HR: 09:00h
AN: H51E-04    [Abstracts]
TI: The Nitrogen Isotopes of Nitrate and Total Nitrogen Between Bermuda and Puerto Rico
AU: * Knapp, A N
EM: aknapp@princeton.edu
AF: Princeton University, Dept. of Geosciences / Guyot Hall, Princeton, NJ 08544 United States
AU: Sigman, D M
EM: sigman@princeton.edu
AF: Princeton University, Dept. of Geosciences / Guyot Hall, Princeton, NJ 08544 United States
AU: Lipschultz, F
EM: fred@bbsr.edu
AF: Bermuda Biological Station for Research, Ferry Reach, St. Georges, GE01 Bermuda
AB: In samples collected along a meridional transect between Bermuda and Puerto Rico in October 2002, nitrogen isotope (d15N) and concentration measurements were made for nitrate and total nitrogen (TN, which is almost entirely dissolved organic nitrogen, or DON, in the upper 100 m). At each station, nitrate d15N decreased from about 5 per mil at 800 m depth to a minimum of approximately 2 per mil at 200 m depth. The minimum in nitrate d15N at 200 m is presumably due to the input of a low-d15N nitrogen source, either locally or elsewhere in the basin. Between 200 and 100 m, nitrate d15N increased to as much as 6 per mil, most likely because of the assimilation of nitrate by phytoplankton. TN concentrations and d15N are relatively uniform in the upper 100 m along the transect, at approximately 5 uM and 4 per mil (vs. N2 in air), respectively, which are similar to the average annual values for the Bermuda Atlantic Time-Series Study (BATS) station. A slight decrease in TN concentration and increase in TN d15N was observed between the upper 100 m and approximately 300 m, which, in conjunction with mixing, may drive a small but potentially significant flux of N from the surface to subsurface. No meridional gradients in the nitrate or TN data are apparent. This lack of north-south change is in contrast to the presence of apparent meridional gradients in parameters such as winter mixed layer depth, iron deposition and nitrogen fixation rate, which might be expected to affect the concentration and isotopic composition of both TN and nitrate. This disconnect suggests that local nitrogen fixation inputs to the TN and nitrate pool are slow relative to the subtropical circulation. This view is qualitatively consistent with our previous observation of a lack of seasonal variation in TN d15N or concentration at the BATS site.
DE: 9325 Atlantic Ocean
DE: 4805 Biogeochemical cycles (1615)
DE: 4845 Nutrients and nutrient cycling
DE: 4870 Stable isotopes
DE: 1050 Marine geochemistry (4835, 4850)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting