The δ15N of nitrate in the southern ocean: Consumption of nitrate in surface waters

We report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern Indian and Pacific sectors of the Southern Ocean, focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton. The δ15N of nitrate increases by 1–2‰ from deep water into the Antarctic summertime surface layer, due to kinetic isotopic fractionation during nitrate uptake. Estimation of the nitrate uptake isotope effect from Antarctic depth profiles yields values in the range of 5–6‰ in east Indian sector and 4–5‰ in the east Pacific sector. Surface transect data from the Pacific sector also yield values of 4–5‰. The major uncertainty in the profile-based estimation of the isotope effect involves the δ15N of nitrate from the temperature minimum layer below the summertime Antarctic surface layer, which deviates significantly from the predictions of simple models of isotope fractionation. For the Subantarctic surface, it is possible to distinguish between nitrate supplied laterally from the surface Antarctic and nitrate supplied vertically from the Subantarctic thermocline because of the distinctive relationships between the δ15N and concentration of nitrate in these two potential sources. Our Subantarctic samples, collected during the summer and fall, indicate that nitrate is supplied to the Subantarctic surface largely by northward transport of Antarctic surface water. Isotopic data from the Pacific sector of the Subantarctic suggest an isotope effect of 4.5‰, indistinguishable from the Antarctic estimates in this sector.

[1]  A. Piola,et al.  Circumpolar properties of Antarctic intermediate water and Subantarctic Mode Water , 1982 .

[2]  D. DeMaster,et al.  A Cooperative Study of Upper-Ocean Particulate Fluxes in the Weddell Sea , 1988 .

[3]  J. Sharp,et al.  Isotopic fractionation of ammonium and nitrate during uptake by Skeletonema costatum: Implications for δ15N dynamics under bloom conditions , 1996 .

[4]  M. Whitehouse,et al.  Spatial variability of inorganic nutrients in the marginal ice zone of the Bellingshausen Sea during the Austral spring , 1995 .

[5]  J. D. Hays,et al.  Evidence for lower productivity in the Antarctic Ocean during the last glaciation , 1991, Nature.

[6]  M. Brzezinski,et al.  Silicon-isotope composition of diatoms as an indicator of past oceanic change , 1998, Nature.

[7]  U. Siegenthaler,et al.  Rapid atmospheric CO2 variations and ocean circulation , 1984, Nature.

[8]  C. Garside.  A chemiluminescent technique for the determination of nanomolar concentrations of nitrate and nitrite in seawater , 1982 .

[9]  D. M. Nelson,et al.  Phytoplankton bloom dynamics of the western Ross Sea ice edge—II. Mesoscale cycling of nitrogen and silicon , 1986 .

[10]  J. Klinck,et al.  The physics of the Antarctic Circumpolar Current , 1986 .

[11]  J. McCarthy,et al.  Natural isotopic composition of dissolved inorganic nitrogen in the Chesapeake Bay , 1990 .

[12]  A. Gordon,et al.  Winter mixed layer entrainment of Weddell Deep Water , 1984 .

[13]  G. Gong,et al.  The nitrogen isotopic composition of nitrate in the Kuroshio Water northeast of Taiwan: evidence for nitrogen fixation as a source of isotopically light nitrate , 1996 .

[14]  T. Yoshinari,et al.  Isotopic composition of nitrate in the central Arabian Sea and eastern tropical North Pacific: A tracer for mixing and nitrogen cycles , 1998 .

[15]  S. Neshyba,et al.  Evidence for counterflow to the west wind drift off South America , 1980 .

[16]  J. Farrell,et al.  Glacialá¤-interglacial changes in nutrient utilization in the equatorial Pacific Ocean , 1995, Nature.

[17]  A. Devol,et al.  Isotopic fractionation of oxygen and nitrogen in coastal marine sediments , 1997 .

[18]  B. Mitchell,et al.  Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean , 1991 .

[19]  E. Boyle,et al.  Late quaternary paleochemistry of high-latitude surface waters , 1989 .

[20]  D. Sigman,et al.  The isotopic composition of diatom-bound nitrogen in Southern Ocean sediments , 1999 .

[21]  I. Kaplan,et al.  Isotopic fractionation of dissolved nitrate during denitrification in the eastern tropical north pacific ocean , 1975 .

[22]  J. McCarthy,et al.  Temporal and spatial variations in the natural abundance of 15N in PON from a warm-core ring , 1985 .

[23]  C. McClain,et al.  Phytoplankton standing crops within an Antarctic ice edge assessed by satellite remote sensing , 1988 .

[24]  A. Orsi,et al.  On the meridional extent and fronts of the Antarctic Circumpolar Current , 1995 .

[25]  E. Boyle,et al.  Last Glacial Maximum paleochemistry and deepwater circulation in the Southern Ocean: Evidence from foraminiferal cadmium , 1997 .

[26]  R. Fairbanks,et al.  Glacial to Interglacial Changes in the Isotopic Gradients of Southern Ocean Surface Water , 1990 .

[27]  G. Pickard.  Some Physical Oceanographic Features of Inlets of Chile , 1971 .

[28]  W. Howard,et al.  Late Quaternary CaCO3 production and preservation in the Southern Ocean: Implications for oceanic and atmospheric carbon cycling , 1994 .

[29]  M. Suter,et al.  Increased biological productivity and export production in the glacial Southern Ocean , 1995, Nature.

[30]  J. Toggweiler,et al.  A new model for the role of the oceans in determining atmospheric PCO2 , 1984, Nature.

[31]  E. Wada,et al.  The dynamic changes of stable isotopic ratios of carbon and nitrogen in suspended and sedimented particulate organic matter during a phytoplankton bloom , 1992 .

[32]  W. Brand,et al.  Automated analysis system for coupled .delta.13C and .delta.15N measurements , 1992 .

[33]  S. Fitzwater,et al.  Iron deficiency limits phytoplankton growth in Antarctic waters , 1990 .

[34]  C. S. Wong,et al.  Nitrogen isotope variations in the subarctic northeast Pacific: relationships to nitrate utilization and trophic structure , 1997 .

[35]  F. Chavez,et al.  The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific , 1999 .

[36]  M. McElroy,et al.  Changes in atmospheric CO2: Influence of the marine biota at high latitude , 1984 .

[37]  P. Tréguer,et al.  Dynamics of nutrients and phytoplankton, and fluxes of carbon, nitrogen and silicon in the Antarctic Ocean* , 1992 .

[38]  J. Cullen.  Hypotheses to explain high-nutrient conditions in the open sea , 1991 .

[39]  R. Michener,et al.  Natural abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method , 1997 .

[40]  G. Jacques.  Primary production in the open Antarctic Ocean during the Austral summer. A review , 1989 .

[41]  M. Brzezinski,et al.  Fractionation of silicon isotopes by marine diatoms during biogenic silica formation , 1997 .

[42]  P. Schlosser,et al.  Helium-3 balance of the upper layers of the northwestern Weddell sea , 1987 .

[43]  Andrew J. Watson,et al.  Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment , 1993, Nature.

[44]  Walker O. Smith,et al.  9 – Polar Phytoplankton , 1990 .

[45]  M. J. Deniro,et al.  Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals , 1978 .

[46]  E. Yu.  Variations in the particulate flux of 23TÌh and 231Pa and paleoceanograhpic applications of the 231Pa/231Th ratio , 1994 .

[47]  T. Whitworth,et al.  Water masses and currents of the Southern Ocean at the Greenwich Meridian , 1987 .

[48]  S. Macko,et al.  Isotopic Evidence for Reduced Productivity in the Glacial Southern Ocean , 1993, Science.

[49]  J. McCarthy,et al.  Isotopic fractionation during nitrate uptake by phytoplankton grown in continuous culture , 1995 .

[50]  M. Altabet.  Variations in nitrogen isotopic composition between sinking and suspended particles: implications for nitrogen cycling and particle transformation in the open ocean , 1988 .

[51]  H. Elderfield,et al.  Planktonic foraminiferal Cd/Ca: Paleonutrients or paleotemperature? , 1999 .

[52]  W. Eitaro,et al.  Nitrogen isotope effects in the assimilation of inorganic nitrogenous compounds by marine diatoms , 1978 .

[53]  A. Gargett.  Physical processes and the maintenance of nutrient‐rich euphotic zones , 1991 .

[54]  J. Montoya.  Nitrogen Isotope Fractionation in the Modern Ocean: Implications for the Sedimentary Record , 1994 .

[55]  R. Anderson,et al.  231Pa/230Th ratios in sediments as a proxy for past changes in Southern Ocean productivity , 1993, Nature.

[56]  M. Altabet,et al.  Glacial to interglacial changes in surface nitrate utilization in the Indian Sector of the Southern Ocean as recorded by sediment δ15N , 1992 .

[57]  K. Arrigo,et al.  A simulated Antarctic ast ice ecosystem , 1993 .

[58]  M. Altabet,et al.  Seasonal and depth-related changes in the source of sinking particles in the North Atlantic , 1991, Nature.

[59]  E. Yu.  Variations in the particulate flux of Th-230 and Pa-231 and paleoceanograhpic applications of the Pa-231/Th-231 ratio , 1994 .

[60]  J. Goering,et al.  Seasonal variability of stable carbon and nitrogen isotope ratios of organisms in a North Pacific Bay , 1990 .

[61]  P. Harrison,et al.  Nitrogen isotope fractionation during the uptake and assimilation of nitrate, nitrite, ammonium, and urea by a marine diatom , 1998 .

[62]  R. Braman,et al.  Nanogram nitrite and nitrate determination in environmental and biological materials by vanadium (III) reduction with chemiluminescence detection. , 1989, Analytical chemistry.

[63]  G. Rau,et al.  δ13C and δ15N variations in Weddell Sea particulate organic matter , 1991 .

[64]  A. Singer,et al.  Climatically linked carbon isotope variation during the past 430,000 years in Southern Ocean sediments , 1995 .

[65]  M. Altabet,et al.  Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization , 1994 .

[66]  Kon‐Kee Liu,et al.  The eastern tropical Pacific as a source of 15N‐enriched nitrate in seawater off southern California , 1989 .

[67]  Carolyn A. Miller,et al.  Nitrogen isotope fractionation by oceanic zooplankton , 1989 .

[68]  M. Altabet,et al.  The Use of Nitrogen Isotopic Ratio for Reconstruction of Past Changes in Surface Ocean Nutrient Utilization , 1994 .

[69]  D. Sigman,et al.  Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period , 1997, Nature.