the southern Ross Sea typ- face). The locations of the three mooring sites ically occurs in the westward direction. This cir- contrasted areas exhibiting differing sea-ice con- culation is best developed in surface waters but ditions during the austral summer and as such extends to depth as well. Tidal currents are an were expected to support different levels of pri- important process for mixing water masses in the mary production. Aanderaa current meters were study area, and current-meter records reveal dis- placed just below each of the particle traps to tinct oscillations at tidal frequencies. Energy monitor current speed and direction• In the bot- spectra reveal peaks at both semidiurnal (0.08 cy tom current meter at each site, a 25-cm-path- h -l) and diurnal (0.04 cy h -I) frequencies, but the length Sea Tech transmissometer was placed in diurnal signal dominates the current records. The the directional vane to monitor particle concen- data generally show a fortnightly modulation as tration. At each of the mooring sites a string of well. three floating traps (0.2-m 2 cross-sectional area, Current speeds can reach 25 cm s -1 at 240 m depths 50-250 m) was deployed for a period of below the surface but are more typically ~ 10 cm 24 h. During a second occupation of the southern s -1. Maximum speeds are somewhat less in near- transect, Moorings A and B were retrieved and bottom records (40 m above the seabed), but typ- OCEANOGRAPHY°Vol. 5, No. 3o1992 147 ical speeds are also ~ 10 cm s -~. These speeds are o , / ,,*,,,, ,,- ,, ,* .... : : *',, : probably insufficient to erode sediment from the seabed but could transport fine particles already in suspension. The deep currents at Mooring B have a net northwest displacement of ~7 km d -1, whereas at Mooring A, the deep currents have a ~ola, ',. ~ .,",, ; .,",':,.i;\ , ,'F'[. •.. this bloom had net northeastward displacement of ~6 km d ~. In the Ross Sea, suspended particulate matter developed within a lens has its highest concentrations near the water sur- of low-salinity face (~3 mg 1-1). Values abruptly decrease with °I t . depth (to <0.1 mg 1 1) but can show a small in- Si (OH)4 • • meltwater... crease near the seafloor (0.5 mg l-l). The surface 80 b , ,. ., • ,\ values generally result from biogenic material, whereas the bottom values reflect predominantly lithogenic material. The time-series records from transmissometers mounted on the moorings in- dicate uniformly low values for turbidity (_<0.5 ~z N% ,,' ',, "zs,,_--"/'~'; .... / ,'-' mg 1-1). Distributions of Biogenie Silica, Nutrients, Chlorophyll, and Primary Production o During the summers of both 1990 and 1992, the spatial distributions of phytoplankton biomass 8o ...
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